A method of making a high purity stratified tissue

Abstract: A foam-formed nonlaminated stratified paper tissue 300 and method of making the same includes a first zone 301 of foam-formed paper tissue formed from a furnish which is relatively rich in hardwood fiber. A second zone 401 of foam-formed paper tissue is formed from a furnish which is relatively rich in softwood fiber. The second zone is formed unitary and entangled with the first zone to form a nonlaminated stratified paper tissue. A high softness integument is defined adjacent to an outer surface of the first zone and a substratum is defined adjacent to a surface of the second zone spaced away from the integument. The integument 320 on the outer surface of the first zone includes an enriched region 330 wherein the hardwood fiber content is greater than that obtainable in the corresponding region of a tissue obtained from furnishes having the same compositions under the same conditions but employing water-forming instead of foam-forming. The substratum 420 on the surface of the second zone includes a region enriched in softwood fiber. The enriched region 430 of substantially pure hardwood fiber provides an extremely soft and smooth surface as detected by human somatic sensibility.

Description: This invention relates to a method of forming a novel foam-formed nonlaminated stratified paper tissue. Hitherto, paper tissues have been constructed and formed by water-forming processes wherein a fiber blend is suspended in an aqueous medium and delivered from a headbox on to a twin wire, Fourdrinier, suction breast roll former or crescent former machine. The water-formed tissue may be produced to include one or more layers formed by multiple headboxes. In addition, a headbox may be utilized having a single slice outlet or a plurality of slice outlets, depending on the number of regions in the product desired to be produced. The water-formed tissues which are formed as a multilayer product by means of multiple headboxes tend to delaminate. Delamination is a phenomena whereby individual regions may be separated from one another, the interface between the regions being not so highly entangled as to prevent the separation of the various regions in a multilayer water-formed tissue. If a two-slice outlet is provided in the headbox with each slice being supplied from a different section of the headbox, two distinct furnishes may be supplied one to each of the two portions of the headbox, so as in theory to form in the paper tissue product regions which are predominantly composed of the fibers of the particular furnish which is supplied to the corresponding section of the headbox. In such a water-formed process, each furnish supplied to the headbox includes typically 0.15% by weight of fibers. In paper tissues intended for intimate contact with the most delicate parts of the body in use, aesthetic and tactile qualities, particularly softness, are important. However, strength is also required. It is known that tactile qualities such as softness are associated with hardwood kraft whereas the greater strength is provided by softwood kraft. Thus, the presence in the tissue of both hardwood and softwood fibers is desirable if a suitable combination of strength and softness is to be achieved. (Hardwood kraft is formed from wood fibers which are relatively short. Softwood kraft is produced from wood fibers which are relatively long.) Since a surface containing significant quantities of softwood fibers feels rough to the touch, to achieve the optimum desired combination of properties ideally the surface of the tissue would consist largely or entirely of hardwood fiber to provide the desired softness while the interior would consist primarily of softwood fiber to provide the required strength. This has been attempted by using a headbox with two or three slices. Using a two slice headbox, one slice is supplied with hardwood pulp and the other with softwood pulp and two plies of the resultant tissue may be assembled back to back so that the surface layers of the product are formed from the hardwood pulp. With a three slice headbox, the two outer slices would be supplied with hardwood pulp and the central slice would be supplied with softwood pulp. Unfortunately, however, in conventional water-formed paper making processes, a substantial proportion of the furnish supplied to the tissue-forming machine (generally about 40-50%) is not retained on the wire. It will be understood that where a multi-slice headbox is used, the fiber material which passes through the wire and is collected in the collection pit for re-use comprises fiber from each slice and is therefore a mixture. Thus, when this recycle material is returned to the headbox, to be combined with the make-up fiber being supplied to each slice, it will modify the composition of the furnish actually being delivered from each slice to the papermaking wire. For example, where the make-up supplied to each slice is essentially 100% hardwood fiber or essentially 100% softwood fiber, as the case may be, the actual furnish supplied from the slice will comprise hardwood fiber (or softwood fiber) diluted by the recycle which will comprise a mixture of both hardwood fiber and softwood fiber. Thus, where, for example, about 50% of the furnish supplied to the papermaking machine is not retained on the wire, the furnish delivered through the slice supplied with make-up which is essentially 100% hardwood fiber may contain in theory only about 75% hardwood, and likewise for the slice supplied with essentially 100% pure softwood fiber make-up. In fact, the actual compositions will not be those calculated theoretically because the composition of the recycle is not necessarily the same as the overall composition of the furnishes delivered through the headbox slices, e.g. because relatively more of one type of fiber than of another is retained on the wire. In any event, it will be recognised that because of the recycle the desired aim of producing a tissue with a substantial purity of hardwood fiber in a surface zone and a substantial purity of softwood fiber in an interior zone is not readily achievable using conventional water-forming processes employing a headbox with a plurality of slices. While in theory the desired aim is achievable if multiple headboxes are used, together with separate collection pits and recycles, the apparatus is more complicated and expensive and the multi-layer tissues formed thereby tend to delaminate. US-A-4498956 describes an apparatus and method for the manufacture of a non-woven fibrous web such as paper from a dispersion of fibers in a foamed liquid. This is referred to as a foam-forming process. According to the present invention it has now been found unexpectedly that if a foam-forming process is employed with a multi-slice headbox, wherein a furnish rich in hardwood fiber is supplied to one slice and a furnish rich in softwood fiber is supplied to a second slice, tissue may be obtained having an outer, or surface, zone wherein the concentration of