Preparation
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The most common method to prepare polyurethanes is by reacting a polyol with a di or polyisocyanate.

R-OH + R1-NCO ®  R-OCONH-R1

2 R1-NCO + H2O ®  R1-NH-COOH ®  R1-NHCONH-R1 + CO2

R1-NCO + R-NH2 ®  R1-NHCONH-R

3 R-NCO ®

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 Isocyanurate ring
The polyurethanes can be light stable if the isocyanates used are aliphatic or cycloaliphatic in nature. In addition the polyol has to be light stable. For lower cost applications or where light stability is not important aromatic isocyanates can be used. Aromatic isocyanates are faster reacting under most conditions than aliphatic isocyanates. The urethane linkage is very polar and strongly hydrogen bonded. This hydrogen bonding leads to an increase in the glass transition temperature (Tg) of the polymer and to a change in solubility of the polymer. The high cohesive energy if the urethane groups is 6294 cal/mol or 26370 J/mol. The different solubility parameter of the urethane linkage leads also to a phase separation and to the formation of polar hydrogen bonded domains. Polymers high in urethane groups can be dissolved in solvents also strong in hydrogen bonding capability such as 1-methyl-2-pyrrolidone (CAS # 872-50-4). Crosslinked polymers and foams can be swollen in this solvent. The reaction of a polyol with an isocyanate requires in most applications a catalyst. Organotin and non-tin compounds can be used for the catalysis of aliphatic and aromatic isocyanates. Aromatic isocyanates are catalyzed with t-amines.
Urea linkages can be formed by the reaction of  water with isocyanates, as a side reaction also carbon dioxide is produced. This reaction can be a problem if void a free polyurethanes is to be prepared. This reaction can be used to advantage in the preparation of foams. In the preparation of foams blown with carbon dioxide special t-amine catalysts are used to achieve the high reaction rate of the aromatic isocyanate with water. The urea linkages formed  from isocyanates and water is a extremely polar and a strongly hydrogen bonded group. This group is even more prone to phase separation and to be incompatible with the remaining polymer. The cohesive energy of the urea group is 10,000 cal/mol or 41,900 J/mol. Solubility of urea group containing polyurethanes is even poorer than only urethane containing polymers. The high glass transition temperature of polymers prepared with urea groups leads to a  high heat distortion temperature of polymers. The linkage is also very stable to hydrolysis under basic and acidic conditions. If the urea groups is formed from aliphatic isocyanates the polymers are also very light stable. A high content of urea groups can lead in coatings or moldings to lower gloss. If the formation of urea is a side -reaction and has not been considered in the molar ratio of polyol to isocyanate it can lead in low functional polymers to poor resistance properties.

Direct formation of polyureas is also possible by reacting a polyamine with an isocyanate. The reaction is extremely fast and with primary sterically non-hindered amine almost explosive. Using sterically hindered amines or amines with a lower pKa value the reaction speed is more manageable. Because of the high reaction speed the application of such as  system is in plural component spray applications where the components are mixed in the spray zone. Also RIM (reaction injection molding) compounds can be prepared by this reaction. Polymers prepared by this reaction have a high heat distortion temperature.

The trimerization of isocyanates in the presence of certain catalysts leads to an isocyanurate ring which is a very temperature stable link. This reaction can be used to create temperature stable polymers with high flame resistance. By combining conventional polyurethane reaction with isocyanurates flame resistant foams can be prepared. The heat distortion temperature of polyurethane RIM formulations can also be improved with isocyanurate crosslinks.
The reaction rate of a polyol with an isocyanate is very slow in the absence of a catalyst. Many compounds can catalyze the reaction of an isocyanate with a polyol. Typically in polyurethane systems, dialkyltin, lead octoate, zinc octoate and amines are used. For aromatic isocyanates amines are very effective catalysts. Dibutyltin dilaurate (DBTDL) is a standard catalyst for aliphatic and cycloaliphatic isocyanates. Catalysts which catalyze the hydroxyl reaction also catalyze the water reaction. DBTDL shows some preference for the hydroxyl over the water reaction. Amines on the other hand preferentially catalyze the water over the hydroxyl reaction.

Last edited on:

 November 14, 2006

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