Human Practices

1 About Biodegradable plastic

1.1 Concept and Types

Biodegradable plastic is plastic material that can degrade in nature. In a natural burial or composting environment with sufficient humidity, oxygen and appropriate microorganisms, it can be metabolized and decomposed by microorganisms to produce water and carbon dioxide or methane

However, not all plastic materials produced from renewable biomass sources are biodegradable plastics. According to Jayarathna S et al. (2022), biomass-derived plastics are called bioplastics，they can be divided into three main categories: 1) Bio-based or partly bio-based non-biodegradable plastics, 2) Both bio-based and biodegradable plastics and 3) Fossil-based biodegradable plastics (Fig. 1). Of these three types of bioplastics, only those made from both bio-based and biodegradable plastics and Fossil-based biodegradable plastics are considered as biodegradable plastics.

Fig. 1 Categories of bioplastics

1.2 General advantages

According to Luo DJ , biodegradable plastic possesses several noteworthy advantages.

1. It is ecologically sound given that its degradation byproducts are innocuous to the environment
2. It mitigates reliance on petroleum resources and thereby lessens energy consumption
3. It demonstrates effectiveness in tackling the effects of plastic waste on marine ecology
4. (4)In select domains, biodegradable plastic may prove a more cost-effective option in terms of packaging and waste disposal.

2 Application status of biodegradable plastics

2.1 Current market size of biodegradable plastics

Based on calculations, it is projected that by the year 2025, the demand for biodegradable plastics in four specific areas including disposable plastic tableware, plastic shopping bags, agricultural plastic films, and express packaging will result in a market potential of approximately 2.5 million tons and an estimated market value of approximately 500 billion yuan (Guangming, 2021). These figures indicate that the current market size of biodegradable plastic products is comparatively minuscule when juxtaposed with traditional plastic products.

According to a study conducted by the Frontier Industry Research Institute, China experienced a surge in plastic product manufacturing in 2019, reaching a staggering 818.4 million tons, constituting approximately 25% of the global demand for plastic. In stark contrast, the consumption of biodegradable plastics in China during the same period stood at a mere 5.2 million tons, accounting for a paltry 4.6% of the global average. This figure represents a significant disparity when compared to the worldwide average consumption rate (Guangming, 2021). Furthermore, in 2020, China witnessed a total output of 76.032 million tons of plastic products, while the production of biodegradable plastics was a meager approximately 5 million tons (Wu CX).

By contrast, it is apparent that the present market demand for plastic products is resonantly sizeable and is poised for continued expansion. This observation is confirmed by a recent report released by the Plastics Industry Association of Europe and MarketsandMarkets, which indicates that the global market size for plastic products exceeded 468.3 billion in 2020 and is anticipated to grow to 596.1 billion by 2025, demonstrating an average annual compound growth rate of 4.94%. Furthermore, data from the National Bureau of Statistics of China illustrates that China's plastic product output amounted to 7.77 million tons in 2022. Over the course of the last 13 years, China's domestic plastic product output growth rate has remained constant at 4.33%, reflecting a consistent and sustained expansionary trend in the industry (Wu CX).

2.2 Analysis of the market size of biodegradable plastics

2.2.1 People's willingness to use biodegradable plastics

Through a public questionnaire survey, it was discovered that a significant proportion of individuals possess a comprehensive understanding of the deleterious effects of plastic products on the environment (93.87%). 86.63% of respondents indicated their contemplation of utilizing biodegradable plastic products and 93.04% of them expressed a stronger inclination towards selecting biodegradable plastic products in comparison to conventional plastic materials (Fig. 2).

Fig. 2 Result of our public questionnaire survey

However, compared to the experience of using traditional plastic products, people also have a relatively clear demand for plastic substitute products (Luo DJ)：

1. Environmental performance: It is able to degrade in natural environments and eventually be decomposed by microorganisms
2. Substitution performance of traditional plastics: it is able to comprehensively replace traditional plastic products in terms of utility and performance
3. Extensive application scope: it can be applied in various fields, including food packaging, agricultural covering film, medical equipment, daily necessities, etc., meeting the different demands of plastic products from different industries
4. High quality: The product needs to have sufficient strength, transparency, heat resistance, stability, etc., and possess excellent quality
5. Economic Feasibility: It possesses a reasonable cost and supply chain system, and has the ability to compete with traditional plastic products.

