Introduction

Plastics have become an essential aspect of modern life. They play a valuable role in packaging, consumer products, industrial products, transportation, infrastructure and agriculture sectors. However, the uncontrolled accumulation of waste plastics in the environment is a matter of deep concern.

It may be recalled that prior to the COVID-19 pandemic, there was a concerted global initiative to ban single use plastics. However, during the pandemic, plastics proved their utility in production of PPEs, medical equipment, medical disposables and other packaging, thereby establishing that currently, there is no readily available alternative for these applications.

The petroleum-based plastics which dominate the market are intentionally designed to withstand biodegradation, considering their use in packaging, consumer durables and industrial products. However this attribute, considered desirable in most plastics applications, becomes undesirable when it comes to disposal of plastics waste. The problem of persistent plastic wastes in the environment was not addressed seriously by the plastics industry till recent times and left largely to civic authorities to handle. Most waste plastics, therefore, have found their way into municipal landfills or, in less organized communities, randomly disposed of in dumping yards, eventually cluttering up terrestrial and marine environments.

According to the United Nations Environmental Programme (UNEP), plastics pollution, which was around 2 million tonnes in 1950, rose exponentially to 348 million tonnes in 2017. If left uncontrolled, this will double by 2040. Marine life has been  affected by plastics pollution, through ingestion, entanglement, and other dangers, causing health and livelihood issues in many coastal communities. Around 11 million tonnes of plastic wastes enter the oceans of the world each year. Many communities indulge in open burning of plastic wastes, generating toxic air pollutants. In March 2022, 175 countries, represented by Heads of State, Environment ministers and other representatives, endorsed a resolution at the UN Environment Assembly in Nairobi, to end plastic pollution and forge an international legally binding agreement by the end of 2024 [1].

The manufacturing and processing of plastics contributes billions of dollars to the global GDP. The problem of persistent plastic wastes, however, has resulted in a popular backlash which jeopardizes the future of this multibillion dollar industry. We are at a point in history where sustainability drives the global economic and social development agenda.

Recognizing the imminent threat to business, the plastics majors have formed an industry grouping called the Alliance to End Plastic Waste [2]. This alliance has about 90 member companies, who are pooling their expertise and resources to create and scale innovative solutions. The focus is mainly on promoting recycling, and developing new products and technologies that make it easier to recycle single use plastics.

In this scenario, “bio-based” and “biodegradable” plastics are experiencing a surge of research and commercial interest. Such new plastics are already beginning to substitute non-biodegradable varieties in packaging, agriculture, medical and fishing sectors.

Mechanisms of Plastic Degradation

From a user perspective, all plastics have a limited-service life and eventually lose their required mechanical and chemical properties. Once their useful lives are over, some types of plastics may be recycled, but most plastics end up as waste.

The mechanisms by which plastics degrade can be categorized into [3]:

  • Physico-chemical degradation
  • Biological degradation

Physico-chemical degradation

Physical degradation occurs due to weathering processes such as sunlight, wind or waves. Chemical degradation may be due to hydrolysis or oxidative degradation. Plastics that degrade due to oxidation are termed Oxo-degradable while those that degrade by hydrolysis are called Hydro-degradable. Multiple mechanisms may be at play simultaneously. Both Oxo and Hydro-degradable types of plastics are Petrochemicals and are formulated using special additives that allow them to degrade by these mechanisms. A sub-category of Oxo-degradable plastics are Photodegradable plastics which undergo oxidative degradation induced by UV light. Both Oxo- and Hydro-degradable plastics are largely responsible for the problem of microplastics pollution [3].

Biological degradation

This form of degradation is caused by the action of microorganisms such as bacteria and fungi on plastics. The end product of biodegradation may be gases like Carbon Dioxide and Methane, depending on whether the pathway is aerobic or anaerobic. Plastics may also be designed to be compostable, which mean that the end product is a compost that can be used to enrich soil for agriculture.

There are various terminologies prevalent in the plastics trade, such as “bio-based plastics”, “biodegradable plastics” and “compostable plastics”. There are differences between these categories and it is important to distinguish between them from commercial and statutory perspectives.

