866
INDUSTRIAL AND ENGINEENRINGCHEMISTRY
Summary The present article is restricted to the empirical facts relating to resinification considered alone-i. e., as applied to the products of the preliminary combination in which the formaldehyde-phenol ratio is already fixed. Consideration of direct resinification of phenol-formaldehyde mixtures, where the ratio of combination is determined by the resinification conditions, and the speculative question of molecular structure is reserved for later articles. Much of the material presented is stated collectively in the form of generalizations covering numerous individual observations. The principal items are the following: Resinification is promoted slowly by heat alone and accelerated by alkalinity or acidity, giving rise t o the two contrasting general methods of condensation. The characteristics of each are described. Under alkaline conditions, the velocity of resinification is largely independent of the concentration of catalyst (hydroxyl ion) but varies greatly with the parent phenol and ratio of combination; under acid conditions the reverse is the case. Three types of resins are obtained-noncurable, semicurable, and highly curable. When the ratio of combination is already fixed, the degree of curability is independent of the method of condensation and is determined by the ratio of combination and the number of free ortho and para positions in the molecule of the parent phenol.
VOL. 29, NO. 8
The more elaborate experiments, made possible by the availability of the pure dialcohols of ortho- and paracresol, are described specifically in detail. In addition to ilIustrating various items of the eneral discussion, these furnish special data, such as the ernpirica! formulations of resinification reactions and resin molecules, and the extent of ultimate dehydration, in every instance closely approximatingtwo molecules of water per molecule of dialcohol, or one per molecule of formaldehyde originally combined.
Literature Cited (1) Baekeland, J. IND. ENQ.CHEM.,1, 149, 545 (1909); 3, 478, 518,932 (1911); 4, 737 (1912);5, 506 (1913); U.S. Patent 942,849 (1909). (2) Baekeland, J. IND.ENG.CHEM.,1, 159, 160 (1909). (3) Baekeland, U.S. Patents 1,088,677and 1,088,678(1914). (4) Baeyer, voii, Ber., 5, 266, 1094 (1872). (5) Granger, IWD. ENG.CHEM.,24, 442 (1932). (6) Granger, U. S. Patent 1,956,530(1934). (7) Holmes and Megson, J . SOC.Chem. I n d . , 52,415 (1933). (8) Manasse, Ber., 27, 2409 (1894); Lederer, J. prakt. Chem., [21 50,223 (1894); Green, Am. Chem. J., 2, 19 (1880). (9) Meneies, J. Am. Chem. Soc., 43, 2309 (1921); Meneies and Wright, Ibid., 43,2314 (1921). (10) Morgan and Megson, J . Soc. Chem. Ind., 52,418 (1933). (11) Morgan, Megson, and Drummond, Ibid., 49, 245 (1930). (12) Piria, Ann., 48, 75 (1843); 56, 35 (1845); Beilstein and &elheim, Ibid., 117,84 (1861); Moitessier, Jahresber., 1866,677. R~CEIVE July D 31, 1936.