2.2.2 Market demand and restrictions

The conventional plastic industry encompasses a vast array of sectors. Presently, biodegradable plastics primarily serve as substitutes for low-grade plastic products in packaging and agricultural sectors. However, this merely constitutes a minuscule fraction of the plastic market. In numerous other domains, there remain irreplaceable plastic products. It is projected that in the future, the proportion of plastic products that can be substituted will amount to only approximately 7.5% of the overall plastic products (Financial Community, 2021).

Currently, polylactic acid (PLA) is the primary constituent of biodegradable plastic. However, the exorbitant price of PLA results in a production cost three times higher than traditional plastic items. Most mid and downstream enterprises engaged in conventional plastic production are small-scale private businesses. As such, there is a large number of such entities while being inadequately concentrated. These conditions, coupled with the high expense of degrading materials and the intense segmentation and competition of plastic goods, add multiple obstacles to the passageway of degradable transformation faced by the aforementioned small- to medium-sized enterprises. Typically, the profit margins of plastic plants are usually less than 10%, and during off-season, it could even fall as low as 5%. Furthermore, the high costs of raw materials, the elevated prices of ingots, and the attenuated reception in the market also extensively impact the willingness of traditional plastic production companies to undergo such transformation (China Economic, 2020).

The plastic industry is anticipated to undergo significant expansion in the coming years. However, at present, the sector remains relatively fragmented and limited in scope. Additionally, in the immediate future, it is unlikely that biodegradable alternatives will completely supplant single-use plastics due to inadequate supply. (Guangming, 2021).

2.2.3 Technical limitations

under a multitude of environmental conditions. However, the optimal approach is through industrial high-temperature composting, where swift and complete degradation can be achieved in a relatively truncated span of time. In accordance with this notion, it would be prudent to categorize biodegradable plastic waste as either kitchen waste or wet waste and proceed to submit it to industrial composting equipment for appropriate treatment. Regrettably, the requisite conditions for such treatment are not presently operational

In 2021, waste classification authorities in Beijing and Shanghai were interviewed by journalists. They recommended treating existing biodegradable plastics as dry waste or other waste, which ultimately leads to their incineration. Professor Liu Jianguo from Tsinghua University explained that industrial composting conditions are required for the degradation of existing biodegradable materials. These conditions entail high temperatures (above 55 degrees Celsius), sufficient oxygen, a moisture content of about 60%, and a decomposition time of 1 to 2 months. In simpler terms, aerobic composting is the most suitable treatment method for biodegradable plastic products. However, the current situation in China reveals that approximately 80% of kitchen waste or wet waste treatment facilities employ anaerobic fermentation technology, while only about 10% utilize aerobic composting technology. Moreover, due to the vast amount of waste that needs processing, the available time for composting treatment is insufficient. Consequently, the majority of biodegradable plastics in China end up being incinerated or landfilled. Tsinghua University's research data indicates that less than 0.007% of discarded biodegradable plastics are included in industrial composting or anaerobic fermentation systems. This is primarily because kitchen waste sorting equipment lacks the capability to differentiate between biodegradable and non-biodegradable plastics, resulting in the sorting of all these plastics together.（CRI, 2021; Guokr, 2023）.

2.2.4 summary

In summary, combined with the opinions of experts (Wu CX et Luo DJ), although biodegradable plastics have great environmental advantages and people have a strong willingness to use them, the main reasons why they are rarely seen in the current biodegradable plastic market are:

1. Limited market demand: Due to reasons such as molding process and performance, the range of traditional plastic products that biodegradable plastics can replace is not large, resulting in poor consumer experience
2. Cost issues: High raw material prices and manufacturing costs make biodegradable plastics less competitive in the market, resulting in low consumer willingness to purchase and low manufacturer willingness to produce them.
3. Technical difficulties: High requirements for degradation conditions require specialized degradation treatment equipment, resulting in high end-of-life treatment costs.

3 starch-based biodegradable plastic

3.1 Production principle and technology of starch-based biodegradable plastics

The fundamental approach in preparing starch-based biodegradable plastic involves treating starch physically or chemically in order to enhance its thermoplastic processing capabilities. This enables the starch to be molded into films with commendable performance while also promoting rapid degradation in suitable environments, ultimately leading to complete biodegradation. Starch can be combined with polymers like poly lactic acid and cellulose, resulting in successful utilization in the manufacturing of fully biodegradable plastic tableware and packaging materials(Tang C). What are made solely from starch and other biodegradable additives are called full-starch bioplastics and what are made by blending starch with other biodegradable polymers are Blend/composite starch-based bioplastics(Anjum F et al., 2018; Zhang Y et Yan S,2016).

