Лако-красочные материалы — производство

(i) Acrylic Resins

The formulation of acrylic resins for electrodeposition is not very different from formulating them for any other use. There are so many acrylic monomers available that almost any property desired can be incorporated, so it is just a matter of selecting the right monomers, initiators and solvent.

Solvents should preferably be water miscible — butanol is commonly used but it is only partly water soluble and so can lead to problems with film resistance and rupture voltage. The propanols are water miscible, but their low boiling point can give problems with choice of a suitable initiator. Probably the best solvents to use are the glycol ethers, which have reasonable boiling points and good water miscibility.

Reproducibility and reaction control are easier if the reaction is run at reflux, which, once the solvent has been selected, fixes the temperature. It is then a matter of choosing the initiator type and amount and, if necessary, the chain transfer agent to produce a polymer with the right molecular weight. As a rough guide, a viscosity of 5 to 10 stokes at 50% solids is where to aim, depending, of course, on the solvent viscosity and the end use of the product.

Acid values for anodic resins should be 60 to 100 mg KOH/g and hydroxyl values for melamine cure about the same. Amine values used in cathodic resins rend to be a little lower than the acid values used for anodic resins, but amine values below about 40, normally lead to problems with getting the resin to disperse when neutralised.

The backbone monomers to use will depend, as usual, on the end use. Styrene and methyl methacrylate are more or less interchangeable as regards Tg, but styrene is usually cheaper, whereas methyl methacrylate has better weathering. Similarly, butyl acrylate is usually cheaper than ethyl acrylate, but the stain resistance of the latter is better.

Two typical formulations for acrylic resins are as follows:

FORMULATION 2-15:

ANODIC ACRYLIC FOR SINGLE COAT WHITES

Procedure

1. Pre-mix the monomers with the initiator.

2. Charge the solvent and raise to reflux.

3. Add the monomer mixture uniformly over four hours, maintaining the batch at reflux.

4. Continue to hold at reflux after the end of the monomer addition. Sample after one hour and then hourly, and hold for a non-volatile content of 69%. If this is not reached after two hours, add a further 0.5 parts of initiator,

FORMULATION 2-16:

CATHODIC ACRYLIC FOR CLEAR COATS

Procedure

1. Premix the monomers without the initiator.

2. Charge the isopropanol and raise to reflux.

3. Slurry the AZDN in some isopropanol. Add l/6th of this AZDN slurry, then commence feeding the monomer mixture at such a rate that it will be added over two hours.

4. Add l/6th of the AZDN at half hour intervals during and after the monomer addition.

5. Hold for a non-volatile content of 69%, adding further AZDN if this is not reached two hours after the completion of monomer addition.

Avoidance of contamination, particularly electrolytic contamination is essential during

the manufacture of the resin and the coating.