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Rheology of Filled Polymers
The flow characteristics (or rheology) of molten polymers have a profound effect on the constructional design and operational performance of most polymer processing machinery influencing levels of shear stress developed (including mixing capability), power consumption, and throughput rate. Other important considerations such as dimensional changes occurring in extrudate (die swell) and flow defects (melt fracture and sharkskin) are also directly related to the rheology of the polymer. Of relevance to the present discussion, however, is the additional effect on flow behavior of introducing fillers to polymer melts and the resulting consequences to compounding machinery design.
In general, the addition of rigid fillers to thermoplastics results in an increase in melt viscosity, which, in common with unfilled polymer, is shear rate dependent. The level of viscosity increase can be very significant depending on the filler loading and the size and shape of the filler particles. In this regard, anisotropic particles, which are fibrous or platelike, may undergo flow-induced orientation and change the shear rate dependency of the filled polymer.
The presence of very high levels of filler commonly results in a critical yield stress, which must be exceeded for flow to occur, due to strong interaction between the filler particles. At stresses below this threshold value, the viscosity of the filled polymer is unbounded, behaving like a solid only deforming elastically, whereas at higher applied stress levels, it undergoes flow.
A phenomenon, commonly found with filler polymers undergoing melt flow, is the wall effect caused by a nonhomogeneous distribution of the disperse phase, resulting in the formation of a melt layer depleted of filler at the wall surface. Having relatively low viscosity, this layer gives rise to lubrication effects, or apparent wall slippage, at the melt boundary.
An important consideration relevant to both compounding and secondary processing of filled polymers is the presence of filler surface treatment. This is generally applied not only to influence interfacial bonding between the filler and matrix to enhance mechanical properties but also to ameliorate the adverse effects of filler on melt viscosity and processability. Coating of fillers with surface modifiers, such as metal stearates and titanates prior to processing, results in a significant reduction in melt viscosity relative to untreated material.
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