Rheological properties of talc-filled polypropylene
In this study polypropylene was modified by a range of talc (Mg3(OH)2Si4OlO) fillers. Three different types of surface modifiers were also used, two types of silanes (octyltriethoxysilane and y-aminopropyltriethoxysilane) and maleic anhydride modified polypropylene plus polyethylene wax as an external lubricant. A series of compounds were prepared in an APV twin screw compounder, based on talc addition level, morphology, particle size, surface coatings and coupling agents. The study showed that addition of talc to PP increases the shear viscosity and the blend system generally follows the power law in the shear rate range studied. The experimental values obtained with talc-filled PP were compared with theoretical data obtained from the Maron-Pierce type equation. It was found that theory predicts a higher value of relative viscosity for some talc fillers. A model equation is proposed for talc-filled PP that predicts accurately relative viscosity of the compound for different volume fractions of talc filler. PP-filled with talc having predominantly acicular particles (high surface area), shows a highest shear viscosity values, since high surface area fillers tend to increase the occluded polymer increasing the effective filler volume. The addition of a high level of coating decreasess hearv iscosity of the system,a ddition of a reactivec oupling systems lightly increasess hearv iscosity. Wall slip for unfilled PP was consistently evident. The slip velocity increases systematically with shear stress, but there is a critical value below which slip velocity was negligible. Addition of talc decreased the slip velocity at a given stress. A mechanism of slip has been proposed whereby a 'slip layer' on the die surface provides a sharp, low energy interface over which, at sufficiently high shear stress, the fluid PP can slip. A new empirical model has been proposed to predict the slip velocity of filled PP, as a function of talc volume fraction. PP filled with 10 % wt. talc having predominantly acicular particles, showed a higher slip velocity than PP filled with talc of other morphologies. It was observed that slip velocity increased by coating talc (either by use of coating or coupling agents) due to modification of velocity distributions in the flow channel. Extrudate swell increases with increasing shear rate for unfilled and talc-filled PP compounds. Inclusion of talc generally decreases the swell over the entire range of shearr ates studied,b ecauset he addition of talc to PP increasesth e shear/extensional modulus and thereby decreasede lastic strain recovery. No appreciablei nfluence of particle size on swell ratio was observed. Coating talc with octyltriethoxysilane produces higher swell relative to uncoated filled PP, and higher coating levels consistently produced higher swell values. However, coupling agent 7- aminopropyltriethoxsilane and maleic anhydride modified polypropylene both reduced the swell of the filled polymer system, with a more pronounced effect in extension than in shear. Swell decreased with an increase in capillary length-todiameter ratio and increased with increase in shear rate, (or shear stress) with or without coating/coupling agent. Addition of talc fillers delayed the onset of melt fracture and higher loadings of talc completely suppressed the melt fracture in the range of shear rate studied. Melt fracture for unfilled and filled PP has been studied and quantified by the frequency of the melt distortion. A mechanism for polymer melt fracture has been proposed to explain the results, based on the tensile failure of the material in extensional deformation. It was found that shear rate and die L/D ratio were the major causes of changesin severity of melt fracture for the unfilled and talc-filled compoundss tudied. However, talc morphology and particle size did not have significant effects on the severity of melt fracture.