Effect of atmospheric pressure plasma treatment on surface physicochemical properties of carbon fiber reinforced polymer and its interfacial bonding strength with adhesive

To study the effect of surface physicochemical properties of carbon fiber reinforced polymer (CFRP) on its interfacial bonding strength with adhesive, atmospheric pressure plasma treatment (APPT) using three gases (i. e., argon (Ar), nitrogen (N-2) and air (Air)) was conducted on surfaces of CFRP substrates. Atomic force microscope (AFM) observation indicates that APPT only changes the surface morphology of CFRP in a depth of tens of nanometers, and this minor change contributes little to the interfacial bonding strength of CFRP joints. X-ray photo electron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) analyses reveal that the release agent (mainly the -Si-CH3 group) and other contaminations on CFRP substrate surfaces were effectively removed by APPT, which improves the shear strength of CFRP joints from 8.6 MPa to 23.1 MPa. In addition, polar chemical functional groups (i.e., -NH2, -OH, -COOH) were generated on the CFRP substrate surfaces after APPT when nitrogen and air were used as plasma gases, which not only enhances the surface free energy of CFRP substrate but also combines with adhesive molecules through the form of hydrogen bonds and covalent bonds. With this mechanism, the shear strength of CFRP joints increased further to 28.7 MPa and 31.5 MPa when nitrogen and air were used as the plasma gases, respectively. After a quantitative study, the interfacial bonding strength between CFRP substrate and adhesive is expressed as a function of the concentration of -Si-CH3, -NH2 and -OH groups.