ultraviolet radiation; they have a broad useful temperature range (up to 550

o

F) and a high

dielectric constant; they exhibit a low coefficient of friction; they have anti stick properties;

and they possess a greater coefficient of thermal expansion than most other plastics and

metals.

A variety of fluoropolymer materials are marketed under a number of different

trademarks. Polytetrafluoroethylene (PTFE) was discovered by E. I. Du Pont de Nemours in

1938. PTFE's properties include an extreme temperature range (from  400

o

F to +550

o

F in

constant service) and the lowest coefficient of friction of any solid material (Hamilton, 1985).

PTFE is by far the most widely used and produced fluoropolymer. Fluorinated ethylene

propylene (FEP) was also developed by E. I. Du Pont de Nemours and is perhaps the second

most widely used fluoropolymer. It duplicates nearly all of the physical properties of PTFE

except the upper temperature range, which is 100

o

F lower. Production of FEP finished

products is generally faster because FEP is melt processible, but raw material costs are higher.

Perfluoroalkoxy (PFA) combines the best properties of PTFE and FEP, but PFA costs

substantially more than either PTFE or FEP. Polyvinylidene fluoride (PVDF) is tougher and

has a higher abrasion resistance than other fluoropolymers, and is resistant to radioactive

environments. PVDF also has a lower maximum temperature limit than either PTFE or PFA.

Care should be exercised in the use of trade names to identify fluoropolymers. Some

manufacturers use one trade name to refer to several of their own different materials. For

example, Du Pont refers to several of its fluorocarbon resins as Teflon

, although the actual

products have different physical properties and different fabricating techniques. These

materials may not always be interchangeable in service or performance.

Aller et al. (1989) provide an excellent summary of the research on PTFE materials

performed by Hamilton (1985), Reynolds and Gillham (1985), Barcelona et al. (1985a), Lang

et al. (1989), Dablow et al. (1988), and Barcelona et al. (1985b). The following advantages

and disadvantages of PTFE are highlighted in Aller et al.'s (1989) summary and by Nielsen

and Schalla (1991).

Advantages of PTFE well casing and screen materials:

Can be used under a wide range of temperatures;

Inert to attack by the environment, acids, and solvents;

Fairly easily machined, molded, or extruded;

Most inert casing for monitoring metals; and

In terms of chemical inertness, best overall choice if only metallic analytes are

of concern (Hewitt, 1992).

November 1992

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