Лако-красочные материалы — производство

The principles of amine and cosolvent selection apply broadly across all types of polymer. However, the specific details of the apparent ‘solvent strength’ of an amine and optimum choice of cosolvent will vary to some extent from one polymer type to another, and will often be influenced by the application method and conditions of cure.

Water reducible acrylic resins are available to the coatings formulator as both thermoplastic, and thermosetting types. The thermoplastic acrylics film form by simple solvent evaporation and, therefore, cosolvent and amine selection are important. One of the cosolvents used must be a true solvent for the system and be less volatile. It must evaporate more slowly than water. This is to minimise the potential for blushing or flocculation that could be caused by having water as the last ‘solvent’ to leave the film. The amine should also be volatile at ambient temperatures to minimise the retained water solubility, or sensitivity of the film after drying. This is particularly important since there is no curing reaction with thermoplastics.

Thermosetting acrylics are produced by the inclusion of crosslinking functionality in the polymer. This can be hydroxyl, acid, amine, amide, alkoxy amide from modified acrylamide, and glycidyl, with most of the monomers being available as both acrylate or methacrylate permitting a wide formulating latitude to balance Tg and flexibility.

The synthesis techniques of water reducible acrylics, of either type, is essentially the same as those normally employed®, and are described earlier in this book. The principle differences in the preparation of water reducible acrylics are the use of only water miscible solvents, and the use of (meth)acrylic acid, or one of the amine functional (meth)acrylate monomers to provide the means of water solubility through salt formation with acidic or basic neutralising agents. It is, of course, possible to use glycidyl (meth)acrylate as one of the functional monomers and modify this after the polymer has been formed, using one of the techniques described, to render the polymer water reducible. This, however, is not a common procedure, because of the price of the monomer.

There are also a number of hydrophilic monomers available. A common use is in the preparation of ’surfactant free’ emulsions where a small quantity of the monomer can be used as the surface active component. These monomers, such as (meth)acrylonitrile, methoxy (meth)acrylamide and ethylene oxide modified hydroxy monomers, when used in solvent based polymerisation are less effective at conferring water solubility unless they are used in high concentrations. The methoxy (meth)acrylamides also have the disadvantage that they are prone to condensation reactions with themselves or with acid or hydroxyl functional groups at temperatures of 100-120°C, and they can also transesterify with alcohols or glycolethers which might be used as cosolvents. They also need lower processing temperatures to avoid unwanted side reactions, because some are self-crosslinking comomomers. The advantage of these (meth)acrylamide derivatives is that after curing the hydrophilic grouping is lost, unlike the other monomers mentioned above, thus reducing the water sensitivity of the cured film.

The advantage of acrylic resins over alkyd or polyester resins is their excellent hydrolytic stability. The disadvantages associated with acrylic resins are: