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Glass

Introduction

A glass is an inorganic non metallic material that does not have a crystalline structure.  Such materials are said to be amorphous and are virtually solid liquids cooled at such a rate that crystals have not been able to form.

Typical glasses range from the soda-lime silicate glass for soda bottles to the extremely high purity silica glass for optical fibers.  Glass is widely used for windows, bottles, glasses for drinking, transfer piping and recepticles for highly corrosive liquids, optical glasses, windows for nuclear applications etc. etc.

In history most products have been blown glass.   In recent times most flat glass has been produced using the float process.   Mass produced bottles and decorative products are made using industrial scale blown glass process.  Hand blown glass items are made in art/craft centres throughout the UK.

Normal Glass

The main constituent of glass is silicon dioxide (SiO 2).  The most common form of silica used in glassmaking has always been sand.

Sand by itself can be fused to produce glass but the temperature at which this can be achieved is about 1700o C.  Adding other chemicals to sand can considerably reduce the temperature of the fusion.   The addition of sodium carbonate ( Na 2 CO 3), known as soda ash,in a quantity to produce a fused mixture of 75% Silica (SiO 2) and 25% of sodium oxide (Na 2O), will reduce the temperature of fusion to about 800o C.   However, a glass of this composition is water soluble and is known as water glass.   In order to give the glass stability, other chemicals like Calcium Oxide (CaO) and magnesium oxide (MgO) are needed.   The raw materials used for introducing CaO and MgO are their carbonates, limestone (CaCO 3) and dolomite (MgCO3), which when subjected to high temperatures give off carbon dioxide leaving the oxides in the glass.

Borosilicate glass:

Borosilicate glass is produced using 70% - 80% Silica (SiO 2) and 7% - 13% Boric oxide (B2O3 ) with small amounts of the alkali Sodium Oxide (soda) (Na 2O) and Aluminum Oxide (AI2O3).  Glassware is often used in laboratories where repeated exposure to water vapour at high temperatures can leach out alkali ions.   Borosilicate glass has a relatively low alkali content and with a resultant high resistance to attack by water.   Borosilicate glass has exceptional resistance to thermal shock because it has a low coefficient of expansion (3.3 x 10 -6 K-1) and a high softening point.   The maximum recommended working temperature (short time) for Borosilicate glass is 500oC

Borosilicate glass has good optical properties with the ability to transmit light through the visible range of the spectrum and in the near ultra-violet range. It is therefore widely used in the field of photochemistry.  Because of its thermal and optical properties it is widely used for high intensity lighting applications.

This glass is used in the manufacture of glass fibres for used in plastic and textile reinforcement-- see below

In the home Borosilicate glass is familiar in the form of oven-ware and other heat-resisting domestic receptacles e.g Pyrex.  These items are generally used at temperatures up to 250oC

Borosilicate glass has a very high resistance to attack from water, acids, salt solutions, halogens and organic solvents.   It also has a moderate resistance to alkaline solutions.   Only hydrofluoric acid, hot concentrated phosphoric acid and strong alkaline solutions cause appreciable corrosion of the glass.  This glass is therefore widely used in chemical plants and for laboratory apparatus.

General Properties Of Glass

Mechanical Strength

Glass has great inherent strength. It is weakened only by surface imperfections, which give everyday glass its fragile reputation. Special surface treatment can minimize the effect of surface flaws.

The practical tensile strength of glass is about 27MPa to 62 MPa.   However, glass can withstand extremely high compressive stresses .   Therefore, most glass breakage is due to tensile strength failure.  The reason that glass is weak in tensile strength is that it is normally covered in microscopic cracks which generate local stress concentrations.   Glass does not possess mechanisms for reducing the resulting high localised stresses and so it is subject to rapid brittle fracture. There are two methods of reducing /eliminating this problem :

  • Treating the glass thermally or chemically such that the outer surfaces are compressively stressed at relatively high levels, the middle region between the surfaces being under tensile stress.  The cracks are therefore "held closed by the continuous residual stress...This is tempered/ toughened glass.   The strength of the glass can be improved by a factor of up to 10 using this method.
  • Ensuring that the glass surfaces have no cracks and ensuring that the glass in use is not in mechanical contact with anything which could scratch the surface.   Glass produced with no surface flaws have strength values approaching the theoretical tensile strength values of 6,5 GPa. These have been produced using very fine fibres of glass.
Hardness

Borosilicate glass is about 2,3 x the hardness of plate glass.   On the Moh's scale plate glass has a hardness value of about 5,7.   Glass is harder than most grades of unhardened steel.

Elasticity

Gives under stress - up to a breaking point - but rebounds exactly to its original shape.  Glass has virtually zero ductility.    Youngs Modulus for fused Quartz glass is about 72 GPa

Chemical Resistance

Affected by few chemicals. Resists most industrial and food acids.

