The melt‐viscosity of some phenol–formaldehyde novolak resins

Abstract

AbstractThe melt‐viscosity and density of a series of phenol–formaldehyde novolak resins of molecular weights varying between 400 and 900 have been studied in the temperature range 100–250°. Similar studies have been carried out for phenol (50–100°) and 2 : 2′‐dihydroxydiphenylmethane (125–190°).The melt‐viscosity results have been examined in the light of Eyring's theory of viscosity and it is seen that the energy of activation for viscous flow falls rapidly as temperature increases and tends to steady values at high temperatures. These steady values are almost linearly related to molecular weight.The rapid fall in energy of activation as temperature increases is attributed to the breakdown of association forces, mainly hydrogen bonds. At high temperatures flow is believed to take place by the movement of single molecules, but at low temperatures the large energies of activation suggest that flow takes place by associated dimers, trimers etc. breaking away from the general association field that surrounds them.The coefficient of cubical expansion α is found to decrease rapidly from phenol to a resin of molecular weight about 400 and then more slowly as molecular weight increases; α also increases slightly with temperature.