In recent years, conventional petroleum-based polymers have grown increasingly unpopular amidst prevalent environmental concerns [1
]. Biopolymers, such as polysaccharides, proteins, and lipids, have attracted considerable interest for their biodegradations in terms of relieving the overdependence on petroleum resources [2
]. Among all biopolymers, soy protein isolate (SPI) has shown several notable advantages such as renewability, biocompatibility, biodegradability, and film-forming capacity [5
]. However, the inferior mechanical properties and high moisture sensitivity of existing SPI-based films have limited their practical application [6
]. The films can be modified, such as by physical treatment, chemical cross-linking, or block copolymerization, to enhance their applicability, but there is no perfect solution [7
]. To date, chemical cross-linking has been proven to be the most effective and facile approach to enhancing the performance of SPI films [9
In terms of potential applications as green, environmentally friendly composites, biocompatibility and high mechanical strength are critical factors for a given packaging film. Several researchers have explored 1,2,3-propanetriol-diglycidyl-ether (PTGE), a novel and biocompatible cross-linking agent, for this purpose [11
]. The tensile strength (TS) of film crosslinked by PTGE was increased by 197% in one of the previous studies, but the elongation at break (EB) was reduced by 67% due to the cross-linkage effect [11
]. Therefore, research on SPI-based films, to this effect, is focused on increasing the TS without compromising the reduction of EB.
Recently, a great breakthrough was made in the bio-nanocomposites field, which was the blended biopolymers with inorganic/organic nano-particles [13
]. Incorporating fillers as reinforcements in the composites, such as montmorillonite, nano-SiO2
, starch nanocrystal, carbon nanotubes, and rectorite nanoplatelets were the methods commonly used [14
]. Unfortunately, most of the nanoparticles mentioned above showed bad compatibility with SPI, and need further modification.
Halloysite nanotubes (HNTs), firstly reported by Berthier in 1826, are natural aluminosilicates (Al2
O) with nano-tubular structures [18
]. Compared to other nanosized materials, naturally occurring HNTs are easily obtained and are much cheaper than other nanoparticles such as carbon nanotubes (CNTs) or boron nitride nanotubes [19
]. HNTs are composed of siloxane (Si-O-Si) groups on the external surface and gibbsite octahedral sheet (Al-OH) groups on the inner surface [20
]. The length of two-layered HNTs is typically 0.2–2 mm, with an external outer diameter of 40–70 nm and an inner diameter of 10–30 nm; these are especially promising candidates as reinforcing fillers due to their different chemistry at the inner and outer surfaces of the nanotube [21
]. This difference allows for the selective modification of the material based on electrostatic interactions or specific chemical reactions [23
]. Due to their unique tubular structure, high surface area, large aspect ratio, good biocompatibility, and low manufacturing cost, HNTs have recently received much attention in many applications [25
Previous studies have shown that poor dispersion and lack of interfacial adhesion to the polymer matrix are the most problematic aspects of HNTs in terms of practical applications [27
]. Carboxymethylated chitosan (CMCS), a water-soluble chitosan derivative, is water soluble under acidic and alkaline environments—an important physiological difference from chitosan alone, which is only soluble in acidic solutions [28
]. CMCS has been widely studied due to its ease of synthesis, ampholytic characteristics, and numerous application prospects. CMCS has many reactive functional groups (e.g., amino, carboxyl, and hydroxyl groups). The interaction between carboxyl and amino groups in the CMCS structure can favor the formation of cationic–NH3+
groups, leading to a stronger electrostatic interaction with the negatively charged materials [29
]. Therefore, CMCS is expected to effectively improve the dispersibility of HNTs, and increase its interfacial adhesion.
The ultimate goal in conducting this study was to prepare a green and highly effective bionanocomposite packaging material via the casting method. Films with a combination of SPI, PTGE, HNTs, and CMCS were prepared and characterized by attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), and UV-Vis spectroscopy. The films’ mechanical properties and water resistance were also investigated.