Plastics. Introduction. Plastics. Development of plastic products

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1 Plastics Introduction In the twentieth century plastics were developed to such an extent that they replaced many natural materials. The term plastics is now used to describe many products which are artificially made and chemically produced. Plastics are made up of giant carbon-based molecules and during their manufacture, pass through a mouldable stage which allows them to be shaped into a limitless variety of shapes. It is this ability to be moulded which makes plastic products suitable for many applications in the building industry. Glues and adhesives have been made since ancient times and many of the materials were naturally occurring; for example, bitumen and tree resins. The growth of the plastics industry has resulted in the discovery of many new adhesives from synthetic resins. Many older type glues are still used today but are generally restricted to use on natural materials whereas the modern adhesives will adhere to a wide variety of materials; for example, glass and steel. Plastics The term plastics as it is commonly used today, refers to a large group of synthetic materials which may be derived from coal, natural gas or other petroleum products, cotton, wood and waste organic products such as oat hulls, corn cobs and sugar cane. From these substances, relatively simple chemicals, known as monomers, are produced. Monomers are capable of reacting with each other and are built up into chain-like molecules called polymers. Rubber products, which are derived from a naturally occurring organic base, have in some cases been superseded by plastic products which can have similar or superior properties. Development of plastic products Plastics have had a profound effect on nearly every facet of our society and the proliferation of plastic products has meant that practically everyone is in almost daily contact with plastics in one form or other. In the building industry, like everywhere else, plastic products have taken over from many traditional materials. Topic 2.8 Plastics CPCCBS6001 Ed 1 1

2 Types of plastics Plastic materials fall into two groups: thermoplastics thermosetting plastics. Thermoplastics These become soft when heated and harden again on cooling, regardless of the number of times the process is repeated. However, there are practical limits to the number of times that thermoplastics can be heated and cooled; too many times affect the appearance and strength of the product. Thermosetting plastics (thermosets) These undergo an irreversible chemical change during production, in which the molecular chains cross-link so that they cannot subsequently be appreciably softened by heat, while excessive heating will cause charring. General properties of plastics Plastics vary considerably in behaviour and specific differences will be discussed under individual plastics. Some properties common to most plastics are: strength thermal conductivity electrical insulation combustibility durability non-biodegradability. Strength Most plastics have tensile strength-to-weight ratios which are higher than many metals but their greater elasticity precludes plastics from most structural applications. Also, plastics tend to creep and degrade at elevated temperatures, resulting in reduced strength. Thermal expansion can be as much as ten times that of steel. Thermal conductivity Expanded plastic materials have relatively low thermal conductivity hence the suitability of foamed plastics, which contain air bubbles, as insulation material. 2 Topic 2.8 Plastics CPCCBS6001 Ed 1 New South Wales Technical and Further Education Commission, 2015 (TAFE NSW WSI

3 Electrical properties Plastics do not conduct electricity and are therefore excellent insulators but electrostatic charges can build up on plastic surfaces and attract dust, and sparking could be hazardous in some situations. Combustibility Many plastics are combustible and the spread of flame over some plastic surfaces is high. When burning, plastics produce a great deal of smoke and it is the noxious gases emitted and the tendency of some plastics to melt rapidly which present the major safety hazards. Durability Although plastics do not rot or corrode, in many cases they have not been around long enough for their durability to be adequately assessed. Ultraviolet radiation from the sun is responsible for breakdown and colour change in some plastics, especially in the presence of heat. Some pigments behave better than others in exposed conditions and advice from manufacturers should be sought regarding suitable colours for outdoors. Some plastics, acrylics and PVC, in particular, have performed well outside for a number of years. Environmental hazards Plastics are not biodegradable and the disposal of plastic products is of environmental concern. In the past, and to some extent at present, plastics were disposed of by burning which causes serious atmospheric pollution. Plastics have also been disposed of by burial which causes problems because they do not break down for many years. Today many plastics are recycled. Properties and uses of specific plastics in building Plastics can be formed by a variety of processes according to the type of plastic and the end product required. The applications of plastic products in buildings are numerous, as are the number of plastics available. Although the list below might seem endless, only the most frequently used plastics are described and since plastics are being used so widely you should be familiar with the properties of at least the most common varieties. Thermoplastics Polyethylene (polythene) This is available in low density and high density forms. It has a high degree of impermeability to water and water vapour. Its toughness and chemical resistance make it suitable for Topic 2.8 Plastics CPCCBS6001 Ed 1 3

