J Polym Environ (2009) 17:203–207 DOI 10.1007/s10924-009-0139-6
ORIGINAL PAPER
Study on Biodegradable Starch/OMMT Nanocomposites for Packaging Applications Penggang Ren Æ Tingting Shen Æ Fang Wang Æ Xing Wang Æ Zhengwei Zhang
Published online: 27 August 2009 Ó Springer Science+Business Media, LLC 2009
Abstract The thermoplastic starch (TPS) and nanocomposite(TPS/OMMT) was prepared with 15% carbamide, 15% ethanolamine and different contents of organic activated montmorillonite (OMMT) by twin-screw extruder with a 130 °C barrel temperature. Fourier transforms infrared spectroscopy and wide angle X-ray diffraction shown that the alkylamine in dodecyl benzyl dimethyl ammonium bromide could react with MMT via cation exchange reaction. After treated, the d(001)space distance of MMT increased from 1.5 to 1.7 nm. Scanning electron microscope revealed that the lower contents of OMMT could disperse well in the matrixes of TPS. The carbamide, ethanolamine and the OMMT could destroy the crystallization behavior of starch, but only the OMMT restrained this behavior for long-term storing. Mechanical properties investigation indicated that the tensile strength and modulus of TPS/OMMT nanocomposites were better than those of TPS, while the elongation at break was descended with the increasing of OMMT contents. When the content of OMMT was 4%, the tensile strength and modulus of TPS was improved from 4.2 and 42 MPa to 6.0 and 76 MPa, respectively. Keywords Thermoplastic starch Carbamide Ethanolamine Montmorillonite Nanocomposites
P. Ren (&) T. Shen F. Wang X. Wang Z. Zhang Institute of Printing and Packaging Engineering, Xi’an University of Technology, Xi’an, Shaanxi 710048, People’s Republic of China e-mail:
[email protected]
Introduction In the past 20 years, the environmental was deteriorated for the rapidly use of plastics packaging [1, 2]. In order to develop an environmentally friendly material, most of the research attention is focused on the replacement of petrobased commodity plastics in a cost-effective manner by biodegradable materials [3–5]. Biodegradable materials was defined as those materials which can be degraded by the enzymatic action of living organisms, such as bacteria, yeasts, fungi, and ultimate product CO2, H2O, and biomass under aerobic conditions or hydrocarbons, methane and biomass under anaerobic conditions [6]. In the family of biopolymers, starch has been considered as most promising candidate because of its inherent biodegradability, ready availability, and relatively low cost. Starch is a semicrystalline polymer stored in granules as a reserve in most plants. It is a mixture of linear amylose (poly-a-1,4-Dglucopyranoside) and branched amylopectin (poly-a-1,4D-glucopyranoside and a-1,6-D-glucopyranoside). The amylose was linked by a (1,4)-linkages; the amylopectin has a (1,4)-linked backbone and a (1,6)-linked branches. Amylose (approximately 20%) forms the amorphous regions whereas the amylopectin are predominantly responsible for the crystalline [7, 8]. This crystalline property of amylopectin has lead to the poor process capability. So a mass of plasticizers were often introduced to make the starch flowed in the processing. Unfortunately, the widespread application of starch as packaging was still prevented for its poor mechanical properties (compared with most petroleum-based polymers) and strong hydrophilic behaviour (poor moisture barrier) [9–13]. Former researchers have reported that montmorillonite’s (MMT’s) homogeneous dispersion in a continuous polymer matrix in nanosheets could greatly improve the properties
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of polymers, and even could produce new properties that cannot be derived from other composites counterparts [14, 15] the polymer/MMT nanocomposites developed rapidly in the recent decade, and the methods of polymer intercalation and intercalative polymerization resolved the dispersion and interface problems in the preparation of polymer nanocomposites. However, the study on the natural polymers was few [16, 17]. In the present study, biodegradable nanocomposites have been successfully prepared from carbamide/ethanolamine plasticized thermoplastic starch and dodecyl benzyl dimethyl ammonium bromide(12-OREC)-activated montmorillonite. The aims of this study were to evaluate the effect of plasticizer type and montmorillonite concentration on mechanical and crystalline properties of sweet potato starch films.
Experimental Materials Sweet potato starch (11.6% moisture) was obtained from Da-Heng Ltd Co. (Chengdu, Sichuan, China). Carbamide was purchased from Tianjing Chemical Reagent Third Factory (Tianjin, China); ethanolamine was purchased from Tianjing Bo-Di Chemical industry Ltd Co. (Tianjin, China); Sodium montmorillonite (Na-MMT) with a cation exchange capacity (CEC) of 90–100 mmol/100 g was supplied by Zhejiang Feng-Hong clay Ltd Co. (Zhejiang, China). Dodecyl benzyl dimethyl ammonium bromide(12OREC) was obtained from Xi’an chemical reagent and glass apparatus factory.
J Polym Environ (2009) 17:203–207 Table 1 The composition and doping content of TPS/OMMT nanocomposites No.
