The production of brake linings for trains currently depends on aramid fibres such as Kevlar, phenolic resins and an extremely energy-intensive manufacturing process, which makes it costly both in environmental and financial terms. The pads also incorporate significant amounts of heavy metal compounds that are discharged into the environment as they wear.

With funding from the UK government’s Technology Strategy Board, the Ecobrake project aims to introduce a marketable alternative composite product employing hemp fibre and cashew nut oil by 2011. Bringing together European Friction Industries, Aptec Products and Hemp Technology, along with specialists at the University of Exeter, the goal of the project is to develop a new technology to create greener brake pads for mass urban rail transit vehicles, such as those employed on the Docklands Light Railway (DLR) in London.

In describing progress to date at the recent Natural Fibres conference held in London, project researcher Will Newby explained that before 1990, the only fibre employed for such brake linings was asbestos in phenolic resin. "Since asbestos was banned, no fibre has completely replaced it," he said. "Aramids are now commonly employed, but also metallic and mineral fibres. But with the cost of aramid fibre around £20 per kg, any alternative low-cost fibre that can perform some of the work of such an expensive material in composite structures would be significant. "Hemp goes some way to meeting requirements in respect of a stable coefficient of friction and resistance to friction heat. It has adequate compression and shear strength - and it is cheap."

Industrial hemp is a fast-growing, disease-resistant plant that absorbs carbon dioxide and self-fertilises the land upon harvest. Bast fibres extracted from the stem of the plant exhibit specific mechanical properties comparable to E-glass, and have been used in the manufacture of automotive composites for some time. The hemp for the Ecobrake project is prepared by Hemp Technology based in Norwich, UK, and processed by Aptec based in West Auckland, County Durham, UK, using a Laroche opening machine. It is the resin, meanwhile, that dominates the mechanical properties of the pad. It must withstand cyclic loading of high temperatures and stresses but maintain a stable coefficient of friction throughout its working life. Currently, phenolic-based resins synthesised from non-renewable resources are used to fulfil this requirement, but a naturally occurring alternative has been discovered in the husk of the cashew nut shell.

Cashew nut shell liquid (CNSL) is the viscous, dark brown substance found in the husk of the cashew nut shell. It is a sustainable and naturally occurring source of phenolic compounds that can be used to produce thermally stable resins suitable for use in brake pads. At present, this liquid is considered a waste product by the cashew nut industry.

A new aggressive mixing process will be the key to the success of the Ecobrake project, Newby revealed. "Aramids fibrillate and form a dense structure and it is this we want to replicate with hemp in our process," he said. "We are also now exploring surface treatment of the fibre. Aptec has so far had some success with blends of aramid and hemp and we are now looking at fibre surface treatments to increase the hemp percentage. "The current market for such brakes in Europe and North America is an annual £200m (US$322m) and the goal is that 20% of the 1.5m sets required each year will be Ecobrake pads by 2013."