Polytetrafluoroethylene surface tension data

Two common liquid membrane support materials, polytetrafluoroethylene and polypropylene, have critical surface tensions of 18 mN/m and 35 mN/m, respectively. Manufacturers often supply critical surface tensions for their porous films. Liquids with a surface tension, y, less than the critical surface tension will probably wet the surface. Therefore, hydrocarbons will wet polypropylene, but water (y = 72 mN/m) will not. Shafrin and Zisman (30) have summarized critical surface tension data for many materials and correlated the data such that critical surface tensions may be estimated from knowledge of the functional groups in the chemical structure of the surface. 

Complete wetting of a solid is only possible if a drop of the liquid spreads spontaneously at the surface, i.e. for 9 = 0 or cos 9=1. The limiting value cos 6 = 1 is a constant for a solid and is named critical surface tension of a solid y... Therefore, only liquids with yl < Vc have the ability to spontaneously spread on surfaces and wet them completely. Tab. 4.2 gives an overview of critical surface tension values of different polymer surfaces [40]. From these data it can be concluded that polytetrafluoroethylene surfaces can only be wetted by specific surfactants with a very low surface tension, e.g. fluoro surfactants. 

The present discussion is restricted to two of the surfaces studied by Bascom and Singleterry-i.e., polytetrafluoroethylene and polyethylene. Data for two hydrocarbon liquids-n-decane and isopropylbiphenyl-which had nonzero contact angles on each of these surfaces are given in Table in. Also given are the appropriate one-liquid adhesion tensions from Table I. 

Adhesion Tensions. Among the surfaces chosen for discussion are the glassy fluorocarbon studied by Fowkes and Sawyer [31] and poly-tetrafluoroethylene,for which extensive data have been reported by Fox and Zisman [33]. For the latter material values of the adhesion tensions with a series of alkylnaphthalenes and a series of alkyl biphenyls have also been presented by Bascom and Singleterry [10]. These data, along with the glassy fluorocarbon data and a selection of the Fox and Zisman polytetrafluoroethylene data, are given in Table I. 

---