Лако-красочные материалы — производство

Acrylics have been used in clear metal coatings for the protection of brass, copper and bronze surfaces. Acrylic lacquers keep the surfaces bright, and low application solids allow thin films of less than 15 microns to be formed so that the original appearance of the article is retained. Such coatings are best made using methyl methacrylate copolymers modified with acrylic — or methacrylic acid to aid adhesion to the metallic substrate.

Two-pack and moisture cured polyurethanes, which offer better chemical and solvent resistance, are increasingly being used. Two-pack epoxy (epoxy-amine) lacquers are also used but are not favoured for exterior use because of their poorer durability. Epoxy resins are notorious for their poor chalking characteristics. A typical example of a clear acrylic lacquer would be the following formulation recommended by the International Copper Research Association in the USA. It is listed in the Rohm & Haas booklet on thermoplastic acrylics:

FORMULATION 3-16:

CLEAR ACRYLIC LACQUER FOR COPPER

Paraloid B44 is a relatively soft, flexible, MMA copolymer from Rohm + Haas. It has now been superceded by Paraloid B48N.

Benzotriazole is incorporated to prevent blackening of the metal in sulphur-rich atmospheres where copper sulphide might be deposited. This is less of a problem since the introduction of pollution controls, but it may still cause problems in some environments.

This metal clear coat would normally be applied by conventional air spray or HVLP (High Volume Low Pressure) air spray. The incorporation of an additive may be necessary to permit electrostatic spray which offers a higher transfer efficiency on complex shapes.