Patent Publication Number: US-2021163698-A1

Title: Flexible, porous, dissolvable solid sheet articles having large pores and method of making same

Description:
FIELD OF THE INVENTION 
     The present invention relates to flexible, porous, dissolvable solid sheet articles having large pores on their top surfaces as well as a method of making the same. 
     BACKGROUND OF THE INVENTION 
     Flexible dissolvable sheets comprising surfactant(s) and/or other active ingredients in a water-soluble polymeric carrier or matrix are well known. Such sheets are particularly useful for delivering surfactants and/or other active ingredients upon dissolution in water. In comparison with traditional granular or liquid forms in the same product category, such sheets have better structural integrity, are more concentrated and easier to store, ship/transport, carry, and handle. In comparison with the solid tablet form in the same product category, such sheets are more flexible and less brittle, with better sensory appeal to the consumers. 
     In order to deliver a sufficient amount of surfactant(s) and/or other active ingredients to achieve the desired product function, it is desirable to use multiple layers of such flexible and dissolvable sheets, and it is further desirable to assemble such multiple layers into a unitary dissolvable solid article, which can then be sold as a unitary finished product. However, various challenges may be encountered when trying to assemble multiple layers of these flexible and dissolvable sheets into a unitary article, including significantly slower dissolution rate in water, in comparison with a single layer structure. 
     To improve dissolution, some studies has developed porous sheets with open-celled foam (OCF) structures characterized by a Percent Open Cell Content of from about 80% to 100%. Particularly, WO2010077627 discloses a batch process for forming such porous sheets with OCF structures that comprises vigorously aerating a pre-mixture of raw materials and then allowing the aerated pre-mixture to be heat-dried in batches (e.g., in a convection oven or a microwave oven) to form the porous sheets with the desired OCF structures. WO2012138820 discloses a similar process as that of WO2010077627, except that continuous drying of the aerated wet pre-mixture is achieved by using, e.g., an impingement oven (instead of a convection oven or a microwave oven). Although such OCF structures in these studies significantly improve the dissolution rate of the resulting porous sheets, there is still a visibly denser and less porous region (i.e., at the top surface) with thicker cell walls in such sheets. Such high-density region may negatively impact the flow of water through the sheets and thereby may adversely affect the overall dissolution rate of the sheets. 
     There is therefore a continuing need for improving pore structures in flexible, porous, dissolvable sheets and enhancing dissolution profile thereof. 
     SUMMARY OF THE INVENTION 
     The present invention provides a flexible, porous, dissolvable sheet having a further improved pore structure, especially at the top surface as well as a method for making such sheet. Particularly, prior to the present invention, it was believed that air bubbles in the aerated pre-mixture might gradually collapse over time, and thereby long-term storage of the aerated pre-mixture might adversely affect the pore structures in the sheets as well as dissolution profile of the sheets. As such, it was suggested that the aerated pre-mixture was immediately dried after the aerating step. Surprisingly, inventors of the present invention have unexpectedly discovered that the introduction of an aging step (i.e., maintaining the aerated pre-mixture for a while after stopping the aerating) before the drying step may bring about a significantly improved pore structures and thereby a significantly improved dissolution profile. As such, the present inventors have successfully prepared a flexible, porous, dissolvable sheet having a further improved pore structure that has not been obtained prior to the present invention. 
     In one aspect, the present invention relates to a process for making a sheet article, comprising the steps of: a) preparing a wet pre-mixture comprising a water-soluble polymer and a surfactant and having a viscosity of from about 1,000 cps to about 25,000 cps measured at 40° C. and 1 s −1 ; b) aerating the wet pre-mixture to form an aerated wet pre-mixture having a density of from about 0.05 to about 0.5 g/ml; c) aging the aerated wet pre-mixture for at least about 5 minute; d) forming the aerated wet pre-mixture into a sheet having opposing first and second sides; and e) drying the formed sheet for a drying time of from about 1 minute to about 60 minutes to make the sheet article. Preferably, the step c) may be conducted for a duration from about 5 min to about 300 min, preferably from about 5 min to about 200 min, more preferably from about 10 min to about 150 min, for example 5 min, 6 min, 7 min, 8 min, 9 min, 10 min, 15 min, 20 min, 25 min, 30 min, 35 min, 40 min, 45 min, 50 min, 55 min, 60 min, 70 min, 80 min, 90 min, 100 min, 110 min, 120 min, 130 min, 140 min or any ranges therebetween. Preferably, the drying in the step e) may be conducted at a temperature from about 70° C. to about 200° C. along a heating direction that forms a temperature gradient decreasing from the first side to the second side of the formed sheet, wherein the heating direction is substantially opposite to the gravitational direction for more than half of the drying time. 
     The sheet formed by the aerated wet pre-mixture may be dried on a heated surface that preferably has a controlled surface temperature of from about 70° C. to about 170° C., preferably from about 75° C. to about 150° C., more preferably from about 80° C. to about 120° C. Preferably, the heated surface may be a primary heat source for the sheet during drying. More preferably, the heated surface may be the only heat source for the sheet during drying. 
     Particularly, the heated surface may be the outer surface of a rotary drum dryer that preferably has an outer diameter ranging from about 0.5 meters to about 10 meters, preferably from about 1 meter to about 5 meters, more preferably from about 1.5 meters to about 2 meters and is rotated at a speed of from about 0.005 rpm to about 0.25 rpm, preferably from about 0.05 rpm to about 0.2 rpm, more preferably from about 0.1 rpm to about 0.18 rpm, during the drying step. Alternatively, the heated surface may be the outer surface of a heated moving belt (for example, a conveyor belt) that is preferably moving at a speed of from about 0.1 m/min to about 50 m/min, preferably from about 0.15 m/min to about 20 m/min, more preferably from about 0.2 m/min to about 10 m/min, for example, 0.1 m/min, 0.2 m/min, 0.3 m/min, 0.4 m/min, 0.5 m/min, 0.7 m/min, 1 m/min, 2 m/min, 3 m/min, 5 m/min, 10 m/min, 15 m/min, 20 m/min, or any ranges therebetween, during the drying step. 
     Further, in the step d), the sheet may be formed by using a spinning bar that is rotating at a speed of from about 5 to about 80 rpm, preferably from about 6 to about 60 rpm, more preferably from about 8 to about 50 rpm, most preferably from about 10 to about 40 rpm. Preferably, the spinning bar may be positioned so that the distance between the spinning bar and the outer surface of the rotary drum or the heated moving belt is from about 3 mm to about 15 mm, preferably from about 4 mm to about 12 mm, more preferably from about 5 mm to about 10 mm, most preferably from about 6 mm to about 10 mm. Alternatively, in the step d), the sheet may be formed by a feeding die having a feeding speed of from about 0.1 m/min to about 50 m/min, preferably from about 0.15 m/min to about 20 m/min, more preferably from about 0.2 m/min to about 10 m/min, for example, 0.1 m/min, 0.2 m/min, 0.3 m/min, 0.4 m/min, 0.5 m/min, 0.7 m/min, 1 m/min, 2 m/min, 3 m/min, 5 m/min, 10 m/min, 15 m/min, 20 m/min, or any ranges therebetween. Preferbaly, the feeding die may have a feeding thickness of from 0.5 mm to 10 mm, preferably from 1 mm to 6 mm, more preferably from 1.5 mm to 4 mm. Preferably, the feeding die may be positioned so that the distance between the feeding die and the outer surface of the rotary drum or the heated moving belt is from about 0.1 mm to about 15 mm, preferably from about 0.2 mm to about 12 mm, more preferably from about 0.3 mm to about 10 mm, most preferably from about 0.5 mm to about 5 mm. 
     Still further, the wet pre-mixture may be characterized by: (1) a solid content ranging from about 15% to about 70%, preferably from about 20% to about 50%, more preferably from about 25% to about 45% by weight of the wet pre-mixture; and (2) a viscosity ranging from about 3,000 cps to about 24,000 cps, preferably from about 5,000 cps to about 23,000 cps, more preferably from about 10,000 cps to about 20,000 cps as measured at 40° C. and 1 s −1 . 
     Still further, the wet pre-mixture may be preheated to a temperature of from about 40° C. to about 100° C., preferably from about 50° C. to about 95° C., more preferably from about 60° C. to about 90° C., most preferably from about 75° C. to about 85° C., before aeration; and/or the wet pre-mixture may be maintained at a temperature of from about 40° C. to about 100° C., preferably from about 50° C. to about 95° C., more preferably from about 60° C. to about 90° C., most preferably from about 75° C. to about 85° C., during aeration; and/or the aerated wet pre-mixture may be maintained at a temperature of from about 10° C. to about 100° C., preferably from about 15° C. to about 70° C., more preferably from about 20° C. to about 50° C., most preferably from about 20° C. to about 40° C. in the step c). 
     Still further, the drying time may be from about 2 to about 30 minutes, preferably from about 2 to about 25 minutes, more preferably from about 2 to about 20 minutes, most preferably from about 2 to about 15 minutes; and/or wherein the drying temperature is from about 80° C. to about 170° C., preferably from about 90° C. to about 150° C., more preferably from about 100° C. to about 140° C.; and the heating direction may be substantially opposite to the gravitational direction for more than about 55%, preferably more than about 60%, more preferably more than about 75% of the drying time. 
     Still further, the aerating in the step b) may be accomplished by introducing a gas into the wet pre-mixture by using a mechanical processing means, including but not limited to: a rotor stator mixer, a planetary mixer, a pressurized mixer, a non-pressurized mixer, a batch mixer, a continuous mixer, a semi-continuous mixer, a high shear mixer, a low shear mixer, a submerged sparger, or any combinations thereof. 
     In another aspect, the present invention provides a flexible, porous, dissolvable solid sheet article comprising a water-soluble polymer and a surfactant, wherein the solid sheet article is characterized by: (i) a thickness ranging from 0.5 mm to 4 mm; and (ii) a Percent Open Cell Content of from 80% to 100%; and (iii) an Overall Average Pore Size of from 100 μm to 2000 μm; wherein the solid sheet article has opposing top and bottom surfaces, the top surface having a Surface Average Pore Diameter that is greater than about 300 μm. Particularly, the top surface may have a Surface Average Pore Diameter that is from about 300 μm to about 2 mm, preferably from about 350 μm to about 1.5 mm, more preferably from about 400 μm to about 1 mm. 
     The solid sheet article may comprise a top region adjacent to the top surface, a bottom region adjacent to the bottom surface, and a middle region therebetween; wherein the top, middle, and bottom regions have the same thickness, and each of the top, middle and bottom regions is characterized by an Average Pore Size. Particularly, the ratio of Average Pore Size in the bottom region over that in the top region may be from about 0.6 to about 1.5, preferably from about 0.7 to about 1.4, more preferably from about 0.8 to about 1.3, most preferably from about 1 to about 1.2; and/or the ratio of Average Pore Size in the bottom region over that in the middle region may be from about 0.5 to about 1.5, preferably from about 0.6 to about 1.3, more preferably from about 0.8 to about 1.2, most preferably from about 0.9 to about 1.1; and/or the ratio of Average Pore Size in the middle region over that in the top region may be from about 1 to about 1.5, preferably from about 1 to about 1.4, more preferably from about 1 to about 1.2. 
     The solid sheet article may comprise from about 5% to about 40%, preferably from about 8% to about 30%, more preferably from about 10% to about 25%, of the water-soluble polymer by total weight of the solid sheet article. Preferably, the water-soluble polymer may have a weight average molecular weight of from about 5,000 to about 400,000 Daltons, more preferably from about 10,000 to about 300,000 Daltons, still more preferably from about 15,000 to about 200,000 Daltons, most preferably from about 20,000 to about 150,000 Daltons. More preferably, the water-soluble polymer may comprise a first water-soluble polymer having a first weight average molecular weight and a second water-soluble polymer having a second weight average molecular weight, in which the first weight average molecular weight may be from about 5,000 to about 50,000 Daltons, more preferably from about 10,000 to about 40,000 Daltons, still more preferably from about 15,000 to about 35,000 Daltons, most preferably from about 20,000 to about 30,000 Daltons; and/or the second weight average molecular weight may be from about 20,000 to about 400,000 Daltons, more preferably from about 30,000 to about 300,000 Daltons, still more preferably from about 40,000 to about 200,000 Daltons, most preferably from about 50,000 to about 150,000 Daltons. Preferably, the water-soluble polymer may be a polyvinyl alcohol characterized by a degree of hydrolysis ranging from about 40% to about 100%, preferably from about 50% to about 95%, more preferably from about 65% to about 92%, most preferably from about 70% to about 90%. 
     The solid sheet article may comprise from 5% to 80%, preferably from 10% to 70%, more preferably from 30% to 65%, of the surfactant by total weight of the solid sheet article. Preferably, the surfactant may be selected from the group consisting of: anionic surfactants, non-ionic surfactants, cationic surfactants, zwitterionic surfactants, amphoteric surfactants, polymeric surfactants and any combinations thereof. 
     Further, the solid sheet article may further comprise from 0.1% to 25%, preferably from 0.5% to 20%, more preferably from 1% to 15%, most preferably from 2% to 12%, of a plasticizer by total weight of the solid sheet article. Preferably, the plasticizer may be selected from the group consisting of glycerin, ethylene glycol, polyethylene glycol, propylene glycol, and combinations thereof. More preferably, the plasticizer may be glycerin. 
     Still further, the solid sheet article may contain one or more additional ingredients, such as fabric care actives, dishwashing actives, hard surface cleaning actives, beauty and/or skin care actives, personal cleansing actives, hair care actives, oral care actives, feminine care actives, baby care actives, and any combinations thereof. 
     The flexible, porous, dissolvable solid sheet article of the present invention may further be characterized by:
         a Percent Open Cell Content of from about 85% to 100%, preferably from about 90% to 100%; and/or   an Overall Average Pore Size of from about 150 μm to about 1000 μm, preferably from about 200 μm to about 600 μm; and/or   an Average Cell Wall Thickness of from about 5 μm to about 200 μm, preferably from about 10 μm to about 100 μm, more preferably from about 10 μm to about 80 μm; and/or   a final moisture content of from about 0.5% to about 25%, preferably from about 1% to about 20%, more preferably from about 3% to about 10%, by weight of the solid sheet article; and/or   a thickness ranging from about 0.6 mm to about 3.5 mm, preferably from about 0.7 mm to about 3 mm, more preferably from about 0.8 mm to about 2 mm, most preferably from about 1 mm to about 1.5 mm; and/or   a basis weight of from about 50 grams/m 2  to about 250 grams/m 2 , preferably from about 80 grams/m 2  to about 220 grams/m 2 , more preferably from about 100 grams/m 2  to about 200 grams/m 2 ; and/or   a density of from about 0.05 grams/cm 3  to about 0.5 grams/cm 3 , preferably from about 0.06 grams/cm 3  to about 0.4 grams/cm 3 , more preferably from about 0.07 grams/cm 3  to about 0.2 grams/cm 3 , most preferably from about 0.08 grams/cm 3  to about 0.15 grams/cm 3 ; and/or   a Specific Surface Area of from about 0.03 m 2 /g to about 0.25 m 2 /g, preferably from about 0.04 m 2 /g to 0.22 m 2 /g, more preferably from about 0.05 m 2 /g to about 0.2 m 2 /g, most preferably from about 0.1 m 2 /g to about 0.18 m 2 /g.       

