Cure Terms Glossary
Oxygen inhibition occurs when dissolved oxygen can act as a comonomer in a free-radical cure. Addition of oxygen to the growing polymer radical results in the formation of a relatively inactive peroxy radical.
Oxygen inhibition is a potential problem of cures conducted in the open air. It can be seen as an incomplete surface cure on the surface or an undercure of coatings. It is particularly prevalent in systems lacking in the active hydrogen which is able to destroy the peroxy radicals.
Allylic (C=CCH2) hydrogens are amongst the most active in this respect. Systems lacking allylic hydrogen (and hence prone to oxygen inhibition) include: acrylates, maleates and styrene. The list includes the key components of common unsaturated polyesters (i.e. maleate resins and styrene).
On the other hand, methacrylates contain allylic hydrogen and methacrylates are less susceptible than acrylates to oxygen inhibition. Drying oils also contain allylic hydrogen and these oils provide key ingredients of air-drying alkyds. Allylic comonomers can also be introduced into unsaturated-polyester formulations to counteract oxygen inhibition.
Increasing the allylic content may help resist oxygen inhibition, but it may not help with respect to overall cure activity. In general, the order of reactivity for the C=C functionality in free-radical polymerization generally decreases as:
acrylate > methacrylate > allyl > styrenic > vinyl
Another form of active hydrogen is that adjacent to an ether oxygen group (OCH2) and the incorporation of ether linkages helps minimise oxygen inhibition of acrylate formulations. Ether-functional acrylates, in various guises, are commonly used in UV-curable inks and coatings. Examples include:
dipropylene glycol diacrylate (DPGDA), CH2=CHCO2(CHMeCH2O)2COCH=CH2
tripropylene glycol diacrylate (TPGDA), CH2=CHCO2(CHMeCH2O)3COCH=CH2
Other examples include the products of reaction of a hydroxyalkyl acrylate (e.g. HEMA) with an isocyanate prepolymer based on a polyether polyol. These are so-called acrylated urethanes.
Methacrylates show some susceptibility to oxygen inhibition, and the presence of ether oxygen is beneficial here also. The monomer,
tetraethylene glycol dimethacrylate (TEDMA), CH2=CMeCO2(C2H4O)4COCMeC=CH2
is one used in UV-curable dental resins.
Increasing functionality is yet another tool for combating oxygen inhibition. Acrylic esters of tri- and tetrafunctional alcohols provide monomers with functionalities of six and eight respectively. Suitable levels of such monomers provide scope for fine-tuning the overall functionality - which must be carefully optimised to provide the right balance of flow and cure.
Other approaches to
reducing oxygen inhibition include increasing the cure temperature (to
reduce the solubility of oxygen in mix) or attempting to introduce a physical
barrier to oxygen ingress. A suitable level of filler can provide a something
of a barrier. The incorporation an additive which can bloom to the surface
is another possibility. The addition of paraffin wax, at around the 0.1%
level, serves the latter purpose in unsaturated-polyester resins.