Current situation and Prospect of plant-based fully degradable bioplastics
Petroleum derived plastics (traditional petroleum based plastics) have many properties, such as lightness, firmness, durability and resistance to degradation. They replace many other substances in the form of disposable gears, packaging, furniture, mechanical frames, accessories and so on. They are widely used in domestic medical and industrial fields to improve the quality of life and comfort.
In 2016, 335 million tons of plastics were produced worldwide, reflecting the popularity and wide application of plastics. As most of the plastics produced are disposable plastics, about 40% of which are used for packaging, while thermoplastic polyethylene (PE), polyethylene terephthalate (PET), polypropylene (PP) and polystyrene (PS) are the most frequently used plastics in packaging. Most of these petroleum derived plastics have strong resistance to biodegradation, which means that once they reach the environment, they will inevitably accumulate, resulting in negative environmental consequences.
Large plastic waste is a major pollutant in the world's oceans. Hundreds of thousands of turtles, seals, whales and seabirds die from ingestion or entanglement of plastic. In recent years, research has noticed plastic waste in soil and warned of the dangers of microplastics in soil and terrestrial ecosystems. With the acceleration of industrial development and the increase of the manufacture and use of plastics, the problem of plastic pollution has been paid more and more attention.
01 Possible solutions to plastic pollution
Although the research of petroleum derived plastics is still in progress, its degradation is still a major problem. One of the ways to solve the problem of plastic pollution is circular economy, which maintains the value of materials in the cycle by reusing and recycling plastic materials, and prevents them from being released into the surrounding environment uncontrolled. However, the recycling potential of plastic waste has not been developed to a large extent. The global plastic recycling rate is very low, and plastic recycling accounts for only 6% of the total plastic demand.
The second method is to bury and incinerate plastic waste, but it will produce toxic emissions and micro plastics, which will cause new environmental pollution, so it is not advisable. In order to better solve the problem of plastic pollution, more and more research has focused on the production of bioplastics and the development of biodegradable materials. Bioplastics is to use renewable carbon resources (such as starch) instead of non renewable fossil resources as production raw materials. The purpose of developing biodegradable plastics is that when plastics are discharged into the environment, no matter what carbon source is used, it is easy to decompose and biodegrade.
02 Plant based fully degradable plastic
Because plants produce many polymers under natural conditions, including starch, cellulose and storage protein, these polymers can be used in the production of biodegradable plastics, and the plant-based fully biodegradable plastics produced by them are non-toxic, completely biodegradable and renewable, so they are feasible materials to replace petroleum based plastics. The research status of several plant-based fully degradable bioplastics is reviewed.
Starch based degradable plastics
01
Starch is a natural biopolymer with low cost and high availability. It is considered to be a promising material because of its complete biodegradability, low cost and reproducibility. It is used to produce edible biodegradable packaging and is an attractive alternative to synthetic polymers.
Starch based bioplastics account for 85% - 90% of bioplastics on the market. Starch based bioplastics are made of natural starch or slightly modified starch, which are separated or mixed with natural / synthetic molecules.
Cassava root is one of the most important sources of starch in the world. The film made of cassava starch is called tasteless, colorless, non-toxic and biodegradable. Researchers use pure cassava starch to develop biodegradable films. By adding plasticizers and surfactants, the flexibility and ductility of cassava starch can be improved.
HEMA et al. Studied the interaction between biological macromolecules such as cassava starch and biological macromolecules such as glycerol and acetic acid in the preparation of biodegradable polymer films. The response surface method designed by box Behnken test found that among the process variables, the most significant (P < 0.05) factors were cassava starch and glycerol. Under the conditions of 3.6 g starch, 0.9 ml gly.
Cellulose based bioplastics
02
Plant fiber is the largest natural resource in the earth. Beet residue and bagasse are important agricultural by-products, which can be used for the preparation of composite materials and films.
Š Imkovic and others used total cellulose in beet and bagasse to prepare films. Through Carboxymethylation of bagasse total cellulose, bagasse total cellulose films with better mechanical properties were obtained. It is concluded that the surface structure of the whole cellulose film has nothing to do with its mechanical properties, and mainly depends on the interaction between the linear structure of cellulose and the structure related xylan chain.
