Winter 2004: Society for Adhesion and Adhesives One Day meeting on 'Silane Coupling Agents, Fundamentals and Industrial Applications'
One day meeting on "Silane Coupling Agents, Fundamentals and Industrial Applications", held at the Society of the Chemical Industry, 15, Belgrave Square, London, on 8th December 2004
This one-day meeting was very well attended and showed a considerable level of interest in this topic. The range of papers summarised below show useful progress in this field. It was very encouraging to find that silane surface treatments for aluminium alloys that are slightly better than phosphoric acid anodising in terms of strength and long-term durability can be produced. These will be of particular interest for repair work and also for production line use.
The first paper was entitled ‘Silanes and their main applications’, and was presented by Jarek Monkiewicz of Degussa.
He explained that silanes have been used in many industrial applications for over 50 years. The basic large-scale products are tetrachlorosilane and trichlorosilane. Chlorosilanes serve as raw materials for the production of optical fibres, fumed silica and many electronic materials. Trichlorosilane is use as a raw material for the manufacture of a range of organofunctional silanes. These are mainly used as adhesion promoters between inorganic materials and polymers, as surface modifiers of inorganic surfaces and mineral fillers, as cross-linking agents in moisture curing processes, and as co-monomers and co-binders in coatings and sol/gel formulations. In practice, organofunctional silanes are applied in aqueous and /or solvent solution as primers or neat as additives. Silanes form "molecular bridges" to create stable, water and chemical-resistant bonds between two dissimilar materials. The hydrolysis of silicon functional groups allows bonding to the inorganic surface by condensation and reaction with the organic matrix via the organofunctional group. The hydrolysis of silanes is strongly affected by their structure, pH, type of catalyst, temperature, solvent and other factors. Special silanes are used for anti-graffiti treatments for buildings and others are used in sealants and pigments.
The second paper entitled, ‘An overview of the International Collaborative Programme on Organosilane Coupling Agents’ was presented by Prof. Steve Shaw.
This project involved Australia, Canada, France, New Zealand, UK and USA and was very successful. The objective was to find a silane surface treatment for aluminium alloys that was as good as or better than Chromic Acid Anodising. In the event the best silane treatment was found to be slightly superior to Phosphoric Acid Anodising.
This was a very good result and should be useful for aircraft repairs and manufacture. Wedge testing was used initially and when an optimum process was found further testing in lap shear, double lap shear, creep and fatigue was done to meet the needs of the aircraft industry. The variables studied were, nature of the solvent, solution pH, silane concentration, hydrolysis time, film drying temperature and time between application and bonding. The process must be followed precisely and training of staff is necessary to ensure that correct procedures are followed.
The third paper by Prof. Tony Kinloch of Imperial College London had the title: ‘Test Methods and Novel Silanes for Assessing and Increasing Joint Durability’.
Attack of water on bonded joints was stated to be the topic of interest. The methods of attack in humid and aqueous environments were studied. Tests were made on Aluminium Alloy and Steel joints and some hybrids where one side was steel and the other aluminium. Tests were run at 25° to 30°C to avoid unrealistic mechanisms of failure. DCB tests were used and were given a grit blast and degrease before application of silane GPS. The mixed joints of aluminium alloy and steel always failed more quickly and at lower loads than the steel alone. It was considered that a corrosion process at the crack tip was the cause of this. The silane treatment gave chemical bonds which were more durable than the interfacial secondary bonds produced by grit blast and degreasing. This is a very useful paper worth reading in full. Further work on these lines is in progress.
Paper 4, entitled ‘The Interfacial Chemistry of Adhesion: Surface Analysis and Organosilanes’ was by John Watts of Surrey University.
In this paper it was stated that both the strength of bonds and the number of bonds in a given surface area are important. As in the previous paper covalent bonds were found to be more durable because they are not displaced by water like intermolecular bonds. XPS has been found to be both reliable and useful. ToF-SIMS gives semi-quantitative molecular data. Adsorption isotherms were used to define the adsorption type: i.e. chemisorption or physisorption. The Si-O-Al+ peak is indicative of a covalent bond between aluminium oxide and the organosilane adhesion promoter.This technique was used to study silane treatments. Please contact John if you need more detail.
Paper 5 was also from Surrey University and was given by Marie-Laure Abel. It was entitled, ‘The Incorporation of Silanes within Formulation: Effect on Durability and Interfacial Chemistry’.
Previous papers have considered the treatment of surfaces prior to bonding whereas this paper discusses the incorporation of silanes into adhesive formulations as an alternative method of use. They may be added to adhesives, paints and coatings, partly to avoid the need to apply a primer.
This paper also used the wedge test, to ASTM-D 3762, as the test method to assess the value of adding silanes to an adhesive. Durability results indicate an improvement upon incorporation of an epoxy type silane. GPS was used in this case and APS was used for tests with a polyamide coating. Again it was found that silanes result in the formation of covalent bonds which are more durable.
Paper 6 was given by Francois de Buyl of Dow Corning. This paper had the title, ‘The Importance of Diffusion and Interface Enrichment in Moisture-Cure Silicone Adhesives’.
The full paper should be read by those with a special interest in this topic. When an alkoxy silicone sealant is exposed to ambient air, a surface skin is formed, which becomes thicker with time. At a constant temperature and humidity, the thickness (cure depth) is initially proportional to the square root of time, but later the gradient of such a plot increases. There are thus two regions if cure, an outer region and an inner one. Swelling in toluene and functionality testing, which has been measured for samples taken at various depths, shows that the crosslink density is greater in the outer regions. Single lap joints in aluminium or glass have been cured in these two regions and joints from the outer region are always stronger than those from the inner region. Young's Modulus is always greater in the outer regions. These behaviours are due to the mobility of the low molecular weight silane and titanium ester species used as crosslinking agents and adhesion promoters.
During the initial stages of the cure they migrate into the outer region of the joint, but once cure has passed into the inner region there is now a paucity of these compounds. It was also shown that the extent of silane migration was influenced by their initial concentration in the system.
Paper 7, entitled ‘Silanes as a Pretreatment of Metals Prior to Painting’, was the final paper of the day and was given by Mr Heitman of Chemetall, GmbH, Frankfurt.
The author explained that silanes are useful as a pretreatment for metals but as with most processes optimisation is required to achieve the best cost/benefit/reliability result. It is first necessary to select the most suitable silane then hydrolysis has to be considered and a stable concentrate has to be developed. After this the best surface treatment must be selected and then the coating formation process. The coating thickness must be decided and the bath concentration defined. They have the existing OXSILAN product range and new developments are being studied.
Silanes are much safer in use than some existing methods and this gives them an advantage.
This was a very successful meeting and a useful update on the progress of silane technology.