Wall & Ceiling Linings
Introduction
In 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.
Process
The 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 cement
4 to 5 sand
0.10 lime
Cement: lime: sand
Gypsum plasters
1 cement
5 to 7 sand
0.10 lime
Gypsum plasters
1 plaster
2 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 cement
4 sand
0.10 lime
Cement: sand
1 cement
1 lime
5 sand
Cement: lime: sand
1 cement
1 to 2 lime
6 to 9 sand
Concrete background
Cement: lime: sand
(undercoat)
Gypsum plaster
1 lime
0.25 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 plasterboard
Plasterboard 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 waterresistant 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 insulation
The 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 Rvalues 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 materials
Thermal 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 resistance
Some 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 insulate
Because 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 insulate
Although 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 insulate
If 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
Introduction
For 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.
Introduction
Today we expect a surface coating or covering to contribute to or
provide any or all of the following:
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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.
Paints
Oil-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 paints
These binders are dissolved in a solvent which evaporates leaving a solid film, such
as lacquer and chlorinated rubber.
Chemically cured paints
These 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 substrate
The 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:
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water-resistant paints
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low-odour paints
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chemical-resistant paints
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quick-drying paints
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fire-retardant paints
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stoving paints
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heat-resistant paints
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insecticidal paints
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fungus-resistant paints
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permeable paints
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anti-condensation paints
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floor paints
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luminous paints
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multi-colour paints
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fluorescent paints
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textured paints
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phosphorescent paints
•
metallic paints
•
radioactive paints
Applying the paint
On site, paint can be applied by:
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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 quickdrying 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 timber
The 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 timber
The 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 timber
Short-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 timber
Clear 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, semigloss, 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 coverings
Sheet 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 coverings
Sheet 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 nonabrasive 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.
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
self-extinguishing. 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.
Thermosets
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.
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 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)
•
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, 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 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.
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 (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.
Cyanoacrylates
These 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.
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.
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
Summary
Plastics 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.