News -

About fiberglass (Definition and much more )

How is fiberglass made?


Fiberglass refers to a group of products made from individual glass fibers combined into a variety of forms. Glass fibers can be divided into two major groups according to their geometry: continuous fibers used in yarns and textiles, and the discontinuous (short) fibers used as batts, blankets, or boards for insulation and filtration. Fiberglass can be formed into yarn much like wool or cotton, and woven into fabric which is sometimes used for draperies. Fiberglass textiles are commonly used as a reinforcement material for molded and laminated plastics. Fiberglass wool, a thick, fluffy material made from discontinuous fibers, is used for thermal insulation and sound absorption. It is commonly found in ship and submarine bulkheads and hulls; automobile engine compartments and body panel liners; in furnaces and air conditioning units; acoustical wall and ceiling panels; and architectural partitions. Fiberglass can be tailored for specific applications such as Type E (electrical), used as electrical insulation tape, textiles and reinforcement; Type C (chemical), which has superior acid resistance, and Type T, for thermal insulation.

Though commercial use of glass fiber is relatively recent, artisans created glass strands for decorating goblets and vases during the Renaissance. A French physicist, Rene-Antoine Ferchault de Reaumur, produced textiles decorated with fine glass strands in 1713, and British inventors duplicated the feat in 1822. A British silk weaver made a glass fabric in 1842, and another inventor, Edward Libbey, exhibited a dress woven of glass at the 1893 Columbian Exposition in Chicago.

Glass wool, a fluffy mass of discontinuous fiber in random lengths, was first produced in Europe at the turn of the century, using a process that involved drawing fibers from rods horizontally to a revolving drum. Several decades later, a spinning process was developed and patented. Glass fiber insulating material was manufactured in Germany during World War I. Research and development aimed at the industrial production of glass fibers progressed in the United States in the 1930s, under the direction of two major companies, the Owens-Illinois Glass Company and Corning Glass Works. These companies developed a fine, pliable, low-cost glass fiber by drawing molten glass through very fine orifices. In 1938, these two companies merged to form Owens-Corning Fiberglas Corp. Now simply known as Owens-Corning, it has become a $3-billion-a-year company, and is a leader in the fiberglass market.

Raw Materials

The basic raw materials for fiberglass products are a variety of natural minerals and manufactured chemicals. The major ingredients are silica sand, limestone, and soda ash. Other ingredients may include calcined alumina, borax, feldspar, nepheline syenite, magnesite, and kaolin clay, among others. Silica sand is used as the glass former, and soda ash and limestone help primarily to lower the melting temperature. Other ingredients are used to improve certain properties, such as borax for chemical resistance. Waste glass, also called cullet, is also used as a raw material. The raw materials must be carefully weighed in exact quantities and thoroughly mixed together (called batching) before being melted into glass.

The Manufacturing Process


  • Once the batch is prepared, it is fed into a furnace for melting. The furnace may be heated by electricity, fossil fuel, or a combination of the two. Temperature must be precisely controlled to maintain a smooth, steady flow of glass. The molten glass must be kept at a higher temperature (about 2500°F [1371°C]) than other types of glass in order to be formed into fiber. Once the glass becomes molten, it is transferred to the forming equipment via a channel (forehearth) located at the end of the furnace.

Forming into fibers

  • Several different processes are used to form fibers, depending on the type of fiber. Textile fibers may be formed from molten glass directly from the furnace, or the molten glass may be fed first to a machine that forms glass marbles of about 0.62 inch (1.6 cm) in diameter. These marbles allow the glass to be inspected visually for impurities. In both the direct melt and marble melt process, the glass or glass marbles are fed through electrically heated bushings (also called spinnerets). The bushing is made of platinum or metal alloy, with anywhere from 200 to 3,000 very fine orifices. The molten glass passes through the orifices and comes out as fine filaments.

