Лако-красочные материалы — производство

Most emulsions are low viscosity liquids. Even with pigments and extenders, the viscosity is still too low for nearly all application methods. Thus, it is necessary to add materials to increase the viscosity of the finished products and possibly to change its rheology.

Traditionally, hydroxy ethyl cellulose was the most commonly used thickener (viscosity increaser ) for emulsion systems. The cost was low and delayed action grades were easy to incorporate. Very small additions can produce large changes in viscosity. However, the rheology of these solutions is far from Newtonian and can lead to thixotropic final products. Solvents also have a marked effect on the viscosity of hydroxy ethyl cellulose,
leading to low viscosities which can rise very rapidly, if any solvent is lost during manufacture, use or application. Being cellulosic in origin, they are susceptible to microbiological degradation, leading to a corresponding drop in viscosity. The water resistance of the coating is also adversely affected.

With the increased interest in water based finishes throughout the surface coatings industry, several alternative ways of changing viscosity and rheology have been developed, which are an improvement on hydroxy ethyl cellulose. Internal thickening often arises from swelling of the polymer particle brought about by the inclusion of a coalescing solvent. In applications where a greater viscosity increase is required, an external thickener is utilised. Alkali soluble thickeners (acrylic resins) are materials that come as dispersions, but are activated by changes in pH. Copolymerised acid at or near the surface of each molecule is neutralised with triethylamine (very aggressive towards aluminium pigments) or dimethyl ethanolamine. The carboxyl functionality is changed to a salt, which, when solvated by the water in the formulation, swells the particle and increases the viscosity. Addition levels of between 0.25 and 1% are usually sufficient to produce the desired viscosity and, because these materials are acrylic solutions, they can have beneficial effects on other properties, such as pigment wetting, gloss and flow.

The recently developed urethane and acrylic associative thickeners are more stable and offer a wide range of rheological profiles. Associative thickeners are low viscosity dispersions of specially produced polymers which are solubilised by the addition of amines. The optimum pH range for this type of thickener is 7.5 -10.5. However, they differ from the above thickeners in that the molecules have a tendency to associate with one another and with other materials in the formulation by means of surfactant groups in the molecules. This combination gives rise to very useful rheological properties in that, at low shear rates, the viscosity is relatively low, so the applied lacquer will flow out, but under high shear conditions, the viscosity increases, which can help in many application methods such as brushing or roller coating. Addition levels would be in the region of

These synthetic thickeners are much less susceptible to fungal attack than cellulose based ones, and therefore, are less prone to viscosity changes brought about by such degradation. Once dried, films thickened with these materials are far less water sensitive than their equivalents thickened with hydroxy ethyl cellulose. They are easier to handle and to incorporate than the cellulosics, as they need no pre-mixing or dissolving, and they can be added at the final stages of production if required.

Commonly used thickeners include :