Rheological properties of molten flax- and Tencel^®-polypropylene composites: Influence of fiber morphology and concentration

Abstract

The rheological properties of short fiber reinforced polypropylene were investigated. Flax and Tencel V R are two cellulose based fibers used in this study. Flax fibers are extracted from the bast of plants. They are composed of thin elementary fibers and rigid thick bundles made of elementary fibers "glued" together. Tencel V R is a man-made cellulosic fiber spun from cellulose solution, with a uniform diameter, thin, and flexible. First, fiber dimensions before and after compounding were analyzed. Both types of fibers were broken during compounding. Flax shows larger length and diameter than Tencel V R , but aspect ratio of flax is smaller. The reason is that after compounding flax remained in bundles. Dynamic viscosity, elastic and viscous moduli were studied as a function of fiber type, concentration (from 0 to 30 wt. %), and composite temperature (from 180 to 200 C). All Tencel V R-based composites showed higher apparent yield stress, viscosity, and moduli compared to flax-based composites at the same fiber concentrations. The results are analyzed in terms of the influence of fiber type, aspect ratio, and flexibility. The importance of considering fiber morphology is demonstrated as far as it controls fiber flexibility and fiber-fiber interactions. V

Influence of fiber content on rheological and mechanical properties of pineapple leaf fibers‐polypropylene composites prepared by twin‐screw extrusion

“…The yield stress σ 0 appears for a volume fraction ϕ of PALF between 6.2% and 13% and increases above. The increase of σ 0 with ϕ can be described by a power law with an exponent of 3.5, close to the theoretical value of 3 suggested by Bennington et al and measured by Abdennadher et al on flax/PP composites. The characteristic times λ are longer for the composites than for the matrix, indicating a slower relaxation process, and the power law index n tends to decrease with ϕ , indicating a more shear thinning behavior.…”

“…The values of the activation energy are provided in Table . E increases quite linearly with the fiber content, with values very close to those measured by Islam and Begum on coir/PP composites and by Abdennadher et al on flax/PP composites.…”

“…While the matrix exhibits a classical behavior with a Newtonian plateau a low frequency and shear‐thinning at high frequency, the composites have much higher viscosities (two decades at low frequency for PALF 30), with the disappearance of the Newtonian plateau, due to the development of a melt yield stress. Similar results have been observed for other types of lignocellulosic fibers . These viscosity curves can be accurately described by a Carreau‐Yasuda law with a yield stress: in which σ 0 is the melt yield stress, ω is the angular frequency, η 0 is the Newtonian viscosity, λ is a characteristic time, n is the power law index, and a is the Yasuda parameter.…”

“…Finally, even though there are numerous papers on the rheological properties of polymers reinforced by lignocellulosic fibers (see, for example, Refs. ), those on PALF‐based composites are very seldom. To our knowledge, George et al were the only ones to characterize the viscosity of PALF‐polyethylene composites using capillary rheometry.…”

Influence of the polarity of the matrix on the breakage mechanisms of lignocellulosic fibers during twin‐screw extrusion

“…The presence of a yield stress in lignocellulosic fiber composites above 20-30 wt% has been reported by many authors. [44][45][46][47] It is due to the interactions between the fibers, which are more significant for flax because of higher aspect ratio (23, instead of 15 for hemp). Similar tendencies are observed for PBS-based composites (Figure 8), except that flax is now more viscous that hemp.…”