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Reactive blending of thermoplastic starch, epoxidized natural rubber and chitosan
European Polymer Journal 84 (2016) 292–299
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European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Reactive blending of thermoplastic starch, epoxidized natural rubber and chitosan Kittisak Jantanasakulwong a,⇑, Noppol Leksawasdi a, Phisit Seesuriyachan a, Somchai Wongsuriyasak a, Charin Techapun a, Toshiaki Ougizawa b a b
School of Agro-Industry, Faculty of Agro-Industry, Chiang Mai University, Mae-Hea, Mueang, Chiang Mai 50100, Thailand Department of Chemistry and Materials Science, Tokyo Institute of Technology, 2-12-1-S8-33, O-okayama, Meguro-ku, Tokyo 152-8552, Japan
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Article history: Received 22 June 2016 Received in revised form 12 September 2016 Accepted 21 September 2016 Available online 22 September 2016 Keywords: Thermoplastic starch (TPS) Epoxidized natural rubber (ENR) Chitosan (CTS) Reaction Epoxy
a b s t r a c t Thermoplastic starch (TPS) was prepared by melt blending cassava starch and glycerol (70/30) at 140 °C. Chitosan (CTS) was incorporated during TPS preparation. The TPS/CTS sample was melt blended with epoxidized natural rubber (ENR) at 140 °C. In the TPS/ENR/CTS blend, adding CTS and ENR improved the tensile strength and elongation at break, respectively. Morphology of the TPS/ENR blend showed the dispersion of largesized ENR particles in the TPS matrix. Adding CTS reduced the size of the ENR particles. Incorporating CTS also enhanced the melt viscosity of the blend, which suggested a reaction between CTS and ENR. FTIR confirmed that the amino groups of CTS reacted with the epoxy groups of ENR. This reaction between the CTS amino groups with the ENR epoxy groups improved the mechanical properties of the TPS/ENR/CTS blend. Ó 2016 Published by Elsevier Ltd.
1. Introduction Thermoplastic starch (TPS) can be prepared by blending starch with a plasticizer, such as water, glycerol or sorbitol [1]. The plasticizer penetrates into the granules of the starch, interrupting its crystalline structure and inducing the formation of an amorphous structure when subjected to high temperatures and shear forces during the melting process [2]. Therefore, the amorphous starch, following the addition of a plasticizer, behaves like a thermoplastic polymer during the melt stage. Some polymers that have been blended with TPS to improve its mechanical properties include: polypropylene [3], polyethylene [4,5], poly(lactic acid) [6–8] and poly(butylenesadipate-co-terephthalate) [9]. Natural rubber (NR) that contains epoxy groups can be modified into epoxidized natural rubber (ENR). Natural rubber can be epoxidized to various degrees: 25%, 50%, and 75% epoxidation is referred to as ENR-25, ENR-50, and ENR-75, respectively. A few studies have reported using ENR to improve the toughness of PLA [10–13]. The improved toughness of PLA by 20% epoxidized natural rubber has been reported [10]. Akbari et al. reported that adding ENR improved the impact toughness of a PLA/talc blend [11]. Wang et al. reported that dicumyl peroxide (DCP) improved the compatibility between PLA and ENR, as well as the impact toughness [13]. Little research exists on the blending of TPS with rubber. Carvalho et al. investigated the morphology of a TPS and natural rubber latex (NRL) blend and found that the phase morphology of the blend depended on the glycerol content [14]. ENR is used in various fields, such as polymer blends [15], polymer modification [16] and polymer composites [17]. ⇑ Corresponding author. E-mail address: [email protected] (K. Jantanasakulwong). http://dx.doi.org/10.1016/j.eurpolymj.2016.09.035 0014-3057/Ó 2016 Published by Elsevier Ltd.
