Technical Notes
Low Refractive Index Coating, Adhesive and Encapsulant
n-1.32 - 1.48

Choosing between UV and moisture cured products
UV curing is normally preferred due to the speed of curing and depth of curing that may reach 3mm and even more. However, UV curing is also associated with some drawbacks. The major of which is the need for inerting which results from the competition of oxygen with the polymerization reaction.

Moisture curing requires no UV facilities and is not influenced by oxygen. Its major advantage is the better adhesion, especially to glass, and stronger film properties. It is however limited to thin sections of 1 mm or less and shows slow curing that becomes slower with depth. It also requires open face to allow diffusion of moisture in and the escape of methanol. The MC should be preferred for open face coating applications and where slow curing is permitted. It is suitable for spin coating. Thin coating can be achieved with dilute solutions. Solubility chart is available.

UV curing is fast with the MY-140 series but lower with the MY-133. MY-133 is also more sensitive to oxygen than normal UV formulations because of the low level of cross-linking. Nitrogen blanketing is recommended for all UV formulations and is a must in the case of MY-133.

Curing UV adhesives
Curing can be achieved by any source of UV at 300-400nm. Typically, a dose of 1000-2000 mJ/cm2 is necessary. Higher doses are advantageous. Curing sources of various types can be found. They differ in geometry and irradiated area, energy delivery, wavelength and type of source. In general we recommend the strongest possible source (highest dose rate per unit area). Stronger sources will show less sensitivity to oxygen but inerting may still be necessary.

The MY-140 series (MY-145 to MY-147) as well as MY-1375 can be cured with no inerting but some residual surface tack may be left. In some applications the curing may be carried out between two layers; At least one of which must be transparent to the UV light. Regular glass transmits light only above 330 nm. Our adhesives are normally designed for a wide response in the 300-400nm. Where necessary we can modify the response of the adhesive and shift it up or down.

When curing between two layers, inerting is no longer necessary.

Certain polymer, and particularly silicones, have a high rate of oxygen permeability and are not recommended as barriers against oxygen in the above laminates.

Inerting
Inerting with nitrogen is the most common. If done properly, it is simple and inexpensive.

Inerting with nitrogen requires a semi-closed chamber. Blowing nitrogen directly on the target is not very effective since the stream of nitrogen acts as a Venturi pump and drags air with it. An inerted curing chamber can be built as a box with a transparent window. Quartz plate or a film such as Teflon FEP or Teflon PFA are ideal transparent windows. Polyethylene, polypropylene and other transparent polyolefins are also good but with powerful lamps they may melt down.

For more specific design of your inerting unit, please provide the geometry of your set up and type of lamps and we shall help you with the design.

In certain applications the use of nitrogen can be replaced by covering the adhesive layer with water. Water acts as a barrier to oxygen the same as a polymeric film. This method is particularly suitable with MY-133 series which are highly inert with regard to water.

All our products, when properly cured must have no liquid layer or a tacky layer on top. They must be completely tack free. If they are not, the likelihood is that inerting is not functioning properly or that the dose is not sufficient.

Adhesion Promotion
Most UV adhesive may benefit from proper surface preparation. We have developed special chemical procedures for glassy substrates. In addition we offer simple to use primer solutions.

Adhesion Primers
Adhesion primers are available for MY-133 and MY-132. Other products do not need primers. Different types of primers are offered. Of particular interest is our Primer 133H which has the same low RI of 1.33, same as MY-133. Primer 132H is similarly made to match the refractivity of MY-132 resin.