This value is termed the critical surface free energy. A liquid with surface free energy below the critical value will wet and spread over the solid surface, whereas a liquid with surface energy above the critical value might wet but won't spread. A liquid with surface energy above the critical value may leave microvoids at the fiber-matrix interface because complete, intimate molecular wetting may not occur. This example plot is for a variety of liquids wetting solid Teflon. Using the concept of critical surface energy, it is possible to characterize a wide variety of polymers and correlate the critical surface energy with polymer structure.
Fluorinated materials have low values of critical surface energy.
Hydrogenated materials, such as polyethylene and polypropylene, have slightly higher values. However, resins used in RTM, such as such as vinyl esters and epoxies, have a fairly high liquid surface free energy, so it can be hard for them to wet fiber substrates. From the critical surface energy data just given, fluorinated and hydrogenated materials will be among the least wettable.