Effect of starch types on properties of biodegradable polymer based on thermoplastic 1
starch process by injection molding technique 2
Yossathorn Tanetrungroj1* and Jutarat Prachayawarakorn1,2* 3
1 Department of Chemistry, Faculty of Science, King Mongkut’s Institute of Technology 4
Ladkrabang, Ladkrabang, Bangkok 10520, Thailand 5
2 Advanced Materials Research Unit, Faculty of Science, King Mongkut’s Institute of 6
Technology Ladkrabang, Ladkrabang, Bangkok 10520, Thailand 7
* Corresponding author 8
Email address: [email protected] Telephone number : +668-3298400 ext 6235 9
Abstract 10
Due to limitations of thermoplastic starch (TPS), effect of starch types on properties 11
of biodegradable polymer based on TPS was investigated. Different types of starch; 12
containing different contents of amylose and amylopectin, were used, i.e. cassava starch, 13
mungbean starch and arrowroot starch. The TPS polymers were compounded and shaped 14
using an internal mixer and an injection molding machine, respectively. It was found that the 15
amount of amylose and amylopectin contents on native starch influence the properties of the 16
TPS polymer. The high amylose starch of TPMS led to higher strength, hardness, degree of 17
crystallization than the high amylopectin starch of TPCS. In addition, function group analysis 18
by Fourier transforms infrared spectrophotometer (FTIR), water absorption and 19
biodegradation by soil burial test were also examined. 20
Keywords : Arrowroot starch, Biodegradable polymer, Injection molding, Mungbean starch, 21
Thermoplastic starch 22
1.Introduction 23
Due to environmental considerations, starch is now an attracting raw material in the 24
production of biodegradable materials because of its abundant, low price and availability. 25
Starch has a granular structure, which is built by two main carbohydrate polymers; the linear 26
amylose and the highly branched amylopectin. Both high molecular weight polymers are 27
formed by (1,4)-α-D-glucopiranosyl repeating units. Amylose is essentially linear; whereas, 28
amylopectin is highly branched with (1,6) glucosidic branching points occurring every 20–25 29
glucose units. The short chains of amylopectin are thought to be arranged as double helices, 30
in clusters of semicrystalline character (Leszczynski, 2004). Generally, starch sources have 31
been shown the influence the differences in amylose and amylopectin contents. 32
Starch can be processed under the action of both high temperature and shearing action 33
and this is called thermoplastic starch (TPS) (Stepto, 2000). TPS from various sources has 34
been studied, including corn and wheat starch (Naguleswaran et al., 2014) potato starch (Guo 35
et al., 2001) maize starch or wheat starch (Schirmer et al., 2013). Traditionally, the 36
difference in water content and the ratio of amylose to amylopectin of various native starches 37
have been attributed to the physical and barrier properties of thermoplastic starch (Zhou et 38
al., 2008). Many researchers have demonstrated on the influence of several starches on the 39
mechanical properties and biodegradability characteristics of biodegradable plastics produced 40
mainly by the extrusion, compression molding and casting techniques (Stepto, 2003; Rosa et 41
al., 2007; Soest et al., 1996; Altskar et al., 2008; Lopez et al., 2014). 42
In addition, there are many been investigations of the effects of starch type or 43
amylose/amylopectin ratio on the final properties of starch-based materials. It was reported 44
that a blend of TPS from wheat starch (0–22% w/w) with LDPE exhibited a decrease in the 45
Young’s modulus (Pierre et al., 1997). Moreover, high amylose maize starch and potato 46
starch were reported to produce strong films, probably due to amylose crystallization 47
(Durejaa et al., 2001; Soest et al., 1996; Cano et al., 2014). The starch films prepared from 48
the low amylose corn starches also showed low degree of crystallinity (García et al., 2000). 49
However, the effect of starch types on properties of biopolymers based on 50
thermoplastic starch prepared from arrowroot starch (TPAS), cassava starch (TPCS) and 51
mungbean starch (TPMS) and processed by an injection molding technique has not yet been 52
reported. Arrowroot is a powdery product made from the arrowroot plant, a starchy tropical 53
root. It is white and powdery just like cornstarch. Arrowroot starch also makes an excellent 54
substitute for talcum powder in cosmetics and it was once used in the several industries 55
