Fig. 7 shows the evolution of redness of PP_20%SP samples. For PP no change at all was found at T2. At T1 the values decreased insignificantly. PP_20%SP on the other hand shows an increase. The high value for sample PP_20%SP T1 number of extrusion one can result from poor particle distribution. The following significant linear increase of redness however matches the expectations. T2 shows higher values, i.e. more oxidation of iron, for first extrusion, followed by a sharp increase. It can be seen that a higher processing temperature promotes the oxidation of iron particles in the PP matrix. Reis and Silvano [48,86] found an increasing redness of samples containing MMT. They explained it by the oxidation of iron impurities enclosed in all commercial MMT.