Surface esterification of corn starch films: Reaction with dodecenyl succinic anhydride
Introduction
Thermoplastic starch (TPS), a homogeneous starch melt obtained through disrupting the starch granular structure by mechanical and thermal energy with presence of water or other plasticizers, can be processed with existing plastic processing technologies (Lorcks, 1998, Shogren et al., 1993). TPS has attracted significant interest in recent years because of its biodegradability, renewability and low cost (Chandra and Rustgi, 1998, Gandini, 2008, Mohanty et al., 2000, Yu et al., 2006) and has found its applications in the plastic market to replace non degradable petrochemical-based materials especially in packaging industry. Loose-fill (foamed chips for filling space around goods within a packing box) and packaging trays made from TPS are commercially available. However, the hydrophilic nature of TPS, mainly due to the hydroxyl groups on starch molecules, renders TPS unsuitable for many high humidity applications.
On the other hand, modifications of starch to meet requirements of desirable properties for various applications have been of interest for many years in the starch industry. Esterification is one of the modifications which can impart hydrophobicity to starch products (Aburto et al., 1999, Neumann et al., 2002, Sagar and Merrill, 1995, Shogren, 2003, Tessler and Billmers, 1996). Starch esters with moderate to high degree of substitution (DS, 3 is the highest) may find applications as substitutes for petroleum-based plastic materials especially in the packaging industry (Albertsson and Karlsson, 1995, Bisws et al., 2008, Riward et al., 1995, Xu et al., 2005). However, the market penetration has been low due to relatively high production costs of starch esters (Bisws et al., 2008). Several studies have been done in an attempt to prepare starch esters more rapidly and environmentally friendly (Bisws et al., 2006, Bisws et al., 2008, Miladinov and Hanna, 2000, Shogren, 2003).
Surface chemical modification is a promising method to reduce the surface hydrophilic character without changing their bulk composition and properties of TPS products. The superficial hydroxyl groups of TPS can be substituted with hydrophobic groups or react with cross-linking agents to form starch molecule networks, so that the surfaces of TPS products become less sensitive to moisture.
It was reported that surface hydrophobicity of TPS samples was improved greatly when the TPS surface has been treated with pre-polymers having NCO groups (Yu & Liu, 2002). Bengtsson, Koch, and Gatenholm (2003) reported that after surface esterification with octanoyl chloride and pyridine, the surface energy of high-amylose potato starch films was lower and water vapor transmission and water absorption decreased with the degree of starch substitution. Carvalho, Curvelo, and Gandini (2005) used several reagents, i.e., phenyl isocyanate, a phenol blocked polyisocyanate, stearoyl chloride and poly(styrene-co-glycidyl methacrylate) to react with the superficial hydroxyl groups of TPS films in the medium of methylene chloride or xylene, and found that all the treatments were effective in decreasing the hydrophilic character of the TPS surfaces. Cross-linking technology was also used for surface modification of thermoplastic starch. Surface of corn starch sheets was modified by cross-linking through ultra violet (UV) irradiation by using sodium benzoate as photosensitizer, the results showed that surface photo-cross-linking modification significantly reduced the hydrophilic character of the starch sheet surface and enhance the water resistance of the starch sheets (Zhou, Zhang, Ma, & Tong, 2008).
Compared to the esterification modification of native granular starch, surface esterification of thermoplastic starch should be easier. Since starch crystalline structure has been disrupted in TPS, the reactant reagents can diffuse into material and combine with the hydroxyl groups on starch molecules more easily. In addition, the amounts of reactant reagents used in a surface modification would be significantly lower than that in a bulk modification. Moreover, as surface modification can be carried out after the formation of TPS products, it will not affect the processing properties of TPS.
In this study, surface of corn starch films was modified through esterification using dodecenyl succinic anhydride (DDSA). Investigation was carried out to determine the preferred parameters such as esterification reaction temperature and time, concentration of alkaline aqueous solution for activating starch, concentration of ethanol diluted DDSA, and treating time in alkaline aqueous solution. The influence of surface esterification on some physical properties, such as moisture absorption behavior and water contact angle, of the films was presented.
Section snippets
Materials
The corn starch was supplied by Changchun Jincheng Corn Development Co. Ltd., Da Cheng Group (Changchun, China). The reactant used in the esterification reaction was DDSA (dodecenyl succinic anhydride, 90% purity) 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 of reagent grade and used as received without any further purification.
Film preparation
Films were fabricated by solution
Surface esterification of starch film
The starch films were surface modified through esterification using DDSA as reactant. Fig. 1 showed the schematic representation of the esterification reaction. The esterification of starch with dodecenyl succinic anhydride could take place in an aqueous slurry system or even in its powder state under temperature of 25–40 °C, and the reaction could last several hours (Chen et al., 2000, Jeon et al., 1999). For the surface esterification in a solid state starch film, a reaction condition at 35 °C
Conclusions
The surface of thermoplastic starch films can be modified through esterification by dipping the films in ethanol diluted DDSA and then heating at 30–40 °C in air for several hours with pretreatments of immersion in sodium hydroxide aqueous solution for a while and reconditioning at 95% RH to equilibrium. The surface esterification modification with DDSA significantly reduced the moisture sensitivity and surface hydrophilic character of the starch films. After surface esterification modification,
Acknowledgment
The authors are grateful to the National Natural Science Foundation of China (No. 50673037) for financial support.
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