Лако-красочные материалы — производство

Solutions of Polymeric Substances and Resins in Organic Solvents. A distinction is be drawn between adhesives that set by evaporation of the solvent and form an adhesive joint by adhesion of the dissolved polymer to the substrate surfaces, and adhesives for plastics, the solvent of which dissolves and swells the plastics surfaces and joins them by means of migration processes. This allows bonding of plastics that are otherwise difficult to join. The polymer solutes in solution adhesives act primarily as thickeners for establishing certain flow properties and retaining the solvent for the duration of the dissolution process. These solid components also may perform a limited gap-filling function.

Adhesion Adhesives. The best known solvent-containing adhesives are the multipurpose adhesives. They can not join every type of substrate, but can bond a wide range of substrates with different results. They contain as binders high molecular mass com­pounds that adhere to many different surfaces, preferably nitrocellulose and poly (vinyl acetate). The solvent mixtures consist of esters and ketones and, in some cases, also
contain alcohols. The composition of the solvent is determined by the polymers used, by the required fluidity of the adhesive, and by the required open time or setting time. The solvents may dissolve certain plastics.

Solutions of natural and synthetic rubber, polyurethane rubber, ethylene-vinyl acetate copolymers, polyacrylates, and other polymers with additions of resins and plasticizers are also used. These polymers can be used in solvents or as dispersions in water.

Trade Names. In Germany. UHU Alleskleber, Pritt Alleskleber; in Japan: Saivinol, Cevien-A; in the USA: Elmer’s School Glue, Wilhold Glu-Bond.

Solvent Adhesives. In solvent adhesives for bonding plastics, the composition of the solvent is particularly important with regard to dissolution and swelling of the plastics to be bonded. Synthetic resins from the same class or from a related class are used for thickening.

The PVC adhesives have been used since the 1930s for bonding PVC resins. They are solutions of chlorinated polyfvinyl chloride) in acetone, cyclohexanone, THF, or mix­tures thereof. The THF-containing adhesives are solutions of PVC in mixtures of THF, 2-butanone, and cyclohexanone. Organic thickeners are used to obtain pseudoplastic solutions that can be applied in sufficiently thick layers. By virtue of their stronger dissolving and swelling effect, THF adhesives can bridge certain gaps.

Pure solvents, such as aromatic hydrocarbons, halogenated hydrocarbons, solvent mixtures, or even solutions of polystyrene in these solvents are used for bonding polystyrene, ABS, and other styrene copolymers.

Trade Names. Solution adhesives for rigid PVC: Tangit, Dytex (Germany); Hamatide, S-dine (Japan); Bostik, Datey (USA).

Solutions of Natural and Synthetic Polymers in Water. Aqueous Starch — and Dextrin-Based Adhesives. The starch — and dextrin-based adhesives reached their zenith between 1920 and 1935. Since then, they have been increasingly superseded by syn­thetic resin-based adhesives, and they are now important only in certain applications. However, they are still used in considerable quantities for applications such as labeling adhesives, paper-sack adhesives, and envelope gums.

The raw materials used for the various types of starch — and dextrin-based adhesives are chiefly potato starch and cornstarch and, to a far lesser extent, tapioca starch and wheat starch. Potato starch is preferred for liquid glues, at least in Europe, where it is widely available.

Native starch is not soluble in water. To obtain soluble starches, the starch granule must be gelatinized by the prolonged heating of an aqueous starch suspension above the gelatinization temperature (e. g., 62.5 °С for potato and cornstarch, 67.5 °С for wheat starch) or by treatment with alkali at lower temperature. The gelatinization temperature and also the gelatinization rate can be varied by adding electrolytes.

To obtain certain solution viscosities and structures, the starch molecule must be degraded before and after gelatinization. A starch suspension is exposed before gelat — inization to the action of acids, primarily nitric acid, or oxidizing agents, such as hydrogen peroxide, at elevated temperature. The greatest degradation is carried out in the case of dextrination. Because of the different degrees of degradation, the various gelatinization processes, and additives that influence hydration, the adhesives obtained differ considerably in their rheology, adhesive strength, and application properties; they are marketed as ready-to-use adhesives and as cold-water — or warm-water-soluble powdered adhesives. All aqueous starch — and dextrin-based adhesives must be protected against biological degradation by additions of preservatives [37].

