As a material class, polyurethanes have unique properties over many other polymeric materials. Ease of reaction is one inherent property of both aromatic and aliphatic diisocyanates (ADIs), as they react in a 1:1 NCO:OH ratio with the numerous types of polyols available in the commercial market. The formed polyurethane bonds have high chemical resistances against many environmental conditions. Depending on the reactive polyol and the structure of the diisocyanate, polyurethanes can provide excellent abrasion resistance, superb elasticity, and high hardness.
Polyurethane chemistry provides many options in choosing systems that give soft-feel coatings and tough elastomers and rigid foams. But choosing the right systems requires a fundamental understanding of the makeup of the diisocyanates used in the crosslinking. Recent data explore the substitution of an aromatic diisocyanate by an aliphatic diisocyanate, with particular emphasis on the formulation of thermoplastic polyurethanes (TPU) and some additional information on the performance of polycarbodiimides (PCDI).
The use of aromatic vs. aliphatic diisocyanates differs dramatically; that difference has not changed much over the previous decades. It is estimated that only 3% of all diisocyanates sold this year will be aliphatic.1 While ADIs are used in adhesives and sealants, their principal use is in coatings (chiefly aromatics) and—to a small extent—elastomers. Some elastomer uses could benefit from increased outdoor durability. But how should ADIs be formulated to replace aromatic diisocyanates and retain the desired elastomeric properties?