Mechanism of Formation of Cream in GR-S Latex

Mechanism of Formation of Cream in GR-S Latex L. H. HOWLAND AND ALFRED NISBNOFF Yuugatuck Chemical Dieision, U . S . Rubber Co., .Vaugatuck, Conn.

ANY high-solids GR-S latices, especially those made a t temperatures near 0 " C., form a thick cream on the surface if allowed to stand without agitation for a sufficient length of time. Although this material can be redispersed, the tendency toward cream formation is undesirable because it results in nonuniformity if the latex is not cont,inuously agitated; furthermore, the presence of cream is deleterious to the physical properties of films formed from latex (1 ). From calculations based on Stokes' law it is readily evident that the particles const'ituting cream must be relatively huge, since the numerical bulk of the particles in cold, high-solids GR-S lat,ices, which are usually in the range of diameters 800 t,o 2000 A., will not rise fast enough under the influence of gravity to form significant amounts of cream. If a typical high-solids GR-S latex is diluted to about 57, solids, so that it,s viscosity approaches that of water, particles 100,000 A in diameter will rise a t the rate of about 1 em. per hour; particles 20,000 A. in diameter will rise about 1 cm. a day; if the particle diamet,eris 1000 .%., the theoretical rat,e of rise is roughly 1 cm. per year. Since a particle 20,000 A. in diameter weighs 8000 times as much as a particle 1000 A. in diameter, the formation of cream is indicative of the presence of comparatively enormous particles. Cream may well be considered as incipient' coagulum; however, cream can be redispersed, while coagulum cannot. This difference makes possible the use of latices containing as much as 5 or even 10% by weight of the larger but microscopic particles of the type which form cream, while no more than 0.2 or 0.37, of coagulum is ordinarily tolerable. Thus, cream is undesirable in the sense t,hat, it is harmful to the properties and uniformity of the latex, but it can often be tolerated in appreciable concentrations, whereas coagulum must be removed. It is therefore an important factor to be considered in latex polymerization, and a study of the mechanism of formation of large particles has appeared desirable. Among the possible mechanisms of cream formation which were considered were: that the agglomeration t,o giant particles in cold, high-solids latex occurs on the surfaces of the reaction vessel, which in large reactors are at a lower temperature than the polymerizing mixture; that polymerizat,ion within the relatively large monomer droplets accounts for the formation of the giant particles; or t,hat under certain conditions an insufficiency of soap during the polymerization results in the formation of large particles, rather than coagulum. The work described in this paper was carried out with a single type of latex, and mild agitation n-as used in the polymerizations, as contrasted with the vigorous agitation in plant reactors. However, some of the principles governing cream formation under these mild condit,ions of agitation have been applied successfully to large scale production. It appears somewhat unlikely that entirely different mechanisms of cream formation are operative in different high-solids GR-S recipes, which a,re basically very similar. However, the exact measures to be employed in eliminating cream may well vary considerably. 31ECIWKISM OF CREAM FORMATION

T o study the mechanism of cream formation, certain variables were investigated in small scale polymerizations carried out in 24-ounce pressure bottles. A method for determination of par-

tide size distribut,ion based on Stokes' law ( 5 ) provided a rapid and quantit,ative means of estimating the creaming tendency of various latices. Essentially the method consists in determining the decrease in solids content a t various heights, and after various time intervals, in t,he highly diluted latex. Particle size distribution is calculated from the data by means of Stokes' law. Khile it is necessary t o centrifuge for analysis of small particle Pisee, gravitational separation sufficed for the determination of the very large particles of int,erest here. The only significant change in technique (6) was the use of 1% ammonium alginate rat'her than 0.4%. The 1% alginate is mixed with an equal volume of latex for the purpose of obtaining separation between serum and polymer in order to determine polymer density. This higher concentration was found necessary to give complete separation in many of the latices prepared for this work. 5-2820, a cold, high-solids latex act'ivated by ferrous sulfide, mas chosen as t'he basis for the work because, as produced a t the time of these experiments, it ordinarily contained a relatively high concentration of very large particles ( 5 to 10% by weight). The polymerization recipe for 5-2820 latex is given in Table I. As indicated, 1.75 parts of potassium oleate, per 100 parts of charged monomers, are ordinarily added as an 18% solution when the conversion of monomers has reached 35%. The data in Table I1 show that a large proportion of the weight of polymer in the latex consists of giant particles when the stabilieing soap is

TABLE I. PoLYMERIZ.4TION

FORMCIULA FOR

5-2820

LATEX

Butadiene 70 Styrene 30 Water 70a Diiso ropylbenaene hydroperoxide 0.22 Mixex goapb 1.25 Dispersing agent 0 1.5 Ferrous sulfate heptahydrate 0.09 Sodium hydrosulfide 0.039 Potassium sulfate 0.4 Potassium pyrophosphate 0.25 lert-Dodecyl mercaptan 0.1 Except where otherwise indicated, polymerizations were carried out for 48 hours in 24-ounce pressure bottles. I n most cases 200 grams of monomers were charged: 1.75 parts of potassium oleate are ordinarily added during t h e polymerization, as a n 18% solution, when conversion of monomers has reached 35%. Polymerization temperature was 5' C. a I n large scale production GO parts of water are used. Potassium salt of a heat-treated, distilled fraction of tall oil: ratio of fatty to rosin acid, about 5 t o 3 by weight. c Potassium salt of condensed alkyl naphthalenesuifonic acid.

