Optical Adhesives FAQ

NTT-AT provides detailed solutions that meet customers' needs for various types of optical adhesive.

Optical Adhesives FAQ

This FAQ introduces many of the frequently asked questions about our Optical Adhesives List

This FAQ does not necessarily apply for other resins and sealant products. If you have a question about other resins or sealants, please contact us from here.

**Purchase**
Q1	Where can I purchase adhesive products? How can I get a quote?
A1	You can purchase our products from our distributors. Please obtain a quote from one of our distributors. If you are not sure which distributor to contact, please contact us from here.

**SDS** (Safety Data Sheet), **Manual**
Q2	Where can I get SDS and Curing/Storage manual?
A2	You can obtain SDS and manuals from our distributors. If you have more detailed questions, you may contact us from here.

**Sample**
Q3	Can I get samples?
A3	We do not provide free samples, but you can purchase small quantities, 5g or 10g, for evaluation.

**Delivery format**
Q4	What types of containers are used?
A4	Most of our products are packed in brown glass jars.

Q5	Can I purchase adhesive in syringes? Is it possible to place adhesive in a specific syringe that I would provide?
A5	If we pack adhesive in syringes, there is always a risk of leakage, so we do not recommend that method of packing. However, if a customer is willing to take a risk, we can pack in syringes for you. We offer syringes made by Musashi Engineering made in Japan (A product number PSY-EU, no scale indication type. Size ; 3ml, 5ml or 10ml.) Or if you provide and mail your own syringes to us to Japan, we can pack in your syringes. But again, you need to take a risk of leakage and if leakage happens during shipment, NTT-AT does not hold responsibility.

Can I purchase adhesive in syringes? Is it possible to place adhesive in a specific syringe that I would provide?

If we pack adhesive in syringes, there is always a risk of leakage, so we do not recommend that method of packing. However, if a customer is willing to take a risk, we can pack in syringes for you. We offer syringes made by Musashi Engineering made in Japan (A product number PSY-EU, no scale indication type. Size ; 3ml, 5ml or 10ml.) Or if you provide and mail your own syringes to us to Japan, we can pack in your syringes. But again, you need to take a risk of leakage and if leakage happens during shipment, NTT-AT does not hold responsibility.

Q6	Are adhesive products already de-gassed?
A6	We de-gass adhesives before packing and shipping, so you can use them without de-gassing. However, if you decide to de-gass again, we recommend a centrifuge or a vacuum. We do not have a specific time or parameters to recommend, so please follow the instructions recommended by an equipment manufacturer.

**Preprocessing**
Q7	Do I need any type of surface treatment before applying adhesive?
A7	Please make sure there is no debris or oil on surface. If you clean surface by an organic solvent, evaporation of the solvent may lower the temperature and attract water on the surface (condensation), so please air-dry (by air-gun etc.) before applying adhesive. Also, depending on the material of your surface, a coupling agent, ozone treatment or plasma treatment can help improve adhesion strength.

**Curing**
Q8	Are adhesives UV-curable? Or thermal-curable?
A8	Our standard adhesive products on our website and in our brochures are UV-curable. During UV-curing, strains maybe created. After UV curing, a thermal treatment is recommended to remove strains to enhance long term reliability. For more details about curing mechanism differences between epoxies and acrylates, please refer to Q14.

Q9	There are "standard curing conditions" written in the product manual. What are standard curing conditions? Can I modify curing parameters?
A9	The recommended curing parameters written in a product manual are created based on the minimum amount of UV irradiation strength & time length needed to cure adhesive. To minimize the strains created during UV-curing, we recommend a slow curing with minimum irradiation strength. If you need to shorten a curing time, please refer to Q11.

Q10	What kind of ultraviolet curing device is best to use to cure UV-curing adhesives ?
A10	Please use an ultraviolet curing device with a high pressure mercury lamp or metal halide lamp, capable of radiating 365nm wavelength. You can also use LED with a peak at 365 nm. However, LED has a few peaks where the wavelength's total energy generated is lower than with broad range lamps. Thus, to have the same amounts of total energy, you need to adjust the exposure time or irradiation strength with LED (Approximately you need 3 times longer time or 3 times stronger irradiation strength with LED, compared to high pressure mercury lamps or metal halide lamps).

Q10

What kind of ultraviolet curing device is best to use to cure UV-curing adhesives ?

