Adhesive bonding - a guide to best practice

Surface pre-treatment: overview & specific methods

Overview:

• Introduction
• Purpose of pre-treatment
• Main types of pre-treatment
• Overview of mechanical, chemical & energetic pre-treatment

Introduction

Synthetic adhesives have been used successfully for many years in diverse sectors of industry including aerospace, automotive, biomedical/dental, construction, electronic, marine, sport & leisure.

Although in a few applications no surface pre-treatment is necessary (the automotive industry has been using specially designed adhesives for bonding oily metal adherends since the 1950s), it is accepted that to obtain the optimum performance from an adhesive joint a pre-treatment is required. The type of pre-treatment is often a compromise between the best surface preparation, economics of component manufacture and health and safety issues.

If the economic and engineering advantages of adhesives are to be realised, the adherends must be given a suitable surface pre-treatment. The benefits of using appropriate surface pre-treatments are:

• enhanced mechanical performance of joint
• improved joint durability in aggressive environments
• increased service-life of component
• ability to bond difficult adherends, e.g. polyolefins and polytetrafluoroethylene

Purpose of pre-treatment

Pre-treatment is carried out to achieve one or more of the following:

• To remove completely, or to prevent formation of, what are often referred to as weak boundary layers. A useful analogy to describe this concept is applying a pressure sensitive tape (e.g. Sellotape) to an adherend coated with a powder (e.g. talc). The Sellotape will simply adhere to and pull off the talc from the adherend. No bond will form between the tape and adherend. Examples of weak boundary layers include weak oxide scale on metallic substrates, plasticisers which have migrated to the surface of polymers, mould release agents from processing of composites. Other surface contaminants are dust, dirt, grease, oils and even finger marks!
  
  
• It is well established that to form an effective bond, intimate molecular contact between adhesive and adherend is required. The correct surface pre-treatment will optimise this degree of contact, which may be brought about by chemical modification of the adherend surface.
  
  
• To protect the adherend surfaces before bonding. This is often necessary, particularly with metals which after surface pre-treatment have a surface that is highly reactive not only towards adhesives but also to atmospheric contaminants. To preserve the integrity of the adherend surface it is usually necessary to bond the surface within a few hours of treatment, or to coat it with a primer which is compatible with the adhesive to be applied later. A primed surface can protect the adherends for up to several months.
  
  
• To produce a specific adherend surface topography, thereby altering the surface profile, and possibly increasing the bondable surface area, that is, to roughen the surface.

Main types of pre-treatment

There is a wide range of surface treatments available. Many adhesives or adherend suppliers give advice. Best practice is collected in Standards; ISO 4588 (for metals) and ISO 13895 (for plastics) are currently under revision. Techniques can be classified into five groups, according to the nature of treatment:

Cleaning/Degreasing: Removal of loose solids can be accomplished with a clean brush or blast of clean, dry air. Organic solvent or alkaline aqueous solution removes organic materials such as grease, oil and wax from adherend surfaces. This can be accomplished by wiping, dipping or spraying.

Surface Roughening: Techniques where abrasive materials are employed to remove unwanted layers and generate a roughened surface texture.

Chemical Treatments: Immersion of the adherend in an active solution which has the power to etch or dissolve a part of the adherend surface or change it in such a way that the treated surface becomes chemically active. An electrochemical reaction can also be included where current is transferred through an electrolyte between an auxiliary electrode and the adherend surface, e.g. anodising, and this introduces several additional factors to be controlled.

Physical Treatments: Techniques where the adherend surface is cleaned and chemically modified by exposure to excited charges or species. Techniques such as corona discharge, plasma, flame or exposure to ultraviolet/ozone are examples in this group.

Primers: Alternative surface treatments, often simpler than chemical or physical methods, applied by dipping, brush or spray. They can chemically alter the surface (e.g. silane coupling agents, chromate conversion coatings), or protect the preferred surface already generated by another method (e.g. aerospace primers after anodising).

In general, the above types of surface pre-treatment can be divided into three major categories: mechanical; chemical; energetic. Each of these can be further subdivided into a given technique or method of surface preparation, as illustrated in Table 1.

