406
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 43, No. 2
6.2 to 6.5. The esterase might be involved in this process, Further experiments will have to be carried out in order to elucidate this matter.
( 5 ) Haan-Romans, L. N. S.de, ibid., XXV, 346 (1950). (6) Iwamoto, J., J. h‘oc. Chem. Ind. Japan, 33, mppl. 409-11 (1930). (7) Klein, G., “Handbuch der Pflanzenanalyze,” Vol. IV., Part 2,
Literature Cited
(8) Oppenheimer, “Die Fermente und ihre Wirkungen,” p. 510, Leipzig, Georg Thieme, 1924. (9) WillstBtter, R., Waldschmitt-Leita, R., and Memmen, F., 2.physiol. Chem., 125, 93 (1923).
Dunstan, H. W., Proc. Chem. Soc., 23, 168 (1907). (2) Gerber, M., Compt. rend., 152, 1611 (1911). (3) Gils, G. E. van, Arch. Rubbercult. Nederland.-lndit?,25,383 (1941). (4) Gils, G. E. van, Trans. I n s t . Rubber Ind., XXIII, 74 (1947). (1)
Vienna, J. Springer, p. 942, 1933.
RECEIVED October 3, 1950.
Creaming Latex with Ammonium Alginate INFLUENCE OF PARTICLE SIZE lEErnst Schmidt and R. H. K e l s e y The Firestone Tire & Rmbber Co., Akron, Ohio
T
the various synthetic latices. IIE process of conoenI n view of the great variety of particle sizes exhibited by The first requirement in the modern synthetic latices it was desirable to obtain more trating natural rubber study of creaming as a funcextensive and systematic information regarding the inlatex with the aid of creamtion of particle sine wm a set fluence of particle size on aided creaming than is available ing agents is of considerable of latex sample8, which varied in the literature. technological importance on in particle size but in no other It is shown that the concentration of ammonium algithe rubber plantations in the respect. nate required forreversible clustering and creamingof GR-S Middle and Far East. This It has been shown (1, 8, 9, latex increases regularly with decreasing particle size of process has gained interest in f f ? ) that the particle size of the latex. Modifying the stability of the latex without recent years in this country GR-S latex can be increased changing the particle size is of little consequence in creamas a practical means for conafter polymerization by addiing. The observed relation between particle size and centrating synthetic latices, tion of e l e c t r o l y t e s . The creaming agent requirement holds for GR-S latices made many of these cannot be conelectrolyte concentrations rein different emulsifier systems, for ammonia-preserved centrated by centrifugation quired to produce the desired Hevea latex, and for silica hydrosol, over a wide range of on a practical scale, primarily changes in particle size of the particle sizes (200 to 9000 A. in diameter). It is shown because of their small particle G R S latex used in the greater that Hevea latex can be fractionated according to particle size. The manner in which part of this study, were desize by successive creaming operations. creaming of natural latex is termined in a preliminary exThe results of this study suggest the general principle influenced by various factors, that the influence of particle size on the reversible agglomperiment. Theresultsshowed such as time, mechanical that the desired variation of eration and creaming of colloidal dispersions is predomiagitation, temperature, connant over that of stability and the chemical nature of the particle size could be acconicentration and type of creamplished by using a constant dispersion. ing agent, rubber content of quantity of salt and varying the latex, pH, and addition of only the contact time. various substances, has been A 60140 butadiene-styrene GR-S latex D-229 (Type X-435) the subject of a great number of investigations (10). However, emulsified with 6.6 parts (per 100 polymer) of sodium Dresinate less extensive information has been published regarding the in731 (the salt of a disproportionated wood roein) was