I
FERDINAND RODRIGUEZ and CHARLES C. WINDING
Riser t o
.
School of Chemical and Metallurgical Engineering, Cornell University, Ithaca, N. Y.
Mechanical Degradation of Polyisobutylene Solutions Degradation of polymer measured in parameters may point the way to its control
WHEN
work is put into a polymer solution, there is a possibility that some of the energy will cause chain scission with a consequent degradation of polymer molecular weight. This mechanical shear degradation can occur whenever a polymer solution must be agitated to prepare such solutions or whenever any other stirring or pumping operation is required. The work on this problem can be divided into three modes of operation. 1. Ultrasonic irradiation. The major factor appears to be cavitation of the solvent although the microscopic interpretation of the process is obscure (5). 2. High speed flow through capillaries (7) and orifices (6). 3. High speed stirring or shaking. Turkel (70) used a disk spinning near a stationary wall so that the rate of shear between the two could be calculated. T h e free-radical character of all these methods is shown by the fact that vinyl polymerization can be initiated by the mechanical degradation of a small amount of high polymer in the same solution as the monomer (7). Johnson and Price (8) found that nearly every chain scission is followed by incorporation of iodine when radioiodine is present during stirring of a polymer solution. Several points are common to all solution investigations. In every case, there is a n initial precipitous drop in molecular weight followed by a gradual leveling off to a value which appears constant or changes very slowly with time. The purpose of this investigation is to relate the degradation of polymer to measurable parameters in order to elucidate the theory of solution degradation, and to point the way to its control. Polymer molecules are distorted into ellipsoids of long major axis as they enter an area of high linear velocity. O n leaving the area of orderly flow, they become entangled with one another. O n entering an area of high turbulence, molecules, or parts of the same molecule, are caught in eddies moving in different directions. T h e kinetic energy is concentrated in polymer bonds which then rupture. The area of orderly flow would
Figure 1. To convert the rotating dlsk cell (shown in exploded view) to one with a high-rate-of-shear zone the stationary plate was added and disk A used. Disks B, C, D, and E were used in other runs
be identified with the radial flow from axis to tip of disk. The turbulent region would probably be above the surface of the disk near the tip. Degradation can be prevented by: avoiding high speeds in stirring or transporting solutions; using the highest temperatures possible commensurate with the prevention of thermal degradation; and using good solvents (those with high [ V I ) of low viscosity.
High Bate-of-Shear Zone Apparatus. From Turkel’s work (70) it was thought that the rate of shear was the controlling variable and that it was necessary to have a zone where the rate of shear would reach 50,000 to 100,000 set.-'. Bestul (7) based his conclusions also on the asTable
Apparatus
Rotating Disk. T o study the effect of high speed rotation of a disk in a polymer solution (Figure l), a cell holding about 125 ml. of solution was used. T h e disk was driven by a Dumore Series 5 Tool Post Grinder (Catalog No. 5-021, the Dumore Co., Racine, Wis.) at 700 to 11,000 r.p.m. Disks of various shapes were used. Centrifugal force of the liquid in the cell circulated the solution continuously through a reservoir of 5 to 10 ml. of liquid to compensate for changes in volume when samples were taken. T h e entire cell was immersed in a stirred bath maintained at a set temperature f 0 . 5 ’ C.
Fraction No. 1 3 10
1.
Polyisobutylene Fractions Studies
% ’ of “hole Polymer 5.0 14.0 16.3 3.9
Intrinsir Viscosity in Cyclohexane,
DI./G. at 30’ C. 9.52 8.74 5.59 2.73 5.89
M, X
10-0
4.1 3.6 1.9 0.68 2.0
sumption that the average rate of shear in a capillary tube was a controlling parameter using values up to 100,000 sec. - l . The apparatus for the present research was originally designed to incorporate a zone of intense shear by using disk A in conjunction with a stationary face (Figure 1). Solution is carried from the area X by centrifugal force through the shear
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2.25 inches inside to 2.73 inches outsidc,
an average value of 2.50 inches is used. Thus, with a gap width of 0.0035 inch and w of 2750 r.p.m., the rate of shear in the zone is:
H
=
(2750 X 2.50 X T J ; (60 X 0.0035) = 103,000 SCC.
Experimental Procedures
- t 0.61
05
I
Polystyrene s o l u t i o n , ultrasonic irradiation
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1
2
.
