Abstract:
A gelled solid adhesive for coating substrates, typically paper. The adhesive can be made available in stick form and is easily applied in even coats to any surface area of the substrate. The adhesive has sufficient tack enabling the coated substrate to instantly adhere to essentially any free contact surface upon gently pressing the substrate to the free surface. The adhesive coated substrate is easily removable from the contact surface by manually lifting it therefrom. The adhesive permits readherence of the adhesive coated substrate to the same or different free contact surfaces. An adhesive coated paper substrate will readhere many times to free paper contact surface. The preferred gelled adhesive product is formed of an aqueous dispersion of components which include a natural rubber latex component, a friction reducing agent, a tackifier, an antioxidant and a gelling agent. Preferred classes of tackifiers are anionic aqueous dispersion of hydrogenated esters of rosin and anionic aqueous dispersion of esters of hydrogenated rosin.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a readhering and removable adhesive composition. The present invention relates to a readhering and removable adhesive in a solid stick form which can be evenly coated onto a surface. 
     2. Description of the Prior Art 
     Prior art adhesive available in solid form are known in the art. These adhesives, however, are commonly intended for use as a permanent adhesive, and therefore once a substrate is coated with the adhesive and the coated substrate applied to a receiving surface, it is not intended to be removable therefrom. One common adhesive of this type is often referred to as a &#34;glue stick&#34; or alternatively, an &#34;adhesive applicator crayon&#34;. Solid or semisolid permanent adhesives available in a stick or crayon form are typically composed of water soluble or water dispersible adhesive components. 
     U.S. Pat. No. 3,576,776 is illustrative of the prior art. This reference discloses an &#34;adhesive applicator crayon&#34; which contains a water soluble film forming adhesive component which is typically polyvinyl-pyrrolidone. The formulation disclosed in this reference also includes a gel-forming salt, typically sodium stearate and aqueous liquid medium which may be water or mixtures of water and water miscible organic solvents. Constituents such as glycerine or ethylene glycol are typically included in the formulation disclosed in this reference to aid in moisture retention and to facilitate the ease with which the adhesive may be applied to a receiving substrate. The active adhesive component in the formulation is designated a water soluble or water dispersible film forming adhesive component. It would be recognized by those skilled in the art that the list of water soluble or water dispersible film forming adhesive components as disclosed in this reference, e.g., at column 2 lines 35-61 including the preferred polyvinyl pyrrolidone are permanent adhesive components, not intended for reusable, removable and readhering application. There is, furthermore, no intent expressed in this reference for use of the adhesive crayon formulation as a removable and readhering type of adhesive. Rather the adhesive formulation disclosed in this reference will form a permanent bond between two substrates. 
     Readhering adhesives, in contrast, when applied to a substrate should permit adherence of the coated substrate to a free contact surface, permit removal of the coated substrate and allow readherence of the coated substrate to another free surface. 
     Readhering adhesives are available, but heretofor are not successfully been commercialized in a semisolid or solid stick form from which even coatings of adhesive may be directly applied to a solid surface. Although attempts in the past may have been made to formulate a removable adhesive in a solid gelled form, such prior art formulations have been difficult to produce for a number of reasons. In practice, it has been found quite difficult to produce a formulation in jelled form which has just the right amount of tack that an adhesive bond between the coated substrate and a contact substrate may be obtained but yet exhibit the proper degree of removability that the adhesive coated substrate, may be removable from the contact substrate without tearing the substrates and without leaving behind residual adhesive. Additionally, it has proved difficult to formulate a removable adhesive in solid form, which exhibits the required degree of tack that enables initial bonding between the coated substrate and the contact surface, but yet permits readherence of the adhesive coated substrate once it has been removed from the contact surface. 
     Paper substrates which have been pre-coated with removable, readhering adhesives have, in recent years, become commercially available. One commonly known product of this type is marketed under the trade designation POST-IT note pads from the 3M Company. Although the POST-IT note pads exhibit sufficient tack, removability and readhering characteristics, they have the disadvantage to the user that they are readily available only in precoated, predesignated sizes and also are considered to be relatively quite expensive. 
     Accordingly, it is an object of the present invention to provide a gelled solid or semisolid adhesive which is readily coatable onto substrates typically paper, and exhibits sufficient tack, such that the coated surface easily adheres to a contact surface. 
