Quantitative Analysis of Polyolefine Blends

Introduction 

Plastic blends represent the largest fraction of plastic materials that occurs in the recycling of plastics from waste packaging. These mixed plastics consist mainly of polyolefines, i.e. polyethylene and polypropylene. For material recycling purposes, the fraction can be processed via dissolution. Recycling via dissolution is in fact a process that results in recycled material with properties superior to those obtained with the usual melting procedure [1].
To produce material of high quality with reproducible material properties, it will however be necessary to develop new methods that can separate the polyolefinic material into its individual components, i.e. into low-density (LDPE) and high-density polyethylene (HDPE), and polypropylene (PP) [2].

The composition of the recycled plastic is of course a measure of the efficiency of the separation. We have therefore developed a method to determine the composition of polyolefinic samples. The method uses DSC measurements and is particularly interesting in that it not only distinguishes between polyethylene and polypropylene, but also between high and low-density polyethylene. The basic principle of the method is to describe the melting enthalpies (measured in the melting curve) of a polyolefinic sample by the mass fractions (which follow from the sample composition) and the standard melting enthalpies of the components involved. The melting curve of a mixture cannot however be described exactly by the sum of the pure melting curves. Correction factors have to be introduced. These are determined iteratively from the measurement results of a series of polyolefinic samples of known composition.

Experimental Details

First of all, a series of samples of known composition, i.e. calibration samples, were prepared. The compositions of these calibration samples were chosen so that the three distribution ratios (in weight percent) of 5:5:90, 10:10:80 and 20:20:60 corresponded to compositions in which each of the three plastics was the main component. These nine mixtures are

Summary

This article describes a method that allows the composition of polyolefine samples to be determined using DSC. The method is based on the principle that the enthalpies of the three components LDPE, HDPE and PP can be derived from their standard melting enthalpies and their concentration in the mixture. Since the components of the mixture do not behave ideally, the relationships described above have to be corrected with correction factors. These are determined using sample mixtures of known composition. The evaluation of the calibration mixtures and the samples under analysis must be performed reproducibly.

_{Quantitative Analysis of Polyolefine Blends | Thermal Analysis Application No. UC 154 | Application published in METTLER TOLEDO Thermal Analysis UserCom 15}