Handbook of Materials for Product Design Part 11 pps

08Seward Page 96 Wednesday, May 23, 2001 10:16 AM 8.96 Chapter refractory materials themselves Crowns are often self-supported refractory structures The refractories must withstand or resist high temperatures, heavy loads, abrasion, and corrosion Types of oxide refractories used in glass melters are as follows: Clay refractories (generally used for insulation) I Fireclay: kaolinite (Al2O3 · 2SiO2 · 2H2O) plus minor components; classified as low-, medium-, high-, or super-duty; (25 to 45% Al2O3) I High alumina (50 to 87.5% Al2O3) Nonclay refractories (generally used for glass contact or furnace superstructure) I Magnesia (MgO) I Silica (cristobalite and trydimite) I Stabilized zirconia I Extra-high alumina (>87.5%) I Mullite (3Al2O3 · 2SiO2) I AZS (alumina-zirconia-silica) containing 30 to 42% zirconia I Zircon (ZrO2 · SiO2) I Chrome-magnesite and magnesite-chrome (combinations of Cr2O3 and MgO) Classifications based on methods of of refractory manufacture (in order of increasing density) are as follows: I Bonded I Sintered (sometimes densified by cold isostatic pressing before firing) I Fusion cast (cast as blocks from arc-melted raw materials); also referred to as fused or fused cast Many factors affect the choice of refractory for a particular application These include melting temperature, thermal conductivity, mechanical strength, creep resistance, and resistance to corrosion and spalling, to name a few Generally, different refractories are used in different regions of a glass-melting tank, because the requirements are different However, the ultimate design consideration is resistance to corrosion by molten glass and by the hot gas atmosphere within the melter Corrosion determines tank lifetimes and affects the rates at 08Seward Page 97 Wednesday, May 23, 2001 10:16 AM Inorganic Glasses 8.97 which certain glass defects (such as stones) are generated In selecting a refractory the considerations are, in order, glass quality tank lifetime initial cost Many different corrosion mechanisms operate within a melter The types and severity depend on the glass composition, the composition and microstructure of the refractory, and the temperature Corrosion rates tend to increase dramatically with temperature This is an important reason why different refractories are often selected for different portions of the melter It is also an important reason why the harder (higher melting temperature) glasses are generally more expensive to manufacture Corrosion types include: I Front surface attack (frontal attack) This is direct attack at the glass/refractory interface Its mechanisms include alkali diffusion into the refractory with consequent fluxing and dissolution of the refractory crystals Porous refractories are generally more susceptible I Melt line corrosion (attack where glass surface, air and refractory meet) Corrosion at this location is enhanced by localized convection currents and fluctuations in glass level within the melter I Upward drilling This form of corrosion occurs where bubbles form under horizontal refractory surfaces, such as throat cover blocks or submerged horizontal refractory joints Corrosive vapor species concentrate in the bubbles As with melt line corrosion, the greatest corrosive activity is believed to take place where the vapor, refractory, and glass touch I Downward drilling This type of corrosion results when droplets of molten metal settle on the bottom of the tank Sources of metal can be contaminants in batch raw materials or cullet, chemical reduction of certain glass components (such as lead oxide), and even tools or metal parts accidentally dropped into the tank Glass contact refractories The most common glass contact refractories include the following: I Fused AZS (alumina-zirconia-silica) This is the most common today (41% ZrO2 in high-wear areas and electrode blocks, as opposed to less expensive 34% variety) The oxidation state is critical (the refractory contains a residual glassy phase which, if produced in reduced condition, will oxidize in use, swell, and exude from the brick) 08Seward Page 98 Wednesday, May 23, 2001 10:16 AM 8.98 Chapter I Dense sintered zircon (ZrO2 · SiO2) This is used in some low expansion borosilicate melters I Clay, fused alumina, bonded AZS, and dense sintered alumina These are used for lower-melting specialty glasses Typical glass contact refractories used for melting various glass types are: I Container glass Fused AZS, life to 10 years; also, fused α-β alumina; finer bottoms are sometimes bonded AZS, zircon, and clay I Float glass Fused AZS in melting zones; fused alumina in conditioner zones, life 10 to 12 years I Hard borosilicates tered zircon I Fiberglass wool Highly corrosive, melted electrically in fused chrome-AZS or fused alumina-chrome refractories (coloration due to chromium is of little consequence in this application) I E-glass (textile fiber) chrome oxide I Lead crystal Fused AZS and zirconia, sometimes dense sin- Less corrosive, melted in dense sintered Tendency to electric melting in AZS Several refractory sidewall design considerations are based on corrosion concerns First, the thicker the wall, the lesser the heat lost and the lesser the energy consumed But thicker walls create a smaller temperature gradient, allowing chemical attack to penetrate more deeply into the refractory Thus, the design thickness of a wall must be a compromise between heat loss and wear To give all sections of the wall approximately equal lifetimes, thickness, and type of refractory are often varied from location to location These techniques are sometimes referred to as zoning by thickness and zoning by type To avoid melt infiltration of horizontal refractory seams and the consequent increased opportunity for upward drilling of corrosion, glass contact wall refractories are often large, full-height blocks arranged adjacent to each other in a “soldier” course fashion However, if multiple courses are required, close-fitted diamond ground horizontal joints can help minimize melt infiltration Similarly, to avoid horizontal joints, the paver blocks composing the top layer of the tank bottom are butted up against the side blocks, not placed under them Superstructure and crown refractories The superstructure, which includes all furnace walls above the melt line and the crown, are subject to corrosion by aggressive vapor species such as NaOH, KOH, PbO and HBO2, batch dust particles, liquid condensates and liquid reaction 08Seward Page 99 Wednesday, May 23, 2001 10:16 AM Inorganic Glasses 8.99 products running down from refractories higher up Superstructure temperatures in fuel fired furnaces are often 60-100°C hotter than the glass Typically, walls and ports made of fused AZS from the back wall (where the batch enters) to the hot spot; fused β-alumina is used downstream But these are not hard and fast rules Crowns typically consist of sintered silica