Short communicationInfluence of surface esterification with alkenyl succinic anhydrides on mechanical properties of corn starch films
Introduction
There has been a growing interest in development of thermoplastic starch (TPS) because of its biodegradability, availability from renewable resources and low cost (Gandini, 2008, Liu et al., 2009). The hydrophilic nature of TPS leads to its moisture content changing with ambient humidity, which results in its mechanical properties sensitive to humidity. This disadvantage renders TPS unsuitable for many high humidity applications.
Surface modification of TPS products is an effective and promising approach that can lower humidity sensitivity without affecting TPS bulk composition. The superficial hydroxyl groups of TPS products can be substituted by hydrophobic groups or react with cross-linking agents to form starch molecule networks, so that surfaces of TPS products become less sensitive to moisture, moreover, the hydrophobic surface layer formed by surface modification could prevent or delay moisture exchange between environment and TPS products. (Yu and Liu, 2002, Bengtsson et al., 2003, Carvalho et al., 2005, Zhou et al., 2008, Zhou et al., 2009a).
Author's previous works showed that surface esterification modification of corn starch films significantly decreased surface hydrophilic character and moisture absorption of the films especially at higher relative humidity (Zhou et al., 2009b, Zhou et al., 2009c). Since mechanical properties are important criterion for many practical applications of TPS, it is necessary to investigate the influence of surface esterification modifications on mechanical properties of TPS. This article presented the research results.
Section snippets
Materials
The corn starch was supplied by Changchun Jincheng Corn Development Co. Ltd. (Changchun, China). The DDSA (90% purity) and OSA (2-octen-1-ylsuccinic anhydride, 97% mixture of cis and trans) were purchased from Sigma–Aldrich (St. Louis, USA). Glycerol, sodium hydroxide and ethanol were from Beijing Beihua Fine Chemicals Co. Ltd. (Beijing, China). All these chemicals were used as received.
Film preparation and surface modification
TPS films with thickness about 0.18 mm were fabricated by solution casting method (Zhou et al., 2009b). The
Result and discussion
Author's previous work showed the surface esterification modifications, by either soaking starch films in 0.7% NaOH aqueous solution for 60 s and dipping in DDSA diluted five times by ethanol (v/v) (Zhou et al., 2009b) or soaking the films in 1.0% NaOH aqueous solution for 10 s and dipping in OSA diluted three times by ethanol (v/v) (Zhou et al., 2009c) and then heating at 35 °C for 6 h, significantly reduced the equilibrium moisture content (EMC) of the starch films at high relative humidity.
Conclusion
The surface modification through esterification by using DDSA, OSA or a mixture of DDSA/OSA can significantly increase the strength, stiffness and toughness of starch films at higher relative humidity (95% RH); but at 75% RH, only strength and Yong's modulus can be enhanced. The effects of NaOH aqueous solution treating time on mechanical properties of starch films were more notable at 75% RH than at 95% RH. The films modified with DDSA were stronger and more rigid than OSA modified one, while
Acknowledgement
The authors are grateful to the National Natural Science Foundation of China (no. 50673037) for financial support.
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