Quantitative polymer-additive analysis using Pyrolysis-GC/MS
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Abstract
In the automotive industry, as in most branches of modern life, polymers have belonged
to the most relevant material classes for several decades and continuously
grow more important every year. One of their most important properties for use
in everyday applications is their capability to resist oxidation. For most polymers,
especially the extremely widespread class of polyolefines, this property is mostly
dependent on stabilization by antioxidant additives. The analysis of this class of
additives as well as their antioxidative effect and influential factors thereof are the
focus of this study.
The main objective was the development of a quantitative analysis method employing
gas chromatography/mass spectrometry augmented with a pyrolysis module.
After optimizing pyrolysis time and temperature, a mass spectral library was
created for the additives used in the reference samples, made from polyamide-6 and
polypropylene. With this, single ion monitoring methods were developed, allowing
reproducible direct quantification of most additives within their polymer matrix.
The method’s efficacy could be proven while monitoring additive concentrations
during oven-aging experiments.
150 _C oven-aging experiments with subsequent mechanical tests on standard
polypropylene test specimens, which represent the state of the art in lifetime prediction
based on accelerated aging, were accompanied by oxidation induction time
measurements in a DSC instrument. A comparison served to evaluate the possibility
of performing lifetime predictions solely with information gathered from the
DSC/OIT measurements. It was shown that this is not possible due to different effective
stabilization temperatures of certain common antioxidant classes, leading to
varying orders of oxidation resistance at 200 _C compared to the 150 _C of the oven
test.
Finally, the influence of carbon black on the oxidation resistance of polypropylene
was examined using the same methods on test specimens with and without
the colorant. A reduced stability was observed for the compound containing carbon
black, which displayed an approximately 20% shorter induction period before
degrading. The focus of these projects lay solely on thermo-oxidation processes.