Effect of monomer concentration on adhesive strength of PTFE films treated with atmospheric-pressure nonthermal plasma graft polymerization

Abstract The effect of monomer concentration on the adhesive strength of polytetrafluoroethylene (PTFE) films treated with atmospheric-pressure nonthermal plasma (NTP) graft polymerization is investigated. In this study, we use acrylic acid (CH2=CHCOOH) as the monomer, and two types of evaporation vessels (named vessel I and vessel II) are prepared to generate the monomer vapor. Vessel II consists of a pair of evaporation containers that have half the evaporation area of vessel I. The monomer concentration in the treatment chamber is adjusted by controlling the monomer temperature. After purging the chamber with Ar, a jet of NTP is applied to the PTFE film surface at an Ar flow rate of 40 L/min. The treated PTFE films are then adhered to stainless steel plates using double-sided tape, and 90° peeling tests are conducted. It is found that the peeling strength increases with decreasing monomer concentration. When the acrylic acid temperature in vessel II is set to 55 °C, the monomer concentration is estimated to be 980 ppm, and the highest peeling strength per unit width of 4.85 N/mm is obtained under this condition. Despite the same monomer temperature used (60 °C), the peeling strength of the PTFE films treated in vessel II is higher than that of films treated in vessel I. Additionally, smaller data variation is observed when vessel II is used. This result is due to the uniform distribution of monomer concentration achieved when vessel II is used to generate the monomer vapor. Furthermore, information regarding the chemical bonding and elemental composition of the untreated and treated films is obtained using X-ray photoelectron spectroscopy measurements and the graft-polymerized surface morphologies are studied using scanning electron microscopy.