Reinforcing fillers particle size

It is well known that the particle size, structure, and surface characteristics are important parameters that determine the reinforcing ability of filler particle size is important because a reduction in size... 

Figure I. Reinforcement of silicone gums by carbon blacks (including nonrreinforcmg NR), silica powder (both surface-treated T and untreated NT), and titanium dioxide. The increase in modulus per unit dose and filler concentration depends on filler particle size.

The dependence of the reinforcement on particle size can be viewed in two ways, first is the surface area, and second is particle mass. The surface area of a particle is inversely proportional to its particle size. If all particles are spheres of the same size, the surface area per gram of filler is given by Equation (3.21) ... 

Polymer composites can be classified according to their particle size. Macro-filled composites contain filler particle size more than 10 pm. Midsize fillers are less than 10 pm and more than 1 pm while nanofillers have a particle size less than 1 pm and more than 0.1 pm. The micro-filled composite contains filler particle size less than 0.1 pm. The macrofiller generally is non-reinforcing filler and generally used for cost reduction particularly. 

The reinforcing ability of fillers is influenced by three primary characteristics of the filler particle size, polymer-filler bonding, and particle shape complexity [30]. 

NMR relaxation has been used to estimate the molecular basis behind the reinforcement of polymer melts by surface modified particles. The results show that both chain molecular structure and filler particle size have effects on the properties of the melt and enable an estimate of the proportion of the bound, perturbed and free segments to be obtained. Chemically modified surfaces can be use to modify the impact of the filler on the dynamics of the host polymer. 

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). 

A decrease of crystallite size may be considered a decrease of filler particle size in the amorphous matrix. Theories of particulate reinforcement predict no influence of the filler size on the mechanical properties, although frequently an increase is empirically found [74]. On the other hand, Sumita et al. [75] report that for polypropylene also the opposite behavior can be observed. The reason may be that below a certain crystallite size further reduction will probably lead to weakening of the filler particle. 

With many synthetic elastomeric polymers, the strength properties obtained from a non-reinforced crosslinked polymer are very low and generally unsuitable for industrial applications. Silicones are no exception and although carbon black can be used for reinforcement, fine particle size fume silica is the usual choice for property enhancement. The incorporation of these highly surface-active silicas into silicone gums is a difficult process due to the rapid interaction between polymer and filler resulting in a pseudo-vulcanised mass. For this reason a variety of siloxane based filler treatments are generally used to control viscosity and other properties. 

When employed in elastomeric systems it is commonly observed that the finer the particle size the higher the values of such properties as tensile strength, modulus and hardness. Coarser particles will tend to give compounds less strong than compounds with the filler absent, but if the particle size is sufficiently fine there is an enhancement in the above-mentioned properties (at least up to an optimum loading of filler) and the phenomenon is known as reinforcement. The particle shape also has an influence for example, the somewhat plate-like china... 

For equivalent particle size the carbon blacks are the most powerful reinforcing fillers. However, fine particle size silicas can be very useful in non-black compounds whilst other fillers such as aluminium hydroxide, zinc oxide and calcium silicate have some reinforcing effect. 

Calcium silicate produced by precipitation is a fine powder with particle sizes down to 1 uni. It is a reinforcing filler with a reactivity greater than aluminium silicate. It requires the use of additional accelerator as it slightly retards the vulcanisation reaction. 

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. 

The furnace process involves injecting low end fraction of cmde oil, eg, Bunker Fuel C, into a heated chamber. The temperature, shape of the injectors of the oil, rate of injection, and other factors are controlled to produce black fillers of different particle size and structure. The particle size and structure control the reinforcing character of the carbon black. There are 30 common grades of carbon black used in the mbber industry. There are numerous specialty grades produced, and several hundred are used in plastic, conductive applications, and other uses. 

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