TABLE11. CONCEKTRATION BILIZED J-2820 LATEXPI/IADE AKD

Mixed Soapa Parts' 1.1 1.2 1.25d 1.3 1.4 1.2s 1.25

O F L.4RGE P.4RTICLES IS UNST.4WITH VARYING h O U S T S O F S O A ?

DISPERSIKG AGEST

Dispersing .4genta, Parts 1.5 1.5 1.5d 1.5 1.5 1.3

2580

Final Conversion of Monomers, 70 b 38 46

Tt. yo of Particles

over 20,000 A . in DiarneterC 29.8 33.8 44.0 46 39.8 46 31.8 49 36.8 47 34.7 1.4 45 33.7 1.25 l.G 47 30.3 1.25 1.7 41 a Per 100 parts charged monomers. For chemical description see Table I. b Polymerized 24 hours instead of usual 48. 0 Mean of duplicate results on st,ripped latices. Lees than 2% coagulum present in each latex. d Amount ordinarily present in 5-2820 latex.

KO.

INDUSTRIAL AND ENGINEERING CHEMISTRY

December 1954

B

A

P

1,

I

1

2581 C

5 X 106A. I

Figure 1. Photomicrographs of Unstabilized 5-2820 Latices Shortstopped at Various Conversions

SCALE

A. B.

C.

14% conversion 34% conversion 53 YOconvorsion

Latices diluted t o solids content of 2 % before photographing Magnification, 850X Unstrippcd latices Lower row. Stripped latices

Upper row.

ization proceeds, so that more and more material appears in the range of diameters greater than 100,000 A. The fact that thr concentration of particles in the 20,000 to 100,000 A. range remains nearly constant in the later stages of polynierization, despite the growth of many particles to a diameter greater than 100,000 A., indicates that continuing growth or agglomeration of small particles occurs as the polymerization proceeds, thus replenishing the quantity of particles in the intermediate range. A second possibility is that the growth or agglomeration t o sizes between 20,000 and 100,000 A. occurs preferentially in the early part of the polymerization, while direct growth of small particles to sizes greater than 100,000 A. is favored in the later stages. As there is no particular reason for assuming such a change, with increasing conversion, in the mechanism of growth, the hypothesis of a gradual and continuous increase in size appears somewhat more plausible. The approximate time of formation of large particles, and the increase in particle size with increasing conversion, are confirmed by the photographs in Figure 1 taken of latices shortstopped a t 14, 34, and 53% conversion. A sample of each latex, after dilution to a solids content of 2%, was photographed on a Levy microscope slide of the TABLE 111. VARIATIOK O F LARGEPARTICLE CONCENTRATION WITH C O N V E R S I O N I N UNSTABILIZED 5-2820 TYPELATICES type used for blood counts. The use of this type of slide Particles, Wt. % Final Between Wt. of Polymer, Grams ensures t~he inspection of a Conversion of Over 20,000 a n d Between 20 000 Monomers, 100,000 A. 100,000 A. Over 100,000 A. a n d 100,006 A. Over 20,000 A. 'Onstant Of latex, and NO. % in diametera in diameters in diameters In diameter in diameter the rulings - -permit ready cali1 14 0 0 0 0 0 bration of the magnification. 2 18 0 0 0 0 0 Another fact evident from the 3 34 5.4 26.8 3.7 18.2 21.9 4 38 1.5 20.5 1.1 15.6 16.7 photographs is that a large 5 41 21.5 19.1 17.6 15.7 33.3 6 48 15.4 15.9 14.8 15.3 30.1 group of particles, about 5000 7 53 23.5 19.1 24.9 20.2 45.1 to 15,000 A. in diameter, pres0 Mean of duplicate results on stripped latices. Les? t h a n 2% coagulum present in each case. b 200 grama of monomers charged in each polymerization. ent in the unstripoed latex -of 14% conversion, is not

omitted from the recipe, and that small variations in the amount of the mixed soap derived from tall oil, and of dispersing agent present, have little effect. A more complete description of the soap and dispersing agent is given in Table I . The soap is referred to hereafter simply as mixed soap. In order t o determine a t what stage in the reaction these large particles are formed, a series of polymerizations was carried out in which the reaction was shortstopped after varying degrees of polymerization had taken place (Table 111). The particles over 20,000 A. in diameter begin to appear between 18 and 34% conversion of monomers. The data in the fifth and sixth columns show that, a t 34% conversion, these particles are mainly between 20,000 and 100,000 A. in diameter. Particles smaller than 20,000 A. were not investigated. As the polymerization proceeds, increasing amounts of polymer particles over 100,000 A. in diameter appear, but the amount of material between 20,000 and 100,000 A. in diameter remains nearly constant. The total concentration of large particles, therefore, increases with increasing conversion. There is a continuous formation of particles in the 20,000 to 100,000 A. range, which continue to grow as the polymer-


