A Symposium on
UNIT PROCESSES Preeented before the Division of Industrial and Engineering Chemistry at the 122nd Meeting of the AMERICAN CHEMICAL SOCIETY, Atlantic City, N. J.
A VINYL CHWRIDE POLYMERIZATION PROCEDURE
............. Z.2'-DlPHBNIC ACID FROM PHENANTHRENE Willi.m F. O'coMm .nd E d 1 J. Moriaoni . . . . . . . . . S.G.&nLoB.ndR.No.ri.Sbrer..
270 2T7
PRODUmON OF ACETONITRILE AND OTHER l l l W MOLECULAR WEIGHT NITRILES W U U v 1. Danand R i o h d R. Bishop
..........
282
MONONITRATION OF BENZENE K d A. Koba and J. John Mills.
.............
287
SUSPENSION POLYMERIZATION OF STYRENE Walter 5. K..bin and R. N d s Shmrc.
...........
A C F I V I T Y OF SOWD CATALYSTS W. B. A** md J. M. Smith
..........
OF
C. C. D e W i t t and P. D. ShroB.
298
. . . . .DYES . . . . . . . . . . . 302
~ ~ ~ ~ c ? & , ~ u ~ ~INi THE & ~ i ~ m D s K-ah
292
....
A. Kobs .ndJam- A. W i l k i m n
..........
EFF'ECI' OF SOLVENT PROPERTIES IN THERMAL DECOMPOSITION OF OIL SHALE KEROGEN Werner D. Sahnsckenbug and Chule. H. Men.
.......
307
$15
A Vinyl Chloride Polymerization Procedure A lauroyl peroxidepolyvinyl alcohol suspension polymerization system was chosen for a study of polymerization factors that influence polyvinyl chloride properties. No activator was found which did not adversely affect heat stability. Heat stability was signilieantly affectedby the degree of wnversion, catalyst wncentration, and, to a lesserextent, by temperature of polymerization. Molecular weight was prineipally affected by temperature and was relatively independent of catalyst or surfactant concentration. Conversion at equal reaction times was proportional to catalyst concentration. Exeept for minor differences, ksults support the theory that suspension polymerization may be viewed as bulk polymerization in small droplets. S. G . BANKOFF Rose Polytechnic Imtitute, Term Route, Ind.
R. NORRIS SHREVE . Purdue University, Lajoyette, I d .
A
LTHOUGH a large body
of patent and journal literature
(4, IO) dealing with the polym-ation
of vinyl chloride, either alone or with m l l amounts of a second monomer, has a p pesred, relatively little has been published about the actual etlect of variablm in the polymerization prosuch BB nature of c a b Ivat. MtUre of the surfactant.. temoerature. and concentrationson . &'important factors of polymer quality and reaction rate. This situation is encountered as a rule rather than an exception in the development of a commercial prooesa. It is, therdore, u a d l y necessary to conduct a large number of scouting teste in order estimate the qualitative etlects of a number of variables and, aithio the limits of the time and m a p o m r available, to choose from thew scouting teats the best combination of variables for further intensive study.
to
270
The objective of thia research wan the development of a polym-ation procedure for vinyl chloride which would yield a bigh quality polymer in a commercially feasible proce88. Scouthg teete were made to establish the laboratory techniques, and a particular recipe was then choaen from the patent literature an a likely mint of departure. This recipe waa found to dve very highr-tion rat& and acceptable molecular weight butinferior heat stability. A number of acouting tests were made in an attempt to improve thia deficiency. These efforta were unauccesaful, but in the w m of the work it wan observed that the particle siae during polymerization had a diatinct Uuence on the heat stability. This led to a study of suspension veraua emulsion polymeriaation. In the courae'of this study a number of surfactants, catalyst, activators, and miscellaneous additives were
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 45, No. 2
.
Dorr Continuous Recausticizing System (center) Delivers 30,000 Cubic Feet of White Liquor Daily to Digesters a t the Pulp and Paper Mills of Coosa River .Newsprint Co., Coosa Pines, Ala.
