INDUSTRIAL A N D ENGINEERING CHEMISTRY
792
For oils from other types of crude more work will have to be done, but even in this case it is believed that the error involved in using these curves will be much less than for any other existing method of extrapolating to high temperatures. Since the curves of Fig. 2 appear t o be nearly straight in the neighborhood of 500’ F., it is probable that they could be extended to 600” F. without the introduction of any serious error. The ordinary Saybolt determinations are always accessible,
Vol. 16, No. 8
so there is no particular need for extrapolation to lower temperatures. Except for a small range, such extrapolation is likely to be in error due to the arbitrary nature of the scale, since the temperature scale extends to negative infinity as the temperature approaches 0” F. These is also a tendency for the introduction of larger errors in the viscosity determination with the heavier oils as they approach their cold test temperature. For these reasons extrapolation to lower temperatures is not recommended.
The Acceleration of Vulcanization’ I-Influence
of the Acetone-Soluble Constituents of Rubber on the Physical and Chemical Properties of Accelerated Rubber Stocks By L. B. Sebrell and W. W. Vogt GOODYEAR TIRE& RUBBERC o . , AKRON,OHIO
A
All accelerators require soluble zinc in order to produce the best ~CCeleratorson the physical amount Of data has physical properties, also in stocks containing the same accelerator Properties of vulcanized rubber, have recorded some been Published from the chemicalcure is no index of the physical properties. time to time during the last The production of maximum physical properties seems to be deinteresting data On the effect of the resins on the ten O r twelve Years On the pendent upon the formation of a zinc-accelerator compound. ,!hch a compound has not been isolafed in any case, except with the merrate of cure. These workers influence Of the resins and capfo accelerators, where it is believed to be [he zinc salt of the acselected two samples of Proteins on the VulcaniZation of rubber. This has celerator. pale crepe rubber, extracted been done to determine Some accelerators. such as diphenylguanidine and dimethylthem with acetone for 36 aminodimethyldithiocarbamate,are able to react directly with zinc hours, and vulcanized them their effect both upon the oxide in order to form the necessary zinc-accelerator compound, with suitable controls for chemical and Physical Propwhile others are dependent upon the presence of a rubber-soluble equal lengths of time in a erties of the rubber. Rezinc salt, either formed by the reaction of the resin acids on zinc oxrubber-sulfur mix* A desuits have been Obtained termination of the coeffifor Pure gum mixes, and ide or added directly to the mix. for those containing certain cients of vulcanization inorganic accelerators, such s h o w e d t h a t one exas litharge and magnesia, but in only a very few cases for tracted sample contained more combined sulfur than the unextracted control, while in the other case there was no compounds containing organic accelerators. Seid12 in 1911 and Beadle and Stevens3 in 1912 made a difference. They further found that, if magnesia was used study of the nature of the resins contained in rubber and in the mix under the same conditions as just previously detheir relation to the quality of the vulcanized product. scribed, the rate of cure was greatly reduced if extracted rubThe latter workers used a simple compound of rubber with 5 ber was used, and that this was equally true of both samples. per cent of sulfur and in some cases with the addition of zinc Magnesia would then seem to function in exactly the same oxide. They found that the removal of the acetone-soluble way as litharge. If, however, a small amount of an organic material retarded the cure and that the tensile properties of accelerator (termed “Accelerator A”) was added to each of the rubber were greatly reduced. At about t,he same time the extracted samples and their controls and cured as before, Weber4 announced the results of work that he had done on then the resin-free rubber again showed an increased rate of the action of the resins, He had used a litharge stock and cure in both cases. The reason for the increased rate of cure with extracted rubber had obtained such poor results that he in this last case was not considered, nor has it since been stated that no vulcanization had taken place. Stevens5 re- satisfactorily explained or further investigated. The effect peated Weber’s work and found that in a litharge stock pre- of the resins on the action of organic accelerators has also been pared with acetone-extracted rubber the physical properties mentioned by Maximoff.’ He was unable to obtain any were indeed poor, that the rubber was more susceptible to accelerating effect with the zinc salt of dimethyldithiocarbaging, but that it did contain some combined sulfur. There- amic acid unless zinc oxide was present or an excessive fore some vulcanization had taken place. I n a 90 : 10 rubber- amount of the accelerator was used. This fact he attributed sulfur mix he found that the removal of the resins retarded the to the acid nature of the resins, which he thought retarded the rate of cure, as shown by the reduced coefficient of vulcaniza- rate of vulcanization. Zinc oxide by neutralizing this acidity allowed the vulcanization to proceed a t the normal rate. We tion and lower physical properties. Kratz and Flower,B in a study of the effect of certain now know that this explanation is not the correct one, since a more plausible explanation of the inactivity of this acceleraPresented under the tit,e ,,The Acceleration of Vulcanization I--The Influence of the Nonrubber Constituents on the Physical and Chemical tor in the absence of zinc oxide has been set forth by Bedford Properties of Rubber Stocks” before the Division of Rubber Chemistry a t and Gray.8 the 66th Meeting of the American Chemical Society, Milwaukee, Wis., Stevensg in some later work has also found certain rubbers September 10 to 14, 1923. which, after removal of the resins, vulcanized a t a faster rate * Gumini-Ztg : 88, 710, 748 (1911).
