Compounding Natural Rubber with Lignin and Humic Acid F. J. TIBENHAJI Dunlop Research Centre, Birmingham, England
N. S . GRACE Dunlop Tire & Rubber Goods Co. L t d . , Toronto, O n t . , Canada
A
CONDITIONS OF COPRECIPITATION
S POINTED out by DaHson ( S ) , the use of lignin in con-
junction with rubber \vas known a t least as long ago as 1929 ( I S ) . The considerable interest in the possible application of lignin to rubber technology which has been evident in both the United States and the United Kingdom in recent years dates, however, from the publication in 1947 of results obtained by Keilen and Pollak (11). These workers uscd a lignin derived from pine wood and obtained from the waste liquor of the sulfate wood pulp process. By coprecipitating a mixture of lignin and GR-S from a GR-S latex containing the appropriate amount of an alkaline solution of the lignin, a masterbatch was obtained, which, a t a loading of 50 parts of lignin per 100 parts of GR-S by weight, yielded a vulcanizate with tensile strength, hardness, and tear resistance similar in magnitude to those of easy processing channel (EPC) black compounds a t the same volume loading. In the case of a compound containing equal parts by weight of lignin and GR-S, both tensile strength and tear resistance were found to be considerably higher than those of the EPC black compounds of the same volume loading while hardness was about the same, Elastic modulus and resistance to abrasion-other criteria of reinforcement-obtained with lignin were appreciably lower at both loadings. Keilen and his coxorkers (9) later extended this investigation to the examination of the effect of sulfate lignin on the properties of other synthetic rubber and natural rubber vulcanizates and showed that by roprecipitating the lignin and the rubber the same type of reinforcement n-as obtained as in the case of GR-S. Thus, 50 parts by wright of lignin in natural rubber imparted a tensile strength somewhat higher than that given by EPC black a t the same volume loading. The resistance to tearing, however, was appreciably lower. At the 100-part loading, tear resistance x a s considerably superior to the black-reinforced compounds. At both loadings, as evidenced by elastic modulus and abrasion resistance, lignin v a s inferior to carbon black in its reinforcing properties. In the studies recorded in the present paper the behavior of lignin in natural rubber alone has been investigated a t a rather lower loading than those mentioned-via., 36 parts by >\eight relative to 100 parte of rubber, x-hich is the sitme loading by volume as that of carbon black in a compound containing 50 parts by weight. In all cases the mixture of rubber and lignin has been obtained by coprecipitation from the latex. Except in the case of the experiments dealing vith ion exchangc, two types of lignin have been employed, pine wood lignin obtained by the sulfate process and a straw lignin obtained as a byproduct of a straw pulp manufacture by the alkali process. A.1though it has been shown that lignins from different sources may differ in the properties they impart to rubber ( I ? ) , the differences observed between vulcanizates containing these two lignins are no greater than those observed between different batches incorporating the same lignin, provided the same method of preparing the lignin-rubber coprecipitate is used. 824
Results published by Keilen, Dougherty, and Coolc (10) indicate that as well as affecting the readiness with which a coprecipitate can be filtered, the conditions under which coprecipitaof stirring, temperature, et(!.-also intion takes place-rate fluence the properties of the final vulcanizate. \T-hile the rcsults reported by Keilen and his coworkers indicate only a relatively minor influence, a comparison of the physical propert,ies of vulcanizatcs obtained from the lignin-rubbcr mast,erbatches prepared according to t'lie two following methods demonstrates that this influence can be considerable. MIWHODI. Twenty parts of lignin and 78 parts of w t e r were ground together in a ball mill for 2 hours. Two parts of caustic soda were then added and the grinding was continued for 2 hours. The resulting solution (180 parts) was mixed with 167 parts of concentrated latex (dry rubber content, BO'%), arid the mixturc was allowed to pun into an cxcess of dilut'e formic acid (20%! with vigorous stirring, yielding a filterable crumb containing 36 parts of lignin for each 100 parts of rubber. The crumb was washed free from acid and dried at 40' C. METHODI1 (1%). A. mixture of lignin solution and latex was prepared in the manner described in Method I. With vigorous stirring of the mixture, formic acid, dilut'ed to 3%, was added until an excess of 30% over that required to neutralize the alkali was present. At this point the pH had fallen to 4.2 and the mixture had thickened t o a smooth paste. The pasl'e was then heated in a jacket'ed container by means of boiling wat,cr until the lignin-rubber mixture separated as a coarse crumb, which m ~ s then removed by filtration, washed free from acid, and dried. The resulting mixtures of lignin and rubber prepared according to the two different methods were then compounded according t o the formula given in Table I. Typical values of the physical properties of the vulcanizates a t maximum resilience are given in Table 11. Values for the corresponding properties of a pure gum compound and of compounds containing an equal volume loading of typical carbon blacks are given for comparison,
TABLE I.
