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Table 1.2 Effect of Pressure on Viscosity for Polystyrene, Equation 1.46
However, the relative measure of viscosity can be obtained from [9,15,16]
where
P
bar
30
100
200
300
500
1000
3000
rjp = viscosity at pressure p and constant shear stress T0
T]0 = viscosity at constant shear stress T0
ap = pressure coefficient
For styrene polymers rjp is calculated from [14]
"> =thesp kL)
where p - pressure in bar.
(1.45)
(L46)
Thus the change of viscosity with pressure can be obtained from Equation 1.46. Table 1.2
shows the values of viscosity calculated according to Equation 1.46 for a polystyrene of
average molecular weight. It can be seen that a pressure of 200 bar causes an increase of
viscosity of 22% compared to the value at 1 bar. The pressure coefficient of LDPE is less
than that of PS by a factor of 3-4 and the value of HDPE is again smaller by a factor of 2
compared to LDPE. This means that in the case of polyethylene an increase of pressure
by 200 bar would enhance the viscosity only by 3 to 4%. Consequently, the effect of
pressure on viscosity can be neglected in the case of extrusion processes which generally
use low pressure. However, in injection molding with its high pressures, typically the
dependence of viscosity on pressure has to be considered.
1.3.7.7 Dependence of Viscosity on Molecular Weight
1.03 TJ0
1.105 TJ0
1.221 TJ0
1.35 TJ0
1.65 TJ0
2.72 TJ0
20 TJ0
The relationship between viscosity and molecular weight can be described by [12]
(1.47)