HEATS OF VULCANIZATION OF &
P. L. BRUCE, R. LYLE, AND J. T. L A K E Sinpl*x WL. I Cable Company, Cmbridsa, k s s .
* * Y
PREVIOUS paper' reported that the heat of reaction be-
A
twean~veaNhberandsulfur~mdbyfobwin& the temperstun,at the center of a ruh cylmd dming v u I 6 t i o n . It waa found that with all p%&%mlfnr used, tbera is an evolution of h t .Below about 2 of sulfur per 100 of rubber, the heat d v e d in relatively small. Above tbis re&ion the evolution h o m e s m v e l y tsrger and in-u the &lmatas d U I incream. It wea nugpestsd that this might be explained hy asmming two reactions betwem Bulfur and rubber. The &.t, which ia the PrlnQinfJ lsaotion at hw aalfur percentyea, farms soft vulcpniwd rubher aeoompanied by lit* or PO bagt evolution. h v e the low sulfur region, a aecond reaction preaominstes which forms ebonite and produces a & t i d y kge quantity of beat.
By the use oforgenio socderstors,the rate offormationof soit vulwraisad ruhber isgrestky but the rata of ebonite formation is practically d e c t e d . By acceabting the soft rubher nurotion, the two nurotions may he mom compl&ely sep uated. T k p r a e o e e o f ~ ~ d a n e s l e d t h e h a e t w o l v e d 8t8gim of &, m & a t Whioh would be prediotsd from the theory oftwo dintinct reactions The two reactions may be still more completely separated by meaM of selenium. AlthO* this Inabru wiu form soft donnised ruhber, it diEm fmm d n r in ita inability to produce &nit% 8ine.e vul&tion of Nbber with &m prodwed & detectable heat evolutmn, hvther support is given to the theoryof tworeactions. HEAT EVOLUTION
The p-nt paper ia eoncBrned nith the h t during the vulcsnisation Of Synthetic rubbers. BUMS,BUM N, Butyl, 1Lpd Neoprene GN wera investigated. B I . L D . I ~ . E * ~ . ~ . . ~ ~ I ~ ~ ~ ( I D ~ .
The aotive curve is plotted fmm-data obtained during the 5m
"p
of a Buna 8 (Firestone GRS) compound oontaining 6 prta o sulfur per 100 of Buna 8. At the oonclusion of thia heatmg the resotion hctwaen the Buna 8 and the sulfur is complete and the is thermally inactin A b m nuve may then be obtained hy ooollng and again h e a t i i in sluline vapor. The maxhum tempersture diffmce between the active and blank curvedis taken as a meanue of the h t evdved. A more wnvenifmo method of-d this madmum tem-
c nature of heatpem$ure Werance mskeg use of the 1 mng anrres. If a th w t i v e maas is heated by a constant temperature muMem&thm of the Unaocompliahed temp"t? abspse a t any polnt &in the man is a atmi$tJine unc&on of tima Deviation from a atraiuhthw funahon aiu indicate themurl aetivio of the snmple. In Figure 2 a s e n d itbmic plot of the blank hea% curve in The Smt wrtion of the heatum cwva ah- to be a strainh%e.
that no heat ia baiw evd+ At a p p m x b & 25; C.M o w thg bat4 temperature, the actrve c w c kgins to d m a t e Imm the strarght h e and thua ahom the evolution of heat. A blank he& ins cuwe for MY oarti& commition msv therefore be ob.
