Microwave curing of composites
Curing large fibre-reinforced plastic components used in the shipbuilding, construction and energy industries is a challenging process. Researchers at the Fraunhofer Institute for Chemical Technology (ICT) in Pfinztal, Germany, have developed a microwave curing technique that may facilitate future production.
Currently, parts are made by manual-lamination or die-casting – both methods suffer drawbacks. The former releases styrene, a volatile hydrocarbon that is carcinogenic, while, in the latter, if the casting mould is not completely filled the first time, it cannot be topped up and the component has to be discarded.
To further complicate matters, the polyester resins cure at different rates depending on the ambient temperature.
‘The variable [external] temperature during production has made it impossible for us to reproduce the chemical process with any degree of accuracy,’ says Dr Rudolf Emmerich, Project Manager at ICT.
Emmerich and his team have therefore collaborated with research institutes and companies from Germany, Slovenia and Spain on a US$2 million project – part funded by the EU – aimed at ‘enabling the plastic curing process to be accurately controlled’.
The new microwave technology enhances the speed and ‘uniformity’ of curing plastics. Polymer resins are heated volumetrically, initiating polymer cross-linking simultaneously throughout the material regardless of its geometry.
'Heating large components, such as boat hulls, in the traditional way used to be a problem as we had no way of transporting heat uniformly and deeply enough into the material. We use the absorption of microwaves by the resin to achieve just that,’ says Emmerich.
Furthermore, the new polymer resin mixture is of low viscosity so that it does not harden at ambient temperature. This allows fibres – glass fibre or foam fabric – to be embedded in the resin while still leaving sufficient time to make adjustments.
Only when the components are correctly positioned in the mould does curing take place, by means of moveable antennae that transmit microwaves and distribute energy as required.
The new method may also have added environmental benefits as it reduces the production of waste material and the extent of pollution by volatile hydrocarbons. The process is now being adapted for industrial scale applications and a prototype has already been completed.
Fraunhofer Institute for Chemical Technology (ICT), Pfinztal, Germany.