PTFE

WHEN IT'S PTFE, THICKER IS DEFINITELYÂ NOT BETTER!
Recently ads have been appearing in trade magazines talking about thick PTFE liners and how vacuum resistant these thicker liners are. Since vacuum services are few and far between, thicker is not better, especially with PTFE. These "thick minded" people are so concerned about an ill-conceived selling point they forget about "creep" or "cold flow". In an articleÂ in Chemical Engineering, November 1992, on "Sealing Tips for Plastic Lined Pipe", the authors from Dow Plastic Lined Piping Products point out that "any creep relaxation experienced by the raised plastic face will affect the integrity of the plastic-lined flanged joint. The extent of plastic creep is dependent upon time, load, temperature and stress levels. Creep or "cold flow" can be significant even at room temperature, especially with PTFE." DuPont in their Teflon Mechanical Design Data book, section III page 6 states: "When materials which creep or cold flow are used as gaskets in flanged joints, the phenomenon of stress relaxation is generally encountered. In flanged, bolted connection, parts of PTFE will cold flow between the flange faces with a resultant decrease in bolt pressure. Such relaxation in gasket stock may result in a leaky joint. To minimize creep and stress relaxation specify the thinnest possible PTFE gasket that will accommodate flange roughness. Gaskets thicker than approximately 1/16" increase the amount and rate of stress relaxation." Never forget that PTFE lined pipe flares are gaskets. Keeping joints tight is extremely important in chemical process piping. To hold joints tight, thinner is much better as vacuum leaks in but liquid under pressure squirts out. Thicker PTFE is also much more prone to environmental stress cracking, especially at the flange.
More permeation resistance is also going to be used as a selling point. Increasing the liner thickness does reduce permeation but it still won't eliminate the vent holes.
The only permanent solution to joint cold flow, permeation and vacuum service is not thicker PTFE, but one of the much tougher fluoroplastics such as dense, vent free ETFE, ECTFEÂ or PVDF.