C 0 N S I DE RAB L E
8th Mfernal. Cong. A g p l . Chcm., 1912, pp. 25, 581. Ibid., 1912, pp. 9, 95. I J . SOC.Chcm. I n d . , $6, $74 (1916). 6 T ~ I JOUR,NAL, S $2, 971 (1920). 8
4
7 8
0
Rubber A g e , 10, 53 (1921). THISJOURNAL, 15, 720 (1923). J . SOC.Chem. I n d . , 41, 3261’ (1922).
August, 1924
INDUSTRIAL AND ENGINEERING CHEMlXTRY
than the unextracted sample; other samples were either unaffected or the rate of cure was decreased. Martin and Daveylo have found that the acetone extract does not play an important part in determining the rate of combination of rubber and sulfur in the presence of thiocarbanilide, but that it does have a decided effect on the nature of the rubber-sulfur aggregate. They concluded, therefore, that the resins do not affect materially the rate of combination of rubber and sulfur, but that they do alter to a marked degree the physical properties of the rubber mix. Their action, however, depends upon the nature of the rubber compound; thus, in a straight sulfur mix they tend to increase rather than to decrease the vulcanization coefficient.ll I n a litharge or magnesia mix they have a decided effect, as has been previously shown.s#6 I n a mix containing an organic accelerator they may increase or decrease the rate of vulcanizing action, but, as will be shown later in the present paper, they have a far-reaching effect upon the nature of the resulting physical properties. Whitby and DolidI2 have recently succeeded in isolating a new acid from the rubber resins which they named “heveic acid.” [n another paper by Whitby and Cambron13 the significance of the resin acids with regard to the action of various accelerators has been discussed. From the results obtained they have concluded that in some cases accelerators react, not only on or with the sulfur through the aid of zinc oxide, but also with the resin acids to form soaps. These soaps then cause an increase in the rate of vulcanization by increasing the degree of dispersion of the rubber. An explanation of the function of the resins in promoting vulcanization in a rubber-sulfur-litharge mix has recently been given by Redford and Winkelman.l4 They found that the resin acids soluble in acetone dissolved the litharge and thus rendered it available for use as an accelerator. The addition of soluble zinc compounds to rubber mixes has recently been patented by Russell.ls This was done for the purpose of eliminating the variability of crude rubbers. The patent appeared a t about the same time that the present paper was read,’ but it does not set forth the fundamental reasons underlying the results obtained with such soluble zinc compounds. It has been claimedlo that diphenylguanidine requires hydrogen sulfide in order to exert its accelerating action, while mercaptobenzothiazole should function best in the absence of hydrogen sulfide. It occurred to the writers that if these two accelerators should be compounded using rubber from which the acetone-soluble constituents had been removed, some light might be thrown on the correctness of this mechanism. It was expected that the acetone extraction, by removing the resins, would so far decrease the amount of available hydrogen sulfide that could be formed, that a slight decrease in the activity of diphenylguanidine would be noted, while the mercaptobenzothiazole would remain unchanged. The results showed just the opposite to be the case. The extracted rubber stock containing diphenylguanidine proved to be superior in physical properties and amount of combined sulfur to the stock made from the unextracted rubber, while the extracted rubber stock containing mercaptobenzothiazole was very inferior to the control stock. These facts suggested a new field of research on the action 10
J . SOC.Chem. Ind., 42, 103T (1923).
