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 Moore, C . , Indiu Rubber World, 116,No. 6, 789 (1947). ( 2 ) Maron, S. H., Ulevitch, I. N., and Elder, M. E., method for determination 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
162
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 41, No. 1
heatzng wzres
liquid-air increased the total yield from about 83 to about 947,. These experimenk appear to indicate: (1) A higher rate of distillation rcsults in a higher total yield of dist'illate, by reducing breakdown into carbon and fixed gaseous products (hydrogen, methane, acetylene, etc.). (2) Higher temperatures give a sniallcr total yicld at constant distillation velocity. (3) The experiments under diminished pressure allow higher temperatures and show a markedly higher isoprene content in the distillate. The contact time of the vppors a t cracking temperature has paramount influence o n the product; a longer t,ime results in more thorough brealidown to uncondensable gases. With too short a contact no further breakdown occurs, and the distillate Figure 1. Discontinuous Quartz Cracking Apparatus docs not differ much from the distillate obtained simply by evaporating rubber and condensing. The purpose of the present investigation \\-as bo investigate the Vacuum brings about a very short contact time and also facilipyrolysis of rubber more thoroughly in order to find the best tat'es the distillation of the rubber itself, as will be discussed later. The cracking tube was filled with a contact material which conditions for depolymerization into isoprene. proved beneficial for t'he isoprene yield. When quartz was reDISCOSTINUOUS 1'ROCESS placed by copper, better results could be obtained. With copper precipitated on pumice (from copper nitrate solutions by h e a t i w Figure sholTsthe first apparatus set up. The rubber in a hydrogen), the yields were again higher! and and reducing small open container placed in t,he left-hand part and quic]cly a spmific CatalYst appeared to have been found. Although distilled. A streanl of nitrogerl T ~ a sfed into tile evaporation chromia on pumice was also very effective, its specific: catalytic eone and mixed TTith the rubber vapors to carry them off action could not be confirmed in later experiments. It became through a high temperature cracking zone. apparent that the heat conductivity of the contact mat'erial is The heating of tllc evaporation and zones reguCopper in the form Of Wire turned Out very lated electrically: the tenlperatures of the evaporating rubber alld to bc quite satisfactory* the cracking zone m-ere controlled by t.hermocouples. The crackThe shape of t'he a,pparatus, the dimensions of the cracking ing products were led into receivers cooled to -80" C., onc of the tube, and the conditions of the process Tere varied in many 1yays. receivers containing an electrical Cott,rell precipitnt,or. Later The best results were obtained by rapid distillation in a similar when vacuum \?-asapplied and it became necessary to cool one vessel liquid air, t,he receiver the Cot,trell conclenser apparatus v d h a shorter craclcing zone (6 ~ h i c hx i s heated from the outside and from within by means of a core. Immedibecame superfluous. The distillate was carefully rectified; in alely after cracking, the gases impinged on a quartz wall cooled t,he fraction boiling below 60" C., isoprene vias estimated by with carbon dioxide-alcohol mixture, so as t o give the cracked means of a modified Farmer-Tarren procedure (4). product,s as little time as possible for secondary reactions. Results of 40 to 44y0 of isoprene were obtained (Table 11); DETERVINATIOS O F ISOPRENE COSTEST. Ralf a gram of the crude isoprene fraction (boiling below 60" C.) was heated at the initial temperature of the cracking zone TTas 850" to 900" C., 100 C. with one gram of maleic anhydride in benzene for 2 hours. measured with a chromel-alumel thernlocouple, and the pressure Then the benzene was distilled off in vacuum t,o dryness. The was around 10 inm. mercury. This pressure could not be lowered gain in 7veight by the maleic anhydride gave the isoprene because of the small capaxity of the pump. The manometor was A correction for the quantity of maleic anhydride distilled with the benzene was made bv acid titration in the distillate. This isoprene determination is correct within about l?, as experiments with pure isoprene have TABLE I. RESULTS OF PYROLYSIS OF RUBBER shown. Olefin and acetvlene derivatives did not interfere. The crude isoTeinv. -. Isourene Yield of Rubber Total 7'0 V 1 s t . lI'olll in Fracof prene fraction contained about. 