1.2 Selection of raw materials for NASC
The main constituents of NASC are non-asbestos reinforcing fibers, elastic binding materials, filling materials and chemical additives. There are great differences of the physicochemical properties between non-asbestos fibers and asbestos fibers. Therefore, in the design of NASC, the effects of the properties and the content of the non-asbestos reinforcing fibers on the performances and manufacturability of the NASC should be paid more attention.
The suitable non-asbestos reinforcing fibers are usually selected according to the performance requirements of NASC. Heat resistance, aspect ratio, medium-resistance, compatibility with rubber, performance-price ratio and source of the reinforcing fibers
should be considered, too. The commonly used non-asbestos reinforcing fibers include carbon fiber, aramid fiber, glass fiber, mineral wool, plant fiber, and so on.
Elastic binding material binds reinforcing fibers and other filling materials together, makes up elastic network structure, and provides compression-resilience and medium-resistance performances of NASC. Natural rubber, nitrile rubber, styrene-butadiene rubber, neoprene rubber, ethylene propylene rubber, fluorine rubber and silicone rubber can all be used as the elastic binding materials of NASC. In consideration of the product performances and price, natural rubber and nitrile rubber are preferably selected.
The filling materials of rubber products include compatibilization filler, reinforcing filler and functional filler. Commonly used filling materials include brucite fiber or sepiolite fiber, carbon black, carbonate, sulphate, metallic oxides, and silicon oxides. In NASC, large grained filler must pair up with short grained filler to maintain proper inter space among rubber bulk material, fibers and filler particles, and ensure sufficient combination between rubber and filler particles.
The selectable accessory ingredients include vulcanizing agents, vulcanization accelerators, vulcanizing activators, anti-aging agents, plasticizing agents and colouring agents etc.. The mixture of dibenzothiazyl disulfide (40 wt%) and tetramethylthiuram disulfide (60 wt%) can be used as a vulcanization accelerator. Zinc oxide (3-5 wt%) and stearic acid (0.5-2 wt%) can be selected as a vulcanizing activator. N-phenyl-n-isopropyl ursol (4010NA) is usually used as the anti-aging agent of rubbers, and the amount is about 1-4wt% (related to rubber weight). Plasticizing agents, colouring agents, solvent, blowing agents, dusting agents and reinforcing resins etc. can also be adopted as accessory ingredients.