Bio-based plastics need not be biodegradable. They are made from biomass or organic wastes and have the same chemical structure as the corresponding petroleum based molecules. They are intended to be drop-in solutions to conventional plastics and have a lower Carbon footprint, due to the use of renewable feedstocks. Some examples include, bio-polyethylene (bio-PE), bio-polyethylene terephthalate (bio-PET) and bio-polyamides (bio-PA or bio-Nylon).

Biodegradable plastics need not be biobased. In fact, fully biodegradable plastics have been developed from petroleum based feedstocks. PolyButylene Adipate co-Terephthalate (PBAT) and Polyvinyl Alcohol (PVA) are examples of biodegradable plastics currently manufactured via conventional petrochemical based synthesis routes.

Some attempts have been made to formally define biodegradable plastics. A legally non-binding definition, used in European Union documentation, states that biodegradable plastics are those that biodegrade in certain conditions at their end of life. Compostable plastics are considered to be a subset of biodegradable plastics and decompose in industrial composting facilities. Both biodegradable and compostable plastics may be synthesised from biobased or fossil-based raw materials [4].

Another way to decide if  a plastic is biodegradable, is to perform standard tests which assess biodegradability of plastics under optimized industrial and municipal composting conditions [5]. Examples of applicable tests are:

  • ASTM D6400: Standard Specification for Compostable Plastics.
  • ASTM D5338: Standard Test Method for Determining Aerobic Biodegradation of Plastic Materials Under Controlled Composting Conditions, Incorporating Thermophilic Temperatures.
  • ASTM D5929: Standard Test Method for Determining Biodegradability of Materials Exposed to Source-Separated Organic Municipal Solid Waste Mesophilic Composting by Respirometry.
  • EN 13432: Requirements for packaging recoverable through composting and biodegradation. Test scheme and evaluation criteria for the final acceptance of packaging.

Since the core issue is that plastic residues should not persist in the environment, the expectation from any plastic termed as “biodegradable” is that it will be fully mineralized within a reasonable period of time under controlled conditions. In this context, consumers need to be aware that in the natural environment, these same materials generally take much more time to fully biodegrade and the degradation process still generates large quantities of potentially harmful micro-plastics [5].

Commercially Significant Biodegradable Plastics

Bioplastics currently constitute about one percent of the total global production quantity of Plastics. In 2020, the global production capacity of bioplastics was 2.11 million metric tons, and it is forecast to continue growing year-over-year, up to 2.87 million metric tons by 2025 [6].

In spite of researchers identifying an impressive array of biodegradable molecules that can be used in plastics applications, only the following five types of biodegradable plastics have been able to successfully enter and survive in global markets. Investment in these five plastics is witnessing impressive growth, particularly in the Asian region.

  • Thermoplastic Starch (TPS)
  • Polylactic Acid (PLA)
  • Poly-Butylene Adipate co-Terephthalate (PBAT)
  • Poly-Hydroxy Alkanoate (PHA)
  • Polybutylene Succinate (PBS)

Asia contributes more than half of the world’s production of bioplastics, with most of the capacity located in China [6]. The other important producing regions are Europe and North America. The total global market size for bioplastics is valued at around USD 7 billion USD,  corresponding to about 2 million metric tonnes per annum of production.

In the USA, the biodegradable plastic market size was estimated at USD 4.7 billion in 2022 and is expected to expand at a compound annual growth rate (CAGR) of 9.7% from 2023 to 2030 [7]. The global production capacity of bioplastics is projected to be around 7.59 million tonnes in 2026 [8]. Figure 1 illustrates recent and projected  market trends in the USA. Only the commercially significant bioplastics have been shown. Starch-based plastics constituted the largest product segment for biodegradable plastics in 2022.

Figure 1

The following sections of this article focus upon the five commercially important bio-degradable plastics.

Starch Based Plastics: Thermoplastic Starch (TPS)

TPS is the most widely used form of biodegradable plastic in the world today. The starting material for TPS is Starch, which is a staple constituent of the human diet in many parts of the world. Starch is cheap and abundantly available from various renewable sources and has been an important industrial product for decades. Some examples of crops with high Starch content are Rice, Wheat, Corn, potato, Cassava and Sorghum.

