Rheological, morphological, mechanical, and water-barrier properties of agar/gellan gum/montmorillonite clay composite films

doi:10.1016/j.ijbiomac.2019.09.021

International Journal of Biological Macromolecules

Volume 141, 1 December 2019, Pages 538-544

https://doi.org/10.1016/j.ijbiomac.2019.09.021 Get rights and content

Highlights

•
Ternary composite films were prepared from agar, gellan gum and MMT clay.
•
MMT clay improved the rheological properties of the composite films.
•
Thermal properties and water barrier properties were improved by MMT.
•
Increasing MMT content increased the dynamic viscosity.

Abstract

Agar (A), gellan gum (G) and montmorillonite (M) based ternary nanocomposite films were prepared via the solution-casting method for food packaging applications. The prepared nanocomposites were investigated for the effect of MMT clay on the structure–property relationships by studying the microstructural, rheological, mechanical, thermal, ultraviolet, and water-barrier properties of AG hydrogel composites. The results indicated that the thermal stability (T_5%: 119.4–174.7) and tensile strength (29.9–44 MPa) were significantly enhanced in the reinforced MMT nanoclay. The water barrier (1.9–1.7) and contact angle (56.8°–49.4°) were reduced by the incorporation of MMT clay whereas the rheological properties improved. The AGM composite solution exhibited shear thinning behavior and viscosity reduction at a high rate. Additionally, the composites exhibited significantly higher storage and loss modulus at high frequencies. The complex viscosity differed from the shear viscosity and remained higher than the shear viscosity. The nanocomposite structure, molecular interaction, and interaction in the multicomponent were investigated by FT-IR, XRD and SEM analysis. The AG and AGM nanocomposites exhibited a synergistic reinforcement effect. The results of this study might introduce a new route for enhancing the nanocomposites for sustainable materials.

Introduction

The consumption of large amounts of non-degradable and non-renewable packaging materials is a significant challenge and presents environmental problems. It is not feasible and wise option to use non-biodegradable polymers for applications which are short-term [1]. Alternatively, biodegradable material in packaging application is a good substitute for non-biodegradable packaging materials [2]. Therefore, to overcome the drawbacks of non-biodegradable materials, biodegradable packaging materials have been developed from natural sources like polysaccharides, proteins, and lipids [3,4]. These natural sources are of plant, animal, and microbial origin [5]. However poor properties, including barrier and mechanical restrict the application of biopolymers in various field when compared to non-biodegradable materials [[6], [7], [8]].

Polysaccharides are widely researched materials for various applications in biomedical field. Agar is made of agarose and agar pectin. It is composed of D-galactose and 3, 6-anhydro-l-galactopyranose along with a heterogeneous combination of smaller molecules [9]. Agar is more stable in different environment such as low-pH and high-temperature. Sphingomonas paucimobilis secretes gellan gum which is an exopolysaccharide. It is made up of repeated linear chain of tetrasaccharide units (L-rhamnose, d-glucose, and D-glucuronic acid). With proper cation concentrations, high-acryl gellan gum forms weak gels, and low-acyl gellan gum forms strong gels [[10], [11], [12]].

Biopolymer-based composite films have significant limitations; e.g., they are brittle and have poor water-barrier properties. For overcoming these drawbacks, various methods have been employed. Different nano fillers are used to improve the physical properties of gellan gum and agar-based films. Montmorillonite (MMT) is a commonly used type of clay in biopolymer-based nanocomposites owing to its swelling properties and water holding nature between the platelet layers [[13], [14], [15]]. The clay particles augment physical properties of nanocomposites due to their reinforcing effects. The transparency of MMT based composite is usually maintained with a perfect dispersion of nanosized clay into the polymer matrix [[16], [17], [18], [19]].

Clay is a naturally available mineral and due to its non-toxic nature it is suitable as components in the packaging applications of food, medical and cosmetic industries. Moreover, clay is ecofriendly and inexpensive [1,20]. Many reports are available on clay materials with various natural biopolymers [[21], [22], [23], [24]]. However, as far as we know, there has been no comprehensive study on the effect of MMT nanoclay with gellan gum and agar materials on the rheological and physical and chemical properties of the packaging applications.

To sum up MMT clay's compatibility with gellan gum and agar based films, viability, economic, non-toxic nature and its ability to improve physical properties of composite films makes it an ideal candidate for studies. In this experiment, gellan gum, agar, and MMT composites were prepared via a solution-casting method, and the effects of the components on the rheological, mechanical, thermal, and water-vapor permeability (WVP) properties of the composite films were investigated.

Section snippets

Materials

Agar (viscosity of 40–100 cps at 1.5% in water at 60 °C), gellan gum (low acylation degree), and MMT clay were purchased from Sigma–Aldrich, South Korea. Calcium chloride, glycerin was purchased from Dajang Chemicals, South Korea.

Preparation of agar/gellan gum/MMT clay composite films

AGM (with 0, 2.5, 5.0, 7.5, and 10 wt% MMT) nanocomposite films were prepared via the solution-casting method. First, agar (1 g) and gellan gum (1 g) were mixed with glycerin (1 g) and calcium chloride (0.5 g). Then, the mixture was added to distilled water (66 ml)

Ft-IR

The FT-IR spectra of the AG and AGM nanocomposite films are shown in Fig. 1. The broad bands at 2935 and 3287 cm⁻¹ were due to the stretching of the hydroxyl (O single bond H) group and CH methyl groups present in the agar, gellan gum, and MMT clay materials. The characteristic stretching at 930 cm⁻¹ was associated with the CO groups of 3, 6-anhydrogalactose. The peak at 1394 cm⁻¹ was attributed to the ester sulfate group of agar [9]. The peak at 1605 and 1394 cm⁻¹ in the spectrum of AG composite ascribed

Conclusions

Different weight percentages of MMT clay were used to prepare agar and gellan gum based nanocomposite films. FT-IR characterization indicated that the agar and gellan gum were miscible and that an intermolecular interaction occurred. Rheology analysis revealed that the MMT improved the properties of the AG solution. The composites exhibited significantly high storage and loss modulus at a high frequency as the clay content increased. The AGM exhibited shear-thinning behavior over the entire

Declaration of competing interest

There are no conflicts to declare.

Acknowledgements

This work was aided by the Technology Innovation Program (or industrial strategic technology development program, 10076409, establishment of solution for optical/light design/dynamic concept hybrid 2D/3D thermoplastic prepreg for design performance) funded by the Ministry of Trade, Industry and Energy (MOTIE Korea).