I
A. W. HANSON and R. L. ZIMMERMAN The Dow Chemical Co., Midland, Mich.
Continuous Recycle Copolymerization Copolymers of known, predictable, and homogeneous composition, can be prepared by a simple technique with a relatively high percentage of solids D E V E L O P M E N T of the theory of copolymerization (7, 72) has emphasized the need for a simple technique whereby two or more monomers can be polymerized to a relatively high perdentage of solids and yet maintain homogeneity of copolymer species. Practical application of copolymerization has been limited to composition ranges having a low degree of difference between composition of monomer mixture and polymer formed from it (called disproportionation in this discussion). If three or more monomers are involved, the possibilities of disproportionation are increased because azeotropic copolymers (copolymers with the same composition as the monomer mixture from which they are formed) are severely limited (75), and the problem of determining the copolymer composition becomes more complex. Several methods (3, 4, 70) have been offered for solving the problem of composition drift in copolymerization-for example, adding the more reactive monomer continuouslY during the Polymerization. However, these methods apply generally to e m d sion Or susPension Polymerizations and require and skill to make them reproducible. The coPolymerization system d e ~ ~ i b e d (8)comprises first a Polymerization reactor to which is fed a monomeric mixture of the desired CoPolymer ComPosition containing little Or no inert diluents* The reactor must be Of mixing the incoming mono* mer and the Polymerizing mixture* A partially Polymerized is removed from the reactor and forwarded to a devolatilizer, where the copolymer is extruded and the un- . reacted monomers are volatilized. After condensation, the monomers are recycled directly to the polymerizer. This system produces copolymers of predictable and homogeneous composition.
pressure or for polymerizations that tend to form partially insoluble gels or become very viscous, a head-to-tail arrangement of two extruders feeding each other (8) would provide a better polymerizer. Conversely, for lower temperatures and lower pressures a stirred kettle-type reactor can be used. For devolatilization the Plastruder (7, 9), a roll-type extruding devolatilizer, was used. The volatiles were removed a t reduced pressures and the condenser was refrigerated to provide good condensation. One pump was used for the recycled monomers and a second for the new feed or “make-up” monomer. This new feed may, of course, be regarded as merely a replacement of the monomers which were extruded from the system as the copolymer product. The weight ratio, 7 , of recycle to new feed was fixed by the per cent solids-that is, r = -1 - f
The viscosity controller then called for new feed to maintain the per cent solids at the desired control point and automatically make up for polymer removed. The apparatus as outlined can be operated as a “single-pass” polymerization system by not returning the devolatilized monomers to the polymerizer. This, however, gives a considerably different result in terms of polymer composition: The Composition depends upon the per cent solids (conversion) and is predictable only by calculation from reactivity ratios or Q and e values ( 7 ) . By recycle copolymerization the polymer composition is predictable, being the same as the new feed composition or a t least closely related to it-any discrepancy is known by keeping a material balance. Equilibrium The copolymerization of styrene and acrylonitrile a t 150° C. will be used to illustrate the recycle copolymerization technique. For these tests the equipment was started up by polymerizing and recycling styrene alone. The feed was then changed to 5, I O , 15, or 20 weight % acrylonitrile-styrene mixtures and for each composition the system was allowed to come to equilibrium or steady-state operation. A plot of the refractive index of the recycle monomer as in Figure 2 shows that equilibrium was achieved after a change-over time of about 16 hours in each case. In general, the half life of nonsteady-state conditions is directly related to the total inventory of the system and the degree of disproportionation of the monomers and is inversely related to the average rate of
I’
where f equals the weight fraction of solids in the partially polymerized solution going into the devolatilizer. A fluid viscosity recorder-controller, installed in the polymerization reactor, controlled the make-up feed pump. The recycle pump was controlled by a level controller in the recycle line. Therefore, the recycle was returned to the reactor a t the rate a t which it was separated from the product, For startup, the reactor and recycle system were filled with styrene. As the styrene polymerized and was removed from the system by the devolatilizer, the quantity of recycle being pumped was insufficient to keep the per cent solids from rising.
NewFeed
~
!
I
Equipment and Control The reactor design is illustrated in Figure 1. A gear pump circulates the polymerizing mixture a t a rate of about 800 passes per hour through a coil heat exchanger and mixes the combined recyde and new feed monomers with the viscous polymer solution. This reactor is particularly desirable for polymerizing a t high temperatures or high pressures, although for very high temperature and
Copolymers of known composition are prepared by continuous recycle copolymerization VOL. 49, NO. 11
Figure 1. polymerizer NOVEMBER 1957
Coil
1803.
