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Numerous techniques have been used to mitigate oxygen inhibition. As shown in Table 3.1 these oxygen mitigation techniques offer several methods that work only to a certain degree [6].

Figure 3.11 Chain termination of the free radical initiator (see PI in Figure 3.2.) by oxygen. When the free radical is formed, oxygen in the environment quenches the PI to an unexcited non-reactive state. This quenching lowers the number of polymeric chains and cross-linking within the system. As can be seen in the first step the reaction forms an oxygen based free radical which then seeks another free radical and then chain terminates as shown in the second step. This classically results in the formation of an uncured surface.

Table 3.1 Methods to Mitigate Oxygen Inhibition showing Advantages and Disadvantages. These techniques can include, but are not limited to, the following methods: uses of inert gases, waxes, coatings, increased PI concentration, increased light intensity and use of thiols, amines and ethers.

Method	Advantages	Disadvantages
Inert Gas	Does not adversely affect coating properties	Expensive; difficult to implement
Waxes	Inexpensive	Affects final coating properties; time needed for migration
Films	Good solution when film becomes part of product	Cost/disposal of film when not part of product
Increased PI Concentration	Easy to implement	Increased residuals/ by-products; reduced coating properties
Increased Light Intensity	May not affect coating properties	Part of existing equipment; cost
Thiols	Improved thermal resistance; reduced moisture absorption; improved adhesion	Odor
Amines	Inexpensive; possible improved adhesion	Yellowing upon or after cure; residual odor; moisture sensitivity
Ethers	Can be used in large quantities	Affects coating properties; reduced temperature resistance; possible reduced water resistance

Inert gases work quite well since the coating is in absence of oxygen so no oxygen inhibition can occur. In fact, electron beam curing will only work when done in an inert atmosphere. Paraffinic waxes work well also but complicate the issue with gloss since paraffinic waxes migrate to the surface and are low gloss. Two additional steps are needed to bring the UV cured coating back to its original gloss by first grinding away the wax and then buffing and polishing the surface back to a high gloss. Barrier coatings work but again add an additional step in the process and are problematic on contoured surfaces. Increasing the PI concentration is also an easy fix but it is costly and can reduce the coating properties. Also increasing the UV light intensity can help override the oxygen inhibition issues but could result in coating degradation. Thiolene based acrylate (mercapto acrylate) is currently being used in the UV cure industry but results in ‘post odor’ of the UV cured coating. Amines are also used but result in yellowing and ‘post odor’ of the UV cured coating. Ether acrylate-based chemistries are used but might result in poor performance properties of the cured coating.