Influence of the mixing process on the graphitization of phenolic resins

Abstract Phenolic resins are usually incorporated into carbon-containing refractories (CCRs) to play a role as binder compounds. Among the benefits associated with the role of these organic compounds, the following can be pointed out: (i) the production of consolidated pieces with improved green mechanical strength, (ii) minimal emission of toxic substances during their heating treatments and (iii) their likelihood to undergo graphitization at intermediate temperatures and in the presence of catalytic agents, increasing the overall carbon content of the CCRs. Knowing that the graphitization process of novolak and resole resins should be influenced by various parameters (i.e., resin composition, mixing, heating schedule, etc.), this work addresses the evaluation of two mixing procedures (M1 = vertical + ultrasonic mix and M2 = two axes mixing) and their influence on the phase transformations and graphitization trend of two novolak resins containing 0 or 3 wt% of ferrocene (graphitizing agent), after setting and firing steps up to 1000 °C. Some techniques such as Nuclear Magnetic Resonance (13C NMR), X-ray Diffraction and Transmission Electron Microscopy - TEM, were applied for the samples' characterization. According to the obtained results, the analyzed novolak resins (commercial and modified-one) presented significant structural differences and the evolution of the chemical groups identified via 13C-NMR changed, depending on the applied mixing procedure. M1 resulted in a higher number of ortho-ortho or ortho-para methylene bridges in the benzene rings of Rc and Nv-based compositions, resulting in higher mobility for the linkage rearrangement during pyrolysis and, consequently, improving their graphitization level. The synthesized resin (Rc) containing 3% wt% of ferrocene showed the highest graphitization level (70 or 58% for M1 and M2 mixing procedures, respectively) and the formed graphitic phase presented onion-like morphology.