Stability of High Polymer Latexes to Acidification R. S. BARROWS
ANI)
G. W. SCOTT
E. I . d u Pont de Nemours C Company, Inc., Wilmington, Del. Acidification is necessary in order to reduce the pH and to facilitate coagulation of high polymer latexes in, which alkaline agents have been used as dispersing agents. When the alkaline soaps are neutralized complete coagulation sometimes occurs and to offset this result acid-stable dispersing agents must be incorporated into the latex. The apparatus and procedure for the evaluation of the dispersing agents are given and the results of the addition
of stabilizing agent after polymerization and polymerization in presence of the stabilizing agent are discussed. The commonly used agent is the condensation product of formaldehyde and naphthalenesulfonic acids. The effectiveness of the 1- and 2- isomer of naphthalenesulfonic acid as dispersing agents and the method of calculating the cross-sectional area of a dispersing agent from the molecular dimensions and Avogadro's number are presented.
T
corporated before or after polymerization. A number of such agents are known; a few of which are listed below.
HE reduction of the p H of high polymer latexes in which
sodium rosinate, sodium oleate, or other alkaline agents have been used as dispersing agents is frequently desirable-for example-polychloroprene latexes which are t o be coagulated by freezing are usually acidified t o facilitate coagulation and t o gain such advantages i n the resultant polymer as greater film strength, more rapid drying, and greater resistance t o swelling in water (IO, 19). I n order to avoid complete coagulation which occurs when the alkaline soaps are neutralized, acid-stable dispersing agents must be incorporated into the latex. These may be in-
BE
1. The sodium salt of the condensation product of naphthalenesulfonic acids and formaldehyde. 2 . Sodium isopropyl naphthalenesulfonate. 3. The sodium salt of the Condensation product of isopropyl naphthalenesulfonic acid and formaldehyde. 4. Aliphatic sulfonates. 5 . Polybenzyl sulfonic acid.
The latex t o be acidified should contain usually not more than 40% solids and be free of amines. More concentrated latexes coagulate, and certain amines rcact with several of the acid dispersing agents destroying their protective power. It is obviously important that coagulation be minimized t o avoid loss of polymer and plugging of pipe lines and screens in transferring the acidified latex. A commonly used agent is the condensation product of formaldehyde and naphthalene sulfonic acids. Variation in the stabilizing action of different samples of the condensation products of formaldehyde and naphthalenesulfonic acids led to a study of the components of the condensate. These differences were found to result from variation in l-naphthalenesulfonic content of the acid mixture which was condensed. APPARATUS AND PROCEDURE
The following method was used to evaluate the dispersing agents. Chloroprene was polymerized at 40 C . under nitrogen with the following recipe: 2-Chloro-1 ,a-butadiene Nancy wood rosin Water Sodium hydroxide Dispersing agent Potassium persulfate
AGITATOR SHAFT SEALED OFF WITH RUBBER TUBING
100.00 4.00 160 00
0.83 As shown 0 60
At the 90% conversion point a small amount of a short stopTHERMoMETER2 H ping agent was added t o stop polymerization. The latex was
NITROREM
lNLEi
I GLASS AGITATOR 3-1/8' LG ROTATED AT 8 0 R.RM
Figure 1. Polymerization Apparatus
acidified with 10% acetic acid t o a p H of 6.0 t o 6.5. Polymerizations and coagulating tests were carried out in an apparatus shown in Figure 1. The hydrometer served as a means of following the progress of the polymerization which was arrested a t a specific gravity reading of 1.068 to 1.070. Stabilizing agents were added after polymerization but before acidification as 5% aqueous solutions t o 50 ml. of latex in a 125 ml. Erlenmeyer flask. The latex was acidified with 2.5 ml. of 10% acetic acid and shaken (100 times). The coagulum was removed, washed, squeezed, and weighed. Values reported are grams of wet coagulum (containing about 4047, water) per 100 grams of 2-chloro-1,3-butadiene. Latexes giving 1.0% or less of coagulum were considered satisfactory. Latexes were also prepared by polymerization in the presence of the stabilizing agents (as well as by adding the agents after
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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K i
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0
.
