Rubber fillers, semi-reinforcing

Silica, fumed Wollastonite filler, semi reinforcing rubber Sodium magnesium aluminosilicate... 

The proven compounding practice for the natural rubber layer is to load it with 30 phr of graphite, 30 phr of semi-reinforcing carbon black filler and 50 phr of inert filler, china clay. A low sulfur curing system is adopted for heat resistance with a suitable antioxidant to prevent flex-cracking. A typical chlorine resistant soft natural rubber compound formula is given in Table 3.2. 

The subject of filler reinforcement on vulcanized rubber is very wide and complex. Fillers can be classified as reinforcing, semi-reinforcing and non-reinforcing. All fillers increase the hardness, modulus and stiffness of vulcanized rubber whether or not they are reinforcing or non-reinforcing. It has been established for a very long time that the term reinforcement has been widely used by the rubber technologist to denote the enhancement in the tensile... 

Non- or semi-reinforcing fillers These are usually added to reduce cost. In natural rubber or polychloroprene, they may be used alone, but with non-crystallizing polymers such as butadiene - acrylonitrile or styrene - butadiene copolymers, they can only be used in conjunction with a reinforcing filler. Their effect is to reduce tensile strength and elongation, tear resistance and resistance to set. The effect on modulus varies according to choice of filler, but it is always much weaker than that of a reinforcing filler. 

Examples of semi-reinforcing and non-reinforcing fillers are given in Table 3. The ranking of such fillers is imprecise because it depends upon the use that may be made of them. Certain clays perform well in insulation compounds, and precipitated whitings are more than adequate to extend rubbers for components that are not subjected to high levels of stress. 

China clay is a widely used white filler in the rubber industry. Depending on particle size, it can be used as a semi-reinforcing filler (hard clay) or a non-reinforcing filler (soft clay) in such applications as chemical liners, bicycle tyres, conveyor belts, shoe soles, gaskets and flooring. Its use in plastics is much more limited. In thermoplastics it is used for speciality antiblocking, in thermosets it is used in urea-, phenol- and melamine formaldehyde, in unsaturated polyesters, and in epoxy resins. 

From all of these systems, there remains the disposal of the process waste treated fabric, from seams, ends, etc. These can be used for certain applications, by shredding the treated textile and using it as a semi-reinforcing and bulking filler in rubber products such as dustbin lids, traffic cones, and so on. 

The barrier properties of 70/30 acrylonitrile-butadiene mbber/ethylene propylene diene monomer rubber (NBR/EPDM) vulcanizates, when loaded with carbon black fillers [e.g., I SAP (intermediate super-abrasion furnace), HAF (high-abrasion furnace) and SRF (semi-reinforcing furnace)] and using benzene, toluene and xylene as penetrants, have been examined with reference to the type of filler employed [66]. The filled samples were found to exhibit a better resistance to uptake of the three organic solvents when compared to the respective unfilled blends for any given blend ratio. With regards to the three types of carbon black used, solvent uptake was in the order SRF-> HAF-> ISAF-filled samples. The reason for this order was attributed to the better filler reinforcements and enhanced crosslink densities of the matrix as the size of the carbon black particles used was decreased. A similar behavior was also identified for NR/EVA composites [52]. 

The anisometry (platelet shape) and particle size of the clays account for their affect on modulus and hardness. More hard clay than soft is used in rubber because of its semi-reinforcing effect and its utility as a low cost complemenent to other fillers. It is used to improve the tensile and modulus of ground calcium carbonate eompoimds and will substitute for a portion of the more expensive carbon black or precipitated silica in certain compounds without sacrificing physical properties. 

FIG U RE 11.1 Typical contour plot to abbreviate compounding data (Vistalon 3708/Ethylene-Propylene Terpolymer, Enjay Chemical Company, 1968). Note that a compound with 200 parts each of oil and filler per 100 parts of rubber by weight (phr) still has a strength of about 1000 psi (7 MPa), quite acceptable for many mechanical applications. The compound, based on a high-molecular-weight ethylene-propylene terpolymer, also contains 5 parts zinc oxide, 1 part stearic acid, 1.5 parts sulfur, 1.5 parts tetramethylthiuram disulfide, and 0.5 parts of benzothiazyl disulfide. Cross-linking takes place at 160°C for 20 min. SRF, Semi-reinforcing furnace. 

---