Figure 3. Refrac . .. _. . ., . . acrylonitrile mixtures
Figure 2. Relation of operating time to refractive index of recycle monomer h o d 0 5% rn 10% 0 15% A 20%
0 Pure somplsr Mas rpsmal analylir of recycle
'
ACWhikilE
in Feed,Wt. %
Table
L
Time, Hr. 0 4 7
10
Recycle Monomer Analysis B t m , Aarylonitrile. wt. % wt. %
In&, wt. ,%
..
4.6 4.4 4.4 4.5 4.5 4.5 4.5
11 19 27 35
95.4 94.3 93.7 93.2 92.1 91.9 92.1 96.5 92.6 91.1 90.5 89.6
1.3 1.9 2.3 3.1 3.6 3.4 2.3 5.8 6.5 6.8 7.1
20
15 23 39
86.5 85.0 86.0
12.0 13.0 12.0
1.5 2.0 2.0
30
24 29
71.0 70.0
28.0 28.0
3.0
11 15 22 34 3
1.4 1.8 2.4 2.7 3.3
2.0
~~
Table II. Aaryloniile, wt. % 5.5 9.8 14.0 21.0 27.0
~
Physical Properties of Styrene-Acrylonitrile Copolymers Imoaot Tensila strdnsth, Heat a m . Elongstion. Ft.-Lb/Inch Distortion, Lb./Bq. Inoh % Notah 0 c. 8,130 7.920 Si320 9,260 10.510
1.6 2.1 2.2 2.5 3.2
72 82 84
0.49 0.48
0.G 0.50 0.50
88
88
Table Ill. Steady-State Continuous Copolymerization at 150°
C.
Mmomer-Pohner
... ...
47 32.4 8.3 6.9 54 11.1 10 55 36.2 12.1 10.7 15 81 33.4 17.1 13.0 20 76 31.5 19.5 18.5 30 133' 25.7 % of pobmf&m oapscitu per hour. 10% solution in methyl ethyl ketone. .BY Kjaddhl nisndysis. 8 d e r PolYmeriaer wed. 0 5
... ...
0.8 4.0 1.6 1.8 2.0 2.0
'
1804
INDUSTRIAL AND ENQINEEUNO CHEMImRY
0
44
5.5 9.8 14.2 21.0 27.0
49 52 59 67
I2@
93 92 94 % 88
90
2.3
2.5 2.3 2.5 11.0 3.9
..jure 1. ..dation of operating time to build-up of acrylonitrile in copolymer product Feed 0 5 % 10% 015% AZO%
polymerization. The inventory of recycle monomer was not actually measured, but, because the four experiments of Figure 2 required about the same time to reach equilibrium, it appears that the total inventory was the predominant variable in determining change-over time. Not included in the figure is a Mth experiment in which the equipment was started directly with a feed of 30 weight yo acrylonitrile. In this case the refractive index of the recycle leveled out at 1.486. Table I lists the results of l~lkssspectral analysis of recycle samples and Figure 3 gives the relationship of refractive index to monomer composition. The recycle monomer mixture contained small amounts (from 1 to 5%) of inert diluents which also tended to establish an equilibrium level. Figure 4 gives the changes in acrylonitrile content of the product with time, again showing that a steady state was achieved. Physical property data on polymers made a t equilibrium are indnded in Table 11; Table I11 summarizes rate, per Cent solids, solution viscosity, and other data. In work reported in Table 111 the rate of copolymer removal is substantially the same as the new feed rate. Material Balance A close material balance is highly desirabl; in recycle copolymerization. .&nomer lost as volatile in the product, out of the condenser, or from leaking pumps should be kept a t a minimum. In the examples of Table I11 the losses were on the order of 2 to 4% except in ,the case of 20% acrylonitrile. Here the recycle rate was too fast for the condenser and considerable monomer was lost from the condenser. Inert impurities in the feed monomer should be kept low, so they will not build up in the recycle. Provision can be made in the recycle system to remove such inerts by continuous fractionation.