hardwood fiber is significantly higher than that obtainable from a conventional water forming process using the same furnishes and operating under the same conditions, thereby providing a tissue with a surface which is extremely soft and smooth as detected by human somatic sensibility. Thus, either improved softness can be achieved by using the same proportion of hardwood and softwood fiber employed in a water-forming process or the same level of softness can be achieved from a lower concentration of the expensive hardwood fiber. While this can be explained in part by the fact that we have observed that in a foam-forming machine a substantially greater amount of the furnish is retained on the wire, this of itself cannot account for the very significant increase in hardwood purity that is obtained in the surface layer. While we do not wish to be bound by any particular theory, it is thought that other factors that may contribute to the unexpected results that have been observed include a change in the relative proportions of softwood fiber and hardwood fiber that are retained on the wire and a change in the degree of mixing that may occur in the recycle look, both as compared with water forming. Another unexpected discovery is that by use of foam-forming, the level of entanglement in the tissue of the fibers delivered from adjacent slices of the headbox is such that a nonlaminated and non-delaminatable stratified tissue product is obtained. According to one aspect of the present invention there is provided a method of forming a multi-layer paper tissue web of improved surface softness from a pulp furnish wherein a first layer of the web is deposited from a first foamed furnish and a second layer of the web is deposited on the first layer from a second foamed furnish, each furnish includes recycle fiber recovered from both furnishes, the fiber content of said first furnish excluding recycle fiber is relatively richer in hardwood fiber than said second furnish and the fiber content of said second furnish excluding recycle fiber is relatively richer in fibrous material selected from softwood kraft, secondary fibers and anfractuous cellulosic fibers than said first furnish. The hardwood fibers may be selected from hardwood kraft fibers and hardwood sulfite fibers. In one embodiment of the invention, the overall amount of hardwood fiber supplied to form the first layer is no more than 8.1 g/m 2 (5 lbs/3000 sq.ft. ream). According to one embodiment of the invention, the overall hardwood fiber content of the first and second furnishes may be approximately 20% to 35% by weight of the paper tissue and the overall content of fibrous material selected from softwood kraft, secondary fibers and anfractuous cellulosic fibers in the first and second furnishes may be approximately 65% to 80% by weight of the paper tissue. The first foamed furnish may consist essentially of at least 90% by weight hardwood fiber and the second foamed furnish may consist essentially of at least 70% by weight of softwood fiber or of fiber chosen from softwood kraft, secondary fibers and anfractuous cellulosic fibers. In a further embodiment of the invention, the method further includes depositing a third layer from a third foamed furnish on said second layer, the recycle fiber comprises fiber from all three furnishes and the fiber content of said third foamed furnish excluding recycle fiber is relatively richer in hardwood fiber than said second foamed furnish. In this embodiment the first and third foamed furnishes may each consist essentially of at least 90% by weight hardwood fiber and the second foamed furnish may consist essentially of at least 70% by weight softwood fiber or fiber chosen from softwood kraft, secondary fibers and anfractuous cellulosic fibers. The first furnish is preferably supplied to a yankee side of a paper making machine. In one embodiment where the first and second furnishes are dried to form said web, the web is creped off said web off a drying means. The tissue is preferably formed by crescent forming. The method of the invention enables the provision of a nonlaminated stratified paper tissue comprising a first zone of foam-formed paper tissue formed from a first fiber furnish and a second zone of foam-formed paper tissue formed from a second fiber furnish, and wherein said first fiber furnish is rich in hardwood fiber relative to the second furnish and second fiber furnish is rich in softwood fiber relative to the first furnish, said second zone being formed unitary with said first zone with fibers of said second zone and fibers of said first zone being entangled together and said tissue including a high softness integument adjacent to an outer surface of said first zone and a substratum adjacent to a surface of said second zone spaced away from said integument, said integument including an enriched region the hardwood fiber content of which is greater than the overall hardwood fiber content of the first zone and is greater than that obtainable in the corresponding region of a tissue obtained from furnishes having the same compositions under the same conditions but employing water-forming instead of foam-forming. The enriched region of the high softness integument provides an extremely soft and smooth surface detectable by human somatic sensibility. It will be understood that in the tissue of the invention the enriched region of the substratum will normally have a softwood fiber content greater than that obtainable in the corresponding region of a tissue obtained from furnishes having the same compositions under the same conditions but employing water-forming instead of foam-forming. In addition to the hardwood and softwood fibers, the tissue of the invention will also contain surfactant retained from the foam employed in the foam-forming process. Generally, the surfactant will be present in the tissue in an amount of from about 200 to about 600 ppm, by weight. A foam-formed nonlaminated paper tissue is obtainable by the method of the invention containing overall 20 to 80% by weight hardwood fiber and from 80 to 20% by weight softwood fiber and from 200 to 600 ppm surfactant by weight, and having a stratified structure comprising:- a first zone of foam-formed paper tissue which is relatively rich in hardwood fiber, and a second zone of foam-formed paper tissue which is relatively rich in softwood fiber; said zones being joined together by entanglement of fibers of the first zone with fibers of the second zone whereby to form a unitary structure; said first zone having adjacent an outer surface thereof a high softness integument including an enriched region wherein hardwood fiber forms at least 91% of the total fiber content; and said second zone having adjacent to a surface thereof spaced away from said integument a substratum which includes an enriched region wherein softwood fiber forms at least 80% by weight of the total fiber content. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings wherein: Figure 1A is a schematic view illustrating two furnishes supplied to a foraminous support means for forming a paper tissue; Figure 1B is a partial schematic view illustrating a headbox divided into three sections with appropriate flow paths for supplying three furnishes thereto; Figure 2A is a perspective enlarged schematic illustration of the characteristics of the hardwood kraft surface purity; Figure 2B is a perspective enlarged schematic illustration of a two-zone stratified paper tissue product; Figure 2C is a perspective enlarged schematic illustration of a three-zone stratified paper tissue product; Figure 3 illustrates data showing the layer of purity for the first region of the paper tissue; Figure 4 illustrates data showing the purity of the third region of the paper tissue; Figure 5 illustrates data showing the layer of purity of the sixth region of the paper tissue; Figure 6 illustrates data showing the layer of purity of the eight region of the paper tissue; Figure 7 illustrates data showing the composite of hardwood kraft as compared to softwood kraft of the material tested; and Figures 8A and 8B are an enlarged photographs showing surface purity in a water formed paper tissue; and Figures 9A and 9B are an enlarged photographs showing surface purity in a foamed formed paper tissue. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT As illustrated in Figure 1A, a papermaking machine 10, generally referred to as a crescent former, includes a web-forming end or wet end with a liquid permeable foraminous support member 11. The foraminous support member 11 may be constructed of felt, fabric or a synthetic filament woven mesh base with a very fine synthetic fiber batt attached to the mesh base. The foraminous support member 11 is supported in a conventional manner on rolls, including breast roll 15 and couch roll or pressing roll 16. The particular papermaking machine illustrated in Figure 1A is generally referred to as the crescent former. However, foam-formed paper, towels and tissue, may also be formed on a twin wire, Fourdrinier, suction breast roll former or other types of paper, towel and tissue making equipment. The present invention provides a degree of purity of stratification on crescent forming machinery beyond that previously demonstrated on this machinery and even surpassing that of the twin wire configuration which has been considered state of the art. A foam-formed paper, towel or tissue machine requires a save-all mechanism for permitting a recycling of the foam and surfactant which is utilized in supplying the furnish to the paper, towel or tissue making machine. Pressing wire 12 is supported on rolls 18 and 19 which are positioned relative to the breast roll 15 for pressing the press wire 12 to converge on the foraminous support member 11 at the cylindrical breast roll 15 at an acute angle relative to the foraminous support member 11. The foraminous support member 11 and the wire 12 move in the same direction and at the same speed which is the same direction of rotation of the breast roll 15. The pressing wire 12 and the foraminous support member 11 converge at an upper surface of the forming roll 15 to form a wedge-shaped space or nip into which one jet with two zones of foamed liquid-fiber dispersion is pressed between the pressing wire 12 and the foraminous support member 11 to force fluid through the wire 12 into a save-all 22 where it is collected as foamed liquid having an air content in the range of 45% to 80% by volume for reuse in the process. A wet web W formed in the process is carried by the foraminous support member 11 to the pressing roll 16 where the wet web W is transferred to the drum 26 of a yankee drier. Fluid is pressed from the wet web W by pressing roll 16 as the web is transferred to the drum 26 of the yankee drier where it is dried and creped by means of a creping blade 27. The finished web is collected on a take-up roll 28. Foamed liquid is collected from the foamed fiber furnish which is pressed between the pressing wire 12 and the foraminous support member 11. The foamed liquid is supplied to the save-all 22 and is returned through line 24 to a recycling process generally indicated by box 50. Box 140 generally indicates a supply of softwood kraft in the form of a furnish having e.g. approximately 3% by weight of softwood kraft fibers. The furnish is supplied from the box 140 through the conduit 142 to the conduit 40. A portion of the recycled foam and fiber from the recycling process 50 is also supplied to the conduit 40 and thence to section 20' of a headbox 200. Similarly, box 141 illustrates hardwood kraft which is supplied in the form of a furnish having e.g. approximately 3% by weight of hardwood kraft fibers. The furnish in the box 141 is supplied through the conduit 143 to the conduit 41 for supplying hardwood kraft furnish to a section 10 of the headbox 200. Foam and fibre from the recycling box 50 is also supplied to the conduit 41 to form a portion of the furnish supplied to the headbox 20. A pit 44 is provided for collecting water, foam and surfactant squeezed from the furnish by the press roll 16 and a Uhle box 29. The water, foam and surfactant collected in the pit 44 may be collected into a flow line 45 for separate processing to remove surfactant and fibers from the water and to permit recycling of the water and the surfactant back to the papermaking machine 10. The foam-formed nonlaminated stratified paper tissue of the present invention may be formed on a paper making machine 10 as described hereinabove. The softwood kraft furnish is supplied through the conduit 40 to a section 20' of the headbox 200. The hardwood kraft furnish is supplied through the conduit 41 to the section 20 of the headbox 200. Approximately 70% to 90% of the furnish supplied to the foraminous support member 11 is retained thereon to form a portion of the paper tissue. Only 10% to 30% of the fibers in the softwood kraft and the hardwood kraft are provided to the save-all 22 for recycling through the recycling process generally indicated by box 50. Thus, the supply box 140 will supply 70% to 90% of the softwood kraft to the section 20' of the headbox 200 with 10% to 30% of the furnish being supplied from the recycling process 50. Similarly, 70% to 90% of the hardwood kraft will be supplied from the supply box 141 with 10% to 30% of the furnish being supplied to the conduit 41 from the recycling process 50. In this way, approximately 70% to 90% of the furnish supplied to each of the sections 20 and 20' of the headbox 200 will be substantially pure hardwood kraft or softwood kraft, as the case may be, with only 10% to 30% of the furnish being a mixture of hardwood kraft and softwood kraft fibers which are recycled through the recycling process 50. The high retention rate of 70% to 90% is made possible by utilizing a foam-formed papermaking machine. Figure 2A illustrates an enlarged perspective schematic view of a portion of a tissue web 300 formed by a foam-formed process according to the present invention. A first zone 301 is a foam-formed paper tissue formed from a furnish