Preparation of high-strength microporous phenolic open-cell foams with physical foaming method

Abstract

A series of high-strength and highly microporous phenolic open-cell foams modified by different epoxy resins were fabricated using physical foaming method. The new compound emulsifiers consisting of anionic and nonionic surfactants were physical blended at high speed with modified resols, foaming agent, and mixed acid curing agent. The modified synthesis of epoxy-modified phenol-formaldehyde (PF) resin was conducted via the etherification mechanism. In order to verify the occurrence of etherification mechanism by co-reaction between resole methylol, hydroxyl group, and ring-opening epoxy group and to characterize the structure successfully, Fourier transform infrared spectroscopy, carbon-13 (13C) and proton nuclear magnetic resonance (1H NMR) had been used to characterize the structure successfully. The phase of resols and curing behavior were characterized. Fabricated foams were characterized for cellular, water absorption, apparent density, and mechanical and thermal properties. The results show epoxy modified resol demonstrated single uniform phase. The open-cell micropore structure revealed by scanning electron microscopy (SEM) demonstrates that smaller and more homogeneous cells existed with increasing dosage of epoxy. When the content of epoxy increased from 0 wt% to 10 wt%, the thermal conductivity, water absorption, and porosity of PF gradually increased. The highest open-cell porosity reached up to 90.959%. The differential scanning calorimetry and gel time data demonstrated that modified resols exhibited less gel time and lower endothermic and curing exothermic heat. Anionic surfactant, sodium dodecyl sulfonate, which was better compatible with resol, promoted the formation of open cells and produced good homogeneous and moderate micropores. In addition, the high-strength open-cell PF possessed excellent filterable and separation performance and absorbency. The porous micropore structure controlled by the content of epoxy resin and the ratio of sodium dodecyl sulfate /Tween80 greatly improved the mechanical strengths of PF.