Effect of plasticiser on the morphology, mechanical properties and permeability of albumen-based nanobiocomposites

https://doi.org/10.1016/j.fpsl.2020.100499Get rights and content

Highlights

  • Molar mass of plasticisers affects properties of protein/clay nanobiocomposites.

  • Water and glycerol favour a more disordered morphology in nanobiocomposites.

  • Better dispersion is achieved if plasticising with water and glycerol instead of PEG.

  • Gas permeability diminishes with higher nanoclays exfoliation degree.

Abstract

This paper delves into the role plasticisers play in the formulation and processing of bioplastics and nanobiocomposites, trying to understand their effect on nanoclays dispersion and, consequently, on mechanical and gas barrier properties of protein-based nanobiocomposites.

Egg white protein/montmorillonite clay nanobiocomposites were obtained by thermomechanical processing plasticised with varying molar concentration of different components (water, glycerol, polyethylene glycol). The extent of dispersion of the filler was evaluated by X-ray diffraction and transmission electron microscopy. Tensile tests and solid-state rheological measurements were conducted to evaluate glass transition temperature and thermomechanical behaviour of plasticised protein-clay nanobiocomposites, whereas gas permeability tests were used to study their gas barrier properties. The results showed that the samples plasticised by a blend of 1:1 glycerol/water presented the most exfoliated structures, resulting in an improvement in gas barrier and mechanical properties. Morphological analyses combined with tensile and permeability tests have shown a lesser effect of polyethylene glycol of 300 molecular weight (PEG 300) on the exfoliation extent into such nanobiocomposites. Moreover, the larger size of PEG 300 does not allow the formation of a structure as compact as in the case of water and glycerol, as a consequence of an apparent phase separation, leaving more spaces that facilitate the diffusion of gases through the material.

Introduction

The management of plastic waste has long been a matter of concern to the scientific community and industry. However, the magnitude of this problem affecting both public health and nature makes society more and more aware that this is one of the greatest challenges to be faced by humanity during the 21 st century. The need to find a truly sustainable alternative is imperative and in this the context, the option of using biodegradable bioplastics from renewable sources as an alternative to conventional plastics of fossil origin is gaining strength every day. Raw materials such as carbohydrates, lipids and proteins have been the main sources for obtaining biodegradable and renewable bioplastics for many years (Cuq, Gontard, & Guilbert, 1998; Song & Zheng, 2008). In particular, protein-based bioplastics turned out to be among the rates of fast-degrading polymers (Domenek, Feuilloley, Gratraud, Morel, & Guilbert, 2004). Moreover, proteins have been extensively used in the development of bioplastic materials because of their capability to form three-dimensional structures with different interactions and bindings that lead to a wide range of functional properties (Cuq et al., 1998). An example of this is egg white protein (albumen), which is composed mainly of ovalbumin. When this protein is heated, the free sulfhydryl sulphides located in its nucleus are exposed and oxidized to form disulphide bonds. Through covalent bonds such as these and other weak ones (ionic, hydrogen bonding and Van der Waals), the three-dimensional structure of the polymer matrix is created (Fernández-Espada, Bengoechea, Cordobés, & Guerrero, 2013; Jerez, Partal, Martínez, Gallegos, & Guerrero, 2007; Pommet, Redl, Morel, Domenek, & Guilbert, 2003). Albumen has been demonstrated to be a very interesting raw material for obtaining bioplastics focused on applications where transparency is important, without neglecting mechanical performance (Cuq et al., 1998; Diañez, Martínez, & Partal, 2016; Jerez, Partal, Martínez, Gallegos, & Guerrero, 2007). In addition, egg white protein can be easily processed by moulding and both the mixing and compression stages are carried out at temperatures lower than those required for processing most polymers and biopolymers (Jerez et al., 2007a).

However, despite the many advantages of these materials, they still have some deficiencies that prevent them from being competitive with conventional plastics in many of their applications. For this reason, the addition of certain types of nanoparticles to the polymer matrix for obtaining nanocomposites is a way of providing bioplastics with even more interesting and valuable features, without affecting their biodegradability and renewability.

It is already well known that the morphology obtained with the addition of nanoparticles is totally determinant in the mechanical (Lee & Kim, 2010), thermal (Mohanty & Nayak, 2012) and barrier properties (Sanchez-Garcia, Lopez-Rubio, & Lagaron, 2010) of the materials obtained. Specifically, in the case of laminar nanoparticles, the most significant improvements are observed when an exfoliated structure is achieved, in which the clay plates have been completely separated from each other and are uniformly dispersed through the polymeric matrix (Sharma, Malik, & Jain, 2018; Zhu et al., 2019).

