Process for Concentrating Latex (:ONCENTRATIOIV OF TYPE I11 GR-S LATEX WITH SODIUM CHLORTDE S. ET. AIARON, C:. MOORE, J. G. KLNGY'L'ON, I. N. ULET'I'I'CFI. J. C. TRIXASTLC, AXI) E. JI. BORNEMAN Ctise Institute of Technolog> , Clerdand,
i process for rapit1 coricen~ru~icm GH-s ~ a ~ ehas x heen developed which itiFolves gelabioii bj addi tig electroI r t e and cooling, and subseqiteni processing of the m i x t o yield a fluid and rapidly filterahlt- mass. The filter va1.e of polymer gel ohfained reverts readily, on warming t o room temperature, to stable latic'eq of 60-10'30 solids vrintents. When processing i s properlj done, the s e r u m remo+ed is free of polymer. This paper describes thc .ipplicatioii of the process t o T y p e 111 G K - S latex and w d i i i i i i chloride as the added electrol! tr. With this l a t e x . t n d elrctrolyte, concentrates have h e n prepared which .tre exceptionally stable atid, a~ 60'3'0 si)lids, show all the vtiaracteristics of a good I ~ t e u . \rial> sis of the p r o d u c i ~ - h o w 4 that i-20c/, of the hWLp in Lhr procebsed latices pas5e4 into the Serurii, as well as 60-805?0 o f the added eleclrolj Le. As a r e s i i l t of the sniall quantity of residual c4t.ctroly te i n such concwiLratci, I hcy ruhihit gel points Iwlow r o o m temperatrir~.
!9YXTHETIC
latices of higli solid, coritcnt can be obtaiiicrl either by direct polymerization to high concentration or by concentration of lower solids latex through serum removal. The yeparation of the medium from lower solids latices has heretofore h n accomplished by (a) evaporation, ( b ) creaming, and (c) the C h m a n Stockpunkt procedurc. Evaporation involves mcrclo t he removal of water without loss of either polymer or nonpolymer congtituents. Creaming, in turn, entalls the use of certain addition agents to latex with the aid of which the latex is segregated into a concentrate cream and a more or lew clear serum. The (veam separation is obtained on alloxing the later mixture t o -rand or by centrifuging. The StocLpunkt method, on the othei hand, involves a different approach, and utilizes gelation and c-ooling of latex N i t h agitation a5 a means of biinying aboiii roncentrtttion. The ewntial diitaili of the process have &cad\ Iwen given (1). Thc process to bo deqcribed heic ii an outgrowth of investigict i o w conducted in this laboratory on the German method of latex concentration. It involves soiiie fundamental modification. C J the ~ German procedure, and perriiits a more rapid conccntriiion of GR-S latices t o solidi contents considerably higher than \\ere achieved by the Germans. T h b paper summarizes tlic development and details of the piocoiq a9 applicd to Tvpe 111 -1
1
cri-s ixic3\1.
DEYELOP\IE\'I'
Ob PROCESS
Iri the suiiinier of 1945 an invc~bligationwas uiitlertaben on the %ckpunkt method of concentrating sgnthrtic latires. The initial tupeiiments involved study of the process on a Ckwiisn late\ ilcsignated as D-62. In the courw of this ~ o r kthe variouc y-pecb of the procedure as practiced by the Germans were dui)licated, and a number of variationu, including the ccntrifuging of gelled latex, were worked out (1). Thew reqearclics a l a showed 1 Tspe I11 latex is a n aqiieoua-mt diriiii \ J iitlietir latex prrpared from 1,iitadione-styiene in 1 1 proportion-, and nitit nota-7iilrii soap of K ~ o o ( 1 rosin aq emulsifier.
