feather fiber-based thermoplastics: effects of different plasticizers on material properties
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Feather Fiber-Based Thermoplastics: Effects ofDifferent Plasticizers on Material Properties
Aman Ullah, Jianping Wu*
Poultry feather fiber is transformed into biothermoplastics using a twin screw extruder, andthe plasticizing effect of four different plasticizers on the material properties is investigated.Conformational changes, viscoelastic behavior, thermal degradation, and phase transitionsare assessed by means of FTIR spectroscopy, DMA,TGA, and DSC, respectively. The mechanical prop-erties of the plasticized resins are assessed bytensile measurements, while optical transmit-tance is recorded using UV-Vis spectropho-tometry. The water uptake behavior of the fiberkeratin and plasticized resins is also investigated.
1. Introduction
After their birth in the beginning of 20th century, synthetic
polymers have been growing tremendously due to avail-
ability of large number of cheap chemicals, suitable for the
production of a variety of durable macromolecular
materials.[1] In addition to remarkable and progressive
increase in prices, the distinct durability of the petro-
plastics which makes them ideal for several applications, is
now leading to waste disposal problems, as these materials
are not biodegradable.[2] Due to these facts, the develop-
ment of biodegradable and environment-friendly bioplas-
tic materials from renewable resources has attracted
increasing attention, as means to substitute petroleum-
based plastic materials, which present several concerns in
terms of environmental pollution and sustainability.[3–5]
Renewable resources, because of their pervasive character,
are inherently valuable in this domain and may provide
sustainability with respect to polymeric materials, espe-
cially attention has now been focused on the utilization of
by-products from agricultural, forestry, agronomy, and
marine activities for producing new polymeric materials
Dr. A. Ullah, Prof. J. WuDepartment of Agricultural, Food and Nutritional Science,University of Alberta, Edmonton, Alberta T6G 2P5, CanadaE-mail: jwu3@ales.ualberta.ca
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instead of using food resources or other natural materials.[1]
One of such examples is poultry feathers, which are
ineluctably produced in large quantities as byproduct of the
poultry industry. It is estimated that more than 4 billion
pounds of feathers are annually generated as a byproduct of
poultry industry in the United States[6] and more than 157
million pounds in Canada. Currently, in addition to animal
feed, and very limited use of feathers in industrial
applications[6,7] the major part of the poultry feathers is
disposed in landfills.
Feather contains about 90% protein called keratin.
Feather keratin is biodegradable, renewable, and poten-
tially valuable biopolymer. Feather consists of 50 wt% fiber
and 50 wt% quill. Quill fraction is composed of more b-sheet
than a-helix while the feather fiber has a higher percentage
of a-helix compared to b-sheet.[8] Utilization of this
valuable biomass will not only be beneficial for poultry
industry, but will also reduce health hazards, and benefit
the environment, by reducing solid wastes being sent to
landfills.[9]
In the recent years, some efforts have been made to
modify poultry feather fibers, either by surface grafting of
synthetic polymers or blending with plasticizer, to trans-
form them into films by using casting or compression
molding techniques. Native chicken feather fiber was
modified by grafting methyl acrylate, using K2S2O8/
NaHSO3 as redox system and films were prepared by
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Table 1. Carbon numbers, molecular weights, and boiling pointsof the plasticizers used.
Plasticizer Carbon
number
Molecular
weight
[g mol�1]
Boiling
point
[-C]
ethylene glycol 2 62 197
propylene glycol 3 76.09 188
glycerol 3 92 290
diethyl tartrate 8 206.19 280
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A. Ullah, J. Wu
compression molding of grafted feathers with glycerol as a
plasticizer.[10] Authors reported higher tensile properties
than soy protein isolate (SPI) and starch acetate (SA) based
films. Schrooyen and coworkers carboxymethylated
extracted feather keratin and prepared films by casting
blends of modified keratin with various amounts of
glycerol.[11] Avian feather keratin-based films, prepared
by compression molding without reducing or oxidizing
agents were developed by Barone et al.[12] Observed elastic
modulus, stress at break, and strain at break values were
40–500 MPa, 6–15 MPa, and 8–50%, respectively. The effect
of different barrel and die temperatures on the extrusion of
feather keratin, using glycerol, water, and sodium sulfite as
processing aids was also investigated.[13] Authors observed
that when lower barrel and die temperatures were used,
keratin polymer softened just before the die, leading to
higher viscosities. On the contrary, while using high barrel
temperatures polymer softened earlier inside the barrel
leading to lower apparent viscosities.