A10

Please use an ultraviolet curing device with a high pressure mercury lamp or metal halide lamp, capable of radiating 365nm wavelength. You can also use LED with a peak at 365 nm. However, LED has a few peaks where the wavelength's total energy generated is lower than with broad range lamps. Thus, to have the same amounts of total energy, you need to adjust the exposure time or irradiation strength with LED (Approximately you need 3 times longer time or 3 times stronger irradiation strength with LED, compared to high pressure mercury lamps or metal halide lamps).

Q11	Can I shorten the curing time?
A11	Slow curing is recommended to avoid strains and stresses, but you can shorten the curing time by gearing up UV irradiation strength. As long as adhesives receive the same amount of total energy, it gets cured. For example, you can cut the curing time by half if you double the UV dosage. However, fast curing tends to create strains & stresses which may potentially lead to cracks and delamination. If curing time must be shortened, please at least start with a recommended UV dosage written in the manual, then apply a step curing which you gear up UV irradiation strength step by step. Or another way is a quick tack on an adhesive surface by a high UV intensity, then move to a next location and take the time for batch curing. Refer to Q16 for more details about "step curing."

Q11

Can I shorten the curing time?

A11

Slow curing is recommended to avoid strains and stresses, but you can shorten the curing time by gearing up UV irradiation strength. As long as adhesives receive the same amount of total energy, it gets cured. For example, you can cut the curing time by half if you double the UV dosage. However, fast curing tends to create strains & stresses which may potentially lead to cracks and delamination. If curing time must be shortened, please at least start with a recommended UV dosage written in the manual, then apply a step curing which you gear up UV irradiation strength step by step. Or another way is a quick tack on an adhesive surface by a high UV intensity, then move to a next location and take the time for batch curing. Refer to Q16 for more details about "step curing."

Q12	Do I need to apply heat? What is a recommended way to apply heat?
A12	We recommend applying heat after UV-curing to improve the long term reliability. If you want to check on bonding ability only, you don't need a heat treatment. During UV-curing, strains occur and the strains can lower the adhesion strength ("delamination") during a long term reliability test. To ease the strains, it is best to apply the heat at Tg temperature or above, however in case you cannot apply heat at Tg temperature, please apply at a temperature specified in our manual. If you do not have a manual, please contact us from here. When you apply heat, please increase the temperature slowly to avoid a rapid change. After the heat process, please bring down the temperature slowly. It is ideal to increase and decrease the temperature by 2°C to 3°C per minute. Please see a recommended temperature curve below. If you need to shorten a heat ramp up time, you may increase the rate of temperature change. However, you should not increase the rate of temperature change when you cool down after the heat process. It is ideal to bring the temperature down slowly to room temperature before removing the part from the oven. If you really need to shorten a cooling down time, please at least wait until it reaches to 40°C, before taking it out from the oven. Temperature Profile

Q12

Do I need to apply heat? What is a recommended way to apply heat?

A12

We recommend applying heat after UV-curing to improve the long term reliability. If you want to check on bonding ability only, you don't need a heat treatment. During UV-curing, strains occur and the strains can lower the adhesion strength ("delamination") during a long term reliability test. To ease the strains, it is best to apply the heat at Tg temperature or above, however in case you cannot apply heat at Tg temperature, please apply at a temperature specified in our manual. If you do not have a manual, please contact us from here.
When you apply heat, please increase the temperature slowly to avoid a rapid change. After the heat process, please bring down the temperature slowly. It is ideal to increase and decrease the temperature by 2°C to 3°C per minute. Please see a recommended temperature curve below. If you need to shorten a heat ramp up time, you may increase the rate of temperature change. However, you should not increase the rate of temperature change when you cool down after the heat process. It is ideal to bring the temperature down slowly to room temperature before removing the part from the oven. If you really need to shorten a cooling down time, please at least wait until it reaches to 40°C, before taking it out from the oven.

Temperature Profile Graph

Temperature Profile

Q13	I have shadowed areas where UV light does not reach. What can I do?
A13	Shadowed area(s) remain uncured which may cause delamination. Please irradiate UV-lights from multiple angles to ensure UV light reaches everywhere on adhesive. But for epoxies, if your shadowed area is very small, the curing effect may reach to the shadowed area, and also the heat treatment after UV-curing can help to cure the small shadowed area (dark curing). Please note that dark curing does not apply to acrylate adhesives. Please refer to Q14 for more details.

