2.6. Surface Tension Measurement

There is a tight relationship between the surface tension and contact angle of materials. The contact angles between probe liquids and polymers were monitored by a sessile drop apparatus named “Digidrop” manufactured by GBX. In order to ensure a smooth surface for contact angle tests, hot pressing technology was utilized to make molded plates. Neat PP or PVA pellets were placed between Teflon plates with heating over their melting temperatures without pressure. Then a high pressure between the two plates (40 bars) was offered and lasted for some time. Afterwards, the pressure was maintained and the plates were water-cooled to the room temperature. After the stress reliever of the plates, the neat polymer plates with smooth surfaces were generated for contact angle measurement.

The contact angles of the liquid on the polymer surface enable us to obtain the polar and dispersive contributions to the surface tension of PP and PVA by the method of Owens and Wendt [28] (Equation (2)):

where θ is the contact angle, and γl is the liquid surface tension (mN/m), γsd and γsp are the dispersed and polar component of the solid surface (mN/m), γld and γlp are the dispersed and polar component of the liquid surface tension (mN/m). Two probe liquids were utilized, one apolar liquid (diiodomethane, γ = 50.8 mN/m, γ^p = 2.3 mN/m, γ^d = 48.5 mN/m) and one polar liquid (distilled water, γ = 72.8 mN/m, γ^p = 51.0 mN/m, γ^d = 21.8 mN/m) [29].