Vol. 37, No. 1
INDUSTRIAL AND ENGINEERING CHEMISTRY
70
pleasure to give special thanks to 0. D. Cole of Firestone and to C. F. Prutton of Case School of App l i e d S c i e n c e for their many valuable suggestions. LITERATURE CITED (1) Bierer, J. M.,and
Davis, C. C., T r a n s . Znst. Rubber Id., 3, 151 (1927);
Rubber Chem. Tech., 1, 148 (1928). (2) B ' r e c k l e y , J . ,
Rubber Age (N.,
Y.),53, 331 I
3.2l 20
298'E Effect of Cure
Minutes Cure At
Figure 2.
Figure 3.
Although a n improvement of 60% falls far short of that required to make a GR-S stock comparable to natural rubber, these results do show that, even in the relatively small time required for the De Mattia flexing test, oxygen reacts with GR-S a t a sufficiently rapid rate to affect the results markedly. With a less drastic test-for example, in actual service-more time would be available for oxidation and the effect upon flex resistance would be correspondingly greater. The reaction is probably localized a t the points of mmimum stress where high temperatures are produced by flexing. This behavior is consistent with the work previously reported ( I $ ) , where it was demonstrated that oxygen played a n important part in the hardening of GR-S stocks at elevated temperatures. ACKNOWLEDGMENT
The authors are pleased to express thanks to The Firestone Tire and Rubber Company for sponsoring this work and for permission to publish certain portions of the data. Hugh Winn is Firestone Fellow a t Case School of Applied Science. It is also a
I
40 60 Temperature, 'C.
1
80
Effect of Testing Temperature
(1943); Rubber
C h e m . Tech., 16,901 (1943). (3) Cadwell, S. M., Merrill, R. A.,
Sloman, C. M., and Y0st.F. L., IND. ENCI.CHEF., ANAL.ED., 12, 19 (1940); Rubber Chem. Tech., 13, 304 (1940). (4) Carlton, C. A,, and Reinbold, E. B., India Rubber World, 108, 141 (1943); Rubber Chem. Tech., 16,897 (1943). (5) Caasie, A. B.D., Jones, M., and Naunton, W. J. S., Trans. Inst. Rubber Znd., 12, 49 (1936); Rubber Chem. Tech., 10, 29 (1937).
(6) Davis, C. C., and Blake, J. T., "Chemistry and Technology of
Rubber", A.C.S.Monograph 74, p. 199,New York, Reinhold Pub. Corp., 1937. (7) GehmaA, S. D.,Jones, P. J., and Woodford, D . E., IND.ENQ. CHEM.,35,964(1943). (8) Juve, A . E.,and Garvey, B. S.,Jr., Ibid., 36,212 (1944). (9) Neal, A. M.,and Northam, A. J . , Zbid., 23, 1449 (1931); Rubber Chem. Tech., 5,90 (1932). (IO) Prettyman, I. B., IND.ENO.CHEM.,36,29 (1944). (11) Rainier, E. T., and Gerke, R. H., IND.ENQ.CHEM.,ANAL.ED., 7,368 (1935);Rubber Chem. Tach., 9,178 (1936). (12) Shelton, J. R.,and Winn, H., IND.ENO.CHEM.,36,728 (1944). (13) Vila, G. R., Ibid., 34, 1269 (1942); Rubber Chem. Tech.. 16, 184 (1943). (14) Winkler, L.W., Ber., 21,2843(1888).
PBE~ENTED M a part of a Sympoeium on Synthetic Rubber before the Engineering Beotion of the American Association for the Advancement of Science a t Cleveland, Ohio.
LIGNIN ETHERS AND ESTERS Preparation from Lead and Other Metallic Derivatives of Lignin
