Plasticizer-Filler Mixtures and Their Dispersion in Rubber FRITZ S. ROSTLER’ Lniversity of Delaware, Newark, Del.
HUBERT I. DU POST W i l m i n g t o n Chemical Corporation, W i l m i n g t o n , Del.
T
Inorderto obtaingoodfiller HIS paper dcals w i t h the Conieritional methods of incorporating plasticizers and dispersion in a rubber comuse of fillers and plasticifillers into rubber mixtures have many shortcomings inpound by conventional millzers in rubber compounding herent in the method of separate incorporation of these ing methods, two properties of and, more specifically, n ith two ingredients. The inFestigation reported in this paper the rubber matrix are most methods of incorporating deals with a method of simultaneous incorporation of fillimportant: [ a )The matrix has rhese ingredients into rubber ers and plasticizers in the form of premixed preparations. to bestiff enough to break up -oinpounds. (The terms It has been found that the ratio of filler to plasticizer of the filler pcllets or beads or ‘filler” and “pigment” are such premixed preparations is the deciding factor in their natural agglomerates of loose 11 s e d i n t e r c h a n g e a b 1y usefulness. A fixed ratio of filler to plasticizer exists for fillers, and also to grind them throughout this paper and ineach filler limiting the amount of plasticizer. The relainto the smallest individual slude all powdered insoluble tion between the plasticity and this ratio has been invesparticles; and ( b ) the matrix oading materials, reinforcing, tigated, and the experimental data are presented in tables has to be sufficiently plastic to tnd nonreinforcing fillers and graphs. It has been found Mith most fillers that the accept the filler readily and to rhe dividing line is too arbiratio of filler to plasticizer in a premixed preparation give a uniform distribution of *rary to wariant strict adshould be such that the addition of the mixture to the the filler throughout the mix. herence to special termimasticated rubber does not increase its plasticity o%er The prerequisite of a plastic riology.) P l a s t i c i z e r s a n d that of the masticated rubber itself. Examples are given matrix for successful uniform pondered fillers are used b j for this general rule and for exceptions to the rule. filler dispersion n as recogthe rubber industry in large nized intheearliest days of the m~ounts.and their handling tnd skillful application are of great importance for the economics rubher industry. Hancock’s invention of masticating rubber ranks equal in importance to Goodyear’s vulcmization. ,f rubber compounding. The dust problem, the bulk volume of the filler, and the free The prerequisite of stiffness is taken into account in the general flowing propel ties of both filler and plasticizer are the principal assumption that a stiff matrix !Till give better dispersion than a soft matrix because of the greater shearing action of the more ’actors in handling. The difficulties in haiidling plasticizers have viscous material. brought about the trend to prefer products of low viscosity which These two opposing principal requirements for obtaining best (’an easily be pumped, shipped in tank cars, and stored in tanks, tlthough the higher viscosity products have lon-er volatilitj and dispersion of fillers in rubber (stiffness sufficieni for grinding, and All-around better compounding properties. This fact points to plasticity sufficient for even distribution of fillers) are n-ell known; the desirability of having plasticizers in a physical state such that consequently, it has been generally stipulated that the best mixthey are all equally easy to handle; this would enable the coming procedure is to masticate, but not to overniasticate, the rubpounder to choose a compounding material in accordance Kith ber, to incorporate the filler into the rubber immediately after bretlkdown, and to add the plasticizer last. This general rule for performance in the compound and not as to its behavior in the -hipping container. One purpose of this work was to investigate compounding applies to both natural and synthetic rubbers, alrhis possibility. though the latter-for instance, GR-S-are, as a rule, stiffer and As to fillers, good progress has been made in reducing dust by also less sensitive to overmilling. The sequence of incorporating fillers first and plasticizers later the production of densified and pelletized fillers. These densified is, however, impractical for many manufacturing purposes, for it fillers, M hich are available in the form of pellets or beads, also have consumes too much time and power If medium or small amounts *headvantage of free flow under gravity. Hon-ever, considerable of plasticizer are used, they are usually incorporated before the amounts of fillers are still being used in loose form, and many of filler, and if large amounts of fillers and plasticizers are used, alrhe densified or pelletized fillers are subject to breakage during ternate incorporation is the routine procedure. The addition of handling, and are consequently not really dustless. A review of development in the field of improving the handling of carbon small amounts of plasticizer before incorporation of filler is sometimes an aid to reaching the plastic stage. Reduction of heat +lacks has been given recently by Drogin (3). The pigment industry has provided the rubber compounder generation during milling is another important purpose of plas*-iith highly developed pigments designed to serve specific purticizer addition in the early stages of the mixing process. Some plasticizers actually extend the hydrocarbon basis of the composes. The full use of characteristic properties of the individual pound and, through their swelling action, equalize and smooth fillers requires choosing of not only the right ingredients but also the rubber matrix, GR-S, to which has been added a small +heiraccurate application. amount of higher viscous plasticizer of the extender type, forms a It is well known and accepted that the dispersion of fillers in the rubber hydrocarbon matrix is one of the deciding factors for band around the mill which is smoother and more uniform in thickness than straight GR-S. Small amounts of plasticizers added the physical properties of a vulcanized rubber product. A poorly to natural rubber counteract the sensitivity of natural rubber dispersed high tear channel black will not give the desired high to overmilling which is especially detrimental to tear resistance. tear resistance of the compound; a poorly dispersed titanium One disadvantage of any of these methods of mixing is the unoxide will not give the desired whiteness. even consumption of power during the milling cycle. This dis1 Present address, Golden Bear Oil Co., Oildale, Calif. d
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INDUSTRIAL A N D ENGINEERING CHEMISTRY
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Vd. 39, No. 10
lumps, which, because of local lubrication, escape the shearing action exerted by the rubber and thus resist dispersion. Furthermore, dumping of plasticizer received in drums into the filler in the compounding room does not eliminate shipping and handling difficulties. I n a word, the conventional methods of incorporating plasticizers and fillers into rubber mixtures have many shortcomings inherent in the necessity of separate incorporation of these two ingredients. This applies to mixing both in internal mixers and on open mills. Mixing in internal mixers does not alter the influence of sequence of incorporation; it merely shortens the mixing cycle. PURPOSE AND SCOPE OF INVESTIGATION
PARTS BY WEIGHT PER I00 GR-S
Figure 1.
