The scientific reasons for heat-tempered glass, long a laboratory curiosity, were determined in 20th century. With process control came greatly expanded utility of stronger glass for architectural, automotive, and other uses.

Fiber glass, also an early glass oddity, expensively produced by crude winding or pulling machines for centuries, came into its own, and joined the great industrial expansions known to the world. In 1892, along with other items determined to put glass, including the fibrous form, on an everyday common usage basis, a fiberglass dress and parasol were exhibited at the Columbian exposition in 

Chicago

. The two exhibitors were responsible for the greatest single revolution in glass, and this event competes with or stands high on the list of all-time industry revolutions anywhere in the world.

Continuous filaments, the then known form, were produced in strands from a heated platinum box or orifices instead of the single hole, which their predecessors had used.

 

Blown glass fiber was discovered also during this series of researches. It was noted that a bead blown off the end of a glass rod thrust in a jet flame carried a fine thread of glass with it due to the high surface tension and viscosity of the hot glass. This mass of bulky glass, it was determined, had better thermal insulating qualities than the slag wool manufacturing at the time.

Since the end applications are distinctly different, the two general fields, blown and continuous fibers, are treated separately herein; methods of manufacturing plus resultant products and end application for each are detailed and copiously illustrated. Also given are technological particulars responsible for the success of each application.

 

 

Manufacturing

Glass fiber manufacturing is the high-temperature conversion of various raw materials (predominantly borosilicate) into a homogeneous melt, followed by the fabrication of this melt into glass fibers. The 2 basic types of glass fiber products, textile and wool, are manufactured by similar processes. Glass fiber production can be segmented into 3 phases: raw materials handling, glass melting and refining, and wool glass fiber forming and finishing, this last phase is being slightly different for textile and wool glass fiber production.

 

 

Raw Materials Handling

The primary component of glass fiber is sand, but it also includes varying quantities of     Feldspar, sodium sulfate, anhydrous borax, boric acid, and many other materials. The bulk supplies are received by rail car and truck, and the lesser-volume supplies are received in drums and packages. These raw materials are unloaded by a variety of methods, including drag shovels, vacuum systems, and vibrator/gravity systems. Belts, screws, and bucket elevators accomplish conveying to and from storage piles and silos. From storage, the materials are weighed according to the desired product recipe and then blended well before their introduction into the melting unit. The weighing, mixing, and charging operations may be conducted in either batch or continuous mode.

Typical chemical composition for mineral wool, insulation type, and high temperature fiberglass are shown in below table. Important test parameters for valuating and controlling glass composition are liquid us temperature (point of initial crystal formation out of the melt upon cooling), softening point (temperature at which glass, a thermoplastic, softens and flows under its own weight), density (weight per unit volume determined after controlled thermal history or annealing), rate of flow at the fiber-forming temperature (a viscosity test), and seen count (either entrained or dissolved gases being released or incomplete melting).

 

                                    Formulations for insulation-Type Glasses

	Formula for typical mineral or slag wool	Typical fiber glass insulation composition	Typical high-temperature fiber composition
AL2O3 Fe2O3	10 1	}   6	} 40
SiO2	50	63	50
CaO	25	7	6
MgO	14	3	4
Na2O	-	14	-
K2O	-	1	-
B2O3	-	6	-
F2	-	0.7	-

 

Glass Melting and Refining

In the glass-melting furnace, the raw materials are heated to temperatures ranging from 1500 to 1700°C (2700 to 3100°F) and are transformed through a sequence of chemical reactions to molten glass. Although there are many furnace designs, furnaces are generally large, shallow, and well-insulated vessels that are heated from above. In operation, raw materials are introduced continuously on top of a bed of molten glass, where they slowly mix and dissolve. Mixing is effected by natural convection, gases rising from chemical reactions, and, in some operations, by air injection into the bottom of the bed.

 

Glass melting furnaces can be categorized by their fuel source and method of heat application into 4 types: recuperative, regenerative, unit, and electric melter. The recuperative, regenerative, and unit melter furnaces can be fueled by either gas or oil. The current trend is from gas-fired to oil-fired.

 

Recuperative furnaces use a steel heat exchanger, recovering heat from the exhaust gases by exchange with the combustion air. Regenerative furnaces use a lattice of brickwork to recover waste heat from exhaust gases. In the initial mode of operation, hot exhaust gases are routed through a chamber containing a brickwork lattice, while combustion air is heated by passage through another corresponding brickwork lattice. About every 20 minutes, the airflow is reversed, so that the combustion air is always being passed through hot brickwork previously heated by exhaust gases.

