Wall & Ceiling Linings

IntroductionIn previous units in this subject, building materials were divided according to their nature of origin (eg clay products). Because both wall and ceiling linings and insulation materials can comprise any number of different base materials or combinations of materials, it seems more logical, in this case, to approach this unit differently—according to the function which the materials perform rather than the nature of the raw material.

This unit, therefore, is divided into two sections: the first deals with wall and ceiling linings and the second with insulation.

Learning outcomes

On completion of this unit, you should be able to:

• describe the types of wall and ceiling lining and insulation materials most commonly in use in this country

• compare and contrast properties associated with the various alternatives

• recognise suitable applications for the materials discussed.

Wall and ceiling linings

The terms ‘wall lining’ and ‘ceiling lining’ refer to the internal wall and ceiling covering of the building as opposed to ‘cladding’ which refers to the external wall covering or, sometimes, roof covering. Additionally, in this unit wall and ceiling lining are defined as being distinct from finishes (such as ceramic tiles, wallpapers and paints) which are usually applied to the wall or ceiling lining.

The most common forms of wall lining used in Australia are gypsum plasterboard, fibrous cement, timber or composite lining boards or sheets, plastic coated wall sheeting and solid plaster.

Timber and composite lining boards and sheets are covered in Unit 2. Timber and plastic coated wall sheeting is mentioned in Unit 9. In this unit, we will concentrate on the other alternatives.

Plaster

The term ‘plaster’ refers to a jointless and usually smooth lining applied to the base wall or ceiling structure.

Solid plaster was one of the first lining materials to be used in buildings. The plaster which was made of lime and sand, often with hair included, was applied in situ to the masonry wall or, in the case of a timber stud wall or ceiling, to timber laths which are thin battens fixed close together to provide a base.

Today, solid or in situ plaster is reserved for solid masonry walls; timber stud walls are lined with plasterboard. However, in situ plastering is a wet and messy process and often internal masonry is left unplastered (face brickwork, for example).

Composition

Plaster comprises a binder, clean sand and fresh water, which sets to a comparatively hard, dense layer. The properties of the final product depend largely on the type and quantity of the binder used.

The binders most commonly used in Australia are gypsum plaster, Portland cement and lime (either quicklime or hydrated lime—refer to Unit 5) or organic binders.

Gypsum plaster

Calcium sulphate or gypsum plaster can be used for undercoats and finishing coats. (Plaster of Paris is one type of gypsum plaster.) It is derived from naturally occurring gypsum rock which has been pulverised and heated to drive off most of the chemically combined water, resulting in a white, pink or grey powder. When water is added to gypsum plaster it sets and hardens into a crystalline solid, giving off heat and expanding slightly in setting.

Two other similar binders are derived from gypsum plaster: ‘hard wall plasters’ which provide a harder finish and Keene’s cement, which is the hardest of the gypsum plaster mixes.

Portland cement

Portland cement is sometimes used as a binder in undercoats and finishing coats where an exceptionally hard surface is required. Too rapid drying increases the likelihood of cracking, and shrinkage must be substantially complete before a further coat is applied. Plasters in which limes are the only binders are rarely used today as the final strength is very low.

Lime

Workability agents or plasticisers, based on non-hydraulic lime or organic materials, are used to improve the workability of the mix and distribute shrinkage stresses, thus reducing visible cracking.

Limes

Plasters in which limes are the only binders are rarely used today as the final strength is very low.

Workability agents or plasticisers, based on non-hydraulic lime or organic materials, are used to improve the workability of the mix and distribute shrinkage stresses, thus reducing visible cracking.

ProcessThe process of applying solid plaster to a base structure is known as rendering. Solid plasters are usually applied in two coats. The undercoat is often referred to as the ‘scratch coat’ and the finishing coat as the ‘set coat’. If the base is particularly smooth and the suction uniform, a single coat only may be required; alternatively, a particularly irregular base may require three coats.

In some applications the coats may not be of the same composition but it is important that each coat be well matured before another coat is applied, especially if cement is used. A general principle to be followed is that each successive coat should be weaker than the preceding one.

The choice of a plastering system depends upon the base to which the plaster is to be applied, the performance of the required finish and the texture desired.Cement-sand or cement-lime plasters are moisture-resistant plasters, while gypsum-based plasters should be used internally in dry situations only.

Mixes containing Portland cement make the hardest plasters, and have the greatest resistance to impact damage. Keene’s plaster is the hardest of the gypsum plasters, while lime plaster is the softest. Tables 6.1 and 6.2 indicate suitable plaster mixes for two- and three-coat internal plasterwork.

