Лако-красочные материалы — производство

Emulsion-based adhesives are adhesives that contain aqueous synthetic resin disper­sions or emulsions or rubber latices as their principal raw material or active binder [41]. They are milky white, odorless liquids with solids contents of 40 — 60 %. In quantitative terms, emulsion-based adhesives are one of the most important groups of adhesives. They set by the release of water and the formation of a film from the emulsified or dispersed resin or rubber droplets.

The number of resin emulsions and rubber latexes suitable for use as raw materials for adhesives is very large. Emulsions of homopolymers and copolymers of vinyl acetate are the most important. The comonomers for vinyl acetate include maleic esters, acrylates, ethylene and vinyl chloride, unsaturated carboxylic acids, and vinyl esters of relatively long-chain fatty acids, such as vinyl laurate. Apart from these products, polyacrylate homopolymers and copolymers and also styrene copolymers are particu­larly important in adhesives [42].

Additions of plasticizers, solvents, and resins improve the specific adhesion of the synthetic resin emulsions, and also are capable of lowering the film-forming temper­ature and increasing the tackiness of the wet adhesive. Poly(vinyl alcohol), cellulose ethers, and other hydrocolloid additives prolong the open time. Polyfvinyl alcohol) also
can improve wet tack. Certain applications require fillers, which may be used in such large quantities that pasty mortars and cements are formed.

The elasticity of the set films can be increased by plasticizers and by copolymeriza­tion. Redispersion powders, that is, spray-dried synthetic resin emulsions, are added to synthetic resin emulsions to increase their solids content and their setting rate.

Synthetic resin emulsions containing reactive groups also have appeared on the market. The reactive groups provide for subsequent cross-linking, making it possible, for example, to increase the resistance of the joints to water. Mixed adhesives consisting of synthetic resin emulsions and starch — or dextrin-based adhesives have advantages in certain fields and are generally cheaper.

Latex adhesives are based on natural rubber or on various types of synthetic rubber. Occasionally, mixtures of natural rubber and synthetic resin latices are also used. In general, resins or solvents also are used in the production of latex adhesives.

In the production of latex adhesives, overvigorous stirring can affect the rheological properties of the adhesives, which are often pseudoplastic and differ in their mechanical stability. Characteristic properties of emulsion-based adhesives include viscosity and flow properties, the setting and breaking behavior of the emulsion, the wet tack or green strength of the film, the minimum film-forming temperature, storage behavior at low temperatures (freezing temperature), and specific adhesion to various materials. Films of emulsion-based adhesives are tested only occasionally for tensile strength and elasticity. The redispersibility of the set adhesive films is important with regard to the cleaning of machine components and recycling of paper.