BIO-BASED POLYETHYLENE/ RICE STARCH COMPOSITE Sathaphorn O-suwankul a, Kittima Bootdee a, Manit Nititanakul* a a The Petroleum and Petrochemical College, Chulalongkorn University, Bangkok, Thailand Keywords (not more than 5): composite material, plastic starch composite, polyethylene composite, LDPE grafted maleic anhydride ABSTRACT Starch/low density polyethylene composites have been widely studied because of their biodegradability. The properties of starch/ldpe composites were inferior to neat plastics because starch is not compatible with polyethylene because of the difference in their polarities. In this research, glycerol (GC) were used as plasticizer and polyethylene grafted maleic anhydride (PE-g-MA) was used as a compatibilizer. Rice starch content was varied between 10, 20, and 30 %wt. Results showed that increasing amount of starch reduced tensile, flexural, and impact properties of materials. Addition of PE-g-MA improved tensile, flexural and impact properties, except elongation at break of the materials. GC the improvement of flowability but they reduced the stiffness; therefore, they increased impact strength of the materials. The morphology from SEM images showed that PE-g-MA and GC promoted compatibility between starch and LDPE matrix. Email:*manit.n@chula.ac.th INTRODUCTION There are many studies relating to plastic composites and it has been becoming an interesting topic because it gives combination of performance between matrix and filler. Biodegradable polymer composite is another field that has been popular researched because polyolefin plastic such as polypropylene and polyethylene are difficult to decompose in the environment. Even though some polymer such as polycaprolactone and poly(lactic acid) can totally be decomposed, they are not widely used because their cost are not competitive compared to polyolefin materials. Nowadays, biodegradable materials are being used to make composite material with polyolefin, for example corn starch and rice starch, in order to get partial biodegradable property, cost competitiveness, and performance improvement. Although corn starch is widely studied in biodegradable material because of its availability and cost competitiveness, corn starch containing material trends to show inferior mechanical properties because corn starch normally has bigger particle size. In Comparison, the particle size of rice starch (4 µm) is smaller than corn starch (25 µm) (J. Jane et al., 1992); thus, rice starch trends to provide better mechanical properties. Rice is widely cultivated in Thailand and made up of biopolymer which is starch; therefore, it is sustainable, inexpensive, and renewable source of biopolymer. Starch can be used to partially replace petroleum based plastics so reducing the dependence on petroleum can be achieved. In addition, it also Petrochemical and Materials Technology Tuesday May 23, 2017, Pathumwan Princess Hotel, Bangkok, Thailand Page 1
contributes to the biodegradability of the composite. However, the performances of starch/polyolefin composites normally have poor mechanical property compared to conventional plastics because starch is immiscible in polyolefin due to differences in polarity. In addition, starch is more brittle and sensitive to water. In order to improve the compatibility and mechanical properties of the composite, dispersion and interfacial properties between starch and polyolefin can be improved by addition of compatibilizer and plasticizer. (R. Chandra et al., 1996). Polyethylene grafted maleic anhydride (PE-g-MA) can be used as a compatibilizer in order to increase interfacial adhesion between polyethylene and starch via an esterification reaction between the hydroxyl group of the starch chain and the anhydride group of the maleic anhydride. Tensile strength and tensile modulus of the starch/polyethylene composite materials were also improved compared to a system without compatibilizer (R. Chandra et al., 1996). Glycerol is a hydrophilic substance which is a common plasticizer of starch because of its high boiling point, availability and low cost. (Kasee et al., 2012). The plasticizer interacts through hydrogen bonding with the starch chains. The interaction increases at higher temperatures, probably due to H-bond formation. As a consequence, the material behaves like a rubber, with a rise in matrix mobility, and a decrease in viscosity. Wang and coworkers (2005) prepared composite material of low density polyethylene (LDPE), glycerol, and starch. Glycerol was added at 25% of starch weight. The results showed that the distribution of rice starch particle in LDPE matrix became homogenous after the addition of glycerol. Because of the good distribution of starch, tensile strength and elongation at break were improved compared to a material without glycerol. (Adeodato et al., 2011). In this study, LDPE/rice starch were prepared. Glycerol was used as a dispersing agent in order to increase dispersion and reduce agglomeration of rice starch in low density polyethylene (LDPE) matrix. Increase of compatibility between starch and LDPE was achieved by adding polyethylene grafted maleic anhydride (PE-g-MA). The effects of rice starch content were also studied. In this study, morphology, mechanical properties, and processability of the obtained composite were also investigated. EXPERIMENTAL A. Materials and Equipment Materials: Low density polyethylene (LDPE) Low density polyethylene grafted maleic anhydride (LDPE-g-MA) Rice starch Glycerol All the chemicals were used as received Equipment: 1. Twin screw extruder 2. Scanning electron microscope (SEM) 3. Attenuated total reflectance (ATR) - FTIR Spectroscopy Petrochemical and Materials Technology Tuesday May 23, 2017, Pathumwan Princess Hotel, Bangkok, Thailand Page 2