I

1.0 1.5 2.0 2.5 PARTS K-OLEATE STABlLlZER /IO0 MONOMERS Figure 2. Effect of I'arying -4mounts of Stabilizing Soap on Large Particle Content of 5-2820 Latices

0.5

Stabilizer added when conversion of monomers had reached 36 k 6% in each case

observed in the same latex after stripping, or in the stripped or unstripped latices of 34 and 5357, conversion. The particles are probably eniulsified droplets of monomer which are removed on steam stripping, or which dissolve in the polymer particles a t higher conversion (at lo^ conversion there is insufficient polymer present t o dissolve all the monomer, since the polymer can dissolve only one t o two times its own weight of monomer). T h e data plotted in Figure 2 sho\y t,he efl'ect of varying amounts of added stabilizing soap on the formation of very large particles. In each case the soap was added a t a conversion of 36 6%. It is evident that the proportion of such particles decreases as the quantit,y of stabilizer is increased. No appreciable weight of part,icles over 20,000 A. in diameter was found Tvhcn the amount, of stabilizing soap exceeded 1.73 parts-i.e., when the total soap content exceeded 3.0 parts. The conversion of monomers at t'he time of addition of the 1.75 parts of soap was 31%. This quant,ity of stabilizer is ordinarily used in making 5-2820 latex. The data present,ed so far shoiv t,hat large amounts of giant particles form during polymerizations carried out without the addition of stabilizing soap, that the formation of these particles begins between 18 and 3470 conversion, and that it can be minimized by addition of a sufficient quantity of stabilizer when the conversion of monomers has reached 36 f 6%. These data suggested that the very large particles are formed by agglomeration of growing small particles, as t,heir total surface area increases during polymerization to an extent, where it cannot adequately be protected by the soap initially present. This would explain the fact that tlie large particles do not appear until polymerization has attained sufficiently high conversion and that the addition of stabilizing soap prevents the formation of such ary surface coverage). particles (by providing thP n T h e data in Table IT' shov the effect of adding 1.75 parts of potassium oleate as st,abilizer a t varying conversions in a series of polymerizations. Few very large particles form if the stabilizer is added early enough in the polymerization ( a t a conversion of 19% or less). An appreciable quantity of large particles, 8.37, by weight, was present after stripping when the st,abilizer was added a t 29% conversion, and this quantity increased to a maximum of 31.67, as t,he extent of conversion a t the time of addit,ionof stabilizer increased.

*

Vol. 46, No. 12

While the final percentage conversion of monomers was increased when the stabilizing soap was added in the early &ages of the polymerization, the corresponding reduction in t,he amount of large particles formed does not appear to be related to conversion. If these latices had been shortstopped when the conversion wa8 lower, say 607,',, it seems certain that t,he same result-the absence of giant particles would have been obtained. The alternat'ive-that hrge particles are rediapersed int a small particles a t high conversions-appears very improbable. These results are consistent with the data obtained by varying the extent of conversion in unstabilized latices. The fact that it is necessary to add st,abilizer early in tlie polymerization in order t o prevent formation of giant particles is consistent 17-ith. t,he finding that such particles do not form until conversion has reached 18 to 347,. Thus, if the stabilizer is added too late, agglomeration has already occurred, and the stabilizer is ineffective. There is some inconsistency between there data and the data in Figure 2. Some large particles xere formed in this series of experiments when the stabilizer was added a t 29 yo conversion, whereas the data in Figure 2 indicate that the same amount of stabilizer entirely prevents the formation of the large particles, even vhen added soineTvhat later in the polymerization (317, conversion in this case). Honever, the percentage of large particles formed in the former case \vas not very large ( 8 . 3 % ) , and the discrepancy is readily attributable to normal variations among replicate polymerizations. The significant features of the data are the absence of large particles a t lon. conversions in uiistabilized latices, or when stabilizer is added early, and the larycx quantity of such particles which are present a t high conversions in unstabilized latex, or Ti-hen stabilizer is added latr. EFFECT O F ROSIN SOAP OY LARGE PARTICLE FOR;\I.i'ITOU

The above data indicated that prevention of cream formation is essentially a matter of providing sufficient soap to prot'ect the surfaces of the growing particles. I t v a s therefore corieidered that the formation of giant particles might be prevented, without adding stabilizing soap during the polymerization, by the use of a soap which initiates ferver particles per unit, of surface-covering capacity than does the mixed soap ordinarily used in 5-2820 latex. Then. if t,he amount of this new soap is increased t,o such an extent that the same number of particles a,re initiated as are formed in normal 5-2820 latex, more surface protection Trould be provided per part,icle. As the t,otal eurface area at a given conversion l d in a higher depends on the number of particles, this ~ ~ o u result percentage coverage of the total surface area during polymerization. i f sufficient, improvement were obtained, it might be possible t o polymerize !vi1 liout additional stabilizer. This would be of practical, as u ; ~ l as l theoretical interest, since the addition of stabilizer a t a definite conversion adds t o the difficulties in large scale polymerizations. Previous studics of reaction rates and latex viscosities have suggested that a unit weight of the potassium salt, of a dispropor-

TABLE iV. VARJATION I N TIXE OF ADDITIOKO F STABILIZING SOAPIS J-2820 TYPEL.~TICES Conversion at Time of Addition of Stabilizer", XO.

