Fillers dispersibility

Surface Area. Surface area is the available area of fillers, be it on the surface or in cracks, crevices, and pores. The values obtained from different methods for measuring the surface area of a filler may vary significandy. These variations are because of the nature of the methods and in many instances yield information related to the heterogeneity of the surface. Understanding the surface area is important because many processing factors are dependent on the surface area, eg, ease of filler dispersion, rheology, and optimum filler loading. 

Plasticizers and Processing Aids. Petroleum-based oils are commonly used as plasticizers. Compound viscosity is reduced, and mixing, processing, and low temperature properties are improved. Air permeabihty is increased by adding extender oils. Plasticizers are selected for their compatibihty and low temperature properties. Butyl mbber has a solubihty parameter of ca 15.3 (f /cm ) [7.5 (cal/cm ) ], similar to paraffinic and naphthenic oils. Polybutenes, paraffin waxes, and low mol wt polyethylene can also be used as plasticizers (qv). Alkyl adipates and sebacates reduce the glass-transition temperature and improve low temperature properties. Process aids, eg, mineral mbber and Stmktol 40 ms, improve filler dispersion and cured adhesion to high unsaturated mbber substrates. 

Hydrophilic/hydrophobic properties. In water-based systems, the filler should be compatible with water because filler dispersion occurs in an aqueous medium before a polymer emulsion is added. In general, most fillers are hydropho-... 

Other components in PVAc-formulations are defoamers, stabilizers, filler dispersants, preservatives, thickeners (hydroxyethylcellulose, carboxymethylcellu-lose), polyvinyl alcohols, starch, wetting agents, tackifiers, solvents (alcoholes, ketone, esters), flame retardants and others. 

Solution blending Polar as well as nonpolar solvents can be used in this method. The polymer is solubilized in a proper solvent and then mixed with the filler dispersion. In solution, the chains are well separated and easily enter the galleries or the layers of the fillers. After the clay gets dispersed and exfoliated, the solvent is evaporated usually under vacuum. High-density polyethylene [24], polyimide (PI) [25], and nematic hquid crystal [26] polymers have been synthesized by this method. The schematic presentation is given in Scheme 2.2. 

Visualization of Nano-Filler Dispersion and Morphology in Rubbery Matrix by 3D-TEM... 

D-TEM gave 3D images of nano-filler dispersion in NR, which clearly indicated aggregates and agglomerates of carbon black leading to a kind of network structure in NR vulcanizates. That is, filled rubbers may have double networks, one of rubber by covalent bonding and the other of nanofiller by physical interaction. The revealed 3D network structure was in conformity with many physical properties, e.g., percolation behavior of electron conductivity. 

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. 

Oils of the three types are offered in a range of viscosities and this will influence their processing character to some extent, although there is little evidence that it will have much influence on the ultimate compound physical properties, at least in natural rubber compounds. The small additions of oil to a compound help with filler dispersion by lubricating the polymer molecular chains and thus increasing their mobility. There will also be some wetting out of the filler particles which enables them to achieve earlier compatibility with the rubber and improve their distribution and dispersion speed. 

A process additive is an ingredient which is added in a small dosage to a rubber compound solely to influence the performance of the compound in factory processes, or to enhance physical properties by aiding filler dispersion. 

Filler surface chemistry is clearly important, although the effects vary widely according to the exact nature of the filler, polymer and surface modifier. Some of the factors that can influence toughness and are, at least in part, controlled by filler surface chemistry include the level of filler polymer interaction [40], the structure of heterophasic polymers [41], the amount of polymer degradation during compounding [42], filler dispersion [43] and polymer crystallinity arising from altered nucleation processes [44]. 

Table 2. Surface Tension of Selected Polymers and Fillers Dispersion (f ) and Polar (yP) Components ...

Further factors influencing rheological characterization of filled polymers include changes in the degree of filler dispersion or inter-particle structure forma-... 

Coalescence is another reverse process in mixing. Whereas this is a major issue in the formation of polymer blends, it is considered of less significance with carbon black or other solid filler dispersions in polymers [83]. 

Fig. 27. Progressive dispersion of calcium carbonate in polypropylene within a co-rotating intermeshing twin-screw extruder. Filler dispersion is expressed in terms of a mean volume diameter determined by image analysis...

Physical Effects of Filler. Dispersion of any hard particulate matter in a soft matrix will yield a composite with quite different properties. The two main causes for these effects are load sharing of the filler particles and strain dilatation of filled elastomers. 

The two systems discussed above demonstrate two mechanisms whereby the tensile strength of elastomers can be reinforced by the presence of rigid fillers. For the polymeric fillers dispersed within a vulcani-zate, the filler operates by raising the viscosity of the matrix, analogous to a decrease in temperature, but without affecting the dynamic, high frequency response (there is ample experimental evidence of the independence of Ty on presence of filler). There is also some indication that the rigidity of the filler affects the extent of reinforcement. 

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