Following isocyanate-polyol curing by volume shrinkage induced pressure decrease - considering reaction volume, activation volume, compressibility and diffusion control

To know the completion of the curing reactions used to create elastomers as HTPB-isocyanate is an important aspect in controlling the curing process and in considering the ageing of the elastomer. The so-called post-curing must be separated from real ageing. The reaction kinetic description of curing reactions gives activation parameters and heat of reactions with thermal methods. The tracking of curing reaction via volume shrinkage is an interesting aspect, because normally calorimetric methods cannot be applied with real systems. To use the volume shrinkage is possible in a constant volume device, filled up with the reaction mixture and held under a certain overpressure during the monitoring. By the curing, the reaction mixture shrinks, and the pressure drops. This enables kinetic analysis of the curing. At this point, one has to consider the pressure influence on the reaction rate constant and the change of bulk compressibility or the compression modulus with the increasing degree of curing, means cross-linking. It is reasonable to assume that the uncured reaction mixture has a lower isothermal compression modulus than the cured mix. This paper gives the analyses of the pressure influence on the reaction rate constant in connection with the reaction kinetic description and the effect of change of compression modulus on the kinetic evaluation via pressure decrease. A further topic with curing reactions is the diffusion control during the later curing process. A solution is offered, which works with any probing method, thermal, number of species, or volume shrinkage.