block Although silica crowns are attacked by alkali vapors, the drips are homogenized into melt Crown life is more of a concern This is especially true with gas-oxy firing (more aggressive vapors, see Sec 8.3.3.5), in which case more costly refractories may be justified Silica and alumina blocks should never be in direct contact, for example at the joint where the crown and superstructure walls meet (they will react); zircon is used as a buffer Regenerative heat exchanger refractories Special refractory considerations are needed because of the large temperature gradient (top–bottom) within the checker chambers and the corrosive nature of the exhaust gases As the gases cool, a temperature is reached at which the corrosive vapors condense on the refractory surfaces, enhancing the corrosion Fine batch particles carried over with the exhaust gases also tend to react with and corrode the regenerator refractories High thermal conductivity and heat capacity are also important characteristics Checker construction for a typical soda-lime-silica melting tank is: top third, bonded 95 to 98% MgO bricks; middle third, lower magnesia content bricks; bottom third (where alkali vapors condense), sintered chrome (chromic oxide) or magnesium-chrome bricks A relatively new approach to checker construction, especially in Europe, is with special interlocking shapes (cruciforms) of AZS or highalumina fused cast refractories Electric boosting and all-electric melting Crucibles, pots, and day tanks for glass melting can be heated in electric furnaces where the heat is generated by resistance heating in windings or bars In come cases, heat is produced by the flow of electricity through the metal crucible itself, and in others by the flow of electricity through the molten glass, which is a moderately good ionic conductor at high temperatures, between submerged electrodes These principles are applied to varying degrees in large continuous melters as well In some fuel-fired furnaces, electrodes are installed in the walls below the glass line so as to provide a source of heat below the batch layer, thus the batch is actively melted from below as well as above By such means, the melting rate for the tank can be increased, or boosted, leading to the term electric boosting Resistance heating by external windings or bars is often used to control temperatures at the orifice or delivery tube by which the molten glass leaves the melter 8.3.3.4 08Seward Page 100 Wednesday, May 23, 2001 10:16 AM 8.100 Chapter Sometimes, all the heat required for melting is supplied electrically within the molten glass In this case, electrodes are positioned as plates at the walls of the furnace, or as water-cooled metal rods extending upward through the bottom of the furnace The electric current passes from electrode to electrode, through the glass, with the amount of heat generated depending on the applied voltages, the shape and spacing of the electrodes, and, very importantly, the electrical resistivity of the molten glass In this case of all-electric melting, the batch components are melted solely by heat flow up from below It is possible and desirable to maintain a continuous layer of batch across the top of the melt, eliminating the need for a refractory roof or crown to contain and reflect the heat However, in many cases, the crown is there for other reasons, but it is cold in temperature Hence, all-electric melting in this manner is sometimes referred to as cold-top or cold-crown melting It should be noted that the electrical resistivity of a pile of nonmelted batch, or even a glass melt at temperatures well below 1,000°C, is too great to allow for efficient heat generation Consequently, all-electric melters are started in a more or less traditional way using fossil-fuel burners and a hot crown Once the molten glass has reached sufficient temperature, the burners (and sometimes the crown) are removed Cold-top melting is valuable for two reasons First, the batch layer acts as a thermal insulating blanket (the batch blanket), which helps reduce heat loss out the top of the melter (thus enabling it to operate cold-top) Second, the top layers of the batch blanket, being much cooler than the molten glass below, act to condense volatile vapor species that might otherwise escape into the atmosphere This is especially valuable for fiberglass and other specialty glass melting where the compositions contain fluorides and other very volatile, and sometimes unhealthy, components Electrodes are made of materials such as carbon, tin oxide, molybdenum, and platinum, with the choice depending on the temperature of operation and the composition of the glass being melted Melt temperatures, temperature gradients, and convection currents are greater at and near the electrodes; therefore, better refractories (higher temperature, more corrosion resistant) are required at these locations Oxygen for combustion Over the past decade, there has been a trend toward the use of oxygen instead of air, in combination with natural gas, to heat fuel-fired furnaces This is called oxy-fuel firing It has several advantages First, since one is not using air with its 80% nitrogen content, much less polluting NOx gases are produced In the 8.3.3.5 08Seward Page 101 Wednesday, May 23, 2001 10:16 AM Inorganic Glasses 8.101 face of increasingly stringent air quality legislation, this factor alone is often sufficient to justify conversion to oxy-fuel firing Second, since there is not the large volume of nitrogen to heat and expel from the furnace, much less waste heat is generated than with gas-air Some operations have been able to eliminate the massive, costly regenerators as a consequence Third, higher flame temperatures are possible Fourth, as claimed by some manufacturers, more stable furnace operation, with an associated improvement in glass quality, is achieved This is especially the case when regeneration, with its inherent periodic reversals of gas and heat flow, is eliminated There are some disadvantages to oxy-fuel firing One is the need for liquid oxygen storage or oxygen generation on site A second is that, without the large volumes of air moving through the furnace, the concentrations of water vapor (a product of combustion) and corrosive volatile species from the melt (e.g., NaOH) are much higher, in some cases leading to increased deterioration of the refractory superstructure 8.3.3.6 Furnaces for specific applications Furnaces designed for spe- cific applications include the following: I Container glass Typically cross-fired regenerative; maximum melt temperatures ~1600°C; large, up to 500T/day I Float glass Typically cross-fired regenerative; no bridge wall, but rather an open surfaced narrow region called a waist to keep inhomogeneities running parallel to the surface of the glass sheet; maximum melt temperatures ~1600°C; larger, up to 800T/day I Fiberglass I Lead crystal I Hard borosilicates Tending to