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Chitosan (CTS) is a linear polymer of a (1 ? 4)-linked 2-amino-2-deoxy-b-D-glucopyranose, which is derived from N-deacetylation [18]. Chitosan, a natural polymer, has remarkable properties, including its high mechanical properties, biodegradability, bio-compatibility, antimicrobial activity, and non-toxicity. Chitosan easily reacts with other reactive groups, due to the presence of NH2 groups. Chitosan’s structure has been modified by chemical modification, graft reactions, and ionic interactions. Chitosan is highly hydrophobic, or insoluble in water, but becomes soluble in water with the addition of acids. Because of its physical and chemical properties, chitosan has been used in a range of applications, including pharmaceutical, food, textile, agricultural, biomedical, and cosmetic products [18]. The objective of this research was to develop TPS blending with ENR rubber and CTS. We investigated the effect of adding chitosan to a TPS and ENR blend on its mechanical properties, morphology, rheological properties, and reaction mechanism. 2. Material and methods Cassava starch (Dragon Fish brand, amylose/amylopectin content 17%/83%, moisture content of 11% total weight and molecular weight of 1.34 108 g/mol) was purchased from Tong Chan registered ordinary partnership, Thailand. Glycerol and chitosan (CTS) (deacetylation degree of 85% and molecular weight of 500 kDa) were purchased from Union Science Co., Ltd., Thailand. Epoxidized natural rubber (ENR) with 25% epoxidation was purchased from Muang Mai Guthrie Public Co., Ltd., Thailand. Lactic acid (99%) was produced by Merck (Darmstadt, Germany). 2.1. Sample preparation Cassava starch was premixed with glycerol (70/30) and distilled water (100 mL/50 g starch) by overhead stirrer at 500 rpm in a water bath at 90 °C. Chitosan is highly hydrophobic, or insoluble in water, but becomes soluble in water with the addition of acids. Chitosan was added during the premixing process along with lactic acid (2% v/v of aqueous lactic acid solution, 100 mL), then melt-blended by a two-roll mill (Pirom-Olarn Co. Ltd., Thailand, PI-140) at 140 °C for 10 min to prepare the TPS and TPS/CTS blend. TPS or TPS/CTS were melt-blended with ENR at 140 °C for 5 min by a brabender internal mixer (Labo Plastomill, Toyoseiki Co. Ltd., Japan). Table 1 shows the composition of TPS, ENR and CTS of the blends. The samples were put in a mold and compressed into sheets for tensile tests and into a film for FTIR by a hot-compress machine with pressure of 500 psi at 140 °C for 3 min. 2.2. Scanning electron microscopy (SEM) Morphology of the samples was observed by scanning electron microscopy (SEM) (SM-200, Topcon Corp., Japan). The samples were prepared as sheets by compression molding at 140 °C for 3 min. The length, width and thickness were 30 mm, 5 mm and 0.5 mm, respectively. Liquid nitrogen was used to break the samples, and then the fractured surface of the samples was extracted by immersing samples into toluene at 60 °C for 24 h. The extracted surface of the samples was coated with a thin layer of gold, and observed with an acceleration voltage of 10 kV. 2.3. Tensile properties measurements Tensile properties of the samples were measured using a tensile tester (Tensilion UTM-II-20; Orientec Co. Ltd., Japan) at 2 mm/min crosshead speed. The bone-shaped samples were prepared as sheets by compression molding at 140 °C for 3 min. The gage length, width and thickness of the sample were 10 mm, 3 mm and 0.5 mm, respectively. 2.4. Contact angle Water droplet contact angle was observed by drop shape analysis (DSA30E, Krüss Co. Ltd., Germany) to estimate the wettability of samples. The samples were prepared as sheets by hot-compression at 140 °C for 3 min. Water was dropped onto the surface of samples and images were recorded at 1 min, 2 min and 3 min.
Table 1 The composition and codes of blends prepared from thermoplastic starch (TPS), epoxidized natural rubber (ENR) and chitosan CTS. Sample
TPS/ENR TPS/ENR/CTS1 TPS/ENR/CTS2.5 TPS/ENR/CTS5
Composition (wt%) TPS
ENR
CTS
90 89 87.5 85
10 10 10 10
– 1 2.5 5
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2.5. Rheological measurement The complex dynamic viscosity (g⁄) was measured by a rheometer (Dynamic Analyzer RDA II, Rheometric Scientific Corp., USA). The samples were prepared as sheets by compression molding to a disc with thickness of 1 mm and diameter of 25 mm at 140 °C for 3 min. Measurements were carried out at frequencies from 0.1 rad/s to 1000 rad/s at a strain rate of 10% and constant temperature of 140 °C. 2.6. Fourier transform infrared spectroscopy A Fourier transform infrared spectrometer (FT/IR-480plus, Jasco Corp., Japan) was used to observe the reaction between the CTS and ENR. The samples were prepared as thin-films with thickness of about 100 lm by hot compress at 140 °C for 3 min. IR spectra were measured from 600 cm 1 to 4000 cm 1, with a resolution of 4 cm 1. 3. Results and discussion 3.1. Morphology The morphology of a bio-based polymer by TPS blending showed TPS matrix phase and rubber as dispersed particles. Fig. 1 shows SEM images of the fractured surface samples with the rubber phase extracted by toluene at 60 °C for 24 h. The TPS/ENR blend showed large holes resulting from the extraction of rubber particles of about 5–20 lm (Fig. 1(a)), due to the distribution of large ENR particles in the TPS matrix. The TPS/ENR/CTS blends with CTS 1 wt%, 2.5 wt% and 5 wt% showed smaller holes resulting from the extraction of rubber particles of about 1 lm, 0.5 lm and 0.3 lm, respectively (Fig. 1(b–d)). It was indicated that the decreasing rubber particle size by the addition of CTS was due to the reaction occurring between the amino groups (ANH2) of CTS and the epoxy groups of ENR. The decreasing size of rubber particles in the polymer blend following a reaction between amino groups (ANH2) and epoxy groups has also been reported [19]. 3.2. Mechanical properties Fig. 2 shows the stress-strain curves of TPS, TPS/ENR (90/10) and TPS/ENR with chitosan 1 wt%, 2.5 wt% and 5 wt%. TPS showed high slope at early state of stress-strain curve which related to the elastic modulus of the sample. The tensile strength, elongation at break and elastic modulus were 4 MPa, 50% and 157 MPa, respectively. The TPS/ENR blend showed lower elastic modulus (47 MPa) and tensile strength (2.5 MPa), but the elongation at break (101%) was higher than for neat
Fig. 1. SEM images of the extracted fracture surface by toluene of (a) TPS/ENR, (b) TPS/ENR/CTS1, (c) TPS/ENR/CTS2.5 and (d) TPS/ENR/CTS5.
