Chemistry, polymer dynamics and mechanical properties of a two-part polyurethane elastomer during and after crosslinking. Part I: dry conditions

Abstract

Due to their chemical and morphological versatility, polyurethanes may be susceptible to fabrication conditions (curing procedure, moisture). Industry reports ‘maturation’ phenomena, i.e. unstable mechanical properties of PU elastomers and PU based adhesive joints right after fabrication. This paper elucidates the impact of fabrication conditions and possible causes of maturation. Chemical and physical bulk properties of a crosslinking two-part PU are monitored during and after cure in dry atmosphere (at room temperature followed by a post-cure). The influence of curing procedure is revealed by processes characteristic of the post-cure, i.e. chemical side reactions (allophanate, carbodiimide) and a new dielectric relaxation. Tensile tests demonstrate the practical relevance of the findings (mechanical modulus). Properties of the fully crosslinked, dryly stored PU are stable during a three-week observation, indicating that maturation is preventable by complete cure and dry conditions. In contrast, chemical crosslinking after an insufficient cure likely causes maturation phenomena.