24M (8)
INDUSTRIAL AND ENGINEERING CHEMISTRY Kirschbsum, E., "Distillation snd Reotificeation," p. 363. tr. by M. Wulfinghoff. New York, Chemioal Publiehing Co..
1948. (91 Kimohbeum, E., and Geretner, Verbhrmalechnik, 1, 10 (1939). (lo) k g d a n , W. M.. and K ~ S ~ D. S , B., x m . EW. cIIEM., 34, 938 (1942). (111 NaW. Bar. Standards, Circ. 19 (19241. (12) Othmer. D. F., A d . Chem., 20, 763 (1948). (13) Othmer. D. F.. IRD. E m . CHEW., 20,743-6 (19281.
Vol. 43, No. 11
"Chemioai Engineer'@Handbook." 2nd ad.. p. 391. New York. McCraar-Kill Book Co., 1941. X.M., and Thompson. A. X., IND.Em. CBEW.,41,
(14) Perry, J. H.,
(151 Xieder,
2905 (19491.
(161 Werkman. C. R.. snd Oshm. 0.Z.. IND.Em. CHSM..ANAL. Eo.. 3, 387 (1931). R ~ c n v r oSune 8. ISM. P m n t s d before the Division of Indwtrid and Eodneerins Chsrniatry a t the 117th Xeetine of the A%rseicmC ~ ~ U I C I L S O ~ B nouston. ~ ~ , T ~ ~ .
Carbon Gel as Revealed by the Electron Microscope W. A. LADD AND M. W. LADD Hesearch Laboratories,
Colunbian Carbon Co., New York l i , Y. 1. T h e carbon gel mmplex described by Sweitzer e t et. plays P basic role in t h e reinforoement of synthetic a n d natural rubhere, particularly for tire tread use. Its existence bas heen arrived a t by deductive reasoning. I t was eonaidered desirable t o photograph this complex u)establish i t s identity definitely. New techniques h a d to be developed t o differentiate between sol rubber, unpigmented gel rubber, a n d t h e oarhon gel m m p l e i itself. T h e photogzaphs demonstrate clearly t h e aggregation of carbon a n d its associated gel rubber a n d m a r k another step in t h e rationalization of esrhon cireots i n rubber.
F i ~ s r e1.
Low Temperature Polymer Gel (X7500)
T
HE elwtrm miarmcope, thiough its nhiliiy to %ee" B carbon black particle, h w bcoome B vital tool for the study of oarban in its fieIda of appliciltion. By its UEB, it wy&8possible to conceive (B), and later prove, the existence of reticulate ohsin StNotUre in carbon blscks and the degree to which this structure persisted through various stagea of processing of B carbon-reioforoed rubber compound (6). These persistent eliiiins were shown t,o be the primary factor the development of hardness properties. As each mom&Ious behavior ia resolved snd rstionslieed it opens the way to new concepts. One such concept ia the oarhon gel complex deeeribed by Swritzer, Goodrich, and Burg=* (7) in 1949. This oarbon gel cornplrx UILC demonstrated 88 m important, if not essential, element Tor the development of modulus, resilienoe, and actwill road wear of cold rubber treads. Therefore, it is natural that again the electron microscope should be called an to give an actual picture of this carbon PPI eomplen. Electron photomicrographs of microgeis, shoim by Baker (I), were concerned with gelstion within h t e a particles and showed oontoured rniorogel structures naainst a structureleas 801 heckground. This method of ditferentktiog between sol snd g d has been used in this paper. Honever, because the problem here was concerned with large units of macrogel found after eoegulation and intimately acsoeiated with carbon black, new preparation techniques were required. TECHNIQUES USED IN STUDY
VWLCAN~ZED FILMS.This technique, developed io 1944 (S), consists essentially of squeeeing uncured stoeka into thin film in special molds and then owing the films. Both molding disks should be of sluminurn for 10%. ternperstwe polymer etoeks in-
Figure 2.
Shadow Cast Low Temperasure Polymer Gel (XiSOO)
stead of the aluminum and steel disks p~+:viouslyused.
This technique wm used mainly in emluating structure persistence snd proving that structure persistence done could not prediot modulus 89 previously supposed (7). HIGHSPEED Micao~om,. The high-speed microtome described in 1945(4) has been improved and has been a valuable tool in thia study. The main improvement has been the change from 8 knife mounted radially in the centriiuge head to a knife mounted
Norembar 1951
Figure 3 (Ab*)).