Starch is a Carbohydrate polymer made by the linking of Glucose units end-to-end, into very long chains. These linkages are termed Glucosidic bonds. Two types of Starch polymers exist, depending on the type of Glucosidic bonds.  Amylose is the name given to Starch having an  unbranched, polymeric structure of 500 to 2000 Glucose subunits linked by Alpha-1,4 Glucosidic bonds. The other structure has Alpha-1,6 Glucosidic bonds forming a branched Glucose polymer called Amylopectin. The relative proportion of Amylose and Amylopectin varies, depending on the source of Starch. The relative amounts of these two types of molecules influences the properties of Starch. Figure 2 illustrates the two molecular structures [8].

What is plasticization of Starch? Actually, Starch powder is not a plastic substance, and is insoluble in cold water, and most organic solvents.  This is due to Hydrogen bonding within the Starch molecules and with adjacent molecules. However, this stability makes it brittle and difficult to process in extruders. In order to process it effectively in  polymer processing equipment, Starch must be plasticized, by application of heat and shear. This results in plasticized Starch (PS) which is subsequently blended with tougher polymers or biopolymers to form Thermoplastic Starch (TPS).

Low molecular weight Plasticizers such as Glycerol, Glycol, and Sorbitol can also be added to Starch to improve its thermo-plastic characteristics. The process of plasticization of Starch involves different physico-chemical changes caused by Water diffusion, Starch granules’ expansion, gelatinization, and polymer melting [8].

Figure 2

TPS dominates the biodegradable plastics market. It is primarily used in packaging applications, due to its  film-forming ability, easy recyclability, and relatively low cost. Starch-based films are made mainly from Starch blended with thermoplastic polyesters to form biodegradable and compostable products. Examples are Starch blended with Polylactic acid, or with PBAT (PolyButylene Adipate co-Terephthalate). Starch-based films are especially suited  for food products packaging, due to  transparency and good gas barrier properties.

TPS formulations with various additives can be processed using conventional polymer processing methods such as extrusion, injection moulding, compression moulding and solution casting [9].  Table 1 lists some of the major producers of TPS products, including blends and composites [8]:

Table 1

Polylactic Acid (PLA)

Also called “Polylactide”, PLA is a rigid thermoplastic polymer that belongs to the family of aliphatic polyesters. PLA is mainly derived from renewable resources, particularly Sugar and Starch [10].

The raw material used in the synthesis of PLA is the high purity monomer, Lactide, which occurs  as two optical forms, namely L-lactide and D-lactide. These two types of monomers can polymerize to form different forms of PLA, which can be crystalline or amorphous in structure.  PLA with different properties can be synthesized depending on the ratio of L and D forms of Lactide.  The synthesis of PLA involves three steps as follows:

  • Production of Lactic Acid (LA) by microbial fermentation.
  • The purification of LA followed by Lactide preparation.
  • Polycondensation of LA or alternatively, Ring Opening Polymerization (ROP) of Lactides.

PLA is a brittle material that lacks toughness. However, by manipulating various parameters such as crystalline structure, molecular weight, type of blending and additives, the mechanical properties of commercial PLA can be adjusted to obtain a wide range of properties, ranging from elastic soft to stiff, high-strength materials [10]. PLA is generally used  for the production of films, fibres, plastic containers, cups, and bottles. PLA has also been used in various medical applications such as sutures and implants. The erosion of PLA used in biomedical applications can be controlled by manipulating PLA’s average molecular weight.

Commercially,  PLA is comparable with  TPS in terms of its widespread  industrial production and use [6]. China is the world’s largest producer of PLA. One Chinese company, the BBCA group, is, projected to produce almost double the current global demand volume of Polylactic acid. There are at least 18 companies in China that make PLA.