80-907, PresCFackDistn. Yield Rubber tioii Isoprene of pure isoprene, the higher percentage sure, ing Rate, of B.P. B.P. from being obtained in the more effective Contact Material in Mm. Zone, G./ Dist., B.P. 60< 60° C., K u k b e r , cracking experiments. Cracking Tube Hg C. IIin. 70 < 60° C. 150° C. 70 %; I -
I
I
Rubber began t o distill at about 310" C. The temperature rose steadily during the process; at 360" C. a residue of 10% was left. in the evaporation tube and at 380" C. 77,was still left, so that most of the rubber hydrocarbon had then evaporated. The end of the process was marlied by a quick rise in temperature. On applying vacuum, the first evaporation was observed a t about 280" C. Some results are given in Table I. The receivers were generally cooled with alcohol-carbon dioxide mixture. I n the vacuum experimcnt,s, cooling .ir-ith
Xone
760 760 760 760
760
None Fragments of quartz
Fragments of copper metal
Co per pptd. on pumice Zagments
CrnOa pptd. on pumice fragments
i60
150 12-14 20 20 20
20
20 20 20
Supermax Glass Apparatus 610 0.25 90 18 0.42 91 1: 0.2 83 13 630 0.4 91 15 630 630 0.8 Q5 17
63 62 57
62 64 64
61 68
68
20 17 14
62 56
610
660 660 630 740
0.98 0.3 0.2 0.5 0.5 0.5
740 765
0.7 0.5
89 82
36
765
0.5 0.5
82 89
35
710 740
740
83 94
91 80 78 84
27 30 35
25
35
61 I
.
.. .. ..
.. ..
69
68 84
85 87 85 88
11
11 8.5 10 12 14 14 12 22 23 26
82 86
29 31
84
30 31
88
INDUSTRIAL AND ENGINEERING CHEMISTRY
January 1949
connected with the condensing vessels, the pressure in the actual cracking zone being probably higher. The velocity of distillation was regulated by means of the heaiing current of the evaporation zone. I n the later apparatus this velocity amount,ed to 4 grams of rubber a minute. Rapid distillation is important, since the isoprene content of the cracking products of the first half of a slow distillation appeared to be about twice the isoprene content of the products of the second half. ApIOcm., parently the rubber changes durFigure 2. Iron Cracking the heating period, since rubing Apparatus for Rubber is partially converted into ber Solution cyclo-rubber when heated above 260 O C. This cyclo-rubber gives very little isoprene when pyrolyzed (8). Therefore, during evaporation the rubber is exposed to a temperature which decreases the yield, and it is import,ant to accelerate evaporation.
..
.
163
OF R U B B E R SOLUTIONS TABLE111. PYROLYSES
Solvent Toluene
Mesitylene
Concentration,
%
25 25
0 25 25 25 25 25 25 25
0 0
Pressure, Mm. H g 20-40 20-40
20-25 13 9.5 8 5-6 4
1-2 1-2
8 1
Isoprene Yield Total 70of Yield, % Rubber 88 13 93 19
94 96 94 95 96
98 92 93 99 99
0 49 53 51 47 45 38 38 0 0
Contact Material (Temp. 750' C . ) Quartz Quartz coated with metallic
Same Same Same Same Same Same Same Same Sa me Same
ceivers and the vacuum pump. The results varied somewhat according t o the nature of the solvent used for preparing the rubber solution; a higher boiling solvent makes possible a lower pressure. Some results are shown in Table 111. The first part of this table indicates t h a t it is not the metallic surface but the massive metal of the contact which counts. A massive copper contact gives double the yield of copper-coated quartz; this must be attributed t o the heat conductivity of massive copper. Toluene as a solvent has too high a vapor pressure t o permit TABLE11. OPTIMAL CRACKING RESULTSIN DISCONTINUOUS very low pressure experiments. With mesitylene solutions the QUARTZ APPARATUS optimum pressure is about 9 mm. mercury; a t the lowest presPressure in Is0 rene Receivers, Temp. of Crack- Distn. Rate, Total Yield Yielcf % of sures the isoprene yield is again reduced. Mm. H a ing Zonea, ' C . G./Min. % of Rubbgr Rubber The yield of 52% is far better than the yield obtained in the 11 900 3.9 89 43 discontinuous process. Evidently this is a result of the fact t h a t 12 860 39 3.9 88 12 860-900 6.5 40 89 the rubber drops are heated so quickly t h a t cyclization of the 12 6.1 830-900 39 91 8 900 41 88 3.9 rubber is reduced to a minimum. 