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.