Table 6.1: Mixes for undercoats for internal two-coat and three-coat work

Finishing coat Undercoats (by volume)

Cement setting 1 cement4 to 5 sand0.10 lime

Cement: lime: sandGypsum plasters

1 cement5 to 7 sand0.10 lime

Gypsum plasters 1 plaster2 to 3 sand(or 1: 3 to 1: 4.5 by weight)

1 gypsum plaster:1.5 sand: 0.10 lime(or 1: 2 by weight, plus lime 5% of weight of plaster)

Table 6.2: Mixes for finishing coats for internal work

Background or undercoat Finishing coats (by volume)

Brick, block, or concrete 1 cement4 sand0.10 lime

Cement: sand 1 cement1 lime5 sand

Cement: lime: sand 1 cement1 to 2 lime6 to 9 sand

Concrete backgroundCement: lime: sand (undercoat)Gypsum plaster

1 lime0.25 to 4 gypsum plaster

Preparation

Porous bases, such as clay bricks and concrete blocks, which have a comparatively high suction rarely require much preparation other than raking of the joints and the removal of loose material.

Smooth, dense materials, such as concrete, have little suction and offer no mechanical key and are either hacked or else treated with a spatter-dish, sand-cement mix, often including a PVA adhesive, to provide a key.

Rough textured surfaces, such as rough concrete, provide a good mechanical key and require little preparation.

Fibrous plaster

Fibrous plaster is made of gypsum plaster reinforced with sisal hemp fibre. Nowadays it has been replaced by plasterboard for sheeting applications but is still used for the more complicated decorative mouldings.

Fibrous plaster is dimensionally stable and easily decorated but is not satisfactory in moist conditions.

Gypsum plasterboardPlasterboard is the most commonly used lining for timber-framed construction and brick veneer. It comprises a core of gypsum plaster reinforced with two outside layers of kraft paper, one on each face. Some are available with an aluminium foil on the back which improves thermal insulation performance.

Plasterboards are easily decorated and are reasonably tough and strong in normal grades but are not satisfactory in damp situations. A water-resistant board is available which is designed to be used in areas where high humidity persists and in wet situations where they are protected with tiles or a similar impervious material.

Sizes: Sheets are available in a broad range of sizes. Thicknesses commonly used in domestic applications are 10 mm for walls and 13 mm for ceilings. However, a 10 mm thick board is now available for ceilings also.

Fixing: The boards are fixed to the studs or ceiling joists by gluing or nailing with special flat-headed nails. Boards are available with either square or recessed edges, the latter being used where a flush surface is required. For a flush joint, a strip of perforated reinforcing paper is embedded in bedding compound in the recess and the area is covered with a topping cement (see Figure 6.1).

Figure 6.1: Fixing

General properties of plaster and plasterboards

Thermal insulation: Plaster linings are relatively thin and make a correspondingly small contribution to the thermal insulation of a building.

Fire resistance: Normal plasters are non-combustible, have no ‘spread of flame’ and do not produce smoke. Special fire-rated plasterboards are available for applications which require a fire rating. Often, the addition of a specified thickness of plaster or render on internal masonry walls is used to achieve a required fire rating according to the Building Code of Australia.

Sound absorption: Ordinary plasters have fairly low sound absorption values but special acoustic plasters and plasterboards are available.

Sound insulation: As plaster linings are relatively thin, they contribute significant sound insulation to lightweight components only. However, plaster can improve sound insulation by sealing the surface to porous base structures.

Hardness: In housing, a fairly soft finish may be preferred but harder surfaces are often required in public buildings and the choice of system should take this into account. Metal angles are used to protect vulnerable corners and provide a line for the plasterer to work.

General properties of plaster and plasterboards

Durability: Gypsum-based products are not usually waterproof and the durability of the finish depends largely on the composition of the plaster.

Texture: Smooth-trowelled surfaces comprising either neat gypsum or gypsum with admixtures are most common but texture can be provided by special trowelling or by including sand in the finish. ‘Bagged’ finishes are popular on masonry walls. These comprise a thin sand-cement mix which is wiped over the wall surface with a piece of hessian. The resultant thin coat allows the form of the masonry units to show through.

Check progress 1

Fibrous cement

Fibrous cement sheeting has replaced asbestos cement as a lining and cladding material due to the health hazards associated with materials containing asbestos.

Composition

Fibrous cement is made from a mixture of Portland cement, sand, cellulose fibre and water, compressed into sheets, boards or other shapes.

Sizes

Sheets are available in a number of sizes. Thicknesses for domestic use are generally as follows: as lining material for eaves, verandas or carports—4.5 mm or 6 mm sheet; for internal wall and ceiling linings—6 mm; compressed fibrous cement for wet area floors is 15 mm or 18 mm thick.

Fixing

Sheets can be glued or fixed with special galvanised flat-head fibrous cement nails to timber frames; joints can be covered with fibre cement cover moulds or PVC sheet holders (see Figure 6.3).

Figure: 6.3: Cover and junction moulds for fibrous cement sheets

Exposed internal linings can be flush jointed. Special recessed-edge sheets are taped with a perforated paper reinforcing tape and finished in a similar way to plasterboard sheets, with a topping cement.

Uses

Externally, fibrous cement products can be used as cladding in the form of boards, sheets or shingles. However, internally, because they are waterproof, fibrous cement sheets are used primarily as a base lining for other finishes (such as tiles) in wet areas. Compressed fibrous cement sheeting is also used as a base floor material for ceramic tile floors in wet areas.

