Abstract
Polypropylene and polystyrene are petroleum-based thermoplastics which are commonly used and disposed of in the environment after their service life, leading to environmental degradation. There is a need to recycle polypropylene and polystyrene, but the effect of recycling on thermo-mechanical properties is not well understood. This study aims to determine thermo-mechanical properties of the recycled polypropylene and recycled polystyrene and compare them with corresponding virgin polypropylene and newly produced polystyrene (general purpose polystyrene 1540 and high impact polystyrene 7240). The study was carried out by preparing bar-shaped samples of recycled polypropylene, recycled polystyrene, general purpose polystyrene 1540, and high impact polystyrene 7240 by compression molding using a hot press and thermally characterizing them to determine glass transition temperature and melting temperature using differential scanning calorimetry. The changes in Young’s modulus, tensile strength, hardness, and toughness due to recycling activities were determined at room temperature (24 °C), 40 °C, 60 °C, and 80 °C. The thermo-mechanical properties of recycled polystyrene (PS) were found to be comparable to those of high impact polystyrene (HIPS) 7240. The study revealed that the hardness and toughness for the recycled polymers were higher than those of corresponding virgin polymers. On the other hand, tensile strength and Young’s modulus for the recycled polymers were lower than those of the virgin polymers. Understanding the thermo-mechanical properties of the recycled polymers will contribute to more industrial applications hence increase the rate of recycling, resulting in a reduction in environmental pollution.
Highlights
Polymers are categorized into thermoplastics, thermosets, and elastomers according to their thermal and mechanical properties
The thermal behavior of the recycled polymers and virgin polymers were probed via differential scanning calorimetry (DSC) to determine glass transition temperature
The results further showed that toughness for recycled PP was higher than that of virgin PP by 11.0–215.7%, which was similar to the results obtained by Bourmaud et al [47] in which it was shown that impact energy increased as the number of injection cycles increased
Summary
Polymers are categorized into thermoplastics, thermosets, and elastomers according to their thermal and mechanical properties. Scientists prefer polymer thermoplastics more than thermosets because of their low production cycle, lower processing costs, and high thermoplastic reparability [1,2]. Global polymer production has increased enormously to more than 330 million tons over the last five decades because polymers are widely used in industry [3,4,5,6]. In 2015, there were around 322 million tons produced worldwide, which was about 90 times higher than polymers produced in 1960. It was reported that in the European Union, 58 million tons of polymers were produced and utilized, and only around 30% were recycled [3,4]. Low density, Recycling 2019, 4, 33; doi:10.3390/recycling4030033 www.mdpi.com/journal/recycling
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