used in the fluence of particle size on the creaming of latex, particularly the following experiments. This latex, produced on the Firestone creaming of synthetic latices. Defense Plants Corporation plant, was polymerized a t 5 ” C. in That the creaming of natural latex by means of creaming agents is influenced by particle size was recognized by McGavack (4), the presence of cumene hydroperoxide, iron pyrophosphate, and dextrose. Portions of the latex were mixed with increasing quanwho studied the particle size distribution of concentrates of pretities of sodium chloride and simultaneously diluted to a constant served Hevea latex prepared by creaming with different amounts polymer content of 10.4%. Samples were withdrawn after varyof creaming agent. ing time intervals and analyzed for particle size by means of the McGavack ( 4 ) found that incomplete creaming of normal electron microscope. It was found that particle growth had taken Hevea latex gives concentrates of larger average particle size than place in those samples which contained more than a certain critical that of the original latex and that the average particle size of the concentration of sodium chloride. The resulting particles were cream is increased as the amount of creaming agent is reduced. spherical in shape and showed no evidence of being simple cluster8 A number of recent patents ( 1 , d , 7, 9) reveal that increasing of the original small particles. the particle size of synthetic latices leads to creamed concentrates The rate of particle growth was slow a t about the critical salt of increased rubber content. concentration and increased as the salt concentration was inIt was believed that further systematic contributions to the creased. Below the critical salt concentration no particle growth knowledge concerning the influence of particle size on creaming of ww observed over a period of several weeks. These results are latex, should be of interest, particularly in view of the great illustrated in Figure 1. variety of particle sizes and particle size distributions exhibited by
INDUSTRIAL AND ENGINEERING CHEMISTRY
February 1951
40'1
100
y;!
80
I" w
N o CI ON AWEWS PHASE H O U R S
60
a
a
2
40
W
0
g *
v)
f
20
0
0,I
I
TIME
Figure 1.
10 (HOURS)
1000
100
1000
0
2000
3000 DIAMETER
Particle Growth of GH-S Latex D-229 a t Various Salt Concentrations
Figure 2.
4000
5000
6000
7000
[A.U.)
Additive Mass us. Particle Diatfieter of GR-S Latex D-229
be
Thus, by selection of a suitable salt concentration a conveniently slow rate of particle growth could be obtained. The particle growth could be arrested a t any desired stage by diluting the latex t o a salt concentration below the critical value. A set of latex samples of varying particle size, but constant composition, was prepared by this method and used in the following creaming experiments. G R S latex D-229 was diluted with a 1.64% spdium chloride solution to B polymer content of 10.4%, resulting in a final salt concentration of 0.176 gram equivalents per liter of aqueous phase. T h e mixture was gently agitated in a sealed bottle for 264 hours. Portims were withdrawn at given time intervals, and determinations were made of particle size and of surface tension. Cumulative mas9 distribution curves for these samples are shown in Figure 2. The mass median diameter and surface tension as a function of time are given in Table I The particle growth was accompanied by a n increase of the soap (sodium Dresinate 731) content of the aqueous phase of the latex as found by analysis of the latex filtrate obtained with the aid of ceramic filters. Soap desorption from G R S latices under the influence of salts had previously been suggested by Maron (6),who found that the surface tension of GR-S latices decreased on addition of salts. The surface tension data in Table I confirm this observation.
Table I.