Effect of Plasticizer on Plasticity
advantage is especially noticeable where many mills or mixera are on the same power line. The method occasionally practiced of adding plasticizers and fillers a t the same time to the rubber gives satisfactory compounds in some cases, but in the majority of cases dispersion is poor. The reason is that the fillers and plasticizers form wet
0
IO0
50
PARTS
Figure 2.
BY WEIGHT P C ?
This paper suggests a method of general applicability consisting in simultaneous incorporation of fillers and plasticizers in the form of premixed preparation? containing any desired filler and plasticizer. The aim of this investigation was to test the possibilities of making premixedpreparationsoffillersandplasticizers3 and to find the conditions to be observed in production and use of such mixtures. The main objective was to find conditions which do not impair filler dispersion but which eliminate the shortconiings of separate incorporation and allow shortening of mixing time The shortening of mixing time is of great economic importance in the manufacture of rubber goods. As in every industry thew are few greater improvements in the economy of production than an increase in output of goods with the same manufacturing facilities. The ratio of filler to plasticizer of such premixed preparations is of considerable importance. The literature does not reveal any systematic investigation of this problem; no references exist in the literature regarding premixing of filler and plasticizers giving quantitative correlations and considering the characteristics of the reinforcing properties of the filler. An early patent of C. J Randall (6), dealing with dust elimination of blacks, suggests the
150 I00 G R - S
200
Effect of Nonblack Fillers on Plasticity
250
300
October 1947
Figure 3.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Effect of Carbon Blacks on Plasticity
mixing of black with tar dissolved in a volatile solvent, and evaporating the solvent. Dating from a time (patent application filed in 1917) when practically nothing was known about specific filler properties, all blacks are considered equal to lampblack in this patent. The ratio of black to plasticizer is not considered critically. The only exception where both handling and dispersion are considered is in the ink industry, where black preparations are used containing the black already dispersed. The dispersions are made, for instance, by grinding black in a nitrocellulose solution and evaporating the solvent. Preparations of this kind, being black dispersions, eliminate for the user the necessity of dispersing the pigment. The principle of this method is as old as the Chinese ink sticks (I). General industrial experience, as well as laboratory tests, however, have shown that, where dispersion of fillers in rubber during milling is to be accomplished, ordinary premixing of plasticizer and filler, in proportions called for by the compound or in proportions to give a dustless product, does not always give satisfactory results in terms of physical properties of the vulcanizate as compared with separate incorporation. From this general observation and a number of preliminary tests it was clear that the problem was one of obtaining good dispersion of the filler in the compound, and that certain preferred ratios exist for each fillerplasticizer combination to give the most desirable preformed misture. The basic concept of this study was, as indicated earlier, that the proportion of plasticizer and filler to be used in order to produce a satisfactory filler-plasticizer preparation is governed by the stiffening effect of the particular filler and the softening effect of the particular plasticizer on the rubber matrix. The assumption was also made that the shearing action of the matrix is fundamentally dependent on the viscosity of the masticated rubber itself, since the shearing action of the uncompounded rubber
1313
matrix must be responsible for dispersion of at least the first addition of filler. That a rubber mix containing poorly dispersed pigments can be only Eartially repaired by remilling or refining justifies this assumption. As the startingpoint of thisstudy theinvestigation of filler dispersion, which can be accomplished by milling a t the plasticity inherent in the rubber, seemed important; it appeared to be the first logical step in this investigatiou to find the ratio in which filler and plasticizer neutralize each other’s effect on plasticitj and then to premix filler and plasticizer in this ratio to obtain preparations which can be milled into rubber without altering the plasticity of the rubber matrix. The experimental approach to the probleni nil1 be clear from the description of the test and discussion of the results GR-S n as used as the rubber hydrocarbon for the principal pari of this investigation since it does not soften to any great extent on p-olonged milling as does natural rubber. Based on the considerations outlined above, experiments were carried out along the following lines: ( a ) plasticity measurements on series of GR-S compounds with gradually increasing amounts of plasticizers. ( b ) plasticity measurements on series of GR-P compounds with gradually increasing amount6 of fillers, (c) preparation of plasticizer-filler mixtures containing the t n o ingredients in various proportions, ( d ) measurements of plasticity and physical propertiw of GR-S compounds containing the preparations of (c) in comparison with separate incorporation of the components of the preparation, and ( e ) testing the results obtained vith GR-S as to their applicability to other rubbers. TEST METHODS
The Scott parallel plate plastometer provided with two mechanical stops and dial was found a satisfactory instrument for follotying up the gradual increase or decrease of stiffness of a compound with gradual increase of filler or plasticizer. The plasticity data obtained with the Scott plastometer plotted against amounts of filler give a straight line relation, in contrast to curves resulting from plotting the values obtained n ith other instruments (Nooney or Killiams) . The plastometer used was Model P-1 with both platens flat ( 7 ) . .I11 plasticity measurements were made at a temperature of212OF. in the platens, and under a load on the spindle of 15 pounds. Figures are reported as the compression in 0.001 inch of a standard cylindrical specimen approximately 1 7 / 1 6 inches in diameter 13.65 cm.) and 0.6 inch high (1.5 cm.) in 2.5 minutes under these conditions of test. Specimens for plasticity tests were prepared as follows: The rubber lvas broken down on the mill for 10 minutes and the stearic acid incorporated. (Sulfur and zinc oside, where used, nere added during the last part of the breakdown.) The plasticizer or filler under investigation was then incorporated, and the stock sheeted off the mill and allowed to rest for 24 hours. After resting 24 hours, the mixtures were sheeted out on a laboratory mill into slabs slightly thicker than 0.1 inch (2.54 mm.). Kith a punch l’/lsinches in diameter six disks were cut out and piled up to form the test specimen. The test specimen between two pieces of Holland cloth was then placed in the middle of the lower platen of the plastometer, and the weighted upper platen released from the first stop and allon-ed to compress the sample to 0.600 inch. If after 0.5 minute the platen had not reached the mechanical second stop a t 0.600
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INDUSTRIAL AND ENGINEERING CHEMISTRY TABLE
Ingredient Plasticizer
Parts by Wt. 100 00 As indicated
Filler Stearic acid
As indicated 0.50
GR-S
I. TESTFORML-LA Ingredient Zinc oxide B e n z o t b i b y l disulfide Diphenylguanidine Sulfur
Parts by K t 5.00 1.50
0 25 Varied according t o demand of rubber a n d plasticizer
TABLE11. EFFECT OF PLASTICIZER ox PLASTICITY
Plasticizer Tested Pine t a r
Saftolen LV
Saftolen R-100
Compression, 0.001 In. .~ Relative ComPn. of hlixture, Measured Measured c o r y r e s P a r t s b y mt. on mixture on GK-s s o n GR-S Plasticizer (Ci) (C2) (Ci - C2) 100 5 266 220 45 100 10 290 220 70 100 15 310 220 90 100 20 320 220 100 100 10 280 225 OJ 100 20 340 226 115 100 30 355 226 130 100 40 380 225 155 100 5 300 275 25 100 10 320 2i5 45 100 15 340 275 05 100 330 20 240 90 100 340 25 240 100 100 360 30 240 120 35 100 370 240 130 100 385 40 240 145 100 50 390 2-10 150 100 50 435 275 160 100 410 60 240 170 100 410 60 235 175
__
Saftolen hIV
Naftolen HV
100 100 100
10 20 30 40
100 loo 100 100 100 100 100 100 100
20 30 40 10
100
Paraffin oil
Mineral rubber m.p. 2 8 5 O F.