Starch (g)
Carbamide (g)
Ethanolamine (g)
OMMT (g)
1
210
45
45
2
204
45
45
6
3
198
45
45
12
4
192
45
45
18
5
186
45
45
24
0
premixed (3000 rpm for 2 min) in the High Speed Mixer GH-100Y (made in China), and stored in the tightly sealed plastic bags for 36 h. Then these mixtures were manually fed into the twin screw extruder SJ-25 (diameter 30 mm and L/D25:1) with a screw speed of 30 rpm, the temperature profile along the extruder barrel was 120, 130, 130, and 110 °C, these mixtures were cut into small particles. At the second step, those TPS/OMMT particles were molded between two metallic sheets to form film with 2 mm thickness at hot-press. The temperature was hold 160 °C for 10 min, and then the two metallic sheets with TPS/ OMMT nanocomposites were removed from hot-press to a cool presser. When the temperature was cooled to room temperature at natural rate, the TPS/OMMT nanocomposites film was removed from the metallic sheets. Fourier Transforms Infrared (FT-IR) Spectroscopy The mixture of KBr powder with MMT or OMMT particles molded to pellet. Spectra were obtained in an optical range of 400–4000 cm-1 by averaging six scans at a resolution of 8 cm-1 to minimize the effects of dynamic scanning with FTIR-8400S spectrometer.
Activation of MMT MMT and appropriate water was added into a three-necked flask equipped with a magnetical stirrer, the MMT–water solution was heated to 50 °C slowly, and then the dodecyl benzyl dimethyl ammonium bromide(12-OREC) was added into and heated the solution to 90 °C for 5h, then the solution cooled to room temperature under natural condition. The high speed stirring was keep through the whole process. After being filtrated, dried and ground, the organic activated MMT (OMMT) was gained. The Preparation of TPS/OMMT Nanocomposites The TPS/OMMT nanocomposites were prepared through two steps. At the first step, the carbamide (15 wt%) was dissolved in the ethanolamine (15 wt%), the solution, starch and different OMMT (as shown in Table 1) were
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X-ray Diffractometry The TPS/OMMT nanocomposites, OMMT, MMT and TPS were carried out using a XRD-7000 X-ray diffractometer (40 kV, 40 mA) equipped with a Ni-filtered Cu radiation and a curved graphite crystal monochromator at a scanning rate of 2°/min. The diffractometer was equipped with 18 divergence slit, a 0.3 mm receiving slit and a 1° scatter slit. Mechanical Testing According to the GB1040-79 standard of China, the tensile stress, strain, yield stress, yield strain, Young’s modulus and breaking energy of nanocomposites were measured using Testometric Materials Testing Machines. The crosshead speed was 10 mm/min. All measurements were performed for five specimens and averaged.
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Result and Discussion Activation of MMT MMT,d001=1.50nm O
intensity (cps)
4.5
O
4
OMMT,d001=1.70nm
2
MMT
OMMT
2835 2927 3000
2500
2000
1500 -1
Wavenumber /cm
Fig. 1 FT-IR spectra of MMT and OMMT
1000
500
5
6
7
8
o
Fig. 2 WXRD patterns of MMT and OMMT
TPS
ST 17.2 15.2
The X-ray diffraction pattern of pure sweet starch (ST) and thermoplastics starch (plasticized by 15 wt% carbamide and 15 wt% ethanolamine) (TPS) were shown in Fig. 3. From the pattern, it can be found that there are three
3500
4
2Theta ( )
TPS/OMMT Nanocomposites
4000
3
intensity (cps)
The FT-IR absorption spectra of MMT and OMMT were shown in Fig. 1. A comparison of spectra displays that the absorption peaks of bands at 2927 and 2835 cm-1 were appeared when MMT was activated by dodecyl benzyl dimethyl ammonium bromide(12-OREC). The band at 2927 and 2835 cm-1 is related to –CH3 and –CH2– stretching vibration, respectively. It indicates that alkylamine can react with MMT via cation exchange reaction and produced OMMT. The X-ray diffraction pattern (XRD) of diagram of MMT and OMMT were displayed in Fig. 1. It was shown that the diffraction peak of montmorillonite(001) crystal plane moved from 4.5 to 4° after it activated by dodecyl benzyl dimethyl ammonium bromide(12-OREC). So the distances d(001) between the layers can be obtained according to the Bragg diffraction equation (2dsinh = k). The data of MMT and OMMT were 1.50 and 1.70 nm, respectively. The increase of distances between the layers indicated that the dodecyl benzyl dimethyl ammonium bromide had intercalated into the layers of MMT. Thus the OMMT can act as nano-scale filler to modify polymer materials due to its increasingly swollen gallery spacing (Fig. 2).
