Eco-Friendly natural rubber composites reinforced with untreated bristle coir fiber: Mechanical and morphological analysis

The use of renewable reinforcements in natural rubber (NR) composites is gaining attention as a sustainable alternative to petroleum-derived fillers. This study evaluates the reinforcing efficiency of untreated bristle coir fibers in NR composites fabricated through two-roll milling and hot-press vulcanization. The effects of fiber length (5, 10, and 15 mm) and fiber loading (10–30 wt.%) on mechanical performance and fracture morphology were systematically investigated. While tensile strength decreased moderately with increasing fiber content due to limited interfacial adhesion, significant improvements were observed in tear strength (up to 63%), Shore A hardness (up to 76%), and abrasion resistance (5.2% reduction in wear loss). Scanning electron microscopy revealed fiber bridging, pull-out, and mechanical interlocking as the dominant reinforcement mechanisms. Among the studied configurations, composites containing 10 mm fibers at 20 wt.% exhibited the most balanced performance. The results demonstrate that untreated bristle coir fibers can effectively enhance durability-related properties of NR without chemical modification, making them suitable for eco-friendly, medium-load, and wear-resistant rubber applications.