Page 278 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Copolymerization 241

be rapidly distributed to the stiffer outer i-PP shell. The closeness of structure between the two
components, the i-PP impregnated particles and the PE/i-PP copolymer allows miscible mixing
of the two phases.
The actual conditions and concentrations and ratios of monomer were developed by research
scientists such as Edward Kresge over a long period of time.

7.9 FLUOROELASTOMERS
Fluoroelastomers are specialty copolymers containing a high amount of fl uorine-containing units.
These copolymers can be divided into four groups. Viton A (Equation 7.39) is a copolymer of vinyl-

idene difluoride and hexafluoropropylene. It is used as a general-purpose sealing material in the
automotive industry and with aerospace fuels and lubricants.
H H F F
H F F F
R
+ m n
F R F (7.39)
H F F
F F F
F
F F F
Viton B (Equation 7.40) is a terpolymer produced from the polymerization of tetrafl uoroethyl-

ene, vinylidene fluoride, and hexafluoropropylene. It is used in power-utility seals and gaskets.
F F F F
F
+ H 2 C + Viton B and F (7.40)
F
F
F F F
F
F

Viton F is also a terpolymer of tetrafluoroethylene, vinylidene fluoride, and hexafl uoropropylene.

It is used as seals for concentrated aqueous solutions of inorganic acids and for oxygenated auto-
motive fuels. Finally, Viton Extreme is a copolymer of tetrafluoroethylene and propylene and the

terpolymer of ethylene, tetrafluoroethylene, and perfl uoromethylvinylether.

Viton O-rings are used in SCUBA diving when gas blends called Nitrox are employed. Viton has
a low flammability even in the presence of high amounts of oxygen often found in Nitrox. It also

stands up well in such increased oxygen conditions. As part of the move toward green fuels, Viton-
lined hoses are often employed when biodiesel fuels are used.
These copolymers are often employed as mixtures with other polymers. They are compatible
with many hydrocarbons but not compatible with ketones and organic acids.

7.10 NITRILE RUBBER

The copolymer of acrylonitrile and butadiene is referred to by a number of names, including nitrile
rubber, Buna-N, and nitrile butadiene rubber (NBR). The composition of the copolymer varies
depending on what the intended end product is. Since butadiene is employed, the resulting copoly-
mer has sites of unsaturation that are often subsequently reacted forming thermosets. The greater
the amount of cross-linking the more rigid the product along with the corresponding decreased
porosity and diffusion properties. The greater the proportion of acrylonitrile incorporated into the
copolymer, the greater the resistance to oils, fuels, and other chemicals and the stiffer the material.
Also, the form of the butadiene monomer can vary.

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