Bio-degradable plastics – development and use are the key for improvement of environment:
A. At present, we make almost 100% of plastics of our requirement from oil and natural gas. Petroleum-based plastics are basically non-degradable. As concern grow about the potential bad effects of petroleum-based non-degradable plastics on the environment, the viability of petroleum-based plastics are in question. At the same time, the increased dependence on oil and gas imports due to manufacture of such petroleum-based products, make us think about the possible solution. In this respect, searching for suitable degradable polymers for various applications as per the need, have become very important aspect in today’s science and technological affair for research.
B. As per reports of various environment protection agencies, plastics alone account for more than 25% (by volume) of municipal waste generated. Plastic’s low density and slowness to decompose makes them a visible pollutant of public concern. Some of the techniques adopted for integrated waste management, which include recycling, source reduction of packaging materials, composting of degradable wastes, incineration etc., may help reduce waste disposal problem; but this will not solve the importation of petroleum products and problem with non-degradability of plastics. As per statistics, about 80% of post-consumer plastic waste is sent to landfill – degrading land masses and causing water pollution, 8% is incinerated – causing unwanted emission and only 7% is recycled. The situation is so acute in some countries of Europe of Japan that today few sites left that can be used for landfill. Since the main bulk of domestic waste is made up of plastics there is a great deal of interest in recycling plastics and in producing plastic materials that can be safely and easily disposed of in the environment.
C. The option to get rid of the adverse effects of non-degradable petroleum-based plastics may be to make bio-degradable plastics suitable for our various applications. Some of the manufacturers in developed countries have already developed some type of degradable plastics made from agricultural products such as corn, potato etc. In fact, bio-degradable plastics can be made from lactic acid. Lactic acid is produced (via starch fermentation) as a co-product of corn wet milling, which can be converted to polyactides (PLA). Alternatively, it can be produced using the starch from food wastes, cheese whey, fruit or grain sorghum.
D. The properties of the plastics changes as per the applications for which it is needed. Some plastics need to be durable like the parts in a car. Yet, there are many plastics that are only used once or have a limited life before being thrown into a landfill or incinerator. Plastics, unlike most organic polymers, are poorly degraded by microbes (although recently some genetically engineered microbes / bacteria have been invented to transform plastic waste into useful eco-friendly plastics – but it is still in research stage). Environmentally degradable polymers are one potential solution to replacing petroleum-based polymers. Potential uses for these polymers are plastics intended for one-time or limited use, for example those used as fast-food wrappers and water-soluble polymers in detergents and cleaners, and for use in the printing industry. Thus, an ideal degradable product would:
(a) Perform the intended task effectively;
(b) Produce little or no side effects in any non-intended target;
(c) Break down, along with any residues of its activity, over a reasonably short time scale;
(d) Produce no harmful substances when it breaks down.
E. Waste disposal: The question now arises, how best to dispose of domestic wastes. The ways of disposing of waste and time required for degradation is very important factors in development of bio-degradable plastics. Current bio-degradable polymers are designed to degrade either biologically or chemically, depending on the disposal environment that they will encounter after use. Ideally, degradation pathways should ultimately lead to the bio conversion of the polymer into carbon dioxide (aerobic) or carbon dioxide/methane (anaerobic) and biomass. Environmental laws and regulations and consumer demands for environmentally friendly products are beginning to have an impact on the use of degradable polymers. As a result degradable polymers, when combined with other degradable plastics, will begin playing a crucial role in helping to solve our waste disposal problems and reducing petroleum imports.
F. Properties of bio-degradable polymers: These new polymers developed from agricultural products described above are truly degradable. These polymers may be used in many applications as well. Some are impervious to water, moisture etc., and retain their integrity during normal use, but readily degrade when they are kept in a biologically rich environment. The amazing part is the full biodegradability can occur only when these materials are disposed of properly in a composting site or landfill. Today, there are three major degradable polymers groups that are either entering the market or are positioned to enter the market. They are
(a) polyactides (PLA),
(b) polyhydroxybutyrate (PHB) and
(c) starch-based polymers.
G. Design for Bio-Degradation of Polymer: Following few points are given to attain bio-degradability.
(a) Some organic chemicals degrade only very slowly, and so the level in the environment can rise steadily. These are the persistent organic pollutants (or "POPs").
(b) In contrast, all chemicals produced in nature are 100% degradable and understanding why this is the case is an important part of being able to design synthetic degradable materials.
(c) For example, natural polymers such as carbohydrates, proteins and nucleic acids usually have oxygen or nitrogen atoms in the polymer backbone. If these atoms are included in synthetic polymers, the material is more easily degraded. A carbon-oxygen double bond (carbonyl group) absorbs light energy, and so can make a substance photodegradable.
(d) These features can be seen in the structures of some degradable polymers that are already in use.
H. Bio-degradable polymers are quite new. Only during last five years some bio-degradable polymers for applications have been in use in some of the developed world. Although they are degradable, the industry has not promoted them. One reason is these new polymers are higher priced than the commodity polymers typically in use in plastics applications. However, producers are currently working toward bringing down the price of degradable polymers by increasing production capacity and improving process technology.
I. Price competitiveness and future growth of bio-degradable polymers: The trend observed regarding bringing down the prices of degradable polymers in last five years is quite encouraging. In US, five years ago PLA and PHB sold for more than USD 25.00 per pound. Today PLA, depending on quantities, is between USD 1.50 and USD 3.00 per pound and PHB, in large quantities is near USD 4.00 per pound.
Though recent advances in production technology have helped lower prices of some degradable resins, prices are still higher than for petroleum-based plastics. This suggests that in the short term, companies making degradable polymers will continue to focus on niche markets. As production capacity increases it is expected that future prices to fall to roughly USD 1 per pound. Moreover, due to sharp increase in prices of petroleum-based plastics in recent time, the prices of bio-degradable polymers will become very much competitive soon.
J. Further, several factors, besides cost, will be important in determining the future growth of degradable polymers. One major obstacle is a lack of a composting infrastructure. Large-scale composting would provide the ideal disposable environment for spent degradable. Future legislation will depend not only on the environmental awareness of planners and politicians but also on their perceptions of how degradable polymers may affect the development of plastics recycling.