Post-crosslinking modification of thermoplastic starch/PVA blend films by using sodium hexametaphosphate
Highlights
► Thermoplastic starch (TPS)/PVA blend films were modified by post-crosslinking. ► The post-treatment consists of soaking the films in aqueous solutions and heating. ► The effects of treating conditions on the properties of the films were investigated. ► The described method can be used to optimize properties of TPS/PVA based products.
Introduction
Starch is a natural polymer of d-glucose containing amylose and amylopectin. The development of thermoplastic starch (TPS) has received considerable attention over the last two decades due to its biodegradability, renewability and low cost (Averous, 2004, Gandini, 2008, Mohanty et al., 2000, Yu et al., 2006). However, the hydrophilic nature of starch causes the moisture content in TPS to change with environment and leads to the mechanical properties of TPS to be jeopardized in damp environment, which renders TPS unsuitable for many high humidity applications (Gandini, 2008).
Poly (vinyl alcohol) (PVA) is well suited for making blends with natural polymer starch because of its biodegradability, good film-forming capability and water solubility (Follain et al., 2005, Shogren et al., 1998, Solaro et al., 2000, Sreedhar et al., 2006, Yun and Yoon, 2010). Although TPS/PVA blends presented a lower water uptake compared to TPS, due to existence of the hydroxyl groups on the molecules of starch and PVA, the hydrophobic characteristics of the TPS/PVA blends still need to be improved.
Crosslinking modification is an efficient and commonly used approach to increase the water resistance of starch and PVA. Research results showed that surface photo-crosslinking modification of TPS sheets and TPS/PVA films significantly reduced the surface hydrophilic characteristics and improved the water resistance of the materials (Zhou et al., 2008, Zhou et al., 2009). The photo-crosslinking modification needs a UV source to initiate and finish the crosslinking reaction. Alternatively, the crosslinking modifications of both TPS and PVA can be also implemented by heating, and heating seems easier and more convenient for practice applications. For the crosslinking modification of starch or PVA through heating, glutaraldehyde (El-Tahlawy et al., 2007, Parra et al., 2004, Ramaraj, 2007, Wang and Hsieh, 2010, Yoon et al., 2007), epichlorohydrin (Jiang, 1992, Rioux et al., 2002, Sreedhar et al., 2006), citric acid (Reddy and Yang, 2010, Shi et al., 2008), boric acid (Yin, Li, Liu, & Li, 2005), borax (Sreedhar, Sairam, Chattopadhyay, Rathnam, & Rao, 2005), sodium trimetaphosphate (Mao, Wang, Meng, Zhang, & Zheng, 2006) and trisodium trimetaphosphate (Li et al., 2009) were used as crosslinking agent to react with the hydroxyl groups in starch or PVA.
In this paper, the crosslinking modification of TPS/PVA blend films was carried out through heating after the films were soaked in sodium carbonate aqueous solution and sodium hexametaphosphate aqueous solution sequentially. Unlike other crosslinking modifications for TPS/PVA blend system (Ramaraj, 2007, Yoon et al., 2007), the method used in this study is a post-crosslinking technique that could be applied after the products were formed instead of during the products formation. Some of the physical properties of the crosslinking modified TPS/PVA films were characterized to investigate the influences of the post-crosslinking modification.
Section snippets
Materials
Corn starch was obtained from Changchun Jincheng Corn Development Co. Ltd., Da Cheng Group (China). PVA with polymerization degree of 1750 ± 50 was supplied by Shenyang Dongxing Reagent Factory (China). Sodium hexametaphosphate (SHMP) was purchased from Xilong Chemicals Co. Ltd. (China). Glycerol and sodium carbonate (Na2CO3) were from Beijing Beihua Fine Chemicals Co. Ltd. (China). All these chemicals were reagent grade and used as received without any further purification.
Film preparation
TPS/PVA blend films
Crosslinking reaction
Because the intact TPS/PVA blend films are soluble in DMSO, therefore, the GM of the crosslinking modified TPS/PVA blend films is related to the amounts of starch and PVA macromolecules involved in formation of the crosslinking networks. The SD of the modified TPS/PVA blend films is related to the crosslink density of the newly created networks. The lower the normalized SD was, the higher the crosslink density was.
Fig. 1 showed the values of the normalized SD and normalized GM of the specimens
Conclusions
The TPS/PVA blend films can be post-modified by crosslinking through soaking the films in sodium carbonate and sodium hexametaphosphate aqueous solutions, sequentially, and then heating in an oven. The results of the moisture absorption measurements indicated that the post-crosslinking modification significantly reduced the equilibrium moisture content of the TPS/PVA blend films, i.e., lowered the hydrophilic characteristic of the films. The results of the mechanical properties measurements
Acknowledgments
The authors are grateful to the National Natural Science Foundation of China (No. 50673037), Natural Science Foundation of Jilin Province of China (No. 20101538) and “985 Project” of Jilin University for financial support.
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