# Bio-Based Polyester Fiber Substitutes: From GWP to a More Comprehensive Environmental Analysis
## 2 Materials and Methods
On the other hand, the main raw material for PTA, para-xylene, is obtained cheaply and in large quantities from the crude oil's aromatic stream [17; 24]. Para-xylene (PX) is subject to catalytic partial oxidation, after which the crude form of treephthalic acid is purified [17; 18]. This process, also known as the Amoco process, is currently the leading technology for PTA production on a global scale with very high process efficiency parameters [17]. Reaction 1 does not depend on the origin of the monomers, and thus equally applies to the partially and fully bio-based PET production as well. Partial bio-content means that only one of the two monomers is derived from bio-based feedstock, while the other is taken from fossil resources. Monomers alone always stem from just one feedstock, making them either fully bio- or fossil-based. The bio-based C content of a polymer is then calculated as the weighted average of the C contents of the two monomers. Namely, as Reaction 1 demonstrates, MEG accounts for 20% of C content of PET, while the remaining 80% comes from PTA. Partially bio-based PET consists of bio-based MEG and fossil-based PTA, and therefore has 20% bio-based C content; the reverse combination of monomers is not known as of yet. PET can also be 100% bio-based when both monomers are derived from biogenic sources. In the case of bio-based MEG, instead of sourcing ethylene from the natural gas, it is obtained indirectly through bio-ethanol (see Figure 1). Bio-ethanol is industrially produced in the microbial fermentation of natural sugars present in sucrose (e.g., sugarcane) and starchy (e.g., corn) biomass, notably in the United States of America (USA) and Brazil [35; 38]. The USA uses corn to produce 63% of the global bio-ethanol volumes, while Brazil uses sugarcane and accounts for 24% [35; 38]. Second-generation feedstock for bio-ethanol production is yet to be commercialized [38] and as such is far from relevant industrial-scale production. Corn-based production predominantly uses the dry milling technology [39] in which starch is extracted from the corn grain and separated into glucose units. This simple sugar is then fermented and ethanol is distilled. Conversely, ethanol from sugarcane can be obtained in two modes--in direct fermentation of raw sugarcane with cogeneration (energy recovery) of lingo-cellulosic by-products [24]; or via fermentation of molasses, a by-product in sugar mills, where crystal sugar is extracted for human consumption. Regardless of the exact production method of ethanol, this compound is dehydrated into ethylene [40]. All the subsequent steps, downstream of ethylene, then mimic those of the petrochemical chain of MEG (see Figure 1). New pathways to produce PTA from diverse non-fossil sources have already been considered in the literature and practice [41]. The only industrial-scale production of PTA to date involves bio-sourced iso-butanol, which is converted into bio-PX using the Gevo process in a bio-refinery in the USA [17; 18]. Its production also requires the extraction of glucose from corn, followed by its fermentation into iso-butanol, conversion into iso-butylene, and several intermediary steps to arrive at the bio-PX [42]. Finally, the bio-based PX is converted into PTA using the Amoco process, just as for the conventional PX. PTT polymer is produced in an esterification reaction between 1,3-propanediol (PDO) and PTA whose conditions are similar to PET polymerization [12; 13]. Monomers are again synthesized separately and brought together during the polymerization. As attested by Reaction 2, PDO accounts for 27% of C in PTT. Depending on whether one or either monomer is bio-based, PTT can therefore have a bio-based C content of 27% (partially bio-based) or 100% (fully bio-based). Unlike PET, where the production of bio-MEG implies a change for the intermediary product, in PTT, fossil- and bio-based PDO have vastly different production pathways (see Figure 1).