Community Research and Development Information Service - CORDIS


The project results suggest that by monitoring the early stages of a PU cure with a device that can monitor changes in both viscosity and elasticity, such as the SVNC, then both the delay characteristics of the cure and the final properties of the cured material could be estimated. Further, by assessing cure of PU formulations in the presence of a catalyst the character of that catalyst could be assessed.

The outputs of the SVNC also have also enabled the selectivities of catalysts to be compared for actual curing formulations. This has revealed that the type of catalyst used has a major influence on this selectivity. A comprehensive number of catalysts were used in the project work and has resulted in an approximate ranking with respect to their influence on their selectivity.

This selectivity has been seen to influence cure in two important ways:

1. The delay character of the cure; and
2. The final properties of the cured product.

The work carried out on PU elastomers has also shown that the profile of a cure and the final properties of the cured product of a given formulation are significantly influenced by the relative rates of chain extension and crosslinking processes.

The shape of the SVNC plots provided a 'fingerprint' of the balance between the reactions of chain extension and crosslinking as the cure develops. Thus, in respect of a PU cure, which progresses by both chain extension and crosslinking, it may be visualized that premature crossliking can ultimately compromize the crosslink density which is attainable. Therefore, for example, in cast elastomer formulations studied, it was possible to distinguish reactions of chain extension (MW growth) and crosslinking. By profiling the balance between chain extension and crosslinking as cure progresses, a road map of the direction/route taken by the cure is obtained.

The results obtained by using different temperatures and concentrations of catalyst, support the hypothesis that reduced early chain extension can compromize the level of crosslinking density ultimately attained. So some catalyst seems to be more selective toward chain extension reactions whereas others appear to be more efficient for crosslinking reactions. Some appear to have intermediate character. Therefore at last the means to probe not only the destination of the cure, but how it got there, is available.

With respect to PU foams the research work showed that not only does the catalyst type have an important effect on the blowing/gelling selectivity, but also the amount of catalyst used. In addition the type of formulation used (e.g. the amount of chemical and physical blowing agents present) can affect the ranking of the various catalysts.

Developing computer models of the effect of catalyst type, catalyst level and temperature proved very difficult to conceive. Even a general model for predicting cure, given the complicated reaction kinetics of a curing PU and the plethora of different PU formulations types possible, proved beyond the scope of this project. The conclusion is that the simplest model for predicting PU cure is complex and on the recommendations of the SMEs greater emphasis was placed on generating more cure data using the SVNC instruments.

Reported by

Rapra Technology Limited
Shawbury SY4 4NR
United Kingdom
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