# Bio-Based Polyester Fiber Substitutes: From GWP to a More Comprehensive Environmental Analysis
## 2 Materials and Methods
Fossil-based PDO used to be produced from ethylene via the Shell process [43; 44]; however, such a monomer is no longer commercially available due to the high costs of production [21]. For the production of bio-based PDO, only one fully commercialized route exists, i.e., the fermentation of corn-derived glucose for a direct production of bio-PDO using genetically engineered _E. Coli_[14, 19, 20, 44]. Corn cultivation and glucose extraction via starch hydrolysis therefore precede bacterial fermentation [19, 20] like in all other corn-based monomers. Last but not least, PLA is a polymer with a 100% bio-based carbon content [26, 45]. From the point of view of life sciences, PLA is a biopolymer, hence biodegradable [45, 46]. As it does not have a fossil-based counterpart, it is classified as a novel/dedicated polymer [10] and a new generic fiber [4]. PLA is predominantly produced in ring-opening polymerization of lactides [47], used by NatureWorks in the USA, which is currently its most notable producer [26, 48]. Here, corn-derived glucose is fermented into crude lactic acid by diverse bacteria; several continuous extraction and purification steps are applied to obtain an aqueous polymer-grade lactic acid [26]. The acid is transformed into lactides followed by an open ring polymerization and pelletization by which PLA granulate is obtained [26, 47]. As a final step, conventional polyester fibers are produced by melt spinning the PET granulate. Melt spinning is a thermo-mechanical, solvent-free process, possible only if the decomposition temperature of the polymer is above its melting point [12], which yields continuous or staple fiber (cut into predetermined length for a cotton-like fiber). Melt spinning can equally be applied on PTT and PLA polymers [13, 45]. Based on the state-of-the-art production processes, and to account for different levels of bio-based content and feedstock, a total of seven different scenarios for PET fiber, two for PTT fiber and one for PLA fiber are modeled here as presented in Figure 1. Scenario 0 is the baseline scenario for fossil-based polyester; Scenarios 1, 2A, 2B combine the bio  
Figure 1: The production pathways of polyester and its bio-based substitutes. Polymers are transformed into fibers via melt spinning. Note that in this graph grey stands for fossil-based, green for bio-based and orange for partially bio-based products. Scenarios refer to the system setup in modeling. Scenario 1 contains corn-based monoethylene glycol (MEG); MEG in Scenario 2A and 2B stems from sugarcane, whereas the ethanol production processes differ between the two. Same explanations apply for Scenarios 3, 4A and 4B, respectively. (MEG: monoethylene glycol; PTA: purified terephthalalic acid; PET: polyethylene terephthalate; PDO: 1,3-propanediol; PTT: polytrimethylene terephthalalate; PLA: polylactic acid).) MEG and fossil-PTA; Scenarios 3, 4A, 4B combine the bio-MEG and bio-PTA; Scenario 5 combines bio-PDO and fossil-PTA; Scenario 6 combines bio-PDO and bio-PTA; Scenario 7 refers to PLA. Firstly, fossil-based MEG (Scenario 0) is derived from ethylene in existing production facilities around the world. All the steps from ethylene to MEG happen on one site. Its global production and transport are modeled using ecoinvent datasets without modifications. Further, three versions of bio-based MEG are modeled:  
* Bio-MEG derived from corn grain in the USA. The entire production from ethanol to MEG is assumed to happen on one production site. Ethanol is obtained using dry milling technology (for Scenarios 1 and 3). * Bio-MEG derived from sugarcane in Brazil. The ethanol is obtained in an autonomous plant in the direct fermentation of sugarcane, where bagasse is incinerated for energy recovery (for Scenarios 2A and 4A). * Bio-MEG derived from sugarcane in Brazil. The ethanol is obtained in an annexed plant through the fermentation of sugar molasses from the crystal sugar production (for Scenarios 2B and 4B). In all three versions, the dehydration of ethanol into ethylene is assumed based on [18; 40].