Continuous-filament process

  • A long, continuous fiber can be produced through the continuous-filament process. After the glass flows through the holes in the bushing, multiple strands are caught up on a high-speed winder. The winder revolves at about 2 miles (3 km) a minute, much faster than the rate of flow from the bushings. The tension pulls out the filaments while still molten, forming strands a fraction of the diameter of the openings in the bushing. A chemical binder is applied, which helps keep the fiber from breaking during later processing. The filament is then wound onto tubes. It can now be twisted and plied into yarn.

Staple-fiber process

  • An alternative method is the staplefiber process. As the molten glass flows through the bushings, jets of air rapidly cool the filaments. The turbulent bursts of air also break the filaments into lengths of 8-15 inches (20-38 cm). These filaments fall through a spray of lubricant onto a revolving drum, where they form a thin web. The web is drawn from the drum and pulled into a continuous strand of loosely assembled fibers. This strand can be processed into yarn by the same processes used for wool and cotton.

Chopped fiber

  • Instead of being formed into yarn, the continuous or long-staple strand may be chopped into short lengths. The strand is mounted on a set of bobbins, called a creel, and pulled through a machine which chops it into short pieces. The chopped fiber is formed into mats to which a binder is added. After curing in an oven, the mat is rolled up. Various weights and thicknesses give products for shingles, built-up roofing, or decorative mats.

Glass wool

  • The rotary or spinner process is used to make glass wool. In this process, molten glass from the furnace flows into a cylindrical container having small holes. As the container spins rapidly, horizontal streams of glass flow out of the holes. The molten glass streams are converted into fibers by a downward blast of air, hot gas, or both. The fibers fall onto a conveyor belt, where they interlace with each other in a fleecy mass. This can be used for insulation, or the wool can be sprayed with a binder, compressed into the desired thickness, and cured in an oven. The heat sets the binder, and the resulting product may be a rigid or semi-rigid board, or a flexible batt.

Protective coatings

  • In addition to binders, other coatings are required for fiberglass products. Lubricants are used to reduce fiber abrasion and are either directly sprayed on the fiber or added into the binder. An anti-static composition is also sometimes sprayed onto the surface of fiberglass insulation mats during the cooling step. Cooling air drawn through the mat causes the anti-static agent to penetrate the entire thickness of the mat. The anti-static agent consists of two ingredients—a material that minimizes the generation of static electricity, and a material that serves as a corrosion inhibitor and stabilizer.

    Sizing is any coating applied to textile fibers in the forming operation, and may contain one or more components (lubricants, binders, or coupling agents). Coupling agents are used on strands that will be used for reinforcing plastics, to strengthen the bond to the reinforced material.

    Sometimes a finishing operation is required to remove these coatings, or to add another coating. For plastic reinforcements, sizings may be removed with heat or chemicals and a coupling agent applied. For decorative applications, fabrics must be heat treated to remove sizings and to set the weave. Dye base coatings are then applied before dying or printing.

Forming into shapes

  • Fiberglass products come in a wide variety of shapes, made using several processes. For example, fiberglass pipe insulation is wound onto rod-like forms called mandrels directly from the forming units, prior to curing. The mold forms, in lengths of 3 feet (91 cm) or less, are then cured in an oven. The cured lengths are then de-molded lengthwise, and sawn into specified dimensions. Facings are applied if required, and the product is packaged for shipment.

Quality Control

During the production of fiberglass insulation, material is sampled at a number of locations in the process to maintain quality. These locations include: the mixed batch being fed to the electric melter; molten glass from the bushing which feeds the fiberizer; glass fiber coming out of the fiberizer machine; and final cured product emerging from the end of the production line. The bulk glass and fiber samples are analyzed for chemical composition and the presence of flaws using sophisticated chemical analyzers and microscopes. Particle size distribution of the batch material is obtained by passing the material through a number of different sized sieves. The final product is measured for thickness after packaging according to specifications. A change in thickness indicates that glass quality is below the standard.

