Pretreatment of Kapton-coated cable for epoxy adhesion

Preliminary testing of a new system for protecting bonded strain gages that will be attached to the MFTF magnets indicated falling electrical resistance to ground, attributed to the infiltration of moisture. The most likely infiltration route seemed to be along the Kapton lead cable, which has an outer surface of FEP fluorocarbon resin. Samples of the cable were pretreated with a fluorocarbon etchant, Tetra-Etch, for periods of 10, 25, and 40 s at room temperature, followed by rinsing with demineralized water. The treated ends were embedded in the proposed epoxy sealant, Hysol EA 934, a compound containing 70 wt % of asbestos. The tensile-shear stresses required to pull the wires out of these embedments were measured. Results show that the three levels of treatment are equally effective in raising the bond strength from 377 psi for the untreated cable to about twice that, 763 psi. The 40-s exposure to Tetra-Etch appears to have penetrated the 0.5-mil fluorocarbon coating and attacked the Kapton film and the conductor coatings inside it.