Production and Characterization of Gelatin-starch Polymer Matrix Reinforced With Cellulose Fibers
Production and Characterization
of Gelatin-starch Polymer Matrix
Reinforced With Cellulose Fibers
W. Rodríguez-Castellanos1, D. Rodrigue2, F. Martínez-Bustos1,
O. Jiménez-Arévalo3, and T. Stevanovic4
1Centro
de Investigación y de Estudios Avanzados del Instituto Politécnico Nacional,
Unidad Querétaro, Libramiento Norponiente # 2000. Fraccionamiento Real de
Juriquilla, C.P. 76230 Santiago de Querétaro, Qro., México
2Centre
de recherche sur les matériaux avancés. Département de génie chimique,
Université Laval, Quebec City, Canada, G1V 0A6
3Universidad
Aeronáutica en Querétaro, Carretera Estatal 200 Querétaro Tequisquiapan No. 22154 C.P. 76270 Colón, Querétaro, México
4Département
des Sciences du Bois et de la Forêt, Faculté de Foresterie, de
Géographie et de Géomatique, Université Laval, Quebec City, Canada G1V 0A6.
Received : 19 January 2005, Accepted : 21 April 2015
Summary
In order to produce gelatin-starch polymer matrix reinforced with cellulose
using twin-screw extrusion and compression molding, gelatin-potato starch
and gelatin-corn starch formulations were plasticized with glycerol (25% w/w)
and reinforced with cellulose (0.34% w/w and 0.68% w/w dry basis) to produce
flat sheets. According to the analysis of variance performed, adding TEMPOcellulose in the formulations decreased tensile strain at yield, but increased
Young’s modulus depending on the type of starch used in the formulation.
Scanning electron microscopy and thermogravimetric analyses showed that the
formation of a polymer matrix from gelatin/potato starch/TEMPO-cellulose and
gelatin/corn starch/TEMPO-cellulose was successfully achieved.
Keywords: Starch, Gelatin, Twin-screw extrusion, Sheets, TEMPO-oxidized cellulose
Introduction
Starch is a biodegradable material available in nature which can be used to
produce polymer matrices and be processed using extrusion technology
©Smithers
Information Ltd, 2015
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W. Rodríguez-Castellanos, D. Rodrigue, F. Martínez-Bustos, O. Jiménez-Arévalo, and T. Stevanovic
with the aim of replacing synthetic polymers commonly used as packaging
materials [1-3].
However, starch cannot be processed thermally without water, due to increasing
fusion temperature with decreasing water content [4, 5]. In order to enhance the
properties of starch-based materials, other components must be added during
processing, such as plasticizers (mainly glycerol and water) [6-8], or other polymers
like polylactic acid (PLA) [9], modified starch [10], and lately gelatin [11, 12].
Gelatin is an animal protein with the ability to produce films by casting [13-17]
as well as presenting bioadhesive properties [18, 19]. In recent investigations,
it was shown that gelatin can be processed by direct extrusion [20], or by
extrusion compounding combined with compression molding [16].
Some authors reported that modified starch [12], and compatibilizers such
as polyethylene glycol (PEG) [21], as well as mono- and diglycerides of fatty
acids [11] can be added to improve the properties of gelatin-starch materials,
especially for medical and food applications. However, no comparative study
with native starches has been made, as well as twin-screw extrusion and
compression molding processing conditions optimization.
Nanocrystalline cellulose and microfibrillated cellulose have been used as
reinforcements in starch-based materials, improving the mechanical properties
due to chemical compatibility, small sizes and large surface areas [22-25].
However, producing a homogeneous dispersion of nanocellulose inside a
polymer matrix is the main limitation, since concentrations higher than 1%
produce agglomeration [23].
The aim of this work was to produce gelatin-starch materials using extrusion
and compression molding. In particular, native starches (corn and potato) were
used and optimization of the processing conditions was made. Also, the effect
of TEMPO-oxidized cellulose gel addition on the mechanical and thermal
properties of these biodegradable composite materials was investigated.
EXPERIMENTAL
Materials
Gelatin from animal source was provided from Norland Products (Cranbury, NJ,
USA), while potato starch and cornstarch were acquired from Almex (Mexico
D.F.). Glycerol from Sigma Aldrich (St-Louis, Missouri, USA) was used as a
plasticizer. Cellulose gel (3% weight content) was provided by the Centre
de recherche sur les matériaux lignocellulosiques (CRML) of Université du
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Production and Characterization of Gelatin-starch Polymer Matrix Reinforced With Cellulose Fibers
Québec à Trois-Rivières (UQTR). By an alkaline cellulose oxidation in NaOCl/
NaBr/TEMPO [26-29] the material was obtained. This process enables to get
oxidized cellulose fibers, which are thermally stable.
