edited by
real world of indwtr i d chemi~ try
W. C. FERNELIUS Kent State University Kent, OH 44242
HAROLD WITTCOFF Koor Chemicals Ltd. Beer-Sheva, Israel P.O.B. 60
Glass Fibers-Are They the Solution? R. L. Tiede Owens-Corning Fiberglas Corporation Technical Center, Granville, OH The fact that glass could he drawn into fibers has long been known, perhaps even since before recorded history. However, significant commercial production of glass fibers did not hegin until the 1930's, when the Owens-Illinois Glass Company and Corning Glass Works both began work on fibers on a small scale. In 1938, these two companies merged their efforts to form Owens-Corning Fiherglas. Since that time, OwensCorning has become independent of the parent companies, and its business has grown to the point where sales are over two billion dollars a year. In addition, anumber of competitors have entered the field-hoth in the United States and overseas. Although the term "glass" can be applied to a number of types of compositions ("organic" or "metallic" glasses, for example), for purposes of this discussion, the term will he used only to denote commercial oxide glasses based on silica. Silica-silicon dioxide-normally occurs in any of several crystalline forms, all characterized hy a definite structure in which each silicon ion is surrounded by, and linked to, four oxygen ions. Such a structure is indicated schematically in Figure 1. For simplicity, only two dimensions of the crystal are indicated. The layer drawn is actually puckered, and each silicon ion is associated with another oxygen ion in another plane. The entire crystal is essentially a single molecule. If silica is heated to its melting temperature, it is thought that thermal energy distorts the bonds hetween the ions and reduces the regularity of the structure. However, the bonds persist and this causes the melt to have a high viscosity. When molten silica or any other glass-forming mixture is cooled, its
0s; 00 Figwe 1. Two-dimensional representation of the crystalline silica structure.
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Journal of Chemical Education
viscosity increases further, according to a relationship that can be expressed by the equation Log V = - A
+ Bl(T - C)
where V is viscosity, T is temperature, and A , B, and C are constants. There are no significant breaks in the curve, so that there is no point that can be identified as a melting or freezing point. The high viscosity of the melt makes the rearrangement of the ions into a crystal so slow and difficult that the melt can become rigid before crystallizing, thus forming a glass. The resulting glass is believed to have a structure similar to that of the melt. Silica glass might thus he called a supercooled liquid. Figure 2 indicates the sort of irregular structure a silica glass is believed to have. Pure silica glass has useful properties, but it is so difficult and expensive to produce that it is used for only high cost specialty items. Melting and forming can be made easier by adding to the silica other materials which tend to open some of the silicon to oxygen linkages and hreak or weaken parts of the network. Although essentially any element in the periodic tahle could he introduced into a glass (some to only a very small extent) the principal additives used in most commercial glasses are the oxides of a relatively few elements-sodium, potassium, calcium, magnesium, aluminum, and boron. Figure 3 shows the possible structure of a common s o d a - l i e glass, such as might be used t o make windows or bottles. Not indicated on the drawing are negative charges carried hy the silicon-oxygen aggregates to neutralize the positive charges of the added ions. The added ions need not he present in any definite, proportion t o make a glass. Thus, such a glass, which contains
0
5;
00 Figure 2.Twodimensional representation of th? structure of silica glass
k ..
.. Figure 5 . The centrifugal process for making glass fibers.
Figure 3. Twodimensional representation of the structure of soda lime silica glass.
BATCH
Figure 6. Production of glass fibers by use of high velocity gases.
SIZE APPLICA TlON
Making Fibers from Glass
Figure 4. How continuous glass fibers are made.
There are three principal methods of converting glass into fihers and very many modifications of each method. The three methods are discussed in the sections below. The Continuous Process
more than one oxide, could he loosely considered to he a supercooled solution. Glass for conversion into fihers is produced by melting a mixture which contains the oxides desired in the finished glass, or compounds that thermally decompose into oxides. These include sand for the silicon dioxide, soda ash (sodium carhonate) for Na20, limestone (calcium carhonate) for CaO, borax, horic acid or minerals such as colemanite (a hydrated calcium borate) for B203, and often silicate minerals such as feldspars or clays for A1203. The glass batch is carefully weighed, mixed, and conveyed to a melting unit called a tank, constructed of special refractory materials, usually made of oxides of silicon, aluminum, zirconium, or chromium, or combinations of these. Molten glass is an excellent solvent, and it has been pointed out that almost any element might he incorporated into it. Glass will thus attack and slowly dissolve any of the refractories commonly available for constructing a tank. Indeed, the problem of melting glass has facetiously heen described as similar to the problem one would face in trying to boil water in a pot made of ice. By proper selection of refractories, control of temperatures and melting conditions, and cooling of the external parts of the refractories, a tank may he made to last up to ten years. Tanks are constructed in various sizes, hut a typical one might produce a hundred tons of glass a day and hold twice that amount. Heat tomelt the hatch is supplied in either of two ways: (a) by burning gas or oil over the melt, under a "crown" which confines the flames and radiates heat hack down to the melt: or (h) by electrical energy applied directly to the molten glass, which conducts electricitv when hot and serves as a resistor. Temperatures in the ran& of 1400-1500' C are typical. Usually, the glass is delivered to the fiher forming apparatus in molten form, after being cooled somewhat to the point at which it acquires the proper viscosity.