centration was greatly influenced by the particle size of the latex. The extent t o which the skim was depleted by any given concentration of creaming agent was greater, the larger the particle size of the latex. A higher concentration of creaming agent wan required to cream a given proportion of a small particle size latex than of a large particle size latex. Figure 3 shows that a creaming agent concentration of 0.1% on the aqueous phase, which caused about 75% depletion in the skim of the largest particle size latex, caused practically no creaming of the smallest particle size latex. Na GI Content of Cle@m!npMixtures 0.0808 Mol/ Liter
MASS MEDIAN PARTICLE DIAMETER ( A U
Particle Growth of GR-S Latex D-229 i n Presence of 0.176 N NaCl (on Aqueous Phase)
Time, Hours 0 24 48 72 96 168 216 264
Particle Siqe (Mass Median Diameter), A. 780 850 1140 1140 1370 2650 3100 3600
Surface
Tension, Dyne/Cm. 57.9 62.0 47.8 42.4 38.2 35.3 3i:7
0.1
0.2
0.3
0,4
0.5
AMMONIUM ALGINATE (% ON AQUEOUS PHASE)
Figure 3. Effect of Particle Size of GR-S Latex D-229 on Creaming Sodium dresinate emulsified 60/40 butadiene styrene
4
copolymer
Creamlng Experiments Portions of the salted GR-S D-229 latex were withdrawn after same time intervals at which the particle size determinations were made. Each of these portions was then divided into subportions which were diluted with water and various amounts of a 3% ammonium alginate (Superloid) solution to a polymer content of 5.15% and a salt concentration of 0.0808 gram equivalents per liter of aqueous phase. This salt concentration was sufficiently below the critical salt concentration t o cause no further particle growth during the following creaming procedure: The mixtures were stirred for 5 minutes and allowed to cream for 3 days. Cream and skim were separated and analyzed for polymer content. The cesults are given in Figure 3. The lower family of curves illustrates the change of polymer content of the skim layer with increasing concentration of ammonium alginate. The concentration of polymer obtained with a given creaming agent con-
To obtain information regarding the possible effect of changing the emulsifier system of GR-S latex on the creaming behavior, the following experiment was carried out: The particle size of a 70/30 butadiene-styrene GR-S latex No. 826, emulsified with 5 parts of potassium castor oil soap and polymerized at 50' C. in the presence of potassium persulfate, was modified by allowing the latex t o age for 3 weeks in the presence of 0.355 mole per liter of sodium chloride (on the aqueous phase). This treatment increased the mass median diameter of the particles from 720 t o 1100 A. Cumulative mass distribution curves for these latices are shown in Figure 4. A higher sodium chloride concentration was required to increase the particle size of the castor oil soap emulsified latex to a given extent, than was required for the sodium Dresinate e m u l i fied latex. Samples of the fresh and aged latex were diluted t o a polymer content of 5.15% and a sodium chloride concentration of
INDUSTRIAL AND ENGINEERING CHEMISTRY
408
P
J"
._.
TIME OF AGING WITH 0 3 5 5 M NoCl
ON AOUEOUS PHASE
I-
5
20
the creams; thus moderate changes of the latex stability are of little consequence in creaming, if the particle size remains constant. To obtain further information regarding the influence of particle size on creaming, for a different polymer and a wider range of particle sizes, natural latex was used in the subsequent experiments. I n order t o prepare modified Hevea latex fractions of large particle size, McGavack ( 4 ) used varying quantities of creaming agent. He found that concentrates prepared with smaller quantities of creaming agent than those required for complete creaming had larger average particle diameters than the original latex and
I A.U,)
DIAMETER
Additive Mass us. Particle Diameter of GR-S Latex 826
Figure 4.
0.163 iV, and creamed with varying amounts of creaming agent. Figure 5 shows that less ammonium alginate was required to cream the large particle size latex than the small particle size latex. It will be shown later that the creaming agent requirement was about the same as found for Dresinate emulsified latex of same particle size. The quantity of sodium Dresinate present in GR-S latex D-229 covered only 64% of the surface area of the latex particles. T h e surface coverage was calculated from the known value (6) for the surface area occupied by one molecule of sodium Dresinate (42.6 A.2) and the specific surface of the latex part,icles determined with the electron microscope. Titration of the latex with sodium Dresinate, using surface tension as a criterion for surface saturation (5)gave good agreement with the calculat'ed value. Since the particle growth in the presence of a constant amount of sodium chloride was accompanied by desorption of soap from the latex particles, their surface coverage with soap, and as a consequence their surface charge and stability, was probably altered. Hence the possibility had to be considered, t,hat the observed differences in crea.ming of the GR-S latices of different particle size were due not primarily to their particle sizes but rather t o differences in stability caused by other factors. I n order to study this question, the stability of the original GR-S D-229 latex was
NoCl
Vol. 43, No. 2
MbSS MEDIAN PARTICLE DIAMETER
Content of Creoming
Mixtures: 0.163 M o l / L ~ I e r on
( A.
