The importance of consistent materials quality for brake – and car – manufacturers

The second in a series of blogs highlighting the most important trends facing car, brake system and brake pad manufacturers.

Brake pads are a complex mix of 20 or more different raw materials, typically divided into abrasives, lubricants, binders, fillers and reinforcing fibres (see my colleague Mark Segeren’s ‘Trends in automotive braking’ article). Car manufacturers are increasingly demanding that brake pads will last the lifetime of the car. Compounders are therefore having to make pads that last longer, while also facing stricter quality and environmental requirements.

Reinforcing fibres like those produced by Lapinus (part of Rockwool) gives mechanical strength to the brake pad, and contributes to the plateau formed on the pad surface during braking. Fibre anchor points contribute to third body layer formation (loose debris between pad and rotor while braking). The fibres improve porosity and compressibility, and the formation of fibre nests improves wear and damp ing. Different fibre lengths can adjust performance and mechanical strength depending on the requirements. Bound together with the other brake pad constituents, the fibres ensure a stable friction film under all braking conditions.

Consistent quality – the essential component

Consistent quality – the essential component
Car drivers must be able to trust their brakes, even when those brakes are over a decade old, corroded and working in poor weather. Consistency between different production runs and consistency over an individual brake’s life span are both important. For brake pads to give the same braking performance over time it is essential to use raw materials which have a consistent quality. Materials must have the same properties and the same chemical composition. Manufacturing needs an ultra-stable spinning process (Figure 1), with consistent fibre diameter. The resulting fibres must also be engineered to give stable fibre length and shot content (shot consists of mineral droplets thrown off during spinning).

20180913 RW-LF PHO 197 ^{Figure 1: Consistent processing is the basis for highest quality friction materials.}

Lapinus uses an online and offline quality check programme to validate product quality and performance. Shot content and fibre length, diameter and chemical composition (Figure 2) are always checked and matched against product specifications. The chemistry is also checked by independent external parties.

20200212 RWLF ILLU 007

^{Figure 2: Key mineral fibre properties must be tightly controlled during manufacture.}

For almost all Lapinus fibre products, the shot content is specified as any material that isn’t a fibre and is coarser than 125 microns (Figure 3).

20200212 RW-LF PHO 300

^{Figure 3: Mineral fibre shot under a microscope}

High shot content influences coefficient of friction, wear and NVH (noise, vibration and harshness). Figure 4 shows the impact of shot content on these brake pad parameters. The blue stacks show the shot content of five Lapinus fibres. Between low and medium shot content, the parameters are fairly constant. Outside that, a small difference in shot content in the raw material can have a large influence on brake parameters.

The two stacks on the left show Extra Low Shot (ELS) products. These are known in the market for excellent friction coefficient, wear and NVH. High shot in the raw material typically degrades wear and NVH.

20200212 RWLF ILLU 005

^{Figure 4: Brake parameters for different shot content.}

Variations in fibre length have a significant impact on friction properties

Another factor that can influence friction properties is the fibre length. In Figure 5, the blue stacks again show different Lapinus products. Between short and long fibre lengths, CoF and reinforcement performance both decrease relatively linearly.

Fibres with medium length and above are increasingly difficult to disperse homogeneously, and are known to form fibre nests. This has – to some extent – a positive effect on pad wear since it will collect wear debris more easily. Pad wear for long fibres is highest of all the products.

Inconsistent fibre lengths between various production batches will also influence the friction properties of the brake pad. That influences the braking behaviour of the car, so mineral fibre length and consistency are both key.

20200212 RWLF ILLU 006

^{Figure 5: Impact of fibre length on key brake pad parameters}

Quality and consistency of mineral fibres

For consistent performance of brake pads, the raw materials in mineral fibre products must be carefully selected for high performance and biosolubility (Lapinus has RAL & EUCEB certified biosolubility). The resulting product must be highly stable and resistant in a wide variety of environments, even with extreme brake temperatures (600°C or even higher). Producing fibres with specific and consistent fibre properties like length, diameter, and purity (low shot content) requires years of experience in engineering fibres. That is the only way to guarantee performance in different applications.

Comprehensive quality evaluations must ensure that product performance always fits the specific requirements. That allows continuous improvement of products and processes.

Lapinus derives its core technology for fibre production from the Rockwool R&D department, which concentrates fundamental research for fibre technology. A strong technical department is needed for application development, and customer needs must drive the development of new or improved fibres and products.

Friction test equipment at Lapinus includes an NVH dynamometer and a Krauss machine for testing brake components and friction materials. These give the detailed information needed to select the most suitable materials.

To summarize

Key fibre parameters include length, diameter, chemical composition, compatibilizer, coating and shot content. All these must be tailored to the different brake pad requirements.

Friction levels must be stable and consistent both over a brake pad’s life and from pad to pad in the production line. That increases brake safety and comfort, and decreasing wear helps to reduce brake pad fine dust emissions.

Tested and certified product quality

The inorganic property of stone wool makes it highly chemical resistant and durable under any extreme climate conditions. It is dimensional stable and does not degrade over time. Being made of stone in a production process with temperatures up to 1500 ⁰C the dimensional stability of our fibres is maintained at the most extreme temperatures that can occur in the brake process. All Lapinus products undergo comprehensive tests during and after the production process. Lapinus and its products are fully certified for bio-solubility, quality and processes (ISO).