Life cycle assessment of bio-based synthetic fibers: the case of polyester substitutes

Numerous studies have shown that the textile sector has a high fossil-fuel dependency, emits excessive amount of greenhouse gases and uses vast amount of water. Currently, polyester (fossil-based) makes up about a half of the industry's fiber production and the volumes are expected to increase. Polyester fiber is traditionally made out of polyethylene terephthalate (PET) polymer. In order to break from the fossil-fuel dependency, textile industry has the possibility to use alternative feedstock for the polyester by reverting to bio-sourcing – strategy where the product's carbon backbone comes partially or fully from the bio-based feedstock. The polyester fiber can be entirely substituted in its current uses by three such bio-based substitutes: bio-based PET fiber as a drop-in substitute (chemically identical to the fossil counterpart); polytrimethylene terephthalate (PTT) fiber as an intra-family substitute with properties comparable to PET fiber and nylon; and polylactic acid (PLA) fiber as a novel fiber without a fossil counterpart and with properties comparable to PET fiber. All of these fibers are produced from the homonymous polymers via melt spinning and their carbon backbone comes partially or entirely from bio-based feedstock. The environmental effects of this feedstock substitution have not been studied abundantly, in spite of the existing political will to prioritize bio-based products, e.g. as the European Green Deal does. This project therefore performs a comparative, cradle-to-gate life cycle assessment of the conventional polyester fiber and these substitutes taking into account state-of-the-art production process. In order to do so Life cycle inventories (LCI) were modelled for 6 different scenarios for bio-based PET, 2 for bio-based PTT and 1 for PLA and were based on public literature and ecoInvent data. Impact assessment was performed by applying two methods – Swiss Ecological Scarcity and European Environmental Footprint, representative of the Swiss environmental policy and of a more scientific classification of environmental and health impacts respectively. The study finds that all three bio-based fibers are produced from the first-generation feedstock (crops). Currently, only the partially bio-based PET and PTT polymers are commercialized, such that the prevailing monomer, purified terephthalic acid (PTA) remains fossil-based for both. Then, monoethylene glycol (MEG) is produced from sugarcane or corn and 1,3-propanediol (PDO) from corn respectively. PLA is a fully bio-based polymer, produced from corn. Bio-sourcing for polyester is found to offer a limited improvement in a small number of impacts (with negligible contribution to the overall score) while causing substantial additional environmental burdens elsewhere (per kg of fiber). Namely, none of the bio-based alternatives perform better than the fossil-based PET fiber on the overall score. However, the partially bio-based PTT fiber may be a viable substitute for nylon (based on lower overall score). In the absence of carbon crediting for bio-based feedstock, climate impacts are found to be worse for the alternatives than the conventional polyester. Eutrophication, acidification, water scarcity impacts and land use rise with fiber's bio-content and largely stem for the agricultural practices (up to 88%, 74%, 97% and 100% respectively). It was also seen that PTA necessitates more precise modeling to raise the certainty of conclusions for the respective fully bio-based PET and PTT scenarios. At this stage, bio-based alternatives have a worse environmental performance than fossil-based polyester. The environmental performance of the bio-based fibers may be increased with alternative feedstock options or with improved agricultural practices.