Q13

I have shadowed areas where UV light does not reach. What can I do?

A13

Shadowed area(s) remain uncured which may cause delamination. Please irradiate UV-lights from multiple angles to ensure UV light reaches everywhere on adhesive. But for epoxies, if your shadowed area is very small, the curing effect may reach to the shadowed area, and also the heat treatment after UV-curing can help to cure the small shadowed area (dark curing). Please note that dark curing does not apply to acrylate adhesives. Please refer to Q14 for more details.

Q14	What is a mechanism of UV-curing?
A14	Epoxies and acrylate cure differently. Epoxies cure by "cationic polymerization." When epoxy receives UV light, organic acids are generated from the photo-initiator which starts the curing process. Even after UV exposure stops, the organic acids remain and continue curing some un-cured parts of the epoxy. Applying heat after UV exposure helps it as well. On the other hand, acrylates cure by "radical polymerization." When acrylate receives UV light, radicals are generated from the photoinitiator which starts the curing process. When you turn off UV-light, radicals are no longer generated and curing stops all at once. Note ; Radicals react with oxygen. Surfaces of acrylate exposed to the air may not cure. For more details about oxygen inhibition, please refer to Q15.

Q14

What is a mechanism of UV-curing?

A14

Epoxies and acrylate cure differently. Epoxies cure by "cationic polymerization." When epoxy receives UV light, organic acids are generated from the photo-initiator which starts the curing process. Even after UV exposure stops, the organic acids remain and continue curing some un-cured parts of the epoxy. Applying heat after UV exposure helps it as well. On the other hand, acrylates cure by "radical polymerization." When acrylate receives UV light, radicals are generated from the photoinitiator which starts the curing process. When you turn off UV-light, radicals are no longer generated and curing stops all at once. Note ; Radicals react with oxygen. Surfaces of acrylate exposed to the air may not cure. For more details about oxygen inhibition, please refer to Q15.

Q15	I cannot cure acrylate adhesives well, or it remains wet/tacky. What is causing under-curing? How can I cure acrylate adhesives?
A15	If an acrylate adhesive is exposed to air during UV-curing, oxygen in the air inhibits curing process, thus the surfaces which are exposed to the air do not get cured. Inside the acrylate gets cured since it is not exposed to the air. After UV-curing, you may wipe the wetness on surface by organic solvents. Or we recommend to cure in a nitrogen envronment to avoid oxygen inhibition. We also offer epoxy adhesvies. Epoxies do not have an oxygen inhibition.

Q15

I cannot cure acrylate adhesives well, or it remains wet/tacky. What is causing under-curing? How can I cure acrylate adhesives?

A15

If an acrylate adhesive is exposed to air during UV-curing, oxygen in the air inhibits curing process, thus the surfaces which are exposed to the air do not get cured. Inside the acrylate gets cured since it is not exposed to the air. After UV-curing, you may wipe the wetness on surface by organic solvents. Or we recommend to cure in a nitrogen envronment to avoid oxygen inhibition. We also offer epoxy adhesvies. Epoxies do not have an oxygen inhibition.

Q16	How can I do a step curing to shorten a curing time?
A16	Adhesive gets cured as long as it receives the same amount of total energy from UV light. For example, when a recommended curing condition is 100mW/cm² for 5 min, the total energy is 100mW/cm² x 300 sec = 30,000mJ An example of " 5 teps curing" to achieve 30,000mJ, shortening the curing time from 5 min. to 2 min. Step 1; 100mW/cm² x 45 sec (4,500mJ) Step 2; 250mW/cm² x 30 sec (7,500mJ) Step 3; 350mW/cm² x 20 sec (7,000mJ) Step 4: 400mW/cm² x 15 sec (6,000mJ) Step 5: 500mW/cm² x 10 sec (5,000 mJ) Total energy 30,000mJ

Q16

How can I do a step curing to shorten a curing time?

A16

Adhesive gets cured as long as it receives the same amount of total energy from UV light. For example, when a recommended curing condition is 100mW/cm² for 5 min, the total energy is
100mW/cm² x 300 sec = 30,000mJ

An example of " 5 teps curing" to achieve 30,000mJ, shortening the curing time from 5 min. to 2 min.