10 900 44 4.3 90 T h e same apparatus can also be used to crack dipentene. -4s 7 850 44 4.1 91 7 850 39 4.4 89 a matter of fact dipentene or limonene gave yields of 20 t o 3570, 6 850 42 3.9 92 5 850 41 2.8 90 depending on the purity of the dipentene, under the same condia I n later experiments, when a more even heat distribution was possible tions as did rubber-Le., a temperature of 700-750" C. and s, lower temperatures were used. Therefore these are probably wall tempressure of about 10 mm. mercury. peratures, a n d the actual gas temperatures may be lower. The higher boiling fraction of the rubber pyrolyzate containing dipentene gives, after recracking, some 5-10y0 of isoprene, depending chiefly on the effectiveness of the previous cracking. The pressure is more or less a function of the distillation rate. Effective cracking with high isoprene yield gives a high boiling Under constant conditions more rapid distillation, giving off more fraction which forms very little isoprene upon recracking. This vapor per second, leads to higher pressure. As it is difficult to fraction may be used instead of toluene t o prepare the rubber regulate the temperature accurately, there is little difference besolution. tween experiments at 850' and 900 ' C. CRACKING OF LATEX. Hevea latex was dropped in a cracking Similar results were obtained with an apparatus made of iron apparatus of the type shown in Figure 2. The cracking zone or heat-resisting chromium-nickel stJeel, so these materials had no had to be somewhat larger than i t was for rubber solutions, beeffect on the reaction. Heat-resisting steel is the best material cause much heat is needed t o evaporate the water in latex. Two for this purpose. types of latex vere used, ordinary hevea latex with a 33% rubber content and concentrated latex (Jatex) with a 607, rubber conCONTINUOUS METHODS tent. Under the conditions just described rubber was exposed for a n Table IV shows that yields of over 50% isoprene can be obappreciable time t o a temperature favorable to cyclization. To tained from rubber in latex or Jatex. The high yield at ordinary avoid this, it is necessary t o supply the rubber continuously into pressure (26 to 30%) compared with the 17% obtained from solid the cracking zone of t h e evacuated apparatus. The best theoretical method, spraying of latex against a heated wall, was discarded after a few experiments because of complications. The TABLEIV. CRACKING OF LATEX following simple methods for continuously supplying small quantiR a t e of Yield of ties of rubber were tried. Pressure Supp!y, Isoprene, Temp., C. Mm. H i G./hlin. % DROPPING OF RUBBER SOLUTIONS.From a funnel high oonLatex 750 4 1.7 50 centrated rubber solution (about 25%) w ~ l sdropped into the 750 6 1.8 48 700 44 10 3.0 cracking zone of a vertical apparatus (Figure 2). This apparatus 725 26 1.5 760 is made of iron, but experiments carried out in a quartz apparatus 675 26 1.5 760 675 2.5 30 760 gave the same yields. The cracking gases can be sucked away Jatex 700 4 1.8 51 through one of the two side tubes. As a rule the upper one was , 750 6 1.6 53 connected with a manometer, and the lower one with the re-
164
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 rubber (Table I) proves the importance of a fine division of the rubber which makes it possible, as in the case of low pressure, to pass the harmful isomerization temperature range of 250500" C. comparatively quickly. Experiiiients Kith latex in vacuum met with difficulty as latex tends to splash against the wall and to coagulate when heated
