FAQ about Optical Adhesives

Heat treatment is recommended for improving reliability. Heat treatment is not required when using only to check adhesion.

During curing, internal stress is generated, which causes a decrease in adhesive strength (exfoliation) during reliability testing. Heat treatment above Tg is required to alleviate the occurrence of internal stress, but the heat treatment conditions are described in the instruction manual in consideration of cases where heating above Tg is difficult. If you need an instruction manual, please contact us using the inquiry form. Raise the temperature slowly during heat treatment to avoid rapid heating, and lower the temperature slowly after heat treatment to avoid rapid cooling. A change of 2-3°C per minute is ideal.

 

Please refer to the figure below (example of temperature rise and fall pattern). If you want to shorten the heat treatment time, you can increase the heating rate. But don't skip the cooling process. Ideally, it should be slowly lowered and allowed to come to room temperature before removal. If you don't have time to bring it back to room temperature, you can remove it from the heater as soon as it cools down to 40°C.

Any shadowed areas will leave uncured adhesive and cause delamination. Consider applying the UV light from different angles so that the UV light reaches all parts of the adhesive, essentially without shadows. However, with epoxy glue, if the shaded area is very small, it will cure to some extent. A post-irradiation heat treatment helps the curing (dark reaction) progress further. (This is not applicable for acrylate adhesives. Please refer to Q14 for details.

Epoxies and acrylates have different curing reactions.

Epoxies undergo cationic polymerization. When exposed to UV light, an organic acid is generated from the photopolymerization initiator and the curing reaction of the epoxy resin occurs. Even if the UV irradiation is stopped, the curing reaction continues because the organic acid remains. After UV irradiation, you can proceed with the curing of the uncured part by heat treatment.

On the other hand, acrylates undergo radical polymerization. When UV light is applied, radicals are generated from the photopolymerization initiator, causing a curing reaction of the acrylic resin. When the UV irradiation is stopped, the generation of radicals stops and the curing reaction also stops. Care must be taken in areas where the resin is in contact with oxygen, as the radicals react with oxygen and hinder the curing of the resin. Please refer to Q15 for measures against oxygen inhibition.

Under standard curing conditions and the same total amount of UV light energy, it will cure.

For example, if the standard curing condition is 100mW/cm ² ×5 minutes, the total energy is
100m/ ^Wcm2 x 300 seconds = 30,000mJ
becomes.
 

<An example of “5-stage step cure” to achieve 30,000 mJ, shortening 5 minutes to 2 minutes>

Step 1: 100mW/ ^cm2 x 45 seconds (4,500mJ)

Step 2: 250mW/ ^cm2 x 30 seconds (7,500mJ)

Step 3: 350mW/cm ² × 20 seconds (7,000mJ)

Step 4: 400mW/cm ² × 15 seconds (6,000mJ)

Step 3: 500mW/cm ² × 10 seconds (5,000mJ)

→Total energy 30,000mJ

If the glass does not move after bonding, it is cured. Or, if you dispense adhesive onto the glass and cure it, and then touch it with a needle, it is cured if it does not penetrate to the glass.
If the glass moves or the adhesive becomes gel-like and shifts, it is not yet cured.

Please be careful when making your decision as acrylate adhesives are anaerobic and do not harden on the surface.

Hardness measurement and adhesion strength measurement are relatively easy methods to check the cured state.

It depends on the type of adhesive, usage and design.

​Click here for data on changes in refractive index, adhesive strength, and transmittance after curing samples of several adhesives with excellent heat resistance after high temperature treatment (left on a hot plate at 260C for 5 minutes) (PDF 403 KB) ) can be viewed.