COMPOSITIOX O F
LIGNIN-RUBBER TESTM I X I N G Parts by Wt.
Natural rubber Lignin Zinc oxide Stearic acid Sulfur hlercaptobeneothiazole Tetramethylthiuramdisulfide
100 36 10
4 3 1 0.1
At the loading of lignin employed, the lignin-rubber stock prepared according to Method 1yields a vulcanized compound very similar in the properties listed to a pure gum compound, except for a rather lower resilience. The lignin behaves, in fact, not as a reinforcing filler, but as an extender. However, vulcanhates based on the lignin-rubber coprecipitate obtained according to Method I1 are appreciably higher in modulus and hardness and, more notably, in tear resistance.
April 1954
825
I N D U S T R I A L AND E N G I N E E R I N G C H E M I S T R Y
TABLE11. PHYSICAL PROPERTIES OF LIGXIN-RUBBER COMPOUNDS AND TYPICAL VULCANIXATES Lignin ConlPds. Method Method I I1
Tensile modulus (300%), Ib./sq. inch 350 Tensile strength (rings), ib./sq. inch 2650 Elongation at break, % 680 Tear strength (crescent) Ib./inch 2i o Indentition b r d ness B.S.”, 43 Pendulum resili85 ence (50’ C.), o/o a British Standard.
Pure Gum Compd.
Black-Reinforced Compds. Containing 50 Parts by Wt./ 100 Parts Rubber -Type of BlackMPC HAF SRF
740
370
1300
1850
1300
2650
2600
3500
3250
2550
600
640
550
480
480
400-750
260
900
850
450
50
46
63
65
61
83
94
67
73
85
The best figures obtained for tear strength do not fall far short of those obtained with the fully reinforcing blacks, but unfortunately the testing of a large number of batches has shown a greater variation in resistance to tearing than in any other property-the average value lies between 500 and 600 pounds per inch, An explanation of the different properties obtained by the two methods of coprecipitation may lie in the manner or the strength of the association between the lignin and the rubber. Lignin is an active filler, as shown bv the fact that an uncured ligninrubber mix prepared by coprecipitation, even after prolonged milling, is insoluble in benzene. It has been shown that in the cold the pH of a dilute solution of lignin in alkali can be reduced to below 4 without separation of lignin taking place, and although the pH of precipitation has been found to rise with increasing concentration, it is thought that a tendency for the latex to flocculate before the lignin may be greater when preparing a coprecipitate by Method I than by Method 11. When following the latter procedure it would appear possible that a t a pH of 4 the rubber particles have lost the stabilizing effect of their natural stabilizers, but the system is stabilized by the lignin. On heating, however, the lignin itself flocculates with simultaneous precipitation of the rubber. ION EXCHANGE O F SULFITE LIGNIN
The lignin recovered from the waste liquor of the sulfite or acid process exists in a sulfonated form and, as obtained, is soluble in aqueous acid. By controlled hydrolysis, however, partially sulfonated lignins may be obtained which can be precipitated by acid (6). By virtue of their phenolic character (albeit weak) and the presence of the sulfonic groups, these lignosulfonic acids bear some analogy to the sulfonated phenolic ion exchanging resins and may therefore be expected to show some ability to undergo ion exchange. It is known that colloidal clays, exemplified by the bentonites, are capable of taking part in ion exchange reactions whereby the inorganic clay cations may be replaced by organic cations derived from substituted ammonium or other “onium” salts. Moreover, when the organic cation contains a sufficiently long carbon chain-10 atoms or more-the clay acquires marked organophilic properties. I t no longer swells when placed in water but is capable of forming gels with organic liquids such as nitrobenzene
