Expanded polystyrene
Material
Expanded polyvinyl chloride (P.V.C.)
Foamed
urea-formaldehyde (U.F.) resins
Foamed
phenol-formaldehyde (P.F.) resins (phenolic foam)
Foamed polyurethane (urethane foam)
Expanded ebonite
+
Table 1
CELLULAR PLASTICS
Description
White rigid material formed by the fusion together of expanded beads of polystyrene. Also obtainable in a pre-compressed form that is more flexible and resilient, and in granular form as unfused expanded beads.
Yellow to deep brown rigid material, form- ed by foaming P.V.C, when in a plastic state, and cooling to solidify,
White friable material, usually formed by foaming an aqueous dispersion of resin and curing it in this condition,
Usually deep red rigid material, somewhat friable, made by foaming and subsequently curing phenolic resin.
Colourless to deep brown material, some- times artificially coloured. Prepared from two liquid components which are sometimes pre-expanded before complete chemical interaction and curing occurs,
Deep brown or black rigid material, which usually has a surface skin.
ap-
used for insulating boards plication of a gloss paint or a co- coon-skin
are not suitable, be- cause of the possible effects of sol- vents on the polystyrene. This dif ficulty can be overcome by first ap- plying a water-based undercoat or a lining paper. A film of polythene or aluminium foil by itself will also provide a vapour barrier, though it is not always easy to obtain a neat finish with these. Water-based coat- ings and emulsion paints do not affect the material but by themselves contribute little to a vapour barrier. Bitumen emulsions. while providing effective vapour barriers, are general. ly unattractive in appearance.
It is possible also to stick expand- ed polystyrene to wall surfaces with a uniform layer of cement mortar. Rough-cut grades of the material are available and are claimed to provide a better bond than smooth surfaced boards to both mortar and plaster. A skim-coat of plaster for providing a surface finish on the boards has been found in practice to form an adequate bond with any of the types of surface normally available, though treatment with a polyvinyl acetate emulsion gives an improved bond. A heavy cement rendering does not bond well to the surface. A normal 3/16-in. skim-coat of plaster will
114
Availability
in,
In sheets a few mm, thick and slabs and more thick. Also in shaped forms for special insulating jobs. Densities normally 1-2 lb/ft3.
In boards up to a few in, thick, with cells sized 1 mm upwards. Densities available 2-8 lb ft*.
up-
In slabs of density from
2 lb, ft3 wards. For building application, is generally foamed in situ to produce a 'cotton-wool' texture and density as low as 6 oz 'ft3.
In sheet and block form with density 2-4 lb 'ft3.
Rigid or flexible foam, in sheet and block form and in shaped forms for special jobs. May also be foamed in situ to produce a rigid foam.
Sheets of density 4 lb/ft3, also in denser forms and in special shapes.
probably have adequate impact resis tance for most purposes, but where impact blows are likely to be severe. it is advisable also to use a bonding coat of sanded plaster under the skim-coat.
Roof insulation
Expanded polystyrene boards have been laid directly on flat concrete roofs and bonded either with cement mortar or low-temperature bitumen. Tapered boards are available for this application to provide falls. This type of structure requires further treatment to make it weather-proof and to enable it to carry even the normal light traffic of a roof. Some care may also be needed to prevent undue build-up of heat in sunny weather, as the normal working tem- perature of 175°F may frequently be exceeded.
Where a sand-cement screed of at least 1 in. thickness is applied over the polystrene, with expansion joints as appropriate. normal roof finishes laid by the usual techniques are ae- ceptable, but it must be realised that adequate structural support for the extra weight is necessary. The technique is suitable to replace other insulating screeds on continuous decks.
Various methods of felt-laying which dispense with the expense of a screed have been suggested, using either emulsion-type adhesives in the cold or hot bitumen at temperatures that will not cause collapse of the polystyrene. The first layer of felt may be applied to the board in the factory, where control is easier, and the joints subsequently taped and further layers of felt applied on site. There is much to be said for a finish of white grit. or, if traffic is ap preciable. of white asbestos or con- crete tiles, to reduce softening in hot weather. To few roofs have been laid so far for the success of this method to be evaluated under all conditions, but the expanded poly- styrene has the valuable advantage that it entraps no water. and full in- sulation is obtained without the need for a drying-out period.
One interesting application of ex- panded polystyrene in roof construc- tion is as 'permanent shuttering' for concrete roofs, for which thermal insulation can be provided by placing boards of the material in the formwork before pouring the con- crete. When this sets. it holds the boards firmly in position. A novel development of this idea in the United States has been the use of wire supported expanded polystyrene
THE HONG KONG & FAR EAST BUILDER—VOLUME 18. NUMBER 6
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