Лако-красочные материалы — производство

Control of molecular weight is more difficult with acrylic resins than with alkyd or polyester resins, especially in terms of achieving low polymer molecular weight with good film performance. This exacerbates the problems encountered with viscosity control in water reducible systems, caused by the hydrogen bonding setup between the polymer, solvent, and water in the solution. To overcome this, one can carry out a solvent selection exercise, although solvent choice is often restricted by application techniques and the need for at least one higher boiling solvent to control ‘lay down’. Also where applicable, the use of styrene as a proportion of the monomer mix is helpful in controlling the complex relationship between polymer solubility. Another option is to create a colloidal dispersion instead of a solution. This is achieved by only partially neutralising the polymer before solubilising it. Neutralisation levels of 40-70% are commonly used, the degree of neutralisation used being a function of the polymer molecular weight, e. g. lower molecular weight polymers need a lower degree of neutralisation to achieve dispersion. It has also been found that dispersions are more readily formed when the water used for dilution is warmed to the same temperature as the polymer solution prior to thinning, and when a portion of the solvent is only partially miscible, i. e. n-butanol or the isobutylether of propylene oxide (Dowanol PiBT).

Another technique for overcoming the solids/viscosity conundrum is demonstrated in a patent by American Cyanamid(4). Here a conventional water soluble acrylic is prepared as a dispersion and is then used as a colloid stabiliser for further emulsion polymerisation. This polymerisation occurs within the dispersion droplets that have already been formed. This allows higher solids solutions to be formed with low viscosities, and low cosolvent contents(5).