Aqueous Phase 0
u.
)
Ill0 720
To-\ k0 I
0.I
02
>c-
* LI e . ,
0.3
0.4
l
z
o
o
6.6 6.6
0,027
A
ry
v)
3 2 I
0
1
COSOS
,
,
0.1
0.2
6 6 IC. 3
LA0.3
0.4
&~
0.5
AMMONIUM ALGINATE(% ON AQUEOUS P H A S E )
Figure 6. Influence of Salt and Sodium Dresinate on Creaming of GR-S Latex D-229 of Constant Particle Size
that the average diameter of the creamed particles depended O L I the amount of creaming agent. From this, McGavack concluded that it would be possible t o fractionate latex by selective creamin,< into portions of different average particle diameter. H e considered only particles above 4500 A. in diameter, which was taken as the limit of resolution of his light microscope. Fractionation oi normal Hevea latex was studied in greater detail by preparing a greater number of fractions, by investigating the whole range of particle bizes, and analyzing the particle size distributions b j means of the electron microscope. Normal ammoniated Hevea latex v a s diluted with a 0.6% solution of ammonia and a small quantity of a 3% ammonium alginate solution (0.031%) to a rubber content of 5.6%. The mixture was allowed t o cream for 3 days. After this period of time a distinct cream layer had formed with a visible boundary separating it from tlie milky skim. The layers were separated and analyzed for rubber content. To the skim portion was added
0.5
AMMONIUM ALGINATE(% ON AQUEOUS P H A S E )
Figure 5 .
Creaming of GR-S Latex 826 of Different Particle Size
Castor oil snap emulsified 70130 butadiene-styrene copolymer
Table 11. Particle Size Fractionation of Diluted Normal Hevea Latex by Creaming with Ammonium Alginate Rubber R ~ Size~of Creamed ~ e4mmonium D r y Rubber in Particles, A. Alginate, Cont,ent,, %-yo on Prior t o % of Mass Numb7 Frac- Aqueous creamTotal median modal tion Phase ing Cream Skim Rubber diameter diameter 4.32 27.8 L60 62.2 12,800 11,600 1 0,031 41.1 GO.0 1.07 8,100 5,800 2 0,062 4.28 1.02 15.1 4,400 3,500 1.96 62.3 3 0.103 0.38 9 . 9 2,900 2,600 4 8 . 2 4 0,164 1.01 0.04) 5.4 0.37 46.0 2,300 2,100 5 0.224 G 0.343 , . .. 1,600 1,300 7 0.463 .. ., 0.7 1,280 1,150 8 0.583 .. ,. .. 1,160 870
cream,
varied in the follo~~-ing experiments without changing its particle size: one portion of the latex was stabilized with just enough sodium Dresinate to raise the surface coverage of the latex particles from 64 to 100%; and the stability of another portion of the same latex was diminished by adding various quantities of sodium chloride, but less than that required for particle growth. The results (Figure 6 ) shorn-that none of these means had any marked influence on the creaming agent requirement or the solids content of
.. ..
)
~
~
INDUSTRIAL AND ENGINEERING CHEMISTRY
February 1951
Control
2
1
6
7
Figure 7.
409
8
Fractionation of Hevea Latex by Creaming. 64OOX
QI
a small increment of ammonium alginate solution, and the mix-
dure ture was Ras subjected repeated eight to a similar times increaming succession operation. as described This in proceTable obtained in these eight creaming opera11. The tiqns were analyzed for particle size by means of the electron microscope. The electron micrographs in Figure 7 show t h a t pronounced fractionation according t o particle size had taken place. Light microscope examination of the skim layer obtained in the first creaming step, which still contained 72% of the original rubber, showed only individual particles and complete absence of clustered latex particles. The corresponding cream, on the other hand, was observed t o contain reversible clusters of particles, which redisperse on dilution, as previously noted and described by many workers (11). Similar observations were made with the cream and skim obtained in the three subsequent creaming steps.
