Adhesives have been around for a long time. The ancient Egyptians used natural substances to attach veneers to furniture. Now we have many synthetic resins and polymers at our disposal which offer engineers and designers clear advantages over mechanical forms of connecting.
Perceptions over the reliability of using adhesives in industry are still mixed. Adhesive bonds can be extremely robust, reliable and aesthetically pleasing form of bonding, but it is important to realise that in order to get the best performance from an adhesive bond, a component must ideally be designed for bonding, rather than attempting to bond a design originally intended for mechanical fixing.
Key benefits of adhesive bonding often translate into economic advantages resulting from improved design, easier assembly, lighter weights and longer life in service. Consideration should be given to specifics such as the ability to avoid the local stress concentrations present in spot welded or mechanically fastened joints. Fatigue cracks are less likely to occur and, under load, with a more uniform distribution of stress, properly designed structures will offer a longer life. The weight of a structure can often be decreased by using adhesives whilst still retaining the required stiffness. Where aesthetics are important, the use of adhesives often eliminates unsightly mechanical fixings.
Adhesives will bond dissimilar materials including those which differ in composition, tensile modulii or thickness. Adhesive bonding often simplifies assembly by replacing several mechanical fastenings with just one single bond or by joining several components in one operation. Complex assemblies such as composite sandwich structures are feasible with adhesives.
Golf club assembly is a typical example of the benefits of adhesives, bonding metal alloy heads to CRP shafts. Here, welding would be impossible because of the dissimilar substances; rivets would weaken the shaft and the durability would be drastically reduced by a screw head at the end of the shaft. From an aesthetics point of view, a smooth bond line is preferable to an ugly mechanical fixing.
Additional benefits of adhesive bonding include the reduced likelihood of corrosion delivered by a continuous bond line, coupled with its ability to act as an electrically insulating barrier, or sound or vibration damper.
Admittedly, resin based adhesives do have some limitations. Depending on the adhesive, bond strength will normally decrease between 70 degrees C and 200 degrees C. However there are some formulations which are designed for use in very high temperature applications such as brake and clutch bonding – up to 300 degrees C for short periods.
There are four major groups of modern industrial adhesives used for load bearing applications: epoxies, polyurethanes, acrylics and phenolics. There are many other types of adhesives including rubber based products, PVAs and so on, which are used for lighter duty applications.
Epoxies are one of the most widely used types of adhesive. Consisting of an epoxy resin plus a hardener, formulated as one or two component adhesives, the types and ratios of these can be varied to produce a huge range of properties. Room temperature curing, high temperature curing, flexibility, rigidity, clarity (for glass bonding), chemical resistance, creep resistance and high peel resistance are just a few of the characteristics achievable from epoxy adhesives. They have the ability to form an extremely durable bond with most materials and can be supplied as liquids, highly thixotropic products capable of filling large gaps, or even as films.
Polyurethanes provide a more flexible bond and are useful for bonding of thermoplastics and GRP. They can be formulated to be fast or slow curing.
Toughened acrylics or methacrylates are two component fast curing adhesives which can typically achieve 75% bond strength in under an hour.
Specialist UV curing, one component acrylic adhesives are currently being specified in, for example, Digital Versatile Disc (DVD) production to bond the various combinations of aluminium, polycarbonate and gold layers.
Cyanoacrylic adhesives, one component products curing by contact with moisture on the bonded articles, form an important part of the DIY, light industrial and medical sectors.
Phenolics were the first adhesives developed for bonding metals and now, as modified phenolics, are successfully used in metal to metal and metal to wood applications.
FACTORS FOR SUCCESS
Just as important as careful selection of the correct adhesive for the substrate is using the appropriate pre-treatment and correct manufacturing conditions. The life expectancy of the component and the environment in which it will be used should also be considered at the design stage.
The component dimensions and the joint design are obviously fixed elements, as are the adhesive properties if used correctly. However it is important that assembly operatives are trained to work correctly and consistently and to understand the implications of extended open times, foreshortened cure times, contamination following pre-treatment and such like.
Appropriate pre-treatment is critical to the success of a bonded assembly.
Processes may extend from simple removal of surface dust through to a carefully planned sequence. The many plastic and metal substrates used in manufacturing today are all prone to contamination by lubricants, release agents, and plasticisers, during their own manufacture. These contaminants will all need to be removed before successful bonding can be achieved.
Degreasing will generally be the first stage in pre-treatment. Then, depending on the substrate, degreasing may be followed by abrading and the subsequent removal of loose particles. Chemical or electrolytic pre-treatment may be required to obtain maximum strength or long term resistance to deterioration and the correct acid treatment will depend upon the metal substrate. Phosphoric or chromic acid anodising, which deposits an oxide layer on the substrate which is ideal for bonding, is a method of pre-treatment exploited extensively by the aerospace industry for aluminium and titanium alloys.
Priming is often used as part of the pre-treatment process and there are also modern pre-treatments, such as low pressure plasma systems.
It is evident that new manufacturing disciplines and workshop practices may need to be established – and tightly controlled – to achieve the economic and performance benefits of a bonded assembly. However benefits obtained often far outweigh the time and costs incurred by change. Designers can instigate these changes – and be applauded for the results – but must be aware of all the implications at the outset.