Mailbox Letters to the
Mailbox Letters to the editor ! ! I think the definition of biodegradable plastics vs compostable plastics (in issue 02/2006) is correct, but it is written to sound like: “compostable is better than biodegradable, or, a compostable plastic is certainly biodegradable.” Instead, I would like to stress the fact that a biodegradable plastic is, as you say, completely assimilated, in ordinary conditions of temperature and pH, with forms of life typically present in everyday soil. And, in brief periods of time, i.e. weeks at the most. Composting conditions are easier, so to speak: 60°C, defined microrganisms. I would say that biodegradable plastics are necessarily and readily composted, NOT viceversa. For example, PLA is degraded only in a very specific, industrial composting site. It is as biodegradable as PET ! In the sense that, if kept in regular soil, nothing will happen to it for years. And like PLA, (this applies to) many other compostable plastics. These compostable bioplastics risk becoming a dangerous factor of confusion in the consumers’ mind, and therefore could contribute to environmental litter. Dr.-Ing. Michelle Marrone R&D Application Projects Europe M&G Group, Italy www.gruppomg.com Ramani Narayan, Professor of Chemical and Biochemical Engineering, Department of Chemical Engineering and Materials Science, Michigan State University basically agrees with this comment. He wrote: I would like to clarify the issue and put the subject on a more sound scientific footing because there seems to be confusion. • Biodegradation or bioassimilation (assimilated as food by microorganisms) has no meaning unless you define the environment and time for complete biodegradation. So one needs to present the subject as: - Biodegradation under composting conditions (compostable); - biodegradation under anaerobic digestion conditions, - biodegradation under soil or marine and so on in other words one must define the disposal environment when discussing biodegradation. • Time is the second important defining element – the rate and time required for complete biodegradation (or better bioassimilation) in the defined disposal environment! – the element of completeness in a short defined time frame (one season) is essential because hydrophobic breakdown fragments released into the environment has been shown to have serious environmental consequences (if they are not completely assimilated by the microorganisms in the disposal environment in one crop growing season). • Both these points are covered in detail in my presentations (e.g. 1st European Bioplastics Conference, Brussels, 2006) or in my publications (see one example at www.bioplasticsmagazine.com/200701) • The National (ASTM D6400, EN 13432) and International (ISO 17088) specification standards are in complete harmony with the above definitions and understanding 38 bioplastics MAGAZINE [01/07] Vol. 2