     These and other aspects of the present invention will become more apparent upon reading the following detailed description of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a prior art convection-based heating/drying arrangement for making a flexible, porous, dissolvable solid sheet article in a batch process. 
         FIG. 2  shows a prior art microwave-based heating/drying arrangement for making a flexible, porous, dissolvable solid sheet article in a batch process. 
         FIG. 3  shows a prior art impingement oven-based heating/drying arrangement for making a flexible, porous dissolvable solid sheet article in a continuous process. 
         FIG. 4  shows a bottom conduction-based heating/drying arrangement for making an flexible, porous, dissolvable sheet in a batch process, according to one embodiment of the present invention. 
         FIG. 5  shows a rotary drum-based heating/drying arrangement for making another flexible, porous, dissolvable sheet in a continuous process, according to another embodiment of the present invention. 
         FIG. 6A  shows a Scanning Electron Microscopic (SEM) image of the top surface of a flexible, porous, dissolvable sheet containing fabric care actives, which is made by a process employing a rotary drum-based heating/drying arrangement.  FIG. 6B  shows a SEM image of the top surface of a flexible, porous, dissolvable sheet containing the same fabric care actives as the sheet shown in  FIG. 6A , but which is made by a process employing an impingement oven-based heating/drying arrangement. 
         FIG. 7A  shows a SEM image of the top surface of a flexible, porous, dissolvable sheet containing hair care actives, which is made by a process employing a bottom conduction-based heating/drying arrangement.  FIG. 7B  shows a SEM image of the top surface of a flexible, porous, dissolvable sheet containing the same hair care actives as the sheet shown in  FIG. 7A , but which is made by a process employing an impingement oven-based heating/drying arrangement. 
         FIG. 8A  shows a photo of bubbles in the wet pre-mixture after a 70-min aging step.  FIG. 8B  shows a photo of bubbles in the wet pre-mixture before the 70-min aging step (i.e., immediately after an aeration step). 
         FIG. 9A  shows a photo of bubbles in the wet pre-mixture after a 120-min aging step in a drum drying process.  FIG. 9B  shows a photo of bubbles in the wet pre-mixture having the same formulation as that shown in  FIG. 9A  without an aging step after aeration in a drum drying process. 
         FIG. 10A  shows a SEM image of the top surface of Article 1 (an inventive flexible, porous, dissolvable sheet article having large pores on its top surface) in Example 3.  FIG. 10B  shows a SEM image of the top surface of Article 2 (a comparative flexible, porous, dissolvable sheet article having relatively small pores on its top surface) in Example 3. 
         FIG. 11  shows the dissolution profiles over time in the dissolution test for Articles 1 and 2 in Example 3, in which Article 1 shows significantly improved dissolution profile compared to Article 2. 
         FIG. 12A  shows a SEM image of the top surface of Article 3 (an inventive flexible, porous, dissolvable sheet article having large pores on its top surface) in Example 4.  FIG. 12B  shows a SEM image of the top surface of Article 4 (a comparative flexible, porous, dissolvable sheet article having relatively small pores on its top surface) in Example 4. 
         FIG. 13A  shows a SEM image of the top surface of Article 5 (an inventive flexible, porous, dissolvable sheet article having large pores on its top surface) in Example 5.  FIG. 13B  shows a SEM image of the top surface of Article 6 (a comparative flexible, porous, dissolvable sheet article having relatively small pores on its top surface) in Example 5. 
         FIG. 14  shows the dissolution profiles over time in the dissolution test for Articles 5 and 6 in Example 5, in which Article 5 shows significantly improved dissolution profile compared to Article 6. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     I. Definitions 
     The term “flexible” as used herein refers to the ability of an article to withstand stress without breakage or significant fracture when it is bent at 90° along a center line perpendicular to its longitudinal direction. Preferably, such article can undergo significant elastic deformation and is characterized by a Young&#39;s Modulus of no more than 5 GPa, preferably no more than 1 GPa, more preferably no more than 0.5 GPa, most preferably no more than 0.2 GPa. 
     The term “dissolvable” as used herein refers to the ability of an article to completely or substantially dissolve in a sufficient amount of deionized water at 20° C. and under the atmospheric pressure within eight (8) hours without any stirring, leaving less than 5 wt % undissolved residues. 
     The term “solid” as used herein refers to the ability of an article to substantially retain its shape (i.e., without any visible change in its shape) at 20° C. and under the atmospheric pressure, when it is not confined and when no external force is applied thereto. 
     The term “sheet” as used herein refers to a non-fibrous structure having a three-dimensional shape, i.e., with a thickness, a length, and a width, while the length-to-thickness aspect ratio and the width-to-thickness aspect ratio are both at least about 5:1, and the length-to-width ratio is at least about 1:1. Preferably, the length-to-thickness aspect ratio and the width-to-thickness aspect ratio are both at least about 10:1, more preferably at least about 15:1, most preferably at least about 20:1; and the length-to-width aspect ratio is preferably at least about 1.2:1, more preferably at least about 1.5:1, most preferably at least about 1.618:1. 
     As used herein, the term “bottom surface” refers to a surface of the flexible, porous, dissolvable solid sheet article of the present invention that is immediately contacting a supporting surface upon which the sheet of aerated wet pre-mixture is placed during the drying step, while the term “top surface” refers to a surface of the sheet article that is opposite to the bottom surface. Further, such solid sheet article can be divided into three (3) regions along its thickness, including a top region that is adjacent to its top surface, a bottom region that is adjacent to its bottom surface, and a middle region that is located between the top and bottom regions. The top, middle, and bottom regions are of equal thickness, i.e., each having a thickness that is about ⅓ of the total thickness of the sheet article. 
     The term “open celled foam” or “open cell pore structure” as used herein refers to a solid, interconnected, polymer-containing matrix that defines a network of spaces or cells that contain a gas, typically a gas (such as air), without collapse of the foam structure during the drying process, thereby maintaining the physical strength and cohesiveness of the solid. The interconnectivity of the structure may be described by a Percent Open Cell Content, which is measured by Test 3 disclosed hereinafter. 
     The term “water-soluble” as used herein refers to the ability of a sample material to completely dissolve in or disperse into water leaving no visible solids or forming no visibly separate phase, when at least about 25 grams, preferably at least about 50 grams, more preferably at least about 100 grams, most preferably at least about 200 grams, of such material is placed in one liter (1 L) of deionized water at 20° C. and under the atmospheric pressure with sufficient stirring. 
     The term “aerate”, “aerating” or “aeration” as used herein refers to a process of introducing a gas into a liquid or pasty composition by mechanical and/or chemical means. 
     The term “heating direction” as used herein refers to the direction along which a heat source applies thermal energy to an article, which results in a temperature gradient in such article that decreases from one side of such article to the other side. For example, if a heat source located at one side of the article applies thermal energy to the article to generate a temperature gradient that decreases from the one side to an opposing side, the heating direction is then deemed as extending from the one side to the opposing side. If both sides of such article, or different sections of such article, are heated simultaneously with no observable temperature gradient across such article, then the heating is carried out in a non-directional manner, and there is no heating direction. 
     The term “substantially opposite to” or “substantially offset from” as used herein refers to two directions or two lines having an offset angle of 90° or more therebetween. 
     The term “substantially aligned” or “substantial alignment” as used herein refers to two directions or two lines having an offset angle of less than 90° therebetween. 
     The term “primary heat source” as used herein refers to a heat source that provides more than 50%, preferably more than 60%, more preferably more than 70%, most preferably more than 80%, of the total thermal energy absorbed by an object (e.g., the sheet of aerated wet pre-mixture according to the present invention). 
     The term “controlled surface temperature” as used herein refers to a surface temperature that is relatively consistent, i.e., with less than +/−20% fluctuations, preferably less than +/−10% fluctuations, more preferably less than +/−5% fluctuations. 
     The term “age” or “aging” as used herein refers to a process of maintaining an aerated wet mixture or pre-mixture for a while without further introducing a significant amount of gas. Preferably, the aging may be conducted under the conditions of being essentially free of mechanical energy input and/or being essentially free of heat input. More preferably, the aging may be conducted under the ambient temperature without any stirring. 
     The term “essentially free of” or “essentially free from” means that the indicated material is at the very minimal not deliberately added to the composition or product, or preferably not present at an analytically detectible level in such composition or product. It may include compositions or products in which the indicated material is present only as an impurity of one or more of the materials deliberately added to such compositions or products. 
     II. Overview of Processes for Making Solid Sheet Articles 
     WO2010077627 discloses a batch process for forming porous sheets with open-celled foam (OCF) structures characterized by a Percent Open Cell Content of from about 80% to 100%, which functions to improve dissolution. Specifically, a pre-mixture of raw materials is first formed, which is vigorously aerated and then heat-dried in batches (e.g., in a convection oven or a microwave oven) to form the porous sheets with the desired OCF structures. Although such OCF structures significantly improve the dissolution rate of the resulting porous sheets, there is still a visibly denser and less porous bottom region with thicker cell walls in such sheets. Such high-density bottom region may negatively impact the flow of water through the sheets and thereby may adversely affect the overall dissolution rate of the sheets. When a plurality of such sheets is stacked together to form a multilayer structure, the “barrier” effect of multiple high-density bottom regions is especially augmented. 
     WO2012138820 discloses a similar process as that of WO2010077627, except that continuous drying of the aerated wet pre-mixture is achieved by using, e.g., an impingement oven (instead of a convection oven or a microwave oven). The OCF sheets formed by such a continuous drying process are characterized by improved uniformity/consistency in the pore structures across different regions thereof. Unfortunately, there are still rate-limiting factors in such OCF sheets, such as a top surface with relatively smaller pore openings and a top region with relatively smaller pores (i.e., a crust-like top region), which may negatively impact the flow of water therethrough and slow down the dissolution thereof. 
     During the drying step in the above-described processes, the OCF structures are formed under simultaneous mechanisms of water evaporation, bubble collapse, interstitial liquid drainage from the thin film bubble facings into the plateau borders between the bubbles (which generates openings between the bubbles and forms the open cells), and solidification of the pre-mixture. Various processing conditions may influence these mechanisms, e.g., solid content in the wet pre-mixture, viscosity of the wet pre-mixture, gravity, and the drying temperature, and the need to balance such processing conditions so as to achieve controlled drainage and form the desired OCF structures. 
     It has been a surprising and unexpected discovery of the present invention that the direction of thermal energy employed (i.e., the heating direction) during the drying step may also have a significant impact on the resulting OCF structures, in addition to the above-mentioned processing conditions. 
     For example, if the thermal energy is applied in a non-directional matter (i.e., there is no clear heating direction) during the drying step, or if the heating direction is substantially aligned with the gravitational direction (i.e., with an offset angle of less than 90° in between) during most of the drying step, the resulting flexible, porous, dissolvable solid sheet tends to have a top surface with smaller pore openings and greater pore size variations in different regions along the direction across its thickness. In contrast, when the heating direction is offset from the gravitation direction (i.e., with an offset angle of 90° or more therebetween) during most of the drying step, the resulting solid sheet may have a top surface with larger pore openings and reduced pore size variations in different regions along the direction across the thickness of such sheet. Correspondingly, the latter sheets are more receptive to water flowing through and are therefore more dissolvable than the former sheets. 
     While not being bound by any theory, it is believed that the alignment or misalignment between the heating direction and the gravitational direction during the drying step and the duration thereof may significantly affect the interstitial liquid drainage between the bubbles, and correspondingly impacting the pore expansion and pore opening in the solidifying pre-mixture and resulting in solid sheets with very different OCF structures. Such differences are illustrated more clearly by  FIGS. 1-4  hereinafter. 
       FIG. 1  shows a convection-based heating/drying arrangement. During the drying step, a mold  10  (which can be made of any suitable materials, such as metal, ceramic or Teflon®) is filled with an aerated wet pre-mixture, which forms a sheet  12  having a first side  12 A (i.e., the top side) and an opposing second side  12 B (i.e., the bottom side since it is in direct contact with a supporting surface of the mold  10 ). Such mold  10  is placed in a 130° C. convection oven for approximately 45-46 minutes during the drying step. The convection oven heats the sheet  12  from above, i.e., along a downward heating direction (as shown by the cross-hatched arrowhead), which forms a temperature gradient in the sheet  12  that decreases from the first side  12 A to the opposing second side  12 B. The downward heating direction is aligned with gravitational direction (as shown by the white arrowhead), and such an aligned position is maintained throughout the entire drying time. During drying, gravity drains the liquid pre-mixture downward toward the bottom region, while the downward heating direction dries the top region first and the bottom region last. As a result, a porous solid sheet is formed with a top surface that contains numerous pores with small openings formed by gas bubbles that have not had the chance to fully expand. Such a top surface with smaller pore openings is not optimal for water ingress into the sheet, which may limit the dissolution rate of the sheet. On the other hand, the bottom region of such sheet is dense and less porous, with larger pores that are formed by fully expanded gas bubbles, but which are very few in numbers, and the cell walls between the pores in such bottom region are thick due to the downward liquid drainage effectuated by gravity. Such a dense bottom region with fewer pores and thick cell walls is a further rate-limiting factor for the overall dissolution rate of the sheet. 
       FIG. 2  shows a microwave-based heating/drying arrangement. During the drying step, a mold  30  is filled with an aerated wet pre-mixture, which forms a sheet  32  having a first side  32 A (the top side) and an opposing second side  32 B (the bottom side). Such mold  30  is then placed in a low energy density microwave applicator (not shown), which is provided by Industrial Microwave System Inc., North Carolina and operated at a power of 2.0 kW, a belt speed of 1 foot per minute and a surrounding air temperature of 54.4° C. The mold  30  is placed in such microwave application for approximately 12 minutes during the drying step. Such microwave applicator heats the sheet  32  from within, without any clear or consistent heating direction. Correspondingly, no temperature gradient is formed in the sheet  32 . During drying, the entire sheet  32  is simultaneously heated, or nearly simultaneously heated, although gravity (as shown by the white arrowhead) still drains the liquid pre-mixture downward toward the bottom region. As a result, the solidified sheet so formed has more uniformly distributed and more evenly sized pores, in comparison with sheet formed by the convection-based heating/drying arrangement. However, the liquid drainage under gravity force during the microwave-based drying step may still result in a dense bottom region with thick cell walls. Further, simultaneous heating of the entire sheet  32  may still limit the pore expansion and pore opening on the top surface during the drying step, and the resulting sheet may still have a top surface with relatively smaller pore openings. Further, the microwave energy heats water within the sheet  32  and causes such water to boil, which may generate bubbles of irregular sizes and form unintended dense regions with thick cell walls. 
       FIG. 3  shows an impingement oven-based heating/drying arrangement. During the drying step, a mold  40  is filled with an aerated wet pre-mixture, which forms a sheet  42  having a first side  42 A (the top side) and an opposing second side  42 B (the bottom side). Such mold  40  is then placed in a continuous impingement oven (not shown) under conditions similar to those described in Example 1, Table 2 of WO2012138820. Such continuous impingement oven heats the sheet  42  from both top and bottom at opposing and offsetting heating directions (shown by the two cross-hatched arrowheads). Correspondingly, no clear temperature gradient is formed in the sheet  42  during drying, and the entire sheet  42  is nearly simultaneously heated from both its top and bottom surfaces. Similar to the microwave-based heating/drying arrangement described in  FIG. 3 , gravity (as shown by the white arrowhead) continues to drain the liquid pre-mixture downward toward the bottom region in such impingement oven-based heating/drying arrangement of  FIG. 4 . As a result, the solidified sheet so formed has more uniformly distributed and more evenly sized pores, in comparison with sheet formed by the convection-based heating/drying arrangement. However, the liquid drainage under gravity force during the drying step may still result in a dense bottom region with thick cell walls. Further, nearly simultaneous heating of the sheet  42  from both the may still limit the pore expansion and pore opening on the top surface during the drying step, and the resulting sheet may still have a top surface with relatively smaller pore openings. 
     In contrast to the above-described heating/drying arrangements (convection-based, microwave-based or impingement oven-based), the present invention provides a heating/drying arrangement for drying the aerated wet pre-mixture, in which the direction of heating is purposefully configured to counteract/reduce liquid drainage caused by the gravitational force toward the bottom region (thereby reducing the density and improving pore structures in the bottom region) and to allow more time for the air bubbles near the top surface to expand during drying (thereby forming significantly larger pore openings on the top surface of the resulting sheet). Both features function to improve overall dissolution rate of the sheet and are therefore desirable. 
       FIG. 4  shows a bottom conduction-based heating/drying arrangement for making a flexible, porous, dissolvable sheet, according to one embodiment of the present invention. Specifically, a mold  50  is filled with an aerated wet pre-mixture, which forms a sheet  52  having a first side  52 A (i.e., the bottom side) and an opposing second side  52 B (i.e., the top side). Such mold  50  is placed on a heated surface (not shown), for example, on top of a pre-heated Peltier plate with a controlled surface temperature of about 125-130° C., for approximately 30 minutes during the drying step. Heat is conducted from the heated surface at the bottom of the mold  50  through the mold to heat the sheet  52  from below, i.e., along an upward heating direction (as shown by the cross-hatched arrowhead), which forms a temperature gradient in the sheet  52  that decreases from the first side  52 A (the bottom side) to the opposing second side  52 B (the top side). Such an upward heating direction is opposite to the gravitational direction (as shown by the white arrowhead), and it is maintained as so throughout the entire drying time (i.e., the heating direction is opposite to the gravitational direction for almost 100% of the drying time). During drying, the gravitational force still drains the liquid pre-mixture downward toward the bottom region. However, the upward heating direction dries the sheet from bottom up, and water vapor generated by heat at the bottom region arises upward to escape from the solidifying matrix, so the downward liquid drainage toward the bottom region is significantly limited and “counteracted”/reduced by the solidifying matrix and the uprising water vapor. Correspondingly, the bottom region of the resulting dry sheet is less dense and contains numerous pores with relatively thin cell walls. Further, because the top region is the last region that is dried during this process, the air bubbles in the top region have sufficient time to expand to form significantly larger open pores at the top surface of the resulting sheet, which are particularly effective in facilitating water ingress into the sheet. Moreover, the resulting sheet has a more evenly distributed overall pore sizes throughout different regions (e.g., top, middle, bottom) thereof. 
       FIG. 5  shows a rotary drum-based heating/drying arrangement for making a flexible, porous, dissolvable sheet, according to another embodiment of the present invention. Specifically, a feeding trough  60  is filled with an aerated wet pre-mixture  61 . A heated rotatable cylinder  70  (also referred to as a drum dryer) is placed above the feeding trough  60 . The heated drum dryer  70  has a cylindrical heated outer surface characterized by a controlled surface temperature of about 130° C., and it rotates along a clock-wise direction (as shown by the thin curved line with an arrowhead) to pick up the aerated wet pre-mixture  61  from the feeding trough  60 . The aerated wet pre-mixture  61  forms a thin sheet  62  over the cylindrical heated outer surface of the drum dryer  70 , which rotates and dries such sheet  62  of aerated wet pre-mixture in approximately 10-15 minutes. A leveling blade (not shown) may be placed near the slurry pick-up location to ensure a consistent thickness of the sheet  62  so formed, although it is possible to control the thickness of sheet  62  simply by modulating the viscosity of the aerated wet pre-mixture  61  and the rotating speed and surface temperature of the drum dryer  70 . Once dried, the sheet  62  can then picked up, either manually or by a scraper  72  at the end of the drum rotation. 
     As shown in  FIG. 5 , the sheet  62  formed by the aerated wet pre-mixture  61  comprises a first side  62 A (i.e., the bottom side) that directly contacts the heated outer surface of the heated drum dryer  70  and an opposing second side  62 B (i.e., the top side). Correspondingly, heat from the drum dryer  70  is conducted to the sheet  62  along an outward heating direction, to heat the first side  62 A (the bottom side) of the sheet  62  first and then the opposing second side  62 B (the top side). Such outward heating direction forms a temperature gradient in the sheet  62  that decreases from the first side  62 A (the bottom side) to the opposing second side  62 B (the top side). The outward heating direction is slowly and constantly changing as the drum dryer  70  rotates, but along a very clear and predictable path (as shown by the multiple outwardly extending cross-hatched arrowheads in  FIG. 4 ). The relative position of the outward heating direction and the gravitational direction (as shown by the white arrowhead) is also slowing and constantly changing in a similar clear and predictable manner. For less than half of the drying time (i.e., when the heating direction is below the horizontal dashed line), the outward heating direction is substantially aligned with the gravitational direction with an offset angle of less than 90° in between. During majority of the drying time (i.e., when the heating direction is flushed with or above the horizontal dashed line), the outward heating direction is opposite or substantially opposite to the gravitational direction with an offset angle of 90° or more therebetween. Depending on the initial “start” coating position of the sheet  62 , the heating direction can be opposite or substantially opposite to the gravitational direction for more than 55% of the drying time (if the coating starts at the very bottom of the drum dryer  70 ), preferably more than 60% of the drying time (if the coating starts at a higher position of the drum dryer  70 , as shown in  FIG. 5 ). Consequently, during most of the drying step this slowing rotating and changing heating direction in the rotary drum-based heating/drying arrangement can still function to limit and “counteract”/reduce the liquid drainage in sheet  62  caused by the gravitational force, resulting in improved OCF structures in the sheet so formed. The resulting sheet as dried by the heated drum dryer  70  is also characterized by a less dense bottom region with numerous more evenly sized pores, and a top surface with relatively larger pore openings. Moreover, the resulting sheet has a more evenly distributed overall pore sizes throughout different regions (e.g., top, middle, bottom) thereof. 
     In addition to employing the desired heating direction (i.e., in a substantially offset relation with respect to the gravitational direction) as mentioned hereinabove, it may also be desirable and even important to carefully adjust the viscosity and/or solid content of the wet pre-mixture, the amount and speed of aeration (air feed pump speed, mixing head speed, air flow rate, density of the aerated pre-mixture and the like, which may affect bubble sizes and quantities in the aerated pre-mixture and correspondingly impact the pore size/distribution/quantity/characteristics in the solidified sheet), the drying temperature and the drying time, in order to achieve optimal OCF structure in the resulting sheet according to the present invention. 
     Furthermore, it has been a surprising and unexpected discovery of the present invention that the introduction of an aging step (i.e., maintaining the aerated pre-mixture for a while after stopping the aerating) before the drying step may bring about an even further improved pore structures and thereby a further improved dissolution profile. Prior to the present invention, it was believed that air bubbles in the aerated wet pre-mixture might gradually collapse as time passes by, and thereby long-term storage of the aerated wet pre-mixture might adversely affect the pore structures in the sheets as well as dissolution profile of the sheets. As such, it was suggested that the aerated pre-mixture was immediately dried after the aerating step. Surprisingly, inventors of the present invention have unexpectedly discovered that, although the air bubbles in the aerated wet pre-mixture indeed gradually collapse after long-term storage (for example, 6-8 hours), an aging step for an appropriate time period (for example, less than 5 hours) would benefit to the pore structure, especially pores at the top surface. Particularly, the introduction of an aging step before the drying step may provide larger pores at the top surface of the flexible, porous, dissolvable sheet compared to the sheets obtained by a process without such aging step. 