General Properties

Thermal insulation: Fibrous cement sheets are relatively thin and make a correspondingly small contribution to the thermal insulation of the building.

Fire resistance: Fibrous cement products will not burn, have a zero ‘spread of flame’ index and do not produce smoke.

Sound absorption: Unless special acoustic material is used, fibrous cement lining contributes little to the sound absorption characteristics of a room.

General Properties

Sound insulation: The sheets have a greater density than plasterboard but are thinner and therefore do not significantly affect sound insulation.

Hardness: Care should be taken during handling and storage to prevent edges from chipping since the material is particularly brittle. When painted or otherwise finished, however, a hard surface finish can be obtained.

Durability: Fibrous cement sheets are unaffected by sunlight, moisture or termites and should not split or rot. Hence its suitability for external and wet area applications.

Check progress 2

Thermal insulationThe question of thermal insulation really forms part of the problem of energy efficient design of the building as a whole, which includes consideration of the following points:

• orientation of the building to maximise the use of solar energy (see Figure 6.4)

• location in relation to summer breezes (see Figure 6.5)• protection from winter winds (see Figure 6.6)• location and treatment of windows (see Figure 6.7)• use of wide eaves or pergolas which shade windows and walls

from summer sun but allow entry of winter sun (see Figure 6.8) • use of solar energy in the design to heat floors or walls (see

Figure 6.9)• interior planning (see Figure 6.10)• prevention of heat loss through unnecessary gaps (see Figure

6.11)• design of floors (see Figure 6.12)• the colour of the exterior of the house.

Figure 6.4: Paths of the sun in winter and summer

Figure 6.5: Location in relation to summer breezes

Figure 6.6: Protection from winter winds

Figure 6.8: The use of wide eaves or pergolas

Figure 6.10: Interior planning

Thermal insulation

Thermal insulation can assist by improving the thermal efficiency of the structural components of the house by reducing heat loss or gain through the major surfaces, such as the walls and ceilings.

Heat transfer

Heat is transferred by:

• conduction—heat is ‘led’ from the side of the material at a higher temperature to the side at a lower temperature

• convection—when air is heated it expands and begins to circulate and heat up colder surfaces by losing some of its heat to them

• radiation—when air comes in contact with a warm object, heat is transferred to the atmosphere.

Thermal resistance

A material’s ability to resist the flow of heat is called its thermal resistance or ‘R-value’. The higher the R-value of a material, the greater its ability to resist the flow of heat.

The Energy Authority of NSW provides data on recommended R-values for different areas in NSW. For instance, if you live in Coffs Harbour the recommended minimum level of thermal insulation is R1.5 but if you live in Cooma, which is colder, the recommended minimum level is R3.0 (see Figure 6.13).

The heat flow through a wall or ceiling is not reduced in direct proportion to the R-value of any insulation added above the recommended level: in fact the extra benefit to be gained diminishes fairly rapidly beyond this level. Thus, there is not much point in installing insulation to a value beyond the recommended R-value for your area.

Types of Insulation

Thermal Insulation

This type of insulation uses the heat-reflective properties of aluminium foil which prevents heat transfer by radiation. The following types are available:

• Foil laminated to reinforcing membranes, supplied in rolls of varying widths. This is used for roof sarking and wall sheathing.

• Laminated foil layers separated by partition strips. When the foil is installed over ceiling joists the partition strips separate the two layers and provide an additional air space to increase the effectiveness by decreasing conduction.

• Foil laminated to bulk insulation.• Foil-backed plasterboard.• Solar reflective film which can be applied directly to glass

panes.• Metal reflective-treated fabrics for blinds, curtains and so

on.

Bulk Insulation

This is normally a cellular material with entrapped air bubbles which slow down heat transfer by conduction. Several forms are available.

Batts and blankets

Insulation batts and blankets are available in the following materials:

• Mineral wool (fibreglass or rockwool), manufactured from inorganic raw materials that are melted at above 1000°C and spun into fibres which are then bonded together to form flexible sheets.

• Urethane foam sheet, made from foamed polyurethane.

• Expanded polystyrene sheet (EPS), made from foamed polystyrene.

Loose fill

• Cellulose fibre, manufactured from waste paper.

• Exfoliated vermiculite, manufactured from a micaceous material.

• Mineral wool, manufactured as explained above.

In situ foam

• Urea formaldehyde is pumped in as a mixture of chemicals using special equipment. The mixture foams up in situ and forms a rigid foam filled area.

• Urethane foam is pumped as fluid foam into the space where it sets chemically to form a rigid insulation.

• Expanded polystyrene beads are mixed on site with a bonding agent and injected into the cavity.

Structural and decorative insulation

This type of insulation comprises a complete wall or ceiling lining system combining thermal insulation and often acoustic modification with a decorative lining. Several forms are available:

• Fibreglass panels laminated with decorative finishes.