A solution of 36 parts of the sodium salt of a partially desulfonated lignosulfonic acid in 100 parts of water was mixed into 167 parts of natural latex (dry rubber content, 60%). A solution of cetyl trimethyl ammonium bromide in water was then added, followed by sufficient acetic acid to cause coagulation. The mixture was heated to 70” C. with the formation of a coarse crumb which was readily removed by filtration. After watersoluble matter u-as removed by washing, the precipitate was dried. Vulcanizing ingredients wcie incorporated on the mill and the material was cured. It was found that the treated lignin gave a vulcanizate with a tensile modulus three times greater and a tear strength twice as great as the untreated lignin. Unfortunately, the level of these properties in vulcanizates containing unmodified lignosulfonic acid is relatively low, so that those of the product incorporating the cation-exchanged material are no better than can be obtained from alkali lignin by straightforward coprecipitation. Improved results might be obtained by the use of a lignosulfonic acid of a lower degree of sulfonation. It might then be possible to make a more complete replacement of the hydrophilic inorganic cations in the surface of the lignin particles without introducing an unduly large number of long carbon chains, which probably tend to act as softeners in the vulcanizate. INFLUENCE OF IMETHYLENE DONORS
As ordinarily available, lignin contains relatively few phenolic hydroxyl groups. In consequence, it would not be expected to show much reactivity in reactions of the phenol-aldehyde resinforming type. Lignin does exhibit a measure of such reactivity, however, as indicated by its ability to extend phenol-formaldehyde molding powders, especially u-lien used to replace part of the phenol in the preparation of the thermosetting resin (16). Phenolic rrsins of suitable types can be used to vulcanize rubber, while others can be used as reinforcing agents. I t is of interest, thereforc, t o examinr the effect on a lignin-rubber vulcanizatr of pretreating the lignin with, for example, formaldehvde. One method which has been used with interesting results employs the following steps: A masterbatch of lignin and rubber was repared by coprecipitation according to Method I. The driesproduct was sheeted on a cool mill and 10% of hexamethylenetetramine (calculated on the amount of lignin) was incorporated. The temperature of the batch was then raised by admitting steam to the mill rolls a t a pressure of 60 pounds per square inch and the working of the batch was continued for 20 minutes. The material was then removed from the mill and allowed to cool. It was then replaced on the mill and the usual vulcanizing ingredients m r e incorporated. Vulcanizates obtained from the lignin-rubber treated in this way show marked differences from the untreated lignin-rubber compound, as shown by the test results recorded in Table 111. The change in the properties of the cured rubber as a result of the hot-milling with hexamethylenetetramine is evident. A volume loading of lignin equal to that of black in a compound containing 50 parts by weight per 100 of rubber results in a modulus. tenyile strength, and hardness of the same order as those obtained with typical carbon blacks, but a resilience equal to that of a pure gum compound. The mechanism by which the effect is achieved is by no means clear. In their work on the effect of metallic oxides in promoting
TABLE 111. EFFECTOF HOFMILLINGLIGKIN-RUBBER MASTERBATCH WITH HEXAMETIIYLENETETRAMINE
(6, ?I.
It is not unreasonable to suppose, therefore, that if the sodium ions associated with the lignosulfonic anions in a sodium lignosulfonate can be replaced by organic cations similar to those mentioned, the affinity between the lignin particles and the rubber chains might be enhanced. There are several ways in which a procedure such as this might be carried out; the following describes one method:
Tensile modulus 300Y0) Ib./sq. inch Tensile strength (rings). ‘lb./sq. inch Elongation a t break % Tear strength (oresoelst) Ib./iny!h Indentation hardness (B:S.a) Pendulum resilience (50° C.): % British Standard.