N
0 GENERAL attempt has been made to prepare ethers
F. E. BRAUNS AND H. F. LEWIS The Institute of Paper Chemistry, Appleton, Wis.
lignin by diasoethane and diethyl sulfate and the preparation of mixed methyl-ethyl ethers. The of lignin as such, except for E. B. BROOKBANK usefulness of these reigents as those needed in the study of the The Mead Corporation, Chillicothe, Ohio g e n e r a l a l k y l a t i n g agents is structure of lignin. Methyl ethers havd been used as resins or limited by the fact that diasoalkyl fillers in redwood molding compositions and appear to be superior derivatives higher than diazoethane and dialkyl sulfates above the butyl derivatives are hard ,to prepare. Other means must to lignin alone; likewise, the reactions of methylation have been studied to determine the nature and number of hydroxyl groups therefore be utilized in making long-chain lignin ethers. The alcohol lignins may be considered RS lignin ethers. They in the lignin structure. Diazomethane has been used to methylate acidic hydroxyl groups; dimethyl sulfate and sodium hydroxdiffer, however, from the methyl and ethyl ethers mentioned beide methylate all other hydroxyl groups except the carboxyl fore in that two alkyl groups have entered the lignin building unit in an acetal-type reaction, and they are split off again when hydroxyls. Recently Jones and Brauns (8) described the ethylation of the alcohol lignin is treated with strong mineral acid. Tho fol-
J.nuuy, 1Q4S
INDUSTRIAL A N D ENGINEERING CHEMISTRY
lowing alcohol lignins are reported in the literature: methanol (a), ethanol (7),butanol (4), isobutanol (6), and isoamyl (6).
.
71
Butanol lignin was described by Bailey (I), but this is not a true butanol lignin (4). The so-called primary lignin described by Friedrich (6)is an ethanol lignin. I n view of the possible industrial application of lignin ethers, studies have been made to develop general methods for their preparation. A partial answer was found by Jones and Brauns (8),based upon the reaction between the thallium salt of lignin and an alkyl halide. The thallium compound of lignin was prepared by reacting an aqueous solution of thallium acetate with the sodium salt of alkali spruce lignin. The thallium alkali
cake after coagulation and cooling, before the acid treatment. The preparation and properties of Meadol esters have already been discussed (9). Inasmuch as the majority of the experiments in the present report were carried out with leltd Meadol, the preparation of this compound will be described in some detail, Meadol contains some water-insoluble material, which is removed by dissolving 300 grams of the product as received in 2500 cc. of distilled water, stirring the solution, and centrifuging. The supernatant dark reddish-brown solution is filtered, the residue washed with water, and the solution made up to 3 litera. One and a half liters of this solution containing 111 grams of oven-dried sodium Meadol were
An economical process for the preparation of lignin ethers has been developed. This is based upon the reaction between the lead salt of the water-soluble alkali hardwood lignin (Meadol) and an alkyl halide. The lead salt is prepared by treating the aqueous lignin solution with either neutral or basic lead acetate. The salt thus formed appeare to be a mixture of a lead lignin containing one lead atom for two hydroxyl groups per building unit and one containing three lead atoms for each two lignin-building units. Using the reaction between this lead salt and the appropriatehalide, the following ethers of alkali hardwood lignin have been prepared: methyl, propyl, isopropyl, butyl, amyl, heptyl, decyl, lauryl, stearyl, and benzyl. In general, the iodide reacts more readily than the bro-
mide, and the longer-chain halides require a longer time for condensation than the shorter-chain halides. Stearyl bromide reacts at temperatures of approximately 210' C. in 5 to 6 hours; stearyl iodide, on the other hand, reacts readily at 180' C. A similar technique has been applied to the preparation of lignin esters by reacting the lead salt with the acid chloride. The following esters of alkali hardwood lignin have been prepared: propionyl, butyryl, caproyl, pelargonyl, stearoyl, and benzoyl. In this case the reaotion appears to be complete, and an ash-free ester results. Other metal derivativesof alkali hardwood lignin have been prepared and used for the preparation of esters. They include divalent mercury, divalent tin, and divalent copper.
spruce lignin is soluble in water but is precipitated by the addition of alcohol. The same end result is obtained by reacting alcoholic solutions of the two reagents, but in this case the thallium alkali lignin separates immediately. Such a thallium compound contains about 32% thallium and about 11% methoxyl when prepared from spruce alkali lignin. The thallium compound reacts with an alkyl halide, such as heptyl iodide or benzyl bromide, at 140-160' C., whereupon the thallium halide is precipitated and the lignin ether is dissolved in the solvent used as the reaction medium (dioxane or dimethyldioxane). One fundamental difficulty found by Jones and Brauns was the use of the methoxyl content of the ethers as the criterion for establishing the theory of etherification. The alkyl radicals are split off in the Zeisel methoxyl determination and appear, a t least in part, as an apparent methoxyl. This, however, was not the case with the benzyl ether. Consequently more satisfactory means for characterizing lignin ethers must be developed. The high price of thallium likewise limits its use to the research laboratory. The work described in this report deals with the preparation of other metallic lignin derivatives which will condense with alkyl halides to produce ethers. Among the metals investigated are: lead, divalent mercury, divalent tin, divalent copper, and monovalent nickel. The lignin used was that type of alkali hardwood lignin known as water-soluble Meadol (WS Meadol). The term Meadol will be used in the following discussion in place of the term "water-soluble alkali hardwood lignin". The work has been extended to include the preparation of the lignin esters through the reaction of lead Meadol with an acyl halide. An earlier article (IO) described the preparation and properties of lignin esters produced through the reaction of the acyl halides on alkali hardwood lignin in pyridine solution.
