Effects of Oxygen Inhibition in Acrylic and Non-Acrylic Free Radical Systems

The adverse effect of oxygen on the curing of acrylic systems is well known and referred
to as oxygen inhibition. Molecular oxygen has, due to its ground state "biradical" nature, a
high reactivity towards radical species which yields peroxy radicals of rather low reactivity
in respect of initiating capacity. An effectively propagating radical, e.g. an initiator radical
or a growing polymer chain radical, is thus scavenged by oxygen resulting in an
ineffectively propagating radical. General kinetic mechanisms for the generation of free
radicals and termination by oxygen have been postulated(1,2) and a comprehensive study
using real time methods has recently been presented(3). The result of oxygen inhibition is
observed as an induction period, a decreased rate of polymerization and a reduction of the
surface properties(2). The molecular oxygen arises from dissolved oxygen in the
formulation and from oxygen diffusion through the surface of the film from the
surrounding curing atmosphere(3). The polymerization will not commence until the
monomer radicals can compete efficiently with oxygen for the initiator radicals(2).
A steady state will eventually be reached where the consumption of oxygen is in equilibrium
with the diffusion of oxygen into the film. Several routes to reduce the effects of oxygen
inhibition have been presented(4,5). These include eliminating oxygen through nitrogen
blanketing (most effective way), addition of waxes to the formulation (6), addition of a
hydrogen donor compound such as a tertiary amine which consumes the peroxy radicals
while concurrently yielding active free radicals or lastly, by increasing the cure speed
through the application of higher light intensities or increased photoinitiator levels(5).

Free radical systems based on vinyl ethers (possessing electron-rich double bonds) and
maleate or fumarate unsaturated polyesters (with highly electron-deficient double bonds)
have been developed(7). These systems are interesting alternatives to acrylate based