Influence of the melting point on the deblocking reaction of methyl ethyl ketoxime‐blocked aromatic polyisocyanates

Blocked polyisocyanates are used in single component (1 K) water‐based and powder coatings to simplify coating synthesis and reduce waste. However, the relationship between the melting point and deblocking behavior of blocked polyisocyanates is not clear. We aim to determine the influence of the melting temperature on the deblocking of methyl ethyl ketoxime (MEKO)‐blocked aromatic polyisocyanates. A toluene‐2,4‐diisocyanate trimer grafted with different mass ratios of methoxypolyethylene glycol (MPEG) is used to achieve compounds with different melting points and is blocked with MEKO. The deblocking behaviors of the polymers are investigated using Fourier transform infrared spectroscopy, differential scanning calorimetry, scanning electron microscopy, and thermogravimetric analysis. The results show that the deblocking temperature and average activation energy of the blocked polyisocyanates decrease with increase in melting point. Further, MEKO is more likely to be lost from blocked polyisocyanates having lower melting points. Furthermore, polyacrylic resins cured with blocked polyisocyanates having low melting points exhibit better properties than those having high melting points. The results reveal that the reduction in melting point induced by grafting with MPEG can significantly reduce the deblocking temperature of blocked polyisocyanates. The improvements in these properties may help achieve the low‐temperature crosslinking of blocked polyisocyanates for coating technologies. Fourier transform infrared spectra of different blocked polyisocyanates at the same temperature (100°C) and heat treatment duration (2 min).