The process of producing biodegradable plastic made from starch is similar to that of traditional plastics. The process includes preparing the raw materials, coloration of the mixture, heating and melting of the mixture, molding the plastic using a mold through casting and injection methods, cutting the molded product, and removing water and excess material Huang K et Huang WQ Preparing the raw materials involves mixing starch with additives, plasticizers, and stabilizers to achieve a desired composition(Luo DJ).

3.2 Advantages of starch-based biodegradable plastics

The advantages of starch-based biodegradable plastics are manifested in the following aspects:

1. Easy availability and low cost of raw materials. Natural starch, a product of photosynthesis in plants, is a widely accessible resource, and numerous plants possess high starch yields (Tang C).
2. Low energy consumption for processing performance. The processing temperature for starch-based plastics (generally below 150 degrees Celsius) is lower than traditional plastics (around 200 degrees Celsius) (Huang K).
3. Low equipment investment. The production process and equipment are basically consistent with traditional plastics, without the need for additional equipment and technology investment. (Huang K et Huang WQ).
4. Fast decomposition rate. Complete degradation of nanoclay reinforced satarch-based bioplastics could be achieved on the 6th day (Wahyuningtiyas NE et al., 2018), and starch-based packaging materials with a content of 78/19.5/2.5 starch/glycerol/water reaches the limit of swelling after 5 days, after which it begins to decompose slowly (Nagy EM et al., 2015).
5. Harmless decomposition products. biodegradable plastics primarily composed of starch seldom release any harmful substances, making them relatively environmentally friendly (Tang C).

Due to its advantages, in 2019, the global biodegradable plastic production capacity was approximately 1.077 million t, with starch-based bioplastics accounting for the largest proportion, reaching 38% (Fig. 3) (Qianzhan, 2021).

Fig. 3 Type distribution of global usage of biodegradable plastics in 2019 (Qianzhan, 2021)

3.3 Improvement in the properties of starch-based biodegradable plastics

Owing to the physicochemical properties inherent to starch and the incomplete development of current technologies, starch-based biodegradable plastics, particularly those without additional additives, exhibit some performance limitations. These include a relatively low hardness, low tensile strength, and low water resistance, resulting in brittleness and a significant reduction in mechanical properties upon contact with water(Huang K; Tang C; Wu CX et Huang WQ).

For these items, the following methods can be used to improve:

1. Physical or chemical treatment of starch. Pure starch has poor solubility and exhibits strong thermoplastic processing properties. Through physical or chemical modification, it can possess the ability to be molded into films (according to Dr. Huang Kun).
2. Blending with other biodegradable polymers. Starch can be blended with polymers such as poly lactic acid fiber gelatin or nanocellulose to improve the strength, processability, film ability, and anti-fragmentation properties of starch-based plastics (according to Dr. Huang Kun).
3. Adjusting the ratio of amylopectin to amylose in raw starch.

In the production of starch-based biodegradable plastics, amylose affects the compactness and crystallinity of the material and it tends to form crystals that have more substantial mechanical properties than amylopectin, causing starch molecules to become brittle when used as raw material for making bioplastics. The stability of bioplastics was influenced by amylopectin and the greater the Amylopectin content, the higher the value of the tensile strength of Biodegradable plastic elongation.（Utami, M. R., 2014; Kamsiati, E., 2017; Gabriel, A. A., 2021; Luqi, M., 2021）。

3.5 Research trends in starch-based biodegradable plastics

In summary, starch-based biodegradable plastics have outstanding advantages compared to other biodegradable plastics, but their performance limitations narrow their application scope and lead to poor user experience. Therefore, further development research on starch-based biodegradable plastics can focus on the following directions (Tang C; Huang K et Wu CX):

1. Modifying starch to improve its thermoplastic processing performance;
2. Adjusting the ratio of amylopectin to amylose in raw starch to meet different mechanical strength and tensile strength requirements.
3. Modifying starch by grafting hydrophobic groups to prepare starch-based biodegradable plastics with good hydrophobicity;
4. Modifying starch by grafting long polymer chains to improve the mechanical properties of starch-based biodegradable plastics;
5. Blending starch with other film-forming materials and reinforcing agents to prepare biodegradable plastics;
6. Filling nanomaterials (such as cellulose) into biodegradable plastics to increase their toughness and strength;
7. Using specific catalysts.

