Oxidation pottants

The situation Is different for a substrate module however, which will employ a weatherable plastic-film front cover. Because all plastic films are permeable to oxygen and water vapor (the only difference Is permeation rate), the pottant is exposed to oxygen and water vapor, and also to UV If the plastic film is non-UV screening. Because Isolation of the pottant from oxygen and water vapor is not practically possible in this design option. It becomes a requirement that the pottant be intrinsically resistant to hydrolysis and thermal oxidation, but sensitivity to UV is... 

Chemical Requirements. The pottant must be stable that is, chemically resistant to oxidation and hydrolysis unless protected in a hermetic package, to reduction by metals, and to outgassing of dissolved gases or liquids or decomposition products under normal operating conditions of -itO°C to +90°C for 20 years. The need for chemical stability is especially stringent when a lower cost non-hermetic design is used. Even when a hermetic package is... 

As explained above, the pottant should contain little or no plasticizer since It can generate electrical problems. The last chemical requirement for the pottant Is that Its melt equilibrium contact angle with all the surfaces to which It bonds be as low as possible below 90°C. This speeds processing as well as maximizing adhesion and minimizing the collection of water and oxygen at the Interfaces to reduce metal corrosion and metal oxide catalyzed polymer changes to form color centers. 

Candidates. The only commeroially available oriented films known at this time which fit the weather resistance requirements are polyvinylidene fluoride (PVF2)i polyvinyl fluoride (Tedlar), polymethyl methacrylate (PMMA), and polybutyl aorylate/methyl methacrylate copolymer (PBA/MMA). PVF2 is currently expensive. PBA/MMA is inexpensive but in clear form does not appear to be sufficiently oxidatively stable for our purposes. It is also too water sensitive and too easily softened in many laminating processes. PMMA appears to be somewhat more chemically stable than PBA/MMA and is also relatively Inexpensive, but has the same dimensional stability problems at 150°C, the normal pottant processing temperature. Both acrylics maintain excellent optical clarity on heat aging, however. 

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