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Miomtomed Section of L o w Temperatnr0 Polymn Gel
Figure 4 (Upper Right). Soluble Part of L o w Temperalure Polymer (x7m) with cutting edge a t sn sngle of about 60' to the radius. Thili han improved both the quality and quantity of usable mctiona
.
cut. "SPREAD~NG" TECHNIQUE. .4e carbon pel is defined 8 8 "all the beneeneinsoluble rubber formed in unvulesni8ed oarbon black-rubber compounds," the specimens are generally obtained 88 B jellylike mass suspended in benzene. The first step in preparing a miormoope specimen is to use the standard procedure of dipping a miwascope slide into a 0.5% solution of Formvnr in ethylene dichloride and then allowing the slide to dry. A piere of gel (or oarban gel) is taken out of the benzene, plaoed on the Formvsr, then apread thinly on the film by strokes of B spatula. The Formvar film is then floated off an B water surface and tranaf e m d to 2M)mesh .ween8 ready for electron mioroacope inveetiption. SEADOW CASTING.The coating of the carbon gel with B thin layer of chmmium (9) has helped in the interpretdon of carbon gel pictures. The 2CQ-mesh soreen supporting the Formvar film with "apread" gel is plaoed in a vmuum chamber and coated with vaporized chromium applied in such B manner w to ea& shadows of the gel on the Formvsr film. Carbon Gel Study. It haa long been recognized that the copolymers of butadiene snd styrene, psrtioularly those varirties now known aa GRS, has-e introduoed new faotora and a whole new voonbulary to the rubber oompounder's language. Polymer gel has become a reoognised md slmost inesoapable part of these synthetic Nbbers. A% the GR-S induetry developed it became expedient to produce polymersahioh left thefsctory in aaubstsnt i d y gel-free oondition, if they were to be capable of processing in the volume demanded by the ruhber indurtry. The differ-
+ Figure 5.
Cacbon Gel (xl@,OOO) 1o-pm ,osdin.
2565
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
Voi. 43, No. 11
ae shown by Sweitzer el al., is m important factor in this gel formation, it seemed advieable to examine the sol and gel fractions of a polymer (LTP) originslly gel-free in which statie heating wa4 the agenay forgelation. P O L ~GEL. R Cold rubber gel WBR formed by heating the raw polymer in an oven at 400" F. for 1 hour. The gel and aol portions of the polymer were eepsrated using benzene io B Soahlet extrsctor. The gel WRB then spread on Fomvar w described shave. In order to eliminate the danger of false interpretstion due to field asmplmg in the eleotron miemsoope, over 1oM) photomiom. graphs were taken in thie gel study. The limited number of photomicrogrsphs shown here were chosen to illustmte the conelusions drawn from the study of the complete set of pho* micmgrnph8. An electron photomicrograph of polymer gel is shorn in Figure 1. T h e tendency of the material to "shred" is sppsrant. This e5ect in due to the stroking action of the spatula. Figure 2 shows gel that hae been shadow oaet. A thinly micmtomed section of sulfur chloride cured ge; is shown in Figure 3. Discounting the soom msrke due to irregulsritiee in the knife edge. this indicstes that the shredding ehown in Figure 1 is due to the method of spreading the pel.
Figure 6. Caibon Gel (x7500) *put
lOSdi0.
