FLOW CHARACTERISTICS OF ORGANOPOLYSILOXANE FLUIDS AND GREASES C. C. CURRIE AND B. F. SMITH Dow Corning Corporation, Midland, Mich.
Some of the rheological properties of organopolysiloxane fluids and greases are described. Data are presented to show that methylpolysiloxane fluids exhibit either Newtonian or pseiidoplastic f l o w depending on niolecular weight and that the rate of shear has little effect on the calculated viscosity-temperature slope. The existence of thixotropy at high shear rate is established. The rheological behavior of the siloxane greases is determined by both the concentration of soap and the type organopolysiloxane fluid. The type fluid is the major factor in the temperature dependence of siloxane greaNes under shear. FIE: expanding use of organopolysilo~ariegreases and fluids as lubricants and damping media necessitated a study of their flow characteristics. The first part of this paper describes the effect of shear, the effect of temperature, and the effect of s h w r and temperature on the methylpolysiloxane polymers. The serond portion deals with the flow*propertiesof the siloxane greas~s. The effect, of temperature and rate of shear and the roles played by the soap :tnd siloxane fluid in various greesc' formulations are discussed. Pertinent to both portions of the paper. is ~tbrief review of the. gcweral chemistry of the siloxanes. The methylpolysilosanes studied are polymers of the general formula
T
nhere n can be varied from 0 to some urikiiown number in excess of 2000. These polymers do not consist of a single molecular species but rather of a statistical distribution of various nioIecdar sizes. The number average molecular weight is most easily expressed by the coefficient of viscosity determined at 25" C. Barry (3)establkhed a relation between these two properties in the following tlmpirical equation : log 7 = 1.00
+ 0.0123 d
M
Either 1)) or n can he varied from 0 to an indeterminately large number so that many degrees of phenylation may be obtained. Again kinematic or absolute viscosity is used t o indicate relative degree of polymerization within a given copolymer series. APPARATUS
Unlew otherwise noted the tests were run at 25' C. No :tttempt was made to correc-t for the temperature rise of the fluid film caused by shear. The authors believe that heat dissipation is rapid enough through the instruments so that this temperature effect ia minor. Most of the data were obtained with a pressure viscometer in accordance with accepted methods. The apparent viscosity was calculated by means of Poiseuille's equation :
PR
The dimensions of the capillaries were determined with mercury. These data are given m Table I. The volume of material extruded by this instrument was measured and found to be 0.0834 ml. er second for the gear with 40 teeth and 0.133 ml. per second for %e 64tooth gear.
The methylphenylpolysilosanesevaluated here conform to the following general formula: Table Capillary 1 2 3 4 5
The fluids represented by this formula also consist of a distriI~ution of varying molecular sizes. The correlation between average molecular weight and viscbosity has not been definitely established, but variations in degree of polymerization itre ronvrniently expressed by viscosity in centistokes or centipoiaes Lit 2J" C. It is also possible to synthesize polymers in which t)oth dimethylsilosane and methylphenylsilosarie units are ctontxined in the same molecule. These copolymers were used in the formulation of the greases which are discussed later. A typic-a1 formula for these materials is
6 7 8
1.
Dimensions of Capillaries Length, Cm. Radius, Cm, 15.44 9.86 7.42 6.10 4.88 4.06 2.54 1.788
0.187 0.116 0.0929 0.0738 0.0559 0.0497 0,0322 0.0251
In order to check the results obtained in the pressure viscometer a few determinations were made in a rotational viscometer. The dimensions of this instrument, described in detail by Smith and Applegate (8),are given in Table 11. Apparent viscosity was calculated with the equation 9.55
ST
?I@ = -
r.p.m.
2457
INDUSTRIAL AND ENGINEERING CHEMISTRY
2458 Table
II. Dimensions of Rotational Viscometer
Height, Cm.
Bob A C D
5.0
1.0 2.5
Length, Cm. 1.95 1.95 1.50
Clearance. Cm. 0.05 0.05 0.50
Instrument, S 0.0001968 0.0008268 0.003226
Constants.