scouted. A particular combination of catalyst and surfactant was chosen from these scouting tests and the effects of temperature, catalyst concentration, surfactant concentration, and degree of conversion were more intensively studied. From the results of these tests conditions were chosen which would yield polymer in a physical form suitable for further processing and a t the highest rate consistent with a high standard of product quality. I n the process of making these tests a number of interesting relationships between the process conditions and the product characteristics were found. It should be emphasized, however, that the number of variables in this process was too large to make an exhaustive study of all the interactions. REAGENTS
tL
Reagent purity is of the utmost importance in obtaining reproducible results in emulsion and suspension polymerization. Even minute traces of contaminants, such as iron or calcium, can affect the reaction rate appreciably. Unless otherwise specified the reagents used throughout this work were of C.P. or reagent grade. Distilled water was employed in all runs. Vinyl chloride was supplied in cylinders by Dow Chemical Go. It was washed with a concentrated caustic solution in the vapor phase to remove the shipping inhibitor (phenol) and was redistilled im-
Typical Peroxide Assay, %
Catalyst a n d Composition
.~
Benzovl " nerouide .. p-Chlorobeneoyl peroxide Alperox C (technical lauroyl peroxide) Hydroxyheptyl peroxide Luperoo ET.C (dibenzaldiperoxide compounded with tricresyl phosphate) Cyclohexanone peroxide technical , Lupersol F D M (meth 1 isobutyl ketone peroxide in dimethyl pxthalate) &Butyl hydroperoxide Di-t-butyl peroxide Di-t-butyl diperphthaiate ~
February 1953
RR
n
95.0 95.0 95.0
Active Oxygen,
%
fix 4.9 3.76 5.8 6.5
50.0 96 0
13 0
80.0 60.0 97.0 95 0
11.0 10 8 10.6 9 8
mediately before charging into bottles. Each new batch of monomer was tested as to polymerization rate before use. The catalysts listed in the preceding table were obtained from the Lucidol Division of the Novadel-Agene Corp. The principal emulsifier used was Duponol ME, which is a technical grade sodium lauryl sulfate, supplied by Du Pont. Suspending agents investigated included Elvanol 50-42 (Du Pont), which is a high viscosity polyvinyl alcohol produced by 86 to 89% hydrolysis of polyvinyl acetate; and Methocel 15 (Dow Chemical Co.), which is a methyl ether of cellulose having a viscosity of 15 cp. in 2% aqueous solution a t 20" C. For specific viscosity determinations, cyclohexanone (b.p. 154" to 156' C.) was obtained from Eastman Kodak. EXPERIMENTAL PROCEDURE
The polymerizations were conducted in 6-ounce, crown-cap beverage bottles rotating end-over-end in a constant-temperature bath thermostated to f 0 . 2 " F . Rigid precautions were taken to exclude oxygen from the bottles, because of its known inhibitory properties. The procedure used was as follows: 1. The suspending agent and other water-soluble ingredients were weighed into the required amount of distilled water and dissolved. It was usually necessary to use water a t about 80' F. to produce a clear solution of polyvinyl alcohol. 2. The aqueous solution was deaerated in the polymerization bottle by boiling under high vacuum for 5 minutes. The bottle was then repressured with water-pumped prepurified nitrogen (National Cylinder Gas), capped, and frozen. 3. The uninhibited monomer was redistilled and weighed into the frozen bottle. A stainless steel pan balance, sensitive to 0.1 gram, was used for this weighing. The bottle was warmed to boil off a slight excess of monomer, in order t o remove oxygen from the monomer and vapor space. The bottle was capped 'in a hand-capping press using an ordinary crown cap with a cork gasket and an aluminum foil liner. 4. The bottles were stored for no more than 12 hours a t -10" C. prior to polymerization, They were placed in wire screen guards and simultaneously immersed, after thaw-ing, in
I N D U S T R I A L A N D EN G I N E E R I N G C H E M I S T R Y
271
TABLE I. SCOUTIKG TESTS-CATALYSTAND ACTIVATOR IN
riun No. Catalyst P a r t s catalyst Activator P a r t activator Time of run hours % Conversibn Heat stability
319
A
0.50
G
0.50 20.3 1.6 Poor
A = Benzoylperoxide B = Potassium persulfate C = Lauroyl peroxide
320 A 0.50 H 0.50 20.3 99.5 Poor
Charge: Vinyl chloride, parts Water, parts Catalyst Activator Duponol M E 321 322 323 A *4 A 0 60 0.50 0.50 I J K 0.50 0.50 0.50 20.3 20.3 20.3 1.4 11.2 3.0 Poor Poor Poor
the constant-temperature bath. The starting time of run was counted as the time of immersion. 5. When the run was completed, the bottles were refrozen in the refrigerator and-vented. After thawing, the contents of the bottle were filtered on a Buchner funnel and washed with distilled a-ater. Occasionally, a considerable proportion of the polymer w-a8 sufficiently finely divided to pass through the filter paper. This latex was coagulated by heating and adding potassium alum solution until a filterable precipitate was obtained. The wet polymer was dried overnight a t 50' C. At this point, it was a white, finely divided, free-flowing solid.
336 C 2 51
H
0.50 24
99.5 Fair
337
338
2.51
3 80
c
H
0.60 24
99.0 Fair
D
H
0.50 50 99.2 Poor
341
343
1.00 H 0.50 50 0.2
150
E
..
,
F
,.. .,.
24
41.3 Fair
J = Tetraethylenepentamine K = Sodium thiosulfate L = Potassium ferrocyanide
This test was quite reproducible. It showed good correspondence with oven heat stability tests made with films compounded on a laboratory mill. Particle Size. For particle size analysis of the polymer as produced, it was necessary to wet-screen the contents of the polymerization bottle. After venting the bottle, it was emptied into a nest of U. S. Standard screens, and sieved for 3 minutes in a Combs gyratory sifter. As many as seven screens could be used a t one time. The fractions were dried at 80" C. and weighed. It was usually necessary to add a small quantity of potassium alum to the -200 mesh fraction in order to produce a filterable precipitate. Conversion. The per cent conversion was calculated from the dry weight of the polymer and the weight of monomer charged to the bottle. The average rate of polymerization was then calculated, based on the elapsed time of reaction.
