Adhesives and Sealants
Surface pretreatment
Introduction
• Inadequate or improper surface treatment is
probably the main reason why adhesive bonds
fail.
• Adhesives are not surface-selective, in that they
will bond to most uncontaminated surfaces
• The exceptions being surfaces which lack polar
groups and are, hence, of low surface freeenergy, such as the polyolefins,
polytetrafluoroethylene (PTFE) and
poly(dimethyl siloxane).
Bonding
• The theories of adhesion suggest strong
bonding will be obtained by the introduction
of dipoles to surfaces which increase van
der Waals forces, the removal of weak
boundary layers and roughening of surfaces.
• Particularly when bonding metals, the desired
gain is not primarily to increase the strength
of the newly made joint, but to increase
resistance to water.
Surface treatment
• Surface treatment of an adherend can cause the following
(i) remove contaminants or weak boundary layers;
(ii) modify the surface chemistry by introducing new chemical
groups;
(iii) change the surface geometry.
• Contamination can take the following forms:
(i) oils and greases on metals;
(ii) weak or loose oxide layers on metals;
(iii) mould-release agents such as silicones, fluorocarbons and waxes on
polymers;
(iv) additives and low molecular weight material on the surfaces of
polymers, which have a tendency to expel foreign matter from the
bulk to the surface.
ABRASIVE METHODS
• Abrasive methods include blasting with sand
and other particulates in air, blasting with
alumina in water and the use of abrasive
papers and cloths.
• They are able to remove contamination and
also roughen surfaces.
• A method for treating mild steel is to gritblast and then apply a silane coupling agent
USE OF SOLVENTS
• Surfaces may be cleaned by wiping with tissues
soaked in solvent or by vapor degreasing.
• Lax wiping methods can result in the
redistribution rather than the removal of
contaminants.
• Solvents are very effective at removing oils and
greases, but health and safety, and
environmental considerations, weigh much
against their use.
FLAME AND CORONA-DISCHARGE
• Exciting gas molecules, which then attack
surfaces, causing chemical modification.
• The excited species can be ions, electrons or
neutrals.
• In a gas-air flame the region just above the
blue cone contains excited species.
(Polyethylene squeezy bottles are rotated in this region of a
broad flame for about 1 s to make them wettable by
printing inks.)
FLAME AND CORONA-DISCHARGE
• A corona-discharge in air at atmospheric
pressure has a purple glow, and polyolefin
film material used to make printed carrierbags is treated in this way.
• Coronae are generated using high voltage,
typically 20 kV, and high frequency (10-20
kHz).
FLAME AND CORONA-DISCHARGE
• Both these treatments introduce new chemical groups
such as -OH, >CO and -COOH, which are polar, to
polyolefin surfaces.
• Corona- discharge can also be used to treat composites
and metals for bonding; it is possible that with metals,
oils and greases are volatilized.
• Newer methods include the use of plasmas and excimer
lasers.
• Chromic acid etch baths can also be used to treat
polyolefins, and similar chemical groups are introduced as
in flame and corona-discharge, but the method is not used
commercially because of the environmental issues and
because dry methods are preferred.
ETCHING OF PTFE
• If sodium metal is added to a solution of naphthalene in
dry tetrahydrofuran (THF), sodium naphthalenide is
formed, giving the solution a dark green color.
• Only one electron is transferred from sodium to
naphthalene, and the resulting naphthalenide is a radicalanion.
• When PTFE is immersed in such a solution, the electron is
transferred to the polymer, resulting in defluorination and
the formation of a carbonaceous surface.
• An alternative treatment is a solution of sodium metal
in liquid ammonia.
ETCHING OF METALS
• Abrasion or solvents can give adhesive joints
which are strong in dry conditions, this is not
the case when joints are exposed to water
or water vapor.
• Acid etched are preferred to solvent
degreased metal joints.
• Aluminum and its alloys can be etched in
chromic-sulfuric or phosphoric acids.
Etching Aluminum
• procedure recommended by Ciba Composites:
1. Vapor degrease in halocarbon solvent and/or alkaline
degrease, e.g. for 10 min in an aqueous solution of
Turco T 5215 at 70 "C, followed by a spray rinse in clean
water.
2. Etch in chromium trioxide 250 g (or sodium dichromate
375 g),concentrated sulfuric acid 750 ml and water to 5
L, at 60-65 "C for 30 min.
3. Immerse in a tank of ambient water.
4. Spray rinse with cold water.
5. Dry in an air-circulating oven at no greater than 45°C.
6. Bonding should take place within 8 h.
Etching Mild Steel
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Remove rust or mill scale by brushing.
Vapour degrease.
Grit-blast.
Etch in 4% solution of hydrofluoric acid at room temperature for
10 min.
Rinse immediately in tap water.
Immediately remove smut (carbon) in a bath composed of
chromium trioxide 100 g, concentrated sulfuric acid 57 ml and
water to make 1 L at 70 "C. This is complete within a few minutes.
