One Day Symposium on Avoiding Failure with Adhesives and Sealants, held on 7th December 2006, at the Society of Chemical Industry, 15, Belgrave Square, London
This meeting brought together a significant number of people from The British Adhesives and Sealants Association and the Society for Adhesion and Adhesives. Many attendees were from the Construction Industry, although Automobiles and Aerospace were also represented. Sealants and paint, like structural adhesives, also require good surface preparation, correct application and the right choice of materials for each specific purpose.
The first paper entitled: ‘Ensuring correct application through training’ was given by Philip Bell of All Mastic Ltd.
This was an excellent paper and it was encouraging to know that the Construction Industry Training Board (CITB) is supporting Apprenticeships, for any one over the age of 18, to encompass 2 year courses leading to Level 2 and Level 3 NVQ’s. These cover: sealants and their application, Health and Safety, First Aid at work and the handling of access equipment that is often needed to get to the location where sealant needs to be applied. This training includes a good balance of direct teaching and mentored practical application. Philip said that 90% of all sealant failures have been found to be due to bad application, incorrect material specification or bad joint design. Training aims to get the job “Right first time”. Many other industries could usefully copy this sound approach.
The second paper: ‘Surface Treatment to Avoid Failure’ was presented by Professor Tony Kinloch of Imperial College, London.
In his excellent presentation, he reminded us that most adhesive bond failures are at the interface and that the surface free energy of the surfaces being bonded needs to be higher, preferably much higher, than that of the adhesive used to join them. Thermoplastic surfaces were first mentioned; here there was a need to use specialized surface treatments to raise their surface energy to a level high enough to ensure a good bond. This is necessary because their natural surface energy is lower than that of the epoxy and acrylic adhesives that we often wish to use to join them. Tony presented a graph of fracture energy versus the time of corona discharge treatment given, which showed a linear improvement until a peak was reached after which further treatment gave little improvement. He then discussed the durability of bonded metal joints and quoted a case where the aluminium oxide layer had failed because a suitable primer had not been used to protect the oxide. The use of an epoxy-phenolic primer containing a silane was shown to give a great improvement in durability. Durable bonding, like many other activities, requires great attention to detail.
The third paper: ‘Quality Assured Metering of Adhesives and Sealants’ was given by Laurie Penn of Failsafe International. This paper showed the great detail in which the metering of sealants and adhesives has been studied. His Puls Meter system mixes small amounts of resin and hardener in very rapid pulses: like tiny machine gun bullets. These are accurately measured and this system is claimed to be the most reliable mixing system for two-part sealants, adhesives and resins, so far developed. It inevitably relies on the products themselves being uniformly made and supplied before entry into the Puls Meter system itself. This mixing system first ensures the correct mix ratio of the two materials and then thorough mixing before application to the bonding surface. Experience has shown that the older method of weighing the two parts and then mixing them thoroughly by hand requires accurate weighing and then at least three minutes of careful mixing to ensure a good product. The Puls Meter system is now in use in a wide range of industries with great success. It prints out details of the process thus providing an audit trail and has a failsafe shutdown system. It has been proved in use on aircraft composite panels and in structural glazing and door panels.
The fourth paper, and the last of the morning session, was entitled: ‘Stress and Strain Limits’ and was presented by Bob Adams of Bristol University.
This paper discussed a considerable amount of testing of sealants. Bob reported work to obtain basic properties of sealants and adhesives: such as strain to failure, stress at failure and elastic modulus. Sealants must bond to the surfaces to be sealed and this will require the right treatment and they must also cope with any movement of structures and components in service. The 180° peel test was discussed and also curves of peel force versus sealant thickness. Peel energy and rate effects on peel test results were also mentioned. A strain to failure of ten percent or more was recommended for adhesives but 300% or more was recommended for sealants. Strength is important, 40+ MPa for adhesives but 2+ MPa for sealants. High adhesion strength is always good for both adhesives and sealants. A very useful paper; contact Bob if it could be helpful to you.
The fifth paper: ‘Why Floors Fail’ was given by Bob Mabbutt of Laybond Products Ltd.
He stated that very few floors do fail and this he attributed to excellent codes of practice, good technical support, the use of site tolerant products and over-engineered designs. He went on to discuss the shrinkage of cement products, thermal expansion and moisture effects, levelling compounds and floor coverings and the need for compatibility in thermal expansion and moisture absorption. Various test methods were also described. All in all, this was a very detailed study of this subject. The full paper is well worth reading.
The sixth paper 6: ‘Test methods and Standards’ was given by Bill Broughton of NPL.
Bill stated that most of the commonly used test methods are incapable of providing reliable engineering data because the test geometry induces a complex state of stress in the adhesive layer, thus invalidating the test results. He described ISO test methods for tensile strength, compression strength and shear strength of adhesives. These enable strength, stiffness, Poisson’s Ratio and strain-to-failure to be measured. The plate twist method of obtaining shear modulus was also discussed. Casting bulk adhesive specimens has its problems: exothermic reactions, porosity, residual thermal stresses and surface scratches. He also mentioned fracture toughness tests in some detail (DCB and Boeing wedge tests), the importance of crack propagation resistance and the thick adherend shear test. Tensile butt joints were also discussed.
This paper gave a good coverage of the many test methods available to assess adhesives and adhesive joints for engineering uses. See also the Adhesive Design Toolkit website: (http://www.adhesivetoolkit.com)
The seventh and final paper of the day was entitled: ‘A Catalogue of Disasters’ and was given by John Comyn of Loughborough University in his usual entertaining style.
Much to everyone’s surprise he said that most of the failures he investigated, over many years, were due to the wrong choice of adhesive for the purpose rather than poor surface preparation, which is a common cause of failure.
The first case quoted was automotive headlights. In this case lamps failed by steaming up. This was due to polyethylene glycol as a plasticiser in the adhesive. Although it dissolves in the monomer, it separates out from the cured adhesive.
The second case was the corrosion of aluminium foil bonded to wooden houses with a solvent–based polychloroprene adhesive. BHT anti-oxidant evaporates with the solvent. The polymer degrades with release of chloride ions which corrode aluminium.
A third case was due to thermal mismatch of the adherends being bonded. Yet another failure was due to premature cure when open time and room temperature were not controlled before mating the parts. Other failures were also discussed and the paper is good reading for anyone investigating adhesive bond failures; especially in the field of double glazing.
This was a useful day as many of the topics were new to those of us not familiar with industrial uses of adhesives and sealants. An excellent Christmas lunch was provided. The amount of work done on mechanical properties of sealants was impressive. It would be very helpful to the progress of adhesives in industry if the example of the Construction Industry in providing training and apprenticeships could extend to other fields of work.