EFFECTS OF RESORCINOL ON THE
MECHANICAL PROPERTIES OF SOY
PROTEIN ISOLATE FILMS
D. Jeevan Prasad Reddy and A.Varada Rajulu*
Department of Polymer Science & Technology
Sri Krishnadevaraya University, Anantapur – 515 055, India
V. Arumugam, M. D. Naresh and M. Muthukrishnan
Biophysics Division, Central Leather Research Institute
Chennai – 600 020, India
ABSTRACT: Soy protein isolate (SPI) films were formed by cross linking with
resorcinol. The resorcinol content was varied and its effect on mechanical properties was measured. The variation of tensile creep modulus with time for different
loadings of resorcinol indicated a higher value when the resorcinol content was 10
and 20% of the weight of SPI. The addition of 20% resorcinol led to an overall
increase in the tensile strength from 4.9 to 24.7 MPa and modulus increases from
801 to 1358 MPa than SPI films. The percent elongation was also increased from
3.2 to 8.4 when compared to unmodified SPI film. The impact strength was at a
maximum and moisture content at a minimum for 20% resorcinol content.
KEY WORDS: soy protein isolate, resorcinol, mechanical properties,
biodegradable films.
INTRODUCTION
C
is available in three forms: soy protein
isolate (SPI), soy protein concentrate (SPC), and soy flour (SF). SPI
is a highly refined or purified form of soy protein with a minimum 90%
protein content on a moisture-free basis. Pure soy protein polymer is
very brittle when dry [1,2]; the authors noted that both SPC and SPI
OMMERCIALLY, SOY PROTEIN
*Author to whom correspondence should be addressed. E-mail: [email protected]
JOURNAL
OF
PLASTIC FILM & SHEETING, VOL. 25—JULY-OCTOBER 2009
8756-0879/09/3-4 0221–13 $10.00/0 DOI: 10.1177/8756087910365030
ß The Author(s), 2009. Reprints and permissions:
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221
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D. J. P. REDDY ET AL.
displayed rigid and brittle plastic properties, which make them difficult
to process. Several researchers modified soy protein to improve its
mechanical and physical properties and thermal stability, to reduce the
moisture absorption, and to improve processability. SPC was modified
by glutaraldehyde [3,4], and stearic acid [5] to improve its physical and
mechanical properties, and in the system, glycerin and sorbitol [6], and
polyhydric alcohols [7] were used as plasticizers.
In this study, the authors used resorcinol as a cross linking agent to
prepare SPI films and then studied their tensile properties. Resorcinol
has two hydroxyl groups, which can crosslink with carboxyl, amine, and
amide groups present in SPI. External plasticizer was not used in the
present case. According to material safety data sheet, both glutaraldehyde and resorcinol have a health risk index of 2 which is moderate on a
scale of 0–4 and resorcinol–formaldehyde mixture is widely used in
endodontic therapy [8]. As resorcinol has a 127.28C flash point and a
607.88C auto ignition temperature the cross linking reactions were
carried out at temperatures below 1108C.
MATERIALS
SPI powder was obtained from Honeyville Food Products, Salt Lake
City, Utah, USA. According to the supplier, the SPI used in the present
case has the following analysis:
Percent
Protein
Total Fat
Iron
Sodium
Calcium
Ash and other minor constituents.
85.7
3.6
1.25
1.25
3.6
Bal.
Analytical grade sodium hydroxide (NaOH) was purchased from
MERCK Chemicals and resorcinol was obtained from FINARÕ
Chemicals Ltd., Ahmedabad, India.
METHODS
Tensile and Creep Properties
The films’ mechanical strength is represented by its tensile and creep
properties. The tensile test was performed as per ASTM D 882-02
Effects of Resorcinol on SPI Films
223
standard method. Specimens with dimensions of 100 20 0.2 mm3
were selected. The tensile properties such as maximum stress, Young’s
modulus and percent elongation at break were determined using an
INSTRON 3369 Universal Testing Machine at a crosshead speed of
20 mm/min maintaining a gauge length of 50 mm. The toughness was
calculated from the area under the tensile curve. In the tensile test,
which is a short term test, the applied stress and resulting strain were
measured. In order to study the time dependent tensile performance, a
creep experiment was conducted. In this creep test, for a fixed stress, the
strain was measured with time. In the present case, the creep
experiment was carried out as per ASTM D 2990-01 method. In this
case, a constant stress of 0.5 MPa was applied. For both tensile and creep
tests and for each category, five samples were tested at 50% RH and
238C and the average values are reported.
