Price: 10.00
INTRODUCTION
Most materials capable of photopolymerizatiàn contain initiators
which are sensitive only to light at a wavelength below 430 nm.
These materials can be processed without difficulty under daylight,
conditions, of course; however, a portion of the radiation emitted
in'the visible range of the usual types of light sources (e. g.
high pressure mercury vapour lamps) is not utilized. The
application of laser technology, for example, for the manufacture
of printing plates without film by direct computer control of
lasers requires initiator systems which are sensitive to
wavelengths above 400 nm. Initiators of this kind are likewise
favourable for photohardening of pigmented, fibreglass-reinforced
or otherwise filled resins thick layers (thicker than 0.5 cm).
The use of dyes as light-absorbing components in photoinitiator
systems potentially permits the initiation .of polymerization by
means of visible light. Dyes exhibitvery high extinction
coefficients. The initiator systems can be adjusted to specified
sensitivity ranges in a simple manner by varying the dyestuffs.
Numerous dyes are soluble in water, and thus allow processing in
aqueous solutions or suspensions.
A wide variety of photoinitaiator systems which contain dyes have
been described in the literature 1-3); nevertheless, systems of
this kind-have not yet received widespreadattention as far as
commercial utilization is concerned.
Dyes frequently contained in these systems are heteroatom
containing anthracene dyes, e.g. acridine, xanthene, phenazine or
thiazine dyes, also cyanine dyes and ketocoumarins (flgure 1).
Polymerization is initiated exclusively by a radical
mechanism. The radicals are generated by electron transfer
processes between excited dye molecules and so-called
coinitiators. Two fundamental pathways are possible;
photoreduction of dyes by'electron donors, on the one hand, and
photooxidation of dyes by electron acceptors, on the other hand
(figure 2).