all-electric or heavily boosted regenerative; melt temperatures > 1600°C I Aluminosilicate glass-ceramics Regenerative gas-fired; temperatures near 1700°C required for efficient fining I Optical glass Small fuel-fired or electric heated; fining and conditioning often done in platinum tubes to avoid refractory contact and resulting inclusions and inhomogeneity Smaller gas-fired recuperative or all-electric Small electric boosted or all-electric With the advent of oxy-fuel, or more specifically gas-oxygen, firing, many of the above listed regenerative and recuperative furnaces have been converted to use this new technology However, as of this writing, float glass manufacturing is just beginning to convert to gas-oxygen firing 08Seward Page 102 Wednesday, May 23, 2001 10:16 AM 8.102 Chapter Trends As is typical in most industries, new designs are aimed at lower overall cost of operation (the calculation of which includes initial cost, melter lifetime, and costs of repairs as well as the daily operating costs) less energy consumption and less overall environmental impact Lower cost almost always must be achieved in combination with improved glass quality and less adverse environmental impact 8.4 Glass Making II—Glass Forming The term forming collectively refers to all the processes of glass making used to form a solid object or product from the molten glass Historically, all glass objects were formed by hand using relatively simple implements Over time, the techniques were modified, automated, and scaled up While several glass forming methods in use today have no precedent in early glass history, most still bear important resemblance to their forbearers Due to space limitations here, this section will describe only processes used in today’s manufacturing plants and, when relevant, the early hand-forming operations Little attention will be paid to the many processes that have intervened We will first discuss processes involving molds 8.4.1 Blowing By far, containers (bottles and like products) account for the largest volume of glass production Almost all these products are manufactured using some form of a blowing process Historically, glass containers have been blown to shape by gathering a gob of molten glass on the end of a hollow iron pipe, the blowpipe or blowing iron, and blowing a puff of air into the soft glass to form a bubble, which is gradually expanded and worked into shape by the combined effects of gravity and the forces of tools pressed against it Generally, the blowing iron, with the soft glass attached, is rotated to balance the effects of gravity and provide an axial symmetry to the product While useful containers of remarkably repeatable shapes and dimensions can be created in this manner, for rapid and precise production, it is preferable to use a two-step process First, a hollow preform, called a parison, is prepared using a simple blowing process Second, the parison is blown to the final shape in a mold This process has been automated to a very high degree in modern times, to the point where more than a dozen containers per minute can be generated from each mold Generally, rotating split molds are used for shapes involving bodies of revolution whenever visible seam lines from the molds are undesirable, such as for light bulbs or highquality drinkware Stationary split molds must be used for containers 08Seward Page 103 Wednesday, May 23, 2001 10:16 AM Inorganic Glasses 8.103 having handles, flutes, or other nonrotationally symmetric shapes The rotating split molds are generally paste molds, called that because their molding surface is coated with a thin layer of cork or similarly permeable substance, which is saturated with water after each molding cycle When the mold surface is contacted by hot, molten glass, a steam layer results, which provides a low-friction layer between glass and mold, giving the product a highly polished appearance without seam lines The stationary molds are generally hot iron molds These metal molds are operated at a temperature hot enough to keep the molten glass from being chilled so quickly that surface cracks or checks result, but cold enough to quickly extract heat from the glass and allow it to become rigid before removal Any metal mold surface defects, as well as the mold seam lines, are transferred to the ware, but production rates can be much faster than with paste molds Also, on the plus side, intentional designs such as logos can be molded or embossed into the glass surface When blowing by a hand-type operation, the final product must be separated from the blowing iron, usually by cracking it off This leaves a rough surface that must be properly finished by grinding or fire polishing, a process step that involves locally reheating the glass to a point at which it will flow to a smooth surface under the influence of surface tension In modern automated container production, free gobs of glass are handled in the molds, so separation from a blowing iron is not required Two common processes are called blow-and-blow and press-and-blow, depending on the method used to form the parison Blow-and-blow is generally used for narrow-neck containers such as beverage bottles The parison is blown in one mold in a way that forms the neck and then, held by the relatively cold newly formed neck, it is transferred into a second mold to blow the body of the container One of the more common machines featuring these operations is Hartford Empire’s (now Emhart Corporation’s) Individual Section (IS) machine This mechanism may have as many as 12 sections driven in tandem by a cam with overlapped timing or, more recently, by electronically synchronized operation Each section operates on as many as four gobs Processing speeds are about 10 s per section In addition to speed, an advantage of the IS machine (as opposed to a rotating turret machine) is that the machine can be programmed to run the remaining sections while one is being repaired The operation of a single two-mold IS section is shown in Fig 8.12 Press forming of the parison before blowing to final shape is used for wide-mouthed containers such as food jars Press forming will be described in the next section For container manufacture, while pressing of the parison is complicated by the need for an additional tool (the 08Seward Page 104 Wednesday, May 23, 2001 10:16 AM 8.104 Chapter Figure 8.12 The H.E IS (individual section) blow-and-blow machine The gob is delivered into a blank mold, settled with compressed air, and then preformed with a counter-blow The parison or preform is then inverted and transferred into the blow mold where it is finished by blowing.27 plunger), this disadvantage is offset by yielding a product of more uniform wall thickness, hence a more efficient utilization of glass and a lighter-weight product than produced by blow-and-blow A very high-speed process for blowing light bulb envelopes and the like, known as the ribbon machine, was developed