Cshows the L*, a* and b* colour coordinates (CIELAB
colour space) corresponding to the series of samples
described previously. The representation of the colour
coordinates in the CIELAB space is convenient for accurate
specification of object colours (two colours which appear
similar to a human observer are close together in the L*a*b*
space). The figure includes the evolution of the lightness L*
and the a* and b* colour coordinates as the vitamin E
content in the samples increased, together with the colorimetric
effects of the g-irradiation.
Note that the lightness L* was not significantly affected
by the incorporation of vitamin E in the polyethylenes,
disregarding the irradiation level. However, both the a* and
b* colour coordinates varied with the vitamin E doping, and
to a lesser extent with the dose of gamma irradiation. The
a* coordinate decreased by about 7 colour units and the b*
increased by 25 colour units as the vitamin E doping
increased up to 3 wt%. It is also worth mentioning that
these results were not dependent on the roughness of the
samples (identical results were obtained for samples with
average roughness of 0.15 and 0.80 mm).
Colour differences DE were evaluated as Euclidean
distances in the CIELAB three-dimensional space. Fig. 5
shows the correlation between colour differences DE of
non-irradiated and 90 kGy irradiated vitamin E doped GUR
1050 UHMWPE samples and their percentage of vitamin E.
Note that DE increased linearly with the percentage of
vitamin E for each set of samples. Disregarding the surface
roughness, identical results were obtained. The same figure
also includes the linear regression (dashed lines) obtained
for each set of samples. It can be seen that irradiation
caused a displacement of the results to higher colour
Fig. 1. Photograph of several series of polyethylene samples: for nonirradiated
(a) and 90 kGy gamma irradiated (b) UHMWPE samples with
increasing vitamin E doping levels, and undoped UHMWPE with increasing
gamma irradiation doses (c).
Fig. 2. Total reflectance spectra of a series of non-irradiated vit.E doped
UHMWPE samples.
Fig. 3. Total reflectance spectra of a series of irradiated UHMWPE samples
with and without vit.E doping.
M.J. Martínez-Morlanes et al. / Polymer Testing 31 (2012) 841–847 843
differences with similar slopes for the corresponding least
square fit, but different intersections with the y-axis.