UV cure epoxy adhesives rely on a fundamentally different chemical reaction than traditional one and two-part epoxy systems. This unique chemistry allows adhesive and sealant formulators to create some very interesting products from both a design and assembly-efficiency perspective.
Whereas traditional epoxies generally rely on the addition of amine species (and sometimes heat) to drive the epoxy polymerization, a true UV epoxy will nearly instantly polymerize in the presence of a UV activated acid. This cationic reaction not only allows for more efficient (less costly) assembly processes than traditional epoxies, but also results in polymer products with very useful physical/ chemical characteristics.
This post builds on several previous articles that touch on cationic UV cure epoxy products and aims to comprehensively address cationic polymerization/ polymer products. Specifically we'll answer the following questions:
- What are the different types of UV cure epoxies?
- How does cationic polymerization work?
- How do cationic epoxies compare to traditional epoxies?
- How do cationic epoxies compare to free radical systems?
What Are The Different Types Of UV Cure Epoxies?
Something that we have to immediately explain is that the term "UV cure epoxy" could really apply to two different types of products:
- UV Acrylate-Epoxy Composites - These systems are comprised of a UV acrylate adhesive mixed into the resin side of a two-part epoxy. These mixtures will not reach 100% polymerization with exposure to UV light alone, however, due to the acrylate functionality the resin will advance enough to "gel" which will eliminate any flow or further movement. Although these types of products might be described as "UV cure epoxies," the epoxy reaction isn't actually influenced by the UV exposure at all (only the acrylate is UV sensitive). For our purposes, we will not considered these to be true "UV cure epoxies."
- Cationic Epoxies - Epoxy resins can be formulated with photo-acid generators (also known simply as photoinitiators) to form true UV cure epoxies. When exposed to the correct radiation wavelengths the photo-acid generator will convert into a strong acid that drives the epoxy in a chain-transfer polymerization reaction. Like acrylate-epoxy composites, cationic epoxies can be formulated first UV gel and finish polymerization with a secondary heat-cure. However, the polymerization reaction that a cationic epoxy undergoes is fundamentally different than both a traditional epoxy and the free radical polymerization of a UV acrylate system.
How Does Cationic Polymerization Work?
UV cationic polymerization breaks down into two main components:
- Initiation - As we have already indicated, UV cure epoxy products are formulated with photo-sensitive chemicals that will convert into acids with UV light exposure. This acid chemically attacks nearby epoxy molecules, changing them to become highly reactive positively charged cations, and initiating the subsequent polymerization reaction. Cationic initiation parallels UV acrylate initiation which involves a photoiniator releasing a free radical (instead of creating an acid) which subsequently drives the free radical polymerization. However, it is important to note that free radicals are so highly reactive that they only last several seconds, whereas the photo-generated acids of cationic epoxies can potentially last several days. We'll touch on the importance of this difference later.
- Propagation - Once the reaction has been initiated and several molecules within the resin hold a positive charge, these epoxy cations will spread (or propagate) the reaction. These highly reactive cationic epoxy molecules will react with nearby neutral epoxy molecules, combining the two epoxy chains while still holding the positive charge. Because the positive charge is not lost, this now larger epoxy cation can undergo yet another chain-extending reaction, and another, and another, until all the epoxy molecules have been combined into a macro-molecule polymer. In this way cationic epoxies naturally grow once initiated, and thus are occasionally referred to as "living polymers." Below you can see a schematic of this epoxy propagation process.
How Do Cationic Epoxies Compare To Traditional Epoxies?
Cationic vs. Traditional Epoxy - The Pros
- Cationic epoxies provide physical and chemical properties similar to tradational epoxies - Because cationic epoxies are still built using an epoxy backbone, the resulting polymer products are able to achieve the exceptional chemical and physical resistant properties epoxies are known for.
- UV cure epoxy products allow for a much simpler and faster (less expensive) assembly processes - The biggest difference a manufacturer will see in a cationic epoxy vs. a traditional epoxy is that the assembly efficiency is drastically increased due to the fast UV curing process. Cationic epoxies are normally able to achieve sufficient polymerization within 60 seconds, compared to 1 hour with a heat cure system, or 24 hours with a two-part room temperature cure system. And of course we all know the very simple equation Time = Money... The graphic below is taken from a section in our UV Adhesives & Sealants Infographic and outlines some of the different ways a UV process can reduce overall production costs. Our Ultimate UV Resin Guide explores this concept even further and even compares a few real-life examples.
