Tissue products are in almost constant use in daily life. Toilet tissue, facial tissue, tissue wipes, and paper towels are examples of tissue products used throughout home and industry. Tissue products can be made of a single lamina (or ply) or can be laminates (or multi-ply, or else laminated) formed from two or more plies. As used herein a “lamina”, or “ply”, refers to a single sheet of tissue paper, and the term “laminate”, or “multi-ply”, or “laminated”, is used to characterize a paper made by uniting several layers of single sheets or plies together or to another woven or nonwoven substrate to yield a unitized material. The phrase “tissue laminate”, as used herein, refers to a laminate including at least one ply of tissue paper unitized with another substrate or ply, and these are well known and recognized by those skilled in the art. “Tissue paper” or “tissue” as used herein, refers to woven or nonwoven substrates that are relatively low weight sheets for use as sanitary products such as facial and bathroom tissues or paper towels. These also may be fabricated in condenser, carbonized, wrapping and cleaning grades. General characteristics may include one or more of softness, adequate strength and absorbency, clean appearance, and freedom from coarse and abrasive particles. See, e.g., G. A. Smook (1990), Handbook of Pulp & Paper Terminology; A Guide to Industrial and Technological Usage, chap. 16; Angus Wilde Publications, Vancouver, BC. ISBN 0-9694628-0-8. In typical embodiments, a tissue may have a sheet basis weight of up to about 50 grams per square meter, more typically about 13 to about 46 grams per square meter.
A single ply tissue product has several drawbacks. For example, a single lamina tissue product will be stiffer than a dual laminae tissue product having the same total basis weight. This increased stiffness results in a consumer perception that the single lamina tissue product is not as soft as a multi-ply tissue product and may, therefore, be less preferred than the multi-ply tissue product. A laminated tissue having the same total basis weight as a single lamina tissue product can be made to have greater caliper. This increased caliper results in the consumer perception that the product has thickness (bulkiness) and high quality. Also, to provide consumers with the convenience of using a predetermined length of material, tissue laminates may be perforated at pre-selected lengths.
However, tissue laminates are subject to the phenomenon of skinning. Skinning occurs when the laminae separate from one another and no longer remain intact to form a unitary laminate. Skinning may occur, for example, when trying to reduce tissue product packaging and transportation costs. For example, U.S. Pat. No. 4,886,167 discloses packages of toilet tissue and paper towels compressed orthogonal to the cores to reduce the core volume. This compression of the tissue product may impart shear forces to the laminae and result in skinning. Skinning may also occur during a tissue converting operation when the top sheet of a roll breaks off as a result of friction in the area of contact between the surface in the production line and the roll of paper. It can also occur during handling by the consumer. In the case of perforated toilet tissue skinning can lead to mismatched plies along the perforation lines when a portion of the top sheet of a roll is torn or lost.
Several attempts have been made in the art to join tissue laminae in a manner to reduce or minimize skinning. One approach involves the application of mechanical forces (friction and compression) to interlock the sheets and keep them from separating from each other. These forces are generated by piercing pinholes in the laminates, with the edges of the knurls engraved in a steel roll, to marry the sheets, or by pressing wavy lines in the tissues during embossing. Another approach uses an adhesive to bond laminae together. This approach has been described, for example, in U.S. Pat. Nos. 5,143,776; 4,885,202; 4,806,418; 4,770,920; 4,481,243; and 4,513,051.
Some special, water dispersible or soluble hot-melt adhesives, like the ones described in U.S. Pat. Nos. 6,448,463B1; 6,087,550; 6,103,809; and 5,663,286 and in WO99/35189, all assigned to H. B. Fuller Company, can be used. Tissue laminates made with water-soluble adhesives readily break down upon wetting, as, for example, might occur when toilet tissue or facial tissue is disposed of in a sewer system. Consequently, the adhesive used to bond tissue laminae together is desirably aqueous-based (solution, dispersion, emulsion, latex or the like) and at least water-soluble (solution) or water dispersible to some degree.
Notwithstanding the success of hot-melt adhesives, it may be desirable to use aqueous-based adhesives for economical reasons. However, using aqueous-based adhesives to manufacture tissue laminates is problematic due, at least in part, to the water transmissivity and delicate nature of the individual tissue plies when wetted. Water-based formulations tend to bleed through (or strike through) the ply(ies) to which the formulations are applied. Bleed through leads to unplanned bonding among laminae beyond the desired design of the laminate, for instance. During production, efforts are made to minimize or eliminate this undesired phenomenon. Such efforts include reducing the amount of adhesive used as well as pre-wetting of the plies. These kinds of efforts can reduce bond strength between plies.
Moreover, when coated onto a delicate tissue substrate, the tissue is susceptible to damage when impacted by adhesive at conventional application pressures. The water in the adhesive composition also can distort or even destroy aesthetic embossing. Wet plies are also more susceptible to damage, complicating manufacture. Drying also takes a relatively long time, decreasing productivity, when using aqueous adhesives.
Accordingly, the tissue industry could benefit from a better way to use aqueous adhesives to manufacture delicate tissue laminates that would provide the advantages offered by such compositions while reducing or even eliminating one or more of bleeding through, undue loss of bonding strength, distortion or destruction of embossed or other three-dimensional features, loss of strength during manufacture when plies are wetted, and relatively long drying times. It would also be advantageous to apply a process that would allow for waste reduction of the adhesive during manufacturing of the laminated tissues as well as the reduction of the adhesive consumption. These benefits, particularly waste reduction, are also applicable to non-tissue products where lamination and/or bonding are desirable.