4 Integration and our work

4.1Project basis

Compared to other types of biodegradable plastics, starch-based biodegradable plastics have obvious advantages. There are also numerous methods and approaches for enhancing one's performance.

As the physical and chemical properties of starch-based plastics are differently affected by amylose and amylopectin (Utami, M. R., 2014; Kamsiati, E., 2017; Gabriel, A. A. , 2021; Luqi, M. , 2021), we can obtain starch-based plastics with different performance characteristics to meet the needs of various industries, by adjusting the ratio of two starches in raw materials. This requires using high-purity amylose and amylopectin. However, natural plant starch is a mixture of amylose and amylopectin. Currently, the laboratory mainly obtains high-purity amylose or amylopectin through various isolation and purification methods. Due to the complexity of the process and the high cost of equipment, it is not only difficult to use for large-scale production in factories but also increases the cost of starch-based biodegradable plastics.

By incorporating the traits of synthetic biology, we can modify the starch synthesis pathway in plants using genetic engineering methods to directly produce high-purity amylose and amylopectin. This approach, starting from starch biosynthesis, enables us to adjust the proportion of the two components in raw starch based on requirements for biodegradable plastic products. This practice will allow us to enhance the performance of the end products and to promote the comprehensive application of starch-based biodegradable plastics.

4.2 Sweet Potato: The starch feedstock we chose

Natural starch is often accumulated in plant photosynthesis. For the synthesis of high-purity amylose and amylopectin by bioengineering methods, a plant with high yield, environmental friendliness and social friendliness is extremely important.

In addition to primary food commodities like wheat and rice, numerous crops commonly encountered in our daily lives display a notable abundance of starch. These include grains such as corn, barley, and sorghum, as well as tubers and roots like potatoes, sweet potatoes, and cassava. Among these, sweet potatoes stand out due to their impressive yields, short growth cycles, adaptability to diverse climates, tolerance to infertile and drought-stricken conditions, and low requirements for water and fertilizer. Consequently, sweet potatoes emerge as a viable candidate for industrial starch production (Li EP). By selecting sweet potatoes as our primary starch feedstock, we can avoid competing with humans for food resources and mitigate the strain on land and water supplies that would otherwise be necessitated by food crops

4.3 Bio-synthesis of high-purity amylopectin through genetic editing

We plan to apply the CRISPR-Cas9 gene editing technology to knock out or silence the genes controlling the synthesis of amylose and amylopectin in plants, so that the engineered plants only synthesize either amylopectin or amylose, enabling direct extraction of high-purity starch products of different types from the plants.

However, we have learned that in the starch biosynthesis pathway of plants, the synthesis of amylose is relatively simple and is primarily regulated by granule-bound starch synthase (GBSS), while the synthesis of amylopectin is relatively complex and is regulated not only by starch branching enzymes (SBEs) but also by soluble starch synthase (SS) as well as debranching enzymes (DBEs) including isoamylase (ISA) and pullulanase (PUL) (Fig.4).

Fig.4 A simplified starch synthesis system in cereal (Tian ZX et al., 2009).

Eighteen genes are involved in or play distinct roles in different steps of starch synthesis. AGP, ADP-glucose pyrophosphorylase; AGPlar, AGP large subunit; AGPiso, AGP large subunit isoform; AGPsma, AGP small subunit; GBSS, granule-bound starch synthase; SS, soluble starch synthase; SBE, starch branching enzyme; ISA, isoamylase; PUL, pullulanase; ISA and PUL belong to starch debranching enzyme (DBE).

Given the feasibilities such as technological intricacy and the Limited project duration, our current approach aims to knock out the GBSS gene in sweet potato so that it only synthesizes amylopectin and high-purity amylopectin products can be extracted directly from the storage roots of sweet potato. We will knock out or silence the genes that governs amylopectin synthesis at a later stage, once the technology has matured sufficiently to facilitate the production of high-purity amylose products.