The global production capacity  of PLA is currently round 250,000 tonnes per annum but this will change once planned Chinese capacity comes on stream [11]. Apart from Chinese manufacturers, the world’s major producers of PLA are as follows:

  • NatureWorks, USA: Originally founded jointly in 1997 by Dow Chemical and Cargill, NatureWorks is currently the largest Polylactic acid manufacturer in the world. The company produces around 150000 tonnes of PLA
  • Total Corbion, Thailand: This is a is a joint venture company established by Total and Corbion. Total Corbion’s PLA plant in Thailand has an annual capacity of 75000 tonnes.
  • Synbra, Netherlands: Synbra has a production capacity of 5000 tonnes per annum. It makes a form of PLA that has higher heat resistance than traditional PLA.
  • Futerro, Belgium: Futerro was founded in September 2007; it has a PLA plant with an annual output of 1500 tonnes.

3.3 PolyButylene Adipate co-Terephthalate (PBAT)

PBAT  is a biodegradable & compostable polymer, which is used to manufacture compounds primarily used for producing extrusion blown films for end applications like carry bags, shopping bags, grocery bags, compostable garbage bags, packing of fruits & vegetables, packing of garments & apparel,  mulch films etc. PBAT is produced from petrochemicals, but is completely biodegradable and compostable. This gives PBAT a unique advantage over other biodegradable plastics, since it can be produced on a large scale, similar to conventional petroleum based plastics.

The polymer is a co-polyester of Carboxylic acids (Adipic & Terephthalic acids) and a Diol or Alcohol like 1,4-Butanediol. It is produced by random co-polymerization of 1,4- Butanediol, Adipic acid and Terephthalic acid monomers. The monomers combine to form oligomers i.e., Butylene Adipate and Butylene Terephthalate via esterification reactions. The oligomers undergo polycondensation polymerization in multiple steps, to form the co-polymer. Organometallic compounds, based on Zinc, Tin and Titanium can be used as polycondensation catalysts. Figure 3 illustrates the synthesis of PBAT [12]:

Figure 3

The biodegradability of PBAT is due to the aliphatic component in the molecule chain, while the aromatic unit from PTA imparts resistance to hydrolysis and improved mechanical properties. Compared to most biodegradable polyesters such as  PLA and PBS, PBAT is less brittle and its properties are similar to those of low-density PE (LDPE). The main thing is that  PBAT is a fully biodegradable alternative to LDPE, having similar properties including high flexibility and toughness, allowing it to be used for various packaging applications. The only problem is its relatively high cost. To reduce costs and improve properties as needed, PBAT is normally blended with other biodegradable polymers such as Thermoplastic Starch (TPS) & Polylactide (PLA) as well as natural & mineral fillers along with processing additives.

While Starch reduces the overall cost without affecting biodegradability, blending with PLA imparts some stiffness to the product. BASF’s “Ecovio” resin is one such blend of PBAT and PLA. A compostable shopping bag made from this blend is typically 85% PBAT and 15% PLA.

Figure 4

In the year 2020, the global production capacity for PBAT was 279,000 tonnes per annum (TPA). The world’s leading producer in 2020 was BASF, with a production capacity of 74,000 TPA. Novamont is the other big European player, with an existing capacity of 40,000 TPA, which is being expanded to 100,000 TPA by modifying an existing PET plant in Patrica, Italy. In South Korea, LG Chem is implementing a 50,000 TPA capacity PBAT plant, set for commissioning by 2024. However, the global PBAT market is expected to be completely dominated by China, since Chinese companies are implementing enormous new capacity additions. Kingfa Technology is the largest producer of PBAT in China at present, with an annual production capacity of 70,000 tons of PBAT, and with a new production line of 60,000 TPA under implementation. Hengli Petrochemical has recently completed a 33,000-ton/year PBAT project. Ruifeng High Materials  and Tongcheng New Materials are  putting up PBAT plants of 60,000 TPA each. Xinjiang Wangjinglong New Materials Co., is putting up a 100,000 TPA capacity PBAT plant at  Korla Petroleum and Petrochemical Industrial Park in Bazhou, Xinjiang, with eventual plans to expand capacity to 1.3 million TPA [13]

Polyhydroxyalkanoate (PHA)

Poly Hydroxy Alklanoate (PHA) biopolymers are a family of polyesters produced naturally by a number of micro-organisms. Many PHA polymers have excellent mechanical properties, comparable to conventional polymers produced by the petrochemical industry. PHAs are fully biodegradable.