1
acid with formaldehyde increases its efficiency but condensation of the 1-isomer does riot improve its stabilizing action (Figure 3). The presence of 1-naphthalenesulfonic acid in a sulfonic acid mixt,ure is objectionabic becauw t,he result,ing formaldehyde condensate has pronounced coagulating power. The l-napht,halenesulfoiiic acid content should not exceed 15% if excessive coagulation is to he avoided. The effect of the 1- isomer in condensates is illustrated in Figure 4. It ail1 be not,ed that, a mixture containing 18 t o 2OLX 1- isomer n-hich gives complete coagulation a t 2.07; concentration may be used~a t lower concrntration t o yield reasonably stable acid latexes. The rate as n-ell as the extent of coa,gulation increases with increasing concentration of a formaldehyde naphthalenesulfonic acid condensation product made from an acid mixture high in 1- isomer. Rates rvere determined by dividing the weight of coagulum at t,he termination of the experiment by the time and therefore may not be uniform throughout the coagulation period.
0
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w a 0.01 1.0
2.0
3.0
4.0
% STABILIZING AGENT Figure 2. Stabilizing Action of Sodium Salts of SUIfonic Acid and Their Formaldehyde Condensates Added after Polymerization polymerization). I n such cases the acidified latexes n-ere allowed to stir 18 hours, and the coagulum was removed and treated as above. This test givcs less precoagulum than the test described above and more closely approaches conditions encountered in the manufacture of polychloroprene. An acidified lates was considered stable when less than O.5yOcoagulum was obtained by this procedure. Procedures for making and purifying the 1- and 2-naphthalenesulfonic acids were similar t o those already described in the literature (8). Preparation of the condensation products with formaldehyde has been disclosed also ( 1 ) . Light transmittance was determined on diluted latexes by means of a Fisher electrophotometer. ADDITIOK OF STABILIZING AGENT AFTER POLYMERIZATIOY
About 0.7% of sodium 2-naphthalenesulfonate is required to give acid stable dispersions when added after polymerization. Sodium 1-naphthalenesulfonate is less active than the 2- isomer when employed in this manner; concentrations of 2g/, and more are, however, effective. The condensation product from formaldehyde and 2-naphthalenesulfonic acid is slightly more effective than sodium 2-naphthalenesulfonic acid. These data are shown in Figure 2. POLYMERTZATION f Y PRESEhCE OF STARILIZI\G AGENT
Somewhat different results are obtained when the polymer is prepared in the presence of the stabilizing agent. Sodium 2naphthalenesulfonate is again a good stabilizing agent; the 1isomer is, however, ineffective in concentrations up to 2.OY0 based nu the polymer. Condensation of thc 2-naphthalenesulfonic I
100
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Vol. 40, No. 11
TABLE I.
EFFECT O R
1- I S O I I E R
O U It4TE OF COkGUL4TIOT
7" of Condensation Product of
Formaldehyde and a Mixture of 65% 2- and 357, 1Xaphthalenesulfonic Acid 0.25 0.5 0.75 1.0
Grams Coagulum per Minute 0.015 0.57 20 72
Time a t Termination of Experiment, 19 hours 14 minutes 2 minutes 0 . 5 minute
TABLE I1
7%
Dispersing Agent, Condensation Product of Formaldehyde a n d SOaH
co
70 Coagulum
I
...
0.5 0.5 0.78 0.75
0.5
0.75
0.21 1.16 0.10 Very high
Further evidence of the effect of 1- isomer may be had by examination of the effect of a 50-50 mixture of the 1- and 2-naphthalenesulfonic acid formaldehyde condensates. A preliminary evaluation of l.OG0 of selected aromatic sodium sulfonates present during polymerization illustrates the effect of structure variation on thc cffcctivcncss of sulfonic acids as stabilizing agents (Table 111).
