Fiberglass

Bundle of fiberglass

Fiberglass, (also called fibreglass and glass fibre), 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 invention 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. This was first worn by the popular stage actress of the time Georgia Cayvan.

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 Fiberglas, which has become a genericized trademark. A somewhat similar, but more expensive technology used for applications requiring very high strength and low weight is the use of carbon fiber.

Fiber formation

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. The technique of heating and drawing glass into fine fibers has been known for millennia; however, the use of these fibers for textile applications is more recent. Until this time all fiberglass had been manufactured as staple (a term used to describe naturally formed clusters or locks of wool fibres). 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. When the two companies joined to produce and promote fiberglass, they introduced continuous filament glass fibers.^[1] Owens-Corning is still the major fiberglass producer in the market today.^[2]

The types of fiberglass most commonly used are mainly E-glass (alumino-borosilicate glass with less than 1 wt% alkali oxides, mainly used for glass-reinforced plastics), but also A-glass (alkali-lime glass with little or no boron oxide), E-CR-glass (alumino-lime silicate with less than 1 wt% alkali oxides, has high acid resistance), C-glass (alkali-lime glass with high boron oxide content, used for example for glass staple fibers), D-glass (borosilicate glass with high dielectric constant), R-glass (alumino silicate glass without MgO and CaO with high mechanical requirements), and S-glass (alumino silicate glass without CaO but with high MgO content with high tensile strength).^[3]

Chemistry

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 at 2,000 °C (3,630 °F), where it starts to degrade. At 1,713 °C (3,115 °F), most of the molecules can move about freely. If the glass is then cooled quickly, they will be unable to form an ordered structure.^[4] In the polymer, it forms SiO₄ groups that 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 1,200 °C (2,190 °F) for long periods of time.^[1]

Molecular Geometry 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 that 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; this is an alumino-borosilicate glass that is alkali free (<2%).^[5] 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 that destroy E-glass.^[5] 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 are very dependent on pH. Chloride ions will also attack and dissolve E-glass surfaces.

Since E-glass does not really melt, but soften, the softening point is defined as "the temperature at which a 0.55–0.77 mm diameter fiber 235 mm long, elongates under its own weight at 1 mm/min when suspended vertically and heated at the rate of 5°C per minute".^[6] The strain point is reached when the glass has a viscosity of 10^14.5 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.^[6]

Properties

Glass fibers are useful because of their high ratio of surface area to weight. However, the increased surface area 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 the order of 0.05 W/(m·K).^[7]

The strength of glass is usually tested and reported for "virgin" or pristine fibers—those that have just been manufactured. The freshest, thinnest fibers are the strongest because the thinner fibers are more ductile. The more the surface is scratched, the less the resulting tenacity.^[5] Because glass has an amorphous structure, its properties are the same along the fiber and across the fiber.^[4] 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.^[4] There is a correlation between bending diameter of the filament and the filament diameter.^[8] 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.

Safety

Fiberglass has increased in popularity since the discovery that asbestos causes cancer and its subsequent removal from most products. However, the safety of fiberglass is also being called into question, as research shows that the composition of this material (asbestos and fiberglass are both silicate fibers) causes similar toxicity as asbestos.^{[9][10][11][12]}

1970s studies on rats found that fibrous glass of less than 3 micrometers in diameter and greater than 20 micrometers in length is a "potent carcinogen".^[9] Likewise, the International Agency for Research on Cancer found it "may reasonably be anticipated to be a carcinogen" in 1990. The American Conference of Governmental Industrial Hygienists, on the other hand, says that there is insufficient evidence, and that fiberglass is in group A4: "Not classifiable as a human carcinogen".

The North American Insulation Manufacturers Association (NAIMA) claims that fiberglass is fundamentally different from asbestos, since it is man-made instead of naturally-occurring.^[13] They claim that fiberglass "dissolves in the lungs", while asbestos remains in the body for life. Although both fiberglass and asbestos are made from silica filaments, NAIMA claims that asbestos is more dangerous because of its crystalline structure, which causes it to cleave into smaller, more dangerous pieces, citing the U.S. Department of Health and Human Services:

Synthetic vitreous fibers [fiber glass] differ from asbestos in two ways that may provide at least partial explanations for their lower toxicity. Because most synthetic vitreous fibers are not crystalline like asbestos, they do not split longitudinally to form thinner fibers. They also generally have markedly less biopersistence in biological tissues than asbestos fibers because they can undergo dissolution and transverse breakage.^[14]

A 1998 rat study found that the biopersistence of synthetic fibers after one year was 0.04–10%, but 27% for amosite asbestos. Fibers that persisted longer were found to be more carcinogenic.^[15]

Glass-reinforced plastic

Glass-reinforced plastic (GRP) is a composite material or fiber-reinforced plastic made of a plastic reinforced by fine glass fibers. Like graphite-reinforced plastic, the composite material is commonly referred to by the name of its reinforcing fibers (fiberglass). Thermosetting plastics are normally used for GRP production—most often unsaturated polyester (using 2-butanone peroxide aka MEK peroxide as a catalyst), but vinylester or epoxy are also used. Traditionally, styrene monomer was used as a reactive diluent in the resin formulation giving the resin a characteristic odor. More recently alternatives have been developed. The glass can be in the form of a chopped strand mat (CSM) or a woven fabric.^[3][16]

As with many other composite materials (such as reinforced concrete), the two materials act together, each overcoming the deficits of the other. Whereas the plastic resins are strong in compressive loading and relatively weak in tensile strength, the glass fibers are very strong in tension but have no strength against compression. By combining the two materials, GRP becomes a material that resists both compressive and tensile forces well.^[17] The two materials may be used uniformly or the glass may be specifically placed in those portions of the structure that will experience tensile loads.^[3][16]

Uses

Uses for regular fiberglass include mats, thermal insulation, electrical insulation, sound insulation, reinforcement of various materials, tent poles, sound absorption, heat- and corrosion-resistant fabrics, high-strength fabrics, pole vault poles, arrows, bows and crossbows, translucent roofing panels, automobile bodies, hockey sticks, surfboards, boat hulls, and paper honeycomb. It has been used for medical purposes in casts. Fiberglass is extensively used for making FRP tanks and vessels.^[3][16] Fiberglass is also used in the design of Irish stepdance shoes.^[18]

Role of recycling in fiberglass manufacturing

Manufacturers of fiberglass insulation can use recycled glass. Owens Corning's fiberglass has 40% recycled glass. A recycling program begun in 2009 in Kansas City, Kansas, will ship crushed recycled glass, called cullet, to the Owens Corning plant that will use it as raw material for fiberglass making.^[19][20]

See also

Notes and references

External links

• Fiberglass and health
• International Geosynthetics Society, information on geotextiles and geosynthetics in general.