     It is an important object of the present invention that the gelled adhesive exhibits sufficient tack which permits the adhesive coated substrate to adhere to a contact surface, but yet has the property that permits the adhesive coated substrate to be readily removable therefrom. 
     Another objective of the invention is to provide a gelled solid or semisolid adhesive which exhibits a high degree of removability and readhering characteristics permitting a substrate coated with the adhesive to be applied, removed and reapplied a number of times to and from a free contact surface without adding additional adhesive. 
     Another objective of the invention is to provide a gelled solid or semisolid adhesive which is readily coatable onto a substrate and which easily adheres to a contact surface, but yet does not leave behind residual adhesive on the contact surface when the adhesive coated substrate is removed therefrom. 
     SUMMARY OF THE INVENTION 
     In accomplishing the foregoing and related objects, a gelled, semisolid adhesive is provided having the following composition and properties: The adhesive composition of the invention is a gelled emulsion which is essentially a dispersion of solids in an aqueous medium. The resulting gelled adhesive product has a solid or semisolid consistency which is non-flowing and retains its shape at ambient temperature. The gelled adhesive product is, in effect, a solid, but yet penetrable by manual insertion of a pointed instrument. The term &#34;solid&#34; as used hereinafter shall be construed as including the solid and semisolid state and shall be understood to be non-flowing and shape retaining at ambient conditions. The gelled adhesive product of the invention is preferably made available in a stick form which can be conveniently housed in conventional containers which permit the user to adjust the amount of exposed adhesive. The user simply coats any area of a substrate, typically paper with the exposed end of the adhesive product. The adhesive product of the invention has the property that it is easily coatable over the surface of a substrate such as paper and readily permits application of even, uniform, coatings to the substrate by manually brushing the substrate with the exposed end of the adhesive product. Typically one or two strokes of the adhesive product over the surface of the substrate is sufficient to apply an even, uniform coating of adhesive on the substrate. Although the adhesive may be coated onto a variety of different substrate materials, including, paper, plastic, polymer film and glass, the adhesive is most advantageously used for coating paper substrate. The adhesive coated substrate will readily adhere to virtually any solid surface, including paper, wood, porcelain, plastic and glass surfaces as well as on painted plaster walls and panels. The adhesive coated substrate will essentially adhere to practically any solid contact surface by simply manually pressing the coated substrate to the contacting surface. 
     The adhesive coated substrate, which has been coated with the gelled solid adhesive composition of the present invention has a unique combination of properties. The adhesive composition of the present invention has the property that gives the coated substrate, such as adhesive coated paper, enough tack that it will readily adhere to virtually any solid contact surface including paper, wood, porcelain, plastic, glass and plaster by simply pressing the adhesive coated substrate to the solid surface. However, the adhesive does not form a permanent bond between the coated substrate and the contact surface. Rather the adhesive coated substrate is readily removable from the contact surface by manually lifting the substrate therefrom. The adhesive nevertheless exhibits sufficient tack to permit the coated substrate to remain adhered to the contact surface until the user wishes to remove the coated substrate. The adhesive coated substrate is very easily removed from the contact surface by simply easily lifting the adhesive coated substrate. The adhesive composition of the present invention has the important additional property that permits the user to reapply the adhesive coated substrate as many as eight to ten additional times to the same or another free solid surface. The adhesive coated substrate will readhere each time it is manually pressed onto the solid contact surface which may be the same surface to which it was previously applied or it may be an entirely different one. 
     Illustratively, a user may advantageously apply the adhesive of the present invention to a minor portion of the backside of note paper and then simply manually press the coated note paper to a contact surface, typically paper. The coated note paper will immediately adhere to the contact surface. Subsequently the note paper may be readily removed from the contact surface by simply lifting it therefrom. If desired, the user may reapply the same adhesive coated note paper to the same or different contact surface at least eight more times without application of additional adhesive. 
     The gelled adhesive composition of the present invention has the property that permits release of the adhesive coated substrate from the contact surface essentially without leaving behind any noticeable adhesive residue on the contact surface. It has been determined that essentially no adhesive residue will be left behind on the contact surface unless the user applies a very heavy coating to the initial substrate. The application of unusually heavy coating to the initial substrate is not within the realm of ordinary prescribed use of the adhesive. In the event the user applies an unusually heavy coating of adhesive to the substrate, any adhesive residue that does remain on the contact surface upon removal of the adhesive coated substrate can be easily and completely removed by gently rubbing the residue. Upon gentle rubbing, any residue left behind will easily peel off from the contact surface, leaving the contact surface clean and unharmed. 
