# Bio-Based Polyester Fiber Substitutes: From GWP to a More Comprehensive Environmental Analysis
## 1 Introduction  
Numerous studies from public agencies, the scientific community and businesses have shown that the current practices throughout the textile supply chain go against the sector's long-term sustainability. Business-as-usual in this resource-intensive sector involves a very high fossil-fuel dependency, produces excessive greenhouse gas (GHG) emissions, uses high quantities of water, and causes substantial water pollution and other burdens to the environment [1, 2, 3]. A major component in the textile industry is fibers. As intermediate products, they are a common denominator for all subsequent products such as yarn or garments. On the fiber market, polyester fiber accounts for little over half of the entire annual global fiber production [4]. In the textile industry, polyester conventionally refers to a fiber produced by melt spinning the amorphous polyethylene terephthalalate (PET) polymer, even though polymer science groups many compounds, including PET, into polyesters. Hence, it is a fossil-based product, where the entirety of the embedded carbon (C) comes from thefossil-based feedstock. In order to mitigate the environmental impacts of its products, to decouple from the dependency on fossil resources, and to meet the sustainability-minded consumer trends, the textile industry needs to rethink its current supply chains. At the same time, \"_the ever-increasing consumption of textiles and clothing is only expected to intensify the social and environmental issues_\" associated with their production [5]. Bio-sourcing is a solution, which supposedly has an effect on all the aforementioned environmental impacts. It is particularly relevant for polyester fiber due to fiber's market dominance. Bio-sourcing relies on feedstock diversification, whereby so-called renewable or bio-based feedstock such as crops (edible, first-generation) or lignocellulosic mass (non-edible, second-generation) displaces the conventional fossil-based inputs [6; 7]. In short, bio-sourcing rests on the idea that changing the provenance of C in the product's backbone results in a decrease in the environmental impacts of the said product. The share of total carbon in the product coming from the bio-based feedstock is expressed as % of bio-based C content, ranging from 0% bio-C (in case of fossil-based polyester) to 100% bio-C (when the polyester is entirely produced out of bio-based feedstock), with \"partially bio-based\" capturing everything in between. Due to the current lack of standardization in the bio-based field, both entirely and partially bio-based products are simply called \"bio-based\". Moreover, the European Commission (EC) particularly supports this strategy with its recent European Green Deal [8]. Through these activities, the EC aims to stimulate the development of new markets for climate-neutral and circular products, especially in resource-intensive sectors, such as textiles and plastics (with relevant overlaps in polymer production) [8]. In this light, polyester fibers could largely be substituted by three bio-based alternatives, minding the necessary functionalities of the fiber in different applications [9]. By accounting for the technical substitution potential of the total polyester put on the market, [PERSON] et al. [9] report that 65% of polyester can be substituted by bio-polyester, 20% by polytrimethylene terephthalate (PTT) fibers, and 10% by polylactic acid (PLA) fibers. In the first case, the substitute fiber is chemically identical to polyester, but the production chain has been revised and slightly modified to accommodate bio-sourcing for the two underlying PET monomers. Such an approach is referred to as drop-in substitution [10]. Secondly, PTT fiber, also known as Triexta (with the Federal Trade Commissions in the United States), is a fiber made out of the homonymous polymer with favorable material properties in substituting the conventional polyester fiber [11].