A study of viscoelasticity and extrudate distortions of wood polymer composites

Natural fiber composites exhibit a characteristic surface tearing and extrudate distortions upon exiting from extrusion dies. This type of defect is characterized by highly rough, cracked, and distorted extrudate surface. In this study, the extrudate distortions and viscoelastic nature of metallocene-catalyzed polyethylene (mPE)/wood flour composites have been investigated. As the wood flour loading increases the region of linear viscoelasticity shortens. The first normal stress difference decreases, while the storage modulus increases. It was observed that increasing the wood flour loading up to 50 wt% aggravated the surface tearing; however, 60 wt% wood flour in mPE completely eliminated the surface defect. It was also found that increasing the shear rate improved the surface appearance of the filled compounds. This is due to the increased wall slip velocity of the composites at high shear rates and wood filler loadings. Increasing the diameter of the die at the same aspect ratio generally provides more severe surface tearing.