Effects of heating rate on the foaming behavior and pore structure of carbon foams derived from phenol-formaldehyde resin

The effect of heating rate on the foaming behavior of phenol-formaldehyde resin and the pore structure and compressive strength of the carbon foams produced were investigated. Results indicate that the viscosity of the resin changes little with temperature between 135 and 225℃, but increases abruptly above 225℃. The foams are formed between 200 and 300℃ by gas released during pyrolysis and the foaming behavior follows the hot-spot nucleation mechanism. By increasing the heating rate from 0.5 to 3℃/min the average pore size decreases from 304 to 267 μm, and the density and compressive strength increase from 0.34 to 0.51 g/cm³ and 6.1 to 12.5 MPa, respectively. The heating rate affects the expansion velocity of the bubbles formed, and the average size and homogeneity of the pores in the foams.