Page 450 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Inorganic Polymers 413

TABLE 12.4
General Properties of Silicate Glasses
High transparency to light Permanent (long-term) transparency
Hard Scratch resistant
Chemically inert Low thermal expansion coeffi cient
Good electrical insulator High sparkle
Good luster Low porosity
Good ease of reforming Easily recyclable
(Relatively) Inexpensive Available in large amounts

structure where each octahedral contains four silicon and four oxygen atoms. There is also a number
of “dangling” hydroxyl groups. This amorphous structure allows the glass to be melted and some,
but not much, fl exibility.
Glass has many useful properties, as listed in Table 12.4. It ages (changes chemical composition

and physical property) slowly, typically retaining its fine optical and hardness-related properties for
centuries. Glass is referred to as a supercooled liquid or a very viscous liquid. Indeed, it is a slow-
moving liquid as attested to by sensitive measurements carried out in some laboratories. Concurrent
with this is the observation that the old stained glass windows adorning European cathedrals are
a little thicker at the bottom of each small, individual piece than at the top of the piece. For most
purposes though, glass can be treated as a brittle solid that shatters on sharp impact.
Glass is mainly silica sand (SiO ) and is made by heating silica sand and powdered additives
2
together in a specified manner and proportion much as a cake is baked, following a recipe that

describes the items to be included, amounts, mixing procedure (including sequence), oven tempera-
ture, and heating time. The amounts, nature of additives, and so on, all affect the physical properties
of the fi nal glass.
Typically cullet, recycled or waste glass (5%–40%), is added along with the principle raw materi-
als (mostly SiO ). The mixture is thoroughly mixed and then added to a furnace where the mixture
2
o
is heated to near 1,500 C to form a viscous, syrup-like liquid. The size and nature of the furnace
corresponds to the glasses’ intended uses. For small individual items, the mixture may be heated in
a small clay (refractory) pot.
Most glass is melted in large (continuous) tanks that can melt 400–600 metric tons a day for pro-
duction of different glass products. The process is continuous with the raw materials fed into one
end as molten glass is removed from the other end. Once the process (called a campaign) is begun,
it is continued indefinitely, night and day, often for several years until the demand is met or the fur-

nace breaks down.
A typical window glass will contain 95%–99% silica sand with the remainder being soda ash
(Na CO ), limestone (CaCO ), feldspar, and borax or boric acid along with the appropriate coloring
3
2
3
and decolorizing agents, oxidizing agents, and so forth. As noted previously, 5%–40% by weight
of crushed cullet is also added. The soda ash, limestone, feldspar, and borax or boric acid all form
oxides as they are heated, which become integrated into the silicon structure:
Na CO → Na O + CO 2 (12.10)
2
2
3
CaCO → CaO + CO 2 (12.11)
3
R OAl O • 6H O → R O + Al O + 6SiO 2 (12.12)
2
2
2
3
3
2
2
H BO → B O 3 (12.13)
2
3
4
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