The molten glass is delivered to a bushing, an electrically heated device constructed of high temperature metal, having a large number of orifices at the bottom. Glass flows through the orifices by gravity and is then caught and pulled mechanically into a bundle of fihers by means of rolls, wheels, or drums. Continuous fibers are produced in diameters ranging . the fibers are pulled at very from about 4 to 25 j ~ Typically, high speeds, which may he on the order of 2 milmin. It is interesting to note that a hushing with 2000 tips could produce enough fiher in an hour to reach the moon. Figure 4 illustrates schematically how most continuous fihers are made. The Centrifugal Process
In this process a stream of glass from the melting tank flows into a rotating cylindrical spinner with many holes in its periphery. Centrifugal force impels glass through the holes and forms i t into fihers. High velocity gas streams (air, steam, or combustion products) may be used to supplement the centrifugal forces in completing the fiberization process and to aid in the collection of the fihers. Figure 5 shows schematically how elass fihers are made hv the centrifueal - mocess. . Fibers so produced are not continuous and have various leneths and fiber diameters. Thev are collected as hlankets of g h wool which is processed in& various forms, usually for use as thermal insulation and sound control products. Processes Employing High Velocity Gases
Glass issuing from one or more orifices can he directed into streams of high velocity gases (air, steam, or combustion products) which will draw them into fibers. This process is used for making staple textile fihers and mats used for some reinforcement applications. I t may also be used to make rock or slag wool. In that case, often a single large stream of molten material is employed. Figure 6 indicates schematically how the steam blowing process works. Volume 59
Number 3
March 1982
199
The Uses of Glass Fibers
properties of glass fibers that have providkd unique solutions to a few interesting problems. 1) Glass fibers d o not burn This led to their use in noncombustible suits for astronauts and for curtains and drapes, partleularly in public buildings, hospitals, etc. For instance, when the new second tower of New York Clty's World Trade Center is complete, the structure will have 21,800 windows with draperies made from
necessary to maintain exact ratios of components for material to remain a glass. Consequently, glass com~ositionscan be varied over wide ranges,-with resulting wide variations in properties. Thus, solutions to problems can sometimes be achieved by changing glass compositions. For instance, special glasses have been developed (a) with low dielectric constant to serve in certain radar installations; (b) with high lead content for radiation shielding; and (c) with exceptionally high strength and modulus of elasticity for critical reinforced plastics applications. Glass fibers are seldom used as bare fibers. Instead, they may be coated with such things as sizes, to facilitate later processing, coupling agents, to improve performance in subsequent use as reinforcements, or hinders to hold fibers in proper relationshi~sin mats or wool Dacks. The ~erformance
as cement factories and power plants where hot exhaust products must be filtered. Specially treated glass fibers are also used in the insulating tiles on the surface of the space shuttle where high temperature resistance is required. 3) Glass fibem are strong. Single fibers of E-glass,the most common textile glass, commonly give values of 3450 megapascals (500,000 psi) for tensile strength. For comparison, high quality steel piano wire might have a strength of about 2070 megapascals (300,000 psi). In addition, steel is much heavier. These considerations have led to the use of glass fibers to reinforce plastics for almost countless applications. 4) Glass fibers haw good electrical properties. This has led to their use in electrical panels and wire insulation. An interesting early demonstration involved rewiring a 5 HP electric motor with glass fiber insulated wire, mountina it on a stand with a standard 10 HP motor. and ao~lvinea 10 HP Toad to bath. The hieher temoerature capability ofthe gl&s fiber insulation enahled t i e smaller motor to perform as well as the larger one.
treatment. Readers interested in learning more about glass fihers can find several books dealing with their production and applications. Two which come ti,mind are fib erg la^"^ and "The SPI Handbook of Technology and Engineering of Remforced Plasti~slComposites."~ In conclusion, it may be said that it is pertinent to use the term "solution" in connection with glass fibers for two reasons: (a) glass itself may be loosely described as a solution; and (b) glass fibers have unique properties which have permitted them to provide the solution to a wide range of problems.
The strength, chemical durability, fire resistance, and translucencv. of elass fihers in cloth form have rather recentlv led to another interesting use-roofing materials for large structures. such as worts arenas. etc. The lareest such inj stallation is the cable-supported fabric roof of t h e ~ aterminal in Saudi Arabia which will cover 105 acres. Since glass may he considered to be a solution, it is not
'Thomas, J. H., "Glass Fibers Used Today in Some 33.000 Products," American Glass Review, 30-32,(July 1966). Mohr, J. Gilbert, and Rowe, William P., "Fiberglas," Van Nostrand Relnhold Co., New York, 1978. Mohr, J. Gilbert, Oleesky, Samuel S., Shook, Gerald P., Meyer, Leonard S., "SPI Handbook of Technology and Engineering of Reinforced Plastics/Composites," Second Edition, Van Nostrand Reinhold Co., New York, 1973.
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Journal of Chemical Education
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