(Jyothi and Sheriff, 2009). Mungbean has been used as raw material for producing starch 56
noodle. Mungbean starch is admired as the best of starch due to its high amylose content, 57
leading to strong gel strength. It is consumed mainly for its rich protein content (24%) but 58
carbohydrate (62–63%) is its major component (Ohwada et al., 2003). Besides, cassava 59
starch is one of the most staple foods in the developing world. It contains significant amounts 60
of rich in starch and poor amounts of other nutrients. It is an abundant, purity, clearness of its 61
paste and cheap agricultural source of starch and flour production. Moreover, it can be enable 62
to blend with other expensive synthesis polymers required to producing biodegradable 63
polymers. (Satyanarayana et al., 2009). 64
The objective of the present study was to evaluate the effect of starch types with 65
different amylose and amylopectin contents on properties of biodegradable polymer prepared 66
from the different types of selected thermoplastic starch, i.e. TPAS, TPCS, and TPMS and 67
processed by injection molding technique. The polymers were, then, characterized by 68
function group analysis by FTIR and crystallinity tested by X-ray diffraction; as well as, 69
morphology, water absorption, tensile properties and biodegradation properties. 70
71
2. Materials and Methods 72
2.1 Materials 73
Arrowroot starch was purchased from Chaopraya Phuchrai 1999 Co., Ltd. 74
(Kamphaengphet Thailand) and containing with 26.5 wt% amylose and 73.5 wt% 75
amylopectin. Mungbean starch was obtained from Sittihinan Co., Ltd. (Bangkok, Thailand), 76
contained approximately 37.9 wt% amylose and 62.1 wt% amylopectin. Cassava starch was 77
obtained from Thaitam (Chonburi, Thailand), composed approximately 16.4 wt% amylose 78
and 83.6 wt% amylopectin. The chemical compositions of arrowroot, mungbean and cassava 79
starches were shown in Table 1 (Erdman, 1986; Habibullah et al., 2003; Pushpadass et al., 80
2010). Glycerol (plasticizer) was purchased from Lab System Co., Ltd. (Bangkok, Thailand). 81
Low-density polyethylene (LDPE, DNDV-0405 R) with MFI of 32 g/10 min. was obtained 82
from Global Connections Public Co., Ltd. (Samuthprakarn, Thailand). Maleic anhydride-83
grafted-polyethylene (MAPE, MB100D) compatibilizer was obtained from Chemical 84
innovation Co., Ltd. (Bangkok, Thailand). 85
2.2 Sample preparation 86
Each kind of starch, i.e. arrowroot starch, cassava starch or mungbean starch was 87
pre-mixed with glycerol (plasticizer) in closed container at room temperature and kept 88
overnight. The weight ratio of starch and glycerol was maintained at 70:30. The TPS 89
polymers were firstly compounded with LDPE and MAPE compatibilizer. The weight ratio 90
of the TPCS and LDPE was kept at 80:20 and The MAPE content was maintained at 5% by 91
weight of the LDPE. The minor content of LDPE polymer was added in order to increase 92
flowability during injection process. Different TPS polymers were compounded using an 93
internal mixer (Lab-Tech Engineering, Bangkok, Thailand) at the temperature of 140°C at the 94
screw rotation speed of 40 rpm for 12 min in order to obtain a homogeneous sample and the 95
specimens were then shaped by the injection molding technique using injection pressure and 96
injection flow speed at 30 bar and 50 rpm, respectively. The injection temperature was varied 97
from 150-170°C (from feed zone to die end). 98
2.3. Fourier transforms infrared spectrophotometer, FTIR 99
FTIR spectrum of a sample was recorded on a Spectrum 2000 GX spectrometer 100
(Perkin Elmer, USA) using KBr disk technique with a resolution of 4 cm-1 in a spectral range 101
of 4000–600 cm-1 using 16 scans per sample. 102
2.4. X-ray diffraction (XRD) 103
X-ray diffraction measurements were carried out using a D8 Advance X-ray 104
diffractometer (Bruker, Madison, USA) with CuKα radiation (wavelength 0.1542 nm) 105
operating at 40 kV and 35 mA. The scattering angle (2θ) covered the range from 3o to 60o (θ 106
is the Bragg angle) with a step size of 0.02o and a sampling interval of 10 s. Percentage 107
crystallinity of a sample was determined by the following equation 108
Crystallinity (%) = Ac/(Ac + Aa) × 100 -----(1) 109
where Ac and Aa were the area of crystallinity region and the area of amorphous region of a 110
sample determined from a diffractogram, respectively. 111
2.5 Morphology 112
A JEOL-6400 scanning electron microscope (Tokyo, Japan) was employed to study 113
the morphology of a sample. Each tested sample was sputter-coated with a thin layer of gold 114