• Rinse in running tap water.
• Dip in bath of propan-2-ol, and then in a second bath of dry
propan-2-ol before drying. This prevents rusting.
• Apply a primer at once.
Etching Titanium
Alloys
(a) Vapour degrease.
(b) Wet blast with alumina.
(c) Immerse for 20min at 65-70°C in sodium
hydroxide 20g,
(d) Wash in hot water for at least 10min.
(e) Dry in warm air.
(f) Preferably apply primer coat immediately.
ANODIZING OF METALS
• Anodizing aluminum and its alloys gives
the most water-durable adhesive joints, and
is used by aircraft makers.
• Anodizing in chromic acid is favored by
European makers, and in phosphoric acid in
the USA, but there are also differences in the
alloys used.
ANODIZING OF METALS
• Phosphoric acid anodize procedure used in the
USA:
(a) Vapour degrease for 10min.
(b) Immerse for 5 min at 40 "C in trisodium phosphate
25 g, Teepol detergent 5ml and water to make 1L.
(c) Anodize in phosphoric acid 60ml and water to make 1
L at room temperature. Raise voltage to 1O V over 2
min. and maintain for 5 min.
(d) Spray rinse with cold water.
(e) Dry in an air-circulating oven at no greater than 45
"C.
ANODIZING OF METALS
• A chromic acid anodizing procedure:
(a) As for chromic acid etching.
(b) Anodize in chromium trioxide 500g and water
to 10 L at 40°C. Raise to 40 V over 10 min and
maintain for 20 min. Raise to 50 V over 5 min and
hold for 5 min.
(c, d and e) As for chromic acid etching.
(f) Bond within 4-6 h.
Honeycomb Structures
• Etching and anodizing of aluminum creates a
thick honeycomb structure of aluminium oxide.
• The precise morphology varies with the
treatment procedure.
• In honeycomb structure proposed for phosphoric
acid anodizing there are some whisker-like
protrusions at the top of the honeycomb.
• Chromic acid anodizing can also be used with
titanium alloys, but anodizing in sodium
hydroxide solution is preferred because of the
toxicity of chromium compounds.
Wood
• A freshly cut wood surface is ideal for
adhesive bonding because of its porosity, but
care should be taken to remove sawdust.
• Old timber is best resawn.
Glass
• The surface of glass can be readily bonded when
clean and dry.
• Probably the largest commercial bonding of glass
is in double glazing units, where the surface is
cleaned with water and the sealant is most often
a polysulfide.
• The use of silane coupling agents is
recommended where there is exposure to water
or humid air, as would be the case with double
glazing.
CONCRETE
• It become necessary to stiffen many reinforced concrete bridges.
• This can be done by bonding external panels of steel or fiber
reinforced composite.
• Concrete surfaces can be prepared for bonding by first removing
loose material and exposing coarse aggregate, followed by cleaning
and drying.
• The surface can be etched with 10-15% hydrochloric acid but only
after thorough washing, so that the acid does not penetrate.
• Thorough rinsing with water or dilute alkali must follow.
• The surface must be dried so that its water content is preferably
below 4%.
• It is considered that mechanical interlocking and physical
adsorption on the high-energy surface contribute to adhesion.
COMPOSITES
• Modern composites mainly consist of glass or carbon fibers
in an epoxide or polyester resin.
• The resins are applied as liquids that subsequently set, and
which have adhesive properties.
• To prevent the resins bonding to the mould, surfaces of the
latter are coated with a mould-release agent such as a
silicone or fluorocarbon [non-stick (abhesion)].
• It is essential that these are removed from the composite
before any bonding and solvents or abrasives may be used.
• An elegant alternative is the use of peel plies, which is a
layer of material built into the face of a composite, which
can be stripped away before bonding.
NON-STICK (ABHESION)
• With a few exceptions all materials can be bonded with
adhesives.
• The exceptions are the polyolefins (polyethylene and
polypropylene), silicones and some fluoropolymers.
• The features which these materials share is a low
surface-energy.
• two well-known examples being the use of
polytetrafluoroethylene (PTFE) in non-stick kitchen
utensils, and backing papers for sticky labels, which are
impregnated with poly(dimethyl siloxane) (PDMS).
NON-STICK (ABHESION)
• Release agents in aerosol cans are based on
either silicone oils or dispersions of PTFE in
water.
• Their use in aerosols means that release
agents may drift beyond their intended target,
and this is a common cause of failure for
adhesive bonds.
Silicone Release Papers
• Silicone release papers are made by impregnation with
poly(dimethyl siloxane) with hydride and vinyl end groups,
which co-react in the presence of about 5 ppm of a platinum
or rhodium catalyst.
• The reaction proceeds at room temperature but can be
accelerated by heating.
THE END
Next Chapter: Primers and Coupling Agents