Characterization of impurities in recycled polystyrene

Problem:
The recycled high-impact polystyrene (HIPS) does not meet the required mechanical properties, especially with regard to impact resistance.

Analysis and results:
Two thermal analysis methods were used to identify and quantify foreign substances:

1. Differential scanning calorimetry (DSC):
   • Detection of foreign polymers in the recyclate.
   • Melt transitions of HDPE (0,7%) and PP (4,2%) could be identified.
2. Thermogravimetry (TGA):
   • Determination of inorganic components in the material.
   • An inorganic content of 7,2% was detected and identified by FTIR spectroscopy as chalk (CaCO₃), which serves as a filler.

Conclusion:
The recyclate contains foreign substances that have a negative effect on the mechanical properties.

Recommended measures:

• Monitoring the filler content in the recyclate, as inorganic fillers above 3 m-% are not tolerated.
• Acceptance of admixtures of PE and PP up to 20 m-%.
• Further investigation of the impact strength depending on the foreign matter and filler content using a notched bar impact pendulum.

Figures: DSC curve (left) and TGA curve (right), each with reference material (blue).