Falling Weight Impact Test
Falling weight impact tests of unmodified and resorcinol modified SPI
(MSPI) films were carried out at 238C and 50% RH as per ASTM D 1709
procedure. For this test weights from 250–1000 gm were used. In each
case, five samples were tested and the average values were reported.
Moisture Content
The moisture content of the specimens was determined as per the
ASTM D 1576-90 method, a procedure recommended for protein (wool)
fibers. In this method, pre weighed samples in aluminum foil were dried
in a circulating air oven at 1058C for 24 h, and the weight loss was then
calculated.
FTIR Study of SPI Films
The normalized infrared spectra of the SPI and MSPI film samples
were recorded on an Analect RFX-65A Fourier transform infrared
(FTIR) spectrophotometer. The samples were dried in an oven and
ground into powder. In each case 2 mg of the powered samples were
mixed with 98 mg of KBr to prepare sample pellets. FT-IR analysis was
performed with a resolution of 2 cm1 in the range of 4000–400 cm1.
Differential Scanning Calorimetry
The differential scanning calorimetry analysis of SPI, MSPI and
resorcinol samples were scanned using SDT-Q 600, TA instrument
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D. J. P. REDDY ET AL.
under a nitrogen atmosphere. The samples were scanned from 308C to
4008C at heating rate of 108C/min.
EXPERIMENTAL PROCEDURES
Processing and Modification of Soy Protein Isolate Resin
SPI resin films were prepared using a casting method. To prepare
these films, SPI powder was mixed with 15 times (by wt. of SPI powder)
distilled water and the mixture was stirred in a water bath at 758C
initially for 20 min at the end of which 1 N NaOH solution was added to
adjust the pH value to 10. The stirring was continued at 758C for
another 20 min. Then 10–50% resorcinol was added and stirred for 5 min
at 758C. This stir-heating process denatures the SPI and is called ‘precuring’. To obtain cured resin films, the pre-cured suspension was
poured onto the TeflonÕ covered glass mould (150 150 3 mm3) to
form a sheet with desired thickness. The glass mould with the pre-cured
SPI was dried for 24 h at room temperature. The dried SPI film was
peeled from the mould, sandwiched between two aluminum sheets and
cured by hot pressing at 1008C for 20 min under a pressure of 2 MPa.
The hot-pressing was carried out on a hydraulic hot press (Model PFM15, Technosearch Instruments, Kolbad, Maharashtra, India). It was
very difficult to form pristine SPI film due to its brittle nature.
RESULTS AND DISCUSSION
Creep Properties
The tensile creep curves were plotted to evaluate the optimum
resorcinol content for optimum cross-linking the SPI films. The
unmodified SPI film creep behavior could not be studied as they failed
in the grips. Figure 1 shows the creep strain and modulus with time
versus different resorcinol contents in SPI films.
Figure 2 shows the variation of creep modulus at designated time
(1000 s in the present case) with resorcinol loading in SPI films and the
data is presented in Table 1. From Figure 2, it is evident that the creep
modulus was the highest for resorcinol loading of 10% and 20%. Further
the creep modulus was found to decrease with increase in resorcinol content
thereafter. In order to identify the exact optimum loading of resorcinol as
curing agent, the tensile properties of these films were studied.
Effects of Resorcinol on SPI Films
225
(a) 0.20
50% Resorcinol
Strain (mm/mm)
0.15
0.10
40% Resorcinol
0.05
30% Resorcinol
0.00
(b)
10% Resorcinol
35
20% Resorcinol
10% Resorcinol
20% Resorcinol
Creep modulus (MPa)
30
30% Resorcinol
25
20
15
40% Resorcinol
10
50% Resorcinol
5
0
0
200
400
600
Time (s)
800
1000
Figure 1. Effect of resorcinol content on (a) creep strain and (b) creep modulus of soy
protein isolate films using tensile creep method.
Tensile Properties
The resorcinol cross linked SPI film tensile parameters are presented
in Table 2.
The variation of ultimate tensile strength, modulus and percent of
elongation at break of SPI films with resorcinol loading is shown in
Figure 3. From this figure, it is evident that both tensile strength and
modulus of SPI films were at a maximum when resorcinol content
was 20%. Thus the tensile studies indicate that the resorcinol content
of 20% (w/w) was required in SPI films for optimum cross-linking.
226
D. J. P. REDDY ET AL.
35
Creep modulus (MPa)
30
25
20
15
10
5
0
10
20
30
40
50
Wt% of resorcinol
Figure 2. Variation of creep modulus (computed at 1000 s) of soy protein isolate films
with resorcinol content using tensile creep method.
Table 1. Creep modulus of resorcinol modified soy
protein isolate films at 1000 s.
