Page 70 - Handbook of Plastics Technologies

THERMOPLASTICS

2.10 CHAPTER 2

the presence of any organic matter during the sintering process will result in poor proper-
ties as a result of the thermal decomposition of the organic matter. This includes both poor
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visual qualities and poor electrical properties. The final properties of PTFE are depen-
dent on the processing methods and the type of polymer. Both particle size and molecular
weight should be considered. The particle size will affect the amount of voids and process-
ing ease, while crystallinity will be influenced by the molecular weight.
Additives for PTFE must be able to undergo the high processing temperatures required.
This limits the range of additives available. Glass fiber is added to improve some mechan-
ical properties. Graphite or molybdenum disulphide may be added to retain the low coeffi-
cient of friction while improving the dimensional stability. Only a few pigments are
available that can withstand the processing conditions. These are mainly inorganic pig-
ments such as iron oxides and cadmium compounds. 78
Because of the excellent electrical properties, PTFE is used in a variety of electrical ap-
plications, such as wire and cable insulation and insulation for motors, capacitors, coils,
and transformers. PTFE is also used for chemical equipment such as valve parts and gas-
kets. The low friction characteristics make PTFE suitable for use in bearings, mold release
devices, and antistick cookware. Low-molecular-weight polymers may be used in aerosols
for dry lubrication. 79
2.2.4.4 Polyvinylindene Fluoride (PVDF). Polyvinylindene fluoride (PVDF) is crystal-
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line with a melting point near 170°C. The structure of PVDF is shown in Fig. 2.7. PVDF
has good chemical and weather resistance, along with good resistance to distortion and
creep at low and high temperatures. Although the chemical resistance is good, the polymer
can be affected by very polar solvents, primary amines, and concentrated acids. PVDF has
limited use as an insulator, because the dielectric properties are frequency dependent. The
polymer is important because of its relatively low cost compared to other fluorinated poly-
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mers. PVDF is unique in that the material has piezoelectric properties, meaning that it
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will generate electric current when compressed. This unique feature has been utilized for
the generation of ultrasonic waves.

FIGURE 2.7 Structure of PVDF.

PVDF can be melt processed by most conventional processing techniques. The poly-
mer has a wide range between the decomposition temperature and the melting point. Melt
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temperatures are usually 240 to 260°C. Processing equipment should be extremely
clean, as any contaminants may affect the thermal stability. As with other fluorinated poly-
mers, the generation of HF is a concern. PVDF is used for applications in gaskets, coat-
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ings, wire and cable jackets, and chemical process piping and seals.
2.2.4.5 Polyvinyl fluoride (PVF). Polyvinyl fluoride (PVF) is a crystalline polymer
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available in film form and used as a lamination on plywood and other panels. The film is
impermeable to many gases. PVF is structurally similar to polyvinyl chloride (PVC) ex-
cept for the replacement of a chlorine atom with a fluorine atom. PVF exhibits low mois-
ture absorption, good weatherability, and good thermal stability. Similar to PVC, PVF

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