Fiberglass insulation manufacturers also use a variety of standardized test procedures to measure, adjust, and optimize product acoustical resistance, sound absorption, and sound barrier performance. The acoustical properties can be controlled by adjusting such production variables as fiber diameter, bulk density, thickness, and binder content. A similar approach is used to control thermal properties.

The Future

The fiberglass industry faces some major challenges over the rest of the 1990s and beyond. The number of producers of fiberglass insulation has increased due to American subsidiaries of foreign companies and improvements in productivity by U.S. manufacturers. This has resulted in excess capacity, which the current and perhaps future market cannot accommodate.

In addition to excess capacity, other insulation materials will compete. Rock wool has become widely used because of recent process and product improvements. Foam insulation is another alternative to fiberglass in residential walls and commercial roofs. Another competing material is cellulose, which is used in attic insulation.

Because of the low demand for insulation due to a soft housing market, consumers are demanding lower prices. This demand is also a result of the continued trend in consolidation of retailers and contractors. In response, the fiberglass insulation industry will have to continue to cut costs in two major areas: energy and environment. More efficient furnaces will have to be used that do not rely on only one source of energy.

With landfills reaching maximum capacity, fiberglass manufacturers will have to achieve nearly zero output on solid waste without increasing costs. This will require improving manufacturing processes to reduce waste (for liquid and gas waste as well) and reusing waste wherever possible.

Such waste may require reprocessing and remelting before reusing as a raw material. Several manufacturers are already addressing these issues.

Where To Learn More


Aubourg, P.F., C. Crall, J. Hadley, R.D. Kaverman, and D.M. Miller. "Glass Fibers, Ceramics and Glasses," in Engineered Materials Handbook, Vol. 4. ASM International, 1991, pp. 1027-31.

McLellan, G.W. and E.B. Shand. Glass Engineering Handbook. McGraw-Hill, 1984.

Pfaender, H.G. Schott Guide To Glass. Van Nostrand Reinhold Company, 1983.

Tooley, F.V. "Fiberglass, Ceramics and Glasses," in Engineered Materials Handbook, Vol. 4. ASM International, 1991, pp. 402-08.


Hnat, J.G. "Recycling of Insulation Fiberglass Waste." Glass Production Technology International, Sterling Publications Ltd., pp. 81-84.

Webb, R.O. "Major Forces Impacting the Fiberglass Insulation Industry in the 1990s." Ceramic Engineering and Science Proceedings, 1991, pp. 426-31.

[Article by: Laurel M. Sheppard]

Laminating layers of glass and resin to make a boat hull
The most popular boatbuilding material is a mixture of fine glass strands and cured polyester resin known as fiberglass, glass-reinforced plastic (GRP), or fiber-reinforced

Common fiberglass materials include the following (top to bottom): Chopped-strand mat, in which randomly oriented glass strands of irregular length are either glued to a scrim backing or loaded into a chopper gun for professional application. Woven roving comprises bundles of continuous glass strands assembled into a coarse weave. In unwoven roving the bundles are stitched together parallel to one another to give great strength along the axis of the bundles. If the unwoven roving shown here were cross-stitched to a second layer oriented 90 degrees to this one, the result would be a biaxial roving with greater strength in two directions than even a woven roving of the same weight provides. The fiberglass cloth at bottom has a finer weave and a lighter weight than woven roving. Professional builders use it in specialized applications but rarely in hull construction. For do-it-yourself repairs, however, you’ll have an easier time and get better results if you use mat and cloth rather than mat and roving.

