Adhesives are here to stay

Engineers are always on the lookout for better and more efficient ways of designing and producing new structures. The choice of the most suitable joining methods is as important as the design and choice of the composite materials themselves. Adhesive bonding allows for the most efficient use of composite substrates which can maximise their potential. From prehistoric times to the present, humans have always tried to assemble or develop new techniques to improve their tools and daily lives. Adhesives have been part of our lives ever since the days when men bonded their bone arrow points using materials such as asphalt. Adhesives will always be with us, but it's an ever-changing world as performance and environmental demands grow. Why adhesives? The advantages of using adhesives are well documented and numerous. Bonding allows the assembly of dissimilar materials and higher weight loads to be carried as well as improving the uniform distribution of the stresses and strains across bonded joints. Adhesives also maintain the integrity and strength of materials as there is no hole, rivet or fastening elements to weaken the structure. No bolts, nuts or screws are necessary and so there is no detraction from the beauty of the finished piece allowing wider design and aesthetic possibilities. There are three main types of structural adhesives, all of which have a different chemistry meaning that each adhesive type has a different structure and a different physical characteristic. The three main modern day adhesive categories are epoxy, polyurethane and methylmetacrylate systems: • Epoxy (EP) adhesives are generally known and used for their excellent adhesion to metals and rigid substrates such as thermoset plastics or composites. They will provide excellent durability, chemical and temperature resistance with low shrinkage
• Polyurethane (PU) adhesives are more suitable for thermoplastics, and together with their flexible properties are perfectly suited to softer materials
• Methyl Methacrylate (MMA) adhesives provide excellent adhesion onto metal, composites and thermoplastics. They are generally faster setting products and thus enable significant productivity gains New applications The load-bearing and high strength epoxy-based adhesives, together with the more flexible methacrylate adhesives with their excellent adhesion properties on various substrates, already cover a broad area of composite bonding applications. However, there are more and more applications in which genuine elasticity is required, such as for the Araldite Highly flexible composite material which received a JEC Asia Innovation Award in 2008. The silyl-terminated polymers of Araldite 2060 and Araldite 2061 have been developed to meet the requirements of this emerging technology. In contrast to the high strength epoxy and methacrylate-based systems, these one-component adhesives offer outstanding flexibility and elastic recovery combined with high adhesion properties due to their silane modification. These properties are combined with good ‘paintability', weather resistance and outdoor durability, as well as ease of application. The silyl-terminated polymers are free of any isocyanates and do not carry any hazard symbols. How an adhesive performs in a particular situation is a complex subject - an intricate mix of various mechanisms are involved. For example, one has to consider the mechanical anchoring, what physical interactions are involved and the precise nature of the chemical bonds that are formed. Also engineers and designers have to keep in mind other external factors that will strongly influence the end performance and durability of the final assembly. Having made the right choice of adhesive, there are always three key factors to take into consideration: • Cleanliness, preparation and pre-treatment of the substrate before bonding. Different techniques are available (degreasing, abrasion, sandblasting, peel ply, etching, surface conversion, plasma, corona, flame, primer, etc). They all play a key role in increasing the number of ‘active' centres and enlarging the effective bonding surface
• The operable conditions and temperature at the start of the procedure. These will affect the final strength and properties of the adhesive. Also the customer needs to know in what conditions the product will be used. Will it be used inside or outside? Are there temperature changes? What is the humidity? Will it be used in a hot environment?
• The design of the joint. From the start of an assembly, every aspect of the design of the bonded joint should be looked at to ensure that any factors which can affect the performance of the adhesive are avoided. A good design has the ability to transform a peel induced stress joint into a more favourable shear applied stress Adhesives in aerospace Composite materials are now used across the industry, but this technology comes with new demands for structural adhesives. Furthermore, aerospace structural adhesives not only have to meet new technical specifications, but they also have to comply with tougher environmental and safety criteria. Examples of where Huntsman's range of aerospace structural adhesives are currently being used include bonding of honeycomb sandwich panels for lightweight and rigid interior panels; core and edge filling materials for local reinforcement on primary structures and structural components; and for engine components where vibration absorption is critical and where honeycomb panel structures are used. New nano-toughened epoxy paste adhesives with outstanding mechanical properties for aircraft structural metal and composite bonding and repairs now far exceed aircraft specification requirements at low, ambient and relatively high temperatures. Engineers are always on the look out for better and more efficient ways of developing and producing new structures. Adhesives are key to new design methods where the conditions in service, the preparation of the substrates to bond and the type of stress and forces applied during the service are critical. Whatever the future brings, and of course the care of the environment will influence how we move forward, adhesives will always have a role to play. Adhesives allow the use of new concepts, more advanced technologies and enable the potential of composites to be maximised. What we are seeing today is just the tip of the iceberg. Adhesive manufacturers are always developing new products to serve new market needs and challenges. Developments range from improved toughness to better and longer-term performance with new features such as flame retardance and electrical features. The demands for better performance and more environmental and sustainable products create new challenges for the next generation of chemists, and one that the industry needs to meet. www.huntsman.com