Studies in the Vulcanization of Rubber VII. Unsaturation of Rubber Vulcanized with Nitro Compounds and Benzoyl Peroxide’
R
UBBER is primarily an JOHN T. BLAKE AND PHILIP L. BRUCE from their results that “Ordinary vulcanization is a n unknown or unsaturated hydrocarbon. Simplex Wire & Cable Company, Boston,Mass. undetermined type of change in Sulfur adds to its double the hydrocarbon involving no change in the unsaturation bonds during vulcanization to form soft and hard vulcanizates. and that chemical union of sulfur is a secondary reaction.” I n the formation of soft vulcanized rubber, unsaturation is If true, this evidence is a serious blow to the chemical theory reduced several per cent by the chemical addition of sulfur. of vulcanization which postulates that a chemical reaction Spence and Scott (10) showed that the proportion of comis essential for the process. The present paper offers evidence bined sulfur corresponds exactly to the decrease in unsaturathat the vulcanization of rubber with these reagents does tion, and therefore that sulfur combination consists entirely involve change in its unsaturation. of addition to the double bonds of rubber. The studies of one of the authors (1) on the vulcanization Experimental Procedure of rubber with nitro compounds indicate that these subThere are a number of methods of determining the unsaturastances or their decomposition products combine with rubtion of rubber. The early ones involved reacting it with an ber. Van Rossem (8) also showed that vulcanization with excess of bromine and titrating the excess reagent with pobenzoyl peroxide is accompanied by a decrease in extractable tassium iodide and thiosulfate after a definite time period. material, indicating that either the reagent or its decomposiVarious authors have recommended differing conditions tion products combine chemically with the rubber. The natural expectation would be that addition to the double bonds (6, 9, 11). The reaction is complicated, however, by the substitution of some of the hydrogen of the rubber by brooccurs and that the unsaturation decreases correspondingly. mine. Lewis and McAdams (7) suggested that the hyVan Rossem’s proposed mechanism for the vulcanization with benzoyl peroxide would involve no change in unsaturadrogen bromide formed by the substitution be titrated with potassium iodate and sodium thiosulfate. If this correction tion. Fisher and Gray (2) applied Kemp’s method (4) in is applied to the total bromine absorption, values are obdetermining the iodine numbers of a single vulcanizate of tained that are approximately correct, but conditions have each of four compounds. The vulcanizing agents involved to be controlled carefully if precise results are to be obtained. were m-dinitrobenzene, trinitrotoluene, and benzoyl peroxide. Kemp suggested the reaction of rubber with iodine chloride Their unsaturation values agreed closely with calculated a t 0”. Under these conditions satisfactory results are obvalues for the unvulcanized compounds. They concluded tained, and this is probably the best method yet offered. 1 Previous papers in this series appeared in 1980 (pages 737,740,744,7481, Nitro compounds react with rubber so weakly that only 2932 (page 649), and in 1934 (page 1283).
AUGUST, 1937
INDUSTRIAL AND ENGINEERING CHEMISTRY
soft vulcanized rubber can be formed. Ebonite cannot be produced with this reagent. It was thought a t first that, since iodine chloride is sufficiently active to react completely with rubber at 0" C., the reagent might displace any combined nitro compound to give the impression that no combination had occurred. This might explain the anomalous results of Fisher and Gray. The first part of this paper contains work with iodine bromide, a reagent less vigorous than Iodine chloride, with the idea of overcoming the above difficulty. The second part contains work with iodine chloride. Substantially the same results were obtained in both cases.
Iodine Bromide
867
50 cc. of 10 per cent aqueous potassium iodide were added, and the excess halogen was titrated with aqueous sodium thiosulfate. As the end point approached, starch solution was added and small amounts of acetic acid were used t o destroy the emulsion. With practice, reproducible results can be easily obtained. A series of determinations was made on 0.1-gram sam les from a single batch of smoked sheet with varying times otreaction and with different excesses of reagent. Each sample was extracted with acetone after weighing, and the acetone was removed in a vacuum desiccator before solution in the solvent. It is generally accepted that rubber hydrocarbon is composed of C6H8 units, each containing one double bond. The theoretical iodine number of this hydrocarbon is then 372.8. Since the hydrocarbon content of rubber is 93 to 96 per cent, the iodine number of raw rubber should be 347 t o 358:
