Direct analysis of polypropylene compounds by thermal desorption and pyrolysis—mass spectrometry

Abstract

This report describes the mass spectrometric identification of components (particularly residual volatile chemicals, organic additives, and degradation products) in five polypropylene compounds of known composition. Programmed direct probe heating of the compounded polymer was carried out over the range 20–400°C. Survey mass spectra were acquired with three different methods of ionization: 70 eV electron impact (EI-MS), isobutane chemical ionization (CI-MS), and field ionization (FI-MS). While this general analytical procedure may be referred to as pyrolysis—mass spectrometry (Py-MS), it should be understood that both thermal desorption and thermal decomposition are taking place. For the polypropylene compounds, residual volatile chemicals and most organic additives were thermally desorbed at lower temperatures (less than ≈300°C), while polymeric components were thermally decomposed (pyrolyzed) at higher temperatures (greater than ≈300°C). It was found that field ionization gives a very useful overview of the composition of the evolved chemicals, since essentially all of the ions observed are molecular ions for the various components. In some cases, tandem mass spectrometry (MS/MS) and/or high resolution mass measurements were carried out (in EI or CI mode) to obtain detailed chemical structure information. Pyrolysis products from polypropylene were observed at every carbon number to masses well above 1000 Da, and the volatile pyrolyzates were interpreted in terms of free radical decomposition mechanisms. Overall, the direct mass spectral analysis of the polypropylene compounds was found to be a very effective means for detection and identification of various organic additives and polymeric components.