Formulating car brake pads: the five materials categories

January 1, 1

Over 500 raw materials are used in today’s automotive brake pads, with a typical compounder’s formulation containing up to 30 separate components. This complex mix of raw materials is needed to cope with the extreme stresses and conditions that brake pads experience.

A major aim is to reduce wear. Compounders are aiming towards supplying a single set of brake pads that don’t need replacing over a car’s life. That will also reduce environmental effects. Over 90% of PM10 particulate matter emissions from traffic are now estimated to come from non-exhaust emissions, with up to half of that from brake wear. Some of the materials – like the Lapinus mineral fibres used as a raw material – are certified biosoluble and therefore environmentally friendly and safe for humans.

Zinc, copper and other materials are however shed from car brakes and tires, and this is particularly unwelcome in urban areas. Health, Safety & Environmental concerns are becoming more and more important worldwide, and that includes raw materials being used in friction materials. For example, copper content in friction materials will be restricted in several US states to a maximum of 5% in 2021 and 0.5% in 2025, and antimony trisulphide is not allowed in European OE passenger car disc pads.

In the short term, filters or vacuum pumps can help. The long-term solution must however be to continue to reduce wear, and use only benign materials in the pads themselves. Disc manufacturers are also working on durable coatings and temperature or chemical treatments to reduce wear.

Reducing brake wear and environmental emissions

Asbestos was once used as a standard raw material (Figure 1), but has now been phased out in most of the world. It was replaced in the 1990s by a semi-metallic formulation for higher performance, and non-asbestos organic (NAO) for lower noise and smoother braking.

Two major formulations now dominate. A low steel (LS) formulation based on steel fibres has higher performance and is used in most of Europe and South America. Less abrasive NAO formulation friction materials dominate the United States and the Far East. The eventual aim is to use a global NAO or hybrid formulation throughout the world to simplify production and logistics in the OE market. 20201028 RW-LF PHO 25

^{Figure 1: The future seems set for global NAO to avoid having different brake pad formulations for car models used in different geographic areas.}

The basic functions of materials in brake pads

There are five basic categories of materials. Of course, the materials all interact with each other, and adding a little more of one material can affect the functionality of the others. The actual raw materials used in a brake pad depend largely on individual requirements.

Abrasives increase the friction coefficient, but may also raise the rate of wear of the disc. They also remove iron oxides from the disc, as well as other unwanted surface films formed during braking. These mild abrasives form a third body layer, maintain surface integrity, and stabilize friction performance. Most commonly used are:

Potassium titanate (PoTi)
PROMAXON ® -D
Iron oxides (Fe₂O₃ , Fe₃O₄)
Calcium compounds (Ca(OH)₂)

Lubricants stabilize and maintain the friction level while reducing wear during braking, particularly at high temperatures. Examples of these materials:

Graphite
Copper (fibre, powder)
Metallic sulphides

Binders maintain the structural integrity of a brake pad when it is subjected to mechanical and thermal stresses. They prevent the constituent parts from crumbling and coming apart. Typical binders:

Phenolic resin
Other resins

Fillers improve a brake pad’s manufacturability as well as reducing the overall cost of the brake pad. They also play an important role in modifying certain characteristics of brake friction material including NVH. Filler materials include:

Barites
CaCO₃

Friction materials can also include raw materials for damping and to stabilise friction. Materials like friction particles and rubber maintain structural integrity and improve NVH performance.

Reinforcing fibres provide mechanical strength to the friction material and stabilize the friction performance. They contribute to the plateau and third body formation for a stable friction film under all braking conditions. Organic fibres reinforce brake pads at lower temperatures and provide easier processing. With increased temperatures (> 150-300°C) they will degrade, and inorganic fibres like the mineral fibres made by Lapinus come into play:

Aramid
Poly(acrylonitrile): PAN
Cellulose
Mineral fibres
Steel fibres

Reinforcing fibres that also improve friction properties

Lapinus’ mineral fibres are used in nearly half of all passenger car brake pads, improving the friction performance and reinforcement properties of the friction material. They give mechanical strength to the brake pad and contribute to the plateau formed on the pad surface during braking. Bound together with the other brake pad constituents, the fibres ensure a stable friction film under all braking conditions.

Lapinus fibres are sustainable and safe for humans and the environment. They are certified bio-soluble due to their unique chemical composition and strict process control.

EUCEB and RAL certification means that the fibres’ bio-soluble chemistry is subject to random testing by external laboratories. Studies have shown that this important bio-solubility remains unchanged even in wear debris, demonstrated by Dynamometer testing in the Lapinus test centre.

Different brake pad formulations are needed for different types of car (sports cars, SUVs and smart cars, for example) and platforms with very different requirements. They also use different mineral fibres: with different mineral fibre lengths and diameters, shot percentages, and coatings/surface treatments. They give various anchoring effects, reservoir effects, and bonding, damping, surface reinforcement and porosity values. That means a product to best suit each type of formulation. It also means that formulations can be tailored to specific needs (for example for particular local markets).

Materials consistency is key

Brake manufacture needs to be seen as an integrated whole. The fibres are particularly important because they play a critical role in obtaining consistent braking performance, low noise and vibrations, and limited wear. It is therefore essential that their properties are stable. Car brakes need consistent performance between the first and last brakes produced on a line. That’s even true for aftermarket replacement pads, which preferably offer the same performance as the pads they replace. That demands strict and constant quality control.