in the 1920s by 08Seward Page 105 Wednesday, May 23, 2001 10:16 AM Inorganic Glasses 8.105 Corning Glass Works (now Corning Inc.) and is still in use worldwide In this machine, a stream of molten glass is continuously fed between a set of rollers, one flat and the other with pocket-like indentations These rollers form a ribbon of glass several inches wide, containing regularly spaced circular mounds of glass down the centerline The parison for each light bulb is formed by inserting a synchronously moving blow head (analogous to a blowpipe) into each mound of glass and blowing it through a synchronously moving orifice plate As the ribbon travels horizontally along the machine, the parison is enclosed in an also synchronously moving rotating paste mold, and the blowing process is completed The moving molds open and swing away to allow the finished glass envelope to be cracked off the ribbon at the machine exit The operation of the ribbon machine is illustrated in Fig 8.13 Incandescent lamp envelopes (for example A-19, 60-W bulbs) can be made at speeds in excess of 1,200 per minute on a single machine using this technique Small automotive and other specialty lighting bulbs can be made at rates exceeding 2,000 per minute 8.4.2 Pressing In simplest terms, pressing or press forming of glass involves placing a gob of molten glass in a hot metal mold and pressing it into final shape with a plunger Sometimes a ring is used, as illustrated in Fig 8.14, to limit the flow of glass up the side of the mold and produce a rim of well Figure 8.13 The “ribbon machine” used for light bulb envelope manufacture U.S patent 1,790,397 (Jan 27, 1931), W J Woods and D E Gray (to Corning Inc.) (Courtesy of Corning Inc.) 08Seward Page 161 Wednesday, May 23, 2001 10:16 AM Inorganic Glasses I Automobile and aircraft instrument panels I Flat-panel display backlighting I 8.161 Future automotive lighting (interior and driving lights) Image transmission, magnification, and inversion I Medical (diagnostics and surgery; endoscopes and the like) I Technical (e.g., chimney, pipeline, and chemical system inspection; remote sensing) I Optoelectronic devices (including vacuum-tight electron tube faceplates for TV cameras and CRTs; advantages include flat fields and greater image intensity) Process Single filaments of clad fiber may be made by redraw of rod in tubing or, alternatively, by a double-crucible method, which is essentially a Vello-type delivery system where a second glass rather than air is delivered through the center of the bell A version of the latter method is illustrated in Fig 8.43 A flexible fiber optic is formed by bundling many of these clad filaments or monofibers together Often, the entrance and exit to the bundle are made more mechanically strong by locally fusing the fibers together, either by heating under pressure, with or without a glass sealing frit as a binder, or with a suitable organic polymer The fused ends can then be polished to a smooth optical surface Scrambling and unscrambling devices may be made by disarraying the fibers at some location along the length of the bundle, fusing them together at that location, then cutting the bundle at the fused point Fused fiber optic devices are formed by stacking together in parallel alignment many (hundreds of) individual monofibers, then fusing them together under heat and pressure These fused multifiber stacks are redrawn (to smaller cross-sectional dimensions), cut, stacked, fused, and redrawn again This process is repeated until the target core diameter (or number of fibers per unit area) is achieved The final multifiber elements are then cut, stacked, and fused (but not redrawn) to give whatever size device is required Sometimes an EMA (extramural absorbing) cladding is used to improve image contrast This prevents leakage of light from one fiber core to another and attenuates light that enters the fiber stack through the cladding; a light-absorbing second cladding can be made part of the original (first-draw) fibers More often, absorbing (black) glass rods are substituted for some of the fibers in the first stacking, in either a random or designed pattern This is often sufficient to provide the extramural absorption needed for contrast enhancement 08Seward Page 162 Wednesday, May 23, 2001 10:16 AM 8.162 Chapter Figure 8.43 The double-crucible method for mak- ing clad optical fiber.39 Fused fiber optic tapers can be used to magnify (or demagnify) images Tapers are made by locally heating the central region of a length of fused fiber optic and stretching it to form an hourglass shape From this hourglass, two tapered sections can be cut Each section is capable of transferring an image from one surface to the other, the magnification being determined by the ratios of the two diameters (assuming a circular cross section) Image rotators or inverters are made by rotating one end of the hourglass with respect to the other as it is being drawn; not much necking is required Near-optical-quality glass is generally required for both core and cladding glasses Examples of refractive index combinations and the corresponding fiber characteristics are: n1 = 1.700; n2 = 1.512; NA = 0.78; θ1 = 51° n1 = 1.650; n2 = 1.560; NA = 0.54; θ1 = 32° 08Seward Page 163 Wednesday, May 23, 2001 10:16 AM Inorganic Glasses 8.163 The thermal expansions of core and cladding glasses must be designed to match from set point of softer glass to room temperature 8.7 8.7.1 Optical Communications Fiber Introduction The subjects of optical communication of information and all the associated applications of glass are beyond the scope of this chapter But because the invention, manufacture, and application of low-loss optical communications fiber have been so important to the development of worldwide telecommunications and computer networking for the past three decades, and promise to remain so well into the new century, we devote the final section of this chapter to the materials and techniques used to manufacture low-loss optical fiber The use of glass as the medium for long distance communication of information by optical signals was pioneered by Corning Glass Works (now Corning Inc.) and AT&T Bell Telephone Laboratories (now Lucent Technologies) It was Corning that first demonstrated in the early 1970s that optical fiber could be made with an optical signal loss rate less than 20 dB/km (approximately 1% of the input light transmitted through km of fiber), the upper limit imposed by the practicalities of telecommunications system design This was done using fiber made from titania-doped synthetic silica glass of otherwise extremely high purity Each company, Corning and AT&T, proceeded to develop different approaches to manufacturing the fiber, and soon others entered the business, sometimes with new techniques as well In this chapter section, we describe the various key methods of optical fiber manufacture 8.7.2 Materials Silica glass is the