TEST METHODS
Specific Viscosity. The specific viscosity of the polymer was determined in cyclohexanone using a 0.4870 solution. Ostwald pipets with a flow time in excess of 150 seconds were used. It was found t o be desirable to chill both solvent and polymer to 0' C. before mixing t o minimize formation of clumps of gel particles. Aside from this variation standard techniques were used. Heat Stability. Several methods of determining the heat stability of small samples of polymer were tried. One technique involved grinding the resin, stabilizer, and plasticizer in a jar mill to form a plastisol which was then cast into a film for oven testing. The method, however, proved to be too laborious and required an extremely fine initial resin particle size. The second method was to dissolve the polymer sample in tetrahydrofuran or cyclohexanone and to add plasticizer and a soluble stabilizer, such as Advance SN, to the solution before casting a film. This method also had several disadvantages. First, undissolved gel particles were frequently present in the polymer solution. These particles were frequently transparent and difficult to detect except by filtration. Second, reactions between the solvent and polymer sometimes produced color bodies. Thus, in making up the tetrahydrofuran solution a pink to red color was sometimes obtained, whereas with cyclohexanone a green color was obtained. Moreover, when using freshly distilled tetrahydrofuran, a colorless solution was obtained, but on allowing the tetrahydrofuran to stand overnight a pink solution Ras produced. This color reaction, which may have been due to the presence of a tetrahydrofuran or cyclohexanone peroxide, made the procedure unattractive. A simple test of unstabilized polymer was then tried; it consisted of the following procedure:
272
EMULSIOX SYSTEM .4T 39" c.
G = Hydrazine sulfate H = Sodium bisulfite I = Ferrous pyrophosphate
D = Luperoo E T C E = Di-t-butyl peroxide F = Benzoyl peroxide
The samples of finely ground polymer were brought to constant weight in a desiccator, overnight, with a sample of Geon 101. The polymer samples and Geon 101 were packed level into the outer cavities of a porcelain titrating dish. The dish was then heated a t 160" C. for 1 hour in an air-circulating oven. The color of the polymer samples was rated as compared to the color of Geon 101 control. Under these conditions the Geon turned a light pink. The heat stability was reported excellent if better than Geon; good if as good as Geon; fair if somemht inferior to Geon (pink t o orange); poor if markedly inferior to Geon (deep pink to brown); and very poor if the sample turned a deep brown or black.
AN
100 250 Variable Variable 2.6 324 326 A B 0.60 0.20 L H. 0.50 0.20 20.3 20.3 5.2 99.5 Poor Very poor
PER SU LFATE CATALYSTS
*
I n view of the rapid reaction rates reported by Coffman and RIcGrew ( 3 ) for persulfate salts as catalysts for the emulsion polymerization of vinyl chloride this class of catalysts was investigated first. It was found that extremely high reaction rates could be obtained, especially in the presence of sodium bisulfite as an activator. Thus, using ammonium persulfate and sodium bisulfite in amounts equal to approximately 0.5% based on monomer, it was possible to obtain substantially complete conversions in less than 4 hours at temperatures as low as 7" C. The activity of this combination was far greater than that of any other catalyst-activator combination scouted, such as hydrogen peroxide or benzoyl peroxide, a-ith sodium bisulfite or tetraethylenepentamine. The molecular weight of this polymer was approximately in the range of commercial polymer (Geon 101), as determined by specific viscosity measurements. However, the heat stability was quite poor both in the presence and absence of stabilizers. The polymer was isolated from the latex by either freezing or coagulation with a dilute solution of potassium alum or v-ith concentrated hydrochloric acid. It was then filtered on a Bdchner funnel and washed with distilled water and methanol. When this polymer was heated in a circulating oven a t 100" C. for 1 hour, it rapidly discolored. The initial coloration was a light blue which on further heating became a bluish gray and finally black. This was in contrast to the behavior of Geon 101, which did not discolor a t all a t 100' C. after 1 hour and when heated to 160" C. turned pink and then eventually a dark bronn. At no stage, however, was a blue coloration observed. The persulfate polymer discolored when held overnight a t temperatures as low as 50" C. Washing with numerous washes of distilled water, acetone, or methanol did not appreciably improve the heat stability. Washing with a 10% solution of lead nitrate, however, improved the heat stability greatly, although it did not bring it to the level of Geon 101. Sodium carbonate washes also improved the heat stability. This indicated that the discolora-
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 45, No. 2
Unit Processes
.
