Rubber filler surface area

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 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. 

It has been well established that wear resistance of filled rubber is essentially determined by filler loading, filler morphology, and polymer-filler interaction. For fillers having similar morphologies, an increase in polymer-filler interaction, either through enhancement of physical adsorption of polymer chains on the filler surface, or via creation of chemical linkages between filler and polymer, is crucial to the enhancement of wear resistance. In addition, filler dispersion is also essential as it is directly related to the contact area of polymer with filler, hence polymer-filler interaction. 

The surface area of a filler per cubic centimetre coming into interfacial contact with the rubber. Extraction... 

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. 

Adsorption Properties. Due to their large specific surface areas, carbon blacks have a remarkable adsorption capacity for water, solvents, binders, and polymers, depending on their surface chemistry. Adsorption capacity increases with a higher specific surface area and porosity. Chemical and physical adsorption not only determine wettability and dispersibility to a great extent, but are also most important factors in the use of carbon blacks as fillers in rubber as well as in their use as pigments. Carbon blacks with high specific surface areas can adsorb up to 20 wt% of water when exposed to humid air. In some cases, the adsorption of stabilizers or accelerators can pose a problem in polymer systems. 

Most elastomers require reinforcing fillers to function effectively, and NMR has been used to characterize the structures of such composites as well. One examples is the adsorption of chains onto filler surfaces,334,335 and the strong absorption of these chains into bound rubber, for example PDMS immobilized onto high surface area silica.316,320 Another example is the use of NMR to image the filler or polymer itself.336-342... 

The last few years have seen the extensive use of nanoparticles because of the small size of the filler and the corresponding increase in the surface area, allowing to achieve the required mechanical properties at low filler loadings. Nanometer-scale particles including spherical particles such as silica or titanium dioxide generated in-situ by the sol-gel process (4-8), layered silicates (9-12), carbon (13) or clay fibers(14,15), single-wall or multiwall carbon nanotubes (16,17) have been shown to significantly enhance the physical and mechanical properties of rubber matrices. 

The characteristic time ro=(q/E0) represents the effects of modulus as well as hysteresis arising from the rubber and/or the filler-rubber interfacial slippage. The inverse particle diameter, 1/d, is proportional to the specific surface area, S, and (dl4 /zg)u being the so-called junction shape factor. The proportionality to h 1A is almost identical to the correlation found by Wolff et al. [89] between tanS and the reciprocal of interaggregate distance, Saa 1. Combined with Eq. (51), Eq. (54) can be written as... 

Fig. 11. The apparent number of elementary chain units ( ) between network junctions (line) and contribution in it from chemical (a) and adsorption (b) Junctions, and topological hindrances near the filler surface (c) as a function of the volume fraction of Aerosil (300 m g" ) in bound PDMS rubber [14] the apparent number of elementary chain units between transient entanglements is shown by an arrow the absolute error for the determination of the n value is shown by the dashed area...

Carbon blacks (c.b.s) have been known since ancient times, for preparing Indian ink. From the 1920s, c.b. has been fabricated industrially on a large scale by the thermal decomposition of hydrocarbons (natural gas) or aromatic hydrocarbons. Of the total production, 90% goes into the rubber industry, and most of this is employed for the reinforcement of tires. Production capacity is at present 7.2 million tonnes/y and the armual production is 6.1 million tormes/y [244] 95% of this global fabrication is by the furnace c.b. process [245]. The specific surface area As (nr/g) in this case covers a range from a few tens up to more than 1500. It should be mentioned that c.b. is used as a filler for conducting polymers [246]. 

This equation was used to estimate the interfacial adhesion in comparison with the acid-base properties of glass fibers in LDPE. The effect of surface treatment of glass beads on their interfacial adhesion to PET was also estimated from a mechanical property measurement. A mathematical model describing the adsorption of polymers on filler surfaces related coupling density to the average area available for coupling between rubber and filler surface. ... 

Figure 7.21 shows that bound rubber increases as the surface area of carbon black increases.This is a classical experiment which shows that the amount of bound rubber depends on the surface area of the filler. High structure carbon blacks... 

Chemical modification of filler surface reduces the surface area available for interaction. This reduces bound rubber (Figure 7.26). The quantity of adsorbing additives on the filler surface must be strictly controlled because these additives compete with the reinforcing effect of the bound rubber. Thermal treatment of rubber increased the quantity of bound rubber but only when rubber was added prior to the addition of low molecular processing additives. " This shows that there was competition between the low molecular additive and the rubber for adsorption sites. 

Figure 8.21. Rubber hardness vs. surface area of silica filler. [Adapted, by permission, from Evans L R, Meeting of the Rubber Division, ACS, Montreal, May 5-8, 1996, paper D.]...

A filler with a high surface area increases the interaction with the matrix and thus increases tear strength (Figure 8.23) When rubber is filled with silica the large surface area of the silica interacts with the rubber and adheres to it. This adhesive interaction allows energy to be stored or dissipated. 

Compression set is an important property of elastomers which is affected by the choice of filler. Studies were conducted on silica in silicon rubber vulcanizates. Figure 8.60 shows the relationship between the surface area of silica and compression set. As the surface area increases compression set increases. The increase surface area contributes to an increase in the number of functional groups on the surface of silica. These groups can potentially react with siloxane. When they do, there is a good interaction of filler with matrix which contributes to reduction of compression set (Figure 8.61). ... 

A study of twelve silica fillers in rubber compounds showed that all of the fillers contributed to the formation of nitrosamines. There was a substantial difference in the amounts of nitrosamines detected in the presence of different fillers (from 1.1 xl 0 to 14.8x10 mol/kg) but this difference could not be correlated with properties such as their structure or surface area. ... 

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