4.4 Risk management of bio-synthesis of high-purity amylose or amylopectin

The conventional approach to genetic modification involves the introduction of an external target gene into an organism via plasmids that may persist within the organism and facilitate the incorporation of additional external genes, thereby potentially precipitating unpredictable outcomes. On the contrary, the CRISPR-Cas9 methodology avoids the incorporation of external genes, thus minimizing the safety risks. Its most significant advantage is its precision in producing mutations by merely cleaving genes with minimal collateral effects (Li EP).

4.5 Application of high-purity amylopectin in the production of starch-based biodegradable plastics

According to the previous studies, bioplastic stability was influenced by amylopectin, the greater the Amylopectin content, the higher the value of the tensile strength of Biodegradable plastic elongation (Utami, M. R., 2014; Gabriel, A. A., 2021; Luqi, M., 2021). In the proccesses of starch-based biodegradable plastics, we can add amylopectin into the raw materials to improve the tensile strength of the products. Therefore, high-purity amylopectin is a high-quality starch raw material in the production of starch-based bioplastics, especially in the production of bioplastic products that require low hardness but high flexibility, e. g. bag films, packaging films and agricultural films.

5 Appendix:

(1)Interviewees

Huang, Kun. PhD. Professor, East China Normal University.

Huang, Weiqi. Assistant to the President, Huayue Packaging Group.

Li, Enpeng. PhD. Professor, Yangzhou University.

Luo, Dijun. Manager of Environmental Consulting Department, Shanghai Environment Science & Technology Co., Ltd., Orient International Group.

Tang, Chao. PhD. Associate Professor, Huaiyin Normal University.

Wu, Chunxu. PhD. Legal Representative, Beijing Huachengxinchuang Environment Science & Technology Co., Ltd.

(2)Literature reference：

Anjum F, Rehan M et Ullah H. (2018) Starch-based biodegradable polymers: A review. Polymers, 10(11): 1276.

Gabriel AA, Solikhah AF et Rahmawati AY. (2021) Tensile strength and elongation testing for starch-based bioplastics using melt intercalation method: A review. Journal of physics: Conference Series, 1858: 012028

Jayarathna S, Andersson M et Andersson R. (2022) Recent Advances in Starch-Based Blends and Composites for Bioplastics Applications. Polymers, 14(21): 4557.

Kamsiati E, Herawati H et dan Purwani EY. (2017). Potensi pengembangan platik iodegradable berbasis pati sagu and ubi kayu di Indonesia. Jurnal Litbang Pertanian, 36 (2): 67-76.

Luqi M et Rahman ED. (2021) The effect of amylopectin on plastic quality. ejurnal.bunghatta.ac.id,18 (4): Summary Executive Jurusan Teknik Kimia Wisudawan ke 76 Nagy EM, Todica M, Coța C et al. (2015) Water degradation effect on some starch-based plastics. Proceedings of the 43rd International Symposium on Agricultural Engineering, Actual Tasks on Agricultural Engineering, Opatija, Croatia, 24-27 February 2015: 755-762 ref.5

Tian Z, Qian Q, Liu Q et al. (2009) Allelic diversities in rice starch biosynthesis lead to a diverse array of rice eating and cooking qualities. PNAS, 106 (51): 21760-21765.

Wahyuningtiyas NE et Suryanto H. (2018) Properties of Cassava Starch based Bioplastic Reinforced by Nanoclay. Journal of Mechanical Engineering Science and Technology, 2 (1): 20-26.

Utami MR, Latifah L et Widiarti N. (2014). Sintesis plastik biodegradable dari kulit pisang dengan penambahan kitosan dan plasticizer gliserol. Indonesian journal of chemical science, vol. 3 (2): 163-167.

Zhang Y et Yan S. (2016) Review on research and development of starch-based biodegradable composites. Carbohydrate polymers, 148: 271-280.

(3)Website reference：

China Economic, 2020：https://baijiahao.baidu.com/s?id=1681028722231965651&wfr=spider&for=pc

CRI, 2021: https://baijiahao.baidu.com/s?id=1688673246295544318&wfr=spider&for=pc

Financial Community, 2021: https://baijiahao.baidu.com/s?id=1690365134639988996&wfr=spider&for=pc

Guangming, 2021 https://m.gmw.cn/baijia/2021-04/06/1302212248.html

Guokr, 2023 https://new.qq.com/rain/a/20230716A05UMT00

Qianzhan, 2021 https://bg.qianzhan.com/trends/detail/506/210330-1be63c70.html