Chemistry Reference
In-Depth Information
rubber, the result is a product with super-high-impact strength. Also, the thermo-
plastic rubber can be used to carry fire retardants into the mixture without loss of
impact strength.
The major current applications of block copolymers in blends involve styrene
diene
polymers, but other block polymers are also useful. Siloxane
alkylene ether
block copolymers are widely used as surfactants in the manufacture of polyure-
thane foams, for example.
5.6
Reinforced Elastomers
The service performance of rubber products can be improved by the addition of
fine particle size carbon blacks or silicas. The most important effects are improve-
ments in wear resistance of tire treads and in sidewall resistance to tearing and
fatigue cracking. This reinforcement varies with the particle size, surface nature,
state of agglomeration and amount of the reinforcing agent and the nature of the
elastomer. Carbon blacks normally are effective only with hydrocarbon rubbers. It
seems likely that the reinforcement phenomenon relies on the physical adsorption
of polymer chains on the solid surface and the ability of the elastomer molecules
to slip over the filler surface without actual desorption or creation of voids.
5.7
Reinforced Plastics
Particulate fillers are used in thermosets and thermoplastics to enhance rigidity
and, mainly, to reduce costs. Examples are calcium carbonate in poly(vinyl chlo-
ride) and clays in rubber compounds. Fiber reinforcement is more important tech-
nically, however, and the main elements of this technology are reviewed briefly
here. Fibers are added to plastics materials to increase rigidity, strength, and
usage temperatures. Fiber-reinforced plastics are attractive construction materials
because they are stiff, strong, and light. The specific stiffness (modulus/density)
and specific strength (tensile strength/density) of glass-reinforced epoxy polymers
approximate those of aluminum, for example.
Many reinforced thermoplastic articles are fabricated by injection molding.
(This is a process in which the polymeric material is softened in a heated cylinder
and then injected into a cool mold where the plastic hardens into the shape of the
mold. The final part is ejected by opening the mold.) Thermosetting resins that
are frequently reinforced are epoxies (p. 11) and unsaturated polyesters, of which
more is said below. Glass fibers are the most widely used reinforcing agents,
although other fibrous materials, like aromatic polyamides (1-23), confer advan-
tages in special applications.
The improved mechanical properties of reinforced plastics require that the
fiber length exceed a certain minimum value. The aspect ratio (length/diameter)