• Wood wool panels—decorative boards made from wood straw bonded with a cement-like adhesive.

• Compressed straw panels, manufactured from pine or straw fibres which are compressed and bonded together.

• Expanded polystyrene, as above with decorative finishes.

General properties of insulation materialsThermal performance The type and thickness of the insulation is selected according to the required R-value and the application. Reflective foil as insulation in horizontal applications should be laid face down as settling dust renders the upper face ineffective. The R-value should be marked on the product and manufacturer’s product information should comply with SAA Standards and Test Methods.

Acoustics Some insulation will also contribute to the acoustic performance of the room, especially in the case of some of the decorative panels.

Fire resistanceSome insulation materials are combustible. Urethane foam, expanded polystyrene and cellulose fibre insulation must contain fire-retardant chemicals. Combustible insulation should be covered with an appropriate non-combustible lining such as gypsum plasterboard.

General properties of insulation materials

Safety

Most bulk insulation materials should be handled with care to avoid dust formation. Gloves and long clothes should be worn when installing fibreglass to avoid contact with glass fibres, which may irritate the skin. In all cases it is advisable to wear a mask covering the mouth and the nose.

Suitability

The type of construction will limit your choice of insulation system. For instance, loose-fill insulation is generally only suitable on flat surfaces. In situ insulation may make access to the roof space extremely difficult. Loose-fill insulation is good for difficult corners.

Where to insulateBecause heat rises, most heat loss occurs through the ceiling. Figure 6.14 illustrates the proportion of heat loss through

(Note that the figures given have been calculated specifically for the Canberra region and may not apply to other areas although the general pattern these figures reveal would apply for this type of construction elsewhere.)

Figure 6.14: Heat loss through a building

Where to insulateAlthough the percentage figure for heat loss through the walls is the highest, in terms of unit area the diagram suggests that (for this type of construction) the greatest heat losses are in fact through the ceiling and, next, the floor. Consequently, the first place to consider insulating is above the ceiling (see Figure 6.15).

Figure 6.15: Insulation above the ceiling

Where to insulateIf the floor is a raised timber floor the sub-floor space should be enclosed, allowing for the required ventilation, and bulk insulation can be supported between the joists or reflective foil can be placed over the joists (see Figure 6:16).

Figure 6.16: Insulation below the floor

Where to insulate

In extremely cold climates rigid foam insulation around the edges of the slab is advantageous (see Figure 6.17).

Figure 6.17: Insulation around the edges of the slab

Where to insulate

In timber walls bulk insulation can be placed between studs (see Figure 6.18).

Figure 6.18: Insulation between the studs

Where to insulate

Foam in-situ insulation can significantly increase the thermal performance of cavity brick walls (see Figure 6.19).

Figure 6.19: Insulation between walls

Where to insulate

The thermal performance of windows can be increased dramatically with double glazing or even triple glazing in extremely cold climates.

Full length drapes with pelmets will also greatly reduce heat loss.

Figure 6.20: Drapes and pelmets

Check your progress 3

Where to insulate

Although materials can be introduced to improve the thermal performance of the building, total energy efficiency requires attention to the design of the building as a whole. Some of the aspects which deserve attention—mainly those which can be easily attended to—have been touched upon in this unit.

Summary

You should now be able to list the types of wall and ceiling lining and insulation commonly used in Australia and be able to compare and contrast the properties associated with each and the applications they are suited to. Now go to Unit 7 which covers metals and glass.

Paints

IntroductionFor hundreds of years people have been finishing the internal and external walls of their buildings with various mixtures or fabrics to decorate, preserve or waterproof them. Very early on, kalsomine (made from powdered limestone) was used to paint interior walls and varnishes and shellac were developed to preserve and decorate timber.

Lacquers, made from resins, came from China originally and became very popular in late seventeenth and eighteenth century Europe for furniture and wall panels. In sixteenth century France painted hessian was popular as an interior wall finish, later superseded by exotic materials such as brocades. Wallpaper, as we know it, did not become really popular until the middle of the nineteenth century when printing processes made available brightly coloured and patterned wallpapers at prices many people could afford.

These days many coatings and coverings are now made either entirely or partially from plastics.

IntroductionToday we expect a surface coating or covering to contribute to or provide any or all of the following:

decoration preservation waterproofing hygiene improved lighting safety.

Surface finishes may only represent up to 5 per cent of the initial building cost but contribute greatly to the maintenance costs of the building. Selection of the correct system and adequate preparation of the surface is, therefore, important.

Learning outcomes

On completion of this unit, you should be able to:

• distinguish between the alternatives available in the range of surface finishes

• select a suitable finish, taking into account the background, location and durability requirements

• describe suitable preparation and application techniques.

Paints

Composition

Broadly speaking, paint is a mixture of:

• the binder

• pigments

• additives and extenders

• the medium.

Binder

The binder, as the name suggests, binds the other ingredients together, forming a solid, elastic film which must adhere to the surface, sometimes penetrating and sealing it as well. A paint is classified according to the type of binder.