 

Electric furnaces melt glass by passing an electric current through the melt. Electric furnaces are either hot top or cold top. The former use gas for auxiliary heating and the latter use only the electric current. Electric furnaces are currently used only for wool glass fiber production because of the electrical properties of the glass formulation. Unit melters are used only for the "indirect" marble melting process, getting raw materials from a continuous screw at the back of the furnace adjacent to the exhaust air discharge. There are no provisions for heat recovery with unit melters.

 

 

 

EMISSION FACTORS

In the "indirect" melting process, molten glass passes to a fore hearth, where it is drawn off, sheared into globs, and formed into marbles by roll forming. The marbles are then stress-relieved in annealing ovens, cooled, and conveyed to storage or to other plants for later use. In the "direct" glass fiber process, molten glass passes from the furnace into a refining unit, where bubbles and particles are removed by settling, and the melt is allowed to cool to the proper viscosity for the fiber forming operation.

 

 

Wool Glass Fiber Forming And Finishing

Wool fiberglass is produced for insulation and is formed into mats that are cut into bats. (Loose wool is primarily a waste product formed from mat trimming, although some is a primary product, and is only a small part of the total wool fiberglass produced. No specific emission data for loose wool production are available.) The insulation is used primarily in the construction industry and is produced to comply with ASTM C167-64, the "Standard Test Method for Thickness and Density of Blanket- or Bat-Type Thermal Insulating Material".

 

Wool fiberglass insulation production lines usually consist of the following processes:

(1) Preparation of molten glass,

(2) Formation of fibers into a wool fiberglass mat,

(3) Curing the binder-coated fiberglass mat,

(4) Cooling the mat, and

(5) Backing, Cutting, and packaging the insulation.

Fiberglass plants contain various sizes, types, and numbers of production lines, although a typical plant has 3 lines. Backing (gluing a flat flexible material, usually paper, to the mat), cutting, and packaging operations are not significant sources of emissions to the atmosphere.

 

The trimmed edge waste from the mat and the fibrous dust generated during the cutting and packaging operations are collected by a cyclone and either is transported to a hammer mill to be chopped into blown wool (loose insulation) and bulk packaged or are recycled to the forming section and blended with newly formed product.

 

During the formation of fibers into a wool fiberglass mat (the process known as "forming" in the industry), glass fibers are made from molten glass, and a chemical binder is simultaneously sprayed on the fibers as they are created. The binder is a thermosetting resin that holds the glass fibers together. Although the binder composition varies with product type, typically the binder consists of a solution of phenol-formaldehyde resin, water, urea, lignin, silane, and ammonia. Coloring agents may also be added to the binder. Centrifugal force causes molten glass to flow through small holes in the wall of a rapidly rotating cylinder to create fibers that are broken into pieces by an air stream. This is the newer of the 2 processes and dominates the industry today. In the flame attenuation process, molten glass flows by gravity from a furnace through numerous small orifices to create threads that are then attenuated (stretched to the point of breaking) by high velocity, hot air, and/or a flame. After the glass fibers are created (by either process) and sprayed with the binder solution, they are collected by gravity on a conveyor belt in the form of a mat.

 

The conveyor carries the newly formed mat through a large oven to cure the thermosetting binder and then through a cooling section where ambient air is drawn down through the mat. Figure -3 presents a schematic drawing of the curing and cooling sections. The cooled mat remains on the conveyor for trimming of the uneven edges. Then, if product specifications require it, a backing is applied with an adhesive to form a vapor barrier. The mat is then cut into bats of the desired dimensions and packaged.

 

Textile Glass Fiber Forming and Finishing

Molten glass from either the direct melting furnace or the indirect marble melting furnace is temperature-regulated to a precise viscosity and delivered to forming stations.

 

At the forming stations, the molten glass is forced through heated platinum bushings containing numerous very small openings. The continuous fibers emerging from the openings are drawn over a roller applicator, which applies a coating of a water-soluble sizing and/or coupling agent. The coated fibers are gathered and wound into a spindle. The spindles of glass fibers are next conveyed to a drying oven, where moisture is removed from the sizing and coupling agents. The spindles are then sent to an oven to cure the coatings. The final fabrication includes twisting, chopping, weaving, and packaging the fiber.