Sebastien Lichtle, Huntsman Advanced Materials Technical Support Adhesives, Composites and Tooling, explains why adhesives have always been with us and why they continue to offer competitive advantage in the wind energy industry.

Engineers are always on the look out for better and more efficient ways of designing and producing new structures. The choice of the most suitable joining methods are equally important as are the design and the choice of the composite materials themselves. Adhesive bonding allows for the most efficient use of composite substrates which can maximise their potential.

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Where we began

From pre-historic times to the present time, humans have always tried to assemble or develop new techniques to improve their tools and daily life. Adhesives have been part of our lives since pre-historic days when men bonded their bone arrow points using materials such as asphalt. Adhesives will always be with us, but it is 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 strain and stresses across bonded joints. Adhesives also maintain the integrity and strength of the material or composite 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.

Adhesives today

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.
• Methylmethacrylates (MMA) adhesives which provide excellent adhesion onto metal, composites and thermoplastics. They are generally faster setting products thus enabling significant productivity gains.

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, e.g. bonding applications for the Araldite Highly Flexible Composite Material which received the JEC Asia Award in 2008.

The silyl-terminated polymers 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 what is the mechanical anchoring, what physical interactions are involved and what is 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. But 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 “warm” environment for example close to motors or engines?
• 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 factor such as face peeling or cleavage forces which can affect the performance of the adhesive is avoided. A good design has the ability to transform a peel induced stress joint into a more favourable shear applied stress!

Wind energy

Governments and energy utilities across the world are embracing wind power not only because it is environmentally friendly, but also because it can compete with conventional energy sources such as oil and gas – not just in terms of the environment, but also on the grounds of cost. Renewable energy is one of the fastest growing global markets.

Huntsman offers a range of new composite structures for wind blades with lengths from the standard 30 meters of today to new wind blade technology with increased lengths of up to 60 metres which because of their increased length puts a much greater demand on the adhesive.

Several epoxy and polyurethane adhesives are used to bond the inner reinforcement (spar or shear webs) as well as for the two shells. The requirement is for an adhesive with high flexural strength, high toughness to avoid cracks during service and a service temperature from -40 to +80°C and gap filling up to 50 mm. Figure 3 below shows a section cut view of a blade with the adhesive use locations.

Huntsman has applied its expertise in the sports and aerospace markets to the energy sector. It has developed a range of adhesives for securing wind turbine blades which includes new generation nano-technology materials based on truly sub-micron particles that can form bonds with dramatically improved strength when compared to traditional solutions. Processing of the products is also facilitated, thanks to Huntsman patented “Chemical Thixotropy” that allows the manipulation of very liquid components before mixing. A new development, with a patented nano-toughened formulation is shortly to be launched which will provide outstanding impact and fatigue resistance in combination with an improved pumping process and a chemical thixotrophy.

The Future

Engineers are always on the look out for better and more efficient ways of developing and producing new structures. Adhesives are a key player in 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 enables the potential of composites to be maximised. What we are seeing today is just the tip of the iceberg – there is much more to come.

Adhesives 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 creates new challenges for the next generation of chemists. It is a challenge that Huntsman intends to meet.

Huntsman is a global manufacturer and marketer of differentiated chemicals. Its operating companies manufacture products for a variety of global industries, including chemicals, plastics, automotive, aviation, textiles, footwear, paints and coatings, construction, technology, agriculture, health care, detergent, personal care, furniture, appliances and packaging. Originally known for pioneering innovations in packaging and, later, for rapid and integrated growth in petrochemicals, Huntsman today has more than 11,000 employees and operates from multiple locations worldwide. The Company had 2009 revenues of approximately $8 billion. For more information about Huntsman, please visit the Company’s website at www.huntsman.com.

Source: Huntsman