Due to a variety of intermolecular interactions, proteins
generally have softening temperatures either very close or
above their decomposition temperatures.[14] Therefore, for
successful processing of proteins and to control protein/
protein interactions, the addition of plasticizer is required. A
plasticizer is a small molecule substantially of low volatility
and high boiling point which, when added to polymeric
material, changes certain physical and chemical properties
of that material.[15] Several theories, including ‘‘lubricity
theory,’’ ‘‘gel theory,’’ and ‘‘free volume theory,’’ have been
proposed about the mechanism of plasticizer action.[16]
Without going into details, which theory is most felicitous,
the plasticizers being small molecules improve processa-
bility by interposing themselves into the polymer chains
and altering the forces holding the chains together.[17]
Water and glycerol are the most commonly studied
plasticizers of keratin and other proteins; even chemically
modified keratin needs glycerol as plasticizer to develop
thermoplastics.[10,18] However, it has been noted that water
and glycerol are unstable and migration of these plastici-
zers during storage of protein-based plastics have been
reported.[19] In addition, glycerol increases moisture
sensitivity, and significantly reduces tensile properties,
especially at high humidities.[20,21]
In present work feather fiber keratin was extruded, using
30% of various possible plasticizers. Poly(ethylene glycol)
(PEG 200 and PEG 500), sorbitol, lactic acid, ethylene glycol
(EG), propylene glycol (PG), glycerol, and diethyl tartrate
(DET). However, no cohesive blends were obtained in the
presence of PEG, sorbitol, and lactic acid, showing that these
components were ineffective as plasticizers for fiber keratin
therefore these materials were not considered as suitable
materials for detailed study. The efficient plasticizers
adopted for the study were, EG, PG, glycerol, and DET.
The objectives were first, to investigate the effect of these
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plasticizers on the structural, thermal, mechanical, and
viscoelastic properties of fiber keratin-based resins, and
then to explore effects of these plasticizers on the water
sensitivity of the fiber-based bioplastics.
2. Experimental Section
2.1. Materials
EG (Sigma-Aldrich, 99þ%), PG (Aldrich, 99.5%), glycerol (Sigma,
99þ%), DET (Aldrich, 99þ%), sodium sulfite (Sigma-Aldrich,
98þ%, MW¼126.04 g �mol�1), and petroleum ether ACS reagent
(Sigma-Aldrich, 99.5%, boiling range 30–60 8C) were used as received.
The main properties of the plasticizers are given in Table 1, and
chemical structures of the plasticizers are shown in Figure 1.
2.2. Fiber Processing
White chicken feathers from broilers supplied by the Poultry
Research Centre (University of Alberta) were washed several times
with soap (Palmolive, antibacterial) and with a plenty of hot water.
The cleaned feathers were dried by first spreading under a closed
fume hood for 4 d to evaporate water and then in a ventilated oven
at 50 8C for 8 h to completely remove remaining moisture. The
cleaned and dried feathers were processed with scissors, and the
fiber portion was separated from the quill portion. Fiber was
ground using a Fritsch cutting mill (Pulverisette 15, Laval Lab. Inc.,
Laval, Canada), at a sieve insert size of 0.25 mm. The batches of
ground fiber material (30 g each) were then treated in a Soxhlet
(extraction tube with 50 mm internal diameter) for 4 h with 250 mL
of petroleum ether to remove grease. The petroleum ether was
evaporated and the dried fiber was stored at room temperature
until used.
2.3. Sample Preparation and Extrusion
Blends of ground fiber with selected plasticizers, [including EG, PG,
glycerol (G), and DET], and sodium sulfite were prepared in a
Laboratory heavy duty blender (Waring Commercial, 120 volt,
Torrington, CT, USA). In a typical blend, 70 g of fiber, 30 g of
plasticizer, and 3 g of sodium sulfite were used. Sodium sulfite was
added into the system in order to dissociate disulfide bonds
between the cysteine residues of the keratin chains to achieve
efficient mixing among keratin and plasticizers. Desired amounts
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(a) (b)
(c) (d)
Figure 1. Chemical structure of (a) EG, (b) PG, (c) glycerol, and (d) DET.
