NDTnet 1998 Aug, Vol.3 No.8

Full-Text - Abstract

Polycondensating thermosets (phenolic, melamin and urea formaldehyde resins) belong to a very widely used class of thermosets - they are manufactured in amounts of about 150 000 tons per year in Germany. Polycondensating thermosets are softened at temperatures of about 80 °C, at temperatures above 100 °C the chemical reaction starts leading to the formation of a polymer network (curing). The quality of the products are mainly influenced by the degree of curing. As well undercuring (no mechanical stability) as overcuring (too brittle) are critical. Therefore methods are needed for effective curing control. But it is problematic to detect the degree of curing of the final product it is more easy to monitor the curing process on-line.

Curing reaction control by the dielectric method
The applicability of the dielectric method - monitoring of permittivity and dielectric loss changes, which is known from epoxy resins since many years, was investigated. We found out, that phenolic and melamine formaldehyde resins are influenced strongly by the byproducts of the polycondensating reaction - ammonia and water - but urea formaldehyde resins showed the curing process very clearly. Fortunately urea resins are widely used in electric industry. Conse-quently we developed dielectric sensors (high pressure and high temperature stable) and a plug in measuring card for computers for industrial applications both now commercially available.

Curing control on industrial conditions
The dielectric method was successfully applied in industrial production of socket plates for electric switches. For every production cycle the softening and curing behaviour was on-line monitored. The figure gives an example for typical process signals. The influence of changes in the curing time is also illustrated. As well changes in softening behaviour by the change in cycle time (longer war-ming up times) as the optimal points for stopping curing reaction are in evidence.

Fig.

Abstract Source:
Book of Abstracts, 7th European Conference on Non-Destructive Testing, 26-29 May 1998, ISBN: 87-986898-0-00

Full-Text Source:
Proceedings of the 7th European Conference on Non-Destructive Testing, 26-29 May 1998, ISBN:

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