156
Ohio
ilia( foariiiIig arid granulatloii i i i c not c Shi Ipunlct process, and that t h e phenomenon of gelat io11
iii
gc.ncral was due t o presence of electrolvtc in the latex. Thc success in concentrating tlie 11-52 latry hv tlic (:ernmi technique and by centrifuging h t ? X I d t(J at tell^^^^^ 1 0 nl)ply these processei to GR-S la Since GR-S latire, do not contain sufficient electrolyte to gelation on cooling, it x a i neccsary t o introduce a suitable salt into thc fiiiiilicd late\ to cause gclation. Sodium salts werc selected bccau~eof ihcit ong gclling properties and weir in1roduced slowly with agit:t. Since the rcquizitc quantity of clectrolyic could not bv added to inost latices without causing some coagultttion or ~ u ~ I ~ J ( > I , it war found evpedient to increase soap content of the laic.\ l)t.forc addition of salt solution. With such latoy-map-salt mi\turcs gelatioa could readily be produccd on cooling. Iloivvrvei , it \\ai, found not to bo quite so rc\crsible a i wa\ thr (we witli (:erru:cn latex, and frequently tlie soap coritcrit tiati t o txl 111crcaqed further, particularly in 1:ttiws containing f a to innkc thr latex withstand gelling slid cooling. Them latex-soap-salt mixes showed serum separatiori i>\.hciit lit. mivtures were gelled, cooled to 0 " C. \tith agitation, ant1 11 $11 either allowed t o stand or centrifuged ivhilc cold. Howvr in no caw did bhe total solids content of a conceiitratc exc ,50Vc, nor r a s the serum clear in all iiistance~. Ful t h w , ill( procem wa, s l o when ~ settling v ILL used nud cuinber~o~iiclmi crntvifuging was attempted with thc available equipnieiit Severthelcri, this phase of tlic worh dio\~etlthat conccntraliori ol GR-S latices is possible b j this method, and that! vxriow i~lectrolytes, such as ?odium cliloride, sodium sulfate. sodiuni monohvdropcn orthophosphaf e, sodium acetate and sodiuni ritrhonatc mav bo used for the purpose. Ou the other hand, aniinoiiiuin chloride or ?odium a1gin:itc proveti ineffrctive a5 cnircentrating agents. 11 this stage of the work ii nas dcciiled io attempt a differrnt tc~hniquein order to acceleiate thc removal of the amount removed, and to avoid the difiiciilties involved in crntrifuging cold and gelled latex. The pi occdure envi4oned TVR\ t o filter a cooled, gelled latex mix by suction through 11 cw)lcd Riichncr funnel with paper as tlie filter medium. 11 ininicdiatdy found that such inixos could be filtered, aliliougli Irlatively slowly, to yield a dear sci-uin and a concentralc of c*onsidei*ablyhigher solids contcnt than I d bcrn poisiblc t)\ ixithcr sedimentation OP rentrifuging. Tri view of the proniibiiig wsults, lurther work on iinprovcnietil of thik filtration Icxchniqiw 17 tts initiated. Samples for filtralion were a t first prepared ill the iaim InLtlilit'r HS for the experiments in which the serum \?as removed bv settling or centrifuging. However, it w&s observed that the speed of filtration and the amount of seruni renioved could be increased l q - introducing certain modifications. Beating the latex into n lieav7; foam, aa suggested by the Germans, proved to be detrimental, since the foam cut down the filtration rate. Furttiei, I)\, properly adjusting soap content, pII, and electrolyte content, and by Eolloning a particular procedure in preparing the latex for filtration, rapid-filtering mixtures could be obtained nhich g a w rlrar sera and concentrste, of solids conkilts above 6O04
I N D U S T R I A L AND E N G I N E E R I N G CHEMISTRY
January 1949
0288
YE& SOAP/
OM.
RUE0ER
0307 MEQ. SOAP OM. RUBEER
_I
n 0 W
B W
0
8
0
.,EM,\
W 0
SOAP PE OM. RUBBER
157
and s h o ~ sall the characteristics of a good late.;. There is, however, one essential respect in which the concentrate differs from the initial latex. Due to a small quantity of residual electrolyte in the concentrate, the latex exhibits a gel point below room temperature. This gel point depends on the quantity and type of electrolyte present, and is lower, the smaller the concentration of electrolyte. Extended study of the proccsr a;3 outlined showed that a number of variables must be controlled for rapid and satisfactory concentration of Type I11 latex. These can be subdivided into two classes --(a) those elitering into the adjustment of the original latex for processing and (b) thosc operative in the concentratioii procedure itself. In the firbt category are the soap content and pH of the latex and the type and amount of electrolyte added. I n the iecond are the temperature a t which the electiolyte is added anti to which the latex mix i3 finally cooled, type and amount of agitation, prepence of foam, aging of the latex-salt mixtures, rate of cooling, and pl1\4c*xlstructure of thc gel and mi\ resulting from the manipulation-.