Feather Fiber-Based Thermoplastics: Effects of Different Plasticizers . . .
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of fiber, plasticizer, and sodium sulfite were mixed in a blender at
high speed (2200 rpm) for 20 min; with 1 min break (for removing
the material stuck to the blender walls) after every 3 min blending.
These blends were sealed in plastic bags and placed at room
temperature overnight so that plasticizers could sufficiently
incorporate into the ground fiber material.
Extrusion was performed using a twin screw extruder (Plasti-
corder Digi-system, PL 2200, Brabender Instruments, Inc South
Hackensack, NJ, USA). The screws were single flighted and had
uniform pitch. The barrel length was 35 cm with a diameter of 31.8/
20 mm. A 7 mm die was used. Extrusion was accomplished at
temperatures of 90, 100, 110, 120, and 120 8C, as well as a screw
speed of 50 rpm. After extrusion, samples were cut and cooled to
room temperature.
2.4. Film Preparation
Films of plasticized materials, for mechanical testing, optical
transmittance measurements, and water uptake (WU) studies were
prepared by compression molding the resins for 5 min at 110 8C and
3500 psi pressure using a Carver press (model 3851-0, Wabash,
IN, USA).
2.5. Fourier-Transform Infrared (FTIR) Spectroscopy
FTIR spectra of solid samples in KBr pellets were obtained on an FTIR
spectrophotometer (Thermo Nicolet 750, Madison, WI, USA). Very
thin slices of extrudates were cut and equilibrated at 0% relative
humidity in a desiccator containing P2O5 for 2 weeks prior to FTIR
investigation. The spectra were collected within the frequency
range 4000–400 cm�1. All sample spectra were recorded at 32 scans
and 4 cm�1 resolution, and spectra of two replicate measurements
for each sample were averaged. The infrared spectra were acquired
using Thermo Scientific OMNIC software package (version 7.1).
Second derivative was used to locate the positions of peaks in amide
I region.
2.6. DSC and Thermogravimetric Analysis (TGA)
DSC was performed under a continuous nitrogen purge on a Perkin-
Elmer (Pyris 1, Norwalk, CT, USA), calorimetric apparatus. The
instrument heat flow and temperature were calibrated using a
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sample of pure indium. Samples having a mass of (�5 mg) were
scanned at 10 8C �min�1 from 25 to 275 8C.
TGA was performed on a Perkin-Elmer (Pyris 1, Waltham, MA,
USA), thermogravimetric analyzer. About 10 mg of the sample was
heated at 10 8C �min�1 over a temperature range of 25–600 8Cunder a nitrogen atmosphere.
2.7. Dynamic Mechanical Analysis (DMA)
A dynamic mechanical analyzer Perkin-Elmer (DMA 8000, Wal-
tham, MA, USA) was used to measure dynamic mechanical
properties in tensile mode at an oscillatory frequency of 1 Hz with
an applied deformation of 0.05 mm during heating. Analyses
were performed on rectangular specimens dimensions of
�11� 6� 0.8 mm (length�width� thickness). The exact thick-
ness and width of the samples was measured with digital calipers
at three different places and averages were used. Each sample was
analyzed at least in duplicate. Temperature scans between 0 and
160 8C were performed at 2 8C �min�1 heating rate. Specimens were
equilibrated (2 weeks) at 0% relative humidity in a desiccator
containing P2O5 prior to analysis. The storage modulus (E0) and tan d
(E00/E0) were recorded as a function of temperature.
2.8. Tensile Tests
Mechanical properties (tensile strength, breaking elongation, and
Young’s modulus) of the films were determined at room
temperature on an Instron 5967 (Norwood, MA, USA) equipped
with a 50 N load cell at a crosshead speed of 50 mm �min�1. The data
for each sample were obtained from an average of testing at least
five specimens with an effective length of 80 and width of 10 mm.
