Petroleum-derived plastics (traditional petroleum-based plastics) are lightweight, strong, durable, and resistant to degradation, replacing many other substances in the form of disposable gears, packaging, furniture, machinery frames, accessories, etc. They are widely used in medical and industrial applications to improve quality of life and comfort.
In 2016, 335 million tons of plastics were produced globally, reflecting the popularity and widespread use of plastics. Since most plastics produced are disposable and about 40% are used in packaging, with thermoplastic polyethylene (PE), polyethylene terephthalate (PET), polypropylene (PP) and polystyrene (PS) being the most frequently used plastics in packaging, most of these petroleum-derived plastics are highly resistant to biodegradation, meaning that once they reach the environment, they will inevitably accumulate and thus have negative environmental consequences.
Big plastic waste is a major pollutant in the world's oceans, and hundreds of thousands of sea turtles, seals, whales and seabirds have died from ingesting or becoming entangled in plastic. In recent years, research has noted plastic waste in soil and warned of the dangers of microplastics in soil and terrestrial ecosystems. As industrial development has accelerated and the manufacture and use of plastics has increased, plastic pollution has become an increasingly common concern.
Possible solutions to the plastic pollution problem
Although research on petroleum-derived plastics continues, their degradation remains a major challenge. One of the solutions to the plastic pollution problem is the circular economy, which maintains the value of materials in the cycle by reusing and recycling plastic materials and preventing their uncontrolled release into the surrounding environment.
However, the recycling potential of plastic waste remains largely untapped, and global plastic recycling rates are low, with plastic recycling accounting for only 6% of total plastic demand.
The second method is to incinerate plastic waste in landfills, but it will produce toxic emissions and microplastics, thus causing new environmental pollution, so it is also undesirable. To better address the problem of plastic pollution, more and more research is focused on producing bioplastics and developing materials with biodegradable properties. Bioplastics are designed to use renewable carbon resources (e.g., starch) as a raw material for production instead of non-renewable fossil resources, and biodegradable plastics are developed with the aim of easy decomposition and biodegradation of whatever carbon source is used when the plastic is released into the environment.
Biodegradable plastics are those that can be completely degraded by the action of naturally occurring microorganisms in landfills, composting plants or wastewater treatment plants.
Truly biodegradable plastics degrade without leaving toxic, visible or distinguishable residues. Therefore, biodegradable plastics are considered to be environmentally sound alternatives to synthetic petrochemical polymers. Completely biodegradable bioplastics have gained much attention due to their excellent performance of being completely degraded by microorganisms.
Plant-based fully degradable plastics
Since plants produce many polymers under natural conditions, including starch, cellulose and storage proteins, all of these polymers can be used in the production of biodegradable plastics, and the plant-based fully degradable plastics produced using them are non-toxic, completely biodegradable and renewable, making them a viable material to replace petroleum-based plastics.
The current status of research on several major plant-based fully degradable bioplastics is divided.
1. Starch-based degradable plastics
Starch is a low-cost, highly available natural biopolymer and is considered a promising material because of its complete biodegradability, low cost and renewable nature. It is used to produce edible biodegradable packaging and is an attractive alternative to synthetic polymers.
Starch-based bioplastics account for 85% to 90% of bioplastics on the market. Starch-based bioplastics are made from natural starch or slightly modified starch, separated or blended with natural/synthetic molecules.
Cassava root is one of the world's most important sources of starch, and films made from cassava starch are known to be odorless, colorless, non-toxic and biodegradable. Researchers have used pure tapioca starch to develop biodegradable films that can improve the flexibility and extensibility of tapioca starch by adding plasticizers and surfactants.
Hema et al. investigated the interaction of biomolecules such as tapioca starch with glycerol and acetic acid in the preparation of biodegradable polymer films. The response surface method of Box-Behnken experimental design was used to find that the most significant (p < 0.05) factors among the process variables were tapioca starch and glycerol, and the resulting biodegradable film had the best uniform and transparent performance at 3.6 g starch, 0.9 mL glycerol, and 0.16 mL acetic acid.
Ashok et al. studied the biodegradability and mechanical properties such as hardness and impact strength of starch bioplastics and found that the presence in the form of a mixture of fibers or other polymers helped to improve the properties of starch-based bioplastics.
Chandra et al. used starch in combination with other natural polysaccharides (okra gum and cellulose) to prepare stable bioplastic films. Different types of starch (wheat, corn, potato) can be used as plasticizers such as glycerol or blended with degradable polymers to produce bioplastics with excellent mechanical properties at a relatively low cost.
2. Cellulose-based bioplastics
Plant fibers are the most abundant natural resource in the earth's reserves. Sugar beet residue and bagasse are important agricultural by-products that can be used in the preparation of composite materials and films.
Šimkovic et al. used whole cellulose from sugar beet and bagasse to prepare the films. By carboxymethylation of bagasse holocellulose, bagasse holocellulose films with better mechanical properties were obtained. It was concluded experimentally that the surface structure of all-cellulose films is independent of their mechanical properties and depends mainly on the interaction of the linear structure of cellulose with the structurally related xylan chains.