DROPPINGOF MOLTEA RUBBER. Strongly niasticated rubber wa5 heated to about 200' C. The rubber then became suffiriently fluid t o be sucked through a n opening in tlie evacuated cracking tube of a modified pyrolysis apparatus (Figure 3). The influenrr of thc preheating tempcvature of the rubber is shown in Figure 4. il jield of 55% of isoprene was obtained at a cracking temperatme of Figure 3. Quartz Cracking 725" C. and a pressure of Apparatus for Moltori about 10 iiini. mercury, but Kubher above a preheating temperature of 200' C. this isoprene yield declined rapidly as a I C S U ~ L of cyclization anti oxidation processes in tmherubber. EX~RUSION OF RUBBICR.The best method proved to be the pressing of rubber in the vacuum craclcing tube by a n extruding machine as shown in Figure 5. The extruder is heated electricaily to a temperature (about 100" C.) such that, the rubber can be extruded smoothly. The rubber thread which usually has a dianieter of 5 mm., melts in the cracking tube about 2 cm. over tlie hot, contact mass, and the rubber drops fall on this mass. The cracking tube is made of iron. With this apparatus tlie best yield, using liquid air cooling, was found ijo be 58 170 of purc isoprene from rubber (first la,tex sheet or cwpe), which is an I average of nine esj perimentv varying
Vol. 41, No. L
I)EPOLY4IEKI%ATION CONDITIONS
hlATERI.4L. S O SpeCific Catalyst W a S f0llIld. Various materials-e.g., copper, iron, nickel, pumice, quartz, and soint: usual petroleum cracking materials such as chromia on magnesia or pumice and copper-nickel on magnesia under optimal conditions-gave comparable results. Metals are preferable because their higher heat conductivity niakcs it homogeneous cracking temperature easier to maintain. It is probable that the cracking of rubber into isoprene can bc! eonsidered as purely thermal, at least under these conditions. The niaiiiier in which the contact material is divided in the cracking tube is very important as this material may be only slightly resistant t o the flow of crackiiig gas to maintain the redrired pressure. I n most, cases copper turnings were used, filling the cracking tube o ~ awlength of 5 em. At a distillation rate of one gram of rubber per minute a muc:h shorter cracking ZUIILL gives insuffichit cracking and a much longer zone gives too much cracking:. Copper has the advantage of a high heat conductivity but oxidizes easily. Probably the best conhct surface can be m i ~ d e of chromium-nickel steel. TX:~~PB:HATUHFII OF CRACKING.Figure 6 shows that thew is LL wide range of cracking temperat,ures between about 675 and 800" C. in which good results are obtained. The maximum yield depends on pressure and on the length mf the cracking zone; a higher pressuro or a longer craoltirlg zone (giving greater crackingj iiiay be partly compensated by a loiver t,emperature. Optiniurn cracking t,cniperatures are in the range 700" t o 775" C. For a maximum isoprene yicld the optimum presI'RIMSL~RIC. sure lies betn-ecn .5 and 13 mm.mercury, as Figure 7 shows. A t a still lovier pressure the cracking of the rubber seems to be insufficient, although this also depends on the length of the cracking zone. At higher pressure the yield drops sharply. Tlic! favorable influelice of low pressurc on blle isoprene yield may have several causes. lsoprene may have insufficient stability under cracking conditions, SO that it is necessary to shorten the coiltact time by lowering the piessin'e. Cracking experiments with isoprene show that a second cause is dircctij. connected with t h o depol!inerizatit,n reaction of rubhc,, : t ~dimillCOXTACT
O
59%. As rubber contains 7 7 , nonrubber constituents,
from the rubber hydrocarbon. By reFigure 4. Influence of Preheating craclcing the higher Temperature of Rubber on 160boiling fraction of prene Yield the distillate (boiling below 60" C.) it is possible to raise this figure with no more than 0.87c. Some typical results are given in Table V. In each experimenl, about 20 grams of rubber were used. Table V shows once more that at higher pressures more noncondensable pyrolysis products are formed. Tn addition, t,he pure isoprene content of the crude isoprene fraction is higher in the more effective cracking experiments. Data obtained with the ext,rusion apparatus will be discussed in the next section. Temp.
u thermocouple t o receiuer
Figure 3 .
