URETHANE COATINGS Introduction Polyurethanes (qv) are polymers containing urethane ( NH COO ) linkages; carbamate is a synonym for urethane. Urethanes are usually formed by reaction of an alcohol with an isocyanate, but they can also be made by other methods. Commonly, urethanes made by a nonisocyanate route are called carbamates. To add to the confusing terminology, the terms urethane and polyurethane are applied to almost any binder derived from isocyanates even though only part, if any, of the reaction products are urethanes. The article on polyurethanes discusses synthesis and sources of various polyisocyanates. An important advantage of most urethane coatings is the superior abrasion resistance of their films. The abrasion resistance results from the strong intermolecular hydrogen bonds between urethane groups. Under mechanical stresses, energy (about 20–25 kJ/mol of hydrogen bonds) may be absorbed by separation of hydrogen bonds, which re-form when the stress is removed (though probably in different positions). Energy absorption by this reversible process reduces the likelihood of irreversible breaking of covalent bonds, leading to degradation. This permits design of polyurethanes that are abrasion resistant while still resisting swelling with solvents. The isocyanate group is highly reactive, and so polyisocyanates can be used to make coatings that cure at ambient temperature or at moderately elevated temperatures. Coatings based on aliphatic diisocyanates exhibit exceptional exterior durability when stabilized with hindered amine light stabilizers (HALS). Resistance of urethane coatings to environmental etching is superior to that of many melamine–formaldehyde (MF) cross-linked coatings (1). The principal limitation of urethane coatings is cost relative to many other binders. 252 Encyclopedia of Polymer Science and Technology. Copyright John Wiley & Sons, Inc. All rights reserved.

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Isocyanates react with alcohols and phenols to form urethanes. In general, rates of urethane formation decrease in the following order: primary alcohols > secondary alcohols > 2-alkoxyethanols > 1-alkoxy-2-propanols. Isocyanates can react with urethanes to form allophanates. This reaction is much slower than the reaction of isocyanate with alcohol. Isocyanates react rapidly with primary and secondary amines to form ureas. The reaction is much faster than the reaction of isocyanates with alcohols. Isocyanates can react with ureas to form biurets. Biuret formation is slower than urethane formation, but faster than allophanate formation. Isocyanates react with water to form unstable carbamic acids, which dissociate into carbon dioxide and an amine. The amine is so much more reactive that it reacts with another isocyanate (in preference to water) to form urea. The reactivity of water with isocyanates is somewhat slower than that of secondary alcohols, but much more rapid than that of uncatalyzed reaction with urethanes or ureas. Imines act as blocked amines because they hydrolyze to yield free amines, which react with an isocyanate. Ketimines also react directly with isocyanates to yield a variety of products, depending on the particular reactants and conditions. Aldimines react analogously with isocyanates to yield unsaturated substituted ureas. Since aldimines are more stable to hydrolysis than ketimines, the fraction undergoing direct reaction with isocyanate in the presence of water is greater than that with ketimines. Carboxylic acids react relatively slowly to form amides and CO2 . Hindered carboxylic acid groups, such as in 2,2dimethylolpropionic acid (3-hydroxy-2-(hydroxymethyl)-2-methylpropanoic acid) [4767-03-7], react very slowly. Isocyanates also react with each other to form dimers (uretdiones) and trimers (isocyanurates). Formation of uretdiones is catalyzed by phosphines. Formation of isocyanurates is catalyzed by quaternary ammonium compounds; trimerization of aromatic isocyanates is catalyzed by tertiary amines. Uretdiones decompose thermally to regenerate isocyanates and are used as blocked isocyanates. Isocyanurates are stable and isocyanurates of diisocyanates are extensively used as multifunctional isocyanates. Reactions of isocyanates with alcohols are catalyzed by a variety of compounds, including bases (tertiary amines, alkoxides, and carboxylates), metal salts and chelates, organometallic compounds, acids, and urethanes. The most widely used catalysts in coatings are organotin (IV) compounds, most commonly dibutyltin dilaurate (DBTDL) (dibutylbis[(1-oxododecyl)oxy]stannate) [77-58-7] and tertiary amines, commonly diazabicyclo[2.2.2]octane (DABCO). Combinations of DABCO and DBTDL often act synergistically. DBTDL is soluble in a wide range of solvents, comparatively low in cost, colorless, and, in general, highly effective at levels of the order of 0.05 wt%. DBTDL promotes urethane formation without promoting allophanate formation (2) or trimerization (3). While aromatic isocyanates are more reactive than aliphatic isocyanates in uncatalyzed reactions with alcohols, the reactivity of aliphatics can be roughly equal with DBTDL. On the other hand, amine catalysts are more effective with aromatic than aliphatic isocyanates. Carboxylic acids inhibit catalysis by organotin compounds. Isocyanates also react with water; if this happens in a solventborne coating during application, CO2 is generated, which may reduce gloss or result in

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bubbling. Catalyst selection affects the relative rate of reaction of isocyanates with hydroxyl groups and water. A zirconium acetoacetate complex [Zr(AcAc)4 ] catalyst is reported to be more selective than DBTDL (4).

