Rubber filler particle size

The principal characteristics of rubber fillers - particle size, surface area, structure, and surface activity - are interdependent in improving rubber properties. In considering fillers of adequately small particle size, reinforcement potential can be quaUtatively considered as the product of surface area, surface activity, and persistent structure or anisometry (planar or acicular nature). 

This property depends on filler particle size and filler loading. Higher filler loading gives lower resilience. Rubber type plays a great role here in that no synthetic rubber can match the characteristic of high resilience of natural rubber. 

Equation (70) is a scaling invariant relation for the concentration-dependency of the elastic modulus of highly filled rubbers, i.e., the relation is independent of filler particle size. The invariant relation results from the special invariant form of the space-filling condition at Eq. (67) together with the scaling invariance of Eqs. (68) and (69), where the particle size d enters as a normalization factor for the cluster size only. This scaling invariance disappears if the action of the immobilized rubber layer is considered. The effect of a hard, glassy layer of immobilized polymer on the elastic modulus of CCA-clusters can be de-... 

Fine powders being composed of particle aggregates such as precipitated silicas or white carbon are made by the decomposition of sodium silicate by sulfuric acid in an aqueous phase at high temperature. This material has microporosity as well as macroporosity due to a rapid growth and aggregation of primary particles as will be discussed later. These are used as rubber filler, paper sizing and etc. 

R. Omranipour, L.H. Meyer, S.H. Jayaram, E.A. Cherney, Tracking and erosion resistance of RTV silicone rubber Effect of filler particle size and loading, 2002 Annual Report Conference on Electrical Insulation and Dielectric Phenomena. 

Carbon blacks are the most widely used fillers for elastomers, especially vulcanised natural rubber. They cause an improvement in stiffness, they increase the tensile strength, and they can also enhance the wear resistance. Other particulate fillers of an inorganic nature, such as metal oxides, carbonates, and silicates, generally do not prove to be nearly so effective as carbon black. This filler, which comes in various grades, is prepared by heat treatment of some sort of organic material, and comes in very small particle sizes, i.e. from 15 to 100 nm. These particles retain some chemical reactivity, and function in part by chemical reaction with the rubber molecules. They thus contribute to the crosslinking of the final material. 

The most challenging part of rubber mixing is the dispersion of the filler The filler agglomerates have to be broken into smaller particles, the aggregates, but not completely to the level of primary particles. An optimal particle size distribution has to be achieved in order to obtain the best properties of the final rubber product [14]. 

Liquid-solid transitions in suspensions are especially complicated to study since they are accompanied by additional phenomena such as order-disorder transition of particulates [98,106,107], anisotropy [108], particle-particle interactions [109], Brownian motion, and sedimentation-particle convection [109], Furthermore, the size, size distribution, and shape of the filler particles strongly influence the rheological properties [108,110]. More comprehensive reviews on the rheology of suspensions and rubber modified polymer melts were presented by Metzner [111] and Masuda et al. [112], respectively. 

Silica used as a filler for rubbers is silicon dioxide, with particle sizes in the range of 10-40 nm. The silica has a chemically bound water content of 25% with an additional level of 4-6% of adsorbed water. The surface of silica is strongly polar in nature, centring around the hydroxyl groups bound to the surface of the silica particles. In a similar fashion, other chemical groups can be adsorbed onto the filler surface. This adsorption strongly influences silica s behaviour within rubber compounds. The groups found on the surface of silicas are principally siloxanes, silanol and reaction products of the latter with various hydrous oxides. It is possible to modify the surface of the silica to improve its compatibility with a variety of rubbers. 

This filler is mined, ground and sieved to a particle size less than 100 mesh and used as an inert diluent and cheapening filler for rubber compounds. It is usually off-white to cream in colour. Depending upon source, the filler can be contaminated with metal ions, e.g., iron, copper, manganese, which can catalyse oxidation. It can be used in very high loadings with great effect on compound hardness. 

The rubber compound usually requires an inert inoiganic filler and small particle size carbon particle for reinforcement. The rubber polymers vary in inherent tensile strength from very high in the case of natural mbber to almost nonexistent for some synthetic polymers, eg, SBR. The fillers most commonly used for mbber compounds include carbon black, day, calcium carbonate, silica, talc (qv), and several other inorganic fillers. 

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