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Fibreglass

2007 Schools Wikipedia Selection. Related subjects: Engineering

   Bundle of fiberglass
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   Bundle of fibreglass

   Fibreglass 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 fibre-reinforced
   polymer (FRP) or glass-reinforced plastic (GRP), is called "fibreglass"
   in popular usage.

   Glassmakers throughout history have experimented with glass fibers, but
   mass manufacture of fibreglass 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
   "fibreglass" 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 (sic), which has become a
   genericized trademark.

Formation

   Glass fibre 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 fibre, 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 fibre.
   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."

   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 fibreglass 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 fibreglass had
   been manufactured as staple. When the two companies joined together to
   produce and promote fibreglass, they introduced continuous filament
   glass fibers. Owens-Corning is still the major fibreglass producer in
   the market today.

Chemistry

   The basis of textile grade glass fibers is silica, SiO[2]. In its pure
   form it exists as a polymer, (SiO[2])[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. In the
   polymer it forms SiO[4] groups which are configured as a tetrahedron
   with the silicon atom at the centre, 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.
   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 fibre 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 . This was the first glass formulation used
   for continuous filament formation. E-glass still makes up most of the
   fibreglass 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.
   T-glass is a North American variant of C-glass. A-glass is an industry
   term for cullet glass, often bottles, made into fibre. AR-glass is
   alkali resistant glass. Most glass fibers have limited solubility in
   water but it is very dependent on pH. Chloride ion 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, “the temperature at which a 0.55 – 0.77 mm diameter fibre
   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”. 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 10^13 poise.

Properties

   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 fibre 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. Because glass has an amorphous
   structure, its properties are the same along the fiber and across the
   fibre. 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 fibre, glass can undergo more elongation before
   it breaks.

   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 fibre.

Manufacturing processes

   There are two main types of glass fiber manufacture and two main types
   of glass fiber product. First, fibre 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 sheared
   and rolled into marbles which are cooled and packaged. The marbles are
   taken to the fibre manufacturing facility where they are inserted into
   a can and remelted. The molten glass is extruded to the bushing to be
   formed into fibre. In the direct melt process, the molten glass in the
   furnace goes right to the bushing for formation.

   The bushing plate is the most important part of the machinery. This is
   a small metal furnace containing nozzles for the fibre 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 fibre formation. In the marble melt process,
   the bushing acts more like a furnace as it melts more of the material.

   The bushings are what make the capital investment in fibre 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 fibre 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. 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.

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

   In the continuous filament process, after the fibre 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 fibre is to be used in a composite. Size is
   usually added at 0.5–2.0% by weight. Winding then takes place at around
   1000 m per min.

   In staple fiber production, there are a number of ways to manufacture
   the fibre. 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.

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

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