Dextrin-based adhesives are available as powdered dextrins, which are dissolved in water before application, and ready-to-use aqueous dextrin adhesives with a dextrin content of 50 — 70 % and viscosities of 3 x 104 to 3 x 107 mPa s.

Dextrins are produced by the thermal and acid-hydrolytic degradation of native starch, generally cornstarch and potato starch. Dextrin-based adhesives are produced by dissolving powdered dextrins or dextrin mixtures of differing degrees of degradation in water. Mixtures of 60 — 70 % dextrin and 30 — 40 % water are heated in glass-lined vessels to 80-100°C until a homogeneous solution is formed. Commercial dextrin — based adhesives differ in the type of dextrins or dextrin mixtures used, the solids content, and in the additives present, such as hygroscopic salts (CaCl2, MgCl2), borax, glycerol, sugar or sugar syrup, and similar substances).

The viscosity of dextrin-based adhesives can be reduced by vigorous stirring. How­ever, the viscosity of dextrin-based adhesives generally undergoes a marked increase after production, so that they have to be left to mature. In most cases, dextrin-based adhesives are marketed only after the maturing process is complete.

Mixed Glues. Starches and dextrins also are used in combination with other raw materials in adhesives. For example, the resistance to water of starch-based adhesives can be improved by the addition of melamine-formaldehyde or urea-formaldehyde resins. Mixtures of starch-based adhesives or dextrin-based adhesives with synthetic resin dispersions are also used.

Casein glues are also still important. Casein is obtained by precipitation from milk protein. The dried powder is mixed with water prior to application and hardens by loss of water at room temperature, with some chemical conversion of protein to more insoluble calcium derivatives. This adhesive is unsuitable for outdoor use but is more resistant to temperature changes and moisture than other water-base adhesives. Casein adhesives tolerate dry heat up to 70 °С and have good resistance to organic solvents. Although they are susceptible to biodegradation, chlorinated phenols can be used to inhibit this behavior. Typical applications include packaging (paper labels for glass), woodworking, and fabrication of interior-grade plywood.

Other Aqueous Adhesive Solutions. Besides starches and dextrins, cellulose ethers, pri­marily methyl cellulose and carboxymethyl cellulose, and in some cases also starch ethers, are important as wallpaper pastes and poster glues. The products are generally marketed in the form of powders that are dissolved in water by the user. Aqueous
solutions of poly(vinyl alcohol) can be used to a limited extent. In most cases, fillers are added to these solutions.

Poly(vinyl pyrrolidone) has acquired significance as a binder in glue sticks. In addition to the polymer, these sticks consist primarily of a soap/water gel as builder. After application, the water evaporates, leaving the solid behind as bonding agent [39].

Aqueous Glutins (Animal Glues). The significance of animal glues or glutins has steadily diminished in the last decades. Like gelatin, glutines are obtained by acidic or alkaline hydrolysis of the collagen present in animal bones, hides, etc. The degra­dation product, which forms colloidal solution in water, is called glutin. Particularly pure glues are known as gelatin glues. In some cases, gelatin waste is also made up into glues.

The glutin solutions that accumulate in the processing of bones and hides are concentrated to a solids content of 30 — 55 %. They solidify on cooling to form firm, cuttable jellies. The slabs or blocks are further reduced in size, dried to a solids content of 85 %, and marketed in various forms. Hide glues are stronger than bone glues and often exceed the strength of wood. These adhesives are primarily used to bond wood, leather, paper, and textiles. Use is limited to interior applications. The setting of glutins is determined primarily by solidification of the gelatin or glutin/water mix on cooling and secondarily by evaporation of the water.

The reversible uptake and release of water by glutins has advantages and disadvan­tages. The possibility of activating layers of glue jellies by moistening is utilized, for example, in the production of moistenable gum tapes [40].