     The gelled adhesive of the present invention, having the above described properties, includes as principal components, a natural rubber latex component, a tackifying agent and a gel forming agent. The preferred formulation for the gelled adhesive of the present invention is composed of: (A) a natural rubber latex component, (B) a friction reducing agent, (C) a tackifier, (D) an antioxidant and, (E) a gelling agent. All of the aforementioned components are preferably water soluble or water dispersible. The natural rubber latex component is preferably supplied to the formulation in the form of an aqueous natural rubber latex dispersion (emulsion). The friction-reducing agent is advantageously selected from the class of polyhydric aliphatic alcohols, preferably dihydric and trihydric aliphatic alcohols. Preferred polyhydric aliphatic alcohols for the friction reducing agent are glycerine (e.g.1,2,3 propanetriol) and propylene glycol. The tackifier is advantageously selected from the class of hydrogenated ester of rosin or the ester of hydrogenated rosin. The preferred tackifier is an aqueous tackifier resin dispersion (emulsion) advantageously an anionic aqueous dispersion of hydrogenated methyl ester of rosin. Another preferred tackifier has been determined to be an anionic (ion-tolerant) aqueous resin dispersion prepared from the glycerol ester of a highly hydrogenated rosin. The antioxidant component in the above formulation, may be selected from a variety of known antioxidants. A preferred antioxidant is a ball-milled dispersion of 4,4 butylidene BIS powder in an aqueous base. The gelling agent is advantageously selected from known gel-forming salts, which are aliphatic carboxylic acid salts, typically aliphatic carboxylic acid salts containing an alkali metal cation. It has been determined that the preferred gel-forming salts for use in the present formulation is sodium stearate. The above listed components may be present in the gelled adhesive of the present invention in the following composition ranges as measured on an aqueous-free basis: the natural rubber latex may be present in a range between 5 to 20% by weight; the friction-reducing agent may be present between about 1-25% by weight; the tackifier between 15 and 70%/weight; the antioxidant between about 3.8-10% by weight; and the gelling agent between about 8 to about 25% by weight. 
    
    
     DETAILED DESCRIPTION 
     The gelled adhesive product of the present invention can be made available in a solid stick form which the user can easily apply to evenly coat any portion of the surface of a substrate, such as paper and the like, by simply gliding an exposed end of the adhesive stick over the substrate. The principle components which applicant has discovered in combination form the essence of the present gelled adhesive formulation, are (a) the natural rubber latex component; and (b) tackifying agent; and (c) gel forming agent. These components are desirably all water soluble or water dispersible. Applicant has discovered that the combination of natural rubber latex and certain classes of tackifier primarily account for providing the required degree of tack to the adhesive product, but yet impart a release property which permits removal of the adhesive coated substrate from a contacting surface and reapplication of the adhesive coated substrate to the same or other free surface. Although other elastomeric polymers have been examined in the present formulation, surprisingly natural rubber latex has proved to provide markedly superior properties in combination with the preferred classes of tackifier. The natural rubber latex component has been determined to be a significant factor in imparting important properties to the adhesive product. It has been determined that the natural rubber latex component provides an excellent base for the formulation. The tackifier, if used alone, would not be suitably coatable onto a substrate. The combination of natural rubber latex with the tackifying agent, makes the formulation as a whole, easily coatable onto the substrate. The natural rubber latex component also imparts a releasability characteristic to the adhesive, such that adhesive coated substrate, after it has been pressed onto a contat surface may be easily removed therefrom by simply manually lifting the substrate from the contact surface. The natural rubber latex component has the additional property in combination with the tackifying agent, that little, if any, adhesive residue is left behind on the contact surface when the adhesive coated substrate is lifted therefrom. Small amounts of adhesive residue can be left behind on the contact surface only if the user coats the substrate very heavily with adhesive prior to applying it to the contact surface. This would be outside the realm of ordinary, recommended usage of the adhesive, since only one or two strokes of the adhesive stick need be applied to the substrate in order to evenly coat the substrate with an adquate amount of adhesive. However, in the event that the user inadvertently coats the substrate very heavily with adhesive, the natural rubber latex component included in the formulation assures the removability of any residual adhesive which is left behind on the contact surface. Such residual adhesive may be removed from the contact surface by simply manually rubbing the surface gently. Any residual adhesive left behind will thereupon easily peel off from the contact surface leaving the contact surface undamaged. It has been surprising to find the natural rubber latex component in combination with select classes of tackifying agents, provides just the right degree of tack necessary to assure adherence of the adhesive coated substrate to a contact surface and yet assures easy removability of the coated substrate from the contact surface so that the adhesive coated substrate may be readily applied over and over again. 