4 waterproof membranes, for cold water cisterns, for bath, basin and sink wastes and cold water pipes. Its high thermal movement, however, makes it unsuitable for hot water pipes. Polyethylene is suitable for waterproof membranes, for cold water cisterns, for bath, basin and sink waste pipes and cold water pipes. It is unsuitable for hot water pipes. Polyvinyl chloride (PVC) PVC is produced in several forms. In its rigid or unplasticised form (UPVC) it is used for soil and rainwater pipes and for electrical conduits and accessories. In transparent, translucent and opaque sheets it is used for roofing or wall cladding. The plasticised or flexible form is used in vinyl floor coverings, electrical cable insulation and sarking. PVC burns only with great difficulty and is self-extinguishing, which makes it suitable for airconditioning ducts. Polyvinyl acetate (PVA) Because of its low softening point, PVA is limited to use in adhesive for joinery, emulsion paints, bonding agents for plaster, cement screeds and in situ floor coverings. Polymethyl methacrylate (acrylic) Because of its high transparency in the clear form (92 per cent compared with 90 per cent for glass) and high resistance to impact (greater than glass), acrylic is used extensively for corrugated sheeting, roof lights and light fittings. However, large areas of acrylic burn rapidly and the melting plastic drops from roofs. It should, therefore be avoided for large areas of roofing. Polystyrene In its unmodified form, polystyrene tends to be brittle, easily attacked by certain organic solvents and readily burnt. It is low in cost and is used for cisterns, light fittings and concrete formwork and in some paints. Expanded or foamed polystyrene is used for building boards, and both rigid and loose-fill insulation. Polytetrafluoroethylene (teflon) Teflon is highly resistant to heat and has very low friction characteristics; however, it is extremely expensive and is used only for special applications such as PTFE (plumber s) tape which is used to give a tight friction fit mainly between threaded brass connections. Polyamide resins (nylons) There are many forms of nylon. They are tough, very strong and hard wearing and have low friction characteristics. Unlike other plastics, they absorb up to 2 per cent of water, swell slightly and burn only with difficulty. Apart from use as a fibre in carpets and upholstery 4 Topic 2.8 Plastics CPCCBS6001 Ed 1 New South Wales Technical and Further Education Commission, 2015 (TAFE NSW WSI

5 materials, nylons are used for nuts and bolts, castors, curtain rails and sliding door fittings and ball valve assemblies. Polycarbonates Extremely high in cost, but with remarkable properties, polycarbonates are dense and hard with a high ductility and tensile strength like metals. They are transparent (86 per cent light transmission), with a high softening point, and are virtually self-extinguishing. They are used for roof glazing and vandal-proof and bulletproof glazing. Thermosets Phenol formaldehyde (bakelite) One of the oldest of the plastics, first produced commercially in 1910, bakelite is also the cheapest thermosetting plastic. It is usually dark in colour and because it is a good insulator and resistant to ignition, its uses include electrical and door furniture mouldings, and in adhesives, paints and foamed applications. Urea formaldehyde Urea formaldehyde products are usually white or brightly coloured and it is selfextinguishing. It is used for electrical accessories, paints, stoving enamels, adhesives and foamed products. Melamine formaldehyde Melamine formaldehyde can be made in a wide variety of bright, permanent colours; it is resistant to hot and cold water and cigarette burns. Its major use is as a surface to paper laminates such as laminex or formica, which creates a durable sheeting material suitable for high-wear horizontal or vertical surfaces such as kitchen benchtops and waterproof cupboard and wall linings. It is also used for mouldings and in adhesives. Resorcinol formaldehyde This is a dark red resin used as a waterproof and boilproof adhesive for wood. Polyester resins These have a wide range of properties including high thermal resistance. They harden without heat or pressure and are used in glass-fibre reinforced plastics (GRP or fibreglass), paints and clear finishes. Polyester films are used to improve shatter resistance and solar control. Polyurethanes Polyurethanes have even wider ranging properties than polyesters and are used in paints, clear finishes, sealants and foamed products, among other things. Topic 2.8 Plastics CPCCBS6001 Ed 1 5

6 Epoxide resins (epoxy) Usually provided as a two-part pack consisting of resin and hardener (or curing agent) epoxide resins are extremely tough and durable, with very good resistance to chemicals. Because they adhere well to most materials, they are frequently used as coatings for metal surfaces. They are also used in paints, clear finishes, fibreglass and adhesives. Silicons Silicons are water repellent and, hence, frequently used in transparent waterproof coatings for masonry, in paints and in mastics. In addition, silicone-based products can be injected into walls to prevent rising damp. Additives for plastics Not all the properties of plastics are positive ones, and in many cases it is necessary to add a material to a plastic compound which will help it to overcome some tendency to failure or to reinforce another property. This was mentioned briefly in the previous paragraph. Among the materials which are added to plastics to improve their performance in some way are: anti-oxidants anti-static agents colouring agents coupling agents fillers reinforcing agents flame retardants foaming agents lubricants plasticisers preservatives processing aids ultraviolet stabilisers For details concerning the need for these additives, their reactions with specific plastics and the materials in common use which perform these various functions, consult the Modern Plastics Encyclopaedia a McGraw-Hill publication. 6 Topic 2.8 Plastics CPCCBS6001 Ed 1 New South Wales Technical and Further Education Commission, 2015 (TAFE NSW WSI

