A New Process of Thermoplastic Polypropylene Reinforced by Interlayered Activated Carbon Fiber Treated by Electron Beam Irradiation under Nitrogen Gas Atmosphere with Oxygen Prior to Assembly and Hot-Press

Abstract

A new process of internal activation of carbon fiber reinforced thermoplastic polymer (CFRTP) of polypropylene (PP) by applying electron beam irradiation (EBI) under oxygen (O₂)–rich nitrogen gas (N₂) 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 N₂ gas with O₂ concentrations between 200 ppm and 200,000 ppm mostly improved the bending strength (σ_b) while reducing strain at the bending strength (σ_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, σ_b was apparently improved by the 200 ppm and 2,000 ppm O₂ atmospheres. Namely, 0.22 MGy-EBI under N₂ gas atmosphere with 200 ppm to 2,000 ppm-O₂ improved σ_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 induced by EBI and oxygen assisted by concentrating the O₂ gas molecules from 200 ppm to 2,000 ppm-O₂ in N₂ atmosphere, rather than weak molecular bonding CF-(H₂O, N₂, O₂)-PP for the untreated samples. Moreover, the action of the EBI apparently acts to clean residual H₂O, N₂, and O₂ 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 O₂-rich N₂ atmosphere appears to be a viable method to increase carbon fiber-thermoplastic polypropylene adhesion enhancing reliability and safety of the PP-CFRTP.

Fig. 6 Oxygen concentration dependent bending strength (P_f = 0.06) of PP reinforced by CF treated with 0.22 MGy-EBI under protective N₂ gas atmosphere with each O₂ concentration (solid lines), together with CF untreated with (thick broken line) and without (thin broken line) sizing film.