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 free-energy, 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 grit-blast 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 carrier-bags 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 radical-anion.

• 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• 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 TitaniumAlloys

(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