material of choice for long distance optical communications applications A significant reason is that silica is one of the few optically transmitting materials that can be made with sufficient purity that light absorption in the near-IR spectral region can be kept at an acceptably low level The silica glass for optical communications fiber is not manufactured by the traditional melting approaches used for most other oxide glasses but rather by methods based on those described in Sec 8.2.9 above for Types III, IV, and V fused silica Other glass composition types, such as fluorides, oxynitrides, and chalcogenides, have been considered, mainly because of their ability to transmit even longer infrared wavelengths, but they are not now manufactured in great volume for long-distance communication This is 08Seward Page 164 Wednesday, May 23, 2001 10:16 AM 8.164 Chapter partly because of difficulties met in obtaining the extremely high purity needed for low optical loss, and partly because of manufacturing obstacles yet to be overcome 8.7.3 Types of Optical Fiber Design There are basically two types of optical communications fiber design, each having many variations The first type, called step-index fiber, is similar in construction to that described in Sec 8.6 for traditional fiber optics, with several key differences Communications fiber dimensions are generally much smaller (The outside diameter is now a standard 125 µm; the core diameter can be as small as about µm.) The refractive index difference (∆n) between core and cladding is much smaller, on the order of 1% or less Much greater precision is required for the critical dimensions, including core concentricity within the fiber The second type is called graded index (sometimes GRIN) fiber Here, the refractive index is varied from the central axis location gradually out into the cladding Often, the index gradient is parabolic in shape, highest at the axis and decreasing outwardly toward the cladding, but sometimes it is more complex in profile The core region occupies about 50 to 70 µm of the fiber diameter; the outside diameter is generally, again, 125 µm Light propagates through an optical communications fiber as electromagnetic waves in a manner similar to how microwave energy propagates through a hollow microwave waveguide Consequently, optical fibers are sometimes referred to as optical waveguides, and fiber types are correspondingly classified according to whether they are single-mode or multimode This waveguide terminology refers to the manner in which the light energy is transmitted down the fiber, in particular how the energy is distributed in space, not to how many different signals the fiber can carry In fact, a single-mode fiber can carry information at much higher rates over a given distance than can a multimode fiber 8.7.4 Manufacturing Processes The manufacturing techniques for all the above types of fiber are generally similar We list the key steps here as follows: Vapor generation 08Seward Page 165 Wednesday, May 23, 2001 10:16 AM Inorganic Glasses 8.165 Preform preparation (silica deposition, including refractive index adjusting elements) Blank preparation (glass rod with required index gradient) Fiber drawing Coating Testing Step As discussed previously, extremely high-purity chemical precursors are required and used—even better than semiconductor manufacturing quality The precursor for silica is often liquid SiCl4 The refractive index controlling dopants, germania and fluorine, are provided by GeCl4 and SiF4, respectively (Other precursor chemicals can be used; there is a trend away from the chlorides, partly for environmental reasons.) These liquids are converted to the vapor phase by bubbling a carrier gas through the liquids at controlled temperatures, as illustrated in Fig 8.44 Alternatively, the needed chemicals may be sublimed from the surface of a solid in the presence of a car- MF To reaction station MF TC TC O2 SiCl4 MF TC TC GeCl4 MF TC TC MF Mass flow controller TC Thermal conductivity cell POCl3 Figure 8.44 Reactant delivery system for vapor-phase techniques.40 08Seward Page 166 Wednesday, May 23, 2001 10:16 AM 8.166 Chapter rier gas In either case, the materials must have a high vapor pressure at reasonable process temperatures This requirement actually limits the dopants that can be incorporated easily into the silica During preform preparation, the ratio of the mix of chemical precursors is varied over time to control the eventual refractive index gradient within the fiber Steps and In these steps, the glass blank, the material that eventually will be drawn into fiber, is generated It is here that the two major approaches to optical fiber manufacture first differ in a very significant way The first approach is called the outside process In step 2, preform preparation, the chemical precursors are reacted in the flame of a specially designed gas-oxygen burner, the combustion gas being either natural gas or hydrogen, just as for all the Type III synthetic silicas described in Sec 8.2.9.1 The doped silica is generated as a fine white amorphous (molten glass) soot that is progressively deposited on a rotating bait rod, as shown in Fig 8.45a The deposition begins with the material that will eventually lie at the axis of the fiber core and ends with the material that will form the outside of the cladding In other words, in the outside process, the preform is created from the inside outward The resulting preform, inherently containing the desired chemical gradient, is porous and moderately fragile To consolidate this porous preform into a solid glass, in Step 3, the bait rod is removed and the remaining preform heated to temperatures near the softening point of fused silica to allow sintering of the soot via viscous flow To ensure that air does not become trapped in interstices and thereby create bubbles in the glass, the preform is consolidated in a helium gas atmosphere The rapid diffusion of helium through the silica glass assures that all voids shrink to zero dimensions under the forces of surface tension As mentioned also in Sec 8.2.9.1, Type III silicas contain water in their structure in the form of OH bonds Light absorption by such OH is extremely detrimental to communications optical fiber performance, so it is removed from the glass by flowing chlorine (Cl2) gas through the preform at high temperatures just prior to consolidation This “outside” process is sometimes referred to as outside vapor deposition (OVD) This is somewhat a misnomer, because the glass is not deposited directly from the vapor phase but rather by the progressive aggregation of submicroscopic particles that were themselves created from the vapor phase The process has also been called radial flame hydrolysis (RFH) In a variation of the outside process, called the axial process, the soot is deposited at the end of the rotating preform as it is gradually pulled 08Seward Page 167 Wednesday, May 