Low-impact friction materials for brake pads

State-of-the-art friction materials for applications in disc brake systems are constituted by composite materials, specifically formulated to ensure proper friction and wear performances, under the sliding contact conditions of braking events. The bases of typical friction compound formulations usually include 10 to 30 different components bonded with a polymeric binder cross-linked in situ. Main requests to be fulfilled during braking are an adequate friction efficiency and enough mechanical resistance to withstand the torque generated by forces acting on the disc brake. Generally, each component confers distinctive properties to the mixture and their primary function can be classified in the following categories: binders confer mechanical strength to friction material guaranteeing pad compactness during use, abrasives increase friction efficiency and improve compound wear resistance, solid lubricants are responsible for stabilizing friction coefficient and contrasting the build-up effect, reinforcements increase mechanical strength improving wear minimization and stabilization. Furthermore, other modifying components such as fillers and functionalizers are involved which are not directly related to friction efficiency, e.g. cheap materials, pigments, etc. Organic brake pads for disc-brake applications are based on phenolic resin binders, generally it requires three main manufacturing steps: raw material blending, where friction compound components are mixed by blenders. Hot-molding, where blended friction mix is pressed against a metallic support at controlled high pressure (>2kN/cm2), temperature (150-200 °C) and pressing time (3-10 minutes). Brake pads post-curing, to complete the hardening of polymeric binder. This last step for phenolic resin is usually performed in a batch convective oven at temperature above 150 °C for 4-12 h, or alternatively using a continuous process, such as IR in-line tunnel ovens where the process time is 10-15 min, the oven heater temperature is between 500 and 700 °C and brake pad superficial temperature is easily above 300 °C. Such kind of formulations and manufacturing process reflects the generally acknowledged state of the art as regards organic friction materials for passenger cars and light trucks. In this panorama the idea of introducing a completely inorganic binder matrix would represent nowadays an extremely appealing topic in the field considering potential improvements of this alternative approach. The complete elimination of the organic binder would reduce emission of phenol-formaldehyde hazardous derivatives generated at high-temperature e.g. volatile organic compounds, highly toxic polyaromatic hydrocarbons etc… Nature and toxicity of the organic compounds released at high temperature was investigated on brake pads manufacturing and compared with preliminary studies recently published. Introducing an inorganic hydraulically bonded matrix in place of the traditional organic-based binders would lead to a substantial reduction of the total embodied energy and water of brake pads considering low-temperature manufacturing process and inorganic binders properties. Primary production embodied energy for phenolic resin is estimated in the range of 75 - 83 MJ/kg (cradle to gate), while primary production water usage (embodied water) is in the range of 94 - 282 l/kg. As a matter of comparison, examples of the embodied energy for inorganic binders typically used for concrete construction are: Portland cements 4.9 MJ/kg, fly ash 9.3 MJ/kg, metakaolin 1.4 MJ/kg, silica fume 0.036 1.4 MJ/kg. The embodied water for these raw materials usually is less than 0.048 l/kg. Well-known properties of such peculiar inorganic materials exploiting the hydraulic activity of binders when exposed to water or alkaline environment. The only energy demanding compound was the alkaline solution (e.g. for sodium hydroxide and sodium silicate the embodied energy is respectively of 22MJ/kg and 16 MJ/kg). New brake pad manufacturing process allowed the substitution of commonly implied highly energy-consuming procedures with low-temperatures steps. Friction material components except binders were blended together with conventional plow-blade blender forming a dry friction-mix, then this dry friction-mix is blended with the inorganic binder and water or alkaline activators in a planetary mixer forming a wet friction-mix. Eventually wet friction mix is cold-pressed onto a metal back-plate without the need for further treatments at high temperature. It immediately emerges the energetic benefit connected to the manufacturing process of this inorganic binder-based brake pads. After brake pad production, the behavior of these inorganic materials was compared to traditional phenolic-based friction materials. Brake pads were tested on a full scale automotive brake dynamometer and on a real vehicle (in terms of performance and particle emission) following custom and international standard procedures. The aim of this work was to produce brake pad prototypes with friction material based on an inorganic hydraulic binder at performance comparable to commercial brake pads with organic-matrix based friction materials. The results obtained so far resulted particularly promising and paved the way to further developments of these novel class of friction materials.