Spring 2008: Society for Adhesion and Adhesives, One Day Symposium on Bio- and Nano-inspired Adhesion
Society for Adhesion and Adhesives, One Day Symposium on Bio- and Nano-inspired Adhesion, held at the Society of the Chemical Industry, 15, Belgrave Square, London, 10th April 2008.
This meeting was the first one held by SAA to discuss these topics and we were fortunate to have so many excellent speakers. We often find we can learn a lot from nature and this meeting certainly confirmed that experience. Attendees will certainly remember one paper when they make their next visit to their dentist!
Stanislav Gorb from the Max –Planck Institute in Stuttgart gave the first paper, entitled: Insects did it first: a micropatterned adhesive tape for robotic applications.
He presented some excellent pictures and explained how insects manage to climb walls and walk on ceilings. One problem is how to copy insects in making bonds and then breaking them as required. Another is that these bonds need to be able to be broken and re-made thousands of times.
He quoted flies, spiders and geckos as examples and went on to describe a biomimetic mushroom–shaped fibrillar adhesive microstructure that he tested and compared with a control flat surface of the same material. Results revealed that the pull-off force and peel strength of the structured specimens was more than twice that of a flat specimen of the same material. In contrast to the control system, the structured one was found to be very tolerant to contamination and was able to recover its adhesive properties after being washed in a soap solution.
He stated that useful progress was being made and that, based on the geometrical principles found by studying insects and their biological attachment devices, he considered that an industrial dry adhesive might be developed. This was a very interesting paper.
The second paper entitled: Switchable adhesion between oppositely charged poly electrolytes, was given by Mark Geoghegan from the University of Sheffield.
Mark started by saying that tuning the interactions between different surfaces is an important area of research with technological benefits in many applications. Polyelectrolytes are good candidates for this kind of study because changing the environmental pH can cause them to undergo conformational transitions which, if controlled, can be used in a wide range of applications in fields of nanotechnology such as, microfluidics, targeted drug delivery and controlled wetting and adhesion. Data was presented on the interaction between a polybase - poly[2-(dimethylamino) ethyl methacrylate] (PDMAEMA) - chemically grafted to planar silicon substrates by atom transfer radical polymerisation, and a hydrogel of poly(methacrylic acid) (PMAA).
He said that the interaction between PDMAEMA and PMAA is of great interest because it shows that the interaction between a positively charged polybase in contact with a negatively charged polyacid can be controlled simply by changing the external environment. In aqueous media there is a strong interaction and in acidic media the interaction is negligible.
A JKR–type experiment showed that the process could be both switchable and reversible. The adhesion can be switched off by lowering the pH to less than 1 and can be re-achieved by re-immersing the components in water at pH 6 and bringing them into contact. Mark said that hydrogen bonding was a function of pH.
Another very interesting paper.
The third paper was given by Jeff Sargent of BAE Systems, ATC-Sowerby, Filton, Bristol and was entitled: A practical approach to the development of a synthetic gecko tape.
He described the fabrication and assessment of the adhesion of bio-mimetic Gecko hair arrays. The objective of this work was to assess the performance of these materials and to characterise their behaviour with multiple attachment on smooth glass surfaces and on glossy and rough painted surfaces likely to be found on aircraft.
It was found that hair size, shape at the contact ends and material properties were important. As expected, contaminated surfaces gave reduced pull off loads. Bond strengths in air and in a vacuum chamber were measured and it was found that the mushroom-shaped PDMS (polydi-methylsiloxane) structures showed adhesion on glass with a contribution from both molecular and a “suction-pad” atmospheric component.
This material showed super-hydrophobic properties and simple cleaning experiments showed that washing with water after contamination with dust and foreign hairs could restore a proportion of the strength. Jeff considered that further optimization of the present mushroom–shaped structures and also the time and strength–dependent material properties could be improved.
Paper four was entitled: Adhesion in dentistry-why and how, and was given by Ric van Noort from the Department of Adult Dental Care at the University of Sheffield. This was a very practical and interesting paper as we all visit dentists from time to time.