PaintsOil-based paints These are based on oils which react with the oxygen in the atmosphere to solidify. Straight oil paints based on naturally drying oils, such as linseed oil, are rarely used today and have been largely supplanted by paints modified with synthetic binders called alkyds. These paints are often called enamels or alkyd enamels.

Water-based paints These binders comprise small globules of resin which are suspended or dispersed as an emulsion in water. As the water evaporates, the globules coalesce to form a solid film. Paints based on this type of binder are commonly known as plastic or latex paints and the resins used include PVA, acrylic, polyurethane or combinations of these. They are often referred to as emulsion paints.

Solvent-based paintsThese binders are dissolved in a solvent which evaporates leaving a solid film, such as lacquer and chlorinated rubber.

Chemically cured paintsThese are usually two-pack paints and the binder forms as the two compounds are mixed together and react chemically. Once mixed, the paint must be applied within a few hours. Epoxy (epoxide) resin paints are examples.

Pigments

Pigments are used to make the paint opaque, to hide the background, and to provide the required colour. For instance, titanium dioxide is used for opacity and another compound such as iron oxide might be used to impart the colour.

Additives and extenders

Additives and extenders are included in varying quantities and have a great influence on the properties of the paint. The roles of additives and extenders tend to merge but basically they are as follows.

Additives might include fungicides and driers in oil and alkyd paints or dispersing and emulsifying agents in latex or plastic paints.

Extenders are used to achieve the required viscosity, body and surface appearance.

Medium

The medium can either be a solvent in which the binder is dissolved or a dispersing medium in which it is suspended. Examples of solvents include mineral turpentine or benzine derivatives. The dispersing medium most commonly used for plastic and latex paints is water.

Thinning and cleaning up depends on the nature of the dispersing medium. Oil-based paints require turpentine or white spirit whereas water-based paints can be thinned and cleaned up with water. Special solvents are required for other types of paints.

Paint systems

Most paint systems include the following: primer or sealer undercoat(s) finishing coat(s).

The choice of system depends on the nature of the surface to be painted and the finish required (see Figure 8.1).

Each component of the system performs a particular function but in some cases, as with plastic paints, a paint can perform more than one function. The type of coat selected must be compatible with the substrate (background) and with adjacent coats.

Primer

The primer can fulfil a number of functions including: providing a key to improve the adhesion of the next coat sealing porous surfaces which would otherwise absorb part of the

next coat and spoil the finish minimising ‘bleeding’ of surfaces such as bitumen and timber.

Primers which etch the surface and inhibit corrosion are available for use on metals.

Undercoats

Undercoats must cover the original colour of the surface and fill in any small depressions.

Finishing coats

Finishing coats provide the final colour and texture and offer the final protection against weather, chemical and mechanical damage. Finishing coats are available in gloss, semi-gloss or satin, flat or matt and in various textures.

gloss is highly reflective, resistant to moisture and easy to clean but shows up surface irregularities

semi-gloss is less reflective and shows fewer surface imperfections

flat has low light-reflection, is usually permeable to moisture and tends to collect grime more easily.

Figure 8.2 demonstrates how, on a microscopic level, the medium evaporates leaving various amounts of pigment exposed, thus forming the various finishes.

Figure 8.2: Microscopic cross sections showing how light is reflected, giving characteristic shiny or matt appearance

Choosing a paint system

The nature of the substrateThe substrate is the surface which is to be painted.

Alkalinity, porosity and loose particles on the surface to be painted can affect the adhesion and durability of a paint system.

Materials such as concrete, cement render, mortar and solid plaster contain small amounts of alkaline materials (mainly from the lime) and some paints, such as the alkyd enamels, are susceptible to alkali attack, which causes breakdown of the film. The gloss and semi-gloss enamels are more susceptible than the flat enamels and must be separated from the substrate by an alkali sealer.

Gloss and semi-gloss alkyd enamels are also adversely affected by porous surfaces which absorb the medium and binder unequally. The use of a suitable undercoat will prevent unequal absorption of the finishing coats. Plastic or latex paints are not affected by porous surfaces because the globules of resin are not absorbed but sit on the surface.

Loose surface material can reduce adhesion. Enamel paints tend to penetrate the loose material and bind it together but plastic or latex paints just tend to sit on the surface. For this reason, loose material should be removed with a brush or scraper before painting with a plastic or latex paint. If the surface is particularly loose, treatment with a 15 per cent solution of phosphoric acid may be required.

Recommended paint system

In addition to consideration of the nature of the substrate, the choice of a paint system ultimately depends upon:

The performance specification whether you require a fully impervious surface or a porous

surface finish which can breathe whether you require a high wear, abrasion resistant surface whether the surface is to be washable whether the surface is inside or exposed to weather and

pollution.

Experimental Building Station Note on the Science of Building No 148 provides information on paint systems which is summarised in Table 8.1.

