J Polym Environ
Fig. 9 Scheme of photo
sensitized degradation of
cellulose acetate (CA)
according to [54]
___
Ph2-C=O
(2)
CA
(3)
Fig. 10 Schematic description
of the photo oxidation
mechanism of anatase titanium
dioxide in the semiconductor
model according to [61]
(1)
CA-O-O-H __
(1)
TiO2
hν
___> [Ph2-C=O]T ___ +CA ___> Ph2-C -OH + CA
__ +O2 ____> CA-OO
hν
_______ >
____> CA-O
___+CA ___> CA-OO-H + CA
+ HO
___+CA –H2O ___> CA-OH
TiO2 ( hVB+ + eCB- )
λ <390nm
(2)
e- + TiO2 – O2ads
______ >
TiO2 + O2-.
__+H+___>
(3)
hVB+ + TiO2 - H2Oads
______ >
TiO2 - H+ +
HO.
(4)
CA + HO. / HO2.
the photo oxidation mechanism of anatase titanium dioxide
in the semiconductor model according to [61]. Irradiation
of the anatase crystal with near UV-light (\390 nm)
excites electrons from the valence band in the conductivity
band (1); which could reduce oxygen by electron transfer
(2) and the remaining ‘‘holes’’ in the valence band (h? )
VB
might oxidize water adsorbed on the surface (3) generating
various radicals. These formed radicals might start any
degradation of the surrounding polymer by hydrogen
abstraction with subsequent radical chain reaction of the
polymer, particularly at higher temperatures if the activation energy for the cleavage of peroxides is achieved.
The influence of temperature and humidity on the titanium dioxide catalyzed degradation of acetate yarns was
studied by Egerton et al. [55] who found that higher temperatures and particularly a high humidity will enhance the
degradation. These findings can be understood with the
above mentioned mechanism which includes the temperature required for the cleavage of peroxides in the chain
radical mechanism, the moisture adsorbed on the titanium
dioxide surface to be oxidized, and to avoid the recombination of the excited states.
Degradation catalyzed by titanium dioxide pigments can
be identified with microscopy as shown by the holes in
films, see Fig. 11a, or fibers irradiated in a weatherometer
(see Fig. 11b).
Haynes and coworkers [66] studied the photo degradation of cigarette filters wi Ѡ