en- between macro and miaro gel, snd the relative oharseter of gel formed in the reactor and formed thmugh tho agency of heat The present m d processing, could lesd thie discussion far &Id. paper is confined to s discamion of low temperature polymer (LTP) gel formed through &tic heat, through processing, or thmugh the preaenoe of carbon during processing. Beoause heat,
The sol portion of the hat-gelled polymer is shown in Figure 4. This specimen was prepared by first sllowiog the sol portion to dry after removal from the oxtmotor. The swelling action of the benzene on the gel wae simulated by swelling the sol in a 1 t o 9
November 1951
INDUSTRIAL AND ENGINEERING CHEMISTRY
Figure 10. Carbon Gel (X7500) mixture of benzene and aloohol. A piece of the swollen m w w s then spread on Formvar. The picture of the sol shorn 8 diffuse type of material in marked eontraat to the welldefined boundariee of the gel. Thii “spreading” technique thue provides a ready mof identifying gel and sol in electron photomioroPPh. CABBONGEL OF Low CABON L~ADINO. Low temperature polymer mseter batchen containing 20 parte of very fine particle furnace esrbon (VFF, Statex K) were mixed in B Bsnhury at a stmk temperature of 2W’ F. The stocka were then heatad in sn oven at UX)’ F. for 1 hour to develop gel. After the sol ww extracted, the carbon gel was epresd on Fomvsr filme and examined In the electron microscope. A photograph of the w h o n gel is shown in Figure 5. Two kin& of gel are evident. Large piecea of highly pigmented carbon gel are readily seen along with shredded polymer gel relatively free of carbon. Tmm STOCK MABTEBB n w m s M m o AT Hma TmmuT . Regular W-psrt carbon low temperature polymez master batehe8 were mixed hot (4OO’to 425’ F.). The sol wm then extraeted and the carbon gel spread on Formvsr. Figure 6 shows the eame type of picture aa found io the 20part carbon stock. A large highly pigmented carbon gel unit is ohsewed to the right of the plste. Unpigmented polymer gel is seen in the center. T h e percentage of this unpigmented mtUial ia low,moat of the material beiDg highly pigmentad. Coneiderable oar8 in the interpretation of these “etresked” area8 was neoesaary. It ie paseihle, by epstulstion, to produce folds in the film itself whioh might he mistaken for clear gel. Such 8 region ia rhown in Figum 7, where the fold%can he readily identified hy the fact that carbon gel is supported on the fold m at A. However, the f s o t that all a t r e a h are not folds is demonntrated by Figure 8. The carbon gel for thin photograph WBB shadow cart with chromium. One ‘‘shred” o m be seen a t the right with end raised from the supporting film, thus csating the pointed shsdow below. The mud procedure of printing sa a negative h e 8 been followed here 80 as to give dark shadows. This picture differentiates hetween shretle of gel sod folds in the supporting film. A w b o o ael w i t is men in Figure 9. Large white meas show where the sol hm been leached out. The graynese around the hlaok particles iadicstea the prssenoe of gelled polymer. This is more apparent ib Figure 10, the specimen for whioh was shadow caat. Dark arw indieate removal of the 801, whereas the white indieate the gel oomplex m which the when particle is e n o l d . Thia eonsewently shows oarbon gel sa B physicsl entity.
Figure 11. Hot Mix of Carbon Low Temperature Polymer Master Batch (Xl5,OW) %p.uf lo-dlos
Carbon maswr batrhw of low wmperatum polymer mixed hot hut not extracted were also exan~ned. AfLer miXio8, the s:wk w m BAOIIPU in s I w 9 mixturn of benzene and alrahal aod then spread on Fonnvar. Exnrnination of :lie phow,gnrphs mveals t h t in addition w the highly pigmented carbon gel there is 801 p w s ~ n trontaioing either B much lower ronccolration of free carbon or s n d l ~ ~ b gel o nnggregam. durh an a m is s e n io Rguw 11. TBEAD Mhsren B n c e s e M i x v ~COLD. Rgure 12 shows the diepmion found in a $?-part carbon mmter barch of low wmpera-
Figure 12. G l d a M i x of Carbon l a w Temperature Polymer Master Batch (X15,OOO) %put lordins
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
2568
ture polymer mixed a t 2W"F. and treated 88 above. The soncentrstion of black is muoh higher than that found io the 801 partion of the high temperature mixed stoolie but lower than h t of the cerbon gel. W e dispersion is comparatively homogeneous. The white are= represent the supporting Foimver Nm. The result of hot mixing is therefore to change the mix mta weaf of high and low carbon concentration aud rea as free of blsck.