Cracking Tube with Extruder
60 50
23 tnq&
-
x,
//-\
A
g 40$30 s
::: p-p q 760 m m.ny
1
I
I
760rnmJ-g 1
I
I
I
cracking tube. This is not necessary in this case, as i t will be shown later that isoprene under these cracking conditions is reasonably stable. However, at still higher pressures-e.g., a t mospheric-where isoprene is not stable enough, a diluent doea have some influence. As mentioned before, a higher cracking pressure also has a bad effect on the depolymerization reaction of rubber; pressure caused by a diluent has the same harmful effect as pressure caused by the cracking gases. KIND OF RUBBER. First latex crepe or sheet, strongly masticated or unmasticated, gave the same maximum yield of 58% of isoprene. Deproteinized crepe gave a slightly higher yieldLe., 61Oj, of isoprene. C0,MPOSITXON OF RUBBER DISTILLATE
I
60 -
10 0
100
200
300
400
500
600
700
Pressure, M m . Hg
F i g u r e 7.
Influence of P r e s s u r e on Isoprene Yield at Different Temperatures
INDUSTRIAL AND ENGINEERING CHEMYSTRY
166 TABLE VII.
CRACRIXG PRODUCTS OF R t n n m 10 Mv. RIERCITRV
AT
450' C .
Isoprene after Recraoking (7; of Hydrocarbon Fraction
Weight
%
Hz and olefins b.p.
< loo C .
Hvdrocarbons b.
60--140
D.
AND
10
Ash
0.3
Figure 8 show6 the influence of pressure on the stability 01 isoprene a t three temperatures, The rate of isoprene supply amounted to about one gram a minute. Thus, under conditions for optimal rubber cracking (below 15 mm. mercuiy piessurd with a contact time for cracking less than 0.2 second, isoprene is lOOV, atable. This contact tirnr of the isoprene molecules can be calculated by the equation:
where t p
method was applied to the gases boiling below 10" C. Table VI gives an estimate of the results of this analysis, calculated on a 100% yield basis. The fraction boiling above 60' C. probably contains dipentene, as it yields 4% of isoprene on recracking (this means 0.8% froni rubber). According t o the investigations of Gaade (6)aromatic compounds are also present, such as benzene and toluene. The difference between cracking at 725" C. and a t much lower temperatures is very great. Table VI1 reports the composition of a distillate obtained after cracking rubber a t 450" C. and 10 mm. mercury pressure. I n this case a low yield of isoprene and a remarkably high yield of a dipentene fraction (boiling point, 140" C. at 760 mm. to 100' C. at 10 mm.) is obtained. This result is in agreement with calculations of Bolland and Orr (2) who showed t h a t the tendency for rubber to be cracked into dipentene is much greater than into isoprene. The next higher boiling fraction and the residue also give isoprene upon recrackIng. Probably, therefore, not only dipentene, but several lower polymers of isoprene are present-e.g., sesqui-, di-, and triterpenes These lower polymers tend to isomerize under the influence of high temperature into products which cannot form isoprene.
Vol. 41, No. 1
V .If
=
contact time, niinutcs
= pressure, atm.
volume of cracking zone, cc. mean molecular weight of cracking gases S = rate of distillation, granis/minute T = absolute temperature = =
The calculation is nicxe 01 less arbitrary, as Jf is not a constant. It is obvious from this equation that a higher pressure gives a longer contact time for cracking. The instability of isoprene at higher pressures is caused by the longer cracking contact time. However, the low isoprene yield obtained from rubber a t higher pressures (Figures 6 and 7 ) can be explained only partly by the instability of isoprene. At 75 nim. mercury pressure and 725 C. 88% isoprene can be recovered after the cracking treatnimt; under the same conditions the yield from rubber is only 605%of optimum. At 760 mm. pressure and 725 O C. 40% isoprene can be recovered, whereas the yield from rubber is 14% of optimum. This proves t h a t there is still a second cause for the favorable influence of low preseure, as discussed previously. Figure 9 shows that at ordinarypressure the stability of isoprene begins to decrease a t 550" to 600" C. This is in close agreement with the results of Figure 6, where the isoprene yield from rubber a t one atmosphere is decreasing -temperature above these temperatures. A4nalysishas shown that the Figure 9. Influence of cracking products of isoprene Temperature o n Stability after treatment at 7 2 5 0 of Isoprene at One Atmosand 760 mm. pressure consist phere of hydrogen, methane, acetylene, many olefins, and other hydrocarbons from methane to conipounds boiling above 180 C. It is remarkable that the cracking products contain only 4qGof a dipentene fraction (boiling from 140" to 180" C.). As Table 1-11shows, rubber gives under mild conditions (450' C. and 10 mm. mercury) 26 5 7 , of a dipentene fraction, so it appears Chat this dipentene is formed dirrctly from rubber and not by dimerization of isoprene.
