Editing Glass fiber (section)

==Chemistry==
The basis of [[textile]]-grade glass fibers is [[silica]], SiO<sub>2</sub>.  In its pure form it exists as a [[polymer]], (SiO<sub>2</sub>)<sub>n</sub>.  It has no true [[melting point]] but softens up to 1200&nbsp;°C, where it starts to [[Chemical decomposition|degrade]].  At 1713&nbsp;°C, most of the [[molecules]] can move about freely.  If the glass is extruded and cooled quickly at this temperature, it will be unable to form an ordered structure.<ref name="Gupta">{{cite book  | last = Gupta  | first = V.B.  |author2=V.K. Kothari  | year = 1997  | title = Manufactured Fibre Technology  | publisher = Chapman and Hall  | location = London  | pages = 544–546  | isbn = 978-0-412-54030-1}}</ref>  In the polymer it forms SiO<sub>4</sub> 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 [[crystal]]line 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&nbsp;°C for long periods of time.<ref name = "Lowenstein" />

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 ("A" for the alkali it contains). It is not very resistant to alkali. A newer, [[alkali]]-free (<2%) type, E-glass, is an alumino-borosilicate glass.<ref name="Volf">{{cite book|last = Volf|first = Milos B.|year = 1990|title = Technical Approach to Glass|publisher = Elsevier|location = New York|isbn =978-0-444-98805-8}}</ref> C-glass was developed to resist attack from chemicals, mostly [[acid]]s that destroy E-glass.<ref name="Volf" /> T-glass is a North American variant of C-glass. 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.

E-glass does not actually melt, but softens instead, the softening point being "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".<ref name="Lubin">{{cite book|editor-last = Lubin|editor-first = George |year = 1975|title = Handbook of Fiberglass and Advanced Plastic Composites|publisher = Robert E. Krieger|location = Huntingdon NY}}</ref> The strain point is reached when the glass has a [[viscosity]] of 10<sup>14.5</sup>&nbsp;[[Poise (unit)|poise]]. The [[annealing (glass)|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<sup>13</sup>&nbsp;poise.<ref name="Lubin"/>