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Fig. 2. Stress-strain curve of (a) TPS, (b) TPS/ENR, (c) TPS/ENR/CTS1, (d) TPS/ENR/CTS2.5, (e) TPS/ENR/CTS5 and (f) ENR.
TPS (50%). Adding chitosan (CTS) to the TPS/ENR blend enhanced its tensile strength and increased the elastic modulus compared to the TPS/ENR blend alone. The decrease in elastic modulus of the TPS/ENR blend may reflect the low elastic modulus of ENR rubber and the connection between the chains of TPS and ENR through the reaction between the AOH groups of TPS and the epoxy groups of ENR. Reaction between AOH groups and epoxy groups has been previously reported [20]. The decreasing of modulus in a polymer and rubber blend with some reactions occurring between them has been reported [21]. Adding CTS 5 wt% increased the tensile strength, elongation at break and elastic modulus to 8.2 MPa, 80% and 104 MPa, respectively. The improvement in tensile properties of the TPS/ENR/CTS blend indicated some CTS remaining in the TPS, the miscibility of TPS/CTS, the increasing reaction by the addition of CTS and the changing of ENR chemical structure by reaction between CTS and ENR. It was considered that adding CTS improved the tensile properties of the blend. 3.3. Contact angle Wettability of samples was observed by water drop contact angle with recording time at 1, 2 and 3 min after dropping the water droplet on to the samples’ surface, as shown in Fig. 3. The TPS showed water contact angle 90° at 1 min and decreased
Fig. 3. Water droplet contact angle of TPS, TPS/ENR, and TPS/ENR/CTS5 at 1–3 min.
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to 62° at 3 min. The decreasing water contact angle with recording time indicated the low surface tension of hydrophilic TPS. The TPS/ENR also showed a similar trend with TPS from the water contact angle reducing from 81° to 60° at 1 min to 3 min, respectively. The water contact angle of TPS showed almost the same degree with the TPS/ENR (60° at 3 min) which suggested to the same surface tension. The TPS/ENR/CTS presented low wettability with high water contact angle from 96° at 1 min to 90° at 3 min. The high water contact angle of TPS/ENR/CTS blend was suggested to miscibility of hydrophobic CTS with TPS phase. The increasing water contact angle of TPS/CTS blends has been reported [22,23]. The interfacial reaction between TPS and ENR was suggested to occur during the melt blending process by the addition of CTS; the rheological properties and FTIR confirmed this reaction. 3.4. Rheological properties The morphology and melt viscosity of the blended polymers are affected by interfacial reaction between the component polymers, due to the formation of chemical bonds during the melt blending process. The complex melt viscosity (g⁄) of neat polymers and the blended samples was observed, as shown in Fig. 4. The TPS/ENR blend showed a melt viscosity value between neat TPS and ENR. The TPS/ENR/CTS blends at all of the tested CTS concentrations showed higher melt viscosity values than the neat TPS, ENR, and the TPS/ENR blends. However, the TPS/ENR/CTS5 blends presented lower melt viscosity
Fig. 4. Complex melt viscosity of TPS, ENR, TPS/ENR and TPS/ENR/CTS blends at 140 °C.
Fig. 5. FTIR spectra of (a) CTS, (b) ENR, (c) TPS/ENR, (d) TPS/ENR/CTS5, and (e) TPS.