i
EWOIIIPIIWO Solution, suspension, or emulsion wpolymerization, though somewhat leas practical, can be carried out by using the continuous recycle system. For solution polymzrization, a d e h i t e quantity of the solvent is preferably added to the system and allowed to &tribute itself throughout the polymerizer, devolatilizer, and recycle lines. It is then continuously recycled and only a small p o d o n of solvent is added to the new feed to make up for loascs. In another way, the solvent m y be introduced in the new feed, but i t must then be fract i o ~ t c dout of the recycle, so that it w i l l not build u p and stop the polymerization. A diqxming pbase such as water might be handled in either of the two ways mentioned for a solvmt. Copolymer Composition In Table I11 the average acrylonitrile content of the copolymers corresponds with the feed composition. The si@cance of t h i s result should be considered in the light of the fact that for many copolymerizations it is difIicult, if not i m m b l e to get an accurate analysis of the copolymer. The same average composition Can be expected even a t high per cent solids of the partially polymerized solution being removed from the polymerizer. This is indicated by simple material balance considerations. Copolymer composition curves d a t e the average composition of the polymer to the monomer compnsition from which the polymer is formed. The usual p d u r e is to carry a hatch polymerization to low conversion, isolate-and analyze the polymer; and use either the differential or integral form of the wpolymer composition Equation 1 ( 1 , l Z ) for calculating reactivity ratios and plotting the copolymer composition curve. In the cax of continuous wpolymerization in a thoroughly mixed polymerizer (described in detail for a coil polymerizer below) the monomer composition in the parrially polymerized solution continuously removed from the polymerizer represents the average monomer composition in thc reactor. Thmfore, the recycle monomer composition may be plotted against the polymer composition for wpolymer composition curves. The myde monomer compositio~ and the corresponding polymer compositions fmm nimgen determination for the styrene-acrylonitrile wpolymerizations have becn plotted in Figure 5 for comparison with data of Fordyce and Chapin (5). As the data presented here are taken at 150‘ C. instead of 75’ C., it is surprising to find a lower value for rl of about 0.26 f 0.03. In the case of a-methylsfyrene and
ASPECTS OF POLYYKR PROCPSSKS
acrylonitrile, the data obtained by wntinuw recycle copolymerization a g r c well (Figure 6) with those of Fodyce, Chapin, and Ham (6). .Styrene and a-methyhymne r e p sent a copolymudzation w k analysis of the copolymer p ~ s e n ed i 5 c u l h . Morthland and Brown (73)determined the fallowing values on the basis of three expcrimenta with radioactive styrene: r1 = 0.71, rs = 0.14. The d t a obtained byrecyde copolymerization (Figure 7), assuming the polymer had the same compwdtion aa the feed h which it was made, indicate a value of rl greater than unit (1.25 =t0.05) with no azeotmpic composition and 12 = 0.25 0.1. Calibration of the infrared analysis of styrene-a-methylstyrene copolymers mentioned M o w were based upon samples made by recydc copolymerization.
*
Figure 5. Copolymer composition curve for styrene (MJ-auylon~ile
fM3
h = 0.41.m = 0.03 Points are from w l i n u w s recycle copdymerizah,ulng mcycls monomer ~npos11Ion
Copolymer Homogeneity The authors have used the exprrsaion “per cent solids” instead of conversion in refening to the partially polymerized solution as it is removed from the polymerizer. This is necesary because the polymer is not formed by conversion of a single monomer composition to a h a l monomer-polymer solution as in batch polymerization, but may be thought of as bzing formed by sumwive incremental conversions followed by readjustment of the monomer concenwations before the next incremental conversion. For example, in the coil of Figure 1, the solution emeqing from the circulation pump has a cenain concenwation of MI,M . etc., and polymer. As. the solution makes one pass around the coil, judicious choice of temperature, circulation rate, and coil capacity has ensured less than 5% couvenion of monomer to polymer (for severely dinproportionating monomers considerably less than 5%), so that the copolymer formed correspnds e m t i a l l y to the copolymer ratio described by the insmntaneous form of the copolymer composition Equation 1 :
.
Figure 6. Copolymer composifion curve for a-melhylstyrene (M~)-aaylcnitrile (&)
-
rz = 0.10. m 0.06 Point4 ar* from eanlhwu, + r
copdynai..tion, wlnp w c l e lllMIomer mpolltion
.
After this single pass the removal of partial polymer, introduction of monomer, and mixing by the cirmlation pump restore the concen!mtions to the same values as before the previous pass. This diecussion shows the importance of adequate mixing and, for a reeirculation polymerizer, the n d t y of su5icient circulation to keep the conversion p” pass low. Furthermore, it indicates that the recycle composition represents the average monomer concenwation from which the polymer has been formed and can be used for plotting copolymer composition curves.
Figure 7. Copolymer Composition for a-methylstyrene [ M k t y r e n e
curve
(Mi) rn = 0.71,m = 0.14 PDinb are frm wlinuour recycle copdym.IcImtion. using recyde mononer -paition and ouuming polymw compmiNon same as feed
VOL 49, NO. 11
N O ~ B E E 1957 I
inns
By recycling unpolymerized monomer, the copolymer eomposition is made independent of the per cent solids. This combined with the incremental conversion of monomer to copolymer means that not only is the average copolymer composition predictable, but the copolymeric species present will be extremely uniform in composition.
tated by addition of methanol. This would be expected to separate the sample by differences in molecular weight and by differences in composition. Table IV shows that the fractions obtained dere very uniform in a-methylstyrene content. Few copolymerizations are as severely disproportionating as styrene (MI) and maleic anhydride (Mz) where r l = 0.042 ( 2 ) or 0.01 (77) and r2 = 0. The copolymer composition curves from these data are shown in Figure 10. In spite of this degree of disproportionation, transparent copolymers containing from I to 45 weight yo maleic anhydride have been prepared by continuous copolymerization. In this case, of course, essentially no maleic anhydride can be recycled, but over most of the composition range so little of the monomer is present in the polymerizer that this affords no difficulty, In copolymers containing up to 427, maleic anhydride no 50-50 copolymer was detectable by solubility tests. For example, the copolymers were soluble in styrene, whereas the 50-50 copolymer is not.
