Effect of Mineral Fillers on the Pyrolysis of Polystyrene

Objective
Many consumer plastics contain minerals as functional fillers to improve processability, reduce costs, or adjust physical properties [1]. Plastic recycling can be categorized into two main types: mechanical and chemical recycling. While mechanical recycling requires pure polymers, chemical recycling methods such as pyrolysis can tolerate impurities, including mineral fillers and are therefore better suited for material recovery [2]. However, the influence of mineral fillers on pyrolysis has so far received only limited systematic investigation.
This study focuses on the pyrolysis of polystyrene (PS) in the presence of minerals. Most PS is used in building insulation, where mineral contaminations are predominant due to fillers and adherences. As PS depolymerizes during pyrolysis, one of the main products is styrene, the monomer of PS, which therefore is a valuable product [3,4]. To provide a recycling route for this waste stream, this study describes the decomposition behavior of PS/mineral composites or mixtures. The focus is on the decomposition temperature, complemented by the semi-quantitative composition of the primary pyrolysis products of different composites.
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Materials and Methods
Filled plastic samples in the form of binary mixtures of PS and one mineral filler were prepared by extrusion. The investigated fillers are calcite (CaCO3), portlandite (Ca(OH)2), anhydrite (CaSO4), and anatase (TiO2). Both the composites and the individual components were characterized by X-ray diffractometry (XRD).
To investigate the effect of the mineral fillers on PS decomposition, thermogravimetric analysis (TGA) was performed on both the individual components and the prepared binary composites. Weighted averages of the pure PS and each mineral filler were calculated, providing the baseline against which the mixtures were evaluated. Shifts in the decomposition temperature or changes in the TGA profile are interpreted as evidence of interactions between the polymer and the fillers.
The effect of mineral fillers on styrene yield during the pyrolysis was likewise studied. Primary pyrolysis products were identified using a microgram-pyrolyzer directly coupled to gas chromatography-mass spectrometry (GC-MS). Here, 100 mg samples are heated almost instantaneously in a Helium atmosphere. The released pyrolysis vapors are immediately directed into the GC-MS, allowing for the separation and identification of components. The semi-quantitative evaluation of product spectra allows assessment of the influence of the mineral filler type on styrene yield.
Results and discussion
TGA results show that not all mineral fillers behave similarly. The presence of anatase appears not to influence the decomposition rate of PS, and leads to TGA profiles that closely align with those of pure PS. Calcite and anhydrite, on the other hand, show a shift to 15°C higher decomposition temperatures of PS. In the presence of portlandite, the decomposition of PS occurs at essentially the same average temperature as in pure PS, but within a broader temperature interval.
GC-MS analysis of primary pyrolysis products indicates that styrene monomer, dimer, and trimer are the predominant primary products of PS pyrolysis, consistent with the literature [4,5]. This observation also holds for the binary samples containing mineral fillers. However, although styrene monomer, dimer, and trimer are the dominant products, their relative composition shows minor variations, which cannot be conclusively explained and are subject to further investigation.
A comparison of TGA and GC-MS analyses indicates that anatase does not affect PS pyrolysis. The results of portlandite lead to a similar conclusion, even though TGA shows a broader decomposition interval. With calcite and anhydrite, the deviation in decomposition shown using TGA is not evident using GC-MS analyses. Only minor differences in the product composition are detected with the minerals present. Conclusively, calcite and anhydrite seem to stabilize the PS polymer chain and suppress its scission, leading to a higher decomposition temperature. However, calcite and anhydrite seem not to influence specific mechanisms that would lead to a different primary product composition. The role that the investigated minerals play in promoting or inhibiting certain decomposition mechanism paths is the subject of further research.
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