plastic (FRP). Its success stems from the fact that one set of molds can produce hundreds of identical hulls, decks, cockpits, and cabintops.Most boatbuilders use the same laminate—alternating layers of chopped strand or mat (1? to 3 oz. per square yard) and woven roving (24 oz. per square yard) wetted out with standard polyester resin. Vinylester resin is more expensive than polyester but less permeable to moisture and more resistant to osmotic blistering or “boat pox.” Some expensive boats use vinylester throughout the layup; in other boats, it’s used only in the surface layer of the laminate, where it bonds very well to the polyester resin beneath it. A skin coat (just beneath the gelcoat) consisting of 3 ounces of chopped strand or mat wet out with vinylester resin greatly reduces the odds of blistering. A skin coat is always a good idea no matter the resin because it prevents “print-through” of the coarse woven-roving pattern onto the gelcoat.Epoxy resin, which is stronger and more flexible than polyester, is not normally used for production hulls because it is more expensive, but it’s widely used for repair work and special projects. Epoxy adheres well to the old polyester resin in boat hulls and gelcoats, but polyester does not fare so well when applied to old epoxy.The two layers mentioned previously (mat and roving) constitute one ply and measure about 3/32 inch (2.4 mm) in thickness. Such a laminate weighs about 94 pounds per cubic foot (1,506 kg per cu m). The glass fibers should comprise about 35 percent of that weight.The thickness of a fiberglass hull in inches should roughly equal the waterline length in feet divided by 150, plus 0.07. The topsides are usually 15 percent thinner than average, whereas the hull at the waterline and below is 15 percent thicker. Powerboats need hulls that are thicker than normal by 1 percent for every knot in speed over 10 knots.Despite its popularity, GRP has some major drawbacks. It lacks the warmth and personality of wood and weighs about three times as much for the same volume of material. It is the floppiest of all boatbuilding materials; therefore, to provide the stiffness a boat requires, most solid fiber-glass hulls should have five or more longitudinal stringers on each side of the inside of the hull. Bulkheads also provide stiffness, as do the molded, ribbed fiber-glass grids often placed under the cabin sole in fiberglass construction.Early fiberglass boats of the 1960s and 1970s were often over-built by manufacturers who were just making the transition from wood and still unsure of fiberglass’s strength. Most of those early boats are still around, and when they are well maintained and updated, they can make great bargains on the used-boat market. Recent boats—especially at the high end—are not just built but also engineered. Most now have foam or balsa cores sandwiched within inner and outer laminates of fiberglass for stiffness and lightness. High-performance boats may have super-lightweight honeycomb cores encased in laminates of vinylester or epoxy resin in which the fiberglass reinforcement is replaced in high-stress areas with Kevlar or carbon fiber. Such hulls are usually laminated with vacuum bagging to remove all excess resin for the very lightest weight.Fiberglass decks are almost always cored construction these days because stiffness in that location is at a premium and cannot be gained from convex curvature in a relatively flat expanse. A “squishy” deck that yields underfoot is often an indication that moisture has invaded and rotted a balsa core—often through fastener holes. Spider-web crazing or local swelling in the deck gelcoat may indicate the same thing. Beware: the repair can be expensive.Fiberglass is long-lasting but not maintenance-free. The gelcoat surface eventually chalks, degrades, and requires painting. Water vapor passing through the outer layers of the underwater hull can cause blistering (osmosis) and delamination—expensive repairs.

Bundle of fiberglass
Bundle of fiberglass

Fiberglass or glassfibre is material made from extremely fine fibers of glass. It is used as a reinforcing agent for many polymer products; the resulting composite material, properly known as fiber-reinforced polymer (FRP) or glass-reinforced plastic (GRP), is called "fiberglass" in popular usage.

Glassmakers throughout history have experimented with glass fibers, but mass manufacture of fiberglass was only made possible with the advent of finer machine-tooling. In 1893, Edward Drummond Libbey exhibited a dress at the World's Columbian Exposition incorporating glass fibers with the diameter and texture of silk fibers. What is commonly known as "fiberglass" today, however, was invented in 1938 by Russell Games Slayter of Owens-Corning as a material to be used as insulation. It is marketed under the trade name Fiberglass, which has become a genericized trademark.


