Page 243 - Carrahers_Polymer_Chemistry,_Eighth_Edition

206 Carraher’s Polymer Chemistry

friction of moving bodies is generally less than that of a static body. The kinetic friction is considered
independent of the velocity. The coefficient of friction is defi ned as F/L. Polymers show a wide range of

coefficients of friction so that rubbers exhibit relatively high values (BR = 0.4–1.5 and SBR = 0.5–3.0)

whereas some polymers such as PTFE (0.04–0.15) and PVF (0.10–0.30) have low values.

The low coefficient of friction for PVF results in materials coated with it remaining somewhat
free of dirt and other typical contaminants, allowing PVF-coated materials to be less frequently
cleaned. It is essentially self-cleaning as rain carries away dust and other particulates, including
bird droppings, acid rain, and graffiti. The low friction also results in longer lifetimes for materials

coated with the PVF and for the PVF coating itself.
o

PVF has a T of about –20 C remaining flexible over a wide-temperature range (from about
g
o
o

–20 C to 150 C), even under cold temperatures. Because of its low coefficient of friction and tightly
bound structure, it retains good strength as it weathers. Films, in Florida, retain much of their
thickness even after about a decade losing less than 20% of their thickness. To increase their useful

lifetimes, relatively thick films, such as 1 mil, are generally employed. The “slickness” also acts to
give the material a “natural” mildew resistance.

Unlike PVC that requires plasticizers to be flexible, PVF contains no plasticizers and does not
“dry out” like PVC. PVF, because of its higher cost in comparison to PE and PP, is used as a coating

and selected “high end” bulk applications such as films. Films are sold by DuPont under the trade
name of Tedlar. Tedlar is used in awnings, outdoor signs, roofing, highway sound barriers, commer-

cial building panels, and solar collectors. It is used as a fabric coating, protecting the fabric from
the elements. PVF is resistant to UV-related degradation and unlike PVC, it is inherently fl exible.

While transparent, pigments can be added to give films and coatings with varying colors. Protective
coatings are used on plywood, automotive parts, metal siding, lawn mower housings, house shutters,
gutters, electrical insulation, and in packaging of corrosive chemicals. PVF has pizeoelectric prop-
erties generating a current when compressed.

The processability of fluorine-containing polymers is improved by replacement of one or more
of the F atoms. Replacing one of eight fluorine atoms with a trifluoromethyl group gives a prod-

uct called FEP or Viton, actually a copolymer of tetrafluoroethylene and hexafl uoropropylene

(6.53). Polytrifluoromonochloroethylene (PCTFE, Kel F; 6.54), in which one fluorine atom has

been replaced by a chlorine atom has a less regular structure and is thus more easily processed.
Poly(vinylidene fluoride) (PVDF; Kynar; Equation 6.55) is also more easily processable but less

resistant to solvents and corrosives.
F R
F
F F F
R
F
R Cl F
F F
F
F R
F F
(6.53) (6.54)
Gore-Tex is named for its inventors W. L. Gore and R. W. Gore and the nature of the material, a
textile. Gore-Tex materials are based on expanded PTFE and other fluoropolymers. They are used

in a variety of areas, including gaskets, sealants, insulation for wires and cables, filter media, and
medical implants and their most widely used area is as high-performance fabrics. Gore-Tex is a
thin, highly porous fluoropolymer membrane bonded generally to a nylon or polyester fabric. The

membrane has about 9 billion pores per square inch. Each of these pores are about 20,000 times
smaller than a drop of water, preventing liquid water from penetrating but allowing individual
water molecules to pass through. This results in a material that is breathable but also waterproof
and wind resistant. This is in contrast to most raincoats that are waterproof but not breathable.
Along with garments, Gore-Tex is also used to make tents and other outdoors goods.

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