Iodine is too inactive a reagent to add completely and Reaction Excess Iodine Reaction Excess Iodine Time IBr No. Time IBr No. saturate all the double bonds in rubber. Iodine bromide is Mzn. % Min. % intermediate in activity between iodine and iodine chloride. 15 39 265 60 110 348 64 290 12.5 34.5 Gorgas (3) claimed that iodine bromide makes a very satisfactory agent for determining unsaturation. The regular Hanus reagent consists of a solution of iodine bromide in glacial acetic acid. Gorgas found that this gave low values, perhaps because of the coagulating effect of the acid on the rubber solution. With carbon tetrachloride as the solvent, From these results i t may be concluded that, if less than better results were obtained, but a rapid increase occurred in 100 per cent excess reagent is used and if the reaction time is unsaturation values with reaction time. Kemp found that at 0" C. the rate of addition of this reagent was too slow to less than 60 minutes, the action is incomplete. Increasing the excess beyond 200 per cent, and the time beyond 60 minmake its use feasible. At room temperatures following utes gives values which are too high. Gorgas' procedure, Kemp and Mueller (5) obtained low Compound A (smoked sheets 100, litharge 5, and m-dinivalues, but by using a larger excess of reagent and a longer trobenzene 10) was vulcanized reaction time they o b t a i n e d for varying periods of time a t better results. The results were sufficiently variable, however, so 132"C. (270"F.). Unsaturation values were determined as above, that they believed the method Previous work in this field could not be relied upon for a using 200 per cent excess reaindicated that the vulcanizagent, B 60-minute reaction time, high degree of accuracy. tion of rubber with dinitroand p-dichlorobenzene as the benzene and benzoyl peroxide Gorgas used 0.2 N solutions but solvent. Experiments s h o w e d in the present work reagents of half is not accompanied by a dethat neither m-dinitrobenzene nor this strength were used. The carl i t h a r g e is affected by iodine crease in unsaturation. This bon tetrachloride was purified by bromide. treating with alcoholic sodium hyimplies that a c h e m i c a l droxide, washing with water, dryIn order to compare the change reaction is unnecessary for ing with c . P. calcium chloride, in unsaturation with the chemical and distilling fractionally. Exactly the formation of vulcanized effect of the reaction with dini0.1 N solutions of iodine and brorubber and deals a serious mine in this solvent were then pretrobenzene, combined nitrogen pared and standardized with aquevalues were determined on other blow to the chemical theory o u s sodium thiosulfate. Equal samples of the same cures after of vulcanization. Other evivolumes were mixed to form the similar acetone extraction. The iodine bromide solution. This was dence indicates that nitro Standardized against s u b 1i m e d nitrogen values were corrected compounds and benzoyl perarsenious oxide. for the blank on the raw unvulThe determination of the unoxide do react with rubber canized rubber after acetone exsaturation of unvulcanized rubber traction and calculated on the during vulcanization, was carried out by refluxing the sample for 2 hours in c. P. carbon base of rubber in the compound: The unsaturation of such tetrachloride. Vulcanized samples vulcanizates has been studied m o o F. were not dissolved sufficiently by l'ulaanizaIodine Combined this treatment. p-Dichlorobenzene by means of iodine chloride tion NO. N was used as the solvent since it Min. % and iodine bromide. The disintegrates most v u 1 c a n i z e d 10 338 0.027 s a m p l e s a t i t s b o i l i n g point results indicate that vulcan46 329 0.082 (172' C.) and does not react with 60 326 0.197 ization with these agents is 90 313 0.483 the reagent. Since it is a solid Y 20 306 0.720 at room t e m p e r a t u r e , carbon accompanied by a decrease I80 299 0 822 tetrachloride must be added after in unsaturation and that it cooling t o maintain a liquid solution. There seems to be a progressive A study of the suitability of the is proportional to the commethod involved an investigation decrease in unsaturation during bined material. Calculations with a substance of known bevulcanization as determined by havior. Samples of smoked sheet of the number of double this method, and this is accomof known weight were swelled in bonds saturated per molecule panied by an increase in comthe solvent in covered Erlenmeyer flasks. A measured amount of the bined nitrogen. In Figure 1both of reagent give results that are iodine bromide solution was added, sets of values are plotted against not readily explained. and) the mixture was allowed to t,he time of vulcanization. If the stand in darknessfor a definite period theory is correct that the two a t room temperature (22' t o 25' C.),
INDUSTRIAL AND ENGINEERING CHEMISTRY
868
VOL. 29, NO. 8
reagent was substituted for the tetrachloroethane in the present determinations.
