The main goal of this study was to analyze the effect of process additives, that is, maleated polypropylene (MAPP), and a nucleating agent on the viscoelastic properties of different types of extruded polypropylene (PP) wood plastic composites manufactured from either a PP homopolymer, a high crystallinity PP, or a PP impact copolymer using dynamic mechanical thermal analysis. The wood plastic composites were manufactured using 60% pine wood flour and 40% PP on a Davis-Standard Woodtruder(TM). Dynamic mechanical thermal properties, polymer damping peaks (tan delta), storage modulus (E'), and loss modulus (E") were measured using a dynamic mechanical thermal analyzer. To analyze the effect of the frequency on the dynamic mechanical properties of the various composites, DMA tests were performed over a temperature range of -20 to 100degreesC, at four different frequencies (1, 5, 10, and 25 Hz) and at a heating rate of 5degreesC/min. From these results, the activation energy of the various composites was measured using an Arrhenius relationship to investigate the effect of MAPP and the nucleating agent on the measurement of the interphase between the wood and plastic of the extruded PP wood plastic composites. (C) 2003 Wiley Periodicals, Inc.