2.9. Water Uptake
Water absorption behavior of the fiber keratin and the plasticized
materials was determined by using controlled humidity and
temperature chamber ETS 5518 (Glenside, PA, USA). Rectangular
specimens of �10�6�1 mm (length�width� thickness) were
conditioned at 0% relative humidity in a desiccator with P2O5 as
desiccant at room temperature until constant weight of films was
reached which was termed as an initial weight (W0). The moisture
content of the sheets was determined by conditioning the samples
at 25 8C and 98% relative humidity in controlled environment
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chamber. The specimens from the chamber were removed at
specific intervals and weighed on a five digit balance to obtain the
weight Wt. The WU of the specimens was determined using[22]
Figpladatcla
rly V
WUð%Þ ¼Wt �Wo
Wo� 100 (1)
2.10. Optical Transmittance
Transparency of the plasticized materials was determined by using
a UV-Vis spectrophotometer Evolution 60S (Thermo Scientific,
Nepean, ON, USA). Films of 1 mm thickness were cut into
rectangular shapes and placed on the internal side of spectro-
photometer cell. The transmittance (%) was measured using
wavelength between 250 and 800 nm at 5 nm intervals. Air was
used as blank (100% transmittance). Duplicate measurements were
performed with individually prepared films and average transmit-
tance (%) values were plotted against wavelength.
3. Results and Discussion
3.1. Conformational Changes
FTIR investigation can be used as an effective tool to assess
the structural changes in proteins. In Figure 2, the IR spectra
of fiber and plasticized resins exhibit typical amide
vibrations including amide A (N�H stretching,
3300 cm�1), amide I (C¼O stretching, with a minor
contribution from N�H bending and C�N stretching,
1600–1700 cm�1), amide II and amide III (N�H bending
and C�N stretching, at around 1540 and 1240 cm�1,
respectively).[23,24] Significant changes can be seen in
amide A region of resins formed with different plasticizers.
A broad absorption band of neat fiber keratin appearing at
3307 cm�1 (Figure 2A) is mainly due to hydrogen bonded
N�H stretching vibrations,[25] as in native secondary
ure 2. FTIR spectra of (A) fiber, (B) EG plasticized, (C) PGsticized, (D) glycerol plasticized, and (E) DET plasticized extru-es. Spectra are offset and curves are shifted vertically for
rity.
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structure the peptide N�H groups make hydrogen bonds
with amide C¼O groups. A shift in this band toward higher
wavenumbers as a function of plasticizer type has been
observed, which becomes sharp particularly in the presence
of glycerol and EG (Figure 2B and D). This shift to higher
wavenumbers can be attributed to the disruption of the
internal hydrogen bonds of the peptide groups by
plasticizers and formation of new bonds between protein
and plasticizers. Polyols disrupt internal hydrogen bonds of
proteins by making new hydrogen bonds between the O�H
groups of alcohol and N�H and C¼O groups of polypep-
tides.[26] It is also well known that the absorption peak due
to free O�H in alcohols appears at around 3600 cm�1, while
hydrogen bonded O�H groups absorb at lower wavenum-
bers between 3200 and 3500 cm�1.[27] The positions of these
bands reflect the strength and type of hydrogen bonding,
while another general characteristic of these hydrogen
bonds is that the stronger the hydrogen bond, the greater
the intensity of the corresponding peak.[28]
This trend in bonding was further confirmed by changes
in amide II region of the FTIR spectrum (Figure 3). The amide
II band is related with N�H bending and C�H stretching
vibrations. Although it is much less conformationally
sensitive than amide I, it is much more sensitive to the
environment of the N�H group.[29] Therefore, the amide II
band can be used to deduce changes to the environment of
the N�H groups and respond to differences in hydrogen
bonding environments.[30]
In general, stronger hydrogen bonded N�H groups
absorb at higher frequencies. As compared to the neat
fiber (Figure 3A), decrease in absorption intensity centered
Figure 3. Amide II region spectra of (A) fiber, (B) EG plasticized,(C) PG plasticized, (D) glycerol plasticized, and (E) DET plasticizedextrudates. For easier comparison, intensities have been normal-ized in all spectra at 1540 cm�1. Spectra are offset and curves areshifted vertically for clarity.
