Fillers dispersion characterization

In very recent years, filler dispersion characterization has been brought again into light because of the difficulties encountered to disperse silica in rubber [93,94]. 

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

The response of unvulcanized black-filled polymers (in the rubbery zone) to oscillating shear strains (151) is characterized by a strong dependence of the dynamic storage modulus, G, on the strain amplitude or the strain work (product of stress and strain amplitudes). The same behavior is observed in cross-linked rubbers and will be discussed in more detail in connection with the dynamic response of filled networks. It is clearly established that the manyfold drop of G, which occurs between double strain amplitudes of ca. 0.001 and 0.5, is due to the breakdown of secondary (Van der Waals) filler aggregation. In fact, as Payne (102) has shown, in the limit of low strain amplitudes a storage modulus of the order of 10 dynes/cm2 is obtained with concentrated (30 parts by volume and higher) carbon black dispersions made up from low molecular liquids or polymers alike. Carbon black pastes from low molecular liquids also show a very similar functional relationship between G and the strain amplitude. At lower black concentrations the contribution due to secondary aggregation becomes much smaller and, in polymers, it is always sensitive to the state of filler dispersion. 

It is seen in the figures that the magnetoelast shows ideal mechanical behavior in the studied deformation range. Similar ideal mechanical behavior was observed for other magneto elasts characterized by different cross-linking densities and different amount of fillers dispersed randomly in the elastic matrix. It has to be mentioned that within the experimental accuracy (5%) no hysteresis has been found. 

Even if the relationship between filler dispersion and abrasion resistance is well established, relatively few studies have been done on the characterization of filler dispersibility. This is mainly due to the fact that carbon black dispersibility was commonly judged satisfactory, partly because it is indeed high, but more probably because all mixing apparatuses were designed for dispersing carbon blacks. 

The most commonly used technique to qualify filler dispersibility is to study light reflectivity of clean-cut mixes. Some apparatuses have been developed to evaluate filler dispersion using a calibrated set of reference mixes (Dispergrader). However, such characterization mainly detects dispersion defects of a few tens of microns, and direct comparison of carbon black and silica mixes has to be done cautiously. In any case, it is necessary to make a mix, which means choosing a formula, a mixer, and mixing conditions thus the result cannot be considered as an intrinsic dispersibility measurement of the filler, but just reflects the dispersibility of the filler in one mix with a set of mixing conditions. 

Recently, a method has been patented to determine filler dispersibility. It consists of measuring continuously the size of the filler by laser granulometry during an ultrasonic desagglomeration (W09928376 et al., 1997). This characterization can be applied to any filler and is an intrinsic property however, the use of water as a desagglomeration medium can be a problem because of its high polarity compared to elastomers. 

Filler dispersion is defined at different levels macro-dispersion, which characterizes the incorporation of fillers into the matrix, micro-dispersion, which describes the random distribution of aggregates, tactoids and primary particles within the polymer matrix, and nano-dispersion, which is fundamental for obtaining structural information on nanofillers. 

From a practical point of view, an accurate morphological characterization of the filler dispersion allows us to ascertain the homogeneity of the filler network, responsible for the material s performance, as well as to detect structural differences in the filler aggregates and to relate them to the final properties of the composite material. 

The morphology characterization of hybrid membranes is very important to identify the possible interfacial morphology and particle dispersion in the final membrane matrix. Electron microscopy is typically used to investigate the filler dispersion and the hybrid membrane morphology. Scanning electron microscopy is the most frequently used technique, and it allows the characterization of the sample surface. The sample s cross-sections can be examined to analyze the inner morphology. Transmission electron microscopy is also a very useful technique because it allows a direct evaluation of the inner morphology of the sample. 

The use of stearic acid as a modifier for silica and other fillers like CaCOs and Mg(OH)2 has been reported. The authors found that the presence of adsorbed stearic acid on the filler surface reduces the hydrophilicity of the silica surface and enhances the compatibility between filler and matrix, which may lead to an improvement in filler dispersion and the related mechanical performance of composites. Kosmalska et al also investigated the adsorption of DPG, ZnO and sulfur on the silica surface and reported that the bonding of DPG/ZnO and ZnO to silica causes a reduction in the surface energy of silica from 66 mN/m to 28.75 mN/m and 35.49 mN/m, respectively. A similar effect of ZnO on the surface tension of silica was also found by Laning et alP and Reuvekamp et al. The adsorption of that additive and its impact on the scorch time and reduction of the crosslink density in silica-filled rubber compounds have been frequently characterized. ... 

Abstract A CaCOs filler was coated with various mono- and dicarboxylic acids in a dry-blending process. The coated fillers were characterized by various techniques, including dissolution experiments, thermal analysis (differential scanning calorimetry) and inverse gas chromatography (IGC) to determine the amount of surfactant needed to achieve mono-layer coverage IGC proved to be the most convenient, reliable and universal method for this purpose. The dispersion component of the surface tension and the specific interaction potential of the coated filler can be derived from the results, but indirect conclusions can be also drawn from them about the orientation of the molecules on the filler surface and the structure of the layer formed. The coverage of the filler with an organic compound leads to a... 

Polymer Filler Dispersant Solvent Fiber d (nm) Characterization Reference... 

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