Development of Karl Fischer Reagents

7. Plastics
Key Points
Chapter III: Applications - 2
Plastics reach high temperatures during the molding process and trace
amounts of moisture can cause bubbling or seriously affect the
performance of a plastic by reducing its molecular weight through
pyrolysis. This means that moisture management is a very important
aspect of the drying processes for plastics. Karl Fischer titration is the
ideal method for measuring trace moisture content in plastics. The
moisture vaporization method permits accurate and relatively fast (10-
30 minutes) determination of moisture content.
When using the moisture vaporization method, it is important to set the
heating temperature with reference to the type of plastic. The optimal
temperature range must be determined for each plastic. The generally
accepted view is that plastics should be heated to around melting
point. Figure 24 provides guidelines on the appropriate heating
temperatures
[If the analytical result is low]
If the heating temperature is too low, moisture vaporization will be delayed
and the analytical time will be longer. The measurement process may end
before the moisture has been vaporized, leading to a low analytical result.
The result may also be low if vaporized moisture is absorbed by additives or
degraded plastic constituents that have condensed in the tubes of the
moisture vaporization system.
[If the analytical result is high]
Plastics that contain OH bases, such as polyvinyl alcohol, phenol resin and
polyvinyl acetate, may produce high readings if the heating temperature is
too high. It may be impossible to complete titration in some cases. Such
phenomena appear to be caused by the formation of water through heating.
Moisture content of urea resin cannot be measured normally at temperatures
above 120 � C because ammonia produced through the breakdown of urea
resins reacts with iodine. Melamine resins appear to cause interference in
the same way.
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