Phenolic resins are the most common binders used in friction materials for automotive disc brake systems, due to their affordability and satisfactory thermal properties. Benzoxazine resins present themselves as possible alternatives thanks to some characteristics related to their different crosslinking mechanism. Oppositely to the condensation polymerization of phenolic resins, benzoxazines crosslink through a ring-opening mechanism, which requires the reaching of a temperature threshold for its activation and minimizes the formation of gaseous byproducts. Therefore, this curing mechanism should provide some attractive characteristics from an industrial point of view, such as longer shelf-life and lower moisture sensitivity. Furthermore, these resins promise enhanced thermal stability, which is related to the production of airborne emissions, especially during harsher braking events. Brake airborne emissions have long been neglected by legislative restrictions, but are increasingly gaining institutional and public interest due to their harmful effects on the environment and on the human health. Consequently, the European commission proposed EURO 7 legislation is going to regulate non-exhaust emissions such as those from brake systems. In this work, two different types of Benzoxazine resins were characterized by DSC and TGA analysis and compared to a commercial phenolic resin. Multiple DSC analyses were carried out both in nitrogen and air atmosphere, allowing for a thorough analysis of the curing process and thus for an optimization of the production parameters. Moreover, it was possible to use the Kissinger-Akahira-Sunose method to calculate the activation energy of the crosslinking process. TGA analyses were carried out on the neat resins and on friction material compositions containing them as binder component. The production process of these brake pad friction materials consisted in the addition of 7 wt% of each resin to a binder-free commercial composition, which was then mixed, hot pressed and finally cured. The produced friction materials were subjected to tribological testing using a pin-on-disc tribometer, which was modified with some equipment that allowed for the airborne emission analysis. A scanning electron microscope was used for the surface characterization of the samples. The results obtained from these tests are promising, and suggest a possible application of benzoxazine resins as brake pad friction material binder.
Dr. Davide Carlevaris, PhD Student, University of Turin; Prof. Cinzia Menapace, Professor, Università degli studi di Trento; Prof. Giovanni Straffelini, Professor, Università degli studi di Trento; Prof. Luca Fambri, Professor, Università degli studi di Trento