'"
c,
I ot L
c
.
100
-.
I
,
zoo
I
300
. 400 I
I
sooi~&I~uI
pressure
YO0
m m.Hg
Figure 8. Influence of Yressui*eon Stability of Tsoprene
T h e cracking process of rubber a t high ternprraturrs can be divided in two steps: I n the first step a t 300" to 400" C. the solid rubber is "evaporated" or cracked mainly into lower polymers of isoprene, which may be quite instable a t this temperature; in the second step at 675' to 800' C. these primary products are cracked further into isoprene. To obtain a high yield of isoprene, it is necessary to pass this first cracking step as quickly as possible t o avoid cyclization and isomerization reactions. As mentioned before, this can be accomplished with good results by applying low pressure and by using rubber in a finely divided state-e.g., in the form of latex or solution. STABILITY OF lSOPRENE
The stability of isoprene uiider cracking contlitions \vas studied by dropping this compound in The iron rraclcing apparatus (Figure 2), which was filled as usuai n-ith 5 em. o f capprr coritact material.
!K I N G O F
VARIO U S HIGHPOLYhIERS
Teml:I.,
Pressure, Mm. Hg
First latex creQe or sheet Dcproteinized crepe Polyisoprene Gutta-percha (first quality) Methyl rubber
725 750
8
Oppanol (poiyisobutene) Neoprene G Polyvinyl chloride GR-S Buna 8 Perhunan Trolitul (yolystyreiie) Plexigum (polyinrt,hsl methacrylate)
775
c.
7Z0
750 729
750 750 725 725 72.5 Gi5 .,00
5 5 5 7 15 15 10 10 5 10 15 2
Yield of Monomw, yc Isogiene 58 Isoprene 61 46 Isoplene Isoprene 59 Dimethylhutadiene 58 Isobutene 46 0 Butadlsne Butadiene Riitadicnr St3iriie
Xethvl methaciylate
0 12
14
t3 70
78
January 1949
161
INDUSTRIAL AND ENGINEERING CHEMISTRY LITERATURE CITED
CRACKIhG OF OTHER HIGH POLYMERS
A series of high polymeric materials was subjected t o pyrolysis in the extrusion cracking apparatus with a 5-cm. copper cracking zone. The results are collected in Table VIII. Polyisoprene was prepared by polymerizing isoprene in bulk with diazoaminobcnzcnc as catalyst (6). I t s iodine number was 283 in contrast to 355 for first latex crepe. The isoprene yield is comparatively high, an indication of a rather regular macromolecular pattern. Keoprene and polyvinyl chloride split off hydrogen chloride. ACKNOWLEDGMENT
The authors wish t o express their gratitude to A. van Rossem, director of the Research Department of the Rubber Foundation for permission to publish this paper.
(1) Bassett, H. L., and Williams, H. G., J . Chem. Soc., 1932, 2324. ( 2 ) Bolland, J. L., and Orr, W. J. C . , Trans. Inst. Rubber Ind., 21, 122 (1945). (3) Davis, E. L., Goldblatt, L. A , and Palkin, S., IND. ENG.CHBM., 38, 53 (1946). (4) Farmer, E. H., and Warren, F. L., J . Chem. Soc., 1931, 3221. (5) Gaade, W., unpublished data. (6) Koningsberger, C., and Salomon, G . , J . Polymer Sci., 1, 200 (1946). (7) Midgley, T., and Henne, H. L., J . Am. Chem. Soc., 51, 1216 (1929). (8) St,audinger, H., and Geiger, E., Helv. Chim. Acta, 9, 549 (1929). (9) Staudinger, H., and Klever, H. W., Ber., 44, 2212 (1911); 75, 2059 (1942). RECEIVED October 28, 1947.
Effect of Moisture Variations on Curing Rate of G IAN C. RUSH AND S. C. I
