Enhancement of Adhesion in Composites

Polymeric adhesives function by providing a bond between the two adherends. This bond is obtained by the adhesive wetting both surfaces. Wetting occurs when the adhesive has both a low-enough viscosity to flow over the surface and an attraction to the surface which causes it to be pulled into intimate contact with the surface topography. While in theory it is desirable to obtain chemical adhesion (covalent bonding), in practice this rarely occurs. Usually, mechanical bonding and attraction forces dominate between the adherend and the adhesive. 

Various surface pretreatments, often referred to as primers, are put on fibers and other textiles by the manufacturers to enhance subsequent bonding. Depending on the subsequent use of the textiles, the change in adhesion can be negative, nonexistent, or positive. In interlaminar shear strength tests of untreated and oxidative surface-treated polyacrylonitrile-based carbon fiber/epoxy composites the shear stress went from 14.9 to 22.1 MPa. 

Most discussions and analyses of composite materials assume that the bonding between the matrix and reinforcement is perfect (i.e., adhesion strength greater than either of the two constituents). However, in reality, adhesion is imperfect. This results in reductions in stiffness and strength of the composite. Additionally, imperfect adhesion results in an increase in hysteretic losses at the interface. 

Tensile test of silation of fiberglass. (Images courtesy of Dow Coming 

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