7 Plastics in construction Plastics, with their wide range of properties - light weight, strength, durability, corrosionresistance and weatherability, to list a few - are ideal for construction applications. With the development of manufacturing processes, any size and shape can be produced quickly and accurately. Plastics are now being used as structural and non-structural components, in composite applications, and as auxiliary materials. Structural applications For structural purposes, a family of plastic materials produced by reinforcing the plastic with a fibrous material are commonly used. About 90 percent of all reinforced plastics use glass fibre but cotton, asbestos as well as synthetic and metallic fibres are also used. Polyester resins are used in about 85 percent of all reinforced plastics, the others being epoxy, acrylic, melamine, silicone, nylon, polystyrene and polyvinyl chloride resins. Corrugated panels One of the more common structural products is the glass-fibre-reinforced acrylic corrugated panel, which is not only transparent or translucent but also highly resistant to discolouration. Panels are available in various widths 450, 600 and 850 mm depending on the application and with 32- or 64-mm corrugations. Lengths vary from 2.4 to 4.3 m. Panels are available in a variety of colours. Because these panels have good dimensional stability, as well as being strong and corrosion resistant, their applications are many. They are used as roof panels, skylights, wall cladding, room dividers as well as carport and patio covers. Flat sheets Flat sheets of clear acrylic and polycarbonate plastic are used extensively as glazing materials. They are available in a variety of thicknesses, sizes, and finishes. Because of their light weight, high impact strength and high light transmission values, these sheets are used in skylights, curtain walls, passive solar applications, pool enclosures and greenhouses. In double glazing, they are about 12 percent more effective than glass when considering heat loss and 20 percent more effective when considering heat gain. When sheets are laminated, they are very resistant to impact and are used where security must be considered. Thermoformed shapes Sheets may also be moulded by the thermoform process into roof shapes. Tinted acrylic sheets may be used as sunscreens to reduce solar heat, or the material may be used to make light-control lenses for glare-free lighting. Transparent or translucent sheets may also Topic 2.8 Plastics CPCCBS6001 Ed 1 7

8 be used for dome skylights. Acrylic sheets are textured or formed to produce spandrel panels, enclosure panels, and other exterior facings. Plastic materials are widely used in the manufacture of sandwich panels used in curtainwall construction. In some cases only the sandwich core is a plastic material, while in others both the core and one or both faces are of plastic. Some sandwich panels are made by bonding plastic sheets to an aluminium grid core (see figure 1). Plastics used for such panels include acrylics, polypropylene, polycarbonates and polyesters because of their dimensional stability, high impact resistance, ability to withstand weathering, corrosion resistance and, in the case of coloured products, their ability to resist fading. Figure 1 Plastic-faced sandwich panel Reinforced acrylic is used to make the dome pans employed in forming a two-way rib or waffle-type concrete floor slab. Glass-fibre structural shapes A relatively new use for fibre-reinforced plastic is in the manufacture of structural shapes (see figure 2). These come in a wide variety of standard and non-standard sizes, and special shapes are produced upon special request. Flat sheets are available in seven standard thicknesses ranging from 3 to 25 mm. Non-standard thicknesses and sizes can be made to order. 8 Topic 2.8 Plastics CPCCBS6001 Ed 1 New South Wales Technical and Further Education Commission, 2015 (TAFE NSW WSI

9 Figure 2 Structural shapes These shapes are produced by the pultrusion process. Figure 3 The pultrusion process The resin used can be tailor-made to suit particular requirements, but the three basic systems are: isophthalic polyester vinyl ester epoxy. The types of reinforcing fibres used are: glass-fibre rovings and continuous strand mat graphite fibres aramid fibres. Sections produced by this method are most useful in areas where corrosion is a problem, water and sewage treatment plants, pulp and paper mills, as well as chemical plants. In addition to being corrosion-resistant, the material is non-conductive, non-magnetic and lightweight. Topic 2.8 Plastics CPCCBS6001 Ed 1 9

10 The material is extremely strong, stronger than steel when compared on a mass basis. Standard shapes have an ultimate tensile strength of 258 MPa, while flat plate can develop 172 MPa when loaded in the longitudinal direction. It will not creep under long-term loads. The material can be sawn, drilled, routed and turned on a lathe, but sections cannot be bent, rolled, or pressed as steel can be. Connectors used to connect sections are screws, bolts and adhesives. A combination of bolts and an adhesive produces the best results. The material may be painted, but since it comes in standard colours and painting is usually not required. If painting is necessary, surfaces should be prepared by sanding or sandblasting. Most paints may be used with good results. Plastic foam boards Rigid slabs of foamed plastic are finding increasing use for structural purposes. Because of their flexibility, the slabs can be readily fitted to the contours of the roof. Later, the deck is covered with a layer of concrete. Research is being carried out continually on the possibilities of further uses of plastics for structural components. Polystyrene is one material ideally suited for such purposes. It has rigidity, lightness, good impact resistance and structural strength. Structural polystyrene items are stronger than comparable wood or metal ones. Like most plastics, it has the added advantage of being able to be moulded into almost any desired shape. This could mean a reduction in the number of component parts required to form a given structure. Also, like many other plastics, polystyrene is resistant to shattering, is weather- and corrosionresistant, and is dimensionally stable. Fabrics Polystyrene and nylon fabrics, coated with PVC, provide an alternate method for enclosing large open areas. These fabrics, a result of space-age technology, are light, strong, durable and remain pliable over large temperature changes. A combination of glass-fibre fabric and a Teflon fluorocarbon resin has been used in fabric roof applications with much success. The glass fibre fabric provides the strength for sustaining the necessary design loads, while the Teflon provides the flexibility, durability and weather resistance. This type of coated fabric has been given a life expectancy of more than 20 years. Foamed insulation One of the most important uses of plastics in the construction industry is insulation. Polystyrene, polyurethane and vinyl resins are the most common material used in the manufacture of insulating materials. 10 Topic 2.8 Plastics CPCCBS6001 Ed 1 New South Wales Technical and Further Education Commission, 2015 (TAFE NSW WSI