1
00
ALPHA 'ACID AND IO
I .o
0.1
l I
1.0 1.5 2.0 3.0 % STABILIZING AGENT Figure 3. Stabilizing Action of Sodium Salts of Sulfonic Acids and Their Formaldehyde Condensates Present during Polymerization
0.5
J
0.5
I,O 1.5 2.0 3.0 % STABILIZING AGENT Figure 4. Effect of 1-Naphthalenesulfonic Acid in Sulfonic Acid Condensed with Formaldehyde Present cliiring Polymerization
I N D U S T R I A L ' A N D E N G IN E E R I N G C H E M I S T R Y
November 1948
POWER OF SELECTED SODIUM AROMATIC TABLE 111. DISPERSING
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by a given amount of the agent may be calculated from the following equations:
SULFONATES
Agent
To Coagulum
Sodium benzenesulfonate Sodium o-toluenesulfonate Sodium p-t oluenesulf onat e Sodium 1 2-xylene-4-sulfonate Sodium 1' 4-xylene-3-sulfonate Sodium 1hxylene-4-sulfonate Sodium 1-naohthalenesulfonate (added after polymerization) Sodium 2-naphthalenesulfonate Potassium anthracene-1-sulfonate
100 100 100 0.46
Where V is volume of the polymer, c is weight of polymer in a given weight of latex, d the density, r t h e radius of the polymer particle, and n the number of particles, and
4.9 100
10.0
A =4 d n
0.80 0.91
It may be concluded from these examples that a n aromatic sulfonic acid must have either alkyl or aromatic substituents on the benzene ring in order t o be effective as a n acid stabilizing agent. Furthermore, it appears t h a t the position of the groups in relation to the sulfonic acid group is important. Decreasing effectiveness is observed as the position of the substituent changes from meta t o para t o ortho. Examination of certain other isomers, unavailable at the time this work was done, would be required to substantiate these conclusions.
(2)
where .i is the total surface area of the particIes. Since c and d are known and A is assumed t o be equal t o the total surface area available from the stabilizer, (I) and (2) may be solved for n and T. For a dispersion protected by 1.0% of sodium 2-naphthalenesulfonate, the smallest possible average radius is 0.44micron. The particle radius should vary inversely with the concentration of stabilizing agent. That this is the case is shown by measurements of light transmittance on dilute acidified latexes containing sodium 2-naphthalenesulfonate. The light transmitted at a given polymer concentration is greater with an increase in the amount of dispersing agent, owing t o a d r n r a s e in the size of the polymer particles (6).
LIGHT TRANSMITTANCE OF LATEXES
The addition of sodium 2-naphthalenesulfonate or sodium I-naphthalenesul fonate after polymerization reduces the opacity of the acidified latexes. The sodium %naphthalene sulfonate has a much greater effect than the 1- isomer. These data are shown in Table IV.
DISCUSSION
It is believed that upon acidification, small particles agglomerate, so that the ultimate size of the agglomeiatss is governed by the nature and concentration of the stabilizing agent which forms a monomolecular oriented !ayer around the particle. If the particles are kept small enough the acidified latex is stable, otherwise it coagulates. This hypothesis is developed in more detail TABLE IV. EFFECT OF NAPHTHALENE SULFONIC ACIDSADDED AFTER POLYXERIZATION ON LIGHT EXTINCTION OF DILUTED below. ACIDIFIED LATEXES CASE1. STABILIZING AGENTADDEDAFTER POLYMERIZATION. Concentration of Diluted Latex Grams The stability of the latex t o acidification is dependent on the Polymer per 100 MI. of Ladeex following factors: Stabilizing Agent 0.00056" 0.00028" 0.00014° 0.00007" Sodium Z-naphthalenesulfo1. The structure of the stabilizing agent nate. % 2. Concentration of the stabilizin agent 0.5 74.5 39.5 23 1.0 79 44 25 12.8 8. The particle size in the unacidiaed latex 1.5 :s 27.8 14.0 ... 4. The molecular weight of the polymer Sodium l-naohthrtlenesulfo5. Concentration of the latex nates % 0.5 1.0 1.5
.. ..
