As indicated already, the fiber-matrix interface quality is very important to promote successful reinforcement of composite materials. There are a number of physical and chemical methods to optimize the interface for varying degrees of efficiency for the adhesion.43

14.4.4 PHYSICAL METHODS

Physical methods for improving the adhesion of surfaces attempt to enhance or im­prove existing surface chemistry or functionalities. For example, surface fibrillation is one method that has been previously used; this is a type of refining or homogeni­zation (NFC, vide supra) that takes a top-down approach in increase the surface area of the materials under study. What this means is that the macrofibers are “opened” up to reveal more of the micro and nano-fibrils that compose the fibers. Such an approach allows for the native high surface energy and area to be increased and promote adhesion. Another method is electric discharge that includes corona or cold plasma. Cold plasma has been used to increase bondability without significantly im­pacting the bulk properties of the substrate treated. Carlsson44 was able to show via a hydrogen plasma treatment of a pure cellulose substrate that the hydroxyl content on the surface was diminished to provide lower molecular weight fragments that were more hydrophobic.