Lie

8 12

12 19 29 31 41 46

Find, Conversion of Uonomers.

70

94 89 89

98 61 56

59 59

_~_ particles, _

Wt. 96 Betxecn Over 20,000 arid Over 100,000 A . in 100,000 -1. in 20,000 A . diaineterb diameter b in diameter 3.0 0.: 2 5 2.6

1.5 1.9 1.0 0.5 2.0 4.5

0.9 0 .5

2.0 7.3 8.1 14.2

17 5

3.4

2.0 3.0

8.3

8.6

16.2 22 0

5.0 20.6 31.6 50 53 Usual stabilizer, 1.75 parts of pot,assium o!eate, added as 15% solution. b Mean of duplicate results on stripped latices. Less than 2 % coagi~lum present in each latex.


December 1954

I

E

D 5 X 1ObA

t

c

R

A

0 1

2583

I

Figure 3.

SCALE A. B.

e.

F

Photomicrographs of 5-2820 Latices

1.75 parts of potassium oleate added as stabilizer a t various conversions 12% D . 31% Latices diluted t o solids c o n t e n t of 2 % before 19% E. 41q0 photographing. Magnification, 925 X 29% F . 53%

tionated rosin acid (rosin soap) initiates only about one fourth t o one third as many particles, in 5-2820 latex, as does the mixed soap ordinarily used. Also, it was found, by soap titration (b, S) of duplicate samples of the same latex, that the surface coverage of polymer per unit weight of mixed soap is only 25% greater than that of the rosin soap. This indicated that the surfacecovering capacity of the mixed soap per particle initiated is considerably less than that of the rosin soap. Therefore, t o test the effectiveness of rosin soap in preventing large particle formation in unstabilized 5-2820 latex, a series of 24-ounce bottles of latex was prepared, in which the rosin soap was substituted for varying amounts of mixed soap. I n the first series, substitution was made on the basis of 3 weight units of rosin soap for each unit of mixed soap removed. S o stabilizer was added during the polymerizations. As shown in Table V, A, the percentage by weight of particles over 20,000 A., in diameter decreases as more rosin soap is substituted for mixed soap on this basis-Le., as the total amount of soap increases. This percentage is zero when the total initial content of soap in the recipe is 2.85 parts or more. Table V, B, shonb the effects of substituting rosin soap for mixed soap on a 4 to 1 basis. Again, the percentage of very large particles decreases as increasing amounts of rosin soap are used, and is zero when the total soap content exceeds 2.75 parts. The use of larger quantities of mixed soap alone is not practical because it results in very viscous latices of small particle size. However, two polymerizations were carried out in which the initial concentration of mixed soap vias increased to 1.85 and 2.25 parts. The latices were extremely viscous and contained 27.7 and 26.77, by weight of particles over 20,000 A. in diameter, respectively, thereby indicating little improvement in stability. The use of a soap such as the rosin soap, which is relatively less effective in initiating particles, thus affords a practical method of eliniinating cream without the addition of a stabilizing soap during the polymerization.

RELATION BETWEEN CREAM AND COAGULUM

The data presented so far may well be interpreted as indicating that cream is incipient coagulum. The same factor which most frequently causes the formation of coagulum during polymerization-the absence of sufficient soap relative to the total surface area of the latex particles-appears t o be responsible for cream formation. However, the problem remains: what factors determine whether cream or coagulum will form in a given polymerization? I n order to vary the severity of the conditions of instability, a series of polymerizations was carried out in which the amount

TABLE1 '.

A.

SUBSTITUTION O F ROSIN S o a p a FOR MIXEDSOAPb IN UNSTABILIZED 5-2820 TYPELATICES

Substitution on 3 to 1 basis

R

Siihstitntion

Mixed Soap08 c , No. Parts 1.25 1.15 1.05 0.95 0.85 0.75 0.65 0.55 0.45 0.35 10 11 0.25 12 0 1 1.28

Rosin Soapa, b , Parts 0 0.3 0.6 0.9 1.2 1.5 1.8 2.1 2.4 2.7 3.0 3.6 0 0.8 1.2 1.6 2.0 2.8

Final Conversion TVt. 70over of Monomers, 20,000 A. in % Diameterd 61 64 67 70 71 66 64 56 55 54 55 55 61 69 66 87 58

61

37.5 27.2 33.4 28.3 23.3 20.4 5.2

11.6 0

0

0 0 37.5 30.9

8.3 4.2 0.0 0.2

85 0.0 Potassium salt of disproportionated rosin acid. For chemical description, see Table I . c Per 100 parts charged monomers. d Mean of duplicate results on stripped latices. Lese than 2 % coagulum present in each oase. 3.6