11 Polystyrene and polyurethane are foamed by a patented process to about forty times their original volume. Slabs are formed either by extrusion or by the moulding process and a variety of sizes are manufactured, the most common being mm in thicknesses of 25, 50, 75, and 100 mm. Both materials have extremely good insulation qualities and relatively high compressive strength and are flexible enough to be formed around curved surfaces. Due to their high insulation values and resistance to moisture, these panels are used as perimeter insulation around foundation walls as well as for wall and roof insulation. These panels can also be used below grade for foundation protection against frost penetration without any detrimental effects to their insulation value. The panels are usually applied to walls with either cement mortar or a special plastic adhesive. Panels used as built-up roofing insulation are sometimes enclosed in a stiff paper covering so that asphalt adhesive can be used to cement them into place. Another type of foamed insulation is one that is formulated on site using polyurethane or epoxy two-part resin. The process consists of injecting controlled amounts of resin liquid, a foaming catalyst and a curing agent into the space to be insulated. The reaction forms a foam which expands and sets, filling the entire space. This type of insulation application is extremely useful when existing structures or installations are being retrofitted or upgraded. Plastic pipe Several types of plastic are used to produce rigid and flexible pipe. Probably the largest single use for plastics in the construction field is in the manufacture of rigid drain waste, and vent pipe and fittings (see figure 4). They are light in weight, easy to cut to length, and easy to assemble and install, using the proper ABS solvent cements. Rigid pipe is also made from PVC, fluoroplastics, and phenylene oxide. Flexible pipe and tubing are made from polyethylene and the vinyls. For example, polyvinylidene fluoride pipe is used in chemical processing equipment because of its resistance to most chemicals and solvents. Extensive use is being made of both rigid and flexible pipe in water systems, with flexible pipe having some advantages because of the long lengths available and because its use may eliminate some fittings. Some plastic fittings for flexible pipe are made to fit inside the pipe, with a clamp around the end to hold the fitting in place. A more recent development for small pipe fittings is the quickconnect type of fitting, for which no clamps are necessary. Pipe and fitting are merely pushed together, and pulled to make the connection. Topic 2.8 Plastics CPCCBS6001 Ed 1 11

12 Figure 4 Plastic pipe and fittings Other uses Other uses of plastic in the manufacture and use of construction materials cover a wide field. These are uses in which the plastic plays a hidden but important role. Forms for precast concrete One important use of plastics in construction is in making of forms for precast concrete products. Such forms are particularly useful in situations in which a considerable number of identical shapes are required, as is often the case in architectural precast concrete design, where a number of identical wall panels will be used in a structure. Such forms are light in weight, have greater strength per unit weight than most other materials, have almost unlimited design possibilities, are resilient and have a high resistance to impact. The form model may be made of plaster of Paris, clay, steel or wood, with plaster of Paris being a favourite medium because it lends itself to the development of complex forms. 12 Topic 2.8 Plastics CPCCBS6001 Ed 1 New South Wales Technical and Further Education Commission, 2015 (TAFE NSW WSI

13 On completion, the pattern is first sprayed with a non-bonding agent, such as polyvinyl alcohol, to prevent the resin from sticking to it. A plastic gel coat is then applied, which accurately reproduces the exact configuration of the mould. Next, the mould body is built up to the desired thickness with glass fibre, a polyester resin, and a catalyst and allowed to cure, after which it is removed from the master pattern, ready for use. Adhesives Adhesives are used in the manufacture of plywood, chipboard, particle board and hardboard. Phenolics, urea formaldehyde, epoxy and amino resins are used for bonding veneer layers, chips, or wood particles into solid sheets. Similar adhesives are used extensively in the manufacture of glue-laminated beams and in the fabrication of wood trusses. Epoxy, phenolic, and amino resin are used as adhesives where metal, glass, concrete, porcelain or rigid plastics are involved. They are also used when adhesives, rather than nails, are used to attach plywood or gypsum board to stud walls. Material coatings Another important auxiliary use of plastics is as a coating for another material. Plywood, hardboard, and particle board are produced with a tough coating of diallyl phthalate, amino resin, epoxy resin, phenolic resin, polyurethane or polyvinyl chloride to render them more waterproof and to produce a smooth, grainless paint surface. Similar resins are used to coat concrete forms to protect plywood, hardboard and particle board from damage and to prevent the concrete from adhering to them. Plywood and laminated cedar siding are produced with a surface layer of melamine-impregnated kraft paper to produce a smooth waterproof paint surface. Silicone, which has extremely good water-repellent properties and good chemical and physiological inertness, is used as a coating for exterior masonry to protect it from water penetration and to render it easier to clean. Amino resins are used in the treatment of textiles, for example, on draperies to provide shrinkage control and render them water-repellent and heat resistant. Lights and sunscreens Acrylics, polycarbonate, polyvinyl chloride, ionomer resins and polyesters are used in the manufacture of lighting fixtures, diffusers, lenses and shields used in lighting systems. Sunscreens of coloured acrylic or polycarbonate may reduce solar heat gain, and thereby the air-conditioning load in a building, by as much as 50 percent. Topic 2.8 Plastics CPCCBS6001 Ed 1 13

