Eco-friendly Polybenzoxazine (PBZ) resins to address the age-old challenge of replacement of phenolic resins in friction materials

Phenol-formaldehyde resins (Ph-Rs) have been invariably used for the last few decades as a binder/matrix in friction materials (FMs) in the industry owing to the ease of processing, excellent balance of brake-performance properties, commercial availability, and cost. However, Ph-Rs pose persistent environmental concerns due to emissions of harmful volatiles during processing (NH3, formaldehyde), apart from the challenge of limited shelf life (∼2–6 months). The present work aimed to address such issues by exploring the potential of five types of in-house synthesized Polybenzoxazines (PBZ) resins to develop brake pads for passenger cars. The composite brake pads' performance (physical, mechanical, and tribological) was compared with those based on Ph-R containing other identical ingredients in the same amount. PBZ-based brake pads exhibited notable advantages compared to PhR-based ones, such as an 8–87% increase in the fade ratio, a 9–41% higher wear resistance, and a 0.5 to 5.4 dBA reduction in noise levels. Interestingly, dimethyl substituted PBZ (PBZ-dma) based brake-pads showed the best tribological (10% lower sensitivity of μ to speed and pressure, 15–87% higher fade ratio, 35% higher wear resistance) and noise vibration performance (lower vibration and sound pressure levels by 2.8% and 0.51 dBA, respectively) apart from its 7% higher thermal stability and thermomechanical properties (∼67% higher storage modulus and 50% higher tanδ). Present work confirms the potential of PBZs (having unlimited shelf life) as the next-generation binders for FMs with superior braking performance and eco-friendliness.