Film-Forming Properties of Cellulose Acetate Propionates EFFECT OF SOLVENTS, DILUENTS, AND PLASTICIZERS A. G. ROBERTS AND S. G. WEISSBERG National Bureau of Standards, Washington 25, D. C.
This investigation was undertaken to determine the suitability of cellulose acetate propionate as the filmforming base for airplane dopes. Preliminary tests had indicated that this material might be superior to cellulose acetate butyrate dope in adhesion and low temperature flexibility. This report describes relationships found between the chemical constitution of cellulose acetate propionate esters and such properties as solubility in organic solvents, dilution tolerance for aromatic and aliphatic diluents, compatibility with plasticizers, and the shrinkage, flexibility, and clarity of films formed from mixtures of cellulose acetate propionate with various solvent-diluentplasticizer combinations. Selected formulas appear promising as airplane dopes. Mechanisms of film shrinkage and plasticizer action are discussed. Comprehensive data on the solution and film properties of cellulose acetate propionate should not only save much valuable investigative time for airplane dope designers, but should make possible the selection of formulations for many applications directly from the data presented.
A
T T H E request of the Bureau of Aeronautics, Department of the Navy, the National Bureau of Standards conducted a
systematic investigation of the film-forming properties of cellulose acetate propionate and its suitability for use in airplane dopes. Preliminary experiments with this material indicated that i t might be superior to the currently used cellulose acetate butyrate with respect to adhesion to airplane fabric and low temperature flexibility. This report describes the relationships which were found to exist between the chemical constitution of cellulose acetate propionates of various acyl and hydroxyl contents and such properties as solubility in organic liquids, dilution tolerance for aromatic and aliphatic diluents, compatibility with plasticizers, and the shrinkage, flexibility, and clarity of films cast from mixtures of cellulose acetate propionates with various solvent-diluentrplasticieer combinations. Promising experimental dopes formulated on the basis of the data obtained are currently undergoing outdoor weathering tests. Nine cellulose acetate propionate esters having various acetyl, propionyl, and hydroxyl contents were furnished by the Tennessee Eastman Corp., together with the analytical data given in Table I. A practically unhydrolyeed cellulose tripropionate of low viscosity was furnished by the Celanese Corp. of America. The cellulose acetate propionates were purified by centrifuging the insoluble mattar out of 10% solutions of each cellulose ester in acetone and then reprecipitating the cellulose ester by pouring the solutions in a thin stream with continual agitation into a large volume of cool water. The precipitated cellulose esters were washed twice with distilled water. After the excess water was gently squeezed out, the residues were dried in a circulating-air oven for approximately 12 hours a t 65' C. in trays of large area '
to permit free access of air. The low viscosity cellulose tripropionate was completely soluble in acetone and did not require purification. Seventeen different solvents and diluents (Table 11),selected for their potential suitability in dope formulations with respect to solvent power, evaporation rate, toxicity, and availability, were tested singly and in combination. Eighteen commercially available plasticizers (Tables 111 and IV) exhibiting various degrees of compatibility with the cellulose a c e h t c propionates were evaluated. TEST METHODS
SOLUBILITY. Solubility waa judged visually in liquid mixtures of cellulose acetate propionate containing 8% by weight of cellulose ester. A measure of the amount of insoluble material present was given by the height of the insoluble layer after a proximately 5 hours on a mechanical shaker followed by 3 f a y s of settlin . Based on an original liquid mixture height of 40 mm., the folfowing arbitrary but convenient rating scale w a s I I R ~ . Rating
Height of Ineoluble Layer, Mm.
Soluble Partially soluble Large1 insoluble Insolu&e
4 t o 15
10 1.83 2.93 1.05 0:75 0 :7 2 1.68 1.27 0.90 1.28 0.92 0.62 0.38 0.58
F o r composition of cellulose ester, see Table 1. CP-e-1 is cellulose triproplonate. b Expressed as parts by volume of diluent per part of original solution. Values were determined within 0.05. Original solutions before dilution contained 8% by weight of cellulose acetate propionate.