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at high frequency than adding CTS 1 wt% and 2.5 wt% because the higher amount and deformation of CTS at 140 °C reduced chain length and increased melt flow rate of the blend at high frequency. The higher melt viscosity of the TPS/ENR/CTS blends than the TPS/ENR blend was likely due to a reaction between the epoxy groups of ENR and the amino groups of CTS. The increasing melt viscosity in the polymer blends due to a reaction between epoxy groups and amino groups has been reported [21,24]. 3.5. Reaction mechanism FTIR was used to observe the reaction between CTS and ENR. Fig. 5 shows the FTIR spectra between wave number 600–4000 cm 1 of neat TPS, CTS, ENR, the TPS/ENR and the TPS/ENR/CTS5 blends. In the FTIR spectrum of ENR, the absorption band at 870 cm 1 represented the asymmetric epoxide ring stretching groups of ENR [25,26]. In the FTIR spectrum of CTS, absorption bands at 1639 cm 1 and 1561 cm 1 were related to the vibrations of carbonyl bonds (C@O) of the amide group and to the vibrations of the protonated amine group (ACONHA), respectively [27,28]. Starch showed characteristic peaks at 3360 (AOH stretching) 1643 cm 1 (AOH bending), 1016 cm 1 and 929 cm 1 (ACO stretching) [29]. In the TPS/ENR blend, the epoxide peak at 870 cm 1 of ENR was not observed due to a small amount of ENR; also, it joined with
Fig. 6. FTIR spectra of (a) TPS/ENR, (b) TPS/ENR/CTS1, (c) TPS/ENR/CTS2.5 and (d) TPS/ENR/CTS5.
Fig. 7. FTIR spectra of (a) neat ENR and (b) the extracted ENR phase from TPS/ENR/CTS5 by toluene at 60 °C for 24 h.
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Fig. 8. Expected reaction between amino groups of CTS and epoxy groups of ENR rubber.
the shoulder peak of TPS at 900 cm 1. The TPS/ENR blend showed peak intensity at 3360 lower than TPS. The decreasing of peak intensity at 3360 suggested to a reaction between AOH groups of TPS and epoxy groups of ENR. The TPS/ENR/CTS blend showed new peaks at 1713 and 1746 cm 1, which were not observed in neat TPS, ENR, CTS and the TPS/ENR blend. Therefore, high intensity of the peaks at 1713 and 1746 cm 1 indicated new peaks occurring due to a reaction during melt blending of the TPS/ENR/CTS blend. The TPS/ENR/CTS blend with CTS 1 wt%, 2.5 wt% and 5 wt% also presented peaks at both 1713 and 1746 cm 1; these two peaks were not observed in the TPS/ENR blend, as shown in Fig. 6. The ENR phase was extracted from the TPS/ENR/CTS blends by toluene to confirm the reaction between ENR and CTS. FTIR spectra of neat ENR and the ENR extracted from the TPS/ENR/CTS blends by toluene are presented in Fig. 7. The FTIR spectra of ENR showed a small peak at 1746 cm 1; this peak was observed with high intensity in the spectrum of the extracted ENR from the TPS/ENR/CTS blends. The increasing peak intensity at 1746 cm 1 of the extracted ENR from the TPS/ENR/CTS blends sample was indicative of a new CAO vibration spectra peak formed by a reaction between the epoxy groups of ENR and the amino groups of CTS. The reaction between the epoxy and amino groups has been previously reported [21,24,30–33]. Our results confirmed that CTS formed a chemical bond with ENR through a reaction between the amino groups of CTS and the epoxy groups of ENR (the expected reaction is shown in Fig. 8). Therefore, the occurrence of this reaction by the addition of CTS into the TPS/ENR blend and the miscibility between CTS and TPS were indicated to improve the mechanical properties and wettability of the TPS/ENR/CTS blend. The miscible blending between TPS and CTS has been reported [34–37]. 4. Conclusions A new TPS composite formed by blending TPS/ENR/CTS, with high mechanical properties and low wettability, was successfully developed by reactive melt blending. Incorporating CTS into TPS/ENR blend improved the tensile properties, morphology and wettability of the blend. The TPS/ENR/CTS blend showed phase-separated morphology, with the rubber particles dispersed in the TPS matrix. CTS increased the melt-viscosity of the TPS/ENR/CTS blend, because the increased reaction between CTS and ENR enhanced the chain length of the blend. FTIR results confirmed that the amino groups of CTS reacted with the epoxy groups of ENR. This reaction and the miscibility between TPS and CTS indicated improved properties of the TPS/ENR/CTS blend. Acknowledgments The authors gratefully acknowledge the financial and/or in-kind support from the Project of the National Research University (NRU) from the Office of Higher Education Commission (OHEC), Ministry of Education, Thailand, the National Research Council of Thailand (NRCT), Bioprocess Research Cluster (BRC), School of Agro-Industry, Faculty of AgroIndustry, Chiang Mai University (CMU), and the Tokyo Institute of Technology (TIT). References [1] L. Yu, G. Christie, J. Gray, U. Dutt, T. Harvey, Effect of additive on gelatinization, rheological properties and biodegradability of thermoplastic starch, Macromol. Symp. 1441 (1999) 371–374. [2] L. Moscicki, M. Mitrus, A. Wojtowicz, T. Oniszczuk, A. Rejak, L. Janssen, Application of extrusion-cooking for processing of thermoplastic starch (TPS), Food. Res. Int. 47 (2012) 291–299. [3] D.S. Rosa, C.G.F. Guedes, C.L. Carvalho, Processing and thermal, mechanical characterization of post-consumer polyolefins/thermoplastic starch blends, J. Mater. Sci. 42 (2007) 551–557.