23, 2001 10:16 AM Inorganic Glasses Figure 8.45 Schematic representation of (a) OVD, (b) MCVD, and (c) VAD preform fabrication routes (From A.J Bruce, Ref 10, p 44) 8.167 08Seward Page 168 Wednesday, May 23, 2001 10:16 AM 8.168 Chapter away (withdrawn) from the burners, as illustrated in Fig 8.45c The preform is thus created from the end, the core and cladding materials being deposited essentially simultaneously This process is sometimes referred to as vapor axial deposition (VAD), axial vapor deposition (AVD), or even axial flame hydrolysis (AFH) In the second major approach, the so-called inside process, the preform is built up on the inside of a silica glass tube, beginning with the materials that will compose the bulk of the fiber cladding, and ending with what will become the innermost region of the core (So, for the inside process, the preform is generated from the outside inward.) In this process, the heat-providing burner traverses the outside of the rotating silica tube and the reactive gases travel down the inside, as shown in Fig 8.45b The gases inside the tube react to form silica soot in the space within the tube The soot migrates to the walls under thermophoretic forces (diffusion down a temperature gradient) The inner surface of the glass tube is sufficiently hot that the soot particles immediately flow and consolidate into a void-free molten layer (just as in Type III and Type IV synthetic fused silica manufacture) The water vapor generated by the gas-oxygen burner never reaches the inside of the tube, so the deposited glass is very dry, requiring no further drying steps After the deposition is complete, in process Step 3, the resulting thick-walled tubing is collapsed under heat and vacuum to form a solid rod The outside silica layer (the starting tube) is left in place and becomes part of the cladding This inside process is sometimes called inside vapor phase oxidation (IVPO) or inside vapor deposition (IVD) However, it is perhaps most commonly referred to as modified chemical vapor deposition (MCVD) In one variation, perhaps not currently in commercial use, an argon/ oxygen plasma is generated within the tube by microwave radiation to provide the thermal energy for the chemical reaction rather than relying on heat from a burner outside the tube Another technique proposed for the inside process is plasma chemical vapor deposition (PCVD) In this version of the process, a low-pressure plasma is generated inside the tube that does not lead to soot generation but rather allows a heterogeneously nucleated chemical reaction to occur at the inner surface of the tube so that the glass is built up in molecular-scale layers This is a true chemical vapor deposition (CVD) process More layers are required than with soot, but the process can be controlled more precisely Unfortunately, the process has not yet proven to be commercially economical Step Fiber drawing from the preform follows the general procedures described in Sec 8.4.5 Notable differences may be higher temperatures, greater cleanliness, greater speed, and more precise di- 08Seward Page 169 Wednesday, May 23, 2001 10:16 AM Inorganic Glasses 8.169 mensional control Large commercial draw towers can be 20 m high, with process preforms of dimensions exceeding 10 cm diameter and m length They draw at speeds exceeding 10 m/s to produce more than 50 km of fiber from a single preform A fiber draw tower is illustrated schematically in Fig 8.46 Step 5—Coating To protect the fiber’s freshly created (pristine) glass surface and thus preserve its inherent strength, the fiber is coated on the draw tower with a polymeric layer as soon as it is cool enough to so Often, dual UV-curable coatings are used: a soft inner coating and a hard outer coating Sometimes, thin hermetic coatings (metal, ceramic, or amorphous carbon) are applied by special equipment just prior to applying the polymeric coatings Step 6—Testing Often, the coated fiber is continuously proof-tested for strength on the draw tower or as it is rewound onto other spools Optical testing is also critically important For multimode fibers, the tests include attenuation, bandwidth, numerical aperture, and core diameter; for single-mode fibers, they include such additional characteristics as chromatic dispersion, cutoff wavelength, and mode field diameter Figure 8.46 Schematic representation of a draw tower for high-silica preforms (From A.J Bruce, Ref 10, p 44) 08Seward Page 170 Wednesday, May 23, 2001 10:16 AM 8.170 8.8 8.8.1 Chapter Notes and Acknowledgments Notes Glass manufacturers generally not sell bulk quantities of glass, that is, chunks of glass by the pound, but rather they sell glass products, e.g., beverage bottles, electric lamp envelopes, television bulbs, optical lens blanks, and telescope mirror blanks However, some specialty glass manufacturers sell glass in limited forms such as sheet, tubing, rod, frit, and bars of optical glass Often, these items are not stocked in inventory and must be special ordered There are several specialty glassmakers that will melt glasses and fabricate shapes to customer’s specifications, within the range of their capabilities Glass manufacturers generally specify their glass types and products by their physical and chemical properties; it is the properties that they control and sell, not the compositions (although careful composition control is needed to assure the desired properties) Often, the same properties can be produced by compensated variations in composition, i.e., increase of one alkaline earth ion at the expense of another Such choices are often made on the basis of batch component cost or availability For these reasons, many manufacturers not disclose the compositions of their products In other cases, the compositions are considered proprietary Thus, in handbooks such as this one, compositions for some glass types are not known, not given, or only approximately given; others are only known because of independent chemical analysis, sometimes by competitors and sometimes by scientific researchers 8.8.2 Acknowledgments One of the authors (TPS) wishes to acknowledge that much of his perspective on glass forming processes came through working with several Corning Inc process engineers: G Clinton Shay, Stuart Dockerty, Richmond Wilson, Leonard Anderson, William Lentz, William Pardue, and Frank Coppola Similarly, his thinking about glass composition has been strongly influenced by close working relationships with Corning researchers Roger Araujo, Nicholas Borrelli, George Beall, and George Hares References R W Douglas and Susan Frank, A History of Glassmaking, G T Foulis & Co Ltd., Henley-on-Thames, England, 1972 08Seward Page 171 Wednesday, May 23, 2001 10:16 AM Inorganic Glasses 8.171 D R Uhlmann and N J Kreidl, eds., Glass: Science and Technology, vol 2, Processing I, Academic Press, Orlando, 1983 2a D R Uhlmann and N J Kreidl, eds., Glass: Science and Technology, vol 