Fluoropolymer barriers to stress corrosion in optical fibers
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James R. Griffith Naval Research Laboratory, Washington, DC 20375 (Received 12 February 1987; accepted 21 July 1987) Some recent studies on fluoropolymers that exhibit substantially increased barrier properties against the migration of moisture when compared to conventional fiber coating materials, and as a result may offer unique advantages for use in optical fiber applications, are reported. Moisture vapor permeability studies of cis-trans fluoropolyol polyacrylate indicate that it can offer up to an order of magnitude reduction in H 2 O permeability relative to commercially available UV curable acrylate coatings. Experimentally determined permeability coefficients were of the order o f l O ~ 1 2 g c m ~ 2 s ~ 1 cm/mm Hg and diffusion coefficients were of the order of 10~ 10 cm2 s ~ \ In addition to improved barrier properties, incorporation of fluorine into the polymer structure lowers the index of refraction of the polymer enabling a material to be tailored for use as a cladding in conjunction with silica glass. The index of refraction of cis-trans fluoropolyol polyacrylate was determined, using index matching oils, to be 1.437 (lower than the silica core) permitting its use as a cladding. Other material parameters relevant to fiber coatings have also been measured. The elastic modulus E of the material was determined to be 1010 dyn/cm 2 with a slow drop to 108 dyn/cm 2 at the glass transition temperature ~ 45 °C. In addition, an improved formulation of cis-trans fluoropolyol polyacrylate is presented that allowed in-line coating of the optical fiber in an oxygen environment.
I. INTRODUCTION Organic polymer coatings play a decisive role in determining the overall performance of optical fibers. They protect the pristine silica glass surface from abrasive damage and environmental effects such as stressinduced corrosion. In addition, they act as a buffer in reducing the sensitivity of the optical fiber to microbending loss.' Application of the coating must be done in line with the drawing of the glass fiber; thus low viscosities (10 3 -10 4 cP), rapid solidification, and fast cross-linking reactions are required. Ultraviolet cures are preferable, but rapid thermal cures are possible.2 In addition, a polymer coating having a low index of refraction (less than that of the silica glass core, nr = 1.458), may be used as a cladding. Stress-induced corrosion by H 2 O is associated with stress concentrations at surface flaws under tensile deformation. The only ways to protect the glass fiber against stress corrosion are to limit the applied stress that it experiences or to prevent the buildup of H 2 O and OH at the fiber surface. Polymer coatings for optical fibers must thus be designed to reduce permeation of H 2 O through the polymer matrix. Moisture may permeate through the polymer coating(s) while it is being stored under tension (wound on drums) or during deployment on hydrous terrain. The process of permeation of H 2 O through a polymer film involves wetting of the polymer surface, sorption of H 2 O into the polym
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