Polyurethanes ammonolysis

Figure 2.6 Flow diagram of a process for polyurethane ammonolysis by treatment with supercritical ammonia.93...

Polyurethanes (PUs, PURs), 197-258 ammonolysis and aminolysis of, 556 analytical techniques for, 241-246 applications, properties, and processing methods for, 198, 202-205 application testing of, 244-245 blood contact applications for, 207 chemical structure of, 5 chemistry and catalysis of, 222-236, 546... 

An interesting ammonolysis process has recently been developed93 based on the reaction of polyurethane with ammonia under supercritical conditions, which favours both the degradation reactions and separation of the polyols produced. The flow diagram of this process is shown in Figure 2.6. 

Two different polyurethanes were used as starting materials a solid elastomer based on a trifunctional polyethertriol, 1,4-butanediol and methylenebis(phenyl isocyanate) and a flexible foam where the diol was replaced by water. The ammonolysis reactions were carried out at 139 °C and 140 atm for 120 min, and with a polyurethane/ammonia weight ratio of 1. Under these conditions the polyurethane conversion was practically total. The ammonolysis reaction transforms the CO group into urea and the ester groups and derivatives of carboxylic acids into amides, whereas ether and hydroxy groups are inert towards ammonia. Scheme 2.7 illustrates the stoichiometry proposed by the authors for the ammonolysis of the polyether urethane. 

After the reaction, urea is separated by extraction with water whereas the polyol remains as a residue in the reactor. Therefore, under supercritical conditions the polyether polyols are separated from the mixture at the same time that the ammonolysis reaction progresses. The diamines and the diol can be separated by distillation or precipitation. The phosgenation of the amine leads to the corresponding diisocyanate, which together with the polyol and the diol may be used in the recovery of the raw polyurethane. 

Chapter 2 discusses depolymerization processes based on the chemical cleavage of polymer molecules to convert them back into the raw monomers. The latter can be reused in the manufacture of new polymers, with properties similar to those of the virgin resins. However, this alternative is mainly used for condensation polymers, and is not successful for the degradation of most addition polymers. Glycolysis, methanolysis, hydrolysis and ammonolysis are the main treatments considered. Chemical depolymerization of polyesters, polyurethanes and polyamides is reviewed. 

After ammonolysis, ammonia is evaporated and can be reused after liquefaction, while degradation products of polyurethane hard segments (e.g., amines and chain extenders) and urea are removed by extraction. The pure polyol is left in the reactor. It can be removed mechanically or by extraction with liquid ammonia in which it is soluble. The recovered amines can be converted to the corresponding isocyanates and can be reused, along with polyols, in the same applications as before. A flow scheme of the recycling process is shown in Figure 6.9. 

---