UV curing adhesives use ultraviolet (UV) light or other radiation sources to initiate curing and form a permanent bond without heating.

Ultraviolet (UV) or visible light cures or polymerizes a combination of monomers and oligomers in a UV adhesive. UV adhesives are also known as radiation curing or radcure adhesives because UV is a radiant energy sources.

Electron beam curable adhesives are another type of radiation curable adhesive. The light or radiation sources used in the UV or visible light curing process include medium pressure mercury lamps, pulsed xenon lamps, LEDs or lasers.

UV curable adhesives consist of monomers, oligomers, photoinitiators, additives, and sometimes UV transparent fillers. The UV photoinitiator component generates free radicals or actions when exposed to UV light, which initiate crosslinking between the unsaturation sites in the monomers or oligomers.

Benefits of UV Curable Adhesives

The ability of UV adhesives to rapidly cure without heat provides energy savings and productivity gains. Energy savings occurs because capital expense and production floor space required for the costly ovens required for thermally curing adhesives is eliminated. Thermally cured adhesives can take hours to cure compared to UV adhesive, which can cure in seconds to minutes.

UV curing adhesives can be “100% solids”, which means all of adhesive is converted to solidified bond joint. As a result, no water or solvents are released to the environment, which is better for the environment (no emissions) and safety (no flammable fumes) as well as water or solvent sensitive parts.

Material

They are compatible with substrates made of the following materials:

• Ceramic
• Composite material
• Concrete
• Fabric
• Glass
• Metal
• Paper
• Plastic
• Rubber
• Wood

UV curing adhesives use various chemical systems, most of which are polymer-based. Choices include:

• Acrylic
• Epoxy polybutadiene (EP)
• Polyester
• Silicone
• Styrene copolymer
• Vinyl

Some UV curing adhesives contain additional modifiers such as fillers, pigments, or chopped fiber reinforcements. Others contain solvent-based adhesive resins which use a volatile organic compound (VOC) to thin or alter viscosity. Typically, solvent-based UV curing adhesives pose greater environmental or regulatory issues.

Video Credit: Ellsworth Adhesives /You Tube

Specifications

Physical Properties

Processing and physical properties include gap fill, the spacing or gap between the substrate and adhesive or sealant; and viscosity, a measurement of a fluid’s resistance to flow.

Cure time is another important property to consider. With UV curing adhesives, setting or curing time is the time required for fully curing or setting a bond system. In thermosetting, hydraulically or other chemically-setting systems, the time will vary depending on the actual curing temperature. In UV curing adhesives, however, cure time can be quite rapid.

Some styles of UV curing adhesives bond instantly upon the application of UV light. Others, which are more common, begin tacking when contacted by UV, but still require a given length of time to fully set. Longer cure times will be required for lower-curing temperatures.

Thermal Properties

Use temperature - The range of temperatures to which products can be exposed without the degradation of structural or other required end-use properties.

Thermal conductivity - The linear heat transfer per unit area through a material for a given applied temperature gradient.

Coefficient of thermal expansion (CTE) - The amount of linear expansion or shrinkage that occurs in UV curing adhesives with a change in temperature.

Electrical & Optical Properties

Dielectric strength - The maximum voltage field that UV curing adhesives can withstand before electrical breakdown occurs.

Resistivity - The longitudinal electrical resistance (ohm-cm) of a uniform rod of unit length and unit cross-sectional area.

Dielectric constant - The relative permittivity of a material compared to a vacuum or free space.

The index of refraction - A measure of the speed of light in a material. This is the amount of light transmitted through a material.

Standards

9985979 - This specification covers a family of single component, solvent free, fast setting, acrylic adhesives that cure when exposed to light, either in the visible or ultraviolet range.

MIL-PRF-24793 - This specification covers the requirements of a UV curable liquid adhesive for bonding optical fibers to glass fiber optic splice, connector, and terminus ferrules.

HN 1711 - UV curing glass adhesive

References

Radiation Curable Coatings

The UV & EB Curing Process

Glossary of Terms (.pdf)

Image credits:

LINTEC of America, Inc. | Henkel North America | Permabond