# Bio-Based Polyester Fiber Substitutes: From GWP to a More Comprehensive Environmental Analysis
## 1 Introduction
The industry has known PTT fiber for a long time (i.e., it was discovered in 1941 as a fossil-based fiber), but the production cost of one of its two monomers rendered it too expensive for any mass-market application [12; 13; 14]. When a biotechnology solution for producing a bio-based version of this monomer emerged, PTT, now in its bio-based form, was put back into consideration due to favorable economic factors, i.e., smart drop-in substitution. Finally, PLA fiber is a representative of novel products, with a \"dedicated\" production pathway [10] and no fossil-based counterpart. It is made of a homonymous polymer. It may be misleading to think that bio-based products are immune to the use of fossil resources since the approach concerns only the product's mass without any other particular modification in the supply chain (e.g., energy production, transportation). Bio-sourcing is not without risks either. Competition between food supply and biomass production, the over-exploitation of natural resources, and the loss of biodiversity are just a few of the possible undesired effects related to this strategy [15]. Whatever the case may be, whether bio-sourcing is a good solution for synthetics or not, the actual impacts of polyester substitution have not been studied abundantly to provide a conclusion. In order to obtain a more adequate and complete picture of the environmental implications of such a solution, the life cycle assessment (LCA) methodology is applied since it accounts for the aggregate environmental impacts of a product [16]. All the resource consumption and environmental emissions from feedstock (cradle) to fiber (factory gate) are thereby accounted for. Thus far, published LCAs are in fact only indirectly related to polyester because its underlying polymer (PET) is well established in the plastics and pack aging sectors, driving bio-based innovation. As to our knowledge, there are no published LCAs on PLA or PTT fibers available either. Thus far, published studies address bio-based PET and PTT monomers [17; 18; 19; 20; 21; 22], bio-based PET polymer [23; 24; 25; 26; 27] or bio-based PET and PLA bottles [28; 29; 30; 31]. While they act as very valuable proxies for an impact assessment and a degree of overlap indeed exists between the polymers and fibers, further comparison to other synthetic or natural fibers is not possible if studies stay on the lower product levels. Recent reviews found that LCA studies related to bio-based synthetics through the aforementioned proxies do not follow the same methodology (impact assessment method, inclusion of transport, etc. ), have vastly different scopes (cradle-to-gate, cradle-to-grave, variant) and functional units, and are often fully aggregated and thus not entirely transparent [32; 33]. Further, existing studies are found to focus mainly on the global warming potential (GWP) or fossil resource depletion, largely ignoring other environmental impacts [34]. In fact, the goal of the only paper related to bio-based polyester fibers (including PET and PLA polymer) was to study GHG emissions and non-renewable energy use assuming incineration at the end of life [35]. On the topic of decreased climate impacts of bio-based products, [PERSON] and [PERSON] [32] found that present LCAs have been favoring bio-based polymers by applying carbon crediting for the uptake of carbon (C) as carbon dioxide (CO\({}_{2}\)) from the air during feedstock growth. In the absence of immediate photosynthesis, a carbon credit is not applied to the fossil-based products since the method is founded on the so-called short C cycle (the uptake of the non-fossilized C from air). They point that carbon crediting is a \"_controversial and potentially misleading method when used in cradle-to-gate studies where the end of life emission of CO\({}_{2}\) is not included_\". If the end of life (EOL) is included, studies are found to report much higher GWP results for polymers than those which limit the scope to granulate, i.e., to the gate [34].