Plastic’ is the collective name for synthetic or semi-synthetic organic materials that is today inextricably related to everyone’s life. The origin of the term ‘plastic’ is not known with certainty – perhaps it is Greek for ‘plastikos’ meaning ‘fit for moulding’ or from ‘plastos’ meaning ‘moulded’. Plastics consist of long chain molecules (amorphous solids) or polymers of high molecular mass which do not break apart when flexed and are easily shaped and moulded with heat or pressure. During manufacture of industrial products, this malleability of plastics allows them to be cast, pressed or extruded into a variety of shapes such as fibres, filaments, films, plates, tubes, bottles, boxes, coatings, adhesives and much more. Certain other properties of plastics like light weight, strength, production cost, easy fabrication and accessibility have made it a raw material of choice for plastic manufacturers. Plastics primarily consist of artificial (petroleum-based) resins, but can also be created from natural substances as in certain cellular derivatives and shellac.
Despite its convenience, plastic disposal and recycling is practised unsustainably. India and many other developing nations suffer from hazards related to plastic disposal wherein plastic litter presents unhygienic conditions, blocks drainage systems and enhancing run off. Plastics also, often contain a variety of toxic additives like adiapates, phthalates, PVCs, etc. that pollute soil and water.
Bioplastics, on the other hand are created from biodegradable polymers derived from plant sources such as soyabean/hemp oil, corn starch, etc., or from microbial sources. Bioplastics, usually amorphous lipid granules, accumulate as storage materials in cells and help in the survival of the nurturing cell under stressful conditions. Chemically, bioplastics are class of polymer known as polyhydroxyalkanoates (PHA). These polymers are produced by certain plants and a range of microorganisms cultivated under various growth and nutrient conditions as carbon and energy reserve. PHAs are polyesters – polymers containing the ester functional group in their main chain. Although there are many polyesters, the term ‘polyester’ as a specific material commonly refers to polyethylene terephthalate (PET). Polyesters include naturally occurring chemicals, such as in the cutin of plant cuticle, as well as synthetics such as polycarbonates and polybutyrates.
Poly 3-hydroxy butyric acid (PHB) is the most common PHA. It occurs in many bacteria as reserve food material. PHB is water insoluble and relatively resistant to hydrolytic degradation. This quality differentiates PHB from most other currently available biodegradable plastics, which are either water soluble or moisture sensitive. PHB shows good oxygen permeability as well as ultra-violet resistance. However, it has poor resistance to acids and bases. PHB is biocompatible and hence is suitable for medical applications.
However, the homopolymer PHB is brittle and not acceptable in the production of raw polyester. To overcome the problem a co-polymer poly-3-hydroxy butyrate-co-3-hydroxy valerate (PHBV) was created which is far more flexible due to their reduced crystallinity and is well suited for commercial applications.
Also, polylactic acid (PLA) a transparent plastic is produced from cane sugar or glucose. At first, lactic acid is fermented from sugar and then it is converted to polylactic acid using traditional polymerisation processes. PLA is used in the production of bottles, cups, foils, moulds, tins and other plastic goods. Polyamide 11(PA11) is another biopolymer derived from natural oil. PA11 is used in making electronic devices, gas pipes, sports shoes, catheters and so on. Bio-derived polyethylene is now available in the market for commercial application.
Bioplastics are not as new as most people think. Dating back to 1888, noted microbiologist Martinus W Beijerinck first observed PHAs as refractile bodies inside bacterial cells, though the composition of most common microbial PHA, i.e., poly-3-hydroxy butyric acid (PHB), was established by Maurice Lemoigne from Bacillus megaterium (a Gram-positive bacterium) in 1926. Model T Ford had coil cases made from a wheat gluten resin reinforced with asbestos fibres. Plant based plastics, however, have changed considerably since then.
The possible exploitation of biopolymer was seriously considered only in the late 1970s. In 1976, Imperial Chemical Industries of England explored if PHB could be satisfactorily produced by microbial fermentation. In 1993, Zeneca Bioproducts took over ICI’s activities and in 1996 Monsanto of United States bought the bioplastics production business from Zeneca. Monsanto made considerable effort for large scale production of bioplastics from various plant sources. Although Monsanto closed down its activities in 1998, many new multinational companies became active in this field since then. Some prominent ones are Du Pont, General Motors, Metabolix, Proctor & Gamble and Toyota, etc.
Bioplastic pros and cons
Disposal of bioplastic has some foibles too. Such plastics use considerably more fossil fuels than do petrochemical based plastics – in the extraction of the plastic from the plants and microbes – making production expensive. Degradable plastics also cannot be recycled because there is no easy way to measure its remaining life span. Again the plant-based materials actually contaminate the recycling process if not separated from petrochemical based plastics. Besides, bioplastics are designed to be composted, not recycled – but home composting may not be an option. Some bioplastics cannot be broken down by the bacteria in our backyards. Polyethylene made from cane sugar is one such example. The rest require the high heat and humidity of an industrial composting facility. Thus most plastics including bioplastics end up in landfill. Landfill is notorious for not providing the right conditions for decomposition. Waste is compressed and sealed under tonnes of soil, thus minimising oxygen and moisture, which are essential requirements for microbial decomposition.
The bioplastic technology is still at a nascent stage and needs further experiments to ascertain its feasibility as replacement for the incredibly versatile polypropylene. Yet, bioplastic bears a huge potential that in near future can be translated into innovative schemes for the development of bioplastic.