Troubleshooting UV Curing Adhesives

UV curing adhesives are extremely convenient as they provide a way to rapidly cure a product in specific applications. There are two cure mechanisms that are extremely different from one another – cationic and free radical.

Understanding Cationic UV Systems

In regards to cationic UV cure products, such as Vividcure 86011, photo-generated acids catalyze polymerization of epoxides. Cationic UV systems have less shrinkage and will not experience oxygen inhibition.

See below for a few key points:

Cure Speed – Cure speed is dependent on the thickness, so expect a longer cure time if the layer is thick. You could possibly increase the cure speed time by increasing the movement of the molecules. This can be done by warming the material prior to the curing process.

Skin-Over – In order to increase depth of cure through thick layers, you may need to reduce the intensity of the lamp and cure for longer. When a thick layer’s surface is affected by high-intensity irradiation, only the top will polymerize creating a hard skin that will act as a barrier from further light penetration. This barrier inhibits the depth of cure of the material.

Moisture Sensitivity –Since the photo-initiators in cationic systems are acidic, moisture and bases will neutralize them. We highly recommend against curing cationic systems in humid environments. The cure speed, interestingly, can be increased in 30-60% relative humidity, but will stall at 70% or greater.

Substrate Considerations – As mentioned, bases will neutralize the acid catalysts that induce polymerization of the adhesive. Certain substrates, such as some metals and treated glass, are basic. Substrate poisoning will look like a small, uncured layer against the substrate. This problem can be solved by treating the substrate to neutralize the basic properties or switching out the substrate. Testing with normal curing parameters on a neutral substrate can help determine if the incomplete cure is due to skin-over or poisoning.

Mechanical Properties – While the mechanical properties of cationic cured systems are great, the reaction rate is slower than acrylate systems. The acids created by the photo-initiator during the radiation process will continue to crosslink the adhesive even after the radiation is removed. As such, we advise you measure the mechanical properties 24 hours after cure.

Post-Cure – Time can be reduced for a cationic system to reach full cure through the use of a thermal post-cure. The post-cure speeds up the reaction rate of polymerization.

Stress – As mentioned, compared to free radical systems, cationic systems have noticeably less shrinkage and therefore lower stress. Shrinkage and stress can be reduced by curing at a lower light intensity for a longer period of time.

Understanding Free Radical UV Systems

Curing in seconds rather than minutes, free radical cure systems such as Vivid Cure 71141, are widely known for their rapid cure. Decomposition of the photo-initiator into free radicals by UV light starts a chain reaction curing mechanism that allows free radical cure systems to cure within seconds.

Take a look below at some important traits:

Cure Speed – Once the UV irradiation is complete, free radical cure systems can reach their full degree of cross-linking shortly after. This is due to their significantly quick cure speed. Cure speeds can be slowed down slightly by decreasing light intensity, and similarly increasing light intensity can slightly increase cure.

Oxygen Inhibition – One of the major downfall for free radical cure systems is oxygen inhibition. The growing-chains and photo-initiator radicals can be quenched by the presence of oxygen in the curing environment. This could cause short chain segments which lead to tacky surface layers as well as poor mechanical and physical traits. Using substrates on either side of the adhesive will isolate it from oxygen in the environment making them less susceptible to oxygen inhibition. Higher light intensity induces higher cure speeds that can decrease the effect of inhibition. This is because quicker chain formation permits the polymerization to proceed to completion faster than the quenching can happen. Light intensity can be modified by increasing the power from the light or decreasing the distance between the light and the adhesive.

Light Considerations – Over time lights will not function at 100% efficiency and will not be as bright. As such adhesives that used to cure in a process will start to fail. As such we highly recommend that companies, for quality purposes, measure and record light intensity and power when initially qualifying an adhesive. If power and intensity fall below threshold numbers you know that the light will need to be repaired or replaced.

