Improving spread of strain and stresses across bonded joints

Paul Boughton

The transport industry will see many changes over the next few years as new legislation is introduced to help improve fuel efficiency and greenhouse gas emissions. A key driver of our development work is to look for innovative ways that will speed up the production process of high performance composite parts whilst keeping costs down.

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.

- 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.

Robotics in automotive manufacturing

There have been a number of new additions to the Araldite 2000 PLUS range. Araldite 2031 is proving popular in robotics in the automotive industry. The automotive industry needs adhesives that can cope with the stress of rotational speed of the tool, axial load to the tool and loading time - there is no additional peripheral equipment.

Bilsing Automation, one of the leading suppliers of flexible tooling and handling systems for press shop and body applications, is using Araldite 2031. They were looking for an adhesive to join carbon fibre structural components with aluminium fixtures for its robot arm and chose Araldite 2031."

These are industries where there is a continued requirement for more advanced and improved developments and where composites are playing an ever greater role. Trains, buses and cars are now using new materials and adhesion to replace standard soldering or fastening solutions. Today's locomotives have outsides panels made of polyester GRP bonded onto the chassis and the car body structures.

Araldite 2015 is one example of an adhesive that is increasingly being used in the transport sector because of its excellent adhesion to substrates, its durability and fatigue resistance as well as its high toughness. New buses using the same approach of GRP panels bonded onto car bodies produced in the Netherlands will be soon available.

Innovation in adhesives is not just focussing on improving mechanical properties, but also on adding new important features such as flame retardance, low smoke density and low toxic emissions which are so important in the transport sector. Huntsman has developed new products to meet transport requirements based on its long experience of working in the aerospace sector.

A recent development from Huntsman now allows the possibility to use an adhesive with self-extinguishing properties. The new product (Araldite 2033) is a two component structural epoxy adhesive which exhibits flame retardance (UL 94 v-o), low smoke density and low toxic emissions.

Adhesives have an essential safety and operational part to play on boats which have to perform in some of the toughest of conditions. The Safran Open '60' boat, constructed with the use Araldite 2000 Plus adhesives, has taken top honours in the prestigious 2009 Transat Jacques Vabre race. It won the race in a little over 15 days and took the race record completing the course 2 days faster than the previous race record set in 2005.

The pioneering monohull racing boat withstood some of the toughest sea conditions thanks to the construction team's decision to chose Araldite 2031 (a black epoxy adhesive) and Araldite 2022 to bond the boat's bulkhead to its hull, machined from carbon-nomex sandwich panels.

On the race track

Using Araldite, the front bonnet of the ORECA racing car is made of more than 15 parts, ut looks like one part. The ORECA car has won podium finishes throughout the 2009 season. The ORECA 01 racing car took the chequered flag at the 2009 Silverstone races in the UK beating off tough competition from competitors such as Aston Martin with the help of Araldite adhesives.

Every car manufacturer is looking for new materials that will enable them to make lighter weight cars with improved safety features at a lower cost. Huntsman has developed a new highly flexible composite, Araldite HFC with outstanding flexibility and strength that will not shatter in an accident which will cut the cost of car panel production. It has won major awards scoring best marks in the materials category for its outstanding flexibility and mechanical characteristics.

As a result of a thermosetting resin system Araldite Highly Flexible Composite (HFC) has similar material characteristics to elastomers - excellent impact, abrasion and tear resistance with good elongation and tensile resistance - but is very different in terms of material and production time costs. Only a few consumable materials are required - resin and fibre - and, most importantly, just a single shell mould is required for component production. As opposed to elastomers, where it is difficult to incorporate fabric re-inforcement, and the processing is complex and expensive, the processing of low viscosity Araldite Highly Flexible Composite is simple.

Standard, well established direct composite production processes such as wet-lay up and infusion can be used to produce parts from a single shell mould quickly and cost effectively. The parts can then be cured for 24 hours at room temperature or in an oven for 1 hour at 60°C and even faster at higher temperatures. Highly detailed automotive parts can be rapidly produced using no specialist equipment and only a single mould.

Every major automotive manufacturer in the world is considering how best to utilise fuel cell technology to reduce carbon dioxide emissions and the reliance on fossil fuels, making the fuel cell component market a potential boom market. Since the conversion of fuel to energy takes place via an electrochemical process, the process is much cleaner, quieter and up to three times more efficient than burning fuel and without CO2 emissions. The fuel cell's bipolar plates have superior corrosion resistance, lower contact resistance, higher thermal conductivity and a longer operating life.

Huntsman's automotive fuel cell resin is based on benzoxazine which is suitable for high temperature PEM fuel cells. The resin which operates at a continuous temperature of 120°C has a phenolic-like backbone which gives it very good flame retardant properties together with excellent glass transition temperatures and mechanical properties.

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Sebastien Lichtle is with Huntsman Advanced Materials Technical Support Adhesives, Composites and Tooling, Basel, Switzerland.

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