Fundamentals of Adhesion and Adhesives symposium report
A full report on the Society for Adhesion and Adhesives/BASA one-day symposium on Fundamentals of Adhesion and Adhesives held at the Society of Chemical Industry in London on 15 April 2010
This meeting was a combined meeting organised jointly by the Society for Adhesion and Adhesives and the British Adhesives and Sealants Association. The attendance was good and the meeting covered a vast range of topics.
Our first paper was given by Barrie Hayes, a Consultant who, amongst other tasks, lectures on adhesives for Henkel-Loctite. Barrie has been working with adhesives for a very long time and covered a wide field of topics. He began by saying that one great advantage is that dissimilar materials can be joined more easily by this method than any other. He then described hot-melts, solvent-based adhesives and thermosets.
The first application covered was the bonding of boat decks of moderate size using acrylics of the resin and activator type. He went on to discuss the bonding of thermal insulation to the hulls of Liquid Natural Gas tankers. He then discussed the assembly of skis, loudspeakers and car body structures. These led to an explanation of the need for a wide range of adhesives. Some need to cure quickly on assembly lines and others need to cure slowly to allow time for the assembly of large parts. Compatibility with the substrate and the use of corrosion inhibitors was also discussed together with the need to cope with the full temperature range in service.
Adhesives for application and use under water are a very special type.
Body armour, wind turbine blades and medical uses for joining flesh wounds indicate the variety of problems considered. When choosing an adhesive for a particular task it isvery much a matter of "horses for courses". Adhesives can be very useful but it is essential to find the right one for each task.
The second paper, entitled ‘Fundamentals of Adhesion;was prepared by Derek Brewis of Loughborough University but presented by John Comyn as Derek was unable to attend at short notice.
John began by describing adsorption, contact angles, kinetics of wetting and loci of failure in detail. He then discussed the theories of adhesion and discussed the Electrostatic Theory, the Diffusion Theory and the Mechanical Keying Theory. Factors affecting adhesion such as surface chemistry, rheology and mechanical properties of adhesives and the effects of temperature and humidity were also covered. He then mentioned adhesives that can absorb a limited amount of lubricant such as those used by the motor industry. John went on to discuss bonding conditions such as temperature, pressure and cure time and he concluded with considerable detail on treatments for plastics and metals to ensure good and durable bonds. The full printed paper should be read as a vast amount of material is covered.
The third paper on ‘Non-structural adhesives' was given by Jim Palmer, a Consultant. Jim showed that even within one group ofadhesive types it remains important to choose the right one for each job both on grounds of performance and cost. He discussed water-borne, solvent-borne and carrier-free types. Jim explained that non-structural adhesives are characterised more by their peel strength than their mechanical strength. He mentioned the importance of substrate compatibility and then continued to describe the formulation of some non-structural adhesives.
Jim mentioned approximate formulations of solvent-based types, water-based types and carrier-free hot-melts. He then discussed environmental issues and limits on the use of VOC's or volatile organic compounds. At least one of these, in general use, is much better in the solvent-based version than the water-based version.
Our fourth paper was given by Professor Steve Shaw of DSTL. He is a regular contributor to these meetings. His topic this time was Sealants. He gave a new definition of sealants: "A compound capable ofresisting passage of extraneous materials through to a substrate whilstaccommodating structural movement often in a potentially damaging environment."
He began by reminding us that pre-treatment of the surfaces to be sealed and the use of adhesion promoters is just as important when using sealants as it is when using adhesives. It is often forgotten that adhesives, sealants and paints all need to bond to the surfaces to which they are applied for long periods of time Steve pointed out that sealants have to be carefully formulated. They also need base polymers, fillers, curing agents,thixotropic agents, plasticizers, anti-oxidants, UV absorbers, pigments and adhesion promoters. He listed many types of base polymer and the need for sealants to resist penetration by water, fuel, hydraulic fluids, solvents and gases in addition to stress and temperature extremes. The design of sealants is a complex process requiring considerable development testing. Steve then discussed applications in building, motor manufacture and aerospace. Aircraft fuel tanks are one special application that require careful design of the structure and the sealant. He concluded with a description of several tests and some green issues such as the removal of asbestos and the elimination of toxic curing agents and surface treatment methods. He also mentioned the use of nanoparticles to improve sealants. There is much very useful material in the written paper, which should be read for full details.
