The Influence of Reprocessing on
the Structure-Property Characteristics of a Plasticised Poly(vinyl chloride)
C. Dehennau and
Synopsis: We have conducted a detailed investigation of the
change in deformational, structural, molecular and rheological characteristics
that are attendant on a PVC-p compound experiencing higher processing
temperatures. Our study reflects an experience that occurs in the PVC
industry and for which a loss of processing performance accompanies
the excursions to higher temperatures. It is apparent that as this process
continues the material will eventually become unprocessable, unless
changes are made.
The change that accompanies the material as it passes from an experience
of low processing temperatures to high processing temperatures has been
thoroughly mapped. However, it has been difficult to make precise observation
of the molecular and structural changes occurring. This has not been
without application of a number of likely candidate methodologies, including
DSC, TGA, WAXS, GPC and fluorescence spectroscopy. It would seem that
some additional crystallization is accompanying the increase in gelation
that occurs at higher processing temperatures. At the same time, the
observations on the influence of annealing hint that this is not the
only consideration and that diffusion of the plasticiser into the PVC
particles might also be playing a role or even residual stresses in
the materials might be important.
This study has not only been concerned with an in depth materials characterisation;
it has also explored ways of alleviating the loss of processability.
Two particular lines of study involving pre-shear history and blending
with higher plasticised compounds have provided interest albeit inconclusive
results. On the other hand, there are strong implications from the DSC
study that reversibility of the "process aging" is not likely.
In conclusion, it is interesting to observe that by conducting this
study through an IUPAC Working Party it has been helpful to pool the
resources and experience of an international community of scientists
which has likely given us an opportunity of conducting this study at
much lower costs than might have otherwise been the case.
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