Vol. 43, No. 11
microeeope BS a shredded type of material. The sol portion of the polymer under the asme treatment s p p m 8s e. disuelse type of material. Cold mixing of a low temperature polymer tread compound 88 against high temperature mixing gives B comparatively hamoge nmw dispereion. High temperature mining produces carbon gel, which is highly pigmented. Polymer gel free of carbon is also produced. T h e
Figure 13. Cold Mix of Tread Stock tXZo0)
Bekw ( I ) , io commenting OLL his Figure 9 (Eleotro!~ Miemgrsph of Wetting of Fine C w b n Black by GR-S Polymer), states, "Thickened regions where the polymer is associated with denser csrhon black are elearly evident.. . . Similar diagmm in which microgel like that in Figure 2 was 8180 present indicate that even fine carbon blsek will not penetrate microgel." Io the p-t studies where oerboo is incorporated into ungelled rubber and the carbon gel complex is formed either simultaneously or subsequently, the oarbon does reinforce the gelled rubber. whsreaa Baker states, "the volumes of &%in sepmmts comprising the microgel fraction of 8 Byathetic rubber receive reduced reinforcement from fillers, psrticularly carbon black. Tenaoity will aoeordingly deelino." Gel must therefore be generated i n the presence of carbon black forgood reioforoement. X-RAYE ~ D E NA~further . support of this type of dispersion in high temperature mixed a t w b is shown in Figwes 13 m d 14. These x-ray photographs were taken by Brock %cording to the technique described in his paper on miororadiographr~( d ) . This teohnique de oda
OD
differential absorption of x-rays.
A piece of stook c u r between 3 sheets of ecllophane and moseurin 0.375 x 0.01 inch is Btretched 3W% sa8
with extremely fine-gratned tilnh (Eastman sample is then exposed to a beam of soft x-rays (15,000 v0lt.r). The film is used its the object in a light microscope and enlargements are made at 200 to 300 dismeters.
Figure 13 shows the diepersion in a odd mixed stock (200" F.). Figure 14 shows the dispersion in a ataek mixed st 430" F. The atock mixed s,t high temperature shows many mare carbon " ~ g gregstea" (white apots indicate greater absorption of x-rays). Them aggregates are OS the m e order of magnitude as the carbon gel units observed in the electron microscope. The lsrge white partioles represent ingredients other than carbon black in the tread compound.
sol rubber reiaairriag contains A muah lower wxxentratiorr of free carbon or small carbon gel sggregatea. We thus arrive st the picture of highly pigmented carbon get units separated by
polymer gel and area9 of low carboo concentration. This view i i supported by x-ray atudies. It is felt that theas highly pigmented unite repurated by "soft" unpigmented areas and arena of Ian earboo concentration give the high modulus properties previously associated chiefly with highly loaded stocks, hut without incurring the pensltios of shortneea and cracking. The improvement in wear ii rexilt of this has been confirmed in road twts (8).
-
ACKNOWLEUGM w r
The authors make grateful aekoowledgmerrt to H. A. Braendle for his invaluable seaistance in this work; and to W. 8.Wiegand, vice president of Columhisn Carbon Co., for his helpful oriticiams snd permiasion to publish this p&per. Acknowledgment is also made to M. J. Brock of the Firestoire Tire sod Rubber Co. for taking the x-ray pictures here show., LITERAPURE CITED
(1) Baker. W.O., 1x0. E m . CHEM., 41.511 (1949). (21 Brock. M. J.. Division of Rubber Chemistry, 116th Meeting. Ax.Caela. Soc., Atlantis City. 1949. (3) Ladd, W. A., IND.Erie. CHEM., ANAT,.Eo..16,642 (1944). (4) Ladd. W.A.. and Brsendle. FI. A.. Rabbm Age ( N . Y.1, 57. W1
(1945). (51 Ladd. W. A . , and Wiegand, W. 8.. Ibid.. 57, 299 (1945). (01 Steffan, H.C . . India Rubber World, 120, 60 (April 1949). (71 Sweituer, C. W., Goodnoh, W. C.. and Burmas. K. A.. Rubber Are (N.Y.I.65, a51 ( 1 ~ 4 9 ) . (8) Wieaand, W. B.. Con. Cham. P m e a a I d . . 25. 579 (1941). (9)Williama, R. C.. snd Wyokoff. R. W. 0.. I. Applied Phys.. 17, 23 (1946).
SUMMARY AND CONCLUSIONS
The application of hest to low temperature polymer develops gel whioh after ewellisg and spreading appeam under the electron
REoSrVED April 24, 1950. Prascnted before the Divirion of Rubber Cbsmb. try et the 117th Meeting of the A i a ; e m ~ nCsmncc~bBocimr, Damit.
Mi&