     If the gelled adhesive product of the invention composed of natural rubber latex, tackifying agent and gelling agent is coated onto a portion of a paper substrate the coated paper substrate will adhere immediately to virtually any contact surface including paper, wood, porcelain, plastic, glass and plaster walls. The adhesive coated paper substrate nevertheless may be readily lifted and removed from the contact surface and can be reapplied at least eight additional times to paper contact surfaces without need of replenishing the adhesive coat. 
     It is sufficient to coat only a minor portion of the substrate surface with the adhesive product. For example, a sheet of paper 81/2 inch×11 inch or smaller, including note paper 4 inch×5 inch or smaller, need only be coated along one edge with a strip of adhesive of about 1/2 inch to assure sufficient tack to permit adherence of the substrate to a contact surface and exhibit all of the aforementioned removability and readhering characteristics. 
     A preferred natural rubber latex for use in the present formulation is available in the form of an aqueous natural rubber latex dispersion (emulsion). The natural rubber latex component is preferably of low ammonia content. A preferred natural rubber latex of low ammonia content is available under the trade designation, Natural Rubber Latex NC407 from the Uniroyal Company. Commercially available natural rubber latex dispersions normally includes ammonia as a preservative which prevents the natural rubber from coagulating and putrefying from bacterial or enzymatic action after the latex has been obtained from the Hevea tree. Applicant has determined that the ammonia present in the commercial natural rubber latex dispersion, however, could interfere with the gelling of the adhesive composition if present in large amounts. Therefore, natural rubber latex having the lowest ammonia content is desirable. A preferred aqueous rubber latex dispersion available under the trade designation Natural Rubber Latex NC407 from the Uniroyal Company, has a low ammonia content and has been determined to be very suitable for use in the present adhesive formulation. 
     The tackifying agent for use in the present formulation is water dispersible or water soluble. Preferred tackifying agents, which applicant has determined, in combination with the natural rubber latex to yield the above described properties may be selected from the classes of hydrogenated esters of rosin and esters of hydrogenated rosin. A preferred tackifying agent of the former class is an anionic aqueous dispersion of hydrogenated methyl ester of rosin. An aqueous dispersion of hydrogenated methyl ester rosin, which has been determined to be suitable, is available under the trade designation HERCOLYN D-55WKX available from Hercules, Inc., Wilmington, Del. The HERCOLYN D-55-WKX tackifier resin dispersion is a 55% solids content anionic (ion-tolerant) aqueous dispersion of hydrogenated methyl ester of rosin. This particular tackifying resin has a total solids content of 54-56% solids, a viscosity at 25° C. of 500-2,500 centipoise and a pH of 9.5-11.5. The HERCOLYN D-55WKX aqueous dispersion, as commercially available, has the following typical properties: total solids content, 55%; viscosity at 25° C., 1500 centipoise; pH 10.5; base resin-colored, gardner #2; particle size 1 micron; emulsifier type, anionic. 
     An alternative tackifying agent selected from the class of ester of hydrogenated rosin which has proved advantageous in combination with the above described natural rubber latex, is an anionic (ion-tolerant) aqueous resin dispersion prepared from the glycerol ester of a highly hydrogenated rosin. A preferred anionic aqueous resin dispersion prepared from the glycerol ester of highly hydrogenated rosin is available commercially under the trade name FORAL 85-55WKX. The FORAL 85-55WKX tackifying agent has a total solids content of between about 53.5 to 56.5%; a viscosity of about 1450 to 3050 centipoise at 25° C.; a pH of about 9.02-11.0 and an average resin particle size of less than about 1 micrometer. 
