Лако-красочные материалы — производство

This group includes heat-sealing adhesives and high-frequency (HF) sensitive heat — seal coats; these are coating adhesives that are heat-activated during bonding. Contact adhesives and pressure-sensitive adhesives bond by the same principle but without the action of heat.

Heat-sealing adhesives are applied to the materials to be sealed in the form of solutions, emulsions, or, preferably, in melt or powder form. The solvent-free layer is intended to be nontacky and only to melt and to wet the adherent under the action of heat during sealing. Solidification takes place after cooling. In many cases, sealing is carried out on a coating-to-coating basis.

The choice of raw materials for sealing coatings is very wide and is influenced by the sealing temperature and the sealing behavior of the materials used, by the requirements the joint has to satisfy, and by the adhesion of the sealing layers to the substrates. In many cases, copolymers of vinyl chloride or vinylidene chloride are used in solution, although they are also used in emulsion in conjunction with resins and other polymers. In addition, copolymers of vinyl acetate and polymethacrylates, polyurethanes, and polyesters are also used. Ethylene-vinyl acetate copolymers are used preferably for heat-sealing coatings applied as melts. Heat-sealing powders also are used occasionally; they can be based on fine polyamide or polyethylene powders and also on vinyl acetate — vinyl chloride copolymers.

High-frequency-sensitive heat seal coats are a special type of heat-sealing adhesive [35]. They are also applied in the form of a solution or emulsion to the surface of a substrate and dried. However, welding to the partner substrate is not obtained by the application of heat through the substrate; instead the sealing temperature is generated by high frequency within the adhesive layer itself. Hence, the adhesive layer must have a high dielectric loss factor.

High-frequency sealing auxiliaries generally contain vinyl chloride copolymers in addition to proportions of other polymers, such as polyacrylates, vinyl acetate copol­ymers, plasticizers, and resins. High-frequency heatable coatings have solids contents of
from 30% (solutions) to 50% (emulsions). Their activation temperature is in the range of 120 -150 °С. High-frequency sealing powders consist predominantly of finely di­vided polyamides.

Contact cements must be applied to both substrate surfaces to be joined. The adhesive layers are joined only when the solvents have almost completely evaporated. A brief, but as high a contact pressure as possible is required for bonding. The strength of the adhesive joint is relatively high immediately after contact bonding and may amount to 50 % of the final strength. In principle, the contact adhesion effect may be obtained with natural rubber solutions, as known, for example, from the repair of bicycle inner tubes with rubber solution. However, the actual contact cements are solutions of synthetic rubbers with resins or solutions of high molecular mass polyurethane elas­tomers. The rubber components are mainly polychloroprene, nitrile-butadiene, and styrene — butadiene rubbers, and the resins used are primarily phenolic resins, rosins, and hydrocarbon resins. In the case of polychloroprene adhesives, metal oxides are added as stabilizers. In some cases, the rubber starting materials are high molecular mass rubbers that must be masticated on rolls or in internal mixers before dissolution, but mainly directly soluble rubbers are used today. The rubber, resin, and additives are dissolved in the appropriate solvents or solvent mixtures in closed stirring vessels to form the end product. The solid contents of the products varies from 20 to 30 %; the more sprayable types have lower solids contents.

The highest strengths are obtained with a uniform, thin coating of adhesive coupled with a high contact pressure. It is important to allow for adequate evaporation of the solvents and to observe the open time of the contact adhesive.

Contact cements based on nitrile rubber and polyurethane are more resistant to oils and plasticizers than polychloroprene adhesives. To improve adhesion and to increase thermal stability, contact adhesives may be applied together with isocyanate hardeners. Today solvent-free contact adhesives based on aqueous polychloroprene latexes are also available.

Trade Names. In Germany: Pattex, UHU Greenit; in Japan: Cemedine, Bond G, Sunstar; in the USA: Pliobond, Fastbond, Elmer’s.

Pressure-sensitive adhesives are permanently tacky substances that adhere spon­taneously to the surfaces of most materials with only light pressure [36]. Their adhesiveness is relatively nonspecific, although the individual products show different adhesion values on different substrates. Pressure-sensitive adhesives are used mostly for coating supports, for example, paper and film tapes, adhesive labels, or self-adhesive decorative sheeting. The adhesives are applied to the supports as solutions, dispersions, or melts. After the adhesive has set, the pressure-sensitive adhesive is present as a thin film on the support and, until it is used, is covered by suitable antiadhesively finished materials, for example silicone paper or siliconized polyethylene film. In some cases, the pressure-sensitive adhesive is also applied to the masking material, and the film of
pressure-sensitive adhesive formed is transferred to the actual carrier by the reverse process. The dry application weight varies between 4 and 40 g/m2, depending on the end use and the roughness of the carrier or substrate to be bonded.

In general, bonds formed with pressure-sensitive adhesives show relatively high resistance to brief and rapid loads. Under permanent load or stress, however, they tend to creep, particularly at elevated temperature. The property spectrum of pressure — sensitive adhesives is characterized by adhesiveness and resistance to heat, aging, and plasticizers.

The raw materials used for pressure-sensitive adhesives are natural and synthetic rubbers in conjunction with modified rosins, phenol-formaldehyde resins, or hydro­carbon resins. In addition to rubber, polyacrylates, polymethacrylates, poly(vinyl ethers), and polyisobutenes also are used frequently, again mostly in combination with resins. Silicone resins are used for special applications.

Pressure-sensitive adhesive dispersions are based primarily on special acrylic ester copolymers, again generally in combination with resins. In addition to suitable resins, two polymer bases are used primarily for hot-melt pressure-sensitive adhesives, namely, ethylene-vinyl-acetate copolymers and styrene — butadiene or styrene — isoprene block copolymers, which also are known as thermoplastic rubbers.

It is said that a pressure-sensitive adhesive always must be composed of a high — polymer base resin, which determines cohesion and specific adhesion, and of tackifying resins (tackifiers). In some systems, the tackifier may be replaced by low molecular mass fractions of the base polymer. To increase cohesion, the base resin in some systems is cross-linked or, in the case of rubber-based pressure-sensitive adhesives, vulcanized after application.