J Polym Environ
Fig. 11 a Photo micrograph of CA film with TiO2 pigments
irradiated in a weatherometer with near UV light. b Scanning
Electron Microscopy (SEM) of acetate fibers from cigarette filters
after few months outdoor exposure
ultrafine titanium dioxide in fibers and demonstrated the
enhanced degradation in the accelerated test and under
outdoor exposure conditions. In the humid and sunny climate of South Florida a weight loss of 65% for acetate
cigarette filters (without wrapping paper) could be
achieved within 6 months exposure using 0.7% ultrafine
titanium dioxide in the fibers, compared to 17% for the
standard material [70, 71].
Itoh et al. have also evaluated the use of TiO2 to enhance
CA degradation with a DS\2.15 adding TiO2 with particle
sizes 0.01 to 1 lm which will provide more degradable
material [72]. In US Patent 5,804,296, Itoh et al. disclose
the benefits of CA fibers with high surface area TiO2
additives, (C50 m2/g) that provide enhanced photo degradation rates [73].
Mechanism Synergy and Disintegration
In the outdoors environment, the degradation of CA
materials can include multiple mechanisms. Deacetylation
seems to be the first step in both, photochemical and biological degradation [69]. In cases where the DS is too high
for esterases, the first step of biodegradation can be initiated by UV light irradiation or by chemical hydrolysis,
with the latter being subject to pH change. Also in already
partly hydrolyzed CA, like commercial acetate with a DS
of 2.5, the critical deacetylation step can be accelerated by
non-biological mechanisms under the influence of sunlight,
temperature and water. With decreasing DS there is a better
chance for biological systems to attack CA, especially if
the DS reaches a region of about 1.5 to 1.8. Acetyl esterases, designed by nature for acetylxylan and chitin biodegradation, are the first enzymes which remove the acetyl
substituents until a DS *1 is reached. When the DS
approaches a region B0.9, CA becomes water-soluble and
the polymer readily disperses [2]. In addition, at this point
CA becomes available to endoglucanases, shortening the
CA chain in a random fashion preferably at those regions
where acetyl substituents do not hinder the contact to the
substrate. A synergistic process between acetylesterases
and endoglucanases starts with the result of a quick liberation of free sugars and acetic acid.
Since deacetylation is the rate determining step in biodegradation of CA, followed by the comparatively faster
enzymatic hydrolysis of the cellulose backbone, the overall
DS of the remaining material will stay nearly unchanged
during degradation. But the specific surface area of the
material increases with ongoing degradation, due to pitting
caused by different rates of degradation at different spots
depending on local conditions at micro scale. This
increase