Photo-initiators – A single photo-initiator cannot absorb all wavelengths of light. Specific photo- initiators that are used in adhesives are tailored towards either LED, metal halide, or mercury lights. We recommend understanding the spectral output of your light to make sure the emitted wavelengths line up with absorbance of the adhesive’s photo-initiator. Your light supplier can provide you with the spectral output of the light.

Post Cure – A thermal post cure will not have a negative effect on free radical systems nor will it benefit them.

Cationic Epoxies – Advantages

Overview

 As discussed in our previous article, cationic and free-radical are the two most popular mechanisms for UV adhesives. Approximately 93% of the UV market is in free radical chemistry  while cationic has about 7%. Although a minority of the market, cationics provide strategic advantages over traditional free-radical acrylate chemistries. These advantages include:

  1. Shadow Cure
  2. Delay Cure
  3. Reduced Shrinkage
  4. No oxygen inhibition
  5. Higher operating temperatures

Shadow Cure

When energy from light is introduced into a cationic system the photoinitiator releases a strong acid. This acid acts as a catalyst starting the polymerization. It is important to note, for the polymerization to occur only the acid is necessary, not the light energy producing the acid. This strong acid has two benefits over a free-radical. The first advantage is that the active life of the acid catalyst is much longer than a free radical. Free radicals’ active life is measured in seconds while the acid can survive for days. The second benefit is that acid catalysts are not consumed in the polymerization reaction whereas free radicals are. These two differences allow cationic reaction to continue curing after UV exposure unlike free-radical reactions.  This is the shadow cure effect. As long as the system has had UV exposure it will continue to cure after the light source is removed.

Delay Cure

The delay cure is a functional use of the shadow cure phenomenon. There are techniques, such as incorporating polyols into epoxy cationic reactions, which play on the shadow cure effect and delay the polymerization of the species. By delaying the polymerization there is a lag time between light exposure and the substance hardening.

Delay cure cationics can provide a unique strategic advantage to manufacturing processes. For example:  a process that requires a one part adhesive to bond two, light blocking substrates. Historically a UV PSA could be used. These free radical adhesives cure instantaneously but when polymerized are partially sticky in nature (think Gluedot or the clear sticky material on the back of a new credit card). Due to the tackiness of these products, they will mate well between two substrates after polymerization. The problem is the PSA will never be a tough polymer. This limits the upper boundary of the adherence strength and may eventually lead to creep. Cationics can solve this problem as the substrates can be mated similarly to a PSA before the adhesive is completely polymerized. Strong controls, however, on UV exposure is paramount so the reaction rate of the cationic is consistent.

Reduced Shrinkage

Large percentages of shrinking can induce stress between bonded substrates. Cationics shrink less than typical free-radical systems. The mechanism behind polymerization causes shrinkage for both free radical and cationic reactions. Unique to cationic reactions, however, is the epoxy ring opening step before molecule to molecule interaction. This step actually lengthens the molecule and can offset the shrinkage caused by polymerization. The ring opening step is unique to cationic epoxy and does not occur in free radical acrylate reactions.

No Oxygen Inhibition

Free radical UV polymerization can be terminated early by oxygen in the air. The oxygen itself will react with the free radical stopping chain initiation and also preventing further chain extension through premature chain termination. This leaves the surface of the material unfinished and tacky. Cationics will not be inhibited in this fashion. This allows them to cure in ambient conditions with better ease than free radical UVs.  It is important to know that humidity can slow the cationic reaction. Relative humidity must be 70% or higher to see this effect. Since the cationic reaction is initiated by an acid, basic molecules or moisture can lessen the effect of the catalyst.  If the moisture level is decreased, however, the reaction will proceed as normal.

Higher Operating Temperatures

Typical free radical acrylate products will not survive temperatures above 120°C and, even if they do, the product could possibly yellow and diminish in physical properties. Cationic epoxies, however, can survive temperatures as high as 200°C. Resin Designs’ Vividcure 86011 is capable of handling extreme operating conditions. Contact us if you have a unique problem that needs a unique solution. Next blog we will discuss UV systems that utilize on a secondary cure mechanism for those applications where UV chemistry is just not enough.