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Plastic

Rapid decomposition

Researchers develop a recycled polymer made from PET that is not harmful to the environment

eduardo cesarPlastic bottles decompose slowly in the wildeduardo cesar

A biodegradable plastic that decomposes in the soil in just 45 days has been developed by Brazilian and French researchers using post-consumption PET packaging, a polymer made from polyresin (polyethylene terephthalate). The secret for development of the new polymer was to use another type of plastic in its composition, in this case an aliphatic polyester (a type of polymer with open chains of molecules), in order to accelerate the degradation process. Because of its molecular structure, composed of aromatic rings, formed by six carbon atoms and six hydrogen atoms in a special alternating single and double bonds, PET is considered to be a non-biodegradable polymer, which means that under environmental conditions of pH, pressure and temperature it does not degrade in nature. Aliphatic polyesters, on the other hand, are easily consumed by the micro-organisms found in soil. “In mixing the two we managed to formulate a highly biodegradable product”, says chemist and co-author of the work Ana Paula Testa Pezzin, from the Biotechnology Laboratory of the Universidade da Região de Joinville [University of the Joinville Region] (Univille), in Santa Catarina; the other institutions involved in the study are the Pontifícia Universidade Católica [Pontifical Catholic University] of Rio Grande do Sul (PUC-RS) and the Pierre and Marie Curie University of Paris.

Various products may be manufactured from the new biodegradable plastic, depending on their properties (mechanical and thermal strength, porosity, etc.), which vary according to the level of PET and aliphatic polyester used in their preparation. “The applications of this new material will not be as refined as those of virgin PET, because when it is recycled it loses some of its original properties. We imagine that its greatest use will be in disposable products, including  plastic containers for plant seedlings, toothbrush handles, telephone cards and packaging for cosmetics”, says the researcher. According to Ana Paula, a Brazilian PET manufacturer has already expressed an interest in using the process on a major scale, but negotiations are still on-going and the company would rather remain anonymous.

At the end of 2006, the work entitled “The chemical recycling of post-consumption PET packaging: the synthesis of new co-polymers”, won the EcoPET Encouraging Recycling Prize, in the “Research and Innovative Processes” category, organized by the Associação Brasileira da Indústria do PET [Brazilian PET Industry Association ] (Abipet). The aim of the competition is to recognize the good ideas and initiatives relating to recycling that are necessary to reduce the impact on nature caused by the huge quantity of discarded PET objects. Every year, for example, Brazil manufactures about 374,000 tons of products, packaging and plastic PET bottles and only 47% of this total is recycled. The rest accumulates in sanitary landfill sites, waste dumps, rivers and lakes, where it takes a century or more to fully disappear.

univilleElectronic microscopy shows PET-PTS polymer before biodegradationuniville

Best results
The team tested three different types of aliphatic polymer (PTS, PES and PEA) that are known to be highly biodegradable. The co-polymer made with a mixture of PET and PTS, the acronym for poly (trimethylene sebacate), was the one that produced the best results. Before this, the researchers had already synthesized a polymer using PET and PEA polyester, or poly (ethylene adipate), which began biodegrading in the soil in seven months, a considerably shorter period of time than the 100 years of PET on its own. The biodegradability of the co-polymers formed from PET and PTS, like the other two, depends to a great extent on the composition and the catalyst used. “After 45 days in the ground samples with just 20% aliphatic polyester were already in the early stages of degradation, while those with 40% had already deteriorated considerably. The increase in the amount of PTS was a determining factor when it came to accelerating biodegradability”, says the chemist from Univille.

The process of synthesizing the biodegradable polymer is relatively simple and occurs as the result of a normal polymerization reaction. PET bottles and products are washed, cut up and placed, together with traces of the selected aliphatic polyester, in a 500 ml glass reactor in an inert nitrogen atmosphere, at a high temperature and are stirred mechanically.

The researchers subsequently add a catalyzing agent. “The plastic resulting from this chemical reaction has mechanical, thermal and biodegradation properties that are different from those of PET”, explains chemist Sandra Einloft, Director of the School of Chemistry of PUC in Rio Grande do Sul, who was responsible for the studies relating to the chemical synthesis of the polymer.

To prove the excellent decomposition qualities of the new material, the study’s authors carried out various biodegradation tests in which the co-polymers were buried for an undefined amount of time at the bottom of suitable recipients, called beakers, containing aged soil and with humidity between 85% and 95%. From time to time the samples were removed for morphological analysis. This characterization was carried out using a sweep electronic microscope (MEB) and various other techniques, such as differential exploratory calormetrics, thermogravimetric analysis, gel permeation chromatography, etc.

According to the researchers the process was not patented because the main objective was to try to show that there are solutions to the problem of the accumulation of PET packaging in landfill sites and waste dumps. “Besides which, we also have an academic interest in preparing human resources  knowledgeable about  the subject and at the same time we want to understand these reactions and how the composition of co-polymers influences their properties and biodegradability”, says Ana Paula Pezzin. Various Masters’ degree dissertations at PUC-RS and one from the Universidade do Estado de Santa Catarina [Santa Catarina State University] (Udesc) took as their subject the synthesis of co-polymers from the chemical recycling of post-consumption PET.

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