was practically inactive a t this temperature. It was interesting to note that, whereas a completely dispersed latex had been obtained with a persulfate catalyst, a combination of beads and latex was obtained with the organic peroxides. I n each case the beads were separately washed, and in each case the beads were more heat stable than the coagulated latex from the same bottle. It was therefore decided to investigate the use of peroxide catalysts in a suspension system (Table 11). I n runs 362 to 366 benzoyl peroxide was tested as the catalyst a t 39" C. with methyl cellulose and polyvinyl alcohol in the presence and absence of sodium bisulfite. The sodium bisulfite exerted a strong activating effect but also resulted in lower heat stability and, rather interestingly, a coarser product. There seemed to be little to choose between the two surfactants. None of the polymers was equal to Geon 101 in thermal stability. At this point polyvinyl alcohol was chosen for further suspension polymerization studies. Three promising catalysts (benzoyl peroxide, lauroyl peroxide, and p-chlorobenzoyl peroxide) in equal concentrations of active oxygen were tested in the presence of sodium bisulfite at 39' C. The order of preference for both increased reaction rate and heat stability appeared to be lauroyl peroxide, followed by benzoyl peroxide and p-chlorobenxoyl peroxide (runs 369 to 374). However, the heat stability was still below that of the commercial resin. Since sodium bisulfite had previously been shown to have a deleterious effect on thermal stability, it was omitted from the next series of runs (383 to 392). The temperature was increased to 47" C. t o offset the absence of any activator. A variety of catalysts were tested, including hydroxyheptyl peroxide, cyclohexanone peroxide, di-kbutyl perphthalate, t-butyl hydroperoxide, Lupersol FDM, and cumene hydroperoxide. Of this group lauroyl peroxide, benzoyl peroxide, and p-chlorobenzoyl peroxide were by far the most active. Of these three catalysts lauroyl peroxide gave the best heat stability, and was therefore selected for further study. The use of a methanol-water solution as the suspending medium for vinyl chloride polymerization has been described (6). This variation was tested using Methocel as the suspending agent and lauroyl peroxide as the catalyst (runs 393 t o 396). A fine suspension of dispersed solids was obtained with excellent heat stability. It seems quite possible that one reason for this excellent heat stability was the increased solubility of the catalyst residues in the suspending medium. A number of inorganic and organic reducing agents and heavy metal salts were scouted as activators in the polyvinyl alcohol-
tion was probably due t o degradation of the polymer accompanied by splitting out of the hydrogen chloride. A sample of polymer was reprecipitated from tetrahydrofuran solution by addition of methanol. The reprecipitated material was considerably more heat stable, indicating that small amounts of occluded catalys and activator may have been responsible for poor heat stability. This polymer tended to become grayish after standing a t room temperature for several weeks, indicating that some breakdown was occurring at room temperature. Assuming that the degradation was a free radical chain mechanism this pointed to residual traces of the highly active persulfate catalyst as the principal cause of degradation. Washing the polymer with reducing agents, such as sodium bisulfite, did not improve the heat stability greatly, indicating the difficulty of completely removing the catalyst residues from the polymer. High speed agitation in a Waring Blendor was tried without greatly improving the efficacy of washing. It became apparent, therefore, that it would be advisable t o investigate other catalysts. PEROXIDE CATALYSTS
A series of runs (Table I ) were made in an emulsion system a t 39' C. in an effort to find an active peroxide system which would give polymer of good heat stability. I n runs 319 to 326 a number of reducing agents, including hydrazine sulfate, sodium bisulfite, ferrous pyrophosphate, tetraethylenepentamine, sodium thiosulfate, and potassium ferrocyanide, were tested in combination with benzoyl peroxide. Sodium bisulfite was by far the most active promoter. Tetraethylenepentamine was the only other reducing agent which gave a conversion greater than 60%. For comparative purposes a test with potassium persulfate and sodium bisulfite was made. With less than half the concentration used in the peroxide experiments this run went substantially to completion. However, the polymer discolored on drying, giving a green color as compared t o the blue color which was characteristic of the ammonium persulfate-catalyzed polymer. The heat stability of the peroxide-catalyzed material was greater than that of the persulfate polymer, although not as good as Geon 101. Several additional peroxide catalysts were scouted in an emulsion system at 39" C. (Table I). These included Alperox C (technical Iauroyl peroxide), Luperco ETC (dibenzal diperoxide compounded with tricresyl phosphate), and di-tbutyl peroxide. Of these three, only lauroyl peroxide appeared to be as attractive as benzoyl peroxide for this polymerization. Di-&butyl peroxide
TABLE 11. SCOUTING TESTS-CATALYSTAND ACTIVATOR IN SUSPENSION POLYMERIZATION SYSTEMS __
R.iin . N o-. -.
Surfactant P a r t surfactant Catalyst Parts catalyst Activator P a r t activator P a r t methanol Time of run. hours Temperature C. % Cpnversioh Particle size Heat stability Run No. Surfactant P a r t surfactant Catalyst Parts catalyst Activator P a r t activator P a r t methanol Time of run, hours Temperature, O C. % Conversion Particle size Heat stability
362
A 0.24 D G
A 0.24 D 1.35 G
0.45
0.45
ii
17 39 86.0 LC
1.35
'
39
LC
Poor
F5 0.058 G
1.50
... ...
