Adhesion to fluoropolymers

Adhesion of copper and solution-deposited polymers to fluoropolymers was studied using films of polytetrafluoroethylene (PTFE) and a TFE-perfluoroalkylvinylether co-polymer (PFA) with different manufacturing histories (cast, skived, laminated). These films were exposed to three surface modification techniques (lamination, sodium naphthalenide etching, acid-stripping). The depth of defluorination from the sodium etching was measured gravimetrically and the nature of the resultant chemical functionalities determined using internal reflectance (FTIR-IRS) and x-ray photoelectron (XPS) spectroscopies. The surface functionality produced by the treatments was independent of fluoropolymer type. Surface area was the major variable affecting defluorination rate. For all but the skived PTFE films, there was little change in surface texture after one hour sodium etch or after subsequent acid stripping of the defluorinated layer.^ Adhesive testing with three test geometries (90 degree peel test, torsional shear test, and stripping of transmission electron micrograph replicas) revealed that a combination of chemical and physical surface modification is required to produce good peel adhesion, with the relative importance of each dependent upon the specific adhesion test used. For relatively smooth films, the Na/Naphth appeared to function by increasing both the chemical functionality and the mechanical integrity of the surface layer. Polymer replicating films which were solution-deposited onto untreated PTFE and PFA gave interfacial failure and negligible adhesion. Smooth PTFE films which were superficially modified frequently failed by fibrillation of the fluoropolymer surface. For short etch times, adhesion improved and failure was interfacial. For long etch times, a mixed mode of failure occurred. Adequate adhesive strength could only be achieved by an increase in surface roughness.^ The best adhesion was achieved by surface roughening followed by Na/Naphth treatment. For such PTFE surfaces plated with copper, peel and shear tests gave a mixed mode of failure, with copper and fluoropolymer found on both fracture surfaces. Extensive fibrillation occurred at the locus of failure. Provided chemical modification was adequate to allow wetting, the roughness of the surface (mechanical adhesion) dominated the properties of the adhesive bond. Prolonged Na/Naphth treatment (one hour) caused a reduction in peel strengths. ^