Plastics in construction

The broad range of thermoplastic, thermosetting and elastomeric plastics are collated into families in Figure 10.12. Typical uses in construction are listed in Table 10.2. (Glass-fibre reinforced polyester is described in Chapter 11; foamed plastics as insulation materials in Chapter 13; and plastics used primarily as adhesives in Chapter 14.)

THERMOPLASTICS Polythene (polyethylene)

Polythene (PE) is one of the cheapest plastics and is available both in the low density (LD) (softening point 90°C) and high density (HD) (softening point 125°C) forms. Polythene is resistant to chemicals, tough at low temperatures, but is rapidly embrittled by ultraviolet light unless carbon black is incorporated. Polythene burns and has a relatively high coefficient of thermal expansion. Low-density polythene is used widely for damp-proof membranes, damp-proof courses and vapour barriers. High-density polythene, which is stiffer than the low-density material, is used for tanking membranes to basements. Polythene is used for the production of cold-water cisterns, but is only suitable for cold-water plumbing applications due to its high thermal expansion; for mains water pressure it requires a significant wall thickness due to its relatively low tensile strength. Cross-linked polyethylene (PEX), manufactured by the action of peroxide catalyst on normal polyethylene, is used for domestic hot water and underfloor heating systems as it can withstand operating temperatures up to 90°C.

Polypropylene

Polypropylene (PP), with a softening point of 150°C, is slightly stiffer than polythene, to which it is closely related chemically. Like polythene, it is resistant to chemicals and susceptible to ultraviolet light, but unlike polythene becomes brittle below 0°C. However, the block copolymer with ethylene does have improved low-temperature impact resistance. Polypropylene is used for pipes, drainage systems, water tanks, DPCs, connecting sleeves for clay pipes and WC cisterns. Polypropylene fibres are used in fibre-reinforced cement to produce an increase in impact resistance over the equivalent unreinforced material. Certain breather membranes used for tile underlay and timber-frame

Plastics

Thermoplastic

Thermosetting

Elastomeric

Rubber Neoprene EPDM Butyl rubber

Phenolic resins

Amino resins

Polyester

Phenol Urea Melamine GRP

formaldehyde formaldehyde formaldehyde

Polyolefins PTFE ETFE Acrylic Polycarbonate Polyamides ABS Vinyls

Polythene Polypropylene Polybutylene

Nylons PVC PVC-U PVC-UE CPVC

HDPE LDPE Nylon 6 Nylon 66

(Insulation materials are described in Chapter 13 and adhesives in Chapter 16)

Fig. 10.12 Plastics used in construction construction are manufactured from multi-layer systems incorporating polypropylene with polyethylene and glass-fibre reinforcement. Such products are wind and watertight, but vapour-permeable. Many geotextiles for soil stabilisation are manufactured as a mat material from non-woven heat-bonded polypropylene continuous fibres. The material may be reinforced by woven polyester fibres.

Polybutylene

Polybutylene is used for pipework as an alternative to copper. It has the advantage of flexibility and the very smooth internal surface is resistant to the build-up of scale and deposits. It can withstand continuous operating temperatures up to 82°C.

Polyvinyl chloride

Polyvinyl chloride (PVC) is the most widely used plastic material in the construction industry. It is available both in the unplasticised form (PVC-U) and as the plasticised product (PVC). In both forms polyvinyl chloride is combustible giving off noxious hydrogen chloride fumes; however, the unplasticised form tends to burn only with difficulty. PVC begins to soften at 75°C, and therefore cannot be used for hot water systems, although chlorinated PVC (CPVC) can be used at higher temperatures. PVC is soluble in certain organic solvents which, therefore, can be used for the solvent welding of joints, but PVC is unaffected by acids and alkalis.

Plasticised PVC is extensively used in the manufacture of floor coverings, either as individual tile units or as continuously jointed sheet. It is also the most widely used material for single-layer roofing systems due to its durability, colour range and ease of application. It also offers an alternative to bitumen felt for sarking. Plasticised PVC is the standard for electrical cable insulation, and many small building components are made from injection moulded PVC.

PVC-U

PVC-U is widely used for rainwater goods, usually in white, grey, black or brown ,and similarly for

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MATERIALS FOR ARCHITECTS AND BUILDERS

Table 10.2 Typical uses of plastics in construction

Material

Examples of plastics in construction

Thermoplastics

Polythene

(Low density)

DPC, DPM, vapour checks, roof sarking

(High density)

Cold-water tanks, cold-water plumbing

Polypropylene

Pipework and fittings, drainage systems, water tanks, WC cisterns, DPCs, fibres in fibre-reinforced

concrete

Polybutylene

Hot and cold-water pipework and fittings

Polyvinyl chloride

(PVC-U)

