Increasing Cathode Potential of Homogeneous Low Voltage Electron Beam Irradiation (HLEBI) to Increase Impact Strength of Carbon Fiber Reinforced Polycarbonate and Characterization by XPS C1s and O1s Peaks

In an interlayered carbon fiber reinforced polycarbonate (CFRPC) composite constructed of nine CF plies alternating between ten PC sheets, designated [PC]₁₀[CF]₉, applying homogeneous low voltage electron beam irradiation (HLEBI) at 200 kV cathode potential, with <i>V</i>_c setting at a 43.2 kGy dose, to both finished sample surfaces resulted in a 47% increase in Charpy impact strength and <i>a</i>_uc at median fracture probability (<i>P</i>_f) of 0.50 over that of untreated, from 118 kJm^-2 to 173 kJm^-2. Increasingly higher <i>V</i>_c settings of 150, 175, and 200 kV successively increased <i>a</i>_uc at median-<i>P</i>_f of 0.50 to 128, 155, and 173 kJm^-2, respectively. Strengthening is attributed to increasing the HLEBI penetration depth, <i>D</i>_th, into the sample thickness. Since the [PC]₁₀[CF]₉ has an inhomogeneous structure, <i>D</i>_th is calculated for each ply successively into the thickness. Scanning electron microscopy (SEM) photos showed a hierarchy of fracture mechanisms from poor PC/CF adhesion in untreated; to sporadic PC adhesion with aggregated CF at 150 kV; to high consolidation of CFs by PC at 200 kV. X-ray photoelectron spectroscopy (XPS) examination of the CF surface in the fracture area showed C1s carbonate O-(C=O)-O and ester O-(C=O)-R peak generation at 289 to 292 eV to be non-existent in untreated; well-defined at 150 kV; and increased in intensity at 200 kV, after which a reduction was observed at 225 kV. Moreover, the 200 kV yielded the largest area sp³ peak at 49.5%, signifying an increase in graphitic edge planes in the CF, apparently as dangling bonds, for increased adhesion sites to PC. For O1s scan, 200 kV yielded the largest area O-(C=O)-O peak at 34%, indicating maximum PC adhesion to CF. At the higher 225 kV, increase in <i>a</i>_uc at <i>P</i>_f of 0.50 was less, to 149 kJm^-2, and XPS indicated a lower amount of O-(C=O)-O groups, apparently by excess bond severing by the higher <i>V</i>_c setting.