S I L ICON ES
Water Repellancy
4
---< High Temperature Hydraulic Systems
N O R M A N KIRK, General Electric Research Laboratory, Schenectady, N. Y.
Process for Making Silicones
A
N e w catalyst makes possible the switch from batch to continuous operation in making silicone oils and gums. This is of potential interest in other polymerization reactions where the catalyst is a problem in the finished product
Low
molecular weight silicones obtained b, the hydrolysis of methylchlorosilanes such as dimethyldichlorosilane, (CH3)2SiCIz, can be polymerized to form useful oils and gums. A continuous silicone polymerization process was suggested by the discovery of a transient catalyst (7), tetrabutylphosphonium hydroxide (TBPH), which will bring about rapid rearrangement and polymerization of siloxanes at temperatures u p to about 110' C. After the desired polymerization has been obtained, the phosphonium base can be readily and completely decomposed to render it inactive, by heating. the polymer above 130" C. The decomposition products are n-butane and tributylphosphonium oxide. (n-CdHg)JPOH
130-1 70 ' C.
n-CdHlo
+
(n-CdHg)3PO
These catalyst decomposition products have no catalytic effect on the polymer. Hence, no further polymerization or depolymerization traceable to the original catalvst can occur. Because the cat-
alyst is initially present in concentrations of only a few hundredths weight per cent, its decomposition products need not be removed from the polymer. The polymers made by this process are clear, colorless, and stable. Need for
Q
Continuous Process
Siloxanes are customarily polymerized in agitated batch kettles using a strong base, such as potassium hydroxide, as catalyst. The reaction requires several hours. I n making high molecular weight gums the difficulty of obtaining satisfactory mixing can lead to a nonhomogeneous product. I t is difficult to remove (or otherwise deactivate) the catalyst that is retained in the polymer. The presence of appreciable amounts of catalyst may, under some conditions, reduce the stability of the polymer. Many of these processing difficulties can be avoided if the polymerization is done continuously by pumping the feed through a heat exchanger (polymerizer) having a high length-to-diam-
eter ratio. The rapid polymerization rates attainable with the new TBPH catalyst make a continuous process feasible. Very good control of reaction time and temperature can be achieved by such a continuous process. A scrapedsurface heat exchanger can be used where highly viscous oils and gums are to be made. It is possible to monitor the consistency of the finished product as it leaves the apparatus, and, if the expected polymer viscosity has not been obtained, the relative amounts of feed constituents (base, chain stopper, and catalyst) can be adjusted to give the desired product. As the system holdup is small, these quick adjustments involve little wastage of silicone. Reaction Materials
All the polymerizations described here were done using octamethylcyclotetrasiloxane (tetramer) as the chain-building material. Small amounts of decamethyltetrasiloxane (MDzM) (2) were incorporated in the feed to act as chain VOL. 51, NO. 4
APRIL 1959
515
?S:!P :YIS GI 7 0 *C
+
9 STABILIZER
I dI
nor
WLTER OUT OIL OUT
STEAM 6i lP0S. ICG
TOORAbN
m VICUUY PUMP
POLYMERIZATION REACTOR
PURGE
Ni?
VOLATILE PRODUC
STORAGE
HOT OIL IN 205.C
TO DRblN GLASS
TO VLCUUY PUYP
FEED M ~ T E A I N G PUMP
POLYMER PRODUCT RECE I V E R
A coil-type heat exchanger was used in the continuous polymerization process for silicone oils stopper. The amount of chain stopper determines the ultimate size of the polymer molecule. The catalyst was in the form of a stable silanolate solution obtained by equilibrating the phosphonium base with tetramer to give a clear oil containing about 8 weight % tetrabutylphosphonium hydroxide. This oil had a viscosity of about 370 cps. at 25' C. In effect, this is a low molecular weight silicone polymer chain-stopped n i t h the base.
