Velez - Forensic Science Communications - July 2003
July 2003 - Volume 5 - Number 3
Research and Technology
Identification Characteristics of PLA Fibers: A New Generic Fiber Type
Heather A. Velez
Crime Laboratory Analyst
Florida Department of Law Enforcement
The Federal Trade Commission recently awarded generic fiber status to polylactide, commonly known as PLA. This is the first new generic fiber designation since the fluoropolymer category was established in 1998. According to Cargill Dow LLC (Limited Liability Company [Minnetonka, Minnesota]), target markets include apparel, fiberfill, nonwoven goods, household and institutional furnishings, and carpeting. Because it is gaining a wide variety of end-uses, this fiber type is now likely to be encountered in forensic casework.
Five samples of 100 percent PLA fiber were obtained to establish a criterion for identification before this material shows up as evidence. All five samples were examined microscopically to obtain interference colors, birefringence by compensator and refractive index oils, melting point, solubility behavior, and FT-IR (Fourier transform infrared) spectra. By using FT-IR and solubility testing, PLA can be reliably identified and easily distinguished from rayon and polypropylene, which can have similar birefringence/interference colors and/or melting point behavior. This study shows that solubility testing can be used as a potential rapid screening tool, because the samples tested were observed to dissolve in HFIP (hexafluoroisopropanol) in less than five seconds.
On January 11, 2002, the Federal Trade Commission awarded PLA, also known as polylactide or poly(lactic acid), generic fiber status (Federal Trade Commission 2002). The Commission defines PLA as follows:
|A manufactured fiber in which the fiber-forming substance is composed of at least 85% by weight of lactic acid ester units derived from naturally occurring sugars.|
This is the first new generic fiber designation by the Federal Trade Commission since DuPont’s (Wilmington, Delaware) application for Teflon®‚ resulted in the formation of the fluoropolymer category in 1998 (Heschmeyer 2002).
|With careful analysis,
it is possible
to identify PLA
and distinguish it
from other generic
fiber analysis protocol.
PLA is a biodegradable synthetic fiber made from lactic acid obtained from the purification and fermentation of sugars from corn, sugar beet, or wheat starch (CBS News 2002; CBS News 2000; FiberNews 2001; Heschmeyer 2002; Industrial Fabrics Association International 2002; Kanebo; Woodings 2001). Although this idea is not new, only recently have production difficulties been overcome to make its manufacture practical. In 1932, DuPont scientist and inventor Wallace Carothers investigated the direct polymerization of lactic acid in solvent under high vacuum. However, this method of production produced a polymer that had a melting point too low to be useful as a textile, so it was abandoned in favor of nylon (Woodings 2001). Many years later, melt extrusion (CBS News 2000; Heschmeyer 2002; Industrial Fabrics Association International 2002; Woodings 2001) followed by hot drawing (CBS News 2000) were used to improve the mechanical properties of PLA. This method also allowed film, fiber, spunbond, and meltblown products to be manufactured on existing factory equipment (Woodings 2001). Dry spinning can also produce PLA fiber, but because of low-spinning and drawing rates, this method is unsuitable for commercial production (CBS News 2000).
|Figures 1A and 1B.|
|Front and Back
of an Advertisement
As a textile fiber, PLA combines the most favorable physical characteristics of both petroleum-based synthetics and natural fibers (Heschmeyer 2002; Industrial Fabrics Association International 2002; Kanebo). It has the excellent hand, drape, and wicking ability of natural fibers such as cotton, wool, and silk (Cargill Dow LLC 2003; CBS News 2000; FiberNews 2001; Industrial Fabrics Association International 2002; Woodings 2001) while incorporating the easy-care, wrinkle-, soil-, and stain-resistant, and lustrous qualities of conventional synthetic fibers like nylon and polyester (Cargill Dow LLC 2003; CBS News 2000; FiberNews 2002; Kanebo; Woodings 2001). Although PLA fibers are new to the United States, the Japanese company, Kanebo, Limited, has been manufacturing poly-L-lactide fibers under the trade name Lactron®‚ since 1994 for use in agriculture and since 1998 for use in apparel (Kanebo; Woodings 2001). According to the literature, shirts woven of PLA have been well-received by the Japanese public (CBS News 2000; Fambri et al. 1997). Cargill Dow LLC began marketing this fiber under the brand name NatureWorks™ in 2002 (Industrial Fabrics Association International 2002).
In addition to fiberfill and nonwoven goods, target markets for textiles made of PLA fiber also include apparel (knit products, fleece, and denim), household and institutional furnishings, and carpets. PLA can also be blended with other fiber types such as wool, silk, rayon, and cotton (Cargill Dow LLC 2003; Kanebo) to further expand the versatility of products that can be fabricated.
Methods and Materials
Five samples of 100 percent PLA fiber were obtained from Cargill Dow LLC for the purpose of elucidating an identification protocol before this material is submitted as evidence. The sample PLA fibers received from Cargill Dow LLC are described in Table 1. Samples 1 5 varied in diameter from approximately 10 to 60µm and had round-hollow, off-round, polygonal, trilobal, and triangular cross-sections, respectively. Sample 4 was determined to be a bicomponent fiber with sheath-core construction. Cross-sections were prepared according to the procedure described in SkiriusÂÂÂ article (1986). Figure 2 depicts Sample 3 with crossed, uncrossed, and partially crossed polars. First-order white, straw, and orange-red interference colors were observed in Samples 2, 3, and the outer edge of Sample 4. Sample 1 produced second-order blue, green, and yellow interference colors caused by its hollow cross-section. In Sample 5, interference colors were masked by dense pigment and/or delustrant particles.
