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