Facile and green fabrication of silk sericin films reinforced with bamboo-derived cellulose nanofibrils

Highlights

• “Green” nanocomposite films were prepared using sericin/cellulose nanofibril.

• Cellulose nanofibrils were prepared by an eco-friendly ultrasonication process.

• The tensile properties nanocomposites were enhanced compared to the raw sericin.

• The hydrophilic properties were easily tunable via nanofibril amount control.

Abstract

As the interest in the recycling of byproducts and resources derived from agriculture has increased, research on the application of sericin abandoned in the silk industry has been continuously carried out, but there is a limit due to the weak mechanical properties. Recently, nanocellulose has been attracting attention as an optimal reinforcing material to improve the deficient physical properties of natural polymers. In the present study, bamboo-derived cellulose nanofibrils (B-CNFs) were prepared and used as a facile reinforcer to improve the mechanical properties of the sericin film. B-CNF was prepared via simple and eco-friendly ultrasonic treatment and whole bio-nanocomposite film fabrication process conducted under aqueous solution condition. The prepared B-CNF was well distributed into the glycerol-plasticized sericin matrix without agglomeration until 10 wt% B-CNF loading, while agglomerated cellulose nanofibrils appeared after 20 wt% B-CNF loading. This uniform distribution of B-CNF not only greatly improved the properties of the sericin film but also affected the hydrophilic properties of bio-nanocomposite film. Moreover, the prepared B-CNF reinforced bio-nanocomposite films had notable antioxidant activity without any additional antioxidant ingredients. These findings support the potential use of B-CNF reinforced sericin films in active food packaging, drug delivery carriers, and in wound dressing materials. Silk and bamboo-derived natural polymeric raw materials and aqueous solvent-based film manufacturing processes will strongly provide cost efficiency and environmental friendliness and can be a suitable replacement of petroleum-based polymer industry and its related application fields.

Introduction

Synthetic polymers have provided undeniable benefits to industrial and social development. However, non-biodegradable petroleum-based plastics threaten the environment and have contributed to the reduction of petrochemical resources (Iwata, 2015). Natural polymer resources are abundant in nature and have excellent biodegradability; therefore, interest in bio-based natural polymeric materials is rising (Saba et al., 2014). The use of natural polymer materials can prevent not only environmental pollution but also the depletion of petroleum resources.

Generally, numerous nature-derived by-products exist, which are generated from processes that utilize natural resources. For example, various straws are produced in agriculture (Wilson et al., 2014), abundant amounts of chitosan and chitin can be obtained from aqua culture (Nisticò, 2017), and large amounts of lingo-cellulosic resources are produced in the pulp industry (Osong et al., 2016). So far, these by-products have been used as feed and fuel, or have even been wasted (Santos et al., 2013). These naturally abundant by-products could be good candidates for sustainable development technology (Reddy and Yang, 2005). As a part of this research, various attempts have been actively conducted for the conversion of mechanical pretreated-agricultural byproducts into biogas (Josef Maroušek, 2013) and the utilization of biomass-derived biochar as a soil remediation process (Josef Maroušek et al., 2018).

Sericin is the secondary silk protein produced by Bombyx mori (Sparkes and Holland, 2018). In silk-based textile industrial activities, the extraction of sericin from fibroin fibers, which is called “degumming,” can be performed for the efficient reeling and dyeing of the fibroin fibers (Zhao et al., 2018). Generally, approximately 25–27% of the original weight of sericin is discarded during the degumming process. Recently, sericin has been found to have various functionalities including antioxidant (Sangwong et al., 2016), antibacterial (Zhao et al., 2014), moisturizing (Padamwar et al., 2005), and wound-healing activity (Lamboni et al., 2016). Because of the variety of sericin functionalities, sericin-based composite films do not require or require fewer functional additives, which can lower the cost of their production and make them price competitive with existing materials.

For these reasons, studies on the utilization of sericin-based materials have been actively underway. However, the inherent drawback of sericin, i.e., its fragile nature, has made its use difficult (He et al., 2017, Yun et al., 2013). The brittleness of sericin makes it difficult to obtain integrated sericin formulations and limits the practical applications of it. Therefore, to overcome these drawbacks and expand sericin's applications, it is necessary to improve its mechanical performance. In general, polymer blends (He et al., 2017), cross-linking agents (Nayak et al., 2012), and plasticizers (Zhang et al., 2011) have been used to reduce the brittleness of sericin. Yun et al. prepared a glycerol-plasticized silk sericin film and showed improvements of the mechanical properties (Yun et al., 2016). They concluded that glycerol and absorbed water molecules in the sericin film have a synergistic effect on the flexibility of the film. However, the addition of glycerol to sericin sacrifices the tensile properties of it. Previously, Zhang et al. reported that the addition of glycerol improved the elongation-at-break from 0.73 to 354% but decreased the tensile strength and modulus from 13.7 to 600.5 MPa to 8.2 and 57.3 MPa, respectively (Zhang et al., 2011). A similar behavior of the tensile properties was observed in Yun et al.’s glycerol-plasticized film (Yun et al., 2016). These deteriorating tensile properties could be a limitation to the practical application of sericin.

Using nano-sized reinforcing materials in the natural polymer matrix is an effective method to obtain high-performance composite materials (Dufresne Alain and Castaño Johanna, 2017). More recently, there have been numerous attempts to use cellulosic nanomaterials as load-bearing reinforcements in natural polymer matrices (Kargarzadeh et al., 2017). Cellulose-based nanomaterials are efficient and eco-friendly reinforcements due to their abundant supply, low cost, biodegradability, and excellent mechanical properties (Julkapli and Bagheri, 2017). Jang et al. prepared a cellulose nanofibril-reinforced sericin composite film and investigated its mechanical properties (Jang et al., 2015). They used formic acid to obtain β-sheet structured sericin; however, the mechanical properties of their sericin film reinforced with cellulose nanofibrils could not be enhanced, even though 15 wt% of cellulose nanofibrils was introduced.

In the fabrication of composite materials, solvents and additives are important factors that affect environmental sustainability and cost competitiveness. In this regard, ionic liquid solvent systems have attracted great attention due to their environmental friendliness and strong dissolving power (Andanson et al., 2015). However, there are many considerations when using an ionic liquid solvent system including purity control of the ionic liquid and recovery of used solvent. Nanocomposite fabrication processes under an aqueous solvent system can provide cost effectiveness and reduce the environmental pollution created by related organic solvent systems.

In this study, an environmentally friendly green nanocomposite film with antioxidative effect was prepared and its characteristics were analyzed. The sericin aqueous solution, which is abundantly obtained in the sericulture industry, was used as raw material for film production. The bamboo-derived cellulose nanofibrils (B-CNFs) with high aspect ratio were isolated through an eco-friendly ultrasonic treatment process. To produce “green” bio-nanocomposite films, we fabricated a silk sericin/glycerol/B-CNF (nano-reinforcement) bio-nanocomposite film under an all-aqueous solvent system. The morphology, chemical and crystalline structure, and mechanical performance of the resulting bio-nanocomposite films were investigated through field emission-scanning electron microscopy (FE-SEM), UV–vis spectrometry, Fourier-transform infrared spectroscopy (FT-IR), X-ray diffractometry (XRD), and measurements of the mechanical properties. Moreover, the hydrophilic properties and antioxidant activity of the composite films were also evaluated.