98 79 63
66 56.4 31.4
40.5
...
The geometric configuration of the stabilizing agent is of great importance. Examination of models shows that 2-naphthalene100 log sulfonic acid should orient more readily than the 1-isomer because it has more tendency for the hydrocarbon portion of the molecule t o be drawn away from the water interface and towards the polySodium 1-naphthalene sulfonate when present during polymer particle. The 2- isomer should, therefore, be a more effective merization causes a marked increase in opacity of the alkaline agent, as the authors' experiments have shown. Further support latex whereas the 2- isomer does not (see Table V). Addition of of the hypothesis is offered by comparison of the stabilizing effect the I- isomer after polymerization was completed did not change of xylenesulfonic acids with their structure-for example, sodium the opacity of the alkaline latex. 1,2-xyIene-4-sulfonate is a n effective stabilizing agent whereas the 1,3-xylene-4-sulfonate is ineffective. The excellent stability of latexes stabilized with potassium anthracene-1-sulfonate may TABLE V. EFFECT O F SODIUM SALTSO R NAPHTHALENESULFONICbe accounted for by the extra benzene ring which helps to orient, ACIDS PRESENT DURIKG POLYMERlZATIOK ON L I G H T EXTINCTION in that it draws t h e hydrocarbon group away from the water OF DILUTED ALKALINE LATEXES interface and towards t,he polymer particle. Furthermore, the Concentration of Diluted Latcx, Grams Polymer per 100 R41. of Latex clectrophotometer reveals t h a t the particle sizes are larger with Stabilizing Agent 0 . 0045a 0 0022" 0 0011" 0.00056" sodium 1-naphthalenesulfonate than with equivalent amounts None 46 23.5 .. .. of the 2- isomer. Increased amounts of 1- isomer d o give, how2 . O r 0 2-salt 57 30.5 .. .. I-salt 97 60 30 .. ever, small particle size and consequently stable latexes. This 1-salt .. ... 68 3.5 indicates t h a t a smaller fraction of the 1-naphthalenesulfonate 100 than of the 2- isomer forms a monomolecular sheath around the a loolog % light transmitted' particles. The greater the concentration of the stabilizing agent the CALCULATIONS greater is the available surface-coating area and the smaller are From the molecular dimensions and Avogadro's number, the the particles on the average. The smaller the particle the easier cross-sectional area of a dispersing agent may be calculated. it is t o prevent agglomeration. Once the polymer particles in a For sodium 1-naphthalenesulfonate it is approximately 5.4 x 102% latex are formed, their size cannot readily be reduced. There square Angstroms per gram, assuming complete absorption, and a fore, the original particle size of the latex (before acidification) monomolecular layer. The average size of particles stabilized must be smaller than thp limiting size which the stabilizing agent Q
, .
100 light transmitted'
~
L;p
...
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INDUSTRIAL A N D E N G I N E E R I N G C H E M I S T R Y
can protect if stability is bo be obtained. I n the case of most neoprene latexes, t'he average particle radius is about 0.1 micron or well within the size capable of stabilization by these agents. The more concentrated the latex t o be acidified the higher the frequency of collisions becomes and the greater is the ease of coagulation. It is more difficult to stabilize latexes of low molecular weight polymer. The reason for this has not been investigated. CASE 2. STABILIZIXG A G E N T PRESENT DURING POLYJ1ERIZ.LTION. This case is subject to the same treatment as that given above except t'hat the effect of the stabilizing agent on the formation of the polymer particle must be considered. Sodium 1naphthalenesulfonate is tot'ally ineffective when present throughout polymerization despite the fact t h a t it has moderate protective power when added after polymerization, whereas the 2- isomer is somewhat more effective when present during polymerization. This is a result, a t least in part, of a n increase in the latex particle size caused by the 1- isomer. Light transmitt~anceexperiments indicate a marlied