Cationic epoxies offer a unique set of advantages: they're UV curable, but aren't inhibited by oxygen like free radical alternatives. However, cationic epoxies have their own airborne weakness: moisture. Fortunately, several methods exist to prevent and even reverse cationic moisture inhibition.
This article aims to address these methods, and specifically answers the following questions:
- Why does moisture inhibition occur?
- What are some practical ways to reduce or reverse moisture inhibition?
Why Does Moisture Inhibition Occur?
You may be familiar with the basics of free radical polymerization and oxygen inhibition. Most UV curable adhesives and sealants are formulated to cure via free radical polymerization in which photoinitiators produce free radical species in the presence of UV light. These radicals then initiate a chain growth reaction in which radicals grow larger and larger by reacting with nearby monomers. Radicals, however, are highly reactive and may react with airborne molecular oxygen, somewhat poisoning the reaction. This is known as oxygen inhibition.
Cationic moisture inhibition is similar, but also somewhat different.
Cationic polymerization begins in a similar fashion. Photoinitiators within the polymer formulation produce a Brönsted or Lewis acid (instead of a free radical). A Brönsted or Lewis acid is a proton donator and will give a proton to nearby epoxy monomers, creating epoxy cations. These epoxy cations will attack other epoxy monomers, continually extending the epoxy chain in process similar to the schematic below
However, because water is nucleophilic (electron donator) it can temporarily neutralize the proton donating acid. As a result, cationic polymerization can be substantially slowed.
Fortunately, unlike oxygen inhibition, moisture inhibition will not change the ultimate properties of the polymer. Moisture may slow, or even temporarily stop the polymerization process. But, once the moisture has been removed, the reaction can continue.
A RadTech report describes an experiment in which a cationic reaction is completely halted in a saturated environment. However, when the system is purged with dry nitrogen the moisture inhibition is observed to reverse, and the cationic reaction is able to complete.
This reversible reaction mechanism is mainly a result of Brönsted/ Lewis acids being more stable than free radicals. As we have already explained, free radicals are highly reactive species and normally will only exist for a few moments. Brönsted/ Lewis acids, on the other hand, can last for up to several days, which is also why cationic epoxies are able to shadow and dark cure.
What Are Some Practical Ways To Reduce Or Reverse Moisture Inhibition?
- Creating a moisture controlled environment - Ventilation systems can be utilized to create a moisture controlled environment. This method might be useful in manufacturing facilities that experience a wide range of humidity levels either due to location or seasonal changes.
- Purging with dry nitrogen - A dry nitrogen purge is a great way to quickly reduce moisture inhibition. At 87% relatively humidity a dry air purge could improve cure speed by up to 50%. This method can also be used to prevent oxygen inhibition of free radical systems. This method actually not only prevents but also reverses oxygen inhibition.
- Utilizing a UV lamp that produces heat - Many cationic photoinitiators are not only light sensitive but heat sensitive as well (dual curable). As a result utilizing a UV lamp that produces heat will help to increase the cure speed and offset the speed reduction that results from moisture inhibition. Utilizing a heat sensitive photoinitiator will also allow for a secondary heat cure that can ensure complete polymerization.
- Trying a different formulation - Some cationic epoxies are affected by moisture much more significantly than others. Epoxide-acrylate hybrids, for example, display low levels of both moisture and oxygen inhibition. These systems utilize both a free radical and a cationic cure, and the resulting system is much more resistant to environmental conditions.