Effect of Fillers

The term fillers refers to solid additives, which are incorporated into the plastic matrix.7 They are generally inorganic materials and can be classified according to their effect on the mechanical properties of the resulting mixture. Inert or extender fillers are added mainly to reduce the cost of the compound, whereas reinforcing fillers are added to improve certain mechanical properties such as modulus or tensile strength. Although termed inert, inert fillers can nonetheless affect other properties of the compound besides cost. In particular, they may increase the density of the compound, reduce the shrinkage, increase the hardness, and increase the heat deflection temperature. Reinforcing fillers typically will increase the tensile, compressive, and shear strengths, increase the heat deflection temperature, reduce shrinkage, increase the modulus, and improve the creep behavior. Reinforcing fillers improve the properties via several mechanisms. In some cases, a chemical bond is formed between the filler and the polymer; in other cases, the volume occupied by the filler affects the properties of the thermoplastic. As a result, the surface properties and interaction between the filler and the thermoplastic are of great importance. A number of filler properties govern their behavior, including the particle shape, the particle size and distribution of sizes, and the surface chemistry of the particle. In general, the smaller the particle, the greater the improvement in the mechanical property of interest (such as tensile strength).8 Larger particles may give reduced properties compared to the pure thermoplastic. Particle shape can also influence the properties. For example, plate-like particles or fibrous particles may be oriented during processing, resulting in anisotropic properties. The surface chemistry of the particle is also important to promote interaction with the polymer and to allow for good interfacial adhesion. The polymer should wet the particle surface and have good interfacial bonding so as to obtain the best property enhancement. Examples of inert or extender fillers include: china clay (kaolin), talc, and calcium carbonate. Calcium carbonate is an important filler, with a particle size of about 1 µm.9 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.10 It is a natural, hydrated magnesium silicate with good slip properties. Kaolin and mica are also natural materials with lamellar structures. Other fillers include wollastonite, silica, barium sulfate, and metal powders. Carbon black is used as a filler primarily in the rubber 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 include 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.

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