14 Paints and varnishes Plastics form the basis of various paints, enamels, lacquers, and varnishes. Vinyl, polyurethane, acrylics, epoxy resins, cellulose acetate and phenolic resin are all used in the manufacture of coatings of this type. Vinyl, epoxy resins, allyl resins, fluoroplastics, polypropylene and polyurethane are used to coat the inside and/or outside surfaces of metal pipe to make it more resistant to rust and corrosion. Caulking and sealing compounds Many of the caulking, sealing and glazing compounds, as well as expansion joint fillers, are manufactured from synthetic resins. Premoulded, foam-rubber-like expansion joint filler is made from expanded polyvinyl chloride and expanded polyurethane by the extrusion process. Sealing, glazing, and caulking compounds are made from acrylics, liquid polymer polysulphide and butyl compounds (styrene, butadiene), normally applied by a caulking gun. Polybutene-based glazing mastic is produced in ribbon form by extrusion. Other sealing compounds are made in two parts which are mixed on the job and poured as a liquid, which cures into a pliable sealer, into the space to be sealed. Adhesives Substances which glue one surface to another have been in use for centuries. In the past most glues or cements have been based on naturally occurring animal and vegetable substances, but recently a range of synthetic adhesives has been developed which give rapid, high strength bonds. Insufficient time has elapsed to thoroughly test the durability of such adhesives but indications are that the durability is very high in exposed conditions, making these newer adhesives suitable for structural applications. General properties Properties of adhesives vary considerably with their constituents. For instance, some are highly flammable during application due to volatile solvents, but are inflammable when cured; some are not waterproof or resistant to chemicals or micro-organisms; others are both waterproof and boilproof. Different adhesives have: a different shelf life (the length of time the adhesive can be stored without deterioration) 14 Topic 2.8 Plastics CPCCBS6001 Ed 1 New South Wales Technical and Further Education Commission, 2015 (TAFE NSW WSI

15 a different pot life (the length of time the adhesive can be used after opening or preparation) a different closed assembly time (the time during which the materials to be bonded can be adjusted in position). Adhesives set in a number of ways: jelling on cooling, which can be reversed by reheating (eg animal glues) evaporation or absorption of solvent (eg starch pastes, PVA and rubber-based adhesives) loss of moisture with some chemical change (eg casein and the thermosetting adhesives) irreversible chemical reaction, accelerated by a catalyst or hardener (eg epoxies) hardening on cooling (eg hot-melt adhesives). Types of adhesive Adhesives from natural products Adhesives derived from starch (like old-fashioned flour-and-water paste), cellulose (eg methyl cellulose, which is a wallpaper paste), animal by-products (used for wood-wood bonds) and casein (made from soured milk curds and used for wood-plasterboard, woodlinoleum bonds) are only suitable for interior use as they tend to lose their strength when wet and, in the case of animal glues, are susceptible to attack by micro-organisms even with the addition of fungicides. The one exception is bituminous adhesives which are based on bitumen or coal tar. This group has good resistance to water and many chemicals but they do tend to flow at high temperatures. They are used for laying various flooring materials, such as parquet blocks and vinyl and linoleum sheets and tiles, and for bonding roofing felt. Rubber-based adhesives These adhesives can be based either on natural or synthetic rubber. In general, they are not suitable for external application but have the advantage of a degree of flexibility which can accommodate slight movements between the glued surfaces. This can be useful when bonding wall boards. They may be used as a one-part system or as a two-part contact adhesive where both surfaces are coated and then brought together to achieve an instant bond after enough time has elapsed for the solvent to evaporate. Contact adhesives are very suitable for bonding plastic laminates and sheet floor coverings but there is no margin for error you must get it Topic 2.8 Plastics CPCCBS6001 Ed 1 15

16 right the first time. They are not generally suited to wood joints as the adhesive tends to flow under constant load. Thermoplastic adhesives These adhesives fall into two groups, those based on polyvinyl acetate (PVA) and those which are described as hot-melt adhesives. Polyvinyl acetate (PVA) Used mainly for wood working but suitable for a wide range of materials, these adhesives are white liquids which become transparent on setting and generally do not discolour materials, except in some cases in the presence of ferrous metals. PVAs are easy to use, they set at room temperature and do not blunt cutting tools. PVA is generally suitable for joints which will not be required to undergo high continuous stress. Usually available as a single-part system, PVA is slightly more waterproof than animal glues but is restricted to interior applications, nevertheless. Hot-melt adhesives As the name suggests these adhesives are usually applied in a hot molten state. They are suitable for continuous flow production, are not flammable and the bond is formed in seconds. Sealing wax is an example of this type of adhesive, but modern varieties are usually based on ethylene vinylacetate (EVA) copolymers. Thermosetting adhesives Capable of extremely high strengths, even for metal-metal bonds, these adhesives harden essentially by heat action in conjunction with a catalyst or hardener which allows reasonable curing times at room temperature. Disadvantages are that they are combustible and require special cutting tools. They are available either as a one-part or two-part system. Urea formaldehyde These adhesives are colourless and inexpensive and are widely used in building but are unsuitable for external applications. Phenol formaldehyde These adhesives are not affected by weather or boiling and are therefore suitable for manufacturing marine ply. Melamine formaldehyde Relatively expensive and colourless, these adhesives are suitable for work such as veneering where increased durability and heat resistance is required. 16 Topic 2.8 Plastics CPCCBS6001 Ed 1 New South Wales Technical and Further Education Commission, 2015 (TAFE NSW WSI

