Лако-красочные материалы — производство

Water soluble resins will not be considered further. Two distinct types of waterborne acrylics find use in surface coatings, namely:

• emulsions

• inverted dispersions

Acrylic emulsions used in surface coating applications are almost invariably polymerised in water as described in Chapter II.

Water dispersible or inverted vinyl or acrylic polymers can be produced by incorporating carboxyl groups along the chain and by limiting the molecular weight by means of a chain transfer agent. These polymers may be made either by the emulsion process or the solvent solution process. For solubilisation after neutralisation, a minimum acid value of 50-60 mgkOH/g is required, as a general rule. It is also possible, but less common, to copolymerise amine functionality into the resin and neutralise with a volatile acid.

It is also possible, but much less common, to produce a waterborne acrylic from a water insoluble acrylic by converting hydrophobic groups to hydrophilic pendent groups. For example, esters, amides or nitrile groups can be hydrolysed or unsaturated groups oxidised.

Carboxyl containing acrylic polymers are often stabilised in aqueous solutions by adding ammonia or amines to form salts. These amines or ammonia can be largely driven off after film formation. However, even after the loss of the neutralising agent, the films remain water sensitive and some form of crosslinking is necessary for water maximum resistance.

The most useful crosslinking agents are the N-methylol group containing resins such as the urea or melamine formaldehyde resins and the phenol formaldehyde resins which react with hydroxyl or carboxyl groups on the polymer.

Emulsion polymerisation is normally undertaken by the resin manufacturer. Inversion can be undertaken ‘in house’. Acid (or amine) rich resins can be purchased and subsequently inverted by the formulator. Some people refer to these inverted dispersions as water reducible resins.