     More detailed descriptions of the processes for making the flexible, porous, dissolvable sheets according to the present invention, as well as the physical and chemical characteristics of such sheets, are provided in the ensuring sections. 
     III. Inventive Process of Making Solid Sheet Articles 
     The present invention provides a new and improved method for making flexible, porous, dissolvable solid sheet articles, which comprises the steps of: (a) forming a pre-mixture containing raw materials (e.g., the water-soluble polymer, active ingredients such as surfactants, and optionally a plasticizer) dissolved or dispersed in water or a suitable solvent, which is characterized by a viscosity of from about 1,000 cps to about 25,000 cps measured at about 40° C. and 1 s −1 ; (b) aerating said pre-mixture (e.g., by introducing a gas into the wet slurry) to form an aerated wet pre-mixture; (c) aging the aerated wet pre-mixture for at least 5 min; (d) forming the aerated wet pre-mixture into a sheet having opposing first and second sides; and (e) drying the formed sheet for a drying time of from 1 minute to 60 minutes at a temperature from 70° C. to 200° C. along a heating direction that forms a temperature gradient decreasing from the first side to the second side of said formed sheet, wherein the heating direction is substantially offset from the gravitational direction for more than half of the drying time, i.e., the drying step is conducted under heating along a mostly “anti-gravity” heating direction. Such a mostly “anti-gravity” heating direction can be achieved by various means, which include but are not limited to the bottom conduction-based heating/drying arrangement and the rotary drum-based heating/drying arrangement, as illustrated hereinabove in  FIGS. 4 and 5  respectively. 
     Step (A): Preparation of Wet Pre-Mixture 
     The wet pre-mixture of the present invention is generally prepared by mixing solids of interest, including the water-soluble polymer, surfactant(s) and/or other benefit agents, optional plasticizer, and other optional ingredients, with a sufficient amount of water or another solvent in a pre-mix tank. The wet pre-mixture can be formed using a mechanical mixer. Mechanical mixers useful herein, include, but aren&#39;t limited to pitched blade turbines or MAXBLEND mixer (Sumitomo Heavy Industries). 
     It is particularly important in the present invention to adjust viscosity of the wet pre-mixture so that it is within a predetermined range of from about 1,000 cps to about 25,000 cps when measured at 40° C. and 1 s −1 . Viscosity of the wet pre-mixture has a significant impact on the pore expansion and pore opening of the aerated pre-mixture during the subsequent drying step, and wet pre-mixtures with different viscosities may form flexible, porous, dissolvable solid sheet articles of very different foam structures. On one hand, when the wet pre-mixture is too thick/viscous (e.g., having a viscosity higher than about 25,000 cps as measured at 40° C. and 1 s −1 ), aeration of such wet pre-mixture may become more difficult. More importantly, interstitial liquid drainage from thin film bubble facings into the plateau borders of the three-dimensional foam during the subsequent drying step may be adversely affected or significantly limited. The interstitial liquid drainage during drying is believed to be critical for enabling pore expansion and pore opening in the aerated wet pre-mixture during the subsequent drying step. As a result, the flexible, porous, dissolvable solid sheet article so formed thereby may have significantly smaller pores and less interconnectivity between the pores (i.e., more “closed” pores than open pores), which render it harder for water to ingress into and egress from such sheet article. On the other hand, when the wet pre-mixture is too thin/running (e.g., having a viscosity lower than about 1,000 cps as measured at 40° C. and 1 s −1 ), the aerated wet pre-mixture may not be sufficiently stable, i.e., the air bubbles may rupture, collapse, or coalescence too quickly in the wet pre-mixture after aeration and before drying. Consequently, the resulting solid sheet article may be much less porous and more dense than desired. 
     Particularly, viscosity of the wet pre-mixture ranges from about 3,000 cps to about 24,000 cps, preferably from about 5,000 cps to about 23,000 cps, more preferably from about 10,000 cps to about 20,000 cps, as measured at 40° C. and 1 sec −1 . The pre-mixture viscosity values are measured using a Malvern Kinexus Lab+ rheometer with cone and plate geometry (CP1/50 SR3468 SS), a gap width of 0.054 mm, a temperature of 40° C. and a shear rate of 1.0 reciprocal seconds for a period of 360 seconds. 
     Preferably, the solids of interest are present in the wet pre-mixture at a level of from about 15% to about 70%, preferably from about 20% to about 50%, more preferably from about 25% to about 45% by total weight of said wet pre-mixture. The percent solid content is the summation of the weight percentages by weight of the total processing mixture of all solid components, semi-solid components and liquid components excluding water and any obviously volatile materials such as low boiling alcohols. On one hand, if the solid content in the wet pre-mixture is too high, viscosity of the wet pre-mixture may increase to a level that will prohibit or adversely affect interstitial liquid drainage and prevent formation of the desired predominantly open-celled porous solid structure as described herein. On the other hand, if the solid content in the wet pre-mixture is too low, viscosity of the wet pre-mixture may decrease to a level that will cause bubble rupture/collapse/coalescence and more percent (%) shrinkage of the pore structures during drying, resulting in a solid sheet article that is significantly less porous and denser. 
     Among the solids of interest in the wet pre-mixture of the present invention, there may be present from about 1% to about 75% surfactant(s), from about 0.1% to about 25% water-soluble polymer, and optionally from about 0.1% to about 25% plasticizer, by total weight of the solids. Other actives or benefit agents can also be added into the pre-mixture. 
     Optionally, the wet pre-mixture is pre-heated immediately prior to and/or during the aeration process at above ambient temperature but below any temperatures that would cause degradation of the components therein. In one embodiment, the wet pre-mixture is kept at an elevated temperature ranging from about 40° C. to about 100° C., preferably from about 50° C. to about 95° C., more preferably from about 60° C. to about 90° C., most preferably from about 75° C. to about 85° C. In one embodiment, the optional continuous heating is utilized before the aeration step. Further, additional heat can be applied during the aeration process to try and maintain the wet pre-mixture at such an elevated temperature. This can be accomplished via conductive heating from one or more surfaces, injection of steam or other processing means. It is believed that the act of pre-heating the wet pre-mixture before and/or during the aeration step may provide a means for lowering the viscosity of pre-mixtures comprising higher percent solids content for improved introduction of bubbles into the mixture and formation of the desired solid sheet article. Achieving higher percent solids content is desirable since it may reduce the overall energy requirements for drying. The increase of percent solids may therefore conversely lead to a decrease in water level content and an increase in viscosity. As mentioned hereinabove, wet pre-mixtures with viscosities that are too high are undesirable for the practice of the present invention. Pre-heating may effectively counteract such viscosity increase and thus allow for the manufacture of a fast dissolving sheet article even when using high solid content pre-mixtures. 
     Step (B): Aeration of Wet Pre-Mixture 
     Aeration of the wet pre-mixture is conducted in order to introduce a sufficient amount of air bubbles into the wet pre-mixture for subsequent formation of the OCF structures therein upon drying. Once sufficiently aerated, the wet pre-mixture is characterized by a density that is significantly lower than that of the non-aerated wet pre-mixture (which may contain a few inadvertently trapped air bubbles) or an insufficiently aerated wet pre-mixture (which may contain some bubbles but at a much lower volume percentage and of significantly larger bubble sizes). Preferably, the aerated wet pre-mixture has a density ranging from about 0.05 g/ml to about 0.5 g/ml, preferably from about 0.08 g/ml to about 0.4 g/ml, more preferably from about 0.1 g/ml to about 0.35 g/ml, still more preferably from about 0.15 g/ml to about 0.3 g/ml, most preferably from about 0.2 g/ml to about 0.25 g/ml. 
     Aeration can be accomplished by either physical or chemical means in the present invention. In one embodiment, it can be accomplished by introducing a gas into the wet pre-mixture through mechanical agitation, for example, by using any suitable mechanical processing means, including but not limited to: a rotor stator mixer, a planetary mixer, a pressurized mixer, a non-pressurized mixer, a batch mixer, a continuous mixer, a semi-continuous mixer, a high shear mixer, a low shear mixer, a submerged sparger, or any combinations thereof. In another embodiment, it may be achieved via chemical means, for example, by using chemical foaming agents to provide in-situ gas formation via chemical reaction of one or more ingredients, including formation of carbon dioxide (CO 2  gas) by an effervescent system. 
     In a particularly preferred embodiment, it has been discovered that the aeration of the wet pre-mixture can be cost-effectively achieved by using a continuous pressurized aerator or mixer that is conventionally utilized in the foods industry in the production of marshmallows. Continuous pressurized mixers may work to homogenize or aerate the wet pre-mixture to produce highly uniform and stable foam structures with uniform bubble sizes. The unique design of the high shear rotor/stator mixing head may lead to uniform bubble sizes in the layers of the open celled foam. Suitable continuous pressurized aerators or mixers include the Morton whisk (Morton Machine Co., Motherwell, Scotland), the Oakes continuous automatic mixer (E.T. Oakes Corporation, Hauppauge, N.Y.), the Fedco Continuous Mixer (The Peerless Group, Sidney, Ohio), the Mondo (Haas-Mondomix B.V., Netherlands), the Aeros (Aeros Industrial Equipment Co., Ltd., Guangdong Province, China), and the Preswhip (Hosokawa Micron Group, Osaka, Japan). For example, an Aeros A20 continuous aerator can be operated at a feed pump speed setting of about 300-800 (preferably at about 500-700) with a mixing head speed setting of about 300-800 (preferably at about 400-600) and an air flow rate of about 50-150 (preferably 60-130, more preferably 80-120) respectively. For another example, an Oakes continuous automatic mixer can be operated at a mixing head speed setting of about 10-30 rpm (preferably about 15-25 rpm, more preferably about 20 rpm) with an air flow rate of about 10-30 Litres per hour (preferably about 15-25 L/hour, more preferably about 19-20 L/hour). 
     As mentioned hereinabove, the wet pre-mixture can be maintained at an elevated temperature during the aeration process, so as to adjust viscosity of the wet pre-mixture for optimized aeration and controlled draining during drying. 
     Bubble size of the aerated wet pre-mixture assists in achieving uniform layers in the OCF structures of the resulting solid sheet article. In one embodiment, the bubble size of the aerated wet pre-mixture is from about 5 to about 200 microns; and in another embodiment, the bubble size is from about 20 microns to about 100 microns. Uniformity of the bubble sizes causes the resulting solid sheet articles to have consistent densities. 
     Step (C): Aging 
     After sufficient aeration, the aerated wet pre-mixture is maintained for a while without further introducing a significant amount of air. Such aging step may be conducted in any suitable manners. For example, the aerated wet pre-mixture may be stored in a container such as a bucket or a tank without any stirring. For another example, the aerated wet pre-mixture may be stirred by using a spinning bar to prevent phase separation or sedimentation, in which the rotating speed of the spinning bar is preferably low enough (for example, from about 5 to about 80 rpm) to prevent introduction further air and/or high shear force in the aerated wet pre-mixture. Without being bound by any theory, it is believed that high shear force in the aerated wet pre-mixture might compromise the further expansion of bubbles or even reduce bubbles instead. 
     Particularly, the aging may be conducted for a duration from 5 min to 300 min, preferably from 5 min to 200 min, more preferably from 10 min to 150 min. As mentioned hereinabove, the wet pre-mixture may be maintained at ambient temperature or at an elevated temperature during the aging step, for example from 10° C. to 100° C., preferably from 15° C. to 70° C., more preferably from 20° C. to 50° C., most preferably from 20° C. to 40° C. 
     Step (D): Sheet-Forming 
     After aging, the aged wet pre-mixture forms one or more sheets with opposing first and second sides. The sheet-forming step can be conducted in any suitable manners, e.g., by extrusion, casting, molding, vacuum-forming, pressing, printing, coating, and the like. More specifically, the aerated wet pre-mixture can be formed into a sheet by: (i) casting it into shallow cavities or trays or specially designed sheet moulds; (ii) extruding it onto a continuous belt or screen of a dryer; or (iii) coating it onto the outer surface of a rotary drum dryer. Preferably, the supporting surface upon which the sheet is formed is formed by or coated with materials that are anti-corrosion, non-interacting and/or non-sticking, such as metal (e.g., steel, chromium, and the like), TEFLON®, polycarbonate, NEOPRENE®, HDPE, LDPE, rubber, glass and the like. 
     Preferably, the formed sheet of aerated wet pre-mixture has a thickness ranging from a thickness ranging from 0.5 mm to 4 mm, preferably from 0.6 mm to 3.5 mm, more preferably from 0.7 mm to 3 mm, still more preferably from 0.8 mm to 2 mm, most preferably from 0.9 mm to 1.5 mm. Controlling the thickness of such formed sheet of aerated wet pre-mixture may be important for ensuring that the resulting solid sheet article has the desired OCF structures. If the formed sheet is too thin (e.g., less than 0.5 mm in thickness), many of the air bubbles trapped in the aerated wet pre-mixture will expand during the subsequent drying step to form through-holes that extend through the entire thickness of the resulting solid sheet article. Such through-holes, if too many, may significantly compromise both the overall structural integrity and aesthetic appearance of the sheet article. If the formed sheet is too thick, not only it will take longer to dry, but also it will result in a solid sheet article with greater pore size variations between different regions (e.g., top, middle, and bottom regions) along its thickness, because the longer the drying time, the more imbalance of forces may occur through bubble rupture/collapse/coalescence, liquid drainage, pore expansion, pore opening, water evaporation, and the like. Further, multiple layers of relatively thin sheets can be assembled into three-dimensional structures of greater thickness to deliver the desired cleaning benefits or other benefits, while still providing satisfactory pore structures for fast dissolution as well as ensuring efficient drying within a relatively short drying time. 
     Step (E): Drying Under Anti-Gravity Heating 
     A key feature of the present invention is the use of an anti-gravity heating direction during the drying step, either through the entire drying time or at least through more than half of the drying time. Without being bound by any theory, it is believed that such anti-gravity heating direction may reduce or counteract excessive interstitial liquid drainage toward the bottom region of the formed sheet during the drying step. Further, because the top surface is dried last, it allows longer time for air bubbles near the top surface of the formed sheet to expand and form pore openings on the top surface (because once the wet matrix is dried, the air bubbles can no longer expand or form surface openings). Consequently, the solid sheet formed by drying with such anti-gravity heating is characterized by improved OCF structures that enables faster dissolution as well as other surprising and unexpected benefits. 
     In a specific embodiment, the anti-gravity heating direction is provided by a conduction-based heating/drying arrangement, either the same or similar to that illustrated by  FIG. 4 . For example, the aerated wet pre-mixture can be casted into a mold to form a sheet with two opposing sides. The mold can then be placed on a hot plate or a heated moving belt (for example, a heated conveyor belt) or any other suitable heating device with a planar heated surface characterized by a controlled surface temperature of from about 80° C. to about 170° C., preferably from about 90° C. to about 150° C., more preferably from about 100° C. to about 140° C. Alternatively, the aerated wet pre-mixture can be applied onto the outer surface of a heated moving belt such as a conveyor belt to form a sheet with two opposing sides. Thermal energy is transferred from the planar heated surface to the bottom surface of the sheet of aerated wet pre-mixture via conduction, so that solidification of the sheet starts with the bottom region and gradually moves upward to reach the top region last. In order to ensure that the heating direction is primarily anti-gravity (i.e., substantially offset from the gravitational direction) during this process, it is preferred that the heated surface is a primary heat source for the sheet during drying. If there are any other heating sources, the overall heating direction may change accordingly. More preferably, the heated surface is the only heat source for the sheet during drying. And, if a heated moving belt is employed, it is preferred that the aerated wet pre-mixture is applied onto the outer surface of the moving belt when the outer surface of the moving belt is upward or is changing from downward to upward. Once dried, the sheet may be picked up, either manually or by a scraper, preferably when the outer surface of the moving belt is still upward or is changing from upward to downward. Preferably, the moving belt may move at a speed of from about 0.1 m/min to about 50 m/min, preferably from about 0.15 m/min to about 20 m/min, more preferably from about 0.2 m/min to about 10 m/min, during the drying step. 
     In another specific embodiment, the anti-gravity heating direction is provided by a rotary drum-based heating/drying arrangement, which is also referred to as drum drying or roller drying, similar to that illustrated in  FIG. 5 . Drum drying is one type of contact-drying methods, which is used for drying out liquids from a viscous pre-mixture of raw materials over the outer surface of a heated rotatable drum (also referred to as a roller or cylinder) at relatively low temperatures to form sheet-like articles. It is a continuous drying process particularly suitable for drying large volumes. Because the drying is conducted at relatively low temperatures via contact-heating/drying, it normally has high energy efficiency and does not adversely affect the compositional integrity of the raw materials. The heated rotatable cylinder used in drum drying is heated internally, e.g., by steam or electricity, and it is rotated by a motorized drive installed on a base bracket at a predetermined rotational speed. The heated rotatable cylinder or drum preferably has an outer diameter ranging from about 0.5 meters to about 10 meters, preferably from about 1 meter to about 5 meters, more preferably from about 1.5 meters to about 2 meters. It may have a controlled surface temperature of from about 80° C. to about 170° C., preferably from about 90° C. to about 150° C., more preferably from about 100° C. to about 140° C. Further, such heated rotatable cylinder is rotating at a speed of from about 0.005 rpm to about 0.25 rpm, preferably from about 0.05 rpm to about 0.2 rpm, more preferably from about 0.1 rpm to about 0.18 rpm. The heated rotatable cylinder is preferably coated with a non-stick coating on its outer surface. The non-stick coating may be overlying on the outer surface of the heated rotatable drum, or it can be fixed to a medium of the outer surface of the heated rotatable drum. The medium includes, but is not limited to, heat-resisting non-woven fabrics, heat-resisting carbon fiber, heat-resisting metal or non-metallic mesh and the like. The non-stick coating can effectively preserve structural integrity of the sheet-like article from damage during the sheet-forming process. 
     In order to form a sheet on a heated surface (for example, the outer surface of a rotary drum dryer or a heated moving belt), a feeding mechanism may be provided, independently or as a part of the drying device (for example, the rotary drum dryer or the heated moving belt). Such feeding mechanism is employed to apply the aerated wet pre-mixture of raw materials as described hereinabove onto the heated surface, thereby forming a thin layer of the viscous pre-mixture onto the heated surface. Such thin layer of the pre-mixture is therefore dried by the heated surface via contact-heating/drying. The feeding mechanism may include a feeding hopper, a feeding die, an extruder, or a spinning bar through which the aerated wet pre-mixture is applied onto the heated surface. The feeding mechanism may further include a feeding trough that is employed for containing the wet aerated pre-mixture, an imaging device for dynamic observation of the feeding, and/or an adjustment device for adjusting the position and/or inclination angle of the feeding hopper, the feeding die, the extruder or the spinning bar. 
     In a preferred but not necessary embodiment, a spinning bar is employed for applying the aerated wet pre-mixture. Preferably, the spinning bar may be rotating at a speed of from 5 to 80 rpm, preferably from 6 to 60 rpm, more preferably from 8 to 50 rpm, most preferably from 10 to 40 rpm. Also preferably, the spinning bar may be positioned so that the distance between the spinning bar and the outer surface of the rotary drum or the heated moving belt is from 3 mm to 15 mm, preferably from 4 mm to 12 mm, more preferably from 5 mm to 10 mm, most preferably from 6 mm to 10 mm. Without being bound by any theory, it is believed that if an appropriate rotating speed and/or an appropriate distance between the spinning bar the outer surface of the rotary drum or the heated moving belt are used, it would not introduce further air bubbles and/or cause high shear force that may compromise the formation of large bubbles. As such, it may result in an even larger bubble size of pre-mixture fed onto the heated surface and in turn, an improved pore structure in the formed sheet. 
     In another preferred but not necessary embodiment, a feeding die is employed for applying the aerated wet pre-mixture. Preferably, the feeding die may have a feeding speed of from about 0.1 m/min to about 50 m/min, preferably from about 0.15 m/min to about 20 m/min, more preferably from about 0.2 m/min to about 10 m/min. Also preferably, the feeding die may be positioned so that the distance between the feeding die and the outer surface of the rotary drum or the heating moving belt is from 0.1 mm to 15 mm, preferably from 0.2 mm to 12 mm, more preferably from 0.3 mm to 10 mm, most preferably from 0.5 mm to 5 mm. Without being bound by any theory, it is believed that if an appropriate feeding speed and/or an appropriate distance between the feeding die and the outer surface of the rotary drum or the heated moving belt are used, it may achieve a preferred sheet formation and/or an even larger bubble size. 
     There may also be a heating shield that is preferably installed on the base bracket, to prevent rapid heat lost. The heating shield can also effectively save energy needed by the heated surface, thereby achieving reduced energy consumption and provide cost savings. The heating shield is a modular assembly structure, or integrated structure, and can be freely detached from the base bracket. A suction device is also installed on the heating shield for sucking the hot steam, to avoid any water condensate falling on the sheet-like article that is being formed. 
     There may also be an optional static scraping mechanism that is preferably installed on the base bracket, for scraping or scooping up the sheet-like article already formed by the heated surface. The static scraping mechanism can be installed on the base bracket, or on one side thereof, for transporting the already formed sheet-like article downstream for further processing. The static scraping mechanism can automatically or manually move close and go away from the heated rotatable drum. 
     Preferably, the making process of the flexible, porous, dissolvable solid sheet of the present invention may be as follows. Firstly, the heated rotatable drum with the non-stick coating on the base bracket is driven by the motorized drive. Next, the adjustment device adjusts the feeding mechanism so that the distance between the feeding hopper and the outer surface of the heated rotatable drum reaches a preset value. Meanwhile, the feeding hopper adds the aerated wet pre-mixture containing all or some raw materials for making the flexible, porous, dissolvable solid sheet onto an outer surface of the heated rotatable drum, to form a thin layer of the aerated wet pre-mixture thereon with the desired thickness as described hereinabove in the preceding section. Optionally, the suction device of the heating shield sucks the hot steam generated by the heated rotatable drum. Next, the static scraping mechanism scrapes/scoops up a dried/solidified sheet, which is formed by the thin layer of aerated wet pre-mixture after it is dried by the heated rotatable drum at a relatively low temperature (e.g., 130° C.). The dried/solidified sheet can also be manually or automatically peeled off, without such static scraping mechanism and then rolled up by a roller bar. 
     The total drying time in the present invention depends on the formulations and solid contents in the wet pre-mixture, the drying temperature, the thermal energy influx, and the thickness of the sheet material to be dried. Preferably, the drying time is from about 1 minute to about 60 minutes, preferably from about 2 minutes to about 30 minutes, more preferably from about 2 to about 15 minutes, still more preferably from about 2 to about 10 minutes, most preferably from about 2 to about 5 minutes, for example, 3 min, 5 min, 7 min, 10 min, 15 min, 20 min, 25 min, 30 min or any ranges therebetween. 
     During such drying time, the heating direction is so arranged that it is substantially opposite to the gravitational direction for more than half of the drying time, preferably for more than 55% or 60% of the drying time (e.g., as in the rotary drum-based heating/drying arrangement described hereinabove), more preferably for more than 75% or even 100% of the drying time (e.g., as in the bottom conduction-based heating/drying arrangement described hereinabove). Further, the sheet of aerated wet pre-mixture can be dried under a first heating direction for a first duration and then under a second, opposite heating direction under a second duration, while the first heating direction is substantially opposite to the gravitational direction, and while the first duration is anywhere from 51% to 99% (e.g., from 55%, 60%, 65%, 70% to 80%, 85%, 90% or 95%) of the total drying time. Such change in heating direction can be readily achieved by various other arrangements not illustrated herein, e.g., by an elongated heated belt of a serpentine shape that can rotate along a longitudinal central axis. 