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at 1515 cm�1 can be seen in the presence of EG, PG, and
glycerol plasticizers (Figure 3B–D). However, the relative
intensity at 1540 cm�1 increases and this increase is more
prominent in the presence of EG. EG, consisting of a short
aliphatic chain capped with hydroxyl groups, can form
stronger hydrogen bonds with polypeptide chains, while
weaker hydrogen bonding of glycerol and PG with fiber
keratin compared to EG might actually be due to asym-
metric structures of these plasticizers. The lack of change in
the absorption band of DET (Figure 3E) may be due to its
inability to diffuse in and/or interact with polymer chains
because of less symmetry and longer chain length. It is
expected that as chain length increases, symmetry
decreases, and thus so does the hydrogen bonding.
Among all the amide bands of the backbone peptide
groups in proteins, the most intense and the most widely
used one is the amide I band. This band arises mainly from
the C¼O stretching vibration of the amide carbonyl group,
which is weakly coupled with the in-plane N�H bending
and the C�N stretching vibration and appears in the region
between �1700 and 1600 cm�1.
For the enhancement of resolution, techniques such as
second derivative[31] can be used to locate the positions of
individual amide I bands. This technique can be used as a
sensitive diagnostic tool in illustrating changes occurring in
the secondary structure. Since plasticized keratin is a
complex system, therefore broad absorption peaks appear
due to overlap of absorption bands of various components
with different contents. Usually, in order to amplify the
intricate differences in spectra, second derivative infrared
spectra are used.[32]
Figure 4 shows second-derivative FTIR spectra of neat
fiber and plasticized resins. For clarity, the spectra are
presented on an offset scale. As is evident, second derivative
analysis allowed the direct separation of amide I band into
Figure 4. Amide I region second derivative spectra of (A) neatfiber, (B) EG plasticized, (C) PG plasticized, (D) glycerol plasticized,and (E) DET plasticized extrudates.
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its components and absorption bands in the original
spectrum are disclosed as negative bands in the second
derivative spectrum. The major component bands evi-
denced at 1636, 1653 and 1658, and 1662 cm�1 in neat fiber
and plasticized resins can be assigned to, b-sheets, a-helices,
and 310-helices, respectively.[33] While other peaks at 1675,
1680, and 1684 cm�1 can be attributed to antiparallel b-
sheets/aggregated strands.[34] Significant differences can
be seen between neat fiber keratin and extruded resins
particularly in EG and glycerol plasticized materials. The
decrease in intensity at 1662 and 1653 cm�1 compared to
neat fiber keratin (Figure 4A), and the appearance of new
peaks at 1630 and 1624 cm�1 (Figure 4B) in the EG
plasticized resin suggests that EG promotes the formation
of b-sheet structures at the expense of a-helices. Further-
more, a peak at 1624 cm�1 indicates the presence of
stronger intermolecular hydrogen bonds between fiber
keratin and EG which was also confirmed by sharp peak at
4313 cm�1 (Figure 2B). On the contrary, in case of EG and
glycerol plasticized resins (Figure 4C and D), increase in
intensities of peaks at 1662, 1658, and 1653 cm�1 and
relatively low intensities of peaks at 1630 and 1624 cm�1
compared to EG plasticized material suggests that these
plasticizers promote the formation of higher number of
helices (both a-helix and 310-helix) than b-sheet structures
and make complex interaction with keratin molecules.
3.2. Mechanical Properties
Stress/strain curves from tensile tests are commonly used
to characterize polymer properties. The properties typically
investigated are Young’s modulus E (tensile or elastic
modulus), tensile strength, and percent strain at break (%
elongation). Generally glassy materials have higher tensile
strength values (above 30 MPa) and are highly brittle
(almost no elongation). Materials having tensile strength
values lower than 5 MPa and elongation more than 100%
are considered as rubbery materials, while thermoplastic
materials (lying in the glass transition zone) have inter-
mediate properties.[35] Figure 5 shows the representative
stress/strain curves of extruded materials with different
plasticizers, while their tensile properties are summarized
in Table 2.
A common consensus is that hard and brittle polymers
exhibit high tensile modulus, moderate tensile strength,
and low elongation at break; soft and tough polymers are
characterized by low elastic modulus, moderate tensile
strength, and high percent elongation at break; and hard
and tough polymers are characterized by high elastic
modulus, high tensile strength, and high elongation at
break.[36] The pressed films of neat fiber material (without
plasticizer) were too brittle to perform mechanical testing.