Ric began by explaining the immense cost to the NHS of replacing dental fillings. The old dental amalgam, a mixture of mercury and silver/tin alloy has a good track record for durability but does not form an adhesive bond to teeth. It does not meet EEC safety regulations and can only be used because of a “grandfather” clause that allows existing materials to be used while acceptable new ones are found.
The lack of a good adhesive bond means that filling retention can only be achieved by cutting away more ‘good tooth’ material than would be required to make a bonded repair. It also means that bacteria can enter the tooth at the edges of the repair because no adhesive bond exists.
He discussed some of the adhesives now available and the benefits of using them. Ric also mentioned that training is required in the use of these adhesives because, like metal bonding, a surface preparation and a coupling agent are required for maximum durability. This would also need to include correct mixing, storage and shelf life recording; dentists require some engineering training in the use of adhesives.
He concluded by saying that properly bonded fillings last much longer and could save the NHS vast sums of money each year. By matching the colour of fillings to teeth a much better cosmetic result is also obtained.
Paper five was given by Nick Aldred from Newcastle University and was entitled: Current understanding of bio-inspired adhesives.
This was another good paper discussing attempts to mimic nature when trying to develop new adhesives. He said that although barnacles have been studied for decades, we are only now beginning to understand the processes involved. Attempts to use Blue Mussel glue failed commercially and research has changed to studying the whole system and not just the glue.
So many pictures were shown and the topic is so complex that summarising this paper is difficult. If particularly interested please contact the author for the full paper.
Paper six was entitled: AFM studies of adhesion–related phenomena: Approaches other than the measurement of force–distance curves, and was given by Martin Munz from the National Physical Laboratory, Teddington.
He began by stating that AFM (Atomic Force Microscopy) can be used for studies other than force–distance curves. He also discussed LFM (Lateral Force Microscopy) and nanoindentation measurements.
Martin then went on to talk about the interphase region in an adhesive joint and the use of nanoindentation measurements to check the Young’s Modulus and hardness of the cured adhesive or resin: from the surface being bonded to the bulk of the adhesive.
The system investigated was a sandwich-like sample consisting of a thermoplastic and an amine-cured epoxy. The epoxy resin and the amine curing agent were diglycidylether of Bisphenol A (DGEBA, DER 332 type) and a 4,4’-diaminodiphenylsulfone (DDS), respectively. The thermoplastic adherend was poly(vinylpyrrolidone) (PVP) and the interphasial property variations were found to extend over a comparatively wide range of up to ~235 microns.
It is necessary to see the paper and graphs to appreciate the detail of this work. It certainly confirms that there is an interphase region. He concluded by saying that the resulting knowledge on the spatial variations in the amine–epoxy concentration ratio as well as on the concentration-modulus relationship gives us an idea of the polymer physics governing the observed variations in modulus.
The final paper was given by Ambrose Taylor from Imperial College London and was entitled: Nano-particle modification of epoxy adhesives for increased toughness.
Ambrose began by defining Nanopox as silica nanoparticles in epoxy resin. Rubber particles have long been used to toughen resins and some rigid particles have been shown to be beneficial but their size has been a concern. Resin infusion processes are becoming more common in the manufacture of composite components and for these it is essential that the particles in a resin are small enough to pass through the weave of the fabrics.
The formation of “hybrid-toughened” epoxy polymers, by combining both rubber toughening and silica nanoparticles has been shown to give a synergistic toughening effect. The microstructure and properties of bulk materials, plus the results for fracture tests on glass-fibre reinforced polymer (GFRP) composites produced by a resin infusion under flexible tooling (RIFT) process were reported. These showed that using both silica particles and rubber particles in the resin was beneficial.
This was a good meeting introducing many to new concepts. In most cases it is recommended that the speakers be contacted if you have a direct interest in their topics as it is not possible to summarise many of them adequately. It was clear that adhesives are used in many areas and the subject is of interest to many disciplines other than engineering. Dentistry and Marine biology in particular gave new insights into other people’s problems and opportunities.
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