Rainwater goods, drainage systems, pipes and fittings, underground services, window and door

frames, conservatories, garage doors, translucent roofing sheets

(PVC-UE)

Claddings, barge boards, soffits, fascias, window boards

(PVC)

Tile and sheet floor coverings, single-ply roofing, cable insulation, electrical trunking systems, sarking,

tensile membrane structures, glazing to flexible doors, door seals, handrail coatings, vinyl-film

finishes to timber products

(CPVC)

Hot-water systems, window and door frames

ETFE

Inflated systems for translucent wall and roof membranes

PTFE

Sealing tape for plumbing, tensile membrane structures, low-friction movement joints

Polymethyl methacrylate

Baths, shower trays, kitchen sinks, glazing, roof lights, luminaires

Polycarbonate

Vandal-resistant glazing, spa baths, kitchen sinks

Polystyrene

Bath and shower panels, decorative expanded polystyrene tiles

ABS copolymer

Pipes and fittings, rainwater goods, drainage systems, shower trays

Nylons

Electrical conduit and trunking, low-friction components - hinges, brush strips for sealing doors and

windows, carpet tiles and carpets shower curtains

Thermosetting plastics

Phenol formaldehyde

Decorative laminates

Melamine formaldehyde

Laminates for working surfaces and doors, moulded electrical components, WC seats

Urea formaldehyde

Decorative laminates

Glass-reinforced polyester

Cladding and roofing panels, simulated cast-iron rainwater goods,

(GRP)

cold-water tanks, spa baths, garage doors, decorative tiles and panels

Elastomers

Rubber

Flooring, door seals, anti-vibration bearings

Neoprene

Glazing seals, gaskets

EPDM

Glazing seals, gaskets, single-ply roofing systems

Butyl rubber

Sheet liners to water features and land-fill sites

Nitrile rubber

Tile and sheet flooring

soil and waste pipes. It is also used colour-coded for underground water, gas, electrical and telecommunications systems. PVC-U is used extensively for the manufacture of extruded window frames, door frames and conservatories, usually incorporating sealed double-glazing units. Where insufficient rigidity is achieved with the PVC-U alone, steel inserts within the extruded sections give strength and provide additional protection against forced entry. PVC-U is used in the manufacture of translucent, transparent and coloured profiled sheeting for domestic structures such as carports and conservatories, where an economical product is required, although eventually the products discolour and craze due the effects of direct ultraviolet light.

PVC-UE

Extruded cellular unplasticised PVC (PVC-UE) is used for cladding, fascias, soffits, window boards, barge boards and other components of uniform section. It is manufactured by the co-extrusion of a high-impact PVC-U surface material over a core of closed cell PVC foam. The foaming agent is usually sodium bicarbonate. The high stiffness to weight ratio arises from the combination of a dense outer skin and a cellular core. Both the cellular core and the wearing surface are stabilised with metallic additives to prevent degradation and discolouration. Contact with bitumen should be avoided. The material will char and melt in fire, but with a limited surface spread of flame. The material is described within the standard BS 7619: 1993.

Tensile membrane structures

PVC-coated polyester is the standard material used for tensile membrane structures and canopies. The durability depends directly upon the degree of translu-cency; at 15% transmission, 15 years can be reasonably expected. At greater levels of translucency, the expected serviceable lifetime is considerably reduced; however, fluropolymer lacquers to the top surface of the fabric enhance durability. While white fabric is the standard, coloured and patterned membranes are available to client requirements. PVC-coated polyester membranes are a cheaper alternative to PTFE-coated fabrics, but are not non-combustible. PVC-coated polyester is more flexible than PTFE-coated fibreglass and is therefore the preferred material for temporary structures which may be folded for transport and storage. The thermal insulation afforded by single-layer tensile membrane roofs is negligible.

Tensile membrane structures are manufactured from a set of tailored panels stitched or welded together. They are usually tensioned by wires or rods running through edge pockets, or by fixing directly to structural elements. Accurate tensioning is required to generate the correct form and resistance to wind and snow loads. The use of double curvature within the panel elements imparts structural rigidity to the overall membrane structure. Damage by accident or vandalism can usually be repaired on site.

Polytetrafluoroethylene

Polytetrafluoroethylene (PTFE)-coated glass-fibre woven fabrics are used for permanent tensile membrane structures. In a fire, PTFE gives off toxic combustion products, but only at temperatures above which any fabric would have already failed and vented the heat and smoke. With a fire rating of Class 0, PTFE-coated glass-fibre tensile membranes are more expensive than the Class 1 rated PVC-coated polyesters, but are generally more durable with an anticipated lifespan in excess of 20-25 years. The low friction PTFE surface has good self-cleaning properties.

