I S D C S T R I A L A S D ESGIAL'EERISGCHEMISTRY
J d v . 1930
i3i
Studies in the Vulcanization of Rubber' I-Thermochemistry of Vulcanization of Rubber John T. Blake
Introduction
w
HEX a chemical re-
The heat of vulcanization of rubber by sulfur has been carefully determined over the complete range of combination. The formation of hard rubber is a strongly exothermic reaction, while the formation of soft vulcanized rubber is apparently without heat interchange, even in the presence of an accelerator. The vulcanization of rubber with rn-dinitrobenzene and selenium is also without heat interchange and only soft vulcanized rubber is formed.
action take5 place, it is usually accoinpanied by an absorption or e v o l u t i o n of h e a t . T h e aniount of the heat interchange i. not a direct nieasure of tlie chemical affinity involved in the reaction, nor is it a measure of the free energy of tlie reaction. The heat of reaction, however, is a measure of the total change in internal energy and is of importance, therefore. in calculating the effect of temperature on a reaction and in elucidating the mechanism of it. Historical
There i.s coinparatively little known about the thermocheniistry of vulcanizabion. I n the attempts that have been made trr relate vulcanizaticin to a chemical reaction the ihermochemical side has heen practically neglected. If vulcanization is a chemical reaction, it should follow the laws of niass action and the thermal relations Fhould be of importance. Previow work has been of a qualitative nature only. Weber (11) has discussed the heats of formation of the halides of sulfur and attempted t o correlate thirm with their abilities as vulcanizing agents. He also determined the heat of combustion of purified Para rubber and found a value of 10,669 calories per gram. Keber further suggests that vulcanization is a n endotlierniic reaction. The energy change he believed to consist of two parts: first, a large absorption of heat clue to the dissociation of S 8molecules into S2molecules; and second. a small evolution of heat when these S? molecules added to the rubher hydrocarbon. The net result was an absorption of heat which he estimated to be about 450 calories per graiii i d rubber a t a combined sulfur of 3.8 per cent. Seidl (I(/) tried to explain the accelerating effect of litharge tliermoclieiiiically. He discovered that when a rubbersulfur-litharge mixture is heated the temperatiire in the interior of the mass rises above that of the heating bath. He concluded that the evolution of heat thus indicated was due t,, the reaction of sulfur with the non-rubber constituents of the rubber to forin hydrogen sulfide. The hydrogen sulfide in turn reacted with the litharge with a large heat evolution. He did not consider that the reaction of sulfur with rubber might prciduce any heat interchange. Willianis and Beaver (12) heated mixtures of rubber, zinc oxide, sulfur, ant1 various accelerators in a glycerol bath and plotted the temperature of the center of the mass against time. I n wine cases the internal temperature of the compouncl ro-e 62" C. (112" F.) above that of the bath. They concluded froni their curves that there is a n evolution of heat a t the beginning of vulcanization followed by a slight absorption of heat near the end of it. Some of the heat involved way consicleretl to be due to a chemical reaction between sulfur and
'
Received April 15, 1930. Presented under the subtitles before the Division of Rubber Chemistry a t t h e 79th Xleeting of the American Chemical Society, Atlanta, Ga., April 7 t o 11, 1930.