The fifth paper was given by John Watts from Surrey University as Marie-Laure Abel was unable to come. This paper was entitled, ‘Theimportance of surface analysis'.
John's paper contained many superb slides and needs to be read fully. He explained the value of surface analysis for the characterisation of surfaces, failure analysis and for obtaining details of interfacial chemistry. It enables the study of the effect of minor additives and unwanted contamination. He also illustrated some miniature fracture mechanics tests on glass-fibre-polyester bonding. Of interest for aircraft and motor vehicle manufacture was a graph showing the relative performance of various surface treatment processes for the adhesive bonding of aluminium alloys. This showed the fracture energy, as measured by a wedge test,when using one hour of hydration followed by a silane treatment to give the highest performance. In order of merit then came phosphoric acid anodise, optimised silane, chromic acid anodise, followed by a much lower result for grit blast only. The temperature of test for these results was not given; please checkwith John. It will be interesting to see if this one hour hydration plus silane treatment is widely adopted. For a copy of this excellent paper contact John Watts at: email@example.com
The sixth paper entitled, Structural Adhesives, was given by John Bishopp of Star Adhesion. John has worked on structural adhesives for many years and gave an excellent outline of this important subject. He began by discussing the available chemical types in film form and the factors involved in formulating them. He went on to explain the advantages and disadvantages and reviewed some applications best suited to each type. John then discussed the various adherends most commonly joined with load bearing adhesives. These were aluminium alloys, steels titanium alloys, honeycomb cores and fibre-reinforced composites. He went on to discuss surface protection primers and corrosion inhibiting primers and the formulation of various types of adhesives. John gave considerable detail on formulation to meet the "horses for courses" requirements for many different applications including the need for good filleting characteristics when bonding to honeycomb. He then covered polyimides, bismaleimides and cyanate ester resins. John concluded with a wide range of practical applications and the whole paper needs to be read as it contains much useful material.
Our seventh paper, ‘The Durability of Structural Adhesive joints' was given by Professor Tony Kinloch of Imperial College, another regular contributor to these meetings. This is a very important issue for aircraft and other structures such as car bodies. The problem is that water can permeate into an adhesive joint and lead to loss of strength by weakening the adhesive bond to the substrate or by weakening the adhesive itself. High loads and high temperature may accelerate the effect of moisture. This is usually seen by failure in the joint changing from cohesive failure within the adhesive itself to disbonding from the substrate also known as apparent interfacial failure. He showed that a hot/wet test environment is far worse than a hot/dry one.
A graph was shown illustrating the superiority of phosphoric acid anodising over other methods. Tony then discussed the effect of surface energy of the substrate and the effects of hydration of the oxide layer. He went on to consider the effects of accelerated test methods and that they can be misleading. Double cantilever beam and Boeing wedge tests were then discussed. Graphs were shown illustrating rate of crack growth under different test conditions. He concluded by saying that aerospace chemical treatments are still the best. Coatings developed for aluminium alloys and steels for automotive use have proved successful.
Tony concluded by saying that predicting service life is progressing but while prediction of failure can be made if cohesive failure occurs, it is not yet possible to make a prediction if interfacial failure occurs. Once again the whole paper should be read as the diagrams are very useful.
The final paper, ‘Testing Adhesives-Breaking Modes-Modelling', was give by Professor Bob Adams from the University of Oxford. He previously worked for many years at Bristol University.
Bob presented many excellent diagrams so the whole paper should be studied and began by saying that failure can occur in several places in a bonded joint. It can occur in the substrate (very rarely),at the substrate surface, in an interphase where the adhesive contacts the surface of the adherends and in the adhesive itself. Failure can be affected by moisture, fatigue, creep or thermal effects. Surface analysis is helpful but strength also needs to be known. Mode 1 fracture mechanics, various lap joints and the thick adherend shear test have all been tried. Finite element analysis has also been used to show stress and strain patterns. Shear tests in torsionand various peel tests are also used.
The most scientific test is the double cantilever beam test but this is expensive; a hinged DCB specimen is cheaper version. The cheapest and easiest is the Boeing wedge test.
He concluded that most joints have complex stress situations and that in the laboratory the best test is the tapered double cantilever beam but in industry the best tests are the Boeing wedge test or thehinged DCB.
This was another intensive and very useful day that served as a useful update on progress for those who have been in adhesives work for a long time and a very good introduction for any newcomers.