It was observed that PG and DET plasticized materials
showed higher tensile modulus, moderate tensile strength
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Figure 5. The representative stress/strain curves of extrudatesplasticized with EG, PG, glycerol (G), and DET.
Figure 6. DSC heat flow signals of neat fiber material and extru-dates plasticized with different plasticizer.
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A. Ullah, J. Wu
but lower breaking elongation than both EG and glycerol
plasticized resins. Although, EG and glycerol plasticized
extrudates had no significant difference in tensile strength
but elongation at the break point of EG plasticized plastics
was almost more than five times higher than glycerol
plasticized resin. A general understanding is that a true
plasticizer generally increases the flexibility and extensi-
bility of the plasticized material while its interactions at a
molecular level increase tensile strength and stiffness. The
differences in mechanical properties especially % elonga-
tion might be due to differences in plasticization efficiency.
EG, the smallest molecule, has the greatest ability to reduce
polymer-polymer associations, increase free volume, and
interact with polypeptide chains through hydrogen bonds.
Glycerol, on the other hand, has three hydroxyl groups but
an asymmetric structure, therefore its interactions with
polypeptide chains may be more complex, which may
ultimately affect mechanical properties. Similar results
were observed while studying the influence of plasticizers
on properties of pea proteins.[37] The DET also has two free
hydroxyl groups but its ability to penetrate into polymer
chains and form hydrogen bonds with peptide groups
might be low due to both its highest molecular mass and the
presence of two bulky ethyl groups located at each end of
molecule, thus making it the least effective plasticizer.
Table 2. Comparison of tensile properties of extrudates with differe
Plasticizer Tensile strength
[MPa]
ethylene glycol 17.76� 2.08
propylene glycol 22.25� 1.52
glycerol 15.66� 4.24
diethyl tartrate 19.0� 3.74
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3.3. Thermal Properties
The thermal transitions of the fiber keratin as well as
extruded materials plasticized by different plasticizers
were studied by DSC. Typical DSC thermograms of neat fiber
and extrudates are presented in Figure 6. Fiber keratin
material exhibited two transitions, a broad peak below
100 8C may be due to evaporation of residual moisture of the
protein and a small peak at around 235 8C might be due to
the crystalline melting of the fiber keratin.[12] The moisture
evaporation takes place slowly and gradually in the
presence of EG and glycerol, while absence of this peak
in the case of PG and DET might be due to the lower
hydrophilic nature of these plasticizers. Except DET
plasticized material, all other resins displayed a second
endothermic peak (attributed to melting) at temperature
lower than 275 8C. An endothermic peak can clearly be seen
at lower temperature in EG plasticized resin than other
plasticized materials, which demonstrates higher improve-
ment in thermoplasticity compared to other plasticizers.
More than one peaks observed in case of the glycerol
plasticized material might be due to different types of
interaction of glycerol with fiber keratin: a loosely bound
glycerol (glycerol-rich zones) and glycerol having higher
interactions with protein. Chen and Zhang also observed
nt plasticizers.
Elongation at break
[%]
Young’s modulus
[MPa]
43.8� 2.21 354.0� 10.2
7.6� 4.70 811.2� 46.2
8.5� 3.15 332.3� 11.4
3.3� 1.57 907.9� 91.1
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glycerol-rich and protein-rich domains, while studying
transitions and microstructures of glycerol plasticized soy
protein.[38]
3.4. Viscoelastic Properties
DMA measures the changes in the viscoelastic properties of
the polymers with changing temperature. Thermal transi-
tions are generally associated with chain mobility and the
most important of these transitions is the glass transition
(Tg), which is related to the onset of main chain motions.
Normally, DMA data for solids is displayed as storage
modulus and damping or tan d versus temperature. This
technique is very sensitive to the motions of the polymer
chains and it is a powerful tool for measuring transitions in
polymers. In order to display changes occurring over large
ranges, modulus is generally displayed on log scale. Figure 7
shows changes in storage modulus (A) and tan d (B) values
of fiber material plasticized with different plasticizers as
a function of temperature. The Tg values have been
determined from tan d versus temperature plots.
A clear shift in the onset of E’ drop to lower temperature
can be seen, particularly with EG and glycerol. This drop in E’
of EG and glycerol plasticized materials at Tg is similar to
the synthetic polymers (usually more than 3 orders of
magnitude).[39]
Figure 7. DMA thermograms log E0 (A), and tan d (B) for fibermaterial plasticized with EG, PG, glycerol (G), and DET.
