No_6_April_1961 — Page 35

DITIONS PRESENT THEIR TO NEW MATERIALS

for hot water services for even high density polyethylene embrittles and deteriorates if its temperature regularly exceeds 115 deg. F.-a property which precludes its continued use if exposed directly to the tropical sun, especially in desert and marine atmospheres.

Polyethylene can be usefully employed in sheet form as a vapour barrier, as a temporary protective covering to both buildings and materials, and as a curing membrane for concrete. It is available reinforced with glass fibre. It should be noted that polyethylene is susceptible to slight termite nibbling which limits its possible uses below ground. It is highly resistant to fungal growth which when it occurs, is generally confined to extraneous surface matter.

So far, the principal building use for polyvinyl chloride (pvc) has been as a tough, flexible floor covering in sheet or tile form. For interior use there are no special tropical problems. Pvc in а wide range of colours has been recently introduced for rainwater goods and as a covering to metal and wooden frames.

There is no published experience yet of this use in the Tropics but there are good technical reasons for believing that black, grey and white colours will weather best. The rainwater goods are light, easy to handle, and do not cor- rode. Adequate provision must be made for their high thermal expansion which is some ten times that of steel.

On metal window frames, too, differential thermal expansion of the pvc and of the metal may pose problems.

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Termites known to nibble pvc and, with wooden window frames, there is a possibility of attack on the pvc to reach the timber with the subsequent destruction of the frame. This possible termite attack makes the use of pvc water and gas pipe for use underground unlikely but above ground and indoors it should be satisfactory. Pvc may also be used for waterstops in expansion joints.

Acrylic resins and polyester resins reinforced with glass fibre have been used as translucent corrugated roof sheets for daylighting. Under tropical exposure, they will both suffer some loss in light transmission and can dis- colour. Acrylic sheets are essentially durable and give good service.

Less is known of the durability of polyester resin. With many of the

THE HONG KONG & FAR EAST BUILDER

reinforced polyester resins at present available the glass fibres will slowfy become exposed and the dirt then collected will encourage mould growth.

Expanded polystyrene is a good insulator, essentially free from attack by insects and mould but has a maxi- mum safe working temperature of around 140 deg. E. This limits its use in the Tropies-it could be unwise, for example, to place it immediately under- neath a black bituminous roofing felt.

Plastics, particularly urea-formal- dehyde, are also used in the manufac- ture of resin-bonded wood particle board. The properties of particle board depend greatly upon the type of wood chip used. Random wood waste is undesirable. Forest thinnings are main- ly employed, preferably of coniferous timber whose naturally high resin con- tent prevents undue absorption of the expensive plastics resin.

The chips are bonded under pressure and heat and the boards so formed can be used internally for flooring ceilings and partitions, but are not suitable for prolonged external use.

Phenol-formaldehyde is weather- resistant and is used in the production of opaque corrugated paper-resin lamin- ates for roofing. Save for loss of original gloss such roof sheets durable to tropical exposure.

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Hollow rigid phenol-formaldehyde bonded panes have been used success- fully as internal partitions. Melamine formaldehyde paper laminates have been used for facing hardboard, particle board, and phenol formaldehyde paper laminates and give satisfactory service indoors.

Plastics are also used in the prepara- tion of modern paints-notably the emulsion and alkyd resin paints, Though there are others, the emulsion paints mainly used today are based on polyvinylacetate (pva) emulsified in water which, being a colourless medium, allows clean pastel shades to be pro- duced.

Pva emulsion paints are primarily decorative--not protective and the best results are obtained on concrete, brick, stone and plaster. These surfaces can be painted when slightly damp for the pva paint film is porous and allows moisture to breathe through it. plication is easy by brush or roller and a good finish is obtained showing few joints; drying is rapid. Flaking of pva

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paint is rare, adhesion in multiple coats is good, and fungal resistance is high.

Alkyd resin paints are both decora- tive and protective and can be obtained in many brilliant colours. They dry rapidly to give a tough, elastic film of considerable durability. Early difficul- ties of application have now been over- come. They are reasonably resistant to mould growth but less so when weather- ing has roughened the surface.

Pitch-fibre

sewage pipes consisting primarily of cellulosic fibres impregnat- ed under vacuum with heated coal-tar pitch are also new to the Tropics. They are light (5 lb. a foot run for a 6-in. diameter pipe), tough and resilient, and resistant to soil sulphates, to attack by sewage, and to fungal decay but not to organic pitch solvents and to continuous hot water.

Pipes are of smooth bore with tapered ends and the few joints needed are easily made by driven-on sleeve coupl- ings. These eliminate the need for jointing compounds and so the possibi- lity of obstruction in the pipes and the later entry of tree roots. They can be safely used underground. They should not be stored in direct sunlight for the high surface temperature then reached could seriously reduce their otherwise satisfactory resistance to flattening.

Brief mention may be made in con- clusion of aluminium sheets. They are (BS light and strong, and alloy NS-3 1470: 1955), which is chiefly used, has considerable resistance to corrosion even near the coast. Corrosion can occur, however, through direct contact with dissimilar materials-particularly with copper, lead, concrete, lime and woods-and through crevice conditions.

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This is especially likely in wet regions and special protective called for. Fixing accessories should be of aluminium, or galvanized or stainless steel, and crevices should be packed with a suitable barium chromate joint- ing compound.

To avoid the discomfort from 'glare' from the surface of new sheets these can now be obtained suitably treated. Aluminium may be anodized to thicken artificially the weatherresistant oxide film which forms naturally upon ex- posure.

The anodized layer can be dyed and coloured sheets are available. The main outlet so far, however, has been for window frames and curtain walling.

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