Formation of porosity and change in binder pitch properties during thermal treatment of green carbon materials

The formation of porosity during heat treatment of an extruded mixture of coke particles and a binder pitch was investigated using image analysis of polished cross-sections of shaped samples. The porosity due to pitch devolatilisation takes place during a rather narrow interval of temperature when the sample is heated at 12 K/hour. Only a small fraction of pores is formed when the shaped sample is submitted to an intermediate isothermal step which allows the departure of volatile compounds from the binder. Physico-chemical changes occurring in the pitch during pyrolysis were followed by measuring glass transition temperature in relationship with weight loss. The results indicate that in sufficiently slow heating conditions, low molecular compounds essentially diffuse through the fluid pitch and evaporate at the external surface of the sample during pyrolysis. As a result, no marked porosity is formed during baking. In contrast, a high heating rate allows low molecular mass compounds to accumulate inside the sample and the formation of a significant porosity is due to the evaporation inside the shaped sample of accumulated low molecular mass compounds.