Лако-красочные материалы — производство

I INTRODUCTION

Polyurethanes can trace their commercial origin to the development work undertaken by Otto Bayer et al in the 1930’s(1). Since then, this family of products has adapted extremely well for use in a wide variety of applications, such as fibres, foams, plastics, adhesives, textiles and coatings to name but a few.

In the coatings sector, two pack acrylic isocyanate technology came to prominence in the 1960’s. However, it was during the 1970’s and 80’s that this product family was observed deeply to penetrate the coatings markets in Europe, USA & Japan. This commercial development can be attributed to two main factors. The first is the urethane linkage which is formed at ambient temperatures by the reaction between the acrylic polyol and the isocyanate prepolymer, as shown in Figure 5-1.

t

О О

II II

С — О— СНоСН,— OOCHN — R— NHCOO— СН, СН,—О— С ■

Figure 5-1

Urethane Formation

This urethane linkage is extremely resistant to chemical degradation by acids, alkalis, petrol, oil and detergents, making coatings based on this technology highly suitable for use in aggressive environments.

The second attribute is the formulatory flexibility of the hydroxy functional acrylic. A myriad of acrylics can be formulated by careful consideration of the following parameters:

• backbone monomers

• hydroxyl monomer (type and level)

• functionality

• molecular weight

• molecular weight distribution

• Glass transition temperature (Tg)

When reacted with the isocyanate component, they produce coatings which display a blend of the following performance characteristics:

• ambient/low bake (60-80°C) cure

• exceptional chemical resistance

• water white coatings

• high gloss

• durability (colour and gloss retention)

• mechanical properties

• hardness and flexibility

These performance attributes have contributed to the important role that the two pack acrylic/isocyanate coatings play in the wide range of end use applications including:

• vehicle refmishing

• aircraft coatings

• transport finishes

• floor coatings

• coatings for plastics

• industrial wood finishing

• military applications

• masonry coatings

As can be seen, two pack acrylic isocyanate coatings satisfy fhe requirements of a large range of end use applications. However, during the last ten years continuing development of this technology has been aimed at complying with impending environmental legislation on solvent emissions.

Three main routes to compliance have been pursued:

• low molecular weight resins for high solids systems

• reactive diluents for high solids systems

• water reducible coatings

Higher solids coatings have been achieved by reducing the molecular weight of both components. The functionality of the acrylic has had to be increased to maintain the excellent resistance characteristics of these types of systems. Coating compositions with solids contents of 65-70% (by weight) have been achieved by this route. Altering the molecular weight/functionality to obtain compliant coatings has, however, resulted in shorter pot lives and longer drying times when compared to existing polyurethane coatings.

A large range of reactive diluent products are now available for the modification of polyurethane coatings based on low viscosity castor oil derivates, oxazolidines, acetoacetate. and ketimines. In certain end uses, this approach offers compliant coatings with minor modification to existing formulations.

Water reducible polyurethane systems are also now commercially available. These systems compare favourably with their solvent based counterparts for gloss, resistance properties, mechanical properties and appearance, but they have inferior weathering performance. Nevertheless, they do present a viable commercial alternative to the alkyd based products, which are presently used in a large range of industrial applications.