Winter 2009: Society for Adhesion and Adhesives in conjunction with BASA, One Day Symposium on 'Future directions for Adhesives and Sealants'
Society for Adhesion and Adhesives/British Adhesives and Sealants Association
One day joint meeting on ‘Future directions for Adhesives and Sealants’, held at the Society of the Chemical Industry 15, Belgrave Square, London, on 10th December 2009
This was a combined meeting with the British Adhesives and Sealants Association. We always find this contact useful as it brings Academia and Industry together. It gives industry the opportunity to present current problems and Academia the chance to guide research to meet real industrial needs and to present progress in various fields.
The first paper was entitled ‘Adhesively bonded truss structures: technology for reducing weight in future transport and infrastructure applications’ and was given by James Broughton from Oxford-Brookes University. James described a range of examples where weight saving was needed, such as Military vehicles that need lighter structures to increase payloads and reduce fuel consumption and also need lighter armour for protection. He described a lightweight bridge over the River Leir, weight saving in aircraft wing ribs and vehicle chassis components. He went on to mention 3D weaving processes, resin infusion methods and then some problems found in manufacture. These were surface preparation, air entrapment, sealing, crack propagation pathways and assembly problems for the node joints required for truss structures and space frames. He concluded that "material" solutions offer potential for composite nodes to compete with metallic ones. He also mentioned the important need for repair procedures to deal with various levels of damage.
The second paper was given by David Johnson from Wessex Resins and Adhesives Ltd. He described ‘Modern epoxy adhesives in production boatbuilding’.
David began with the importance of preparing mouldings for bonding with great care. They use a sequence of solvent wash and careful abrasion followed by another solvent wash. For repair work a drying procedure before commencing work would be helpful. He said that the choice of adhesive for each task was driven by the performance needed, the material to be bonded, the gel time to ensure that the adhesive flowed and did not start to cure until the assembly was completed, the cure time to meet production needs and last but not least, the ease of use of the adhesive chosen. The last issue is often found to be important in most areas of adhesive and resin usage. He said that their Pro-Set System 170 (Parts A and B) had proved very successful and a toothed spreader was used to define the adhesive thickness and also to provide airways. They also use a wrinkled plastic sheet as a vacuum bag and this works very well and provides airways for extraction of air and volatiles.
Their primary task for epoxy resins is bonding teak decking panels to glass-fibre reinforced production boat mouldings and their own Wessex Resins have proved very successful for this purpose. It is no surprise that they use resins with a low water uptake.
Our third paper entitled ‘MS polymer developments for a changing future’ was given by Luc Peeters from Kaneka, Belgium NV.
This technology was developed to meet European Health and Safety Legislation and has helped to reduce VOC emissions. Luc mentioned the need to use "greener" materials and that some materials might be banned and therefore not available.
Methoxy silylated polymers, such as MS polymers, have long been known for their "green" properties and enable formulators to produce sealants and adhesives with a low impact on the environment. These polymers combine good workability and excellent mechanical properties with a low viscosity and enable low VOC and solvent and isocyanate free products to be developed for a wide range of applications. A recent development for roofing applications was described. This is for bonding bituminous roof sheeting.
The fourth paper, ‘Eastman 168 and Eastman DBT, as alternatives to ortho-phthalates for the adhesives and sealants markets’ was given by Martin Stimpson from Eastman Company UK Ltd.
This development was also undertaken to meet legislative requirements by the removal of ortho-phthalates from their materials.
Martin explained the need to remove ortho-phthalates from formulations and replace them with alternatives that will do the same job without the VOC or Eco effects that are now regarded as unacceptable. He showed that Eastman 168 and Eastman DBT [di-butyl phthalate] have been evaluated against a common range of plasticizers in a generic acrylic sealant formulation. His graph comparing mechanical properties showed that 168 and DBT gave good results. He said the newer generation of acrylic sealants are more plasticised than earlier versions and have movement capability of plus or minus 25%, which meets most requirements and minimises the likelihood of sealant cracking. This is a very detailed paper giving an excellent presentation of this work and is well worth reading.
