Page 341 - Handbook of Plastics Technologies

PLASTICS ADDITIVES

PLASTICS ADDITIVES 5.21

5.2.1.14 Swelling and Permeation. These are reduced, because fillers and fibers restrict
free volume and mobility of the polymer matrix, making it harder for small molecules of
liquids and gases to dissolve and diffuse through the polymer, and because the small mol-
ecules must permeate around the impervious particles—a “tortuous” route that further im-
pedes permeability. On the other hand, the high polarity of most fillers and fibers may
attract moisture to penetrate along their interface with the polymer, weakening stress
transfer across the interface, and often plasticizing and even hydrolyzing the polymer; this
is particularly noticeable in outdoor weathering.

5.2.1.15 Cost. The cost of simple extender fillers may be lower than polymers on a
weight basis, but their higher density, more difficult processability, and decrease in
strength properties may eliminate any overall economy. Fillers should be chosen primarily
for their beneficial effects on technical properties; if they also decrease cost, this is simply
an added benefit. Reinforcing fibers increase the cost of commodity plastics, but they may
actually reduce the cost of some high-end engineering thermoplastics.

5.2.2 Extender Fillers
Simple inorganic particles are generally added to plastics to increase modulus, friction,
and opacity, and to reduce raw material cost.

5.2.2.1 Glass Microspheres. Glass microspheres range in size from 5000 down to 4 µm
and may be solid or hollow down to one tenth of solid density. Solid spheres improve melt
processability and give smooth surfaces, high modulus, compressive strength, and dimen-
sional stability. Hollow spheres are added to epoxy and other thermoset resins to produce
syntactic foams for deep-sea and low-dielectric applications. For lower cost and lower per-
formance, coal-fire fly-ash is sometimes recommended in place of costly glass spheres.

5.2.2.2 Calcium Carbonate. This is the most widely-used economic extender for poly-
mers. Benefits commonly reported include processability, hardness, dimensional stability,
whiteness, opacity, gloss, and mar resistance. Particle sizes range from 0.125 in for ground
mineral grades down to submicron sizes for chemically precipitated grades that may even
reinforce strength and impact strength; price is generally inverse to particle size. Calcium
stearate surface treatment improves most of these properties.

5.2.2.3 Titanium Dioxide. This is the leading white pigment in coatings and is also
widely used in paper and plastics. (Relative market volumes are coatings 50 percent, paper
25 percent, and plastics 25 percent.) Its high refractive index produces opacity, and its
chemical and UV stability produce weather resistance. (It is important to use the rutile
grade for weather resistance; the less-stable anatase grade is strictly for paints that erode
gradually, producing a chalky surface that is self-cleaning, washing away easily to shed
dirt and mold.)
5.2.2.4 Clays. Clays such as kaolin are finer than calcium carbonate, typically 0.2 to
10 µm, providing more reinforcement. They are used to increase the viscosity of polyester
bulk molding and sheet molding compounds; to give hardness, opacity, and whiteness in
vinyl flooring; and to increase heat and electrical resistance in wire and cable insulation.
They are improved by calcining and by silane surface treatment. New delamination treat-
ments to produce extremely small particle size are the basis of current developments in
nanotechnology (Sec. 5.2.3.6).

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2006 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.