As a result of its low permeability to moisture and gas, good mechanical properties, and high heat resistance, poly(ethylene terephthalate) (PET) is used extensively throughout the textiles and packaging industries.1 One common application of PET is in polyester fabrics. However, in such products, a flame retardant (FR) functionality must be incorporated into the plastic for safety reasons, extending the time of escape should a fire occur.
Despite their life-saving capabilities, PET/FR products pose a significant challenge to the plastic recycling industry as they are not easily degraded and repurposed; the flame-retardant additives alter the recyclability of pure PET. As a result, most products end up as solid plastic waste and accumulate in landfills.2 Thus, to uphold a sustainable circular economy, it is essential to research and develop novel methods to improve the recycling process of PET with flame retardant characteristics.
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Degradation Properties of PET with Eco-friendly Flame Retardants
As part of the transition to recyclable PET, attention has been on developing environmentally friendly flame retardants for use in plastics. The result of such research has led to the search for nontoxic biobased sources of these additives,3 such as retardants made from eggshell waste, which is high in minerals, including phosphorus.3 Phosphorus-based flame retardants are of particular interest as these additives have been found to both enhance the structure and stability of PET during repeated thermomechanical recycling processes and lower the mechanical energy and temperature required to degrade and reform the product.4
A recent collaboration between the Swiss Federal Laboratories for Materials Science and Technology and the University of Leoben has pioneered the search for such alternatives. The researchers performed a variety of thermomechanical and chemical experiments to test the degradation mechanisms of PET containing two types of phosphorus flame retardants: DOPO-PEPA (DP) and Aflammit PCO 900 (AF).2 The main goal was to better understand how to improve the thermomechanical recycling of PET products with flame retardants.
In addition to a variety of analyses and testing of PET structure and stability, NMR spectroscopy was performed to explore interactions during repeated heating cycles at very high temperatures. They obtained 31P{1H} NMR spectra of samples at ambient temperature using a Bruker AV- III400 spectrometer (Bruker Biospin AG, Switzerland).2
Improving Recyclability of Flame-resistant PET Products for a Circular Economy
The study enabled the researchers to identify changes in the degradation mechanisms of PET with DP and PET with AF via temperature and mechanical stress during recycling.2 In particular, the team observed changes that affected the melting behavior and degradation of the flame-resistant plastic materials.
On the one hand, the DP additive was found to reduce the viscosity of PET, improve the structural stability of the plastic for repeated recycling, and lower the temperature of processing via a lubrication effect.2 On the other, the AF additive increased the viscosity of PET, became highly brittle and substandard for repeated recycling, and altered the structural branching behavior that could, in turn, beneficially shift melting temperatures down.2
Thus, a combination of the two qualities of DP and AF as flame retardant additives to PET could optimize the thermomechanical recycling process of plastics. These findings are highly encouraging when it comes to designing recyclable PET/FR products from novel eco-friendly phosphorus chemistry.2
With this information in mind, researchers will now be able to embark on the design of PET products with biobased flame retardants that can be optimally recycled via a thermomechanical recycling process. The study also supports the development of an improved thermomechanical recycling process that takes into account these beneficial compound characteristics, ultimately contributing to the sustainable circular economy.
References and Further Reading
- Venkatachalam, S., et al. “Degradation and Recyclability of Poly (Ethylene Terephthalate).” Polyester, Sept. 2012, https://doi.org/10.5772/48612. Accessed 13 May 2023.
- Bascucci, Christopher, et al. “Investigating Thermomechanical Recycling of Poly(Ethylene Terephthalate) Containing Phosphorus Flame Retardants.” Polymer Degradation and Stability, vol. 195, Jan. 2022, p. 109783, https://doi.org/10.1016/j.polymdegradstab.2021.109783. Accessed 13 May 2023.
- Rhoda Afriyie Mensah, et al. A Review of Sustainable and Environment-Friendly Flame Retardants Used in Plastics. Feb. 2022, pp. 107511–11, https://doi.org/10.1016/j.polymertesting.2022.107511. Accessed 16 May 2023.
- Salmeia, Khalifah A., et al. “Comprehensive Study on Flame Retardant Polyesters from Phosphorus Additives.” Polymer Degradation and Stability, vol. 155, Sept. 2018, pp. 22–34, https://doi.org/10.1016/j.polymdegradstab.2018.07.006. Accessed 13 May 2023.
This information has been sourced, reviewed and adapted from materials provided by Bruker BioSpin - NMR, EPR and Imaging.
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