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European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Reactive blending of thermoplastic starch, epoxidized natural rubber and chitosan Kittisak Jantanasakulwong a,⇑, Noppol Leksawasdi a, Phisit Seesuriyachan a, Somchai Wongsuriyasak a, Charin Techapun a, Toshiaki Ougizawa b a b
School of Agro-Industry, Faculty of Agro-Industry, Chiang Mai University, Mae-Hea, Mueang, Chiang Mai 50100, Thailand Department of Chemistry and Materials Science, Tokyo Institute of Technology, 2-12-1-S8-33, O-okayama, Meguro-ku, Tokyo 152-8552, Japan
a r t i c l e
i n f o
Article history: Received 22 June 2016 Received in revised form 12 September 2016 Accepted 21 September 2016 Available online 22 September 2016 Keywords: Thermoplastic starch (TPS) Epoxidized natural rubber (ENR) Chitosan (CTS) Reaction Epoxy
a b s t r a c t Thermoplastic starch (TPS) was prepared by melt blending cassava starch and glycerol (70/30) at 140 °C. Chitosan (CTS) was incorporated during TPS preparation. The TPS/CTS sample was melt blended with epoxidized natural rubber (ENR) at 140 °C. In the TPS/ENR/CTS blend, adding CTS and ENR improved the tensile strength and elongation at break, respectively. Morphology of the TPS/ENR blend showed the dispersion of largesized ENR particles in the TPS matrix. Adding CTS reduced the size of the ENR particles. Incorporating CTS also enhanced the melt viscosity of the blend, which suggested a reaction between CTS and ENR. FTIR confirmed that the amino groups of CTS reacted with the epoxy groups of ENR. This reaction between the CTS amino groups with the ENR epoxy groups improved the mechanical properties of the TPS/ENR/CTS blend. Ó 2016 Published by Elsevier Ltd.
1. Introduction Thermoplastic starch (TPS) can be prepared by blending starch with a plasticizer, such as water, glycerol or sorbitol [1]. The plasticizer penetrates into the granules of the starch, interrupting its crystalline structure and inducing the formation of an amorphous structure when subjected to high temperatures and shear forces during the melting process [2]. Therefore, the amorphous starch, following the addition of a plasticizer, behaves like a thermoplastic polymer during the melt stage. Some polymers that have been blended with TPS to improve its mechanical properties include: polypropylene [3], polyethylene [4,5], poly(lactic acid) [6–8] and poly(butylenesadipate-co-terephthalate) [9]. Natural rubber (NR) that contains epoxy groups can be modified into epoxidized natural rubber (ENR). Natural rubber can be epoxidized to various degrees: 25%, 50%, and 75% epoxidation is referred to as ENR-25, ENR-50, and ENR-75, respectively. A few studies have reported using ENR to improve the toughness of PLA [10–13]. The improved toughness of PLA by 20% epoxidized natural rubber has been reported [10]. Akbari et al. reported that adding ENR improved the impact toughness of a PLA/talc blend [11]. Wang et al. reported that dicumyl peroxide (DCP) improved the compatibility between PLA and ENR, as well as the impact toughness [13]. Little research exists on the blending of TPS with rubber. Carvalho et al. investigated the morphology of a TPS and natural rubber latex (NRL) blend and found that the phase morphology of the blend depended on the glycerol content [14]. ENR is used in various fields, such as polymer blends [15], polymer modification [16] and polymer composites [17]. ⇑ Corresponding author. E-mail address: [email protected] (K. Jantanasakulwong). http://dx.doi.org/10.1016/j.eurpolymj.2016.09.035 0014-3057/Ó 2016 Published by Elsevier Ltd.
