Poly(Trimethylene Terephthalate): A “New” Type of Polyester Fiber by Houck, Huff, Lowe, and Menold (Forensic Science Communications, July 2001)
July 2001 - Volume 3 - Number 3
Research and Technology
Poly(Trimethylene Terephthalate): A “New” Type of Polyester Fiber
Max M. Houck
Trace Evidence Unit
Rebecca A. Huff
Preston C. Lowe
Ronald E. Menold
(formerly Chemist, Chemistry Unit)
Federal Bureau of Investigation
Shell Chemicals is producing a new type of polyester (Shell Chemicals is the name used to denote the chemical businesses of the Royal Dutch/Shell Group of Companies). This polymer is being used for fibers in the residential and industrial carpet markets. The fiber is poly(trimethylene terephthalate) (PTT) and has the tradename Corterra®. These fibers have a number of traits that lend themselves to carpet products. Shell expected to produce 400 million pounds of Corterra® polymers for the general marketplace by the end of the third quarter of 1999, and thus, it will begin to appear as evidence in cases.
PTT fibers have many similarities to, but some important differences from, the more common polyester, poly(ethylene terephthalate) (PET) fibers. The history of PTT, its optical and instrumental characteristics, and data from known samples are presented in this technical note.
Poly(trimethylene terephthalate) (PTT) was first synthesized and patented in 1941 (Whinfield and Dickson 1941), but it was not produced commercially because of the expense of one of the precursors, 1, 3-propanediol (PDO; Chuah et al.1995A). The production of PDO was halted in the mid-1960s, and ethylene oxide (EO) hydroformylation was developed as an alternative. In the early 1990s, hydroformylation catalysts were created to allow for the economic formulation of PDO through continuous EO hydroformylation. The vast majority of polyester textile fibers are PET. Its sister polymer, poly(butylene terephthalate) (PBT), has a very limited application to textiles (Werny and Chuah 1996). PTT, made by Shell Chemicals and marketed under the tradename Corterra® , has many characteristics that lend themselves to a variety of products—superior elastic recovery, good colorfastness, uniform dye uptake, stain resistance (Chuah et al. 1995B), and low static-charge generation (Chuah et al. 1995A).
|Figure 1. PTT molecule. Click here for enlarged image.|
PTT is synthesized by the polycondensation of trimethylene glycol with either a terephthalic acid or dimethyl terephthalate. Trimethylene glycol is now commercially producible through the hydroformylation of ethylene oxide allowing for the economic production of PTT (Chuah et al. 1995B). The chemical structure of PTT is shown in Figure 1. PTT has an odd number (three) of methylene units between each of the terephthalates, whereas PBT and PET have even numbers of methylene units. The odd number of methylene units affects the physical and chemical structure of PTT, giving it elastic recovery beyond that of PBT or PET and into the range of nylon (Chuah et al. 1995B; Table 1). PTT is also dyeable without a carrier at boiling temperatures under atmospheric conditions because of the open molecular structure, providing colorfastness comparable to nylon with select dyes (Werny 1998). PTT allows for additional tonal shades with pressure dyeing (Anton 2000), giving designers more choices for textile colors. Dispersed dyes work best on PTT fibers, yielding a uniform color with good fastness (Chuah et al.1995B; Yang et al. 1999).
|Figure 2. PTT can be spun to fine deniers for activewear and other specialty clothing. Click here for enlarged image.|
|Figure 3. PTT fiber in polarized light (left) and undercrossed polars (right) 250×. Click here for enlarged images in polarized light.
Click here for enlarged image in undercrossed polars.
|Figure 4. Raman spectra of PET and PTT. Click herer for enlarged image.|
PTT is easily heat-set and can be spun in a PTT/PET bicomponent (side by side) resulting in a crimp (because of differential shrinkage) that yields a high loft but retains the other desirable traits (Chuah et al.1995A). Core-sheath bicomponents are also being produced. Although initially targeted for the carpeting market, PTT can be spun and drawn at high speeds, resulting in a fiber suitable for fine denier applications (Figure 2), such as sportswear, activewear, and other specialty textiles (Werny and Chuah 1996). Its heat-setting properties make PTT particularly useful in nonwoven fabrics (Hwo et al. 2000).
PTT fibers have optical properties like PET fibers (high-refractive indices in n^ and nll but with a lower birefringence (between 0.06 and 0.08). Hopen and Bartek (1999) measured the refractive indices of PTT fibers as 1.626 in parallel and 1.566 in perpendicular (nD = 0.06). Homofilaments of PTT display lower-order interference colors than PET fibers but higher than nylon (Figure 3), which accords with the lower-than-PET birefringence (nD PET = 0.098 to 0.183 (AATCC 1996; ASTM 1996; McCrone et al.1979; Rouen and Reeve 1970). Theoretically, dichroism should be possible in PTT fibers, but to date none has been observed.
Fourier transform infrared spectra have been published on PTT fibers (Hopen and Bartek 1999). To compare PET and PTT fibers, Raman spectra were collected (Figure 4) with a Chromex Raman 2000 dispersive CCD spectrometer at 785 nm excitation with Raman shifts between ~150–3000 cm–1 with a resolution of 4 cm–1. The spectra were white light and bias corrected. A microscope objective was used at 40× magnification with a spot focus at about 70mW power. The sample fibers were taped to aluminum foil-covered glass slides. Sampling time was about 30 seconds. Other methods have also been used to characterize this polymer (Poulin-Dandurand et al. 1979).
Whereas no prediction can be made about the future prevalence of PTT fibers in consumer goods, they are currently present in specific markets, such as carpeting. New products are being designed with PTT’s qualities in mind. Solenium, for example, is a composite flooring material designed for institutional and hospital use that capitalizes on PTT’s elastic regain, durability, and colorfastness properties (Bertolucci et al. 2000). It is important for the forensic fiber examiner to be aware of the analytical properties of PTT fibers and to be able to distinguish them from PET. Additional information is available at www.shellchemicals.com.
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