Лако-красочные материалы — производство

Manufacture of Corrugated Board. For this purpose starch suspensions are used almost exclusively. Some of the starch is gelatinized under the heated corrugating rollers which form the corrugation, thus insuring high initial tack. The use of emulsion — based or hot-melt adhesives is confined to special cases. Hitherto, it has been discour­aged by the relatively high price of these products, which only occasionally is com­pensated by other advantages.

Lamination of Paper and Board. Adhesives based on starch, dextrin, glutin and poly(vinyl alcohol), and also emulsion adhesives, mainly poly(vinyl acetate), are used for the lamination of paper and board. Only when high initial tack and a lay-flat effect are required are hot glues based on glutin still in use. Starch-based adhesives modified with silicate, casein-stabilized copolymer dispersions, and rubber latices are used in the lamination of paper to aluminum foils. Modified polyacrylate solutions and reactive polyurethane adhesives are used mainly for paper-to-plastic lamination.

Adhesive Coating of Paper and Board. Pressure-sensitive coatings, for example, for self-adhesive labels and envelopes include natural rubber-resin combinations in sol­vents, polyacrylate emulsions, and hot-melt adhesives. Polyacrylate emulsions and hot-melt adhesives are becoming increasingly significant because they are solvent-free.

Heat-sealable coatings are applied from emulsions, solutions, or melts and are activated in different ways: heat-sealable coatings with delayed tack mainly by heat in the absence of heavy pressure, and typical heat-sealing coatings by pressure and heat applied to the bond area as contact or high-frequency heat.

Moisture-reactivated coatings are produced from dextrin glues, dextrin emulsions, mixed glues, and remoistenable hot-melt adhesives.

Bags, Sacks, Pouches, and Boxes. Starch-based adhesives and also emulsions of polyfvinyl acetate) homopolymers and copolymers or mixtures thereof are used in the manufacture of bags, pouches, and carrier bags, depending upon the type of materials (e. g., paper, plastic films, etc.), surface treatment, and the speed of the machine. Machine design factors necessitate the use of pastelike starch-based or emulsion-based adhesives for the bottom gluing of block-bottom or cross-bottom bags.

In making paper sacks, starch-based adhesives normally are used for the longitudinal seam and also for the bottom and bottom patching. For paper sacks with PE free film or inliner, emulsion-based synthetic resin adhesives are used. For the longitudinal seam of the inliner, hot-melt adhesives are used.

The manufacture of folding boxes has long been a domain for free-flowing, low — to medium-viscosity emulsion-based synthetic resin adhesives.

Envelopes. In general, the top-flap gumming of envelopes is made with modified conventional dextrin glues and dextrin — emulsion mixed glues, which dry more quickly and show better ability to stay flat for automatic enveloping. To an increasing extent, modified emulsions and remoistenable hot-melt adhesives are being used. Dextrin glues and synthetic resin emulsions normally are used for bottom-flap and window gluing. Gums for self-seal gumming are based on natural rubber. Self-adhesive top flaps are based on polyacrylate emulsions.

Adhesives for Automatic Packaging Machines. In automatic packaging, package production, filling, and closing are often carried out in a single machine. For this reason, very fast setting adhesives are used.

Considerable recovery forces occur in the closing of prefabricated boxes, particularly in seaming machines, so that hot-melt adhesives often have to be used in such cases. Emulsion-based adhesives are normally used for the dust-tight sealing of cases. Slide boxes, deep-freeze packs, etc., can be line — or spot-sealed with hot-melt adhesives.

Cartons are sealed with emulsion-based or hot-melt adhesives, depending on the length of the pressure belt and the quality of the cardboard. On modern, fast-running machines with up to 120 cases to be closed per minute, hot-melt adhesives are preferably used, due to their fast setting. An interesting combination is the simulta­neous use of emulsion-based and hot-melt adhesives. In this case, a spot application of hot-melt adhesive performs the function of the pressure unit or compression section, whereas the actual bonding function is performed by emulsion-based adhesives, gen­erally applied in line form.

