Natural silica siliceous fillers

Natural silicon-based fillers are divided into hydrated silicas and silicon dioxide (silicas) on the one hand, and silicates, on the other. There is no sharp dividing line between these two groups, since the former do not always occur in a pure form and may contain silicates together... 

The hydrated precipitated silica used with general-purpose elastomers in HRH adhesive systems (and also as fully reinforcing filler in rubber compounding) comes from water glass (sodium silicate). This is made from natural silica (sand) that is reacted either with sodium hydroxide or sodium carbonate at very high temperatures see Figure 8.6. 

The effect of polymer-filler interaction on solvent swelling and dynamic mechanical properties of the sol-gel-derived acrylic rubber (ACM)/silica, epoxi-dized natural rubber (ENR)/silica, and polyvinyl alcohol (PVA)/silica hybrid nanocomposites was described by Bandyopadhyay et al. [27]. Theoretical delineation of the reinforcing mechanism of polymer-layered silicate nanocomposites has been attempted by some authors while studying the micromechanics of the intercalated or exfoliated PNCs [28-31]. Wu et al. [32] verified the modulus reinforcement of rubber/clay nanocomposites using composite theories based on Guth, Halpin-Tsai, and the modified Halpin-Tsai equations. On introduction of a modulus reduction factor (MRF) for the platelet-like fillers, the predicted moduli were found to be closer to the experimental measurements. 

Frolov, Shabanova, and co-workers (37-39) studied the transition of a sol into a gel and the aggregate stability of colloidal silica. Their aim was to develop a technology for the production of highly-concentrated silica sols and to use them as binders, catalyst supports, polymer fillers, adsorbents, and so forth. Kinetic studies were made of polycondensation and gel formation in aqueous solutions of silicic acids. At the stage of particle growth, poly condensation proceeds in the diffusion-kinetic region. With changes in pH, temperature, concentration, and the nature of electrolytes,... 

Talc is hydrated magnesium silicate, a nonmetallic mineral, white-colored, chemically inert. Unlike many other minerals, its particles have a distinct platy shape. It has a natural affinity to oil and, therefore, serves as a good filler for hydrophobic plastics, such as polyethylenes and polypropylene. Platy particles of talc are structurally not uniform they have a layered composition, in which a brucite (magnesium-based, tetrahedron-cell atomic structure) sheet is sandwiched between two silica (octahedron-cell atomic structure) sheets. The elementary sheet is of ik (0.7 nm) thick. 

As an example, the graphic representation in Figure 5.13. shows crack growth curves of natural rubber compounds reinforced with layered silicate and silica. It can be seen that the crack propagation behaviour is different in dependence on the filler type and on the content of the layered silicate, too. The slope of the crack propagation curve is decreased by adding 60 phr in comparison to 15 phr layered silicate and to 60 phr silica. This means, the crack growth speed is reduced. 

In the rubber industry, silica is widely used as a non-black reinforcing filler to improve the mechanical properties of NR composites due to its high specific surface area, particularly tensile strength, tear resistance, abrasion resistance and hardness. Silica can be obtained by various methods, such as precipitated silica by precipitation of an aqueous sodium silicate solution, fumed silica by pyrogenic process, silica from natural resources e.g. rice husk ash and fly ash), and in situ silica or silica sol by the sol-gel process. 

Elastomers such as natural rubber (NR) are of great importance in manufacturing industries. However, they possess properties which are less than that required for practical applications. This has led to the use of fillers for reinforcement of their properties. For practical applications, the physical and mechanical properties of elastomers are reinforced with mineral fillers such as silica, CB, clay fibre, short fibre and layered silicate. ... 

Examples of inert or extender fillers include china clay (kaolin), talc, and calcium carbonate. Calcinm carbonate is an important filler, with a particle size of about 1 pm. It is a natural product from sedimentary rocks and is separated into chalk, limestone, and marble. In some cases, the calcium carbonate may be treated to improve interaction with the thermoplastic. Glass spheres are also used as thermoplastic fillers. They may be either solid or hollow, depending on the particular application. Talc is a filler with a lamellar particle shape. It is a namral, hydrated magnesium silicate with good slip properties. Kaolin and mica are also natural materials with lamellar structures. Other fillers include woUastonite, silica, barium sulfate, and metal powders. Carbon black is used as a filler primarily in the rnbber industry, but it also finds application in thermoplastics for conductivity, for UV protection, and as a pigment. Fillers in fiber form are often used in thermoplastics. Types of fibers inclnde cotton, wood flour, fiberglass, and carbon. Table 1.3 shows the fillers and their forms. An overview of some typical fillers and their effect on properties is shown in Table 1.4. Considerable research interest exists for the incorporation of nanoscale fillers into polymers. This aspect will be discussed in later chapters. 

Diatomite di- a-t9- mlt (1887) (diatomaceous earth, DE, kieselguhr, infusorial earth, siliceous earth, and tripolite) n. The naturally occurring deposit of skeletons of small unicellular algae called diatoms, consisting of from 83 to 89% silica. Its many uses include fillers for plastics. 

The existence of a chemical reaction between filler and coupling agent depends not just on the nature of the coupling agent but also on the filler. Silanes react readily with glass beads, metal hydroxides, clay, silica, the silicates, wollastonite, mica and various oxides, but not with calcium carbonate, carbon black or barium sulphate. 

---