The spreading of liquids on low-energy surfaces. II. Modified tetrafluoroethylene polymers

Abstract

1. 1. Data are presented on the equilibrium contact angles of a wide variety of liquids on specially prepared surfaces of halogenated derivatives of polyethylene. The free energy decrease of immersion ( f SL ), the work of adhesion ( W A ), and the spreading coefficients have been calculated for the liquids which do not spread. The free energy decrease on immersion of these solids in the saturated vapor of most of the liquids of this study is shown to be negligible. 2. 2. For each solid surface the contact angle and spreading coefficient follow the relation to liquid surface tension reported previously for polytetrafluoroethylene, i.e., as the liquid surface tension increases, the contact angle increases and the spreading coefficient decreases. 3. 3. Substitution of chlorine or hydrogen for fluorine in these polymers increases W A , f SL , and the spreading coefficient with respect to a given liquid, the increase depending on the proportion of nonfluorine substituent. Substitution of hydrogen for fluorine has a smaller effect in this respect than substitution of chlorine for fluorine. 4. 4. It is shown that when f SL for a given solid in a series of liquids is constant, the plots of W A υ s. surface tension and spreading coefficient υ s. surface tension should be straight lines with positive and negative 45° slopes, respectively. The data presented for the fluorinated polymers and the liquid n-alkanes and di- n-alkyl ethers fall on such lines. The values of f SL for the fluorinated polymers plot as a straight line against mole per cent fluorine substitution. 5. 5. There appears to be a maximum value of W A characteristic of the polymer, which is identical with the work of adhesion given by the liquid having a contact angle of 90° on the given solid. 6. 6. It is shown experimentally that there are many exceptions to the “rule” that nonpolar liquids wet nonpolar solids. 7. 7. From the data it is shown that recent attempts to measure the surface tension of solids by contact angle measurements alone involve unjustifiable assumptions.