to prevent electrical charge during observation and a sample was immersed into liquid 115
nitrogen before being fractured. 116
2.6 Mechanical property 117
Mechanical tests were evaluated according to ASTM D-638 at the temperature of 118
23±1 oC and relative humidity of 60±5 %. The starch mixtures with different starch type as 119
prepared above were stored in a desiccator containing a saturated solution of ammonium 120
nitrate for 1 day to equilibrate the moisture content of 60±5 % before measurements. A 121
mechanical measurement of each dumbbell shaped specimen was carried out using a 122
Universal Testing Machine (Lloyd Instrument, LR 5K, West Sussex, UK) with a 100 N load; 123
and the crosshead speed was maintained at 40 mm/min. It should be noted that the 124
mechanical property results were obtained by averaging the measurement results of ten 125
independent specimens. Hardness test was conducted according to ASTM: D-2240 using 126
Shore A hardness durometer (Intro enterprise, Thailand). The hardness values were recorded 127
from ten different measurements for each samples. In addition, impact test was carried out 128
using a Izod Impact tester (Yazuda 199311, Japan) and was also measured ten independent 129
specimens according to ASTM: D-256. 130
2.7 Water absorption 131
Water absorption test was carried out according to ASTM: D-570. A samples was 132
dried at 105°C for 3 hour and then immersed in distilled water. The amount of water 133
absorbed by a sample was determined daily for 30 days. The percentage of water absorption 134
was calculated as followed: 135
Water absorption = (W2-W1)/W1 × 100 (2) 136
where W2 and W1 were the wet weight and the dried weight of a sample, respectively. 137
2.8 Biodegradable by soil burial test 138
A dumbbell sample was buried at approximately 10 cm under the soil surface. The 139
water content of the soil was maintained in the range of 30-40% and the test was carried out 140
for 0, 4, 8 and 12 weeks. After that, the tensile test of a soil-buried sample was conducted in 141
comparison with an original sample. 142
3. Results and Discussion 143
3.1 IR Spectroscopic Study 144
FT-IR is a powerful technique for identifying types of chemical bonds of polymers. 145
FTIR spectra of the different TPS polymers are shown in Figure 1. The broad band appeared 146
at 3500–3250 cm-1 was attributed to O–H stretching. The peak in the range of 1275–1070 147
cm-1 was characteristic of C–O–C stretching. The bands in the range of 1200–1000 cm-1 was 148
assigned for C–O–H stretching and 1646-1641 cm-1 was attributed to bounded water 149
presented in starch. These peak positions were due to the TPS component (Shi et al., 2007). 150
Other vibrational bands from LDPE component were found in the wavenumber range of 151
3000–2800 cm-1, assigned from C–H asymmetric stretching of –CH2–. The band in the range 152
of 1475–1450 cm-1 was characteristic of –CH2–deformation. Moreover, the wavenumber in 153
the range of 931-855 cm-1 were designed for C-H bending and the band at 718 cm-1 was 154
owing to CH2- rocking from of LDPE and MAPE component (Bower, 1996). 155
It was found from Figure 1 that all the TPS polymers showed the same IR peak 156
positions and no distinct new peak position could be observed since all different TPS 157
polymers consists of the same chemical structure of polysaccharides. 158
159
3.2 X-ray Diffraction 160
X-ray diffraction technique can be used to identify crystal structure and regular 161
molecular arrangement of starch. X-ray diffraction patterns of different TPS polymers are 162
shown in Figure 2. The peak at 2θ of 19.6 o corresponded to the Vh type pattern of starch due 163
to the amylose-lipid complex formation forming by the effective starch gelatinization (Hartel, 164
2001). Besides, the main peaks at 2θ of 21.4°, 23.7° and 36.0° represented the orthorhombic 165
crystal structure of LDPE component (Pereira et al., 1997). It should be noted that the 166
crystallinity of different TPS polymers mainly resulted from the LDPE component since the 167
LDPE component caused the major peak intensity in the diffractograms; eventhough, the 168
LDPE components was introduced into the TPS polymers in very small proportion. 169
Moreover, it was found that no significant difference in XRD patterns could be observed for 170
the different TPS polymers. This could be due to the similarity of the composition of the 171
different TPS polymers which was also consisted of starch and LDPE, leading to the similar 172
XRD peak patterns. 173