of hardwood kraft and recycle. A second zone 401 is a foam-formed paper-tissue formed from a furnish of softwood kraft and recycle. The first zone 301 and the second zone 401 are formed unitary and entangled with each other to form the nonlaminated stratified paper tissue 300. Nonlaminated is defined as an adherence of the first zone to the second zone so as to prevent separation of the two zones. Boundary sections 310, 410 are disposed between the first zone 301 and the second zone 401. The boundary sections used hereinafter refer to a transition zone which is ultra-light and smaller than a water formed transition zone. The boundary sections 310, 410 are entangled sections wherein short papermaking fibers from the hardwood kraft and long papermaking fibers from the softwood kraft are intermingled and entangled to provide an interface zone which will permit a multizone construction while eliminating delamination of the zones. The foam-formed paper tissue 300 does not permit delaminatien. This feature is distinct as compared to multi-layered water-formed paper tissues wherein the layers may actually be separated one from another. The entangled short fibers 303 and the entangled long fibers 403 can be seen in the interface 310, 410 disposed between the first zone 301 and the second zone 401. A high softness integument 320 is defined adjacent to the outer surface of the first zone 301 of the web of paper tissue 300. The integument 320 includes an enriched region 330 having a substantial purity of hardwood kraft. Similarly, a substratum 420 is defined adjacent to a surface of said second zone spaced away from said integument. The substratum 420 includes an enriched region 430 having a substantial purity of softwood kraft. The term integument means an outer covering. As used in this invention, the term refers to a section of the paper tissue defined adjacent to an outer surface of the first zone 301 on one side of the paper tissue 300. In the embodiment illustrated in Figure 2B, the integument is defined adjacent to the outer surfaces of the two-ply tissue on both sides of the paper tissue 300A. In the embodiment illustrated in Figure 2C, the integument is defined adjacent to the outer surface of the multi-zone tissue on both sides of the paper tissue 3000. Similarly, the term substratum means an underlying layer. As used in this invention, the term refers to a section of the paper tissue defined adjacent to a surface of the second zone 401 spaced away from the integument. In the embodiment illustrated in Figure 2B, the substratum is defined as the central portion of the two-ply tissue 300A. In the embodiment illustrated in Figure 2C, the substratum is defined in the central portion of the multizone tissue 3000 and is split into two sections. Figure 2B illustrates a foam-formed nonlaminated stratified paper tissue product which is composed of a first ply 500 and a second ply 600. The first ply 500 includes a first zone 301 which is a foam-formed paper tissue formed from a furnish of hardwood kraft and recycle. A second zone 401 is a foam-formed paper tissue formed from a furnish of softwood kraft and recycle. The discussion of the first zone 301 and the second zone 401 is set forth hereinabove. Similarly, the second ply 600 includes a first zone 301' which is a foam-formed paper tissue formed from a furnish of hardwood kraft and recycle. A second zone 401' is a foam-formed paper tissue formed from a furnish of softwood kraft and recycle. The first zone 301' and the second zone 401' are formed unitary and entangled with each other in the same manner as the first ply 500. Boundary sections 310' and 410' are disposed between the first zone 301' and the second zone 401'. boundary sections 310' and 410' are entangled sections wherein short papermaking fibers from the hardwood kraft and long papermaking fibers from the softwood kraft are intermingled and entangled to provide an interface zone which will permit a multi-zone construction while eliminating delamination of the zones. Entangled short fibers 301' and entangled long fibers 401' can be seen in the interface boundary sections 310', 410' disposed between the first zone 301' and the second zone 401'. A high softness integument 320' is defined adjacent to an outer surface of the first zone 301' of the web of paper tissue 301A. The integument 320' includes an enriched region 330' having a substantial purity of hardwood kraft. Similarly, a substratum 420' is defined adjacent to a surface of the second zone 401' and is spaced away from the integument 320'. The substratum 420' of the second ply 600 is juxtaposed adjacent to the first ply 500. The substratum 420' includes an enriched region 430' having a substantial purity of softwood kraft. The foam-formed nonlaminated stratified paper tissue product 300A illustrated in Figure 2B provides a product having softwood kraft composed of recycled material or other fibers positioned in the central section of a two-ply product. In proprietary testing, this product has elicited wide consumer preference. Figure 1B is a schematic view of another embodiment of the present invention wherein a headbox 200' includes three sections 201, 202 and 203 and forms three slices for providing one jet with three zones of foamed liquid-fiber dispersion to the paper making machine. A return line 24' is connected between a save-all and a recycling process generally indicated by box 50'. Box 146' generally indicates a supply of hardwood kraft which provides a furnish having e.g. approximately 3% by weight of hardwood kraft fibers. The furnish is supplied from the box 146' through the conduit 145' to the conduit 147'. A portion of the recycled foam and fiber from the recycling process 50' is also supplied to the conduit 147' for inclusion in the furnish to the section 203 of the headbox 200'. Box 140' generally indicates a supply of softwood kraft which in the form of a furnish having e.g. approximately 3% by weight of softwood kraft fibers. The furnish is supplied from the box 140' through the conduit 142' to the conduit 40'. A portion of the recycled foam and fiber from the recycling process 50' is also supplied to the conduit 40' for inclusion in the furnish to section 202 of a headbox 200'. Similarly, box 141' illustrates hardwood kraft which is supplied in the form of a furnish having e.g. approximately 3% by weight of hardwood kraft fibers. The furnish in the box 141' is supplied through the conduit 143' to the conduit 41' for supplying hardwood kraft furnish to a section 201 of the headbox 200'. Foam and furnish from the recycling box 50' is also supplied to the conduit 41' to form a portion of the furnish supplied to the section 201 of the headbox 200'. The other elements illustrated in Figure 1B are similar to the elements illustrated in Figure 1A and are not further discussed herein. As illustrated in Figure 2C, a paper tissue 3000 which includes three zones 3100, 4100 and 5100 is provided with a hardwood kraft being disposed on the outer surface of both sides of the paper tissue 