Isocyanates Used in Coatings Both aromatic and aliphatic isocyanates are used in coatings; the former are less expensive and the latter provide films with better color retention and exterior durability. Diisocyanates are used in two broad types of ways: (1) as raw materials in the synthesis of resins and (2) as cross-linkers for resins with multiple substituent groups that are reactive with isocyanate groups. Isocyanates are toxic materials. When used as cross-linkers for coatings, they are reacted to form higher molecular weight isocyanate products with lower toxic hazards. The aromatic isocyanates most widely used in coatings are bis(4isocyanatophenyl)methane [101-68-8] (MDI) and 2,4-toluene diisocyanate (TDI) (2,4-diisocyanato-1-methylbenzene) [584-84-9]. MDI is available in several grades: a mixture of 90% of the 2,4
isomer and 10% of the 4,4
isomer; and several oligomeric (frequently called polymeric) MDI with longer chains of methylene phenyl groups. The cost is lower than TDI. When used as a cross-linker in coatings, the monomeric grade is converted to a low molecular weight prepolymer by reacting with a polyol such as trimethylolpropane. The polymeric grades have lower toxicity. The most common grade of commercial TDI consists of a mixture of about 80% 2,4- and 20% 2,6-diisocyanato isomers. Nearly pure 2,4-TDI is also available at a premium price. Because of toxic hazards, TDI is not used as such in final coating formulations. For coatings in which unreacted isocyanate groups are needed, TDI is converted into derivatives of higher molecular weight and higher functionality. Higher molecular weight reduces the toxic hazard, and the higher functionality yields solvent-resistant films more rapidly. TDI has the advantage of a differential in reactivity between the ortho- and the para-isocyanate groups with alcohols, which makes possible synthesis of isocyanurates and prepolymers with narrower molecular weight distribution than with diisocyanates in which the isocyanate groups are equally reactive. A prepolymer prepared by the reaction of trimethylolpropane (TMP) (2-ethyl-2-(hydroxymethyl)-1,3-propanediol) [77-99-6] with a large excess of 2,4-TDI and then removal of excess TDI by using a vacuum wiped-film evaporator is widely used. The isocyanurate derived from TDI made by trimerizing TDI also has a lower toxic hazard than monomeric TDI; the reaction is almost exclusively at the 4-position. The principal aliphatic isocyanates used are 1,6-hexamethylene diisocyanate (HDI) (1,6-isocyanatohexane) [822-06-0], bis(4-isocyanatocyclohexyl)methane (H12 MDI) [5124-30-1], and isophorone diisocyanate (IPDI) (3-isocyanato-1-isocyanatomethyl)-1,3,3-trimethylcyclohexane [4098-71-9]. Tetramethyl-m-xylidene diisocyanate (TMXDI) [2778-42-9] and m-isopropenyl-α,α-dimethylbenzylisocyanate (TMI) (1-(1-isocyanato-1-methylethyl)-3-(1-methylethenyl)benzene [2094-99-7] are specialty products used on a much smaller scale. Diisocyanates are only used as higher molecular weight derivatives in coatings to increase functionality and reduce toxic hazard.

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HDI is especially hazardous and is handled on a large scale only in chemical plants. The first less hazardous derivative was a biuret, which can be made by reacting HDI with a small amount of water and then removing excess HDI. The presence of oligomeric biurets makes the average functionality higher than 3. These polyfunctional isocyanates give coatings with good color retention and weather resistance. The viscosity of an early commercial product was about 11.5 Pa·s (115 P) at 20◦ C. Grades with lower average molecular weights (and average functionalities nearer to 3) are available with viscosities as low as 1.4 Pa·s (14 P). HDI isocyanurates are used on a larger scale. The isocyanurate gives coatings with greater heat resistance and even better long-term exterior durability than HDI biuret. Commercial products contain oligomeric material, and average functionality is over 3. Grades with lower oligomer content with viscosities as low as 1 Pa·s (10 P) are available. Allophanate derivatives of HDI and IPDI are another type of polyfunctional isocyanate. They are made by reacting an alcohol or diol with excess isocyanate, then removing unreacted diisocyanate with a wiped film evaporator giving an isocyanate-terminated allophanate (5). The properties can be varied by using different alcohols or diols to make the starting urethane. For example, the cetyl alcohol urethane from HDI yields an allophanate diisocyanate that is soluble in aliphatic hydrocarbons. Derivatives with higher functionality are made by reacting a glycol with excess diisocyanate. The uretdione dimer of HDI also has lower volatility with low viscosity and can be used for cross-linking in ambient cure coatings. Very low viscosity grades,