diluted to 2500 cc. and warmed to 50' C. The solution was stirred vigorously, and 35 grams of neutral lead acetate in 175 cc. of water were added; a voluminous dark brown precipitate separated out. The suspension was stirred for 30 minutes and the precipitate allowed to settle. The supernatant solution was siphoned; the precipitate was resuspended in fresh water, stirred, and again allowed to settle, The washing was repeated several times until the supernatant solution was practically free of lead ions. The main part of the supernatant solution was removed by siphoning, and the precipitate was filtered on a Biichner funnel and sucked aa dry as possible. The pasty lead derivative of lignin was then spread on clay plates and dried in an oven a t 50" C. to a black crumbly product easily ground in a mortar and screened on a 100-mesh screen. * Asimilar product with a slightly higher lead content may be made with basic lead acetate. The Meadol used in the preparation of the various ethers normally contains about 22% methoxyl on the ash-free basis. The lead Meadol prepared from neutral lead acetate contains 23.1% lead and 17.1% methoxyl. The lead content of lead Meadol containing one lead atom for two hydroxyl groups of the lignin building unit is calculated to be 19.6%. (The significance of the term "lignin building unit" was discussed by Brauns in an earlier article, 8.)
PREPARATION OF LEAD MEADOL
The h t problem had to do with the preparation of metallic derivatives of the lignin. The Meadol process was described by Plungian (10);water-soluble Meadol represents the first filter
PREPARATION OF ETHERS
The following illustration describes the method used in preparing the lignin ethers. I n preliminary experiments it was determined that the alkyl bromides or iodides could be used to react with the lead salts. I n general, the iodides act a t lower temperatures and in shorter periods of time. For the prepraration of stearyl lignin, a mixture of 21 grams of lead Meadol, 15 grams of stearyl iodide, and 35 cc. of dioxane was heated in a bomb tube a t approximately 180' C. for about 5 hours. A t the end of this time most of the lignin was in solution and lead iodide had settled out. The contents of the tube were filtered, and the crystalline lead iodide was washed with ether and dried;
INDUSTRIAL AND ENGINEERING CHEMISTRY
12
TABLE^ I.
PREPARATION AND
Alkyl Meadol Ether Methyl Propyl
Alkyl Halide Methyl iodide Propyl iodide
Isoeropyl
Iaopropyl iodide
Butyl Amyl
Butyl iodide
Heptyl
Amyl iodide Heptyl iodide
Femp., O
c.
160
160 160
:;1
160
160
(ftf ,---
METHOXYL CONTENT O F ALKALIHARDWOOD ETHERS Time, Hr. 2 2
2 2+
4.6 2 2 6
;\
Me0 ConPrecipitant" Ether Ether (1) Petroleum ether (60-90O) (2) Ether (1) Petroleum ether (60-90") (2) Ether (1) Petroleum ether (60-90") (2) Ether (1) Petroleum ether (30-60') (2) Ether (1) Petroleum ether (30-60") (2) Ether
Content Me0 on
% 22.1 16.3 22.7 19.6 22.0 16.8 20.9 14.8 20.8 13.6 16.2 8.6
18:6
.. ,. .. .. .. ..
1417
....