Table IV. Infrared Analysisof Styrenea-Methylstyrene Copolymer Fractions a-MethylFraction Wt. % styrene, SO. of Sample Wt. % 1
53.0 5.4 7.9 10.2 4.6 7.5 5.4
2 3
4 5 6
7
37.1 38.3 37.7 38.8 37.9 37.4 36.4
Copolymer homogeneity or heterogeneity may be established in several ways, not the least of which is transparency. All of the styrene-acrylonitrile copolymers made by recycle copolymerization were transparent. Figure 8 shows corresponding compositions polymerized in bombs to completion; all but the 30 weight % sample were milky. Skeist (14) has given the mathematical treatment of such copolymerizations and indicated the effect of rl and r2 values on copolymer heterogeneity. Figure 9 is a plot of the differential composition distribution for an 80 mole yo methyl acrylate copolymer with styrene, The two peaks correspond to different copolymer species; a sample of the batch copolymer was opalescent and brittle. A similar copolymer made by the continuous recycle method was essentially transparent. The bulk copolymerization of a 4 0 weight a-methylstyrene-60 7ostyrene copolymer could not be carried to
Acknowledgment Figure 8. Compositions polymerized in bombs to completion Acrylonitrile,
A
5
B C
10 15
D
20 30
E
02
04
Figure 9. Differential distribution of copolymer composition for styrene (&)-methyl acetylate (Mz) r l = 0.75,r2 = 0.20,x = mole fraction of MI in
Styrene,
%
95 90
85
’
Literature Cited
80
70
(1) Alfrey, T., Jr., Boher, J. J., Jr.
completion because of the depletion of styrene and the lack of polymerization of mixtures rich in a-methylstyrene. The copolymer prepared by recycle copolymerization was found by infrared analysis to contain 38yGa-methylstyrene, The copolymer was dissolved in methyl ethyl ketone and fractionally precipi-
’
‘0
%
O
O
m
Mole % MA in Monomer Mixture
Figure 10. Copolymer composition curves for styrene (MI)-maleic anhydride (Mz) __- r l = 0.042
copolymer
1 806 INDUSTRIAL AND ENGINEERING CHEMISTRY
- - - - r] = 0.01,
r2
=0
The authors are indebted to several laboratories of The Dow Chemical Co. for analytical results and testing, particularly the Main Analytical Laboratory, Spectroscopy Laboratory, and Styrene Polymeriza tion Laboratory.
Mark, H., “Copolymerization,” Interscience, New york, 1952. (2) hlfrey, T., Jr., Lanvin, E., J . Am. Chem. Sac. 67,2044 (1945). ( 3 ) deNie, W. L. J., U. S.Patent 2,496,384 (1950). (4) Fikentscher, H., Hengstenberg, Josef, Zbid.. 2.100.900 (1938). (5) Fordyce, R . G., Chapin,‘E. C., J . Am. Chem. SOC.69, 581 (1947). (6) Fordyce, R. G., Chapin, E. C., Ham, G. F,., Ibid.,70, 2489 (1948). (.7 .) Hanson, A. W., U. S. Patent 2,488,189 . . (1949). (8) Ibid., 2,769,804 (1956). ( 9 ) Hanson, A. I V . , Heston, A. L., Buecken, H. E., Ibid., 2,519,834 (1950). (10) Heerema, J., Ibid., 2,482,771. (1950). (11) Mayo, F. R., Lewis, F. M., Walling, C., J. Am. Chem. SOC.70, 1529 (1948’1. (12) &yo, F. R., Walling, C., Chem. Reus. 46, 191 (1950). (13) Morthland, F. W., Brown, W. G., J . Am. Chem. SOG. 78, 469 (1956). (14) Skeist, I., Ibid.,68, 1781 (1946). (15) Walling, C . , Briggs, E. R., Zbid., 67, 1774.
RECEIVED for review April. 25, 1957 ACCEPTED August 26, 1957 Division of Industrial and Engineering Chemistry, Symposium on Engineering Aspects of Polymer Processes and Applications, Joint with Divisions of Paint, Plastics, and Printing Ink and Polymer Chemistry, 131st Meeting, ACS, Miami, Fla., April 1957.
