This characteristic is common tomost widely used
commodity polymers and is the main reason why polymers flow
readily at high processing rates in the melt phase. It can also be
seen that log–log plots of shear viscosity followed a linear relationship
with wall shear strain, allowing a simple power law model to
be used to describe flow behaviour in this shear rate region. This
is an extremely useful flow model as it allows the flow to be characterised
by two constants, n and K. Power law index, n, describes
the shear thinning nature of the melt and is calculated from the
gradient of the shear viscosity versus shear rate plot. K is the consistency
index of the melt and is calculated from the intercept of the
shear viscosity at zero wall shear rate. In practice, the shear flow
behaviour of all polymers is more complex than this simple model
suggests, viscosity being constant in the ‘Newtonian’ region at very
low shear rates (less than approximately 1 s−1) and also deviating
from the power law at very high shear rates (above 106 s−1) but the
power law model is adequate for calculations relevant to most pro-
Fig. 3. Temperature dependence of shear viscosity with power law curve fit (A)
HPC-SSL; (B) HPC-SL and (C) HPC-L.
cessing applications
This characteristic is common tomost widely usedcommodity polymers and is the main reason why polymers flowreadily at high processing rates in the melt phase. It can also beseen that log–log plots of shear viscosity followed a linear relationshipwith wall shear strain, allowing a simple power law model tobe used to describe flow behaviour in this shear rate region. Thisis an extremely useful flow model as it allows the flow to be characterisedby two constants, n and K. Power law index, n, describesthe shear thinning nature of the melt and is calculated from thegradient of the shear viscosity versus shear rate plot. K is the consistencyindex of the melt and is calculated from the intercept of theshear viscosity at zero wall shear rate. In practice, the shear flowbehaviour of all polymers is more complex than this simple modelsuggests, viscosity being constant in the ‘Newtonian’ region at verylow shear rates (less than approximately 1 s−1) and also deviatingfrom the power law at very high shear rates (above 106 s−1) but thepower law model is adequate for calculations relevant to most pro-Fig. 3. Temperature dependence of shear viscosity with power law curve fit (A)HPC-SSL; (B) HPC-SL and (C) HPC-L.cessing applications
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