Characteristics of different types of hydroxyl components in polyurethane coatings

Isocyanate can react with hydroxylate at room temperature to produce urethane, is a typical polyurethane reaction:
when the hydroxyl group is divided into polyols, isocyanate is polycyanate, can be polymerized to produce mesh or stereostructure polymerized polymer compounds.
polyurethane coatings can be divided into single-group polyurethane coatings and two-group polyurethane coatings according to type. Regardless of the type, hydroxyl components are part of polyurethane resins or polyurethane coatings

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different hydroxyl parts due to the main structure of different, the basic performance will also have a large difference. Polyols from small molecules can only be used as raw materials for the manufacture of prepolymers or additives, or polyester resins, and in the practical application of polyurethanes, most hydroxyl-containing polymers are used.
common hydroxygens are: hydroxypropyl acrylic resin, polyester polyol, polyether polyol, epoxy resin, castor oil or its processed products.

hydroxypropyl acrylic
hydroxypropyl acrylic resin with polycyanate can be made with excellent performance of polyurethane paint. Acrylic resin does not absorb more than 300nm of ultraviolet and visible light, so weather resistance is excellent; And high hardness, good gloss, good decoration.
hydroxypropyl acrylic emulsions or dispersions are the most used hydroxyl parts in water-based two-group polyurethane coatings. Generally speaking:
water-based hydroxypropyl acrylic emulsion does not contain VOC, the particle size is slightly larger, the dryness is better, easy to de-light, but the fullness of the paint film, gloss in general, most products difficult to achieve 90% gloss
(60 degrees); Water-based hydroxypropyl acrylic dispersion (secondary dispersion) itself contains a small amount of solvent, smaller particle size, paint film plumpness, gloss are significantly better than the former, most can do more than 90% gloss (60 degrees), but slow drying and not easy to dissipate.
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polyester polyols
hydroxyl polyester and polycyanate with the paint film weather resistance is good, not yellowing, solvent resistance, heat resistance is good. Compared with acrylic resin, polyester has a low molecular weight and good deflection of the paint film, because the oxygen atoms on its ester bonds rotate easily, while the carbon bonds of acrylic resins do not rotate easily. However, polyester is not as hard and dry as acrylic polyurethane paint.
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Polyether polyol
polyether and polyester, ether bonds than ester bonds more easily rotating, so polyether glass conversion temperature is low, so the paint film low gentle toughness is good, alkaline hydrolysis, low viscosity. And there is no hydrolysed ester base in polyethers, so water resistance is better than polyurethane. However, oil resistance, mechanical strength, dryness, and NCO reaction rates are not as high as polyester.
price, polyethers are more competitive than polyester. Many domestic polyether-based, foreign countries are more inclined to apply polyester.
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epoxy resin
epoxy resin has a hydroxyl and epoxy, acrylic can react with isocyanate. Epoxy resin as a hydroxyl composition, prepared paint film adhesion, alkaline resistance are excellent performance, with excellent chemical stability, suitable for chemical resistance, salt water-resistant coatings. However, outdoor weather resistance is poor due to benzene rings and ether bonds in epoxy resins.
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oil or its processing product
castor oil has a hydroxy value of about 163, i.e. 4.94% hydroxyl content. The long-chain non-polar fatty acids give the paint film good water resistance and flexibility, and because of the hydrophobic action of its long-chain fatty acids, the problem of polyurethane paint made with castor oil is easy to foam after long-term immersion. In addition, it is inexpensive and rich in source.
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