rubber. They found that the heat effect was increased by an increase in the amount of sulfur present, an increase in temperature, and the presence of accelerators. Attempts were made to calculate value> for heats of w l canization by evaluating the heat conduction through the sample and into the bath. Such a type of calculation is somewhat uncertain, but they found average exothermic values of the heat of vulcanization of 10 calories per gram of compound a t 10 per cent sulfur. They worked with not inore than 14 per cent sulfur. Kirchhof and Tagper ( 7 ) extended the work of Seidl to include magnesium oxide, calcium hydroxide, lead dioxide, lead carbonate, antimony sulfide, and zinc oxide. Kirchhof ( 5 ) also st'udied the effect of various accelerators on heat evolution. The presence of natural resins in a litharge stock was shown to enhance the effect. Perks ( 9 ) , using approximately the same method as Williams and Beaver, studied mixtures of rubber and sulfur containing froni 0 to 32 per cent sulfur. I n some cases he recorded temperatures in the center of the block of rubber during vulcanization 170" C. (306" F.) above that of the bath. He concluded froni his curves that the combination of rubber with sulfur is accompanied by a slight evolution of heat in the first stages of the reaction, followed by an energetic exothermic reaction. Perks was chiefly interested in a method for evaluating the quality of raw rubber. Bostrom ( I ) has measured directly the heat of vulcanization of rubber with sulfur chloride in benzene solution, using calorimetric methods developed by Hock (4) for the measurement of heats of wetting of carbon black and other fillers by rubber. Bostrom found values up to 5.9 calories per grain of rubber for the heat of vulcanization with sulfur chloride. Method
It is practically iiiipossible to measure directly a, heat of reaction a t high teniperature if the course of the reaction occupies any appreciable time. The law of Hess, which of course is a corollary of the first law of thermodynamics, states that the heat interchange of a reaction is dependent only on the initial and final states of the system m d is independent of the course of the reaction. Heats of combustion nieasurenients may thus be used to measure heats of vulcanization. The course of the reaction may be illustrated thus: (CJIdS,
+
7 J. ' H1
+ 5COa f y SOY where H I = heat of combustion of vulcanized rubber 'jHs
ys i 4 H 2 O
H 2 = heat of combustion of unvulcanized compound H = H , - H I = heat of vulcanization a t temperature at which heats of combustion are determined
To convert' the value of H to the temperature of rulcanization involres calculations based on the specific heats of t'he
INDUSTRIAL AND ENGINEERING CHEMISTRY
738
+YS
=
R
mu 4
A
XJYLFU4 8 12 16
Rubber Parts 100 92 82 76 68
(C5Hs)Su
as y is varied from zero to unity.
20
24
\,6 $8
32
Williams and Beaver (12) attempted some preliminary work on the problem, using the same method. They determined the heat of combustion of a mixture of 6.5 per cent sulfur and 93.5 per cent rubber vulcanized to 0.7 and 5.6 per cent combined sulfur. They found no appreciable difference in the values obtained and concluded that there was no measurable heat of vulcanization between the different states of cure. For these compounds they give a heat of combustion of 5825 calories per gram. Kirchhof and Matulke (6) give a value for the heat of combustion of rubber of 10,070 calories per g a m . This value is of the correct order of magnitude, whereas the values given by Williams and Beaver are apparently seriously in error. Messinger (8) determined the heats of combustion of purified rubber, balata, and guttapercha in an attempt to see if there were enough differences t o throw light on their structural differences. He reports them to be the same within experimental errors and concludes that no deductions as to their respective structures may be drawn from his work. His accuracy was not of the highest, the individual values for rubber varying about 4 per cent.
HEATOF
COMPOUXD
reactants and products of the reaction. The values would not be seriously altered by such a transfer from one temperature to the other. It is desirable to find the heat interchange of the reaction CsH8
Vol. 22, No. 7
Sulfur Parts
..
8 18 24 32
COMBUSTION
Cal. per gram compd. 10547 10055 9434 9061 8568
These values, when plotted against the percentage of sulfur, lie on a straight line, Figure 1, curve A. Rubber-sulfur mixes were made on the mill in the usual manner. They were vulcanized between sheets of aluminum for 6 hours a t 160' C. (320' F.) in the laboratory hydraulic press. Curtis, McPherson, and Scott ( 2 ) have shown that rubber-sulfur compounds when cured to this extent have less than 0.07 per cent free sulfur up to 14 per cent sulfur. From 14 to 32 per cent sulfur the value is somewhat higher, but for ordinary purposes the total amount of sulfur compounded may be considered to have become combined under the conditions of vulcanization. Preliminary w'ork showed that certain of the compounds were very susceptible to oxidation. When allowed to stand exposed to the atmosphere for a month, they gave heats of combustion as much as 400 calories per gram lower than that of a freshly cured sample. All samples, therefore, were vulcanized, kept overnight in a vacuum desiccator, and the heat of combustion determined the next day. COMPOUND HEATOF COMPOUND HEATOF COMBUSTION Rubber Sulfur COMBUSTION Rubber Sulfur Parls Parts Cal. per gram compd. Parts Parts Cal. Per gram compd. 90 10 9849 100 .. 10547 98 2 10395 88 12 9693 96 4 10298 82 18 9275 94 6 10198 76 24 8811 92 8 9978 68 32 8268
N 2
Apparatus
The calorimeter used in the present work was made with an adiabatic jacket of the type suggested by Daniels (3). The calorimeter thermometer was calibrated by the Bureau of Standards and was readable to thousandths of a degree Centigrade. The jacket thermometer was carefully standardized against the calorimeter thermometer. I n burning materials containing less than 4 per cent sulfur, spun-nickel linings were used in the bomb, while for greater amounts of sulfur linings made of fused silica were employed. The standardization of the apparatus was checked by determining the heat of combustion of specially purified benzoic acid supplied by the Bureau of Standards. Sulfur-Rubber System
A sample of 50 pounds (22.7kg.) of smoked sheets was broken down, thoroughly blended, and set aside for the work. Four rubber-sulfur compounds were mixed and heats of combustion determined on the unvulcanized mixtures. A blank determination was made on the smoked sheets.