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Chitosan (CTS) is a linear polymer of a (1 ? 4)-linked 2-amino-2-deoxy-b-D-glucopyranose, which is derived from N-deacetylation [18]. Chitosan, a natural polymer, has remarkable properties, including its high mechanical properties, biodegradability, bio-compatibility, antimicrobial activity, and non-toxicity. Chitosan easily reacts with other reactive groups, due to the presence of NH2 groups. Chitosan’s structure has been modified by chemical modification, graft reactions, and ionic interactions. Chitosan is highly hydrophobic, or insoluble in water, but becomes soluble in water with the addition of acids. Because of its physical and chemical properties, chitosan has been used in a range of applications, including pharmaceutical, food, textile, agricultural, biomedical, and cosmetic products [18]. The objective of this research was to develop TPS blending with ENR rubber and CTS. We investigated the effect of adding chitosan to a TPS and ENR blend on its mechanical properties, morphology, rheological properties, and reaction mechanism. 2. Material and methods Cassava starch (Dragon Fish brand, amylose/amylopectin content 17%/83%, moisture content of 11% total weight and molecular weight of 1.34 108 g/mol) was purchased from Tong Chan registered ordinary partnership, Thailand. Glycerol and chitosan (CTS) (deacetylation degree of 85% and molecular weight of 500 kDa) were purchased from Union Science Co., Ltd., Thailand. Epoxidized natural rubber (ENR) with 25% epoxidation was purchased from Muang Mai Guthrie Public Co., Ltd., Thailand. Lactic acid (99%) was produced by Merck (Darmstadt, Germany). 2.1. Sample preparation Cassava starch was premixed with glycerol (70/30) and distilled water (100 mL/50 g starch) by overhead stirrer at 500 rpm in a water bath at 90 °C. Chitosan is highly hydrophobic, or insoluble in water, but becomes soluble in water with the addition of acids. Chitosan was added during the premixing process along with lactic acid (2% v/v of aqueous lactic acid solution, 100 mL), then melt-blended by a two-roll mill (Pirom-Olarn Co. Ltd., Thailand, PI-140) at 140 °C for 10 min to prepare the TPS and TPS/CTS blend. TPS or TPS/CTS were melt-blended with ENR at 140 °C for 5 min by a brabender internal mixer (Labo Plastomill, Toyoseiki Co. Ltd., Japan). Table 1 shows the composition of TPS, ENR and CTS of the blends. The samples were put in a mold and compressed into sheets for tensile tests and into a film for FTIR by a hot-compress machine with pressure of 500 psi at 140 °C for 3 min. 2.2. Scanning electron microscopy (SEM) Morphology of the samples was observed by scanning electron microscopy (SEM) (SM-200, Topcon Corp., Japan). The samples were prepared as sheets by compression molding at 140 °C for 3 min. The length, width and thickness were 30 mm, 5 mm and 0.5 mm, respectively. Liquid nitrogen was used to break the samples, and then the fractured surface of the samples was extracted by immersing samples into toluene at 60 °C for 24 h. The extracted surface of the samples was coated with a thin layer of gold, and observed with an acceleration voltage of 10 kV. 2.3. Tensile properties measurements Tensile properties of the samples were measured using a tensile tester (Tensilion UTM-II-20; Orientec Co. Ltd., Japan) at 2 mm/min crosshead speed. The bone-shaped samples were prepared as sheets by compression molding at 140 °C for 3 min. The gage length, width and thickness of the sample were 10 mm, 3 mm and 0.5 mm, respectively. 2.4. Contact angle Water droplet contact angle was observed by drop shape analysis (DSA30E, Krüss Co. Ltd., Germany) to estimate the wettability of samples. The samples were prepared as sheets by hot-compression at 140 °C for 3 min. Water was dropped onto the surface of samples and images were recorded at 1 min, 2 min and 3 min.
Table 1 The composition and codes of blends prepared from thermoplastic starch (TPS), epoxidized natural rubber (ENR) and chitosan CTS. Sample
TPS/ENR TPS/ENR/CTS1 TPS/ENR/CTS2.5 TPS/ENR/CTS5
Composition (wt%) TPS
ENR
CTS
90 89 87.5 85
10 10 10 10
– 1 2.5 5
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2.5. Rheological measurement The complex dynamic viscosity (g⁄) was measured by a rheometer (Dynamic Analyzer RDA II, Rheometric Scientific Corp., USA). The samples were prepared as sheets by compression molding to a disc with thickness of 1 mm and diameter of 25 mm at 140 °C for 3 min. Measurements were carried out at frequencies from 0.1 rad/s to 1000 rad/s at a strain rate of 10% and constant temperature of 140 °C. 2.6. Fourier transform infrared spectroscopy A Fourier transform infrared spectrometer (FT/IR-480plus, Jasco Corp., Japan) was used to observe the reaction between the CTS and ENR. The samples were prepared as thin-films with thickness of about 100 lm by hot compress at 140 °C for 3 min. IR spectra were measured from 600 cm 1 to 4000 cm 1, with a resolution of 4 cm 1. 3. Results and discussion 3.1. Morphology The morphology of a bio-based polymer by TPS blending showed TPS matrix phase and rubber as dispersed particles. Fig. 1 shows SEM images of the fractured surface samples with the rubber phase extracted by toluene at 60 °C for 24 h. The TPS/ENR blend showed large holes resulting from the extraction of rubber particles of about 5–20 lm (Fig. 1(a)), due to the distribution of large ENR particles in the TPS matrix. The TPS/ENR/CTS blends with CTS 1 wt%, 2.5 wt% and 5 wt% showed smaller holes resulting from the extraction of rubber particles of about 1 lm, 0.5 lm and 0.3 lm, respectively (Fig. 1(b–d)). It was indicated that the decreasing rubber particle size by the addition of CTS was due to the reaction occurring between the amino groups (ANH2) of CTS and the epoxy groups of ENR. The decreasing size of rubber particles in the polymer blend following a reaction between amino groups (ANH2) and epoxy groups has also been reported [19]. 3.2. Mechanical properties Fig. 2 shows the stress-strain curves of TPS, TPS/ENR (90/10) and TPS/ENR with chitosan 1 wt%, 2.5 wt% and 5 wt%. TPS showed high slope at early state of stress-strain curve which related to the elastic modulus of the sample. The tensile strength, elongation at break and elastic modulus were 4 MPa, 50% and 157 MPa, respectively. The TPS/ENR blend showed lower elastic modulus (47 MPa) and tensile strength (2.5 MPa), but the elongation at break (101%) was higher than for neat
Fig. 1. SEM images of the extracted fracture surface by toluene of (a) TPS/ENR, (b) TPS/ENR/CTS1, (c) TPS/ENR/CTS2.5 and (d) TPS/ENR/CTS5.
