Paper making and fiber liberation – Processes and products – Non-uniform – irregular or configured web or sheet
Reexamination Certificate
2000-07-11
2002-10-08
Chin, Peter (Department: 1771)
Paper making and fiber liberation
Processes and products
Non-uniform, irregular or configured web or sheet
C162S117000
Reexamination Certificate
active
06461474
ABSTRACT:
BACKGROUND OF THE INVENTION
Historically, tissue making has relied on creping technology to provide a paper sheet with adequate softness and bulk. Recently, new methods have been developed for uncreped tissue manufacture with noncompressive drying methods, especially through-air drying, to achieve soft, high bulk, wet resilient structures with novel properties. For practical reasons, these methods utilize woven papermaking fabrics to provide the three-dimensional structure required in uncreped sheets if they are to have excellent mechanical properties such as high bulk, high stretch in the cross direction, and high compressive wet resiliency.
Unfortunately, woven fabrics are limited in terms of height differentials and patterns that can be achieved. There are physical constraints on what can be produced on a loom, and there are further constraints on the runnability of anything so produced. While high surface depth (characteristic peak to valley depth) may be desired in many cases in order to impart bulk, stretch, and texture to a paper web, only a narrow range of surface depths can be achieved practically in existing papermaking fabrics. Further, the surface topography of woven papermaking structures are inherently characterized by precipitous peaks and valleys with step changes in height that are typically some multiple of a filament diameter. Typically, the surface has a series of warps or chutes elevated relative to other filaments, with multiple interstices between the filaments. A probe passing along such a surface will encounter a series of sudden jumps up and down. A papermaking web deformed against such a surface becomes smoothed by the physics of paper deformation, but if the underlying fabric surface is given a high degree of surface depth, the large, precipitous peaks and valleys in the fabric can result in sharp structures in the paper web which can be perceived as grits or abrasive elements by humans using the product, especially if the sheet remains uncreped. Much more desirable would be a substrate for forming paper that could have a high degree of surface depth without precipitous peaks and valleys, but rather less abrupt structures offering more pillow-like topography against which the paper web could be deformed.
A further problem with typical woven structures for papermaking is that the filaments and the surface structure itself are largely incompressible. As a result, highly textured 3-D structures are problematic in operations where one surface contacts another, as in a pressing event or a sheet transfer between two fabrics, because most of the load, shear stress, or friction during the event is borne by a small portion of the web resting on or near the highest filaments, which can result in breaking of the web near the high spots of the substrate or other forms of damage to the web and even to the underlying substrate. In some cases, it would be desirable if the highest elements in a 3-D substrate were deformable to allow the 3-D substrate to perform better in a nip or sheet transfer point such that the integrity of the web is better maintained or the distribution of stress is more uniform as the substrate deforms. This is particularly important when the transfer or pressing event involves a first textured substrate such as a papermaking fabric and a second textured substrate such as a fabric or patterned roll, for damage to the sheet and the textured substrates can occur at contact points involving relatively high spots from both substrates unless one or both such substrates can deform to allow more uniform load or stress distributions to be established.
The use of nonwoven substrates in the formation or drying of paper is known to a limited degree, for monoplanar films and membranes have been taught for the production of tissue. In tissue making, these structures typically offer flat, planar regions for imprinting a web during a compression step in order to provide a network of densified regions surrounding undensified regions, with the densified regions providing strength and the undensified regions providing softness and absorbency. Such structures and processes lack the contoured, non-planar three-dimensionality most desirable for textured and noncompressively dried materials and, due to the lack of a non-monoplanar, 3-D wet molding surface, are incapable of providing the high bulk levels of the present invention. Such processes also result in a sheet with regions of high density and regions of low density, unlike the structures of substantially uniform density provided in the noncompressive drying method of the present invention. Further, substantially planar films are inherently limited in their ability to impart three-dimensional structures to a sheet.
Therefore, it would be desirable to provide a method for improving the degree of wet molding and surface depth that can be achieved in a soft, noncompressively dried tissue.
SUMMARY OF THE INVENTION
It has been discovered that three-dimensional nonwoven structures can be used as the substrate for wet molding or through drying a tissue web, thus greatly increasing the possible geometries and textures that can be applied to the web. The use of three-dimensional nonwoven substrates for wet molding allows higher sheet bulk and higher surface depth to be achieved than is possible even with advanced woven substrates. Further, it has been discovered that a tissue web can be given high bulk and distinct three-dimensional texture by the proper application of differential velocity transfer from a carrier fabric onto an endless belt comprising a three-dimensional nonwoven surface, followed by or simultaneous with a proper air pressure differential across the web and substrate to further control the molding of the sheet. The web can also have high wet resiliency properties if the molding of the sheet occurs while the sheet is still relatively moist, followed by substantially noncompressive drying said web on the molding substrate to a solids level of about 70% or more.
In one embodiment, the nonwoven surface has sufficient compressive compliance to deform substantially in a nip or during sheet transfer, in order to prevent damage to a weak, wet sheet as it is suddenly applied to a highly textured surface. A compliant surface may also be useful in other compressive transfers as in the transfer nip of a can dryer or during other events. Preferably, the nonwoven surface is structured to provide pillow-like contours rather than the sharp, precipitous peaks and valleys that are typical of three-dimensional woven structures, for such precipitous structures often give rise to grittiness in the final product. In a further embodiment, the nonwoven material is extruded onto an existing porous underlayment in a manner that disguises or fills in undesirable structures of the underlayment while providing additional desired structures, allowing the underlayment to be selected for strength, runnability, or other characteristics independent of the topography of the underlayment. Such underlayments can include materials other than traditional papermaking fabrics and can include porous substrates such fabrics, felts, general textiles, reticulated foams, metallic screens, dense extruded plastics and nonwovens, laminated composites, and multicomponent woven and nonwoven structures.
Hence in one aspect, the invention resides in a method for making a high bulk paper sheet comprising:
(a) forming an embryonic web from an aqueous dispersion of papermaking fibers, preferably on a papermaking forming fabric;
(b) transferring the web from the papermaking forming fabric to a wet molding substrate comprising an upper porous nonwoven member and an underlying porous member supporting said upper porous nonwoven member, with the upper nonwoven member defining the paper-contacting surface of said wet molding substrate, preferably wherein
(1) the upper porous nonwoven member comprises a fibrous or foam-based material having a Low Pressure Compressive Compliance (hereinafter defined) greater than 0.05, preferably greater than 0.1; a High Pressu
Burazin Mark Alan
Lindsay Jeffrey Dean
Chin Peter
Croft Gregory E.
Kimberly--Clark Worldwide, Inc.
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