Maitin and Elliot, Ibid., 41, 2261‘ (1922). “Examination of the Emulsifying Properties of Some Constituents of Hevea Latex;’ presented before the Division of Rubber Chemistry at the 65th Meeting of the American Chemical Society, New Haven, Conn., April 10 to 14, 1923. 18 J . SOC.Chem. Ind., 48, 333T (1923). 14 THISJOURNAL, 16, 32 (1924) 1) U. 5. Patent 1,467,197 (September 4, 1923) $ 6 Scolt, TEXIS JOURNAL, 16, 286 (1923). 11
12
793
of organic accelerators, to which little attention has thus far been given. The action of about fifteen of the more common organic accelerators was investigated, both in extracted and unextracted rubber, and the results obtained, together with the conclusions which have been drawn from them, constitute the subject matter of this paper. PROCEDURE
A large lot of selected pale crepe rubber was set aside for the work, part of which was extracted with acetone as required in large Soxhlet extractors for a period of 16 hours. This treatment removes upwards of 95 per cent of the total acetone extractable material. The ordinary procedure of milling, curing, and testing was followed, and wherever possible the regular and extracted compounds were cured simultaneously in the same twocavity mold to secure identical curing conditions. In some cases extracted rubber to which the acetonesoluble constituent had been returned was used to compare with the extracted rubber, thus obviating any differences due to the extraction. This procedure gave the same results as the use of regular rubber. As a matter of facility the regular rubber has been used directly in these tests. Furthermore, the results are plotted on such a scale that the ordinary testing errors are suppressed. All cures were made a t 141.7’ C. (287’ F.) (2.8 kg. per square centimeter [40 pounds per square inch] steam pressure) unless otherwise noted. The physical tests on the cured stocks are recorded by tensile in kilograms per square centimeter, elongation in per cent, and the stretch modulus a t 700 per cent-i. e., the load in kilograms per square centimeter necessary to produce an elongation of 700 per cent. The chemical data are presented i s percentage of combined sulfur on the compound, inasmuch as the data are on comparable formulas. The coefficient of vulcanization may be computed from the data. The combined sulfur was determined directly on the residual rubber after extraction with acetone, according to tKe methods1’ of the AMERICAN CHEMICAL SOCIETY. PRELIMINARY EXPLANATIONS Through this paper frequent mention is made of “activation,” and “best” or “maximum” physical properties. I n order that there may be no misunderstanding of the use of these terms the following explanation is made : It is realized that the conceptions as to desirable physical properties in stocks of the near pure gum type vary considerably among various investigators. For example, a tire friction or skim coat stock may be compounded without any zinc oxide. Such stocks are characterized by their relatively low tensile, high elongation, and softness-i. e., low modulus. When a small proportion of zinc oxide is added to this mixture, stocks of higher tensile, lower elongation, and greater stiffness of the stress-strain curve are obtained. These changes are what are referred to as “activation effects,” and when “best” or “maximum” physical properties are mentioned the authors have in mind the stiffer, high tensile stocks secured by the incorporation of zinc oxide into the mix. Practically all accelerators are so “activated” by zinc oxide. It has frequently been noticed, and has recently been stated in the literature,1E that in a diphenylguanidine stock small amounts of zinc oxide up to 1 per cent will disappear upon curing, giving a transparent stock. This has usually been attributed to some action between the accelerator and the zinc oxide. It was observed that 1 per cent of zinc oxide will disappear in a rubber-sulfur mix, giving a transparent 17
THISJOURNAL, 16, 397 (1924).
INDUSTRIAL A N D ENGINEERING CHEMISTRY
794
stock, whereas if extracted rubber is used the 1per cent of zinc oxide will not disappear and a transparent stock is not obtained. The solubility and disappearance of the zinc oxide is therefore due to the solvent action of the resin acids contained in the rubber. This same explanation applies equally well to the REGULAR RUBBER
0
- --- -- - -
EXTRACTED R U B B E R O - - - - - - -
Vol. 16, No. 8
added to stocks containing hexa, that in the regular rubber good physical properties were obtained with some tendencies toward reversion and the development of a slightly foul odor on overcure. The cured stocks were also transparenti. e., the zinc oxide had disappeared. I n the stocks using extracted rubber much lower physical properties, higher combined sulfur, extreme reversion, and an extremely foul odor were found. The stocks were not transparent. This lack of transparency shows rather conclusively that: 1-Hexa does not react directly on zinc oxide to form soluble zinc compounds. 2-The action of the resin acids on the zinc oxide is the major cause of its disappearance.
