A junction box (without connections and bypass diodes) for a photovoltaic module, encapsulated in ELASTOSIL Solar 2120 UV. The new silicone is characterized by fast curing at room temperature. Curing is initiated by UV light and not, as with conventional silicone elastomers, by heat.

By Christian Ochs

The safety and reliability of a PV system depend on the sensitive electrical components being protected against weathering. Weathering protection is also required for the junction box mounted on the reverse side of each module as an interface between it and the inverter. All the conductors and cables from the solar cells meet up in the junction box. It also contains the bypass diodes required for reliable module operation. The output from the junction box is the DC electricity generated by the solar cells in the module.

The bypass diodes are not the only sensitive components in the junction box that require protection. Another point of concern are the electrical contacts, which are prone to corrosion. The arch enemy is moisture--particularly in the presence of salts or corrosive gases. With the added effect of atmospheric oxygen and the heat that is generated in the junction box during operation, an environment is produced that, over time, can damage the contacts and soldered joints.

One way of protecting the components in the junction box is to pot them with a silicone, which cures chemically to form a soft, elastic material. To achieve curing at room temperature, module manufacturers have had to use two-component addition curing silicones until now. Processing these materials caused difficulties.

By using a highly reactive silicone system, the module manufacturer could achieve rapid curing at room temperature. However, this had the downside of only a very narrow processing window--the silicone potting compound had to be processed within just a few minutes. Even a brief production disruption would result in the material curing in the mixing and metering system. Alternatively, the manufacturer could make more time available for processing--i.e., a longer pot life--by choosing a less reactive system. But then the curing times would be longer, too. Rapid curing and long pot lives were mutually exclusive.

The New Solution: UV Activation

A way out of this dilemma is provided by an innovative silicone product from WACKER. Marketed as ELASTOSIL Solar 2120 UV, the new silicone is a pourable, transparent one-component rubber. It is cured in a platinum-catalyzed reaction initiated not by heat, but by a brief flash of UV light.

That is made possible by a catalyst that is passive in the dark, but activated by UV radiation. Only the active form of the catalyst can initiate curing. Once active catalyst molecules have been generated, the addition curing reaction starts and continues even in the dark--the curing reaction itself does not require UV. That is why the developers of the new technology use the term UV activation, not UV curing.

The new UV-active silicone rubber only needs a few seconds¡¯ exposure to cure completely and homogeneously, through the full thickness, within a few minutes at room temperature. This deep curing is possible because the new material is transparent and the UV radiation can penetrate deep into the potting compound. Layers that are several centimeters thick can be cured.

WACKER¡¯s technical service engineers can adjust the curing rate to customers¡¯ requirements; the processor himself can further modify the curing time by varying the UV dose. A 2-mm-thick layer irradiated for 10 seconds with a UV lamp (140 mW/cm²) normally cures completely within one minute at room temperature.

Simple to Use, Excellent Protection

The new product is much easier to process than conventional two-component silicone rubber grades. ELASTOSIL£— Solar 2120 UV is supplied as a ready-to-use one-component system and, therefore, no mixing step is required. The processor does not need a mixing and metering system. With a viscosity of 1,000 mPa s, the UV-active silicone rubber is just slightly thicker than olive oil and, therefore, easy to pour. It penetrates into even the tiniest cavities and cures without shrinking.

The cured rubber is soft, elastic and compliant; with a hardness of 40 Shore 00, it is as soft as a gel. That is an advantage for potting sensitive components: the soft potting compound conforms to any movement if, for example, the potted components have different thermal expansion from the potting compound itself.

There is, therefore, no mechanical stress on the sensitive components and soldered joints. No thermomechanical strains arise in the potted component.

The cured rubber is also an excellent electrical insulator and highly hydrophobic, i.e., water repellent. It is chemically stable--it neither changes over time nor chemically attacks other materials. In particular, the ready-to-use potting compound--unlike UV-curing organic resins--does not contain photoinitiators and does not release curing byproducts, which could damage the potted devices and contacts.

In addition, as a silicone, it features excellent thermal stability--an advantage for long-term use of the module in high and cyclic temperatures. There is hardly any change in its dielectric, mechanical and elastic properties over a very wide temperature range from -45 to +180-the material does not become brittle.

With its native silicone properties, ELASTOSIL Solar 2120 UV is the material of the future for reliably protecting the sensitive parts and contacts in the junction box against weathering. It ensures long-lasting electrical insulation, contributing to reliable and safe PV systems.

The innovative curing technology of ELASTOSIL Solar 2120 UV makes it economically attractive, too--by allowing short cycles without the need to compromise on the pot life. The next and final step in module production--determining the power rating--can be performed just a few minutes after potting the junction box.

Thus, a potting compound is now available that is the dream of every module manufacturer: a one-component system that remains processable for a long time and cures very rapidly at room temperature--but only on command, by a flash of UV light.

Dr. Christian Ochs is Technical Service Manager of Industrial Solutions at Wacker Chemie AG (www.wacker.com).

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