3000. Figure 2C is an enlarged perspective schematic view of a portion of a tissue web 3000 formed by a foam-formed process according to the present invention. A first zone 3100 is a foam-formed paper tissue formed from a furnish of hardwood kraft and recycle. A second zone 4100 is a foam-formed paper tissue formed from a furnish of softwood kraft and recycle. A third zone 5100 is a foam-formed paper tissue formed from a furnish of hardwood kraft and recycle. The first zone 3100, the second zone 4100 and the third zone 5100 are formed unitary and entangled with each other to form the nonlaminated stratified paper tissue 3000. Boundary sections are disposed between the first zone 3100, the second zone 4100 and the third zone 5100. The boundary sections are entangled sections wherein short papermaking fibers from the hardwood kraft and long papermaking fibers from the softwood kraft are intermingled and entangled to provide an interface zone which will permit a multizone construction while eliminating delamination of the zones. The foam-formed paper tissue 3000 does not permit delamination. This feature is distinct as compared to multi-layered water-formed paper tissues wherein the layers may actually be separated one from another. Entangled short fibers and entangled long fibers can be seen in the interface section disposed between the first zone 3100, the second zone 4100 and the third zone 5100. High softness integuments 3200 and 5200 are defined adjacent to the outer surface of the first zone 3100 and third zone 5100, respectively, of the web of paper tissue 3000. The integuments 3200 and 5200 include an enriched region 3300 and 5300, respectively, having a substantial purity of hardwood kraft. Similarly, substratums 4200, 4200' are defined adjacent to a surface of said second zone spaced away from the integuments 3200 and 5200, respectively. The region 4250 between the substratums 4200 and 4200' is an enriched region having a substantial purity of softwood kraft. As illustrated in Figures 1A, 1B, 2A, 2B and 2C, the combination of foam-forming with a stratified headbox 200 or 200' provides a method and produces a product which has a significantly improved enriched region of purity of the stratified tissue product. Stratification is employed to position fibers in a product to yield the greatest economic and consumer benefits. A high quality fiber such as hardwood kraft can be positioned in a zone over a harsh furnish such as recycled fibers. Currently, hardwood kraft, especially eucalyptus, is an expensive fiber for use in producing paper, towels or tissue. Recycled fibers normally are mixtures of short and long fibers which often have a medium-to-rough texture when touched by an individual. Thus, the present invention provides a product and a process whereby the short fibers of hardwood kraft come in contact with the consumer so that a clean, soft product is produced making it possible to use lower quality recycle while preserving product quality. The hardwood kraft provides a surface coating which will cover and not have the same objectionable color as possibly the under zone of recycled fibers. Alternatively, the stratification can be used to achieve maximum consumer benefit by providing zones with all of the strong softwood fibers in an under zone and putting all of the soft hardwood fibers in the outer zone. In this way, the properties of each of the furnish components are employed in the best way to obtain the maximum consumer value. The combination of foam-forming with stratification technology provides unexpected benefits by increasing the enriched region purity or definition of the zones. This allows the stratification affect to be achieved with less of the premium hardwood kraft pulp. In addition, the total sheet basis weight may be reduced. The present invention permits a harsh furnish such as recycled fibers to be utilized in producing a product without decreasing quality and while providing high consumer perception as to smoothness and softness. A number of phenomena are observed in foam-forming which permits the stratification process to be improved. Foam-forming provides a higher first pass retention, thus reducing the amount of fibers which pass through the wire and are mixed back into the layers through the recycling process. The greatly improved formation of foam products allows the fibers in a zone to be more evenly distributed. Thus, a more uniform coverage of an undesirable zone, such as recycled fibers or other softwood kraft, is permitted with less of the hardwood kraft being added to the product. The increased viscosity of the foam which is mixed in with the furnish as it is supplied to the headbox also may tend to prevent mixing of the zones in the forming step. Further the velocity of the jet of the furnish having two or three zones and the velocity of the wire of the paper making machine also has to be taken into consideration to match the speed of the machine with the speed of the jet. Figures 3-7 illustrate data showing the layer purity of four separate layers of a foam-formed nonlaminated stratified paper tissue according to the present invention. The testing method utilized in layering the paper tissue is a standardized Scotch tape method of extracting layers of fibers from a sheet in order to identify the layers for determination of stratification. A sheet of paper, towel or tissue is selected which is clean and free of folds, wrinkles and blemishes. The yankee side, drainage side and the machine direction of the sheet are determined. The sheet size should be approximately 27.9 centimeters (11 inches) to 35.56 centimeters (14 inches) in the cross machine direction for the length and 5.08 centimeters (2 inches) to 15.24 centimeters (6 inches) in the machine direction of the width. A sample of the paper, towel or tissue is placed on a flat surface with the yankee side up. Thereafter, a strip of tape of approximately 2.5 centimeters (1 inches) in width is removed from a roll of tape. The strip should be approximately 10.16 centimeters (4 inches) longer than the sample. Static is removed from the tape by wiping the smooth surface of the tape onto or with a soft, damp surface or air stream. The static-free sticky-side of the tape is applied to the top surface of the paper, towel or tissue. The tape is centered in the long direction of the sheet and lowered onto the sheet from one end to the other in a gentle touch-down manner. Air pockets are avoided. The tape is not pressed or touched on the surface. This tape is labeled No. 1 "YANKEE" side. Thereafter, the paper, towel or tissue together with the tape is turned upside down. The tail ends of the tape are taped to the flat surface. A second strip of tape is applied to the opposite side of the taped specimen directly above the first strip of tape. This tape is labeled with a number which equals the total number of pulls desired; i.e. the total number of layers into which the sample will be divided. Thus, if it is intended to have 4 pulls (i.e. dividing the sample into 4 layers). This