TWI Knowledge Summary

PCM (Polymer Coated Material) joining

By Roger Wise

The Polymer Coated Material (PCM) joining technique is a method for joining materials using a thermoplastic polymer adhesive. It was first introduced in 1991 as a means to join thermoplastic polymer composites to metals.

Underlying the technology is the concept of pre-coating components with a polymer layer (as little as 5µm in thickness is sufficient), and then joining them to thermoplastic components or other materials coated in a similar manner. The joining operation involves melting the thermoplastic using techniques such as induction, resistive implant or ultrasonic welding. Thus it can be fast, readily controllable and reversible. The technology was developed as a competitor to conventional thermosetting adhesive bonding (e.g. epoxy resins) where the full bonding procedure may take several hours (including pre-treatment) and where joints are not easily disassembled.

Several steps are involved in the PCM joining technique. First, any components not made from thermoplastic polymer are pre-treated; this pre-treatment can comprise, for example, cleaning in the case of some ceramics, or solvent degreasing, followed by etching and anodising in the case of aluminium alloys. Second, the thermoplastic coating is applied using a solution, or by polymerisation in situ, for example. The idea is to introduce the coating in a form in which it adheres well to the substrate, and a low viscosity liquid provides this form.

Third, the components are joined by polymer welding i.e. allowing parts and coatings to fuse together using heat. If it is desirable to disassemble the components, for example to recycle them at the end of their useful working life, then re-melting the coating will allow them to be separated with a small force.

Mechanical properties of PCM joints can compare well with those made by conventional adhesive bonding, provided that a suitable polymer has been employed. For example, single lap shear PCM joints in aluminium alloy made with the thermoplastic polymer polyetherimide (PEI) had strengths exceeding 25MPa when tested in tension. This is a similar performance to that expected of a structural epoxy resin. The reduction in joint strength in PCM joints resulting from prolonged exposure to a humid environment was also similar to, or less than, that observed in joints made with thermosetting adhesives.

Selection of the polymer for the coating is very important to the success of the technique. Some polymers are naturally sticky with respect to polar surfaces (e.g. metals), and these polymers often contain polar groups. Examples of polymers which possess good adhesion characteristics include polyetherimide, polyethersulphone and some thermoplastic polyesters. However, some thermoplastic polymers are not naturally sticky with respect to polar surfaces, and these include polyethylene and polypropylene. In these cases, versions of these polymers which have been made sticky by the addition of certain chemical groups are often commercially available. These modified polymers should then be used to produce the coatings.

There are no known cases where PCM joining technology is being employed commercially yet, although there has been some development for applications in aerospace. The fact that PCM joints can be disassembled means that applications in automotive and electronics, where recycling is particularly important, are anticipated.

Further information

You can use the Weldasearch literature database to supplement what you find in JoinIT.

For specific enquiries please contact the Polymer Group at TWI (e-mail: pol@twi.co.uk).