PHA plays an important metabolic role in bacteria, acting as a Carbon and energy reserve to promote the survival of bacteria under nutrients-scarce conditions [14]. Numerous strains of PHA producing bacteria have been isolated, which are capable of synthesizing PHA from a variety of Carbon sources. These Carbon sources include:

  • Saccharides (e.g. Fructose, Maltose, Lactose, Xylose, Arabinose)
  • n-Alkanes (e.g. Hexane, Octane, Dodecane).
  • n-Alcohols (e.g. Methanol, Ethanol, Octanol, Glycerol).
  • n-Alkanoic acids (e.g. Acetic acid, Propionic acid, Butyric acids, Valeric acid, Lauric acid, Oleic acid)
  • Carbonaceous Gases (e.g. Methane and Carbon Dioxide).

Waste streams with high organic Carbon loading can also be utilized for PHA production. Potential waste streams include waste frying oil, vinegar waste, oil mill effluents, crude Gylcerol from biodiesel production, food waste, landfill gas etc.

The molecular structure of PHA is shown in Figure 4. The PHA nomenclature is based on the number of Carbon atoms including those in the side chain Alkyl group.

Figure 4

The number of monomer units in a PHA polymer molecule may range from 600 to 35,000. Each monomer unit has a side chain alkyl group, denoted as “R” in the structural formula. The Alkyl group may range from Methyl radical, all the way to Tridecyl group. Other Alkyl groups including branched and substituted Alkyls are also possible, though not very common. Due to the number of different Alkyl groups that can be associated, PHA monomers may be of different chain lengths. More than 150 different types of PHA monomers have been identified and more are being discovered or created by genetically modified organisms. Monomers scan be broadly classified on the basis of chain lengths, as:

  • Short Chain length PHA, containing 3 to 5 Carbon atoms
  • Medium Chain length PHA containing 6 to 14 Carbon atoms
  • Long chain length PHA containing 15 or more Carbon atoms.

Given the range of feedstocks, microorganisms and types of monomers, PHA is a versatile biopolymer that can be the basis of many tailor-made products, in sectors ranging from packaging materials to medical applications.

The manufacturing process for PHA typically involves the following steps [15]:

  • Fermentation:

Aerobic fermentation is performed using specified strains of  bacteria with the appropriate Carbon source and Nutrients. Like all pure culture fermentations, sterilization of the broth is generally required to prevent contamination. There are some newer processes that use waste streams as Carbon sources which do not require sterilization. PHA granules form inside the growing  microbial cells.

  • Harvesting and drying biomass:

After completion of fermentation, the microbial cells containing PHA must be harvested. The fermentation is stopped by acidifying the broth, after which the broth is neutralized with alkali and dewatered in centrifuges. The dewatered biomass is then dried in rotary dryer to about 10% moisture,  to allow extraction of PHA.

  • Extraction and recovery of PHA:

Solvent extraction is the most widely used method for PHA extraction. The process involves breaking the cell walls by heat and pressure in the presence of a solvent. This is  done in a steam heated agitated extraction vessel. The PHA dissolves into the solvent and is separated from the insoluble biomass by centrifugation. PHA is subsequently recovered from the solvent by precipitation. Crystallized PHA is then filtered, washed and dried to produce a saleable powdered product. Alternative methods of extraction include chemical or enzymatic digestion, which solubilize the cell walls, releasing PHA which is then separated by centrifugation and purified.

PHA is used in the production of compost bags, agriculture foil and films,  packaging for food, beverages, consumer products and medical applications. The global Polyhydroxyalkanoate (PHA) market is estimated to be USD 62 million in 2020 and is projected to reach USD 121 million by 2025, growing at a CAGR of 14.2% between 2020 and 2025 [16].Short chain length PHAs dominate the market. Even though more than 150 types of PHAs are known, there are only four commercially important PHAs at this time [17]. These four types of PHAs are profiled in detail in the following paragraphs.

