rPET Quality Report

7.0 Analysing the effects of contaminants on rPET discolouration
Recycled PET has classically been more yellow or brown when compared to virgin PET. The
factors that cause discolouration of rPET include:
• Presence of PVC particles;
• Coloured particles from PET and other polymers;
• Adhesive residues;
• Processing of residual rPET fines and dust with PET flake; and
• Thermal and oxidative history.
To measure the discolouration of rPET resin, spectrophotometry was used to measure the
resin‟s L, a, and b values. The „b‟ value is of particular importance for recycled PET as it
denotes the yellowness of an item. A higher „b‟ value represents a more yellow resin.
Figure 7: Measurement of rPET colour is performed by evaluating the L, a, b values.
7.1 rPET discolouration simulation trials
A series of R&D laboratory trials were performed to evaluate the impact of PVC, coloured
PET particles and PET fines on the discolouration and yellowing of rPET. The trials included
mixing of virgin PET with these contaminants in known quantities as well as comparing the
L,a,b values (used as a measure of colour) of hot washed flake from a UK recycler.
Virgin PET was used as the main constituent for the majority of the trials due to its ability to
not mask the yellowing effect of the contaminants. The test matrix used for the study is
presented below.
Improving food grade rPET quality for use in UK packaging 25