We cannot live without plastics having innumerable applications in every facet of life, right from packaging, transportation, reliable storage, clothing, sports, furniture, foams, pipes to industries. However, most conventional plastics such as polyethylene (PE), polypropylene (PP), polystyrene (PS), poly (vinyl chloride) (PVC) and poly (ethylene terephthalate) (PET), are non-biodegradable, and their cumulative buildup in the environment has been a danger to the planet. To overcome all these problems, some steps have been undertaken.

The post-1950 era has witnessed tremendous growth in plastic production because of a variety of reasons. In the year 2019 alone, the plastic production was 368 MMT which was estimated to reach 8,300 MMT between 1950 and 2015, with a total plastic waste of 6,300 MMT. In an ideal circular economy, plastics would be made from renewable or recycled resources (Fig. 1).

However, the traditional life of most plastic materials is linear. Of these, 78% plastic was discharged into landfills, 12% incinerated and only 9% recycled. Although recycling has increased since the 1980s, the recycling of non-fibre plastic remains stagnated at 18% and almost no textile fibres are recycled.  Plastic pollution is seen by one and all and several legislative actions are in place. The first approach involved production of plastics with a high degree of degradability.

Why bioplastics?

Since plastics are an integral part of modern living and with the ever-increasing awareness of the environment as well as the health hazards of petroleum-derived plastics, there is an emphasis on the use of biomass derived alternatives.  As the name suggests bioplastic is a long-lasting polymer made from renewable raw materials that may be used in place of non-renewable petro-polymers.

Bioplastics are misunderstood by many as biodegradable. However, bioplastics consist of either biodegradable plastics (i.e., plastics derived from fossil feedstock) or bio-based plastics (i.e., plastics derived from biomass or renewable carbon resources). Biodegradable plastics can be degraded by microorganisms present in the environment by entering the microbial food chain.

This property does not depend on the origin of the raw materials, which could be either fossil or biomass origin. When plastics are used as substrates for microorganisms, evaluation of their biodegradability should not only be based on their chemical structure, but also on their physical properties (melting point, glass transition temperature, crystallinity, storage modulus, etc.).

Beside the covalent forces of polymer molecules, various kinds of weak forces such as hydrogen bond, van der Waals and coulombic forces, among macromolecular chains affect not only the formation of polymer aggregates, but also the structure and physical properties and function/reactivity of the polymer aggregates and thus the degradability.

Feedstock for bioplastics

The universal feedstocks used to produce bioplastics include corn, wheat, potato, maze, rice husk, palm, sugarcane, etc. which are easily available in many countries across the globe. Compared with fossil petro-plastics, bio-based plastics can have a lower carbon footprint and show advantageous materials properties; moreover, they can be compatible with current recycling streams and some offer biodegradation as an end of life (EOL) option if performed in controlled or predictable environments. The use of bioplastic has the potential to reduce carbon dioxide emissions by 30 to 70%. It yields a 42% reduction in carbon footprints. Bioplastic production requires 65% less energy than the usual petro-plastic.

The global bioplastics market is developing due to new technologies and the emphasis on the net zero goal.  According to Nature Reviews, the annual production of 100% bioplastics is ~2 MMT in which about 2/3 is biodegradable. If partial bio-based Pus and polyamide co-polymers are included in these statistics, then it would be ~7.5 MMT in 2018 which is predicted to reach 9.1 MMT in 2023 with market size of US$ 1.1 billion and 1.7 billion, respectively.

The bioplastic market is categorized into three segments: by type, by application, and by region. Based on type, it is divided into biodegradable plastic and nonbiodegradable plastic. Based on application, the market is classified into flexible packaging, rigid packaging, textiles, coating and adhesives, agriculture and horticulture, consumer goods and others.

Region wise, the market is divided into North America, Europe, Asia-Pacific, South America, Middle East and Africa. Currently, bioplastics cost more than twice the petro-plastics.  As stated, before there is also a confusion about the bioplastics which are derived from biomass, but it does not mean all are biodegradable (Figure 1).  On the contrary, the comparison of fossil-derived and bio-derived plastics is given in Figure 2.

Biodegradable polyurethanes

When petroleum-based reserves are exhausted, biobased polymers will take over. Also biomass derived polymers to obtain vinyl monomers, carboxylic acids, alcohols, amides and rubbers will come to the centre stage. For instance, polyurethanes (PUs) are one of the most widely used plastics, mainly in flexible and rigid foams, with global production of 18 MMT which uses the carcinogenic isocyanate monomer. Instead of the unsafe and fossil-based route, PU can be produced from cyclin carbonates (produced from cycloaddition of epoxides) and diamines derived from vegetable oils.

It should be noted that PU can be synthesized by combining multiple biological materials. Except for turpentine, vegetable oil, cellulose, lignin, phenolic and sugar, there are other bio-renewable materials, like proteins and starches, etc. that have the characteristics of wide sources, low price, large output and no pollution. PU obtained from vegetable oils are hydrophobic. Consequently, by adding proteins, sugars and other natural hydrophilic compounds, hydrophilic polymer materials can be obtained for medical materials and devices. Adding cellulose, lignin and other reinforcing materials to vegetable oil-based polymers can improve the mechanical and thermal properties of the materials.