tape is numbered 4. For 8 pulls, it is numbered 8, and so on. The tape is thus identified as e.g. No. 4 or No. 8 "DRAINAGE" side. Thereafter, a paper cutter is utilized to trim 0.317 centimeters (1/8 inch) off each edge of the sample. A 2000 gram weight is rolled across the length of the tape specimen on the yankee surface and drainage surface, once on each side. Pressure is not exerted on the weight. The weight is moved at a uniform slow speed over the surface of the paper, towel or tissue. Subsequently, the two tapes are pulled apart at approximately a 180° angle at a uniform moderate speed. The tapes are not jerked or yanked. The two fiber tape splits are positioned on a flat surface with the fiber surface up. The tail ends are taped down. A 2.54 centimeter (1 inch) strip of tape is applied to each half as previously done. The steps identified hereinabove are followed to split the 1/2 sheet fibers into 1/4 and 1/8 splits, or more, as desired, producing layers of fiber attached to tapes. The splits are identified in sequence starting from the yankee side of the paper, towel or tissue. One end of the fiber tape splits is positioned in a petri dish half filled with water. A glue brush is utilized to firmly brush the fiber surface of the submerged sheet in one direction. The sample is slowly pulled in the opposite direction. Thus, wood fibers are removed from the tape into the petri dish. This solution is poured into a beaker. The brush and tape are rinsed as clean as possible of fibers into the beaker. The extracted fibers are used as a standard fiber analysis specimen. The Technical Association of Pulp and Paper Industries (TAPPI) publishes guideline T-401 om-88 as the standard by which the hardwood and softwood fibers are analyzed to identify the various fibers as either hardwood or softwood fibers. Utilizing the tape pull fiber extraction process identified hereinabove, the data set forth in Figures 3-7 was generated. The acronyms set forth in Figures 3-7 are identified as follows: Acronym Meaning FF foam-formed WF water-formed HW hardwood kraft SW softwood kraft Of the samples identified in Figures 3-7, the foamformed samples had a nominal basis weight of 4.46, 3.79 and 3.13 kg (10, 8.5 and 7 lbs.) per ream. The water-formed sheets had a nominal basis weight of 4.46 and 3.79 kg per 278.7 m 2 ream (10 and 8.5 lbs. per 3000 ft 2 ream). All sheets were nominally composed of 50% hardwood kraft and 50% softwood kraft. The sheets were made using stratified forming technology with all of the hardwood on the yankee side of the sheet and all of the softwood on the air side of the sheet. The sheets were separated into eight regions by means of tape splits. Four of the regions (1, 3, 6, 8) were analyzed for percent hardwood and percent softwood. Region 1 corresponds to the outer surface of the hardwood kraft on the yankee side of the product. Region 8 corresponds to the softwood kraft on the air side of the second zone. The data illustrated in Figures 3-7 are working examples as set forth numerically in the following table: Zone Purity (weight % HW) Sample ID Region 1 Region 3 Region 6 Region 8 Composite 4.46 kg (10 lb) FF 96.5 87.1 40.3 12.8 59.1 3.79 kg (8.5 lb) FF 96.1 86.8 54.7 17.1 63.1 3.13 kg (7 lb) FF 99.1 88.5 54.5 17.5 59.0 4.46 kg (10 lb) WF 87.5 69.6 48.8 25.0 57.7 3.79 kg (8.5 lb) WF 89.5 86.5 51.4 24.5 56.5 Additional data to demonstrate the enriched regions of the paper tissue according to the present invention as compared to a water formed tissue is set forth numerically in the following tables. In Table IIA and IIB, the tissues were divided into 10 regions. Zone Purity TRIAL WATER Region 1 Region 5 Region 6 Region 10 Composite m.sec -1 (fpm) J/W 1 68 47 40 60 40 33 (6500) .81 2 47 52 46 66 45 30.5 (6000) .88 3 31 36 38 59 37 27.9 (5500) .96 4 40 38 37 49 40 25.4 (5000) .96 5 48 39 32 49 38 22.9 (4500) .96 Zone Purity TRIAL FOAM Region 1 Region 5 Region 6 Region 10 Composite m.sec -1 (fpm) J/W 1 50 34 41 48 36 33 (6500) .81 2 85 39 27 6 29 30.5 (6000) .88 3 92 39 32 6 35 27.9 (5500) .96 4 79 50 30 7 41 25.4 (5000) .96 5 91 39 24 5 33 22.9 (4500) .96 Please note that the percentages indicated hereinabove represent hardwood kraft. The percentage of hardwood kraft and softwood kraft for each of the layers will total 100%. Thus, in order to determine percentage of softwood kraft, one would merely add an appropriate percentage to the percentage identified hereinabove to add up to a total of 100% for the particular regions of the zones. In addition, the acronym fpm refers to the feet per minute speed of the paper machine whereas the acronym J/W refers to the ratio of the jet to wire speed. The test results set forth in Table IIA indicate that a water formed paper tissue in the trial runs consisted of a rather homogenous mixture of the hardwood and softwood fibers throughout the regions of the paper tissue. This result was achieved even though a separate furnish of hardwood fibers was supplied to one section of a headbox and a separate furnish of softwood fibers was supplied to a second section of a headbox. The first trial run of a foam formed paper tissue as listed in Table IIB indicates that good stratification is not achieved when a large speed difference exists between the jet speed as compared to the speed of the forming wire of the machine. In order to compare the results of the present invention, commercial tape splits of various products manufactured by the assignee of the present invention and products on the marketplace manufactured by others were tested to determine the percentage of hardwood kraft and softwood kraft in various regions of the layers of commercial products. The commercial products were manufactured with a water-forming technique. The results of the testing of commercial products are set forth in Tables III and IV. In Table III, the product was divided into 8 regions. The product reported in Table IV was divided into 20 regions. Commercial Product Basis Weight Region 1 Region 3 Region 6 Region 8 Composite Brand A-1 19.4 72 41 35 14 29 Brand A-2 19.7 81 68 59 48 62 Brand A-3 20.0 84 67 33 10 41 Brand A-4 57 37 29 7 27 Brand A-5 84 49 32 10 28 All of the samples set forth above are two-ply paper tissues manufactured by the patentee at three separate plants. Commercial Product Basis Weight Region 1 Region 8 Region 14 Region 20 Composite Brand X-1 17.7 95 69 72 85 72 Brand X-2 90 50 59 92 63 Brand X-3 98 35 60 97 71 Brand X-4 86 53 48 95 58 Brand X-1 15.8 95 63 69 84 80 Brand X-2 90 49 62 92 61 Brand X-3 98 80 42 96 66 Brand X-4 85 50 44 86 58 Brand X-1 17.8 98 67 78 94 81 Brand X-2 92 49 43 92 70 Brand X-3 97 81 30 97 73 Brand X-4 77 54 41 88 58 Brand X-3 25.0 89 34 70 89 51 All of the samples set forth above are one-ply paper tissues with three zones manufactured by a competitor at four separate plants. Please note that the percentages indicated hereinabove represent hardwood kraft. The percentage of hardwood kraft and softwood kraft for each of the layers will total 100%. Thus, in order to determine the percentage of softwood