...
364
365
0.24
D
A 0.24 D
1.35
... ...
1.35
ii
17 39 42.0
A
'
39 28.1
FS
... ... ...
Poor
Fair
FS Fair
386
387
388
B
0 058
G
1 50
B 0 058 H 1 60
B
0.058
D
1 50
...
366
369
370
371
372
373
374
0.24
0.60 D 0.45 G 0.30
0.60
0.60
0.60
0.60
0.60
0.45
0.58 G 0.30
0.58 G 0.30
0.75 0.30
0.75 G 0.30
17 39 69.3
ii'
17 39 42.6
ii'
ii '
ii'
VFS Fair
VFS Fair
391 B 0 058 J 1 50
392 B 0 058
B
D
1.35
G
0.45
...
17 39 82.1 LC Poor
389 B
0 058 E 1 50
...
B
B
...
VFS Fair
390
B 0 058
I
1 50
...
D G
0.30 39 66.8
...
B
E
...
K
1.50
...
B E
B
C G
B
C
384 B B 0.058 0 . 0 5 8 F C 1.50 1.50
383
G
...
...
15.9 47 5.0 .
39 42.6
39 72.0
39 99.8
VFS Poor
VFS
VFS Fair
393
394
0 15 C 0 15
0.15 C 0 15
iQ5 iQ6 0 15 0 15
A
...
Good
B
...
C 0 45
...
. A
Fair
...
... ...
15.9 47 95.1
VFS Good
C 0 45
...
. . I
15.9 47 0.6
...
Poor
A = Methyl cellulose B = Polyvinyl alcohol C = Lauroyl peroxide D = Benzoyl peroxide
February 1953
363
E = p-Chlorobenzoyl peroxide F = Lupersol FDlM G = Di-t-butylperphthalate H = &Butyl hydroperoxide
I = Luperco E T C J = Cyclohexanone peroxide
K = Cumene hydroperoxide L = Sodium bisulfite
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
VFS = Very fine suspension FS = Fine solids LC = Large clumps
273
TABLE 111. EFFECT O F REDUCING A G E N T S AXD HEAVY METAL SALTS I N SUSPENSION POLYMERIZATION SYSTEM .4T 50" c. Charge: Vinyl chloride, parts Water, parts Polyvinyl alcohol Lauroyl peroxide Activator Time, Hours 22.5 22.5 22.5 22.5 22.5 22.5 22.5 22.5 22.5 22.5 22.5 22.5 22.5 22.5 14.7 14.7 14.7 14.7 14.7 14.7 14.7 14.7
Activator
-4 B C D E F G
H
I J
D D K C C
E
E F F G
..
A B C D E F
100 217 0.24 0.15 0.15
%
Conversion 35.4 55.5 87.1 ,..
= Silver nitrate
= Sodium sulfide = Guanidine hydrochloride = Hydroxylamine hydrochl oride = Diphenyl sulfone = Diphenyl sulfoxide
89.0 80.4 99.5 89.0 94.8 95.9 41.7 32.9 78.7 66.6 49.8 26.? 55.0 52.7 33.8 36.0 99.5 50.6 G =
H = I = J =
I< =
POLYVINYL ALCOHOL CONCENTRATION
A series of runs (Table V) \vas made a t 50" C. with a constant Iauroyl peroxide concentration and variable polyvinyl alcohol concentration. Fithin the limits of experimental error the conversion rate was independent of the polyvinyl alcohol concentration. The heat stability ranged from good to excellent, and within the limits of experimental error there did not seem to be any significant influence of polyvinyl alcohol concentration on heat stability. Specific viscosities run on several of these polymeis indicated very little influence of polyvinyl alcohol concentration on molecular weight.
Heat Stability Exc. Exc. Exc. Exc. Good Fair Poor Exc. Fair Poor Good Exc. Exc. Exc. Good Good Fair Fair Poor Poor Very poor Good Sodium bisulfite Copper sulfate Calcium sulfite Thioglyoolic acid Sodium tellurite
lauroyl peroxide systein (Table 111). The compounds tested in eluded silver nitrate, sodium sulfide, guanidine hydrochloride hydroxylamine hydrochloride, diphenyl sulfone, diphenyl sulfoxide, sodium bisulfite, tetraethylenepentamine, copper sulfate, calcium sulfite, thioglycolic acid, sodium hypophosphite, and sodium tellurite. None of these was effective in accelerating the reaction rate as compared to the unactivated recipe except sodium bisulfite, calcium sulfite, tetraethylenepentamine, and copper sulfate. However, these all resulted in polymer of inferior heat stability as compared to polymer from the unactivated recipe. It was therefore decided not to incorporate any activator into the recipe in the interest of maximum heat stability. CATALYST- SURFACTAhT COMBIUATIONS
Upon completion of investigation of miscellaneous additions it appeared that polyvinyl alcohol would be the preferred surfactant and lauroyl peroxide (Alperox C ) the preferred catalyst. As a check on this, a series of runs (Table IV) was made to evaluate combinations of two surfactants, Duponol ME and polyvinyl alcohol, and two catalysts, potasqium persulfate and lauroyl peroxide. The results indicated that potassium persulfate, weight for weight, is a more active catalyst than lauroyl peroxide. The reaction rate was essentially independent of whether Duponol 01 polyvinyl alcohol mas used as the surfactant. The heat stability results, although not extremely clear-cut, favoi ed polyvinyl alcohol over Duponol, and lauroyl peroxide over potassium persulfate. The effect of increasing the Iauroyl peroxide concentration mas t o reduce the heat stability somen hat, al-
TABLEIv.
