Researchers investigate commonplace polymers with an eye to the microplastics crisis
Polyolefins are the most common class of polymers produced globally. Used mainly for single-use applications like food containers and disposable utensils, polyolefins are extremely slow to biodegrade, which puts them at the centre of a major environmental crisis. This has led to a growing interest in biodegradable polyolefin polymers, and the mechanisms behind that process; namely abiotic oxidation, which breaks down the polymers into low molecular-weight products, and biodegradation. Additives, typically based on transition metals, are known to promote this degradation, but the catalytic activity of these pro-oxidants has not been widely reported.
Writing in the latest issue of Polymer [DOI: 10.1016/j.polymer.2022.125455], a group of Japanese researchers have examined the effects of commercially available pro-oxidants on the photo-oxidation of four polyolefin films: high-density PE (HDPE), low-density PE (LDPE), linear low-density PE (LLDPE), and isotactic PP (itPP).
Neat samples of the two chosen additives (Green20 and PP20), described as having “100% biodegradation, and their safety complies with the United States food and drug administration,” were prepared, and acted as comparisons throughout the study. Reference samples of non-oriented films of HDPE, LDPE, LLDPE, and itPP were also prepared, by hot pressing and compression moulding. Finally, additive-blended films were prepared, with a thickness range of 130–180μm.
Test samples were artificially aged in a UV irradiation test chamber for up to 4 weeks, with settings designed to approximate 1-year UV exposure in the Pacific Ocean coastal regions of Honshu, Japan. The original and UV-aged film samples were then subjected to SEM imaging, as well as IR spectroscopy, differential scanning calorimetry (DSC), and both wide- and small-angle X-ray scattering (WAXS and SAXS) experiments.
The UV exposure test led to significant changes in the surface morphology of both neat and additive-containing polymers. Scraping, abrasion, crazing and material loss were all observed on the surface of HDPE, LDPE, and LLDPE. However, itPP’s degradation extended beyond the surface – random cracks and embrittlement were seen throughout that sample. The authors attribute this to the tertiary carbon atoms in the itPP chains, which they say “are more susceptible to abiotic attack than the secondary carbons in the PE backbone.”
IR spectroscopy showed the increasing presence of various oxidative degradation products in both neat and additive-containing HDPE, though the extent of oxidation was significantly greater in the presence of additives. However, both neat and additive-blended itPP samples were observed to degrade similarly, regardless of the presence of pro-degradation additives. The authors say that the stability of polyolefins towards photooxidation is strongly dependent on their molecular structures, particularly the number of tertiary carbons present. As such, they suggest that the hierarchy in oxidation susceptibility of polyolefins is as follows: itPP > LDPE > LLDPE > HDPE
When investigating the melting temperature of their samples, the researchers found that UV ageing had a significant impact on itPP – its melting temperature decreased from 164 to 137°C. Those of HDPE, LDPE, and LLDPE exhibited only slight changes. X-ray scattering tests showed that all the samples exhibited an increase in crystallinity after the oxidative degradation, whereas the crystal structures remained unchanged. This suggests that degradation of these polymers occurred primarily in the amorphous phases.
The authors caution that while these pro-oxidants seem to enhance the degradation of the polyolefin samples, the degraded polymer fragments could “contribute to environmental microplastics if bio-assimilation is not achieved in a reasonable time frame.”
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A. Padermshoke, T. Kajiwara, Y. An, M. Takigawa, T. Van Nguyen, H. Masunaga, Y. Kobayashi, H. Ito, S. Sasaki, A. Takahara. “Characterization of photo-oxidative degradation process of polyolefins containing oxo-biodegradable additives,” Polymer 262 (2022) 125455. DOI: 10.1016/j.polymer.2022.125455