No_6_April_1964 — Page 119

Cellular Plastics for Building

THER

HERE has been a considerable increase in the use of plastics in foamed and expanded forms for thermal insulation and acoustic purposes in buildings. This Digest examines the special properties of these cellular plastics and comments on their suitability as building materials. A short section on their use in sandwich panels is also included.

THE

HE relatively high cost of many plastics has tended to restrict their use in building to applications where a little material goes a long way, particularly in situations where their special properties, such as light- ness or resistance to corrosion, are more important than strength. The comparative ease with which plastics can be manufactured in lightweight cellular forms has, however, reduced costs and at the same time enabled valuable properties arising from their porosity to be utilized. Chief among these properties is thermal insulation, since a high proportion of the total volume of cellular plas- tics is entrapped still air.

The term 'cellular plastics is used for both foamed and expanded ma- terials. Foamed plastics are mainly those made from liquid starting materials; they may have an open or closed cell structure. Expanded plastics have a substantially closed cell structure. In open-cell materials. the cells are mostly interconnected. permitting free movement of air and vapour through the volume of the material. whereas closed-cell ma- terials are substantially air and vapour-tight. Table 1 lists the cel lular plastics at present used to any appreciable extent in building: ex- panded ebonite though not a true plastic material' is included here. because its use for thermal insula. tion purposes compares with that of other cellular plastics.

Table 2 lists some properties of cellular plastics in the more com- monly available forms, likely to be of most interest to builders and de- signers. It has not been possible to obtain all these data at first hand, or to obtain data determined under the same conditions in every case. Το give roughly comparable data, some figures have been estimated from in- formation obtained under somewhat different conditions.

The use and application of these materials will now be discussed in. dividually.

EXPANDED POLYSTYRENE

Although known principally as a white cellular material obtainable as rigid boards and slabs. expanded polystyrene is available in all the forms listed in Table 1. and these are considered in more detail later.

A number of advantages contri- bute to the usefulness of expanded polystyrene in its principal applica- tion

Its for thermal insulation. thermal conductivity is very low; an average k-value of 0.24 deter- mined at building temperatures is better than that of almost any other available material. and can be im- proved by increasing the density to about 2 lb ft", though the benefits of this rarely justify the extra cost.

The advantages include the ex- treme lightness of the material, its attractive appearance, the ease with which it can be handled and fixed. together with surprisingly good me- chanical properties for a material with so little weight. It is odour- less, is chemically very stable, and has a very low water absorption and so retains its thermai insulating pro- perties even under wet conditions. It is also resistant to fungal attack. while no cases are on record of its being attacked by termites. It can be made in a grade described by the manufacturers as self-extinguishing. which has flame-retardant properties. though in the event of a fire the small amount of material makes little con- tribution to the fire load.

The water vapour permeability. while lower than that of most in- sulating boards, is not usually low enough to allow the use of a vapour seal to be dispensed with when con- densation is likely to occur. The re commended maximum working tem-

THE HONG KONG & FAR EAST BUILDER-VOLUME 18. NUMBER 6

perature is 175°F. which is too low for some applications, e.g. insulating hot water systems, and its dimen- sional stability depends on the condi- tions of use. It is softened by some organic solvents, such as are present in many paints and adhesives, and in some petroleum fuels.

The material is manufactured in large blocks. which are then sliced into boards of the required thickness. Ease of manufacture of expanded polystyrene in various thicknesses makes it more competitive with other insulating materials in thicknesses greater than in. than it is in the commonly used in. boards.

The low density of expanded poly- styrene makes it easy to fix with ad- hesives, though, as indicated above. these must be carefully selected so as to avoid risk of chemical attack. Manufacturers generally recommend suitable adhesives. Where the area of contact is limited, as under un- even ceilings or on corrugated sur- faces, it is not advisable to rely solely on the adhesive, and some form of mechanical fixing, such as nailing to battens, should be provid ed. Particularly under corrugated skinroofs, the build-up of heat over the insulating boards in sunny wea- ther can be sufficient to render most of the common adhesives ineffective. In such cases, it is advisable to fix the material with large-headed nails to timber battens taking precautions similar to those for fibre insulating boards. Alternatively, one of the mechanical systems specifically de- signed for use with expanded poly- styrene should be used to fix it either to the purlins. or as a suspended ceiling.

Expanded polystyrene will not by itself constitute a

vapour barrier when used under a skin roof in a warm humid building, and one must be provided. The methods normally

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