Effect of the incorporation of surfactants on the physical properties of corn starch films

Highlights

• The use of surfactant affects the final microstructure of corn starch-glycerol films.

• Size of the crystalline complexes was smaller for the surfactant with the highest HLB.

• Films containing surfactants were less hard, resistant and extensible.

• Films containing surfactants were more permeable to oxygen.

Abstract

The effect of surfactant addition on structural, mechanical, optical and barrier properties of corn starch-glycerol based films was studied. Sorbitan monopalmitate, monostearate or monooleate were incorporated into starch-glycerol (1:0.25) at a surfactant:starch ratio of 0.15:1. The film forming dispersions (FFD) were characterized as to rheology, ζ-potential, particle size distribution and contact angle. Film characterization was carried out at 1 and 5 storage weeks (at 25 °C and 53% relative humidity). Surfactants led to different particle size distribution, zeta potential and viscosity in FFD, and film extensibility, depending on their hydrophobicity and melting properties. Their incorporation to the corn starch-glycerol films produced a coarser film microstructure due to the appearance of free surfactant aggregates or V-amylose inclusion complexes which produce discontinuities in the amorphous continuous matrix. The size of these crystalline complexes was smaller for the surfactant with the lowest hydrophobicity with saturated fatty acid (span 40). This contributed to decrease the WVP values with respect to surfactant-free film. Films containing surfactants were less hard, resistant and extensible, and more permeable to oxygen, than surfactant-free films, but they did not notably affect the film gloss and transparency. Saturated fatty acid compounds with higher melting temperature are recommended to ensure a finer microstructure in the final film which favours water barrier efficiency.

Introduction

The environmental conservation policies, the integral use of natural resources and the reduction of hydrocarbon reserves have generated particular interest in developing alternatives to petroleum synthetic polymers for different industrial uses, particularly for foodstuffs. Several studies focused on the development of biodegradable materials to replace, at least partially, conventional plastics. The materials obtained from natural biopolymers, such as polysaccharides, are an interesting alternative, of which, starch is one of the most promising materials for the manufacture of biodegradable plastics (Ma, Chang, Yu, & Stumborg, 2009). This polymer is a renewable, low cost resource, readily available and has thermoplastic characteristics which permit it to be processed easily by using conventional synthetic polymer methods (Shah, Bandopadhyay, & Bellare, 1995). However, starch presents worse physical characteristics than synthetic polymers. Its main weaknesses are its highly hydrophilic nature, which makes it a poor water vapour barrier, and the fact that it undergoes retrogradation processes, which implies that its mechanical properties vary over time. Nevertheless, their properties can be modified by adding small quantities of chemical compounds (García et al., 2000, Ma et al., 2009).

Plasticizers act by increasing the molecular mobility in the polymeric network, thus improving mechanical properties, but reducing the water vapour barrier properties (Rosen, 1993). Glycerol can be added as a plasticizer to improve the mechanical properties of the film, increasing the flexibility and tensile strength by lowering the glass transition temperature (Vieira, Altenhofen, Oliveira, & Masumi, 2011).

Other components with potential capacity to improve some properties of the starch-based films are surfactants. Some studies have found that surfactants may enhance the wettability and stability of the dispersions (Chen et al., 2009, Ghebremeskel et al., 2007; ), reduce the starch retrogradation (Jovanovich & Añón, 1999) and improve the water vapour barrier properties (Villalobos, Hernández-Muñoz, & Chiralt, 2006). Vieira et al. (2011) have observed that if the surfactant was added without glycerol, it had a significant effect on mechanical properties but did not significantly modify the water vapour barrier properties. Nevertheless, if the surfactant was added with glycerol, it provoked a reduction in the tensile strength and an increase in both elongation and water vapour permeability. Surfactants have been added to different polysaccharide matrices, such as tapioca starch/decolorized hsian-tsao leaf gum films (Chen et al., 2009). When concentration and HLB of surfactant increased, there was an observed improvement in the water vapour barrier properties, although films showed a loss of mechanical resistance. Nevertheless, in matrices of corn starch reinforced with microfibres of cellulose, the mechanical properties were improved with the addition of glyceryl monostearate by the formation of complexes which increase the V-type crystallinity (Mondragón, Arroyo, & Romero-García, 2008).

Working on films of potato starch and glycerol, Rodríguez, Osés, Ziani, & Maté (2006) observed that the presence of surfactants increased the wettability of film forming dispersions, with a synergistic effect between glycerol and surfactants. Similarly, in chitosan films, surfactants showed a synergistic effect with the glycerol on the water vapour permeability (Ziani, Osés, Coma, & Maté, 2008). Zhong & Li (2011) also observed that the addition of surfactants and citric acid diminished the surface tension in the film forming dispersions and the formation of crystalline forms in kudzu starch films.

Despite several works have report the effect of different surfactants on biopolymer films, no previous studies have been published in corn starch films containing this kind of compounds, except that reported by Jiménez, Fabra, Talens, & Chiralt (2012) where the effect of different fatty acids, with very low (between 1 and 2) hydrophilic-lipophilic balance (HLB), was analysed. A low HLB value indicates the predominant lipophilic balance in the molecular structure which can contribute to limit the water vapour permeability of the films, although slightly higher values could improve the better integration of these compounds in the hydrophilic matrix. Likewise, the presence of saturated and unsaturated hydrocarbon chains could also play a relevant role in the component interactions in the matrix. In this sense, the study of the effect of a family of surfactants with intermediate HLB values containing saturated or unsaturated chains could give interesting information about the different effects on the properties of the film forming dispersions and films, which are relevant for both coating applications on a determined product and film formation for packaging ends. Surfactants incorporated to film aqueous forming dispersions can act as carrier vehicles of non-polar bioactive compounds (such as antimicrobials or antioxidants), making their dispersion easy in the non-polar core of the formed micellar structures.

The aim of this work was to study the effect of addition of sorbitan esters of different fatty acids (saturated and unsaturated), with low-intermediate HLB values (sorbitan monopalmitate: 6.7, sorbitan monostearate: 4.7 and sorbitan monooleate: 4.3) on physical properties of film forming dispersions (particle size distribution, ζ-potential, contact angle) and films (mechanical, optical, structural and barrier properties) of corn starch-glycerol based blends. The effect of storage time on film properties was also analyzed.