C
0.00767 0.03222 0.1257
RHEOLOGY OF METHYLPOLYSILOXANES
The methylpolysiloxanes are characterized by extremely flat viscosity-temperature slopes, low freezing points, and a high degree of heat stability. For these reasons they have been used in a variety of applications involving shear such as the torsional vibration damper, the magnetic fluid clutch, shock absorbers, and certain devices used as dashpots. Accordingly, considerable time has been expended in studying the flow characteristics of the met hylpolysiloxanes.
SW
2
;
100
I’
g
;
so
Y
-
=
A graphic representation of the data obtained with the pressure viscometer over a shear rate of 25 to 10,000 reciprocal seconds is given in Figure 1. The actual data are shown in Table IV Several items of interest are illustrated by the curves: first, a t absolute viscosities of 1000 cp. or lower the methylpolysiloxanes are essentially Newtonian in their behavior-that is, the rate of shear of a liquid flowing through a given capillary tube is directly proportional to the activating stress; a t absolute viscosities above 1000 cp. these fluids exhibit, pseudoplastic flow-that is, there is a deviation from linearity when the apparent viscosity is plotted against the rate of shear. Furthermore, the effect of shear becomes more pronounced as the molecular weight, as indicated by the absolute viscosity, is increxscd.
IV. Apparent Viscosities of Methylpolysiloxane Fluids
Table
Rate of Shear, Seo. -* 16.1 25.7 67.4
IOW
17L
Vol. 42, No. 12
108 131 209 265 423 601 855 959 1360 3130 5020 6660 10600
(Pressure viscometer) Fluids, Cp. 90,000 60,000 30,000 12,500 -Viscosities, poises---876 568 309 116 316 121 875 55 1 692 117 471 ... 427 604 110 280 600 456 271 108 250 507 101 3 89 436 236 358 98.9 380 205 95.9 308 191 320 256 88.4 271 232 160 84.0 244 225 151 81.8 223 195 132 74.4 143 130 61.6 97.3 113 100 53.1 78.3 94.3 47.9 56.6 87.7 75.0 66.2 a2.8 41 . O
-
3000
lo00
...
...
...
30.6 30.3 29.4 29.0 28.1 27.5 27.0 24.7 25.2 23.9 21.0 18.5 18.1 16.4
...
...
... lio. 3
gq0
10.3 10.0 10.1 9.9 10.2 10.1 10.0 10.0 10.0
h 1000 C’9
IO
I
r
I
t
so
10
100
Figure 1.
lo00
500 RATE
OF
5000
IO000
SHEAR-% c'urvc! B w-iiirh represents the behavior of R fluid H ith itri absolute viscosity of 32,000 rp. The viscositytemperature dope i.r substantially independent of the rate of s h w r Iwtueen 1 and lo00 reviprocal seconds for this fluid. Althougli rurve R does appear to he somewhat8less steep, this is proI),tt)ly c*aused by ~ r r o r 5in me:iwr(mwt inti oducwi hy the use o f t \ 4 o different instrumrnts. - 7
1II(V
RHEOLOGY
c.. c,,,
100 100 100 100
].ow I,\!.
8
7
Figure 5. Variation of Log of Apparent Viscosity with Log of Shear Rate for Greases Containing a Methylphenylpolysiloxane of High Aromaticity
Lithium
(
b
t
OF SILOXANE GREASES
I n 1!421 Rurhingham ( 4 )published it paper in which he clerivctf :I t t i c ~ o i ~ ~ t i cequation nl for flow through a capillary tube. He ~ h o u e dthat plastic flow in a capillary tube rannot be linear and furthe1 indicated that plug flow cannot be rompletely eliminated until shear is infinite. Since greases fall in the rheological vlassificdation of' plastir flow, data obtained by capillary mrthods are oprn to some question. In spitr of this fart the pressure visconicater h:ib heen a useful tool in the study of flow properties of gr-e:ise% ippiirc'nt viwosities ( 1 , 9, 7 ) obtained uith t h i s instru-
Table Vll.
Variation of Viscosity with Temperature of Methylpolysiloxane Fluids
I emrieratiire,
I .
c.
Vinro-ity", Poises _-____ Fluid A
Fhid B
:1 10
R2 43 50 54.5
148 132
907
111 83
74 75
100
146
98,.-,
70
Determined in an Ubbelohde tribe for tlriid A ; drtcriiiined at 1000 sec.-1 in yressure viscometer for fliiid H. a
I 10
L.c
500
10
RATE
CT
i
9
5030
I 0 0
S