disappearance of zinc oxide in small amounts in diphenylguanidine or hexamethylenetetramine stocks, and can be demonstrated experimentally as described for the rubbersulfur-zinc oxide mix. I n order that the experimental data may be more readily understood, attention shpuld be called to some of the outstanding results which may be briefly enumerated as follows: 1-All accelerators requiring zinc oxide for activation require that it be in a soluble form before any activation takes place. 2-The soluble zinc must be produced by the resin acids of the rubber for all except three of the accelerators tried-diphenylguanidine-dimethylammonium,dimethyldithiocarbamate, and tetramethylthiuramdisulfide. These accelerators have the power of dissolving zinc oxide directly. 3-The chemical cure proceeds entirely independently of the physical cure. Removal of the resins in some cases promotes the rate of addition of sulfur to rubber and in others it seems to retard it. AMMONIA TYPEACCELERATORS
Hexamethylenetetramine gives a most interesting series of results. A direct comparison is given in Fig. 2 for regular and extracted rubber. Here it is found that the absence of resins leads to lower physical properties but to higher combined sulfur. The resin acids act as retarders of the chemical reaction, probably by neutralizing the ammonia liberated during the curing reactions of the hexa. However, the absence of the resins leads to a lack of soluble zinc, which lack causes the lower physical properties, for reasons to be discussed later. Furthermore, in the extracted stock extreme reversion of the stress-strain curve is encountered, with also the attendant development of the characteristic foul odor common in stocks that contain none or too small an amount of zinc oxide. This reversion has been discussed by Twissll* who concludes that some reaction product of hexa and sulfur is a compound which accelerates reversion and that the preaence of zinc oxide is necessary to neutralize this action. The present experiments confirm this viewpoint, except that it now appears that soluble zinc compounds rather than zinc oxide are necessary, because in the absence of resin acids the zinc oxide is not capable of reacting directly with hexamethylenetetramine and sulfur. It was further noted, when one part of zinc oxide was 18
J SOC.Chem. I n d . , 40, 242T (1921).
Furthermore, some experiments have been cited recently to determine the minimum amount of zinc oxide necessary to bring out the maximum activatien of a given accelerator. Inasmuch as most accelerators will not be activated by large amounts of zinc oxide if compounded in extracted rubber, it seems to the writers that the determination of the minimum amount of zinc oxide necessary for activation is a left-handed way of determining the resin acid content of the particular rubber used. Aldehyde ammonia (Fig. 2) is similar to hexa in that in extracted rubber the combined sulfur is higher and the physical tests lower. Here no tendency towards reversion is found. T w i s ~ states * ~ that “aldehyde ammonia does not need the additional presence of other substances for the full development of its power.” The writers are unable to agree entirely with Twiss on this point, finding that stocks containing 5 parts of zinc oxide have much higher tensiles and greater stiffness than those containing no zinc oxide, regular rubber being used in both cases. However, the difference is less marked with aldehyde ammonia than with many other accelerators. The absolute differences are about the same as the differences obtaining between regular and extracted rubber in the presence of zinc oxide and aldehyde ammonia.
Sulzin, an inorganic accelerator of approximate composition ZnSOr.5NHs,is most probably a true ammonia type accelerator. The results are given in Fig. 1. Inspection of the graph shows that here again the extracted rubber gives lower physical properties and higher combined sulfur than the regular rubber. This shows again the necessity for the presence of J . SOC.dhem. I n d . , 41, S1T (1922).
INDUSTRIAL A N P ENGINEERING CHEMISTRY
August, 1924
soluble zinc in order to form the compound that is responsible for the increase in physical properties. Diphenylguanidine gives rather startling differences in behavior depending upon the test formula used. In Fig. 3 is given a direct comparison of regular and extracted rubber in a test fordula containing 1 part of zinc oxide per 100 parts of rubber. This graph shows beyond doubt REGULAR RUBBER
a-
so
90 D P6UANIDlNE
5 D PWMICWE
5
EXTRkCTm RUBBER
so
60
a-
- - -- -
795
sulfide, and also by the interaction of these gases in anhydrous ether according to the methods of Thomas and Riding.20 This compound is comparatively unstable, changing rapidly into ammonium sulfide and hydrogen sulfide. By drying the material carefully, however, it could be coinpounded into rubber without having suffered serious decomposition. The results obtained are as follows: TABLB I
--Formula? 1 2 3 Parts Parts Parts 100 100
50 40
T ~ I O ~ R B ~ N I3L0I D E PlETHILFNEI) IPLUIOINE I O
Regular rubber Extracted rubber Zinc oxide Sulfur Ammonium hydrosulfide
..5
166
6
5 6
1
1
., ..