20
30 40 40
265 320 340 375
220 220 220 220
290 345 360 385 260 270 270 275 280
265 265 265 265 245 245 245 245 240
$ .j
100 120 155 25 80 95 120 15 25 25 30 40
inch, the stop was raised until the platen rested on it, and this dial reading recorded as zero point. At the end of an additional 0.5-minute warming up and conditioning period in this position, the stop was released and the dial reading recorded after a 2.5minute compression. The figures reported are the difference betveen the zero point (usually 0.600 inch) and this last reading. All tests were made in duplicate, and the average was taken of the two values. If the values did not check within loco, the test was repeated. This method was chosen as being simple in principle and operation, and at the same time expressive of the property under investigation-namely, the behavior of a larger rubber sample subjected t o squeezing. Although the experimental error is somewhat greater than TTith some other instruments, the test was found to be sufficiently accurate if duplicate samples were run in each test, and if tests were made on several mixings and results averaged. Stress-strain properties of the cured compounds, determined by the usual A.S.T.N. methods, were used as a measure of dispersion, under the assumption that higher stress strain values will indicate better dispersion, other conditions being equal. I n some cases tear resistance and resilience were also measured. The test formula used is given in Table I. PLASTICITY
EFFECT OF PLASTICIZERS. Figure 1shows the effect of increasing amounts of various plasticizers on tht? plasticity of GR-S. I n each series of tests the plasticity of the rubber hydrocarbon plus
Vol. 39, No. 10
0.5% stearic acid was measured after 10-minute breakdown. Plasticity of plasticizer mixtures is reported in the graphs as the difference between the plasticity of the mixture and the plasticity of the rubber hydrocarbon itself (with stearic acid). These values for relative comprcssion are expressive of the influence of the compounding ingredients independent of variations in the plasticit? of the rubber hydrocarbon; the values found made it possible t,o compare results and series of compounds made with different lots of rubber. I n other words the graphs were made by plotting the parts of the plasticizer on 100 GR-S against the increase of plasticity, using as zero on the scale the plasticity of the GR-S. The line for SRF black, plotted in the same manner, is given as a reference line. The broken lines are extrapolations of the straight portion of t,he plasticity curves made for convenience in reading the slope of the lines. The experimental data and the calculation of the relative compression are given in Table I1 as an example. All values reported as relative compression were obtained in the same manner. Column 1 lists the type of plasticizer tested; columns 2 and 3. the composition of the mixture tested; column 4 , the measured plasticity (compression in 0.001 inch) of the mixture: column 5 . the plasticity of the particular GR-S used; and column6, the rclative compression xvhich is plotted in Figure 1. The graphs in Figure 1, shon-ing the difference in effect on playticity of various pktsticizers, demonstrate that the influence of tht, viscosity of the plasticizer on plasticity of the rubber mix is not great when it comes to plasticizers of the same type. Line 3 , for instance, representing the influence of plasticity of three commercial plasticizer extenders in the medium-to-heavy viscosit? range, indicates that a !Tide variety of plasticizers arc interchanpable as to plasticity, and that the findings on one plasticizer will be applicable to most plasticizers of the same type. The type of’ plasticizer is obviously the deciding factor and the viscosity onlv of secondary influence. EFFECT OF FILLERS. Figure 2 shone the effect of increasing amounts of various nonblack pigments on the plasticity of GR-S. The plasticizer line (3) from Figure 1 is given as a reference line. Figure 3 s h o m data for carbon blacks and again the line for unsaturated hydrocarbon plasticizer. All point,s for the powdered dry fillers investigated fall om straight lines-that is, the change in plasticity is directly proportional to the amount added. PLASTICITY EQUIVALEXT
All lines representing the change of plasticity brought about by incorporating increasing amounts of plasticizer are curved. The
TABLE 111.