Glass fiber is formed when thin strands of silica-based or other formulation glass is extruded into many fibers with small diameters suitable for textile processing. Glass is unlike other polymers in that, even as a fiber, it has little crystalline structure (see amorphous solid). The properties of the structure of glass in its softened stage are very much like its properties when spun into fiber. One definition of glass is "an inorganic substance in a condition which is continuous with, and analogous to the liquid state of that substance, but which, as a result of a reversible change in viscosity during cooling, has attained so high a degree of viscosity as to be for all practical purposes rigid." [1]

The technique of heating and drawing glass into fine fibers has been known to exist for thousands of years; however, the concept of using these fibers for textile applications is more recent. The first commercial production of fiberglass was in 1936. In 1938, Owens-Illinois Glass Company and Corning Glass Works joined to form the Owens-Corning Fiberglas Corporation. Until this time all fiberglass had been manufactured as staple. When the two companies joined together to produce and promote fiberglass, they introduced continuous filament glass fibers. [1] Owens-Corning is still the major fiberglass producer in the market today.


The basis of textile grade glass fibers is silica, SiO2. In its pure form it exists as a polymer, (SiO2)n. It has no true melting point but softens up to 2000°C, where it starts to degrade. At 1713°C, most of the molecules can move about freely. If the glass is then cooled quickly, they will be unable to form an ordered structure. [2] In the polymer it forms SiO4 groups which are configured as a tetrahedron with the silicon atom at the center, and four oxygen atoms at the corners. These atoms then form a network bonded at the corners by sharing the oxygen atoms.

The vitreous and crystalline states of silica (glass and quartz) have similar energy levels on a molecular basis, also implying that the glassy form is extremely stable. In order to induce crystallization, it must be heated to temperatures above 1200°C for long periods of time. [1]

Molecular Structure of Glass
Molecular Structure of Glass

Although pure silica is a perfectly viable glass and glass fiber, it must be worked with at very high temperatures which is a drawback unless its specific chemical properties are needed. It is usual to introduce impurities into the glass in the form of other materials, to lower its working temperature. These materials also impart various other properties to the glass which may be beneficial in different applications. The first type of glass used for fiber was soda-lime glass or A glass. It was not very resistant to alkali. A new type, E-glass was formed that is alkali free (< 2%) and is an alumino-borosilicate glass [3]. This was the first glass formulation used for continuous filament formation. E-glass still makes up most of the fiberglass production in the world. Its particular components may differ slightly in percentage, but must fall within a specific range. The letter E is used because it was originally for electrical applications. S-glass is a high strength formulation for use when tensile strength is the most important property. C-glass was developed to resist attack from chemicals, mostly acids which destroy E-glass. [3] T-glass is a North American variant of C-glass. A-glass is an industry term for cullet glass, often bottles, made into fiber. AR-glass is alkali resistant glass. Most glass fibers have limited solubility in water but it is very dependent on pH. Chloride ions will also attack and dissolve E-glass surfaces. A recent trend in the industry is to reduce or eliminate the boron content in the glass fibers.

Since E-glass does not really melt but soften, the softening point is defined as written next, “the temperature at which a 0.55 – 0.77 mm diameter fiber 9.25 inches long, elongates under its own weight at 1 mm/min when suspended vertically and heated at the rate of 5°C per minute”. [4] The strain point is reached when the glass has a viscosity of 1014.5 poise. The annealing point, which is the temperature where the internal stresses are reduced to an acceptable commercial limit in 15 minutes, is marked by a viscosity of 1013 poise. [4]


Glass fibers are useful because of their high ratio of surface area to weight. However, the increased surface makes them much more susceptible to chemical attack.

By trapping air within them, blocks of glass fiber make good thermal insulation, with a thermal conductivity of 0.04 W/mK.