6
1
TABLEI1 Formula 1 2 3
that the regular rubber yields stocks of much higher physical properties, but with almost identical combined sulfur values, in comparison with the extracted rubber. As previously noted, t,he regular rubber gave transparent cured sheets, while the extracted rubber stocks were translucent. On increasing the proportion of zinc oxide to 5 parts on 100 of rubber, however, entirely differentresults were obtained as shown in Fig. 3. The extracted rubber gives higher physical properties, the stress-strain curve being much stiffer, as shown by the load a t 700 per cent and the reduced elongation a t break. The tensile figures are practically equal a t the proper cure (about 40 minutes), but on overcure the regular rubber gives continually rising tensiles, as contrasted with the extracted rubber, whose tensile curve shows no increase. * The extracted rubber also gives somewhat higher combined sulfur figures. The differences between the results obtained with 1 and 5 parts of zinc oxide were so striking that the comparison was repeated three times in order to be sure that the differences were not due to faulty technic. *Furthermore,the magnitude of the differences found was so great as to exclude any minor testing variations. The explanations of this surprising behavior is probably as follows: Diphenylguanidine will react on zinc oxide directly or through some of its sulfur reaction products to form rubber-soluble zinc-containing compounds. It is therefore not dependent entirely on the supply of soluble zinc soaps ordinarily furnished by the action of the resin acids on the zinc oxide. However, in cases where the quantity of zinc oxide present is abnormally low (say, 1 per cent), the rate of the reaction of diphenylguanidine and zinc oxide t o form the zinc-accelerator compound and the amount formed are materially reduced by reasons of mass equilibria; consequently, low physical properties of the cured stock result. If the amount of zinc oxide is increased, the formation of the zinc-accelerator compound is hastened and good physical properties are secured in the cured stocks.
This rubber-soluble zinc-accelerator compound is of unknown composition a t present, but is most certainly responsible for the excellent physical properties of the stocks in question. Since it was found that diphenylguanidine could dissolve, by some means, sufficient zinc oxide for its activation, it seemed that it might possibly be due to the formation of a compound of the ammonium sulfide type. As a comparative test a quantity of solid ammonium hydrosulfide was prepared both by the interaction of gaseous ammonia and hydrogen
5
Cure a t 40 Lbs. 500 Minutes Per cent 90 28 90 25 90 13
&O-S"L,J"R
29
R E G U L A R RUBBER 0-
---JO
_---
PlPERlDlME--
60
--IO
--
700
Per cent 99 87 18
Tensile Elongation Kg./Sq. Cm. Per cent 180 820 160 770 95 1030
DrePROPYLAPfINE-
--- - - - -- -50 - - - - - _ -b o
EXTRACTED RUBBER 0-
50
- - - - - --60 --IO
pTOLUlDlNE-
- IO
I n the regular rubber stocks using both piperidine and din-propylamine, pronounced overcuring tendencies are observed, as shown by (1) the almost linear rise of the "stress a t 600 per cent" curve; (2) by the "peak" and subsequent drop in the tensile curve; and (3) by the steady downward progression of the elongation curve. The extracted rubber stocks using the same accelerators give no evidences of similar behavior, giving essentially "flat" curves over the curing range. This overcuring tendency in the unextracted rubber stocks 20
J . C h e w SOC.( L o n d o n ) , 124, 1184 (1923).
796
INDUSTRIAL AND ENGINEEWNG CHEMISTRY
is strong evidence of the existence of a cyclic process of formation and reformation of a zinc-accelerator complex, which is the true accelerating agent. Furthermore, the amines seemingly are incapable of reacting, a t least to any great degree, on zinc oxide directly to generate this complex. The presence of rubber-soluble zinc salts is thus apparently necessary. MISCELLANEOUS TYPES Ethylidene aniline (Fig. 1) and methylene p-toluidine (Fig. 3) give results similar to those of the ammonia type in that the combined sulfur is higher and the physical tests lower in the extracted rubber stocks. These compounds also require the presence of soluble zinc in order to produce their best physical properties. It requires a rather wide stretch of the imagination to class these two accelerators as of the ammonia type by any chemical reasoning from their molecular structure, but the compounding results obtained indicate their marked similarity to the true ammonia type accelerators. MERCAPTO TYPEACCELERATORS-CLASS I The comparison of the results obtained with the regular and extracted rubber are given in Fig. 2 for the zinc salt of mercaptobenzothiazole. Inspection shows that : 1-The physical properties of the "extracted" compound are much inferior to those of the regular compound. 2-The rate of combination of sulfur-i. e., the amount combined per unit of curing time (as shown by the slope of the line)is less for the extracted rubber.