PLASTICITY EQUIVALENTS O F NONBL.4CK AND NAFTOLEN R-100 FROM FIGURE 2
Zinc oxide Red iron-bxide Talc Calcium carbonate Hard clay Magnesium sarbonate
P a r t s Filler E q u i v . t o 10 P a r t s Naftolen R-100 330
i_ n _i 95 _
Constant Plasticity hlixture. % Naftolen Filler R-100 97 3 92 _-
90 85 77 74
63 34 28
FILLERS
8
10 15
23 26
TABLE IV. PLASTICITY EQUIVALENTS OF CARBOSBLACKS AKD NAFTOLEN R-100FROM FIGURE 3
hlT black FT black S R F black Lampblack H M F black M P C black Acetylene black
parts ~ l Equiv. to 20 P a r t s Naftolen R-100 190 90 54 42 32 28 26
~
Constant ~ k Plasticity Mixture, % Carbon Naftolen Black R-100 90 10 82 I8 73 27 68 32 62 38 58 42 56 44
INDUSTRIAL AND ENGINEERING CHEMISTRY
October 1947
curves flatten with higher amounts of plasticizers. I n the practical range, however, a straight line (slope indicated by broken lines in Figure 1) can be used graphically to represent the plasticizer curve without introducing very great error. The number of parts of the various fillers equivalent to a given number of parts of a particular plasticizer in terms of influence on plasticity can be read from the graphs. These proportions of plasticizer to filler will result in nonchanging plasticity independent of the amount of the mixture added to masticated rubber, within Ride limits. This ratio of filler to plasticizer 17-hich does not influence the plasticity of the matrix can be called, for the sake of simplicity of expression, the constant plasticity ratio. The constant plasticity ratios read from the graphs for each filler with an extender-type plasticizer are given in Tables I11 and IV. S R F BLACK AND PLASTICIZER
I n order to have a complete picture on one combination of plasticizer and filler, a more detailed study was made with a higher viscous plasticizer and an S R F black. Screened Pelletex was used as S R F black and a laboratory-produced Saftolen R-100 as the higher viscous plasticizer, since exact reproducibility of results appeared essential for a detailed study involving the examination of relatively small variations in composition. The tcsts on this combination are reported in Figure 4,which s h o w the relative compression plotted against parts by weight of totaladdition to 100 GR-S (sum of plasticizer and black). The line labeled “0 Black” is identical with the line for Saftolen R-100 in Figure 1 and the plasticizer line in Figures 2 and 3. The line labeled “0 Naftolen” is identical with the line for S R F black in Figures 1 and 3. The other lines show the influence of combinations of plasticizer and S R F black on plasticity. For instance, all points on the line labeled “30 Saftolen” represent relative plasticities (as compared 11-ithstraight GR-S) of miytures containing 30 parts Saftolen and various amounts of S R F black on 100 GR-S. The in200 tersection of this 30 Xaftolen line with the 100 parts-byn-eight background line represents the plasticit,y of a GR-8 mixture containing 30 Naftolen and 70 S R F black; the intersection with the I50 parts-byweight background line, the plasticity of a GR-S mixture containing 30 Naftolen and 120 SRF black. Figure 4 shows that the in~ ~ - 5 . 0 fluence on plasticity of plasticizer and filler is strictly additive up to very high concentrations.
1315
ure demonstrates that the additivity of plasticity influencch of plasticizers and fillers, shown by Figure 4 for separate. incorporation of these ingredients, also holds for premixed preparations. I t can be seen from this figure that the constant plasticity ratio (preparation containing 73 parts of S R F black and 27 parts of a viscous extender-type plasticizer) does not alter the plasticity of the compound, regardless of how much of the preparation is added to the rubber matrix u p to more than 100 parts of the preparation on 100 of rubber. Corresponding results were obtained with other fillers mixed with plaeticizw in the constant plasticity ratio. COMPARISON O F PLASTOMETERS
Figure 6 shows with a few plasticizers and fillers a comparison of the curves obtained by plotting plasticity values measured with a Scott, TKlliams, and JIooney plastometer against parts addition to 100 GR-S. I n each case the relative plasticity (plasticity of the stock minus plasticity of GR-S) was the figure used in the graph. This rather self-explanatory figure s h o w why the Scott plastometer was used in preference to the other two for the purposes of this investigation. The straight line relation obtained simplified the approach to the problem. [The Goodrich plastometer appears to be another instrument well suited for this type of investigation, as can be seen from the data for plasticity reported by Garvey and Freese ( 4 ) , which can b3 plotted to give straight lines similar to those obtained with the Scott plastometer in this investigation.] However, a filler-plasticizer ratio which does not alter the Scott plasticity value of the mix does not alter the Killiams or the llooney plasticity either. TESTING OF RUBBER COMPOCNDS
The influence of the ratio of filler to plasticizer in premixed preparations on filler dispersion in rubber compounds was inves-
PRElIIXED FILLERPLASTICIZER PREPARATIONS
Based on the findings outlined, a number of mixtures consisting of fillers and plasticizers in various ratios, including the constant plasticity ratio, were prepared, and their effect on plasticity was measured using GR-S as rubber hydrocarbon. Figure 5 shows the results graphically on three ratios of S R F black to plasticizer. The Saftolen and the SRF black lines are given as reference lines. This fig-
0
Figure 1.
