UV Adhesives - Current Status and Applications

Although UV adhesives have existed for 30 years, they still remain a niche technology within the adhesive field. The goal of this technology is to achieve different adhesive properties from a single adhesive by adjusting the UV dose. This new technology aims to gradually replace solvent-based adhesives.

But what exactly is UV crosslinking? In chemistry, crosslinking refers to the process where polymer chains bind together to form a three-dimensional network. This reaction alters the physical properties of the material, typically making it stiffer, harder, less soluble, and more thermally stable.

What is UV Adhesive?

UV adhesives are adhesives that cure when exposed to ultraviolet light. They are composed of polyacrylate containing benzophenone, which polymerizes under UV light. This photochemical process causes the double bonds in the photoinitiator to break, generating free radicals that immediately react with acrylate molecules. The resulting molecules have longer chains and form the aforementioned three-dimensional network. This crosslinking occurs instantly when the adhesive is applied at temperatures up to 140°C.

UV Acrylic Hot Melt Adhesive

The process uses UVC radiation within the 250-260 nm range. UVC radiation is distinguished from UVA and UVB by its shorter wavelength and its absence in natural sunlight. This is why adhesives crosslinked with this type of radiation are resistant to aging and UV light. UVC radiation is emitted by medium-pressure mercury lamps with typical output power ranging from 120 to 240 W/cm, which are used in the printing ink industry. Due to the free radical reaction, the crosslinking process is very rapid. The selected crosslinking dose controls the degree of crosslinking, which in turn determines the balance between adhesion and cohesion, thereby defining the adhesive's bonding performance.

However, short-wavelength UVC radiation can only penetrate the adhesive layer to a certain depth. The deeper it penetrates, the more radiation is absorbed until none remains, meaning no further crosslinking occurs. Therefore, the layer thickness of UV hot melt adhesive is limited.

Stable Dosage

It is important that the UV dosage remains constant throughout the coating process, which requires continuous monitoring. If the dosage decreases (for example, due to UV lamp aging or dust buildup), the adhesive’s bond strength will increase, but shear resistance will decrease. To maintain consistent bonding performance in such cases, the lamp output power can be increased, or the conveyor speed can be slowed.

Modern integrated lamp systems are equipped with measurement and control technology, allowing for simultaneous measurement of UVC output and dose, and automatic correction based on set values. Instruments such as UVPowerMAP, Power Puck, or UVpad radiometers can be used for these checks.

Measuring UV Dose

The crosslinking reaction requires UVC light within the 250-260 nm short wavelength range. The crosslinking dose controls the balance between adhesion and cohesion, determining the adhesive's bonding performance. The dose itself is determined by the lamp's output power and system speed.

The UV dose is measured using a UV dosimeter, which can detect the UVC radiation crucial for crosslinking. Modern measuring instruments are capable of detecting UVA, UVB, UVC, and UVV radiation as well as their associated doses. Modern integrated lamp systems are now equipped with measurement and control technology that can continuously measure UVC output and dose, and automatically correct them based on the set values.

Depending on the type of measuring instrument, measurements are taken across different bandwidths (spectral ranges). The wider the bandwidth, the higher the energy absorption. Some measuring instruments work only within the 200-280 nm range, and the resulting measurements can be up to seven times higher.

Diverse Applications

UV technology supports a wide range of applications, from ultra-removable to high-adhesion levels, which can be achieved by adjusting the base polymer formulation. UV hot melt adhesives can also be used in humid environments.

Advantages Overview

UV adhesives offer several advantages over traditional adhesives that cure through heat or moisture exposure. These include:

1. UV-crosslinked adhesives cure within seconds, making them ideal for fast coating processes.
2. Curing under UV light ensures high strength and bonding capability.
3. No drying process is needed, so drying equipment is not required in the factory.
4. UV-crosslinked hot melt adhesives are made from thermoplastic polymers and, once cured, can resist a wide range of solvents and corrosive chemicals.
5. They have high heat resistance, tolerating short exposures up to 200°C.
6. High UV resistance also makes them suitable for outdoor applications.
7. They are certified for direct contact with food.
8. They are suitable for applications involving contact with human skin.
9. There is no subsequent crosslinking, as crosslinking only occurs in the UVC range.