Step 1; 100mW/cm² x 45 sec (4,500mJ)

Step 2; 250mW/cm² x 30 sec (7,500mJ)

Step 3; 350mW/cm² x 20 sec (7,000mJ)

Step 4: 400mW/cm² x 15 sec (6,000mJ)

Step 5: 500mW/cm² x 10 sec (5,000 mJ)

Total energy 30,000mJ

Q17	I finished a curing process, but I cannot tell whether an adhesive is cured or not. How do I know if the adhesive is fully cured?
A17	After you finish adhering parts, if you move the parts and don't see any misalignment and adhesives are not "jelly-ish," we can say it is cured. Or you can dispense a drop of adhesive on a glass, then cure. Poke the cured adhesive by a needle, and if a needle does not penetrate all the way to the glass, we can say the adhesive is cured. Another way to know if adhesive is cured or not is to check if you are getting enough adhesion strength or enough hardness. Note ; If you cure acrylate adhesive in the air, the surface does not cure and remains wet due to "oxygen inhibition," and you may mistakenly believe that the problem is with the adhesive but actually the problem is that oxgen has inhibited curing. Inside the acrylate, which is not exposed to air, is actually cured.

Q17

I finished a curing process, but I cannot tell whether an adhesive is cured or not. How do I know if the adhesive is fully cured?

A17

After you finish adhering parts, if you move the parts and don't see any misalignment and adhesives are not "jelly-ish," we can say it is cured. Or you can dispense a drop of adhesive on a glass, then cure. Poke the cured adhesive by a needle, and if a needle does not penetrate all the way to the glass, we can say the adhesive is cured. Another way to know if adhesive is cured or not is to check if you are getting enough adhesion strength or enough hardness.
Note ; If you cure acrylate adhesive in the air, the surface does not cure and remains wet due to "oxygen inhibition," and you may mistakenly believe that the problem is with the adhesive but actually the problem is that oxgen has inhibited curing. Inside the acrylate, which is not exposed to air, is actually cured.

**Features**
Q18	Does refractive index have wavelength dependency?
A18	Yes. When you look at our data sheet, please check a refractive index of our adhesive(s) with a specific wavelength that you use. We are equipted to measure wavelengths at Sodium D-line(589 nm), 633nm, 830nm, 1300nm, and 1550 nm. We also offer adhesive products with cusomizable refractive index. If you request a custom refractive index, please let us know your wavelength as well.

Q19	Do you have a reliability test data for high temperature or high humidity?
A19	For our standard adhesive products that are listed on our website and brochures, we have heat and humidity test (121°C/100%) and heat cycle test (-40°C~ 80°C)data available. Please contact us for more details.

Q20	I have a high heat process such as 260°C. Can adhesives tolerate this?
A20	It depends on the adhesive type, how you use it and your design. We selected our several adhesive products that have higher tolerance to high heat and created cured film samples. These were tested at a high temperature (5 minutes on a hot plate preheated to 260C). We checked how refractive index, adhesion strength and transparency changed before and after 260C process. You can see the test result from here. (PDF 220 KB)

Q20

I have a high heat process such as 260°C. Can adhesives tolerate this?

A20

It depends on the adhesive type, how you use it and your design. We selected our several adhesive products that have higher tolerance to high heat and created cured film samples. These were tested at a high temperature (5 minutes on a hot plate preheated to 260C). We checked how refractive index, adhesion strength and transparency changed before and after 260C process. You can see the test result from here. (PDF 220 KB)

**Warranty**
Q21	How long is the shelf life? How should I store product(s)?
A21	The shelf life is 3 months. If you open a bottle, use it, and close it, the shelf life is still 3 months. Each product has different storage requirements. Please refer to the product manual which is included in the shipping box. If you don't have a manual, please contact where you purchased a product from.

Q22	Can I extend the shelf life longer than 3 months?
A22	We do not extend our warranty period. After an expiration date, please use it with your own responsibility. For storing adhesive(s) in a low temperature in a fridge or freezer, please read here for more details. (PDF 184 KB)

Q23	Can I extend a shelf life if I store in a fridge or freezer?
A23	We can expect the extension of quality if a product is stored at a lower temperature. However, it you open it cold, the adhesive may attract and absorb humidity (condensation).

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