50
100 PARTS BY WEIGHT
150
200
W T O L E N PLUS SRF E L A W PER 100 GR-S
Additivity of Influence of SRF Black and Naftolen on Plasticity
250
INDUSTRIAL AND ENGINEERING CHEMISTRY
1316
Vol. 39, No. 10
tion containing 67y0 HMF black and 33yGextender-type plasticizer with those obtained by separate incorporation of Com,pound No. 1 2 3 4 5 the two components. The preparation GR-S. 100.00 100.00 100.00 100.00 100.00 HMF 67 is not exactly at the constant Stearic acid 0.50 0.50 0.50 0.50 0.50 Sulfur 1.80 2.00 2.20 2.61 3.42 plasticity ratio but has a slight stiffening Zinc oxide 5.00 5.00 5.00 5.00 5.00 SRF black ... 18.25 36.50 73.00 146.00 effect on the uncured stock. Philblack Naftolen M V ... 6.75 13.50 27.00 54.00 A was used as HMF black in these tests. Bencothiazyl disulfide 1.50 1.50 1.50 1.50 1.50 Diphenylguanidine 0.25 0.25 0.25 0.25 0.25 Other constant, plasticity ratios might be Filler: plasticizer ratio ... 73:27 73:27 73:27 73: 27 found for other brands of HMF black. Filler-plasticizer incorporation *_ (Concentrations of plasticizer lower than Added premixed, A ... -A- _B* _A - _ B A B A B Added separate1 B the constant plasticity ratio are, for Mixing time after g;eakdown, mi?. 8.5 12.5 15 l4 24 practical compounding purposes, preferScott plasticity, compression I n 0.001 in. 250 260 265 240 270 240 260 190 230 able to t,he constant plasticity concenOptimum cure at 45 Ib., min. 50 50 50 50 50 50 50 50 50 Tensile at break, lb./sq. in. 280 620 700 1310 1280 1710 1680 1660 1700 trations.) The data for mixing time Shore Elongation hardness, at break, 30 sec. % show the advantage of premixed preparations in this ratio. Table VI1 s h o w one soft rubber, one hard rubber compound, and their tigated, using the physical properties of the cured compound as controls employing a premixed clay-plasticizer preparation of criteria. The physical properties of vulcanized compounds made the constant plasticity type. The data indicate, as in the with these filler-plasticizer preparations were compared with preceding tables, that the dispersion of filler did not suffer physical propert'ies of compounds mixed by separate incorporafrom the shorter milling time of the compounds made with tion of the two component$. Mixing times of the compounds the premixed preparation. The mixing of the compound was were noted, and the behavior of the preparations during milling difficult with separate incorporation of plasticizer and clay. observed and compared with separate incorporation of their comThe compound was extremely sticky and ran on both rolls of ponents, in order to judge the practical advantages of simultanethe mill. ous incorporation. First a fex series of tests measuring stressThe mixing of the compound using the premixed preparation strain properties were carried out' comparing dispersion by sepacould be carried out without any difficulties, and the stock rerate incorporation with dispersion of premixed preparations of the mained on the front roll during the entire mixing. constant plasticity ratio. The test' results are recorded in Tables V to VII. Tests as to the effect of variation of ratio are shoyn in Table VIII. Table V gives the results of tests made on compounds containTABLE CLAY-PLAsT1C1ZER IN PLASTICITY RATIO ing increasing amounts of an SRF black-Naftolen mixture of the S o f t Rubber Hard Rubber GR-S constant plasticit'y composition. Compounds 2B, 3B, 4B, and 100,00 100.00 5B are the control mixtures in which the components of the preStearic acid 1.00 1.00
TABLE V.
CONPOUNDS CONTAINING SRF BLACK-PLASTICIZER PREPARATIOKS OF CONSTANT PLASTICITY RATIO
44640 4zg 2';
'2; 4iy '42;
mixed preparations have been incorporated separately. The data show that the same physical properties are obtained using the premixed preparation as with separate incorporation; this indicates the same degree of filler dispersion in spite of t'he difference in time necessary to complet,e the misings. The mixing time report,ed in the tables is the minimum time required after breakdown t,o obtain a uniform mix in accordance with general laboratory practice. Table VI reports three pairs of GR-S compounds comparing the physical properties obtained by compounding TTith a prepara-
TABLE
VI.
PREPARATION HlIF 67
Compound No. GR- S, Stearic acid Sulfur Zinc oxide HMF black Saftolen M V Benzothiaeyl disulfide Diphenylguanidine Filler: plasticizer ratio Filler- lasticizer incorporation ad&d premixed, A Added separately, B Mixing min. time after breakdown Scott plasticity, compression in 0.001 in. Optimum cure at 45 lb., rnin. Tensile Modulusatatbreak 300%lb./sq, in, Elongation at b;eak, % Shore hardness, 30 seo. Tear resistance, lb./in. Lengthwise Crosswise Resilience (Luepke) 1st impact 4th impact
1 100.00 0.50 2.33 5.00 33.50 16.50 1.50 0.25 67: 33
IN
GR-S
2 100.00 0.50 2.66 5.00 67.00 33.00 1.50 0.25 67:33
-------A
B
A
65
280 65
1470 860 420 47
14,0 920 410 50
265 65
210 65
2010 1560 2ooo 1650
l,jjo
400 56
370 59
64 18
62 17
B
290 64
215 65 260
66
iig it; 54 10
53 10
77:OO 23.00
... ...
2.00 0.10 77:23
...
Accelerator 55zb Fil1er:plasticirer ratio
Filler-plasticizer incorporation Added premixed, A Added separate1 B bIixing time after gieakdown, min. Scott plasticity, compression in 0.001 in. Optimum cure, min. At 45 lb. . At 80 lb. Tensile at break, Ib./sq. in. Elongation at break, % Shore hardness Durometer A (30 sec.) Durometer D
42.00
...
, " ~ ~ ~ ~ $ ~ o f ~ n e
b
21
65
;$ 71 26
A
2.50 3.00 77.00 23.00 1.50 0.25
~ ~ ~ ~ ~ x i d e Suprex clay Kaftolen M V Benzothiaeyl disulfide
a
7 -
B
280
iTg :i 72 28
0.50 2.99 5.00 100.50 49.50 1.50 0.25 67:33
18
13 285
3 100.00
2:9 6' ;
77:23 7----7
A
B
11 350
30 340
80
80
950 750
900
39
40
740
......
7--7
A
B
9 340
24 330
...
120 7000 5
iib
6300 3
.85. . . .86.
Butyraldehyde-aniline condensation product. Piperidinium pentamethylene-dithiocarbamate.
The next series of tests dealt with the influence on dispersion of ratios other than the constant plasticity ratio. Table VI11 reports the physical dat,a on compounds containing four ratios of S R F black to Kaftolen MV. In compound 1 the proportion of black to plasticizer is greater than the constant plasticity ratio, in compound 2 the ingredients are in the constant plasticity ratio, and in compounds 3 and 4 the proportions of black are lower than this ratio. In compounds 1 and 2 the use of the premixed preparation (A) and separate incorporation of the ingredients (B) gave equally good results. With compounds 3 and 4 the compounds made by separate incorporation were better than the compounds using the preparations containing plasticizer in concentrations higher than the constant plasticity ratio.