though even a t the higher concentrations the heat stability was nearly as good as Geon 101. On the basis of these results the selection of lauroyl peroxide and polyvinyl alcohol for more extensive study was confirmcd.
cO\fVLPARISON O F TTTO
CATALYSTS
$I
A 0.958 C 0.208 89.6 Very poor
A = Duponol M E
274
A 0.958 C 0.104 61.9 Poor
A 0.958 D 0.416 99.5 Good
.4 0.958 D 0.150 39.4 Good
B
0 233 D 0.416 99.2 Good
B = Polyvinyl alcohol
B
.e 5
.oh0
.015
SCREEN APERTURE, In.
Figure 1. Effect of Polyvinyl Alcohol Concentration on Polymer Particle Size at 50" 6. for 14.7 Hours Using 0.15 Part Lauroyl Peroxide Curve NO. 1 Parts PVA 0.65
2
0.74
3 1.02
4
5
1.57
1.85
6 2.12
The average polymer particle size decreased with increasing polyvinyl alcohol concentration as shown in Figure 1. It seems probable that the polyvinyl alcohol concentration is not as iniportant a factor as the degree of agitation in determining the particle size. Fairly stable dispersions of vinyl chloride in water can be produced by violent shaking even at relatively low polyvinyl alcohol concentrations. It is to be expected, therefore, that pilot plant results might differ considerably from laboratorv results in this regard. LAUROYL PEROXIDE CORCENTRATION
h number of iuns at 50" and 65" C. to deterinine the effect oi lauroyl peroxide concentration are summarized in Figure 2 and Table VI. The average conversion rate varied approximately linearly with the lauroyl peroxide concentration a t constant tiinr of reactioii and temperature. At the lower peroxide concentiatioiis the average reaction l a t e IT as approximately doubled n ith a
.4ND ' k O
S U R F A C T A K T S IT
Charge: Vinyl chloride, parts Watei, parts Surfactant Catalyst Surfactant P a r t surfactant Catalyst Parts catalyst % Conversion Heat stability
.oh
.OZ
c. F O R
14.7 HOURS
216 Variable Vaiiahle
.I
0,233 D 0.150 33.8 Excellent
50'
100
B
B
C
c
0.958
0.233
0.208 61.9 Very poor
0,208 82.3 Good
C
0.233
0.104
53.7 Good
A 0 038 C 0.208 99.0 Very poor
C = Potassium persulfate
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
B 0.233
B 0.233
97.2 Fair
0 833 95.0 Fair
D 0 625
D
13 0 233
D I 04 09.31 Fair
D = Lauroyl peroxide
Vol. 45, No. 2
Unit Processes O F POLYVINYL ALCOHOLCONCENTRATION TABLE V. EFFECT 50' C. FOR 14.7 HOURS Charge:
Vinyl chloride, parts Water, parts Lauroyl peroxide Polyvinyl alcohol
100 217 0.15 Variable
%
Part PVA
Conversion
0.088 0.172 0.265 0.354 0.530 0.619 0.088 0.172 0.269 0.354 0.441 0.530 0.619 0.706 0.187 0.464 0.740 1.020 1.290 1.571 1.848 2.125
35.3 31.0 41.7 45.0 42.7 34.8 44.4 41.5 51.2 39.4 50.5 52.5 38.7 39.4 53.9 37.2 36.0 38.5 36.4 41.6 45.8 44.6
AT
Heat Stability Excellent Good Good Good Good Excellent Good Good Good Good Good Good Excellent Excellent
was a definite trend toward decreasing heat stability with increasing degree of conversion. Below about 60% conversion the heat stability of the polymer was equal t o or better than Geon 101. Above that percentage the heat stability was inferior to that of Geon 101. The specific viscosity was independent of the degree of conversion within the limits of experimental error. The molecular weight as determined by specific viscosity measurements was again lower a t this temperature than at 50' C.
...
... ... ... ... ... ...
...
10" rise in temperature. The percentage increase was somewhat less at higher coneentrations. The heat stability a t low catalyst concentrations was excellent but decreased as the catalyst concentration increased. Within the limits of experimental error the specific viscosity seemed to be independent of catalyst concentration. However, it %*assubstantially lower a t the higher temperature. The polymer was wet-screened using 16-, 45-, and 170-mesh screens. There did not seem to be any correlation between the peroxide concentration and the particle size distribution.
POLYMERIZATION TIME, HRS.
