Building Design and Construction Handbook - Merritt - Ventech!

BUILDING MATERIALS 4.87
on different bases and used in different ways. Some plastics show no yield point,
because they fail before a yield point can be reached. Others have a moderately
high elastic range, followed by a highly plastic range. Still others are highly extensible
and are employed at stresses far beyond the yield point.
More than many other materials, plastics are sensitive to temperature and to the
rate and time of application of load. How these parameters influence the properties
is indicated in a general way in Fig. 4.5, which shows that for many plastics in
increase in temperature, increase in plasticizer content, and decrease in rate of load
application mean an increase in strain to fracture, accompanied by a decrease in
maximum stress. This viscoelastic behavior, combining elastic and viscous or plastic
reaction to stress, is unlike the behavior of materials which are traditionally
considered to behave only elastically.
4.69 FILLERS AND PLASTICIZERS
Fillers are commonly added, particularly to the thermosetting plastics, to alter their
basic characteristics. For example, wood flour converts a hard, brittle resin, difficult
to handle, into a cheaper, more easily molded material for general purposes. Asbestos
fibers provide better heat resistance; mica gives better electrical properties;
and a variety of fibrous materials, such as chopped fibers, chopped fabric, and
chopped tire cords, increase the strength and impact properties.
Plasticizers are added to many thermoplastics, primarily to transform hard and
rigid materials into a variety of forms having varying degrees of softness, flexibility,
and strength. In addition, dyes or pigments, stabilizers, and other products may be
added.
4.70 MOLDING AND FABRICATING METHODS
FOR PLASTICS
Both thermosetting and thermoplastic molding materials are formed into final shape
by a variety of molding and fabricating methods.
Thermosetting materials are commonly formed by placing molding powder or
molded preform in heated dies and compressing under heat and pressure into the
final infusible shape. Or they are formed by forcing heat-softened material into a
heated die for final forming into the hard infusible shape.
Thermoplastics are commonly formed by injection molding, that is, by forcing
soft, hot plastic into a cold die, where it hardens by cooling. Continuous profiles
of thermoplastic materials are made by extrusion. Thermoplastic sheets, especially
transparent acrylics, are frequently formed into final shape by heating and then
blowing to final form under compressed air or by drawing a partial vacuum against
the softened sheet.
Foamed plastics are employed for thermal insulation in refrigerators, buildings,
and many other applications. In buildings, plastics are either prefoamed into slabs,
blocks, or other appropriate shapes, or they are foamed in place.
Prefoamed materials, such as polystyrene, are made by adding a blowing agent
and extruding the mixture under pressure and at elevated temperatures. As the
material emerges from the extruder, it expands into a large ‘‘log’’ that can be cut