17 Cold-setting reactive adhesives Some of the adhesives in this group have remarkable properties which tend to offset their high cost. They also have the advantage of setting at room temperatures. Resorcinol formaldehyde This adhesive can be used at low temperatures and, although it is water soluble until cured, when hardened it is weatherproof and boilproof. It is used for extremely strong and durable joints in timber and is also suitable for plastics, and alkaline materials such as fibrous cement sheets. Epoxide resins (epoxy) Although expensive, these two-part adhesives (e.g. Araldite ) will bond almost any materials. In addition, they are waterproof, resistant to most chemicals, highly electrically resistant and very resilient. Shrinkage is negligible during curing. As they are transparent, they are suitable for frameless glass assemblies, such as show cases. Cyanoacrylates These are costly, one-part adhesives which form an instant bond (e.g. Superglue ). The bond produced is extremely strong but the glue tends to fill gaps between the two surfaces poorly. Instant adhesion to skin can present a serious hazard. Achieving good adhesion Adhesion may be due to molecular attraction between two surfaces (as occurs between two sheets of damp glass), or to bonding agents which key into porous surfaces, or both. Modern adhesives work in both ways. For maximum bond strength it is important not to use too much adhesive so that the surfaces are brought into close contact with a thin glue line. Contact adhesives give instant tack but, generally, surfaces must be clamped together (but not too tightly) until a bond is achieved. Surfaces to be bonded must be clean, dry and free from grease. In some cases they need to be roughened or etched. Factors to consider when selecting an adhesive The following factors must be considered when choosing an adhesive. Materials to be bonded: the adhesive must suit the materials being bonded. Where the bonding is to be done - an adhesive for use on site must be more versatile than one used in a workshop because of the widely different conditions of temperature, humidity and varying contact pressures that may be encountered. Topic 2.8 Plastics CPCCBS6001 Ed 1 17

18 Strength required of the bond -the strength of the joint depends on the type of adhesive selected, the design of the joint, the thickness of the adhesive layer and the state of the surfaces being bonded. Durability of a bond - an adhesive bond must last as long as the materials being joined will last. It should be resistant to chemicals, moisture and mould growth. Working properties - the important working properties required of an adhesive include: o o o ease of spreading pot life rate of strength development. Shelf life is the time which an adhesive can be stored without deterioration. Pot life is the time available for using an adhesive after it has been prepared. Cohesiveness This is the state in which the particles of the adhesive and the materials which are joined are held together. Creep is an example in which cohesiveness gradually fails - either by the particles within the cured adhesive gradually separating, or by the adhesive and the materials joined gradually coming apart. Gap filling A gap filling adhesive is one which will fill spaces of up to 1.3 mm between surfaces without reducing the joint strength. Silicone sealants (see below) are a good example of a gap filling material. Summary of adhesives used in building Animal and casein are inexpensive adhesives used in assembling internal joinery. Starches, dextrines and cellulose adhesives are mainly used as wallpaper adhesives. Lignin adhesives are suitable for fixing porous materials such as linoleum, cork and fabric-backed PVC. Portland cement-based adhesives are used to fix ceramic tiles. Bituminous (asphalt) adhesives are used for laying parquetry floors, linoleum sheets and tiles and ceramic floor tiles. Polyvinyl acetate (PVA) adhesives are thermoplastic and do not undergo chemical changes in hardening. They are used to bond wood, expanded polystyrene and PVC wall coverings. They lack resistance to heat and moisture. 18 Topic 2.8 Plastics CPCCBS6001 Ed 1 New South Wales Technical and Further Education Commission, 2015 (TAFE NSW WSI

19 Rubber adhesives are natural or synthetic rubber-based adhesives provided in a solvent which evaporates leaving a tacky coating which immediately forms a strong bond when joined with a similar coated surface. They are used to fix laminated plastics to benches and walls. Epoxy resins are thermosetting and cure at room-temperatures. They are strong and durable and will bond concrete, glass, steel or aluminium to themselves or each other. Using adhesives Surfaces to be joined must be firm, clean, usually dry and free from grease, so that the glue is not adversely affected either mechanically or chemically. Joints which are subject to shear stress are stronger than those subject to tension. Peel stress should be avoided. The way adhesives set The following list explains the ways in which an adhesive can cure or set. The methods include: gelling on cooling and subsequent loss of moisture, a process which is reversed by reheating - or example, animal glues loss of moisture or solvent by evaporation and absorption into the materials being joined - for example, starch pastes, PVA and rubberised adhesives loss of moisture and some chemical change - for example, casein and thermosetting resins such as urea-phenol, melamine and resorcinol formaldehyde. raising water soluble glue to a critical temperature at which solvents evaporate and it becomes an insoluble solid - for example, protein glues and some synthetic resin adhesives (although here a catalyst is usually required to accelerate the reaction) an irreversible chemical reaction accelerated by a hardener or catalyst - for example, epoxies or polyesters. Sealers The primary purpose of a sealer is to seal the surface of wood and prevent the absorption of succeeding finish coats. It may be applied to bare wood that has been sanded smooth or applied over a stain or filler. (You learned in the unit titled Paint about stains and fillers, lacquers and varnish.) A sealer also tends to: seal in filler Topic 2.8 Plastics CPCCBS6001 Ed 1 19

