Лако-красочные материалы — производство

In the mid 1960s acrylamide functional acrylics began to dominate the domestic appliance or ‘white goods’ finishing industry, replacing alkyd/amino enamels. In particular acrylamide functional acrylics were used since they offered the best combination of hardness and chemical resistance. As organic coatings, the hardness approached that of the inorganic vitreous enamel finishes. At that time the cases of appliances such as home laundry equipment, cookers, fridges and freezers, were constructed of mild steel, subjected to a phosphate pretreatment, primed with epoxy functional primers and finished in acrylamide functional acrylics. The acrylics self crosslinked as described earlier at temperatures in the region of 180°C and cure schedules were typically between 20 and 30 minutes.

Domestic appliance enamels are not required to be flexible, nor indeed to be weather resistant. As a consequence resins are formulated with high styrene contents (for economy and hardness) and high levels of acrylamide in order to maximise the crosslink density of the cured film. High crosslink densities not only help film hardness, but also resistance to chemicals.

Acrylamide based thermosetting acrylics for domestic appliance finishes offer:

Excellent film hardness (pencil hardness 2H-4H)

Excellent film colour Excellent gloss Overbake colour retention

High resistance to household chemicals, solvents, oils, fats.

High resistance to staining, eg mustard, lipstick, fumes

This combination of properties made them ideal enamels for use in many household applications, including refrigerators, ovens, washing machines, tumble dryers and dish washers.

The application equipment in those days was often conventional or electrostatic pneumatic spray guns. Application solids were very low, typically 25% by volume. There was a great deal of overspray. Finishing was very labour intensive, but the finished effect was very good, giving a high gloss (though fairly orange peely), tough, detergent resistant coating. (Although the detergents used in those days were much milder than todays’ aggressive chemicals.)

The technology progressed through the 70’s and early 80’s and has now been almost totally superseded in Europe and North America by either coil coating or powder coating. Powder coatings offer similar performance levels and at the same time eliminate solvent emissions.

By the mid 80s the acrylamide acrylic technology had reached its peak. Substrates were now pretreated zinc coated steels such as Zintec. There were no primers, therefore all corrosion protection had to come from typically 35 microns of finish only! The pretreated appliance cases were now being finished on fully automated conveyorised lines, application of paint was by high speed electrostatic disc such as those made by Soprea of France or Sames of Germany.

The paint formulations were very sophisticated as the automatic application was very unforgiving. Good batch to batch consistency was essential to ensure smooth, trouble free running of the paint lines, which often ran 24 hours a day.

The paint film performance requirements for this industry are high. Adhesion to pretreated Zintec is difficult and all types of acrylics have short comings in this area. Acrylamide acrylics perform the best in this area, but still require boosting by the inclusion of a percentage of epoxy resin, such as Shell’s Epikote 828.The inclusion of epoxy also acts to toughen the coating, as acrylamide acrylics can be very hard, tending towards brittleness. Other film characteristics required are resistance to household chemicals and of course to detergents.

The majority of domestic appliance enamels are white, and most have high opacity in order to cover the substrate in a one coat application. Typical pigment/binder ratios are 1/1 w/w for titanium dioxide. 10-15% of an epoxy resin is normally included to aid cure and adhesion to the substrate.

A typical formulation for a white domestic appliance finish is given below. It is intended for application by high speed electrostatic disc.

FORMULATION 4-5:
DOMESTIC APPLIANCE FINISH

Items 1-3 are high speed dispersed for 15 minutes. Add items 4 & 5 and pass over a horizontal bead mill to achieve a ‘sub micron’ grind. Add items 6-12 to the mill base and adjust the viscosity with 13. The viscosity for delivery will be around 60 seconds, BSB4 flow cup. Item 14 is used to adjust the resistivity of the paint for electrostatic application. The paint must be conductive enough to take an electrostatic charge, but not too conductive to cause earthing problems or too much paint build up on edges of the target article being painted, resulting in sagging or gassing on stoving. Typical resistivities will be 500K — 1MQ.

Synocryl 836S (Cray Valley Ltd) is a methylolated acrylamide resin, 60 %nvc in a 50:50 mix of n-butanol and xylol. It has limited solubility in alcohols and white spirit.

The pigment used is a chloride grade Ti02 for maximum whiteness and a pigment to binder ratio of 0.8/1 gives a balance between opacity and gloss. (The gloss achievable with this formulation will be very dependent on substrate condition and the degree of orange peel obtained, but it should be in the region of 85% on a 60° measurement head.)

Solvesso 100 is an aromatic hydrocarbon. The solvent balance is intentionally very slow evaporating, as the atomization achieved with high speed disc application gives rise to poor substrate wetting and flow, if faster solvents are used. It must also be remembered that acrylamide, as a monomer, gives very high molecular weight copolymers due to its great reactivity, and these big polymers do not flow after the solvent has evaporated.

The BYK 310 additive is used to reduce surface tension to aid substrate wetting and to counter the effects of atomization from the disc which can often give rise to air entrapment that results in a paint film that is full of micro bubbles of air. The trapped bubbles rupture on stoving resulting in a blistered film. The BYK 310 assists these bubbles to burst before all solvents have flashed off.

The paint is further thinned with Solvesso 100 to a viscosity of around 25 seconds for application. This will require typically 15-20% solvent, giving a volume solids at application of circa 33% and this factor, probably above all others, has led to the replacement of acrylamide acrylics by powder coatings for the coating of cabinets of domestic appliances in Europe and North America. If it were not for the issue of VOC reduction, this technology would continue to compete successfully with powder coatings and will probably do so in developing countries where VOC constraints are not yet an issue.

Thermosetting acrylics recommended for domestic appliance finishing include:

Sobral 16 Scott Bader Ltd.

Setalux С1386AX60 Synthese NV