These values are plotted as curve B in Figure 1 and give a fairly smooth curve. This curve coincides with curve A over the range of 0 to 6 per cent of sulfur and then falls below it in a gradually increasing amount. The difference between the two curves is the heat of vulcanization and is calculated for nine points. All previous values have been given on the basis of a gram of the compound. I n the last column they are converted to the basis of a gram of raw rubber. SULFUR Per cent 0 2 4 6 8 10 12
1s
24 32
HEATO F Vulcanized Cal. 0 10395 1029s 1019s 9978 9849 9693 9275 8811 8268
COMBUSTION
Unvulcanized Cal.
HEATOF VULCANIZATION Per gram Per gram compd. rubber Cal. Cat. 0 0 30 30 4 4 18 19 84 77 91 82 130 114 194 159 329 250 442 300
-
-
July, 1930
INDUSTRIAI, A N D ENGISEERIAVG CHEMISTRY
The heat of vulcanization per gram rubber is plotted against the per cent sulfur in Figure 2. Soft Vulcanized Rubber
An accelerator speeds up the formation of soft vulcanized rubber and allows it to be produced with less combined sulfur. The following compound was mixed and cured: smoked sheets 600, zinc oxide 30, sulfur 60, and D.P.G. 4.5. Heats of
739
tion of soft rubber without the complicating hard-rubber reaction being present. The following compound was mixed and cured: smoked sheets 600, litharge 60, and m-dinitrobenzene 18. Heats of combustion were determined on three cures and on the unvulcanized material. TIMEOF CUREAT 145' C (293' F.) Minutes Unvulcanized 60 120 240
HEATOF COMBUSTIOX Cal. per gram c o m p d . 9428 9430 9426 9428
It is quite evident that there is no apparent heat interchange in the vulcanization of rubber with nz-dinitrobenzene, Selenium
Selenium vulcanizes rubber in the presence of certain organic accelerators, giving a vulcanizate with many desirable properties. Apparently, it is also capable of forming only soft rubber. This reagent likewise allows isolation of the soft-rubber reaction for study. The following compound was mixed and cured: smoked sheets 400, litharge 20, selenium 50, and p-nitroPodimethylaniline 12. Heats of combustion were determined on both the vulcanized and unvulcanized material.
combustion determinations were made. An unusually large amount of sulfur was used so that the soft-rubber formation could be completed and the hard-rubber reaction started.