Here we have proof that the chemical as well as the physical properties are dependent on the action of the nonrubber constituents. The resin acids react with the zinc oxide to form zinc resinates (soaps), which then react with the accelerator to form the zinc salt of the accelerator, and a t the same time set free the resin acids again. The zinc salt of the accelerator then functions as a sulfur carrier. The constant formation of the zinc salt depends (1) on a supply of resin acids to form the zinc resinates; and (2) on an excess of zinc oxide to neutralize the resin acids set free, thus insuring a t all times enough soluble zinc to form the zinc salt of the accelerator. The action of the zinc salt of mercaptobenzothiazole is strong, supporting evidence that the process of acceleration is essentially a cyclic one, inasmuch as if it were not we should expect the zinc salt in an extracted rubber stock to be a powerful accelerator while the straight mercaptobenzothiazole would be inactive in a similar stock. As a matter of fact, neither possesses any marked activity in such a stock, thus showing that it is the constant reformation of the zinc salt which is responsible for the accelerating action. The same conchions hold for thiocarbanilide, whose action is shown in Fig. 3. Here again the physical and chemical properties of the extracted rubber are inferior to the regular rubber. It is of interest to record that the "set curing" properties of thiocarbanilide are wholly absent in the extracted rubber stocks, this behavior being additional evidence that the set curing phenomena as well as the physical properties obtained are due to the action of a zinc-containing compound-i. e., the zinc salt of thiocarbanilide. MERCAPTO TYPEACCELERATORS, CLASSI1 Dithiocarbamates and thiuram disulfides-Dimethylammonium-dimethyldithiocarbamate and tetramethylthiuramdisulfide behave peculiarly in extracted rubber. It might be expected that they would act like the mercapto accelerators of Class I, but as a fact the physical properties of the extracted rubber stocks are equal to the regular rubber in the case of the dithiocarbamate, and only slightly less in the case of the thiuram disulfide.
Vol. 16, No. 8
Since these accelerators also function through the formation of their zinc salts, it is evident that, since there are no resin acids present, these compounds must have the property of reacting with zinc oxide to form the zinc salt of the mercaptan. This applies directly for the dithiocarbamate and indirectly for the thiuramdisulfide. It has been shown by Bedford and Grays that dithiocarbamates will react directly with zinc oxide to form the zinc salt. It has also been shown that thiuramdisulfide must first be reduced by hydrogen sulfide to the dithiocarbamate in order that it may react with the metallic oxide. Additional proof has now been obtained that the same reactions take place in rubber. The thiuramdisulfide does not give such good results in extracted rubber as it does in the regular rubber, for the following reasons: 1-The removal of the resins cuts down the amount of hydrogen sulfide available to reduce the disulfide to the dithiocarbamate, although some hydrogen sulfide should still be available by the reaction of sulfur on the protein. 2-Owing to this lack of hydrogen sulfide the thiuram is not completely reduced to the dithiocarbamate so that i t can react with zinc oxide. Consequently, it does not give such good results as the dithiocarbamate.
2-Mercapto-4-phenylthiazole (Fig. 1) should be mentioned here because this compound, owing to its greater acidity, has the power of reacting with zinc oxide directly to a greater extent than the mercaptobenzothiazole, but to a much less extent than the dithiocarbamate previously mentioned. Hence, this compound renders soluble enough zinc to cause the chemical reaction to proceed with equal rate in the extracted rubber, but the physical properties are still low. FURTHER PROOF OF THE ACTION OF RESINS I n Fig. 5 are shown the results obtained with six accelerators in regular and extracted rubber! and, in addition, the effect of zinc stearate on the extracted rubber stocks. I n every case zinc stearate has improved the extracted stocks to such an
,
I
1-
1v
-
"I ,w
I
6"
7"
-
0"
TlMF I N MlNllTFI
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6"
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extent that they are in most cases equal or superior to those using the regular rubber. These results afford additional proof that an adequate supply of soluble zinc is necessary for all these accelerators to exert their maximum physical properties. The exact mechanism between the soluble zinc and the accelerators or their decomposition products is as yet unknown in the case of the ammonia accelerators, or hydrogen sulfidepolysulfide type. I n the case of carbosulfhydryl or mercapto accelerators it can reasonably be said that the zinc-accelerator compound responsible for both the chemical and physical cures is the zinc salt of the accelerator. ACKNOWLEDGMENT The writers wish to acknowledge the assistance of C. M. Carson and E'. L. Shew for valued suggestions and the preparation of compounds.