INDUSTRIAL AND ENGINEERING CHEMISTRY
October 1947
1317
to handle. Higher proportions of plasticizer in the premixed preparation resulted in lower physical properties than did separate incorporation. In subsequent tests the other pigments for which the constant plasticity ratios had been established (Tables 111 and IV) were tested in order to determine whether the observations too made on S R F black offer the possibility of formuw v1 lating a general rule based on plasticity as to disU persion of fillers contained in premixed fillerz plasticizer preparations. It appears practical to consider the blacks and nonblack fillers separately. The findings on SRF black were confirmed by the thermal blacks. The limit to the amount of plrtstioizer which could be premixed with the filler *to give satisfactory dispersion fell a t the constant plasticity ratio (Table IX). The limit to the amount of plasticizer which could be used in a satisfactory preparation with channel blacks and with the structure blacks, acetylene black arid lampblack, did not coincide with the constant plasticity ratio. [The classification of these blarks as “structure” blacks and their abnormal behavior in general are discussed by Riegand and eo-workers (2, 8, 9 ) ] . Channel blacks gave useful preparations only if premised with much smaller amounts of plasticizer than corresponds to the constant plasticity ratio. Acetylene black and lampblack, on the other hand, could be premised with higher 1 I I >O 20c 0 50 100 amounts of plasticizer without detriment to the PARTS BY WEIGHT PER 100 GR-S properties of the vulcanizate. (Whether HhfF Figure 5. Effect of Premixed Plasticizer-SRF Black Preparations on blacks, also structure blacks, have a higher tolerPlasticity ance for plasticizer will have to be determined. Tests carried out so far are contradictorv with SIGNIFICANCE OF CONSTANT PLASTIClTY RATIO various brands of black classed as HhfF black.) These two observations, (a) that higher amounts of plasticizer I t was assumed a t the start of this investigation that there is a can be used with structure blacks than limited by the constant Limit to the amount of plasticizer which can be premixed with a plasticity ratio, and ( b ) that lower ratios of plasticizer to filler have filler and still give good filler dispersion in rubber. With S R F to be used with channel blacks in order to obtain filler dispersion black this limit proved to be a preparation containing the two comequal to the dispersion obtainable by separate incorporation, reponents in such proportions that the addition of the preparation veal that the plasticity inherent in the rubber itself is not the only to the rubber resulted in a mix of the same plasticity as that of consideration, and that other factors will have to be brought in rubber itself. Preparations containing this proportion of plasfor an all-inclusive theory. ticizer or less gave vulcanizates with physical properties not inferior to those made by separate incorporation of the two ingredients, required shorter mixing time, and were considerably easier
l
TABLE Ix.
PREPARATIONS CONTAINING
Compound No.
TABLE
VIII. EFFECTO F DISPERSIONOF VARYING RATIO O F SRF BLACK TO PLASTICIZER ,
Compound No.
GR-S,
dteario acid Sulfur Zinc oxide SRF black Naftolen M V Benzotbiazyl disulfide Diphenylguanidine Filler: plasticizer r a t i o
1 100.00 0.50 2.47 3.00 80.00 22.50 1.50 0.25 78:22
--
Filler-plasticizer incorporation A B Added premixed, A Added separately, B Mixing time after breakdown, min. 12 17 dcott plasticity, compres230 240 sion i n 0.001 i n . Opti,mum cure a t 45 Ib., min. 50 50 Tensileatbreak Ib./sq.in. 1810 1670 420 410 Elongation a t bieak, % Shore hardness, 30 sec. 59 57
2 100.00 0.50 2.67 3.00 80.00 29.00 1,50 0.25 73:27
---A
10
B
3 100.00 0.50 3.25 3.00 80.00 48.50 1.50 0.25 62:38
22
7 340
50 50 50 1680 1700 1230 380 430 430 55 55 47
8: 1.50 0.25 59:41
6tearic acid Sulfur Zinc oxide F T black (P 33) M T black (Thermax) Naftolen M V Bencothiazyl disulfide Diphenylguanidine Filler: plasticizer ratio Constant plasticity ratio
....
2 100.00 0.50 2.60 5.00 70.00
15.00 30‘.00 1.50 1.50 0.25 0.25 85:15 70:30 82:18
-
TIIERXAL BLACKS 3 100.00 0.50 2.16 5.00
....
4 ILOO.00
92.00 8.00 1.50 0.25 92: 8 9O:lO
0 50
2.30 5.00
....
85 00 15.00 1.50 0.25 85:1.5
-
Filler-plasticizer incorpo7 d ration 4 B A B A B A R Added premixed, A Added separately, B e-Mixing time a f t e r breakB A B down, min. 7 13 5 14 9 14 9 16 Scott plasticity, compression i n 0.001 in. 195 220 350 355 220 215 275 280 ODtimum cure a t 45 lb.. 28 11 23 .min. 50 50 65 65 50 50 50 50 Modulus 365 355 365 A t 300% 720 660 380 340 870 860 660 690 50 50 50 A t 500% 1190 1040 710 700 1030 950 Tensile a t break, Ib./sq. i n . 1300 1230 1220 1460 l2iO 1220 1260 1410 1520 1080 1380 Elongation a t break, ?’ & 540 530 660 700 490 480 610 640 500 420 470 Shore hardness, 30 sec. 56 56 54 44 45 46 45 44 59 58 55
----A
270 285
4 100.00 0.50 3.48 3.00
GR-5,
1 100.00 0.50 2.30 5.00 85.00
INDUSTRIAL A N D ENGINEERING CHEMISTRY
1318
GR-S
100 00
100 00
100 00
100 00
100 00
100 00
0.50 2.88 5.00 56.00
0.50 3.00 5.00 50.00
0.60 3.20
0 50 2 48 5.00
0.50 2.64 .5 00
40.00
0 50 2.70 5,OO 15.00
44.00 1,50
50.00 1.50 0.23
60.00 1.50 0.25
....
