Will the plastic degrade?

Loopholes in technology, Delhi government's order

 
Last Updated: Saturday 04 July 2015

questions are being raised about the quality of degradable plastic bags to be used by hotels and hospitals in Delhi following a recent order of Delhi government's Department of Environment see Down To Earth ' Let Delhi Breathe', August 15, 2005). The Delhi Plastic Bag (Manufacture, Sales and Usage) and Non-Biodegradable Garbage (Control) Amendment Act, 2004, also prohibits the manufacture, sale and usage of non-degradable plastic in the National Capital Territory. But apprehensions are being raised about the ambiguities in the recent order and the 'degradability' of different types of degradable plastics. That the government hasn't notified plastic manufacturers about standards for such plastic deepens the doubts.

"We do not need to notify the manufacturers about the standards. We are following American Society for Testing and Materials standards and they have to follow the same," argues Naini Jayaseelan, environment secretary, Delhi government. But how will manufacturers know this? Sources say the department plans to notify manufacturers "at the earliest".

Degradable plastics are designed to degrade in different ways, in different environs. These include biodegradable, compostable, oxidative and photodegradable plastics. The degradation process can be initiated by factors like heat, water, uv light and oxygen. In case of biodegradation, this is followed by microbial decomposition, producing carbon dioxide, water and biomass. But the government hasn't specified the type of degradable plastic in either the 2004 Act or the recent order.

Delhi's plastic manufacturers claim their degradable plastic is biodegradable and decomposes completely. This kind of plastic is a blend of starch and polyester. But these manufacturers mix starch or prodegradant additives with polyethylene. "We mix dana (an additive) in the plastic to make it degradable," says a manufacturer, refusing to disclose the identity of the dana; he admits that his plastic is not biodegradable but oxidative degradable. Such plastics disintegrate through oxidation. If they end up in the litter stream, they disintegrate and might be ingested by fish and marine mammals (see table: Mere form change).

In starch-polyethylene blend plastics, the starch content ranges from 10 to 90 per cent. To qualify as biodegradable, the starch content must be over 60 per cent and the plastic must meet specified criteria about rate biodegradation and maximum residue left. But even biodegradable plastic degrades maximum up to 40 per cent, experts say.

Mere form change
Degradable plastics leave considerable residues

Category Composition End product Degradability
Thermoplastic blend Thermoplastic starch with polycaprolactone (PCL)/ polybutylene adipate terephthalate (PBAT) Through microbial action into H2O, CO2, inorganic compounds, biomass Biodegradable/compostable
Starch polyethylene blend Starch with polyethylene or polypropylene Only starch biodegradable, plastic disintegrates and  remains in environment
Controlled degradation
Polyethylene with prodegradant  additives Polyethylene mixed with prodegradant additives based on metals and metal complexes Plastic disintegrates in the presence of UV light, water or air; remains in the environment as fine particles Controlled degradation (photo or oxidative degradation)
Compiled from Impact of degradable plastic bags in Australia, April 2004, Department of Environment and Heritage, Australian Government, and other sources

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