INDUSTRIAL A N D ENGINEERING CHEMISTRY
38
Vol. 36, No. 1
the amount of heat transferred from the aniline vapor into the sample decreases as the amount of heat evolved increases. For this reason there is a small positive error which becomes greater with increasing heat evolution. This error is believed to be small since the reaction takes place within a short time. N e g l e c t i n g the above error, the calories evolved per gram of rubber may be calculated from the following equation: calories/gram = (S)(max. AT)
f where S
specific heat of compound max.AT = maximum diff e r e n c e between active and bjank curves, C. f = f r a c t i o n of rubber in compound =
The specific heat values used were: organic mateFigure 2. Semilogarithmic Plot rials, 0.5; carbon black, of Typical Heating Curve magnesium oxide, and sulfur, 0.2: zinc oxide, 0.1. Inaccuracies in these values are reflected in the absolute values for heats of vulcanization, but the relative values are not affected. BUNA S
The Buna S (Firestone GR-5) was a copolymer of approximately 75 parts butadiene and 25 parts styrene. Four types of Buna S compounds were investigated: A
Buna S
100
Zinc oxide
..,
Carbon black (Micronex) Stearia acid Santooure (accelerator) Sulfur
.... .. 2-io
B
C
D
100
100 50
100 10-60
...5
1 1 2-10
5
1 1
2-10
-20
-16
-10
-S
0
5
10
Figure 3. Temperature Differences between Heating Curves for Active and Blank Samples
believed that a change in heat conductivity is not a sufficient explanation. The curves suggest that the presence of carbon black in Buna S decreases the heat of vulcanization in the 4-10 per cent sulfur region. The effect of varying the amount of carbon black in a Buna S compound containing 10 per cent sulfur is shown in Figure 6 . The calories evolved decrease linearly with increase in percentage of carbon black. BUNA N (HYCAR OR-1 5)
This synthetic rubber is a copolymer of butadiene and acrylonitrile. Its heat of vulcanization was studied in a series of typical compounds containing carbon black. (Pure gum compounds
5
1 1
10
The temperature differences between the heating curves for the active and blank samples are shown in Figure 3 for various unaccelerated Buna S-sulfur mixtures. The curvw are plotted arbitrarily so that all maximum temperature differences occur a t the same. time. The calories evolved per gram of Buna S, calculated from the maximum temperature difference values, are plotted against percentage sulfur in Figure 4. For comparison, the values for smoked sheets previously obtained' are alvo given. The curve for the accelerated Buna S compounds (type B ) is not given since it was nearly identical with the unaccelerated type. This may be contrasted with values for smoked sheets where the addition of accelerators lowered the heat of vulcanization. The heat of vulcanization curve for Buna S is sZmilar to the curve for natural rubber. Both curves show a definite upward bend in the low-sulfur region which indicates the presence of two reactions. The addition of carbon black to Buna S appears to lower the heat of vulcanization in the region above 4 per cent sulfur (Figure 5). Below this point, the values are identical for both the pure gum and carbon black compounds. This effect might be explained by an increase in heat conductivity caused by the carbon black, or a change in the amount of heat evolved, or both. If the difference between the two curves is due to increased heat conductivity, separate curves should exist in the 0 to 4 per cent sulfur region. Since they are identical in this region, it is
Figure4. Comparison of H e a t Evolution of Buna S and Nrtural Rubber
Figure 5. Effect of Carbon Black on Heat Evolution of Buna S
INDUSTRIAL AND ENGINEFRING CHEMISTRY
January, 1944
proved to be insufficiently plastic to be adapted to the procedure used.) The formula follows: Hycar OR-15 Carbo; black (Micronex) Dioctyl phthalate Zinc oxide No. 550
100 35 20 5
Stearic acid Benzothiaeyl disulfide Sulfur
1
1.5 2-14
39
als. It is common practice, however, to use metallic oxides in neoprene compounds. Heats of vulcanization have been determined for Neoprene GN, alone and with zinc and magnesium oxides. Unusual care was taken by use of low temperatures to avoid any cure during mixing operations.