Extrusion and Compression Molding
Using a twin-screw extruder (Thermo Scientific Haake PolyLab OS system,
Germany) with a cylindrical 5 mm air-cooled die, pellets were produced with
a temperature profile of 25-30-40-50-55-60°C and 30 rpm screws speed
[30]. All the materials were mixed manually before being fed in the extruder.
To optimize the effect of starch concentration, screw speed, and compression
molding temperature on the mechanical properties, an experimental design
with three factors was used. Cornstarch concentration was varied between 45
and 55% w/w, screw speed was varied between 30 and 50 rpm, and molding
temperature between 70 and 90°C. The obtained data were analyzed by
SigmaPlot software (Systat Software Inc.). Glycerol content (25% w/w) and
moisture content (25%) of the samples were selected according to previous
investigation on starch-based materials [1, 31, 32].
Two different concentrations of cellulose gel were used 24.3% (0.34% dry basis
of cellulose) and 12.2% (0.68% dry basis of cellulose). The formulations were
processed at 25% moisture content. Subsequently, plates were fabricated
using compression molding in a laboratory scale press (Carver Mini C press,
Wabash, In., USA), with 2 tons of force for 10 min at 90°C in a mold with
dimensions of 11.5 cm x 11.5 cm x 2 cm.
CHARACTERIZATION
Tensile mechanical tests were performed following ASTM D638 using a universal
testing machine Instron model 5565, with specimens having dimensions of
1.67 x 2.87 x 9.53 mm. The deformation rate was 10 mm/min and seven
replicates per sample were analyzed.
Residual moisture was measured using a moisture analyzer MX-50 (A&D, Elk
Grove Village, Il., USA) and density was determined by a gas pycnometer
Ultrapyc 1200e (Quantachrome Instruments, FL, USA) using nitrogen.
Scanning electron microscopy was performed (Phillips® XL30 ESEM, 47
Eindhoven, Holland) using environmental mode and vacuum mode, with
different magnifications. Cellulose was dried at 50°C for 24 hours, then
placed on a piece of silicon and coated with gold. The samples were placed
on aluminum stub using double-sided adhesive tape.
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W. Rodríguez-Castellanos, D. Rodrigue, F. Martínez-Bustos, O. Jiménez-Arévalo, and T. Stevanovic
Thermal stability was measured with a thermogravimetric analyzer Mettler
Toledo TGA/SDTA 822e (Columbus, Ohio, USA) with samples of 5-10 mg
using alumina crucibles. The tests were performed in nitrogen environment
at a heating rate of 10°C/min between 50°C and 800oC.
RESULTS AND DISCUSSION
Through a three-way ANOVA, a statistically significant difference (p=0.014)
between the mean values of Young’s modulus was found. Nevertheless, no
significant difference between the averages was obtained due to interactions
between starch concentration and screw speed (p=0.761). In the same
way, there was no significant difference due to interactions between starch
concentration and molding temperature (p=0.477). Nevertheless, a significant
interaction between screw speed and molding temperature was observed
(p=0.002). Young’s modulus behavior is shown in Figure 1 where variation
between 13.1 and 36.0 MPa was obtained. Lower Young’s modulus at 40 rpm
with 70°C and 80°C as molding temperature was observed, while at 90°C the
values presented a linear behavior with increasing screw speed. This indicates
Figure 1. Young’s modulus (MPa) as a function of screw speed (rpm) and molding
temperature (°C) for materials extruded and subsequently compression molded
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Production and Characterization of Gelatin-starch Polymer Matrix Reinforced With Cellulose Fibers
that the material was more stable due to humidity change, which decreased
with increasing molding temperature. Gilfillan et al. reported a direct relation
between Young’s modulus and relative humidity for starch-based materials
[33], while Al-Hassan and Norziah reported that Young’s modulus decreased
with the incorporation of gelatin in cast films [34]. However, they did not find
a linear trend when protein content was increased with sorbitol and glycerol
used as plasticizers. The values reported were also much lower (between
1.2 and 2.0 MPa) with glycerol as plasticizer for starch-gelatin films. On the
other hand, Acosta et al. [11] reported decreasing elastic modulus of gelatinstarch based films when improving stretchability by the addition of mono- and
diglycerides of fatty acids.