Overview of mechanical, chemical & energetic pre-treatment

Table 1 Surface pre-treatments for adherends

Selection of a pre-treatment should be based on issues such as cost, production, durability and health and safety. Pre-treatment facilities can include equipment, chemicals and consumables. Key surface features which should be kept in mind are wettability, roughness, soundness, stability, contamination, uniformity and adhesive compatibility.

Surface pretreatment must always be appropriate to performance requirements, and be compatible with manufacturing procedures and costs. Use of aluminium in four industries illustrates this:

• Aerospace (wing skin to honeycomb)
  - complex and high cost; etch, anodise and prime sequence before lay-up with epoxy adhesive
  
  
• Automotive (bodyshell)
  - metal supplier provides material with conversion coating and press lubricant applied, before application of epoxy paste adhesive by manufacturer
  
  
• Rail (carriage panels to frame)
  - solvent clean, then primer or coupling agent applied to localised areas in factory, before application of polyurethane adhesive
  
  
• Automotive (engine-block flanges)
  - alkali cleaning before application of anaerobic acrylic, or silicone sealant in factory

Mechanical pre-treatment

Mechanical abrasion is the most widely applicable surface preparation technique, being suitable for most materials.

Mechanical abrasion will remove weak boundary layers. It will also change the surface topography of the adherend, increasing the bondable surface area on a microscale. Furthermore, mechanical abrasion will enhance the adhesive's ability to 'wet' (when the adhesive readily and completely covers) the surface of the adherend.

The simplest form of abrasion uses silicon carbide paper to abrade/polish surfaces. This method may be carried out dry or in conjunction with a suitable solvent. The quality of the adherend surface obtained with silicon carbide depends upon the grit size and whether the operation is performed manually or mechanically. It is necessary to monitor carefully the abrasion since, if the process is carried out for too long, surface debris initially removed can be re-deposited. For composites, it is important to note that mechanical abrasion may cause fibre damage and impair performance of the joint.

Blasting is another form of mechanical abrasion and includes alumina grit blasting, shot blasting, cryoblast and sodablast. Shot blasting is only appropriate for treating ferrous surfaces. For other metals, grit such as alumina should be used. Cryoblast and sodablast are used for preparing composite adherends. Cyroblasting is less aggressive than alumina grit and consists of solid carbon dioxide pellets.

Sodablasting was initially used in the aerospace industry as a preparative method for painting aircraft. It uses a suspension of sodium bicarbonate in water. A disadvantage of sodablast is that it increases the water content of composite and hence brings a need to dry the component before bonding. The variables associated with blasting are the chosen particle, the particle size, pressure of blast, exposure time, angle of blast and distance between blast nozzle and adherend.

In the peel ply surface preparation method used for composites before curing, a fabric material is used to cover the external surface of the composite. During the cure cycle, part of the matrix will flow and penetrate the fabric and eventually, after curing, becomes part of the laminate. When the laminate is required for bonding, the fabric is peeled off, fracturing the resin between the fabric and the first layer of reinforcement, producing a clean, roughened surface to which the adhesive can be applied. The surface morphology obtained is dependent on the nature of fabric and type of weave used.

Chemical pre-treatment

There are several different chemical methods by which adherends can be cleaned and prepared for bonding including: solvent cleaning, detergent wash, acid etch, anodising and primers.

Adherends are frequently contaminated with oils or grease. An effective method of cleaning ceramics, glass and metal adherends is by an organic solvent vapour degrease. Solvent is boiled in a chamber whereupon it condenses on the cooler adherend, and dissolves the oil and grease before it drips back into the heating tank. Very clean surfaces can be obtained in this way. Consideration must be given to local or national environmental regulations on the use of organic solvents.

Alternatively, if small enough, the adherend can be immersed in an ultrasonic bath containing a solvent. Agitation of the adherend increases the speed of treatment.

Solvent cleaning in its simplest form can be performed by using a suitable cloth to apply the solvent to the adherend. The cloth should be applied so that the surface is wiped in one direction only, to prevent any surface debris from being re-deposited. The cloth should also be replaced regularly.