The fifth paper was given by Andy Crocombe from Surrey University. It had the important title of ‘Designing durable adhesively-bonded structures’.
This has been a problem for years with aluminium alloy aircraft structures but, as adhesives become more generally used, the problem now extends to aluminium alloy and steel motor vehicle structures, composite aircraft structures, bridge reinforcements and many other applications. It also includes the repair of all these items. This project attempts to model the lifetime of these structures and the factors that cause degradation. These are fluctuating loads [fatigue], sustained loads [creep], environment [moisture and temperature variations] and degradation of the adhesive or adherend. It could also consider corrosion due to leaching of chemicals from the adhesive assisted by water uptake or other fluids. The effect may depend partly on the pH value of leached extracts and their effect on the adherend material.
This is a fascinating and complex project. It is almost certainly no surprise that adhesives used under water have the lowest water uptake, although this may also mean the highest chemical toxicity. Many epoxies, for example, have a water uptake at saturation of between 4 and 20 % but those used under water have an uptake of less than one percent. One piece of research I did showed that durability can be related to water uptake.
Andy has studied static loads and a constant ageing environment, fluctuating loads and a constant ageing environment and static loads and fluctuating ageing environment.
He says much work remains to be done and interfacial degradation has received limited attention. This is another excellent paper that deserves to be read in full.
The sixth paper, entitled ‘Future directions for electronic and photonic adhesives’, was given by Norman Stockham from The Welding Institute (TWI).
Norman spoke about the need for adhesives for printed circuits to be pure and non-conductive in some cases. This could also apply to the durability problem mentioned above. I found it a matter of interest and also concern some years ago that an American specification for epoxy adhesives for printed circuits required very low amounts of chlorine, potassium, sodium and ammonia in adhesives for electronics [quite justifiably].
No such requirement is found in specifications for adhesives used on aircraft for the prevention of corrosion in aluminium alloys when bonding aircraft structures. This could well be due to the cost of purification but the cost of repair is also high.
See paper ‘New epoxy adhesives for compliance with MIL-A-87172’ by Justin C. Bolger, and Charles T. Mooney, Amicon Polymer Division and W.R. Grace. SAMPE National Technical Conference, October 22-24, 1985.
Norman also mentioned that because such tiny amounts are used, although millions of items are made, the total consumption is not very large. The added value is nevertheless high. These adhesives need to be designed to suit their end uses especially regarding electrical conductivity or insulation and thermal expansion to match the surfaces being bonded. Some need to be optically transparent. Norman also mentioned the need for low ionic impurities. Maximum service temperature capability is very important for some applications and thermal conductivity in others. This paper gives pictures, detailed illustrations and tables of requirements for different applications and really needs to be read to appreciate all the requirements for each of the many end uses. Rapid cure is necessary in a mass production environment.
Usage of adhesives in electronics is steadily increasing.
The seventh and final paper was given by Steve Shaw from DSTL. His title was ‘Application of adhesives for extreme environments’.
Steve is a regular contributor to SAA meetings. His paper detailed a range of topics that can affect the performance and durability of adhesives. These included extremes of temperature, water, and other fluids that may be encountered such as fuel, icing inhibitors, hydraulic fluids etc. He also discussed creep, fatigue and impact loading, which can be combined with elevated temperatures. Steve then discussed short-term high-temperature effects and embrittling effects due to raising the Tg to improve high-temperature resistance. He also mentioned attempts to use brittle and tough adhesives in combination.
Steve went on to discuss high-temperature effects in the long term and resulting polymer degradation. He said great efforts were being made to improve high–temperature performance but processing was difficult.
Steve then discussed degradation of bond lines due to water and surface treatments to enhance interfacial stability and moisture resistance of adhesives. He concluded by saying that fatigue loading under hot/wet conditions was a real "killer" of adhesives. His paper illustrated a vast amount of detailed work in this area.
Another interesting day and a good Christmas Lunch!