W
r
EFFECT OF LATEX PROFERTIES AND ELECTROLYTE
NaCl GONG.: 0.24N
TOTAL VOLUME: IISMI. I 10
t II
I 12
pH (ROOM TEMPERATURE)
Figure 1. EKect oTpH and Soap Content on F i l t r a t i o n K a t e of Type 111 Latex
Yarious electrolytes were found to be suitable and effective in rapidly concentrating several types of latex. However, this report will be confined to sodium chloride as used with Type 111 late.; GENERAL DESCKIPTION OW PROCESS
The process developed for concentrating Type It1 GR-Y late\ by gelling and filtering the prepared mix involves several essential steps. First, the initial latex must be modified to meet certain conditions optimum for rapid concentration. This involves adjustment of the soap content and $1 of the latex t o limits which will be defined later. Next, a solution of the electrolyte in question is added with agitation to the adjusted latex in quantity sufficient to give the desired electrolyte content in the final mix. With sodium chloride and Type I11 latex the mass is generally gelled by the time all the electrolyte is introduced at room temperature. This mass is cooled then with agitation. On drop in temperature, the gel thickens further until, a t a point about 5-10 O C. below room temperature, the mass suddenly becomes fluid again. The cooling of this fluid mix is continued further t o a temperature between 5' and 10" C. When the latter range is reached, the mass is filtered with suction through a jacketed Btichner funnel through yhich ice water is circulated to keep the funnel cold. The medium used is ordinary filter paper. When conditions are properly adjusted, clear serum is removed rapidly until the solids content of the filter cake rises to about 60%. Thereafter the rate tapers off, and becomes quite slow when 70y0solids and higher are attained. The cold latex retained on the filter is in the form of a fairly stiff and coherent cake a t the higher solids. At lower solids contents it has more of a pasty consistency. When removed and warmed to room temperature, the concentrate reverts to a latex whose viscosity depends on the solids content and on the type and amount of electrolyte added for concentration. The final product is esceptionallv stable on storage and manipulation. A t a concentiation of GOC$ solids it is quite fluid
Although the soap content and thc pLI of various sample* of Type I11 latex may vary somewhat, it is posbible to adjust all laticcs t o give satisfactory concentration by bringing the soap content and pH to certain values found to be optimum. These limits on the soap are 0.25-0.31 milliequivalent of the potassium soap of K wood rosin per gram of polymcr, m-ith the best concentration being a t about the midpoint of thii: mngc. I n certain latices the limit may be somewhat loivcr than 0.25, but in general thc limits specified are best with sodium chloride as electrolyte. Soap contents higher or lgwer than those given tend to decrease the filtration rate. The pH of the latex before addition of electrolyte muqt, a3 a rule, be above 10.5 for rapid filtration. When neces8ai.y the pH can readily be adjusted by adding a small quantity of sodium hydroxide. There appears to be no upper limit for pH, but when the latter approaches about 12, the base present may contribute appreciably along with the sodium chloride to the gelling of the latex; hence a lower concentration of sodium chloride may have t o be used. For best operating conditioris a pH of 11.0-11.5 is recommended. Figure 1 shows how the filtration rate is affected by tho IJHfor scveral soap contents of the latex. The time given is that necessary t o remove 50 ml. of serum from approximately 115-1111. samples of processed latex-salt mixtures. The original latex (Xo. 302) had an initial pH of 9.3, a soap content of 0.0764 N , free rosin acid content of 0.0052 N , and rubber content of 35.9%. This combined soap and rosin acid concentration of 0.231 milliequivaleiit per gram of polymer was increased t o three different values (0.268, 0.288, and 0.307) in these esperiments, and the effect of p R on filtration rates was determined, using 0.24 N sodium chloride in the final mix in every case. Examination of Figure 1 a t a pH of 11.5, for instance, shows that increasing soap content froin 0.268 to 0.288 milliequivalent per gram of polymer decreased the time required for removal of 50 ml. of serum. On the other hand, a further increase in soap content t o 0.307 milliequivalent per gram of rubber slowed down the filtration rate considerably. Lowering of pH caused filtering time to increase in all cases. The latex having 0.288 milliequivalent of soap per gram of polymer was not much affected between a pH of 10.5 and 12, but the time for the other two series increased rather markedly, especially for the latex having the highest soap content. Below a p H of 10.0 all samples filtered very poorly. Samples of other latices studied in the same manner showed results essentially similar to thow given in Figure 1. In practically all instances the time for removal of a definite quantity of serum decreased as the soap content was increased up to about 0.29 milliequivalent per gram of polymer, and then rose again as the soap was increased. However, up t o a soap content of
158
INDUSTRIAL AND ENGINEERING CHEMISTRY
NaCI: 0.24N SOAP: 0.275 MEP./GM,
RUE8ER
pH: 11.4
I
1
I
I
I
10
20
30
40
50
S A L T ADDITION TEMPERATURE
Figure 2.