In order to compare the structure and properties of cellulose films prepared from different cellulose sources, Pang et al. Selected pine, cotton, bamboo cellulose and microcrystalline cellulose (MCC) as raw materials to prepare environmentally friendly regenerated cellulose films. By comparing the morphology, surface and mechanical properties of different kinds of cellulose regenerated membranes, it was found that the cellulose membrane prepared by cotton fiber had a more uniform and smooth morphology and the highest thermal stability. In addition, cellulose films prepared from pine cellulose have good film-forming properties and high tensile strength. The research takes pine and cotton wool as raw materials, opening up a way for the further application of these materials in industry.
David et al. Used cellulose derivatives to prepare biological films. In order to obtain transparent and stretchable films, plasticizers were added to make cellulose derivatives films elastic and ductile. Plasticizers reduce the glass transition temperature and mechanical strength, but increase the extensibility. It is found that glucose, urea and even absorbed water can produce this effect. By controlling the amount of glucose and urea, the tensile properties of the film can be adjusted. These films achieve ductility and environmental sustainability by using only water, an environmentally friendly solvent.
Fatma et al. Studied the extraction of mixture materials including polylactic acid (PLA) and cellulose from Stipa tenuiflora and towel gourd by melt extrusion technology, studied their properties, and found that when these fibers are used at a ratio of 10%, they can improve the mechanical properties of the mixture and obtain plastic films with excellent properties. Cellulose is synthesized with natural degradable polymers such as starch and polylactic acid, and blended with degradable polymers to enhance the mechanical properties of plastics and maintain its biodegradability, so as to obtain biofilm with better performance.
Chitosan based bioplastics
03
Chitosan is a natural polymer composed of deacetylated derivatives of chitin. It is the main component of the exoskeleton of crustaceans and the second largest sugar found in nature, second only to cellulose. In this kind of bio derived membrane, chitosan not only has antibacterial properties, but also has the characteristics of non-toxic, biodegradable, biological function, biocompatibility and so on. In addition, chitosan has become one of the ideal biomaterials for the development of environmental friendly films because of its excellent gas barrier performance, easy film-forming ability, good mechanical properties, biodegradability and low cost.
Chitosan films can be prepared by solution casting, and can also be blended with other materials to improve their properties. Because the physical and chemical properties of the film largely depend on the intermolecular interaction in the film matrix, and at an appropriate pH, there is electrostatic attraction between chitosan and polysaccharide molecules, which is conducive to the intermolecular interaction in the film matrix, so as to improve the performance of the film.
Younis et al. Characterized the blend films of chitosan and high methoxy apple pectin with different mass ratios, and found that the transparency and mechanical properties of the blend films of high methoxy apple pectin and chitosan were better than those of pure chitosan and pure pectin films.
In addition, chitosan can also form a blend film with plasticizers (such as glycerol), lignin, synthetic degradable polymers, etc., so as to increase the flexibility of the film and obtain an improved and optimized bioplastic film.
Protein based bioplastics
04
Protein is a biopolymer composed of amino acids, which is easy to form thin films, low-cost, and has decomposable properties. There are many functional groups in the amino acid chains of different kinds of proteins, which provides a good prospect for the development of protein-based bioplastics. When developing protein based films, plasticizers are usually used to obtain ideal physical and chemical properties such as material flexibility.
Gluten is synthesized from protein, glutenin and gliadin (together with other globulin and albumin components). Because of its unique internal properties, such as viscosity and elasticity, and its excellent film-forming properties, gluten shows great potential in the synthesis of edible films. However, the mechanical properties and moisture resistance of gluten film are affected by gluten content, pH and ethanol content. The gas isolation ability, mechanical properties and moisture resistance of gluten film can be improved by adding plasticizers, binders and enhancers.
In recent years, through the study of the synergistic effect of protein, gum and lipids on the physical and chemical properties or activities of edible films, the barrier and mechanical properties of gluten films have been improved, and the application of gluten films in the food industry has been explored.
Conclusion and Outlook
Plant-based fully degradable bioplastics have broad prospects for development. They have great advantages in edible packaging materials, agricultural mulch, garbage disposal and surgical suture, and play an important role in production and life.
The research shows that fully biodegradable bioplastics is feasible, and the research on plant-based fully biodegradable bioplastics has made some progress, but there are still some problems to be solved, such as high price, mechanical properties still need to be optimized, biodegradation rate control, start-up degradation control, etc. While developing new fully degradable bioplastics, we should explore how to better reduce production costs and achieve industrialization, how to better control the biodegradation rate of plastics to ensure the balance between plastic circulation and environmental degradation capacity, whether we can add light or chemical triggers to make plastics have good mechanical properties in the process of use, and then start degradation after use, Through the in-depth study of fully degradable bioplastics, their properties have been further improved and optimized to play a better role in practical applications.
