Adhesion to Sodium Naphthalenide Treated Fluoropolymers. Part III. Mechanism of Adhesion

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Abstract
Adhesion of fluoropolymers to copper and to other polymers is examined using a range of fluoropolymer types (PTFE, PFA, extruded, skived and cast films), surface modification techniques such as sodium naphthalenide (Na/naphth), acid stripping and lamination to produce surfaces of controlled roughness, and three tests of adhesion (90 degree peel tests, torsional shear tests and stripping of transmission electron microscopy (TEM) replicas). A combination of chemical and physical modification is required to produce good adhesion, with the relative importance of each dependent upon the specific adhesion test used. For relatively smooth-surfaced films, Na/naphth appears to function by increasing both the chemical functionality and the mechanical integrity of a surface layer. Untreated PTFE and PFA show interfacial failure and negligible adhesion. Smooth-surfaced PTFE with superficial surface modification, e.g. after lamination to shiny copper foil or after acid stripping of defluorinated material, often fails by fibrillation of the fluoropolymer surface. For short sodium etch times, adhesion is improved and the failure mode is interfacial. For long etch times, there is a mixed mode of failure. Fibrillation in smooth-surfaced PFA systems was not observed. Adequate adhesive strength in these systems could only be achieved by an increase in the surface roughness. The best adhesion could be achieved by surface roughening, followed by Na/naphth treatment. For such PTFE surfaces plated with copper, peel and shear tests showed a mixed mode of failure, with copper and fluoropolymer found on both failure surfaces by x-ray photoelectron spectroscopy (XPS) and energy dispersive x-ray spectroscopy (EDS). Extensive fibrillation occurred at the locus of failure. Provided chemical modification is adequate to allow wetting, the roughness of the surface dominates the properties of the adhesive bond. Prolonged Na/naphth treatment (e.g. one hour) causes a reduction in peel strengths.