Figure 14: Example of the type of discolouration occurring in moulded plaques when virgin PET is mixed with 1.3% of mixed coloured PET particles. (The top left and bottom left plaques are virgin control samples for comparison) The results obtained from these trials have shown that coloured particles have a major impact on the discolouration of PET. The presence of even very low percentages of coloured PET particles in clear PET flake will discolour PET. Analysis of coloured particles in PET flake from a commercial reprocessing plant has shown that coloured particles in the sub 2mm size flake are present at high levels and were measured to be 1.5%. 7.3 Impact of particle size and presence of fines on rPET discolouration Particle size plays an important role in causing discolouration of rPET resin. Many modern plants sieve and remove particles below 2mm and in some cases below 4mm because these small particles are difficult to accurately sort. This does however impact the reprocessors‟ material yields as losses can range between 5-10% of the overall flake stream. Discolouration due to small particles comes from several factors. Studies on flake from a UK recycler have shown that the majority of PVC present in PET flake is under 2mm in size. The percentage of coloured particles is also typically significantly higher in the small particle fraction i.e.
Figure 15 shows that the majority of PVC particles found in PET flake are under 2mm in size. Removal of particles under 2mm in size therefore facilitates the removal of majority of PVC contamination. Data in Table 6 shows the impacts of small particles and PET fines on the L,a,b values. The graph in Figure 16 demonstrates the significant impact of small particles on the b-value, which is considered the most important indicator of discolouration and yellowing. Table 6: Measured L,a,b values indicating discolouration impacts of small particles (2mm) 89.16 -0.84 3.05 Sieved PET particles (