Vol. 37, No. 1 soluble in dilute alkali and very soluble i n ether, and the long-chain alkyl ethers are even soluble i n p e t r o l e u m ether. The conditions of preparation and the methoxyl contents are given in Table I. OTHER METALLIC LIGNIN DERIVATIVES
Using techniques similar to that for- the (60-90") (2) preparation of lead ligLauryl bromide Lauryl 6.6 180 Lauryl bromide 10 Lauryl ii:5 180 nin, four other metal 60-90') (2) Stearyl bromide Stearyl 6 30-60') 9:2 180 salts of hardwood lignin Stearyl Stearyl bromide 200 12 .. (30-60') (1) 8.0 were investigated. They 3.6 Allyl iodide Allyl 130-140 8 16.6 10:1 include mercuric lignin (30-60O) (2) 16.6 Benzyl bromide Bensyl 13:9 160 6 12.9 (17.6% m e t h o x y l ) , (60-90') (2) .. 14.0 stannous lignin (16.8% . ." a Designations (1) and (2) indicate t h a t the lignin ether was precipitated into ether (1) and the ether-soluble product methoxyl), copper lignin was then recipitated into petroleum ether (2); i n the cme of the stearyl ether, the firet precipitant was petroleum (18.7% methoxyl), and ether and t l e second was water. nickelous lignin (20.87' methoxyl). Based on analyses for methoxyl. a yield of 9.9 grams resulted (theoretical yield, 10.7 grams). The the mercury compound appeared to have one mercury atom per stearyl ether was recovered by evaporating the combined dioxanelignin building unit of 860, the stannous to have two tin atoms per ether solution. lignin unit, the copper to have two copper atoms per lignin unit, The purification of the stearyl Meadol was carried out as foland the nickel to have only one nickel atom per lignin unit. lows: The crude product was dissolved in ether, centrifuged, Further experiments were not carried out with the latter comfiltered, and'evaporated to a volume of about 100 cc. The ether pound because an insufficient amount of material was available. solution was then added dropwise to absolute ethanol which preThe copper lignin derivative was used to prepare the isopropyl cipitated the lignin ether. The ether was washed with alcohol lignin ether (19.5% methoxyl) by condensing 10 grams of the and distilled water, by decantation transferred to a Biichner salt with 5 grams of isopropyl iodide and 40 cc. of anhydrous difunnel, again washed with water, and dried in a desiccator over oxane in a bomb tube for approximately 7 hours between 150' and 160' C The mercuric derivative was condensed with amyl sodium hydroxide and concentrated sulfuric acid. The yield of iodide under about the same conditions to give the amyl ether ether was 12 grams. After drying over phosphorus pentoxide, the stearyl lignin (17.3% methoxyl). The stannous salt was condensed with contained 10.8% methoxyl and about 5.0% ash (as sulfate), benzyl bromide in dioxane for 4 hours a t 180' C. to give a benzyl which corresponded to 3.5% lead. The methoxyl calculated to ether with 10.3% methoxyl. an ash-free basis was 11.2'%. If this methoxyl content is used to Further work is being done on the nature of the metallic lignin calculate the number of stearyl groups entering the lignin molederivatives and also on the etherification reaction. The ethers cule (assuming a molecular weight of 860), three stearyl groups produced in this manner are impure even after some degree of are found for each lignin building unit. Inasmuch as only two purification; they contain some lead, which suggepts either an incomplete reaction of the lead salt with alkyl halide, resulting hydroxyl groups in the lignin are replaced by lead and only enough in the formation of partial ethers, or the inclusion of lead halide stearyl iodide was used to react with the lead, it is evident that a in the product. Those lignin ethers which were soluble in side reaction involving a splitting of methoxyl has occurred. If the same calculations are made on the assumption that there ether were precipitated by pouring into petroleum ether; those are only five methoxyls in the lignin, the results indicate the preswhich were soluble in petroleum ether were isolated by pouring ence of only two stearyl radicals in the lignin ether. into absolute alcohol. Other experiments show that at 160' C. stearyl iodide reacts little, if any, with lead lignin. At 200" C. the reaction is more LIGNIN ESTERS FROM LEAD LIGNIN DERIVATIVES On the other hand, the bromide requires rapid than at 180'. temperatures of approximately 210' C. to achieve the desired The lead lignin derivative was condensed with acyl halides result. This was shown in another series in which the reaction directly to form the lead halide and the lignin ester. The reacwas carried on at 200' C. for about 4 hours with no condensation tion is carried out by refluxing lead lignin in a dioxane suspension and then for an additional 7 hours a t 210' C., whereupon the conwith an acyl halide. As the reaction proceeds, the lead halide densation waa practically complete. begins to separate and the lignin ester goes into solution in the dioxane. The lignin ester can be precipitated from this solution Among the lignin ethers which have been prepared by this by pouring into ether if the acid group is below that of pelargonic procedure are the following: methyl, propyl, isopropyl, butyl, amyl, heptyl, decyl, lauryl, stearyl, and benzyl. The ethers are acid. To precipitate the stearoyl derivatiye, i t is necessary to pour the dioxane solution into water because the lignin ester is light to dark brown powders. The lower ethers are insoluble in soluble in both ether and petroleum ether. Six esters have been ether and petroleum ether and soluble in alcohol; but with increasing chain length of the alkyl group, they become more solupropionyl, butyryl, caproyl, made by this reaction-namely, ble in ether and less soluble in alcohol. They are more or less pelargonyl, stearoyl, and benzoyl. soluble in alkali, depending upon the degree of etherification. The methoxyl contents for the acyl lignins thus prepared may The solubility in organic solution also depends upon the degree be used for calculating the number of acyl groups entering the of etherification. The more highly etherified products are inlignin molecule. If no frectionation occurs during the isolation
Decyl Decyl
Decyl bromide Decyl bromide
180
16'
Ether (1) Petroleum ether Ether Ether (1) Petroleum ether Petroleum ether Petroleum ether Water (2) Ether (1) Petroleum ether Ether (1) Petroleum ether
I
9.4 13.4 7.6 10.6 13.0 8.1
10.6
..
