Chemistry Reference
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radicals destruction should depend on the thickness of the sample. Constant of the rate should
decrease to the value, characteristic for uncatalyzed recombinations, which is limited by the
relay of valency migration, only in rather thin polymer films (-10
-4
- 10
-5
cm).
Thickness of absorbing layer should be as small as possible, as at 1 → 0 the additive does
not change quantity of light absorbed by polymer, that is effect of light diffusion processes
through the polymer is eliminated. In optically thin layer I
abs.
=2,3ecd.
In order to eliminate diffusion processes during investigation of light-stabilizing action of
hexaazocyclanes, it is necessary to use films with small thickness as model object.
As it is rather difficult to obtain film from PETP, then films, obtained from cellulose
diacetate (CDA), modified by hexaazocyclanes were used as samples.
Cellulose diacetate is related, as PETP, to polyesters, that is why effect of
hexaazocyclanes on light-destruction of ester-groups may be investigated by using this
material.
Photolysis of CDA runs with participation of two active centers: acetylalkyl radicals R
and polyene radicals P', which differ hard in reaction ability. Acetic acid is formed as a result
of CDA photolysis, that is why protective action of the additive may be judged by the
quantity of extracted acid. Kinetic curves of acetic acid formation at irradiation of stabilized
CDA films by the light with wave length of 254 nm. In all cases dependence of the amount of
formed acid on the time of irradiation is a strait line from the tangent of slope angle of which
the rate of acetic acid formation has been calculated. As it is shown in Figure 4.1, HC - 2 has
strong protective effect on the rate of CDA - films photodestruction.
C
ONCLUSION
Thus, obtained results allow to consider that search for effective light-stabilizers should
be carried out among macrocyclic aromatic compounds, possessing high degree of
conjugation and having structure close to plane.
Sulfur- and nitrogen-containing hetrocyclic compounds I-IX, presented on pages, that
quite well absorb photochemically active component of sunlight, owing to the system of
conjugated bonds, have been studied as light-stabilizers. That is why they can “shield” hard
ultra-violet irradiation and also may be “suppressors” of excited molecules of polymer,
absorbed quantum of light. Presence of functional groups with labile hydrogen atom also
assumes the possibility of these compounds action as inhibitors of oxidative destruction
(antioxidants), and presence of chromophore groups shows the possibility of their use as
polymer dyes.
Taking principles of “conjugation” as the basis, investigated compounds may be
symbolically divided into four main groups (see appendex):
A - sulfur-containing heterocycles on the basis of benzo/v/thiophen (I-IX);
B - indan-carbazole-sulfoanilide (X-XXX);
C - Polyconjugated azomethine compounds (PAC) (XXXI-XXXVII);
D - naphthoilenebenzimidazoles and Hexaazocyclanes (XXXVIII - LVII).
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