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This specification covers the characteristics and inspection requirements for the soda-lime float glass substrates produced by Glaverbel for use in LCD Applications

The substrates may be divided in 2 different classes:


- in standard quality
- in special quality:lower micro-corrugationvalue and/or extra cutting tolerances 

both either with clean cut edges,either with edge grinding and corner cutting



Typical float glass composition in % of weight is: 

SiO2   Na2O  K2O    CaO    MgO    Al2O3  SO3
70.8    13.9    0.4     8.2     4.4     1.9     0.3 

- Specific weight :                                2.49 0.01 103  K/m3
- Young's Modulus :                              E= 702 G.Pascals or GN/m2
- Poisson's ratio :                                 0.230.01
- Hardness:
         - Vickers scale:                          6.35 G Pascals or G N/m2
         - Moh's scale:                            6.50.5 (between orthoclase and quartz)

THERMAL PROPERTIES  (General tolerances 10 °C)
In discussing the thermal behaviour of glass,the melting point is not a characteristic temperature so as for crystalline substances;various empirically defined temperatures and particular points on the viscosity/temperature curves,have been proposed to replace this important constant.

Annealing range
To prevent strain in the glass,exact control of temperature is important d uring processing and especially during an interval the annealing range between  480 and 585 °C .Above the upper part of the annealing range,the glass is so fluid that it yields practically instantaneously to stress,and a stress cannot persist

Strain Point
The strain point is the temperature from which a piece of glass can be more quickly cooled without introducing permanent strain.The strain point is 490 °C with a corresponding viscosity of 1014.5poises. 

Transformation point:Tg
The transformation point is the temperature at which a sudden change in the coefficient of extension takes place;the change can be located with fair precision and is a characteristic temperature  for a given glass composition.The Tg of Vertec is about 555 °C with a viscosity of 1013.4poises.

Softening point
The softening point,corresponding to a viscosity 107.6poises, is the maximum point reached on the complete thermal expansion curve for glass.The softening point of Vertec is about 585°C.

Mean linear coefficient of thermal expansion
measured with the dilatometer.This value expresses the expansivity of the material;it is the increase in length of the specimen devided by the original length when heated over the considered temperature interval.

0 to 100 °C: 8.00.2 10-6 per °C or °K
0 to 200 °C: 8.1
0 to 300 °C: 8.4
0 to 400 °C: 8.7
0 to 500 °C: 9.0

A typical value used is 8.5x10-6 per °C or °K

Specific heat

Thermal conductivity
at 20 °C:0.95 0.05 W m-1 K

Hydrolytic class
         - according to standard DIN 12.111  :class 3
         - according to standard ISO 719      :class 3

Once installed for use,glass can withstand large amount of water without significant surface damage.However,water accumulated between lites in storage,even if only small amount is trapped,can cause surface deterioration.Storage areas for glass should be maintained at degrees of temperature and humidity that will prevent water vapor deposition on the glass.Waters and chemicals should not be allowed to harden on glass surfaces because it is always difficult to remove the residue after a too long time.

One can advise:
Relative humidity      HR<= 60%     ideally 40%
Temperature              T<=20°C     ideally 15°C
Temp variation           T<=5°


Vertec thick=1mm

- Refractive index n = 1.520.005 in visible range
- Reflexion for one face  =(n-1/n+1)2 with =0.04 for n=1.5 i.e. reflection is about 8% for the 2 faces of one sheet of glass

Example:       Light (375-700 nm) -thickness 1 mm
                            Reflection       8.1%
                            Absorption     0.8%
                            Transmission  91.1%


- Specific resistivity

(at 1000 Hz)

log R ohms cm        25°C = 9.7
                             100°C = 9.1
                             250°C = 6.5

(at 65 Hz)
                               25°C = 11

- Dielectric constant at 25°C/1MHz=7.75



Available thichnesses
nominal 0.55 mm - 0.70 mm - 0.85 mm - 1.10 mm mainly.

Thickness variation
It is measured with a micrometer  fitted with an electronic gauge (0.001 mm) over the whole length of the piece.The tolerance is indicated in the summary table hereafter enclosed.

It is measured with a Caliper fitted with an electronic gauge (0.01 mm).The tolerance is indicated in the summary table hereafter enclosed.

