Page 343 - Handbook of Plastics Technologies

PLASTICS ADDITIVES

PLASTICS ADDITIVES 5.23

parency, and impermeability. Typical studies report flexural modulus increased 126 per-
cent, flexural strength increased 60 percent, HDT increased 87°C, and impermeability
increased fourfold. Since they are smaller than the wavelength of visible light, they do not
reduce transparency. The technology is being extended into commercial practice, includ-
ing a variety of other fillers and polymers.
5.2.3.7 Carbon Black. Carbon black is made by cracking organic oils in a high-temper-
ature furnace, producing particle sizes in the 10 to 100 nm range. The best grades, seen un-
der an electron microscope, are clusters of particles, referred to as high-structure. The
aromatic carbon rings are attracted to the C=C bonds in rubber and may graft to them dur-
ing vulcanization. They give such high-strength reinforcement of rubber that their use is
almost universal. For some reason, they do not reinforce the strength of plastics but are
very useful for UV stabilization and electrical semiconductivity.
5.2.3.8 Fumed Aerosil Silica. This is produced by mixing SiCl with steam. Here
4
again, the particle size is in the nanometer range, and high-structure clusters give good re-
inforcement to silicone rubber. They also give extreme viscosity and thixotropy to liquid
systems such as vinyl plastisols and epoxy resins.

5.2.4 Reinforcing Fibers
Fibers have much higher modulus and strength, and much lower thermal expansion, than
bulk polymers, so dispersing them in a polymer matrix can produce an excellent increase
in modulus, strength, and dimensional stability.
5.2.4.1 Glass. Continuous glass fibers are typically calcium/aluminum/boron/magne-
sium silicate, melt spun at 2400°F (1316°C), and 9 to 18 µm in diameter. When they are
incorporated into plastics, they produce the highest modulus, strength, and impact strength
ever achieved (Table 5.19). However, processing of continuous fiber is limited to special-
ized techniques such as filament winding, pultrusion, and compression molding. For
broader application, glass fibers are chopped 1 to 2 in (~25 to 50 mm) long for sheet mold-
ing compound, 0.5 to 1 in (~13 to 25 mm) for bulk molding compound, and 0.125 to 0.5 in
(~3 to 13 mm) for thermoplastic molding and extrusion. This does permit fairly conven-
tional melt processing, but it certainly sacrifices a good portion of the potential properties,
whether processors admit it or not (Table 5.20). Optimum performance depends on stress
transfer between polymer and fiber, and fiber ends act negatively as stress concentrators.
Furthermore, fiber breakage during melt flow severely reduces the final length of the fi-
bers, reducing properties even further. Nevertheless, it is still possible to improve thermo-
plastic properties considerably by adding glass fibers, so the technique is very popular
(Table 5.21).
5.2.4.2 Mineral Wool. Mineral wool is a low-cost silicate fiber spun from molten slag in
steel refineries. It is widely used as thermal insulation in housing and appliances. Since its
composition and structure are not well controlled, it is not comparable with chopped glass
fibers; however, it is sometimes used as a partial replacement for them. Jim Walters Pro-
cessed Mineral Fiber (PMF) in particular has been reported for such applications.

5.2.4.3 Specialty Fibers. Specialty fibers offer benefits, but, because they are expensive,
they are only used in special high-performance products. Carbon fibers are made by pyro-
lyzing polyacrylonitrile, producing amorphous carbon reinforced by crystalline graphite
fibrils; they offer high strength, lubricity, and electrical conductivity. Aramide fibers are
aromatic polyamides; they offer low density, impact strength, vibration damping, and wear
resistance.

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