FIGURES 1 AND 2. COMPOUND A, m-DINITROBENZENE
5
FIGURES 3 AND 4. COMPOUND B, '?%DINITROBENZENE
One-tenth-gram samples of the vulcanizates were acetone-extracted and added to 50 grams of p-dichlorobenzene,and the mixture was b o i 1 e d u n t i 1 disintegration was complete. After cooling, 50 cc. of c. P. carbon disulfide were added to assure a liquid solution. The m i x t u r e in watch-glass-covered flasks was chilled in ice for 30 minutes. One hundred per cent excess of 0.2 N iodine chloride in glacial acetic acid was added with stirring. After a 2-hour reaction period at 0" C., 25 cc. of 15 per cent aqueous potassium iodide and 50 cc. of water were added. The excess halogen was determined with 0.1 N aqueous sodium t h io s u 1f a t e . Emulsions were broken with small amounts of ethyl alcohol. Tests indicated that ethyl alcohol had no effect on the determination. Blanks were run on the reagents in the usual manner. The iodine numbers on duplicate samples normally checked to within less than four units. It was found that the acetone remaining in the rubber sample after extraction was completely volatilized in the disintegration so that the acetone did not need to be removed previously. It was also determined that during the reaction period with iodine chloride no vapors of halogen were lost from the covered flasks. The nitrogen content of the various cures of compounds B and C was determined by a micro-Kjeldahl method on aretone-extracted samples. The amount of acetone-soluble material was determined on compounds D, E, and F, and from these values the amount of products from the benzoyl peroxide which combined with the rubber was calculated. In the following
,. effects are proportional, plotting the iodine number against combined nitrogen should give a straight line. These data are delineated in Figure 2 and produce a straight line within the limits of the experimental error.
Iodine Chloride Iodine chloride was used in following the unsaturation for several compounds. The composition of these compounds is as follows:
Smoked sheet Litharpe m-Dimtrobenzene sum-Trinitrobenzene Benzoyl peroxide
B
C
D
100 5
100
100
... 6 ...
...
6
...
...
5
FIGURES 5 AND 6. E
...
100
...
...
7
...
...
11.1
COMPOUND c, Sum-TRINITROBENZENE
F 100
... ...
I
IODINE
TUMBER I
'25
These vulcanizates had tensile strengths of, 1000 to 2000 pounds per square inch. In Kemp's procedure for determining the unsaturation with iodine chloride, tetrachloroethane was used as the solvent for disintegration of the samples. A number of blanks were run on this material, heating them exactly as Kemp prescribed. No matter how carefully the tetrachloroethane was purified, enough decomposition was found a t its boiling point t o introduce serious errors. Tests on c. P. p-dichlorobenzene indicated that this material is completely stable a t its boiling point. Tests lasting u p to 5 hours indicated that the time of boiling of this reagent with rubber has no effect on the unsaturation determination. Therefore, this
FIGURE 7. COMPOUNDS D, E, AND F, BENZOYL PEROXIDE
INDUSTRIAL AND ENGINEERISG CHEILIISTRY
AUGUST, 1937
table these values are listed as combined benzoyl peroxide. All analytical drlta are calculated on the basis of the raw rubber in the compound.
--
270' F. rulcanization .Win.
Compound .B -Combined N
%
Iodine NO.
---Compound C 2450 ,F. vuloaniza- Combined Iodine tion N NO. 'Win. %
It is possible that, contrary to the experiments with sulfur, a single molecule of the present vulcanizing agents adds to more than one double bond. The double bonds saturated per molecule of vulcanizing agent may be calculated by the following formula: loo (lo- It)''
=
68 Vo)W
whereI, and It =
M Benzoyl Peroxide Compounds Vulcanized 20 Min. a t 275' F. --Benzoyl Peroxide-Iodine Compound Total Combined No. 2.31 341 D 7.0 333 11.1 5.31 E 311 13.97 25.0 F
These data are plotted in Figures 3 to 7. The procedure used in the present work is somewhat different from that of Fisher and Gray. They swelled the vulcanizates in carbon disulfide for 24 hours a t room temperature and then allowed the iodine chloride to react for 24 hours a t 0" C. in the dark, or twelve times as long as the above experiment. The effect of this long reaction period was examined by allowing iodine chloride to act on vulcanizates of compound B for 24 hours a t 0°C. The iodine numbers showed that the long time of treatment changes the absolute values somewhat, but the slope of the line relating combined nitrogen and unsaturation was almost exactly the same as that in Figure 4. An unusually long time of acetone extraction had no effect on the iodine number or the per cent combined nitrogen. The possibility that the decrease in unsaturation might be attributed to a reaction of the double bonds of the rubber with one another or, in the case of compounds B and C, a reaction of the double bonds with litharge was investigated. The following table contains the results of these experiments: --CompoundRubber
PbO
%
%
100 100 95 95
0 0 5 5
2700 I?. Heat Treatment iWin. None 120 None 120
Iodine No. (Not acetone-Extd.)
