tration can be reduced by suitably rendering walls. the technique is not to be recommended in exposed areas. and particularly in the more westerly parts of Britain. It is signifi- cant that. in Denmark, where the technique originated. it is not now commonly used because of this in. creased risk. One American authori. ty requires that any insulation placed within the cavity shall be placed at least one inch away from the outer leaf. Expanded polystyrene and resin-bonded fibreglass slabs have been used in this way, fixed to the inner leaf of the cavity. and this method of insulation is less likely to cause trouble.
A technique of in situ foaming has been developed for another foamed plastics material foamed polyurethene, and is discussed in the section on that material. below.
FOAMED
PHENOL-FORMALDEHYDE
Foamed phenol-formaldehyde resin has been available for some time but only comparatively recently has the manufacturing technique been de- veloped to produce a useful and uni- form product. It is normally sup- plied in boards and slabs with a rather rough surface. but claims have been made that it can be foamed in situ with the use of special equip- ment. It is usually a deep red. some- what friable material with an in- herently open-cell structure and is most commonly used in densities greater than 2 lb/ft. with an optimum of mechanical strength and thermal insulating properties at around 4 lb/ft3.
It has important advantages in being relatively inexpensive, having self-extinguishing fire characteristics. and showing comparatively good me. chanical strength even for continuous use up to 265°F. Excess of acid hardener used in its production may cause corrosion of metals and it should not be used in contact with metal tanks or hot water pipes. The surface is not one that can easily be painted and it is possible that. in any case, free phenol in the plastics ma- terial might inhibit the drying of paint. It can, however. be used to provide insulation for concrete struc- tures and, since plaster will bond to it satisfactorily. it has been used as permanent shuttering for
for concrete
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and then covered with a skim-coat of plaster.
A most useful application is as thermal insulation on flat concrete roofs. Slabs of the material can be laid and bonded on concrete with any of the normally used adhesives and then, provided care is taken to avoid excessive local pressures which might puncture the felt. a built-up felt cover can be applied using normal techniques with hot bitumen. which will not cause the foam to collapse. The mechanical strength is not great enough to resist puncture if ladders are rested on such a roof covering but. for roofs where only light traffic is expected. it is strong enough. Tile finishes can be laid over the built-up roof where traffic is heavier.
FOAMED POLYURETHANES
Until a very few years ago, these materials were hardly known in Britain. and they first appeared for general use as flexible foamed synthetic rubbers which have been used in carpet underlays, cushioning. household sponges and. more recent- ly as draughtproof sealing strips for use round windows and doors. Flexible foams can be made with a range of properties. In general they have a largely open-cell structure. which gives excellent acoustic ab sorption at high frequencies. They are probably too expensive for gener- al building use though they might be fitted behind perforated panels and find application in hospitals, for example. to control noise. They have been used to line air-conditioning ducts to suppress plant noise that would otherwise be carried by the air stream in the ducts. They can also be produced with a substantial pro- portion of closed cells. giving a ma- terial with somewhat better thermal insulating properties. However, they are not widely used for thermal in- sulation because of their cost and the only building application of any im- portance is as flexible re-formed pipe-lagging, which can be carried round bends and easily shaped. This can be manufactured with a vapour- sealing outer skin, and so finds ap- plication where protection of pipes from condensation is a problem.
Recently, rigid polyurethane foams have been developed and have been recommended for a number of build-
ing applications. For the first ma- terials of this type to appear, the foaming agent used was the carbon dioxide gas that was produced by chemical reaction simultaneously with the hardening process. More recent. ly. volatile fluorinated hydrocarbons have been injected into the mixture to produce even and controlled foam- ing. The product, having pores filled with a vapour that does not readily diffuse through the cell walls. has a thermal conductivity less than that of still air. In time. a certain amount of diffusion does occur but. even so. the eventual thermal conductivity of a properly made foam is likely to be no more than that of still air. and appreciably lower than that of any other solid thermal insulating ma- terial that is at present available,
The principal advantages that poly- urethane foams can offer over other foamed plastics are their excellently low thermal conductivity. fairly good high-temperature resistance (210°F- 225°F). low vapour permeability and. most particularly. their property of being able to be foamed in situ by simply nixing two liquid components in
monitored proportions. These produce a foam which expands after placing to fill any holes and crevices. and which also adheres well to sur- faces. It is possible. for example, to produce the foam round a complicat- ed network of pipes to provide thermal insulation, or to fill complex cavities.
This may be useful in roof-spaces. and the process has also been sug- gested fo filling cavity walls to im- prove their thermal insulation. For this there should be no large voids in the foam because, as already dis- cussed in connection with foamed urea-formaldehyde. the primary pur pose of the cavity is to provide a barrier to water penetration. Such a foam would not permit ingress of water except at the bridge formed by a void where in severe condi- tions,
any penetration would be aggravated. While it is possible in the factory to produce a good and uniform foam. on the building site. in adverse weather conditions and with wide variations in the wetness of surfaces. this requires a great deal of skill. A recently developed techni que produces a 'frothed foam with better flow properties than the mix. ture normally injected into the cavity. and it seems likely that this techni
THE HONG KONG & FAR EAST BUILDER--VOLUME 18, NUMBER 6
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