UV curable epoxies are very unique materials utilizing specialized oligomers and polymers to create fast curing, chemically resistant, highly versatile one-component adhesives. These epoxies are not like traditional two-component or one component heat cure systems, they have a curing mechanism unlike any other adhesive in the market. This cationic mechanism cures through the epoxy-functional group while acrylics cure through a free radical mechanism.
One of the major differences in these two cure mechanisms is that the epoxy continues to cure after the UV light has been removed. This is called the “dark cure” phenomenon. The cationic epoxy reaction is initiated by the UV light and continues to react as much as it can within the matrix until all reactive functional groups are used up. Acrylics on the other hand, only react while they are exposed to UV light. This means if the light is on for a short period of time, the adhesive may not be fully cured while short exposure for a cationic epoxy is only the beginning of the reaction.
The epoxy reaction continues as far as it can at room temperature and then stops. A short post-heat cure from 40°C to 150°C for 60 minutes to 3 minutes is recommended to push the reaction to completion. It is very important to allow these materials to fully cure before testing for ultimate properties. A 24-hour wait before performing physical testing is recommended. Parts can be further processed right away as the epoxy is set, but it is important to keep in mind it may not be fully cured.
A few more important differences between UV epoxy and UV acrylic are:
- Epoxies have no odor.
- Epoxies all cure in safer, long-wavelength UV 320-380nm.
- Epoxies are not inhibited by air when curing, therefore, no sticky surface.
- Epoxies shrink less than 1% with some formulations as low at 0.25% or less.
- Acrylics can shrink anywhere from 2% to 5%.
EMI epoxy formulations are available in a range of hardness and viscosity, Tg and CTE. Custom formulation is available when necessary.
The table below summarizes the differences between UV epoxy and UV acrylic.
| Epoxy | Acrylic | |
| Odor | None | Moderate-High |
| Chemical Resistance | Excellent | Good |
| Solvent Resistance | Excellent | Fair to Good |
| Tensile Strength | Very High | Lower |
| Tensile Shear | Very High | Lower |
| Heat Resistance | Very High | Lower |
| Tg | Can be up to 220°C | Can be up to 125°C |
| Shelf life | 12 months | at 25°F 12 months |
| Dark Cure | Yes | No |
| Air Inhibition | No | Yes |
| Shrinkage | Negligible | High |
| Outgassing | up to 5% Very Low | Much Higher |