4. Universal testing machine (UTM) B. Experimental Procedures Figure 1 Preparation of low density polyethylene/rice starch composite procedure flow diagram. C. Preparation of composite and chemicals Rice starch, glycerol (25%wt of starch weight), PE-g-MA, and LDPE were premixed at room temperature by using a KitchenAid mixer at starch-to-ldpe ratios of 10, 20, and 30% (w/w). These mixtures were then melt-blended in a laboratory scale twin-screws counter-rotating extruder (Labtech) at 150-170 C and 35 rpm. The neat LDPE was subjected to the same procedure as LDPE/rice starch composites. D. Characterization of LDPE/rice starch composite Attenuated Total Reflectance (ATR) - FTIR Spectroscopy ATR-FTIR spectrum was obtained by using Thermo Scientific Nicolet is5.the selected spectrum resolution and the scanning range were 4 cm -1 and 4000-650 cm -1, respectively. FTIR spectra with percentage transmittance versus wavelength (cm -1 ) were acquired after the scanning process Scanning Electron Microscope, SEM Surface morphology of the composite material was characterized using a Hitachi Scanning Electron Microscopy (Model S-4800, Hitachi High Technologies, Japan) at a voltage of 2 kv on a sample which were previously deposited on carbon tape mounted on sample stubs. Petrochemical and Materials Technology Tuesday May 23, 2017, Pathumwan Princess Hotel, Bangkok, Thailand Page 3
Universal testing machine, UTM UTM (Instron 33R4206) was used to study the mechanical properties of the composite. Results for each formulation were obtained from five dumbbell-shaped samples. The constant rate used was 50 mm/min. Load cell was 5 kn. Gauge length was 50 mm. Tensile mode was used to evaluate tensile strength. RESULTS AND DISCUSSION A. Fourier transform infrared spectrometry (FTIR) Figure 2 FTIR Spectra of (a) neat LDPE, (b) LDPE/starch, (c) LDPE/starch/PE-g-MA, (d) LDPE/starch/glycerol. Figure 2 showed FTIR spectra of LDPE/rice starch composite. C-H stretching bands were found between 3000-2840 cm -1 region, medium-strong C-H bending bands were between 1465-1450 cm -1 region, and C-H out-of-plane bending was observed at 740-719 cm -1. These were consistent with polyethylene spectrum and all the characteristic peaks which we observed these peaks in all formulations. The LDPE/rice starch composite showed a broad O-H stretching absorbance in the 3600-3000 cm -1 region. In the formulations which contained glycerol, they showed a stronger board peak of O-H stretching absorbent consistent with O-H in the glycerol. C-O stretching band at 1190-960 cm -1 were observed in composites containing starch. Moreover, we could not see this peak in the samples that contained PE-g- MA which has anhydride functional group because only small amount (1 phr.) was added; therefore, the difference of the graph was not clearly observed. These FTIR results agreed with the studies of R. Chandra et al., 1996 B. Morphological Structure Petrochemical and Materials Technology Tuesday May 23, 2017, Pathumwan Princess Hotel, Bangkok, Thailand Page 4
a) b) c) d) e) f) Figure 3 SEM micrographs of (a) neat LDPE, (b) LDPE/starch (10%wt), (c) LDPE/starch (30 %wt), (d) LDPE/starch (30%wt)/PE-g-MA, (e) LDPE/starch (30%wt)/glycerol, (f) LDPE/starch (30%wt)/ PE-g-MA/glycerol. Figure 3 showed micrographs from a fractured surface of LDPE/rice starch composites. The effect of PE-g-MA and glycerol were also studied. It was found that wettability of polymer matrix was changed according to the incorporated compatibilizer in both systems, with and without glycerol. There was a stronger interfacial adhesion between rice starch and LDPE matrix. This was evidenced by the amount of gap and void. The gaps and void between fillers and polymer interfaces were minimized. In addition, it is also difficult to differentiate fillers from matrix resulted from the good compatibility between them. However, adding only glycerol, which is plasticizer of starch, did not show significantly improvement of starch dispersion and the agglomeration of starch was observed. C. Tensile property Petrochemical and Materials Technology Tuesday May 23, 2017, Pathumwan Princess Hotel, Bangkok, Thailand Page 5
Figure 4 Tensile strength at break of LDPE/rice starch composites. Tensile strength at break of LDPE composite was shown in the figure 4. It was observed that tensile strength at beak of material decreased when starch content was increased. It was interesting to note that composites contained PE-g-MA showed increasing of tensile strength at break. The results showed that tensile strength was equal or increased slightly when compared to the composite without compatibilizer due to the increase in interfacial adhesion between starch and maleic anhydride. When glycerol was added, tensile strength at break was reduced because of the effect of plasticizer. CONCLUSIONS LDPE composites based on LDPE and rice starch were prepared by extrusion and injection process. The amount of starch was varied by 10, 20 and 30%wt. and they were modified by PE-g-MA and glycerol. It was found that increasing rice starch content would reduce mechanical properties of the composites which were observed by the reduction of tensile strength at break. PE-g-MA showed positive effect on mechanical properties which increased in tensile strength and improved compatibility between rice starch and LDPE base as showed by SEM micrographs. The SEM images also support the improvement of tensile strength after using PE-g-MA. Voids and gaps between rice starch and LDPE matrix were reduced. Even though the results showed property improvement by PE-g-MA, FTIR could not detect any reaction between PE-g-MA and rice starch, this is because only small amount of PE-g-MA was added. Glycerol which acted as plasticizer of starch slightly improved tensile strength at break compared to formulation without it because glycerol increase dispersion of rice starch in LDPE matrix. ACKNOWLEDGEMENTS This work is funded by the Petroleum and Petrochemical College and Center of Excellence on Petrochemical and Materials Technology. Petrochemical and Materials Technology Tuesday May 23, 2017, Pathumwan Princess Hotel, Bangkok, Thailand Page 6
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