Properties of Glass (Part II)
 

As we continue from the previous article where in ,We discussed about Physical, Mechanical and Chemical properties of glass. We would now highlight the Optical, Electrical and Thermal properties of Glass.
The properties of any material will depend on its ingredients. Same is true very much for glass .The various properties of glass can be changed in accordance to the requirement by changing the composition of glass.
By regulating the manufacturing process and techniques, the  properties can  be altered. It may also be emphasized that all the properties are interlinked. Change in one property may bring changes in other properties. So ,it is  the best suited combination of these properties that make a glass suitable for a particular application.    


OPTICAL PROPERTIES OF GLASS


How a glass will be have when light is beamed on it, will explain the optical properties of that glass.
When we say light we mean white light or light with all colours. Light is a form of Energy., it cannot be destroyed but can be changed into other form of energy like Heat ,Sound etc.

The light that is visible to us has a wide spectrum of colours. These colours are like a band from (VIBGYOR) Violet to Red at the extreme end.       The reflective indexs.
                                    (=435): 1523 (=645) : 1513
The wavelength is the most important factor to separate these colours bands of the light .
Whereas the Ultra Violet Light has waves shorter than visible light waves. The infrared waves are longer that visible light waves.
When we say transparent glass ,it does not mean that the glass will neither reflect nor absorb any light but will transmit one hundred percent light . No such glass been possible to make. A transparent glass transmit most of the light and a negligible quantity is reflected or absorbed. So the glass can be termed as transparent glass in near absolute terms.
A coloured glass is one that absorbs all colours expect that of its own, from the spectrum band. The yellow light of car indicator absorbs all colours except yellow thus to our eyes it is visible as yellow light.
When light beam is made to pass through a glass, three main things happen

  • A percentage of light is reflected.
  • A part of light is absorbed.
  • A part of light is transmitted

Optical properties of glass are more with behavior of glass, particularly, towards the Ultra Violet Band  and the Infrared Band of the spectrum
As the name suggest  Ultra Violet Band is near the Blue Band and Infrared Band is near the Red Band of the Spectrum.


Infrared Light Absorbing Glass:


These glasses are used to absorb the infrared spectrum of light, which has light high heat he at energy. These glasses help in keeping the system cool and thus avoiding damage of heat sensitive components .
When the light ray passes through the glass,it bends. This is bending of ray of light when passing through different mediums, is called Refraction. The measure of this refraction is called the refractive index of that glass (Lens). The Lens helps to focus these differently refracted light ray to one point (Called Focus).

All colours of light are refracted differently .So the difference of the refractive index of each colour result in dispersion of light .These optical properties are glass taken into consideration while designing Lens ,Eye Glass or Spectacle Glass is one such example.
Common lens and Projector lens have several element to take care of each requirement.
Metallic coating like the looking glass or mirror is one example of glass where a reflection can be seen.
Depending upon the need, coatings can be done to make Low E-Glass ,Reflective Glass (Lens). These types of glass have been discussed in earlier articles.

We conclude that optical properties of a glass are dependent on its composition

Manufacturing process of technique or a combination of all of above conditions.

Thermal properties of Glass :

Thermal expansion is the most important of the thermal properties of glass. The Co-efficient of thermal expansion varies strongly with composition e.g quartz glass 3.2 X 104
/kg. heavy lead flint glass 8.0 X 10-6

Thermal Breakage of Glass :

The total solar energy on a glass is sum of ABSORBED + REFLECTION + TRANSMITTED energy.
Thermal breakage is mainly due to thermal stresses and uneven expansion of the inside and outside surfaces of the glass.
Where the thermal stresses are more , the normal glass (Not toughened) has more breakage than the toughened glass due thermal changes.
Given below is the Co- relation of the strengths of various kinds of processed glass.

Strength Factors (Pressure Factors)
Annealed                    -           1.0
Sand Blasted              -           0.4
Wired                          -           0.5
Laminated                  -           0.8
Patterned                    -           1.0
Insulated units            -           1.5
Heat strengthened     -           1.6
Toughened                  -           2.5

If the core area of the glass becomes hotter than the edges, the risk of breakage increases with increases in the difference of temperature. This is very common in framed glass. Glass expands when exposed to direct sunlight. The center portion becomes heated due to sunlight and the edges being in frame are not heated. Resulting in breakage of glass due to uneven expansion causing thermal stress. The material of frame is also an important factor in phenomenon.

The glasses with metallic frames, are less at risks as metals are good transmitters and absorbers of heat than the wooden frames. Various type of glass have different absorptance of heat.

Risk of breakage due to thermal shock

Type of Glass breakage

Solar Absorptance

Risk of Thermal Shock

Clear

18

Low

Coloured or Tinted

30 – 40

Medium

Colour Glass with Reflective Coating

60-70

High

Tinted or Coloured glass with Reflective Coating

80 - 85

Very High

Coated Glass (High Light) Transmitting Coating

45 – 55

Medium - High

There are other factors also which enhance the thermal breakage. The most important of them are : -

  • Condition of the Edges of Glass :  fine out edge are at less risk than broken or damaged edges.
  • Colours, Films, Paints used on Glass :- Some Colours absorb higher amount of heat.
  • Broader and thick frames covering large edge area of glass. The frames keep the edges cool. Thus creating a higher temperature difference between the center and edges.
  • Clinging films or curtains attached to the glass, not allowing flow of air resulting in heat and higher temperature.
  • Tightly fixed glass of large sizes – the more the size of glass the more risk of breakage.

 

Remedy

Where there is a risk of such breakage the thermal stress analysis must be done. It is also suggested that toughened glass must  be used in areas where there is excessive heating. A typical example of this is , the use of toughened glass on the doors Microwaves and Kiln.

The temperature gradient is the most important factors to understand thermal breakage.

Thermal gradient is defined as the maximum temperature differential between the front surface and back surface of glass that will cause 1000 PSI tensile stress on the cooler surface.

GLASS BULLETINE 2006


 
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