Apparatus and Procedure The experimental work was done with two different prepilot-sized units. The first used a coil-type heat exchanger for polymerization and catalyst deactivation. This unit was used to make fluids having viscosities u p to about 100,000 cps. The second experimental unit employed a scraped surface heat exchanger and a vacuum double-drum dryer for polymerization and deactivation, respectively. This assembly was used for the full range of viscosity to about 30,000,000 cps. The oils were polymerized in a heat
exchanger made of 44 feet of copper tubing having an outside diameter of 0.5 inch and an inside diaineter of 0.436 inch, coiled inside a 3-foot vertical steam jacket of 6-inch pipe. The holdup of this coil \vas about 0.3 gallon. The feed was pumped into the bottom of this coil at a fixed rate by a positive displacement plump. The temperature in the polymerizer steam jacket \.vas kept at 1 10' to 1 1 5 O C. The polymer leaving the top of the reactor coil entered the bottom of a second steam-jacketed heat exchanger and was heated to about 170' C. to deactivate the catalyst, thus stabilizing the product. This stabilizer \vas similar to the reactor: except that it contained only 22 feet of copper tubing. The holdup was therefore about 0.15 gallon, and the stabilizing period was only about half the polymerizing time. The oil leaving the top of the stabilizer was devolatilized in a \vetted-\sall vacuum column 6 feet high and made of 2-inch stainless steel pipe jacketed with circulating heat transfer oil a t 280" C . The polymer, \L,hich normally contained about 12 meight 7 0 volatiles, was fed into a disengaging section 5 inches in
DRY NITROGEN
VACUUM MiUM DRYER
(6'DIA I B'DRUMS OILHEATED TO 28O"C.l -TO CONDENSER
STRlfflNG
Silicone gums were made in a scraped-surface heat exchanger
51 6
INDUSTRIAL AND ENGINEERING CHEMISTRY
diameter and 10 inches long at the top of the column. The feed to the column was directed against the wall of this disengaging section. N o attempt was made a t uniform distribution of the liquid flowing down the column wall. The column was normally evacuated to 20 to 30 mm. Open steam was metered into the bottom of the column to sweep out the volatile silicones as they were removed from the polymer. These volatiles were taken overhead and condensed and recovered. The stripped polymer was withdrawn from the bottom of the column. A4 3-inch section of borosilicate glass pipe a t the bottom of the column permitted visual observation of the product level during this withdrawal. so that a liquid seal could be maintained at all times. I n the second experimental apparatus the continuous polymerizer was a \-etator scraped-surface heat exchanger (Girdler Corp., Louisville~Ky.). The gum was conducted from the exchanger to a vacuum double-drum dryer for stabilizing and devolatilizing. Use of this '1-pe of equipment makes it possible to cover an extremely wide polymer viscosity range in one apparatus. LYith the fred system shoivn, tetramer, catalyst, and chain stopper could be metered into the reactor independently. .Any desired product viscosity could be obtained by adjusting the rate of stopper addition. The catalyst and stopper were each pumped by a piston pump capable of metering reproducibly as little as 5 or as much as 225 cc. per hour. The tetramer was polymerized by pumping it through the laboratory-sized scraped-surface heat exchanger. The inner heat transfer surface of the reactor tube tvas scraped clean by longitudinal scrapers attached to a solid. stainless steel shaft which rotated \vithin the tube at 100 to 200 r.p.m. These fixed scraper blades did not come in contact with the tube wall; a blade clearance of a few mils was used to avoid metallic contact. The variable-speed shaft drive had a 3-hp. motor, and a wattmeter was used to indicate the load on this motor. Two shafts were used in this bvork; one was 2l; 4 inches in diameter and the other was 11/4 inches, giving, respectively, 0.17- and 0.32-gallon holdup in the reactor. The gum leaving the reactor was stabilized and devolatilized in the laboratory-sized vacuum double-drum dryer. The dryer rolls were heated to 280" C. by circulating hot oil through them. Open steam was used to purge the volatiles from the vacuum chamber. L'olatiles were condensed by a water-cooled surface condenser followed by a dry ice trap. -4pressure of 20 mm. of mercury or less was maintained in the dryer. The gum from the reactor \vas fed continuously to the trough formed by the horizontal dryer rolls. The roll
CONTINUOUS SILICONE PRODUCTION 7 -- T------
?. 400,000 !4 -
g 200,000
t 0 2 n
+ 4 ~0,000~ 10
_ 20
_ 30
-- -
50
k
-
L
ME,
500
200
I00
REACTION TI
d
1000
10,000
0.001 0.002
W
Solid points are equilibrium values
OD05
0.010
WT X (n-C,HO)4POH
0
MINUTES
Polymer of very high viscosity was formed first
Figure 1.
