Лако-красочные материалы — производство

by Graham Raynor BSc

Two pack acrylic polyurethane coatings offer some excellent properties including:-

• long pot life

• short touch dry times

• early sandability

• very good chemical and stain resistance

• excellent durability (aliphatic isocyanates)

• good mar resistance

• high solids (lower VOC)

However historically the woodfinishing industry has been dominated by the requirement for some or all of the following from its finishes:

• Fast Drying — quickly handleable and stackable

• Good Wetting — brings out the rich colour of the wood

• Low cost

• Excellent flow — coating penetrates into the grain and does not ‘bridge’ across it.

• Appearance — looks and feels smooth even at low sheen levels.

Nitrocellulose based coatings fulfilled these criteria well. The acrylic urethane coatings fall down on cost and, to some extent, wetting the substrate. However, over the past 20-25 years there has been a steady move towards higher solids coatings with improved physical and chemical properties.

More recently with the advent of the Environmental Protection Act users have had to move towards systems which contain less organic solvent. This forces the industry to look at three altematives:-

• high solids systems — acid catalysed alkyd/amino (а/с), urethane

• solvent free systems — u/v cured unsaturated acrylics

• waterbased systems — emulsions and water soluble

Although hydroxy acrylic/urethane systems have advantages over nitrocellulose, they have never been more than of minor significance in the finishing of wood because advantages are often outweighed by disadvantages. For example better chemical resistance is desirable but the increased cost which goes with it may well be unacceptable. There are specific areas, such as car facias, where these systems come into their own and nitrocellulose or acid catalysed systems fall down.

With the move towards high solids solvent borne coatings, hydroxy acrylic coatings have more to offer, but there are still two main disadvantages:

a) Price — hydroxy acrylic/urethane systems are more expensive than а/с (acid catalysed) systems at the same solids.

b) Health and Safety — the use of isocyanate containing coatings by spray application, the main application method in woodfinishing, requires air fed respiratory equipment, which hinders many potential users who prefer to stay with acid catalysed systems.

There are however specific areas where these two pack systems outshine others, e. g. on drumkits which require high gloss and excellent flexibility, and on cane furniture where a high build, high gloss finish with excellent adhesion is required.

Suppliers of hydroxy acrylics include Bayer, Cray Valley, DSM resins, Synthese, BASF, and Hoechst.