     Applicant has determined that friction reducing agent is advisably included in the formulation, since the combination of natural rubber latex and tackifying agent by itself does not have quite the optimum degree of coatability that is required for consumer application of the gelled adhesive produce. Although a variety of friction reducing agents to improve the coatability of the product can be employed, the preferred friction reducing agents for use in the context of the present adhesive formulation is advantageously selected from a polyhydric aliphatic alcohol. The preferred polyhydric aliphatic alcohol for the friction reducing agent are advantageously dihydric and trihydric aliphatic alcohols. A preferred dihydric aliphatic alcohol is propylene glycol and a preferred trihydric aliphatic alcohol has been determined to be glycerine. In addition to improving the coatability of the adhesive, the foregoing classes of friction reducing agents helps to retain moisture in the gelled adhesive product, thus preventing the product from becoming hard and brittle during prolonged storage. Additionally, the polyhydric aliphatic alcohols tend to protect the gelled adhesive product from freezing if subjected to cold weather during shipment or storage. 
     Preferably an antioxidant is included in the formulation to stabilize the composition against ambient oxidation and oxidative degradation. The antioxidant prevents yellowing or browning of the product adhesive as a result of slow oxidation which in turn gives the product an unattractive appearance. Selection of a suitable antioxidant for use in the present formulation to prevent oxidative degradation is within the experience of those skilled in the art. A preferred antioxidant for use in the present formulation is available under the trade designation HEVEATEX D407A antioxidant which is a ball-milled dispersion of 4,4 butylidene BIS powder in a aqueous base. HEVEATEX D407A dispersion is available from the HEVEATEX Corp. of Fallriver, MA. 
     An alternative preferred antioxidant exhibiting excellent antioxidative properties for the present adhesive product may contain the combination of TI-NOX WLE antioxidant dispersion and TI-ZATE B antioxidant. TI-NOX WLE antioxidant is a high potency, non-staining, non-discoloring antioxidant available in the form of an aqueous dispersion from Technical Industries, Inc., Peach Dale, R.I. TI-NOX WLE product contains a butylated reaction product of paracresol and dicyclopentadiene and a liquid carrier such as ditridecylthidipropionate. (The butylated reaction product component above mentioned is available under the tradename WINGSTAY-L from Goodyear Chemical Division and the ditridecylthidipropionate is available under the tradename CYNOX 711 from American Cyanamide Rubber-Chemical Division). TIZATE B antioxidant is an aqueous dispersion containing ZINC dibutyldithiocarbonate and is manufactured by Technical Industries, Inc., Peach Dale, R.I. 
     The adhesive formulation of the present invention importantly also includes a gelling agent. The gelling agent is necessary in order to cause coagulation of all of the solids in the formulation, thus resulting in a solid gelled adhesive product. Without the gelling agent, the adhesive product would remain as an aqueous solid dispersion in liquid form. Suitable gelling agents for aqueous dispersion are known in the art. Such agents, for example, are disclosed in U.S. Pat. No. 3,576,776 all of which are incorporated herein by reference. A preferred class of gelling agent for use in the context of the present formulation has been determined to be a salt of an aliphatic carboxylic acid wherein the cation consists of an alkali metal. Applicant has determined that the use of sodium stearate is the preferred gelling agent for the present formulation. Sodium stearate causes the solids in the formulation to gel quickly upon cooling, thus permitting easy commercial production of the gelled adhesive product in any desired shape. 
     Ranges of the above described components of the present formulation which have been determined to be suitable is reported on a water-free basis as follows: the natural rubber latex may be present in a range between about 5-20% by weight; the friction reducing agent, e.g., glycerine may be present between about 1-25% by weight; the tackifying agent, for example, HERCOLYN D-55-WKX tackifier may be present between about 15-70 weight percent solids; the antioxidant may be present between about 3.8-10% by weight, and the gelling agent, e.g., sodium stearate between about 8-25% by weight. The gelled adhesive product typically has a penetrometer hardness of between about 11 mm and 36 mm as measured in accordance with ASTM D-1321. 
