Civil Engineering Reference
In-Depth Information
term, on the ability of the oil industry to continue to supply the
raw materials and, in the long term, for scientific development
to open up new sources of supply - e.g. agriculture has been
suggested as a possible source.
skidding large assembly oil rigs from barges on to sub-sea
foundations. In addition, PTFE is used in bearings (particularly
for bridges), pipework, low friction industrial components and
coated tensile fabrics.
PTFE is an expensive material so it is important to keep
wastage to a minimum. Waste in the manufacturing process is
usually cleaned and ground down into powder for reuse. It is
combustible and at high temperature releases toxic chemicals.
PTFE is fully recyclable.
14.8.2.3 Polystyrene
Polystyrene is a clear, non-crystalline, brittle plastic material.
Surprisingly it was first made 135 years ago but was not made
commercially until 1936. By that time, 800 t was produced in
Germany rising to 5000 t by 1942. Polystyrene is produced
by both an expansion and an extrusion process and is usually
referred to by one of the following abbreviations.
14.8.4 Thermosetting resins
The term synthetic resin is used to describe man-made thermo-
setting pre-polymers. Some are solids with a low melting point
and many have the viscous, sticky consistency of naturally
occurring substances similar to those secreted from coniferous
trees. Epoxy resins were first discovered in the late 1980s fol-
lowed by commercial production by CIBA of Switzerland in
the early 1950s.
The scientific terminology can be confusing since both
cross-linked polymers and some pre-polymers are commonly
grouped together and described as resins. Thus, one component
of a two-pack epoxy resin adhesive is an epoxy resin which,
on reaction with the second component (the hardener or cur-
ing agent) gives a cured adhesive that is also referred to as an
epoxy resin. Those systems used in construction that contain
formaldehyde are used in laminates, mouldings, adhesives,
surface coatings and as binders in chipboard. These products
are almost always factory produced as they require both heat
and pressure. Other resins, such as furanes and polyurethanes,
can be used on site where curing takes place at the point of ser-
vice and at ambient temperatures.
Elevated temperatures do not cause thermosets to melt and
flow but do induce softening and changes in properties such as
strength and chemical resistance.
■
PS
- a foam.
■
EPS
- a foam moulded into blocks, boards and other shapes some-
times referred to as
beadboard
.
■
XPS
- an extruded foam manufactured into boards.
Polystyrene has good compressive strength. This allows use
under floors and even in road construction. Although its tensile
strength is modest it is usually sufficient to withstand damage
during transportation. It is widely used in concrete formwork and
can easily be removed after use by melting. In recent times it has
found an application under suspended concrete floors to dwell-
ings in areas subject to swelling and shrinkage of sensitive clays.
As insulation in cavity wall construction there are two
systems:
as panels fixed to the internal leaf;
■
as foam sprayed into the cavity (this is usually a retrofit
■
operation).
Polystyrene is combustible and also toxic to the extent
of breaking down into CO
2
and CO during the combustion
process.
14.9 Glass
14.9.1 Introduction
Flat glass has been made through the centuries since Roman
times. Methods include casting, rolling, spinning, blowing,
floating and drawing. Today a wide range of glass products are
available, 90% of which are based on the float process that was
developed by the Pilkington brothers in the 1950s. This section
focuses on the glass products derived from the float process.
14.8.3 Polytetrafluoroethylene (PTFE)
Well known in domestic applications as a non-stick material
used in saucepans and other kitchenware, PTFE has a number
of uses in engineering. These make use of low friction prop-
erties when associated with other materials. PTFE is produced
by the polymerisation of the monomer tetrafluoroethelene. It
was discovered in 1938 by Dr Roy Plunkett, a chemist working
for DuPont. In working on refrigerant gases he noticed that a
frozen compressed sample of PTFE had spontaneously poly-
merised into a white waxy solid. The trade name Teflon was
registered heralding the start of commercial production.
PTFE has good dialectic properties and is an excellent elec-
trical insulator, is not wetted by water and is non-absorbent.
Its tensile strength is relatively low but has good impact resist-
ance even at sub-zero temperatures. Some characteristics can
be modified by the addition of fillers such as glass or carbon
fibres and metals. The coefficient of friction is between 0.4 and
0.09 making it ideal material for the offshore industry when
14.9.2 Primary manufacture
Float glass used in the building industry is generally referred
to as soda-lime-silica glass. The process involves the following
stages:
1.
Melting of the raw materials 72% silica sand (SiO
2
);
13% sodium carbonate (NA
2
CO
3
); 10% calcium car-
bonate (CA CO
3
) and 4% calcium magnesium carbonate
(MgCa(CO
3
)
2
) in a regenerative furnace at 1500°C. This
can include up to 20% of recycled glass.