To compare the structure and properties of cellulose films prepared from different cellulose sources, Pang et al. selected four types of cellulose, pine, cotton, bamboo cellulose, and microcrystalline cellulose (MCC) as raw materials to prepare environmentally friendly recycled cellulose films. By comparing the morphology, surface and mechanical properties of different types of cellulose regenerated films, it was found that the cellulose films prepared from cotton fibers had a more uniform and smooth morphology and the highest thermal stability. In addition, the cellulose films prepared from pine cellulose have better film-forming properties and higher tensile strength. The study uses pine wood and cotton lint as raw materials to open the way for further industrial applications of these materials.
David et al. used cellulose derivatives to prepare biofilms. To obtain transparent and stretchable films, plasticizers were added to make the cellulose derivative films flexible and ductile. Plasticizers reduce glass transition temperature and mechanical strength, but increase extensibility. Tests have found that glucose, urea and even absorbed water can produce this effect, modulating the film stretch properties by controlling the amount of glucose and urea added. These films achieve stretchability and environmental sustainability by using only water, an environmentally friendly solvent.
Fatma et al. studied the properties of blended materials including polylactic acid (PLA) and cellulose extracted from fine stem needlegrass and lucerne by melt extrusion technique and found that these fibers, when used at 10% ratio, improved the mechanical properties of the blends and could yield plastic films with excellent properties. Cellulose is synthesized by blending with natural degradable polymers such as starch, polylactic acid, and other degradable polymers to enhance the mechanical properties of plastics and maintain their biodegradability to obtain biofilms with better performance.
3. Chitosan-based bioplastics
Chitosan is a natural polymer composed of deacetylated derivatives of chitin, which is the main component of the exoskeleton of crustaceans and the second largest polysaccharide found in nature after cellulose. Among these bio-derived films, chitosan is non-toxic, biodegradable, biofunctional and biocompatible, in addition to its antibacterial properties. In addition, chitosan is one of the ideal biomaterials for the development of environmentally friendly films due to its excellent gas barrier properties, easy film formation ability, better mechanical properties, biodegradability and low cost.
Chitosan films can be prepared by solution casting method and can also be blended with other materials to improve the performance. Since the physicochemical properties of the films depend largely on the intermolecular interactions occurring in the film matrix, and the electrostatic attraction between chitosan and polysaccharide molecules at the appropriate pH facilitates the intermolecular interactions within the film matrix, thus improving the film properties.
Younis et al. characterized chitosan and high methoxylated apple pectin blends with different mass ratios and found that the transparency and mechanical properties of high methoxylated apple pectin and chitosan blends were better than those of pure chitosan and pure pectin films.
In addition, chitosan can also form co-blended films with plasticizers (e.g. glycerol), lignin, and synthetic degradable polymers to increase the flexibility of the film and obtain improved and optimized bioplastic films.
4. Protein-based bioplastics
Proteins are biopolymers composed of amino acids, easy to form thin films, low cost, decomposable properties, and the presence of multiple functional groups in the amino acid chains of different types of proteins offer good prospects for the development of protein-based bioplastics. In developing protein-based films, plasticizers are often required to obtain desirable physicochemical properties such as material flexibility.
Gluten is synthesized from proteins, wheat gluten and alcohol soluble proteins (with other globulin and albumin fractions) and shows great potential in the synthesis of edible films due to its unique intrinsic properties, such as viscosity and elasticity, and its excellent film-forming properties. However, the mechanical properties and moisture resistance of gluten films are affected by gluten content, pH and ethanol content, which can be improved by adding plasticizers, binding agents and reinforcing agents to improve the gas barrier, mechanical properties and moisture resistance of gluten films.
In recent years, the application of gluten films in the food industry has been explored by studying the synergistic effects of proteins, gums and lipids on the physicochemical properties or activity of edible films to improve the barrier and mechanical properties of gluten films.
Mohammad et al. investigated the effect of enzymatic reaction on SPI film-forming solution and membranes at fixed pH conditions (pH 7), on protein denaturation temperature (80 and 90 °C), and enzyme incubation time (1, 2 and 3 h), and showed that glutamine transaminase treatment of SPIs improved protein network cross-linking, water uptake and protein solubility. Enzyme-treated protein solutions containing 7.5% solids content exhibited non-Newtonian, pseudo-plasticity, viscosity increased with enzyme incubation time, and changes in membrane physical properties, such as mechanical properties, surface smoothness, and contact angle, may overlap with enzyme stabilizer effects, but were significantly different from control values. It is thus clear that enzyme treatment can be a useful method to control the physical properties of protein-based biopolymers. SPI-fibrin composites have excellent moisture resistance, tensile strength, thermal stability and flexibility, and retain biodegradability. The fully biodegradable bioplastics produced by protein polymerization have good mechanical properties and may provide ideas for protein-based organics processing.
Conclusion and Outlook
Plant-based fully degradable bioplastics have a promising future, with great advantages in edible packaging materials, agricultural mulch, waste disposal and surgical sutures, and play an important role in production and life.
Studies have shown that fully biodegradable bioplastics are feasible and some progress has been made in research on plant-based fully degradable bioplastics, but there are still some problems to be solved, such as high prices, mechanical properties still need to be optimized, biodegradation rate control, and control of initiating degradation.
While developing new fully degradable bioplastics, we should explore how to better reduce the production cost to make them reach industrialization, how to better control the biodegradation rate of plastics to ensure the balance between plastic circulation and environmental degradation ability, and whether we can make the plastics have good mechanical properties during use and start degradation only after use by adding optical or chemical triggers, etc. Through in-depth research on fully degradable bioplastics, their performance can be further improved and optimized to play a better role in practical applications.