5 B
50
20
10
A double precipitation system ( 2 ) \$.as used to fractionate a polyisobutylenc (PIB), Vistanex L,120, of in average molecular weight of 2.0 X 106 (in diisobutylenr). T\senty fractions ranging in molecular \\-eight from 0.18 to 4.1 X 106 \\-?re recovered (Table I). Intrinsic viscosities were used to estitnate average molecular weights. hZeasurements ivrrc made in Ubbiehode viscometers arranged t o permit dilution in the reservoir. Solvents. Solvents used in this study are listed in Table 11. The mixture of mineral oil and kerosine was chosen for much of the ivork because degradation is marked and easy to follow; volatility is lolv, simplifying handling; viscosity is high enough to s h v down leakage from
, u 100
i
500
200
Time, minutes ( t r c e p t A )
Figure 2. Similar curves are obtained b y degrading with I, ultrasonic irradiation, 11, high speed capillary flow, and 111, high speed stirring. IV is a plot of capillary flow data for a 10 grams/dl. solution a t 40" C. due to Bestul ( I ) . V shows the results of high speed stirring on a fractionated PI9 (Fraction No. 1 ) at a concentration of 0.1 gram/dl. Table 11.
Degradation Characteristics in Various Solvents c = 0.25 g . / d l , 5 5 0 0 r.p.m., 24-31 C. Methylcyclohexane
Cyclo-
hexane Matheson P.2825 1.08
Grade
95 mole y1 purity 0.824
Viscosity, centistokes at 30' C. Intrinsic viscosity for M = 2.0 X 106,30°C.,dl./g. 5.89 121 Vapor pressure, mm. Hg at 30' C. 11 for Eq. [lo] 0.105 2.1 MI, M at0 = 1, x MSCx 10-6 1.14 Rate of degradation for: -If = 1.2 X 10e,minutes i c - 1 X 10' 0.52 .1( = 1.0 x 106 0.090 -if = 0.8 x 106 0.011
zone behi*een disk and the stationary plate (stator). In area Y i t mises and eventually returns to X via the slots in the bottom of the stationary plate. \Vith a shear zone of diameter D inches, a disk rotating omega revolutions per minute.
5.34 58 0.10 21 1.20
Address
1282
7.04
4.41
3.95
23 7.8 2.1
3.92 58 0.18 2.3 0.74
*..
0.14 1.2 0.51 140 38 7.7
5.0 1.8 0.52
(rrDw)>(6OGi
(1 1
Toluene
Heneetic
Nitration
Nitration
0.606
0.648
3.53 12 0.17 1.8 0.65
3.46 37 0.17 1.8 0.65
2.03 118 0.16 1.3 0.485
15
15 5.3 1.4
98 32 7.5
5.3 1.4
apparatus and to act as a shaft seal lubricant; and because solution viscosities can be measured in large bore viscometers not easily fouled by stray particles.
As the shear zone varies in diamrter from Results
For complete manuscript: Rodriguez
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and with the gap betlveen the disk and stator set at G inches, the rat2 of shear, R, is:
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Turkel (10):Price (81,and Bestul ( I ) have assumed that, undcr conditions oC constant rate of shear, there exists a critical molecular weight polymer which \\-ill not be degraded. Any highcr molecular weight pol>-mer is degraded until i t reaches the equilibrium value corresponding to the particular rate of shcar. 111 some runs, data were accumulated bryond the point of equilibrium chosen by previous workers. I n no case did a clear-cut equilibrium value emerge. O n e explanation is that there is no equilibrium or "critical" molecular wright which is resistant to degradation
POLYISOBUTYLENE DPGRADATION 5 .o
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/ /
=I
700 r.pm. ,1315
os
/
/
- hpc
/
0.3L
Figure 3. The rate of degradation, -1(1 /MI . (dm/ de)], obtained from data plotted in Figure 2 is a linear function of M. Solid lines are the experimental range for degradation of fractionated PIB at a concentration of 0.1 gram/dl. in mineral oil-kerosine
/ I
I
I ~4 10-3 10'2 IO" of Dogrodation (or Probability d Osqradatton),
I 0-5
Rote
fracfionol loss in M per minute
a t a given speed. There is only a decreased probability of degradative attack with decreasing molecular weight. If we assume that there is no equilibrium molecular weight and if we assume that the rate at which a polymer degrades depends only on its molecular weight raised to some power (m 1) we can write
+
dM/dB = k'Mt"+l
(2)
where dM/d6 is the rate of change of molecular weight, M, with time$.
"r
I P I 0 Inmineral oil-kerosine
8 10 20 R o l d i o n o l Speed.