     IV. Physical Characteristics of Inventive Solid Sheet Articles 
     The flexible, porous, dissolvable solid sheet article formed by the above-described processing steps is characterized by improved pore structures that allows easier water ingress into the sheet article and faster dissolution of the sheet article in water. Such improved pore structures are achieved mainly by adjusting various processing conditions as described hereinabove, and they are relatively independent or less influenced by the chemical formulations or the specific ingredients used for making such sheet article. 
     In general, such solid sheet article may be characterized by: (i) a Percent Open Cell Content of from about 80% to 100%, preferably from about 85% to 100%, more preferably from about 90% to 100%, as measured by the Test 3 hereinafter; and (ii) an Overall Average Pore Size of from about 100 μm to about 2000 μm, preferably from about 150 μm to about 1000 μm, more preferably from about 200 μm to about 600 μm, as measured by the Micro-CT method described in Test 2 hereinafter. The Overall Average Pore Size defines the porosity of the OCF structure of the present invention. The Percent Open Cell Content defines the interconnectivity between pores in the OCF structure of the present invention. Interconnectivity of the OCF structure may also be described by a Star Volume or a Structure Model Index (SMI) as disclosed in WO2010077627 and WO2012138820. 
     Such solid sheet article of the present invention has opposing top and bottom surfaces, while its top surface may be characterized by a Surface Average Pore Diameter that is greater than about 300 μm, preferably greater than about 310 μm, preferably greater than about 320 μm, more preferably greater than about 330 μm, most preferably greater than about 350 μm, as measured by the SEM method described in Test 1 hereinafter. When comparing with solid sheet articles formed by prior art heating/drying arrangements (e.g., the convection-based, the microwave-based, or the impingement oven-based arrangements), the solid sheet article formed by the inventive heating/drying arrangement of the present invention has a significantly larger Surface Average Pore Diameter at its top surface (as demonstrated by  FIGS. 6A-6B, 7A-7B and 8A-8B , which are described in detail in Example 1 hereinafter). 
     Still further, the solid sheet article formed by the inventive heating/drying arrangement of the present invention is characterized by a more uniform pore size distribution between different regions along its thickness direction, in comparison with the sheets formed by prior art heating/drying arrangements. Specifically, the solid sheet article of the present invention comprises a top region adjacent to the top surface, a bottom region adjacent to the bottom surface, and a middle region therebetween, while the top, middle, and bottom regions all have the same thickness. Each of the top, middle and bottom regions of such solid sheet article is characterized by an Average Pore Size, while the ratio of Average Pore Size in the bottom region over that in the top region (i.e., bottom-to-top Average Pore Size ratio) is from about 0.6 to about 1.5, preferably from about 0.7 to about 1.4, preferably from about 0.8 to about 1.3, more preferably from about 1 to about 1.2. In comparison, a solid sheet article formed by a prior art impingement oven-based heating/drying arrangement may have a bottom-to-top Average Pore Size ratio of more than 1.5, typically about 1.7-2.2 (as demonstrated in Example 1 hereinafter). Moreover, the solid sheet article of the present invention may be characterized by a bottom-to-middle Average Pore Size ratio of from about 0.5 to about 1.5, preferably from about 0.6 to about 1.3, more preferably from about 0.8 to about 1.2, most preferably from about 0.9 to about 1.1, and a middle-to-top Average Pore Size ratio of from about 1 to about 1.5, preferably from about 1 to about 1.4, more preferably from about 1 to about 1.2. 
     Still further, the relative standard deviation (RSTD) between Average Pore Sizes in the top, middle and bottom regions of the solid sheet article of the present invention is no more than 20%, preferably no more than 15%, more preferably no more than 10%, most preferably no more than 5%. In contrast, a solid sheet article formed by a prior art impingement oven-based heating/drying arrangement may have a relative standard deviation (RSTD) between top/middle/bottom Average Pore Sizes of more than 20%, likely more than 25% or even more than 35%. 
     Preferably, the solid sheet article of the present invention is further characterized by an Average Cell Wall Thickness of from about 5 μm to about 200 μm, preferably from about 10 μm to about 100 μm, more preferably from about 10 μm to about 80 μm, as measured by Test 2 hereinafter. 
     The solid sheet article of the present invention may contain a small amount of water. Preferably, it is characterized by a final moisture content of from 0.5% to 25%, preferably from 1% to 20%, more preferably from 3% to 10%, by weight of said solid sheet article, as measured by Test 4 hereinafter. An appropriate final moisture content in the resulting solid sheet article may ensure the desired flexibility/deformability of the sheet article, as well as providing soft/smooth sensory feel to the consumers. If the final moisture content is too low, the sheet article may be too brittle or rigid. If the final moisture content is too high, the sheet article may be too sticky, and its overall structural integrity may be compromised. 
     The solid sheet article of the present invention may have a thickness ranging from about 0.6 mm to about 3.5 mm, preferably from about 0.7 mm to about 3 mm, more preferably from about 0.8 mm to about 2 mm, most preferably from about 1 mm to about 1.5 mm. Thickness of the solid sheet article can be measured using Test 6 described hereinafter. The solid sheet article after drying may be slightly thicker than the sheet of aerated wet pre-mixture, due to pore expansion that in turn leads to overall volume expansion. 
     The solid sheet article of the present invention may further be characterized by a basis weight of from about 50 grams/m 2  to about 250 grams/m 2 , preferably from about 80 grams/m 2  to about 220 grams/m 2 , more preferably from about 100 grams/m 2  to about 200 grams/m 2 , as measured by Test 6 described hereinafter. 
     Still further, the solid sheet article of the present invention may have a density ranging from about 0.05 grams/cm 3  to about 0.5 grams/cm 3 , preferably from about 0.06 grams/cm 3  to about 0.4 grams/cm 3 , more preferably from about 0.07 grams/cm 3  to about 0.2 grams/cm 3 , most preferably from about 0.08 grams/cm 3  to about 0.15 grams/cm 3 , as measured by Test 7 hereinafter. Density of the solid sheet article of the present invention is lower than that of the sheet of aerated wet pre-mixture, also due to pore expansion that in turn leads to overall volume expansion. 
     Furthermore, the solid sheet article of the present invention can be characterized by a Specific Surface Area of from about 0.03 m 2 /g to about 0.25 m 2 /g, preferably from about 0.04 m 2 /g to about 0.22 m 2 /g, more preferably from 0.05 m 2 /g to 0.2 m 2 /g, most preferably from 0.1 m 2 /g to 0.18 m 2 /g, as measured by Test 8 described hereinafter. The Specific Surface Area of the solid sheet article of the present invention may be indicative of its porosity and may impact its dissolution rate, e.g., the greater the Specific Surface Area, the more porous the sheet article and the faster its dissolution rate. 
     V. Formulations of Inventive Solid Sheet Articles 
     1. Water-Soluble Polymer 
     As mentioned hereinabove, the flexible, porous, dissolvable solid sheet article of the present invention may be formed by a wet pre-mixture that comprises a water-soluble polymer and a surfactant. Such a water-soluble polymer may function in the resulting solid sheet article as a film-former, a structurant as well as a carrier for other active ingredients (e.g., surfactants, emulsifiers, builders, chelants, perfumes, colorants, and the like). 
     Preferably, the wet pre-mixture may comprise from about 3% to about 20% by weight of the pre-mixture of water soluble polymer, in one embodiment from about 5% to about 15% by weight of the pre-mixture of water soluble polymer, in one embodiment from about 7% to about 10% by weight of the pre-mixture of water soluble polymer. 
     After drying, it is preferred that the water-soluble polymer is present in the flexible, porous, dissolvable solid sheet article of the present invention in an amount ranging from about 5% to about 40%, preferably from about 8% to about 30%, more preferably from about 10% to about 25%, by total weight of the solid sheet article. In a particularly preferred embodiment of the present invention, the total amount of water-soluble polymer(s) present in the flexible, porous, dissolvable solid sheet article of the present invention is no more than 25% by total weight of such article. 
     Water-soluble polymers suitable for the practice of the present invention may be selected those with weight average molecular weights ranging from about 5,000 to about 400,000 Daltons, more preferably from about 10,000 to about 300,000 Daltons, still more preferably from about 15,000 to about 200,000 Daltons, most preferably from about 20,000 to about 150,000 Daltons. The weight average molecular weight is computed by summing the average molecular weights of each polymer raw material multiplied by their respective relative weight percentages by weight of the total weight of polymers present within the porous solid. The weight average molecular weight of the water-soluble polymer used herein may impact the viscosity of the wet pre-mixture, which may in turn influence the bubble number and size during the aeration step as well as the pore expansion/opening results during the drying step. Further, the weight average molecular weight of the water-soluble polymer may affect the overall film-forming properties of the wet pre-mixture and its compatibility/incompatibility with certain surfactants. 
     The water-soluble polymers of the present invention may include, but are not limited to, synthetic polymers including polyvinyl alcohols, polyvinylpyrrolidones, polyalkylene oxides, polyacrylates, caprolactams, polymethacrylates, polymethylmethacrylates, polyacryl amides, polymethylacrylamides, polydimethylacrylamides, polyethylene glycol monomethacrylates, copolymers of acrylic acid and methyl acrylate, polyurethanes, polycarboxylic acids, polyvinyl acetates, polyesters, polyamides, polyamines, polyethyleneimines, maleic/(acrylate or methacrylate) copolymers, copolymers of methylvinyl ether and of maleic anhydride, copolymers of vinyl acetate and crotonic acid, copolymers of vinylpyrrolidone and of vinyl acetate, copolymers of vinylpyrrolidone and of caprolactam, vinyl pyrollidone/vinyl acetate copolymers, copolymers of anionic, cationic and amphoteric monomers, and combinations thereof. 
     The water-soluble polymers of the present invention may also be selected from naturally sourced polymers including those of plant origin examples of which include karaya gum, tragacanth gum, gum Arabic, acemannan, konjac mannan, acacia gum, gum ghatti, whey protein isolate, and soy protein isolate; seed extracts including guar gum, locust bean gum, quince seed, and psyllium seed; seaweed extracts such as Carrageenan, alginates, and agar; fruit extracts (pectins); those of microbial origin including xanthan gum, gellan gum, pullulan, hyaluronic acid, chondroitin sulfate, and dextran; and those of animal origin including casein, gelatin, keratin, keratin hydrolysates, sulfonic keratins, albumin, collagen, glutelin, glucagons, gluten, zein, and shellac. 
     Modified natural polymers can also be used as water-soluble polymers in the present invention. Suitable modified natural polymers include, but are not limited to, cellulose derivatives such as hydroxypropylmethylcellulose, hydroxymethylcellulose, hydroxyethylcellulose, methylcellulose, hydroxypropylcellulose, ethylcellulose, carboxymethylcellulose, cellulose acetate phthalate, nitrocellulose and other cellulose ethers/esters; and guar derivatives such as hydroxypropyl guar. 
     The water-soluble polymer of the present invention may include starch. As used herein, the term “starch” include both naturally occurring or modified starches. Typical natural sources for starches can include cereals, tubers, roots, legumes and fruits. More specific natural sources can include corn, pea, potato, banana, barley, wheat, rice, sago, amaranth, tapioca, arrowroot, canna, sorghum, and waxy or high amylase varieties thereof. The natural starches can be modified by any modification method known in the art to form modified starches, including physically modified starches, such as sheared starches or thermally-inhibited starches; chemically modified starches, such as those which have been cross-linked, acetylated, and organically esterified, hydroxyethylated, and hydroxypropylated, phosphorylated, and inorganically esterified, cationic, anionic, nonionic, amphoteric and zwitterionic, and succinate and substituted succinate derivatives thereof; conversion products derived from any of the starches, including fluidity or thin-boiling starches prepared by oxidation, enzyme conversion, acid hydrolysis, heat or acid dextrinization, thermal and or sheared products may also be useful herein; and pregelatinized starches which are known in the art. 
     Preferred water-soluble polymers of the present invention include polyvinyl alcohols, polyvinylpyrrolidones, polyalkylene oxides, starch and starch derivatives, pullulan, gelatin, hydroxypropylmethylcelluloses, methycelluloses, and carboxymethycelluloses. More preferred water-soluble polymers of the present invention include polyvinyl alcohols, and hydroxypropylmethylcelluloses. 
     Most preferred water-soluble polymers of the present invention are polyvinyl alcohols characterized by a degree of hydrolysis ranging from about 40% to about 100%, preferably from about 50% to about 95%, more preferably from about 65% to about 92%, most preferably from about 70% to about 90%. Commercially available polyvinyl alcohols include those from Celanese Corporation (Texas, USA) under the CELVOL trade name including, but not limited to, CELVOL 523, CELVOL 530, CELVOL 540, CELVOL 518, CELVOL 513, CELVOL 508, CELVOL 504; those from Kuraray Europe GmbH (Frankfurt, Germany) under the Mowiol® and POVAL′ trade names; and PVA 1788 (also referred to as PVA BP17) commercially available from various suppliers including Lubon Vinylon Co. (Nanjing, China); and combinations thereof. In a particularly preferred embodiment of the present invention, the flexible, porous, dissolvable solid sheet article comprises from about 10% to about 25%, more preferably from about 15% to about 23%, by total weight of such article, of a polyvinyl alcohol having a weight average molecular weight ranging from 80,000 to about 150,000 Daltons and a degree of hydrolysis ranging from about 80% to about 90%. 
     In addition to polyvinyl alcohols as mentioned hereinabove, a single starch or a combination of starches may be used as a filler material in such an amount as to reduce the overall level of water-soluble polymers required, so long as it helps provide the solid sheet article with the requisite structure and physical/chemical characteristics as described herein. However, too much starch may comprise the solubility and structural integrity of the sheet article. Therefore, in preferred embodiments of the present invention, it is desired that the solid sheet article comprises no more than 20%, preferably from 0% to 10%, more preferably from 0% to 5%, most preferably from 0% to 1%, by weight of said solid sheet article, of starch. 
     2. Surfactants 
     In addition to the water-soluble polymer described hereinabove, the solid sheet article of the present invention comprises one or more surfactants. The surfactants may function as emulsifying agents during the aeration process to create a sufficient amount of stable bubbles for forming the desired OCF structure of the present invention. Further, the surfactants may function as active ingredients for delivering a desired cleansing benefit. 
     In a preferred embodiment of the present invention, the solid sheet article comprises one or more surfactants selected from the group consisting of anionic surfactants, nonionic surfactants, cationic surfactants, zwitterionic surfactants, amphoteric surfactants, polymeric surfactants or combinations thereof. Depending on the desired application of such solid sheet article and the desired consumer benefit to be achieved, different surfactants can be selected. One benefit of the present invention is that the OCF structures of the solid sheet article allow for incorporation of a high surfactant content while still providing fast dissolution. Consequently, highly concentrated cleansing compositions can be formulated into the solid sheet articles of the present invention to provide a new and superior cleansing experience to the consumers. 
     The surfactant as used herein may include both surfactants from the conventional sense (i.e., those providing a consumer-noticeable lathering effect) and emulsifiers (i.e., those that do not provide any lathering performance but are intended primarily as a process aid in making a stable foam structure). Examples of emulsifiers for use as a surfactant component herein include mono- and di-glycerides, fatty alcohols, polyglycerol esters, propylene glycol esters, sorbitan esters and other emulsifiers known or otherwise commonly used to stabilize air interfaces. 
     The total amount of surfactants present in the solid sheet article of the present invention may range widely from about 5% to about 80%, preferably from about 10% to about 70%, more preferably from about 30% to about 65%, by total weight of said solid sheet article. Correspondingly, the wet pre-mixture may comprise from about 1% to about 40% by weight of the wet pre-mixture of surfactant(s), in one embodiment from about 2% to about 35% by weight of the wet pre-mixture of surfactant(s), in one embodiment from about 5% to about 30% by weight of the wet pre-mixture of surfactant(s). 
     In a preferred embodiment of the present invention, the solid sheet article of the present invention is a cleansing product containing from about 30% to about 80%, preferably from about 40% to about 70%, more preferably from about 50% to about 65%, of one or more surfactants by total weight of said solid sheet article. In such cases, the wet pre-mixture may comprise from about 10% to about 40% by weight of the wet pre-mixture of surfactant(s), in one embodiment from about 12% to about 35% by weight of the wet pre-mixture of surfactant(s), in one embodiment from about 15% to about 30% by weight of the wet pre-mixture of surfactant(s). 
     Non-limiting examples of anionic surfactants suitable for use herein include alkyl and alkyl ether sulfates, sulfated monoglycerides, sulfonated olefins, alkyl aryl sulfonates, primary or secondary alkane sulfonates, alkyl sulfosuccinates, acyl taurates, acyl isethionates, alkyl glycerylether sulfonate, sulfonated methyl esters, sulfonated fatty acids, alkyl phosphates, acyl glutamates, acyl sarcosinates, alkyl sulfoacetates, acylated peptides, alkyl ether carboxylates, acyl lactylates, anionic fluorosurfactants, sodium lauroyl glutamate, and combinations thereof. One category of anionic surfactants particularly suitable for practice of the present invention include C 6 -C 20  linear alkylbenzene sulphonate (LAS) surfactant. LAS surfactants are well known in the art and can be readily obtained by sulfonating commercially available linear alkylbenzenes. Exemplary C 10 -C 20  linear alkylbenzene sulfonates that can be used in the present invention include alkali metal, alkaline earth metal or ammonium salts of C 10 -C 20  linear alkylbenzene sulfonic acids, and preferably the sodium, potassium, magnesium and/or ammonium salts of C 11 -C 18  or C 11 -C 14  linear alkylbenzene sulfonic acids. More preferred are the sodium or potassium salts of C 12  and/or C 14  linear alkylbenzene sulfonic acids, and most preferred is the sodium salt of C 12  and/or C 14  linear alkylbenzene sulfonic acid, i.e., sodium dodecylbenzene sulfonate or sodium tetradecylbenzene sulfonate. 
     LAS provides superior cleaning benefit and is especially suitable for use in laundry detergent applications. It has been a surprising and unexpected discovery of the present invention that when polyvinyl alcohol having a higher weight average molecular weight (e.g., from about 50,000 to about 400,000 Daltons, preferably from about 60,000 to about 300,000 Daltons, more preferably from about 70,000 to about 200,000 Daltons, most preferably from about 80,000 to about 150,000 Daltons) is used as the film-former and carrier, LAS can be used as a major surfactant, i.e., present in an amount that is more than 50% by weight of the total surfactant content in the solid sheet article, without adversely affecting the film-forming performance and stability of the overall composition. Correspondingly, in a particular embodiment of the present invention, LAS is used as the major surfactant in the solid sheet article. If present, the amount of LAS in the solid sheet article of the present invention may range from about 10% to about 70%, preferably from about 20% to about 65%, more preferably from about 40% to about 60%, by total weight of the solid sheet article. 
     Another category of anionic surfactants suitable for practice of the present invention include sodium trideceth sulfates (STS) having a weight average degree of alkoxylation ranging from about 0.5 to about 5, preferably from about 0.8 to about 4, more preferably from about 1 to about 3, most preferably from about 1.5 to about 2.5. Trideceth is a 13-carbon branched alkoxylated hydrocarbon comprising, in one embodiment, an average of at least 1 methyl branch per molecule. STS used by the present invention may be include ST(EOxPOy)S, while EOx refers to repeating ethylene oxide units with a repeating number x ranging from 0 to 5, preferably from 1 to 4, more preferably from 1 to 3, and while POy refers to repeating propylene oxide units with a repeating number y ranging from 0 to 5, preferably from 0 to 4, more preferably from 0 to 2. It is understood that a material such as ST2S with a weight average degree of ethoxylation of about 2, for example, may comprise a significant amount of molecules which have no ethoxylate, 1 mole ethoxylate, 3 mole ethoxylate, and so on, while the distribution of ethoxylation can be broad, narrow or truncated, which still results in an overall weight average degree of ethoxylation of about 2. STS is particularly suitable for personal cleansing applications, and it has been a surprising and unexpected discovery of the present invention that when polyvinyl alcohol having a higher weight average molecular weight (e.g., from about 50,000 to about 400,000 Daltons, preferably from about 60,000 to about 300,000 Daltons, more preferably from about 70,000 to about 200,000 Daltons, most preferably from about 80,000 to about 150,000 Daltons) is used as the film-former and carrier, STS can be used as a major surfactant, i.e., present in an amount that is more than 50% by weight of the total surfactant content in the solid sheet article, without adversely affecting the film-forming performance and stability of the overall composition. Correspondingly, in a particular embodiment of the present invention, STS is used as the major surfactant in the solid sheet article. If present, the amount of STS in the solid sheet article of the present invention may range from about 10% to about 70%, preferably from about 20% to about 65%, more preferably from about 40% to about 60%, by total weight of the solid sheet article. 
     Another category of anionic surfactants suitable for practice of the present invention include alkyl sulfates. These materials have the respective formulae ROSO 3 M, wherein R is alkyl or alkenyl of from about 6 to about 20 carbon atoms, x is 1 to 10, and M is a water-soluble cation such as ammonium, sodium, potassium and triethanolamine. Preferably, R has from about 6 to about 18, preferably from about 8 to about 16, more preferably from about 10 to about 14, carbon atoms. Previously, unalkoxylated C 6 -C 20  linear or branched alkyl sulfates (AS) have been considered the preferred surfactants in dissolvable solid sheet articles, especially as the major surfactant therein, due to its compatibility with low molecular weight polyvinyl alcohols (e.g., those with a weight average molecular weight of no more than 50,000 Daltons) in film-forming performance and storage stability. However, it has been a surprising and unexpected discovery of the present invention that when polyvinyl alcohol having a higher weight average molecular weight (e.g., from about 50,000 to about 400,000 Daltons, preferably from about 60,000 to about 300,000 Daltons, more preferably from about 70,000 to about 200,000 Daltons, most preferably from about 80,000 to about 150,000 Daltons) is used as the film-former and carrier, other surfactants, such as LAS and/or STS, can be used as the major surfactant in the solid sheet article, without adversely affecting the film-forming performance and stability of the overall composition. Therefore, in a particularly preferred embodiment of the present invention, it is desirable to provide a solid sheet article with no more than about 20%, preferably from 0% to about 10%, more preferably from 0% to about 5%, most preferably from 0% to about 1%, by weight of said solid sheet article, of AS. 
     Another category of anionic surfactants suitable for practice of the present invention include C 6 -C 20  linear or branched alkylalkoxy sulfates (AAS). Among this category, linear or branched alkylethoxy sulfates (AES) having the respective formulae RO(C 2 H 4 O) x SO 3 M are particularly preferred, wherein R is alkyl or alkenyl of from about 6 to about 20 carbon atoms, x is 1 to 10, and M is a water-soluble cation such as ammonium, sodium, potassium and triethanolamine. Preferably, R has from about 6 to about 18, preferably from about 8 to about 16, more preferably from about 10 to about 14, carbon atoms. The AES surfactants are typically made as condensation products of ethylene oxide and monohydric alcohol&#39;s having from about 6 to about 20 carbon atoms. Useful alcohols can be derived from fats, e.g., coconut oil or tallow, or can be synthetic. Lauryl alcohol and straight chain alcohol&#39;s derived from coconut oil are preferred herein. Such alcohol&#39;s are reacted with about 1 to about 10, preferably from about 3 to about 5, and especially about 3, molar proportions of ethylene oxide and the resulting mixture of molecular species having, for example, an average of 3 moles of ethylene oxide per mole of alcohol, is sulfated and neutralized. Highly preferred AES are those comprising a mixture of individual compounds, said mixture having an average alkyl chain length of from about 10 to about 16 carbon atoms and an average degree of ethoxylation of from about 1 to about 4 moles of ethylene oxide. If present, the the amount of AAS in the solid sheet article of the present invention may range from about 2% to about 40%, preferably from about 5% to about 30%, more preferably from about 8% to about 12%, by total weight of the solid sheet article. 
     Other suitable anionic surfactants include water-soluble salts of the organic, sulfuric acid reaction products of the general formula [R 1 —SO 3 -M], wherein R 1  is chosen from the group consisting of a straight or branched chain, saturated aliphatic hydrocarbon radical having from about 6 to about 20, preferably about 10 to about 18, carbon atoms; and M is a cation. Preferred are alkali metal and ammonium sulfonated C 10-18  n-paraffins. Other suitable anionic surfactants include olefin sulfonates having about 12 to about 24 carbon atoms. The α-olefins from which the olefin sulfonates are derived are mono-olefins having about 12 to about 24 carbon atoms, preferably about 14 to about 16 carbon atoms. Preferably, they are straight chain olefins. 
     Another class of anionic surfactants suitable for use in the fabric and home care compositions is the β-alkyloxy alkane sulfonates. These compounds have the following formula: 
     
