Urethane coatings development

We have demonstrated that in styrenic polymers and in a urethane coating that surface carbonyl group development is a major chemical change following prolonged sunlight exposure. 

This paper is a commercial survey covering recent developments in urethane coatings and adhesives with reduced solvent contents. There are many aspects of urethane chemistry which make polyurethanes an ideal base for nonsolvent coatings developments. However, in order to achieve practical handling behavior for coatings adhesives, modifications of classical urethane chemistry are needed. 

We believe it is a good time to initiate a comprehensive classification system. The system would be oriented to the commercial user and not intended to be limited by chemical considerations, because in the practical world frequently more than one composition will perform an equivalent job. We also believe that such a system would encourage a healthier industrial growth situation. The present ASTM urethane coatings classification (l) covers chiefly the solvent types and is thus an extremely limited one from the viewpoint of the new developments in the past ten years. 

However, the use of PU finishes on automotive metal on assembly lines will not come about unless or until the toxicity problem is overcome. Urethane coating producers are active in the development of new PU systems—some involving proprietary blocking agents—to tackle this problem. Their efforts should show some success by 1985. Positive influences here include the growing use of base coat/clear coat systems clear PU topcoats are under active consideration. In addition, vapor permeation curing (VPC) is under investigation. Here, acrylic/urethane topcoats are cured rapidly under ambient conditions in an amine catalyst atmosphere. 

Urethane coatings were first developed by Otto Bayer and his coworkers in the laboratories of the I. G. Farbenindustry, today s Farbenfabriken Bayer, in Leverkusen, West Germany. Although... 

The earliest commercial urethane coatings were based on polyester-polyisocyanate systems that exhibited excellent abrasion resistance, toughness, and a wide range of mechanical strength properties. Most urethane coating systems in this country were first based on tolylene diisocyanate (TDI), while in Europe many systems based on 4,4 -methylene bis(phenyl isocyanate) (MDI) were developed. In order to avoid the use of free TDI, adducts of polyols such as trimethyolpropane or 1,2,6-hexanetriol with TDI were introduced, particularly for two-component coatings (1., 2). 

One-component urethane coatings with "blocked" isocyanate groups were developed by Bayer (1, 3) and Petersen ( ). Application of heat to these "splitters" with regeneration of free isocyanate groups and fast curing has led to the acceptance of these coatings as wire enamels in the electrical industry, as well as for coatings in other industries. 

Similarly, excellent two-component urethane coatings were obtained from the adduct of trimethylolpropane and 4-bis(isocya-natomethyl)cyclohexane (BDI), which was developed by Suntech (Sun Oil Co.) ( ). 

The light stabilizer category, hindered amine light stabilizers (HALS) are very effective stabilizers for urethane coatings. These compounds, developed primarily by Ciba-Geigy, do not act as UV absorbers but act by Inhibiting the degradation of the binder that is already in an excited state. 

In addition to the five groups of urethane coatings described here, other coating systems have been developed that are described as follows. 

Miscellaneous Urethane Coating Systems. Light-stable urethane coating systems have been developed from 3,3-tetramethylenedi(l,4,2-dioxazol-5-one) (ADNC, Arco Chemical Co.). The structure of ADNC is as follows ... 

Such polyurethanes have excellent hydrolytic stability compared to water-reducible polyesters and superior abrasion resistance. In view of the importance of developing low solvent emission coatings, considerable effort is being devoted to new types of water-borne urethane resins (62,63). 

Urethane rubbers have found steadily increasing use for oil seals, shoe soles and heels, fork-lift truck tyres, diaphragms, chute linings and a variety of mechanical applications. Fabric coatings resistant to dry cleaning are a recent development. In many of these applications high elasticity is not an important prerequisite so that the polyurethane rubbers must be compared not only with other rubbers but also with a variety of thermoplastics. 

There is a seemingly infinite variety of tests for specific applications, and some even have alternate adaptations for particular industries or customers. This section contains a few examples of common application tests for urethane foams, coatings, adhesives, and elastomers that may be conducted in addition to those described above. Some of these are just specialized versions of tests discussed in die previous section but are presented here because they have been developed for specific applications. Others are more broadly useful and very common. 

Several commercial products have resulted from our phosphorus oligomer research. Fyrol 99, a 2-chloroethyl ethylene phosphate oligomer, has been successfully used as a flame retardant additive in rebonded urethane foam, in thermoset resins, in intumes-cent coatings, adhesives, paper air filters (13), and related uses. This product is less volatile and has a higher flame retardant efficacy than the parent compound tris(2-chloroethyl) phosphate. A related product was developed especially for use in flexible polyurethane foams. A vinylphosphonate/methylphospho-... 

A recording medium was developed in which photosensitive films (urethane-urea copolymer), nonphotosensitive (polyvinyl alcohol, PVA and poly( methyl methacrylate), PMMA) films, were coated on a glass substrate. Figure 16.13 shows the structure of the multilayered recording medium. 

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