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Table 6. Some general characteristics of materials used for well casing and screens (modified <br />from T.E. Imbrigiotta, U.S. Geological Survey, written commun., 1989) <br />[PTFE, polytetrafluoroethylene; PVC, polyvinylchloride; SS, stainless steel] <br />PTFEa,b <br />- Virgin PTFE readily sorbs some organic solutes (Parker and Ranney, 1994). <br />- Ideal material in corrosive environments where inorganic compounds are of interest. <br />- Useful where pure product (organic compound) or high concentrations of PVC solvents <br />exist. <br />- Potential structural problems because of its low tensile and compressive strengths, low <br />wear resistance, and the extreme flexibility of the casing string as compared to other <br />engineering plastics (Driscoll, 1986, table 21.6; Dablow and others, 1988; Aller and <br />others, 1989, table 25). <br />- Potential problems with obtaining a seal between the casing and the annular sealant <br />because of PTFEs low coefficient of friction and antistick properties as compared to <br />other engineering plastics (Aller and others, 1989, p, 151). <br />- Maximum string length of 2 inch schedule 40 PTFE casing should not exceed about <br />375 feet (Nielsen and Schalla, 1991, p. 262). <br />- Expensive. <br />PVCa,b <br />- Leaching of compounds of tin or antimony, which are contained in original heat stabiliz- <br />ers during formulation could occur after long exposure. <br />- When used in conjunction with glued joints, leaching of volatile organic compounds from <br />PVC primer and glues, such as THE (tetrahydrofuran), MEK (methylethylketone), MIBK <br />(methylisobutylketone) and cyclohexanone could leach into ground water. Therefore, <br />threaded joints below the water table, sealed with o-rings or Teflon tape, are preferred. <br />- Cannot be used where pure product or high concentrations of a PVC solvent exist. <br />- Maximum string length of 2 inch threaded PVC casing should not exceed 1,200 to 2,000 <br />feet (Nielsen and Schalla, 1991, p. 250). <br />- Easy to cut, assemble, and place in the borehole. <br />- Inexpensive <br />30