       
         
         
             
             
         
       
     
     where R 1  is a straight chain alkyl group having from about 6 to about 20 carbon atoms, R 2  is a lower alkyl group having from about 1 (preferred) to about 3 carbon atoms, and M is a water-soluble cation as hereinbefore described. 
     Additional examples of suitable anionic surfactants are the reaction products of fatty acids esterified with isethionic acid and neutralized with sodium hydroxide where, for example, the fatty acids are derived from coconut oil; sodium or potassium salts of fatty acid amides of methyl tauride in which the fatty acids, for example, are derived from coconut oil. Still other suitable anionic surfactants are the succinamates, examples of which include disodium N-octadecyl sulfosuccinamate; diammoniumlauryl sulfosuccinamate; tetrasodium N-(1,2-dicarboxyethyl)-N-octadecylsulfosuccinamate; diamyl ester of sodium sulfosuccinic acid; dihexyl ester of sodium sulfosuccinic acid; and dioctyl esters of sodium sulfosuccinic acid. 
     Nonionic surfactants that can be included into the solid sheet article of the present invention may be any conventional nonionic surfactants, including but not limited to: alkyl alkoxylated alcohols, alkyl alkoxylated phenols, alkyl polysaccharides (especially alkyl glucosides and alkyl polyglucosides), polyhydroxy fatty acid amides, alkoxylated fatty acid esters, sucrose esters, sorbitan esters and alkoxylated derivatives of sorbitan esters, amine oxides, and the like. Preferred nonionic surfactants are those of the formula R 1 (OC 2 H 4 ) n OH, wherein R 1  is a C 8 -C 18  alkyl group or alkyl phenyl group, and n is from about 1 to about 80. Particularly preferred are C 8 -C 18  alkyl ethoxylated alcohols having a weight average degree of ethoxylation from about 1 to about 20, preferably from about 5 to about 15, more preferably from about 7 to about 10, such as NEODOL® nonionic surfactants commercially available from Shell. Other non-limiting examples of nonionic surfactants useful herein include: C 6 -C 12  alkyl phenol alkoxylates where the alkoxylate units may be ethyleneoxy units, propyleneoxy units, or a mixture thereof; C 12 -C 18  alcohol and C 6 -C 12  alkyl phenol condensates with ethylene oxide/propylene oxide block polymers such as Pluronic® from BASF; C 14 -C 22  mid-chain branched alcohols (BA); C 14 -C 22  mid-chain branched alkyl alkoxylates, BAE x , wherein x is from 1 to 30; alkyl polysaccharides, specifically alkyl polyglycosides; Polyhydroxy fatty acid amides; and ether capped poly(oxyalkylated) alcohol surfactants. Suitable nonionic surfactants also include those sold under the tradename Lutensol® from BASF. 
     In a preferred embodiment, the nonionic surfactant is selected from sorbitan esters and alkoxylated derivatives of sorbitan esters including sorbitan monolaurate (SPAN® 20), sorbitan monopalmitate (SPAN® 40), sorbitan monostearate (SPAN® 60), sorbitan tristearate (SPAN® 65), sorbitan monooleate (SPAN® 80), sorbitan trioleate (SPAN® 85), sorbitan isostearate, polyoxyethylene (20) sorbitan monolaurate (Tween® 20), polyoxyethylene (20) sorbitan monopalmitate (Tween® 40), polyoxyethylene (20) sorbitan monostearate (Tween® 60), polyoxyethylene (20) sorbitan monooleate (Tween® 80), polyoxyethylene (4) sorbitan monolaurate (Tween® 21), polyoxyethylene (4) sorbitan monostearate (Tween® 61), polyoxyethylene (5) sorbitan monooleate (Tween® 81), all available from Uniqema, and combinations thereof. 
     The most preferred nonionic surfactants for practice of the present invention include C 6 -C 20  linear or branched alkylalkoxylated alcohols (AA) having a weight average degree of alkoxylation ranging from 5 to 15, more preferably C 12 -C 14  linear ethoxylated alcohols having a weight average degree of alkoxylation ranging from 7 to 9. If present, the amount of AA-type nonionic surfactant(s) in the solid sheet article of the present invention may range from about 2% to about 40%, preferably from about 5% to about 30%, more preferably from about 8% to about 12%, by total weight of the solid sheet article. 
     Amphoteric surfactants suitable for use in the solid sheet article of the present invention includes those that are broadly described as derivatives of aliphatic secondary and tertiary amines in which the aliphatic radical can be straight or branched chain and wherein one of the aliphatic substituents contains from about 8 to about 18 carbon atoms and one contains an anionic water solubilizing group, e.g., carboxy, sulfonate, sulfate, phosphate, or phosphonate. Examples of compounds falling within this definition are sodium 3-dodecyl-aminopropionate, sodium 3-dodecylaminopropane sulfonate, sodium lauryl sarcosinate, N-alkyltaurines such as the one prepared by reacting dodecylamine with sodium isethionate, and N-higher alkyl aspartic acids. 
     One category of amphoteric surfactants particularly suitable for incorporation into solid sheet articles with personal care applications (e.g., shampoo, facial or body cleanser, and the like) include alkylamphoacetates, such as lauroamphoacetate and cocoamphoacetate. Alkylamphoacetates can be comprised of monoacetates and diacetates. In some types of alkylamphoacetates, diacetates are impurities or unintended reaction products. If present, the amount of alkylamphoacetate(s) in the solid sheet article of the present invention may range from about 2% to about 40%, preferably from about 5% to about 30%, more preferably from about 10% to about 20%, by total weight of the solid sheet article. 
     Zwitterionic surfactants suitable include those that are broadly described as derivatives of aliphatic quaternary ammonium, phosphonium, and sulfonium compounds, in which the aliphatic radicals can be straight or branched chain, and wherein one of the aliphatic substituents contains from about 8 to about 18 carbon atoms and one contains an anionic group, e.g., carboxy, sulfonate, sulfate, phosphate, or phosphonate. Such suitable zwitterionic surfactants can be represented by the formula: 
     
       
         
         
             
             
         
       
     
     wherein R 2  contains an alkyl, alkenyl, or hydroxy alkyl radical of from about 8 to about 18 carbon atoms, from 0 to about 10 ethylene oxide moieties and from 0 to about 1 glyceryl moiety; Y is selected from the group consisting of nitrogen, phosphorus, and sulfur atoms; R 3  is an alkyl or monohydroxyalkyl group containing about 1 to about 3 carbon atoms; X is 1 when Y is a sulfur atom, and 2 when Y is a nitrogen or phosphorus atom; R 4  is an alkylene or hydroxyalkylene of from about 1 to about 4 carbon atoms and Z is a radical selected from the group consisting of carboxylate, sulfonate, sulfate, phosphonate, and phosphate groups. 
     Other zwitterionic surfactants suitable for use herein include betaines, including high alkyl betaines such as coco dimethyl carboxymethyl betaine, cocoamidopropyl betaine, cocobetaine, lauryl amidopropyl betaine, oleyl betaine, lauryl dimethyl carboxymethyl betaine, lauryl dimethyl alphacarboxyethyl betaine, cetyl dimethyl carboxymethyl betaine, lauryl bis-(2-hydroxyethyl) carboxymethyl betaine, stearyl bis-(2-hydroxypropyl) carboxymethyl betaine, oleyl dimethyl gamma-carboxypropyl betaine, and lauryl bis-(2-hydroxypropyl)alpha-carboxyethyl betaine. The sulfobetaines may be represented by coco dimethyl sulfopropyl betaine, stearyl dimethyl sulfopropyl betaine, lauryl dimethyl sulfoethyl betaine, lauryl bis-(2-hydroxyethyl) sulfopropyl betaine and the like; amidobetaines and amidosulfobetaines, wherein the RCONH(CH 2 ) 3  radical, wherein R is a C 11 -C 17  alkyl, is attached to the nitrogen atom of the betaine are also useful in this invention. 
     Cationic surfactants can also be utilized in the present invention, especially in fabric softener and hair conditioner products. When used in making products that contain cationic surfactants as the major surfactants, it is preferred that such cationic surfactants are present in an amount ranging from about 2% to about 30%, preferably from about 3% to about 20%, more preferably from about 5% to about 15% by total weight of the solid sheet article. 
     Cationic surfactants may include DEQA compounds, which encompass a description of diamido actives as well as actives with mixed amido and ester linkages. Preferred DEQA compounds are typically made by reacting alkanolamines such as MDEA (methyldiethanolamine) and TEA (triethanolamine) with fatty acids. Some materials that typically result from such reactions include N,N-di(acyl-oxyethyl)-N,N-dimethylammonium chloride or N,N-di(acyl-oxyethyl)-N,N-methylhydroxyethylammonium methylsulfate wherein the acyl group is derived from animal fats, unsaturated, and polyunsaturated, fatty acids. 
     Other suitable actives for use as a cationic surfactant include reaction products of fatty acids with dialkylenetriamines in, e.g., a molecular ratio of about 2:1, said reaction products containing compounds of the formula: 
       R 1 —C(O)—NH—R 2 —NH—R 3 —NH—C(O)—R 1  
 
     wherein R 1 , R 2  are defined as above, and each R 3  is a C 1-6  alkylene group, preferably an ethylene group. Examples of these actives are reaction products of tallow acid, canola acid, or oleic acids with diethylenetriamine in a molecular ratio of about 2:1, said reaction product mixture containing N,N″-ditallowoyldiethylenetriamine, N,N″-dicanola-oyldiethylenetriamine, or N,N″-dioleoyldiethylenetriamine, respectively, with the formula: 
       R 1 —C(O)—NH—CH 2 CH 2 —NH—CH 2 CH 2 —NH—C(O)—R 1  
 
     wherein R 2  and R 3  are divalent ethylene groups, R 1  is defined above and an acceptable examples of this structure when R 1  is the oleoyl group of a commercially available oleic acid derived from a vegetable or animal source, include EMERSOL® 223LL or EMERSOL® 7021, available from Henkel Corporation. 
     Another active for use as a cationic surfactant has the formula: 
       [R 1 —C(O)—NR—R 2 —N(R) 2 —R 3 —NR—C(O)—R 1 ] + X − 
 
     wherein R, R 1 , R 2 , R 3  and X −  are defined as above. Examples of this active are the di-fatty amidoamines based softener having the formula: 
       [R 1 —C(O)—NH—CH 2 CH 2 —N(CH 3 )(CH 2 CH 2 OH)—CH 2 CH 2 —NH—C(O)—R 1 ] + CH 3 SO 4   − 
 
