Characterization of reinforcing effect of alumina nanoparticles on the novolac phenolic resin

Very fine alumina nanoparticles were loaded in novolac type phenolic resin (PF) using solution mixing method. The concentration of nanoalumina in PF was varied between 2.5 to 20 wt%. All the compounds were compression molded and then subjected to scanning electron microscopy (SEM), tensile, flexural, and dynamic mechanical analysis (DMA) tests. SEM analysis showed that the nanoalumina were dispersed uniformly at low concentrations, however, at high concentrations, dispersion was suppressed leading to agglomerates in the composites. Mechanical testing revealed that the nanoalumina had a great influence on the strength and stiffness of PF resin particularly at concentrations below 5 wt%. However, at concentration above 5 wt%, the stress concentrations were developed because of the formation of big aggregates that results in strength reduction. Theoretical analyses based on Pukanszky and micromechanical models of tensile modulus revealed that strong interfacial interaction and thick interphase region around the alumina nanoparticles is formed. DMA results suggested that the nanoalumina increased the crosslinking density of the PF resin, possibly around the interface region. It was also postulated that an apparent percolation state is established above 5 wt% loading of nanoalumina in which interphase region comes to contact before direct contact of particle leading to continuous interphase region. POLYM. COMPOS., 35:1285–1293, 2014. © 2013 Society of Plastics Engineers