5, Elasticity and Strength of Glasses, Academic Press, Orlando, 1980 E B Shand and G W McLellan, Glass Engineering Handbook, 3d ed., McGraw Hill, New York, 1984 F V Tooley, ed., The Handbook of Glass Manufacture, vols I and II, Books for the Glass Industry Division, Ashlee Publishing Co., New York, 1984 D C Boyd and J F MacDowell, eds., Advances in Ceramics, vol 18, Commercial Glasses, The American Ceramic Society, Columbus, Ohio, 1986 N P Bansal and R H Doremus, Handbook of Glass Properties, Academic Press, Orlando, 1986 S J Schneider, Jr., vol chairman, Engineered Materials Handbook, vol 4, Ceramics and Glasses, ASM International, Materials Park, Ohio, 1991 A K Varshneya, Fundamentals of Inorganic Glasses, Academic Press, San Diego, 1994 D C Boyd, P S Danielson, and D A Thompson, “Glass,” in Kirk-Othmer Encyclopedia of Chemical Technology, 4th ed, vol 12, John Wiley and Sons, New York, 1994 9a J R Hutchins, III and R V Harrington, “Glass,” in Kirk-Othmer: Encyclopedia of Chemical Technology, 2d ed., vol 10, John Wiley and Sons, New York, pp 533–604, 1966 10 J S Sanghera and I D Aggarwal, eds., Infrared Fiber Optics, CRC Press, Boca Raton, Fla., 1998 11 H Bach and N Neuroth (eds.), The Properties of Optical Glass, Springer-Verlag, Berlin and New York, 1998 12 J McGaughey, Libbey Inc., personal communication, 1999 13 T R Kozlowski and G A Chase, “Parameters of Chemical Strengthening and Impact Performance of Corning Code 8361 (White Crown) and Corning Code 8097 (Photogray®) Lenses,” Am J of Optometry and Archives of Am Academy of Optometry, vol 50, 1973, pp 273–282 14 Y Nakao, “Next-Generation Glass Substrate,” presented at Electronic Display Forum 98, Yokohama, Japan, April 29, 1998 15 W Holand, “Materials Science Fundamentals of the IPS Empress Glass Ceramic,” Ivoclar-Vivident Report, no 12, December 1998, Ivoclar Aktiengesellschaft, Schaan, Liechtenstein 16 Y Abe, “Glass-ceramics based on calcium phosphates—artificial dental crown and microporous glass-ceramics,” Glastech Ber Glass Sci Technol., vol 73, no CI, 2000 17 D Hoffman, Owens Corning Science and Technology Center, private communication, April 1999 18 D R Hartman, “Evolution and Application of High Strength Fibers,” The Glass Research: Bulletin of Glass Science and Engineering, vol 4, no 2, Winter 1995, NSF Industry-University Center for Glass Research, Alfred University, Alfred, N.Y 19 G Hetherington, K Jack, and M W Ramsay, “The high-temperature electrolysis of vitreous silica: Part I Oxidation, ultra-violet induced fluorescence, and irradiation color,” Phys, Chem Glasses, vol 6, 1965, pp 6–15 20 R Bruckner, “Properties and Structure of Vitreous Silica, I,” J Non-Cryst Solids, vol 5, 1970, pp 123–175 21 H W McKenzie and R J Hand, Basic Optical Stress Measurements in Glass, Society of Glass Technology, Sheffield, England, 1999 22 A G Pincus and T R Holmes, Annealing and Strengthening in the Glass Industry, 2d ed., Ashlee Publishing Co., New York, 1987 23 G H Beall, “Chain silicate glass-ceramics,” J Non-Cryst Solids, vol 129, 1991, pp 163–173 24 R M Klein, “Optical fiber waveguides,” in Glass: Science and Technology, vol 2, Processing I., R R Uhlmann and N J Kreidl, eds., © 1983, Academic Press 08Seward Page 172 Wednesday, May 23, 2001 10:16 AM 8.172 Chapter 25 A J Bruce, in Infrared Fiber Optics, J S Sanghera and I D Aggarwal, eds., CRC Press, Boca Raton, FL, 1998, p 40 His references 21 and 22 are P C Shultz, Wiss Z Friederich-Schiller-Univ Jena, Math Naturwiss Reihe 32, 215 (1983), and H Takahashi, A Oyobe, M Kusuge, and R Setaka, Tech Dig., ECOC ’86, (1986) 26 F E Woolley, p 390, in S J Schneider, Jr., vol chairman, Engineered Materials Handbook, vol 4, Ceramics and Glasses, ASM International, Materials Park, Ohio, 1991 His reference is W Trier, Glass Furnaces (Design, Construction and Operation), trans K L Lowenstein, Society of Glass Technology, Sheffield, England, 1987 27 D C Boyd, P S Danielson, and D A Thompson, “Glass,” in Kirk-Othmer Encyclopedia of Chemical Technology, 4th ed, vol 12, John Wiley and Sons, New York, 1994, p 602 Their reference is W Giegerich and W Trier, Glass Machine Construction and Operation of Machines for the Forming of Hot Glass, trans N J Kreidl, Springer-Verlag, Berlin, 1969 28 J R Hutchins, III and R V Harrington, “Glass,” in Kirk-Othmer: Encyclopedia of Chemical Technology, 2d ed., vol 10, John Wiley and Sons, New York, pp 533–604, 1966, p 558 Their reference is E B Shand, Glass Engineering Handbook, McGraw-Hill, New York, 1958, p 164 29 U.S patent 3,338,696 (Aug 29, 1967), S M Dockerty (to Corning Inc.) 30 R A McCauley, “Float glass production: Pilkington vs PPG,” The Glass Industry, April 1980, 18–22 31 M Cable, “Mechanism of Glass Manufacture,” J Am Ceram Soc 82, 1093–1112 His reference is Ford Motor Co., Float Method of Manufacturing Glass, Br patent no 1,085,010, 1967) 32 A K Varshneya, “Glass Transition Range Behavior,” in Fundamentals of Inorganic Glasses, p 298, ©1994 by Academic Press, reproduced by permission of the publisher 33 R Gardon, “Thermal Tempering of Glass,” in Glass: Science and Technology, vol 5, Elasticity and Strength of Glasses, D R Uhlmann and N J Kreidl, eds., ©1980 by Academic Press, reproduced by permission of the publisher 34 A G Pincus and T R Holmes, Annealing and Strengthening in the Glass Industry, 2nd ed., Ashlee Publishing Co., New York, 1987 35 H W McKenzie and R J Hand, Basic Optical Stress Measurement in Glass, Society of Glass Technology, Sheffield, England, 1999, p 33 36 Man-Made Vitreous Fibers: Nomenclature, Chemical and Physical Properties, Nomenclature Committee of TIMA Inc (courtesy of North American Insulation Manufacturers Assoc [NAIMA]) 37 G R Machlan and F V Tooley, Handbook of Glass Manufacture, vol II, F V Tooley, ed., 1974 38 Loewenstein, K L., The Manufacturing Technology of Continuous Glass Fibers, 3d ed., Elsevier, New York, 1993 39 A J Bruce, in Infrared Fiber Optics, J S Sanghera na dI D Aggarwal, eds., CRC Press, Boca Raton, FL, 1998, p 35 His reference is L G Cohen, J Lightwave Technology LT3, 1985, p 958 40 R M Klein, “Optical Fiber Waveguides,” in Glass: Science and Technology, vol 2, Processing I, D R Uhlmann and N J Kreidl, eds., ©1983 by Academic Press, reproduced by permission of the publisher Bibliography Bamford, C R., Color Generation and Control in Glasses, Elsevier Scientific, New York, 1977 Cable, M., “A Century of Developments in Glassmelting Research,” J Am Ceram Soc vol 81, no 5, pp.1083–1094 (1998) Cable, M., “Mechanization of Glass Manufacture,” J Am Ceram Soc vol 82, no 5, pp 1093–1112 (1999) 08Seward Page 173 Wednesday, May 23, 2001 10:16 AM Inorganic Glasses 8.173 De Jong, B H W S., J W Adams, B G Aitken, J E Dickinson, and G J Fine, “GlassCeramics,” in Ullmann’s Encyclopedia of Industrial Technology, vol A12, VCH Verlagsgesellschaft mbH, Weinheim, Germany, 1989, pp 433–448 De Jong, B H W S., “Glass,” in Ullmann’s Encyclopedia of Industrial Technology, vol A12, VCH