Thermal Shock Resistance

Normal glass has low heat shock resistance but borosilicate glass has very good heat shock resistance, and withstands intense heat or cold as well as sudden temperature changes.

Heat capacity

Retains heat, rather than conducts it. Absorbs heat better than metal.

Optical
  1. Reflects light
  2. Bends light
  3. Transmits light very efficiently
  4. Absorbs light with great accuracy.
Electrical Insulation

Strongly resists electric current.   Stores electricity very efficiently.


Glass Types

..Some of the types of glass available
Glass Type Notes
Float Glass Perfectly flat clear glass made using the Float Process. Glass form most widely used for windows etc. low Cost
Reflective Glass Ordinary Plate glass with a metallic coating on one side to reduce solar heat transfer. The metallic coating produces a mirror effect prevent viewing through glass pane.
Insulating Glass Two or more panes of glass with a hermetically sealed space between. Glass used for double glazing.
Pattern Glass Normal plate glass with pattern molded into the surface by passing plate through engraved rollers
Wired Glass Normal glass which has a wire mesh inserted during the production process. This glass is only as strong as normal glass.   However on fracture the mesh stays in place and holds the glass together.   Can be used for security and for a low cost fire glass
Laminated Glass Laminated glass is a combination of two or more glass sheets with one or more interlayers of plastic (PVB) or resin.   In case of breakage the plastic holds the glass fragments
Fire-Resistant Glass Contains flames and inflammable gas for a short period of time. Does not prevent the conduction of heat through panes.    Includes, wired glass, reinforced laminated glass, Borosilicate glass).
Tempered
Toughened Glass
Tempered (toughened) glass is two or more times stronger than annealed glass. When broken, it shatters into many small fragments which prevent major injuries. Tempered glass has a highly stress surface and cannot be cut as conventional plate glass
Bullet Proof Glass Bullet-resistant laminates consist of multiple plates of glass with a internal polycarbonate plates bonded together by interlayers of polyvinyl butyral or aliphatic urethane.   These laminates can resist bullet penetration from a variety of small arms and rifles.


Special Engineering Applications of Glass

Glass Fibres..

Glass fibres are made of silicon oxide with addition of small amounts of other oxides. Glass fibres are made very small diameter and have a low ratio of surface cracks which are the main cause of the brittle property of glass.  Glass fibres have the properties of high strength, good temperature and corrosion resistance, and low price.

There are two main types of glass fibres: E-glass and S-glass.   The E-Glass type is the most used, and has good electrical   . The S-Glass is very strong , stiff, and temperature resistant.

On its own glass fibre it not used to any serious extent in engineering. However is is widely used as a composite material as a reinforcing material with a matrix of thermosetting plastic.   Typical examples of the composites are glass reinforced phenol composites, glass reinforced epoxy resin and glass reinforced UP resin composites.   Used as reinforcing materials in many sectors, e.g. automotive and naval industries, sport equipment etc.  They are produced by a spinning process, in which they are pulled out through a nozzle from molten glass at rates thousands of meter/min.

Properties of glass fibres.. /
E-glass fibres:
Modulus of Elasticity : about 72.4 GPa
Tensile strength: about 2.400 GPa (typical steel = 400 MPa)

S-glass fibres:
Modulus of Elasticity : about. 85.5 GPa
Tensile strength: 4.500 GPa

Fibre Optics

Electronic communication has improved significantly over recent years following the widespread introduction of fibre optics.  These are fine glass fibres through which light can be transmitted very efficiently over long distances.   This allows information to be transmitted at extremely high data rates.

The fibre itself is a strand of silica based glass, it's dimensions similar to those of a human hair, surrounded by a transparent cladding.   A typical fibre includes a centre core ( 8,5 μm dia ) a surrounding glass cladding (125 μm dia ), a protective buffer coating and an outer jacket (245 μm dia ).

The light propagates along the fibre by the process of total internal reflection.   The light is contained within the glass core and cladding by careful design of their refractive indices.   The loss along the fibre is low and the signal is not subject to electromagnetic interference which plagues other methods of signal transmission, such as radio or copper wire links.

The signals transmitted down optical fibres do degrade and optical amplifiers are required at regular distance intervals to compensate for the losses.



Links Providing information on Glass
  1. British Glass.. Official site of British Glass Confederation
  2. Schott.. A Supplier of High Quality Glass Products
  3. Tech Glass.. Moliterno Technical Glass information
  4. Pilkington Glass.. The website of the British inventor of the float glass process- Useful information from -> downloads
  5. Glass on the Web.. A website devoted entirely to glass

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Last Updated 12/03/2010