Extrudate surface tearing in extrusion of wood plastic composites

In previous publications [1,2] from this laboratory it was shown that the phenomenon of wall slip occurs with HDPE filled with rice hulls and HDPE filled with wood flour. In the present work, metallocene polypropylene (mPP) was used as a thermoplastic matrix to investigate the extrudate surface tearing of wood plastic composites (WPC). Maple wood flour, mesh 100, was predried and used as filler at 50 wt.% loading. Maleated syndiotactic metallocene polypropylene and a thermoplastic silicon elastomer (TPSE) were employed as coupling agent and polymer processing aid (PPA) respectively to investigate their influence on the extrudate surface tearing. At loadings of 50 wt.% wood flour in the mPP the extradâtes come out of the die with significant tearing and surface roughness. Addition of the coupling agent results in considerable improvement of the extradates appearance, especially at high output rates. At low throughputs, however, the extrudate surface is still rough. It was found that TPSE at low concentrations (1-3 wt.%) in the coupled 50% filled mPP composite was able to reduce or even completely eliminate the tearing at all throughputs. Likely, the low viscous PPA forms a lubricating layer on the extrudate surface initiating slip at the wall leading to plug-like flow of the melt flowing through the die. Increasing the screw rotation as well as the concentration of the additive yielded better results.