Chemistry Reference
In-Depth Information
decrease the rate of phototransformation. Great importance acquires determination of initial
photochemical processes to solve the problems of photodestruction of dyed fibrous materials.
Primary photochemical reactions are accompanied by dark processes. Products being formed
during dark and photochemical reactions may be subjected to further transformations. Hence
it follows that influencing directly both photochemical stages and dark processes one may
protect polymer from light ageing.
Such characteristics as compatibility with polymer, volatility, chemical stability under
conditions of processing and usage become very important during selection of light-stabilizer.
It should be noted that efficiency of additives action depends on the testing conditions
(intensity and spectral composition of the irradiation source, humidity, concentration of
additives and so on). This is connected with the fact that stabilizers may at the same time act
according to several mechanisms, but only one of them is realized depending on testing
conditions.
Japanese development engineers have shown that fibres, reflecting ultra-violet rays [262]
are obtained during impregnation of polymer, containing finely grinded powder of ZnO
2
,
MgO, BaSO
4
or MgCO
3
, by butadiene and chloroprene solution. Another method of PETP
light stabilization is introduction of 0,1-2% of finely dispersed inorganic particles [average
diameter
≤
0,01-0,05 nm]. Fibres being obtained have improved light stability [263], [264].
Triphenylphosphate, igranox, diglycol phosphate, trimethyl borate [265], benzotriazole
[266] are used as light stabilizers. Fibres modified by such substances exceed goods from
compositions produced by traditional method to a far greater extent as to their resistance to
ultra-violet irradiation.
In the work [267] ultra-violet absorbers, black and mixture of TiO
2
with BaSO
4
were
used as light stabilizers. Obtained PETP samples were subjected to the action of light-weather
during 1100 hours. Results of the light effect on modified PETP-films were analysed by ultra-
violet spectroscopy, titration, chromatography and intrared spectroscopy. By these methods it
has been found out that among above-mentioned stabilizers the most effective is ultra-violet
absorber.
Good light stabilizers, used in the industry, are o-oxybezophenones [268]. Probably
mechanism of their light stabilizing action is in the fast tautomerism of excited states of
molecule
H
H
H
H
O
C
O
C
O
C
O
C
+
O
O
O
O
hv
Hydrogen atom of OH group, situated in ortho-position, forms hydrogen bond with
oxygen atoms of carbonyl group, thereby creating favourable conditions for photoenolization
or proton transfer. Stabilizing action of o-oxybenzophenones increases because of this fact.
The more labile is H-O bond in a molecule, the higher is the efficiency of their action.
Authors of the work [269], while studying mechanism of the action of benzophenones
derivatives, proved the well-known conclusion [10] that light stabilizing action of these
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