Cationic vs. Traditional Epoxy - The Cons
- UV cure epoxy products can not bond opaque substrates - If UV light can not reach the photoinitiator species either due to opaque substrates or odd product geometries, the cationic epoxy will simply never cure. There are however ways to work around this issue which we will explain later.
- Many of the cost reductions referred to above are only realized in large scale processes - For small manufacturing and assembly processes, for example, making the upfront investment in UV lights and equipment may be prohibitively expensive. In these cases a two-part room temperature curing epoxy may be best.
- Cationic cure is negatively affected by high humidity which will slow the reaction speed - At 87% relatively humidity a dry air purge could improve cure speed by up to 50%. Cationic polymerization will occasionally require a moisture controlled environment.
- Amines and other basic materials have the potential to neutralize the cationic species, resulting in poor polymerization - Equipment that has been used to process amines needs to be cleaned thoroughly or risk poisoning the cationic resin.
- Substrates must be free of nitrogen groups - This means urethane substrates are out of the question.
- Certain fillers also have the potential to retard the polymerization reaction - This limits the range of properties available to cationic epoxies when compared to more traditional systems (thermally and electrically conductive epoxies rely on high filler concentrations).
How Do Cationic Epoxies Compare To Free Radical Systems?
Cationic vs. Free Radical Polymerization - The Pros
- UV cure epoxy adhesives tend to display superior adhesion and reliability - This is mainly a result of the tougher epoxy backbone.
- UV epoxies can be formulated to exhibit substantially less shrinkage than acrylate alternatives - Acrylate products tend to significantly shrink during cure: 10-15%, compared to 1-5% for cationic epoxies. Less shrinkage means less internal stress and better overall adhesion.
- Cationic epoxies do not experience oxygen inhibition - Free radicals can be quenched by airborne oxygen reducing polymerization at air exposed surfaces. Cationic species are not affected by oxygen in this way.
- Cationic epoxies are able to achieve a "dark cure" - As we mentioned earlier, cationic species can exist for several days whereas free radicals normally only last several moments. This implies that UV light is only needed to activate the cationic reaction, not finish it. In practice this means cationic epoxies can be used to bond opaque substrates: 1) the reaction is initiated by UV light exposure, 2) the epoxy is sandwiched between two opaque substrates as the reaction is just beginning, 3) the reaction finishes at the dark interface.
- UV curable epoxies are able to achieve a small degree of "shadow curing" - Another implication of the extended life of cationic species is that they can exist long enough to migrate into sections that weren't originally exposed to UV radiation. This allows the cationic polymerization to creep into shadowed areas (highly geometry dependent).
- Some cationic epoxies can be thermally post-cured to ensure complete polymerization - Many cationic photoinitiators are sensitive to heat in the same way that they are sensitive to UV radiation. Heat will also induce the photointiator to release a super acid that begins the cationic reaction.
Cationic vs. Free Radicaly Polymerization - The Cons
- Free radical polymerization is extremely fast, even faster than cationic polymerization - Some acrylate polymers can cure in 1-5 seconds or less, whereas UV cure epoxy products normally take around 60 seconds to achieve sufficient polymerization.
- Free radical products can be formulated to provide a wider range of properties than cationic epoxies - free radical chemistry is fundamentally more versatile and polymer formulators have more monomer, oligomer and photoinitiator options at their disposal as they create free radical curing products.
- Although free radicals are quenched by airborne oxygen, they are not sensitive to moisture like cationic species are - High relative humidity will further slow the reaction speed of cationic systems in comparison to free radical alternatives.
In conclusion, UV cure epoxy products are very interesting and fundamentally unique in the adhesives and sealant world. Some of the properties that set cationic epoxies apart are:
- Physical and chemical properties comparable to traditional epoxies
- UV cure capabilities similar but slightly slower than free radical curing systems
- Resistance to oxygen inhibition, but sensitive to airborne moisture
- Post thermal cure capabilities
- Dark cure capabilities
- Shadow cure capabilities
- Low levels of shrinkage during cure