Glass Fibers from High and Low Viscosity Melts
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Glass Fibers from High and Low Viscosity Melts Frederick T. Wallenberger1 and Norman E. Weston2 1 University of Illinois, Department of Materials Science and Engineering, Urbana, IL 61801 Present Address: PPG Industries, Incorporated, Fiber Glass Science and Technology, Glass Technology Center, Pittsburgh, PA 15230-2844, U.S.A. 2 Retired, Lewes, DE 19958, U.S.A. ABSTRACT Typical commercial and experimental oxide glass fibers are made from silicate melts, which have viscosities of log 2.5 to log 3.0 poise at the forming temperatures. But, strong fibers were recently also made from melts having viscosities of 0.5 to 2.0 poise, and a sharp melting point rather than a viscosity that gradually changes with temperature. This paper analyzes the various mechanisms of fiber formation, and proposes an answer to the apparent inconsistencies. In this context, the present paper bridges recent advances in the disciplines of advanced inorganic fibers [1], composite reinforcing fibers [2, 3, 4], and fiber glass reinforcements [5]. INTRODUCTION Silica and most silicate melts showing a straight-line rise in viscosity above the glass transition temperature are strong melts. Melts showing a notable deviation from straight-line behavior above the Tg, are fragile melts [6]. The labels “strong” and fragile” as used in this paper apply to the behavior of a melt relative to its liquidus temperature, since the liquidus, where crystals and melt are in equilibrium, is a more practical reference for fiber glass melts than the Tg. Glass fiber forming melts having a sharp melting point are “inviscid” (i.e., non-viscous) melts [1]. They normally afford highly polycrystalline solids, rather than amorphous glass fibers. CONCEPTS The stability of a melt depends on crystallization phenomena which occur in a given time at a given melt temperature. Thus, a melt may yield an amorphous glass structure in a given time at a given temperature or else a polycrystalline solid [7]. The premise for the following analysis was that fiber formation from oxide glass melts depends on the ability of a process to stabilize the melt viscosity in a given time, no matter how transient, in the range between log 2.5 and log 3.0 poise. This is the case with strong melts. And it was of interest to determine if the same viscosity range is also required for fragile and/or inviscid melts during their fiber formation. VISCOUS MELTS Viscous melts from which fibers may be spun can be conveniently characterized as strong or as fragile [1, 8, 9]. The difference in melt behavior is a direct outgrowth of differences in temperature variation of viscosity in the fiber forming range and in liquidus temperature. With strong melts, fibers can be downdrawn directly from a melt or a solid preform. But fibers from fragile melts require updrawing from a supercooled melt or rapid quenching of the fibers.
U5.3.1
Strong Melts Strong melts are preferred for fiber development. Strong melt fibers are continuous and are typified by E-glass, a commercial general-purpose fiber, and S-glass, a commerc
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