STABILIZATION AND MODIFICATION OF POLYCAPROAMIDE - Photochemical Reactions in Heterochain Polymers - page 32

greatest stabilizing effect is found, as it has been observed earlier, in the compound XLJ,
containing oxygen in a bridge group.
It should be noted that when me increase concentration of the additive, stabilizing effect
of bis-aroilenbenzimidazole also increases (Figure1.10). Since compounds XL_XLYI absorb
active light contribution of the effect of shielding into the total effect of stabilization of bis-
(1',8' -benzoilen - 1,2 - benzimidazoles) and bis- 1',8' - naphthoilen - 1,2 -
benzimidazoles) has been estimated.
Figure 1. 10. Dependence of the rate of oxygen absorption by PCA fibres, containing compoundl XLY,
on the additive concentration.
Values of extinction and shielding coefficients on the wave length of active light (365
nm) were calculated according to the spectra of PCA solutions absorption. The polymer itself
absorbs very little in this region.
When comparing coefficients of photooxidation retardation, defined as relations of
stationary rates of oxygen absorption by the polymer in the absence of (W o ) and presence
(W st ) of the additive (Table 9) and coefficients of shielding (iφ), obtained on the basis of
spectral characteristics of dyed PCA it is seen that stabilizing effect, achieved only by
additives shielding is less than observed stabilization effect, obtained during using bis-
aroilenbenzimidazoles. Hence, XL-XLYI being investigated protect PCA not only according
to shielding mechanism, as the value of (W o /W st · iφ) is much more than one.
If protective action of the additive is completely brought to shielding, then W o /W st · iφ=1.
If W o /W st · iφ <1 - the additive acts as photosensitizer; if W o /W st · iφ>1 - additional effect of
super-shielding is found here.
Other possible mechanisms of PCA light stabilization may be: suppression of PCA
excited states and inhibition of radical processes. That is why, possibility of compounds XL-
XLYI participation in the process of deactivation of PCA excited states should be considered.