Synthesis of MWCNTs for successive polymeric and nano modification of a Nigerian bitumen, optimization using response surface methodology

Abstract

Polyethylene (PE) is attractive as a bitumen modifier for several reasons, the most important of which is cost. However, polymeric bitumen is highly sensitive to PE dosages and suffers thermal degradation. To develop a tougher, more ductile crack-resistant binder, a ternary mixture was formulated using PE-modified binder and Multi-Wall Carbon Nanotubes (MWCNTs). The conventional properties of PE-modified bitumen were optimized using a mixture design and Response Surface Methodology (RSM). MWCNTs (0.1 – 0.3 wt%), mixing time (20 – 40 min) and speed (1000 – 15000 rpm) were considered for ternary modification at 140 °C. The softening and penetration points as well as ductility were used to determine the best mix. MWCNT was synthesized using a carbon vapour deposition method and characterized using Transmission Electron Microscopy (TEM), which indicates cylindrical-shaped CNT with numerous walls; Thermogravimetric Analysis (TGA) indicates crystalline graphitic carbon with a weight loss of 25.96 %; X-ray Diffraction (XRD), which indicates graphene-like layers of tube walls at 2θ values of 25.89° and 44.53°. Brunauer-Emmett Teller (BET) shows surface area, pore size and pore volume of 0.62 m²/g, 15.91 nm and 0.00362 cm³/g, respectively. The developed models for the modified bitumen are predictive (R² greater than or equals to 0.94). The optimum mix design for polymeric bitumen was 4.07 wt% PE, and 95.83 wt% bitumen and 0.1 wt% MWCNT, 1500 rpm mixing speed and 37.7 min for the ternary bitumen. The relatively small absolute relative error between experimental and simulation indicated the reliability of the final models with a combined desirability of 0.77 – 0.87.