Untreated Masterbatch (Method I) 450 2650 680 270
43 85
Masterbatch Hot-Milled with Hexamethylenetetramine 1400 3150 530 3iO 59
93
826
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 46, No. 4
the vulcanization of I ubbcr compounds containing oxidized lignin, Griffith and ;\lacGregor (4) showed that the positive effect obtained using the oxides of lead, copper, and bismuth could not be explained on the basis of their neutralizing residual acidity from the coprecipitation process, since oxides such a8 lime and magnesia were ineffective. It is unlikely, therefore, that the effect of hexamethylenetetramine can be explained in this \yay. The conclusion reached by Griffith and MacGregor (4) is that the effectiveness of the oxides is associated mith the insolubility of the sulfides formed by ieaction with the hydrogen sulfide which results from the interaction of sulfur and lignin during vulcanization and which, in the absence of one or other of these oxides, exerts a retarding influence on cure. S o such mechanism can be postulated to explain the effect obtained by hot-milling the lignin-rubber coprecipitate with hexamethylenetetramine. It also appears unlikely that piimary bond formation occurs bettveen the lignin and rubbei. According to both the chroman theory of Hultasch (8) and the methylene o-quinone theory of van der Meer (14), the formation of a chemical linkage between B phenol-formaldehydc resin and rubber requires the presence of a primary alcohol group in a position ortho to the phenolic hydroxyl. Even if formaldehyde reacts with the lignin to form such a primary alcohol gioup, van der Meer has shown that, in the case of a phenolic resin, linkage to the rubber does not occur in the presence of hexamethylenetetramine (14). The formation of primary linkages between resin and rubber is not, however, a necessary condition of reinforcement, as follows from a recent study of Piccini (15), who concludes that in the reinforcement of rubber with resorcinol-formaldehyde resin ( 2 ) the resin is not combined, but is dispersed in a finely divided state; i t would appear probable that the heuamethylenetetraminr treated lignin is also in this condition. Th'it some enhancement of its activity has been achieved, er, appears to be confirmed by the fact that the stiffness of the uncured lignin-rubber stock is increased by the hobmilling with hrxamethylenetetramine~ I n one set of observations the bIooncy viscosity of the treated material was found to be 65, while the lignin-rubber blend submitted to the same procedure -xithout the addition of the hexamethylenetetramine had a Mooney viscosity of only 40. The Mooney viscosity obtained after hot-milling rubber with hexamethylenetetramine in the abspnce of lignin shows no significant difference from that of the rubber hot-milled without cither hexamethylmetetramine or lignin. The possibility of residual hexamethylenetetramine exerting its normal function as an accelerator during vulcanimtion cannot be discounted.
100% of the weight of black), caustic soda (10% of the weight of lignin), and water to yield a finished dispemion containing 20% of black. The slurries were then ball milled to break down aggregates and finally passed through the colloid mill. The resulting dispersions were then mixed with latex (dry rubber content, 60%) to give equal parts of rubber and black with varying proportions of lignin. On acidification with formic acid, the rubber, black, and lignin were t h r o m down as a clean crumb. After the flocculate had been washed and dried, the batches were run up o n the mill, 10% of hexamethylenetetramine (calculated on the lignin) lqas incorporated, and the batch w a ~ mixed for 20 minutc,s v i t h the mill rolls a t a temperature of 153" C. T h r stocks n ere then allowed to cool and were compounded with further rubbcr and the necessary sulfur, etc., to give the required range of black and lignin loadings. The physical test results of vulcanizates prepared from compounds made in this x-ay show a best combination of properties with a black loading of about 35% (by might) and a lignin loading of about 10%. The figures obtained on such a compound are indicated in Table IV.
