Abstract
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.
Introduction
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 DuPonts (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 types
using routine
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. |
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here
for a larger image. |
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here
for a larger image. |
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and Back
of an Advertisement
for NatureWorks
Products |
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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 Skirius’ 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.
Table
1. |
Description
of Fiber Samples
Provided by Cargill Dow LLC (Hazaimeh 2002) |
|
|
|
7
dpf hollow-slickened fiber |
|
Spun
yarn 20/1 |
|
Filament
yarn, false-twist textured |
|
Flat
yarn |
|
Bulked
continuous filament |
|
|
|
Figure
2. |
Sample
3
PLA False-Textured Twist Filament from Cargill Dow
LLC |
|
|
|
|
|
A.
Uncrossed polars |
|
B.
Partially crossed polars |
|
C.
Crossed polars |
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over
for a larger image. |
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over
for a larger image. |
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over
for a larger image. |
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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.
Table
2. |
Melting
Points of Calibration Standards
and Melting Point Ranges of Samples 1 through 5 |
|
Benzophenone
|
Benzoic
acid |
Caffeine |
|
|
|
|
|
|
|
163.8
|
|
166.3
|
|
162.9
|
|
160.6 |
|
163.5 |
|
|
170.7
|
|
170.1
|
|
169.2
|
|
167.4 |
|
170.4 |
|
|
|
|
|
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.
Table
3. |
Solubility
Behavior of PLA Samples 1 through 5 |
Solvent
|
Sample
Number
|
1
|
2
|
3
|
4
|
5
|
|
Formic
acid |
Glacial
acetic acid |
Acetonitrile |
Chloroform |
Cyclohexanone
|
HFIP |
Acetone
|
Nitric
acid
|
Sulfuric
acid
(75%) |
Sulfuric
acid
(100%) |
Water |
|
I
> 30 sec |
I
> 30 sec
|
I
> 30 sec |
S
≈ 1 sec |
I
> 5 min
|
S
≈ 1 sec |
I
> 5 min
|
SW,
G ≈
3 sec |
I
> 10 sec
|
S
< 30 sec
|
Sinks |
|
I
> 30 sec |
I
> 30 sec
|
I
> 30 sec |
S
≈ 1 sec |
I
> 5 min
|
S
≈ 1 sec |
I
> 5 min
|
SW,
G ≈
3 sec |
I
> 10 sec
|
S
< 30 sec
|
Sinks |
|
I
> 30 sec |
I
> 30 sec
|
I
> 30 sec |
S
≈ 1 sec |
I
> 5 min
|
S
≈ 1 sec |
I
> 5 min
|
SW,
G ≈
3 sec |
I
> 10 sec
|
S
< 30 sec
|
Sinks |
|
I
> 30 sec |
I
> 30 sec
|
I
> 30 sec |
S
≈ 1 sec |
S
≈ 1 min
15 sec |
S
≈ 1 sec |
S
≈ 3 min
15 sec |
SW,
G ≈
3 sec |
I
> 10 sec
|
S
< 30 sec
|
Sinks |
|
I
> 30 sec |
I
> 30 sec
|
I
> 30 sec |
S
≈ 1 sec |
I
> 5 min
|
S
≈ 1 sec |
I
> 5 min
|
SW,
G ≈
3 sec |
I
> 10 sec
|
S
< 30 sec
|
Sinks |
|
|
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. |
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here
for a larger image. |
|
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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) |
Discussion
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’ 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).
Table
4. |
Microscopic,
Physical, and Spectral Properties
of PLA, Rayon, and Olefin Fibers
(Identification of Textile Materials 1975;
McCrone et al. 1979; Rouen and Reeve 1970) |
|
Rayon |
Polyethylene
|
Polypropylene
|
PLA
|
|
Birefringence |
Melting
point |
Solubility
in HFIP |
Can
be
identified
by FT-IR |
|
+0.020
- +0.039 |
+0.030
- +0.052 |
+0.028
- +0.034 |
+0.028
- +0.030 |
Does
not melt |
108
- 113°C
(135°C) |
165
- 175°C |
160.6
- 170.7°C |
I
> 8 min |
I
> 8 min |
I
> 8 min |
S
≈ 1 sec |
Yes |
Yes |
Yes |
Yes |
|
|
|
Summary
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.
References
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