Specific surface area, fillers

The single selection of particle diameter for the characterization of a reinforcing filler is, however, not appropriate, because, on the one hand, only fillers exhibiting a very poor reinforcing effect consist of independent spherical particles, and, on the other hand, gum-filler interactions taking place at the elastomer-filler interface are thus conditioned by the accessibility of the surface. The latter may, indeed, be restricted either by the presence of micropores or by the size of the macromolecule. The knowledge of the specific surface area of the filler is thus a prerequisite. Insofar as the determination of the filler specific surface area, performed by low-temperature gas adsorption or iodine adsorption, takes into account its microporosity, the adsorption of larger tensioactive molecules will often be favored 12,13). 

Filler specific surface area (m g ) Burn time (s) Extent bimit (mm)... 

The external surface area of the filler can be estimated from a psd by summing the area of all of the equivalent spheres. This method does not take into account the morphology of the surface. It usually yields low results which provide Htde information on the actual area of the filler that induences physical and chemical processes in compounded systems. In practice, surface area is usually determined (5) from the measured quantity of nitrogen gas that adsorbs in a monolayer at the particle surface according to the BET theory. From this monolayer capacity value the specific surface area can be determined (6), which is an area per unit mass, usually expressed in m /g. 

In case of a polydisperse filler with a specific surface area Sspec, it has been proposed to calculate the mesophase thickness the product Afiller volume per unit of its mass) [50,65]. 

Table 4. Effect of specific surface area of filler and its concentration on concentration shift factors aci and ac for composites copolymer + ash (the reference concentration 10%)...

Filler content, % aci values for ash with specific surface areas, m2/g ... 

Specific surface area, m2/g aCl values for filler concentration of (%) ... 

Fig. 2 a, b. Dependence of the maximum Newton viscosity (t/0) (a) and swelling ratio of the extrudate (D) (b) on the molecular mass of cis-1,4-polyisoprene, unfilled and filled to 33% by mass. Filler 1 — not 2 — CaC03 with specific surface areas 2-3 m2/g 3 — ash PM —15 with specific surface areas 12-18m2/g 4 — ash PM-100 with specific surface areas 90-100 m2/g... 

Vaterite is thermodynamically most unstable in the three crystal structures. Vaterite, however, is expected to be used in various purposes, because it has some features such as high specific surface area, high solubility, high dispersion, and small specific gravity compared with the other two crystal systems. Spherical vaterite crystals have already been reported in the presence of divalent cations [33], a surfactant [bis(2-ethylhexyl)sodium sulfate (AOT)] [32], poly(styrene-sulfonate) [34], poly(vinylalcohol) [13], and double-hydrophilic block copolymers [31]. The control of the particle size of spherical vaterite should be important for application as pigments, fillers and dentifrice. 

Nonmetal electrodes are most often fabricated by pressing or rolling of the solid in the form of fine powder. For mechanical integrity of the electrodes, binders are added to the active mass. For higher electronic conductivity of the electrode and a better current distribution, conducting fillers are added (carbon black, graphite, metal powders). Electrodes of this type are porous and have a relatively high specific surface area. The porosity facilitates access of dissolved reactants (H+ or OH ions and others) to the inner electrode layers. 

The amount that is theoretically needed to completely cover the surface of a filler can be calculated in many instances if the specific surface area of the filler (or better the spacing of coating reactive groups on the surface) and the area oc-... 

In general, the filler industry recognises these limitations, and tries to use a few relatively simple parameters that, taken in combination, give an approximate, working definition of morphology appropriate to the application in mind. The parameters that are most likely to be encountered are specific surface area, average particle size, effective top size and oil adsorption. The measurement and application of these are discussed in more detail below. 

The specific surface area is obviously related to the particle size distribution of the filler and can be used as a guide to this. For materials of the same density and shape a higher specific surface area means a smaller particle size, but again it must be remembered that two distinctly different particle size distributions can give rise to the same value for the specific surface area and so it is not a unique property. 

Aluminium hydroxide has a Moh hardness of about 3 and a specific gravity of 2.4. It decomposes endothermically with the release of water at about 200 °C and this makes it a very useful flame retardant filler, this being the principal reason for its use in polymers. The decomposition temperature is in fact too low for many thermoplastics applications, but it is widely used in low smoke P VC applications and finds some use in polyolefins. For these applications low aspect ratio particles with a size of about 1 micron and a specific surface area of 4-10 m g are preferred. The decomposition pathway can be diverted through the mono-hydrate by the application of pressure, and this may reduce the flame retardant effect [97]. This effect can be observed with the larger sized particles. Although it is chemically the hydroxide, it has for many years been known as alumina trihydrate and by the acronym ATH. 

