Mica particles

Table 17. Effect of the number of processing cycles on the properties of molded PP samples filled with mica particles [374]...

Pearlescent pigments contain small flakes or platelets of the mineral mica that are additionally coated with a very thin layer of titanium dioxide. The simultaneous reflection of light from many layers of small platelets creates an impression of luster and sheen. By varying the thickness of the coating on the surface of the mica particles, pigment manufacturers can achieve a range of colors for the pearlescent effect. 

Thus it is seen that all the mica particles settle at approximately the same velocity. 

The development of the mica-based pigments started with pearlescent colors (Fig. 76 A, TiOz - mica). It was followed by brilliant, mass-tone-colored combination pigments (i.e., mica, Ti02, and another metal oxide) with one color (interference color same as mass tone) or two colors (interference and mass tone different) that depend on composition and viewing angle (Fig. 76 B). In the 1980s further development was made by coating mica particles with transparent layers of iron(III) oxide (Fig. 76 C) [5.222]. 

Mica particles coated with a metal oxide film have three layers with different refractive indices (layer 1 and 3 are identical, layer 2 is mica) and four interfaces (see chapter 16.2). Interference of light is generated by reflections of all six possible combinations of the four interfaces. Some of them lead to equal effects. The thickness of the mica platelets varies according to a statistical distribution. As a con-... 

FIGURE 13.19 SEM micrograph showing a horizontal view of the dip coated layer of mica particles. Photo courtesy of M. Albers and T. A. Ring, LTP-DMX-EPFL, Lausanne, Switzerland. 

Clay-size mica particles, as Figure 6.3 illustrates, release as much as half their almost instantaneously, but resist the subsequent release of the remaining K. It... 

Figure 6.4. Schematic picture of edge weathering of large mica particles and layer weathering of small mica particles.

The electron micrograph of Figure 10 is of particular interest since it apparently shows edges (rectangular shapes) as well as faces and indicates the relative thickness of the particles. Also, it can be seen that the particles appeared layered much in the manner of microscopic mica particles. The electron micrograph of Figure 11, which is of material from solution C, indicates that microcrystalline gibbsite is ultimately formed even if rM value is as low as 0.94 and pH is near 4. 

Kaolinite, on the other hand, has no structural counterpart among the igneous minerals. It is also the most widespread of the crystalline clay mineral. The most likely mechanism for kaolinite formation is the complete breakdown of feldspar or mica particles and the precipitation of kaolinite from Al(OH)3 and Si(OH)4 from the soil solution or from amorphous, less stable intermediates. 

Because the 57Vratio is proportional to the specific surface of the mineral and being higher for mica than for talc, it follows that specific surface would always be lower for mica than for talc particles. Then for the same crystaHine amount of the polypropylene matrix, a higher fraction of amorphous phase involved in the coating of talc particles than in the coating of mica particles would be expected. 

Silica (silicon dioxide) particles, synthetic or natural based, have commonly been used in various polymer systems. Silica has a low coefficient of thermal expansion and high stiffness, translating into increased modulus of the compounded polymer. However, silica filler particles are not flake or plate like as talc or mica particles are, and typically have low aspect ratios. This means that unless the particles are very small, a silica filler addition provides a relatively low surface area for contacting the polymer, and thus it reinforces the resin system less than platy fillers. Researchers have also noted that mica-filled PP, for instance, contains fewer voids than silica-filled PP, which helps explain the higher strengths of mica compounds at 20% filler loadings. This tendency to form voids or cavities increases as silica filler content increases [7-20, 7-21). 

The coats are sintered at a temperature in the range of 400-420 °C. The grain size of the PEEK powder has a mean grain size of about 20 xm. Suitable inert fillers are from the group of metal oxides, silica, mica particles, and flaked fillers. 

Table 14.4 Compounding Effect on Mica Particle Size...

Mica particle size has significant effects on most composite properties. Table 14.23 shows these effects for a 30 wt% filled polypropylene copolymer composite. It is not possible to get good trends from this table based on particle size only. These products were prepared by different methods including wet grinding, dry grinding, and screening of spiral mica. These production processes provide different degrees of and different aspect ratios for particles of similar diameter. 

Table 14.23 Mica Particle Size Effects on Polypropylene Composite Properties...

Effects of mica particle size on 30 wt% filled polypropylene copoiymer, Profax 8623. Source. Ref. 41. 

Naturally occurring micas have a variety of chemical compositions and morphologies. Three major types are commercially available for use in polymers wollastonite, muscovite, and phlogopite. Mica particles are sometimes fiber shaped, but more commonly platelet or flake shaped. Their aspect ratio (width-to-thickness ratio) plays an important part in the reinforcement properties. 

Muscovite (KAl3Si30io(OH)2) mica particles that are less than about 70 Xm in size are used in mica MMCs [84]. Galvanic corrosion between aluminum and muscovite should not be a problem, since muscovite is an insulator with resistivities that range from about lO to fl cm [85]. Muscovite is insoluble in cold water [86] however, it has been reported that muscovite particles have a tendency to absorb moisture and then swell [87]. 

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