T I X EOF CUREA T 149' C. (300' F.) Minutes Unvulcanized 60 120
HEATOF COMBUSTION Cal. per gram compd 9004 8982 8986
To check this conclusively and give more definite evidence that selenium does not undergo the hard-rubber reaction, the following comDound was mixed: HEATOF TIMEOF VULCANIZATION COMBINED smoked 'sheets 400, selenium 500, p-nitrosodiVULCAKIZAHEAT OF Per Per SULFURO N TENSILESTRENGTH TIOS A T coxgram gram RVBBER+ 2 days 0. B at methylaniline 12, and litharge 20. This com142' C. BUSTIOX compd. rubber SULFUR NEW 70" C. pound contains 125 per cent selenium on the Col. per Kg.1 LbsJ Kg. p e r Lbs. per Minutes gram tal. Gal. Per cent sp. o n . sq. W. sp. cm. sq. t n . rubber. It was cured 16 hours a t 160" C. 0 9516 ,.. (320" F.). This is approximately sixty times t,he 10 9506 '0 '6 i:o iii 8 2 2 i 3 66.5 945 20 9510 .. .. 1.97 192 4 2733 103.8 1474 vulcanizing effect necessary to produce good, 40 9515 .. .. 240 2415 ~ 3 . l 825 soft, vulcanized rubber from this mix. The Av. 9512 ... ., product did not resemble ha.rd rubber a t all. 80 9484 28 32 5.82 26 1 370 160 9456 56 65 8.50 24 2 345 Heats of combustion determinations were made on the vulcanized and unvulcanized material. Heats of \dcanization are plotted against combined-sulfur HEATOF COMBUSTION values in Figure 3. The correct cure at the temperature used Cal per gram Unvulcanrzed 5037 is approximately 20 minutes. ilt the 160-minute cure the Vulcanized 5045 hard-rubber reaction is well started and the resclmblance of Figure 3 to the first part of Figure 2 is quite evident. The I t is quite evident that the vulcanization of rutber with heat of vulcanization becomes evident at about 4 per cent selenium also does not involve any heat interchange. combined sulfur. The use of an accelerator has apparently Discussion allowed an earlier beginning of the hard-rubber rt,action with its attendant evolution of heat. The curve illustrating the heat of vulcanization of rubThe comparison of the physical properties of this compound with the tliermocheniical progress of the reaction is illustrated ber with sulfur in Figure 2 is quite striking. Apparently by Figure 4. The decrease in original tensile strEngth begins there is little or no heat interchange in the vulcanization as the evolution of heat due to the reaction becomes appar- of a rubber-sulfur mix until after about 6 per cent sulfur ent. The evolution of heat is associated with the formation has combined. From this point to the complete formation of hard rubber and the decline of physical properties sets in of hard rubber there is a steadily increasing heat evolution. This evolution appears to be approximately propor7~hen this becomes appreciable. tional to the amount of combined sulfur above 6 per cent. This quite agrees with the conclusions arrived at qualitatively m-Dinitrobenzene by Perks, who found that the combination of rubber and m-Dinitrobenzene 1s capable of vulcanizing rubber in the sulfur was accompanied by a slight evolution of heat in the presence of litharge to give a material having a ten3ile strength first stage of the reaction, followed by an energetic exothermic of about 98.6 kg. per sq. cm. (1400 Ibs. per sq. in.). This is reaction. This latter reaction does not commence until 4 to nearly as good physically as the material produced by sulfur 5 per cent sulfur has combined with the rubber. He, howwithout the use of an organic accelerator. m-Dinitrobenzene ever, claimed a maximum heat effect at 21 per cent combined apparently is capable of forming only soft vulcanized rubber. sulfur, which does not agree ivith the present work. The The reaction, therefore, offers a means of studying the forma- formation of hard rubber is accompanied by an evolution of
I N D U S T R I A L A S D ENGINEERING CHEMISTRY
740
Vol. 22. s o . 7
chiefly of rubber and sulfur explode with violence during vulcanization. The average value for the heat of vulcanization of 10 calories per gram as calculated by Williams and Beaver a t 10 per cent combined sulfur is apparently far below the new value. Their statement that there is an evolution of heat a t the beginning of the reaction followed by a small absorption a t the end seems to be the reverse of the present results. The formation of soft vulcanized rubber and of ebonite seems to be two distinct and separate processes and very different thermochemically. Literature Cited
442 calories per gram of rubber. The significance of this high value may perhaps be best illustrated by the fact that if vulcanization of a hard rubber compound could be started and there were no heat lost in the process, the temperature would rise about 1000” C. or 1800” F. (specific heat = 0.4). It is well known that large masses of hard rubber compounded
Bostrom, Kolloidchem. Beihefle, 24, 467 (1928). Curtis, McPherson, and Scott, Bur. Standards, Sci. Paper 560. Daniels, J . A m . Chem. SOL.,38, 473 (1916). Hock, Kaufschuk, 1927, 207. Kirchhof, Gummi-Zlg., 39, 892 (1925). Kirchhof and Matulke, Ber., 5TB, 1266 (1924). Kirchhof and Wagner, Gummi-Zlg., 39, 357, 372 (1925). Messinger, Trons. Inst. Rubber I n d . , 5, 71 (1929). Perks, J . SOC.Chem. I n d . , 45, 142T (1926). Seidl, Gummi-Ztg., 95, 710, 748 (1911). (11) Weber, “Chemistry of India Rubber,” pp. 106, 114. (12) Williams and Beaver, IND. E N G . CHEM.,15, 255 (1923).