- 'C,
Effect of Salt .Iddition Temperature on Filtration Rate
0.31 milliequivalent per gram of rubber, leasoilably rapid filtiation rates could be obtained with minimum amounts of sodium chloride. The influence of electrolyte content on filtration rate is illustrated in Table I. These data show that, when the electrolyte content falls below a certain limit, the latex-salt inives filter either very slon-ly or not a t all. As the clcctrolytc content is increased, the filtration improves. Since 0.24 S sodium chloride, based on the final volume of latex and salt, TT-as found to give a satisfactory filtration rate viith all latices tried, this concentration of electrolyte is recommended. Higher concentrations of electrolyte also work well. However, to keep down the residual electrolyte content of the concentrates, it is advisable to use as low an amount of electrolyte as possible coniineriauiate with good operation. Table I also gives some values for the gel points of various latex-salt mistuies.
0.24 with 3.0 LV sodium chloride solut,ion. I n all instances the salt m-as added to the latex a t 30" C. These mmples all gelled a t 29" C., and the stiff gel broke down int,o a creamy fluid at 18" C. Cooling was then carried below the breakdoxn point, to temperatures between 16" and 3 " C. Figure 3 shows that filtration speed is improved with decrease in temperature. Thus, the time required to filter off 40 nil. of Serum from samples of 110-ml. volume was reduced by half when t,he temperature was lowered from 16' to 10" C. Below 8" C., however, not much change occurred in rate of filtration. Results v-ith other latices have also shoxrii that 8" C. is as lo^ a temperature as is necc'-;sary for good serum removal. \$-ith salt concentration lower than 0.24 S,this final teniperature has to be loTT-ered below 8" C . Anot'her essential variable that cannot be overlooked is agitatioii during cooling. If samples of latex, adjusted to the desirable soap, salt, and pH contents, chilled without agitation, thoy gel and become quite thick. en temperature is lowered still further, the gels tend to crack and synerize to a slight extent, but do not filter with suction. Agitation of such gelled and cooled samples does not' break the gel down to the rapid-filtering form. Some stirring appears t o be necessary during chilling arid gelling in order to produce the deaircd statc for filtration. The thoroughness of stirring also seems to be important in detormining how rapidly serum may be removed. Table I1 givrs mine results of a feiv simple experiments which illustrate how increasing the size of the stirrer paddles decreases the period of time necessary to remove a given quantity of serum from initial samples of the same volume. The rate of stirring xvith aiiy particular paddle does not appcar to change significantly the filtration rate of the cooled mass. Either don- or rapid agitation produces a satisfactory gel breakdown and gives a state of agglonieratiori suitable for speedy filtration. The important fact, is that some agitation is essential to effect brealrdown of the gel. The stirring q x e d normally niaintaiiied iii the laboratmy scale concentration of Type I11 latex is held just under the speed that causes air to be whipped into a sample, Foaming is detrimciital to filtration spccd. Thc Gcrman concentration process required
TABLE I. TTFFECT
EFFECT OF VARI4BLES IR CORCENTRATION PROCEDLRE
The influence of vaiiables in the processing of a latex for concentration is particularly noticeable in regard to the rate of serum removal. Both the temperatuie at nhich electiolyte is added to latex and the temperature to i t liich the yelled material is cooled before being filteied affect the time requiied to remove seium. Figure 2 demonstrates the manner in which filteiing speed changes when the temperature of the latex a t salt addition is vaiied. Samples of latex 303, adjusted to 0.276 milliequivalent of rosin soap per grain of rubber and a pH of 11.4, were taken, and 3 0 S sodium chloride was added to these at variou7 temperatures to give in every case a concentration of 0.24 S sodium chloride in the final mix. The sample3 were then cooled, gelled, thinned, and filtered. From the filtration times obtained (Figure 2 ) , it is evident that addition of electrolyte a t room temperature and above has little effect on the filtration rate. On the other hand, introduction of the electrolyte below room teniperature definitely lowers the speed of serum iemoral. I n all instances the gel point of the salt-latex mixtures was 29" C. Therefore, it appears best t o add sodium chloride at a temperature near the gel point or above. Addition of the electrolyte a t room temperature is satisfactory and works well in all ca3es tried. Figure 3 shows the manner in which the filtration rate is affected by the tcmperature to which a batch is finally cooled. Samples of latex 303 were used, with the soap adjusted to 0.275 milliequivalent per gram of rubber, the pH to 11.4, and the salt to
Vol. 41, No. 1
FII,TR.ITIOSRITE
307
Soap Content AIeq. /G: Rubher 0.245
6 )H 11.5
306
0.279
11.0
Latex NO.