Q

b

2584


Vol. 46, No. 12

to various conversions in the absence of potassium sulfate. LATICESMADEWITH The highest percentage conPotassium Final Coagulum in W t . % between v e r s i o n a t t a i n a b l e before Sulfate Conversion of Unstripped 20,000 and W t . yo over T o t a l Wt. Chargeda, M onoiners, Latices, 100,000 A. 100,000 A. in oyer 20,000 A. coagulum formed was less than No. Part %a % C in D i a m e t e r % d Diameterc>d in Diameter 35%, indicating that the forma1 0 83 40 0.7 0.3 1.0 2 0.1 77 34 0 0.8 0.8 tion of coagulum owing t o the 3 0.2 76 20 0 10.6 10.6 reduction in electrolyte con4 0.3 69 0.4 33.8 4.6 38.4 5 0.4 65 1.3 31.5 0.4 31.9 tent was not caused by the 6 0.6 55 0.2 14.8 0 14.8 7 0.8 38 0.3 1.8 0 1.8 increased extent of conversion 8 1.0 37 0.3 2.6 0 2.6 of monomers. 0 Per 100 p a r t s charged monomers. B e c a u s e c r e a m i s usually b Including monomers converted t o coagulum. C Percentage based on total of latex polymer plus coagulum. associated with the cold, highd Mean of duplicate results on stripped latices. Less than 1% additional coagulum formed o n stripping. solids type of latex-i.e., with relativelv small amounts of water in the charge recipe-it of electrolyte charged was varied. The quantity of potassium was of interest t o see whether coagulum formation could be pyrophosphate in the recipe was held constant a t the usual induced in unstabilized 5-2820 latex by an increase in the level, 0.25 part per 100 parts of charged monomers, while the amount of water present. The effect of variation of the water potassium sulfate was varied between 0 and 1.0 part (0.4 part content is shown in Table VII. It is apparent that when 90 of potassium sulfate is ordinarily used). No stabilizing soap was parts or more of water are present, considerable quantities of added during these polymerizations. ils shown by the data in coagulum form in preference t o large dispersible particles. When Table VI, a considerable amount of coagulum formed when the 85 parts are present, both the large particles and prefloc are potassium sulfate was reduced from the usual 0.4 part to 0.3 found, while less than S5 parts result essentially in the elimination part or less. With 0.4 part or more present, large particles reof prefloc, which is replaced by the large particles. The slow placed nearly all of the coagulum; as the level of electrolyte was reaction rate in the presence of 60 parts of water makes compariincreased above 0.4 part, the percentage of large particles deson with the other data difficult. It is noteworthy that the uncreased and was very small when the potassium sulfate concencoagulated large particles are mainly over 100,000 A. in diameter tration reached 1.0 part. However, the elimination of large when 85 parts of water are used, and between 20,000 and 100,000 particles a t high levels of electrolyte may possibly be due t o the A. in diameter when less water is present. Thus the sizeof agglomlowered Gnal conversion. Thus, coagulum can be caused t o erates decreases from coagulum to particles over 100,000 A . in form in place of cream by reducing the amount of electrolyte diameter, and then to particles between 20,000 and 100,000 -4.; present, thereby increasing the severity of the conditions of in this case, as the m-ater level is gradually decreased. Increasing instability. Both coagulum and cream were formed in the latex the amount of ~ s a t e r ,therefore, may be considered a means of made with 0.3 part of potassium sulfate. This might be considered as an intermediate TABLE VII. VARIATIOK IS L&XOLTNT O F WATER I N uNST.4BILIZED 5-2820 TYPE LATICES condition of instability. The Final Coagulum in Wt. yo b e t a e e n T o t a l Coagulum large particles in this latex were Water Conversion of Vnstripped 20,000 a n d W t . 70over Plus Particles Chargeda, Xonomers, Latices, 100,000 -4. in 100,000 A. over 20,000 A. over 100,000 A. in diameter, SO. Parts %b %b in Diarneterb, C i n Diameterbrc in Diameter, % while the large particles in 1 36 0.6 60 0.9 0 1.5 latices made with more elec2 70 1.1 24.1 31.2 61 6.0 0 . 9 3 75 3 1 . 8 3 7.3 4 . 6 68 trolyte were mainly between 4 26.4 34.6 71 1.6 5.6 80 82 14.1 28.7 5 2.6 12.0 85 2 0 , 0 0 0 a n d 1 0 0 , 0 0 0 ,4.in 37.5 6 90 83 0 1.5 36 diameter. An interesting way 39.1 95 38 0.3 7 89 0.8 38.0 38 0 0 98 8 100 of looking at the data is t o 9 101 0.6 1.4 40.0 38 110 103 0 10 120 38.5 1 . 5 37 consider coagulum as extremely 103 36 1.6 130 11 38.8 1.2 large particles. As the elec28.4 0 102 28 140 12 0.4 trolyte content is increased, a Per 100 parts charged monomers. b Based o n t o t a l polymer formed, including coagulum. Less t h a n 270 additional coagulum formed in each case the size of the large particles on stripping. 0 M e a n of duplicate results on stripped latices. decreases from coagulum t o particles larger than 100,000A., then to particles between O F POTASSIT7lcr SULFlTE I N UNSTABILIZED 5-2820 TYPE TABLE S'III. \'-4RIATION I N AMOUNT 20,000 a n d 1 0 0 , 0 0 0 A. i n LATICESaf.4DE WITH 100 P-4RTs O F WATER diameter. Additional As the total amount of W t . 7% over Coagulum Wt. % between Coagulum in Potassium Final 100,000 A. Formed 20 000 a n d Unstripped Sulfate Conversion polymer produced increased 100,600 A. i; in Latices, on Stripping, Charged", Monomers, with decreasing electrolyte conNo. Parts %b %b %b Diameter Diameter b, 0 centration, it might be argued 3.1 91 11 11 1.1 1 0 4.1 0.4 0 28 93 2 0.4 that the coagulum forms late 2.4 0.2 0.4 32 82 3 0.6 1.4 2 7 1.0 28 87 4 0.7 in the polymerization. There2.9 0.4 5.2 25 66 5 0.8 fore, i t might form t o a greater 9.2 1.5 0.4 24 64 6 0.9 27.6 0.9 5.4 0 . 4 1 . 0 63 7 extent when less electrolyte is 0 . 5 2 4 . 1 1 9.6 0 . 1 73 1.1 8 0.5 28.8 17.8 0.1 1.2 65 9 used because of the higher 26.2 1 2.5 0 . 0 0 . 6 72 10 1.3 conversion, rather than because 11.1 20.2 0.1 0.8 65 1.4 11 22.0 9.4 0 . 2 0 . 9 67 1 . 5 12 of the increase in the severity a Per 100 p a r t s charged monomers. of the condition of instability. b Based on total polymer formed, including coagulum. c Mean of duplicate results on s t r i p ~ e d latices. To check this possibility a series of polymerizations was carried

