L6 - Secondary glass processing

Toughened glass & Laminated glass - how they break. Coatings and colourants. Principal glass types

What are the finishing processes done to glass?

• Toughening or tempering​

• coating

How is toughened or tempered glass made?

• Stronger than ordinary window glass

• By introducing stress through rapid heating & cooling of glass surface

• Resistant to breaking under stress

How does tempered glass break?

• does not shatter​

• fragments into small squares​

• with little splintering

• = small blunt pieces reduces risk of serious injury

What is laminated glass?

• One layer of plastic ​

• Sandwiched between two layers of glass​ permanently bonded with a tough, clear polymer interlayer, such as PVB.

• Used in automotive windshields

How does laminated glass break?

• it cracks in a "spiderweb" pattern, with the broken glass shards remaining stuck to the inner polymer layer.

• the outer layers of glass break but are held together by the strong, adhesive plastic interlayer (PVB)

• No sharp shards = safety

• Continued cracking: Over time, the cracks can spread further as more stress is applied to the damaged glass. 

Where is toughened glass used in automotives?

• Rear & side windows

Where is laminated glass used in automotives?

Windscreens

Why is coating used for glass?

• Decoration​

• Protection ​

• Strengthening ​

How is coating applied ?

= 2 stages

1. Hot end coatings

2. Cold end coatings

When is hot end coatings applied?

• Applied just after the glass has left forming machine​

• Compounds of tin or titanium​

When is cold end coatings applied?

Applied just before the annealed glass leaves lehr​

Organic waxes, polyethylene emulsions, polyethylene glycols, fatty acid esters, fatty acids​

What is the natural colour of glass?

Unless raw materials (silica etc.) very pure ​

Resultant glass normally has green tint​

Primarily caused by iron oxide ​

Has effect in 1 in 1000 part

What are the colourants used in glass?

What are the principal glass types?

Fused Silica​

Soda lime silicate glasses​

Borate silicate glasses​

Lead glass​

Aluminosilicate glasses​

Alkali barium silicate glasses​

Borate glass​

Phosphate glass

What is fused silica glass?

Silica can be melted and cooled to form glass​

Very high processing temperatures​

Products ​

Good temperature resistance ​

Low thermal expansion​

Hence good resistance to thermal shock​

Excellent chemical durability​

What is soda lime silicate glass?

Silica (SiO2) Soda (Na2O) & lime (CaO)​

Used in wide range of applications​

Containers, flat, domestic glassware​

Greatest tonnage for containers (bottles and jars)​

Container glass is made in three colours:​

White (also known as flint)​

Green​

Amber

What is borate silicate glass?

• Mainly silica and boric oxide (B2O3) + smaller amounts of alkalis and aluminium oxide​

• Good chemical stability​

• Low thermal expansion​

• Good resistance to thermal shock​

• Used in kitchenware, laboratory glassware​

What is lead glass?

• Lead oxide (PbO)​

• Excellent hand-working and optical properties​

• Used in manufacture crystal glass​

• Over 24% PbO can be described as Lead Crystal [European Directive 69/493]​

What is insulating lead glass?

• Good insulating and dielectric properties​

• Lead oxide + potassium and sodium oxides​

What is optical glass?

Lead imparts high refractive index without colouring glass

What is Aluminosilicate glasse?

• Small but important group of glasses​

• Contain 20% aluminium oxide (alumina Al2O3)​

• Often include calcium and magnesium oxides ​

• small amounts of boric oxide​

• Very small amounts of soda or potash​

• Able to withstand high temperatures with good resistance to thermal shock​

• Applications: combustion tubes, gauge glasses for high pressure steam boilers, envelopes of halogen-tungsten lamps (operate as high as 750oC​

What is Alkali barium silicate glass?

• Used to shield X-rays (e.g. cathode-ray tubes)​

• Contain heavy metal oxides​

• Lead​

• Barium​

• Strontium