Winter 2008: Society for Adhesion and Adhesives/BASA, one day Symposium on 'Additives for Adhesives and Sealants'

The Society for Adhesion & Adhesives
,
15 Dec 2008

One day joint-meeting on "Additives for Adhesives and Sealants", held at the Society of the Chemical Industry 15, Belgrave Square, London, on 4th December 2008. 

This was a combined meeting with the British Adhesives and Sealants Association (BASA) and covered a range of topics and introduced us to carbon nanotubes in some applications. There seem to be many papers on nano-everything at present and it was useful to have one application of interest to the SAA discussed. 

The first paper, entitled The use of engineered minerals to improve processing efficiency and modify rheology’ was given by David McCann of Imerys Performance Minerals. This was very interesting and showed how much effort goes into designing the required performance into sealants and adhesives. He discussed the use of Kaolin and Calcium Carbonate and left us in no doubt that there was much more involved in choosing the best additives than just reducing cost by filling with a cheaper material. 

He illustrated the talk with some excellent graphs and pictures and explained the importance of particle size, shape and surface characteristics and their use to provide the rheology required.  Techniques for measuring particle size and shape were also illustrated.   

David discussed two particular products in depth. These were Barrisurf® a new Kaolin product range developed by their Paper and Performance Minerals division for use in the paper and board packaging market and Imerseal®. Barrisurf allows water-based coatings to be used and to simplify recycling of packaging materials.  Grease resistance was also mentioned. He said that the shape factor of particles in the pre-coat was more significant than the total weight of particles incorporated. Imerseal is a surface treated Calcium Carbonate designed for applications where excess moisture has a detrimental effect on production processes. 

A most interesting paper: for further information contact David McCann. 

The second paper was given by Dr Tao Wang from Surrey University [deputising for Prof. JL Keddy]. This paper, entitled ‘The influence of molecular friction from nano–fillers on the properties of pressure–sensitive adhesives’, was also well illustrated and showed us that an impressive amount of work is going on to find useful applications for these materials. It was found that carbon nanotubes (CNT’s) could create nanocomposite pressure–sensitive adhesives by blending an aqueous suspension of CNT’s with waterborne polymer colloids i.e. latex. This water-based process ensured that volatile organic compounds (VOC’s) were not detrimentally released into the atmosphere. The process was also found to be effective in obtaining a uniform dispersion of the CNT’s in a polymer matrix. This is a complex paper that should be discussed with the authors by those with a specific interest.    

Paper three was given by Dr Joanne Lloyd of REACH (Registration, Evaluation and Authorisation of Chemicals). Jo reminded us that all companies making , importing or using chemicals need to Register and should have done so already by the date if this meeting. See   www.reachready.co.uk  or enquiries@reachready.co.uk.    

Manufacturers and importers have to register all substances and are required to provide safety information on all their materials. These MUST be supplied to downstream users so that they have all the information they need to manage the risks involved. Companies are required to work with their competitors to share risk information.   They are also required to seek the information they need from their suppliers so that safety and environmental requirements can be met.    

Jo reminded us that Registration is a legal requirement and non-compliance is an offence so if not already registered urgent action is required. Part of the purpose is to protect consumers as end users.    

See also the European Chemical Register website. This refers to the European Chemical Substances Information System, which is a part of the European Commission Joint Research Centre Institute for Health and Consumer Protection. 

The fourth paper, entitled ‘Migration of Additives’ was given by Dr Graham Lawson of the Health and Life Sciences Department at De Montfort University, Leicester. This paper resulted in a good discussion of the Liquid Chromatography/Mass Spec/Mass Spec technique (LC-MS/MS). It raised questions such as, “What is the classical approach to the determination of the migration level of a specified species into a given matrix?”   He also considered what happens if the migrant changes its chemical nature by reaction with the matrix or its components in the timescale of the test.

