Reduction of particulates on car brakes

The problem of particulate matter is a hot topic as can be derived from the introduction of LEZ zones in some cities. The origin of fine dust is complex. Not only do wood and coal stoves cause the emission of soot particles, the increasing car and bus transport also causes quite some pollution in cities.

It is known that older diesel cars in particular emit more particulate matter than petrol cars. The use of particulate filters in modern cars and city buses can partly reduce this problem. However, cars also produce particulate matter from a different source, i.e. the brake dust produced by brakes.

Pollution

The European Commission wants to reduce the pollution caused by brake dust by means of several measures, one of which is improving the composition of the brake pads. Most brake pads contain copper. When braking, small particles are released due to the friction between the brake rotor and the brake pad. In a number of American states the use of copper in brake pads is already prohibited. The same must now be done in Europe. According to research, braking is responsible for 20 percent of the harmful emissions in traffic.

(Source: Mann+Hummel)

Types of brakes

Vehicles are slowed down by means of disc brakes and brake drums, mounted in each wheel. The braking effect is obtained by a mechanical contact between the friction material in the brake pad or brake segment (popularly called ‘Ferodo’, after a well-known trademark) and the disc or drum brake. The friction material usually consists of compressed fibres. In the past, asbestos was also used for this purpose because of its high heat resistance. After all, when braking, the frictional heat must be able to be dissipated over a relatively small surface area, resulting in high temperatures. It is important that this heat is efficiently dissipated via the disc or drum brakes without reducing braking performance. When slowing down a passenger car at a speed of 80 km/h, enough energy is generated to boil 2 litres of water in 3 seconds.

Nowadays, disc brakes are made of lamellar cast iron and fitted with the necessary internal cooling channels (slots or perforations). Brake drums used to be made of cast iron, but modern cars usually have aluminium brake drums because of the better thermal conductivity of the aluminium and because of the lower weight.

The contact of the friction material with the metal disc brakes and, to a lesser extent, the drum brakes, because they are closed, creates a lot of particulates that end up on the road and thus in the environment.

Organic brake pads

The composition of friction materials is very diverse. In case of organic brake pads, the friction material is usually held together by a phenol-based binder. Other binders are cement, graphite and zirconium silicate. Asbestos used to be a material for organic brake pads, but since asbestos is a carcinogenic substance, Kevlar, glass fibres and other mineral fillers are now used.
Because organic brake pads have good friction, less force is needed. An additional advantage is that they produce little brake noise at low temperatures.
Organic brake pads are less suitable for high performance applications: they wear out quickly because they are softer, fade (overheating with a reduction in braking power) and oxidise when exposed to air.

Sintered brake pads

As the name suggests, these brake pads are made of sintered metal, with only a small amount of binder. Sintering means compressing ground powder metal under high pressure and temperatures. This results in a solid mass. Commonly used metals are copper, bronze or a mixture of both. Mixed metals are often used for normal use while the blocks with iron particles are more suitable for high performance (racing) cars because they can withstand extreme temperatures. For extremely high temperatures, such as Formula 1 cars, a ceramic powder is added.
Sintered brake pads are less soft than organic brake pads, which means that more force has to be applied when braking. The blocks often produce a fine black dust, which quickly contaminates the wheel rims.

Filters for brake pads

The German manufacturer Mann+Hummel has now come up with an innovative solution to the problem of brake dust. By placing a filter at the top of the disc brake, fine dust is collected continuously. This filter consists of a plastic housing with built-in filter element. The reservoir is then emptied after a period of time, for example, during a major maintenance.     

(Source: Mann+Hummel)

According to this manufacturer, this system can collect 80 percent of the brake dust. A prototype was successfully tested on a VW Golf 8 in 2019. From 2021 onwards, these filters could be mounted on series cars of certain car models.

In its test lab in Zwijnaarde, Sirris is able to perform comparative wear tests (rubber-wheel-abrasion- and tumbler-test) on small metal parts, such as steel or cast iron.

Sources