RESULTS AND DISCUSSION Processing
When mixed plastics waste is processed, one ofthe main problems is to find the best compromise between homogenization and degradation. In the waste material, there are polymers with different melting points and different thermal stability. Especially, PVC and PET are very difficult to process together, because the melting point of PET is over 250oC, and at this temperature PVC is easily decomposed forming HCl and chary residues. Therefore, the optimal processing
Figure 1. The modulus and impact strength of LDPE/MPW blend (MPW = 25%) vs. the number of extrusions.
conditions must ensure a good dispersion of the materials with high melting point in a continuous phase of molten polymers, avoiding gas bubbles, low molecular weight compounds, and crosslinked residues that are formed by thermal degradation.
In the previous work,2 it has been demonstrated that all mechanical properties reach a maximum or a plateau after two extrusions. As an example, Figure 1 gives the data on the elastic modulus and the impact strength of a blend with 75% of LDPE. The data indicate that a double extrusion gives the best balance between homogenization (which improves with a number of passages through the extruder), and the degradation which is increased by the severity of the thermomechanical treatment. Based on these results, two extrusion steps were used for the preparation of all samples.
During processing, no significant evolution of HCl from the venting port of the extruder was found, only at the die lips some acid vapors were observed. This is a rather surprising fact, because the HCl evolution from PVC is a very common occurrence in the industrial operation of plastics recycling. One possible explanation could be the residence time in the screw, which is much longer for the industrial extruders than used in laboratory extruder. Additionally, it should also be considered that some antioxidants and lubricants were added.
Figure 2. Dimesionless modulus vs MPW content.
The stability of PVC, under the processing conditions of experiment, was confirmed by X-ray analysis with an energy-dispersive Philips apparatus. Significant amounts of chlorine are present in all extruded blends. The only processing difficulty, found during extrusion, was due to the presence of non-polymeric materials (paper, aluminum, etc.) in the MPW which required frequent changes of filter at the end of the screw.
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