Polytrimethylene terephthalate (PTT) is an aromatic polyester produced from 1,3-propanediol (PDO) and terephthalic acid (PTA) via polycondensation, similar to the way commercial polyester is produced from ethylene glycol and PTA. PTT was first patented in 1941, but was not considered a marketable product until a low cost route to high-quality PDO could be developed, which has now been accomplished by Shell Chemical. The polymerization chemistry involves 2 major reaction steps. The first step is the direct esterification of terephthalic acid (PTA) to bis-hydroxypropyl terephthalate (bis-HPT). The second step in the polymerization sequence is the polycondensation of bis-HPT. 
The reaction temperature must be above the melting point of the polymer (228°C) and temperatures between 250°C and 267°C are favored for polycondensation. The removal of glycol vapors (under vacuum) drives the equilibrium toward polycondensation. Because the diffusion rate of by-product propanediol from the molten polymer is rate limiting near the end of the reaction, the propanediol must be separated as quickly as possible. This is accomplished by high vacuum and by mixing the melt so as to continuously expose a large amount of surface. The time of the reaction is at least two hours. By-products produced because of the high temperatures involved and/or the high shear stresses of the process include acrolein and ally alcohol. Polytrimethylene terephthalate (PTT) has properties that make it applicable to three major end-use markets:
- Carpet (predominantly as a substitute for nylon)
- Textiles (predominantly in the high value market as a substitute for nylon and polyester)
- Engineering thermoplastics
The polymer has a chemical structure between that of polyethylene terephthalate (PET) and polybutylene terephthalate (PBT); it is targeted initially as a replacement for nylon in textile and carpet markets for staple and continuous filament products. PTT, trademarked Corterra™ by Shell Chemical and Sorona™ by DuPont, is said to have the chemical resistance of PET and the elastic recovery and resilience of nylon; it therefore combines the best properties of these two most popular textile and carpet polymers, as shown in the table below.FIBER PROPERTIES| Property | PTT | PET | Nylon 6 | Nylon 6,6 |
|---|
| Tenacity (strength) (g/denier) | 3.15 as measured so far | 2.8-6.0 | 4.6-8.8 | 4.6-8.8 | | Loop Tenacity (g/denier) | * | 2-5 | 6-8 | 6-8 | | Degree of fibrillation | None | None | None | None | | Water Absorption (% by wt) - 24 hours | 0.03 | 0.09 | 1.9 | 2.8 | | Water Absorption (% by wt) - 14 days | 0.15 | 0.49 | 9.5 | 8.9 | | Flammability | Low | Low | Low | Low | | (Oxygen index, LOI) | Good rating | 20.6 | 20.1 | | | Elastic Recovery (stretch)at 5% elongation | 99-100% | 75-80% | 99-100% | 99-100% | | Melting point (°C) | 228 | 265 | 220 | 265 | | Glass transition temperature (T(g)°C) | 45-65 | 80 | 40-87 | 50-90 | | Density (g/cm3) | 1.33 | 1.40 | 1.13 | 1.14 |
Source: Shell and DRA from trade sourcesThis new report assesses the economics of PTT production with special emphasis given to the differences of Shell's petrochemical and DuPont's biotech routes to 1,3-PDO.
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