The Inland Revenue Amenity Building in Nottingham (Fig. 10.13) and the Millennium Dome at Greenwich are roofed with PTFE (Teflon) coated glass-fibre tensile membranes. The translucent fabric gives well-lit internal spaces during the daytime, and striking glowing surfaces at night. In the Nottingham building, the membrane roof is suspended from four steel columns, and is linked to the fixed structure below by inflatable elements which absorb any movement.

PTFE tape has a very low coefficient of friction and a high melting point. It is therefore ideal for use as a sealing tape for threaded joints in water and gas pipes. It is also used to form sliding joints in large structures.

Ethylene tetrafluoro ethylene

Ethylene tetrafluoro ethylene copolymer (ETFE) is used as a translucent foil for low-pressure pneumatic metal-framed building envelope cushions. The fluoro-copolymer has the advantages over glass that when used to form two- to five-layer air cushion systems, it offers higher thermal insulation with greater transparency to UV light. ETFE is strong, shatter proof, half the cost, and only one-hundredth the weight of the equivalent glass, thus offering significant economies to the required structural supporting system. ETFE with an anticipated life span of 25 years, can withstand maintenance loads, be easily repaired and is recyclable. It has been used very effectively on the galvanised-tu-bular steel space-frame envelope for the biomes (domes sheltering plants from around the world) at the Eden Centre, Cornwall (Fig. 10.14). The structure is formed from an icosahedral geodesic outer layer, with a combination of hexagons, pentagons and triangles as the inner layer of the three-dimensional space frame. Only a small pumping system, powered by photovoltaic cells, is required to maintain the air-fill of the ETFE cushions. At the National Space Centre at Leicester, also designed by Nicholas Grimshaw and Partners, the ETFE cushion clad tower houses the main space rocket exhibits (Fig. 10.15).

If automatic smoke venting of an ETFE atrium is required, electrical wiring can be incorporated into the cushion frames, which release the cushions, except at one point, in case of fire. An enclosed atrium is thus turned into a fully open lightwell.

Transmitted light levels through ETFE cushions can be constantly adjusted by the use of partially

Fig. 10.13 Tensile Membrane - Inland Revenue Amenity Building, Nottingham. Architects: Hopkins Architects. Photographs: Courtesy of Martine Hamilton Knight
Fig. 10.14 ETFE - Eden Project, Cornwall. Architects: Nicholas Grimshaw and Partners. Photographs: Arthur Lyons and courtesy of Perry Hooper (interior)

printed internal layers within the cushions, which can be moved closer or further apart by changing the pumped air pressure, thus modifying the shadowing effect. Interesting patterns can be created by the use of coloured cushions, whilst aluminium-coated foils will give a highly reflective effect with reduced sunlight penetration.

Polymethyl methacrylate

Acrylic or polymethyl methacrylate (PMMA) is available in a wide variety of translucent or transparent, clear or brightly coloured sheets. It softens at 90°C, and burns rapidly with falling droplets of burning material. Stress crazing may occur where acrylic has been shaped in manufacture and not fully annealed, but generally the material is resistant to degradation by ultraviolet light. Acrylic is frequently used for decorative signs, roof lights and light fittings. Baths and shower trays are manufactured from acrylic as a lighter alternative to cast iron and ceramics. Although not resistant to abrasion, scratches can usually be polished out with proprietary metal polish.

Fig. 10.15 ETFE - National Space Centre, Leicester. Architects: Nicholas Grimshaw and Partners. Photograph: Arthur Lyons

Polycarbonate

Polycarbonates (PC) are used as vandal-resistant glazing, due to their high-impact resistance, good optical transparency and low ignitability. Polycarbonate blocks offer a lightweight alternative to traditional cast glass blocks. Proprietary extruded cellular systems of double or triple walled polycarbonate offer combined thermal insulation and vandal-resistant properties. The protective outer surface prevents ultraviolet degradation for ten years, and sections may be curved on site within the limits of the manufacturers' specifications.

Acrylonitrile butadiene styrene

Acrylonitrile butadiene styrene (ABS) plastics are a range of complex terpolymers manufactured by combining together the two copolymers, styrene-acryloni-trile and butadiene-styrene. ABS plastics are relatively expensive but tough and retain their strength at low temperatures. They are used to manufacture moulded components, rainwater and drainage goods. A special ABS solvent cement is required for solvent welding.

Nylons

Nylons, usually nylon 66 or nylon 6, are used for the manufacture of small components where low friction is required. Nylons are tough and strong but tend to be embrittled and become powdery on prolonged exposure to sunlight. Carpet tiles in nylon 66 are durable and hard wearing.

Kevlar

Kevlar (polyparabenzamide) fibres are produced by extrusion of a cold solution of the polymer into a cylinder at 200°C, which causes the solvent to evaporate. The resulting fibres are stretched by a drawing process, which aligns the polymer molecules along the fibres to produce a very high modulus material used in ropes and composite plastics.

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