303
303
0.286
0.256
10.5
11.0
303
0.256
11.6
303
0.273
11.0
303
0.275
11.4
303
0.275
11.G.5
302
0.288
11.3
302
0.288
11.0
F o r 26 Inl. b F o r 37 ml.
a
O F SALT COUTEYT O X
h-aCI Cont,ent Sormality 0.1s 0.21 0.24 0.26 0.20 0.22 0.24 0.22 0.23 0.24 0.28 0.20 0.22 0.24 0.24 0.22 0.24 0.24 0.22 0.24 0.22 0.24 0.24 0.20 0.24 0.22 0.24 0.22 0.24
GELPoI\-T Gel Point,
c.
23.8 25 28 32
.. I
.
..
.. .. ,
I
.. 27.5
..
29.5 .
I
28 2Y
..
24.6 27.5
..
28 29 24
.. ..
..
\h I1
INDUSTRIAL AND ENGINEERING CHEMISTRY
January 1949
+
Y)
Y
NoCI: 0.24N SOAP: 0.275 MEQ./GM.
P
RUBBER
z
3
P i 0
8 Y
0
n
I 4
TEMPERATURE
I
I
8
OF BATCH
I2
-
Figure 3. Effect of Final Cooling Temperature on Filtration Rate
that air be whipped into the latex during chilling and gelling until the volume had increased severalfold. This procedure was followed in the early work in this laboratory, but further study has shown elimination of foam to be desirable (Figure 4). These data mere obtained by gelling and cooling a larger batch of adjusted latex 303, filtering half of it directly, and then beating the other half into a foam of about twice the original volume betore filtration. The time required to filter the foamed mass was more than twice that for the sample without foam. Although the foamed sample did not filter too well, the concentrate was stable and did not appear to be affected adversely by the beaten-in air. Aging of latex in the presence of saltr was studied particularly a t 2 5 ” and 35” C. Results show that changes x i t h time occur in latex during this aging which affect the filtration process markedly. Table I11 gives data for a series of experiments in which the effect of aging on filterability of the processed latex w ab measured. In eveiy instance electrolyte solution was added to latex of adjusted soap content and ~ € 1 ,and the latex was allowed to stand for given times. The samples of 110 ml. total volume mere then chilled with stirring in the usual manner and filtered on an ice-cold Bdchner funnel. Table 111 shows that the change which impairs the filterability of the final mix occurs betxeen 2 and 3 hours after salt addition. Latices thus aged do not gel and break down as do freshly prepared latices when cooled and stirred in the same manner. Whereas the latter upon cooling display a thin point about 1 0 ” C. below the gel point, aged samples become continually more viscous, no breakdown occurring even with prolonged agitation a t 0 ” C. This gel cannot he filtered, nor can its character be improved by salt addition, pII adjustment, or warming and reprocessing. On the other hand, latex mixes which have been processed can be kept at low temperature for prolonged periods of time without impairing filterability. Under certain conditions these “bad batches” have also been found to occur with freshly prepared mixtures. With some Type I11 latices unfilterable batches occasionally result when the latex temperature at which salt is added is considerably above the gel point of the final salt-latex mix. To avoid the possibility of bad batches, it is best to add the salt to latex at a temperature near the gel point. The gel obtained in bad batches resembles that formed in chilling, gelling, and stirring latices that have insufficient amount of soap, too low a pH, or a salt content bclow the optimum concentration for the particular latex used
159
T o investigate more fully the cause of bad batch formation and to obtain some idea of the effect of the physical structure of the gel, a microscopic study was made of the state of agglomeration , of latex gels and the gelation phenomenon itself. This study revealed the fact that properly processed samples which filter readily possess a characteristic gel structure which is readily observable under the microscope. On the other hand, samples which are not adjusted properly and which either filter poorly or not a t all exhibit a gel structure which is decidedly different, in appearance from the one described. During cooling of samples properly adjusted for concentration, a Type I11 latex containing electrolyte gels a t fairly definite temperature, and the colloidal properties of the heretofore stable material are altered. Viscosity increases, a network of small gel particles forms, and the entire mass becomes sensitive t o temperature change. Observed under a microscope with s u b t a g e illumination, the material consists of a mosaic of light and dark areas. Small and large masses of gel are present, and there is very little serum. However, as the temperature is lowered and the breakdown point of the gel is reached (roughly about 10’ C. below the gel point), a sudden change occurs in which the gel is transformed into a new state of agglomeration. Viscosity decreases sharply, and the stirring and chilling produce small spheres of gel of remarkable uniformity surrounded by clear serum. This oolitic structure is observed only in samples which yield serum rapidly during filtration and is absent in those that do not. I n samples which filter more slowly, the “fish egg” or “caviar” structure is less distinct, the spheres are less uniform, and a considerable number of smaller particles only a fraction the size of the regular spheres is present. The finer material appears to serve as a matrix between the larger spheres, giving thus a more rigid structure to the entire mass. DETAILS OF FIlVAL PROCEDURE
As a result of this work on the effect of variables on the operating efficiency of the concentration process, the following procedure was developed for concentrating Type 111 latex with sodium chloride:
1. Anv sample of Type I11 latex received is analyzed for soap and rubber content to obtain the milliequivalents of soap per gram of rubber present (9). 2. If the soap content is less than about 0.25 milliequivalent per grain of rubber, a solution of the potassium soap of K wood rosin is added to bring the soap content up to, preferably, about 0.28-0.29 milliequivalent of soap per gram of rubber.