Special paints

A variety of paints for special purposes are available, including the following:

• water-resistant paints • low-odour paints

• chemical-resistant paints • quick-drying paints

• fire-retardant paints • stoving paints

• heat-resistant paints • insecticidal paints

• fungus-resistant paints • permeable paints

• anti-condensation paints • floor paints

• luminous paints • multi-colour paints

• fluorescent paints • textured paints

• phosphorescent paints • metallic paints

• radioactive paints

Applying the paintOn site, paint can be applied by:

Brush which provides the best adhesion, desirable in priming coats, but skill is required to avoid brush marks.

Roller which is much quicker but provides a slightly stippled surface finish; edges must be finished with a brush.

Spray equipment is expensive but can be economical on very large areas can be used to achieve metallic and graded effects the only suitable method for quick-drying paints; the hot spray process reduces the viscosity of a paint without the addition of a solvent. In the factory, paint can be applied by:

• dipping smooth—this is rapid and economical, producing a very smooth finish

• flow coating—paint is hosed onto the surface• roller coating (by machine)—used for continuous lengths.

Preparation of surfaces

One of the most common causes of breakdown of painted surfaces is inadequate preparation of the substrate. Sometimes brushing is adequate but in other cases dirt must be removed by washing and scraping, using suitable solvents for oils and stains.

Previously painted surfaces might simply require priming, filling and rubbing down but where a perfect surface is required paint can be removed by burning off and scraping, using solvent and chemical removers or by steam stripping. Water-soluble paints, such as tempera, must be removed before painting as they prevent the formation of a key.

When to paint

Generally speaking, it is best not to paint in wet, damp or foggy weather or below 4°C, in direct sunlight or in dusty conditions. Humid conditions delay drying of ordinary paints.

Each coat should be thoroughly dry before the next is applied.

Good ventilation is required to assist drying and sometimes to remove noxious fumes.

Check progress 1

Clear finishes

Clear finishes are used to enhance the natural appearance of the substrate and in many cases waterproof and protect it as well. They may or may not include some colour pigment and, depending upon the type of compound, may be available in gloss, semi-gloss or matt finishes.

In general, clear finishes lack sufficient pigment to filter out damaging ultraviolet light and are therefore much less durable than paints in exposed conditions. Consequently, the choice is limited for external conditions.

Interior clear finishes have been formulated specially to suit the substrate. We will deal with them according to the nature of the substrate.

Clear finishes for internal timberThe clear finishes currently available include the following:

Oil seal: a type of varnish, used to achieve a water and grease resistant, non-slip finish for floors.

Wax polishes: based on natural waxes, such as beeswax, they can be used as complete system or to maintain other finishes. They are relatively soft and more inclined to collect dirt than other finishes; they discolour when wet and will be stained by ink or heat but are less likely to show scratches and easily are repaired.

Polymer-based emulsions: based on PVA, acrylic and polyethylene resins; they are easy to apply and maintain.

Clear finishes for internal timberThe clear finishes currently available include the following:

French polish: based on applications of shellac and linseed oil in successive treatments, requiring great skill for a good finish. They are considered to be the most beautiful finish for internal timber but are extremely expensive and easily marked by water, heat and solvents.

Cellulose lacquer: based on nitro-cellulose and a plasticiser and showing a similar appearance to French polish but less expensive and easier to apply. It is more resistant to water but eventually cracks and must be completely removed before renewing. Nitro-cellulose is extremely flammable and appropriate precautions should be taken regarding storage and use.

Clear finishes for internal timberShort-oil varnishes: have a low oil and high resin content, producing a high gloss but reduced flexibility. They are easy to apply with a brush but they dry slowly, collect dust and crack.

Spirit varnishes: made with resins, such as shellac, they dry quickly by the evaporation of the solvent. They are cheap but brittle and inclined to crack.

Synthetic resin finishes: made from plastics, such as phenol formaldehyde resins, urea formaldehydes, polyurethane and epoxides. They are available in one-pack or two-pack forms. They are relatively expensive but are very popular because of their ease of application by brush or spraying. They are rapid drying and are extremely hard and flexible, water and chemical resistant and heat resistant. Repairs are difficult because they cannot be removed by normal solvents.

Clear finishes for internal timber

When choosing a clear finish for a timber surface it is important to define your requirements carefully, taking into account the nature of the timber. For instance, the clear finish chosen may actually be harder than the timber substrate and breakdown of the finish has often occurred because an impact has caused denting of the timber below, not the finish itself. The result is a loss of bond between the substrate and the finish. Thus, softer timbers should be finished with the more flexible finishes.

Preparation of internal timber surfaces

•As with painted surfaces, a good finish can only be obtained with adequate preparation of the substrate. In general, the surface must be clean, firm and dry but additional preparation might include:

• bleaching or liming to give a grey effect

• sanding to smooth the surface

• stopping or filling of pores or indentations, usually with a tinted, oil-based wood filler

• staining—this may be applied before the final finish or may be included in the finish (the manufacturer’s advice should be followed regarding the compatibility of a stain with a finish).

