Fiberglass

Glass fiber cuff.

The fiberglass is a material consisting of many extremely fine ceramic filaments based on silicon dioxide (SiO₂).

Throughout history, glassmakers have tested with fiberglass, but the mass manufacture of this material was only possible with the invention of more refined machines and tools. In 1893, Edward Drummond Libbey exhibited a dress at the World's Columbian Exposition in Chicago that had fiberglass with filaments the diameter and texture of a silk fiber. It was first worn by Georgia Cayvan, a well-known stage actress at the time. Glass fibers can also form naturally and are known as Pele's hairs.

However, glass wool, commonly called fiberglass today, was not invented until 1938 by Russell Games Slayter at Owens-Corning as a material that could be used as insulation in the construction of buildings. It was marketed under the trade name Fiberglass, which has since become a popular brand in English-speaking countries.

Fiberglass is commonly known as an insulating material. It is also used as a reinforcing agent with many polymeric products; It is typically used to form glass-reinforced plastic, also known by metonymy as fiberglass, a form of composite material consisting of fiber-reinforced polymer. For this reason, it essentially exhibits behaviors similar to other composites made of fiber and polymer such as carbon fiber. Although it is not as strong or rigid as carbon fiber, it is much cheaper and less brittle.

Fiber formation

Fiberglass is made up of thin strands made from silica or special glass formulations, extruded as tiny diameter filaments and suitable for weaving processes. The technique of heating and making fine fibers from glass has been known for thousands of years; however, the use of these fibers for textile applications is much more recent: only until now has it been possible to manufacture stored glass fibers and strands in standardized, chopped lengths. The first commercial production of fiberglass occurred in 1936; in 1938 the Owens-Illinois Glass Company and the Corning Glass Works merged to form the Owens-Corning Fiberglas Corporation. When the two companies came together to produce and promote fiberglass, they introduced continuous fiberglass filaments to the market. Owens-Corning remains the largest producer of fiberglass in the market today.

The most commonly used types of fiberglass are class E glass (E-glass: alumino-borosilicate glass with less than 1% weight of alkaline oxides, mainly used for reinforced plastic with glass, but classes A are also used (A-glass: alkali-lime glass with few or no boron oxides), class E-CR (E-CR glass: alkali-lime silicate with less than 1% weight/weight of alkaline oxides, with high resistance to acids), class C (C-glass: alkali-lime glass with high oxide content boron, used for example in glass fibers with short filaments), class D (D-glass: borosilicate glass with a high dielectric constant), class R (R-glass: aluminosilicate glass without MgO or CaO with high mechanical properties) and class S (S-glass: aluminosilicate glass without CaO but with a high MgO content with high tensile strength).

Chemistry of fiberglass

Theoretical molecular structure of glass

The fiberglass useful for weaving is based on the compound silica, SiO₂. In its pure form, silicon dioxide behaves like a polymer (SiO₂)_n. That is, it does not have a true melting point but softens at 1,200 °C, at which point it begins to decompose, and at 1,713 °C most molecules are free to move. If the glass has been extruded and rapidly cooled from this temperature, it is impossible to obtain an ordered structure. In its polymer state, SiO₄ clusters are formed that are configured in a tetrahedral structure with the atom silicon in the center, and four oxygen atoms at the tips. These atoms then form a lattice of corner bonds that the oxygen atoms share.

The glassy and crystalline states of silica (glass and quartz) have similar energy levels in their molecular bases, which implies that in its glassy form it is extremely stable; In order to reduce crystallization, it must be heated to temperatures above 1200 °C for prolonged periods of time.

Although pure silica is perfectly viable for making glass and fiberglass, it must be processed at very high temperatures, which is a drawback unless its specific chemical properties are required. It would seem unusual to introduce impurities to the glass, however adding some materials contributes to lower its working temperature; these materials also add other properties to the glass that can be beneficial in different applications. The first type of glass used to make fiber was soda lime glass or Class A glass, which is not very resistant to alkaline compounds; to correct this, a new type known as Class E, was developed as an alumino-borosilicate glass that is free of alkaline elements (<2%); this was the first glass formulation used for filament formation. Class E glass is still the main form of fiberglass production and its particular compounds may have slight variations that must remain within a certain range. The letter E is used because it was developed primarily for electrical applications. Class S glass is a formulation whose main characteristic is high tensile strength and for this reason it receives its letter (from tensile strength). Class C glass was developed to resist chemical attack, mainly from acids that would destroy class E glass (its letter then comes from chemical resistance). Class T glass is a commercial variant of North American Fiberglass from Class C glass. Class A glass is an industry reference for recycled glass, often from bottles, used to make glass wool. The AR class is a glass resistant to alkaline compounds (AR for alkali-resistant). Most glass fibers have limited solubility in water, but this changes in relation to pH. Chloride ions can also attack and dissolve Class E glass surfaces.