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10)
11-Vulcanization of Rubber with Nitro Compounds John T. Blake SIMPLEXWIRE & CABLECo., Bosrox MASS.
The Kjeldahl method for nitrogen analysis can be vulcanized with the:-e maadapted to the determination of combined nitrogen terials had approximately the Ostroniuislensky ( 8 ) in rubber vulcanized with nitro compounds. The same degree of unsaturation showed that a number combination of nitro compounds with rubber has been as the unvulcanized rubber. of organic materials possess followed in several cases. Strong evidence is given T h e y t h e r e f o r e concluded the ability to vulcanize rubthat the vulcanization is a chemical reaction. “that ordinary vulcanization ber. The vulcanization does The density of rubber has been shown to change is an unknown or undeternot develop quite such good during vulcanization with dinitrobenzene. The mined type of change in the physical properties in the rubchange approximates the progress of the combination hydrocarbon i n v o l v i n g no ber as sulfur does, but there is of .the vulcanizing agent with rubber. change in the unsaturation, no question that it does take The vulcanization of rubber with dinitrobenzene and and that the chemical union place. Trinitrobenzene, mtrinitrobenzene is monomolecular. A theory of the of sulfur is a secondary redinitrobenzene, and benzoyl mechanism of the vulcanization is advanced. action producing a further peroxide were shown to be the The value of the stoichiometric method in estimatchangewhich, no doubt, gives most satisfactory of the vuling the molecular weight of rubber is discussed. Values properties that are very inicanizing agents. of this constant are suggested by the data. portant in the manufacture We have never been able to Nitro compounds appear to be incapable of producing of rubber goods but which is produce a material resembling a change of degree only, not hard rubber. The amount of combined reagent is only ebonite by using the above of kind.” a small fraction of that required for ebonite formation. materials a s v u l c a n i z i n g Stevens ( 2 1 ) has also inagents. This suggests that the reagents are capable of undergoing only the soft-rubber vestigated the vulcanization of rubber with trinitrobenzene. reaction. They fall, therefore, in the same class with He found t’hat treating the acetone extract of the rubber selenium. This reaction offers a method of studying the with sodium hydroxide gave no red coloration, indicating formation of soft rubber without the complicating effect of the absence of trinitrobenzene. This would imply that the the hard-rubber reaction. The vulcanization of selenium has trinitrobenxene had either been destroyed during vulcanizabeen shown to be a chemical reaction and to follow the mass- tion or had combined with the rubber and therefore could action laws. Vulcanization of rubber with nitro compounds not be extracted with acetone. should follow the same course and throw more light on the Determination of Nitrogen in Vulcanized Rubber mechanism of vulcanization. If a chemical reaction does take place duying vulcanization Fisher and Gray ( 3 ) have investigated the vulcanization of rubber with dinitrobenzene, trinitrobenzene, and benzoyl with nitro compounds, it is probably best followed by analyperoxide. They have determined the unsaturation of the sis. This may be followed, presumably, by determining the vulcanized rubber by the Kemp-Wijs method ( 6 ) . Briefly, combined nitrogen. Since the amount of this nitrogen is this consists of allowing the material t o react for a definite small, the Dumas method for nitrogen determinations is not length of time with an excess of iodine chloride. The excess sensitive enough to be of value. The Kjeldahl method is very is determined and the amount of iodine chloride absorbed is much more sensitive and is universally used for nitrogen a measure of the unsaturation. They found that the rubber determinations on certain types of materials. It is, however, a
EVERAL years ago
S
