Abstract:
In most of polymer ageing studies conducted at laboratory, there is a trend to
underestimate some of the parameters involved in natural exposure, leading to poor
simulation results and low reliability of the real world phenomena. Moreover, the nature
of polymer and its formulation should be taken into account due to possible synergies
and antagonisms between environment and polymer additives. Among the
underestimated parameters, we can stress the role of ambient humidity and inorganic
fillers. These additives are usually considered to be inert regarding the degradation
induced by radiation.
In this work, four different PVC formulations, designed for outdoor applications,
in the form of calendered films were submitted to artificial accelerated ageing under
different radiation sources, without and with spray of water, and to natural exposure.
The influence of the different photo-oxidation conditions was evaluated by means of Xray
photoelectron spectroscopy (XPS) and infrared spectroscopy (FTIRS).
The photo-oxidation of PVC can be studied by FTIR measurements following the
formation of oxidation products of carbonyl type, due to radical attack to the double
bonds in the molecular structure of PVC, enhanced by radiation and temperature.
Results have shown that relative rates of PVC degradation reactions are different under
different photo-oxidative conditions. XPS results show that, in technical PVC
formulations, the fraction of chloride ion trapped in the inorganic filler is a better
parameter to measure the PVC degradation (measured by the yellowing index variation)
than the amount of total remaining (or lost) chlorine in the surface. When the ageing
method includes water jets, the top superficial layer made from the inorganic filler and
the organic products from the PVC degradation is removed and leaves behind a much
more homogeneous layer which is more resistant towards ageing than the “as
processed” one that contains defects introduced by the calendering process. XPS also
makes clear that zinc-calcium additive is a better thermal stabilizer than the dibutyltinmaleate
and the presence of the tinuvin P retards the PVC degradation. The best
simulation of the natural weathering of such PVC materials in laboratory conditions
needs to consider, besides the light, the intervention of water jets and/or humidity as
well as the existence of light and dark periods.