-
January, 1945
INDUSTRIAL AND ENGINEERING CHEMISTRY
of the acyl ester, the results indicate that, for the lower fatty acids, three groups have with the unit Of lignin' With increasing chain I e w h Of the acyl radical, the number of acyl poups decreases so that, for the stearoyl eater, only the monosbarate is formed. Considerable work still remains to be done in chracteridng this general reaction for the prepamtion of lignin atens. LITERATURE CITED (1) BaiIey, A. J., Paper Trude J., 110, No. 1, 29; 111, No.6,27; No.7,27; No. 9,86 (1940). (2) Brauns, F. E., IW., 111, No. 14,83 (1940).
No. 2, 29 (1940);
73
(3) Brauns, F. E., and Hibbert, H., Can. J . Research, 13B, 78 (1935). (4) Charbonnier, H.Y.,Paper Trade J., 114, No. 11, 31 (1942). (5) Friedrich, A., and Diwdd, J., M o d s h . , 46, 31 (1925). (6) Hagglund, E., and Urban: H., Cdlulosechem., 8, 69 (1927) : 9, 49 (1928). (7) Hibbert, H., and oo-workers, J . Am. Chepn. Soc., 61, 888 (1939). (8) Jones,G . M., and Brauns, F. E., Paper Trade J . , 119, No. 11, 108 (1944). (9) Lewis. Brauns. F. E.. Buchanan. M. A.. and Brookbank. .., H -.F.. - .~ -E. B.,IND. E N ~CRaraa., . 35, 1113 (1943). (10) Plungian, M,, Ibid., 32, 1399 (1940).
.-,
PRBBBINTED before the Division of Cellulase Chemistry at the 107th Meeting of the AMBRIOAN C L ~ E M WSOOZETY AL in Cleveland, Ohio.
Paper Capacitors Containing Chlorinated Impregnants STABILIZATION BY ANTHRAQUINONE D. A. MCLEAN A N D L. EGERTON Bell Telephone Laboratories, Inc., Murray Hill, N. J .
Chemical and electrochemical reactions occurring in paper dielectrics containing chlorinated impregnante were diecussed in a previous paper. The present paper shows anthraquinone to be an effective stabilizer for capacitors cbntaining such dielectrics when aluminum electrodes are used and d.c. potentials are applied. One half per cent of anthraquinone prevents formation of the usual carbonized brown spots in the paper, and diminishes corrosion of electrodes and instability of leakage current. It increases the life under accelerated testing conditions by factors of four to one hundred fold, depending upon materials used and conditions of test. This development has added appreciably to the reliability of paper eapacitors containing chlorinated impregnants, particularly for military equipment where high temperatures and high voltages are often encountered simultaneously. Solubility of anthraquinone in the usual chlorinated impregnants is limited. Where greater solubility is desired, the more wluble chloro and methyl derivatives can be used.
C
HLORINATED aromatics such as chlorinated naphthalene and chlorinated diphenyl are extensively used as impregnants for paper capacitors. Their attractive features are nonfiammability, stability toward heat and oxygen, and high electrical resistance, dielectric strength, and dielectric constant. However, under high operating temperatures and direct-current voltages, capacitors impregnated with these compounds undergo a characteristic type of degradation which results in progressive loss of insulating properties and premature dielectric failure (4). The rate of degradation under fixed test conditions is B function of the chlorinated compound used, the type and quality of paper, the electrode material, and the drying and impregnating process. With a fixed test sample, the rate of degradation is greater, the higher the temperature and the voltage. This degradation has been a serious practical problem, beverely limiting the con-
ditions under which capacitors impregnated with chlorinated compounds could be used. The addition of quinones (3) to the chlorinated impregnant has been found to diminish the degradation markedly. Among the quinones, anthraquinone is particularly suitable owing to ita high effectiveness aa a stabiber, commercial availability in pure form, low vapor preasure, stability toward heat and oxygen, and lack of toxicity, Several years of commercial experience with anthraquinone-stabilized dielectrics in paper capacitors for d.c. use have given uniformly favorable resulta. The purpose of this paper is to present experimental evidence of the stabilizing action of anthraquinone.
c Figure 1. Structure of a Simple Paper Capacitor of the Laid-in Terminal Type