These observations and the electron microscopic particle size analysis of the eight creamed fractions indicate t h a t fractional creaming Of Hevea latex is preceded by selective clustering of the latex particles according t o particle size. Cumulative mass distribution curves prepared from the electron micrographs of the eight fractions (Figure 8) reveal considerable overlapping of the particle size ranges of adjacent fractions. This is, in part, because the clustered creamed latex particles are not dispersed in a rubber-free, aqueous medium, but rather in one which has the same composition as the underlaying skim. The cream layer, therefore, contains a certain amount of unagglomerated particles of all sizes found in the skim. The uniformity and effectiveness of fractionation should therefore depend not only on the number of creaming steps but also on the initial rubber con-
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
410 100-
10,000 DIAMETER
I5.000
20,000
(A.U.)
Figure 8. Additive Mass DS. Particle Diameter of Hevea Latex Fractions
tent of the creaming mix. Consequently, the best results should be obtained with highly diluted latex. Normal Hevea latex and two small particle size fractions, prepared in the same manner as fractions 3 and 4 in Table 11, were diluted t o 5.6% rubber content and creamed with varying amounts of ammonium alginate. The results given in Figure 9 show increasing creaming agent requirement and decreasing optimum cream concentration, with decreasing particle size, as w m found for the GR-S latices. I n plotting Figure 9 correction was made for the residual creaming agent in the cream fractions 3 and 4. The lower limit of the particle size range investigated in the creaming experiments with different latices was about 600 A. It was desirable to extend this range toward lower values. Since latices of such small particle size were not available, a silica hydrosol (Ludox from E. I. du Pont de Nemours and CO., Inc.)
-
5
\.
% O t -A
A '
hydrosol, showing particles of 100 to 200 A. in diameter. No attempt was made to determine whether any separation according to particle size had taken place. The results of the creaming experiments with latices of different type and particle size, presented in Figures 3, 6 , and 9, indicate clearly that the concentration of creaming q e n t required for creaming increases regularly with decreasing particle size of the latex. I n order to obtain accurate information about this relationship, disperse systems of uniform particle size would have ta be studied. The GR-S latices of modified particle size used in the creaming experiments were not uniform in sim (Figurn 2 and 4). Considering a heterodispersc system, any concentration of creaming agent, which is lower than the minimum concentration required for creaming of all particles, should cream only those above a certain size, whereas the particles below this size should remain
30
t
ob
I
I
I
1.0 1.5 AMMONIUM ALGINATE ( % ON AQUEOUS PHASE) 0.5
Figure 10. Creaming of Silica Sol with Ammonium Alginate
0
5
Vol. 43, No. 2
30
0
AMMONIUM ALGINATE(% ON AQUEOUS PHASE)
Figure 9.
Effect of Particle Size of Hevea Latex on Creaming
was used. The silica sol was diluted and mixed with varying amounts of a 3% ammonium alginate solution. The creaming mixtures, which had an initial silica content of about 5% by volume (or 11% by weight), separated into two distinct layers on standing. The silica contents of the separated layers are given in Figure 10. The concentration of ammonium alginate necessary t o cream half of the silica was much higher than for any of the latices investigated. Electron micrographs of silica particles of the concentrated bottom layer and of the dilute top layer had superficially the same appearance as those of the original silica
in the skim. Hence, in expressing creaming of all the dispersions investigated as a function of particle size, the size of the smallest creamed particles of each dispersion was plotted against the creaming agent concentration (Figure 11). The size of the smallest creamed particles in the Hevea latex fractions was obtained from the particle size distribution curves in Figure 8. Correction was made for the known quantity of smallest particles, originating from the skim portion, present in the cream layer. The size of the smallest creamed GR-S particles was obtained indirectly from the particle size-mass distribution of the latex prior to creaming (Figures 2 and 4) and the extent of polymer depletion of the skim, caused by a given concentration of creaming agent, under the assumption that all creamed particles are larger than the largest particles left in the skim. The values for GR-S latices given in Figure I1 actually represent the plot of massmedian particle bize of the uncreamed latex versus the ammonium alginate concentration required for creaming of half of the original mass of rubber. The particle size distribution of the silica sol was not established, as pointed out above. The value plotted in Figure 11 represents only a rough estimation of the mass-median diameter. Figure 11 shows that there is a pronounced, consistent relation between particle size and creaming agent concentration, which holds for a wide range of particle sizes and for different disperse systems. The results illustrated in Figure 11 were obtained with dispersions of constant and relatively low concentration (about 5% by volume), and different results might be obtained with different lat,ices by creaming a t high initial rubber contents.
