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 (NH 3 , 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 inhouse 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 mu 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 tan delta ). 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.