Challenges in Composite Bonding

Composite materials are advancing rapidly. The desire to make aircraft, trains and automobiles lighter, more fuel-efficient, quieter and higher-performing is the most significant driving force behind the composite industry. A major example is the Boeing 787 Dreamliner aircraft fuselage, which contains large composite parts. The transition from metal-based substrate materials, joining and fixing components also means different operational routes. The primary benefit is reduced weight, which lowers the impact on the environment and working costs, and improves energy efficiency. Joining composite materials can be quite difficult. Traditional joining methods such as welding become impossible. How do you weld an aluminium extrusion to a composite sheet? For a plastic joint to plastic, using a solvent to weld parts together may be acceptable, but if you want to join plastic to composite in this way, your chances are slim. Mechanical fasteners can offer a simple and inexpensive solution, but they can create high-stress areas where they are located, or if a hole needs to be drilled, the structural integrity of the part may be compromised. Mechanical fasteners are often unsightly, add to component weight, increase friction, create noise or vibration (which can be very annoying).

Bonding components with adhesive offers a solution to this joining problem and can provide many benefits:

•Better stress distribution between components – there are no "weak points" where stress can concentrate, •A more orderly appearance, •Less processing – no need to use a drill that applies extra pressure to the substrate, •Lightweight and silent, •Lower likelihood of being damaged or tampered with by opportunists, •Creates a seal – there is no leakage path (making it ideal for things like boats), •Application method options, suitability for application, speed, durability and flexibility. However, not everything may be easy. At the design stage, awareness of these issues is necessary for high performance.

Surface Preparation

To achieve the highest strength bonds and best durability, you may need to prepare or prime the surfaces you want to bond. Each substrate from different materials needs to be clean, dry and grease-free, and has different requirements. Polypropylene or polyethylene, like some plastics, are non-adhesive. Significant surface preparation is required before bonding can occur; base resin and composite mixtures may need to be carefully tested to ensure surface energy is high enough to allow bonding. For composites such as carbon fibre or GRP, you may need to abrade them, particularly if you are bonding to the Gelcoat side of GRP, but you can get away with minimal surface preparation. There are numerous variables and a list of different surface preparation techniques, so if you have any doubt, it is recommended that you contact the Permabond technical support line for advice.

Thermal Expansion and Contraction Rates

This can also cause problems if you have a long GRP profile and want to bond it to carbon fibre – when components expand and contract. Of course, this is not just a problem when using adhesives, but also when mechanical fasteners are used. Sometimes this is so severe that materials can bend, buckle or deform, or the bond can fail. Component substrates need to be allowed to move to some degree to cope with different thermal expansion and contraction; thus selecting a flexible adhesive and increasing the adhesive line thickness (gap) between substrates can help reduce expansion and contraction stresses. The table below shows different thermal expansion figures for common engineering materials. To put the graphic figures into perspective, if you bond a one-metre length of polyethylene to carbon fibre and apply a temperature change of +20°C, the PE will expand at least 4 mm more than the carbon fibre. When the adhesive cures, it is a good idea to cure it at the temperature at which it will normally operate (if possible), as this helps keep stresses between different materials to a minimum. A helpful website to help with calculations: http://www.calculatoredge.com/calc/exp.htm.

Joint Configuration

Using adhesive opens up many more options when joint designs are concerned. Complex connection configurations and gap filling can now be considered where previously welding or mechanical fastening, with limited options, would have been thought of. Building in adhesive line thickness control helps minimise stresses between different materials. Normally, a gap of approximately 250 μm will give optimum strength performance while allowing some degree of expansion and contraction between substrates. You can control adhesive line thickness by using very thin wire of the desired gauge, or you can use glass bead spacer inserts (these can be scattered as easily as salt on your chips). Examples of Good and Poor Joint Designs: (Arrows Show Force Direction on Joint)

Service Conditions

To ensure the best adhesive for the job is selected, it is important to consider the service conditions to which the adhesive will be exposed. These include temperature changes, maximum and minimum temperature performance, chemical exposure, environmental exposure (factors such as rain, humidity, sunlight, frost, etc.) and of course a combination of all of the above – a relatively non-aggressive chemical at room temperature can become quite aggressive when heated.

Adhesive Properties

So you have chosen your composite materials, designed your joint, prepared the surfaces, and are confident that service conditions will not be a problem. Now you are faced with over 100 adhesives to choose from.

To help narrow down your options, prepare answers to the following questions:

•What is your preferred cure speed? For example, seconds, minutes, hours? How quickly do you need to assemble and set the parts? •Which cure method do you prefer? Does it need to cure at room temperature? Can you provide oven curing? •If you are using a two-part adhesive, do you prefer to measure and mix, or would you rather use a gun, nozzle, spray or brush? Are you looking at an automated process? Do you want a single-component adhesive? •What viscosity do you prefer – water-thin or thixotropic like sour cream, or high-viscosity paste like peanut butter? If you have a large gap to fill, you will need to use a high-viscosity adhesive. Some adhesives only allow their cure mechanisms to cure with minimal gaps. If applied as a very thick layer, it may not harden. Some adhesives work better with certain composites. The table below shows typical shear strength values of different bonding technologies on some common composites: Eray Atak Chemical Engineer Technical Director Lefa Representation