A new process of internal activation of carbon fiber reinforced thermoplastic polymer (CFRTP) of polypropylene (PP) by applying electron beam irradiation (EBI) under oxygen (O 2 )rich nitrogen gas (N 2 ) atmosphere to CF chopped strand matt (CSM) layers prior to assembly and hot press to strengthen the typically weak CF/thermoplastic polymers (TPs) adhesion was proposed. Samples were interlayered composite with layup of alternating PP and CF plies, [PP] 4 [CF] 3 . Composite fabrication was performed by one directional hot-press under constant pressure of 4.0 MPa at 473 K for 1 min. Results showed applying an optimum 0.22 MGy-EBI under protective N 2 gas with O 2 concentrations between 200 ppm and 200,000 ppm mostly improved the bending strength (· b ) while reducing strain at the bending strength (· b ) apparently increasing the elasticity. The method appears to work well for the weakest samples in the data sets: at low accumulative probability P f = 0.06 by median rank method, · b was apparently improved by the 200 ppm and 2,000 ppm O 2 atmospheres. Namely, 0.22 MGy-EBI under N 2 gas atmosphere with 200 ppm to 2,000 ppm-O 2 improved · b at P f = 0.06 (57 MPa) about 21%, over that of untreated (47 MPa). Strength increase could be explained by mutual entangling of both sizing epoxy film on CF and PP with strong covalent bonding, which formation of direct CF: C:C :PP induced by EBI and oxygen assisted CF: C:O:C :PP by concentrating the O 2 gas molecules from 200 ppm to 2,000 ppm-O 2 in N 2 atmosphere, rather than weak molecular bonding CF-(H 2 O, N 2 , O 2 )-PP for the untreated samples. Moreover, the action of the EBI apparently acts to clean residual H 2 O, N 2 , and O 2 to purify and activate the CF surface increasing polar group and active site density. They most likely contributed to bending strength enhancement. The 0.22 MGy-EBI in O 2 -rich N 2 atmosphere appears to be a viable method to increase carbon fiber-thermoplastic polypropylene adhesion enhancing reliability and safety of the PP
Tensile strength at each accumulative probability of strength (Pf) was obtained for 28mass% carbon fibers (CF) reinforced thermoplastic polypropylene (PP) with and without sizing epoxy film on the fibers prior to making composites (CFRTP) of three cross CF cloth sheets and four PP matts, layer by layer. The sizing film covered on CF apparently improved the tensile strength. Namely, an effect of the sizing epoxy film covered on carbon fiber apparently strengthened the CFRTP. It could be explained by the increasing resistance to pull-out fibers with large friction force because the adhesive sizing film probably increased the interface contact atom pairs of CF and PP. The PP was distorted and twisted polymers more than that of straight polymers of polyethylene, and was probably generated the spontaneous nano-scale rough interface against CF.
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.
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