Quantifying effects of compositional variations on microstructural properties of polypropylene-wood fiber composites by melt rheology and tensile test data

Abstract

In this study, melt-state rheological behavior and solid-state mechanical properties of polypropylene-wood fiber composites were investigated in detail depending on compositional variations such as (i) alkaline treatment on wood fibers, (ii) fiber size, (iii) wood fiber content, and (iv) compatibilizer/wood fiber ratio. Composite samples were prepared in a lab-scale co-rotating twin screw extruder by using a maleic anhydride grafted polypropylene as compatibilizer. Morphological features of composites were examined in a scanning electron microscopy. Viscoelastic behavior and mechanical properties of samples were analyzed by performing oscillatory tests in a rotational rheometer and a universal tensile test machine. It was found that the increasing amounts of wood fiber and compatibilizer/wood fiber ratio led to improve melt elasticity and tensile strength. It was concluded that the amount of compatibilizer into composite formulation was the most important compositional parameter compared to size and chemical treatment of wood fibers for improving the physical properties of composites. The Nicolais-Nicodemo micromechanical model showed that the increasing amount of compatibilizer yielded lower parameters which implied better interfacial adhesion between polypropylene and wood fibers.

A Review of Wood Polymer Composites Rheology and Its Implications for Processing

“…Next, the cone and plate geometry cannot be used due to the high viscosity of the fluid, and this also prevents testing in continuous mode, unless the material contains very low filler amounts. On the other hand, if testing in oscillatory mode is employed, the material must be loaded within its LVR, which for molten WPCs is well known to be very narrow and to occur at very low strains [ 13 , 77 , 78 , 79 , 80 ]. Moreover, the validity of the Cox-Merz rule is anyway questionable, as it should not be applied to concentrated polymeric suspensions [ 81 ].…”

“…On the other hand, at the typical testing temperatures, both dynamic moduli increase with frequency, and, since the storage modulus increases faster than the loss modulus, at a certain frequency a cross-over is reached ( Figure 1 a). This means that the material changes its behavior, i.e., it switches from that of a viscoelastic fluid at low frequencies to that of a viscoelastic solid at higher frequencies [ 14 , 18 , 55 , 72 , 77 , 78 , 79 , 82 , 83 , 84 , 85 , 86 , 87 , 88 , 89 ]. The complex viscosity has a decreasing trend with frequency, similar to a shear thinning fluid, if the Cox-Merz rule was supposed to hold ( Figure 1 b, curve A).…”

“…At low frequencies (i.e., less than 1 rad/s), if the wood content is below 30 wt.%, the complex viscosity curve displays a Newtonian-like plateau [ 14 , 92 ], and the behavior is mostly influenced by the polymer-polymer interactions. At higher wood content, filler-polymer interactions become more important; thus, the Newtonian plateau reduces and tends to disappear [ 13 , 65 , 77 , 89 , 92 , 95 ], as shown in Figure 1 b (curve C). At even higher wood percentages, filler-filler interactions are dominant, and yield phenomena may appear in some systems ( Figure 1 b, curve D).…”

“…At high frequencies, the behavior is qualitatively similar for all wood percentages, with the complex viscosity showing a shear thinning like behavior, that can be modeled as a power law. The effect of the wood particles is limited to the curve becoming steeper at higher filler content ( Figure 1 b) [ 14 , 15 , 53 , 54 , 72 , 77 , 79 , 85 , 86 , 90 , 92 , 93 , 94 , 96 ]. The rheological behavior is, thus, anyway more dependent on the characteristics of the polymeric matrix.…”

“…Particle size is another important factor that was considered. The complex viscosity and the dynamic moduli seem to be slightly influenced by the dimension of the wood particles when the size variation is moderate [ 77 ] and the frequencies are high [ 59 ], while the complex viscosity increases at low frequencies when the filler morphology is constituted by small particles [ 53 , 59 ]. Moreover, this trend becomes more important at high percentages of wood.…”