The composite of different biological materials can not only obtain excellent PU materials, but also obtain good environmental and economic benefits, which should be advocated and applied. 

FDCA based biodegradable plastics

The synthesis of 2,5-furandicarboxylic acid (FDCA) based polymers is getting significance due to their biodegradability (hydrolytic and enzymatic) and compostability, as a replacement for petro-polymers due to their excellent thermomechanical and barrier properties.  FDCA based homopolyesters faced technological and commercial hurdles. Direct manufacture of FDCA from biomass is still limited to 5-hydroxymethyl furfural (HMF) which is in turn derived from glucose, fructose, sucrose, high fructose corn syrup, jerusalem artichoke and starch (Figure 3).

The industrial production of FDCA has not attained its full potential due to its low yield and high cost since the current average for FDCA synthesis is $2,300/kg. To make the process economically viable, FDCA production cost should be below $1,000/ton.  The major processing costs are associated with the HMF formation stage and the long residence time for HMF oxidation. Replacing simple sugars with food wastes could be a sustainable option for HMF production from environmental and economic perspectives. Starch from food waste such as bread wastes, cooked rice, pasta, noodles and cellulose-rich fruit peels can be used to synthesize HMF.

However, the industrial production of FDCA based homopolyesters is delayed due to several factors including poor optical property, low ductility and slow crystallization rate. In order to tune the properties of FDCA-based homopolyesters, FDCA has been copolymerized with several aliphatic (adipic acid, succinic acid, lactic acid, sebacic acid, polyglycolic acid and polyethylene glycol) and cyclic (caprolactam and isohexides) comonomers (Figure 4).

Biodegradability versus plastic chemical recycling

Biodiversity and occurrence of polymer-degrading microorganisms vary depending on the environment, such as soil, sea, compost, activated sludge, etc. Generally, the adherence of microorganisms on the surface of plastics followed by the colonization of the exposed surface is the major mechanisms involved in the microbial degradation of plastics. The enzymatic degradation of plastics by hydrolysis is a two-step process: first, the enzyme binds to the polymer substrate then subsequently catalyzes a hydrolytic cleavage. Polymers are degraded into low molecular weight oligomers, dimers and monomers and finally mineralized to CO2 and H2O.

Therefore, biodegradable plastics are viewed as a potential solution to plastic pollution because they are environmentally friendly. Further, they can be derived from renewables thereby reducing GHG emissions. The much talked about polyhydroxyalkanoates (PHA) and lactic acid (raw materials for PLA) can be manufactured by fermentation using agricultural residues and microorganisms.

Biodegradable plastics offer a lot of advantages such as increased soil fertility, low accumulation of bulky plastic materials in the environment, and decrease in the cost of waste management. Furthermore, biodegradable plastics can be recycled to useful metabolites (monomers and oligomers) by microorganisms and enzymes.

A second strategy involves degradation of some petro-plastics by biological processes. Some aliphatic polyesters such as PCL and PBS can be degraded with enzymes and microorganisms. Polycarbonates (particularly the aliphatic types) possess some degree of biodegradability. It is advisable to recycle non-biodegradable plastics though chemical recycling and all plastic waste can be converted into hydrocarbons using hydrogen and the nasty atoms like Cl, S, N are converted into HCl, H2S and NH3 that can be absorbed by using well established technologies. For instance, polystyrene (used in making some disposable spoons, plates, cups, and some packaging materials) can be recycled and used as filler for other plastics or through solvolysis and hydrogenation.

Conclusion

In order to circumvent the problems associated with the production of plastic goods production and end-of-life (EOL), bioplastics were introduced as a viable substitute to petro-plastics. A variety of materials belong to the category of bioplastics, which largely differ from each other depending on the polymer they are composed of, as well as in respect to the structural characteristics that mainly affect their accumulation in the environment when dumped.    Mechanical, chemical, biochemical recycling, biodegradation (composting) are some of the approaches to deal with bioplastic pollution. Instead of biodegradation, microbes and hydrolyzing enzymes can be employed to depolymerize bio-plastics into monomers instead of producing carbon dioxide on complete biodegradation. 

Moreover, a future challenge in the bioplastics market could be the  production of new blends of biopolymer that are more easily biodegradable without losing the characteristics (such as mechanical strength or flexibility) that make the bioplastics attractive in the first place. Bioplastics will replace petro-plastics in all types provided the costs are similar; currently the bioplastics are 3-4 times more expensive.

The use of food and lignocellulosic wastes as potential low-cost feedstock should be further developed. Also, photosynthetic algae have shown remarkable carbon fixation abilities in producing sugars for bacterial fermentation and subsequently used for the bioplastics production. Algae biomass could also be blended with conventional plastics to reduce the dependency of petroleum-based sources.  Some of the algae species are found to accumulate polyhydroxyalkanoates (PHA) which can be extracted for bioplastics production.

Several nations and international organizations are encouraging circular plastic economy which is bolstered by the policies by UNIDO and G20 nations. It is vital not to overlook the technologies of bioplastics disposal, their impact on microplastic formation in the environment and aquatic life. Therefore, the life cycle assessment of bioplastic is important before bringing them into the industrial sector.