J Polym Environ
The analyses of the gaseous products (methane, carbon
monoxide and dioxide) and the analyses of the DS confirmed and supported the interpretation, that the cleavage
of the side groups is playing a major role in the direct
photodegradation. Interestingly, during the irradiation the
degree of substitution decayed initially rapidly up to a level
of 30% bound acetic acid which corresponds to a degree of
substitution of approximately 1 and slows down thereafter,
see Fig. 8.
If the irradiation was performed through a glass window
with near UV-light ([340 nm) only, no ESR signal could
be detected and the fibers were stable and materials
remained nearly unchanged (tensile strength, intrinsic viscosity, weight loss).
The interpretation of the mechanism with the initial
free-radical intermediates was confirmed by Merlin and
Fouassier in 1982 who performed similar ESR studies on
acetate films [56] and compared the resulting spectra with
those from cellulose and similar monomeric reference
substances.
Degradation by Near UV-Light: Use of Photo
Sensitizers
Content of bound acetic acid [%]
Although far-UV light which is able to deteriorate pure
cellulosic materials is not present in normal sunlight, the
use of photosensitizers will allow and enhance the degradation with near UV-light. These photosensitizers are acting either by energy transfer or by initial radical
abstraction.
Endo and coworkers from the Yamagata University
explored the use of different additives, as photo sensitizers
and so called photo acid generators on the photo degradation of cellulose acetates [54, 57–59]. Acetate films
(40 lm thickness) containing different additives in various
60
50
40
30
20
10
0
0
10
20
30
40
50
60
70
Irradiation time [h]
>340 nm
>280 nm
Fig. 8 Decrease of bound acetic acid content of cellulose acetate
fibers irradiated with far (k [ 280 nm) and near UV light
(k [ 340 nm), taken from Fig. 7 in [53] (recalculated from bound
acetic acid with corrections)
concentrations were irradiated in glass vials with a xenon
arc light (k [ 275 nm) simulating solar exposure.
The irradiations of the photosensitizer benzophenone in
concentration of 1–16% embedded in acetate films were
examined with various analytical techniques [54]. The
yield of the decomposition products of CO, CO2, and acetic
acid increased with higher concentration of benzophenone
in the film. The decrease of the polymeric molecular
weight and the increase of the polydispersity indicated the
scission of the glucosic bonds [59]. The authors concluded
that the irradiation starts with the hydrogen abstraction
from the polymer by the excited triplet state in benzophenone, and passes through similar radical intermediates
as by far UV-light (Fig. 9).
Additional additives explored by this group were so
called ‘‘photoacid generators,’’ substances which generate
strong acids after irradiation (triphenylsulphonium trifluoromethanesulfonate (TPS) [57], and diphenyliodonium
trifluoromethynsulfonate (DPI) [58] which were embedded
in CA films and irradiated with UV light (k [ 275 nm) for
the release of sulfonic acid. DPI was more effective than
TPS and contributes to an effective release of acetic acid
after irradiation [57, 59] and could further be enhanced by
the use of benzophenone, which acts as triplet state sensitizer for DPI.
Degradation by Near UV-Light