LIGNIY IN COY.JUh1ICTION WITH BL4CK
The term humic acid is applied to a wide range of related materials, both artificial and obtainable from natural sources. The artificial humic acids can be prepared by suitable treatment of polyhydric phenols and carbohydrates, but from the point of view of rubber reinforcement, only those obtainable from natural sourccs are of interest. The naturally occurring humic rniLterials are formed from veget,able matter during the coursc of its conversion to coal. The rate of conversion of the plant remains and the rank of the final product are dependent on the prevailing biological and geological conditions. Sugars go rcitdily into solution and proteins are completely decomposed, but the conversion of the cell wall material is slow. I t is principally this cell wall material, consisting largely of cellulose and lignin, that enters into t,he formation of the humic substanecs, yielding humus and peat, and, in the course of geological time, brown coal, bituminous coal, :tnd anthracite, The view was formerly held that the cellulose was the chief raw material from which the humic material and therefore coal was formed. It is now considered, however, that i t iF the lignin from which these are derived, so that humic subst,ances may be regarded as intermediate products in the conversion of
I n spite of the high degrcc of dispersion of the lignin which would be expected in a lignin-rubber masterbatch prepared by eoprecipitation and nhich mi( I oscopic examination < h o w to exist, and notwithstanding the rcirifolcement indicated by the level of other properties, the rcP1stance to abrasion shown b y ligninrubber vulcanizates with 01 nithout the hexamethylenetetramine treatment, remains much inferior to that obtainable by the use of the finer carbon blacks. This and the rather unusual combination of other properties shown by the lignin-rubbcr compounds after the hexamethylenetetramine treatment lead to consideration of the properties of vulcanizates containing both black and lignin. I n view of the knoxm value of alkaline solutions of lignin as a dispersing agent for black, an obvious choice of method for p r e paring rubber masterbatc*hes containing black and lignin is to disperse the black in the lignin solution, mix the resulting dispersion with the latex, and then precipitate the whole. This was the basis of the method employed in preparing a series of compounds ~ i t :hL range of black and lignin loadings. Slurries were prepared rontaining black (H..iF), lignin (from 5 to
TABLE IT'.
PHYSIC.4L PROPERTIES O F COMPOUNDS HAF BLACK AND LIGKIN
Tensile modulus (300%), lb./sq. inch Tensile strength (rings), lb./sq. inch Elongation a t break, % Tear strength (crescent), lb./;nch Indentation hardness (B.S.), Pendulum resilience ( E O o C . ) , %
HAF Black 35 Parts/Lignin 10 P a m 1600 3700 520
740 53 82
CONTAIXING H A P Black, 50 Parts 1860 3250 480 850 65
73
Compared with a vulcanizatc reinforced with 50 parts of HAF black, the compound containing lignin and a reduced proportion of black has a somewhat improved tensile strength, and a slightly lower but good tear resistance. It is softer and more resilient. I t s abrasion resistance is of the order of that given by an H M F black of the smooth-out type. A compound with these properties has potentialities in applications whcre a rubber product is subjected to continuously varying stresses. I n certain rubber products the necessary redience and low heat build-up are usually achieved at the expense of tensile strength, tear resistance, and abrasion resistance. The use of a compound such as that described provides the opportunit>yto redress the balance of properties. HUMIC ACIDS
April 1954
INDUSTRIAL AND ENGINEERING CHEMISTRY