10
15
o
23.1
o
20
o
25
30
o
2Theata ( )
Fig. 3 WXRD patterns of ST and TPS
crystallization peaks in the ST sample. The peaks at 15.2 and 17.2° were attributed to A type crystallization. The peak at 23.1° was the VH type crystallization. These crystallization structures made the starch brittle and process difficult. Compare with the ST, the height of crystallization peaks of TPS descended obviously. It indicated that the introduced carbamide and ethanolamine, as plasticizers, could form hydrogen bonds with starch under the high temperature and high shear stress effect produced by twin screw extruder. So the crystallization structure of starch was destroyed. As a result, the starch mechanical and processing properties were improved. Figure 4 shows the X-ray diffraction pattern of TPS and TPS/OMMT (6 wt%) nanocomposites after being kept for 100 days at RH = 100%. It was found out that at 13.6 and 22.6° there appeared a type A and type VH crystallization peaks in TPS pattern. It indicated that the hydrogen bond
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existing of OMMT. When the content of OMMT was little, the crystallization behavior of starch can hardly be observed. Though the crystallization behavior of starch can still be observed at the high contents of OMMT, The dimension of global crystal in starch descend obviously. It reason may be attributed to the small contents of OMMT can dispersed well in the matrixes of TPS, while the high contents of OMMT tend to agglomerated in starch.
intensity (cps)
TPS/OMMT
TPS
Mechanical Properties 0
10
13.6
o
22.6
15
20
o
25
30
o
2Theata ( )
The stress–strain curves of TPS and TPS/OMMT were shown in Fig. 6. It was found that the tensile strength and modulus of TPS/OMMT nanocomposites increased with the content of OMMT increasing. The tensile strength
Fig. 4 WXRD patterns of TPS and TPS/OMMT nanocomposite after 100 days at RH = 100%
Fig. 5 SEM photographs of starch and TPS/OMMT nanocomposites. a Pure starch, b TPS/2 wt% OMMT, c TPS/ 4 wt% OMMT, d TPS/6 wt% OMMT, and e TPS/8 wt% OMMT
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e d 6
Tension strength /MPa
between carbamide, ethanolamine and starch was weakened by the action of moisture. The carbamide and ethanolamine mixture plasticizers could not restrain the crystallization behavior of starch caused by long storing under the moisture condition. While these peak at the same position in the TPS/OMMT nanoplastics were not appeared. It demonstrated that the OMMT in nanocomposites restrained the growth of global crystal existing between starches in TPS, the results might be attributed to the OMMT particles can form a large of crystal nucleus in TPS. So it is harmed to form integrated global crystal in TPS. The scanning electron microscope (SEM) photographs of starch and TPS/OMMT nanocomposites were shown in Fig. 5. A comparison of photographs demonstrated that the crystallization behavior of starch was restrained for the
c b a
4
a-0% b-2%OMMT c-4%OMMT d-6%OMMT e-8%OMMT
2
0 0
20
40
60
80
strain /%
Fig. 6 Stress–strain curves of TPS/OMMT nanocomposites with different mass contents
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increased from 4.2 MPa for TPS, to 5.2 MPa for 2% OMMT, 6 MPa for 4% OMMT, 6.4 MPa for 6% OMMT and 6.8 MPa for 8% OMMT introduced, respectively. The tensile modulus of TPS was increased from 42 to 62°MPa for 2% OMMT, 76°MPa for 4% OMMT, 88°MPa for 6% OMMT and 102°MPa for 8% OMMT introduced, respectively. When OMMT content is in the range of 2–4%, the mechanical properties of TPS/OMMT nanocomposites are obviously ameliorated. While the elongation at break descended with the increasing of OMMT contents. It descended from 90% for TPS to 50% for the TPS nanocomposites with 8% OMMT introduced. It indicated that the mechanical properties of nanocomposites were better than those of TPS except the elongation at break. This can be explained as follows. On one hand, OMMT can interact with TPS to form physical crosslinking network, so as to reinforce the molecular chains of TPS. As a result, both modulus and strength of TPS were increased, while the elongation at break decreased. On the other hand, the hydrogen bonding formed in TPS became stronger with the increase of OMMT content, leading to the increase of Young’s modulus and stress. However, the impermeability of OMMT would decrease the flexibility of starch molecules and the strain of materials.
Conclusion TPS and TPS/OMMT nanocomposites were successfully prepared by the twin screw extruder. FT-IR spectroscopy revealed that the alkylamine in dodecyl benzyl dimethyl ammonium bromide(12-OREC) can react with MMT via cation exchange reaction. X-ray diffraction shown that the d(001)space distance of MMT increased from 1.5 to 1.7 nm for treated by 12-OREC. The investigation of crystallization behavior displayed that carbamide and ethanolamine can destroyed the crystallization behavior of starch in the course of processing, but only the OMMT can restrained the crystallization behavior of starch for longterm storing. SEM show that the lower contents of OMMT can disperse well in the matrixes of TPS. The mechanical properties of TPS/OMMT nanocomposites reveal that the tensile strength and modulus increased with the content of OMMT increasing. While the elongation at break descend with the OMMT increased. When the content of OMMT was 4%, the tensile strength and modulus of TPS was improved from 4.2 and 42 MPa to 6.0 and 76 MPa, respectively. It was better than TPS.
207 Acknowledgements This work was supported by department of education of Shaanxi province, China (07JK343).
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