Clear finishes for external timberClear finishes which will help to preserve the natural appearance of timber in exposed conditions include the following:

• Preservatives: These help protect the sapwood and heartwood or timber from attack by fungi and discolouration by moulds.

• Water repellents: These are a mixture of linseed oil, paraffin wax and a fungicide, applied by brushing or dipping, especially to end grain. They help preserve the appearance of the timber by reducing surface cracking due to wetting and drying

• Stains: Water-resistant stains can provide a degree of ultraviolet filtration change the colour of the timber and revive bleached timber.

• Varnishes: The only suitable varnishes for exterior use are long-oil marine and exterior varnishes but these require frequent recoating—less than four coats will be unlikely to last more than a year. While intact, varnishes seal the timber against water but it is desirable to apply a preservative as well.

Preparation of external timber

In general, a lower standard of preparation is required for external timber but any stopping or filling must be water-resistant and galvanised nails should be driven well below the surface and filled to avoid rust stains.

External clear finishes on other materials

Clear finishes designed to reduce soiling and make the surface impervious to water are frequently applied to masonry surfaces, finishes based on silicone being the most effective and the most expensive alternatives.

Finishes based on acrylic resins and polyurethane two-pack systems are available to give some protection to metals such as copper. They must be applied by spraying and preferably in a factory.

Other Coatings

Vitreous enamel (often called porcelain enamel) is actually glass which is fused under extreme heat to metal surfaces. The process is expensive but the resultant coating is extremely hard and durable and adheres firmly to the substrate so that where damage exposes the underlying surface, rust will not creep under the rest of the coating.

The finish is applied after fabrication is complete and the number of coats required depends upon the location of the finished component.

A wide range of colours is available and finishes can be gloss, semi-gloss, matt or textured. The latter collect grime easily and are not suitable for external use.Vitreous enamel coatings are used for metal-wall infill panels, mullions, lift panels, steel rainwater components and baths.

Plastics coating

Plastics can be applied in a number of ways to metal, timber and other surfaces and form continuous protective coatings which, in general, are more durable and tough than ordinary painted finishes.

Some are extremely durable (eg polyvinyl fluoride and nylon) but others (eg polyethylene) deteriorate in exterior conditions, fading and becoming brittle.Many colours are available though some are not suitable for external use and the finish obtained is usually warm to the touch, and smooth, easily cleaned and provides electrical insulation.

The coatings are applied to the components or sheet materials in the factory and are used for sheet metal, and extruded components, such as handrails, in particular.

Check progress 2

Sheet coverings

As briefly mentioned at the beginning of the unit, sheet coverings such as wallpapers and fabrics have been used to decorate wall and ceiling surfaces for hundreds of years.

Wallpapers and textiles are still the easiest way to obtain large areas of highly patterned or textured wall surface and in addition can contribute to acoustic modification of the space.

Light-fastness varies and few are suitable in areas receiving strong sunlight.

Types of sheet coveringsSheet coverings used frequently include the following:

• Lining papers: These are used to cover imperfect plaster surfaces which are subsequently painted or wallpapered. They are hung horizontally under wallpaper to minimise coincidence of joins.

• Expanded polystyrene: This is a great deal thicker than wallpaper and it provides some thermal insulation, often sufficient to prevent surface condensation.

Types of sheet coveringsSheet coverings used frequently include the following:

• Wallpapers: These can be machine-made or hand-made—the latter being more expensive, with denser colours but some imperfections. Wallpapers are available in the following types:

– pulps—patterns printed directly onto the paper– embossed—with a raised design– duplex—two-ply papers– ingrain—having fibres incorporated into the surface– washable—coated with a plastic emulsion, vinyl-faced papers

are washable but maximum dirt resistance is provided by PVC coated papers

– shiny—surfaced with mica– flock—raised applied patterns created by blowing fibres onto

patterns printed in adhesive.

Types of sheet coverings

• Wood veneer: This can be mounted on paper, cloth or metal foil backings and is often coated with transparent vinyl.

• Textiles: A wide variety of textiles is available, such as hessian, silk and synthetic fibres, which can be used unbacked in folds or stretched taut on frames or backed with paper, foamed plastic or PVA.

• Leather: Usually backed with padding such as foamed plastic, panel sizes must be limited to available hide sizes.

• Plastic-faced cloths: PVC-impregnated cotton cloths are produced in a wide range of colours, textures and patterns. They are waterproof and can be cleaned with warm water and soap or mild, domestic non-abrasive chemicals.

• Grass cloth: This consists of bamboos or grasses held together with thread and mounted on backings.

• Carpet: Stapled to vertical surfaces, carpets can provide a durable, soft finish with excellent sound modification characteristics.

Hanging wallpapers and other sheet coverings

There are some important considerations when hanging wallpaper:

• It is best if patterns are matched at eye level to minim• ise obvious irregularities in printing or stretch in the paper.• Drying time is important for a good result and paper should be

neither too wet nor too dry.• Care should be taken to avoid paste staining of the paper,

especially flock papers.• Most ordinary wallpapers come pre-pasted with flour, starch or

cellulose pastes which have good slip properties for hanging.• Heavy papers can be hung with special proprietary brand

pastes.• Expanded polystyrene must be fixed with a PVA adhesive as

other adhesives destroy it. If it is to be used as a lining paper it should be painted with plastic paints only.