Smoked sheets Stearic acid Sulfur Zinc oxide Acetylene black Lampblack Kaftolen RIV Benzothiazyl disulfide Diphenylguanidine Mercaptobenzothiazole
56 : 44 _i__
A
B 275
50
50
1 100 00 0 50 3 00 5 00 60.00 40 00
Diphenylgkidine Filler: plasticizer ratio
1 50 0 25 60: 40
....
295
21 315
360
25 380
50
50
50
50
6
4
70:30
1060 1660 1690 600 53
690 840 520 380 1350 1440 1020 820 l e 5 0 l+I@ 1160 1130 0.10 a00 550 610 48 -18 39 41
_--
3 100 00 0 50 2.80 d 00 67 00 33 00 1 50 0 25 07:33
3 100 00 0 50 2.55 ;7.00 75 00 25 00 1.50 0 23 --..,,>.-J
,
F
13 350
6 245
23 250
280
35
50
50
35
3 335
83
.\.IPCHI241’K TO 1’1 4 100.00 0.50 2.40 5.00 80.00 20.00 I . 50 0.25 8O:ZO
010 1050 1170 570
300 ,.. 590 470 35
55
,jig
Compounding tests illustrating the mentioned exceptions are reported in Tables X?nd XI. The compounds in Table X show that a premixed preparation of 30 acetylene black to 70 plasticizer is still a satisfactory preparation in terms of stress-strain properties, although the constant plasticity ratio is 56 acetylene black to 44 plasticizer. (Smoked sheets compounds included in this table were tested in order t o raise the physical properties of the compounds above a level where the experimental error easily falsifies the results.) With lampblack the constant plasticity ratio determined was 68 parts to 32 parts plasticizer, although a preparation in the ratio of 60:40 gave satisfactory results. Various explanations could be advanced for the fact that higher plasticizer concentrations than are in accord n i t h the constant plasticity ratio are permissible in structure black preparations. However, it is premature to make any statements on the hasis of as few data as are presented in this paper, except that the softening effect of the preparation does not seem to interfere with dispersion as is the case with SRF black. Table XI demonstrates that the critical concentration for MPC black contains much less plasticizer than corresponds to the constant plasticity ratio, which is 58 black to 42 plasticizer. The first compound of this series s h o m the influence of poor dispersion very convincingly (tensile strength 1460 for the premixed preparation against 2230 for separate incorporation) The S0:20
~~
6
100.00 0.50 .5 00 65.00
15.00 1.50 0.23
100.00 0.50 2 10 5 00 90.00 10 00 1 50 0.25
8 5 : 15
U!): 10
A-
3i:On
....
....
....
56:i.l
5
630 970 1150 560 .I,>
,
400 ,560 1360 790 43
30: 70
/-
B
.I
B
23 285
8 335
16 350
35 ,
,
380 600 1060 680 46
.
.
I
35
I . .
33
830 850 1660 1700 2000 1960 ,540 560 50 50
--A_-
.I 4
420
~i lii
445
. . . .
3.5
36
360 670 1950 750 83
340 670 2070 760 33
\\TICIZER
_-I-_ ... . . Filler-plasticizer incorpora- -I---n-tion A H A B A H A B A N 4 n Added premixed, A .4dded geparately, B Mixing down, time min. after break11 33 12 29 13 21 E 14 10 li 11 17 Scott plasticity, roinpression in 0.001 in. 260 265 220 220 110 123 110 105 80 60 tiU (io Optimum cure at 4elow 60 mesh, too little. Uniformity of composition is a very important feature of the premixed filler-plasticizer preparations. I t is easy to visualize Lhat local changes in plasticity of the rubber matrix should be kept a t a minmum so that the influence of the small amounts of plasticizer and filler contained in each pellet of the preparation we neutralized continually during the milling process. Lack of uniformity interferes with dispersion and prolongs the milling time. PROPERTIES. The preparations obtained in the pilot plant operation were in the form of small pellets when loose fillers 7% ere used as starting materials, except when very small amounts of plasticizer were used. When starting with fillers in the form of
beads or pellets, the preparations were also in the form of beads or pellets. In all cases the finished product was considerably less dusty than the original filler. \\-ith loose fillers a very substantial reduction in bulk was also achieved. Some of the plasticizer-filler mixtures of .the ratios established in this investigation, produced commercially [Naftex, of \Vilmington Chemical Corporation), have been tested in factory compounding; the laboratory results of this study were then duplicated on large scale. CONCLUSIONS
The aim of this investigation, to find the ratio in which plasti: cizer and filler are to be premixed to give satisfactory results in terms of physical properties of the vulcanizates and easier handling of the ingredients, has been achieved. It \vas found that the ratio in which filler and plasticizer can be premixed for satisfactory use was, in all cases investigated, with the exception of channel black, any ratio which does not increase the plasticity of the rubber matrix above the plasticity of the masticated rubber itself. Best results were obtained when the premised preparation had a stiffening effect. Higher percentage of plasticizer than stipulated by this constant plasticity rule can be used with structure blacks. Channel black preparations should not contain more than 2070 of an extender-type plasticizer. (The 85:15 and 9O:lO channel b1ack:plasticizer preparations, which are the most practical ratios for tire compounding. are in this range. Fifty parts of the latter preparation on 100 rubber, for example, would be 45 black and 5 plasticizer.) The quantitative relation and the reasons for the exceptions from the constant plasticity rule are being investigated, and the results will be reported as soon as the investigation is completed. The following generalizations can, however, be made on the strength of the tests carried out up to date: 1. A critical ratio of filler to plasticizer exists for each pair; this limits the amount of plasticizer n-hich can be mixed with a filler previous to incorporation in rubber. 2. A rule of thumb, applicable to all fillers except channel black and structure black, can be formulated that the proportion of plasticizer in a premixed preparation should be a t least low enough to keep the plasticity constant throughout the mixing process, independent of the amount of premixed preparation added, in order to disperse satisfactorily in a rubber mix and t o facilitate handling. The investigation reported dealt principally with establishing of the frame for further work. The constant plasticity rule as formulated in this paper is only a guide until such time as the particulars for each filler and plasticizer are thoroughly established.