increase in opacity of the alkaline latexes prepared in the presence of sodium l-naphthalenesulfonate. The larger particle size may result from agglomeration, or from interference with polymerization in the micelles ( 2 1 ) . Since the 1-naphthalenesulfonate does not cause agglomeration of an alkaline latex when added after polymerization, it appears likely that simple agglprneration during polymerization is n o t a factor. Sodium 2-naphthalenesulfonate does not cause an appreciable increase in particle size when present during polymerization. I-Naphthalenesulfonic acid brominates and nitrates in the 5 position (3,4,7 ) . It is probable t h a t formaldehyde condenses in the same position. Examination of a molecular model of the product reveals that' the txvo sulfonic acid groups are a t opposite ends of the hydrocarbon chain. If they be forced into positions on the same side of the chain, a considerable part, of the hycirocarbon portion of the molecule must enter the v a t e r phase with the sulfonate group. Examination of other possible positions of condensation 8, 4 (4, 7 ) shows similar undesirable configurations. The 2-napht'halenesulfonic acid, however, with its active positions 5, 6, 8 (2, 6, 9) shows condensate structures for all positions in which the sulfonic acid groups and the hydrocarbon group arc in much better relationship so that orientation a t the polymermater interface should be easier. It is not surprising, therefore, that formaldehyde condensation products of the 1- isomer are ineffective whereas those derived from the 2- isomer are highly
Carotene
Vol. 40, No, 11
effective. To account for the coagulating power of the forrnaldehyde condensates made from sulfonic acids high in the 1isomer it' is only necessary to assume an exaggeration of the effect already demonstrated for 1-naphthalenesulfonate in giving rise to larger particles during polymerization so that the particles become too large to be stable. I n the foregoing discussion bhe condensation of formaldehydc and naphthalenesulfonic acids has been treated as a simple condensat'ion of 1 mole of formaldehyde and 2 moles of naphthalenesulfonic acid. The condensation is more complex, however, particularly for the l- isomer. When this isomer is present in the condensation mixture, there is a tendency for production of resinous products indicating condensat,ion of the I- isomer at more than one position. Therefore, it is possible that a singlc 1-molecule may be linked t,o 2 or more 2-molecules, producing larger amounts of unsatisfactory stabilizing agent than would be >predictedby t h e simple t'reatment. Slt.hough most of the authors' work has been done with polychloroprene dispersed in sodium rosinate solutions, other alkaliric soaps such as sodium oleate, sodium hydrogenated rosin, and sodium disproportionated rosin may be used, and it seems likely t h a t the principles iuvolved should be applicable to other high p o l p i e r dispersions. A C K 3 QWLEDGMENT
The authors wish to express appreciation for the help giveii t)y H. W. Walker in this work. LITERATURE CITED (1)
(2) (3)
(4) (5) (6) (7) (8) (9) (10) (11)
Badische, Brit. Patent 7.137 (1913). Baurn, Ger. Patent 61,730; Friedlander,3, 419 (1893). Cleve, P. T., Bar., 10, 1722 (1877). Ibid.,23, 958 (1890). Clove, P. T., B?LZZ. SOC. chim., [2] 29, 414 (1878). Debye, P., J . A p p l i e d Phys., 15, 338 (1944). Erdmann, H., Ann., 247, 312 (1888). Fiera, H. T,;..and Weissenbach, Hela. Chim. Acta, 3, 315 (1920). Rudolf, Chem. Zentr.. I, 960 (1899). Starkweather, H.W., and Youltor, M.A , , IXD.ENQ.CIxnxf., 31, 934 (1939). Stearns, R. S., and Harkins, ITr. D., J . Chem. P h y s . , 14, 214
(1946). (12) Walker, H. W., J.Phys. & Colioid Chem., 51,451 (1947), RECEIVED June 10, 1947. Presented before the Division of Rubber Chemistry, AXERICANC H E ~ I I C A SOCIETP, L Cleveland, Ohio, N a y 26, 1947. Contribution No. 58 from Jackeon Laboratory, E. I. du Pont de h-emours & Company. Wilmington, Del.
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EFFECT OF BLANCHING, PACKAGING, A N D STORAGE TEMPERATURE WALTER L. SELSOIV, JOHN I