TABLE VI. VARIATION

SULFATE I N UNST.4BILIZED 5-2820 TYPE 70 PARTS OF WATER

I N AJf OUhT O F POTASSIUN

bl

December 1954


increasing the severity of the condition of instability, which in turn results in larger agglomerates. However, the total percentage by weight of coagulum plus large particles (last column, Table VII) does not increase markedly with increasing amounts of water. Therefore, the presence of more water appears to favor a higher degree of agglomeration of a given number of particles, rather than an increase in the number of particles which undergo agglomeration to diameters larger than 20,000 A. T o determine whether the higher conversions attained, when higher amounts of water were used, were responsible for the coagulum formation, a series of polymerizations was carried out in which the degree of conversion of monomers was varied in 5-2820 latex made with 100 parts of water in the recipe. This was done by varying the time of reaction before shortstopping. The highest conversion attainable, before considerable amounts of prefloc formed, was only 31%. This showed that coagulum formation in the presence of increased amounts of water is not caused by the higher percentage conversion of monomers. Because an increase in the amount of water charged, without a corresponding increase in electrolyte content, obviously results in a decrease in the electrolyte concentration, it was considered desirable to determine whether the formation of cream, rather than coagulum, in the presence of more water could be induced by a suitable adjustment in the amount of electrolyte. The results of a series of experiments in which the amount of electrolyte was varied in unstabilized 5-2820 type latices made with 100 parts of water (as compared with the usual 70 parts) are shown in Table VIII. As was found in the experiments in which 70 parts of water were present (Table VI), it is evident that coagulum is replaced by large particles when a sufficiently high concentration of electrolyte is used in the presence of 100 parts of water. However, a minimum of 1.0 part of electrolyte was required in this case, as compared with 0.4 part when 70 parts of water were used. Electrolyte concentrations cannot be directly compared because 0.25 part of potassium pyrophosphate was present in each series, and its effectiveness relative to that of potassium sulfate is not accurately known. It appears that electrolyte concentration determines whether cream or coagulum will form a t a given water level, as a higher amount of electrolyte prevents coagulum from forming when the amount of water is increased. It is also apparent from Table VI11 that when 0.8 or 0.9 part of potassium sulfate is used, appreciable amounts of both coagulum and large particles are formed. At these intermediate levels of electrolyte, the large particles are mainly over 100,000 A. in diameter, whereas most of the weight of large particles appears in the range 20,000 to 100,000A. when more electrolyte is present. Thus once again a decrease in the size of agglomerates i5 observed as the condition of instability is made less severe.

