1.1 Molding process
The molding process of NASC is similar to that of the traditional rubber based composites, as shown in Fig. 1. The plastication should be carried out using a mill mixer or an internal mixer, to improve plasticity of raw rubber. Before this step, raw rubber may be roasted in a hot chamber within 333-343 K to decrease its hardness and improve manufacturability. The roll temperature of the mill mixer should be controlled below 343 K, and the roller space is about 0.5-1 mm.
The gross rubber is obtained in the mixing step where all accessory ingredients and filling materials are evenly dispersed in broken-down rubber by extrusion and shearing actions of rollers repeatedly.
The mixing step can also be carried out in a mill mixer or an internal mixer. During mixing, the reinforcing fibers can be evenly dispersed in gross rubber, and the aspect ratio of the fibers is reduced to an appropriate value. In the vulcanization step, the rubber and a vulcanizing agent will chemically crosslink at sulfurizing pressures and temperatures. The effect of sulfurization parameters on the transverse tensile strength of aramid fiber reinforced NASC is illustrated in Fig. 2.