Vengal and Srikumar studied the mechanical properties of starch-lignin
(from waste wood chips), films and reported that Young’s modulus values
decreased with increasing lignin content in the formulation [35]. On the other
hand, for gelatin-lignin based films, they reported increasing Young’s modulus
with lignin content (1.5-27.3 MPa). In the case of starch-gelatin-lignin films,
Young’s modulus decreased from 67.4 MPa to 26.5 MPa with increasing
lignin in the formulation.
According to the three-way ANOVA, the effect of different starch concentration
on tensile strain at break depends on screw speed (p=0.007). In the same
way, a statistically significant interaction between screw speed and molding
temperature was observed (p=0.001). According to the surface response, the
tensile strain at break increased at lower values of screw speed and starch
concentration (Figure 2). Al-Hassan and Norziah reported that for starch
gelatin-based films, elongation at break increased with gelatin addition in the
matrix, such that gelatin could act as a plasticizer to improve film flexibility
and reduce brittleness [34]. Pranoto et al. reported that a polysaccharide with
long chains can crosslink gelatin, improving tensile strength, and possibly
promote macromolecular relaxation to increase strain at break [36].
Hanani et al. reported no significant differences between mechanical properties
of extruded films derived from beef and fish gelatin sources, as well as changes
in processing temperature from 90 to 120°C [20]. Although the mechanical
properties of the films were improved using higher screw speed, the authors
attributed this result to the molecular size of the components used. Since the
extrusion process contributes heat (viscous dissipation) and melts the gelatin,
this effect promotes plasticization. The values of elongation at break were
between 3.2 and 4.8%, while tensile strength was between 3.3 and 5.4 MPa.
Krishna et al. investigated gelatin films fabricated by casting and extrusion
followed by compression molding [37]. These authors reported significant
differences in physical properties between the films. Particularly, solution
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W. Rodríguez-Castellanos, D. Rodrigue, F. Martínez-Bustos, O. Jiménez-Arévalo, and T. Stevanovic
Figure 2. Tensile strain at break (%) as a function of screw speed (rpm) and molding
temperature (°C) for materials extruded and subsequently compression molded
cast films presented higher values of elastic modulus than extruded films.
This behavior was attributed to lower final moisture contents in solution
cast films, which increased elastic modulus. Materials formed by extrusion
and molding presented different behavior than cast films, to possibly due
to different chain orientations or conformations affecting the mechanical
properties of the specimens.
The three-way ANOVA for tensile stress at yield showed a statistically
significant interaction between starch concentration, screw speed and molding
temperature (p<0.001). The surface response corresponding to tensile stress
at yield is shown in Figure 3. Higher values were obtained at lower screw
speed and lower starch concentration.
According to the results obtained, a starch concentration of 35% and 30 rpm
screw speed gave the highest values of tensile strain at break (75%) and
tensile stress at yield (4.6 MPa), while Young’s modulus indicated that the
optimum molding temperature was 90°C. Although under these conditions
the best results were obtained, according to several authors the addition of
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Production and Characterization of Gelatin-starch Polymer Matrix Reinforced With Cellulose Fibers
Figure 3. Tensile stress at yield (MPa) as a function of screw speed (rpm) and molding
temperature (°C) for materials extruded and subsequently compression molded
cellulose increases the elastic properties of starch-based materials. Therefore
cellulose was added in the optimized formulation [38-42].
CELLULOSE ADDITION
According to the ANOVA, there were significant differences between specimens
due to starch type (α = 0.05), in terms of tensile strain at break, Young’s
modulus and residual moisture values.
Tensile strain at break decreased at 0.34% of cellulose compared to specimens
without cellulose. The same behavior was reported for low nanocellulose
concentrations in thermoplastics starch based films [23]. The decrease in
the tensile stress at yield at 0.34% of cellulose can be caused by increased
porosity inside the specimen, resulting from higher residual moisture (see
density results in Table 1).
