Combinations of Malonate- and Pyrazole-Blocked Isocyanates to Improve Performance
In contrast, it is generally accepted that diethyl malonate (DEM) blocked isocyanates react by way of transesterification, rather than liberation of isocyanate (see Figure 2). The final reaction product differs from that formed from a conventional blocked isocyanate in the following ways: a stoichiometry of 1:2 (nco:oh) is theoretically achievable; and a carbamate (urethane) bond is not formed in the crosslinking reaction.
Furthermore, the transesterification reaction can take place at lower temperature than normal stoving conditions. This is both an advantage and a disadvantage. The reduced temperature cure is desirable but since the reaction can also take place slowly at ambient temperature, the stability of 1K formulations based on DEM-blocked isocyanates is poorer than those derived from other blocked isocyanates. The stability can be improved by incorporating a volatile monofunctional alcohol, which competes with polyol in the transesterification reaction and prevents crosslinking from taking place during storage. Key features of the main blocking agent are shown in Table 1.