Urethane coating components Isocyanate

Raw Materials for Urethane Coatings Isocyanate Components Di- and Polyhydroxy Components Chain Extenders and Cross-linkers Catalysts... 

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. 

There are two specific groups in this series of products (a) neutral urethane polyethers (112) that have to be activated with catalysts when used as second components in urethane coatings, which can be easily transformed into isocyanate-terminated polymers when reacted with additional isocyanate (b) basic urethane polyethers (113. 114) having hydroxyls activated by tertiary amine nitrogen present in the chemical chain of the product and that are very reactive second components of urethane coatings. They can also be converted into isocyanate-terminated polymers, but their stability is very limited (12-24 h). They are stable when immediately blocked by means of phenol, alcohols, or other suitable blocking agents. 

Chain Extenders and Cross-linkers. In addition to the two principal components of most urethane coatings, isocyanate and polyol components, a number of di- or polyfunctional, active hydrogen components may be used as chain extenders or cross-linkers. The most important classes of compounds for this use are diols or polyols (monomers or oligomers), diamines, and alkanolamines. Typical examples of diols are ethylene, dlethylene, dlpropylene glycol, 1,4-butanedio1, 1,5-hexanediol, neopentyl glycol,... 

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. 

Toluene diisocyanate (TDI), methylene diisocyanate (MDI), and related chemicals are industrial components in the poiymerization of urethane coatings and insulation materiais. Most two-part urethane products contain some amount of one of these chemicais, and iesser amounts may oontaminate one-part systems. Methyl isocyanate (the toxin reieased in the Bhopal, India, tragedy) is a carbamate insecticide precursor it is not used in urethanes, has actions different from those of the TDI group of chemicals, and is not discussed here (see Table IV-4, p 593). 

Uses Acrylic for low isocyanate demand urethane coatings ind. low-cost 2-component urethane coatings Properties Solid vise. 3800 cps VOC 4.0f Ib/gal 92% solids Joncryl 819 [S.C. Johnson Pol er]... 

Chem. Descrip. Bismuth carboxylate in 2-ethylhexanoic acid Uses Catalyst for blocked isocyanate and two-component urethane coatings, coil coatings, aerospace, automotive, urethane coatings for plastics... 

One-component urethane coatings are usually based on MDI, TDI or HDI terminated prepolymers containing free isocyanate groups. Pigmenting such systems is delicate due to moisture in the pigment, which can cause premature gelation. Therefore, these systems must contain additives for moisture removal such as zeolites (alumosilicates). Since the chain extension in such systems is carried out with water from air, one of the products of the reaction is CO2, which diffuses from the film without foaming. 

Urethane An important resin in the coatings industry. A true urethane coating is a two-component product that cures when an isocyanate (the catalyst) prompts a chemical reaction that unites the components. 

The main rule to follow in 2 component isocyanate systems is to avoid solvents which contain active hydrogen. For instance, alcohols can react with the isocyanate reducing the crosslink density. In severe cases, this can lead to a decrease in resistance and adhesion characteristics resulting in unacceptable coatings. The water content of solvents has to be closely controlled, again to avoid reaction with the isocyanate component. Where applicable, urethane grade solvents should be used, as these are designed to remove contaminants, such as water and alcohols, which could prematurely react with the isocyanate prior to application. 

Diisocyanates are an important class of chemicals of commercial interest, which are frequently used in the manufacture of indoor materials. such as adhesives, coatings, foams and rubbers (Ulrich, 1989). In some types of particle board, the diisocyanates have replaced formaldehyde. Isocyanates are characterized by the electrophilic -N=C=0 group, which can easily react with molecules containing hydroxy groups, such as water or alcohols. On hydrolysis with water, primary amines are formed, while a reaction with alcohols leads to carbamates (urethanes). Polyurethane (PUR) products are then obtained from a polyaddition of diisocyanate and diol components. Compounds commonly used in industrial surface technology are 4,4 -diphenylmethane diisocyanate (MDI) and hexamethylene diisocyanate (HDI). The diisocyanate monomers are known as respiratory sensitizers and cause irritation of eyes, skin and mucous membrane. Therefore, polyisocyanates such as HDI-biuret and HDI-isocyanurate with a monomer content <0.5 % are used for industrial applications, and isocyanate monomers will not achieve high concentrations in ambient air. Nevertheless, it is desirable to measure even trace emissions from materials in private dwellings. 

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