Лако-красочные материалы — производство

Most acrylic monomers will undergo emulsion polymerisation, although the monomer molecules must be available in a solubilised state, either in the form of surfactant stabilised droplets or in micelles. Where the acrylic monomer contains a long chain alkyl group (e. g. lauryl and stearyl acrylates and methacrylates) polymerisation may be retarded by steric factors resulting in low degrees of conversion of the monomer.

The rate of “slow” reactions can often be improved by the addition of a small amount of water soluble solvent (e. g. glycols, alcohols or ketones) to the aqueous phase to improve the emulsification and hence, the availability of the monomer for reaction.

Emulsion latices film form by evaporation of the water phase followed by coalescence of the polymer molecules. For the polymer to form a coherent film it must be of sufficiently high molecular weight and also the temperature must be above the “minimum film forming temperature” (MFFT) of the system. The MFFT is governed by the Tg of the polymer, and is normally somewhat lower than the Tg due to the plasticising effect of the surfactant and any additives present in the film.

Below the MFFT the film is chalky and non-continuous. At temperatures very much higher than the Tg, the film will be soft with poor adhesion, and abrasion resistance. Most emulsion resins intended for ambient temperature applications have a Tg in the range of 0°C-30°C. Although it is possible to include additives specifically to reduce the MFFT, these could possibly leach out of the film, and it is considered a much more desirable practice to control the Tg (and hence the MFFT) of the polymer by selection of the monomer type and composition. For this reason most emulsion polymers are copolymers. In addition to Tg, other factors dictate the choice of monomer, such as the performance characteristics conferred by a particular monomer. A copolymer is designed to give a balance of properties in the final film, including such parameters as adhesion, flexibility, abrasion resistance, water resistance and ease of film forming (coalescence).

Methyl methacrylate is typical of a “hard” acrylic monomer and considerable amounts of flexibilising monomers are required when making copolymers using this monomer. Flexibilising monomers are, in effect, internal plasticisers, and include acrylic esters. The longer and more branched the aliphatic ester chain, the more effective the plasticising capability for a given concentration. Typical flexible esters are ethyl hexyl acrylate, butyl acrylate and ethyl acrylate. These monomers have to be used with styrene or methyl methacrylate. They can also be used in conjunction with versatate esters (e. g.Veova 10 ex Shell) which confer good alkali resistance to the copolymer. Due to unfavourable reactivity ratios, versatates are not used in conjunction with styrene containing copolymers.

Acrylic and methacrylic acids are often included in the monomer mixture to improve freeze-thaw stability and amino monomers (e. g. amino ethyl methacrylate) are sometimes included as wet adhesion promoters.