     wherein R 1 —C(O) is an oleoyl group, soft tallow group, or a hardened tallow group available commercially from Degussa under the trade names VARISOFT® 222LT, VARISOFT® 222, and VARISOFT® 110, respectively. 
     A second type of DEQA (“DEQA (2)”) compound suitable as a active for use as a cationic surfactant has the general formula: 
       [R 3 N + CH 2 CH(YR 1 )(CH 2 YR 1 )]X −   
     wherein each Y, R, R 1 , and X −  have the same meanings as before. An example of a preferred DEQA (2) is the “propyl” ester quaternary ammonium fabric softener active having the formula 1,2-di(acyloxy)-3-trimethylammoniopropane chloride. 
     Suitable polymeric surfactants for use in the personal care compositions of the present invention include, but are not limited to, block copolymers of ethylene oxide and fatty alkyl residues, block copolymers of ethylene oxide and propylene oxide, hydrophobically modified polyacrylates, hydrophobically modified celluloses, silicone polyethers, silicone copolyol esters, diquaternary polydimethylsiloxanes, and co-modified amino/polyether silicones. 
     3. Plasticizers 
     In a preferred embodiment of the present invention, the flexible, porous, dissolvable solid sheet article of the present invention further comprises a plasticizer, preferably in the amount ranging from about 0.1% to about 25%, preferably from about 0.5% to about 20%, more preferably from about 1% to about 15%, most preferably from 2% to 12%, by total weight of said solid sheet article. Correspondingly, the wet pre-mixture used for forming such solid sheet article may comprise from about 0.02% to about 20% by weight of said wet pre-mixture, in one embodiment from about 0.1% to about 10% by weight of said wet pre-mixture, in one embodiment from about 0.5% to about 5% by weight of the wet pre-mixture. 
     Suitable plasticizers for use in the present invention include, for example, polyols, copolyols, polycarboxylic acids, polyesters, dimethicone copolyols, and the like. 
     Examples of useful polyols include, but are not limited to: glycerin, diglycerin, ethylene glycol, polyethylene glycol (especially 200-600), propylene glycol, butylene glycol, pentylene glycol, glycerol derivatives (such as propoxylated glycerol), glycidol, cyclohexane dimethanol, hexanediol, 2,2,4-trimethylpentane-1,3-diol, pentaerythritol, urea, sugar alcohols (such as sorbitol, mannitol, lactitol, xylitol, maltitol, and other mono- and polyhydric alcohols), mono-, di- and oligo-saccharides (such as fructose, glucose, sucrose, maltose, lactose, high fructose corn syrup solids, and dextrins), ascorbic acid, sorbates, ethylene bisformamide, amino acids, and the like. 
     Examples of polycarboxylic acids include, but are not limited to citric acid, maleic acid, succinic acid, polyacrylic acid, and polymaleic acid. 
     Examples of suitable polyesters include, but are not limited to, glycerol triacetate, acetylated-monoglyceride, diethyl phthalate, triethyl citrate, tributyl citrate, acetyl triethyl citrate, acetyl tributyl citrate. 
     Examples of suitable dimethicone copolyols include, but are not limited to, PEG-12 dimethicone, PEG/PPG-18/18 dimethicone, and PPG-12 dimethicone. 
     Other suitable platicizers include, but are not limited to, alkyl and allyl phthalates; napthalates; lactates (e.g., sodium, ammonium and potassium salts); sorbeth-30; urea; lactic acid; sodium pyrrolidone carboxylic acid (PCA); sodium hyraluronate or hyaluronic acid; soluble collagen; modified protein; monosodium L-glutamate; alpha &amp; beta hydroxyl acids such as glycolic acid, lactic acid, citric acid, maleic acid and salicylic acid; glyceryl polymethacrylate; polymeric plasticizers such as polyquaterniums; proteins and amino acids such as glutamic acid, aspartic acid, and lysine; hydrogen starch hydrolysates; other low molecular weight esters (e.g., esters of C 2 -C 10  alcohols and acids); and any other water soluble plasticizer known to one skilled in the art of the foods and plastics industries; and mixtures thereof. 
     Particularly preferred examples of plasticizers include glycerin, ethylene glycol, polyethylene glycol, propylene glycol, and mixtures thereof. Most preferred plasticizer is glycerin. 
     4. Additional Ingredients 
     In addition to the above-described ingredients, e.g., the water-soluble polymer, the surfactant(s) and the plasticizer, the solid sheet article of the present invention may comprise one or more additional ingredients, depending on its intended application. Such one or more additional ingredients may be selected from the group consisting of fabric care actives, dishwashing actives, hard surface cleaning actives, beauty and/or skin care actives, personal cleansing actives, hair care actives, oral care actives, feminine care actives, baby care actives, and any combinations thereof. 
     Suitable fabric care actives include but are not limited to: organic solvents (linear or branched lower C 1 -C 8  alcohols, diols, glycerols or glycols; lower amine solvents such as C 1 -C 4  alkanolamines, and mixtures thereof; more specifically 1,2-propanediol, ethanol, glycerol, monoethanolamine and triethanolamine), carriers, hydrotropes, builders, chelants, dispersants, enzymes and enzyme stabilizers, catalytic materials, bleaches (including photobleaches) and bleach activators, perfumes (including encapsulated perfumes or perfume microcapsules), colorants (such as pigments and dyes, including hueing dyes), brighteners, dye transfer inhibiting agents, clay soil removal/anti-redeposition agents, structurants, rheology modifiers, suds suppressors, processing aids, fabric softeners, anti-microbial agents, and the like. 
     Suitable hair care actives include but are not limited to: moisture control materials of class II for frizz reduction (salicylic acids and derivatives, organic alcohols, and esters), cationic surfactants (especially the water-insoluble type having a solubility in water at 25° C. of preferably below 0.5 g/100 g of water, more preferably below 0.3 g/100 g of water), high melting point fatty compounds (e.g., fatty alcohols, fatty acids, and mixtures thereof with a melting point of 25° C. or higher, preferably 40° C. or higher, more preferably 45° C. or higher, still more preferably 50° C. or higher), silicone compounds, conditioning agents (such as hydrolyzed collagen with tradename Peptein 2000 available from Hormel, vitamin E with tradename Emix-d available from Eisai, panthenol available from Roche, panthenyl ethyl ether available from Roche, hydrolyzed keratin, proteins, plant extracts, and nutrients), preservatives (such as benzyl alcohol, methyl paraben, propyl paraben and imidazolidinyl urea), pH adjusting agents (such as citric acid, sodium citrate, succinic acid, phosphoric acid, sodium hydroxide, sodium carbonate), salts (such as potassium acetate and sodium chloride), coloring agents, perfumes or fragrances, sequestering agents (such as disodium ethylenediamine tetra-acetate), ultraviolet and infrared screening and absorbing agents (such as octyl salicylate), hair bleaching agents, hair perming agents, hair fixatives, anti-dandruff agents, anti-microbial agents, hair growth or restorer agents, co-solvents or other additional solvents, and the like. 
     Suitable beauty and/or skin care actives include those materials approved for use in cosmetics and that are described in reference books such as the CTFA Cosmetic Ingredient Handbook, Second Edition, The Cosmetic, Toiletries, and Fragrance Association, Inc. 1988, 1992. Further non-limiting examples of suitable beauty and/or skin care actives include preservatives, perfumes or fragrances, coloring agents or dyes, thickeners, moisturizers, emollients, pharmaceutical actives, vitamins or nutrients, sunscreens, deodorants, sensates, plant extracts, nutrients, astringents, cosmetic particles, absorbent particles, fibers, anti-inflammatory agents, skin lightening agents, skin tone agent (which functions to improve the overall skin tone, and may include vitamin B3 compounds, sugar amines, hexamidine compounds, salicylic acid, 1,3-dihydroxy-4-alkybenzene such as hexylresorcinol and retinoids), skin tanning agents, exfoliating agents, humectants, enzymes, antioxidants, free radical scavengers, anti-wrinkle actives, anti-acne agents, acids, bases, minerals, suspending agents, pH modifiers, pigment particles, anti-microbial agents, insect repellents, shaving lotion agents, co-solvents or other additional solvents, and the like. 
     The solid sheet article of the present invention may further comprise other optional ingredients that are known for use or otherwise useful in compositions, provided that such optional materials are compatible with the selected essential materials described herein, or do not otherwise unduly impair product performance. 
     Non-limiting examples of product type embodiments that can be formed by the solid sheet article of the present invention include laundry detergent products, fabric softening products, hand cleansing products, hair shampoo or other hair treatment products, body cleansing products, shaving preparation products, dish cleaning products, personal care substrates containing pharmaceutical or other skin care actives, moisturizing products, sunscreen products, beauty or skin care products, deodorizing products, oral care products, feminine cleansing products, baby care products, fragrance-containing products, and so forth. 
     VI. Conversion of Multiple Sheets into Multilayer Structures 
     Once the flexible, dissolvable, porous solid sheet articles of the present invention is formed, as described hereinabove, two or more of such sheets can be further combined and/or treated to form dissolvable solid articles of any desirable three-dimensional shapes, including but not limited to: spherical, cubic, rectangular, oblong, cylindrical, rod, sheet, flower-shaped, fan-shaped, star-shaped, disc-shaped, and the like. The sheets can be combined and/or treated by any means known in the art, examples of which include but are not limited to, chemical means, mechanical means, and combinations thereof. Such combination and/or treatment steps are hereby collectively referred to as a “conversion” process, i.e., which functions to convert two or more flexible, dissolvable, porous sheets of the present invention into a dissolvable solid article with a desired three-dimensional shape. 
     Conventional dissolvable solid articles have relatively high length/width-to-thickness ratios, i.e., they are relatively thin, in order to ensure fast dissolution of such articles in water. Therefore, such dissolvable solid articles typically are typically provided in form of relatively large but thin sheet products, which may be difficult to handle (e.g., too floppy and easily sticking together and hard to separate upon use) and are not aesthetically pleasing to the consumers. However, there is little or no space for change or improvement of such product form, due to constraints imparted by the dissolution requirement. 
     It has been a surprising and unexpected discovery of the present invention that three-dimensional multilayer solid articles formed by stacking multiple layers of the solid sheet articles of the present invention together are more dissolvable than single-layer solid articles that have the same aspect ratio. This allows significant extension of such solid articles along the thickness direction, to create three-dimensional product shapes that are easier to handle and more aesthetically pleasing to the consumers (e.g., products in form of thick pads or even cubes). 
     Specifically, the multilayer dissolvable solid articles formed by stacking multiple layers of the solid sheet articles of the present invention together is characterized by a maximum dimension D and a minimum dimension z (which is perpendicular to the maximum dimension), while the ratio of D/z (hereinafter also referred to as the “Aspect Ratio”) ranges from 1 to about 10, preferably from about 1.4 to about 9, preferably from about 1.5 to about 8, more preferably from about 2 to about 7. Note that when the Aspect Ratio is 1, the dissolvable solid article has a spherical shape. When the Aspect Ratio is about 1.4, the dissolvable solid article has a cubical shape. 
     The multilayer dissolvable solid article of the present invention may have a minimal dimension z that is greater than about 3 mm but less than about 20 cm, preferably from about 4 mm to about 10 cm, more preferably from about 5 mm to about 30 mm. 
     The above-described multilayer dissolvable solid article may comprise more than two of such flexible, dissolvable, porous sheets. For example, it may comprise from about 4 to about 50, preferably from about 5 to about 40, more preferably from about 6 to about 30, of said flexible, dissolvable, porous sheets. The improved OCF structures in the flexible, dissolvable, porous sheets made according to the present invention allow stacking of many sheets (e.g., 15-40) together, while still providing a satisfactory overall dissolution rate for the stack. 
     In a particularly preferred embodiment of the present invention, the multilayer dissolvable solid article comprises from 15 to 40 layers of the above-described flexible, dissolvable, porous sheets and has an aspect ratio ranging from about 2 to about 7. 
     The multilayer dissolvable solid article of the present invention may comprise individual sheets of different colors, which are visual from an external surface (e.g., one or more side surfaces) of such article. Such visible sheets of different colors are aesthetically pleasing to the consumers. Further, the different colors of individual sheets may provide visual cues indicative of different benefit agents contained in the individual sheets. For example, the multilayer dissolvable solid article may comprise a first sheet that has a first color and contains a first benefit agent and a second sheet that has a second color and contains a second benefit, while the first color provides a visual cue indicative of the first benefit agent, and while the second color provides a visual cue indicative of the second benefit agent. 
     Further, one or more functional ingredients can be “sandwiched” between individual sheets of the multilayer dissolvable solid article as described hereinabove, e.g., by spraying, sprinkling, dusting, coating, spreading, dipping, injecting, or even vapor deposition. In order to avoid interference of such functional ingredients with the cutting seal or edge seal near the peripherals of the individual sheets, it is preferred that such functional ingredients are located within a central region between two adjacent sheets, which is defined as a region that is spaced apart from the peripherals of such adjacent sheets by a distance that is at least 10% of the maximum Dimension D. 
     Suitable functional ingredients can be selected from the group consisting of cleaning actives (surfactants, free perfumes, encapsulated perfumes, perfume microcapsules, silicones, softening agents, enzymes, bleaches, colorants, builders, rheology modifiers, pH modifiers, and combinations thereof) and personal care actives (e.g., emollients, humectants, conditioning agents, and combinations thereof). 
     Test Methods 
     Test 1: Scanning Electron Microscopic (SEM) Method for Determining Surface Average Pore Diameter of the Sheet Article 
     A Hitachi TM3000 Tabletop Microscope (S/N: 123104-04) is used to acquire SEM micrographs of samples. Samples of the solid sheet articles of the present invention are approximately 1 cm×1 cm in area and cut from larger sheets. Images are collected at a magnification of 50×, and the unit is operated at 15 kV. A minimum of 5 micrograph images are collected from randomly chosen locations across each sample, resulting in a total analyzed area of approximately 43.0 mm 2  across which the average pore diameter is estimated. 
     The SEM micrographs are then firstly processed using the image analysis toolbox in Matlab. Where required, the images are converted to grayscale. For a given image, a histogram of the intensity values of every single pixel is generated using the ‘imhist’ Matlab function. Typically, from such a histogram, two separate distributions are obvious, corresponding to pixels of the brighter sheet surface and pixels of the darker regions within the pores. A threshold value is chosen, corresponding to an intensity value between the peak value of these two distributions. All pixels having an intensity value lower than this threshold value are then set to an intensity value of 0, while pixels having an intensity value higher are set to 1, thus producing a binary black and white image. The binary image is then analyzed using ImageJ (https://imagej.nih.gov, version 1.52a), to examine both the pore area fraction and pore size distribution. The scale bar of each image is used to provide a pixel/mm scaling factor. For the analysis, the automatic thresholding and the analyze particles functions are used to isolate each pore. Output from the analyze function includes the area fraction for the overall image and the pore area and pore perimeter for each individual pore detected. 
     Average Pore Diameter is defined as DA50:50% of the total pore area is comprised of pores having equal or smaller hydraulic diameters than the DA50 average diameter. 
       Hydraulic diameter=‘4*Pore area (m 2 )/Pore perimeter (m)’.
 
     It is an equivalent diameter calculated to account for the pores not all being circular. 
     Test 2: Micro-Computed Tomographic (μCT) Method for Determining Overall or Regional Average Pore Size and Average Cell Wall Thickness of the Open Cell Foams (OCF) 
     Porosity is the ratio between void-space to the total space occupied by the OCF. Porosity can be calculated from μCT scans by segmenting the void space via thresholding and determining the ratio of void voxels to total voxels. Similarly, solid volume fraction (SVF) is the ratio between solid-space to the total space, and SVF can be calculated as the ratio of occupied voxels to total voxels. Both Porosity and SVF are average scalar-values that do not provide structural information, such as, pore size distribution in the height-direction of the OCF, or the average cell wall thickness of OCF struts. 
     To characterize the 3D structure of the OCFs, samples are imaged using a μCT X-ray scanning instrument capable of acquiring a dataset at high isotropic spatial resolution. One example of suitable instrumentation is the SCANCO system model 50 μCT scanner (Scanco Medical AG, Bruttisellen, Switzerland) operated with the following settings: energy level of 45 kVp at 133 μA; 3000 projections; 15 mm field of view; 750 ms integration time; an averaging of 5; and a voxel size of 3 μm per pixel. After scanning and subsequent data reconstruction is complete, the scanner system creates a 16 bit data set, referred to as an ISQ file, where grey levels reflect changes in x-ray attenuation, which in turn relates to material density. The ISQ file is then converted to 8 bit using a scaling factor. 
     Scanned OCF samples are normally prepared by punching a core of approximately 14 mm in diameter. The OCF punch is laid flat on a low-attenuating foam and then mounted in a 15 mm diameter plastic cylindrical tube for scanning. Scans of the samples are acquired such that the entire volume of all the mounted cut sample is included in the dataset. From this larger dataset, a smaller sub-volume of the sample dataset is extracted from the total cross section of the scanned OCF, creating a 3D slab of data, where pores can be qualitatively assessed without edge/boundary effects. 
     To characterize pore-size distribution in the height-direction, and the strut-size, Local Thickness Map algorithm, or LTM, is implemented on the subvolume dataset. The LTM Method starts with a Euclidean Distance Mapping (EDM) which assigns grey level values equal to the distance each void voxel is from its nearest boundary. Based on the EDM data, the 3D void space representing pores (or the 3D solid space representing struts) is tessellated with spheres sized to match the EDM values. Voxels enclosed by the spheres are assigned the radius value of the largest sphere. In other words, each void voxel (or solid voxel for struts) is assigned the radial value of the largest sphere that that both fits within the void space boundary (or solid space boundary for struts) and includes the assigned voxel. 
     The 3D labelled sphere distribution output from the LTM data scan can be treated as a stack of two dimensional images in the height-direction (or Z-direction) and used to estimate the change in sphere diameter from slice to slice as a function of OCF depth. The strut thickness is treated as a 3D dataset and an average value can be assessed for the whole or parts of the subvolume. The calculations and measurements were done using AVIZO Lite (9.2.0) from Thermo Fisher Scientific and MATLAB (R2017a) from Mathworks. 
     Test 3: Percent Open Cell Content of the Sheet Article 
     The Percent Open Cell Content is measured via gas pycnometry. Gas pycnometry is a common analytical technique that uses a gas displacement method to measure volume accurately. Inert gases, such as helium or nitrogen, are used as the displacement medium. A sample of the solid sheet article of the present invention is sealed in the instrument compartment of known volume, the appropriate inert gas is admitted, and then expanded into another precision internal volume. The pressure before and after expansion is measured and used to compute the sample article volume. 
     ASTM Standard Test Method D2856 provides a procedure for determining the percentage of open cells using an older model of an air comparison pycnometer. This device is no longer manufactured. However, one can determine the percentage of open cells conveniently and with precision by performing a test which uses Micromeritics&#39; AccuPyc Pycnometer. The ASTM procedure D2856 describes 5 methods (A, B, C, D, and E) for determining the percent of open cells of foam materials. For these experiments, the samples can be analyzed using an Accupyc 1340 using nitrogen gas with the ASTM foampyc software. Method C of the ASTM procedure is to be used to calculate to percent open cells. This method simply compares the geometric volume as determined using calipers and standard volume calculations to the open cell volume as measured by the Accupyc, according to the following equation: 
       Open cell percentage=Open cell volume of sample/Geometric volume of sample*100 
     It is recommended that these measurements be conducted by Micromeretics Analytical Services, Inc. (One Micromeritics Dr, Suite 200, Norcross, Ga. 30093). More information on this technique is available on the Micromeretics Analytical Services web sites (www.particletesting.com or www.micromeritics.com), or published in “Analytical Methods in Fine particle Technology” by Clyde Orr and Paul Webb. 
     Test 4: Final Moisture Content of the Sheet Article 
     Final moisture content of the solid sheet article of the present invention is obtained by using a Mettler Toledo HX204 Moisture Analyzer (S/N B706673091). A minimum of 1 g of the dried sheet article is placed on the measuring tray. The standard program is then executed, with additional program settings of 10 minutes analysis time and a temperature of 110° C. 
     Test 5: Thickness of the Sheet Article 
     Thickness of the flexible, porous, dissolvable solid sheet article of the present invention is obtained by using a micrometer or thickness gage, such as the Mitutoyo Corporation Digital Disk Stand Micrometer Model Number IDS-1012E (Mitutoyo Corporation, 965 Corporate Blvd, Aurora, Ill., USA 60504). The micrometer has a 1-inch diameter platen weighing about 32 grams, which measures thickness at an application pressure of about 0.09 psi (6.32 gm/cm 2 ). 
     The thickness of the flexible, porous, dissolvable solid sheet article is measured by raising the platen, placing a section of the sheet article on the stand beneath the platen, carefully lowering the platen to contact the sheet article, releasing the platen, and measuring the thickness of the sheet article in millimeters on the digital readout. The sheet article should be fully extended to all edges of the platen to make sure thickness is measured at the lowest possible surface pressure, except for the case of more rigid substrates which are not flat. 
     Test 6: Basis Weight of the Sheet Article 
     Basis Weight of the flexible, porous, dissolvable solid sheet article of the present invention is calculated as the weight of the sheet article per area thereof (grams/m 2 ). The area is calculated as the projected area onto a flat surface perpendicular to the outer edges of the sheet article. The solid sheet articles of the present invention are cut into sample squares of 10 cm×10 cm, so the area is known. Each of such sample squares is then weighed, and the resulting weight is then divided by the known area of 100 cm 2  to determine the corresponding basis weight. 
     For an article of an irregular shape, if it is a flat object, the area is thus computed based on the area enclosed within the outer perimeter of such object. For a spherical object, the area is thus computed based on the average diameter as 3.14×(diameter/2) 2 . For a cylindrical object, the area is thus computed based on the average diameter and average length as diameter x length. For an irregularly shaped three-dimensional object, the area is computed based on the side with the largest outer dimensions projected onto a flat surface oriented perpendicularly to this side. This can be accomplished by carefully tracing the outer dimensions of the object onto a piece of graph paper with a pencil and then computing the area by approximate counting of the squares and multiplying by the known area of the squares or by taking a picture of the traced area (shaded-in for contrast) including a scale and using image analysis techniques. 
     Test 7: Density of the Sheet Article 
     Density of the flexible, porous, dissolvable solid sheet article of the present invention is determined by the equation: Calculated Density=Basis Weight of porous solid/(Porous Solid Thickness×1,000). The Basis Weight and Thickness of the dissolvable porous solid are determined in accordance with the methodologies described hereinabove. 
     Test 8: Specific Surface Area of the Sheet Article 
     The Specific Surface Area of the flexible, porous, dissolvable solid sheet article is measured via a gas adsorption technique. Surface Area is a measure of the exposed surface of a solid sample on the molecular scale. The BET (Brunauer, Emmet, and Teller) theory is the most popular model used to determine the surface area and is based upon gas adsorption isotherms. Gas Adsorption uses physical adsorption and capillary condensation to measure a gas adsorption isotherm. The technique is summarized by the following steps; a sample is placed in a sample tube and is heated under vacuum or flowing gas to remove contamination on the surface of the sample. The sample weight is obtained by subtracting the empty sample tube weight from the combined weight of the degassed sample and the sample tube. The sample tube is then placed on the analysis port and the analysis is started. The first step in the analysis process is to evacuate the sample tube, followed by a measurement of the free space volume in the sample tube using helium gas at liquid nitrogen temperatures. The sample is then evacuated a second time to remove the helium gas. The instrument then begins collecting the adsorption isotherm by dosing krypton gas at user specified intervals until the requested pressure measurements are achieved. Samples may then analyzed using an ASAP 2420 with krypton gas adsorption. It is recommended that these measurements be conducted by Micromeretics Analytical Services, Inc. (One Micromeritics Dr, Suite 200, Norcross, Ga. 30093). More information on this technique is available on the Micromeretics Analytical Services web sites (www.particletesting.com or www.micromeritics.com), or published in a book, “Analytical Methods in Fine Particle Technology”, by Clyde Orr and Paul Webb. 
     Test 9: Dissolution Rate 
     Firstly, the solid sheets are stored under ambient relative humidity of 50±2% and ambient temperature of 23±1° C. for 24 hours (i.e., a conditioning step). Following the initial conditioning step described above, 25 mm diameter discs are firstly cut from the large solid sheet using a 25 mm hollow hole punch. The required number of foam discs is set such that the total mass of all foam discs is no less than 0.1 g. 
     The required number of foam discs are then stacked in a head to toe orientation and placed inside an Omnifit™ EZ chromatography column (006EZ-25-10-AF) having 25 mm inner diameter, 100 m length and an adjustable, removable endpiece. The stack of foam discs is placed inside the column such that the direction of flow through the column is perpendicular to the top surface of the foam discs. Once placed inside the column, the endpiece is inserted into the column and adjusted until the perpendicular distance between the two inner frits is equal to the thickness of the stack of foam discs. 
     Masterflex silicone tubing (MFLEX SILICONE #25 25′) and a Masterflex peristaltic pump (MFLX L/S 1CH 300R 115/230 13124) are used to control the flow of water through the column. The system flow rate is calibrated by flowing water through the pump, tubing and an empty column at different pump RPM settings and recording the volume of water collected over a defined period of time. For all experiments a flow rate of 5 litres per hour was utilized. 
     The inlet and outlet tubing are both placed inside a 1 litre beaker containing 500 ml of deionised water at ambient temperature. The beaker is placed on a magnetic stirrer plate, and a magnetic stirrer bar having length 23 mm and thickness 10 mm is placed in the beaker, and the stirrer rotation speed is set to 300 rpm. A Mettler Toledo 5230 conductivity meter is calibrated to 1413 μS/cm and the probe placed in the beaker of water. 
     The flow of water through the system is started. Once the first drops of water can be visibly seen inside the column and in contact with the foam, the data recording function of the conductivity meter is started. Data is recorded for at least 20 minutes. 
     In order to estimate the time required to reach a 90 or 95% percentage dissolution of the foam, a calibration curve is firstly generated where layers of the foam discs are dropped one a time into a stirred beaker of 500 ml deionised water. The mass of each individual foam disc, and the conductivity after 5 minutes are both recorded. This process is repeated for up to 5 discs total. A linear function is fitted to the data, which is then used to estimate the maximum conductivity in each dissolution experiment based on the total mass of the foam discs placed in the column. The percentage dissolution is then calculated as 
       % Dissolution=Experimentally measured conductivity/Maximum conductivity*100 
     The time required to achieve 90 or 95 percentage dissolution is then found from this calculated data. The calibration procedure is repeated for each formula tested. 
     Test 10: Bubble Size 
     The bubble size of aerated pre-mixture is measured as follows: 
     Rectangular glass cover slides, having a width and a length of 2 cm and a thickness of 1 mm are firstly glued onto a glass slide having a width of 6 cm and a length of 2 cm, such that a cavity having a thickness of 1 mm, a length of 2 cm and a width of slightly less than 2 cm is located in the center of the glass slide. The width of the cavity must be kept at less than 2 cm so that an additional cover slide can be placed on top of the cavity. 
     To capture the image for bubble size analysis, the aerated liquid foam is deposited into the cavity using a spatula and another cover slide placed on top and pressed down gently, in order to reduce the thickness of the liquid to 1 mm. 
     A SMZ-T4 Chongqing Optec microscope and RZIMAGE MicroUL300 digital camera were used to capture the images. The glass slide was placed onto the backlit area of the microscope, and the magnification adjusted such that the image area was no less than 16 mm 2 . An additional image was taken with a transparent ruler placed in the image area, such that the graduated lines could be seen and used to determine the pixel to distance ratio. 
     The bubble sizes were calculated using the ‘imfindcircles’ function in the Image Analysis Toolbox of the Matlab 2017b software. For each image, the function was called four times, for pixel size ranges of 21 to 40, 41 to 50, 51 to 100 and 101 to 200, respectively, where 20 pixels corresponds to an approximate length of 60 micron. The sensitivity parameter was set to 0.95. The bubble radii estimated from each call of the function were combined to generate a single distribution, and the radii converted to microns using the calibration image generated with the transparent ruler. 
     EXAMPLES 
     Example 1: Different OCF Structures in Solid Sheet Articles Made by Different Heating/Drying Arrangements 
     Wet pre-mixtures with the following surfactant/polymer compositions as described in Table 1 and Table 2 below are prepared, for laundry care and hair care articles, respectively. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 (LAUNDRY CARE FORMULATION) 
               