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Fig. 2. Stress-strain curve of (a) TPS, (b) TPS/ENR, (c) TPS/ENR/CTS1, (d) TPS/ENR/CTS2.5, (e) TPS/ENR/CTS5 and (f) ENR.
TPS (50%). Adding chitosan (CTS) to the TPS/ENR blend enhanced its tensile strength and increased the elastic modulus compared to the TPS/ENR blend alone. The decrease in elastic modulus of the TPS/ENR blend may reflect the low elastic modulus of ENR rubber and the connection between the chains of TPS and ENR through the reaction between the AOH groups of TPS and the epoxy groups of ENR. Reaction between AOH groups and epoxy groups has been previously reported [20]. The decreasing of modulus in a polymer and rubber blend with some reactions occurring between them has been reported [21]. Adding CTS 5 wt% increased the tensile strength, elongation at break and elastic modulus to 8.2 MPa, 80% and 104 MPa, respectively. The improvement in tensile properties of the TPS/ENR/CTS blend indicated some CTS remaining in the TPS, the miscibility of TPS/CTS, the increasing reaction by the addition of CTS and the changing of ENR chemical structure by reaction between CTS and ENR. It was considered that adding CTS improved the tensile properties of the blend. 3.3. Contact angle Wettability of samples was observed by water drop contact angle with recording time at 1, 2 and 3 min after dropping the water droplet on to the samples’ surface, as shown in Fig. 3. The TPS showed water contact angle 90° at 1 min and decreased
Fig. 3. Water droplet contact angle of TPS, TPS/ENR, and TPS/ENR/CTS5 at 1–3 min.
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to 62° at 3 min. The decreasing water contact angle with recording time indicated the low surface tension of hydrophilic TPS. The TPS/ENR also showed a similar trend with TPS from the water contact angle reducing from 81° to 60° at 1 min to 3 min, respectively. The water contact angle of TPS showed almost the same degree with the TPS/ENR (60° at 3 min) which suggested to the same surface tension. The TPS/ENR/CTS presented low wettability with high water contact angle from 96° at 1 min to 90° at 3 min. The high water contact angle of TPS/ENR/CTS blend was suggested to miscibility of hydrophobic CTS with TPS phase. The increasing water contact angle of TPS/CTS blends has been reported [22,23]. The interfacial reaction between TPS and ENR was suggested to occur during the melt blending process by the addition of CTS; the rheological properties and FTIR confirmed this reaction. 3.4. Rheological properties The morphology and melt viscosity of the blended polymers are affected by interfacial reaction between the component polymers, due to the formation of chemical bonds during the melt blending process. The complex melt viscosity (g⁄) of neat polymers and the blended samples was observed, as shown in Fig. 4. The TPS/ENR blend showed a melt viscosity value between neat TPS and ENR. The TPS/ENR/CTS blends at all of the tested CTS concentrations showed higher melt viscosity values than the neat TPS, ENR, and the TPS/ENR blends. However, the TPS/ENR/CTS5 blends presented lower melt viscosity
Fig. 4. Complex melt viscosity of TPS, ENR, TPS/ENR and TPS/ENR/CTS blends at 140 °C.
Fig. 5. FTIR spectra of (a) CTS, (b) ENR, (c) TPS/ENR, (d) TPS/ENR/CTS5, and (e) TPS.
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at high frequency than adding CTS 1 wt% and 2.5 wt% because the higher amount and deformation of CTS at 140 °C reduced chain length and increased melt flow rate of the blend at high frequency. The higher melt viscosity of the TPS/ENR/CTS blends than the TPS/ENR blend was likely due to a reaction between the epoxy groups of ENR and the amino groups of CTS. The increasing melt viscosity in the polymer blends due to a reaction between epoxy groups and amino groups has been reported [21,24]. 3.5. Reaction mechanism FTIR was used to observe the reaction between CTS and ENR. Fig. 5 shows the FTIR spectra between wave number 600–4000 cm 1 of neat TPS, CTS, ENR, the TPS/ENR and the TPS/ENR/CTS5 blends. In the FTIR spectrum of ENR, the absorption band at 870 cm 1 represented the asymmetric epoxide ring stretching groups of ENR [25,26]. In the FTIR spectrum of CTS, absorption bands at 1639 cm 1 and 1561 cm 1 were related to the vibrations of carbonyl bonds (C@O) of the amide group and to the vibrations of the protonated amine group (ACONHA), respectively [27,28]. Starch showed characteristic peaks at 3360 (AOH stretching) 1643 cm 1 (AOH bending), 1016 cm 1 and 929 cm 1 (ACO stretching) [29]. In the TPS/ENR blend, the epoxide peak at 870 cm 1 of ENR was not observed due to a small amount of ENR; also, it joined with
Fig. 6. FTIR spectra of (a) TPS/ENR, (b) TPS/ENR/CTS1, (c) TPS/ENR/CTS2.5 and (d) TPS/ENR/CTS5.
