Filler agglomeration

Clarke,. 1. and Freakley, P.K., 1995. Modes of dispersive mixing and filler agglomerate size distributions in rubber compounds, Plast. Rubber Compos. Process. Appl. 24, 261-266. 

FIGURE 19.1 Morphology of nano-filler in rubbery matrix Nano-particles are aggregated, and the aggregates also associate to give filler agglomerate in rubber. (From Kohjiya, S., Kato, A., Suda, T., Shimanuki, J., and Ikeda, Y., Polymer, Al, 3298, 2006. With permission.)... 

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]. 

For filler agglomerates, the cohesive energy between aggregates, Wc, can, therefore, be estimated by... 

The macro-dispersion of the fillers can be determined by light-microscopic techniques with computer-assisted image processing on glazed cuttings of the vulcanized samples. At least five picture details have to be evaluated for each specimen. The dispersion coefficient D is calculated from the ratio of non-dispersed filler agglomerates and the volume fracture of the filler in the composites in accordance with ASTM D2663. 

It is apparent from the data that particles of a few nanometers in size can only be made on industrial scale by synthetic methods. On the other hand, these particles are either intentionally or unintentionally aggregated and agglomerated in their powder forms. Thus, for the dispersion of fillers, agglomerate and aggregate size is usually as relevant as the primaiy particle size. Fillers, which are obtained by various milling and classification processes, can also be obtained in the form of small particles, but usually not below 100 nm. 

Heat conductivity of composite materials are severely and adversely affected by structural defects in the material. These defects are due to voids, uneven distribution of filler, agglomerates of some materials, unwetted particles, etc. Figure 15.18 shows the effect of filler concentration on thermal conductivity of polyethylene. Graphite, which is a heat conductive material, increases conductivity at a substantially lower concentration than does quartz. These data agree with the theoretical predictions of model. Figure 15.19 shows the effect of volume content and aspect ratio of carbon fiber on thermal conductivity. This figure should be compared with Figure 15.17 to see that, unlike electric conductivity which does depend on the aspect ratio of the carbon fiber, the thermal conductivity is only dependent on fiber concentration and increases as it increases. 

The dispersion results in progressive spreading of filler particles. This operation involves a reduction in size of filler agglomerates, possibly to their ultimate particle size and, generally takes place between the two power peaks. An attempt in theoretically predicting the dispersive mixing process from fundamental considerations has recently been published (7). 

Thixotropy is the decline of viscosity with time for a rubber under conditions of steady state shearing. This decrease in viscosity can be related to filler loading and the destruction of filler agglomerates [82]. When the shearing is stopped, the viscosity can rise again or recover as the filler particles reagglomerate. 

Fig. 5.8 Fracture resistance of filler-reinforced SBR/BR blends as a function of the macro dispersion index DI DI was determined by a light-microscopic investigation and is a measure for the number of filler agglomerates being larger than 3-5 pm.

Inadequate processing conditions, resulting in filler agglomeration and poor filler dispersion within the matrix... 

More recently nanoscale fillers such as clay platelets, silica, nano-calcium carbonate, titanium dioxide, and carbon nanotube nanoparticles have been used extensively to achieve reinforcement, improve barrier properties, flame retardancy and thermal stability, as well as synthesize electrically conductive composites. In contrast to micron-size fillers, the desired effects can be usually achieved through addihon of very small amounts (a few weight percent) of nanofillers [4]. For example, it has been reported that the addition of 5 wt% of nanoclays to a thermoplastic matrix provides the same degree of reinforcement as 20 wt% of talc [5]. The dispersion and/or exfoliahon of nanofillers have been identified as a critical factor in order to reach optimum performance. Techniques such as filler modification and matrix functionalization have been employed to facilitate the breakup of filler agglomerates and to improve their interactions with the polymeric matrix. 

In the case of HAF N330 filler in its virgin first cycle that the resistivity increased up to 20% strain. They have attributed this initial increase in resistivity to the breakdown of the filler network structure in the rubber. They observed that when the applied tensile strain increases above this 20% strain, the resistivity with strain graph reaches a plateau. They suggested the occurrence of this phenomenon is a consequence of the orientation effects of filler imder strain and also the effect of the reformation of some of the conduction paths. When the load is removed, the resistivity does not return to its original value but increases further. This indicates that some of the breakdown of the filler agglomerate structure is permanent. 

Non-linear mechanical properties were observed for rubber eomposites and referred to as the Payne effect. The Payne effeet was interpreted as due to filler agglomeration where the filler clusters formed eontained adsorbed rubber. The occluded rubber molecules within filler elusters eould not eontribute to overall elastic properties. The composites behaved similarly to rubber composites with higher filler loading. Uniform and stable filler dispersion is required for rubber composites to exhibit linear viscoelastic behaviour. Payne performed dielectric measurements on SBR vulcanizates containing silica or carbon black. The dielectric data were used to construct time-temperature superposition master curves. The reference temperature increased with crosslinking but not significantly with filler. Comparison of dynamic mechanical and dielectric results for the SBR blended with NR was made and interpreted. ... 

The OM analysis is carried out on thin eryoseetions of about 1 mm, as required for light transmittance. In eorrespondenee of filler agglomerates or aggregates the light scatters, giving dark areas. With image analysis techniques, it is possible to calculate the surface of these areas and to evaluate a... 

However, for making a reliable and representative analysis, a large number of thin sections, obtained by cryomicrotomy from different zones of the composite, are required. Moreover, during the preparation of the thin eryoseetions, cutting artefacts can be produced in the case of highly loaded rubber and plastics. They appear as dark striations with the same opacity of the filler agglomerates and they must be eliminated by image analysis software. 

Controlled rheological properties (higher loadings with no viscosity increase) Improved filler dispersion (no filler agglomerates)... 

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