NoCI Q Z 4 N
SOAP 0.275 MEP./GM.
g
a
“;
RUBBER
pn:11.4
Y
z
4-
2-
ML. OF SERUM REMOVED
Figure 4.
Effect of Foam on Filtration Rate
INDUSTRIAL AND ENGINEERING CHEMISTRY
160
Vol. 41, No. 1
The evaluation data, 011 thc coriceiitrittes indicate that a b o i i ~ PADDLE SIZEOX E ' r l A m ~ n o s 7-207, of the soap preqent in the adjusted latex niises and about SPEED 60-80q of the electrolj te added are removed nith the w t i i i n (Latex 303, p H = 11.3, roqin rioap = 0.275 meq. soap/g. r u b h r r , S R C ~ = during filtration. a pciieral rulc, the quantity of soap ~ i n ( l 0.24 A', slow stirring) electrolyte removed iricreaw. with increase in volume of sei u i r i 'relrly. oi Latex at S8lt Filtration taken out- i e , ihr higher the solids to which a latex is coilRxpt. Adiition, 'L'iinr (40 MI.), NO. C. Paddle Min. centrated The aoap rcatnRiiiiiig, which ranges from 0.22 t I i 0 20 2fi Small 2 9 iiiillieyuivalent per grant ol rubber, is e ntiallv the iaiiit: :I' 30 Small 4 1 was found in the original laticcs -namPly, 0 22-0.28. ' I l i ( 1 :t\h YO 1.7 Large Q-2 :30 Large 1 '3 u-3 contents found, I 2 l.7ry, 0 1 1 the concentlated latex, nw NIW essentially the sain(' \\.hen expreajed on the basis of i x l ) l x ~ r present-0.022-0.031 grain per gram of rubber coini)iwwi 1 1 1 TABLEIII. EYWECT OF AGING JATEX-SALT MIXT~-RW o v 0.022-0.030 initially. This applies esbentiallv also 1o tcil t i l R ~ TOF UFILTRATION lionrubber conrtitueritn (SRC), which amounted to 0.M; 0 I I (Latex 303, rosin soap = 0.276 ineq./g. rubber, p R = 11.4) Filtration Time, AIin. . gram pcr grain of rubbei in the original latices and t o 0.09- 0 Temp.. KaC1 Conc., Acing Time, C. hormality Hr. 20 ml. 40 nil. ,XI nil. in thc concentrates. Since the. roncentrates show 0.8 I 9' 25 0.22 0.0 1 .o 3.3 7 .i reGidual salt contents xiid since the ratio of NRC to ~ u h b rt ~i 0.22 2.0 1 .o 3.8 9.0 0.22 3.0 3 9 Poor .. practically the S R I ~ Pin t h r conrcwlrateb ab in the original 1 : i l i i n
'~..LRLE:
11. EFFNCT OF YTIRREIZ
8::
35
0.22 0.22 0.24 0.24 0.24 0.20 0.20 0.22 0.22 0.22 0.24 0.24
24.0 48.0 0.0
2.0
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3. The pH is nieasured; if it is below about 10.3--1I ..i,ctiough sodium hydroxide solution is added to increase it to this ratigc. 4. In the next step 3.0 N sodium chloride solution is slo\vly added t o the lates a t room temperature t o give a concentriition of approximately 0.24. S sodium chloride in the final .i~oluincof latex and salt solution. It. is essciitial t,hat all the additions to lal,ox be made with agitation. 5 . The latex-salt' mix is cooled AS soon as possible R ration by surrounding the vessc,l containing the latex bat,h. During cooling, t,he Inass must be agitated continuously to prevent gel build-up along the walls and to facilitate Iicat transfer. The cooling is continued through the gelled stage mid I h e
ter paper. The cold latex is poured over .the paper and suc1,ion imiiiediatcly applied. Enough serum is removed to gi\.e thc: solids content sought in the filter cake. Generally it is profoidde to overconcentrate the latex mass and then dilute it bank u.it8hwater, for under such conditioiis inore of the electrolytc. initially piwelit idual electrolyt I: uorit ciit of
0.994 0 . ios2 None
.j4,8
78.0 0.987 0.0764 0 . 0052
37.9 34.8 3.6 10.5 48.7 73.4
0.890
Normality of base Meq. soap/=. rubber AIeci. acid.'& rubbcr G. ash/p. rubber CJ. SIZC/g. ritbber
Eleetrolyto Sormality of salt J l e q . soap/g. rubber G. salt/g. rubber Gel uoint of mix. C.