Solvent cleaning is often applied to polymeric materials, but the correct solvent must be chosen with care since solvents are organic based and may attack, be absorbed by, or plasticise the adherend. Plastics material suppliers can usually advise.

Detergents dissolved in water, alkaline or acidic solutions and used at temperatures of about 50-70°C may also be used to supplement or replace the organic solvent cleaning process.

Metal adherend surfaces are rarely of pure metal, but are a combination of oxides, sulphides, chlorides and other atmospheric contaminants. This results in a surface which is mechanically weak and is prone to crack and flake off.

Acid etching is a well established method of removing weak metallic scale, in favour of forming an oxide layer which is mechanically and chemically compatible with the adhesive. Hence, different acid treatments are applied to different metal adherends, for example, chromic acid for aluminium, sulphuric acid for stainless steel and nitric acid for copper. Some metals require an alkaline, rather than an acid, pretreatment, such as alkaline peroxide used for titanium.

Acid pre-treatment can also be applied to certain plastics. Chromic acid is used to surface treat polyolefins. Even PTFE, known as a non-stick material, can be bonded when treated with a solution of sodium naphthalenide in tetrahydrofuran.

Anodising has been exploited extensively by the aerospace industry as a surface pre-treatment for aluminium and titanium alloys. Anodising is performed only after the adherend has been etched. The purpose of anodising is to deposit on the adherend a porous and stable oxide layer on top of the oxide layer formed after etching. The porous oxide layer enables adhesive (or primer) to penetrate the pores readily to form a strong bond and is resistant to environment attack by H₂O.

Anodising is a type of electrolysis where the adherend is the anode, and a typical electrolyte is phosphoric acid. An inert electrode is used for the cathode. The differences in the structures of aluminium oxide layers before and after anodising are illustrated in Fig. 1. A disadvantage of anodising is that it is a time-consuming operation. In addition, there are a number of variables which must be carefully controlled: applied voltage, time of anodising, temperature and concentration of electrolyte.

Fig. 1 Oxide layer on aluminium:

a) Before anodising	b) After anodising

Application of a primer to an adherend is another form of surface pre-treatment mainly used for materials such as metals and ceramics. Generally, the primer is the final stage of a multistage pre-treatment process. The primer acts as a medium which can bond readily to the adherend and adhesive.

Some adhesives have high viscosities and thus do not flow readily over the adherend, or the adherends have 'difficult to bond' surfaces (e.g. copper). The primer, which is formulated so that it represents a solvented version of the adhesive, readily wets the adherend. The primer is then cured on the adherend as desired. The adhesive, when applied to the primed surface, being chemically compatible, will establish a strong joint on curing.

Primers often contain ingredients which enhance the environmental resistance and thermal stability of the joint, as well as protecting the adherend from hydration and corrosion. The cured primer can protect the adherend for several months before bonding.

Energetic pre-treatment

Energetic surface pre-treatments which have been reported in the literature include flame, corona discharge and plasma (FCDP) and excimer laser. All of these procedures cause a change in the surface texture of the adherend, brought about by the interaction of highly energetic species with the adherend surface. These pre-treatment methods have been applied to metals and in particular composites and plastics.

A plasma is an excited gas consisting of atoms, molecules, electrons, ions and free radicals. A plasma is generated by applying a high frequency and high voltage between, for example, parallel plate electrodes in a low pressure chamber. The advantage of this method is that it allows treatment of adherends by different plasmas of argon, ammonia, oxygen or nitrogen.

Plasmas created from inert gases are generally used to clean the surfaces of adherends. The excited species generated can have one or more of the following effects on the adherend:

• surface clean
  - the excited species may have sufficient energy to displace some surface contaminants
  
  
• degradation and ablation
  - the plasma can cause degradation of the surface of polymeric materials and lead to removal of debris from the surface
  
  
• crosslinking
  - the surface of the adherend may become crosslinked and prevent the formation of weak boundary layers
  
  
• oxidation
  - the plasma can lead to introduction of oxygen-containing groups, for example carbonyl, brought about by oxidation of the polymer surface; this can lead to the adherend being readily wetted by the adhesive

If instead, a plasma is created in air at atmospheric pressure, the air when ionised appears as a blue/purple glow with faint sparking, and is termed a corona. The effects which the corona discharge can have on the adherend surface are similar to those described above. Corona treatments are usually applied for preparing thin polymer films and composite laminates.

