Gelest Silicone Fluids - Gelest Inc.

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Gelest, Inc.
Rheological Behavior Under Shear
At shear rates commonly encountered (≤10 4 s -1 ) polydimethylsiloxanes
behave, at viscosities up to 1,000 cSt., like
Newtonian fluids. Viscosity is constant and independent of the
velocity gradient. Apparent viscosity is identical with viscosity
extrapolated to zero velocity gradient.
For oils of a higher viscosity than 1,000 cSt., this ratio is only
constant for velocity gradients below a certain value. Beyond
this value, becoming lower as the product becomes more viscous—the
ratio is no longer constant: apparent viscosity falls
below real viscosity (extrapolated for a zero velocity gradient)
and the behavior is then known as “pseudoplastic.” This
change is perfectly reversible, and behavior again becomes
Newtonian when the velocity gradient falls once more below
the critical value. Viscosity returns to its initial level even after
intense shearing of long duration.
As a guide, the table indicates the “critical” velocity gradients
for polydimethylsiloxanes (where change of rheological
behavior occurs) as well as apparent viscosity measured at
velocity gradient equal to 10,000 s -1 .
CH3
CH3
Si
CH3
O
CH3
Apparent
viscosity
for a velocity
gradient
Critical velocity of 10,000 s -1
gradient (s -1 )
(in cSt.)
1,000 2,500 850
12,500 200 4,700
30,000 150 6,000
100,000 30 8,200
Si
CH3
O
n
CH3
Si
CH3
CH3
Apparent viscosity as a function of velocity gradient
Viscosity in cSt.
2,000,000
1,500,000
500,000
200,000
100,000
50,000
20,000
10,000
5,000
2,000
1,000
2,500,000
500,000
200,000
100,000
60,000
30,000
20,000
12,500
10,000
5,000
2,000
1,000
500
Temperature 25°C
0.1
0.2 0.3 1 2 5 10 20 50 100 200 500 1,000 2,000 5,000 20,000 100,000 500,000
10,000 50,000 200,000 1,000,000
VELOCITY GRADIENT (s -1 )
(215) 547-1015 FAX: (215) 547-2484
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