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Can surface preparation improve bond strength when adhesive bonding composites?


Frequently asked questions

Yes. There are five main surface preparation methods that can be applied to composite materials to improve bond strength -

1. Cleaning or degreasing

Loose material can be removed from the surface using a clean brush or a blast of clean, dry air. Grease, oil or wax can be removed from the surface by wiping, dipping or spraying with an organic solvent or alkaline aqueous solution appropriate for the materials being used, e.g. acetone, or isopropyl alcohol.

2. Surface roughening

Abrasive materials are used to remove unwanted layers and generate a roughened surface texture, e.g. grit blasting. Any abrasion treatment must be followed by operations to ensure removal of loose particles (such as blasting with dry air) and degreasing of the material (see above). In high-performance composites it is important to avoid damage to the underlying reinforcing fibres during any surface roughening.

3. Primers

These are surface treatments, applied by dipping, brush or spray that can chemically alter the surface (e.g. silane coupling agents), or protect the preferred surface already generated by another method.

4. Corona discharge, flame, laser and plasma treatment

For non-polar polymers, or polymers with a low surface energy such as polyethylene or polypropylene, it is necessary to introduce chemically reactant surface functional groups to improve adhesion. Corona discharge, flame, laser and plasma surface treatments are all used to modify the surface of composites and improve adhesion.

5. Peel ply

A peel ply is a sacrificial layer of fabric applied to the surface of a thermoset composite before it is cured. The fabric is impregnated with excess resin during cure, creating a resin-rich surface. The peel-ply fabric is removed before bonding, leaving an exposed surface of roughened resin without damage to the underlying reinforcing fibres. Peel plies are often nylon, and because nylon absorbs moisture they will often leave residual moisture at the surface. Moisture will have a detrimental effect on the strength and durability of an adhesive bond. After the peel-ply is removed it is therefore important that the surface is thoroughly dried before bonding.

When bonding composites to metals there are additional surface treatments that may be used, for example see ' What is Comeld TM ?'.

Selection of an appropriate surface treatment should be based on issues such as cost, compatibility with manufacturing procedures, performance requirements, durability and health & safety.

TWI advises on appropriate surface treatments for specific bonding applications - contact us.

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