Poly(3-hydroxybutyrate):

Abbreviated as P(3HB) or PHB, this  is by far the most widely studied and produced molecule in the PHA family. Sine it is the simplest form of PHA, it is naturally synthesised by the largest number of microbes, from  feedstocks such as Carbohydrates, Alcohols or Fatty acids, with an even number of Carbon atoms. PHB is fully biodegradable, with properties comparable to those of isotactic Polypropylene. It is insoluble in Water and has low permeability for gases like Oxygen, Carbon Dioxide and Water vapor. Unlike petrochemical based polymers, the product does not have  toxic residues of catalysts or Hydrocarbon molecules. PHB, however, is hard and brittle. It is also thermally unstable, so that its molar mass and physical properties change during processing. PHB must therefore be blended with other biodegradable polymers and additives for many applications. Some uses of PHB are as follows:

  • Production of biodegradable articles for use in the food and beverage industry such as cups, spoons, bottles and bags.
  • In pharmacology, PHB can be used in microcapsules or in packaging for tablets.
  • Due to its compatibility with mammalian blood and tissue, it can be used in surgical implants and sutures.

PHB is mainly manufactured from Sugar based feedstocks though a couple of small projects utilize Biogas and Carbon Dioxide as Carbon sources. Some important producers of PHB are listed below[17]:

  • PHB Industrial S.A. (PHB/ISA), Serrana, Brazil: Capacity-100 TPA, using hydrolysed cane sugar as feedstock
  • Bio-On, Bologna, Italy: Capacity-2000 TPA, using beet Sucrose and beet molasses as feedstock. The company faced financial bankruptcy in recent times.
  • Biomer, Schwalbach, Germany: Capacity-900 TPA from Sucrose feedstock.
  • COFCO, Beijing, China: Capacity-1000 TPA from Glucose feedstock
  • Mango Materials, Redwood City, USA: Capacity-0.25 TPA, planning scale-up to 5 TPA, using Biogas as feedstock.
  • Newlight Technologies LLC, Huntington Beach, USA: Utilize Methane and Carbon dioxide as feedstock to make PHB. Capacity information not available.

Poly(3-hydroxybutyrate-co-3-hydroxyvalerate):

Abbreviated as P(3HB-co-3HV) copolyester, this molecule was isolated in nature in the 1970s. It finds use in materials for injection-moulded parts, bottles, extruded sheets, films, fibres, and P(3HB-co-3HV)-coated paper. Major producers of P(3HB-co-3HV) are as below [17]:

  • Tianan Biologic Materials Company, Ningbo, China: Capacity-2000 TPA, utilizes Glucose plus a 3HV precursor as feedstock.
  • PHB Industrial S.A. (PHB/ISA), Serrana, ,Brazil: Capacity-100 TPA, using hydrolysed Cane Sugar as feedstock.
  • Genecis Bioindustries Inc., Canada: Utilizes discarded organic food waste. which is converted to Fatty acids and used as feedstock. Demonstration plant only.

Poly(3-hydroxybutyrate-co-4-hydroxybutyrate):

Abbreviated as P(3HB-co-4HB) Copolyester, this is a highly flexible material unlike  P(3HB) and P(3HB-co-3HV). It also has improved thermal stability. It finds use in surgical sutures and other surgical materials as well as textile fibres. Major manufacturers of this product are as follows [17]:

  • Tianjin GreenBio Materials Co. Ltd., Tianjin, China: Capacity-10000 TPA, Glucose plus 1,4 Butanediol feedstock.(Note: GreenBio was making a loss and has probably shut down operations).
  • CJ, Seoul, Republic of Korea: Glucose plus 1,4 Butanediol feedstock. Capacity information not available.
  • Shenzhen Ecomann Biotechnology Co. Ltd., Guangdong, China: Capacity-10000 TPA, planned expansion to 75000 TPA. Sugar plus 4HB-related precursor feedstock.
  • PhaBuilder, Beijing, China: Capacity-10000 TPA. Glucose, Corn Steep liquor and GBL feedstock.
  • Medpha, Beijing, China: Capacity-100 TPA. Glucose, Corn Steep liquor and GBL feedstock.

