Лако-красочные материалы — производство

I INTRODUCTION

Since they were introduced in the early forties, two pack coatings based on polyurethane technology have begun rapidly to play a dominant role in a large range of industrial applications (see isocyanate/acrylics chapter). For example, they have largely replaced traditional nitrocellulose and air drying alkyd based enamels for vehicle refinishing.

The use of this technology provides coatings which exhibit the following general characteristics:

• Ambient/low bake (60 — 80°C) cure

• High chemical resistance

• High gloss

• Colour stability

• Durability

• Hardness

• Abrasion resistance

Through the proper selection of backbone monomers, control of molecular weight and hydroxyl functionality (level and type), binders with a vast array of properties can be formulated to meet the requirements of a wide range of high performance applications.

However, polyurethane acrylic coatings have continued to be the subject of toxicological concern over the use of isocyanate compounds, although the commercialisation of higher molecular weight polyisocyanates has minimised the risk of exposure to monomeric isocyanates. These systems are only recommended for use in well ventilated areas with efficient air extraction. In the case of spraying, operators must wear masks with a separate air supply.

Throughout Western Europe, where these systems have now been used for a number of years with proper handling and application equipment, few problems have been observed in practice. Nevertheless, there has been a requirement and an interest in the development of alternative ambient cure technologies, which do not give rise to the same toxicological concerns.

The new technologies would, of course, be required to have similar high performance to those characteristics exhibited by the existing two pack acrylic polyurethane systems.

A variety of potential ambient curing characteristics have been studied with this aim in mind and these have been reviewed in A. Noomens paper on ambient-curable coatings based on two pack technoIogy(4 An overview from this paper can be seen in the following table.

Five of these new technologies have been successfully developed into commercially available products. These are:

• Carboxy/epoxy

• Michael Addition

• Ketimine/activated hydrogen

• Anhydride/hydroxyl

• Amine/epoxy

Simplified reaction mechanisms for these technologies are shown in Figure 6-1:

Tertiary amines can also react with the epoxy group via a quaternary ammonium structure in the presence of a proton source, such as acid or moisture, to form a 3D network. In general, however and so far, none of these new technologies has been successful in providing the same overall performance characteristics of acrylic polyurethane technology. However, they have been successful in offering performance improvements to specific properties required for certain end use applications. Overall, the development of non-isocyanate two pack acrylics in the market place has been largely to replace alkyd systems rather than acrylic polyurethanes. They offer the paint manufacturer the opportunity to upgrade their coating performance in terms of dry times, chemical resistance, appearance and durability without the need to install expensive extraction equipment. Secondly, they enable the introduction of two pack technology in end use sectors where the equipment in place would preclude the use of isocyanates.

Since the introduction of these products in the 1980’s, further development of these technologies has been directed towards low VOC coatings to satisfy legislation on solvent emissions which will come into force in the latter part of the 1990’s. To date, only two of the commercially available systems, Michael and amine/epoxy, are reported to have met these legislative requirements on solvent emissions.