The effect of a flame treatment is to oxidise the adherend, which produces polar groups such as -COOH, -C=O, -OH, -NO₂, -NO₃ and -NH₂'. This creates a surface better suited to wetting by the adhesive. This method of surface pre-treatment has been applied successfully to carbon/PEEK and glass/polypropylene composites.

The variables of flame treatment include type of gas, gas/air (oxygen) ratio, the rate of flow of mixture, exposure, time and distance between flame and adherend.

Fig. 2 Effects of surface pretreatment on the durability of aluminium alloy/toughened epoxy joints subjected to accelerated ageing in water at 50°C

Specific methods for pre-treatment

• Introduction
• Standard method for simple surface preparation
• Surface preparation method - SPM
• Preparation of:
  - metals
  - plastics, composites & rubber
  - ceramics, paints & fabrics
  
• Etching solutions

Introduction

Substrate surface pre-treatment is necessary in order to:

• remove chemically incompatible layers
• overcome any low surface energy effects which inhibit bonding
• achieve a surface topography which is receptive to the adhesive
• ensure good durability of the bonded joint

Standard method for simple surface preparation

Simple surface preparation techniques such as cleaning and degreasing are required for most adhesives. Certain adhesives, for example cyanoacrylates, may be used alone on plastics materials as they can penetrate or dissolve away surface debris.

Cleaning and degreasing

• use non-ionic detergents, alkaline cleaners or isopropyl alcohol (IPA)
• do not wipe with fibrous cloths or tissues
• allow to dry

Surface preparation method - SPM

The following method is suitable for most materials. A silane coupling agent may be used to indicate surface wettability and increase the bonded joint's long-term durability.

• Remove oil or grease contamination
• Abrade or shot blast using medium grit (120 - 200 mesh suitable). Grit should preferably be alumina (bauxite). Do not use steel or iron shot of any description on aluminium alloys
• Remove coarse debris (if present) with water jet
• Remove fine debris with solvent impregnated wipes
• Ensure surface is completely dry
• Bond or prime immediately

Preparation of metals

Aluminium and its alloys

Method A
Etching (DEF STAN 03-2/3: Method 0)

• Use simple SPM and degrease in alkaline cleaner
• Immerse in etchant A for 30 minutes at 60-65°C
• Remove and rinse thoroughly in cold distilled or de-ionised water
• Dry with hot air (‹65°C) for 10 minutes
• Bond immediately

Method B
Anodising (Boeing PAA 5555)

• Etch as in Method A
• Immerse in phosphoric acid bath at room temperature (18-30°C)
• Raise voltage to 10V. Keep current low (‹2A) for 20-25 minutes. DO NOT OVER-RUN
• Disconnect power
• Remove and rinse thoroughly in running water
• Dry in oven at 40-60°C
• Prime or bond within 16 hours

Aluminium honeycomb

• Degrease in vapour bath of trichloroethane
• Stand for 2 hours at room temperature, or for 15 minutes at 93°C (200°F)

Note: Alternatively, do nothing. Aluminium honeycomb core has low thermal mass and vapour degreasing may be ineffective.

Chrome plate

• Use simple SPM
• Immerse in etchant B for 1-5 minutes at 93 ±2°C
• Remove and rinse thoroughly in cold distilled or de-ionised water
• Dry in hot air stream (‹55°C) for 10 minutes
• Bond immediately

Copper and its alloys -brass, bronze (DEF STAN 03-2/4)

• Use simple SPM
• Immerse in etchant C, D or E for specified time
• Remove and rinse thoroughly in cold distilled or de-ionised water
• Dry in clean, cold pressurised air (hot air will discolour surface)
• Bond immediately

Lead (and pewter)

Use SPM . Dust must be carefully controlled.