Nevertheless, the calculated percentage crystallinity values of different TPS polymers, 174
obtained from the diffractograms, were approximately 36.2%, 37.4% and 38.7% for the 175
TPCS, TPAS and TPMS polymers, respectively. It can be seen that the highest percentage 176
crystallinity value was found in the TPMS polymer and the percentage crystallinity value of 177
the TPAS polymer was higher than that of the TPCS polymer. The differences in crystallinity 178
values may also be attributed to different contents of amylose and amylopectin of native 179
starch sources as shown by the chemical compositions of different starches in Table 1. 180
From Table 1, it can be seen that the mungbean starch exhibited the highest amylose 181
content. The higher content of amylose led to the higher degree of crystallization. These can 182
be explained that the higher content of linear amylose molecules causes the effective 183
crystallization but amylopectin does not crystallize due to its very highly branched structure 184
and results in the formation of a less firm gel (Marie and Imada, 2004). 185
3.3 Morphology 186
Morphology of different TPS polymers was examined by SEM technique is presented 187
in Figure 3. The TPMS polymer (Figure 3(a)) showed the smoothest surface without phase 188
separation but the TPCS (Figure 3(b)) and TPAS polymers (Figure 3(c)) showed clearly 189
rough surfaces. This could be due to the highest amylose content and elasticity property of 190
the mungbean starch. Amylose content is the most important factor for the resulting of 191
rheological and morphology. The high amylose starch; i.e. mungbean starch, showed smooth 192
surface and better rheological property than the low amylose starches; i.e. cassava and 193
arrowroot starches, it might be due to a comparatively less branching chains and a less 194
compact arrangement of linear short chains. 195
It agrees with the findings by Tan et al. (2006) that the mungbean starch showed 196
smoother than sweet potato starch due to the higher amylose content of the mungbean starch. 197
198
3.4 Water absorption 199
Generally, TPS could absorb some amount of water from the environmental humidity. 200
The water absorption behavior of different TPS polymers is represented in Figure 4. 201
It can be seen that all TPS polymers showed the rapid increase of water absorption during the 202
1 days of testing. After that, the percentage water absorption tended to increase slowly and 203
reached its maximum. It should be noted that the TPMS polymer showed the rapidly increase 204
of water absorption during the 3 days of testing. After that, the TPMS polymer could not 205
absorb water because the TPMS specimen degraded and cracked during the water absorption 206
test. 207
It could be seen from Figure 4 that the lowest percentage of absorption was found in 208
the TPMS polymer. On the other hand, the highest percentage of water absorption was 209
present in the TPAS polymer. 210
From Table1, the mungbean starch showed the lowest amylopectin content, but the 211
cassava starch exhibited the highest amylopectin content. Unexpectedly, the TPCS polymer 212
did not show the highest of water absorption property. It could be explained by considering 213
the carbohydrate content in Table 1. The arrowroot starch consists of higher carbohydrate 214
content when compared with the cassava starch. More content of hydrophilic characteristics 215
of carbohydrate could lead to higher percentage of water absorption. Moreover, the lowest 216
water absorption of the TPMS polymer could be due to lowest carbohydrate content 217
(Table 1) and also the highest crystallization as confirmed by the XRD technique. 218
219
3.5 Mechanical Properties 220
The amount of amylose and amylopectin molecules of the various starch sources 221
(Table 1) also influences mechanical properties of TPS polymers. It can be observed from 222
Figure 5 that the highest stress at max load, Young’s modulus and hardness, the lowest strain 223
at max load and impact strength were found in the TPMS polymer, resulting from the highest 224
amylose content and lowest amylopectin content of the mungbean starch. 225
Additionally, the TPAS polymer presented higher stress at max load,Young’s modulus and 226
hardness, but lower strain at max load and impact strength than those of the TPCS polymer. 227
High amylose starch, generally, presents higher stiffness and hardness than high amylopectin 228
starch due to the comparatively less branching structures (Marie and Imada, 2004). The 229
results also confirms by the good phase morphology (Figure 3) and higher cystallinity (Figure 230
