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Materials engineering

Plastic under control

System developed at UFSCar improves the production of bottles made with polymers

A new industrial process is going to make it possible for disposable bottles for soft drinks and mineral water, amongst other products manufactured with polymers, to be able to be produced in less time, reducing the industrial costs and, who knows, the end price for the consumer. The innovation that is about to come out of the laboratories of the Materials Engineering Department of the Federal University of São Carlos (DEMa/UFSCar) is made up of an optical system for monitoring and controlling the crystallization of polymeric materials during the production process. The system may bring important gains in productivity to the factories specializing in the manufacture of bottles produced with the polymer poly (ethylene terephthalate), better known as PET, besides other items that are injection-molded, a technique of molding something plastic from a molten raw material that is employed in the production of automobile bumpers and cases for computers, printers and cell phones. According to the researchers, the process makes it possible to identify the exact moment when the polymer is crystallized inside the mold, leading to savings of precious seconds in the manufacturing process. Estimates made by the researchers from the DEMa reveal, for example, that a factory with a capacity for producing 160 million PET bottles a year could have an additional gain of US$ 600 thousand, should the system reduce by 1 second the cycle for molding 2 liter PET bottles, which lasts 22 seconds, on average.

The great novelty of the system, the patent registration for which has now been forwarded to the National Institute of Industrial Property (INPI), is to succeed in carrying out the monitoring of the crystallization of the polymer and, at the same time, to control the production of each item, independently of its size, volume or shape. Crystallization is a physical mechanism whereby a semi-crystalline polymer in the molten state is solidified. The monitoring of the crystallization is not currently done in the course of the injection molding process of the polymer, but only after its conclusion, with the use of various techniques, such as optical and electron microscopy. According to chemical engineer Rosario Elida Suman Bretas, the coordinator of the researches that resulted in the development of the new technology, studying the crystallization of the polymer is important, because all the mechanical, optical and electrical properties of this material depend on its crystallinity. “It is by means of the morphology and the volume of crystallinity existing in the polymer, developed during the process of crystallization, that the characteristics of rigidity, flexibility, mechanical resistance and transparency are determined”, the researcher says.

Laser in sapphire
Made up of a metal mold, a laser, two optical fibers, a detector and a laser attenuator, the system can be adapted to any injection machine, which molds the items, making it possible to reduce the working time of the equipment and the process. It works like this: inside the cavity of the mold for the bottle or of any other item, an optical fiber is inserted, which sends a laser beam through a sapphire window. Sapphire is used because it is transparent to the laser and supports the high temperatures and pressures existing inside the mold. The beam crosses through the polymer, which is injected in a molten state, and the signal is collected from the other side of the mold by another optical fiber. The result is shown using software that analyzes the intensity of the light, the pressure and the temperature inside the cavity. This information reveals the exact moment of the crystallization. Today, there is no precision of the moment at which the PET bottle, for example, is ready.

Even when the polymer does not crystallize, or when its crystals are very small, as in the case of PET under some conditions of production, the new system makes it possible to detect the moment the item becomes unstuck inside the mold, or even failures in the filling in of specific points in the cavity, thus preventing defects in the product. The process developed at the DEMa by Professor Rosario and two doctoral students, Marcelo Farah and Alessandra Marinelli, may shortly be available for the market. “A company called Quantum Tech, from São Carlos, has shown interest in developing it for commercial use in transformation industries. It is important to say that there are no like sensors in Brazil or abroad, and even similar ones are still not used commercially”, says Rosario.

The development of the optical system for the injection molding industry was merely one of the advances achieved by Rosario’s team, in the course of carrying out a thematic project financed by FAPESP. Under this project, the group also carried out a study centered on the viability of incorporating into polystyrene, another kind of polymer, rubber devulcanized using ultrasound, to produce blends. Blend is the name given to a polymeric material made from the mixture of two or more polymers. One of the most common uses for this kind of material is the manufacture of bathroom enclosures, which are popularly known as plastic or acrylic enclosures. The purpose of the research was to add devulcanized rubber, originating from used and discarded tires, to polystyrene, to improve its resistance to impact. The devulcanization of the rubber by ultrasound was carried out at the University of Akron, in the United States, by another doctoral student from the group, Carlos Scuracchio.

