Лако-красочные материалы — производство

As discussed above, there are many technical and environmental pollution related issues in using solventbased acrylics, especially solventbased TPAs. This has led to the development of waterborne acrylic resins and coating systems. Commercial waterborne acrylic resins are supplied as dispersions in water. Generally, a polymer dispersion is a two-phase system consisting of spherical polymer particles, usually less than 1 qm in diameter that are uniformly distributed (dispersed) in a continuous water phase. The particles in such dispersion are kept from agglomerating and becoming unsta­ble by different stabilization mechanisms so as to provide long-term stability during their manufacture, storage, transportation and use. Aqueous polymer dispersions are usually milky liquids with a vis­cosity that varies from very low, like that of water, to high, like that of whipped cream. Unlike polymer solutions in organic solvents, the viscosity of polymer dispersions is not dependent on the MW of the polymer particles. This allows formulators to use very high MW acrylic resins without facing the challenges of VOCs or low solid content that are frequently encountered in solution systems. It is also important to note that most polymer dispersions are sen­sitive to such factors as temperature, pH and applied shear rate and can undergo destabilization, irreversibly separating into two phases — called breaking of the dispersion — if not properly formu­lated, handled and stored.

The commercially available acrylic dispersions are broadly of two types: acrylic latex and water-reducible acrylics.

Acrylic latexes are high MW acrylic (co)polymer particles dispersed in water. Synthetic latexes are prepared by a radical polymeriza­tion mechanism using an emulsion polymerization technique. The emulsion polymerization is carried out in water using monomer(s), surfactant (emulsifier) and water-soluble initiator. In a typical manufacturing process, an initiator and a separate emulsion of monomer(s) in water are slowly added to a reaction vessel contai­ning water and emulsifier, at a predetermined rate. Polymerization of monomers occurs within tiny pockets formed by aggregation of emulsifier molecules, called micelles, resulting in formation of stabilized polymer particles. The mechanism of emulsion poly­merization is very complex, and the composition, MW, properties, particle size and morphology, and stability of the latex formed are dependent on several factors, including types of monomers, emul­sifiers, and initiators, rate of addition, stirring speed, pH and tem­perature. Latexes are typically supplied at 35 to 45 % non-volatile content with varying particle size range. Because the stability of the latex formed is dependent on pH, temperature and shear rate, care must be exercised in handling, storage and use of latex to avoid irreversible destabilization.

Commercial latexes are supplied both as thermoplastic and ther­mosetting types. Thermoplastic acrylic (and vinyl) latexes are very commonly used as binders for architectural paints and some spe­cialty coatings. Such coatings are required to dry under ambient conditions without any chemical cross-linking and hence are desi­gned to precise Tg and minimum film formation temperature.

Thermosetting acrylic latexes are prepared similarly to thermopla­stics but using some additional functional acrylate monomers in their copolymer composition. For example, when 2-hydroxyethyl acrylate is used as a co-monomer, the resulting latex will be a hydro­xyl functional acrylic latex. Functional group type and content can be varied by using different functional monomers and by varying copolymer composition. Thermosetting acrylics require appropriate cross-linkers, and their coatings are cured at temperatures dictated by the type of cross-linker. Thermosetting latexes are typically offered with such functional groups as hydroxyl, carboxylic acid, and epoxide, while many other types may be possible. Self-cross­linking acrylic resins are another type of thermosetting acrylic latex finding increasing interest. While the terminology is a bit con­fusing, self-cross-linking acrylic latexes are typically comprised of reactive functional groups (such as — OH, — COOH) and the added cross-linker. The cross-linking reaction is triggered at ambient tem­perature after water has evaporated from the wet film. An example of such a system is a stable carboxylic acid functional latex supplied with a polyaziridine type cross-linker, which after application will undergo a cross-linking reaction at ambient temperature over a period of a few days.