Preparing for adhesive bonding

TWI Bulletin, May/June 1994

David Saunders studied Polymer Science and Engineering at London University, and went on to research structure-property relationships of toughened plastics at the Manchester Materials Science Centre. Obtaining a research fellowship from the Royal Society, he carried out research on mechanical and material characterisation of thermoplastic modified epoxy resins at HIT Japan. Prior to joining TWI in 1993, David worked at the National Physical Laboratory, Teddington and carried out research on structural adhesives. His current research interests include the structure-property relationships of adhesively bonded structures.

Adhesive bonding is now recognised as an alternative method of joining engineering materials alongside other well-established joining technologies like riveting, bolting, soldering and welding. David Saunders looks at some of the surface pre-treatment methods needed to ensure the best performance and durability from an adhesive join.

Synthetic adhesives have been used successfully, for many years in diverse sectors of industry giving rise to aerospace, automobile, biomedical/dental, construction, electronic, marine, sports and leisure applications.

Although in a few applications no surface pre-treatment is necessary (the automobile 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 optimum surface preparation and the economics of component manufacture.

Surface pre-treatment aims 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 grease!
  
  
• 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 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 adherend for up to several months before bonding.
  
  
• To produce a specific adherend surface topography, thereby altering the surface profile, possibly increasing the bondable surface area.
  

Pre-treatment techniques

In general, types of surface pre-treatment can be divided into three major categories:

• Mechanical;
• Chemical;
• Energetic.

Each of the above 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

Mechanical

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

Mechanical abrasion will remove the weak boundary layers. It will also change the surface topography of the adherend, increasing the bondable surface area on a microscale. Furthermore, it is believed that in some circumstances, 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 was performed manually or mechanically. It is necessary to monitor carefully the implements for abrasion; since if the process is carried out for too long surface debris initially removed is re-deposited. For composites, it is important to note that mechanical abrasion may cause fibre damage and impair the performance of the joint.

Blasting is another form of mechanical abrasion and includes alumina grit blasting, cryoblast and sodablast. The latter two are used for preparing composite adherends. Cryoblasting 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 shotblasting are shot 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

There are several different chemical methods by which adherends can be cleaned and prepared for bonding 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 a 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.

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 not readily applicable to polymeric materials, since the majority of solvents are organic based and may attack, be absorbed by, or plasticise the adherend.

Detergents dissolved in water, alkaline or acidic solutions and used at temperatures of about 50-70°C may also be used to supplement the 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, nitric acid for copper and an alkaline peroxide 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 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. Anodising is a type of electrolysis where the adherend is the anode, 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.l. 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.

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 such 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 primer can protect the adherend for several months before bonding.

The most commonly encountered primers are silicones which have the general formula R(CH ₂)n SiX ₃ where R is an organofunctional group and selected for its reactivity with a given adhesive and X is a hydrolyzed group on the silicon which can form a bond with the adherend n is in the range n=0 to n+3.

Primers have been successfully used on aluminium and titanium alloys, copper, steels and silica. A silicone primer, j-aminopropyltriethoxysilicone has been used to aid bonding of alumina plates with a novel hot melt polyethylene adhesive.

Energetic

Energetic surface pre-treatments which have been reported in the literature include corona discharge, plasma, flame 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 carbonyl groups, brought about by oxidation of the adherend surface. This can lead to the adherend being readily wetted by the adhesive;
• Polymerisation and grafting on to the adherend surface - the plasma can result in polymerisation of constitutents of the gas, thus forming a thin layer on the adherend surface. This will alter the surface characteristics of the adherend which may make it more receptive to the adhesive.

Table 2 illustrates how, as a result of plasma treatment, the improvements in lap-shear joint strengths of some polymeric adherends can be realised. It is important to note that the data given in Table 2 can only be compared between the control and plasma-treated for a given material.

Table 2 Typical examples of lap-shear bonding improvement after various plasma treatments

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=0, -OH, -NO ₂, -NO ₃ and -NH ₂. 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.

Excimer lasers have been used to produce a wide band of ultraviolet radiation. When exposed to a polymeric adherend the material will absorb the ultraviolet radiation to a depth of 1µm below the surface, causing rapid bond scission. The dissociated material is then ejected at high speed, dissipating any heat generated. This technique has been successfully applied to polycarbonate and polyetherimide. When it was applied to composite surfaces, the excimer laser radiation was found to etch the organic matrix preferentially. Variables of this process include energy density, laser wavelength and number of laser pulses.

Finally, Fig.2 illustrates the effect that some of the surface pretreatments above have on lap shear 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.

Conclusions

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

• Enhanced mechanical performance of the joint;
• Improved joint durability in aggressive environments;
• Increased service life of component;
• Bonding of difficult adherends, for example, polyolefins and polytetrafluroethylene.
  

Bibliography