Cigarette Manufacture. For gluing cigarette seams, depending on the application system, highly viscous or free-flowing starch glues or emulsion-based adhesives are used. Hot-melt and emulsion-based adhesives are used for cigarette-filter seams, since their viscosity, initial tack, and setting properties are compatible with the special conditions of this manufacturing step, both at low startup speeds and at maximum speeds of 600 m/min. Emulsion-based adhesives are used for bonding the filters to the cigarettes.

Adhesives for Laminated Films. Laminated films are currently produced by two techniques: lamination and extrusion. In lamination, the low-viscosity adhesive solution is applied by rolls to one of the films in a thickness of a few micrometers and dried. The two films are then combined in the laminator. The adhesives used are either two — component or one-component, moisture-cross-linking polyurethane adhesives, depend­ing on requirements (type of film, sterilization resistance, sealing seam strength, etc.). Besides the traditional solvent-based systems developments have been made in the direction of solvent-free systems (high-solids products) and of polyurethane emulsion — based systems.

In extrusion, a polymer melt is extruded onto a primary web. An improvement in adhesion or reduction in the extrusion temperature is obtained partly by applying an
adhesive as primer to the primary web. Adhesives suitable for this purpose are similar to those used in a lamination. In eoextrusion, two polymer melts are combined with one another. If required, a polymer primer is extruded between the two melts.

Bonding of Cellophane. Cellulose hydrate film is used for bag manufacturing or for wrapping operations. Because it readily takes up moisture in its unlacquered state, aqueous emulsion-based adhesives are used primarily for bonding. The addition of glycerol ensures they release their water only slowly so that the film stays flat. Lacquered cellophane is either bonded or sealed. The adhesives do not require any additives to keep the film flat because the film is protected by the lacquer. The adhesives are normally based on polylvinyl acetate) copolymers or other flexible polymers. Where solvent-based or hot-melt adhesives are used, both types of film may be bonded with the same adhesive, because the absence of water eliminates the problem of keeping the film flat.

Manufacture of Polyethylene Sacks. The one-component and two-component polyurethane adhesives used for the production of laminated films (see above) are used for this purpose, depending on the type of machine.

Labeling of Glass. For labeling glass, dextrin, starch, and casein glues are used. With many machines, the adhesive used is required to show pickup ability, that is, the adhesive applied, for example, to a pallet must be tacky enough that it can pick up a label from the label magazine. Furthermore, the label must not slip after application, and any tendency to curl as a result of moistening must be counteracted. Dextrin glues are well endowed with these properties. In addition, by virtue of their high solubility in water, any glue that remains on the machine is easy to clean off. However, dextrin glues cannot be used where resistance to condensation, water, or ice is required (e. g., in breweries). Therefore, casein glues are mainly used in breweries. Although they are not as concentrated as dextrin glues (ca. 60 %), they develop a high initial tack, even on wet bottles, because they undergo a marked increase in viscosity on contact with the cold bottle. An additional reason for the use of casein adhesives is their wider adhesion range compared to dextrin adhesives. The latter adhere reliably to normal glass bottles, but their adhesion to coated or surface-treated glass bottles is not sufficient, so that casein adhesives have to be used.

Modern automatic high-performance labeling machines are capable of labeling between 500 and 1300 bottles per minute. At lower speeds, starch glues, which do not show such pronounced pick-up properties, are still used because of their lower cost. However, their higher water resistance can cause problems in bottle-washing machines for returnable bottles.

Labeling of Plastic Containers. The glues normally used for glass bottles cannot be used for plastic containers because they do not adhere sufficiently to plastic. Plastic bottles normally consist of polyethylene, polypropylene, polyethylene terephthalate),
polycarbonate, poly(vinyl chloride), or polystyrene. Canlike containers and tablet tubes are made of polystyrene or polypropylene. Modified copolymer emulsion-based and hot-melt adhesives are used for labeling.

Although there are no major adhesion problems with PVC, PET, and PS, adhesives with stronger adhesive properties must be used for polyethylene and especially poly­propylene. However, if the polyethylene surface is treated (e. g., exposed briefly to a gas flame) before labeling, it is possible to use the same adhesives as for PET or PVC.