The terms “degradable”, “biodegradable”, “oxo-degradable”, “oxo-biodegradable”, “thermo-degradable” and others are often used when describing plastics.
These designations have raised numerous doubts as to their actual features. Some confusion that has occurred should be clarified.
• Biodegradable plastic: a plastic that undergoes biodegradation (a process where the degradation results from the natural action of microorganisms such as bacteria, fungi and algae) and which meets certain standards. Since 2008, standard specifications have been as follows: ASTM D6400, ASTM D6868, ASTM D7081 (for USA) or EN 13432 (in the EU). According to these standards, there must be complete biodegradation in less than 6 months. The specification of the time required for final biodegradation is an essential requisite for any claim pertaining to biodegradability.
• Oxo-degradation: the main effect of oxidation is not biodegradation, but, rather, the fragmentation into small particles, which remain in the environment for an indeterminate time period, making their disposal and final elimination uncontrollable. No data has been released publically relating to mineralisation rates that supports the claims of complete biodegradation. Therefore the term “oxo-fragmentable” is the most appropriate to describe the final end-of-life state for these materials. The fragmentation results from including oxidising additives to conventional plastic (PE, PP, Pet, PVC). These additives are based on chemical catalysts containing transition metals such as cobalt, manganese, iron, etc., or on biological materials, which could cause fragmentation as a result of chemical oxidation of polymer chains in plastics, as caused by ultraviolet radiation or by exposure to heat. Secondly, it is stated that the resulting fragments eventually undergo biodegradation. Even though there is a chemical theory supporting a very slow biodegradation process, the absence of light, the presence of moisture or very low temperatures act as a decelerator of the process, resulting in a very slow transformation or even its stoppage.
Therefore, the fragmentation of "oxo-biodegradable" plastics is not the result of a biodegradation process, but, rather, the result of a chemical reaction. The resulting fragments will remain in the environment. Fragmentation is not a solution to the problem of waste, but, rather, the transformation of visible contaminants (such as plastic bags) into invisible contaminants (plastic fragments). Normally, this solution is not regarded as viable for solving the problem of plastic waste, all the more since the problem of pollution caused by civic behaviours, such as dumping waste in the environment, namely, it can be further stimulated by this type of products. Furthermore, while conventional plastic products can be collected in the environment, microscopic plastic fragments are impossible to control or collect.
This way, the end disposal for these oxo-degradable products is difficult to solve. On the one hand, they are neither biodegradable nor compostable (none of the standards are met); therefore, they cannot be sent to composting plants. As recycling (the normal and proper disposal for common plastics) is concerned, oxo-fragmentable products could make it difficult to recycle post-consumption plastics. In practice, "oxo-degradable" plastics are traditional plastics. The only difference is that they include additives that affect their chemical stability. Thus, they are identified and classified according to their chemical structure and, together with other plastic waste; they end up in the recycling system. This way, these include additives (which cause degradation) in the recycled raw material. Consequently, recycled materials can be destabilised, thereby making their acceptance difficult and leading to a reduction of their value.
Finally, if placed in a landfill, we could risk being in the presence of conditions (absence of light, low temperatures) which act as a decelerator in the degradation process, resulting in a very slow transformation or even its stoppage, thus creating serious problems for soils.
According to European standard EN13432, a compostable material should comply with the following requisites:
• Biodegradability: determined by measuring the metabolic conversion to carbon dioxide. This property is quantified using the standard test (Standards EN 14046 or ISO 14855). The material is considered biodegradable if, within 6 months, it achieves 90% of the figure obtained by the reference (cellulose).
• Friableness (Ability to disintegrate): this evaluates the fragmentation and loss of visibility in the final compost. It is evaluated using a test according to EN 14045. The material to be tested is degraded, together with organic waste, for 3 months. The waste resulting from the material being tested larger than 2mm is considered as not having disintegrated. This fraction should be less than 10%.
Absence of negative effects in the composting process. Heavy metal contents in the compost should be inferior to the predefined values, and there must not be any negative effects in the quality of the compost.