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
Februray 19frl
i'l
Influence of Particle Size on Pruperties of Cream
D-229 626
0
GR-5
X
GR-S
A
HEVEA COLLOIDAL SILICA
0.8
Q8
i
0.4-
411
\
In actual creaming practice, it is of great importance not only t o recover a high proportion of the total rubber in the eream but also to obtain a cream of the highest possible concentration. It is well known (1, 9, 7, 9) that increasing the particle size of synthetic latices results in creamed concentrates of higher rubber content (Figure 3). Similar results were obtained with Hevea latex fractions of different particle size (Figure 9). Van Dalfsen ( I O ) has pointed out that there is a connection between the viscosity of fresh Hevea latex used as starting material and the rubber content of the creamed concentrates. He found the cream concentration to be the higher, the lower the viscosity of the original latex. A similar relation has been observed in creaming synthetic latices. It is known (8)that synthetic latices of large particle size not only produce creams of higher concentration but are also less viscous than small particle size latices. The effect of particle size increase on the viscosity of GR-S D-229 latex was determined in the following experiment.
DIAMETER OF SMALLEST CREAMED PARTICLES (An U.)
Figure 11. Particle Size of Various Dispersions us. Effective Creaming Agent Concentration
Figures 12 and 13 illustrate how the creaming agent concentration required for creaming is influenced by the initial rubber content of the creaming mix. I n Hevea latex (Figure 12) it is evident that the influence of the initial rubber content on the degree of separation is small. The creaming agent requirement of G R S D-229 latexdecreases markedly as therubber content of the creaming mix is increased (Figure 13). Further, Figure 13 indicates that the creaming agent concentration required for creaming a given proportion of the initial polymer approaches a comtant value for low polymer contents of the creaming mix. (Constant composition of the aqueous phase in the creaming series illustrated in Figures 12 and 13 was maintained by diluting all latices with their own filtrate.)
Particle Size Determination The particle size measurements for all latices investigated were carried out in the following manner:
U
Q.
A droplet of the latex was dispersed in about 1 ml. of water containing three drops of o.5y0 aerosol OT solution Two or three drops of this dispersion were then diluted with 1 ml. of aerosol OT solution &s above. T o this final Sam le there was added, slowly and with vigorous stirring, 5 ml. o f w a t e r which contained three drops of saturated bromine solution. A small amount of brominated, dilute latex was placed on a dry collodion covered specimen screen and dried at 60 C. The screen was then laced in a vacuum chamber and shadowed with a 50/50 mixture o f latinum and palladium. Electron micrographs were made in t f e usual way at a magnification of 5810 diameters. I n the case of the large Hevea particles, the magnification was reduced t o 1930 diameters in order to increase the field of view. Two hundred particles were measured in each case. The scale factor in enlarging the electron micrographs was reduced by 9.6y0 in order t o compensate for the swelling of particles as a result of bromination (18). Since the creaming data were all in terms of mass, the particle sizes are expressed 88 mass average diameters, dm, or as mass median diametem, Dm. The mass average diameter is given by
zds
[XI
118
where n is the number of particles having dm = diameter d, the summation covering all observed size classes. The maas median diameter is the diameter of the particle having median mass-that is, aa much maas resides in the sum of all particles larger as in that of all particles smaller. The maaa median diameter was determined from cumulative mass versus diameter plots (Figures 2,4,8).