Magnesium and its alloys (DEF STAN 03-2/7)

• Degrease. Do not abrade
• Immerse item in sodium hydroxide solution for 10 minutes at 70°C ±5°C
• Wash thoroughly in cold tap water
• Immerse item in etchant F for 10 minutes at room temperature
• Wash thoroughly with cold tap water
• Wash again with distilled or de-ionised water
• Dry in hot air (<65°C)
• Bond immediately

Note: FIRE HAZARD - never place magnesium components in a vapour bath

Nickel and its alloys

• Use SPM
• Immerse for 5 seconds at room temperature in concentrated nitric acid (specific gravity 1.42)
• Remove and rinse thoroughly in cold distilled or de-ionised water
• Dry in hot air stream (<65°C) for 10 minutes
• Bond immediately

Steel - mild and iron (DEF STAN 03-2/2A-B)

• Use SPM
• Immerse for 10 minutes in etchant G at 60°C ±2°C
• Brush off black deposit with a stiff nylon brush under clean, cold running water
• Blow dry with clean air
• Wipe with isopropyl alcohol (IPA)
• Allow to dry
• Heat for 1 hour at 120°C
• Bond immediately

Note: For exterior use and demanding environments, use a silane coupling agent (Permabond SIP or Accomet C)

Steel - stainless

• Use SPM - alumina (120/220) shot blasting is preferable
• Wash in detergent solution for 10 minutes at 75°C ±5°C
• Rinse in cold water
• Rinse in distilled or de-ionised water
• Immerse in etchant H for 5-10 minutes at 60°C ±3°C
• Rinse in cold water
• Rinse in cold distilled or deionised water
• Dry in hot air stream, oven or with infrared lamps for 10 minutes (<95°C)
• Bond immediately

Tin

Use SPM

Titanium and its alloys

Method A

• Vapour degrease
• Use SPM
• Wash in detergent solution for 10 minutes at 75°C ±5°C
• Rinse in cold distilled or de-ionised water
• Dry in hot air stream, oven or with infrared lamps ( <95°C).
• Immerse in etchant I for 5-10 minutes at room temperature
• Wash in cold distilled or de-ionised water
• Oven dry at 75°C ±5°C for 10-15 minutes
• Bond immediately

Method B

• Vapour degrease
• Use SPM
• Vapour degrease again
• Use sodium hydroxide anodiation (SHA) process at 10V and 5M NaOH at room temperature
• Wash and oven dry at 75°C ± 5°C for 10-15 minutes
• Bond immediately

Zinc and its alloys, including hot dipped galvanised steel, electrogalvanised steel and iron-zinc alloy coated steel (DEF STAN 03-2/8)

• Degrease surface
• Immerse for 5 minutes in Bonderite 71 (an alkaline cleaner)
• Wash thoroughly in distilled or de-ionised water
• Treat in 5% volume solution of Accomet C, to yield a dry oxide coating of 0.5g/m²
• Oven dry or in hot air stream at 115°C ±5°C
• Bond immediately

Note: If a dark yellow or green layer appears, the passivate is too thick and therefore weak and friable.

Zinc - passivated

• Use SPM
• Then use the standard method for zinc and its alloys

Preparation of plastics, composites and rubber

Use SPM or FCDP (flame, corona discharge or plasma) techniques.

ABS (acrylonitrile butadiene styrene)

Cyanoacrylate, toughened acrylic and solvent-based adhesives will bond without surface preparation. Use minimum quantity of initiator for two-part acrylics and ensure no surface residue after bonding.