;E

5 10

11

12

Coagulum in Unstripped Latices,

%b

%b

Wt. Yo of Particles over 20 000 A. in Diameterbv c

61 62 62 63 60 60 58 63 59 59 57 64

4.9 1.6 0.5 0.2 0.4 0.03 0.5 0.2 0.08 0.05 0.05 0.2

23.5 32.4 35.8 44.1 28.6 29.6 35.6 36.5 35.4 34.1 42.2 37.7

Less t h a n 1%

EFFECT O F CROSSLINKING IN POLYMER ON FORMATION OF CREAM AND COAGULUiM

The attempt to remove the modifier (tert-dodecyl mercaptan) from 5-2820 latex in large scale polymerizations, and thus produce a latex containing a polymer of high molecular weight, results in the formation of coagulum. Because this latex ordinarily contains some cream, it appeared conceivable that the unmodified latex might tend to agglomerate more completely, for some unknown reason, than the same type of latex containing polymer of lower molecular weight, as made in the presence of modifier. In view of this, an investigation of the mechanism of destabilization in the absence of modifier appeared t o be of considerable interest. To determine the effect of modifier concentration on the formation of large particles and coagulum, a series of polymerizations was carried out in which the modifier concentration was varied between 0 and 0.16 part per 100 of monomers, in 5-2820 latex. (Normally 0.10 part of modifier is present.) No stabilizer was added during these polymerizations. The results recorded in Table I X show that almost 5% by weight of coagulum forms when the modifier is omitted from the formula. A smaller amount of coagulum forms when 0.01 part of modifier is present, and 0.02 part or more reduces the amount of coagulum t o a small value. The data on weight percentage of particles over 20,000 A. in diameter, given in the last column of Table IX, indicate that the formation of coagulum in the absence of modifier occurs a t the expense of very large particles, as there is a decrease in the percentage of such particles when modifier is omitted. One may best interpret these data by assuming that the absence of modifier influences the mechanism of agglomeration in such a way that it tends to continue until some coagulum is formed. The fact that only part of the large particles are replaced by coagulum does not

X. VARIATION IN AMOUNT OF DIVINYLBEN ZEXE ADDEDTO UN~TABILIZED J-2820 TYPELATICES Coagulum in Unstripped Latices,

Additional Coagulum Formed on Stripping,

NO.

%b

%b

%b

1

0.Q 0.1 0.5

0.4 0.4 0.4 0.4 0.4 0.4 0.6 0.6 0.6 0.6 0.6 0.6

61 69 50 59 68

1.1

2

1.0 3.0 5.0 0.0 0.1 0.5 1.0 3.0 5.0

0.01 0.02 0.03 0.04 0.05 0.06 0.08 0.10 0.12 0.14 0.16

Final Conversion of Monomers,

a Per 100 parts charged monomers. b Based on total polymer formed, including coagulum. additional coagulum formed on stripping these latices. 0 Mean of duplicate results on stripped latices.

Final Conversion of Monomers,

12

C

Yo. 1 2 3 4 5 fi 7 8

tertDodecyl Mercaptan, Parts 0

Potassium Sulfate ChargedQ, Parts

11 a b

TABLE IX. VARIATION I N AMOUNT OF MODIFIERIN UNSTABILIZED 5-2820 TYPE LATICES

Divinylbenzene Charged'", Parts

3 4 5 6 7 8 9 10

2585

76

56 52 53 58 65

Per 100 parts charged monomers. Based on total polymer formed, including coagulum Mean of duplicate results on stripped latices.

0.6 0.9 1.0 19.0 28 0.2 0.6 0.4 1.3 13.0 26

Wt. % between 20,000 and 100,000 A. in Diameterb. 0

Wt. % over 100,000 A.