NaCl sac1 NaCl 0.24 0.24 0.24 0,275 0.282 0,288 0.0409 0.0474 0 . 0 4 6 4 27 27 27
c%
blethod of Preparation
.i!i (i .iH , 0
63.3
6.2 1 .Bfi 0.82 0,140 0.261 7.5 0.0311 0 0134 fi2.R 1) 111; 10 7
.5 . 1 I .2Q 0.4fi 0.1311 0."6
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0 9!Ifi 24 2 6
_- H .,,), 60.4
14.7,. 0 ,02dd 0.0O83 82.5 n ,092 10.4 :35.n 0 :9'33 18
41.8 3 8 , :3 3.5 10.4 60.4
89.1
n.mo
0 , 0 8 6 8 0.0911 None None >-one None 0.92 0.80 Kone SUIlt! 0.255 0.240 None Nono 0.0233 0.0240 0.105 0 .0!Ic,O
None 1.13 0.70 o ,0130 S o n e 0.282 n ,230 None 0.015 0.0301 0.0220 0.081 (i,064 None
Solids Orisinal cone. Diluted cone. Rubber content, % W 7. Finally thc filter cake is removed from the paper, ~ I ~ ~ ~ I Nonrubber conatitusnt, ferred preferably L o a closed cont>aint:r,allowed 1.0 warm up to Ash content, o/c room temperature, aiid then blended by agitat,iotl. 1)ut.iiig this Salt content, 5% Meq. s o r ~ p / g .latex blending operation ivai,ev inay hr added to rcducc, thc: mlii1.j OOILMeq. soap/g. rubber trni. and t o obtain 1111: consistcxicy of latox desired. Soap reinored, T, C. auh,'g. rubber C . salt:g. rubher Salt removed, A \ A L Y b l S O k C,ONCS%I'R4TES G. S K C / p . rubiwr IJII (rooin temp.) The concentiatcaubtairied by the pxocetlure dmciibi 11 t i ( - ! t i i i l i ~ i Surface tensiun (2,-)" ('.,> Viscosity (23O C , ) ,cciitx to those obtained by other method5. The final producat I Density f 2 5 O C.) Gel or tltickeniriir p o i n t , smooth, and opaleaceiit. The final soap content and ail1 cuiltt,llt
per gram of polymer are about the Tame as in the late\ twiore concentration. The product is ielatively fluid, particulat 11 i i total solids ale 6OC; or loner. l b o v e approximately 60' 3011clr: the viscosity rises sharply. ,kt 70y0 and above, tlle coIlccIitrates are relatively thick paste3 151iich blend ieadily PI 1111 \$ atcr to stable dispersions R it hout coagulum. The concentrates YI ere prepared from latices obtained F10m several sources. Before use each of the latices nab evaluated for physical charactel istics, and analyzed for solids and r u b h ~ coili~ tents, soap, alkali or acid, and ash (Table IV). Tho>(.lxtwcs were concentrated by the method described, and the ti( ( > i i trates analyzed and waluated in the same manner (Tat)lv 1 )
38.8 35.4 2.9 8.3
t4 I 09.6 ,54 4 ,,i 2 1 32 0.52 0,128 0 230 14.2 0.0243 0 . 009s 79.5 0.045 10.7 34 fi 320 0.993
18
Crcaming 38.fi 54.8
3.8 82 2 II.0
....
1.004 660 4
37.9 35.0 2,!l 9.7
30 . I5 60.4 0.m
n.07.i.i N 0111' Yonl' 0.02 So1rr
0.218 Noni' 0 ,0203 O.ORB0
Y!,('l Xa('l 0.24 ll.Z%T, 0.282 0.0412 I I . O ~ ~ J
0.24 27
2X
January 1949
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
without salt, some of the nonrubber materials present in the original latices must also pass into the serum during filtration. In some physical properties the concentrates do not differ much from ordinary Type 111 latex. The density is close to unity, and the pH ranges from 10.0-10.7 which, if anything, is more constant than the pI-1 of the original latices, 9.3-11.7. The final surface tensions are somewhat lower-35-37 dynes per cm. compared to 44-55 before concentration. The most noticeable difference< are, as expected, in the viscosity, which around 60% solids vaiie- generally between 300 and 600 centipoises, and also in the fact that the concentrates prepared by this method exhibit an inherent gel point due to the small quantity of residual electrolyte present. Only a single measurement of the particle size of a sodium chloride concentrate ha- been made. I t showed tkat latex 303, with an original average particle diameter of 940 A,, gave after concentration an average diameter of 1390A. Table VI compares the properties of a typical concentrate prepared by the method of this paper and two high solids concentrates obtained by either evaporation or creaming. With the exception of the fact that our concentrate has a lower ash content and a fairly high gel point, all the concentrates appeal. to sboa- essentially comparable properties.
