Page 26 - Handbook of Plastics Technologies

INTRODUCTION TO POLYMERS AND PLASTICS

1.12 CHAPTER 1

Fatigue tests can be performed under a variety of loading conditions as specified by the
service requirements. Thermal effects and the presence or absence of cracks are other vari-
ables to be considered when the fatigue life of a material is to be evaluated.

1.4.3 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 im-
prove 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. Re-
inforcing 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 ten-
8
sile strength). Larger particles may give reduced properties compared to the pure thermo-
plastic. 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 car-
9
bonate. Calcium carbonate is an important filler, with a particle size of about 1 µm. 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 ther-
moplastic. 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
10
shape. 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.

1.5 Rheological Properties
Viscosity is the resistance to flow. As shown in Table 1.5, polymer melts have viscosities
11
of 100 to 1,000,000 Pa-s, whereas water has a viscosity of 0.001 Pa-s. These high vis-
cosities result from the long polymer chains and cause the polymer melt to exhibit laminar

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