in Diameterbf

0

Total Coagulum Plus Particles over 20.000 A. in Diameter, %b

2586


Vol. 46, No. 12

mixed soap on a 3 to 1, or a 4 t o 1, basis by weight are shown in Table XI. As exMixed Rosin Final Coagulum in rVt. yo between pected, large amounts of both Unstripped 20,000 a n d K t . % over Soap Conversion of Soap Llonomers, Latices. 100,000 A. in 100.000 A . in ChsrgedC, Chargedc, dispersible giant particles and Dianieterds j Diameterd, f %d 'Zed, e No. Parts Parts coagulum formed when only 70 2 2 1 6 . 3 0 0 7 3 1 1.25 A. Substitution 0.5 0.6 69 28 0.0 on3 to1 2 1,Od t'he mixed soap v a s used. As 1 . 2 17 0 . 5 89 B O basis 3 0.85 increasing amounts of rosin 4 0.65 1.5 57 8 0.0 1.8 2,4 58 1 1 6 5 0.46 3.2 soap were substituted-Le., a8 I 2 0.25 3.0 58 3 5 1.8 3.78 0.0 78 1 0.0 1.0 t'he total amount of soap present increased-the a,mount of agglomerates decreased, and 0 RR 1 .i; 57 2.0 was small when the total soap content exceeded 2.8 t,o 3 parts. 0 For chemical degcription see Table V. d Based on total polymer formed, including coagulum. e LESSt h a n 1% additional coagulum formed on stripping. b For chemical description see Table I . These results are very similar c Per 100 parts charged iiionomers. f Mean of duplicate results on stripped latices. t o those obtained without the use of divinylbenzene (Taljla V), except that prefloc formeti in place of some of the cream when irisufficient rosin soap V-US conflict ivith the observation that some modifier must be used in large scale production of this type of latex, as tolerances for present. The amount, of rosin soap needed to prevent t h c formation of large agglomerates was nearly the same whether or coaguluni are very small as compared with cream. Omission of modifier results in a crosslinked polymer of high not divinylbenzene was present. However, in the presence of the crosslinking agent, the forrnatiori of very large agglomerates molecular weight,. I n order t o st,udg further the effect of cross(coagulum) was favored. linking on the mechanism of agglomeration, a series of 5-2820 These data give addit,iorial support to the hypothesis t h a t t,he type latices was prepared in which varying amounts of a crosseffect of crosslinking in the polymer is to alter the mechanism of linking agent, divinylbenzene, were present in the polymerization agglomerat,ion in some manner so as to prevent it from stopping recipe. 30stabilizing soap was added during t,hepolymerization in tliese experiments. Tn the first half of Table X results are at' the stage where the particles are large, but dispersible. About the same percentage of the polymers appears as agglomeratw recorded which riere obtained with the use of the usual amount when crosslinking is introduced, and no increased difficulty is enof potassium sulfate, 0.4 part. in the charge recipe. Dat,a in the countered in at'tempting to produce a stable latex by suitable second half of the table correspond to the use of 0.6 part of pochoice of soap, but the formation of larger agglomerates is fnvored. tassium sulfate. I n each case little coagulum but large amounts Although no definite conclusion can be drawn as t o the effect of of giant part,icles were present in the latices made without dicrosslinlring, one possibility is that the difference in mechanism of vinylbenzene. However, with 0.4 part of electrolyte present, the agglomeration is related to t,he fact that' a crosslinked polymer i,s use of 1 part of divinylbenxene resulted in the formation of appreless cohesive than a linear polymer of the same type! and that ciable amounts of coagulum, while large quantities of coagulum cohesion is related to the ability of particles to fuse into larger were present vhen 3 parts or more of divinylbenzene were used. spherical particles. As was found earlier, in reducing the modifier content, the forT o determine whether a general increase in particle size is asmation of coagulum was accompanied by a decreaee in the amount sociated with the formation of cream, in each of the ten experiof dispersible large particles, thus giving further evidence that 3 ments represent'ed in Figure 2, the average part,icle size of t,he highly crosslinked polymer tends to continue agglomera,tirig until latex, after removal of cream, was determined in the spectrocoagulum is formed. phot,ometer ( 4 ) . I n t h e first experiment, in which no stabilizing I n a separate series of experiments it was found t h a t 97.57, soap was added, t h e average particle diameter was 1850 A. In of the polymer made on thc 5-2820 recipe as soluble in benzene; the other nine experiments, the average particle size varied within however, the presence of only 0.1 part of divinylbenzene in the a narrow range of diameters, 1480 to 16.10 A , , and showed no charge recipe caused the formation of 61% gel (insoluble polymer) trend with increasing stabilizer. The consistency of this average and, as might, then be expected, the presence of 1 part of divinyldiameter indicat,es that agglomeration, once started, tends t o benzene resulted in the format,ion of a polymer that was almost continue until extremely large particles are formed; any apprecompletely insoluble (9570 gel). Thus, the polymer niust be ciable quantity of particles of intermediate size mould have retightly crosslinked if an appreciable replacement of cream b>sult'ed in marked increases in the average diameter, as obtained prefloc is t'o occur. by measurement of turbidity, which weights large particles It does not appear that the major effect of crosslinking is to heavily. cause more of the small original particles to agglomerate; there is some increase in the total of large particlcs plus coagulum when ACKNOWLEDGMENT 5 parts of divinylbenzene are present, but most, of the coaguluiii This work was performed as a part of the research project formation shon-n in Table X is accompanied by a corresponding sponsored by the Reconstruction Finance Corp., Office of Syiidecrease in t,he amount of dispersible, large particles. One may thetic Rubber, in connection with the government synthet'ic conclude, t,herefore, that the over-all instability is not greatl?, rubber program. affected by crosslinking of the polymer. The important, effect apLITERATURE CITED pears to be on the mechanism of agglomeration. This conclusion ie given additional support by the experiments discussed nest. (1) Brown, R. W., Messer, W. E,, and Howland, L. €JND. I., E.v-n. T o look further into the mechanism of agglomeration of latex CHEM.,45, 1322 (1953). (2) Harkins, W. D., J . Am. Chena. Soc., 69, 1428 (1947). particles containing crosslinked polymer, another series of ex( 3 ) Xlaron, 9. H., Elder, M. W., and Ulevitoh, I. S . , J . Colloid S e i . , periments was performed based on the use of divinylbeiizene. 9,89, 101-(1954). Increasing amounts of rosin soap were substituted for part of the (4) llaron, S. H., and Ulevitch, I. X., privatc communication to mixed soap in unstabilized 5-2820 type Iatices containing diOffice of Rubber Reserve, Recoiistruction Finance Corp, ( 5 ) Xisonoff, A.. Messer, W. E., and Howland, L. H., Anal. Cliem., vinylbenzene. The experiments werc similar to those sum26,8513 (1954). marized in Table V, except t,hat 3 parts of divinylbenzene were ACCIP-~U July ~ J 24, 1054. REUEIVED for review M a y 6, 1954. present in each bottle. The effects of substituting rosin soap for

TABLE XI.

SUBSTITUTION O F R o s n Soapa % O K MIX^ Soapb IY UNST-~BILIZED J-2820 TYPE L4TICES >I %DE T I T H 3 P l R T S O F DIT I S Y L B E N Z E S E C

Mechanism of Formation of Cream in GR-S Latex

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