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.

UV Acrylates- Secondary Cure Mechanisms

Overview

In two of our previous articles, “Understanding UV Curing Adhesives” and “Cationic Epoxies- Advantages” we discussed the benefits of single component light curing adhesives. UV/light curing adhesives use energy from visible light or UV radiation to initiate polymerization. However, areas that are not exposed to radiation will not cure. This weakness has led to hybrid systems that allow for dark sectional curing through a secondary mechanism. Hybrid systems come in two forms, those that continue the use of the acrylate chemistry and those that incorporate new chemistries.

Homogeneous chemistry

Light radiation is not the only avenue for creating the free radicals that causes acrylate polymerization. Free radicals can be created using e-beam curing technology or heat.

Electron beam technology, otherwise known as E-beam, uses an electron accelerator to project an electron at its target. Unlike UV or light radiation, no photoiniator is required. This is because a free radical, used in acrylate curing mechanisms, provides an unpaired electron to initiate polymerization while the e-beam directly provides that unpaired electron. E-beam can penetrate certain substrates that would absorb light. The depth of penetration is directly linked to the energy used by the electron accelerator.

Heat can also generate free radicals.  Heat generating initiators (e.g. peroxides) decompose at high temperatures to produce free radicals.  Due to the elevated temperatures, the acrylate polymerization occurs very rapidly. Heat induced reactions prevent dark section cure issues. Heat polymerization can also be used alongside light induced polymerization to guarantee complete and homogeneous curing.

Heterogeneous chemistry

Isocyanates, which is one of the two functional parts of a urethane adhesive, can react with moisture to become polyureas as discussed in the article “Urethanes”. This technique can be used, and combined with, acrylate chemistry to allow dark sectional cure providing that there is moisture present. After the moisture reaction finishes the adhesive will have strong moisture resistance. As such the adhesive will become a barrier where the initial moisture penetration existed. Aside from moisture resistance and dark section curing, another major benefit to using a secondary moisture reaction is improved bond with difficult substrates.

Vividcure 76011 is one part dual cure UV Urethane. This product bonds well to difficult plastics such as santoprene, delrin, polypropylene, nylon, and PBT. It has had success many markets, including automotive.

Understanding UV Curing Adhesives

Often referred to as UV Curing, Ultraviolet Curing utilizes high-intensity ultraviolet light to generate a photochemical reaction which instantly polymerizes adhesives, inks and coatings. The main two types of photochemical reactions used are acrylate free radical and cationic epoxy. UV curing differentiates itself from other traditional drying methods due to its increased production speed, reduced reject rates, enhanced scratch and solvent resistance and expedited superior bonding.

Originally, UV curable inks and coatings were used as a substitute to solvent-based products. Traditional heat and air drying methods work by solvent evaporation. This particular method shrinks the application of coatings by 50% and causes environmental pollutants. However, in UV curing, there are no environmental pollutants, no loss of volume or coating thickness, and reduced stress on the substrate due to shrinkage. This means that there is a higher productivity in a shortened period of time with a reduction in waste, energy, use and pollutants.

What Industries Use UV Curing

UV curing was first introduced in the 1960’s and became increasingly popular among several industries including electronics, graphic arts, automotive, telecommunications and more. This process has since transitioned into a multi-billion dollar worldwide industry, containing about 4% of the industrial coatings market. UV curing has increased over 10% per year, expelling traditional water and solvent-based thermal drying processes. Why has UV curing seen such dramatic growth? The UV curing process causes an astounding increase in productivity, enhancement of product quality, as well as several environmentally-friendly traits as no evaporation of product occurs unlike solvent-based products. Our UV products are 100% solids.

How UV Curing Works

UV adhesives are an all-inclusive, single component adhesive solution. The base polymer and photosensitive initiator are all contained in one package. To make the product, liquid monomers and oligomers are mixed with photoinitiators.

There are two different types of photochemical reactions, free radical and cationic. In free radical reaction, energy from light radiation is absorbed by the photoinitiator. The excited photoinitiator releases a free radical that initiates the polymerization. Cationic reactions are slightly different as the energy absorbed releases a positively charged ion instead of a free radical. During the polymerization of a cationic reaction more positively charged ions are released. This allows the cure to occur after light exposure unlike free radical polymerization. This phenomenon is often called “dark curing”.

The Benefits of UV Curing

UV curing offers an array of benefits including the following:

  • Physical properties are improved
  • Easy to automate
  • Production speeds and capacity are much faster
  • Reduced clean-up labor and set-up
  • Minimal emissions; ensuring a safer work place
  • Less floor space required
  • Reduced scrap

Improved Physical Properties

Manufacturers typically consider UV chemistry due to the improved gloss, abrasion resistance, enhanced chemical resistance, and unique control of properties such as hardness, elasticity and adhesion.

Quick Production Speeds

Since the process requires less space, this allows for higher production speeds as well as less direct labor. The decreased down time and higher throughput increases machine utilization and directly impacts plant capacity. Essentially, UV curing provides increased productivity and enhanced plant and equipment efficiency.

Reduced Clean-Up Labor and Set-Up

Since UV chemistries will not cure without UV energy exposure, they can be put out overnight without concern for the ink or coating drying in the machine. This is typically a huge advantage for graphic arts printing presses, plastic decorating machines and coding machines.

Not as Much Floor Space Needed

UV curing processes generally require considerably less floor space than drying ovens.  Several drying processes require longer cure times which require large amount of floor space. For instance, adhesives and potting applications using two-part adhesives have cure times which is measured in days.  When the number of parts is large, more floor space is necessary.

Reduced Scrap

Curing issues are detected instantly because UV polymerization only takes a matter of seconds, increasing inspection and scrap removal efficiency. Regarding painting and coating applications, the reduced time it takes to UV cure removes the chance for dust and particles to pollute the part surface.  During some processes, there may need to be an inspection of the ink, coating or adhesive application before beginning the UV curing process. If there are issues, the ink, coating or adhesive can be removed easily.

Choose Resin Designs Products

The chemists at Resin Designs offer an array of products, often used for encapsulation or bonding. Two of our most popular urethane acrylate products include the Vivid Cure 71151 and Vivid Cure 71141.

The Vivid Cure 71151 is a water white, non-yellowing adhesive used as an encapsulate for optical applications. The Vivid Cure 71141 is another high-performance adhesive featuring PET & RPET Bonding. Both products are RoHS & REACH Compliant.

Resin Designs also specializes in cationic epoxy formulations. These homopolymers polymerize due to acid generation during UV initiation instead of free radical initiation. This allows for high glass transition temperature polymers and shadow curing effects. Vivid Cure 86011 is an example of a cationic epoxy with an operating temperature beyond 120°C. Our next blog will discuss the benefits of using cationic chemistry verses traditional acrylate chemistry for UV adhesives.

The Various Methods for Curing UV Adhesives

A majority of manufacturers, including Resin Designs, understands that new technology is frequently misunderstood. For example, let’s circle back to 15 years ago when suppliers of LEDs believed that this light source could cure UV adhesives more cost efficiently than mercury bulbs, metal-halide bulbs and electrodeless bulbs. Manufacturers also believed the following:

  • LEDs last extensively longer
  • LEDs produce significantly less heat
  • LEDs are usable in more malleable configurations than standard bulbs

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