+
where n = l/m and M1 is the value of M a t 6 = 1. According to Equation 3, logarithmic plots of M against 0 should be linear with slope -n. This type of plot has been found to correlate a wide range of data. Both fractionated and unfractionated polymen yield linear plots when subjected to various mechanical degradative influences. Figure 2 illustrates the method applied to disk, capillary, and ultrasonic degradation data. This present method permits good interpolation of data and reasonable extrapolation. All data have been correlated in this manner. Thus, for each run, there are values of n and M1 for Equation 3. Another analysis of the data is to calculate the rate at which polymer is degraded. From the integration, k' is a h m p e d constant which equals --n/(Ml)l/". Therefore ( l / M ) ( d M / d O )- n(M/Mi)"*
0 I */dl
t 6
If M > 5 and M at time 9 is less than about three fourths of the original M , integration and simplification leads to the expression log M = -n log 0 log M I (3)
40
60 8 0 100
c p m a 10.'
Figure 4. The pseudocritical molecular weight (A&) appears to be proportional to the rotational speed raised to the -0.6 power for PIB in both solvents. Some of Turkel's data (TO), obtained by a less direct method, are shown with rotational speed adIf justed to equal tip velocity (111). allowance is made for the difFerence in monomer molecular weight (polystyrene to PIB = 104 to 56), the data are shifted downward (IV)
(4)
Accordingly, the rate of degradation of a polymer [ (1/M) (dM/de)] expressed as the fractional loss in molecular weight per unit time is a function only of the molecular weight and independent of the time in the apparatus of initial molecular weight For comparing results of this work with those from previocls work, the molecular weight a t which the rate of degradation becomes equal to 0.00033 min.-l was chosen as a "pseudocritical" molecular weight, M,, which is reasonably resistant to degradation under the conditions of the test. This degradation
rate corresponds to a change in molecular weight of 2% per hour.
Effect of Experimental Parameters
Rotational Speed. Four fractions of polyisobutylene (PIB) were degraded a t various speeds. The degradation rate is plotted us. M in Figure 3. The range of M for which rate of degradation varies tenfold is relatively narrow. Hence, the arbitrary equilibrium values of M chosen by various workers should be comparable. The pseudocritical M , M,,, chosen at rate of degradation equal to 0.00033, is plotted against the rotational speed of the disk in Figure 4. The slope of -0.6 used to correlate the data is amazingly close to the value of -0.5 predicted by Frenkel ( 4 ) . Data obtained by Turkel (70) in a less direct fashion are also plotted. The rate of degradation might be considered to be the probability that a polymer of molecular weight M will be attacked. Then Figure 3 becomes a plot of probability of degradation us. M . From a knowledge of the rate of degradation us. molecular weight for fractionated polymer plus the molecular weight distribution of the whole polymer, one can predict the rate for M hole polymer with reasonable accuracy. Solvent Character. Each of the following mechanisms by which degradation can be thought to occur has a logical basis. They are not mutually exclusive. 1. Degradation occurs by solventpolymer interactions. Accordingly, varying the solvent viscosity should give the same result as varying the rate of shear. Concentration should play a minor role. 2. Degradation occurs by the interaction of many polymer molecules with each other. One would expect degradation to decrease with lowered concentration. Solvent should be less important except as it affects polymer geometry. That is, whether or not a solvent i s a poor or a good one should outweigh the viscosity effect. 3. The degradative energy is not transmitted by the solvent dragging beads of polymer, in a shear field, but by the collapse of cavities with accompanying high temperatures and pressures. In ultrasonic irradiation, cavitation has been shown to be very important. If it is the causative agent in disk and capillary degradation, one might expect the vapor pressure of the solvent to assume importance as high vapor pressure would give a low energy of collapse. Disk geometry should affect the amount of cavitation, also. The results of runs in eight different solvents are summarized in Table I1 VOL. 51, NO. 10
OCTOBER 1959
1283
\vlicre cadi solvent is characterized by its viscosity, vapor pressure, and intrinsic viscosity for polyisobutylenc of .If = 2.0 X 106 a t 30' C. Derived cluantitirs from each rim aria shoum, also. No pattern seems to emergc from the vapor pressure data. Cyclohrsaiic and methylcyclohexane and toluene and cthylbenzene are two pairs in \vhich vapor pressure is almost the only variable between the mates. ?'here is little variation in behavior ivithin each pnir. .ifpcvaries \vith [ q J to the first po\ver according to Figure 5. Plotting .\IDc against solvent viscosity \vith the data grouped according to intrinsic viscosity yields a low poww dependency of .ifpc on solvent viscosity. Because solvent viscosity has such .\CS, 19.58.
Rodriguez. F., Pt1.D. thesis. (:ornc.ll Univrrsity, Ithaca. N. Y..1958. I10) 'I-urkel, h. M.. Ihid.. 1955. ([I!
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