lignin to hard coal and anthracite. In the course of this conversion and under the influence of oxidation, hydrolysis, and microbiological action, the methovy groups are partially removed and carboxylic acid groups are formed. Carbonyl and phenolic hydroxyl groups arr also present. Peat and brown coal consist largely of humic substances and attempts have been made to use peat as a rubber-reinforcing agent (3). Humic acids may also be obtained from bituminous coals but this entails preliminary oxidation. A particularly convenient source of humic acids is the naturally weathered coal which occurs at outcropping coal seams. By evtraction of this material with caustic>soda solution, relatively high yields of humic acids may bc obt:iined. The following procedure was employed to obtain the material used in the M-ork desvrihed in this paper: Forty parts of a naturally oxidized, weathered, outcrop coal were mixed with 12 parts of sodium hydroxide dissolved in 400 parts of water and the mixture was placed in a metal container equipped with an efficient stirrer. The mixture was heated with continuous stirring a t a ternpcrature slightly below the boiling point for 6 hours. Water was added from time to time to make up the loss caused by evaporation. The solution wa4 allowed to stand undisturbed for approximately 18 hours, and undissolved material was separated by centrifuging. This undissolved residue was extracted with 400 parts of water instead of soda solution, and the solution was separated by centrifuging. The extraction with alkali and with water and the se aration were repeated with the solid material remaining after txe last centrifuging, and all aqueous extracts were combined. After the combined sodium humate solutions were acidified with sulfuric acid, the preci itated humic acid gel was separated by centrifuging and driezfor approximately 24 hours. As the dried product did not redisperse in water, it was washed with water until free from inorganic salts and mineral acid. The yield was 85% of the outcrop coal taken. The final product was a dark brown owder of specific gravity 1.68. Although this product is a compKx mixture of related compounds, for the purpose of this present paper it i s convenient to describe it simply as humic acid. The humic acid so obtained was found by Bailey (1) to behave in rubber in a manner very similar to alkali lignin. The results obtained on incorporation by conventional dry milling are of little interest. As in the case of lignin, coprecipitation of the humic acid and rubber from a mixture of a solution in alkali and latex yields a much superior vulcanizate. The method of preparing the coprecipitate was essentially the same as that of Method I employed in the case of lignin. Table VI gives the test results obtained on a vulcanizate containing humic acid a t the same volume loading that was used in the experiments with ligninLe., the same as the volume loading of a black reinforced compound containing 50 parts by weight. The composition of the mix is given in Table V. As in the case of lignin a t the same volume loading, the figures obtained do not indicate reinforcement so much as the ability to extend the rubber without loss of properties. Tensile strength, elongation, tear strength, and hardness are substantially the same as those of a gum stock and those of the corresponding lignin compound. Tensile modulus is somewhat higher and resilience is somewhat lower. Like lignin, humic acid also responds to hot-milling with hexamethylenetetramine. The treatment is the same as that for lignin-i.e., incorporating 10% of hexamethylenetetramine (calculated on the humic acid) into the coprecipitate on a cool mill, raising the temperature to 153' C., continuing the working for 20 minutes, and then removing the batch to cool before incorporating the necessary compounding ingredients. Typical test figures obtained on the vulcanizate are shown in Table VI. The effect of the treatment is similar to that obtained by the same treatment of lignin coprecipitates. Modulus, tensile strength, tear resistance, hardness, and resilience are all increased. Except that i t is somewhat softer, the vulcanizate is substantially similar to the corresponding lignin compounds.
827
TABLE v. COWPOSITION OF HUMICACID COMPOUND Parts by Wt. 100
Natural rubber Humic acid Zino oxide Stearic acid Sulfur Benzothiazyl disulfide
47 5
3 2.5
0.85
OF HEMIC!d210 VULCANIZATES TABLE VI. PROPERTIES
Tensile modulus (300%). lb./sq. inch Tensile strength (rings), lb./sq. inch Elongation a t break. % Tear strength (crescent), l b . / i p h Indentation hardness (B.S.), Pendulum resilience (50' C.), %
Prepared from Untreated Masterbatch 670 2600
Masterbatch Hot-AIilled with Hexamethylenetetramine 1090
230
3100 570 320
75
51 90
690
45
Humic acil is darker in color than lignin and imparts a darker color to rubber compounds Containing it. Like lignin, it also showe markcd antioxidant properties. SUMMARY
