Citric acid as crosslinking agent in starch/xanthan gum hydrogels produced by extrusion and thermopressing

Highlights

• Mixed starch/xanthan gum hydrogels were produced using CA/SHP as a crosslinker.

• Crosslinking preserves the hydrogel integrity even after immersion in water for 48 h.

• Hydrogels presented lower swelling and higher elongation than the control.

• Interaction among the components and smooth surface were allowed by processing.

• Mechanisms were proposed to explain the possible reactions among the components.

Abstract

Biopolymers based hydrogels could have applications in various fields, such as packaging materials, drug delivery systems, biosensors, and agricultural practices. The current work aimed to develop starch/xanthan hydrogels through extrusion and thermopressing processes, using citric acid (CA) as crosslinking agent and sodium hypophosphite (SHP) as catalyst. The hydrogels were produced with different levels of CA (0.00, 0.75, 1.50 and 2.25 g/100 g polymer). Hydration, mechanical, thermal and microstructural properties of the hydrogels were determined. Swelling behavior of the materials with CA was lower than the control. Additionally, CA ensured the preservation of hydrogels integrity after the swelling process. Gel fraction increased with CA 0.75 g/100 g. Starch/xanthan hydrogels crosslinked with CA demonstrated lower strength than non-crosslinked hydrogels, which may be related to acid hydrolysis of the polymer chains. CA-SHP increased the storage modulus of the hydrogels. Reactive extrusion and thermopressing were efficient methods in the production of crosslinked starch/xanthan based hydrogels.

Introduction

Environmental concerns over the disposal of non-renewable and non-biodegradable plastics have led to increasing interest in the synthesis and production of biopolymer-based materials (Lambert & Wagner, 2017). These include hydrogels, which are three-dimensionally cross-linked structures, generally highly hydrophilic, that have at same time swelling properties and resistance to dissolution (Ali & Ahmed, 2018). Hydrogels have extended applications in various fields, such as packaging materials (Farris, Schaich, Liu, Piergiovanni, & Yam, 2009), controlled drug delivery platforms (Shalviri, Liu, Abdekhodaie, & Wu, 2010), biosensors, agricultural systems, etc (Ali & Ahmed, 2018).

Starch and xanthan gum are polysaccharides obtained from renewable sources and widely used in the synthesis of biodegradable polymeric materials, including hydrogels (Balasubramanian, Kim,& Lee, 2018; Bueno, Bentini, Catalani,& Petri, 2013; Kim, Choi, Kim, & Lin, 2015; Shalviri et al., 2010).

Starch is a homopolysaccharide of glucose, consisting of two structures with great molecular weight: amylose and amylopectin. Amylose consists of essentially linear chains, formed by d-glucose units linked by α-1,4-glycosidic bonds, while amylopectin has a branched structure, with α-1,4 glycosidic bonds in the main chain and α-1,6 at branching points (Du, Jia, Xu, & Zhou, 2007). When used to produce biodegradable materials, starch is generally combined with a plasticizing agent, such as glycerol, originating thermoplastic starch (TPS) (Chivrac, Pollet, & Avérous, 2009).

Xanthan gum is an extracellular polysaccharide obtained mainly through the fermentation of Xanthomonas campestris. Its primary structure is constituted by repeated units of glucose, mannose, and glucuronic acid, in the proportion of 2:2:1. Glucuronic acid and pyruvic acid groups on the side chains are responsible for its anionic character (Li et al., 2016).

Synthetization of hydrogels from two biopolymers generally improves the stability of the material, impacting on its functionality. The different structures of the polymers may result in a synergism caused by possible conformational changes and chemical interactions between the chains (Balasubramanian, Kim, & Lee, 2018; Gong et al., 2019; Liu, Kost, Yan, & Spiro, 2012).

Polymers chains can be crosslinked via chemical (covalent bonds, ionic interactions, and hydrogen bonds) as well physical interactions (electrostatic, hydrophobic, dipole-dipole, and chains packing) (Ali & Ahmed, 2018; Tao et al., 2016). The resulting hydrogels may have different forms (e.g. films, sheets and coatings).

Citric acid (CA) has been widely used as a crosslinking agent in starch (Garcia et al., 2014; Olivato, Grossmann, Bilck & Yamashita, 2012; Olivato, Grossmann, Yamashita, Eiras & Pessan, 2012; Reddy & Yang, 2010) and xanthan gum (Bueno, Bentini, Catalani,& Petri, 2013; Tao et al., 2016) materials. However, starch/xanthan gum based hydrogels crosslinked with CA are not reported in the literature.

Reddy and Yang (2010) used CA as crosslinking agent associated with sodium hypophosphite (SHP), as catalyst, in starch films produced by casting. These and other authors (Garcia et al., 2014; Olivato, Grossmann, Bilck & Yamashita, 2012) highlighted that, besides the crosslinker function, CA also presents hydrolytic and plasticizing action.

Although research on hydrogel production by the casting method is quite extensive in the literature, studies reporting the use of processes such as extrusion and thermopressing are limited. Both extrusion and thermopressing subject the materials to thermal and mechanical energy, resulting in chemical and physical reactions (Dastidar & Netravali, 2012; González-Seligra, Ochoa-Yepes, Goyanes & Famá, 2017). Dastidar and Netravali (2012) report the occurrence of a cure process using a hot press and, consequently, an increase in the extent of esterification using malonic acid as a crosslinking agent.

The objective of the current study was to investigate the crosslinking effect of CA-SHP on starch/xanthan gum hydrogels produced by extrusion and thermopressing processes.