“Normally, a synthetic rubber of virgin polybutadiene is used to reinforce the polystyrene, which originates a blend known as high impact polystyrene. The advantage of using rubber devulcanized by ultrasound is to confer a noble purpose on used tires, which constitute a serious environmental problem”, Rosario explains. “Our researches are still under way, but we already know that the mixture of devulcanized rubber with the polymer has increased its resistance, although not at the same level as the mixture of rubber with polybutadiene. We believe that the size of the particles of devulcanized rubber mixed with the polystyrene was still not sufficiently small to increase its resistance to impact in a significant way.”

Another research relating to the morphology of polymers involved the use of artificial neural networks, a concept of artificial intelligence that aims at working in a similar way to the human brain, accumulating information, processing it and taking decisions. In this work, blends were investigated made up of polyphenylene sulfide, known as PPS, and an elastomer or thermoplastic rubber. PPS is normally used in the manufacture of electrical computer connectors and of certain components in the automobile industry. For being very rigid, this material has low resistance to impact, but, in compensation, it supports very high temperatures, of up to 250°C. Doctoral student Cybele Lotti mixed into the PPS a thermoplastic rubber known as SEBS, a terpolymer, or a polymer made up of three monomers, made of styrene, ethylene and butadiene. “Our objective was to improve the polymeric blend’s flexibility and resistance to impact”, Rosario explains. With the assistance of artificial neural networks, the researchers managed to predict such properties as the morphology, the quantity or volume of the blend’s crystallinity, resistance to impact and the mechanical properties in a situation of traction. “In the conventional process, we would have to inject and test an enormous quantity of pieces to get information on these properties, which would consume time and money. With the use of the neural network system, the number of pieces injected and tested fell to 5% of what would be necessary in a traditional process”, Rosario claims. “Neural networks had never been used before to relate the parameters of the injection molding process to the microstructure and the properties of polymeric blends.”

Basin of snakes
Another strand of the thematic project involved studying the molecular orientation in blends made up of PET and various polymeric liquid crystals, known in short as LCP (for Liquid Crystal Polymer), which are materials resistant to high temperatures and used in the production of coils, electrical connectors, sensors, surgical equipment, packaging for corrosive liquids, and astronauts’ clothing etc. LCP also has the characteristic of a relative molecular organization when melted, unlike the majority of the traditional polymers, which do not have any organization in a liquid state. To use a metaphor, the melted polymers are like a basin full of snakes of different sizes, in permanent movement, while LCP remains organized, with straight and parallel structures, in the shape of a box of pencils. The objective of this research was to improve the impermeability of the blend, because LCP has an extremely low level of permeability, one of the lowest amongst polymers, and is impermeable to the majority of gases. The problem, though, is that it is still very expensive, about US$ 25 a kilo.

“Understanding the molecular orientation of this blend was fundamental for us to understand the mechanical properties and the permeability of this material”, Rosario says. To understand these characteristics, the polymeric blends were submitted to molecular orientation studies at UFSCar and at the National Synchrotron Light Laboratory, in Campinas, by postdoctoral student Marcia Branciforti. To produce the blends of PET and LCP, the group overcame another challenge: constructing an accessory that brings the final format to a product, because there was no specific equipment on the market for this purpose. This matrix was constructed by doctoral student Lucineide da Silva. “We realized that our matrix brought molecular orientation to PET and improved it in LCP”, Rosario says. The team from the DEMa is now carrying out permeability studies to improve this property of PET and LCP films.

The projects
Study and simulation of the development of the microstructure of polymeric blends and compounds during processing (nº 00/06942-8); Modality Thematic Project; Coordinator Rosario Elida Suman Bretas – UFSCar; Investment R$ 183,578.92 and US$ 80,698.52 (FAPESP)
2. Invention of a method and system for monitoring the crystallization of polymeric materials during injection molding (nº 04/00025-4); Modality
Intellectual Property Support Program (Papi); Coordinator Rosario Elida Suman Bretas – UFSCar; Investment R$ 6,000.00 (FAPESP)