Class E glass cannot actually melt, but instead softens, its softening point being defined as "the temperature at which a fiber with a diameter between 0.55 and 0.77 mm of 235 mm in length, it elongates under its own load at a rate of 1 mm/min when suspended vertically and has been heated at a rate of 5 °C per minute". The deformation point is reached when the glass has a viscosity of 10^14.5 poise. The attenuation (quench) point, which is the temperature at which internal stresses are reduced to a commercially acceptable limit of 15 minutes, is determined by a viscosity of 10¹³ poise.

Properties

Sounds

Thanks to the composition of the resin and the direction that the fibers have when the composite material is formed, it is a great acoustic insulator, since it reflects sound waves.

Thermals

Glass fibers are good thermal insulators due to their high surface area to weight ratio. However, an increased surface area makes it much more vulnerable to chemical attack. Fiberglass blocks trap air between them, making fiberglass a good thermal insulator, with thermal conductivity of the order of 0.05 W/(m K)

Tension

Glass stress is usually checked and reported for "virgin" or pristine (those that have just been manufactured). The thinner, newly made fibers are the strongest because they are more ductile. The more its surface is scratched, the lower the resulting toughness. Because glass has an amorphous structure, its properties are isotropic, that is, they are the same along and across the fiber (unlike carbon fiber). carbon, whose molecular structure makes its properties different across and across, that is, they are anisotropic). Moisture is an important factor for ultimate stress; it can be easily adsorbed and cause ruptures and microscopic surface defects, lowering the toughness.

Unlike carbon fiber, glass fiber can withstand more elongation before breaking; there is a proportional relationship between the bending diameter of the filament and the diameter of the filament itself. The viscosity of molten glass is very important for success during manufacturing; during shaping (pulling the glass to reduce the thickness of the fiber) the viscosity should be relatively low; if it is too high, the fiber can break while pulling. However, if it is too low, the glass can form beads instead of becoming useful filaments to make fiber.

Manufacturing processes

Foundry

There are two main types of fiber manufacturing and two types of output. The first is fiber made from a direct casting process and the second is a marble recast process. Both start with the material in its solid form; the materials are combined and melted in a furnace. Then, for the marble process, the molten material is separated by shear stress and rolled into marbles that are cooled and packed. The marbles are taken to the fiber manufacturing facility where they are inserted into containers to be recast; Molten glass is extruded into threaded coils (similar to threaded inserts) to form the fiber. In the direct casting process, the glass melted in the furnace goes directly to the formation of the inserts.

Training

The plate where the inserts are threaded is the main component in the machining of the fiber. It consists of a hot metal plate in which the nozzles are located, through which fiber will be made from the inserts introduced into them. This plate is almost always made of a platinum-rhodium alloy for durability. Platinum is used because molten glass has a natural affinity for making it wet. The first plates used for this purpose were 100% platinum and the glass penetrated them so easily that it soaked the plate and collected as residue at the exit of the nozzles. This platinum-rhodium alloy is also used because of the cost of platinum and its tendency to tarnish easily; In the direct casting process, the plates also have the function of collecting the molten glass. They are used slightly hot to keep the glass at the correct temperature, suitable for the formation of the fiber. In the marble casting process, the plate acts more like a heat distributor, in that it melts most of the material.

These plates represent the largest cost in the production of fiberglass. The design of the nozzles is also important; the number of nozzles covers a range from 200 to 4000 in multiples of 200. One of the most important dimensions to take into account in the production of continuous filaments is the thickness of the walls of the nozzles at their exit; it was found that adding an enlargement of the cavity before the orifice reduced wet-out. Currently, nozzles are designed to have as thin a wall thickness as possible at the end; As the glass flows through the nozzle it forms a droplet that is suspended vertically, and as it falls, it leaves a thread connected by the meniscus to the nozzle, which will be as long as the nozzle design allows. The smaller the nozzle ring (the final part of the walls surrounding the exit orifice) the faster it will allow the formation of the falling drop and the lower the tendency for it to soak the vertical part of the nozzle. surface tension of the glass is what influences the formation of the meniscus; for Class E glass it should be approximately 400mN per minute.