In previous study (Diañez et al., 2016) we determined that the macromolecular structure of egg white-based materials was affected by the nature of the nanoclays added. Moreover, the molecular/macromolecular compatibility between the clay layers and the egg white matrix appeared as the key parameter governing the nanostructure, and therefore, the mechanical properties and the water absorption capacity of resultant nanobiocomposites.

Another indispensable component in the formulation of bioplastics and biocomposites are plasticisers, which have been proved to be a determining factor in the final properties of plastics, bioplastics and nanocomposites (Chivrac, Pollet, Dole, & Avérous, 2010; Hopkins, Stone, Wang, Korber, & Nickerson, 2019; Lara & Salcedo, 2016). Plasticisers are usually small molecules that are located between the polymer chains, facilitating their mobility, increasing free space and reducing interactions between them. This results in the material being easier to process, more flexible and less brittle, lowering its glass transition temperature and considerably increasing its elongation capacity (Athamneh, Griffin, Whaley, & Barone, 2008; Cuq, Gontard, Cuq, & Guilbert, 1997; Lee, Pranata, Ustunol, & Almenar, 2013; Song & Zheng, 2008; Vieira, Da Silva, Dos Santos, & Beppu, 2011). However, beyond all these well-known aforementioned effects, it is of great interest to evaluate the influence of plasticisers, on the dispersion of nanoparticles in the polymeric matrix. To create a packaging concept for optimal preservation of food, different parameters such as the thermal and mechanical properties, humidity uptake, and barrier properties must be taken into account in order to avoid loss of nutritional content, off‐flavors, color changes, oxidation processes, and spoilage. Knowing the influence of plasticizers on those parameters is very important to develop customized packaging materials (El Miri et al., 2018). Bio-based films could be a good alternative to prevent deterioration for many food products because they often possess excellent oxygen barrier properties. Unfortunately, plasticisers usually increase gases permeability. Previous studies have indicated that gas permeability increases proportionally with plasticiser content (Arvanitoyannis, Nakayama, & Aiba, 1998; Sothornvit & Krochta, 2000). However, although the use of plasticiser to modify the permeability of different films has been reported, the interaction of them and albumen-based nanobiocomposites requires additional elucidation, since the finding of a plasticiser that favours the correct dispersion of nanoparticles into this matrix could compensate for this negative effect and help to obtain materials with improved barrier properties.

Cuq et al. (1997) found that differences in plasticising ability of substances with similar chemical nature were actually due to different molar content when comparing on mass basis substances with different molecular weight. Previously, Donhowe and Fennema (1993)b) had stated that these effects of the chemical nature of plasticisers are only significant when the differences are substantial, such as when comparing glycerol with a much higher molecular weight PEG (Donhowe & Fennema, 1993).

The aim of this research was to find a plasticiser with the most suitable formulation to favour the dispersion of nanoclays in albumen bioplastic matrix and, consequently, to improve the mechanical behaviour and barrier properties of the obtained material. To accomplish this objective, two studies were performed. In the first one, the molar content of the plasticiser was varied, keeping the mass content constant. To this end, different proportions of two plasticisers of similar characteristics (and probably the most widely used) such as water and glycerol, were studied. Subsequently, in the second study, the plasticiser capacity of PEG 300, a large plasticiser that has already been successfully used in the plasticisation of other proteins, was evaluated.

Section snippets

Materials

The spray-dried egg white albumen (EW) used was provided by OVOSEC S.A. (Spain). Glycerol from Guinama (Spain), PEG 300 from Manuel Riesgo, S.A. (Spain) and distilled water were used as protein plasticisers. Regarding the nanoparticles, two selected montmorillonites from Southern Clay Products, Inc. (USA) were used: a) Cloisite® Na+ (MMT-Na) (natural sodium); and b) Cloisite® 30B (OMMT) (organo-modified).

Samples formulation

A plasticiser/protein mass ratio of 0.4/0.6 was always maintained (López-Castejón,

Results and discussion

The plasticisers investigated in this study represent different chemical composition and molar mass, thus providing the opportunity to explore the effects of these factors on dispersion of nanoclays and NBC final properties.

Conclusions

The effect of plasticiser composition and its molar concentration (glycerol and/or water and PEG 300) on albumen bioplastics/nanobiocomposites permeability and mechanical properties were compared. In general, for egg white-based-bioplastics and nanobiocomposites, the presence of glycerol favours the disorder or amorphous character of the structure. On the contrary, water gives a more ordered and compact material. Additionally, the best characteristics are obtained when the nanobiocomposites

CRediT authorship contribution statement

Isabel Diañez: Investigation, Visualization, Formal analysis, Writing - original draft, Writing - review & editing. Inmaculada Martínez: Supervision, Conceptualization, Methodology, Writing - original draft, Writing - review & editing. Perla A. Gómez: Investigation.

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    This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

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