            
           
           
               
               
               
            
               
                 Materials: 
                 (Wet) w/w % 
                 (Dry) w/w % 
               
               
                   
               
            
           
           
               
               
               
            
               
                 Polyvinyl alcohol (with a degree of 
                 7.58 
                 21 
               
               
                 polymerization of about 1700, a 
               
               
                 hydrolysis level of 88%) 
               
               
                 Glycerin 
                 1.08 
                 3 
               
               
                 Linear Alkylbenzene Sulfonate 
                 19.12 
                 53 
               
               
                 Sodium Laureth-3 Sulfate 
                 3.61 
                 10 
               
               
                 C12-C14 Ethoxylated alcohol 
                 3.61 
                 10 
               
               
                 Water 
                 Balance 
                 Balance 
               
               
                   
               
            
           
         
       
     
     Viscosity of the wet pre-mixture composition as described in Table 1 is about 14309.8 cps. After aeration, the average density of such aerated wet pre-mixture is about 0.25 g/cm 3 . 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 (HAIR CARE FORMULATION - SHAMPOO) 
               
            
           
           
               
               
               
            
               
                 Materials: 
                 (Wet) w/w % 
                 (Dry) w/w % 
               
               
                   
               
            
           
           
               
               
               
            
               
                 Polyvinyl alcohol (with a degree of 
                 6.85 
                 23.69 
               
               
                 polymerization of about 1700, a 
               
               
                 hydrolysis level of 88%) 
               
               
                 Glycerin 
                 2.75 
                 9.51 
               
               
                 Sodium Lauryl Sulfate 
                 9.52 
                 32.89 
               
               
                 Sodium Laureth-3 Sulfate 
                 3.01 
                 10.42 
               
               
                 Sodium Lauroamphoacetate 
                 5 
                 17.28 
               
               
                 Citric acid (anhydrous) 
                 0.93 
                 3.21 
               
               
                 Water 
                 Balance 
                 Balance 
               
               
                   
               
            
           
         
       
     
     Viscosity of the wet pre-mixture composition as described in Table 2 is about 19254.6 cps. After aeration, the average density of such aerated wet pre-mixture is about 0.225 g/cm 3 . 
     Flexible, porous, dissolvable solid sheet articles A and B are prepared from the above wet pre-mixtures as described in Tables 1 and 2 using a continuous aerator (Aeros) and a rotary drum dryer in which a spinning bar is employed to feed the aerated wet pre-mixture onto the drum dryer, with the following settings and conditions as described in Table 3 below: 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 (DRUM DRYING) 
               
            
           
           
               
               
               
            
               
                   
                 Parameters 
                 Value 
               
               
                   
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Wet pre-mixture temperature before and 
                 80° 
                 C. 
               
               
                   
                 during aeration 
               
            
           
           
               
               
               
            
               
                   
                 Aeros feed pump speed setting 
                 600 
               
               
                   
                 Aeros mixing head speed setting 
                 500 
               
               
                   
                 Aeros air flow rate setting 
                 100 
               
            
           
           
               
               
               
               
            
               
                   
                 Wet pre-mixture temperature before drying 
                 60° 
                 C. 
               
               
                   
                 Spinning bar speed 
                 150 
                 rpm 
               
               
                   
                 Distance between spinning bar and drum dryer 
                 2 
                 mm 
               
               
                   
                 Rotary drum dryer surface temperature 
                 130° 
                 C. 
               
               
                   
                 Rotary drum dryer rotational speed 
                 0.160 
                 rpm 
               
               
                   
                 Drying time 
                 4.52 
                 min 
               
               
                   
                   
               
            
           
         
       
     
     A flexible, porous, dissolvable solid sheet article C is also prepared from the above wet pre-mixture as described in Table 2 using a continuous aerator (Oakes) and a mold placed on a hot plate (which provides bottom conduction-based heating), with the following settings and conditions as described in Table 4 below: 
     
       
         
           
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                 (HOT PLATE DRYING) 
               
            
           
           
               
               
               
            
               
                   
                 Parameters 
                 Value 
               
               
                   
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Wet pre-mixture temperature before and 
                 80° 
                 C. 
               
               
                   
                 during aeration 
               
               
                   
                 Oakes air flow meter setting 
                 19.2 
                 L/hour 
               
               
                   
                 Oakes pump meter speed setting 
                 20 
                 rpm 
               
               
                   
                 Oakes mixing head speed 
                 1500 
                 rpm 
               
               
                   
                 Mold depth 
                 1.0 
                 mm 
               
               
                   
                 Hot plate surface temperature 
                 130° 
                 C. 
               
               
                   
                 Drying time 
                 12.5 
                 min 
               
               
                   
                   
               
            
           
         
       
     
     Further, flexible, porous, dissolvable solid sheet articles I and II are prepared from the above wet pre-mixtures described in Tables 1 and 2 using a continuous aerator (Oakes) and a mold placed on an impingement oven, with the following settings and conditions as described in Table 5 below: 
     
       
         
           
               
             
               
                 TABLE 5 
               
             
            
               
                   
               
               
                 (IMPINGEMENT OVEN DRYING) 
               
            
           
           
               
               
               
            
               
                   
                 Parameters 
                 Value 
               
               
                   
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Wet pre-mixture temperature before and 
                 80° 
                 C. 
               
               
                   
                 during aeration 
               
               
                   
                 Oakes air flow meter setting 
                 19.2 
                 L/hour 
               
               
                   
                 Oakes pump meter speed setting 
                 20 
                 rpm 
               
               
                   
                 Oakes mixing head speed 
                 1500 
                 rpm 
               
               
                   
                 Mold depth 
                 1.0 
                 mm 
               
               
                   
                 Impingement oven temperature 
                 130° 
                 C. 
               
               
                   
                 Drying time 
                 6 
                 min 
               
               
                   
                   
               
            
           
         
       
     
     Tables 6-9 as follows summarize various physical parameters and pore structures measured for the solid sheet articles A-C and solid sheet articles I-II made from the above-described wet pre-mixtures and drying processes. 
     
       
         
           
               
             
               
                 TABLE 6 
               
             
            
               
                   
               
               
                 (PHYSICAL PARAMETERS) 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                   
                   
                 Average 
                   
                   
                 Specific 
               
               
                   
                   
                   
                 Basis 
                 Average 
                 Average 
                 Surface 
               
               
                   
                   
                 Drying 
                 Weight 
                 Density 
                 Thickness 
                 Area 
               
               
                 Samples 
                 Formulation 
                 Process 
                 g/m 2   
                 g/cm 3   
                 mm 
                 m 2 /g 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 A 
                 Laundry Care 
                 Rotary Drum 
                 147.5 
                 0.118 
                 1.265 
                 0.115 
               
               
                 B 
                 Hair Care 
                 Rotary Drum 
                 138.4 
                 0.111 
                 1.254 
                 0.107 
               
               
                 C 
                 Hair Care 
                 Hot Plate 
                 216.3 
                 0.111 
                 1.968 
                 — 
               
               
                 I 
                 Laundry Care 
                 Impingement 
                 116.83 
                 0.118 
                 1.002 
                 — 
               
               
                   
                   
                 Oven 
               
               
                 II 
                 Hair Care 
                 Impingement 
                 212.9 
                 0.111 
                 1.929 
                 — 
               
               
                   
                   
                 Oven 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 7 
               
             
            
               
                   
               
               
                 (OVERALL PORE STRUCTURES) 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                   
                   
                 Percent 
                 Overall 
                 Average 
               
               
                   
                   
                   
                 Open Cell 
                 Average 
                 Cell Wall 
               
               
                 Sam- 
                   
                 Drying 
                 Content 
                 Pore Size 
                 Thickness 
               
               
                 ples 
                 Formulation 
                 Process 
                 % 
                 μm 
                 μm 
               
               
                   
               
               
                 A 
                 Laundry Care 
                 Rotary Drum 
                 90.75 
                 467.1 
                 54.3 
               
               
                 B 
                 Hair Care 
                 Rotary Drum 
                 93.54 
                 466.9 
                 42.8 
               
               
                 C 
                 Hair Care 
                 Hot Plate 
                 — 
                 287.4 
                 19.7 
               
               
                 I 
                 Laundry Care 
                 Impingement 
                 — 
                 197.6 
                 15.2 
               
               
                   
                   
                 Oven 
               
               
                 II 
                 Hair Care 
                 Impingement 
                 — 
                 325.1 
                 18.7 
               
               
                   
                   
                 Oven 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 8 
               
             
            
               
                   
               
               
                 (SURFACE AND REGIONAL PORE STRUCTURES) 
               
            
           
           
               
               
               
               
            
               
                   
                   
                 Surface 
                   
               
               
                   
                   
                 Average Pore 
               
               
                   
                   
                 Diameter 
               
               
                   
                 Drying 
                 (μm) 
                 Average Pore Size (μm) 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Samples 
                 Formulation 
                 Process 
                 Top 
                 Top 
                 Middle 
                 Bottom 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 A 
                 Laundry Care 
                 Rotary Drum 
                 201.5 
                 458.3 
                 479.1 
                 463.9 
               
               
                 B 
                 Hair Care 
                 Rotary Drum 
                 138.2 
                 412.4 
                 519.0 
                 469.1 
               
               
                 C 
                 Hair Care 
                 Hot Plate 
                 120.8 
                 259.7 
                 292.0 
                 309.9 
               
               
                 I 
                 Laundry Care 
                 Impingement 
                 53.3 
                 139.9 
                 213.1 
                 238.7 
               
               
                   
                   
                 Oven 
               
               
                 II 
                 Hair Care 
                 Impingement 
                 60.0 
                 190.7 
                 362.6 
                 419.6 
               
               
                   
                   
                 Oven 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 9 
               
             
            
               
                   
               
               
                 (VARIATIONS BETWEEN REGIONAL PORE STRUCTURES) 
               
            
           
           
               
               
               
            
               
                   
                   
                 Btw-Region Ratios of 
               
               
                   
                 Cross-Region 
                 Average Pore Sizes 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                   
                 Drying 
                 Relative STD 
                 Bottom- 
                 Bottom- 
                 Middle- 
               
               
                 Samples 
                 Formulation 
                 Process 
                 (%) 
                 to-Top 
                 to-Middle 
                 to-Top 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 A 
                 Laundry Care 
                 Rotary Drum 
                 2.31% 
                 1.012 
                 0.968 
                 1.046 
               
               
                 B 
                 Hair Care 
                 Rotary Drum 
                 11.43% 
                 1.137 
                 0.904 
                 1.259 
               
               
                 C 
                 Hair Care 
                 Hot Plate 
                 8.84% 
                 1.193 
                 1.061 
                 1.124 
               
               
                 I 
                 Laundry Care 
                 Impingement 
                 25.99% 
                 1.706 
                 1.120 
                 1.523 
               
               
                   
                   
                 Oven 
               
               
                 II 
                 Hair Care 
                 Impingement 
                 36.74% 
                 2.200 
                 1.157 
                 1.901 
               
               
                   
                   
                 Oven 
               
               
                   
               
            
           
         
       
     
     The above data demonstrates that when the heating direction is offset from the gravitation direction during most of the drying step, the resulting solid sheet article (e.g., Articles A, B and C) may have a top surface with larger pore openings and reduced pore size variations in different regions along the direction across the thickness of such sheet article compared to the solid sheet articles obtained when the heating direction is substantially aligned with the gravitational direction (e.g., Articles I and II). Particularly, the above tables show that Articles A, B and C have Top Surface Average Pore Diameters of greater than 100 μm, while the Articles I and II do not. Specifically,  FIG. 6A  shows a Scanning Electron Microscopic (SEM) image of the top surface of Article A, while  FIG. 6B  shows a SEM image of the top surface of Article I.  FIG. 7A  shows a SEM image of the top surface of Article C, while  FIG. 7B  shows a SEM image of the top surface of Article II. 
     Example 2: Increased Bubble Size in an Aged Aerated Pre-Mixture Compared to an Aerated Pre-Mixture Before the Aging Step 
     A wet pre-mixture (i.e., a slurry) containing the ingredients of the solid sheet article (Formulation 1) shown in the following Table 10 and additional water is prepared, to result in a total solids content of about 35% by weight (i.e., the total water content in the slurry is about 65% by weight). 
     
       
         
           
               
               
             
               
                 TABLE 10 
               
               
                   
               
               
                   
                 Formulation 1 
               
               
                 Materials (Dry), wt % 
                 (Fabric Care) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
            
               
                 Polyvinyl alcohol (with a degree of polymerization 
                 18.00 
               
               
                 of about 1700, a hydrolysis level of 88%) 
               
               
                 Polyvinyl alcohol (with a degree of polymerization 
                 6.00 
               
               
                 of about 500, a hydrolysis level of 88%) 
               
               
                 Glycerin 
                 3.51 
               
               
                 Linear Alkylbenzene Sulfonate 
                 40.00 
               
               
                 Sodium Laureth-3 Sulfate 
                 4.60 
               
               
                 C12-C14 Ethoxylated alcohol 
                 16.00 
               
               
                 Ethoxylated Polyethyleneimine 
                 1.50 
               
               
                 Palm kernel fatty acid soap powder 
                 2.07 
               
               
                 Sodium Aluminosilicate (crystalline)/Zeolite 
                 0.95 
               
               
                 Denatonium Benzoate 
                 0.04 
               
               
                 Water 
                 6.00 
               
               
                 Miscellaneous 
                 Q.S. 
               