Poly(3-hydroxybutyrate-co-3-hydroxyhexanoate):

Abbreviated as P(3HB-co-3HHx) copolyesters, this a hybrid variety of PHA consisting of short chain and medium chain building blocks. P(3HB-co-3HHx) with a 3HHx content of 10–17 mole percent features excellent flexibility, evidenced by very high elongation at break of up to 850%. Specialized industrial applications are being pursued. Manufacturers of this product include:

  • Danimer Scientific, Bainbridge, GA, USA: Capacity-10000 TPA. Uses inexpensive oils from the seeds of plants such as Canola and Soy as feedstock.
  • Kanegafuchi Chemical Industry Co. Ltd.(Kaneka), Tokyo, Japan. Uses vegetable Oils as feedstock, capacity information not available.
  • RWDC Industries Ltd., Athens, GA, USA: Capacity-4000 TPA. Uses waste cooking oil as feedstock.
  • Bluepha Co. Ltd., Beijing, China: capacity-1000 TPA. Uses alternative Carbon sources including crops, kitchen waste and seawater

PolyButylene Succinate (PBS)

PBS is a white semi-crystalline thermoplastic polymer with a melting point of 115 0C. It has good mechanical properties, thermal stability, flexibility  and excellent processability, enabling it to be drawn into textile filaments or converted in injection moulds, extruded and blown products PBS also has good thermal stability. PBS and PBS/PLA blends have been employed used to make mulch films, packaging, compostable bags, catering articles, non-woven sheets,  drug encapsulation systems, orthopaedic applications and coffee capsules. Figure 5 shows some of the products that can be made from PBS.

Figure 5

The direct esterification of Succinic acid with 1,4-Butanediol is the most common way to produce PBS. It consists of a two-step process. First 1,4 BDO is esterified with Succinic acid to form PBS oligomers, with elimination of Water. In the second step, polycondensation of oligomers occurs, to form high molecular weight PBS. The synthesis usually takes place in an agitated, blanketed with Nitrogen gas ( to avoid oxidation during the esterification step), and followed by a distillation column [44]. This reactor is heated to 160–190 0C, to start esterification. When no more Water (or alcohol) is distilled out under normal pressure, polycondensation is continued at a higher temperature (220–240 0C) [18].

The raw materials Succinic acid and 1,4 Butanediol have traditionally been produced by the petrochemical route. However, in recent times the direct synthesis of 1,4 Butanediol by fermentation using Dextrose feedstock and  genetically engineered “E.coli” bacteria has been accomplished commercially (the plant is owned by Novamont and uses technology licensed by Genomatica). Similarly, Succinic acid can also be produced by fermentation and subsequently hydrogenated to 1,4 Butanediol. This route is not economical when compared to the direct Genomatica process. The point is that completely bio-based routes are now available to produce PBS, though there is no green premium in the market for PBS made this way.

Figure 6

The global PBS market was estimated at USD 276.51 million in 2021 and is projected to expand at an impressive compound annual growth rate (CAGR) of 19.7% from 2022 to 2030. The USA market for PBS was around 20.3 million USD in 2021[19].

In 1993 the Japanese company Showa High Polymer, built a PBS plant with a capacity of 3,000 TPA. Marketed under the brand “Bionolle”, the synthesis was done via melt condensation polymerization, followed by chain-extension with a Diisocyanate. This allows more control over polymer chain length and therefore its properties, when compared to the conventional route. In 2016, Showa announced termination of the production and sale of Bionolle, due to lack of commercial success and market conditions. In April 2003, Mitsubishi Chemicals  built a 3,000 TPA plant and launched its PBS under the name “GS Pla” (Green and Sustainable Plastic). They were able to make high molecular weight  polymer without the use of a chain extender. Other prominent PBS producers are Hexing Chemical (Anhui, China), Xinfu Pharmaceutical (Hangzhou, China) and IRe Chemical (South Korea). In 2010 Hexing Chemical established China’s largest PBS manufacturer, with the annual capacity of 10,000 tonnes. They were promptly outdone the same year, by Xinfu Pharmaceutical, who announced the building up of the world’s largest continuous PBS production line with an annual capacity of 20,000 tonnes. At the moment most manufacturers follow the traditional petrochemical route to make PBS [20].