d>
0.020
OD40
0.100
IN FEED
Figure 2. High catalyst concentration greatly increases rate of polymerization Solid points are equilibrium values
speed and nip \\-ere adjusted to maintain a uniform layer or film on the rolls which \vould match the polymerizer discharge rate. T h e doctor blades removed the gum after revolution, and the product fell into stainless steel pans. Sormal roll speed was benveen 0.25 and 1.0 r.p.m., corresponding to film contact times of 120 to 30 seconds. respectively. The nip on the rolls \vas set betxveen 10 and 30 mils for gums. I
Experimental Results with Coil-Type Polymerizer The copper coil apparatus was used to study the effect of feed composition, residence time: and temperature on the polymerization of silicone fluids. I n Figure 1 product viscosity is plotted as a function of residence time \vith the reactor a t 115' C. and for a feed containing 0.01 kveight 76 catalyst (TBPH solids). T h e data indicate that very high viscosity polymer first formed, and that this material. Lvhich may consist of very long chain molecules xvhich are formed initially, approaches an equilibrium state as the chain stopper is incorporated into the molecule. T h e viscosity is only a proportional measure of the degree of polymerization, but it is one of the basic properties used in silicone product specification and its ease of measurement enhances its usefulness. Figure 2 shows the effect of catalyst concentration on the viscosity of the oil for different conditions of residence time. T h e chain stopper concentration was fixed a t 0.60 weight for this group of runs. Although the viscosity a t equilibrium was primarily controlled by the amount of chain stopper, the dotted line shows that equilibrium viscosity was further lowered as the catalyst concentration was increased. This shows that
the catalyst itself functioned to some extent as a chain stopper. T h e broken line (marked Coil) in Figure 3 shows the effect of chain stopper on the polymer viscosity for operation a t 110' to 115' C. with 0.02 \$eight catalyst and a residence time of 27 minutes in this reactor. The data show that it should be possible to make oils u p to 100.000 cps. on a production basis with a viscosity within 10% of any desired value by this process. At the higher viscosities the residence time becomes increasingly uncertain. and minor variations in feed composition, etc., nould cause greater deviations in product properties.
lox 106
a.
I
004
010
040
IO
Tetramer Pentamer Hexamer Higher boiling siloxanes Total
w t . yc 51.5 31.0 8.5
9.0 ____ 100.0
Polymerization in ScrapedSurface Heat Exchanger
I 001
Satisfactory devolatilization in thr \vetted-wall column was obtained for polymers having viscosities between 3000 and 150~000cps. Below 3000 cps. the residence time was too short to permit adequate stripping at 280' C . Above 150,000 cps. a sharp flood point occurred because of bridging in the column, so that little devolatilization was obtained. Within the indicated viscosity range. product having a fairly constant volatile content of 1Yc or lotj-er \vas obtained. T h e volatile silicone stripped from the product from a typical run had the following analysis:
40
I(
WEIGHT PER CENT MD2M IN FEED
Figure 3. Polymer viscosity i s regulated b y chain stopper
Po1)merizations were carried out in the scraped-surface exchanqer to make a series of dimethylsilicone gums covering a wide range in molecular weight. These here made with 0.02 weight yc TBPH and MD2M concentrations of from 0 to 0.15 weight 96. The tetramer feed rates ranged from 3.0 to 9.0 pounds per hour. Reaction time could also be varied independently of the feed rate by a factor of 2 by employing the t\vo different diameter shafts in the polymerizer. T h e capacity of the unit depmded primarily on the time-temperature relationship and was therefore predictable. Feed which had not been preheated bypassed through the exchanger if the residence time was less than 10 to 15 minutes, but not at longer residence times. With the larger shaft ( 0 17gallon holdup) the capacity of about 3 VOL. 5 1 , NO. 4
APRIL 1 9 5 9
517
10,000---/I
5000
7 7
-
lox106
I
'VOTATOR HOLDUP ( 3 LE1
6X106
'
I
I
.