% Resorcinol loading
Creep modulus (MPa)
31.7 0.56
31.7 0.09
27.9 0.25
6.7 1.21
3.1 0.94
10
20
30
40
50
Table 2. Tensile properties of resorcinol modified soy
protein isolate films.
% Resorcinol
0
10
20
30
40
50
Ultimate tensile
strength (MPa)
Modulus
(MPa)
% Elongation
at break
4.9 0.83
22.5 2.35
24.7 1.83
16.3 1.85
7.2 2.12
6.4 0.82
801 33.5
1138 85.3
1358 36.5
1070 76.3
414 81.6
357 55.75
3.2 0.22
4.4 0.89
8.4 0.92
32 0.57
204 27.3
250 25.5
Effects of Resorcinol on SPI Films
227
Maximum stress (MPa)
(a)
25
20
15
10
5
Modulus (MPa)
(b) 1500
1200
900
600
300
% Elongation at break
(c)
250
200
150
100
50
0
0
10
20
30
40
50
% Resorcinol
Figure 3. Variation of (a) maximum stress, (b) tensile modulus and (c) % elongation at
break of soy protein isolate films with resorcinol content.
From Figure 3(c) it is observed that the percent of elongation at break is
increasing with resorcinol content. This behavior indicates that
resorcinol is also acting as a plasticizer.
Impact Strength
Resorcinol addition to soy protein isolate films increased the
toughness and impact strength of these films as shown in Figure 4
and listed in Table 3. The curves for the two tests are quite similar.
Further, the trend of these two curves is similar to the behavior of
228
D. J. P. REDDY ET AL.
(a)
10
Toughness (MPa)
8
6
4
2
0
(b) 21
Impact strength (J)
18
15
12
9
6
3
0
0
10
20
30
40
50
% Resorcinol
Figure 4. Variation of (a) toughness and (b) impact strength of soy protein isolate films
with resorcinol content using falling weight method.
Table 3. Impact strength and toughness (calculated from the area under the
tensile curve) of resorcinol modified soy protein isolate films.
% Resorcinol
loading
0
10
20
30
40
50a
a
Impact strength
(J)
(KJ/m)
1.22 0.13
2.5 0.18
4.9 0.23
7.4 0.15
19.6 1.32
419.6
7.8
15
31.2
49.2
113.7
4113.7
Toughness
(MPa)
0.02 0.01
0.3 0.04
1.6 0.43
4.1 0.48
8.7 1.74
10.1 0.58
This sample did not fail for the highest load and highest height from which the weight was dropped.
It exceeded the limit of the instrument.
Effects of Resorcinol on SPI Films
229
elongation at break in Figure 3(c). With increasing resorcinol loading,
the interaction between the protein chains and resorcinol molecules
leads to increment in impact properties of MSPI films.
Moisture Content
The moisture content values of the films for various resorcinol
loadings are presented in Table 4.
From this table, it is evident that the moisture content was lower
when the resorcinol content was 20%. For other loadings, the moisture
content increased. It can also be observed from Table 4 that the
moisture content of the resorcinol modified SPI films was lower than
that of the unmodified ones. Similar observation was made by Zhichao
et al. [9] in the case of N-hydroxymethyl-acrylamide cross linked with
resorcinol. They observed that hydrophilicity of the polymeric chains
decreased with increasing cross-linking that induced the attraction
between cross-linked chains until the integral network was formed.
FTIR Analysis
In order to probe the cross-linking of the SPI molecules with
resorcinol, the FTIR spectra of pure SPI, cross-linked SPI with different
resorcinol content and pure resorcinol are presented in Figure 5.
The band positions and corresponding assignment of the functional
groups are presented in Table 5.
From Figure 5, it is evident that the characteristic SPI IR bands shifted
on cross-linking with resorcinol. The band position at 3353 cm1
correspond to N–H stretching of SPI and the band at 3260 cm1
correspond to O–H stretching of phenolic group in resorcinol. The
characteristic peak position at 1650 and 1535 corresponds to C¼O
stretching and N–H stretching of amide I and amide II of unmodified SPI
Table 4. Moisture content of resorcinol modified soy
protein isolate films.
% Resorcinol loading
0
10
20
30
40
50
Moisture content (%)
16.5 0.99
9.5 0.09
8.3 0.1
8.5 0.31
9.1 1.2
11.1 0.25
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D. J. P. REDDY ET AL.
% Transmittance
(c)
1544 cm–1
(b)
1535 cm–1
(a)
4000
3500
3000
2500
2000
1500
1000
Wavenumber (cm–1)
Figure 5. FTIR specturm of (a) pure resorcinol; (b) pure SPI; (c) SPIþ 20% resorcinol.
