Epoxidharzsysteme, die mit zunehmendem Aushärtungsgrad und steigender Härtungstemperatur bei Raumtemperatur eine abnehmende Dichte aufweisen

16. Mitteilung über Chemie der Epoxidharze

Abstract

During an investigation of various epoxide resin systems, some cases were found in which the room temperature density decreased with increasing curing temperature and increasing degree of cure. This is an effect of considerable technical interest. In one system which is described here in greater detail, the density of the cured product decreased from 1.223 to 1.202, i.e. by ca.2%, after a 75 to 100% conversion of the epoxide group.
In other systems the density of the cured resins is independent of the curing temperature.
The unexpected behaviour of the first type of resin system can be explained in the following way:

1. When during curing the increasing glass temperature exceeds the curing temperature by a certain value, the chemical conversion comes to a standstill; it “freezes chemically”. Complete curing is possible only at a temperature which is high enough and specific for each system.
2. With isothermal curing shrinkage is always found. The method mentioned initially – to deduce an increase of the chemical conversion from the change in density – is also valid for the reaction temperature.
3. The effect of decreasing room temperature density with increasing curing temperature can be explained in a purely physical manner from the pattern of the density changes during one entire curing cycle. The volume expansion of the non-cured mixture – which is due to the increased curing temperature – outweighs all contraction effects consisting of isothermal chemical and cooling shrinkage.

By means of values that can be measured readily at low temperatures it is possible to draw up diagrams from which the variation of the density at higher temperatures of the curing cycle can be estimated.