It is measured by set square and slip gauges and is expressed in % of the length of the substrate.Limit value :see  summary table hereafter

The first size given is orientated in the length of the pull.
Example:30 x 40 cm  the 30 cm edge is in the direction of the pull


The flatness is considered at different scales
The typical defects of a float are as follows:
At an even lower scale there is for memory the rugosity.
The main criterias to define the flatness of an LCD substrate are the WARP and the MICROCORRUGATION.

Warp is a large surface irregularity;any deviation from a true plane.In particular,this is any twisting or raising of any portion of the cut sheet of glass from a flat inspection surface on which it is resting.The measuring equipment is labeled "dot board" and is schematically detailed in the enclosure.
The measurement for warp is made by first selecting the proper dot size for the warp limit involved.View the image of the dot as reflected from the reference plate and the test part.If the images of a given dot as reflected from the 2 surfaces are coincident,overlapping or tangent,the test part is within the warp limit.If the images of a given dot shows two non tangent and non-touching dots,the test part exceeds the warp limit.
This can be expressed in terms of the ratio:
d/L,where d equals the distance,or depth of variation,and L equals the distance,or length over which the variation occurs,e.g.
No S shaped variation should be allowed.



The max value of warp is given thickness by thickness in the summary table hereafter.


It is a shorter wavelength component superimposed on the overall form of the substrate.It is measured on the tin face in a direction perpendicular to the pull.
The amplitude is measured with a roughness meter,connected to n amplifier;the filter cut-off values of which are 0.8 mm for the short-wave and 8 mm for the long-wave
The equipment is a Taylor Hobson and procedures are as determined in ISO468 This equipment has a 100 mm evaluation length.
Glaverbel has also develloped its own instrumentation allowing a measurement over the whole length of the substrate.
The resulting curve is then "screened" with a 20mm sample length and the maximum amplitude is then selected
Values may also be given for other cut-off values (typically 0.8-25 mm cut-off) as an information.The production datas will be given with cut-off 0.8-8 mm with a 20 mm window.
Max values are summarised in the table hereafter thickness by thickness


Among those defects, one can understand

A bulk bubble is a gaseous inclusion more or less circular depending on the thickness of the glass.It deforms the image in transmission or reflexion.

Solid inclusion (unmolten)

A term used to denote a gaseous inclusion having an extremely small diameter in the glass.

Fine lines with different transmission in the glass.It is glass of different glass composition due to non homogeneities

Inclusions within the glass or layers or strings of the glass which are not homogenous with the main body of the glass.

and others



Adhering surface chips
Glass particles sticked on the glass surface

Surface digs
Bubbles located on the glass surface (open bubbles)

Dammage on the glas surface appearing in lines

Dot scratches
Micro-scratches grouped in dots

Free zone
A zone on the border of the sheet in which any defect is allowed except breakage defects

A chemical attack on the surface by the age of the glass or by storage in bad conditions

Tin speck
A tin inclusion.It deforms the image in transmission or reflection

It is a deformation like a tin spec but without tin inclusion

It is a protusion on the border of the sheet caused by cutting problems. Internal and surface defects are examined after washing under edgelight condition in a 100% visual inspection procedure but also statistically with the "Shadowgraph" evaluation technique-see description in enclosure.


The glass is provided with ground egdes and corner cuts

In this execution,the edges are beautifully pencil edged in a C profile and one corner has a big cut in order to recognize which is the tin face; the 3 remaining corners are dubbed as can be seen from the drawing:



Detailed specifications are enclosed at the end of this document


In order to fullfill the requirements for the STN - LCD quality,one can polish the tin face of glass in order to improve the microcorrugation.
As an alternative for "easy STN's",Glaverbel is developing what is called a "float Å"( in 1.1 and 0.7 mm thick) by improving the settings of the furnace to such an extent that one can foresee to eventually avoid the polishing.In this case,contact should be taken with us to discuss the matter further.

The specification for
Dimensions ,Internal defects ,Surface defects also apply for the polished glass.

More specific are the flatness requirements,essentially the microcorrugation on the polished face.

The orientation of the polished face is the same as described for the ground edge glass.Instead of the "tin face up",one should read "polished face up".The polished substrates are automatically ground edge too.

For  specifications other than µ-corrugation please refer to the “ground  edge“descriptions.This is temporarily not available.