Advancing sustainable packaging: The role of nanofibers in bioplastics

Abstract

In the last few decades, the packaging industry has become one of the fastest-growing industries worldwide, owing to some changes in standards of living, consumption habits, and global trade expansion. Bio-based materials have emerged as one of the most interesting subjects of research in the packaging industry due to the environmental concerns associated with materials derived from petrochemical sources, such as resource depletion, recycling challenges, and biodegradation which have resulted in the development of eco-friendly materials. Polymers are widely used in the packaging industries, and synthetic polymers are extensively employed mainly because of their outstanding mechanical properties, effective barriers against oxygen and water, and ease of processability. However, they present significant downsides, such as poor degradability and challenges in recyclability, and as a result, packaging waste constitutes a large portion of post-consumer solid waste, leading to ecological problems. Therefore, extensive research is being conducted to develop biopolymers in the packaging industry. The challenges associated with the global usage of biopolymers include poor mechanical and barrier properties and high production costs. In order to modify the properties of biopolymers, various methods could be employed, such as reinforcing the polymer matrix with nanomaterials, especially nanofibers. The first goal of this project was to optimize the process of preparing nanofibers derived from hemp. Pre-treatments were applied before the fibers underwent the refining process to reduce the number of steps required for refinement and to investigate their effect on the stability and diameter of the nanofibers produced. This approach not only saves time, energy, and costs but also enhances the overall efficiency of the process, representing a significant step forward for the industry. In this study, mechanical treatment was applied for the fibrillation of hemp fibers. This method has significant advantages over chemical treatment, particularly in terms of reducing the amount of chemicals used. This aligns with one of the most important goals of this project, promoting a more sustainable and cost-effective approach. In order to improve the efficiency of the fibrillation process, several pre-treatments were applied. Among them, the pre-treatment involving fiber hydration by immersing the fiber in water for one hour, subjecting it to a strong vacuum for 30 minutes, and processing it in a pressure cooker at high temperature (≈120°C) and pressure (12 psi) for 10 minutes resulted in the smallest fiber size reduction after eight passes. Furthermore, based on the stability test, this sample exhibited the highest stability, remaining stable after seven days.

Another goal of this project was to improve the mechanical and barrier properties of biodegradable nanocomposite films for packaging applications. Polybutylene succinate (PBS) has high flexibility, high elongation at break, good biodegradability, and water resistance. However, due to its low molecular weight, low stiffness, poor oxygen resistance, and high cost, its potential applications are limited. Therefore, one solution could be the addition of hemp nanofibers (HNF) to PBS in order to enhance biodegradation and reduce costs. In this study, nanocomposites of PBS and HNF, with a ratio of 95/5, were first prepared using an extruder and hydraulic press. Their barrier and mechanical properties were then investigated. Then, these properties were compared with the properties of nanocomposites containing PBS/HNF with the addition of beeswax and sodium dodecyl sulfate (SDS) at different ratios. The moisture content, water absorption capacity, and water solubility tests showed that adding beeswax reduced moisture content, water absorption, and water solubility. These effects became more pronounced with increasing amounts of beeswax. Similarly, introducing SDS as a surfactant resulted in a greater decrease in these properties compared to adding beeswax alone, with further reductions observed as the concentration of SDS increased. Furthermore, the results of the water vapor permeability (WVP) test revealed that the incorporation of nanofibers resulted in a decrease in the film permeability due to its hydrophilic nature. However, beeswax created a barrier that hindered the movement of water vapor molecules through the film due to its hydrophobic nature. The extent of this decrease depends on the amount and distribution of the beeswax. When SDS was introduced to the film’s formulation, its bridging effect could further reduce the WVP amounts of films, though only at low SDS concentrations. Overall, the interactions between all components (PBS, hemp nanofiber, beeswax, SDS) can influence the final film structure. Additionally, mechanical tests demonstrated that adding HNF to PBS films increased tensile strength and modulus. However, this led to a decrease in elongation at break. For samples with beeswax in the formulation, the flexibility of the films increased, resulting in an increase in the film's elongation. In terms of tensile strength and tensile modulus, beeswax improved the compatibility between PBS and HNF. This enhancement led to better dispersion of hemp nanofiber within the PBS matrix, resulting in a more uniform composite and improved tensile strength. Meanwhile, the addition of beeswax to the formulation, due to its plasticizing effect, is expected to reduce the tensile modulus. In the final compositions, SDS was added to the film formulation. At low concentrations, SDS behaves as a surfactant, reducing the surface tension between PBS and HNF. This leads to a better dispersion of the nanofibers throughout the PBS matrix, which could reduce stress concentration. Well-dispersed nanofibers create a more uniform stress distribution within the film. This can help prevent premature failure at specific points and allow for more stretching before breaking, potentially increasing elongation. While SDS aids in dispersion at lower concentrations, excess SDS can interact with the surfaces of both PBS and HNF, disrupting the natural interactions (such as hydrogen bonding) between them, which contribute to the overall strength and integrity of the film and their disruption can make the film more susceptible to breaking under stress, potentially leading to decreased elongation. Meanwhile, the initial addition of SDS enhanced the composite's tensile strength mainly because of improved PBS-HNF adhesion and better stress transfer from the polymer matrix to the fibers. The addition of SDS over the optimal concentration resulted in phase separation, which could be regarded as a weak point in the composite, thereby having an adverse effect on the tensile strength. It was observed that by introducing SDS to the formulation tensile modulus also decreased. Overall, the nanocomposites prepared exhibited promising properties for sustainable packaging applications. Nevertheless, additional research and development are essential to improve and optimize the material properties further for optimal performance.