Verlagsgesellschaft mbH, Weinheim, Germany, 1989, pp 365–432 Doremus, R H., Glass Science, John Wiley & Sons., New York, 1973 Giegerich, G and W Trier, Glass Machines (English translation), Springer, BerlinHeidelberg, Germany, 1969 Kleinholz, R and H Tiesler, “Glass Fibers,” in Ullmann’s Encyclopedia of Industrial Technology, vol A11, VCH Verlagsgesellschaft mbH, Weinheim, Germany, 1989, pp 11–27 Kurkjian, C R and W R Prindle, “Perspectives on the History of Glass Composition,” J Am Ceram Soc vol 81, no 4, pp 795–813 (1998) Morey, G W., The Properties of Glass, 2d ed., Reinhold, New York, 1954 McMillan, P W., Glass-Ceramics, 2d ed., Academic Press, London, 1979 Nakahara, T., M Hoshikawa, T Sugawa, and M Watanabe, “Fiber Optics,” in Ullmann’s Encyclopedia of Industrial Technology, vol A10, VCH Verlagsgesellschaft mbH, Weinheim, Germany, 1989, pp 433–450 Perkins, W W., ed., Ceramic Glossary, 2d ed., American Ceramic Society, Columbus, Ohio (1984) Pilkington, L A B., “The float glass process,” Proc Roy Soc Lond A., vol 314, pp 1–25, 1969 Pilkington, Alastair, “Flat glass: evolution and revolution over 60 years,” Glass Technology, vol 17, no 5, pp 182–193, 1976 Rawson, H., Inorganic Glass-Forming Systems, Academic Press, London, 1967 Standard Terminology of Glass and Glass Products, ASTM Standards 1995, ASTM C162-94c, vol 15.02, pp 27–41 Weyl, W A., Colored Glasses, Society of Glass Technology, Sheffield (Reprinted by Dawson’s of Pall Mall, London) 1951 Wachtman, J B., Jr., ed., Ceramic Innovations in the 20th Century, American Ceramic Society, Westerville, Ohio, 1999 09Izzo Page Wednesday, May 23, 2001 10:27 AM Coatings and Finishes Carl P Izzo Industrial Paint Consultant Export, Pennsylvania 9.1 Introduction Coatings and finishes today are engineering materials From cave dwellers decorating their walls with earth pigments ground in egg whites to factory workers protecting products with E-coat primers and urethane acrylic enamels, these coatings are still composed of filmforming vehicles, pigments, solvents, and additives.1 Unlike Noah, who received divine instructions to paint his ark with pitch; the Egyptians, who mixed amber with oils to make decorative varnishes; or the Romans, who mixed white lead with amber and with pitch, coatings scientists and engineers using these materials design coatings today for specific applications on products Because of concern for environmental regulations, safety in the workplace, performance requirements and costs, the design of these coatings must be optimized The first 50 years of the twentieth century were the decades of discovery Significant changes were made in the vehicles, which are the liquid portions of the coatings composed of binder and thinner.2 Since the 1900s and the introduction of phenolic synthetic resin vehicles, coatings have been designed to increase production and meet performance requirements at lower costs These developments were highlighted by the introduction of nitrocellulose lacquers for the automotive and furniture industries; followed by the alkyds, epoxies, vinyls, polyesters, acrylics, and a host of other resins; and finally the polyurethanes In the 1950s, the decade of expansion, manufacturers built new plants to supply the coatings demand for industrial and consumer products Coatings were applied at low solids using inefficient conventional air-atomized spray guns The atmosphere was polluted with 9.1 09Izzo Page Wednesday, May 23, 2001 10:27 AM 9.2 Chapter volatile organic compounds (VOCs), but no one cared as long as finished products were shipped out of the factories Coatings suppliers were fine-tuning formulations to provide faster curing and improved performance properties In 1956, powder coatings were invented By 1959, there were several commercial conveyorized lines applying powder coatings by the fluidized bed process In the 1960s, the decade of technology, just as coatings were becoming highly developed, another variable—environmental impact—was added to the equation Someone finally noticed that the solvents, which were used in coatings for viscosity and flow control, and which evaporate during application and cure, were emitted to the atmosphere Los Angeles County officials, who found that VOC emissions were a major source of air pollution, enacted Rule 66 to control the emission of solvents that cause photochemical smog To comply with Rule 66, the paint industry reformulated its coatings using exempt solvents, which presumably did not produce smog California’s Rule 66 was followed by other local air quality standards and finally by the establishment of the U.S Environmental Protection Agency (EPA), whose charter, under the law, was to improve air quality by reducing solvent emissions During this decade, there were three notable developments which would eventually reduce VOC emissions in coating operations, electrocoating, electrostatically sprayed powder coatings and radiation curable coatings In the 1970s, the decade of conservation, the energy crisis resulted in shortages and price increases for solvents and coatings materials Also affected was the distribution of natural gas, the primary fuel for curing ovens, which caused shortages and price increases In response to those pressures, the coatings industry developed low-temperature curing coatings in an effort to reduce energy consumption Of greater importance to suppliers and end users of coatings and finishes was the establishment of the Clean Air Act of 1970 An important development during this period was radiation curable coatings, mostly clears, for flat line applications using electron beam (EB) and ultraviolet (UV) radiation sources In the 1980s, the decade of restriction, the energy crisis was ending, and the more restrictive air quality standards were beginning However, energy costs remained high The importance of transfer efficiency, the percentage of an applied coating that actually coats the product, was recognized by industry and the EPA This led to the development and use of coating application equipment and coating methods that have higher transfer efficiencies The benefits of using higher transfer efficiency coating methods are threefold: lower coating material usage, lower solvent emissions, and lower costs The automotive industry switched almost exclusively to electrocoating for the application of ... mathematical design of the tapering cross-sectional profile of the trough which, in combination with the incline of the top of the trough, assures that the volume flow of glass over the pipe is uniform... machined slot to form the root of the draw This block is called the debiteuse The molten glass that forms the root of the draw is forced up through the slot by the buoyant force of gravity The... straightness of the root Viscous 08Seward Page 114 Wednesday, May 23, 2001 10:16 AM 8 .114 Chapter forces confine the drawn sheet to a position over the center of the bar In the Colburn-LOF process, the soft