|Description of Fiber Samples
Provided by Cargill Dow LLC (Hazaimeh 2002)
PLA False-Textured Twist Filament from Cargill Dow LLC
|A. Uncrossed polars||B. Partially crossed polars||C. Crossed polars|
for a larger image.
|Click for a larger image.||Click for a larger image.|
Fiber order and estimated birefringence were determined using a six-wavelength quartz wedge compensator, a Nikon (Japan) Optiphot2-pol polarized light microscope, and the Michel-Lévy chart. Birefringence was estimated to be +0.030 for Samples 2 and 3, +0.045 for Sample 4, and +0.065 for Sample 1. However, these differences can be explained by the hollow cross-section of Sample 1 and the sheath-core bicomponent construction of Sample 4. Therefore, a different method of identification is recommended for samples of these types. Because the pigment/delustrant masked the interference colors of Sample 5, it was not possible to determine its birefringence using the six-wavelength quartz wedge compensator, polarized light microscope, and Michele-Lévy chart. Instead, it was estimated to be +0.055 using the following equation (McCrone et al. 1978):
|Birefringence (nm) =||550 x fiber order|
|1000 x fiber diameter (µm)|
The use of Cargill refractive index oils and the Becke line method provided more uniform and reproducible results. Using this method, the birefringence of Samples 1 through 5 was measured at either +0.028 or +0.030.
The melting points for all fiber samples were to be determined using a Mettler (Mettler-Toledo, Incorporated, Columbus, Ohio) FP82HT hot stage apparatus with a Mettler FP90 central processor. Before the samples were analyzed, the hot stage apparatus was evaluated using three calibration standards: benzophenone (Tf = 48.1°C), benzoic acid (Tf = 122.4°C), and caffeine (Tf = 236.4°C). Tm – Tf was less than 0.5°C, so the apparatus was determined to be in good working order. To set up each sample, a small fiber fragment was placed on a glass microscope slide that was inserted into a slot in the hot stage and observed under crossed polars at 400X magnification. Each fiber fragment melted within a specific temperature range. The beginning of the melting point range was marked by a change in interference colors, and the end of the melting point range was marked by the fiber becoming isotropic. The values of the melting point (ranges) for the calibration standards and for each of the samples appear in Table 2. Woodings (2001) reported the melting point of Cargill Dow Polymers (CDP)-PLA to be 120°C to 170°C.
Solubility behavior was monitored stereomicroscopically (50X) by placing a small fiber fragment under a coverslip on a glass microscope slide. The solvent was added drop-by-drop and was pulled beneath the coverslip by capillary action to contact the fiber. These reactions were timed and compiled as the data in Table 3. Irregularities in the solubility behavior of Sample 4 could be attributed to its bicomponent nature and the inability to test each component individually.
|Solubility Behavior of PLA Samples 1 through 5|
|I = Insoluble S = Soluble SW = Swells G = Gel|
Finally, fibers were prepared for FT-IR analysis by flattening with a roller pen, then placing them on a NaCl disc. FT-IR spectra were obtained for all five samples using a Nicolet (Madison, Wisconsin) Magna-IR 560 spectrometer E.S.P. with Nic-Plan™ IR microscope at 32X magnification. Nicolet also viewed the spectra using OMNIC E.S.P. software version 5.1. These spectra were then compared to a spectrum provided by Cargill Dow LLC (Cargill Dow LLC 2001) and found to agree. Figures 3 and 4 depict the FT-IR spectra from Sample 3 and Cargill Dow LLC, respectively.
|Figure 3.||Figure 4.|
|Click for a larger image.||Click here for a larger image.|
|FT-IR Spectrum of Sample 3,
False-Textured Twist Filament
from Cargill Dow LLC (32X objective)
|FT-IR Spectrum of PLA
(Reproduced with permission
from Cargill Dow LLC)
When attempting to distinguish PLA from the other generic fiber types commonly encountered in forensic casework, most fiber types can be eliminated by polarized light microscope examination of relative refractive index and birefringence. However, two generic fiber types, rayon and olefin, have similar birefringence values to PLA (Identification of Textile Materials 1975; McCrone et al. 1979; Rouen and Reeve 1970). Melting point is a poor choice of confirmatory test for PLA because olefin, specifically polypropylene, will melt in the same temperature range. Additional testing, such as solubility or FT-IR, must be performed in order to distinguish these two fiber types. Both rayon and polypropylene can quickly be distinguished from PLA by the fibers’ÂÂÂÂ solubility behavior. The five PLA samples tested in this report took approximately one second to dissolve in hexafluoroisopropanol (HFIP), whereas the other two fiber types are insoluble in HFIP. There are definitive methods of differentiating PLA from all other generic fiber types, though according to a report published in 2001, only 49 percent of laboratories surveyed in the United States and 63 percent surveyed in Europe routinely use an FT-IR with microscope attachment in casework (Wiggins 2001).
|Microscopic, Physical, and Spectral Properties
of PLA, Rayon, and Olefin Fibers
(Identification of Textile Materials 1975;
McCrone et al. 1979; Rouen and Reeve 1970)
With careful analysis, it is possible to identify PLA and distinguish it from other generic fiber types using routine fiber analysis protocol. Some difficulty can be encountered when using the compensator method to determine birefringence, as with any other large diameter, hollow cross-section, or deeply dyed manufactured fiber type encountered in casework. However, refractive index oils give reliable and reproducible results, even for fibers with a large diameter, dye/delustrant masking, and/or unusual cross-sections. PLA can be distinguished from other fiber types with similar interference colors/birefringence values and melting point ranges, such as rayon and polypropylene (Δn = ≈+0.03, m.p. 165 175°C) (Identification of Textile Materials 1975), using FT-IR and solubility testing. Solubility testing could potentially be used as a rapid screening tool for use in forensic casework where PLA must be distinguished from other manufactured fiber types with similar physical properties.
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