The Oxo-biodegradable Plastics Association is challenging the effectiveness of a recently-developed class of plastics containing additives described as "natural", "enzymatic" or "microbiodegradable".

These additives, which are not oxo-biodegradable, are being marketed as promoting biodegradation of a host of polymers within a few months to several years, even when buried deep in landfill.

An OPA statement (www.biodeg.org) reads: "It seems reasonable to believe that the additives themselves will biodegrade, but will they make the plastic biodegrade? It is difficult to believe on the published scientific evidence that incorporation of these additives into a polymeric matrix will render the resultant plastic article biodegradable at all, and on the basis of known scientific principle it is hard to see how it can. Biodegradation of the additive could give a false reading in a CO2-evolution test, suggesting that the plastic itself is biodegrading."

The new additives appear to consist of a starch or polycaprolactone (PCL) matrix often extended with mineral filler, with no pro-degradant catalyst salts in the composition.

The idea seems to be to help plastic disintegrate but, unlike an oxo-biodegradable additive, it does not change the plastic into a biodegradable material.

Polyethylene and polypropylene do not present a metabolic pathway for enzymes and it is precisely because of these properties that they are useful for food-packaging.

In addition, degradation of PVC may produce toxic residues that are highly dangerous, and which may interfere with the survival of the biodegrading microorganisms.

A film made with the new additive was analyzed and found to contain inorganic filler derived from CaCO3 and some primary antioxidant, and approx. 400 parts per million of a secondary stabilizer. No other chemical compounds were found. Based on this analysis, these products cannot biodegrade.

There seems to be ambiguity in the testing by companies promoting these products. It is for example sometimes unclear whether the data refers to the additive or the final product.

Some of the testing seems to be on blends of material using much higher rates of additive than recommended. This will obviously alter its properties, processing-characteristics and recyclability, but the strength and fitness-for-purpose of products extended in such a manner must be doubted.

Further, if biodegradation is by an enzymatic process, the enzymes are unlikely to survive the processing conditions needed to create the plastic in the first place.

The OPA is not persuaded that these "enzymatic" additives will work as claimed.