The production process is able to produce all three crystal modifications of calcium carbonate and a wide variety of particle sizes and shapes, including plates and acicular forms [107]. However, only the calcite form with a rhombic shape and a low aspect ratio seems to have found much commercial application in polymers. For filler applications the particles have an ultimate particle size of 50-100 nanometers, a specific surface area of 15-25 m g and a low aspect ratio. 

The characteristics of particulate filled polymers are determined by the properties of their components, composition, structure and interactions [2]. These four factors are equally important and their effects are interconnected. The specific surface area of the filler, for example, determines the size of the contact surface between the filler and the polymer, thus the amount of the interphase formed. Surface energetics influence structure, and also the effect of composition on properties, as well as the mode of deformation. A relevant discussion of adhesion and interaction in particulate filled polymers cannot be carried out without defining the role of all factors which influence the properties of the composite and the interrelation among them. 

The specific surface area of fillers is closely related to their particle size distribution however, it also has a direct impact on composite properties. Adsorption of both small molecular weight additives, and also that of the polymer is proportional to the size of the matrix/filler interface [14]. Adsorption of additives may change stability, while matrix/filler interaction significantly influences mechanical properties, first of all yield stress, tensile strength and impact resistance [5,6]. 

Although a number of filler characteristics influence composite properties, particle size, specific surface area, and surface energetics must again be mentioned here. All three also influence interfacial interactions. In the case of large particles and weak adhesion, the separation of the matrix/ filler interface is easy, debonding takes place under the effect of a small external load. Small particles form aggregates which cause a deterioration in the mechanical properties of the composites. Specific surface area, which depends on the particle size distribution of the filler, determines the size of the contact surface between the polymer and the filler. The size of this surface plays a crucial role in interfacial interactions and the formation of the interphase. 

Both adhesive and hydrodynamic forces depend on the size of the particles. The two forces were calculated for CaC03 fillers of various particle sizes homogenized in a PP matrix. The results are presented in Fig. 3. At a certain particle size adhesion exceeds shear forces, aggregation of the particles takes place in the melt. Since commercial fillers have a relatively broad particle size distribution, most fillers show some degree of aggregation and the exact determination of the particle size, or other filler characteristics where aggregation appears, is difficult. Experiments carried out with 11 different CaC03 showed this limit to be around 6 m /g specific surface area [25]. 

The amount of polymer bonded in the interphase depends on the thickness of the interlayer and on the surface area, where the filler and the polymer are in contact with each other. The size of the interface is more or less proportional to the specific surface area of the filler, which is inversely proportional to particle size. In accordance with the above proposed explanation on the relation of the effect of immobilized polymer chains and the extent of deformation, modulus shows only a very weak dependence on the specific surface area of the filler [64]. 

The specific surface area of the filler is an important factor which must be taken into consideration during surface treatment. The proportionally bonded surfactant depends linearly on it [74]. ESCA studies carried out on the surface of a CaC03 filler covered with stearic acid have shown that ionic bonds form between the surfactant molecules and the filler surface and that the stearic acid molecules are oriented vertically to the surface [74]. These experiments have demonstrated the importance of both the type of the interaction and the alignment of sur-... 

Fig. 20. Dependence of the maximum packing fraction on the specific surface area of the filler in PP/CaC03 composites...

Interfacial structure is known to be different from bulk structure, and in polymers filled with nanofillers possessing extremely high specific surface areas, most of the polymers is present near the interface, in spite of the small weight fraction of filler. This is one of the reasons why the nature of the reinforcement is different in nanocomposites and is manifested even at very low filler loadings (<10 wt%). Crucial parameters in determining the effect of fillers on the properties of composites are filler size, shape, aspect ratio, and filler-matrix interactions [2-5]. In the case of nanocomposites, the properties of the material are more tied to the interface. Thus, the control and manipulation of microstructural evolution is essential for the growth of a strong polymer-filler interface in such nanocomposites. 

Hence, it is supplanted by the filler density (p) and its specific surface area (Q to address the shape factors. This can successfully represent the aspect ratio effects because changes in aspect ratio are reflected in the ratio of particle surface area to particle volume, i.e., density x surface area/gram. 

The morphology of a filler, such as carbon black, is usually well estimated by its specific surface area accessible to the rubber molecules and by its density. 

Specific Surface Area. The specific surface area of industrial carbon blacks varies widely. While coarse thermal blacks have specific surface areas as small as 8 m2/g, the finest pigment grades can have specific surface areas as large as 1000 m2/g. The specific surface areas of carbon blacks used as reinforcing fillers in tire treads lie between 80 and 150 m2/g. In general, carbon blacks with specific surface areas >150 m2/g are porous with pore diameters of less than 1.0 nm. The area within the pores of high-surface-area carbon blacks can exceed the outer (geometrical) surface area of the particles. 

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