Лако-красочные материалы — производство

The lamination process (ply bonding) is crucial for the quality of the finished tissue product. Lamination adhesives join two or three (or even more) tissue plies together. There are several ways in which the plies of embossed products are brought together; one way is to apply the adhesive to the tips of the raised elements of one of the two layers, and those tips come into contact with and are bonded to the recessed areas or floors of the other layer (nested laminating). In pin-to-pin, tip-to-tip or point-to-point laminating, the raised elements of each layer contact each other at the tips, which is where the bonding occurs. The paper typically has a weight of 15-25 g m-2 and can be either bleached or unbleached. A typical machine speed is 600-800 m min-1, but even faster equipment is available which runs at speeds of up to 1000 m min-1.

Although, in the past, laminating adhesives were mainly derived from dissolved cellulose, at present they are increasingly being replaced by synthetic polymers, particularly polyvinyl alcohol with its excellent adhesion properties. This is especially the case on tissue papers, which are more difficult to bond due to their wet strength, or because they are impregnated with lotions. The key characteristics of these adhesives are appropriate wet bonding capacity, no or only little foaming, and good processability. Adhesives with fast setting speeds enable the lamination to perform well in the machinery and make it possible to avoid staining on the tissue paper. Laminating adhesives must be flexible enough so as not to impair the softness of the finished product. Water-based products release the majority of their water content into the paper during lamination, which can threaten the embossed structure of the tissue laminate. Recent years have seen the development of hot-melt laminating adhesives that retain the embossing and provide a ‘softer feel’. Higher production speeds and a better fixation of the layers (compared to mechanical fixing, particularly on high-speed machinery) are further benefits of hot-melt laminating adhesives. These solid systems are applied by spraying but, owing to their rheological properties (Figure 8.38) and modern application technology, very low application weights of around 2-10 mg per running meter are possible with production speeds of 600-800 m min-1 and drum widths of 2.7 to 3.5 m. Hot-melt laminating adhesives have been tested at production speeds of up to 1000 m min-1.

Although, most paper-bonding applications have been dominated by hot-melt adhesives for many years, in the production of tissue paper hot melts have only recently begun to acquire popularity. This is due to the fact that neither an appropriate application technology nor water-soluble or easily redispersable polymers suited for the formulation of hot melts, had been available before. Today, the hot-melt adhesives used for most paper-bonding purposes are commonly based on thermoplastic, non-water-

soluble polymers (e. g. EVA or block copolymers). To meet the specific needs of the tissue-producing industry, special raw materials had to be developed; at present, sufficiently hydrophilic polyesters and polyurethanes are available that are redisper — sable or even totally water-soluble. Tissue-laminating hot melts have been successfully developed from these polymers which combine low application quantities with special physical properties that provide for the lamination of soft tissue products, without staining them. These hot melts should be as clear in color as possible (preferably water — white) so as to not to impair the appearance and color of the tissue paper. They must also be stable during the melting process. Water-soluble or redispersable polymers are generally very polar and therefore susceptible to oxidation, which may cause staining of the product. Today’s commercial adhesives remain water-white for hours at their processing temperature, and do not stain the tissue papers.