Urethane coating types systems

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. 

Polyesters derived from e-caprolactone and ethylene glycol and triols, such as trimethylolpropane, have been employed for the preparation of different types of urethane coatings, including nonyellowing systems (88. 89). These types of polyesters exhibit improved hydrolytic stability and low-temperature properties as compared to adipate polyester based urethanes. 

A variety of urethane coating systems has been introduced based on hydroxyl-containing acrylic and methacrylic ester copolymers that contain hydroxyethyl acrylates or methacrylates and that are generally cured with aliphatic di- or polyisocyanates. Depending upon the type and relative amounts, acrylate or other saturated esters, for example, adipate-isophthalic esters, are also used in combination with aliphatic polyisocyanates. 

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. 

A relative new type of moisture-cure urethane coating consists in the formation of terminal oxazolidine group containing systems... 

One major automobile manufacturer was concerned about the use of urethane coatings with regard to solvent safety and emissions regulations. The cost and size of the complex coating supply and solvent evaporation equipment were other issues. Also, a 2 hr oven time limited the type of board it could coat and presented a severe handicap to short-run production and just-in-time delivery systems. 

Chem. Descrip. Org. and inorg. colorant disp. in polyether polyol base Uses Pigment tor polyether-type coatings, RIM systems, and MOCA side ot TDI cast urethanes Properties M.w. 6500 hyd. no. 26 Colormatch ED Series [Plasticolors]... 

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. 

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. 

The acetates of most alcohols are also commercially available and have diverse uses. Because of their high solvent power, ethyl, isopropyl, butyl, isobutyl, amyl, and isoamyl acetates are used in ceUulose nitrate and other lacquer-type coatings (see Cellulose, esters). Butyl and hexyl acetates are exceUent solvents for polyurethane coating systems (see Coatings Urethane polymers). Ethyl, isobutyl, amyl, and isoamyl acetates are frequentiy used as components in flavoring (see Flavors and spices), and isopropyl, benzyl, octyl, geranyl, linalyl, and methyl acetates are important additives in perfumes (qv). 

Many epoxy dispersions are compatible with most types of latex emulsions including acrylic, urethane, styrene butadiene, vinyl chloride, and polyvinyl acetate. The epoxy dispersion can be used as a modifier for these emulsions to alter handling and application characteristics such as emulsion rheology, foaming tendencies, pH sensitivity, wetting properties, and coating coalescence. They can also be reacted into the latex resin either by reacting the epoxy with a functionalized latex or by use of an epoxy with a coreactant. In this way adhesive systems can be formulated that are cured at room or elevated temperatures. 

Most of the important types of modem solvent-borne coatings — epoxies, alkyds, acrylics — are also available in waterborne formulations. In recent years, even urethane polymer technology has been adapted for use in waterborne coatings [1]. However, waterborne paints are not simply solvent-borne paints in which the organic solvent has been replaced with water the paint chemist must design an entirely new system from the ground up. In this chapter, we discuss how waterborne paints differ from their solvent-bome counterparts. 

Secondary hydroxyl groups of the epoxy resin backbone may be reacted with polyisocyanates, aromatic as well as aliphatic, in two-component ambient curing systems that are commonly known as epoxy-urethane hybrid systems. This type of coating combines advantages of epoxy and polyurethane systems. 

Reaction of isocyanates with different active H-compounds that form the fundamental basis for polyurethane coating systems are shown in Figure 2.60. The order of relative rates for these reactions is -NH -OH > H O > -COOH. Many types of catalysts are available that are used to control these reactions. It should be noted that while urethanes are produced only from reaction of isocyanates with alcohols, other reactions are also effectively controlled and used in different polyurethane coating systems. 

Polyurethane coatings are used as a single-pack system or a two-pack system. There are three basic types of single-pack systems and two of the two-pack system. More recently water-borne urethanes have been developed to reduce solvent emmission. 

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