     Preferred formulations employing the above described components, which result in the gelled adhesive product of the present invention, are shown in Table I and II. The formulations set forth in the respective tables are reported in the first column as percent by weight based on the total aqueous dispersion, that is including all water present in each of the listed constituents as supplied by the vendor. The second column is a tabulation of the preferred formulation on a water-free basis, that is essentially on a solid basis with the exception of the friction-reducing agent which is available typically in natural liquid state. The gelled adhesive product made with the formulation shown in the Tables I or II has a penetrometer hardness of about 20 mm as measured in accordance with ASTM D-1321. 
     
                       TABLE I______________________________________       BASIS: Total   INPUT       Aqueous Dispersion                      FORMULATIONADHESIVE    Input Formulation                      Water Free BasisCOMPOSITION Wt. %          Wt. %______________________________________Natural Rubber       15.3            14.9 (Solids)Latex (e.g. NaturalRubber LatexNC-407)Friction Reducing       5.1             8.0 (liq.)Agent(e.g. Glycerine)Tackifier   64.5            56.4 (Solids)(e.g. HERCOLYND55-WKX)Antioxidant 4.4             3.8 (Solids)(e.g. HEVEATEXD407A)Gelling Agent       10.7            16.9 (Solids)(e.g. SodiumStearate)TOTAL       100.0          100.0______________________________________ 
    
     
                       TABLE II______________________________________       BASIS: Total   INPUT       Aqueous Dispersion                      FORMULATIONADHESIVE    Input Formulation                      Water Free BasisCOMPOSITION Wt. %          Wt. %______________________________________Natural Rubber       15.0            14.7 (Solids)Latex(e.g. NC-407)Friction Reducing       5.0             7.9 (liq.)Agent(e.g. Glycerine)Tackifier   63.3            55.7 (solids)(e.g. HERCOLYND55-WKX orFORAL 85-WKX)AntioxidantTI-NOX WLE  3.3             2.5 (solids)TIZATE B    2.9             2.5 (solids)Gelling Agent       10.5            16.7 (Solids)(e.g. SodiumStearate)TOTAL       100.0          100.0______________________________________ 
    
     The preferred formulations of the gelled adhesive product as discussed in the foregoing and as illustrated in the Tables are advantageously prepared in the following manner. With specific reference to Table I and II, all of the listed constituents are mixed in the proportions shown in the column 1 of the respective Tables. All of the components except the gelling agents are first supplied to a closed vessel at room temperature. It has been found desirable to add the constituents to the closed vessel under ambient room temperature in the following order, while continually mixing the components. The natural rubber latex component can be added first to the vessel. While continually stirring at room temperature, the following constituents listed in Table I may be added at about 15 minute intervals: the friction reducing agent, the antioxidant, and then the tackifying agent. The mixture is continually stirred at ambient temperature until a homogenous mix is obtained. The mixture is then gradually brought to a temperature of about 165°-175° F. over the course of about one hour while continually stirring. As the contents gradually reaches a temperature of between about 165°  F.-170° F. the gelling agent, preferably sodium stearate, is added in the proportions shown in the first column of Table I. The contents are continually mixed at about 170° F. for an additional half hour to assure that all of the components become solubilized in the mixture. The mixture is then ready for pouring into individual containers. The mixture is poured hot into individual containers in a continuous assembly line operation. It should be appreciated that the containers may be of any shape, but typically they are of cylindrical shape. The containers, after they are filled with the hot liquid mixture, are left to cool at ambient room temperature. As the liquid cools, the gelling effect occurs gradually. When the liquid reaches room temperature, upon cooling the gelling enters its final state resulting in a gelled solid adhesive product. The solid adhesive product naturally takes the same shape as the container. The container may be fitted with conventional mechanical means which allows the user to expose fresh adhesive for use in coating a substrate as required. It will be apparent that the container may be fitted with a variety of devices which permits the user to conveniently push up fresh solid to an exposed end of the container. Containers of this type are readily available in the ordinary consumer market. 
     Although the adhesive product of the invention has been described within the context of particular embodiments, the invention is not intended to be limited to the specific composition described in the preferred embodiment. The uses for the adhesive product of the invention as discussed herein, have also been provided for illustrative purposes. Similarly it should be recognized that the adhesive product may be supplied in any desired shape, and any referenced shape is not intended to be limiting. The invention, therefore, is not intended to be limited to the description in the specification, but rather is defined by the Claims and equivalent thereof.