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The observation of a single, relatively narrow transition
and the strong plasticization effect of EG reflect a good
compatibility of this plasticizer with fiber keratin. This
strong plasticization effect can be attributed to the fact that
it has the low-molecular-weight as compared to all other
plasticizers investigated, having higher ability to lubricate
by incorporating itself among the polymer chains, and
the formation of polymer/plasticizer interactions at the
expense of polymer/polymer interactions. Its plasticization
efficiency is also reflected by significant depression in glass
transition as compared to other plasticizers, in agreement
with the free volume theory of the plasticization.[16]
According to this theory, the plasticizer efficiency is
predicted from the Tg depression of plasticized polymer.
It is also very interesting that a sharp decrease in the
rubbery modulus (Figure 7A) and increase in tan d peak size
(Figure 7B) is observed, in the presence of the EG as
plasticizer. As size of the tan d peak reflects the volume
fraction of the material undergoing transition, therefore,
from highest variation in E’ and tan d peak, it can be
suggested that EG plasticized material undergoes glass
transition phenomenon to greater extent and interactions
involved between keratin and EG are quite homogeneous.
Above the glass transition, E’ depends highly on the density
of the polymer crosslinks,[40] and it is expected that the
higher the density of polymer/polymer crosslinks the lower
the decrease in rubbery modulus. Therefore, the decrease in
the rubbery modulus and increase in tan d is actually due to
replacement of polymer/polymer crosslinks by polymer
plasticizer interactions.[41] For PG and DET, very broad
transitions (both a and tan d) suggest weaker interactions
between these plasticizers and the keratin molecules. Two
transitions in both the a-relaxation and tan d values of
glycerol plasticized material have been observed. These
may be assigned to glycerol-rich and protein-rich domains.
3.5. Thermal Stability
The TG and DTG curves of neat fiber and the plasticized
materials are shown in Figure 8. Two weight loss steps can
be seen in case of pristine fiber material. The weight loss in
the first stage (near 100 8C) for the neat fiber is assigned to
the evaporation of residual moisture whereas the second
step (between 250 and 600 8C) is mainly due to the
degradation of the fiber keratin. The degradation of each
plasticized resin consists of three weight loss steps. The first
gradual weight loss (below 150 8C) is due to the evaporation
of moisture, the second (between 150 and 250 8C) is
attributed to the plasticizer evaporation, and the final
weight loss beyond 250 8C is due to decomposition of fiber
material. It is important to mention here that plasticizers
act by reducing hydrogen bonding, van der Waals, or ionic
interactions that hold polymer chains together, through
forming plasticizer/polymer interactions,[42] by adding free
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Figure 8. (A) TG and (B) DTG; curves of neat quill material andplasticized resins. The DTG curves have been offset for clarity.
Figure 9. Water uptake (WU) behaviors of neat fiber and plasti-cized resins during conditioning at 98% RH versus time.
Table 3. Water uptake (WU) at equilibrium of fiber and plasticizedresins at 98% RH.
Sample Water uptake at equilibrium
[wt%]
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A. Ullah, J. Wu
volume to the system, causing a physical separation of
adjacent chains and by acting as lubricants between chains.
The temperature at the minimum of DTG curves (Tmax)
corresponds to the maximum weight loss at that particular
temperature.
It can be seen from the TG and DTG curves that the delay
in the onset of loss temperature in the plasticizers loss zone
(Tmax between 150 and 250 8C) is higher in case of EG and PG
plasticized resin compared to glycerol plasticized material.
