Adhesive bonding - a guide to best practice
Surface pre-treatment: overview & specific methods
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:
Pre-treatment is carried out to achieve one or more of the following:
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.
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:
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.
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:
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 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:
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°CFigure 2 illustrates the effect that some of the surface pre-treatments described have on lap shear strengths of aluminium/epoxy joints. Clearly, the best performance is obtained from bonded joints having been prepared by an etchant and post-anodised. In contrast, a simple solvent degrease or gritblast in this type of environment is inadequate.
Substrate surface pre-treatment is necessary in order to:
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
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.
Aluminium and its alloys
Note: Alternatively, do nothing. Aluminium honeycomb core has low thermal mass and vapour degreasing may be ineffective.
Copper and its alloys -brass, bronze (DEF STAN 03-2/4)
Lead (and pewter)
Use SPM . Dust must be carefully controlled.
Magnesium and its alloys (DEF STAN 03-2/7)
Note: FIRE HAZARD - never place magnesium components in a vapour bath
Nickel and its alloys
Steel - mild and iron (DEF STAN 03-2/2A-B)
Note: For exterior use and demanding environments, use a silane coupling agent (Permabond SIP or Accomet C)
Steel - stainless
Titanium and its alloys
Zinc and its alloys, including hot dipped galvanised steel, electrogalvanised steel and iron-zinc alloy coated steel (DEF STAN 03-2/8)
Note: If a dark yellow or green layer appears, the passivate is too thick and therefore weak and friable.
Zinc - passivated
Preparation of plastics, composites and rubber
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
Acetal (also known as polyoxymethylene or POM)
Note: If using cyanoacrylate adhesive, use only alcohol for pretreatment
Cellulose - acetate/butyrate/nitrate
Note: Elevated temperature will increase cure rate
Alkyl phthalate (polyalkyl phthalate)
Epoxy (and composites)
Note: Check for presence of mould release agent. Take care to avoid stress cracking
Polyester - thermoset, including composites
Note: Check for presence of mould release agent within structure
Polyester - thermoplastic
Polymethylmethacrylate (PMMA or 'perspex')
Does not require surface pre-treatment
Does not require surface pre-treatment
Use TetraEtch (an Albright & Wilson product)
Note: See polyethylene
Does not require surface pre-treatment
Polyurethane foam (PU)
Does not require surface pre-treatment
Note: If using a cyanoacrylate, use alcohol only to degrease
Note: Use silicone-based adhesives
Cellulose (i.e. wood)
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)
Note: If limestone, dolomite or carbonate aggregates present, acid etch surface after sand-blasting. Wash thoroughly. Alternatively, use dilute sulphuric acid.
Ceramics (including ferrites, masonry and pottery)
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.
Note: Check solvent is compatible if synthetic fabric
Friction materials (brake pads and linings)
Glass (and quartz)
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.
Paint - cataphoretic
Paint - epoxy powder