Fig. 7. FTIR spectra of (a) neat ENR and (b) the extracted ENR phase from TPS/ENR/CTS5 by toluene at 60 °C for 24 h.
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Fig. 8. Expected reaction between amino groups of CTS and epoxy groups of ENR rubber.
the shoulder peak of TPS at 900 cm 1. The TPS/ENR blend showed peak intensity at 3360 lower than TPS. The decreasing of peak intensity at 3360 suggested to a reaction between AOH groups of TPS and epoxy groups of ENR. The TPS/ENR/CTS blend showed new peaks at 1713 and 1746 cm 1, which were not observed in neat TPS, ENR, CTS and the TPS/ENR blend. Therefore, high intensity of the peaks at 1713 and 1746 cm 1 indicated new peaks occurring due to a reaction during melt blending of the TPS/ENR/CTS blend. The TPS/ENR/CTS blend with CTS 1 wt%, 2.5 wt% and 5 wt% also presented peaks at both 1713 and 1746 cm 1; these two peaks were not observed in the TPS/ENR blend, as shown in Fig. 6. The ENR phase was extracted from the TPS/ENR/CTS blends by toluene to confirm the reaction between ENR and CTS. FTIR spectra of neat ENR and the ENR extracted from the TPS/ENR/CTS blends by toluene are presented in Fig. 7. The FTIR spectra of ENR showed a small peak at 1746 cm 1; this peak was observed with high intensity in the spectrum of the extracted ENR from the TPS/ENR/CTS blends. The increasing peak intensity at 1746 cm 1 of the extracted ENR from the TPS/ENR/CTS blends sample was indicative of a new CAO vibration spectra peak formed by a reaction between the epoxy groups of ENR and the amino groups of CTS. The reaction between the epoxy and amino groups has been previously reported [21,24,30–33]. Our results confirmed that CTS formed a chemical bond with ENR through a reaction between the amino groups of CTS and the epoxy groups of ENR (the expected reaction is shown in Fig. 8). Therefore, the occurrence of this reaction by the addition of CTS into the TPS/ENR blend and the miscibility between CTS and TPS were indicated to improve the mechanical properties and wettability of the TPS/ENR/CTS blend. The miscible blending between TPS and CTS has been reported [34–37]. 4. Conclusions A new TPS composite formed by blending TPS/ENR/CTS, with high mechanical properties and low wettability, was successfully developed by reactive melt blending. Incorporating CTS into TPS/ENR blend improved the tensile properties, morphology and wettability of the blend. The TPS/ENR/CTS blend showed phase-separated morphology, with the rubber particles dispersed in the TPS matrix. CTS increased the melt-viscosity of the TPS/ENR/CTS blend, because the increased reaction between CTS and ENR enhanced the chain length of the blend. FTIR results confirmed that the amino groups of CTS reacted with the epoxy groups of ENR. This reaction and the miscibility between TPS and CTS indicated improved properties of the TPS/ENR/CTS blend. Acknowledgments The authors gratefully acknowledge the financial and/or in-kind support from the Project of the National Research University (NRU) from the Office of Higher Education Commission (OHEC), Ministry of Education, Thailand, the National Research Council of Thailand (NRCT), Bioprocess Research Cluster (BRC), School of Agro-Industry, Faculty of AgroIndustry, Chiang Mai University (CMU), and the Tokyo Institute of Technology (TIT). References [1] L. Yu, G. Christie, J. Gray, U. Dutt, T. Harvey, Effect of additive on gelatinization, rheological properties and biodegradability of thermoplastic starch, Macromol. Symp. 1441 (1999) 371–374. [2] L. Moscicki, M. Mitrus, A. Wojtowicz, T. Oniszczuk, A. Rejak, L. Janssen, Application of extrusion-cooking for processing of thermoplastic starch (TPS), Food. Res. Int. 47 (2012) 291–299. [3] D.S. Rosa, C.G.F. Guedes, C.L. Carvalho, Processing and thermal, mechanical characterization of post-consumer polyolefins/thermoplastic starch blends, J. Mater. Sci. 42 (2007) 551–557.
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