The density of the samples decreased with cellulose addition. This implies that
the components of the polymer matrix formed a less dense material. Benezet
et al. reported that adding fibers in a starch matrix promoted lower density
of the compounds, depending on fiber source [39]. These authors explained
Polymers from Renewable Resources, Vol. 6, No. 3, 2015
111
112
26.1±0.1a
1.23±0.01c
17.7±3.7b
1.92±0.13c
0.68
* Same letters in the column indicates no significant differences between the means at a 0.05 level according to the Holm-Sidak method
26.1±0.1b
1.31±0.01a
31.9±4.8b
1.38±0.17b
0.34
15.3±1.3c
23.3±0.1a
1.40±0.01b
63.3±3.1a
1.94±0.11a
0
Potato
0.68
6.3±1.6b
22.8±0.1a
1.30±0.01a
10.8±1.1a
26.0±0.1b
1.30±0.01a
24.1±3.7b
1.63±0.13c
10.1±1.3c
21.8±3.7b
1.39±0.13b
0.34
10.3±1.3b
21.3±0.1a
1.38±0.01a
77.4±3.1a
2.20±0.11a
Corn
0
11.0±1.1a
Residual moisture
(%)
Density (g/cm3)
Young’s modulus
(MPa)
Tensile strain at
break(%)
Tensile stress at
yield (MPa)
Cellulose
content (%)
Starch type
Table 1. Sample description and properties of gelatin/starch/cellulose composites extruded and subsequently
compression molded
W. Rodríguez-Castellanos, D. Rodrigue, F. Martínez-Bustos, O. Jiménez-Arévalo, and T. Stevanovic
that during extrusion of starch with
fibers, two processes may occur. On
the one hand, the fibers can increase
the viscosity of the material such that
it decreases the ability to expand. But
at the same time, fibers can act as
nucleating agents providing greater
surface contact between starch pellets.
Young’s modulus of samples with potato
starch increased with the addition of
0.68% cellulose, regardless of the
remaining moisture. This type of starch
at this concentration of cellulose had the
best performance and similar results have
been reported for cellulose fibers addition
[38, 42] and different type of fibers [4,
43-45]. Young’s modulus for samples
with cornstarch decreased with cellulose
addition in the formulation. At 0.34% of
cellulose, residual moisture increased
and Young’s modulus decreased as
reported by Gilfillan et al. [33]. Since at
0.68% of cellulose, humidity decreased
as well as Young’s modulus, this implies
that cellulose has better interaction with
potato starch than cornstarch.
Previous studies reported that the main
disadvantage of starch-based materials
is their stability and susceptibility to
absorb humidity from the environment,
and to increase density. Müller et al.
reported that adding cellulose fibers
in starch-based films decreased its
susceptibility to ambient humidity,
and at the same time improved the
mechanical properties of the films
[41]. Previous studies had showed that
cellulose can improve the mechanical
properties of gelatin-starch matrix due
to chemical compatibility [46] reducing
the aging effect of retrogradation.
Polymers from Renewable Resources, Vol. 6, No. 3, 2015
Production and Characterization of Gelatin-starch Polymer Matrix Reinforced With Cellulose Fibers
The micrographs presented in Figure 4 show the formation a polymer matrix
containing starch, gelatin and glycerol as plasticizer with cellulose. Figure 4a
and 4b show the micrographs of cellulose at different magnification. The average
diameter of cellulose fiber was measured as 15.5 µm. Figure 4e exhibits the
sample with potato starch, and Figure 4f shows the presence of cellulose in
the sample with corn starch. Al Hassan and Norzia related the formation of
channels in starch-gelatin films with glycerol as plasticizers [34], and observed
rougher surfaces with the presence of pores or cavities. The porosity in the
matrix could be related to moisture migration typical of hydrophilic materials,
Figure 4. Typical SEM micrographs of different materials. a) and b) cellulose at different
magnification, c) corn starch at 500x, d) potato starch at 500x, e) gelatin/potato
starch/cellulose polymer matrix at 500x, and f) gelatin/corn starch/cellulose at 1000x
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W. Rodríguez-Castellanos, D. Rodrigue, F. Martínez-Bustos, O. Jiménez-Arévalo, and T. Stevanovic
which in turn causes a decrease of density. Similarly in lipophilic gelatin-corn
starch matrix, irregular surfaces and cracks have been reported [12].
The results of thermogravimetric analysis are presented in Figure 5 and Table 2
showing the behavior of gelatin, cellulose gel, potato starch and the polymer
matrix with potato starch. Cellulose gel exhibited the maximum weight loss in
two steps. The first step started at 193°C (32.4% loss) and the second started
at 262°C (52.5% loss). Kaushik et al. have reported an onset temperature of
decomposition for cellulose nanofibrils at 283°C and the peak temperature
of thermal decomposition at 337°C [40].
