Recycling and reusing denim waste is critical to reducing environmental pollution. In this article, a functional fiber-reinforced composite was designed by combining ammonium polyphosphate (APP)-treated denim fabrics with epoxy resin mixed with hollow glass microspheres (HGM) by resin transfer molding (VARTM) using viscose-polyester denim waste as raw material. The effect of the volume fraction of waste denim fibers and the viscose-polyester ratio on the mechanical properties of this composite was investigated, and experimental results revealed that the best mechanical properties of this functional composite were obtained when the volume fraction of waste denim fibers was 25.63%, and the viscose-polyester ratio was 5/5. The addition of denim fibers increases the composite material’s flexural strength (39.31%), compressive strength (29.03%), and shear strength (48%) compared to the material without fibers. In addition to their mechanical properties, the composites exhibit excellent thermal stability, as evidenced by their V-0 rating for vertical flame retardancy and HB rating for horizontal burning. These ratings meet the standards required for applications in the automotive interior field. In terms of thermal insulation performance, the composites have a thermal conductivity range of 0.16–0.21 W/mk when the fiber volume content is between 16% and 32%; reducing the fiber volume content in the composites reduces thermal conductivity. Notably, the composites with the most superior thermal insulation performance in our experimental samples demonstrate a thermal conductivity of only 36% compared to pure epoxy resin, reaching 0.16 W/mk. The study proved that these composites have excellent mechanical and flame retardant properties, and the flame retardant effect reaches the internationally certified flame retardant standard and has high potential for application in manufacturing automobile interior parts.
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