3x10'-
2
0
s z
i"0
Figure 5. Adjustments in operation give flexibility of product control
2000-
a
2 iooob-
1
-100
-60
-30
N
i
&
I
,
0
200 t-
a
i'
100 001
I
/
002 WT %
loo,ooo 60,000
I
-
I id
a f
b IO
1
L
5 a
a z
3
30,000 1
005
MD2M
IN
0 10 FEE0
I
0.20 10,000
Figure 4. A smooth relationship exists between amount of chain stopper and gum penetration
to 5 pounds per hour could be increased to a t least 10 pounds per hour by preheating the feed. Feed rates of about 10 pounds per hour were possible with no preheating when the smaller shaft was used (0.32-gallon holdup). Figure 4 shows the effect of chain stopper in the feed on gum penetration. These penetrations are an indication of the viscosity of the gum. Reproducible operation can be obtained by this process and a smooth relationship exists. The penetrations were measured with a precision penetrometer with a special foot 0.25 inch in diameter by 0.187 inch long on a 0.125-inch diameter shaft. An external load of 100 grams was used. The polymers made by this process contained the 12 to 13% volatiles normally expected for gums made under essentially equilibrium conditions. T h e vacuum drum dryer consistently reduced these gum volatiles to less than 0.5 weight yo. At 250" to 275" C. satisfactory dryer operation was obtained with roll clearances of 10 to 30 mils and film contact times of 15 to 120 seconds. The optimum combination appeared to be in the region of 20 mils and 30 seconds. Dryer feed rates of 10 pounds per hour were possible under these conditions, so that the dryer capacity could be made to match that of the polymerizer. This apparatus has also been used to make oils having viscosities as low as 350 cps. The only change in the method of operation from that used for gums consisted of completely closing the gap between the drum dryer rolls and reversing the direction of rotation. The oil was fed into the trough formed by the rolls as before, but it was carried u p over the top of the rolls and down around the outside, where it was doctored off as it approached the bottom. T h e results of these oil and gum polymerizations are plotted in
518
2 J
0
2
p
I
1
0
5
1 10
I
15
TOTAL
Figure 3, along with the data from the copper coil polymerization apparatus. The useful ranges of the two units overlap smoothly and the viscosity-concentration relationship exists a t least from 350 to 10 X 106cps. The progress of a run employing independent metering of tetramer catalyst and chain stopper can be followed on Figure 5. Initially, sufficient MDzM was pumped to give a n oil having a viscosity of about 25,000 cps. The MDzM rate was then decreased abruptly while the catalyst and tetramer rates were held constant. After a three- or fourfold volume turnover, gum having a viscosity of about 8 X 106 cps. was obtained. This shows how adjustments in the operation can be made to give any desired polymer viscosity. Thermal Stability of Catalyst Some experiments were done to determine the stability of the catalyst equilibrate as a function of temperature. Samples were heated in an oven for different lengths of time a t 90" and 110' C. A nitrogen purge was used to protect these samples from atmospheric carbon dioxide during the tests. The heated samples were then titrated with 0.1N hydrochloric acid to determine their basicity a t the end of each time interval. These measurements showed the catalyst decomposition to be a firstorder reaction. The heat of activation was calculated to be 25.3 kcal. per mole and the specific reaction rate constants a t 90' and 110' C. are 0.17 and 1.06 reciprocal hours, respectively. These constants, as well as those calculated a t several other temperatures, are listed in the table. The catalyst half life a t each of these temperatures was then calculated. From
INDUSTRIAL AND ENGINEERING CHEMISTRY
20
TETRAMER
1
25 FEED,
30
35
. I ' 40
LBS
Decomposition of Tetrabutylphosphonium Hydroxide Catalyst Is a FirstOrder Reaction Temp., O
C.
130 110 100
Specific Reaction Rate Constant, Hr.-1 4.95
90
1.06 0.41 0.17
50
2.2
30 20
x 10-3 1.6 x 10-4 4 . 0 X 10-6
Half Life,
Hours 0.14 0.65 1.7 4.1 312
4,250 17,300
the table it can be seen that a t 130" C. the half life is 0.14 hour (8 minutes) while a t 30" C. it is 4250 hours (5.8 months). Catalyst stored a t 20' C. would have a half life of about 2 years. .4 further check on these measurements was obtained by titrating a sample from a separate batch of catalyst which had been standing at room temperature for one month. The experimental loss in basicity was 1070, while the loss calculated from the rate constants was 10.770. Acknowledgment The author gratefully acknowledges the help and encouragement received from A. R . Gilbert and S. W. Kantor. Literature Cited (1) Gilbert, A. R., Kantor, S.W., Division of Polymer Chemistry, 134th Meeting, ACS, Chicago, Ill., September 1958. 12) E. G.. "Chemistrv of the \ , Rochow. Silicones,'; 2nd ed., p. 79, Wiiey, New York, 1951. RECEIVED for review September 8, 1958 ACCEPTED December 8, 1958 Division of Polymer Chemistry, 134th Meeting, ACS, Chicago, Ill., September 1958.
