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Difficult decomposition

Study shows that oxybiodegradable plastics do not decompose in the wild as expected

MIGUEL BOYAYANIllustration of degradation and decomposition of plastics in soilMIGUEL BOYAYAN

Environmentally aware consumers have probably already noticed that certain plastic bags of the kind used to pack products purchased at supermarkets, drugstores and other stores, have been informed that they are made of oxybiodegradable plastic. This kind of plastic was first manufactured in the late 1980’s and the manufacturers claim it is environmentally friendly because it decomposes very quickly in the wild. As a result, several environmental hazards stemming from the discarding of this product, such as waterproofing of soil and contamination of water tables, are minimized. A recently concluded research study conducted by a Brazilian researcher challenges this allegation. Materials engineer Guilherme José Macedo Fechine, a professor at Mackenzie Presbyterian University, conducted a battery of tests with a kind of oxybiodegradable plastic sold in Brazil and verified that, although the plastic fragmented and turned into dust, it is not eaten by fungi, bacteria, protozoa and other micro organisms – the necessary conditions to be considered biodegradable and disappear in soil or water. According to the researcher, who does not want to specify the commercial names of the products because the related companies were not consulted, the biodegradable properties of oxybiodegradable polymers have been a controversial issue for quite some time among members of the international scientific community. One group of scientists has doubts whether these polymers are actually biodegradable. At the beginning of this year, São Paulo State Governor José Serra vetoed a bill of law proposed by the State Assembly that made it compulsory to use plastic bags with oxybiodegradable additives because of doubts about the actual benefits to the environment. “My research study proved that they are not biodegradable,” says Fechine, who has just returned from Belgium, after having participated in MODEST 2008, (the acronym in English), an international congress on the modification and degradation of polymers.

To understand the controversy about oxybiodegradable polymers, it is important first of all to understand how the biodegradation process of this plastic occurs and learn how this plastic is produced. Oxybiodegradation occurs in two stages. First, the plastic is converted into smaller molecular fragments, through the action of oxygen, temperature or ultraviolet radiation. These fragments then biodegrade, which means they are converted into carbon dioxide, water and biomass by decomposing micro organisms. To foster this characteristic, manufacturers added a pro-oxidant additive to conventional polymers, such as polypropylene, polyethylene and others. These polymers are commonly used to manufacture bags and other plastic products. The pro-oxidant additive allegedly makes the polymer biodegradable. When discarded into landfills or garbage dumps, the additive allegedly breaks up the long molecular chains that form the polymers, providing them with the necessary characteristics for consumption by micro organisms found in the soil.

“According to my study, the only difference in the oxybiodegradable polymers is that fragmentation time is much faster than that of conventional polymers,” Fechine states. “Companies that sell this kind of pro-oxidant additive should warn consumers that the additive alone does not make the plastic biodegradable. For this to occur, the polymer would first have to go through powerful pre-degradation caused by ultraviolet radiation or temperature, for example, and be discarded in appropriate soil, with pH, humidity, temperature and existence of micro organisms that would allow biodegradation to occur.”

Not everybody agrees with the additive’s limitations. “I am not familiar with this study. I don’t know if it was conducted with the additive that I represent and I don’t know which methodology the researcher resorted to. But I can assure you that the tests conducted by Ecosigma, a company with its main office in the city of Campinas, specialized in composting and waste management, such tests conducted in conjunction with Paulista State University/Unesp, State University of Campinas/Unicamp and the Campinas Agronomy Institute/IAC, showed that oxybiodegradable plastics manufactured with the d2w additive, which we represent, are indeed biodegradable, can be made into compost and are not ecotoxic for superior plants, worms and methane-producing micro organisms,” states Eduardo van Roost, director-superintendent of Res Brasil. This company sells the d2w additive to more than 160 Brazilian plastic packaging manufacturers. “The efficiency, performance and safety of our additive is attested to by the fact that this product is sold in more than 60 countries,” he adds.

Comparing samples
The experiment conducted by Fechine, who for the last three years has been the head of a Young Researcher project funded by FAPESP, was held at the Materials Engineering Department at the University of São Paulo/USP’s Polytechnic School. Prior to becoming a professor at Mackenzie, Fechine had compared the degradation of two polypropylene samples, one of them containing the pro-oxidant additive and the other one without the additive. In the first stage of the work, the two samples were previously photo degraded in an aging chamber accelerated with ultraviolet radiation emission. “We simulated the photo degradation that plastics undergo in a landfill or garbage dump because of the absorption of solar radiation,” explains the professor. The samples were exposed to different radiation times; the longest exposure corresponded to 480 hours (or 20 days) in the aging chamber. At the end of this period, the polymer with the pro-oxidant additive was in an advanced state of decomposition. “We measured the molar mass (the quantity of molecules) of both samples before and after testing in the aging chamber and verified that the pro-oxidant additive actually accelerated the photo degradation very intensely when compared to the conventional polymer sample. But we still needed to know whether, in addition to fragmenting, it would become biodegradable,” says professor Fechine.

The two samples were then submitted to biodegradability tests on previously prepared terrain. The samples were buried and collected from time to time to be weighed and to be evaluated in terms of mass loss. “After nearly two months, we verified that the two samples did not undergo any significant loss of mass. This means that neither sample was eaten up by soil micro organisms during this period of time,” says Fechine. “Our experiment showed that the additive accelerates the polymer’s fragmentation, but that it does not make the polymer biodegradable.” An article with the results of the essays was accepted for publication by Polymer Engineering and Science, one of the most highly respected journals in the field of polymers. The title of the article is Effect of UV radiation and pro-oxidant biodegradability. It was written in partnership with researchers Nicole Demarquette, from Poli-USP, Derval dos Santos Rosa and Marina Rezende, from São Francisco University in the city of Itatiba, State of São Paulo. The latter two researchers were responsible for conducting soil biodegradation tests.

The Project
Photo degradation and photo stabilization of polymeric blends and compounds (nº 05/00322-1); Modality Apoio a Jovens Pesquisadores Program; Coordinator Guilhermino José Macedo Fechine – USP and Mackenzie; Investments R$ 59.645,00 and US$ 48.470,55 (FAPESP)