Glass strengths are usually tested and reported for "virgin" fibers which have just been manufactured. The freshest, thinnest fibers are the strongest and this is thought to be due to the fact that it is easier for thinner fibers to bend. The more the surface is scratched, the less the resulting tenacity is. [3] Because glass has an amorphous structure, its properties are the same along the fiber and across the fiber. [2] Humidity is an important factor in the tensile strength. Moisture is easily adsorbed, and can worsen microscopic cracks and surface defects, and lessen tenacity.

In contrast to carbon fiber, glass can undergo more elongation before it breaks. [2]

The viscosity of the molten glass is very important for manufacturing success. During drawing (pulling of the glass to reduce fiber circumference) the viscosity should be relatively low. If it is too high the fiber will break during drawing, however if it is too low the glass will form droplets rather than drawing out into fiber.

Manufacturing processes

There are two main types of glass fiber manufacture and two main types of glass fiber product. First, fiber is made either from a direct melt process or a marble remelt process. Both start with the raw materials in solid form. The materials are mixed together and melted in a furnace. Then, for the marble process, the molten material is shared and rolled into marbles which are cooled and packaged. The marbles are taken to the fiber manufacturing facility where they are inserted into a can and remelted. The molten glass is extruded to the bushing to be formed into fiber. In the direct melt process, the molten glass in the furnace goes right to the bushing for formation. [4]

The bushing plate is the most important part of the machinery. This is a small metal furnace containing nozzles for the fiber to be formed through. It is almost always made of platinum alloyed with rhodium for durability. Platinum is used because the glass melt has a natural affinity for wetting it. When bushings were first used they were 100% platinum and the glass wetted the bushing so easily it ran under the plate after exiting the nozzle and accumulated on the underside. Also, due to its cost and the tendency to wear, the platinum was alloyed with rhodium. In the direct melt process, the bushing serves as a collector for the molten glass. It is heated slightly to keep the glass at the correct temperature for fiber formation. In the marble melt process, the bushing acts more like a furnace as it melts more of the material. [1]

The bushings are what make the capital investment in fiber glass production expensive. The nozzle design is also critical. The number of nozzles ranges from 200 to 4000 in multiples of 200. The important part of the nozzle in continuous filament manufacture is the thickness of its walls in the exit region. It was found that inserting a counterbore here reduced wetting. Today, the nozzles are designed to have a minimum thickness at the exit. The reason for this is that as glass flows through the nozzle it forms a drop which is suspended from the end. As it falls, it leaves a thread attached by the meniscus to the nozzle as long as the viscosity is in the correct range for fiber formation. The smaller the annular ring of the nozzle or the thinner the wall at exit, the faster the drop will form and fall away, and the lower its tendency to wet the vertical part of the nozzle. [1] The surface tension of the glass is what influences the formation of the meniscus. For E-glass it should be around 400 mN per m. [3]

The attenuation (drawing) speed is important in the nozzle design. Although slowing this speed down can make coarser fiber, it is uneconomic to run at speeds for which the nozzles were not designed. [1]

In the continuous filament process, after the fiber is drawn, a size is applied. This size helps protect the fiber as it is wound onto a bobbin. The particular size applied relates to end-use. While some sizes are processing aids, others make the fiber have an affinity for a certain resin, if the fiber is to be used in a composite. [4] Size is usually added at 0.5–2.0% by weight. Winding then takes place at around 1000 m per min. [2]

In staple fiber production, there are a number of ways to manufacture the fiber. The glass can be blown or blasted with heat or steam after exiting the formation machine. Usually these fibers are made into some sort of mat. The most common process used is the rotary process. Here, the glass enters a rotating spinner, and due to centrifugal force is thrown out horizontally. The air jets pushes it down vertically and binder is applied. Then the mat is vacuumed to a screen and the binder is cured in the oven. [5]

End uses for regular fiber glass are mats, insulation, reinforcement, heat resistant fabrics, corrosion resistant fabrics and high strength fabrics.

related keywords