Surface-area fillers

Figure 20.1. NO content vs. filler surface area. [Adapted, by permission, from Gorl U, De Kok J J, Bomal Y, Cochet P, Mueller H, Kaut. u. Gummi Kunst., 47, No.6, June 1994, 430-4.]...

Density measurements were done on different composite samples before the tensile test and immediately after they reached the yield point. In each case we observed density decrement, which was of the order of 0.5 t i% and which could be attributed to microvoid formation during the test the decrement was higher in the case of composites containing SIC treated beads. In the case of yield stress Oy, the effects of the filler should depend on the filler surface area and therefore related to 02/3, Smith and Nicolais and Narkis theoretically suggested lower limit value of Oy, attributing the loading capacity to the polymer only. If n spherical particles of radius r are dispersed in the unit cube the cross section of the continuous phase is A = 1 - IT-(nr) 2 and the volume fraction is 0 = jn(nr). After substitution we get A = 1 - (3/4n) / 0 / =... 

Dispersion is highly influenced by filler surface area the higher the surface area, the lower the dispersion (Sone et al., 1992). This result is probably due to the fact that high surface area usually has smaller aggregates, which will develop more interactions with their neighbors in the dry state. 

A number of studies of high-surface-area fillers (surface area 100 m /g) reveal detectable increases in Tg or other changes in relaxation behavior. 

Surface treatments for Cab-O-Sil also influence the viscosity and thixotropic properties of an adhesive as shown in Fig. 3.18 where MS-5 is untreated Cab-O-Sil, TS-720 is treated with polydimethylsiloxane, and TS-610 and TS-530 are treated with dimethyldichlorosilanes. The silica filler treated with polydimethylsiloxane (TS-720) produces greater van der Waals forces of attraction and thus higher viscosity compared to TS-610 and TS-530.1 The surface area of the filler has a pronounced effect on viscosity. As the surface area of silver-flake filler increases, the frictional forces between particles increase resulting in higher viscosity. Epoxy formulations filled with constant-size silver flakes of various surfaces areas show this effect when viscosity is plotted as a function of the filler surface areas (Fig. 

The modulus at minimum and low strain amplitudes is due to the so-called filler network and it is accepted that the filler surface area, as well as the surface activity, play a major role in establishing a filler network, determining the effective contact area between filler particles and between filler particles and the elastomer matrix. The stress assisted disruption of the filler network causes the reduction of the modulus as the strain amplitude increases, giving rise to the non-linearity of the dynamic-mechanical behaviour of the rubber composite. This phenomenon is known as the Payne effect and it is (to a certain extent) reversible. The disruption and re-formation of the filler network is... 

In an industrial setting, dwell times of only 2-3 min are common. Since the main covalent reaction is completed after about 15-30 min, an additional heating step is recommended to assure fixation of the silane on the mineral. In some cases, a catalyst can help activate a slowly reacting silane or mineral [20]. Typical silane loadings are between 0.7 and 2 wt% relative to the filler and depend on filler surface area and chemistry and application procedure. 

First, it is necessaiy to emphasize that nanocomposite reinforcement comprises several contributions. Above the neat matrix Tg, immobilization of polymer chains becomes the primary effect given by the large nano-filler surface area open for polymer-filler contacts. Thus, the nanocomposite represents a system with high filler-polymer interface area. This is analogous in respect of surface interaction to another system produced by depositing thin polymer layers on solid substrates. 

Nanocomposites represent systems, where specific filler surface area commonly exceeds 100 m /g and filler particle dimensions are easily comparable with the dimensions of polymer chains. Due to the large internal surface area exposed to the filler-polymer interaction, it is reasonable to assume that the... 

The mechanisms responsible for the property enhancements are generally accepted as associated with the inhibition of polymer molecular motions near the filler surface. Macroscopic measurements of composite properties show that for the nylon/clay system, basic mechanical properties such as modulus, strength and impact strength cease to improve b ond a concentration of 5 wt% (equivalent to 2 vol%) [1]. If we accept the primary role of filler surface, and therefore filler surface area, the existence of a ceiling on property enhancement... 

The action of particulate fillers on a thermoplastic is dependent on factors that can be classified as extensity, intensity and geometrical factors. The extensity factor is the amount of filler surface area per m of the composite in contact with the plastic. The intensity factor is the specific activity of this solid surface per m of interface, determined by the chemical and physical nature of the filler surface in relation to the plastic. Geometrical factors are the structure including shape of filler (anisotropic such as such as lamellar, plate, needle and isotropic such as spherical), their particle size and size distribution as well as porosity. Among those the chemical nature of the filler surface plays most vital role in determining the degree of plastic-filler interaction. 

In general terms, the effect of a filler on rubber physical properties can be related mainly to how many polymer chains are attached to the filler surface and how strongly they are attached. Filler surface area and activity are the main determinants, supplemented by structure. Since the filler particles can be considered crosslinks for the elastomer chains, the presence or absence of a coupling agent on the surface of non-black fillers is also important. 

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