Лако-красочные материалы — производство

For process improvement in unsewn binding, two or rarely three different adhesives can be applied one after the other (multilayer binding) [41]. Here, one popular technique is the so-called ‘two-shot system’, which uses two adhesives. The applica­tion of two different dispersions for primer and topcoat has a long tradition; they are frequently derived from the same formulation, their viscosities being adapted to the demands of a primer and a topcoat, respectively. These systems can be processed with or without intermediate drying. When using two-shot systems with two dispersion adhesives, the first dispersion optimizes the adhesion to the paper edge, while the second dispersion ensures a safe pick-up of the covers, especially when these are rather heavy. Two-shot systems also optimize the cold flexibility of the product. Two — shot, hot-melt systems have a similar functionality that is beneficial in the manufac­ture of catalogs, where increased machine capacities and paper qualities with a high share of secondary fibers have transformed the production process. They also provide better page pull and page flex values at consistent production speeds.

Two-shot systems that combine dispersions with hot melts benefit from the advantages of both. A relatively low-viscosity dispersion adhesive is usually used as primer, providing excellent bond strength to the paper edge. The dispersion can be applied in a very thin layer as this adhesive is only responsible for bonding the paper edges. After drying, a hot melt is applied as a topcoat that has been designed to adhere particularly well to the dispersion film. The hot melt provides stability to the back and also for the pick-up of the cover. New combinations of raw materials have better matched up dispersions and hot melts, meeting the requirements perfectly. The low — viscosity primer dispersions show excellent wetting properties, and can be applied either via standard or foam rollers. The hot melts provide very good adhesion to the

dispersion film; indeed, when their film properties act together they provide good lay — flat behavior and very good page flex values. A careful selection of two-shot unsewn binding systems leads to an increased product quality compared to conventional unsewn binding with hot melts, and to increased production speeds compared to perfect binding with dispersions. However, since two different adhesives are employed, the binding process requires more expertise.

Another option is the combination of dispersions, applied as primers, with reactive polyurethane hot melts, applied as topcoats. These systems are processed with intermediate drying via rollers, as in conventional processing, and via nozzles in the case of PU resin hot melts, applying the adhesive, for example, to the cover.

The low processing viscosities and rheological properties of both products allow for a minimum layer thickness, leading to an improved lay-flat behavior and, last but not least, to a more economic production. The economic effect is fostered by the fact that these systems set up high strength within a very short period of time, such that the products are ready for use in a few hours.