GR-S D-229 latex of increased particle size (last sample of Table
I) was diluted with enough water to reduce its sodium chloride
concentration from 0.176 to 0.022 molar (on the aqueous phase). A portion of the original GR-S D-229 latex was mixed with 0.088 gram moles of sodium chloride per liter of aqueous phase, a quantity previously shown to be insufficient t o cause particle growth. These and the salt-free control latex D-229 were concentrated by filtration (using ceramic filters). The filtered concentrates were diluted t o various degrees with corresponding filtrates and their viscosity determined with the Brookfield viscosimeter. a
I m
I
D.R.C
;look
0
I
OF CREAMING MIX ( % )
45.9 22.9
t: P 0 LL
ps
5
3 In E a W
m m
3 a
Od
0.05
0.I
0.15
0.2
AMMONIUM ALGINATE (% ON AQUEOUS P H A S E )
Figure 12. Effect of Rubber Content on Creaming of Hevea Latex
The results (Figure 14) show that the latex of increased particle size is less viscous (particularly at concentrations of more than 35y0) than the latex of original particle size. The same large particle size latex was shown above (Figure 3) to give creams of higher concentration than could be obtained with the control latex. I n the light of these facts one might suspect that the relation between viscosity and creaming capacity of fresh Hevea latex, observed by Van Dalfsen (IO), was a n indirect one and caused by differences of the particle size. A comparison of curves 2 and 3 in Figure 14 shows that sodium chloride in quantities insufficient t o cause particle growth likewise decreased the viscosity of the latex. This effect, however, is less pronounced than that caused by particle size increase. It is obvious that high viscosity is a limiting factor in concentrating latex. However, latex Concentrates obtained by creaming with creaming agents do not contain individual spherical particles but large irregularly shaped clusters of the original latex particles, which should greatly influence the viscosity, depending on their size, shape, and flexibility. Consequently t h e viscosity of creamed, clustered latex concentrates is not necessarily related t o the size of the original, individual latex particles in a manner similar t o that of nonclustered concentrates, obtained without creaming agent.
Vol, 43, No. 2
INDUSTRIAL AND ENGINEERING CHEMISTRY
412
E
=E
1 r
1004-
; eo-
x
20.2
2 60+
A
11.4
0
8.1
-> 5 0 -
60-
0
I
u)
40-
2
20
200
I
MOLILITER ON AOUEOUS PHASE
0
790
%
790
0 088
3600
0.022
-
10-
I
0.2 0.4 AMMONIUM ALGINATE (Y. ON AQUEOUS PHASE )
Figure 13.
0.I
pe, I
I
Effect of Rubber Content on Creaming of GR-S D-229 Latex
In order to establish the influence of particle size on the viscosity of creamed, clustered concentrates, the viscosities of three Hevea latex fractions of different average particle size were determined, after adjusting them t o the same concentration of rubber, creaming agent, and serum constituents (by proper dilution with latex filtrate and addition of creaming agent, taking into account the amount of creaming agent already present in the creamed concentrates). The creamed Hevea latex (fractions 1, 2, and 3, Table 111) were prepared by fractional creaming of normal lates in the manner described previously in this study (Table II). KO particle size determinations \yere made of these fractions. Estimation of their particle size from the known relation between tht. amounts of creaming agent used in their preparation and particle size (Table 11) indicates that the average particle size of the iractions differed widely and decreased in the order of their numbers. Table 111. Viscosity of Hevea Latex Fractions of Varying Particle Size at Constant Rubber (42.3%) and Ammonium Alginate (0.187 %) Concentration
Fraction 1 Fraction 2 Fraction 3
NoCl
30-
>
L a
MASS MEDIAN PARTICLE DIANETER ( A U )
0
, I
5
v
+ ;; 40-
0
E
70 -
POLYMER CONTENT OF CREAMMG MIX 1%)
a
Range of Ammonium Alginate Conon. Used for Fractionation, % ’ on Aqueous Phase