Table 5. Characteristic IR bands and corresponding assignments
of soy protein isolate films.
Sample
Soy protein isolate
SPI þ 20% resorcinol
Pure resorcinol
Wavenumber
cm1
Functional groups
3353
2964, 2930
1659
1535
1399
1236
1165
1079
3429
2960, 2921
1644
1544
1390
1230
1148
965
3270
2924, 2881
1613, 1488
1380, 1299
1155
964
843
773,739
681
N–H st
C–H st of alkanes
C¼O st of amide
N–H st of amide
C–H st and N–H def
C–N stretching
C–O asymmetric stretching
C–O–C stretching
N–H stretching
C–H st of alkanes
C¼O st of amide
N–H st of amide
C–H st and N–H def
C–N stretching
C–O st of alcohol
C–H out of plane bending
O–H st of phenols
C–H stretching
C–C st of benzene ring
O–H in plane bending
C–O asymmetric stretching
C–H out of plane bending
C–H out of plane def of ar ring
C–H are out of plane bending
C–C out of plane bending
Note: st –stretching, ar-aromatic, def-deformation.
Effects of Resorcinol on SPI Films
231
3
Heat flow (W/g)
2
(c)
1
(b)
0
–1
–2
–3
(a)
50
100
150
200
250
300
350
400
Temperature (°C)
Figure 6. DSC curves of (a) pure resorcinol; (b) pure SPI; (c) SPIþ 20% resorcinol.
films, respectively. In case of resorcinol modified SPI films the band at
1535 cm1 decreased its intensity and shifted slightly when compared to
unmodified SPI film. This indicates that the cross-linking was effected by
the NH2 group of SPI and OH groups of resorcinol by hydrogen bonding.
Differential Scanning Calorimetry Study
The DSC endothermic curves of pure SPI, MSPI and pure resorcinol
samples are presented in Figure 6. The thermogram for pure resorcinol
depicts two endotherms, the first one starting at approximately 1108C
(corresponding to its melting point) and another at approximately 2408C
(corresponding to its boiling point). In the case of pure SPI and modified
SPI these two peaks are missing indicating the absence of free resorcinol
in the MSPI.
CONCLUSIONS
SPI films were prepared with resorcinol as a cross linking agent.
In this case, no external plasticizer was used. The optimum resorcinol
level required for effective cross linking was determined using the creep,
tensile and impact data. The data show that the best combination was
obtained when the resorcinol content was 20% by weight of SPI.
232
D. J. P. REDDY ET AL.
Further, the moisture content of these modified films was at a minimum
when the resorcinol content was 20% by weight of SPI. DSC analysis
indicated the absence of free resorcinol present in the MSPI films.
ACKNOWLEDGMENTS
The authors are thankful to Department of Science and Technology
(DST), India for the financial assistance in the form of a Major Research
Project (SR/S1/Gc-04/2006) under Green Chemistry area. We also
acknowledge the assistance of M/s Ananta PVC Pipes Ltd., for the
impact test. The authors also thank STIC, Cochin University, India for
providing some analytical facilities.
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Effects of Resorcinol on SPI Films
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BIOGRAPHIES
Jeevan Prasad Reddy
Jeevan Prasad Reddy received his BS from Layola Degree College and
MS in Polymer Science from Sri Krishnadevaraya University. He is
working as Research Fellow in a DST Major Research Project and also
pursuing a PhD at Sri Krishnadevaraya University on completely
biodegradable polymer films.
Varada Rajulu
Varada Rajulu obtained his BS from VR College and MS and PhD
from Sri Venkateswara University. Rajulu is now Professor of Polymer
Science and also Dean, School of Physical Sciences at Sri
Krishnadevaraya University. He is engaged in research on Polymer
Blends, Composites and characterization of polymers. In his 33 years of
experience he has had more than 140 publications.
V. Arumugam
Arumugam received his PhD in Physics from Madras University. He is
presently working as a Senior Scientist in Biophysics Division of CLRI.
He published about 50 research papers and has 2 patents to his credit.
He is actively engaged in instrumentation and biomaterial testing.
M.D. Naresh
Naresh obtained his PhD in Zoology from Madras University. He is
presently working as Scientist in the Biophysics division of CLRI. He
specializes in microscopic morphology investigation of biomaterials.
M. Muthukrishnan
Muthukrishnan received his BS in Chemistry from Madras
University. Presently, he is working as Technical officer in biophysics
division of CLRI. He specializes in tensile and creep experiments of biofilms and sheets.