161
CONCLUSIONS
The results obtained with sodium chloride show that the method of concentrating Typc 111 latex described here is highly promising, and gives stable concentrates with properties not very different from f hose prepared by other means of increasing solids. Further, the method is simple and fast, and can yield latices of 70y0 solids and above. Pilot plant studies under the sponsorship of the Office of Rubber Reserve have shown that the process works well even in larger scale, and that suitably prepared Type 111 latex can be filtered readily on an Oliver filter. 4CKNOW LEDGMENT
This investigation was sponsored by the Offire of Rubber Reserve, Reconstruction Finance Corporation, as part of the Government Synthetic Rubber Program, and was first reported in September 1945. LITERATURE CITED
(1) Maron, S. I%., aud M o o r e , C . , Indiu Rubber World, 116,No. 6, 789 (1947). ( 2 ) Maron, S. H., Ulevitch, I. N., and Elder, M. E., method for det e r m i n a t i o n of soap in latices. Presented before the Division of Rubber Chemistry at the 112th Mret,ing of the AMERICANCHEVICAL SOCIETY. New Yo1.k. N. 1‘.
RECIIVEDOatober 3, 1947.
Pyrolytic Depolymerization ubber into Isoprene €3.
W. S. T. BOONSTRA
AND
G. J.
VAN
AMERONGEN
K u b b e r Foundation, Del&, Holland r .
I he depolymerization of natural rubber into isoprene under the influence of high temperature was studied. Several methods, discontinuous and continuous were tried on the principle of heating the rubber as rapidly as possible to cracking temperatures. The best method appeared to be the pressing of rubber b y means of a n extruder into a reaction tube o€ the desired temperature and pressure. After systematically varying the conditions for cracking, a yield of 5894 isoprene from crude rubber was obtained a t a cracking temperature of about 750’ C. and a pressure of 10 mm. of mercury. Good isoprene yields were obtained in a rather wide range of temperatures he-
tween 675’ and 800’ C.; however, the yield became rapidly less a t higher pressures. A t lower temperatures-e.g., 450’ C.-dipentene was the main cracking product. By studying the pyrolysis of isoprene itself in contrast with the pyrolysis of rubber, it became evident t h a t low pressure causes quicker has a twofold favorable influence-it “evaporation” of rubber a t lower temperature and shortens the contact time of pyrolysis products. When hevea latex was dropped into a cracking tube, the yield of isoprene from the rubber it contained was about 52%. Several synthetic rubbers and plastics could be depolymerized under almost the same conditions as rubber.
T
limonene vapors several times over a heated coil in high vacuum. Davis, Goldblatt, and Palkin (3) obtained similar results. The most important work on depolymerizing rubber into isoprene as carried out by Bassett and Williams (1). Previous investigators had mentioned isoprene only incidentally as one of the distilllttion products of rubber. Bassett and Williams attempted to obtain a maximum yield of isoprene by various methods of distillation. The best result, 16.7% of pure isoprene, was found by dropping pieces of rubber on a hot surface a t 600 O C. and ordinary piessure. The higher boiling fraction from the distillate, containing dipentene, wm treated in the Harries isoprene lamp, to obtain more isoprene. By combining the two processes, a total yield of 23% of isoprene was obtained from rubber. Bassett and Williams also dropped solutions of rubber on a heated surface, but the yield was then only 5.5% of isoprene.
HE decomposition of rubber by thernial action has frequently been a n object of research ( 7 ) . Mostly a mixture of a large number of unimpartant hydrocarbons has been produced. However one of these, isoprene, can be used as a raw material for the production of synthetic rubber, in the same way as butadiene. Rubber can be partially depolymerized t o isoprene under the influence of heat according to the reaction:
This depolymerization can be compared with the well known preparation of isoprene from dipentene or limonene. According to Staudinger and Klever (9) it is possible to obtain a field of 68% of crude isoprene, starting with pure limonene, by passing the
