A new strengthening process of thermoplastic polypropylene reinforced by interlayered activated sizing film-free carbon fiber treated by electron beam irradiation under oxygen-rich nitrogen gas prior to assembly and hot-press

A new process of activating sizing film free (removed sizing) carbon fiber (SFF-CF) mats directly with electron beam irradiation (EBI) under oxygen-rich nitrogen gas prior to lamination assembly and hot-press was found to increase bending strength of CFRTP (CF reinforced thermoplastic polypropylene (PP)) interlayered composite composed of 3 SFF-CF mats between 4 PP sheets, [PP]4[SFF-CF]3. The hot-press fabrication was performed under 4.0 MPa at 473 K for 1 min. Experimental results showed optimal condition of 0.22 MGy-EBI under 2,000 ppm-O2-rich, protective N2 atmosphere improved bending strength, σb at all accumulative probabilities, Pf over that of untreated. This could be explained by maximum number of the strong covalent bonds of CF:C:C:PP and oxygen assisted CF:C:O:C:PP bonds adhering bare SFF-CF surface to PP, instead of the weak intermolecular bonding of CF(H2O, N2, O2)PP of untreated. The optimal concentration of O2 molecules together with the EBI probably purified the SFF-CF surface simultaneously generating dangling bonds, which already naturally exist in SFF-CF as evidenced by the highest intensity ESR peak whose inflection point was at B = 323 mT. This, along with attaching O groups to the SFF-CF surface can explain the increased SFF-CF to PP adhesion sites and bending strength increase. Fracture analysis showed the optimum condition prevented ply delamination and reduced damage area along sample length appearing to increase cohesion between the difficult to adhere thermoplastic PP and SFF-CF plies to take on higher loads. Although bending strength was increased, carefulness is always highly recommended when investigating unsized CF composites.