Lignin and humic acid are two materials of considerable interest to the rubber compounder. They may be obtained from low cost natural sources in great abundance. Previous workers have shown that the incorporation of lignin into rubber by conventional dry mixing techniques does not yield vulcanizates with the same level of properties as can be obtained when the lignin-rubber stock is obtained by coprecipitation. Even when latex masterbatch methods of preparing the ligninrubber stock are employed, however, the properties of the final vulcanizate depend upon the procedure followed in preparing the masterbatch. One method yields a product in which, a t moderate loadings, the lignin behaves less as a reinforcing agent than as an extender, while a second technique of coprecipitation gives a vulcanizate which exhibits a marked degree of reinforcement. The application of the technique of hot-milling the ligninrubber stock with hexamethylenetetramine yields a vulcaniaate in which modulus, tensile strength, and hardness of the same order as those obtained by the use of typical carbon blacks a t the same volume loading are combined with the resilience of a pure gum compound. The use of lignin in conjunction with carbon black is also capable of yielding vulcantates with unusual combinations of properties of interest to the rubber compounder. ACKNOWLEDGMENT
The authors are indebted to a number of colleagues a t the Dunlop Research Centre who have contributed appreciably to the work described, and in particular to Roy Hickson and A. E. W. Bailey. The permission of the directors of the Dunlop Rubber Co. Ltd., t o publish the paper is also acknoxledged. LITERATURE CITED (1) Bailey, 9.E. W., private communication. (2) le Bras, J., and Piocini, J., IND.ENG.CHEM.,43, 381 (1951). 13) Dawson. T. R.. Trans. Inst. Rubber Ind.. 24. 227 (1949). Griffith, T. R.,' and MacGIegor, D. W., IND. ENG.CHEM.,4 5 , 255 (1953). (5) Harmon, C. (to Marathon Corp.), U. S. Patent 2,371.136 (March 13, 1945). (6) Hauser, E. -4., Colloid Chem., 7, 431-41 (1950). (7) Hauser, E. A., U. S.Patent 2,531,427 (Nov. 28, 1950). (8) Hultzsoh, J., Prakt. Chem., 158, 275 (1941).
(4
INDUSTRIAL AND ENGINEERING CHEMISTRY Keilen, J. J., Dougherty, K.I>k,, the bracket ( 7 ) becomes merely (3/P) - 2. To eliminate F2, one may writc ki
- L,
=
(kj1Ti
- k/272)
(2)
Data to test these relations are scarce. Schncider of the Kational Research Council of Canada recently furnished the writer with density and porosity data from the thesis of Weininger, which made possible testp of the above relations with their data (5’)on alumina and borosilicate glass beds with different gases. The gas thermal conductivities used by the writcr for the computations are froin a compilation by Ileyes (11: 30” C. k X IO’, cal./cm. Hydrogen Helium Xitrogen Carbon dioxide
see. ’C. 4.34 3.58 0.624 0.402
The results are shown in Table I. Xote that reasonably good agreement is found between experimental and theoretical results when kl is several times greater than kp. I n the calculations, k, for alumina was assumed high and the relation 7 = ( 3 / P ) - 2 was used. However, k, = 0.0026 was used for borodicate glass and ri were calculated for each gas. It is tempting to assume that F2 is given by the vacuum con-
TABLE I. EXPERIMENTAL VERSUS CALCULATED CONDCCTIVITIES’
THERMAL
ki-ka
G a 5 ~
Aluniina 16 mesh’ P = 0.432 Esptl. Calod.
Aliimina, 36 mesh P = 0.461 Erptl.
104 cal./cni. sec. 3.76 2.32 16.73 18.37 14.41 14.61 19.47 18.74 15.71 16.42 1.10 2.01
Cdcd.
Borosiliclttc glass, 30 mesh P = 0.689 Exptl. Calcd.
C.
3.43 0.74 2.55 16.75 16.87 5.90 He-&*> 14.32 13.33 5.16 HrC02 18.66 17.75 7.11 16.10 14.33 He-CO? 6.37 1.00 1.21 s2-coz 1.78 lo4 of Table I, Weininger and Schneider ( 9 ) . ‘ om. sec. IIr-He H2-S*
1.04
6.09 5.05 6.57 6.53 0.48
ductivity of the bed. However, in the writcr’s experience nith these data and those cited in previous papers, this is frequently disappointing, and further data probably will be necessary. Reference should be made to the papers cited for details and for statements as to the requirements of data for testing surh relationships. The writer thanks Dr. Schiieider for supplying the density a n d porosity data and for his interest in the problem. LITERATURE CITED (1) (2) (3)
Keyes, T’. G., personal communication, 1949. Strickler, H. S.,J . Chem. Phys., 20, 1333-4 (1952). Weininger, J. I,., and Schneidcr, ‘K. 0.. IKD.ESG. CHEM.,43, 1229-33 (1951).
H. S. S T H I C K l . l ~ R WILLIAVH. SINGERMEMORI.AL R E ~ E A R CL
