Dynamic light scattering study of the curing mechanisms of novolac-type phenolic resins

Abstract

The curing behavior of a novolac resin (NV) cured with hexamethylenetetramine (HMTA), as well as the influence of an excess amount of HMTA on the curing reaction, were investigated by dynamic light scattering and gel permeation chromatography. A two-roll mixing mill process was applied to control the curing reaction degree. The dependence of the number of mixing times on the weight-average molecular weight (M w) and hydrodynamic radius (R h) of NV differed significantly with the HMTA amount. A larger quantity of HMTA resulted in faster growth and larger R h and M w values. The relationship between M w and R h for NV in tetrahydrofuran indicates two different polymer-growth mechanisms irrespective of the excess amount of HMTA (that is, power-law and subsequent deviation corresponding to the chain extension and intermolecular reactions between larger molecules, respectively). These results suggest that structural differences governing the mechanical properties of phenolic resins are initiated in the pregel stage, followed by noticeable differences in the mechanical properties during the subsequent curing process.