Introduction For years, fillers have been used extensively to improve the mechanical properties of polymeric materials. Be- sides the increment usually obtained in stiffness, hard- ness or abrasion resistance, and the reduced cost of the filled material, the addition of fillers to polymers also modify their flow behavior and consequently their processability(Friedrich et al. 1995). The matrix and particles interact thermodynamically according to their surface potentials and hydrodynamically through flowfield interactions (Solomon and Lu 2001). The presence of direct mechanical contacts between embedded particles or fibers can also give rise to many non-linear rheological features, which strongly manifest themselves at high concentrations of filler(Chaouche and Koch 2001). Thus, the presence of filler enhance the non-linear behavior of polymers and can contribute with new non-linear effects especially at high concen- trations. Some of these non-linear responses are shear thinning and yield stress, which appear when the par- ticles can form flocs that are held together by adhesive forces. Shear thinning occurs when viscosity decreases with increasing shear rate, while yield stress appears in suspensions that do not flow until that critical stress value is overcome. The probability of direct interaction between the particles increases as the concentration of filler increases. The formation of separate individual clusters is also possible even at small concentrations, affecting the flow properties of the viscoelastic Norma E. Marcovich Marı´a M. Reboredo Jose´ Kenny Mirta I. Aranguren Rheology of particle suspensions in viscoelastic media. Wood flour-polypropylene melt Received: 18 June 2003 Accepted: 28 November 2003 Published online: 16 January 2004 Ó Springer-Verlag 2004 Abstract The dynamic mechanical behavior of suspensions of wood flour in polypropylene (PP) melts was investigated at varying filler concentrations. The main observed features were related to the visco- elastic nature of the polymer and to the filler aggregation, where the process of formation and destruc- tion of particle clusters is governed by the polymer chain dynamics. The effect of the wood flour particles at low and large deformations was analyzed. The sample containing a wood flour concentration of 50% (by weight) showed a solid like behavior at low frequencies and was identified as the sample closer to a liquid-solid transition (LST). The values of the Newtonian viscosity obtained from sinusoidal oscillations at low frequencies were related to the concentration of filler in the suspensions. Moreover, a filler con- centration scaling was found, that allows to obtain a master curve using the neat polymer as the refer- ence and from which it is possible to calculate the dynamic mechanical behavior of all the suspensions. Apparently, for this system, the relaxation mechanisms of the neat polymer are not changed by the presence of the filler. However, the corresponding relaxation times are increased as a function of the filler concentration. Keywords Polypropylene Æ Wood flour Æ Dynamic mechanical properties Æ Particle concentration scaling Rheol Acta (2004) 43: 293–303 DOI 10.1007/s00397-003-0349-0 ORIGINAL CONTRIBUTION N. E. Marcovich Æ M. M. Reboredo M. I. Aranguren (&) Institute of Materials Science and Technology (INTEMA), University of Mar del Plata—National Research Council (CONICET), Av. Juan B. Justo 4302, 7600 Mar del Plata, Argentina E-mail: marangur@fi.mdp.edu.ar J. Kenny Institute of Chemical Technologies, University of Perugia, Loc. Pentima Bassa 21, Terni, Italy