Improvement of mechanical properties and thermal stability of biodegradable rice starch–based films blended with carboxymethyl chitosan

Highlights

• Rice starch (RS) and carboxymethyl chitosan (CMCh) blend films were produced.

• Increased CMCh content improves tensile strength and flexibility of RS–CMCh films.

• CMCh content improves thermal stability of RS–CMCh blend films.

• Biodegradation rate and extent are lower for RS/50CMCH than for RS film.

Abstract

Biodegradable blend films from rice starch (RS) and carboxymethyl chitosan (CMCh) were produced and characterized. Color, opacity, mechanical properties, thermal properties, swellability, oxygen and water permeability, and biodegradability of the RS–CMCh blend films are reported. Interaction and compatibility of films components were evaluated by using Fourier transform infrared (FTIR) spectroscopy and X-ray diffraction. Increased yellowness, total color difference and transparency, and decreased redness, lightness and whiteness index were observed in the blend films as incorporation of CMCh increased. Addition of 50% w/w of CMCh into the RS matrix increased the tensile strength of the RS–CMCh blend film by 35% and the elongation at break by 28%. Addition of CMCh improved the thermal stability of the RS–CMCh films. Incorporation of 12, 33 and 50% w/w CMCh in the blend films increased the swelling ratio by around 850%, 3985% and 3404% at 24 h, respectively, when compared with the RS film. The oxygen permeability of all the films increased as relative humidity increased. The FTIR spectra suggested that interactions may be present between the OH groups of RS and the COO groups of CMCh. Scanning electron microscopy images revealed that the cross-sectional fracture surfaces of all the films were smooth and homogenous. The RS film exhibited a priming effect in the biodegradation study. The addition of 50% w/w CMCh led to a decrease in mineralization of the blend films.

Introduction

Among the several types of biomaterials, the polysaccharide starch is an attractive material due to its good film-forming capability, biocompatibility, relatively low cost, renewability and abundance. Rice starch (RS) is a by-product of rice processing. RS is considered to be a relatively cheap source of starch. RS contains around 30% amylose, which is the linear and the more readily crystallizable component of starch compared with amylopectin. RS can be used to produce biodegradable films (Janjarasskul and Krochta, 2010; Wittaya, 2012).

Polysaccharide films are good oxygen barriers with suitable optical properties and moderate mechanical properties at low relative humidity (RH) (Bourtoom and Chinnan, 2008; Laohakunjit and Noomhorm, 2004; Whistler et al., 1984). However, starch-based films are brittle and hydrophilic, which affect and limit their processing and application (Mendes et al., 2016). Plasticizers such as glycerol, sorbitol and polyethylene glycol have been used to overcome the brittleness of starch films (Suppakul et al., 2013; Wittaya, 2012), but their use decreases the tensile strength of the films (Tantala et al., 2012a). Blending, grafting, and compounding with other materials have been used as the main techniques to overcome this shortcoming (Al-Hassan and Norziah, 2012; Bourtoom and Chinnan, 2008; Dias et al., 2010; Ghanbarzadeh et al., 2010; Mathew et al., 2006; Saberi et al., 2016).

Chitosan is a cationic polysaccharide containing β (1–4)-2-amino-2-deoxy d-glucopyranose repeating units (Wu et al., 2013). Chitosan is a deacetylated product of chitin, and is abundantly available in nature, nontoxic and biodegradable. Chitosan has high quality film-forming capability (Wu et al., 2013); however, it dissolves in acidic water, which affects the finished product by causing a bad odor (Ferreira et al., 2009). Carboxymethyl chitosan (CMCh) is a water-soluble etherified chitosan. Applications for CMCh have been reported for food (Carolan et al., 1991), drugs (Chen et al., 2004), cosmetics (Wannaruemon et al., 2013), and agriculture (Dau et al., 2016). CMCh has the capability to form films and gels, is biodegradable and soluble over a wide range of pH, has high viscosity, biocompatibility and antimicrobial activity, and low toxicity (Bukzem et al., 2016; Tantala et al., 2012b). The introduction of CMCh in blend films can increase water solubility, improve tensile strength, and provide some antimicrobial capability (Fan et al., 2006; Tantala et al., 2012b). Previous studies have shown that the incorporation of chitosan or CMCh can improve the mechanical properties of rice starch–chitosan blend films (Bourtoom and Chinnan, 2008), corn starch–chitosan blends (Mendes et al., 2016), and pullulan–CMCh blend films (Wu et al., 2013). Tantala et al. (2012b) showed that CMCh has some antimicrobial capabilities against several strains of lactic acid bacteria but not against foodborne pathogens. Thus, incorporating CMCh into rice starch–based films could provide appealing film properties, and it is the focus of this work.

To the best of our knowledge, there is no published research on the development and characterization of RS-based films that incorporate CMCh, or on understanding their end-of-life scenario and biodegradability. Thus, the purpose of this work was to evaluate the effect of incorporating CMCh on the physical and thermal properties, permeability and biodegradability of RS–CMCh blend films. The interactions and compatibility between these two polysaccharides were also studied by Fourier transform infrared spectroscopy and X-ray diffraction.