Elsevier

European Polymer Journal

Volume 36, Issue 9, 1 September 2000, Pages 1875-1884
European Polymer Journal

Characterization of scrap poly(ethylene terephthalate)

https://doi.org/10.1016/S0014-3057(99)00261-XGet rights and content

Abstract

The goals of the investigation were: to indicate the methods of characterization of recycled polymers, to show general tendencies in properties deterioration and characterize recyclates on Central Europe and European Community markets. The properties and composition of scrap poly(ethylene terephthalate) from several sources were characterized by: TGA, DSC, FTIR, tensile properties, dynamic viscosity, intrinsic viscosity and thermo-oxidative stability. We found that all PET regrinds contained admixture of other polymers (0.1–5 wt%). The presence of more than 50 ppm PVC makes PET worthless for advanced application as film forming, because it catalyzes the hydrolysis and reduces the strength of material. Although the individual flakes of recycled PET show almost unchanged molecular characteristics and properties, the processed regrinds always exhibit worse properties. Partial restoration of recycled PET properties can be achieved by careful working, removing the dust fraction and by proper drying. The difference between studied PET's results from different applied recycling procedure. Admixtures of polymers without compatibilizer always deteriorate tensile properties. Various levels of stability of polymer viscous flow during film and tape extrusion were observed, depending on composition of recycled PET from various sources. Microgels were observed in all samples during film extrusion.

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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