ASTM urethane coatings

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. 

One-Package Urethane Alkyd (Oil-Modified Urethanes) Coatings (ASTM Type 1)... 

One-Package Moisture-Cure Urethane Coatings (ASTM Type 2) Single-Package Blocked Adduct Urethane Coatings (ASTM Type 3)... 

Two-Package Catalyst Urethane Coatings (ASTM Type 4) Two-Package Polyol Urethane Coatings (ASTM Type 5)... 

Historically, the most commonly used urethane coatings have been grouped into five ASTM categories. Because these coatings have been reviewed in detail in many articles and chapters (126-130), they are listed below together with some brief comments. 

One-Package Moisture-Cure Urethane Coatings (ASTM Type 2). One-package moisture-cure urethane coatings consist of isocyanate-terminated polymers cured by atmospheric moisture. The rate of cure depends upon the humidity in the air and the presence of certain tertiary amine catalysts that accelerate the N(X)-water reaction. A major difficulty with this type of coating system is pigmentation. 

Two-Package Polyol Urethane Coatings (ASTM Type 5). Two-package polyol urethane coatings consist of isocyanate-terminated adducts of polymers that are cured by reaction with di- or polyfunctional hydroxyl-containing materials. The latter may consist of low- to medium-weight polyols with a polyester, polyether, polyether urethane, or castor oil backbone. When the two components (OH- and NC0-) are mixed together, they have only a limited pot life. Therefore, the components are mixed prior to application. Catalysts may be used to speed up the cure either for room temperature or oven cure. 

One-package urethane alkyd (oil-modified urethanes) coatings (ASTM type 1), alternatively called uralkyds. Here, curing takes place by oxidation of the double bonds present in the fatty acid portion of the coating system. 

Single-package block adduct urethane coatings (ASTM type 3). Here, curing is carried out after deblocking either at elevated temperatures or at lower temperatures in the presence of catalysts, according to the reactions discussed earlier. 

Two-package catalyst urethane coatings (ASTM type 4). Here, curing is carried out either by the addition of reactive catalysts containing hydroxyl groups such as alkanolamines or nonreactive catalysts such as tertiary amines and metal salts of carboxylic acids. These systems are similar to type 2 coatings. 

Two-package polyol urethane coatings (ASTM type 5). Here, the curing involves reaction between isocyanate-terminated adducts and di- or poly-functional hydroxyl-containing prepolymers having different backbone structures. 

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