Viscosity at 25’ C . (Centipoises)
0-0.03 0.03-0.05 0.097-0.187
90 115 19 1
The results in Table 111 show that the viscosity of creamed concentrates (of equal rubber arid creaming agent content) increases with decreasing particle size of the latex used as starting material. It should be recognized that the “viscosities” discussed here do not represent viscosity in the Newtonian sense but correspond to shear stresses, all measured a t the same rate of shear, and serve only for comparison of these various materials. The results obtained, indicate that the causes for the increase of viscosity of latices with decreasing particle size still prevail after clustering of the latex particles under the influence of the creaming agent
Summary 1 set of GR-Ssamples which differed only in particle size was prepared by treating a small particle size GR-S latex with a constant critical amount of sodium chloride for varying periods of time. After the particle gron-th had been arrested by further dilution, the latices of different particle size mere creamed with various quantities of ammonium alginate. It was found that the creaming agent concentration required for creaming decreases with increasing particle size of the latex. Changing the emulsifier system of the GR-9 latex from sodium Dreeinate t o potassium castor oil soap does not alter the relation between particle size and creaming agent requirement. Increasing the stability of the original GR-Slatex, by complet-
ing the surface coverage of the particles with soap or reducing the stability by addition of salt, without change of particle size, has practically no effect on thc degree of separation with any given amount of creaming agent. Kormal Hevea latex was separated into eight fractions of characteristically different particle size distributions by successive creaming operations n.it,h incremental additions of ammonium alginate. A silica hydrosol was concentrated by creaming with varying amounts of ammonium alginate. A relation between particle size and creaming agent requirement was observed; this holds for GR-S latices made in different emulsifier systems, for ammonia preserved Hevea lat,ex, and for silica hydrosol over a wide range of particle sizes (200 to 9000 A. in diameter). Lat,ex fractions of different average particle size, obtained by fractional creaming of normalHevea latex, were examined wit,h respect to maximum cream concentration and viscosity. The maximum cream concentration increases and the viscosity ( a t a given rubber and creaming agent concentration) decreases with increasing particle size.
Aoknowledgment The aut,hors wish to express their thanks to the Firestone Tire and Rubber Company for permission to publish this work and to F. W. Stavely, J. W. Liska, 0. D. Cole, E. M. Glymph, and P. H. Biddison for their interest and helpful suggest,ions.
Bibliography (1) Arundale, E. (to Standard Oil Development Co.), U. 6 . Patent
2,444,801 (1948). (2) Ibid., 2,462,591 (1949). (3) Harkins, W.D., private communication to Office of Rubber Reserve (July 24, 1943). (4) McGavack, J., IND.ERG.CHEX.,31, 1509-12 (1939); E. S. Patent 2,300,261 (1942). (5) hlaron, S. H., Case Institute of Technology, private communication to Office of Rubber Reserve (July 23, Aug. 25, Oct. 8, 1943). (6)I b i d . (Aug. 6, 1946). (7) Rhines, C. E.(toU. &Rubber Co.),U. S.Patent2,481,876 (1949). (8) Rhines, C. E., and McGavack, J., Rubber Age, 63, 599-606 (1948). (9) Rumbold, J. S. (to U. 8.Rubber Go.), U. S. Patents 2,467,053 and 2,467,054 (1949). (10) Van Dalfsen, J. W., Brch. RubbercuZtu.ur, 23, 1 (1939). (11) Van Gils, G. E.. and Kraay, G. M., “Advances in Colloid Science,” Vol. 1, p. 247, New York, S . Y.,Interscience Publishers, 1942. (12) Willson, E.A. (to B. F. Goodrich Co.), U. S. Patents 2,357,861 (1944)and 2,444,689 (1948). (13) Willson, E.A,, Miller, J. R., and Rowe, E. H., private communication to Office of Rubber Reserve (Oct. 30, 1947). RECEIVED September 2 7 , 1950.
