DEGRADATION OF PLASTICS IN THE ENVIRONMENT - Plastics and Environmental Sustainability - page 228

(6.2)
The Equation 6.2 must be applied with caution as the rate must be time
independent for the expression to apply. Kinetics of polymer degradation
can change at T > Tg (°C); therefore, values of E determined below Tg
cannot be assumed at be the same at T > Tg (°C). If the mechanism of
degradation changes with temperature, Equation 6.2 cannot be used to
estimate the value of Δ E . Typical values for Δ E for unstabilized PP was
reported to be 27 kJ/mol (François-Heude et al., 2014).
Temperature effect is especially pertinent to outdoor exposure of plastics.
Nearly 52% of the sun's energy is in the IR range, with most of it in the
near-IRrange(NIR,700-1200nm).NIRradiationinteractswiththeplastic
increasing its bulk temperature 7 well above that of the surrounding air. The
difference in temperatures between a surface painted with white pigment
(reflects >80% sunlight) and a carbon black dispersion (reflects <5%
sunlight) and exposed to solar radiation outdoors is more than 25°C. 8
Coloration of the plastic therefore significantly affects rates of degradation
because of differences in heat buildup. The thermal conductivity of the
backing material used (such as air, metal, and wood) in samples exposed
also makes a difference in the outcome of weathering experiments because
the temperature of samples depend on the backing.
6.2.4 Wavelength-Dependent Photodamage
The efficiency of photodegradation is inversely proportional to the
wavelength of absorbed light; in sunlight, the solar UV-B wavelengths
(290-315 nm) are therefore the most damaging radiation 9 followed by
UV-A. Wavelength sensitivity information for deterioration of selected
relevant properties (such as yellowing or changes in tensile strength) for
common plastic resins and some of their common formulations have been
reported (Andrady, 1997; Torikai and Hasegawa, 1998). These are plots
of the efficiency of photodamage process (the extent of selected modes of
damage per mole of photons incident on the sample) when the plastic is
exposed to monochromatic light) versus the wavelength, λ (nm) (Andrady,
1997). An example of a wavelength sensitivity plot (also called an action
spectrum) for light-induced yellowing of lignocellulose or mechanical pulp
is shown in Figure 6.5 . The data were generated by exposing the plastic to