1322
INDUSTRIAL AND ENGINEERING CHEMISTRY
The practical advantages offered by premixed plasticizer-filler preparations in the proportions determined are: 1. A considerable saving in milling time over the practice of separate incorporation and consequently an increase in output of finished goods without enlargement of mixing facilities 2. Lower heat development in the milling piocess 3. Lower and even power consimption in the milling process 4. Easier handling 5. Cleaner \Torking conditions 6. Elimination of the necessity for the compounder to choose plasticizers as to their handling instead of as to their performance in the compound 7. Elimination of containers for shipping and handling liquid plasticizers. ACKNOWLEDGMENT
Richard Weil of the rubber laboratory of Kilmington Chemical Corporation was most helpful with supervising the testing of rubber compounds. This valuable cooperation is greatfully acknowledged.
Vol. 39, No. 10
LITERATURE CITED
(1) Coluinhian C a r b o n Co., “ C o l u m b i a n Colloidal C a r b o n , ” Vol. 1 p. 14, N e w York, 1938.
(2) Ibid.. Vol. 3 , p. 77, S e w York, 1942. (3) Droein, I., “ D e v e l o n m e n t a n d S t a t u s of C a r b o n B l a c k , ’ ’ UD. 3943, C h a r l e s t o n , \*, Va., U n i t e d C a r b o n Co., 1945. (4) G a r v e y , B. S., Jr., a n d Freese, J. A, Jr., IND.EXG.C H E M . ,34,
__
1277 (1942).
( 5 ) M e m m l e r , K., “Science of Iiuhher,” E n g l i s h t r a n s l a t i o n e d . b> R. F. D u n b r o o k a n d 5’. N. M o r r i s , p. 497, Now York, Reinhold Pub. Corp., 1934. (0) R a n d a l l , C. J., U. S. P a t e n t 1,105,439 ( F e b . 7 , 1 9 2 2 ) . ( 7 ) S c o t t Testers Inc., Catalog 45 (1945). (8) Swveitzer, C. IT.,and Goodrich, W. C.,‘Rubbrr Age ( S . Y.), 55 469 (1944). (9) W i e g a n d , Iy.B.. Can. C h e w Process Inds., 28 (3), 151 (1944).
PRESENTED before the Division of Rubber Chemistry at t h e 110th Sleeting of the AMERICANCHEMICAL SOCIETY,Chicago, Ill.
Solvent Seuaration of Hvdrocarbon I Mixtures by Vapor-Liquid Extraction J
*
RI. R. FENSKE, C. S. CARLSONl, AI\;D D. QUIGGLE The Pennsylvania State College, State College, P a . Vapor-liquid equilibrium data at one atmosphere pressure have been obtained for the system methylcyclohexanetoluene containing various proportions of aniline to alter the volatility of this pair of hydrocarbons. These data illustrate various features of vapor-liquid extraction or extractive distillation. The term “gamma” is used to define the volatilities that exist in the presence of sol\ents in order to prevent confusion with ordinary distillation processes. A compact, rugged, vapor-liquid equilibrium apparatus that is particularly suitable for mixtures which give liquid-phase separation on cooling is described. Simultaneous determinations of equilibrium temperatures and vapor-liquid equilibria are possible with this apparatus.
D
ISTILLrlTIOS has been studied extensively in recent year. as a useful and simple method for separating hydrocarbon mixtures. Laboratory and pilot plant size packed distillation columns, which are usually evaluated in terms of H.E.T.P. (the height of column equivalent to one theoretical plate) have also been investigated in some detail. Data obtained during the past several years a t this laboratory show that, with wire-helix-type packing, H.E.T.P. values essentially equivalent to the column diameter are obtained for column diameters varying from about I to about 10 cm. These figures indicate the ease with which one theoretical plate can be obtained in such equipment. Countercurrent, multistage, liquid-liquid solvent extraction has not been studied so extensively as distillation because it is a relatively new operation, and is usually more difficult to understand and apply properly. Data which have been obtained on the height of a theoretical extraction stage in laboratory, packed, countercurrent extraction columns usually show that the contacting efficiency of such columns is low in comparison with the same packing and column employed in fractional distillation. I n laboratory and pilot plant size countercurrent liquid-liquid extractors, packed columns are commonly used. The packing and several features of such columns are usually similar to laboratory and I
Present address, Standard Oil Development Company, Elizabeth, N. J.
pilot plant size fractional distillation columns. However, FThta extraction and distillation columns with comparable packing and dimensions are evaluated in terms of their separating efficiencics. it is frequently found that the height equivalent to a theoretical stage or plate in the extractor is from five to twenty times a-: large as the height equivalent to a theoretical plate in the distillation column. Depending on the solvent, however, a liquidliquid extractor may produce good separations because the selectivity of the solvent as expressed by bet,a (17, 18) may be considerably better than the relative volatility found in conventional distillation and expressed as alpha (19). Varteressian (17, 18) pointed out that, for the separation of n-heptane and methylcyclohexane, about nine theoretical plates in distillation are needed to give the same separation as one theoretical stage in liquid-liquid extraction when aniline is the se1t.ctivc solvent. Despite such advantages of liquid-liquid separational processes, the problems of accumulating twenty or niorc. theoretical liquid-liquid extraction stages in a single compact arid relatively simple countercurrent operation have not yet bew wholly solved. The purpose of this study was to obtain sufficient physical chemical equilibrium data to establish a procedure for evaluating methods of separating hydrocarbon mixtures which would retain the usual efficiency-Le., relatively low H.E.T.P. values--of fractional distillation and the generally better selectivity of solvent extraction. This, in principle, could be accomplished by scrubbing an ascending vapor stream, rich in hydrocarbons, with :i liquid phase containing a solvent having a greater affinity for one component, or class of components of the hydrocarbon mixture than for the others; the relative volatilities of the various hydrocarbon components \Tould thus be advantageously changed froni the values that normally prevail in Grdinary fractional distillatiori. PRE\ IOUS INVESTIGATIONS
In recent years there has been an increasing number of studieof the vapor-liquid equilibrium relations that exist in various hydrocarbon and norihydrocarbon systems. In a large portion of
