Composite materials are being increasingly used in applications in industries such as rail, but what is the most suitable joining method? Jochem Sauer, Huntsman Advanced Materials Adhesives marketing manager Europe, comments
Composite substrates offer many benefits to applications, including a high strength-to-weight ratio, excellent fatigue and corrosion resistance, good impact resistance, design flexibility and lighter weight. However, selecting the most suitable joining method is just as important as the design and choice of the composite materials themselves. Bonding with adhesives enables the most efficient use of these composites.
Adhesive bonding allows the assembly of dissimilar materials and higher weight loads to be carried, while offering more uniform stress distribution across the entire bonded joint, as well as high levels of fatigue resistance. Furthermore, adhesives also facilitate the integrity and strength of composite substrates as there are no holes, rivets or fastening elements to weaken the structure. There are also no visible joining elements, and greater design flexibility can be achieved. What’s more, adhesives offer the potential to reduce costs in parts production through compensating for dimensional part tolerances.
There are three main structural adhesive categories today. Epoxy adhesives offer excellent adhesion to metals and thermoset composites, durability, and chemical and temperature resistance (up to 190°C) with low shrinkage.
Polyurethane adhesives are more suitable for bonding thermoplastics. Methacrylate adhesives provide excellent adhesion onto metal, composites, thermoset composites and most thermoplastics. These are generally faster setting products.
The load-bearing, high strength epoxy-based adhesives, together with methacrylate adhesives, are capable of delivering excellent adhesive properties to various substrates across a broad range of composite bonding applications.
Rail industry solutions
The land-transport sector as a whole accounts for about 25% of the European composites market. Composites provide lightweight solutions for rail vehicles, a benefit which translates into less energy costs in vehicle operation – important criteria for all vehicles including the recent high-speed railway projects.
A recent report from Lucintel predicted the rail industry demand for composites is forecast to grow at an average rate of 10.5% until at least 2015. This industry is firmly fixed on all the benefits outlined above as well as better flame retardancy, smoke and toxicity (FST) performance.
But here, composites and adhesives continue to play an even greater role by providing a solution to demands for more advanced and improved developments that help to sustain long-term growth.
Composite materials are used in the rail sector for exterior and interior parts as well as infrastructure components, and adhesive bonding is replacing soldering or fastening solutions.
Components manufactured from composites include items such as interior ceiling and side wall covers, corridor adapter frames, luggage bins or racks, driver’s cabins, exterior panels, end cabs and more. With their high strength, impact resistance and combination of high lap shear and peel strength, epoxy adhesives are the type of adhesive most commonly used here.
One epoxy adhesive which offers the advantages of excellent chemical resistance and thermal stability, even in temperatures as high as 140°C, is Araldite AW 4859 / HW 4859, which is suitable for exterior parts.
The methacrylate-based adhesives tend to be used in applications like bonding metal hinges to GRP interior panels, which benefit from their fast curing properties. By contrast, polyurethanes with their flexible properties are the material of choice for joining tough-to-bond engineering thermoplastics, rigid plastics and composites – finding application in frame bonding, for example.
Araldite 2013 epoxy adhesive, for example, is increasingly being used to bond GRP train driver cabins due to its excellent adhesion to substrates, high durability in ambient weather conditions, fatigue resistance and toughness. It also provides a fast and efficient bonding solution for producing the more complex and ergonomic shape of composite driver cabins now seen on the new generation of high-speed trains.
While much of the innovation in adhesives is about improving mechanical properties, it is also about adding new features such as flame retardance, low smoke density and low toxic emissions.
Huntsman has developed products that meet stringent FST safety criteria – including Araldite 2033. Offering excellent adhesion on metals and composites, this UL 94 V-0 and NF F 16-101 I2 and F2 approved two-component epoxy adhesive exhibits flame retardance, low smoke density and low toxic emissions.
As the use of composites across the rail industry increases, structural adhesives will continue to be a key player in supporting new design methods where the conditions in service, greater efficiency in manufacturing, and the type of stress and forces applied during service are critical.
Huntsman Advanced Materials