20 blend stain stiffen any raised wood grain and thus make sanding easier form a bond between the wood and finishing coats. A number of materials are used as sealers, the proper one to use in a specific situation depending on the type of finish required. Shellac is widely used as a sealer, thinned out to a 0.9 to 1.8 kg cut, depending on whether varnish or lacquer is to be used. Lacquer finish requires the thinner sealer. It dries rapidly, does not penetrate and does not soften appreciably under newly applied lacquer or varnish. It does not, however, provide the best adhesion for finish coats and may show crazing tendencies under a thick finishing film. Lacquer sealer is the type of sealer most commonly used under lacquer finishes. It consists primarily of the same type of resins from which lacquer is made, with plasticisers and solvent acid as well as solid content in the form of zinc and calcium stearates. These are called sanding agents because they increase the ease with which the sealer surface may be sanded when dry. Varnish sealer is also available for use under varnish or lacquer. It is similar to varnish cutback until the material contains 30 to 35 per cent solids. This type of sealer dries in air in about 8 hours, or may be force dried in one to two hours at 65 C. The surface must be sanded after the sealer is completely dry. Sealing compounds can be to seal the surface of other materials against the penetration of water or other liquids or in some cases to prevent the escape of water through the surface. To do this they must have some adhesive qualities and the ability to fill the surface pores and form a continuous skin on the surface to which they are applied. In many applications the adhesion should be permanent, while in others it need be only temporary. Sealants A sealant is a material used to fill the joint between two adjoining elements of a structure in order to render it moisture- and air-tight. The development of new building methods and, in particular, the widespread use of prefabricated panels and curtain-wall construction have brought about the development of dynamic materials which will provide permanent sealing of moving joints which occur in metal, glass, and masonry curtain wall exterior cladding systems. The areas in which these material sealants will be used include glazing, curtain walls, deck areas, concrete, masonry, stone, terrazzo flooring, quarry tile, skylights, window units and metal roofs. 20 Topic 2.8 Plastics CPCCBS6001 Ed 1 New South Wales Technical and Further Education Commission, 2015 (TAFE NSW WSI

21 Figure 5 Typical sealant and backup applications To be useful in such a variety of functions, a sealant must have a number of specific properties. These include: the ability to adhere to the surfaces with which it comes in contact the ability to remain workable over a considerable range of temperatures the ability to form a tough, elastic skin over the surface, while the interior of the mass remains flexible the ability to stretch or elongate with changes that may occur in the width of the joint the ability to move - that is, it must be able to move in either extension or compression from its mean position the ability to recover well after having been extended or compressed very low sensitivity to water low volatility Topic 2.8 Plastics CPCCBS6001 Ed 1 21

22 the ability to provide good service performance. Types of sealant There are three basic types of sealant. These are: low performance - these will not tolerate movement. They: - are cheap - are easy to apply - have poor weathering properties - may be oil-based - may be bituminous based intermediate performance. These: - have very limited movement tolerance - have better weathering characteristics, but suffer from limitations as for low performance - may be butyls - may be water-based acrylics. high performance (elastomeric sealants). These: - have the ability to tolerate joint movement - are resistant to the effects of exposure - may be polysulphides - may be silicones - may be polyurethanes. Oil-based sealants (mastics) The oil-type sealants consist primarily of special processed vegetable oils, selected fibres and fillers. The compounds are skin forming, but retain their flexibility and adhesive characteristics for a prolonged period. These materials have a special place in building, since they can be used for numerous purposes where a weatherproof seal is required. They are, however, squeezed out of joints if there is appreciable movement, and being non-elastic do not return to their original shape. Oil mastic sealants with certain improved properties have been produced by modification with butyl rubber. For some applications, such as waterproofing joints in sheet roofing, tape forms of oil mastic are available. 22 Topic 2.8 Plastics CPCCBS6001 Ed 1 New South Wales Technical and Further Education Commission, 2015 (TAFE NSW WSI

