Understanding the Utility of UV Curable Adhesives
UV curable adhesives are a family of adhesives with a unique way of curing. Unlike other polymer systems that can take hours or days to cure, UV curable adhesives cure within seconds. This rapid curing facilitates high throughput when processing large batches.
Curing Behavior of UV Adhesives
To understand how to cure a UV adhesive, it is important to be familiar with some of the key terms.
Ultraviolet Light – UV light comprises wavelengths (known as a band or spectrum) from about 100-400 nm. This range includes UVA (315-400 nm), UVB (290-320 nm) and UVC (100-280 nm). Adhesives react at different wavelengths, so it is important to know which band a UV system reacts to. By understanding the specifics of the UV adhesive, you can be sure to use the proper light source.
Irradiance – Irradiance is a measure of the light intensity striking a surface, measured in W/cm2. Higher irradiance lowers cure times, as high intensity radiation activates the system, which begins to crosslink the resin. A simple radiometer measures Irradiance to monitor a lamp’s output over time.
Dose – The time (in seconds) that a UV adhesive undergoes exposure to light is the dose. The unit for dose is J/cm2. The formula below shows the relation between irradiance and dose.
Time required to cure (s) = Dose (J/cm2)/Irradiance (W/cm2)
|
UVA |
UVB |
UVC |
|
|
Irradiance (W/cm2) |
1 |
0.7 |
0.5 |
|
Dose (J/cm2) |
3 |
4 |
6 |
|
Time of exposure (s) |
3 |
5.7 |
12 |
UV Cure Equipment
UV curable adhesives require special light sources to cure the system. Historically, customers used a broadband light source such as a mercury discharge lamp. These light sources emit a wide spectrum of UV light. These lights can be expensive to run because of high energy output.
Within the last few years, light emitting diodes (LEDs) gained a lot of attention as a light source. With their improved power output, LEDs are now practical and can cure UV systems as effectively as broadband sources. Compared to broadband lights, LEDs are far more energy efficient, last longer and can turn on and off without delay.
The wavelength absorbed by the adhesive penetrates different depths of the liquid and effects the depth of cure. Shorter wavelengths like those found in mercury lamps absorb close to the surface of the adhesive, drying the surface. Longer wavelengths like those from LEDs penetrate deeper within the bulk liquid and provide deep section cure. This feature of LED cure systems allows users to apply more material for gap filling or encapsulation.
Figure 1: Shorter wavelengths stay near the surface preventing tack while longer wavelengths penetrate deep into the material
Limitations of UV Curable Adhesives
Because curing initiates with UV light, UV curable adhesives must be visible to light to work. These adhesives cannot bond two opaque materials as they would block the light. UV adhesives can bond dissimilar materials; however, one material must be clear such as glass, polycarbonate or acrylic.
Figure 2: A UV adhesive bonding dissimilar materials
UV adhesives will sometimes be applied around the outer edge of an object to secure it to a surface. This is a common application in circuit board fabrication, bonding components like resistors to circuit boards.
Industry Use of UV Curable Adhesives
Because UV adhesives cure rapidly, they find wide use where there’s a need for high throughput. For example, medical disposable equipment such as catheters and syringes uses UV adhesives to connect tubing.
They also find wide use for bonding screens on phones, laptops and tablets. Optical equipment such as cameras and telescopes use UV adhesives, which offer high clarity and dimensional stability.
MG Chemicals just launched its first UV adhesive, with more products in the development pipeline. If you have an application requiring a UV curable or specialty adhesive, please contact Technical Support for further assistance.
