Characterization of scrap poly(ethylene terephthalate)
Introduction
The problem of recycling of polymeric materials is not yet solved on satisfactory level. Usually only 30% of polymeric waste is recycled. Recently environmental, legislative and consumer pressures have led to an increase of interest in reusing polymers from waste [1]. Reasonable way of waste treatment of commodity polymers such as polyolefines (HDPE, LDPE, LLDPE, PP) or poly(ethylene terephthalate) (PET) is material recycling. Industrial scraps are relatively easy to deal because the contamination is usually negligible. Much more complicated is processing of municipal plastic waste. The potential application of these materials is often limited by their poor properties.
Efforts to recycle plastics are mostly concentrated on sorting, separation and reprocessing of a single polymeric material [2]. Main problem during material recycling is the segregation of polymers. A polymer after segregation is typically not completely pure. If the contents of polymeric impurities is significant it is in fact a mixture of noncompatible polymers. In such case a phase separation on a microscale level is visible. Usually the minor component forms separate inclusions with size of 10 μm or more. The interfacial adhesion is weak. Scraped commingled polymers, due to their incompatibility and/or presence of non-polymeric impurities, have poor mechanical properties. The possible way for improvement of dispersion and increase in adhesion is reprocessing by blending with carefully chosen compatibilizers.
The second reason for unsatisfactory mechanical properties of recycled polymers is degradation during reprocessing [3]. Some kinds of admixtures may promote degradation of polymers during processing. The main problem in recycling of PET is the elimination of all impurities which may catalyze hydrolysis [4]. The most common enhancing hydrolysis impurities are: PVC, NaOH or alkaline detergent from label removing and washing, ester or acidic glues, like EVA or acrylics, which are used for fixing label on PET bottles. The hydrolysis of PET is an autocatalytic process because the end hydroxyl groups transform into carboxyl groups which catalytically accelerate further hydrolysis. Another important destructing factor for PET is a moisture which may be removed from a polymer by an appropriate drying procedure.
In any case, before selecting a recyclate for a particular application or before finding an application for a recycled polymer, it seems necessary to characterize its composition and properties. Results of our analysis of properties of selected recycled polymers available in Europe are presented below.
There were three goals of our study: (1) to indicate the methods which should be used for characterization of crucial properties, (2) to show general tendencies in properties of recycled polymers, and (3) to characterize recyclates available in Polish market and compare them with selected European Community waste.
Section snippets
Materials
The search in Poland for large producers of recyclates or regrinds of plastics exposed no single large recycling company but several regional companies, collecting and regrinding or pelletizining scrap plastics. Among them GTX Hanex Plastics, a company located in Sokolka near Bialystok, and Ekogeminex, a company located in Lodz county and partially supported by the city of Lodz, are significant producers of recycled PET and polyolefins. Representative samples of recycled plastics were obtained
Characterization of composition
A typical thermogravimetric curves are presented in Fig. 1 and the results of TGA studies are summarized in Table 1. Thermogravimetry of extruder averaged PET Hanex in oxygen free nitrogen shows the onset of mass loss at 380°C. The rate of loss of mass equilibrates at the level of 89.8% at 478°C. TGA traces taken during atmospheric air flow shows the onset of degradation at 310°C with two steps of mass loss: first at the level of 86.1% at 460°C and the second at 572°C at the level of 99.0%.
The
Conclusions
Application of several techniques appeared to be necessary to characterize sufficiently PET rich regrinds for material recycling. All samples of PET waste chosen for the studies contained admixtures of other polymer or polymers, at the concentration from 0.1 to 5 wt%, depending on the type of recyclate. In part of analyzed PETs (e.g. PTM3, and Hanex) the level of PVC contents was unacceptably high. The presence of PVC in concentration >50 ppm makes PET regrinds practically worthless for more
Acknowledgements
This work was supported by European Commission project INCO-Copernicus, No. IC15CT960731.
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