NATURE OF MATERIALS - Materials for Civil and Construction Engineers - page 77

Glasses are a special type of inorganic solids; they do not develop a crys-
talline structure. Commercial glasses are based on silica. In glass, the silica
tetrahedrons link at the corners, resulting in a random or amorphous struc-
ture. Glass can be made of pure silica; it has a high softening point and low
thermal expansion. However, due to the high softening point, pure silica
glass is hard to form. Metal oxides are added to reduce the cross-linking of
the silicates, improving the ability to form the glass into the desired shape.
Although glass has an amorphous structure, it is a very stable compound at
atmospheric temperatures. It does not flow despite the often cited example
of glass windows in European churches.

2.4 Organic Solids

All organic solids are composed of long molecules of covalent bonded carbon
atoms. These molecules are chains of carbon and hydrogen combined with
various radical components. The radical component can be a hydrogen atom,
another hydrocarbon, or another element. These long molecules are bound
together by secondary bonds; in many cases the molecules are also cross-
linked with covalent bonds. There are a wide range of organic solids used in
engineering. These can be classified as follows (Ashby and Jones 1986):

1. Thermoplastics are characterized by linear carbon chains that are not
cross-linked; at low temperatures secondary bonds adhere the chains.
Upon heating, the secondary bonds melt and the thermoplastics be-
come a viscous material. Asphalt is a natural thermoplastic. It is ob-
tained primarily by refining petroleum. In addition, there are many
manufactured thermoplastics that have broad engineering applications.
These include polyethylene, polypropylene, polytetrafluroethylene,
polystyrene, and polyvinyl chloride. Polyethylene and polypropylene
are used in tubing, bottles, and electrical insulation. Polytetrafluroeth-
ylene is commonly known as Teflon. In addition to cookware applica-
tions, Teflon is widely used for bearings and seals due to its very low
friction and good adhesion characteristics. (Polytetrafluroethylene is a
carbon-fluorine chain.) Polystyrene is used for molded objects. It is
foamed with carbon dioxide to make packing materials and thermal
insulation. Polyvinyl chloride is used for low-pressure waterlines.
2. Thermosets are characteristically made of a resin and a hardener that
chemically react to harden. In the formation of the solid, the carbon
chains are cross-linked to form stable compounds that do not soften
upon heating. The three generic types of thermosets are epoxy, poly-
ester, and phenol-formaldehyde. Epoxies are used as glues and as the
matrix material in plastic composites. Polyester is a fibrous material
used in the reinforcing phase of fiberglass. Phenol-formaldehydes are
brittle plastics such as bakelite and formica.