362 350 350 351
An increase in unextractable nitrogen during the vulcanization with nitro compounds might be explained as being due to a reaction of the nitro compound and the nonrubber constituents, or a reaction between the nitro compound and the litharge to form a material insoluble in acetone. These possibilities cannot be rejected definitely. It was shown, however, that on heating a mixture of litharge and m-dinitrobenzene no acetone-insoluble nitrogenous material was formed.
Discussion The data show an approximately linear relation between the per cent combined ,vulcanizing agent and the unsaturation of the rubber in the compound. These results obtained through the use of two independent analytical methods are at variance with those of Fisher and Gray. The present experiments seem to indicate that during the vulcanization of rubber with polynitro compounds or benzoyl peroxide, the reagents or products from them add chemically to the double bonds of the rubber hydrocarbon. There is no evidence in these experiments nor is there any in the case of sulfur vulcanization to indicate vhether the vulcanizing agents add to the double bonds in a single molecule or bridge between double bonds in adjacent molecules, thereby increasing molecular size.
869
=
W =
double bonds saturated per molecule of nitro compound or benzoyl peroxide, or atom of sulfur or selenium iodine numbers at vulcanization times of 0 and t, respectively molecular weight of nitro compound or benzoyl peroxide, or atomic weight of sulfur or selenium weight of vulcanizing agent combined with 100 grams of rubber
The number of double bonds saturated per molecule of vulca,nizing agent was calculated as follows : Vulcanizing Agent m-Dinitrobenzene m-Dinitrobenzene sun-Trinitrobenzene Benzoyl peroxide
Unsaturation Reagent
1Br IC1 IC1 IC1
Double Bonds Satd. per Mol. of Reagent 5.4 7.5 3.2 2.6
Preliminary data indicate that an atom of selenium saturates about 6 double bonds. A molecule of m-dinitrobenzene appears to saturate a fsirly large number of double bonds. It was perhaps to be expected that a molecule of trinitrobenzene would saturate more double bonds than the dinitro compound, but this does not seem to be true. A molecule of benzoyl peroxide also saturates more than one double bond. It is possible that these results may be explained by the theory that vulcanizing agents produce some sort of cyclization. The quantitative implications of the data need more study. The qualitative implications of these data are important, indicating that we have yet to find a vulcanization in which a chemical reaction does not take place and thereby give more support to the chemical theory of vulcanization.
Literature Cited Blake, J. T., IXD.ENG.CHEM.,22, 740 (1930). Fisher, H. L . , and Gray, A. E., Ibid., 20, 294 (1928). Gorgas, A., Kautschuk, 4, 253 (19'28). Kemp, A. R . , IND.ENQ.CHEM.,19, 531 (1927); Kemp, A. R., Bishop, W. S., and Lackner, T. J., Ibid., 20, 427 (1928). ( 5 ) Kemp, A. R., and Mueller, G. S.,I N D .ENQ.CHEM.,Anal. Ed.,
(1) (2) (3) (4)
6, 52 (1934).
(6) Kirchhof, F., Gummi-Ztg., 27, 9 (1913). (7) Lewis, W. K., and McAdams, W. H., J. IND.ENQ.CHEM.,12, 673 (1920). (8) Rossem, A. van, Dekker, P., and Prawirodipoero, R. S., Kautschuk, 7, 202, 220 (1931). (9) Schmitz, W., Cummi-Ztg., 27, 1342 (1913). (10) Spence, D., and Scott, J. H., Kolloid-Z., 8, 304 (1911). (11) Vanbel, W., Cummi-Ztg., 26, 1879 (1912).
RECEIVED April 21, 1937. This paper is a composite of two articles presented before the Division of Rubber Chemistry a t the 92nd and 93rd Meetings of the American Chemical Society, respectively, a t Pittsburgh, Pa., September 7 t o 11, 1936, and Chapel Hill, N. C., April 12 t o 15, 1937 (paper No. 12 on the program of the Division of Rubber Chemistry a t Pittsburgh, paper No. 9 a t Chapel Hill).