Gelatin also presented two important stages of weight lost. The first is around
160-340°C which is associated with the degradation of gelatin chains, and
a second stage around 340-601°C attributed to the decomposition of more
thermally stable structures like amino acids linked by peptide bonds. Similar
results have been reported by Mu et al. [17], as well as Hoque et al. [15] in
gelatin edible films. Potato starch exhibited an onset temperature around
251°C, and similar results have been reported in different studies about
thermal stability of starches [33, 40, 45].
The polymer matrix based on potato starch, gelatin, cellulose and glycerol as
plasticizer exhibited a step between 160-266°C (27.3% loss). This is associated
Figure 5. Thermal decomposition curves of the raw materials and the gelatin/potato
starch/cellulose polymer matrix
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Production and Characterization of Gelatin-starch Polymer Matrix Reinforced With Cellulose Fibers
Table 2. Temperature range and weight loss, during thermal
decomposition of the raw materials and the gelatin/potato starch/
cellulose polymer matrix
Sample
Polymer matrix
(starch/gel/cel)
Cellulose
Gelatin
Potato starch
Temperature
range (°C)
Weight loss
(%)
Temperature
range (°C)
Weight loss
(%)
Temperature
range (°C)
Weight loss
(%)
Temperature
range (°C)
Weight loss
(%)
31-160 160-266 266-491 491-800
8
27
61
4
31-193 193-262 262-350 350-511 511-744
3
32
53
11
1
50-160 160-340 340-601 601-790
8
43
47
2
57-117 117-251 251-299 299-529 529-699
5
1
54
36
3
with the presence of glycerol [37, 44, 45], but it can also be attributed to gelatin.
The second step is around 266-491°C (60.8% loss) which is attributed to the
decomposition of the polymer matrix (gelatin/starch/cellulose). Kaushik et
al. reported variations in onset temperature with 15% cellulose at 255°C in
nanocomposites based on thermoplastic starch and cellulose nanofibrils [40].
Garcia et al. reported that glycerol reduced thermal stability and modified the
degradation in the temperature range between 120 and 300°C for starchbased films [44]. Hoque et al. reported that increased glycerol contents
prevented protein-protein interactions resulting in higher heat sensitivity of
gelatin films [15].
CONCLUSIONS
The aim of this work was to determine the most suitable conditions for
processing a gelatin-starch polymer matrix using twin-screw extrusion and
compression molding. Also, the study proposed to use TEMPO-oxidized
cellulose to improve the mechanical properties of gelatin-starch formulations.
From the results obtained (statistical analysis), the mechanical properties
of gelatin-starch matrix (Young’s modulus, tensile stress at yield and tensile
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W. Rodríguez-Castellanos, D. Rodrigue, F. Martínez-Bustos, O. Jiménez-Arévalo, and T. Stevanovic
strain at break) were maximized for a starch concentration of 35% processed
at 30 rpm in the twin-screw extruder and compression molded at 90°C.
TEMPO-oxidized cellulose addition was found to decrease more the tensile
strain at yield (1.63-1.39 MPa) in gelatin-corn starch matrix compared to MPa
gelatin-potato starch matrix (1.94-1.92). In addition, density decreased from
1.38 to 1.30 g/cm3 in gelatin-corn starch matrix and from 1.40 to 1.23 g/cm3
in gelatin-potato starch matrix. On the other hand, an interaction between
TEMPO-cellulose and gelatin-potato starch matrix was found to better increase
the Young’s modulus (15.3 MPa) than for gelatin-corn starch matrix (10.1 MPa).
Overall, better properties were obtained for gelatin/potato starch/TEMPO
cellulose composite. Also, the addition of TEMPO-cellulose improved the
thermal stability of the composites, but further studies are required to optimize
the amount of cellulose fiber.
Acknowledgments
The authors would like to thank Centre de recherche sur les matériaux
lignocellulosiques (CRML) of Université du Québec à Trois-Rivières (UQTR), Dr
Claude Daneault of UQTR for the samples of TEMPO-oxidized nanocellulose
and Yann Giroux for the development of this work. Also, Consejo Nacional
de Ciencia y Tecnología (CONACyT) for the scholarship provided to develop
this research.
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