GLASS-TYPES

I. SODA-LIME or SOFT GALSS:

• The raw materials are silica (sand), calcium carbonate and soda ash. The approximate composition is Na2O.CaO. 6SiO2. • Its is low in cost, resistant to devitrification and relatively

resistant to water. • The glass melt easily and hence, can be worked easily. • Such glasses are, however attacked by common reagents

like acids.• This glass constitutes 95% of all glass manufactured.

• Uses: They are widely used as window glasses, electric bulbs, plate glasses, bottles, jars, automobile

building blocks, and cheaper table wares, where high temperature resistance and

chemical stability are not required.

• II. Potash-lime or hard glass:• Is obtained from silica (sand), calcium

carbonate and Potassium carbonate. • Their approximate composition is K2O.CaO.

6SiO2.• They possesses high melting point, fuse with

difficulty and are less attacked by acids, alkalis and other solvents than ordinary glasses.

• Uses: These are used for chemical apparatus, combustion tubes etc., which are to be usedfor heating operations.

III. Lead glass or Flint glass:

• Is made by using lead oxide, instead of calcium oxide for fusing together with silica. For dense optical glasses, as much as 80% of PbO is incorporated.

• In addition, K2O is used, instead of sodium oxide.

• So its approximate composition is K2O.PbO. 6SiO2

• It is more expensive to make than ordinary lime-soda glass but much easier to shape andwork with. Lead glass has a lower softening temperature than soda- glass and also high refractive index. It has excellent electrical properties;It is bright, lustrous and possesses high specific gravity.

• Uses: lead glasses are used widely for high quality table wares, optical purposes, neontubing, cathode ray tubes, electrical insulators and in art objects, because of their highluster. High lead content glasses are used for extra dense optical glasses for windows andshields to protect personal form X-rays and gamma rays in medical and atomic energyfields respectively.

IV. Borosilicate glass or Pyrex glass or Jena glass:

• Is the most common of the hard glasses of commerce.

• Such glasses contain virtually only silica and boron, with small amount of alumina and some alkali oxides.

• A typical composition is:

Properties:

• 1. Low thermal coefficient of Expansion• 2. High chemical resistance• 3. High softening point• 4. Excellent shock proof resistivity

uses

• 1. Extensively used in Industry for Pipe lines for transportation of (corrosive)liquids

• 2. Gauge glasses• 3. Laboratory apparatus/glass wares• 4.TV tubes/Electrical Insulators

Fused quartz or fused silica or vitreous silica

• This glass consisting of high purity silica in amorphous (non-crystalline) form.

• It is different from traditional glasses in that no other ingredients are added to lower the melt temperature.

• Fused silica, therefore, has much higher working and melting temperatures than soda-lime or borosilicate glasses.

• Fused quartz is manufactured by fusing (melting) naturally occurring quartz crystals of high purity at approximately 2000 °C, using either an electrically heated furnace (electrically fused) or a gas/oxygen-fuelled furnace (flame fused).

• No fluxing agents are added and extremely difficult to get rid of all air bubbles and shaping is difficult.

• Temp. above melting temp. of pure sand(1750 C)is used.

• Another common method involves adding silicon tetrachloride to a hydrogen–oxygen flame(vapor phase ,high temperature Pyrolysis).

• However use of this precursor results in environmentally unfriendly by-products including chlorine and hydrochloric acid.

• The high temperature of the reaction tends to drive off undesired contaminants.

• Removal of bubbles from glass is difficult because of high viscosity.

Lime soda glass

Pyrex/borosilicate

Vycor/96%silica

Fused silica

Softening point(0C)

715 746 1530 1525

Annealing point(0C)

527 450 1020 1085

Sp.gravity(***)

2.5 2.23 2.18 2.2

(***) low values are desirable for higher transparency for the product glass

• Fused quartz is normally transparent• Blebby(a small bubble in glass) ,has

remarkable properties(as given in above table) • Has lowest ultrasonic absorption.• Because of its low thermal expansion, it is

used for telescopic mirrors.

High Silica/Vycor/96%silica• 1. It contains 96% silica and 3% boric acid; rest

alumina & alkali.• 2. Fused silica: problem in Melting & Forming; this is

avoided in High silica glass by the following method;• 3. Borosilicate glass (75%silica, 20%B2O3) taken(as

Raw glass) , melted ,the desired article is molded and cooled.

• 3. NOW the article is subjected to heat treatment and annealing; this causes the glass to separate into Two distinct physical phases

• One phase (high in boric acid and alkaline oxides) is soluble in Hot acid (10% Hydrochloric acid, 980C) solutions.

• Second phase is rich in Silica; insoluble in Hot acid and attains cellular structure.

• By dehydration of this body, we can make it as: nonporous vitreous glass.

• In this process there will be shrinkage of 14% in linear dimensions.

• The final product can be heated to cherry red and plunged in ice water without any damage.

PRPOERTIES:

• HIGH chemical stability, extremely stable to all acids except HF, retains its original shape

SAFETY GLASS

• TWO GENERAL CLASSES:– i) laminated safety glass– Ii) heat-strengthened or tempered or case

hardened safety glass

I) LAMINATED SAFETY GLASS

• This is most widely used.• Two thin sheets(3mm thick) are sandwiched

with a sheet of non-brittle plastic material in between them.

• The plastic and glass rae washed , and • An adhesive is applied to the glass.• Then glass and plastic sheets are pressed

together under moderate heat to seal the two glass sheets

• This is accomplished in an autoclave with hydraulic pressures.

• Finally the edges are sealed with water resistant compound.

Different plastic sheets useful

• Cellulose nitrate(1st commercially used )• Next is cellulose acetate• Now all safety galsses uses “poly vinyl butyral

resin (more elastic than cellulose acetate, clear and color less, not affected by Sunlight, does not require adhesives) “

• The glass has all physical properties of ordinary glass;

• In addition it possess safety features.• Safety features: • The plastic inter layer , holds the fragments

caused by the accidental breakage of the glass it-self.

FIBER GLASS or GLASS FIBERS

• 1. Are extremeely fine(10 to 5 microns or even less)

• 2. can be drawn in to thread/blown into a mat for insulation tape/tape and Air filtration.

• 3. the fibers are also used for reinforcing various plastics(FRP Lining)

• It is a special glass with low silica content;• Approx.composition:• 55%SiO2,10%B2O3,14%Al2O3,Balance other

oxides.