He said that Bisphenol A diglycidylether (BADGE) has recently been the subject of studies using equipment more frequently associated with “proteomics” to explain why conventional analysis of spiked samples, for example BADGE in tuna or apple puree, showed quantitative recoveries of less than 20%.   

The question then became “Where did the rest go?”   As is sometimes the case, we found more questions than answers.

This is another topic to discuss with the author if it is of special interest. The fifth paper: ‘Volatile Organic Compounds’ was given by Bob Mabbutt of Bostik Limited.  This is a very topical subject and Bob pointed out that there is growing interest in anything that may cause harm to humans or the environment. Europe is following the USA in legislating extensively on these issues. Paints Directive (Directive 2004/42/EC has been introduced to limit emissions from certain paints, varnishes and vehicle re-finishing products. 

Indoor and outdoor definitions of air quality are different and many issues were mentioned. Bob quoted several definitions of VOC’s and listed a large number of groups involved. He also pointed out the great advantages of using them for several important purposes to achieve the required product performance. 

Bob mentioned that as the attention turns to certification of conformity of the adhesive for the more demanding requirements of indoor air, the small fact that the VOC will be encapsulated shortly after application should not be overlooked. He also said that it is wrong to use the same harsh criteria where there is lower risk of harm whether that is for the components or the combination. 

There is no doubt the debate about their value and, where it is safe to use them and where not, will continue for some time. A problem is that they are very useful, difficult to replace, and many have already been taken off the market. This leaves very few remaining to fulfil necessary requirements. 

The sixth paper, entitled ‘Toughening of Structural Adhesives’ was given by Prof. Tony Kinloch of Imperial College, London. As usual, Tony gave a very interesting and well illustrated talk on this important topic. It was interesting to see that in the last few years these adhesives have been made much tougher than they used to be. This is partly to do with their use in cars and the need to meet impact requirements. 

Tony explained that highly cross-linked polymers are normally very brittle and that the incorporation of a second phase of dispersed rubbery particles can greatly increase their toughness without significantly impairing their other desirable properties. He mentioned that simple polymers have a fracture energy of about 100-300 J/m2 but that modern toughened adhesives now give 2,000-4,000 J/m2This is a very useful improvement. 

Tony said that the Airbus A.380 is now using adhesives with a fracture energy of 2,000 J/m2. These are hybrids, toughened anhydride cured epoxies with a CTBN and SiO2 mixture that shows a synergistic effect. The correct epoxy formulation must be used together with the nanoparticles.    

Tony concluded by saying, “Another very new and exciting route to toughening structural adhesives is the formation, in-situ, of nanoparticles of SiO2 in the adhesive to give a “hybrid” toughened adhesive containing both nano-SiO2 particles and micron-sized rubbery particles.  So, watch this space for even tougher structural adhesives becoming a commercial reality.  Much tougher epoxies are the result. 

The seventh and final paper was given by Prof. Steve Shaw of DSTL.   This was entitled, ‘Coupling agents as additives’. He said that “One of the primary factors for the under utilisation of adhesive bonding in engineering applications was the effect of water on bond performance and long-term durability”. This has always been a problem and has led to the need for low water uptake adhesives and very complex surface treatment methods.   

As mentioned earlier the safety in use of various chemicals is being more tightly controlled and the well known phosphoric acid and chromic acid anodising methods for aluminium alloys are considered toxic. Many methodologies have been tried to replace these with safer procedures but none have yet achieved the required performance with regard to long-term durability. 

Chromate-free anodising techniques, laser treatments and organo-silanes have all been tried. The last have given some good results but many test results presented showed that control of the process is important and difficult to ensure for field level repairs. Three critical factors were solution pH, which is critical for aluminium alloys and also depends on the alloy content, hydrolysis time of the silane after it is added to water and the influence of silane concentration. Controlling these at a repair station or for a field repair is likely to be a problem compared to an aircraft manufacturer’s test laboratory.    

The need is for simple, cheap and reliable surface preparations for all the metals likely to be used. This is a tall order and one likely to keep us busy for a long time.   

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