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Monday, 28 December 2015

Properties of glass

Properties of glass:

The following are the main properties of glass:

  1. Solid and hard material.
  2. No definite crystalline structure.
  3. No sharp melting point.
  4. Glass is 100% recyclable and one of the safest material to pack due to its properties.
  5. Absorbs , refracts and transmits light.
  6. Affected by alkalies.
  7. Glass can be cleaned easily.
  8. An excellent electrical insulator at elevated temperature.
  9. Fragile and easily breakable into sharp pieces.
  10. Available in different colours.
  11. Not affected by air or water.
  12. Not easily attacked by ordinary chemical reagents.
  13. Capable of being worked in several ways.
  14. Possible to weld pieces of glass by fusion.


MECHANICAL PROPERTIES

MECHANICAL PROPERTIES
Density
2500 kg/m3
Hardness
470 HK
The hardness of float glass is established according to Knoop.
The basis is the test method given in DIN 52333 (ISO 9385).
Compression resistance
800 - 1000 MPa
The compression strength defines the ability of a material to 
resist a load applied vertically to its surface.
Modulus of elasticity
70 000 MPa
It is either determined from the elastic elongation of a thin 
bar, or from bending a bar with a round or rectangular cross 
section.
Bending strength
45 MPa  
It is a measure of its resistance during deflection. It is
determined by bending tests on glass plate using the double 
ring method according to DIN EN 1288-5.

Strength: Glass is weak in tension and it always fails in tension no matter how stress is applied. However glass is four to five times as strong in compression as it is in tension and it should, therefore, be used under compresive loads. Strength depends upon the factors such as surface conditions, cross-section.

Hardness and Brittle: Hardness of glass can't be measured with Brinel or Rockwell machines because rupture occurs during the test and the specimen will fail. 

THERMAL PROPERTIES:

THERMAL PROPERTIES
Transformation range
520 - 550°C
Temp&Softening 
temperature
approx. 600°C
Contrary to solid bodies of crystalline structure, glass has 
no defined melting point. It continuously transforms from 
the solid state to the viscous plastic state. The transition 
range is called the transformation range and according to 
DIN 52324 (ISO 7884), it lies between 520°C and 550°C.
Tempering and bending require a temperature of a 
further 100°C
Specific Heat
0.8 J/g/K The specific heat (in joules) defines the 
amount of heat required to raise the temperature of 1g
of float glass by 1K. The specific heat of glass increases 
slightly the temperature is increased up to the 
transformation range.
Thermal conductivity
0.8W/mK Thermal conductivity determines the amount
of heat required to flow through the cross sectional area 
of the float glass sample in unit time at a temperature
gradient.
Thermal expansion
9.10-6 K-1 There is a difference in the expansion behaviour 
of a body under the effect of heat between linear expansion 
and volumetric expansion. With solid bodies, the volumetric 
expansion is three times that of linear expansion. The 
temperature coefficient of expansion for float glass is given 
according to DIN 52328 and ISO 7991.


OPTICAL PROPERTIES:

Glass has several strong points concerning optical properties:
- It can be produced in large and homogeneous panes.
- It's optical properties are not affected by ageing.
- It is produced with perfectly flat and parallel surfaces.

Refractive index: n = 1.52
          If light from an optically less dense medium (air) meets an optically denser medium (glass), then the light ray is split at the surface interfaces. The measure of deflection determines the refractive index. For float glass, this refractive index is n-1.52.


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