               
                   
               
            
           
         
       
     
     The method of slurry preparation is as follows:
         1. Water and glycerin are firstly added together into a glass beaker and stirred at 200 rpm using an overhead stirrer.   2. While continuing to stir, the polyvinyl alcohol is then slowly added into the beaker containing water and glycerin, ensuring that no foaming of the solution or clumping of the polyvinyl alcohol occurred.   3. The beaker is then placed in a water bath and heated to 80° C. while continuing stirring. The beaker is covered with clingfilm or tinfoil in order to mitigate water evaporation and left to continue mixing for at least 1.0 hour.   4. The remaining components are weighed and added together in a separate glass beaker. The balance of water required to achieve 65% total water content in the slurry is also added to this beaker.   5. This beaker is placed in a water bath at 80° C., and its contents are stirred using an overhead stirrer at 500 rpm for at least 30 minutes.   6. Once the predefined mixing time is reached in both beakers, the contents of both are added together into a single glass beaker, followed by continued stirring at 500 rpm and the temperature is maintained at 80° C. for at least another 30 minutes.       

     The slurry so formed is then aerated as follows:
         1. An Aeros A20 continuous aerator, consisting of a jacketed hopper (model JCABT10) and A20 mixing head, is preheated to 80° C. using a water bath and pump.   2. The slurry prepared previously is then added to the hopper. The aerator unit is then switched on and the mixing head speed, feed pump speed, and air flow rates were set to 600, 500 and 100 respectively.   3. The aerated slurry is collected from the aerator outlet and its density measured by filling a density cup of known volume and weighing the mass of the aerated slurry. At the aerator settings described above, an aerated slurry density of about 0.225 g/cm 3  is achieved.       

     Then, the aerated slurry collected from the Aeros A20 outlet is aged for 70 minutes in a bucket (open) without any stirring at ambient temperature. At different time points from 0 min (i.e., the sample is taken immediately after the bucket is filled up with the aerated slurry exited the outlet of the aerator) to 70 min (i.e., after the completion of the aging step), a sample of the aerated slurry is taken out from the bucket. Then, the bubble size of the aerated slurry is determined according to Test 10 and is shown in the following table. 
     
       
         
           
               
             
               
                 TABLE 11 
               
             
            
               
                   
               
               
                 (BUBBLE SIZE GROWTH OVER AGING TIME) 
               
            
           
           
               
               
               
            
               
                 Ageing 
                 Average bubble size 
                 % of bubbles having average 
               
               
                 time 
                 (μm) 
                 diameter greater than 100 μm 
               
               
                   
               
            
           
           
               
               
               
            
               
                 0 
                 &lt;60*   
                 0 
               
               
                 10 
                 84.1 
                  7% 
               
               
                 20 
                 86.2 
                 10% 
               
               
                 30 
                 84.3 
                  8% 
               
               
                 45 
                 94.4 
                 28% 
               
               
                 60 
                 95.0 
                 30% 
               
               
                 70 
                 99.8 
                 41% 
               
               
                   
               
               
                 *A minimum threshold of 60 micron was utilized for the bubble detection. For the sample analyzed immediately after aeration, no bubbles above 60 micron were detected. 
               
            
           
         
       
     
     The above table shows that bubbles gradually expand over time during the aging step. Further,  FIGS. 8A and 8B  respectively show a photo of bubbles in the slurry of Formulation 1 at 70 min ( FIG. 8A ) and at 0 min ( FIG. 8B ) of a 70-min aging step under optical microscopy using the same magnification. These data indicate that, surprisingly, the bubbles after the aging step are much larger than that at the beginning of the aging step immediately after the aeration. Thus, the introduction of a quite long aging step (for example, up to 70 minutes) does not result in collapse of bubbles, but significantly increase the bubble size. The increased bubble size would result in larger pores in the solid sheet article formed by the slurry, and in turn, improved dissolution profile of the solid sheet article. 
     Example 3: Improved Pore Structure and Improved Dissolution Profile of the Solid Sheet Article Achieved by Introduction of an Aging Step in a Drum Dryer Process 
     1) Preparation of Solid Sheet Articles 
     Similarly as Example 2, a wet pre-mixture (i.e., a slurry) of Formulation 1 and additional water is prepared and then aerated to provide an aerated slurry density of about 0.225 g/cm 3 . 
     An inventive, flexible, porous and solid sheet article (Article 1) and a comparative, flexible, porous and solid sheet article (Article 2) are produced using a rotary drum dryer system comprising a feeding trough and a spinning bar (for example, the system shown in  FIG. 5 ), in which Article 1 is prepared in a process having an aging step after the aeration, and Article 2 is prepared in a process in which drying is conducted immediately after the aeration (no aging step). 
     The method for preparing Articles 1 and 2 from the wet pre-mixtures are as follows:
         1. The rotary drum dryer (having a drum diameter of about 1.5 m) is pre-heated to about 130° C.   2. The aerated slurry collected from the Aeros A20 outlet is added to the feeding trough of the drum dryer.   3. For the inventive sheet article (Article 1), the aerated slurry is aged in the bucket for a total aging time of 120 minutes. Then, the aged aerated slurry is removed from the bucket to the feeding trough.   4. Articles 1 and 2 are formed by feeding the aerated slurry onto the surface of the rotating drum dryer with the spinning bar that is rotating in an opposite direction to the rotating drum dryer (for example, clockwise vs. counterclockwise) in which the rotating speed of the drum dryer is set so that the slurry residence time on the heated drum is about 15 minutes. For the inventive sheet article (Article 1), the rotating speed of the spinning bar is relatively low (i.e. 30 rpm) and the distance between the spinning bar and the surface of the drum dryer is relatively long (i.e. 8 mm). Under such conditions, the slurry can be fed onto the surface of the rotating drum dryer without further introducing air bubbles. For the comparative sheet article (Article 2), the rotating speed of the spinning bar is relatively high (i.e. 180 rpm) and the distance between the spinning bar and the surface of the drum dryer is relatively short (i.e. 4.5 mm). Under such conditions, the slurry is vigorously stirred at the air interface and as such, air bubbles are further introduced into the slurry. As such, the effect caused by the aging step is reversed or at least compromised by this additional aeration, which may be considered to be comparable with a process without an aging step. A leveling blade that is placed near the slurry pick-up location is employed to ensure a consistent thickness of the sheet (about 0.8-1.5 mm).   5. Once dried, the flexible and porous sheets so formed are peeled from the drum surface and placed in a plastic bag.       

     2) Larger Bubble Size in the Slurry Achieved by Introduction of an Aging Step 
     A sample of the aerated slurry for Article 1 or 2 is respectively taken out from the feeding trough near the feeding location of the slurry. Then, the bubble size of the slurry is determined according to Test 10 and is shown in the following table. 
     
       
         
           
               
               
               
             
               
                 TABLE 12 
               
               
                   
               
               
                   
                 Average bubble size 
                 % of bubbles having average 
               
               
                 Sample 
                 (μm) 
                 diameter greater than 200 μm 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 Article 1 
                 135.2 
                 8.3% 
               
               
                 (Inventive) 
               
               
                 Article 2 
                 106.4 
                 1.4% 
               
               
                 (Comparative) 
               
               
                   
               
            
           
         
       
     
     The above table shows that bubbles in the slurry obtained after an aging step (Article 1) are much larger than that in the slurry obtained without an aging step (Article 2). Further,  FIGS. 9A and 9B  respectively show a photo of bubbles in the slurry for Articles 1 and 2 under optical microscopy using the same magnification. These data indicate that, in the drum drying process, introduction of an aging step after the aeration results in much larger bubbles in the pre-mixture fed onto the drum dryer. On the contrary, if the aerated pre-mixture is fed onto the drum dryer immediately after the aeration, the bubbles remain relatively small. 
     3) Improved Pore Structure of Solid Sheet Articles Achieved by Introduction of an Aging Step 
     SEM testing is carried out according to Test 1.  FIG. 10A  and  FIG. 10B  respectively show a SEM image of the top surface of Articles 1 and 2, and the following table shows the pore structure for Articles 1 and 2. These data indicate that the inventive sheet article (i.e., Article 1) has significantly larger pores on its top surface and also significantly larger average pore size compared to the comparative sheet article (i.e., Article 2). 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 13 
               
             
            
               
                   
                   
               
               
                   
                 Top Surface Pore Structure 
                 Total Pore 
               
            
           
           
               
               
               
               
            
               
                   
                   
                 Surface 
                 Structure 
               
               
                   
                   
                 Average Pore 
                 Average Pore 
               
               
                 Sample 
                 % Pore area 
                 Diameter (μm) 
                 Diameter (μm) 
               
               
                   
               
            
           
           
               
               
               
               
            
               
                 Article 1 
                 57.9 
                 427.3 
                 341.2 
               
               
                 (Inventive) 
               
               
                 Article 2 
                 48.7 
                 169.2 
                 223.2 
               
               
                 (Comparative) 
               
               
                   
               
            
           
         
       
     
     4) Improved Dissolution Profile of Solid Sheet Articles Achieved by Introduction of an Aging Step 
     Dissolution rates for Sheets 1 and 2 are determined according to Test 9. The following Table 14 as well as  FIG. 11  shows the results of the dissolution rate testing, indicating the inventive sheet article (Article 1) has a significantly improved dissolution profiles compared to the comparative sheet article (Article 2). Particularly, the time for the dissolution of 90% of Article 1 is only 301 seconds, while the time for the dissolution of 90% of Article 2 is 928 seconds that is more than three folds of that for Article 1. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 14 
               
               
                   
               
               
                   
                 Article 1 
                 Article 2 
                   
               
               
                 % Dissolution 
                 Time, seconds 
                 Time, seconds 
                 Relative % 
               
               
                   
               
             
            
               
                 90% 
                 301 
                 928 
                 208% 
               
               
                 95% 
                 479 
                 Not reached 
                 — 
               
               
                   
               
            
           
         
       
     
     In conclusion, the introduction of an aging step (i.e., maintaining the aerated pre-mixture for a while after the aeration) in a drum dryer process brings about a significantly improved pore structures and thereby a significantly improved dissolution profile. 
     Example 4: Improved Pore Structure of the Solid Sheet Article Achieved by Introduction of an Aging Step in a Belt Drying Process 
     1) Preparation of Solid Sheet Articles 
     Similarly as Example 2, a wet pre-mixture (i.e., a slurry) of Formulation 2 in the following Table 15 and additional water is prepared and then aerated to provide an aerated slurry density of about 0.225 g/cm 3 . 
     
       
         
           
               
               
             
               
                 TABLE 15 
               
               
                   
               
               
                   
                 Formulation 2 
               
               
                 Materials (Dry), wt % 
                 (Fabric Care) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
            
               
                 Polyvinyl alcohol (with a degree of polymerization 
                 18.00 
               
               
                 of about 1700, a hydrolysis level of 88%) 
               
               
                 Glycerin 
                 9.00 
               
               
                 Linear Alkylbenzene Sulfonate 
                 56.00 
               
               
                 Sodium Laureth-3 Sulfate 
                 6.00 
               
               
                 Ethoxylated Polyethyleneimine 
                 2.00 
               
               
                 Palm kernel fatty acid soap powder 
                 2.00 
               
               
                 Water 
                 7.00 
               
               
                 Miscellaneous 
                 Q.S. 
               
               
                   
               
            
           
         
       
     
     An inventive, flexible, porous and solid sheet article (Article 3) and a comparative, flexible, porous and solid sheet article (Article 4) are produced using a belt drying system instead of the drum drying process. In the belt drying system, the drying surface is a moving stainless-steel belt of approximately 4 m length and 60 cm width which is heated from underneath by hot air convection. The aerated slurry is pumped from the continuous aerator to a 50 litre stainless steel heated vessel, where it was stored for varying amounts of time (i.e., aging time) to allow the bubble size to increase. To feed the slurry onto the drying surface of the moving belt, the vessel was placed above the drying surface and an outlet positioned at the bottom of the vessel was opened to allow a steady flow of the slurry onto the belt surface. A thickness control blade was positioned near this depositing location in order to accumulate excess slurry, and only carry a defined volume (i.e. thickness) of slurry along the drying surface for drying and subsequent removal. Once dried, the flexible and porous sheets so formed are peeled from the drum surface and placed in a plastic bag. The settings of the belt dryer process are shown in the following table. 
     
       
         
           
               
             
               
                 TABLE 16 
               
             
            
               
                   
               
               
                 (BELT DRYER PROCESS SETTINGS) 
               
            
           
           
               
               
               
               
            
               
                   
                 Settings 
                 Article 3 
                 Article 4 
               
               
                   
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Average belt temperature (° C.) 
                 130 
                 130 
               
               
                   
                 Belt speed (m/min) 
                 0.6 
                 0.6 
               
               
                   
                 Aging time (min) 
                 15 
                 0 
               
               
                   
                   
               
            
           
         
       
     
     2) Improved Pore Structure of Solid Sheet Articles Achieved by Introduction of an Aging Step 
     SEM testing is carried out according to Test 1 for Sheets 3 and 4.  FIG. 12A  and  FIG. 12B  respectively show a SEM image of the top surface of Sheets 3 and 4, and the following table shows the pore structure for Sheets 3 and 4 as determined according to Test 1. These data indicate that inventive sheet articles (i.e., Article 3) has significantly larger pores on its top surface and also significantly larger average pore size compared to comparative sheet articles (i.e., Article 4). 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 17 
               
             
            
               
                   
                   
               
               
                   
                 Top Surface Pore Structure 
                 Total Pore 
               
            
           
           
               
               
               
               
            
               
                   
                   
                 Surface 
                 Structure 
               
               
                   
                   
                 Average Pore 
                 Average Pore 
               
               
                 Sample 
                 % Pore area 
                 Diameter (μm) 
                 Diameter (μm) 
               
               
                   
               
            
           
           
               
               
               
               
            
               
                 Article 3 
                 62.1 
                 402.1 
                 366.1 
               
               
                 (Inventive) 
               
               
                 Article 4 
                 30.6 
                 169.2 
                 225.4 
               
               
                 (Comparative) 
               
               
                   
               
            
           
         
       
     
     In conclusion, the introduction of an aging step (i.e., maintaining the aerated pre-mixture for a while after the aeration) in a belt dryer process also brings about a significantly improved pore structure. 
     Example 5: Improved Pore Structure and Improved Dissolution Profile of the Solid Sheet Article Achieved by Introduction of an Aging Step in a Drum Dryer Process Involving a Feeding Die 
     1) Preparation of Solid Sheet Articles 
     Similarly as Example 2, a wet pre-mixture (i.e., a slurry) of Formulation 3 in the following Table 18 and additional water is prepared and then aerated to provide an aerated slurry density of about 0.225 g/cm 3 . 
     
       
         
           
               
               
             
               
                 TABLE 18 
               
               
                   
               
               
                   
                 Formulation 3 
               
               
                 Materials (Dry), wt % 
                 (Personal Care) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
            
               
                 Polyvinyl alcohol (with a degree of polymerization 
                 28.10 
               
               
                 of about 1700, a hydrolysis of 88%) 
               
               
                 Glycerin 
                 9.60 
               
               
                 Sodium Lauroamphoacetate 
                 11.30 
               
               
                 Sodium Lauramidopropyl Betaine 
                 28.10 
               
               
                 Sodium Lauroyl Methyl Isethionate 
                 16.90 
               
               
                 Water 
                 6.00 
               
               
                 Miscellaneous 
                 Q.S. 
               
               
                   
               
            
           
         
       
     
     An inventive, flexible, porous and solid Article 5 and a comparative, flexible, porous and solid Article 6 are produced using a rotary drum dryer system comprising a feeding die, in which the feeding die is positioned at the top of the drum dryer and is used to continuously feed the slurry onto the drum surface. The internal flow channel of the feeding die has a feeding width of approximately 30 cm and a feeding thickness of approximately 2 mm. The slurry is firstly pumped from the continuous aerator to a 50 L vessel where it was kept for a defined amount of time (i.e., aging time) to allow the bubble size to increase, and then pumped from this vessel to the feeding die. Once dried, the flexible and porous sheets so formed are peeled from the drum surface and placed in a plastic bag. The settings of the drum dryer-die process are shown in the following table. 
     
       
         
           
               
             
               
                 TABLE 19 
               
             
            
               
                   
               
               
                 (DRUM DRYER-DIE PROCESS SETTINGS) 
               
            
           
           
               
               
               
               
            
               
                   
                 Settings 
                 Article 5 
                 Article 6 
               
               
                   
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Drum temperature (° C.) 
                 100 
                 100 
               
               
                   
                 Drum speed (m/min) 
                 0.5 
                 0.5 
               
               
                   
                 Aging time (min) 
                 30 
                 0 
               
               
                   
                   
               
            
           
         
       
     
     2) Improved Pore Structure of Solid Sheet Articles Achieved by Introduction of an Aging Step 
     SEM testing is carried out according to Test 1 for Sheets 5 and 6.  FIG. 13A  and  FIG. 13B  respectively show a SEM image of the top surface of Sheets 5 and 6, and the following table shows the pore structure for Sheets 5 and 6 as determined according to Test 1. These data indicate that inventive sheet articles (i.e., Article 5) has significantly larger pores on its top surface compared to comparative sheet articles (i.e., Article 6). 
     
       
         
           
               
               
             
               
                   
                 TABLE 20 
               
             
            
               
                   
                   
               
               
                   
                 Top Surface Pore Structure 
               
            
           
           
               
               
               
            
               
                 Sample 
                 % Pore area 
                 Surface Average Pore Diameter (μm) 
               
               
                   
               
            
           
           
               
               
               
            
               
                 Article 5 
                 37.6 
                 343.4 
               
               
                 (Inventive) 
               
               
                 Article 6 
                 34.7 
                 179.9 
               
               
                 (Comparative) 
               
               
                   
               
            
           
         
       
     
     3) Improved Dissolution Profile of Solid Sheet Articles Achieved by Introduction of an Aging Step 
     Dissolution rates for Sheets 5 and 6 are determined according to Test 9. The following Table 21 as well as  FIG. 14  shows the results of the dissolution rate testing, indicating inventive sheet article (Article 5) have significantly improved dissolution profiles compared to comparative sheet article (Article 6). 
     
       
         
           
               
               
               
               
             
               
                 TABLE 21 
               
               
                   
               
               
                   
                 Article 5 
                 Article 6 
                   
               
               
                 % Dissolution 
                 Time, seconds 
                 Time, seconds 
                 Relative % 
               
               
                   
               
             
            
               
                 90% 
                 129 
                 197 
                 53% 
               
               
                 95% 
                 165 
                 261 
                 58% 
               
               
                   
               
            
           
         
       
     
     In conclusion, the introduction of an aging step (i.e., maintaining the aerated pre-mixture for a while after the aeration) in a drum dryer process with a feeding die also brings about a significantly improved pore structures and thereby a significantly improved dissolution profile. 
     The dimensions and values disclosed herein are not to be understood as being strictly limited to the exact numerical values recited. Instead, unless otherwise specified, each such dimension is intended to mean both the recited value and a functionally equivalent range surrounding that value. For example, a dimension disclosed as “40 mm” is intended to mean “about 40 mm.” Every document cited herein, including any cross referenced or related patent or application and any patent application or patent to which this application claims priority or benefit thereof, is hereby incorporated herein by reference in its entirety unless expressly excluded or otherwise limited. The citation of any document is not an admission that it is prior art with respect to any invention disclosed or claimed herein or that it alone, or in any combination with any other reference or references, teaches, suggests or discloses any such invention. Further, to the extent that any meaning or definition of a term in this document conflicts with any meaning or definition of the same term in a document incorporated by reference, the meaning or definition assigned to that term in this document shall govern. 
     While particular embodiments of the present invention have been illustrated and described, it would be obvious to those skilled in the art that various other changes and modifications can be made without departing from the spirit and scope of the invention. It is therefore intended to cover in the appended claims all such changes and modifications that are within the scope of this invention.