Building Design and Construction Handbook - Merritt - Ventech!

4.88 SECTION FOUR
into desired shapes. The cells are ‘‘closed’’; that is, they are not interconnecting
and are quite impermeable.
Foamed-in-place plastics are made with pellets or liquids. The pellets, made, for
example, of polystyrene, are poured into the space to be occupied, such as a mold,
and heated, whereupon they expand and occupy the space. The resulting mass may
be permeable between pellets. Liquid-based foams, exemplified by polyurethane,
are made by mixing liquid ingredients and immediately casting the mixture into
the space to be occupied. A quick reaction results in a foam that rises and hardens
by a thermosetting reaction. When blown with fluorocarbon gases, such forms have
exceptionally low thermal conductivities.
All the plastics can be machined, if proper allowance is made for the properties
of the materials.
Plastics are often combined with sheet or mat stocks, such as paper, cotton
muslin, glass fabric, glass filament mats, nylon fabric, and other fabrics, to provide
laminated materials in which the properties of the combined plastic and sheet stock
are quite different from the properties of either constituent by itself. Two principal
varieties of laminates are commonly made: (1) High-pressure laminates employing
condensation-type thermosetting materials, which are formed at elevated temperatures
and pressures. (2) Reinforced plastics employing unsaturated polyesters and
epoxides, from which no by-products are given off, and consequently, either low
pressures or none at all may be required to form combinations of these materials
with a variety of reinforcing agents, like glass fabric or mat.
4.71 THERMOSETTING PLASTICS
General properties of thermosetting plastics are described in Art. 4.68. Following
are properties of several thermosetting plastics used in buildings:
Phenol Formaldehyde. These materials provide the greatest variety of thermosetting
molded plastic articles. They are used for chemical, decorative, electrical,
mechanical, and thermal applications of all kinds. Hard and rigid, they change
slightly, if at all, on aging indoors but, on outdoor exposure, lose their bright surface
gloss. However, the outdoor-exposure characteristics of the more durable formulations
are otherwise generally good. Phenol formaldehydes have good electrical
properties, do not burn readily, and do not support combustion. They are strong,
light in weight, and generally pleasant to the eye and touch, although light colors
by and large are not obtainable because of the fairly dark-brown basic color of the
resin. They have low water absorption and good resistance to attack by most commonly
found chemicals.
Epoxy and Polyester Casting Resins. These are used for a large variety of purposes.
For example, electronic parts with delicate components are sometimes cast
completely in these materials to give them complete and continuous support, and
resistance to thermal and mechanical shock. Some varieties must be cured at elevated
temperatures; others can be formulated to be cured at room temperatures.
One of the outstanding attributes of the epoxies is their excellent adhesion to a
variety of materials, including such metals as copper, brass, steel, and aluminum.