23 Bituminous sealants (mastics) Sealing compounds based on bituminous mixtures are available and can be used in many instances as an alternative to oil mastics, but these cannot be successfully overpainted with ordinary paints. Special compounds based on mixtures of bitumen and rubber are used for sealing joints between sections of concrete road slabs and runways. These compounds are applied hot. Because they are very viscous, these materials can be overheated when being softened ready for use. Slight overheating or prolonged heating at the temperature at which they are normally applied can result in a change in properties. Some tend to revert back to the properties of a straight bitumen and become very sticky, while others tend to harden and lose their ductility and adhesiveness. As with oil-type mastics, bituminous materials are squeezed out if the movement is appreciable. They also lose adhesion to materials which can become moist behind the mastic, such as concrete and asbestos cement. Butyl sealants The formulation of synthetic butyl-rubber-based sealants varies with the manufacturer. These sealants can be obtained in special packs for gun application or in tape or strip form. Many of the gun-grade types are very tacky and stick to most clean dry surfaces, but they tend to contain a large proportion of volatile matter, which on evaporation, results in appreciable shrinkage of the mastic. The tape or strip types are less tacky, but have smaller shrinkages. For glazing purposes very thin joints should be avoided, and the sealant which is squeezed out of a joint when fixing glass and other sheet materials or beads should be left untrimmed for as long as possible. These measures will reduce the risk of shrinkage cracking of the sealant. Silicone sealants, polysulphide sealants, polyurethane These sealants differ from oil and butyl types in two respects. They are elastic rather than plastic and they harden and set by a chemical reaction. Two basic types are available - two-part compounds, in which the chemical reaction is initiated by mixing a catalyst or hardener into the polysulphide resin base, and one-part compounds in which the hardening to rubber is achieved by chemical reaction with atmospheric moisture. Some of the more common purposes for which building sealants are used include: curtain walling expansion joints Topic 2.8 Plastics CPCCBS6001 Ed 1 23

24 bedding, and sealing of joints, between external window- and door-frames and the surrounding structure sealing of joints between prefabricated units jointing for rain-water goods, etc. Joint design Often the successful performance of other building components depends on the proper functioning of the joints between them. It is therefore important that careful attention be paid to the size of the joints and the selection, application, and maintenance of the sealants used in them. When a sealant is used a polystyrene rod - called a backer (or back up) rod - is placed behind the sealant in the joint. You can see this in figure 6. The width of the joint to be used in any specific case depends on the calculated amount of movement at the joint and the type of sealant to be used. The amount of movement at a joint is dependent on the coefficient of linear expansion of the panel materials involved and the temperature gradient which causes the movement. The recommended joint width may be determined from the: calculated amount of joint movement anticipated temperature extremes of the completed structure during its life elongation capability of the proposed sealant temperature at the time of application. Figure 6: Joint width and movement 24 Topic 2.8 Plastics CPCCBS6001 Ed 1 New South Wales Technical and Further Education Commission, 2015 (TAFE NSW WSI

25 The type of sealant to be used is important because wider joints are necessary with sealants of low movement capability than with those of higher capability, in order to prevent sealant failure. Points to note Complete adhesion is required between the sealant and adjoining materials. The joint must be dust free, dry and free of oil. The sealant depth must be sufficient for expected joint movement (elasticity of sealant). To achieve non-adhesion of sealant to the backer rod during application, avoid having primer on the rod. Achieve a smoothness in finishing of sealant in movement joints. The sealant must be able to withstand the effects of exposure in exposed joints. Joints should be concave in section to allow for expansion. A suitable sealant in a joint subject to movement must: be able to stick firmly to the surrounding material (that is, joint sides) be able to stretch and recover as required by the movement of the joint have sufficient internal strength to withstand stresses created by the stretching be able to withstand temperature change, ultraviolet light etc. if exposed be finished smoothly be able to maintain these properties over a long period. Joint failures are caused by: joint openings of less than 6 mm dust or oil left an adjoining material too thin a depth of sealant sealant adhered to backer rod causing 'three way stretch' (Use closed cell polyethylene backer rods only.) ultraviolet exposure (Protect exposed joints against UV. exposure by using a metal cover strip.) depth of sealant exceeds joint width (Where width exceeds 6 mm, depth of sealant must not exceed 2:1 ratio.) incorrect primer is used when one is specified, or it is not dry before applying the sealant closed cell polyethylene backer rods are not used timber strip soaked in release oil is used where horizontal joints are to be made in reinforced concrete - a disposable closed cell polyethylene section should be used. Topic 2.8 Plastics CPCCBS6001 Ed 1 25

26 Table summary Sealant type Cure system Movement expected Uses Working life putty dries/oxidation less than 2% glazing up to 5 yrs caulking compound dries/oxidation less than 5% glazing 1, 2, 3 storey expansion joints up to 7 yrs acrylics dries 5 10% glazing up to 10 yrs +15% qualified high rise polysulphides chemical cure +25% glazing high rise up to 20 yrs silicones chemical cure +25% glazing high rise up to 20 yrs 7.50%* urethanes chemical cure +25% glazing high rise up to 20 yrs >50%* hot melts sets +10% glazing up to 15 yrs * special types now commercially available. Joint width Sealants can be used in joints of width 6 mm to 25 mm - ideal joint width from sealant manufacturers and applicator point of view is 12 to 15 mm. Mastics A mastic is a sealant which usually provides little structural support but seals the joint against weather and sound while allowing the different components to move relative to each other. The most common mastic used in domestic construction is linseed oil putty for glazing timber sashes but modern mastics are now available which can be either of the plastic or elastic type. 26 Topic 2.8 Plastics CPCCBS6001 Ed 1 New South Wales Technical and Further Education Commission, 2015 (TAFE NSW WSI