2). Similar observation was reported for high amylose rice film that was stiffer, less elastic 231
than those of low amylose rice film, prepared from casting method (Cano et al., 2014). 232
233
3.6 Biodegradable property 234
Biodegradable property of different TPS polymers, determined by soil buried tests 235
for 0, 4, 8 and 12 weeks is presented in Figure 6. It was found that the stress at maximum 236
load, Young’s modulus and strain at maximum load of all TPS polymers significantly 237
decreased after the biodegradation test. The results of water absorption properties (Figure 4) 238
also related to the biodegradable property. The TPS polymers could absorb water from the 239
soil TPS component, leading to the biodegradation of TPS polymer by the microorganisms 240
existed in soil. The most rapid degradation was found in the TPMS polymer since the 241
mungbean starch consists of high protein content (Table 1) that can be easily decomposed 242
(Diaz, 2008). 243
244
4. Conclusion 245
The influence of starch types on properties of biodegradable polymers based on TPS, 246
processed by injection molding technique, was examined. It was found that properties of 247
different TPS polymers depended on the amylose and amylopectin contents including 248
carbohydrate contents. The high amylose polymer of the TPMS polymer presented higher 249
degree of crystallization and mechanical properties than low amylose polymer of the TPCS 250
polymer. Besides, the high amylose polymer of the TPMS polymer presented the lowest 251
water absorption. The quickest biodegradation was also found in the TPMS polymer. 252
Moreover, the TPAS polymer presented higher degree of crystallinity and mechanical 253
properties including faster degradable property than those of the TPCS polymer. 254
255
Acknowledgements 256
The authors express their sincere appreciation to the KMITL Research Fund for 257
supporting the study financially. 258
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Table captions 338
Table 1 Chemical compositions of different starches 339
340
Table 1 341
Chemical
composition (%)
Cassava starch Arrowroot
starch
Mungbean
starch
Moisture 0.5 8.6 9.3
Carbohydrate
Amylose
Amylopectin
85.0
16.4
83.6
94.4
19.9
75.4
82.3
37.9
62.1
Protein 0.3 4.2 23.3
Fat 1.1 0.2 1.9
Ash 0.2 2.4 3.0
Others 0.6 0.5 0.9
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Figure Captions 353
Figure 1 IR spectra of TPS polymers by different types of TPS (a), TPAS (b), TPCS and 354
(c) TPMS 355
Figure 2 X-ray diffraction patterns of different TPS polymers (a), TPAS (b), TPCS and 356
(c) TPMS 357
Figure 3 SEM micrographs of fracture surfaces of different TPS polymers (a), TPMS (b), 358
TPCS and (c) TPAS 359
Figure 4 Relationship between water absorption and immersion time of different TPS 360
polymers 361
Figure 5 Mechanical properties of different TPS polymers (a) Stress at maximum load and 362
Young’s modulus (b) Strain at maximum load and Hardness 363
Figure 6 Mechanical properties of the different TPS polymers after soil buried test for 0, 4 364
8 and 12 weeks 365
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Figure 1 373
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Figure 1 IR spectra of TPS polymers by different types of TPS (a), TPAS (b), TPCS and 375
(c) TPMS 376
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379
380
381
382
383
384
(a)
(b)
(c)
Figure 2 385
386
Figure 2 X-ray diffraction patterns of different TPS polymers (a), TPAS (b), TPCS and 387
(c) TPMS 388
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390
391
392
393
394
395
(a)
(b)
(c)
19.6°
21.4°
23.7° 36.0°
Figure 3 396
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Figure 3 SEM micrographs of fracture surfaces of different TPS polymers (a), TPMS (b), 400
TPCS and (c) TPAS 401
Figure 4 402
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Figure 4 Relationship between water absorption and immersion time of different TPS 413
polymers 414
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Figure 5 428
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Figure 5 Mechanical properties of different TPS polymers (a) Stress at maximum load and 438
Young’s modulus (b) Strain at maximum load and Hardness 439
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Figure 6 453
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Figure 6 Mechanical properties of the different TPS polymers after soil buried test for 0, 4 476
8 and 12 weeks 477
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