Heats of vulcanization are plotted in Figure 7, together with those obtained for smoked sheets. The rapid change in slope of the curve for Hycar OR-15 does not occur until near 10 per cent sulfur as compared with the change a t about 2 per cent sulfur for natural rubber. In the region from 4 to 10 per cent sulfur, Hycar OR-15 shows a relatively small heat of vulcanization. This may be taken as an indication that the ebonite reaction in this region does not occur so readily in Hycar OR-15 as it does in natural rubber. BUTYL RUBBER (B-1.45)
Butyl is a copolymer of a diolefin and isobutylene. The percentage of diolefin is adjusted in the material used (B-1.45) so that the double bonds available for vulcanization are approximately 1.5 per cent of those available in natural rubber. If one sulfur atom reacts per double bond, the maximum amount that can combine will be 0.72 part of sulfur per 100 of Butyl rubber. If the heat of reaction is similar to that of natural rubber, an excess of sulfur will produce about 1 calorie per gram of Butyl. Approximately this amount of heat was evolved by the following compounds : Butyl rubber Zinc oxide No. 550 Stearic acid Carbon black (Micronex) Tetramethylthiuram disulfide Mercaptobenzothiaaole Sulfur
100 5 2
... ... ... 5
100 5
2 ... 1
100 5 2 40 1
0.5 5
0 5 5
Butyl rubber may also be vulcanized with p-quinone dioxime and lead peroxide. The study of this reaction, however, is complicated by a strongly exothermic reaction between p-quinone dioxime and lead peroxide. A correction for the heat of this side reaction was obtained by determining the heat evolved by these vulcanizing agents when heated in Vistanex. The latter material has substantially the lo 2o 30 40 6o 6 o same structure as Butyl rubber but no Figure 6. Effect of Varying Amounts of unsatuCarbon!Blackion H e a t Evolution of Buna S ration. T h e following table gives the compound formulas and their respective heats of vulcanization:
Figure 7.
Comparison of H e a t of Evolution
of Buna N and Natural Rubber
The following table gives compound formulas and their respective heats of vulcanization : E
Neoprene GN Light calcined magnesia Stearic acid Zinc oxide No. 550 Heat evolved, cal./gram
100
... ... ... 15
100 4 0.5 5 12
100
4 0.5 10 12
The heat of vulcanization of neoprene alone is appreciable and corresponds t o a value for smoked sheets containing 4.5 per cent sulfur. The addition of 5 per cent zinc oxide and 4 per cent magnesia definitely lowers the heat evolution. The amount of heat evolved is not changed by increasing the zinc oxide to 10 per cent. SUMMARY
1. The heats of vulcanization for natural rubber and Buna S
NEOPRENE
are nearly equal. The data for both materials indicate two different chemical reactions during vulcanization. At low sulfur percentages, the principal reaction forms soft vulcanized rubber and is accompanied by little or no heat evolution. Above the 2 per cent sulfur region, a second reaction predominates, forming hard rubber and producing a relatively large quantity of heat. 2. The presence of an accelerator (Santocure) in Buna S has little, if any, effect on heat of vulcanization. 3. The addition of carbon black to Buna S lowers the heat of vulcanization in the region above 4 per cent sulfur. The calories evolved in a 10 per cent sulfur compound decrease linearly with percentage of carbon black. 4. The heats of vulcanization of Buna N (Hycar OR-15) indicate the presence of two chemical reactions. Unlike natural rubber and Buna s, the ebonite reaction does not predominate until the sulfur concentration is raised above 10 per cent, 5. The heat of vulcanization of Butyl rubber with sulfur is equal to the heat evolved with natural rubber containing 0.6 per cent sulfur. If one sulfur atom reacts per double bond, the maximum amount combining would be 0.72 per cent sulfur. During the vulcanization of Butyl rubber with p-quinone dioxime and lead peroxide, a large amount of heat is evolved by a side reaction between the vulcanizing agents. The reaction involving the Butyl rubber produces about 6 calories per gram, a considerably higher value than the 1 calorie produced by sulfur vulcanization. 6. The heat of vulcanization of Neoprene GN without added agents corresponds to a value for smoked sheets containing 4.5 per cent sulfur. The addition of zinc oxide and magnesia decreases the heat of vulcanization.
In contrast to the materials previously discussed, neoprene (polychloroprene) will vulcanize in the absence of other materi-
PIZESENTED before the fall meeting of the Division of Rubber Chemistry. AMERICAN CmncxcaL SOCIETY. in New York, N. Y.,1943.
Butyl rubber Vistanex p-Quinone dioxime Lead peroxide Zinc oxide Stearic acid Heat evolved, cal./gram
100
... 6
10 5 2 24
...
100
6 10 5 2 18
The difference between these values (6 calories per gram) may be taken as an approximation of the heat evolved by the reaction between Butyl rubber and the vulcanizing agents. This value is considerably higher than 1 calorie per gram produced in the sulfur vulcanization.