Conclusion

Biodegradable plastics offer a promising pathway to mitigate the global environmental problem of plastics waste. There has been an explosion of research and redevelopment activity in this area, as the era of fossil-based chemicals is slowly drawing to an end. Biotechnology has advanced significantly in the past few decades and this has facilitated the discovery of numerous new molecules synthesized by microbes, which can be usefully converted into biodegradable plastics products. At present, the biodegradable plastics market share is about 1% of the global plastics market, which is likely to increase since many governments are mandating the use of biodegradable plastics for single-use applications. The Asian region, in particular China, is emerging as  a leader in this area, as demonstrated by the ambitious scale of investments. Currently only five varieties of biodegradable plastics, namely TPS, PLA, PBAT, PHA and PBS have been able to carve out a niche for themselves in the extremely competitive plastics market. Commercial viability has been a concern and many of these ventures have not done well financially, with quite a few having closed down. There have been many successes as well, with some of the products and producers having been in the market for decades. The time is ripe for growth of the biodegradable plastics sector, given the unprecedented global focus on sustainability and elimination of fossil-based products.

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  6. An overview on synthesis, properties and applications of poly(butylene-adipate-co-terephthalate)- PBAT, by Jiao Jian, Zeng Xiangbin, Huang Xianbo, in Advanced Industrial and Engineering Polymer Research, Volume 3, Issue 1, January 2020, pages 19-26; An overview on synthesis, properties and applications of poly(butylene-adipate-co-terephthalate)–PBAT – ScienceDirect
  7. The project is going crazy, what is the prospect of PBAT? ECHEMI.com; The project is going crazy, what is the prospect of PBAT? (echemi.com)
  8. Start a Research on Biopolymer Polyhydroxyalkanoate (PHA): A Review, by Giin-Yu Amy Tan et al., in Polymers2014, 6(3), 706-754; Polymers | Free Full-Text | Start a Research on Biopolymer Polyhydroxyalkanoate (PHA): A Review (mdpi.com)
  9. Techno-economic assessment of poly-3-hydroxybutyrate (PHB) production from methane-The case for thermophilic bioprocessing, by Ian Levett et al., Journal of Environmental Chemical Engineering, Volume 4, Issue 4, Part A, December 2016, Pages 3724-373; Techno-economic assessment of poly-3-hydroxybutyrate (PHB) production from methane—The case for thermophilic bioprocessing – ScienceDirect
  10. Global Polyhydroxyalkanoate (PHA) Market (2020 to 2025)- Increasing Scope in End-use Segments Presents Opportunities – ResearchAndMarkets.com, businesswire, February 11, 2021; Global Polyhydroxyalkanoate (PHA) Market (2020 to 2025) – Increasing Scope in End-use Segments Presents Opportunities – ResearchAndMarkets.com | Business Wire
  11. A New Wave of Industrialization of PHA Biopolyesters, by Martin Koller  and Anindya Mukherjee, Bioengineering 2022, 9(2), 74; Bioengineering | Free Full-Text | A New Wave of Industrialization of PHA Biopolyesters (mdpi.com)
  12. A Brief Review of Poly (Butylene Succinate) (PBS) and Its Main Copolymers: Synthesis, Blends, Composites, Biodegradability, and Applications, by Laura Aliotta et al., Polymers2022, 14(4), 844. Polymers | Free Full-Text | A Brief Review of Poly (Butylene Succinate) (PBS) and Its Main Copolymers: Synthesis, Blends, Composites, Biodegradability, and Applications (mdpi.com)
  13. Polybutylene Succinate Market Size, Share & Trends Analysis Report By Type (Bio-based, Petro-based), By Application (Mulch Films, Packaging, Medicine), By Region, And Segment Forecasts, 2022 – 2030; Global Polybutylene Succinate Market Size, Share Report, 2030 (grandviewresearch.com)
  14. Polybutylene Succinate-Wikipedia; Polybutylene succinate – Wikipedia
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