A frightening quantity of used soft drink plastic bottles floating in rivers and streams in some points of a flooded city. This scene has already become typical in the summer, mainly in large urban centers, where any torrential rain brings up the problem of the discarding of these bottles make from Poli (in Portuguese), but better known as PET (from the English, Poly Ethylene Terephthalate).
If reused, in cities where there is selective collection of garbage or through rubbish collectors who pick through the rubbish dumps, the PET bottles go into the industrial production of bristles for brooms and brushes, sewing thread, ropes, carpets, pillow fillings, blankets and fluffy animals among other products. However, if it were to depend on professor Maria Zanin of the Department of Materials Engineering of the Federal University of São Carlos (UFSCar), the recovered PET would have a nobler destiny, returning to the production of more bottles.
Professor Maria is coordinating the project Development of a Method for the Chemical Recycling of Post Consumption PET by Hydrolysis, which has as its goal the establishing of a system of de-polymerization of the used bottle, allowing the final product (the original raw material of the PET, also called as monomer) to return to the production line of new bottles. “In the petrochemical industry, petroleum derivatives are transformed into plastics. The process worked out by our team goes in the opposite direction.” explains professor Maria. She explains that a method to transform the PET again into petroleum derivatives has been developed, with a degree of sufficient purity to allow its use in the manufacturing of PET without any type of market restriction. “During the process, all of the impurities chemically or physically linked to the polymer are separated, and this way, the recycled product can once again have direct contact with food.” points out the researcher.
A change of route
“The conventional process of recycling, grinding, washing, drying and processing, only allows for the use in products that don’t have direct contact with food.” she remarks. Thus, the recycled product is placed outside of the main market, which consists of bottles for soft drinks, water, cooking oil, vinegar, among others.
The re-use of PET for these packages also brings a benefit to the economy of the country. According to data from the Brazilian Association of the Chemical Industry (Abiquim), in 1999 the country consumed almost 330,000 tones of PET, of which 146,000 tones had to be imported. Besides the importation of the PET itself, in that year a further 27,000 tones of the raw material – monomer – were imported. The total cost reached almost US$ 140 million. Besides the decline in imports, the use of this process of recycling bottle to bottle has other important benefits. It has generated the development of our own technology new jobs, the saving of natural resources (petroleum) and a decrease in the overall waste to be discarded. All this through the good practice of recycling.
The developed world
According to data from the Brazilian Association of PET Packaging Manufacturers (Abepet), in 2000, 67,000 tones of this product, in relation to the previous year, were recycled, what means an increase of 34% over the last year. “This represents an rate of 24.6%, similar to countries such as Japan and the United States where the recycling of PET has been going for more than 20 years. Here the process of recycling is just over seven years old.” compares Alfredo Sette, the President of Abepet. He explains that the system of selective collection and the implanting of cooperatives of collectors are primary factors for increasing these numbers.
The number of PET bottles discarded is impressive for those who are unaware of it. Professor Maria carried out a survey into urban disposal between August of 1997 and January of 1999, in Araraquara, a town with 180,000 inhabitants, in which was verified the throwing out of more than 7,000 PET bottles per day.
During the development of the work, which led to the new system of de-polymerization of PET, Maria’s team came up against various difficulties. One of them was the lack of information, since a good part of the process of chemical recycling is patented. The greatest difficulty in this area is that the processes found in articles or patents are normally incomplete, covering the problem from the beginning to the middle or from the middle to the end. Even then, according to the professor, the emphasis in these cases is on recycling material of industrial residues, which is a reality completely different from that of the recycling of after-consuming use, mainly in Brazil. Here, with few exceptions, domestic garbage contains residues of all types, making difficult the selection and the cleaning of recyclable products.
Thus, professor Maria elected a comprehensive study, using PET in contact with water. In the research for the best conditions for this reaction various parameters were investigated such as the temperature of the water, particle size, catalyst (an agent that accelerates the chemical reaction) pH, stirring, surface treatment and finally pressure. The initial production, done in common glassware in the laboratory, produced 100% of the monomer after three hours. Afterwards, a reactor in stainless steel was built that allowed for the pressurization of the system and the obtaining of higher temperatures. “With the reactor it was possible to reach the totality of the reaction in almost 15 minutes with pressures of 11 atmospheres.” relates the engineer Sandro Donnini Mancini, who is preparing his doctorate thesis on the subject.
As well as producing the monomers with this new process, the researchers still were looking for a level of purity similar to that produced in the petrochemical plants. The greatest problem for the clear development of hydrolysis (the breaking of the chemical bonds by water) as a technique of chemical recycling is the difficulty of obtaining pure terephthalic acid, the main monomer of PET. To achieve this, a method of purification was also developed.
Ten sets of experiments were carried out on characterizing the purified material, whose results were compared to those of the terephthalic acid produced by the petrochemical industry. “Apart from some small differences, the high similarity between them allowed for the use of the terephthalic acid produced from recycled PET in the production of the polymer without the restriction towards food packaging.” analyzes professor Maria Zanin.There are other forms of recycling bottle-to-bottle that are different from the proposal of professor Maria. One of them is the use of recycled material as filler in a multilayer “sandwich” with raw material that covers the conventionally recycled material (this application has already been approved by the Ministry of Health since the end of 1998). Another form is the so-called “super-clean”, a system of cleaning that is highly efficient, which ensures the absence of any element hazardous to health or to the food to be packaged. This process is not in use in Brazil yet, though there is research in this direction.
Companies that are using some of these recycling techniques bottle-to-bottle throughout in the world are applying a small percentage of recovered material in conjunction with virgin material on their production line. However, the tendency is that, with the decrease in price of the recycled PET driven by higher consumption, this percentage can be gradually increased.
To complete her project of recycling bottle-to-bottle, the researcher Maria is working on way of recovering the other monomer of PET, ethylene glycol. According to the professor, the transfer of the method into an industrial scale will depend on experiments done on a pilot scheme and a costs analysis. The full development of bottle-to-bottle recycling certainly depends on the completion of this project which would serve as a stimulus for the adoption of more elaborate systems of collection and recycling products whose destiny is the garbage dump.
The Development of a Method Of Chemical Recycling of PET Post Consumption via Hydrolysis (nº 00/06867-6); Modality Regular line of research assistance; Coordinator Professor Maria Zanin – Department of Materials Engineering of UFSCar; Investment R$ 9,741.25