Glass epoxy tubes and sheets have been used for many years in outdoor applications such as railroad track insulators and oil refinery insulators. They act as electrical insulation preventing electrolytic and galvanic corrosion and are constantly exposed to UV radiation. Convolute & filament wound glass epoxies have also been used for decades as antenna protection and are frequently the body of sonobuoys dropped into the ocean for sonar underwater research or submarine detection.
Inevitably, yellowing, cracking, and reduction of mechanical properties to some degree are aspects of UV exposure. Epoxy resins possess aromatic groups with a strong absorption in the UV range (about 300 nm). This makes epoxy structures vulnerable to UV degradation. When oxygen molecules in the air are exposed to UV light radiation, oxygen radicals are produced. Radicals are also formed from broken bonds, which are highly reactive and have a very short life. These formed radicals attack the surface of the epoxy coatings and react with them. The energy of UV light is usually higher than the chemical bond strength (e.g., C–C, O–O, H–O, and C–N in polymers). Therefore, these chemical bonds are prone to break and photo degradation occurs. Other environmental factors including humidity, oxygen, temperature, and pollutants can intensify the rate of photo degradation.
Mechanical properties can decline as much as 30% under these conditions, with tensile strength most affected. That being said, convolutely wound tubes supplied by Franklin Fibre-Lamitex Corp. are rated to perform in excess of 40,600 psi. A 30% drop in all mechanical properties would result in a product that could still handle about 28,420 psi of constant pressure.
Naturally, the easy solution is to apply a clear lacquer or insulating varnish to tubes & sheets which will protect them from long term degradation and any discoloration that may occur depending on the amount or concentration of UV exposure.
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