Metrohm 763 PVC Thermomat Manual - Manual (Page 8)

1.1 Determination of thermal stability
763 PVC Thermomat
1
1 Introduction
1.1 Determination of thermal stability
The determination of the thermal stability of polyvinyl chloride according
to DIN 53 381, part 1, is based on the fact that PVC decomposes at
higher temperatures with the release of HCl. The hydrochloric acid
formed is taken up by a gas stream (normally N
2
) and transferred into
the measuring vessel where it is absorbed in the measuring solution
(dist. water). The decomposition process is monitored by measuring
the conductivity of the aqueous HCl solution. Both the stability time and
the induction time are determined. The stability time is defined as the
time required to achieve a conductivity difference of 50 µ S/cm. The in-
duction time is the time that elapses before the conductivity starts to
rise sharply. The method is used for testing PVC in all stages of proc-
essing as well as for testing stabilizers.
Fig. 1: Schematic measuring arrangement
Measuring
solution
Conductivity
measuring cell
Measuring
vessel
PVC
Heating block
Reaction vessel
N
2