• Plastic-faced cloths must be fixed with adhesives recommended by the manufacturer.

Preparing the surface to be papered

The wall surface should be dry and chemically neutral with a slight suction. This is achieved by removal of efflorescence by brushing and painting with an alkali-resistant primer. If mouldy, old wallpaper should be removed and the surface treated with a fungicide. Depressions and cracks should be filled and a lining paper could be applied to improve the substrate.

Check progress3

Galvanising

Galvanising is the process of coating steel and iron with zinc to form a protective coating. The steel is lowered into a molten bath of zinc heated to approximately 500°C and emerges with a shiny coating of zinc. The zinc coating acts as a ‘sacrificial’ anode and corrodes to protect the steel. Since its rate of corrosion is slow, the steel can remain protected for hundreds of years, depending on the environment.

Zincalume

Zincalume is a newer protective coating and is a combination of zinc and aluminium (45% and 55% respectively), which is applied in a factory process to sheet steel used for roofing and cladding in the building industry.

Summary

Surface finishes include paint, clear finishes, plastic coating, various types of wallpaper and other sheet coverings.

On steel and iron, galvanising is another method of coating the surface to protect it from deterioration. Surface finishes may be used to protect, preserve or waterproof interior and exterior walls, floors, ceilings and roofs. They are also used for decorative purposes and to improve the lighting in rooms.

If you have completed all the check your progress questions you are now ready to begin the final unit of this module, on plastics and adhesives.

Development of plastic products

Introduction

In the twentieth century plastics have been developed to such an extent that they replace many natural materials. The term ‘plastics’ is now used to describe many products which are artificially made and chemically produced.

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.

Learning outcomes

On completion of this unit, you should be able to:

• differentiate between thermoplastic and thermosetting plastics

• demonstrate a knowledge of the practical uses of plastics and adhesives in the building industry

• describe the different adhesives in general use.

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.

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 affects 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.

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 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 air-conditioning ducts.

Thermoplastics

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.

Polystyrene

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 materials, nylons are used for nuts and bolts, castors, curtain rails and sliding door fittings and ball valve assemblies.

PolycarbonatesExtremely 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.

ThermosetsPhenol 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 formaldehydeUrea formaldehyde products are usually white or brightly coloured and it is self-extinguishing. It is used for electrical accessories, paints, stoving enamels, adhesives and foamed products.

Melamine formaldehydeMelamine 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.

ThermosetsResorcinol formaldehydeThis is a dark red resin used as a waterproof and boilproof adhesive for wood.

Polyester resinsThese 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.

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.

Check progress 1

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 propertiesProperties 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)• 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 adhesives

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, wood-linoleum 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 right the first time. They are not generally suited to wood joints as the adhesive tends to flow under constant load.

Thermoplastic adhesivesThese 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 adhesivesCapable 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.

Cold-setting reactive adhesivesSome 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 formaldehydeThis 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 (eg ‘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.

CyanoacrylatesThese are costly, one-part adhesives which form an instant bond (eg ‘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.

MasticsA 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.

Plastic mastics are often called sealants and are more expensive and more durable than the elastic mastics. They usually remain plastic for a period of time before hardening to a point where loads can be sustained without squeezing out.

Elastic mastics can be based on silicone, polyurethane, butyl rubber or polysulphide rubber. They are used to seal a variety of assemblies including glazing and metal curtain-walling; around baths, sinks and basins and joints in wall tiling.

Selection of mastics

As constituents vary considerably, manufacturers’ recommendations should be studied carefully. Points to consider when selecting a mastic include resistance to moisture penetration, exposure to weather, exposure to chemicals, compatibility with adjacent materials, loading conditions and ease of application.

Check progress 2

SummaryPlastics are synthetic materials chemically produced from coal, natural gas, other petroleum products, cotton, wood, oat hulls, corn cobs and sugar cane.

The two types of plastic are thermoplastics (which are used for waterproof membranes, emulsion paints, in situ flooring, cisterns, lights and roof glazing) and thermosetting plastics (which are used for door mouldings, electrical accessories and durable sheeting).

The characteristics of plastics are their strength, thermal conductivity, electrical properties, combustibility, durability and non-biodegradability.

Adhesives and glues were made in the past from naturally occurring materials such as bitumen and tree resin, but now many adhesives are made from plastics. They can be grouped into thermoplastic and thermosetting types.

Thermoplastic adhesives include polyvinyl acetate (PVA)—white liquids which become transparent on setting—and hot-melt adhesives, which are applied in a hot, molten state.Thermosetting adhesives harden by heat action in conjunction with a catalyst or hardener. They include urea formaldehyde, phenol formaldehyde and, melamine formaldehyde.