kraft in a particular region, one would merely subtract the recorded percentage for hardwood kraft for that region from 100%. The softwood kraft is a particular wood pulp which has relatively long fibers. Softwood trees growing on the western side of the Cascade mountain range in Washington and Oregon States yield fibers of somewhat greater length than those grown on the East. For example, the TAPPI handbook indicates that Douglas Fir from the coast (Western) side of the Cascades is assigned a weight factor of 1.4 whereas Douglas Fir from the inland (Eastern) side of the Cascades is assigned a factor of 0.90 When technicians attempt to determine fiber content, the stains which are used to distinguish hardwood fibers from softwood do not distinguish between East and West side softwoods often leading to inaccuracies in wood fiber content weight percentages. In addition, part of the furnish is supplied from a large quantity of wood from saw mills where the long softwood fibers are broken up when sawdust is used in the furnish leading to further difficulty assigning appropriate weight factors to each fiber. Thus, the test results identified hereinabove with regard to Brand A wherein a small percentage of hardwood kraft produces a large percentage of hardwood kraft on the outer surface is subject to clarification. The composite material which is utilized in Brand A has a unique characteristic wherein it is thus difficult to assign appropriate weight factors to the softwood kraft fibers leading to error in hardwood content estimates as well. The particular wood utilized in producing Brand A tissue does not fit within the normal definitions of softwood kraft and hardwood kraft as employed in the industry. From the above Tables and Figures 3-7, it is clear that the purity of Region 1 according to the present invention is substantially higher, thus producing a soft and smooth surface for the foam-formed nonlaminated stratified paper tissue. The hardwood kraft supplied as a furnish to the headbox comprises 50% by weight of the total of hardwood and softwood kraft supplied to the headbox and the softwood kraft likewise comprises 50% by weight of the total. The amount of hardwood kraft actually measured in the samples tested was found to be in the range of 59% to 63.1% by basis weight. Nevertheless, this relatively small percentage of hardwood kraft yields a surface purity in the range of 96.1% to 99.9% hardwood kraft. The present invention is thus a substantial improvement over the water-formed paper tissues. More specifically, the products manufactured as Brand X require a hardwood kraft percentage in the range 66% to 81% in order to provide a surface purity of hardwood kraft in the range of 95% to 98%. In other words, if a large percentage of the furnish is hardwood kraft, it is inevitable that the percentage on the outer surface of the tissue will also be hardwood kraft. In contradistinction thereto, the present invention permits a smaller percentage of hardwood kraft to be applied as a furnish. The result achieved by the present invention with a smaller percentage of hardwood kraft in the composite material results in a larger percentage in the range of 96.1% to 99.1% surface purity of hardwood kraft. This percentage is substantially higher than the conventional water-formed tissues and produces a soft and smooth tissue which is desirable. Figures 8A and 8B are SEM microscopy surface views of a paper tissue which was constructed by using a water forming method. The enlarged photographs show the composition of the fibers in the outer layer of the product. Figures 9A and 9B are SEM microscopy cross-sectional views of a paper tissue which was constructed by using a foam forming method. The enlarged photographs show the composition of the fibers in the product wherein the enriched regions in the first and second zones have a high degree of purity of hardwood fibers. Comparison with Figures 8A and 8B clearly shows the higher purity of the surfaces of products of the present invention. In one embodiment of the present invention a foam-formed nonlaminated stratified paper tissue may be constructed having a first zone formed from a first foamed furnish consisting essentially of at least about 90% by weight hardwood fiber. A second zone may be formed from a second foamed furnish consisting essentially of at least about 70% by weight of softwood fiber. The second zone may be formed unitary and entangled with the first zone to form a nonlaminated stratified paper tissue. A high softness integument is defined adjacent to an outer surface of the first zone and consists essentially of at least about 80% by weight of fibers chosen from the group consisting of hardwood kraft fibers, or hardwood sulfite fibers, and up to about 10% by weight of strength enhancing softwood fibers. A substratum is defined adjacent to a surface of the second zone spaced away from the integument and consists essentially of at least about 65% by weight of strength and bulk enhancing fibers chosen from the group consisting of softwood fibers, secondary fibers, and anfractuous cellulosic fibers. The integument on the outer surface of the outer zone includes an enriched region having a concentration of hardwood fiber of above the greater of 80% by weight and at least 95% of the concentration of hardwood fiber in the first foamed furnish and the substratum includes an enriched region having a concentration of softwood fiber of at least about 95% of the concentration of softwood fiber in the second foam furnish, wherein the enriched region of substantially pure hardwood fiber provides an extremely soft and smooth surface. In another embodiment foam-formed nonlaminated stratified paper tissue may be constructed having a first zone formed from a first foamed furnish consisting essentially of at least about 90% by weight hardwood fiber. A second zone may be formed from a second foamed furnish which consists essentially of at least about 70% by weight of fiber chosen from the group consisting of softwood kraft, secondary fibers, and anfractuous cellulosic fibers. The second zone is formed unitary and entangled with the first zone to form a nonlaminated stratified paper tissue. A high softness integument is defined adjacent to an outer surface of the first zone and consists essentially of at least about 95% by weight of fibers chosen from the group consisting of hardwood kraft fibers, and hardwood sulfite fibers, and up to about 5% by weight of strength enhancing softwood fibers. A substratum is defined adjacent to a surface of the second zone spaced away from the integument and consists essentially of at least about 65% by weight of strength and bulk enhancing fibers chosen from the group consisting of softwood fibers, secondary fibers, and anfractuous cellulosic fibers. The integument on the outer surface of the first zone includes an enriched region having a concentration of hardwood fiber of at least 95% of the concentration of hardwood fiber in the first foamed furnish with a basis weight of at least 1.12 kg (2.5 lb)/ream and no less than the basis weight of the fiber applied in the first furnish minus 0.67 kg (1.5 lb)/ream, wherein the enriched region of substantially pure hardwood fiber provides an extremely soft and smooth surface. In still another embodiment of the present invention a foam-formed nonlaminated stratified paper tissue may be constructed having first and third zones formed from first and third foamed furnishes consisting essentially of at least about 90% by weight hardwood fiber. A second zone may be formed from a second foamed furnish consisting essentially of at least about 70% by weight of softwood fiber. The second zone may be formed unitary and entangled with the first and third zones to form a nonlaminated stratified paper tissue. High softness integuments are defined adjacent to outer surfaces of the first and third zones and consist essentially of at least about 80% by weight of fibers chosen from the group consisting of hardwood kraft fibers, or hardwood sulfite fibers, and up to about 20% by weight of strength enhancing softwood fibers. A substratum is defined in the second zone spaced between and away from the integuments and consists essentially of at least about 65% by weight of strength and bulk enhancing fibers chosen from the group consisting of softwood fibers, secondary fibers, and anfractuous cellulosic fibers. The integument on the outer surface of each of the outer zones includes enriched regions having a concentration of hardwood fiber of above the greater of 80% by weight and at least 95% of the concentration of hardwood fiber in the first foamed furnish and the substratum includes an enriched region having a concentration of softwood fiber of at least about 95% of the concentration of softwood fiber in the second foam furnish, wherein the enriched regions of substantially pure hardwood fiber provide extremely soft and smooth surfaces. In yet another embodiment foam-formed nonlaminated stratified paper tissue may be constructed having first and third zones formed from foamed furnishes consisting essentially of at least about 90% by weight hardwood fiber. A second zone may be formed from another foamed furnish which consists essentially of at least about 70% by weight of fiber chosen from the group consisting of softwood kraft, secondary fibers, and anfractuous cellulosic fibers. The second zone is formed unitary and entangled with the first and third zones to form a nonlaminated stratified paper tissue. The high softness integuments are defined adjacent to an outer surface of the first zone and consist essentially of at least about 95% by weight of fibers chosen from the group consisting of hardwood kraft fibers, and hardwood sulfite fibers, and up to about 5% by weight of strength enhancing softwood fibers. A substratum is defined in the second zone spaced away from the integuments and consists essentially of at least about 65% by weight of strength and bulk enhancing fibers chosen from the group consisting of softwood fibers, secondary fibers, and anfractuous cellulosic fibers. The integuments on-the outer surface of the first and third zones include enriched regions having a concentration of hardwood fiber of at least 95% of the concentration of hardwood fiber in the first and third foamed furnishes with a basis weight of at least 1.12 kg (2.5 lb)/ream and no less than the basis weight of the fiber applied in the first and third furnishes minus 0.67 kg (1.5 lb)/ream, wherein the enriched regions of substantially pure hardwood fiber provide extremely soft and smooth surfaces.

Claims: A method of forming a multi-layer paper tissue web of improved surface softness from a pulp furnish wherein a first layer of the web is deposited from a first foamed furnish and a second layer of the web is deposited on the first layer from a second foamed furnish, each furnish includes recycle fiber recovered from both furnishes, the fiber content of said first furnish excluding recycle fiber is relatively richer in hardwood fiber than said second furnish and the fiber content of said second furnish excluding recycle fiber is relatively richer in fibrous material selected from softwood kraft, secondary fibers and anfractuous cellulosic fibers than said first furnish. A method as claimed in Claim 1 wherein the overall hardwood fiber content of the first and second furnishes is approximately 20% to 35% by weight of the paper tissue and the overall content of fibrous material selected from softwood kraft, secondary fibers and anfractuous cellulosic fibers in the first and second furnishes is approximately 65% to 80% by weight of the paper tissue. A method as claimed in claim 1 or claim 2 characterised in that said first foamed furnish consists essentially of at least 90% by weight hardwood fiber; and said second foamed furnish consists essentially of at least 70% by weight of softwood fiber. A method as claimed in claim 1 or claim 2 characterised in that said first foamed furnish consists essentially of at least 90% by weight hardwood fiber and said second foamed furnish consists essentially of at least 70% by weight of fiber chosen from softwood kraft, secondary fibers and anfractuous cellulosic fibers. A method as claimed in Claim 1 which further includes depositing a third layer from a third foamed furnish on said second layer, the recycle fiber comprises fiber from all three furnishes and the fiber content of said third foamed furnish excluding recycle fiber is relatively richer in hardwood fiber than said second foamed furnish. A method as claimed in claim 5 characterised in that the first and third foamed furnishes each consist essentially of at least 90% by weight hardwood fiber and said second foamed furnish consists essentially of at least 70% by weight of softwood fiber. A method as claimed in Claim 5 characterised in that the first and third foamed furnishes each consist essentially of at least 90% by weight hardwood fiber and said second foamed furnish consists essentially of at least 70% by weight of fiber chosen from softwood kraft, secondary fibers and anfractuous cellulosic fibers. A method as claimed in any one of Claims 1 to 7 characterised in that the hardwood fibers are selected from hardwood kraft fibers and hardwood sulfite fibers. A method as claimed in any one of Claims 1 to 8 characterised in that the overall amount of hardwood fiber supplied to form said first layer is no more than 8.1g/m 2 (5 lbs/3000 sq.ft. ream). A method as claimed in any one of Claims 1 to 9 characterised in that the tissue is formed by crescent forming. A method as claimed in any one of Claims 1 to 10 characterised in that said first furnish is supplied to a yankee side of a paper making machine. A method as claimed in any one of Claims 1 to 11 wherein the first and second furnishes are dried to form said web and the web is creped off a drying means.

Inventors: JANDA BRUCE W, JANDA, BRUCE W.
Applicants: FORT JAMES CORP, FORT JAMES CORPORATION
Application year: 1992
Main classification: D21H27
IPC codes: D21F11, D21H27, D21F9, A47K7, A47K10