Gar-Seal butterfly valves by Garlock Inc., Newtown,   Pa., have a minimum seal lining 0.125-in. thick and vary in size from   2 to 42 in.

Gar-Seal butterfly valves by Garlock Inc., Newtown, Pa., have a minimum seal lining 0.125-in. thick and vary in size from 2 to 42 in.


Conventionally lined Teflon (PTFE) valves in DuPont Dow Elastomers' Pontchartrain plant, La Place, La., were prone to failure when 18°C liquid chloroprene permeated the Teflon lining and made it swell. But new butterfly valves made with Teflon NXT resist absorption of chloroprene and a whole host of other caustic chemicals and extend valve service life by nearly eightfold.

Teflon NXT is said to have higher creep resistance and stiffness than conventional PTFE as well as a smoother surface. In addition, the inherent lubricity of the NXT material lets the valve operate smoothly unlike metal valves which often see their lubricants washed away by chloroprene and other harsh fluids.

Information for this article was provided by DuPont Fluoroproducts, Wilmington, Del.

Typical properties for Teflon NXT and conventional PTFE
PROPERTY
TEFLON NXT
PTFE
Tensile strength, (psi)
4,500
5,000
Elongation at break, %
450
375
Specific gravity
2.17
2.16
DEFORMATION UNDER LOAD (CREEP), 73°F, %
500 psi
0.2
0.7
1,000 psi
0.4
1.0
2,000 psi
3.2
8.2

Comparative permeation rates for Teflon NXT versus conventional PTFE
 
SPECIMEN
VAPOR
LIQUID
PERMEANT
THICKNESS, IN.
PTFE
TEFLON NXT
PTFE
TEFLON NXT
Perchloroethylene
0.08
1.4
0.1
0.0019
0.005
Perchloroethylene
0.2
0.055
0.05
x
x
Hexane
0.08
3.4
0.2
23.4
0
Hexane
0.2
0.045
0.015
x
x
MEK
0.08
36.3
23.3
49.4
34.2
MEK
0.2
22.6
20.8
35.5
25.2