STABILIZATION AND MODIFICATION OF CELLULOSE DIACETATE - Photochemical Reactions in Heterochain Polymers - page 62

As it is seen from Figure 2.6, experimental data according to post-effect are well
described by the equation (1) at the values: ξ=5,3; K n [RH]/K r = 0,9·10 -5 mole/kg; τ st =300c
and W sg =1,69·10 mole/kg·s. Correspondence of obtained data with kinetic law of
photochemical aftereffect at quadratic break of oxidation chains with equation (1) shows that
chains break at CDA oxidation is quadratic.

2.5. L IGHT S TABILIZATION OF CDA BY H EXAAZOCYCLANES

It has been noted that CDA photolysis proceeds with participation of two active centres,
order 1,5 according to light intensity for the rates of formation of acetic acid and polyene
radicals, that gives the possibility to determine mechanism of CC action.
Since inhibitor of radical processes are low-effective light stabilizers of CA, then it is
necessary to search for compounds with large values of extinction coefficient as CA light
stabilizers. For this purpose we have used hexaazocyclanes (HAC), which have ε=1-
10·10 4 l/mole·s over the range of λ=200-350 nm.
Hexaazocyclanes (HAC), having unfolded conjugated chain, high photostability and
thermal stability, were investigated as CC, and presence of chromophoric groups assumed to
use them as dyes for polymers.
Chemical structures of chosen HAC differ in degree of conjugation and flatness. For
example, HAC on the basis of phthalodinitrile and n-phenyldiamine (HAC - XLIX) - is
conjugated nonflat system [167]. HAC on the basis of phthalodinitrile and 9,9 - bis (4
diamine) phlourene (HAC - LII) is nonconjugated cycle.
We have also investigated substances (HAC - LVII), (HAC - LI), being the products of
condensation of phthalodinitrile with para-phenylene diamine in order to examine the effect
of macrocyclic stabilizer structure on HAC properties.
Light-stabilizers, introduced into polymer, in general case may simultaneously act
according to several mechanisms: ultra-violet shielding, inhibition; and in some cases display
undesirable effect of photosensibilization.
Taking into account coefficient i φ , efficiency of stabilizer A was defined according to the
following equation
А = W 0 / (W st ·i φ ) (4)

If the additive acts only as ultraviolet shield then A=1. In the case when the additive acts
as sensitizer, then A<1. And if А>1 then effect of overshielding.
Value A for all investigated light stabilizers is less than 1. Hence it follows that HAC acts
not only as ultraviolet shields, but as photosensitizer too. It is seen from Table 13A that the
best light stabilizers are HAC-1, P-3 and HAC-4, for which value A is the highest.
Introduction of HAC inhibits the process of photomechanical CDA destruction by 5,1-5,9
times.
It should be noted that HAC-1 does not prevent oxidation after CDA irradiation. Taking
into account coefficient of shielding in the case of stabilized CDA films it has been found,
that kinetics of O 2 absorption after irradiation is described by equation (1) at
K n [RH]/K r =0,9·10 -5 mole/kg and the rest of parameters are the same, as in the case of initial