New insights into high-ortho phenolic novolac: Elucidating dependence between molecular structure, curing kinetics and thermal stability

The molecular structure of phenolic novolacs plays an important role in their processing, properties and applications, yet its exact influences on the cure and thermal decomposition remain insufficiently understood. Herein two type of novolacs are prepared: high-ortho novolacs (HOPF) with high ortho–para ratio, and ordinary novolacs (NPF). Their cure behavior with hexamethylenetetramine (HMTA) investigated using DSC and TGA. The DSC analysis confirmed that HOPF/HMTA exhibited higher reactivity than NPF/HMTA. The model-free isoconversional analysis with the Vyazovkin method HOPF/HMTA has the higher effective activation energy than NPF/HMTA at the first stage, but lower in the latter stage. The isothermal cure is reasonably predicted. Moreover, the model-fitting kinetics using Málek method reveals the autocatalytic reaction mechanisms and yields the curing rate equation which can well predict the non-isothermal cure. Furthermore, TGA reveals the network structures of cured HOPE and NPF moderately affect the thermal decomposition process.