Figure 3. Conversion as a Function of Time of Immersion, Using 0.15 Part Lauroyl Peroxide and 0.23 Part Polyvinyl Alcohol For this series of runs per cent conversion is plotted as a function of polymerization time in Figure 3. Within the range studied the per cent conversion was approximately a linear function of time. SODIUM BISULFITE ACTIVATION
The effect of the presence of water and of sodium bisulfite in the polymerization recipe a t 50" C. was studied (Table VIII). No surface active agent was included in any of the runs. Sodium bisulfite in the presence of water gave an average conversion of 84.2% in 5.75 hours whereas in the absence of water the average conversion was 58.2%. In the absence of sodium bisulfite the
1
1.5
1.2
0.9
OF TABLE VI. EFFECT
% LAUROYL PEROXIDE Effect of Lauroyl Peroxide Con-
Figure 2. centration on Conversion at Constant Time of Immersion and Temperature, Using 0.27 Part Polyvinyl Alcohol CurveNF. Temp., C. Time. hr.
1
50 3
2 50 5
3 60 3
4 50 14.7
5 60 8.7
The heat stability was somewhat lower a t the higher degrees of conversion. For maximum heat stability it appeared desirable to operate a t 50' rather than 60" C. and under 70% conversion of the monomer. It appeared therefore that the catalyst concentration and the degree of conversion were relatively minor influences in determining the average molecular weight whereas the temperature of polymerization was a major influence. DEGREE O F CONVERSION
A series of runs (Table VII) was made at 60' C. using a uniform lauroyl peroxide and polyvinyl alcohol concentration. The time of the run was varied from 2 to 8 hours giving a corresponding variation in the per cent conversion from 10 to 87%. There February 1953
CATALYST CONCENTRATION AT
Charge: Vinyl chloride, parts Water, parts Polyvinyl alcohol Catalyst
Parts Lauroyl Peroxide 0.113 0.150 0.200 0.267 0.067 0.112 0.150 0.200 0.267 0.584 0.067 0.112 0.150 0.200 0.267 0.417 0.584 0.30 0.60 1.20 1.50 0.30 0.60 0.90 1.20 1.50
Time of Run,
Hours 14.7 14.7 14.7 14.7 3.0 3.0 3.0 3.0 3.0 3.0 8.7 8.7 8.7 8.7 8.7 8.7 8.7 3.0 3.0 3.0 8.0 5.0 5.0 5.0 5.0 5.0
50"
AND
60' C.
Temperoatwe,
C.
50 50 50 50 60 60 60 60 60 60 60 60 60 60 60 60 60 50 50 50 50 50 50 50 50 50
INDUSTRIAL AND ENGINEERING CHEMISTRY
%
Variable Heat Stability Excellent Good Excellent Good Poor Good Good Good Good Poor Excellent Poor Poor
Con-
version
26.4 40.6 54.7 89.0 7.5 11.0 14.8 19.8 27.1 52.7 52.3 84.0 92.4 92.4 96.2 95.4 94.3 6.0 12.9 25.5 33.2 12.2 24.4 30.1 50.1 62.4
100 216 0.265
Poor
-
Poor Poor Poor Good Good Good Good Good Good Good Good Good
Specific Viscosity
0.65 0.70 0.65 0.70
...
0.44 0.44 0.44
... ...
0.47
...
0.51
...
0.46 0.46 0 46 ... ...
... ... ...
... ... ...
... 275
TABLE VII.
EFFECT O F DEGREE O F CONVERS~ON AT 60’ C. Charge: Vinyl ohloiide, parts Water, parts Polyvinyl alcohol Lauroyl peroxide
%
Time, Hours
10.2 18.8 28.6 41.5 55.0 76 3 87.1
7.0 8.0
TABLEVIII.
...
ii6
... ...
216
Specific Viscosity 0.37 0.44 0.40 0.41 0.39 0.40 0.40
EFFECTOF WATERASD SODIUXBISULFITEAT 50” C. FOR 5.75 HOURS
Charge: Vinyl chloride, parts Lauroyl peroxide, parts NaHSOa Water
Parts Water 216 216
0 265 0 150
Heat Stability Excellent Excellent Excellent Good Good Fair Fair
Conversion
2.0 3.0 4.0 5.0 6.0
100 216
100 1.50
Variable Variable
Part
%
NaHSOz
Conversion
0.50 0.50 0.50 0 50
83.6 84.9 56.0 60.4 70.5 44.9 54.6 40.8
... ... ... *..
