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Sebastien Lichtle from Huntsman Advanced Materials Technical Support Adhesives, Composites and Tooling has explained how adhesives are offering competitive advantage in the solar 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 methods is 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.

From prehistoric times to the present time, humans have always tried to assemble or develop new techniques to improve their tools and daily life.

Adhesives will always be with us, but it is an ever-changing world as performance and environmental demands grow.

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.

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 methylmethacrylate 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 suited to softer materials.

Methylmethacrylate (MMA) adhesives provide excellent adhesion onto metal composites and thermoplastics.

They are generally faster-setting products, therefore 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 applications in which genuine elasticity is required, for example, bonding applications for the Araldite Highly Flexible Composite Material.

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 high 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 is 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.

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, including degreasing, abrasion, sandblasting, peel ply, etching, surface conversion, plasma, corona, flame primer and so on.

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 will affect the final strength and properties of the adhesive.

In addition, the customer needs to know the conditions in which the product will be used: inside or outside; whether there are temperature changes; humidity; whether it will be used in a ‘warm’ environment, for example, close to motors or engines.

From the start of an assembly, every aspect of the design of the bonded joint should be looked at to ensure that factors such as face peeling or cleavage forces that 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.

Solar heating panel design has become more refined and efficient, to the extent that now solar heating panels can be an integral and virtually indistinguishable part of the roof design.

Adhesives, fixings and sealants specifically designed for these panels will see increased usage.

As part of the sustainable and renewable energy markets, the solar energy and the photovoltaic (PV) are attractive and many new adhesives have been developed to fulfil this fast-growing and demanding industry.

Huntsman now has a range of products to cover the complete value chain from the silicon wafer cutting (to produce PV cells), via the module assembly and encapsulation with adhesives for the bonding of the different layers, including thermally conductive adhesive or the bonding of the frame (Araldite 2029 or 2014-1 or 2015) to connections and inverters and all electric and electronic applications such as printed circuit boards, insulation and casting.

To meet this specific need, Huntsman has developed an adhesive – Araldite 2015 with TUV design and type approval for PV modules.

Araldite 2015 allows a module design and construction with improved heat transfer and efficiency together with excellent strength and bonding properties.

All too often improved heat transfer properties are achieved at the sacrifice of bonding features.

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 beings – 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.

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 create new challenges for the next generation of chemists.

Huntsman Advanced Materials

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