Note: Take care to avoid stress cracking

• Degrease surface
• Immerse in etchant J for 15 minutes at room temperature
• Remove and rinse thoroughly in cold water
• Rinse again in hot water
• Dry in a hot air stream (<60°C)
• Bond immediately

Acetal (also known as polyoxymethylene or POM)

• Stress relieve material
• Follow method for ABS
• Alternatively, use corona discharge

Acrylics

• Use SPM
• Take care to avoid stress cracking

Alkyds (polyalkyds)

Use SPM or FCDP

Note: If using cyanoacrylate adhesive, use only alcohol for pretreatment

Cellulose - acetate/butyrate/nitrate

• Degrease using solvent
• If using with epoxides, heat plastic parts for one hour at 93°C
• Apply adhesive while still warm

Note: Elevated temperature will increase cure rate

Alkyl phthalate (polyalkyl phthalate)

Use FCDP

Epoxy (and composites)

Use SPM or FCDP

Note: Check for presence of mould release agent. Take care to avoid stress cracking

Polyamide (Nylon)

Method A

• Use SPM
• Dry the material (polyamides are hygroscopic)
• Bond immediately

Method B

• Degrease
• Immerse in etchant K for 8 seconds ±2 seconds at room temperature
• Dry in well-ventilated atmosphere at 23°C ±2°C for 30 minutes or less
• Allow to dry
• Bond immediately

Polycarbonate (PC)

Use SPM or FCDP

Polyester - thermoset, including composites

Use SPM or FCDP

Note: Check for presence of mould release agent within structure

Polyester - thermoplastic

Use FCDP

Polyetheretherketone (PEEK)

Use FCDP

Polyethylene

• Use FCDP
• Alternatively, use a sodium naphthalene etch solution for halogenated polyethylenes

Polyimide

Use FCDP

Polymethylmethacrylate (PMMA or 'perspex')

See acrylics

Phenolic

Use SPM or FCDP

Phenolic foam

Does not require surface pre-treatment

Polyphenylene oxide

Use SPM or FCDP

Polypropylene

See polyethylene

Polystyrene

See acrylics

Polystyrene foam

Does not require surface pre-treatment

Polysulphone

Use SPM or FCDP

Polytetrafluroethylene (PTFE)

Use TetraEtch (an Albright & Wilson product)

Note: See polyethylene

Polyvinylchloride (PVC)

See acrylics

PVC foam

Does not require surface pre-treatment

Polyurethane foam (PU)

Does not require surface pre-treatment

Rubber

Use SPM

Note: If using a cyanoacrylate, use alcohol only to degrease

Silicone rubber

Use SPM

Note: Use silicone-based adhesives

Preparation of ceramics, paints and fabrics

Cellulose (i.e. wood)

• Plane surface
• Sand to provide evenly abraded surface
• Remove debris using a brush or air-blast
• Bond immediately

Note: Wood with >20% moisture must be dried prior to bonding - ideal moisture content is 8 - 10% by weight. Cellulose has a slightly acidic surface; if bonding with cyanoacrylate adhesive, use activator to neutralise surface.

Cementitious (including concrete, mortar)

• Degrease using 2% non-ionic detergent
• Wash thoroughly with water
• Sand-blast to remove 1-2mm of bonding surface
• Remove debris
• Dry surfaces
• Bond immediately

Note: If limestone, dolomite or carbonate aggregates present, acid etch surface after sand-blasting. Wash thoroughly. Alternatively, use dilute sulphuric acid.

Bituminous concrete

• Scrub surface with detergent solution
• Rinse with high pressure water jet.
• Dry
• Bond immediately

Ceramics (including ferrites, masonry and pottery)

Use SPM

Note: Abrade bricks/non-glazed ceramics with a wire brush. With glazed surfaces, use emery paper or sand blast (medium grit). Ferrites can bond well without abrasion.

Fabrics

• Remove oil or grease
• Ensure material is dry
• Bond immediately

Note: Check solvent is compatible if synthetic fabric

Friction materials (brake pads and linings)

Use SPM

Glass (and quartz)

• Degrease
• Immerse in etchant L for 10 - 15 minutes at room temperature
• Rinse thoroughly with distiller/de-ionised water
• Dry for 30 minutes at 98°C ±1°C
• Bond immediately

Note: Cyanoacrylates may be inhibited by an etched surface. Silane coupling agents, e.g. Permabond's SIP may be used as an alternative to etching.

Leather

Use SPM

Paint - cataphoretic

Use SPM

Paint - epoxy powder

Use SPM

Etching solutions