Urethane coating components

Other Urethane Coating Components Types of Urethane Coatings... 

Moisture-curable urethane systems (one-pack) can be considered as two-component systems which use atmospheric moisture as the second component. One-pack urethane coatings can be produced that are similar in physical properties to the two-pack systems for almost all applications. These highly complex systems can have a great deal of flexibility. Claimed advantages are a one-pack system, rapid cure, even at low temperatures, excellent chemical and abrasion resistance and good flexibility. Although these systems have been available for some time in other countries of Europe, they are only recently beginning to be of interest in the UK. 

Blocked polyisocyanates are inert compounds at ambient temperature, and they generate free polyisocyanates at elevated temperatures by thermal dissociation (208). Blocked polyisocyanate technology is used in one-component urethane coatings. 

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. 

In more recent years one- and two-component urethane coatings based on polyethers have made a significant penetration of the urethane coatings market. Due to low cost and wide latitude in physical properties ranging from very flexible to hard, tough and solvent-... 

The principal components of commercially available urethane coatings are di- or polyisocyanates and di- or polyhydroxy compounds. Active hydrogen-containing compounds, especially diols and diamines, as well as alkanolaraines, are also employed as chain extenders. In addition, various cross-linking agents such as neutral or tertiary amine based triols or tetrols are also used. 

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.) ( ). 

Pi- and Polyhydroxy Components. By far the most important active hydrogen-containing components of urethane coatings are di- and polyhydroxy compounds, often referred to briefly as polyols. 

Castor Oil and Transesterification Derivatives of Castor Oil and Other Oils. Castor oil, consisting mainly of a triglyceride of ricinoleic (12-hydroxyoleic) acid (90% ricinoleic acid, 10% nonhydroxy acids, largely oleic and linoleic acids), is a frequent hydroxyl component of urethane coatings. On the basis of the hydroxyl number, castor oil is 70% trifunctional and 30% difunctional. The presence of castor oil in urethane polymers... 

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. 

Smetankina and Karbovskaya (196) have studied the effect of introducing carbofunctional organosilicon glycols into urethane coatings. The hydroxy-containing components had the general formula... 

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... 

Chem. Descrip. Zirconium chelate in 2,4-pentanedione Uses Catalyst for two-component urethane coatings (automotive topcoats, high performance maintenance coatings, aerospace coatings, coatings for structural plastics)... 

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