Лако-красочные материалы — производство

It is possible here to select only typical examples for each subheading.

1.9.1. Bookbinding

Bookbinding with adhesives has played an important role since the beginning of the production of graphical products [185]. Originally, vegetable and animal raw materials were the main ingredients of adhesives, but a steady development of the adhesives took place to fulfill manufacturing requirements. Today, a range of adhesives, tailor-made for particular applications, is available. These applications include perfect binding, case making, casing in, back-lining and gluing-off of thread-stitched book blocks, as well as side-gluing and end-sheet tipping. The most important field is perfect binding.

Perfect Binding of Books. It is impossible to imagine the production of books, brochures and catalogues without perfect binding. New paper qualities such as high — weight coated (HWC) papers, lightweight coated (LWC) papers and recycling papers, , high-quality papers such as matt-varnished papers and the increase of the production speed up to 300 books per minute require new bookbinding adhesives. Coated papers can have more than 50 % coat and therefore are difficult to bond. In general, the fiber structure of recycled papers is deteriorated by the recycling process, and these papers must be sized strongly, for instance, to withstand the pulling strain in offset printing, which causes a reduction of absorbency. Therefore, a longer setting time is needed for aqueous adhesives. Strong sizing can also lead to a change of the surface properties, which may cause adhesion problems. Occasional adhesion problems occur because of the often lavish designs of the graphic products. Varnishing, laminating, embossing, and other refinements have the result that the adhesives can not reach the paper fibers, which are otherwise easy to glue.

Four adhesive types are used for perfect binding:

— Glutin glues (animal glues)

— Emulsion adhesives

— Hot-melt adhesives

— Reactive polyurethane hot-melt adhesives

Glutin adhesives are used in form of jellies which are processed at 50-70 °С. Due to a sol-gel transition, very fast setting is achieved and a high production speed is reached. However, the adhesion spectrum of glutin glues is normally not very wide.

In contrast, many synthetic polymers possess excellent adhesion characteristics. For example, polyfvinyl acetate) homopolymers combined with external plasticizers show high adhesion, sufficient cohesion, and are easy to use [186]. The improved productivity of the machines has led to the development of fast-drying systems (by high-frequency, IR heating, or gas dryers). The book block can be further processed after cooling for 4-5 min.

Hot-melt adhesives are especially useful when high production speeds are required. On rapid cooling, they set fast and achieve the necessary strength quickly. Hot-melt adhesives based on ethylene-vinyl acetate (EVA) copolymers are most commonly used in the graphical industry, although occasionally other thermoplastic polymers are used. Reactive polyurethane (PUR) hot-melt adhesives have proved to be especially useful for high-quality perfect bindings [187]. PUR hot-melt adhesives, which have a very good adhesion characteristics and excellent strength, are applied at 120-130 °С. After cooling (physical setting), they react with moisture (chemical setting) to give high — molecular compounds which are practically unmeltable. Besides good adhesion char­acteristics, they have high chemical resistance (e. g., to the oil of printing inks), high temperature resistance, and good aging resistance. PUR hot-melt adhesives require special premelters and application equipment because of their reactivity. The PUR adhesives combine the advantages of dispersions (e. g., good adhesion, high aging resistance) and of hot-melt adhesives (e. g. high production speeds) in a single system.

Another method of optimizing adhesion and cohesion is to use two different adhesives, one immediately after the other (two-shot application) [188]. The two adhesives can be of the same type (e. g., two dispersion or two hot-melt adhesives), or two different systems can be used (e. g., a dispersion primer followed by a hot-melt or PUR hot-melt adhesive).

Case Making. In case making, when the covering material consists of textiles, plastics film, or paper, as in the backlining of books, hot animal glues are used because they set rapidly by gelatinization and rapidly bond the narrow overlap of the coating material around the board, even under the brief contact pressure normally applied in the machine. In special cases, also emulsion-based adhesives are also used.

Casing In. Depending on the case material and the machine speed, normally emulsion adhesives [poly(vinyl acetate) or copolymer-based] with good flowing prop­erties are used for casing in.