A broad weight loss step in the plasticizer loss zone can
clearly be seen in glycerol plasticized resin, potentially due
to glycerol which is loosely bound with protein (glycerol-
rich zone) and glycerol which is more strongly bonded with
protein. Similar degradation patterns were observed while
studying degradation behavior of glycerol plasticized
cottonseed proteins.[3] On the other hand, the relatively
higher stability of DET plasticized resins compared to other
plasticized materials may result from the comparatively
high molecular mass of the DET. DET also has the lowest
ability to plasticize and break protein/protein interactions.
fiber 12.28
EG plasticized 8.36
PG plasticized 7.92
G plasticized 19
DET plasticized 5.48
3.6. Water Uptake Studies
In WU experiments, the mass of sorbed moisture is
measured as a function of time. The WU during exposure
to 98% RH of the fiber and plasticized resins versus time was
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evaluated. The WU curves of the fiber and plasticized resins
are shown in Figure 9. It was observed that all specimens
absorbed water during the experiment. The diffusion of
water is remarkably influenced by the microstructure of the
polymeric materials, and type, and mass of the plasticizers
as well as water affinity of the components.[43] There are
two well-separated zones for all the curves as displayed in
Figure 9. As reported previously in plasticized soy protein
and starch systems,[4,44] the kinetics of absorption was fast
at lower times, whereas at longer conditioning times the
kinetics of absorption became slow. The maximum relative
WU, or WU at equilibrium corresponding to plateau values
are presented in Table 3. The highest WU in the presence of
glycerol suggests that the affinity between glycerol and
water is higher than even fiber keratin and water.
This pronounced rate of WU may be due to the presence
of loose glycerol-rich domains as evidenced by DMA and
DSC characterization. Similarly Chen et al.[20] evidenced
higher WU by glycerol plasticized SPI due to the presence of
glycerol rich domains especially when glycerol concentra-
tion was more than 25 wt% of SPI. The lower moisture
uptake of EG plasticized material compared to neat fiber
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Figure 10. (A) Dependence of optical transmittance (%) of extruded resins on the type of plasticizer used at different wavelengths and (B)digital photographs of their corresponding 1 mm thick specimens.
Feather Fiber-Based Thermoplastics: Effects of Different Plasticizers . . .
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and glycerol plasticized fiber can be explained due to the
stronger hydrogen bonding between fiber keratin and EG
and water adsorption capacity of EG-fiber keratin networks
is lower than fiber keratin and glycerol plasticized resins.
The least WU by PG, and DET plasticized samples might be
due to more hydrophobic nature and higher molecular mass
of these plasticizers compared to EG.
The opacity or transparency of materials can also be used
as an auxiliary criterion to judge the compatibility and
homogeneity of blends.[45] Figure 10A shows the depen-
dence of percent optical transmittance at different
wavelengths on type of plasticizer used. The transmittance
values of the plasticized resins in the visible region (400–
800 nm) are in the order of DET plasticized<G plastici-
zed< PG plasticized< EG plasticized resin. Interestingly,
the EG plasticized film was the most transparent among all
which was also confirmed by the visual inspection of the
samples (Figure 10B). The relatively darker appearance of
glycerol plasticized film may be due to the degradation of
keratin in protein-rich zones during thermal processing.
4. Conclusion
Feather fiber keratin can be processed into thermoplastics
of different transparencies and physical properties by
extrusion processing with the addition of different
plasticizers. The fixed concentration of plasticizers
(30 wt%, dry basis of fiber) was used. Among the various
plasticizers investigated, PEG, sorbitol, and lactic acid were
found to be ineffective to plasticize fiber keratin. Suitable
materials were obtained in the presence of EG, PG, glycerol,
and DET as plasticizers. The highest compatibility and
strongest H-bonding was seen for EG, while glycerol had
complex interactions with the fiber keratin. Relatively low
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Macromol. Mater. Eng. 2012, DO
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H-bonding occurred between the fiber keratin and PG and
the lowest with DET. Both PG and DET transformed fiber
into relatively hard and brittle bioplastic compared to EG
and glycerol. All plasticizers were able to plasticize fiber
keratin; however, the best mechanical properties, transpar-
ency, flowability, and processability were seen with the
addition of EG. Addition of all plasticizers increased water
resistance of fiber keratin except glycerol. The glycerol
plasticized material appears as a complex heterogeneous
system composed of glycerol-rich and protein-rich
domains.
Acknowledgements: The authors gratefully acknowledge thefinancial support for current work from Alberta Innovates –Biosolution Corporation and the Biorefining Conversions Network.The authors also acknowledge Prof. Thava Vasanthan andDr. Anastasia Elias for providing extruder and DMA facilitiesand helpful discussions.
Received: January 12, 2012; Published online: DOI: 10.1002/mame.201200010
Keywords: extrusion; fiber keratin; plasticizers; thermoplastics;water resistance
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