average conversion was 57.770. In the absence of both water and sodium bisulfite the average conversion mas 47.5%. These results are quite interesting. First of all, it is worthy of note that sodium bisulfite acts as an activator a t all in the absence of a surfactant. I n this instance, a water-soluble reducing agent is activating a monomer-soluble peroxide in the absence of a solubilizing surfactant. It suggests that the locus of this reaction is the relatively small phase interface. It is also seen that the water phase serves as something more than an inert, heatabsorptive suspending medium. It affects the polymerization rate, presumably through its slight solubility in vinyl chloride. It also increases the activating effect of sodium bisulfite, possibly also by a solubility effect. Further work is required t o elucidate this mechanism, but in any case it is clear that bulk and suspension polymerization of vinyl chloride differ in a t least one important respect. DISCUSSION OF KINETIC DATA
Several investigators ( 1 , 2, 6, 9) have studied the bulk polymerization of vinyl chloride, using peroxide catalysts. An acceleration period was found in all cases at low conversions, which Bengough and Norrish ( 1 ) attributed to the autocatalytic effect of “dead” polymer. The specific viscosity, measured in tetrahydrofuran solution, decreased with increasing polymerization temperature, but was practically independent of the peroxide concentration and of the degree of conversion. The independence of mean polymerization degree on the peroxide concentration was considered to be evidence that the average molecular weight of the polymer is determined by a chain transfer between the growing chain and the monomer. This view was advanced by Nozaki (8)based on a consideration of the stability of radicals derived from various monomers. I n general, the experimental results agree with bulk polymerization results, showing the close similarity between bulk and suspension polymerization noted by Hohenstein and Mark ( 5 ) . However, the average rate of polymerization over equal polymerization periods was found to be proportional t o the first power of the catalyst concentration (Figure 2 ) , whereas in the absence of water the instantaneous polymerization rate has been found ( 1 ) to be proportional, a t equal extents of conversion, to the square root of the catalyst conGentration. These observations are not neceprily inconsistent, since the suspension system shows the
276
small increase in polymerization rate with time noted previously for bulk systems ( 1 , 6). The theoretical significance, if any, of the first-order dependency of conversion on catalyst concentration is uncertain. However, the practical significance of this relationship is obvious. CONCLUSIONS
The heat stability of polyvinyl chloride depended on a number of factors. These included: 1, Particle size during polymerization: Suspension polymerization gave improved heat stability as compared to emulsion polymerization,. possibly because of the greater ease of removing the catalyst residues. 2. Nature of catalyst: Organic peroxides in general gave better results than persulfates; of those tested lauroyl peroxide was the preferred catalyst. 3. Nature of surfactant: Polyvinyl alcohol gave better heat stability than Duponol 4. Presence of activators: S o reducing agent or heavy metal compound was found which was effective in accelerating the reaction without lowering the heat stability. 5 . Degree of conversion: Below 70% conversion the heat stability was somexhat improved as compared to substantially complete conversion. 6. Polymerization temperature: The heat stability a t 50’ C. was better than a t 60’ C. 7 . Catalyst concentration: Low lauroyl peroxide concentrations gave better stability than higher concentrations.
The particle size of the polymer depended on the polyvinyl ai- ’ coho1 concentration and on the degree of agitation. The average reaction rate a t 60’ C. was approximately double that a t 50” C. It increased linearly with catalyst concentration at equal times of reaction. The specific viscositv was essentially independent of the catalyst concentration but was lower at 60’ than a t 50” C. I n general, the observed results were in agreement with bulk polymerization data, lending support to the theory that suspension polymerization can be viewed as a case of bulk polymerization in small droplets, Howwer, a water-soluble activator was shown to be much more effective in accelerating the reaction in the suspension rather than bulk system. I n addition, there is some question as to whether the termination reaction in the suspension system is first- or second-order. ACKh-OWLEDGMENT
This work was carried out in the laboratories of Rose Polytechnic Institute under the sponsorship of the Detrex Corp., Detroit, Rfich., whose financial support is gratefully aclcnowledged. Helpful suggestions were received from W. L. hIcCracken and C. E. Kircher, Jr. Appreciation is also due to Robert J. Jones and a number of senior engineering students of Rose Polytechnic Institute who performed most of the experimental work. REFERENCES
Bengough, W. I., and Xorrish, R. G., Proc. Rog. SOC.( L o n d o n ) , A200, 301 (1950).
Breitenbach, J. IT., and Schindler, A , , Sitzber. Ahad. Wiss. Wien., Math.-nnturw. K l . , 158, 429 (1949).
Coffman. D.. arid RIcGrew. F.. U. S. Patent 2,404,791 (July ~~
1946).
De Bell, John hI., and others, “German Plastics Practice,” Box 240, Springfield, Mass., De Bell and Richardson, 1948. Hohenstein, IT. P., and Mark, H., J. Polymer Sci.,1, 127 (1946).
Jenckel, E., Ecknians, H., and Rumbach, B., MukromoZ. Chem., 4, 15 (1949).
Naps, M.. U. 6. Patent 2,494,517 (January 1950). Nozaki, K., Discussions Faraday SOC.,2, 337 (1947). Prat, J., M e m . Senices chim. etut (Paris), 32, 319 (1945);, Schildknecht, C. E., “Vinyl and Related Polymers, New York, John Wiley & Sons, 1952. RECEIVEDfor review September 12, 1952.
INDUSTRIAL AND ENGINEERING CHEMISTRY
ACCEPTED December 1, 1952.
Vol. 45, No. 2
