Rheological aspects of wood polymer composites extrusion.

摘要

Wood polymer composites (WPC) have received considerable industrial and academic interest due to the fact that natural fillers are an abundant, inexpensive and renewable raw material. One of the challenges is to optimize the processing operations of highly filled WPC. In this thesis, viscosity, viscoelasticity, wall slip, entry flow, and melt flow instabilities of WPC containing 5-70 wt% wood flour have been investigated. An emphasis was given towards investigation of the surface tearing in extrusion of WPC and its relation to wall slip. This interesting phenomenon has been known in the WPC industry for years, but it has not received much research interest. It was observed that increasing the wood flour loading up to 50 wt% aggravated the surface tearing, however, addition of 60 wt% wood flour completely eliminated the surface defect due to strong slip leading to plug-like flow. It was also found that use of lubricants, increasing the shear rate, and length-to-diameter ratio of the die improved the surface appearance of the filled compounds. Strong wall slip or plug-like flow at high shear rates was found responsible for disappearance of the extrudate defects.;The effect of different commercial coupling agents on the interfacial morphology and rheological properties of WPC has also been investigated in this research. The melt flow properties of the composites containing coupling agents were found to depend on both matrix and coupling agent molecular weight and structure.;Molecular weight and molecular weight distribution of the polymer matrix influence the melt flow properties of the composites. Viscosity significantly increased with wood flour loading. Low molecular weight and narrow molecular weight distribution polyethylene matrix provided larger increase of the viscosity of the composites than high molecular weight polymers. The significance of the entrance pressure loss measurement and its usefulness for quantitative assessment of filler-matrix interactions in composite materials is also demonstrated in this thesis.