The rate of attenuation (cooling) is important in nozzle design. Although lowering this speed would allow harder fiber to be made, it is not economically viable to operate at low speeds and for which the nozzles are not designed.

Continuous filament process

In the continuous filament process, after being attenuated, a special sizing is applied to the fiber that allows it to be wound or rolled. The addition of this compound may also be related to its intended use, since some of them are co-reactive (pre-impregnated) with certain types of resin when the fiber is going to be used to form a composite material. Added usually has a ratio of between 1 and 2% by weight. Subsequent winding is done at a rate of 1000m per minute.

Standard fiber process

For the production of ordinary fiberglass (also useful for making "glass wool"), there are various manufacturing methods. Glass may be blown or sprayed with heat or steam after leaving the forming (cast) machine; usually this fiber is turned into a certain type of textile, similar to a felt. The most common process is the rotary process, in which the glass enters a rotor that, by centrifugal force, shoots the glass into pieces horizontally while jets of air push it downwards, where it receives a binder. This plush is then sucked into a curtain that shapes it, and the binder is cured using an oven.

Health

Fiberglass has become very popular since asbestos was found to cause cancer, and it has been removed from many products. However, the safety of fiberglass has also been called into question because research shows that the composition of this material (both asbestos and fiberglass are silicate fibers) can cause asbestos-like toxicity..

Studies conducted on rats in the 1970s showed that glass fibers less than 3 microns in diameter and greater than 20 microns in length were a "potential carcinogen". Likewise at the International Center for Research on Cancer CIRC it was found that "could be reasonably anticipated as a carcinogen" in 1990. On the other hand, in the American Conference of Governmental Industrial Hygienists, it is stipulated that there is insufficient evidence and that fiberglass is on the association's list within group A4: "Not classified as a carcinogen human".

The North American Insulation Manufacturers Association (NAIMA) claims that fiberglass is fundamentally different from asbestos, in that it is a man-made material rather than a product of nature. Members of this association claim that fiberglass "dissolves in the lungs" while asbestos remains for life within the body. Although fiberglass and asbestos are made from silica filaments, NAIMA says asbestos poses a greater risk because of its crystalline structure, which causes the material to delaminate into smaller, more dangerous pieces, citing the Department of Health. and United States Social Services:

Synthetic glass fibers (glass fiber) differ from asbestos in two forms that can provide at least partial explanations of why their low toxicity. Because most synthetic vitreous fibers are not crystalline asbestos, they cannot be separated longitudinally to produce thinner fibers. They also present a marked lower biopersiste in living tissues than asbestos fibers because they can dissolve and suffer transverse ruptures.

In 1998, a study was carried out using rats, which showed that the biopersistence of synthetic fibers after one year was 0.04 to 10%, but it was 27% for asbestos fibers from the grunerite variety. These fibers that stayed longer proved to be more carcinogenic.

Fiberglass reinforced plastic

Fiberglass-reinforced plastic is a composite material or fiber-reinforced plastic (FRP) made of a polymer armed with fine glass fibers. Like carbon fiber reinforced plastic, it suffers from a synecdoche that simplifies its enunciation as fiberglass, when referring to the composite material. The fiber presented in CSM (chopped strand mat) can be used, which is essentially a fabric in rolls made of loose pieces (different from glass wool that is characterized by its cotton appearance, much more fluffy), or as a woven cloth (sometimes called a mat).

Like many other composite materials (for example reinforced concrete) the two materials act at the same time, each complementing the properties of the other. While polymeric resins are strong under physical compressive loads, they are relatively weak in tension; or tensile loads; fiberglass is very strong in tension but tends not to resist compression; so by combining both materials, the FRP Fiber Reinforced Plastic becomes a material that resists both compression and tension in acceptable ranges.

Uses

The normal use of fiberglass includes acoustic insulation, thermal insulation and electrical insulation in coatings, as reinforcement to various materials, tent poles, sound absorption, heat and corrosion resistant fabrics, high-density fabrics. resistance, pole vault poles, bows and crossbows, translucent skylights, auto body parts, hockey sticks, surfboards, boat hulls, and lightweight honeycomb structural fillers. It has been used for medical purposes in splints. Fiberglass is widely used for the manufacture of composite tanks and silos.

Importance of recycling glass to make fiber

Manufacturers of fiberglass insulation can use recycled glass. The fiber produced by Owens Corning is 40% from recycled glass. In 2009, this company began a glass recycling program to send recycled glass waste from Kansas City to the Owens Corning plant to be used as raw material to make class A fiberglass.