FILMS
FROM
SOLVENT-DILUENT MIXTURES O F MINIMUM ACTIVE SOLVENT CONTENT
Films of cellulose acetate propionate were cast from solution in the various solvent-diluent mixtures resulting from the previous dilution tolerance work. The properties of the films are shown in Table VII. They are characterized by much greatrr shrinkage on drying than are those films cast from single solvents (3). This is a consequence of the gel structure set up in such solutions early in the drying cycle, due to the progressive increase in the concentration of the medium boiling nonsolvent constituent as the more volatile active solvent evaporates. The mechanism whereby the gel structure is set up may be pictured in the following manner:
2093
which arise when the shrinkiig film is constrained by the fabric to which i t is applied. SHRINKAGEVERSUS CHEMICALCOMPOSITION.The greatest shrinkages generalIy were obtained with the medium propionyl esters of medium or low hydroxyl content. There are a few exceptions t o this generalization, as in the case of the low propionyl esters (CAP-a-24, 25, and 26) with Cellosolve as the diluent. Shrinkage is noticeably inferior with the high hydroxyl esters This may be due more to the effect of their better solubilities on the gel stage structure than to an inherently lesser tendency for hydroxyl groups to promote shrinkage. Theoretically, it should be possible t o obtain good shrinkage with any cellulose acetate propionate provided the proper solvent balance with respect to gel stage is achieved. FLEXIBILITY VERSUS CHE~KICAL COMPOSITION.Flexibility of the films improves with increased hydroxyl content and, usually, with increased propionyl content. The apparent brittleness of some of the films when folded is probably more the result of the increase in thickness due t o the high shrinkage undergone rather than t o a n inherent lack of flexibility. Also, the effect of wrinkles in causing stress concentrations probably tends t o affect flexibility adversely. CLARITYVERSUS CHEMICALCOXPOSITION.Although the solubility of the cellulose esters in the various acetone-liquid mixtures improves with increasing propionyl content and with increasing hydroxyl content, there are no definite correlations between cellulose ester composition and clarity of the final film as had been noted in the case of films cast from single solvcnts. Cloudy, opaque, or two-phase films were produced in many instances when the solvent balance was such as t o cause precipitation of the cellulose ester during the drying cycle. FLEXIBILITY VERSUS CLARITY. Flexible films were usually clear, but clear films were often brittle. It appears, therefore, that clarity is usually an essential condition for good flexibility but does not ensure it. Practically all the cloudy films were brittle. FLEXIBILITY VERSUS SHRINKAGE. Good shrinkage, to the extent of 30% of the original dimensions of the film when poured, was usually accompanied by poor flexibility. Films which shrank 40% during drying were usually very brittle; film which shrank 3s much as 50% were very brittle in almost every instance. The brittleness of high-shrinkage films may br a n ap-
Active centers of attractive force (van der Waals forces) are present at intervals along the solute molecule chain. There is a continual competition between solvent molecules and solute molecules for bare active sites on the cellulose ester macromolecule, with the indifferent nonsolvent diluent molecules interfering only by filling space. As the concentration of active solvent in the volatile vehicle diminishes during the evaporation process, greater opportunity is afforded for polymer-polymer unions. The linking of polymer chains eventually results in thrir becoming partially immobilized in an entangled three-dimensional network, or gcl, which is highly mvollen because of the spare-filling presence of nonsolvent molecules whose concentration has greatly increasrrl at the expense of the more volatile cactivt. solvent during the drying prorws. The deswelling of polymer chains when the residual volatiles (nonsolvent and Polvent) are evaporated is accompanied by a loss in volume which is manifest ~ h sa shrinkage of the entire film mass. A convenient measure of this change in volume is provided by the change in TABLE VI. PERCEKTACETONE REQUIRED IN SOLVENT-DILUENT MIXTURESTO YIELDFILMdiameter of B test film when FORMING SOLUT~ONS OF CELLUWSEACETATE PROPIONATES~ dried in a cellophane-lined Petri % Methyl dish which permits unhindered Acetate ehrinkage. If 4 is the volume Cellulose in Final TriproCAP-8-24 fraction of nonvolatiles in the pionate, Cellulose hcetate Yropionate CAP-b Solvent dish at the instant of gel formaDiluent CP-e-1 a-18 a-19 a-20 8-21 a-22 a-23 'a-24C a-25 a-26' Mixture tion, then shrinkage, S, as Methyl ethyl ketone 6 S S S S S S 8 6 S S 47 characterized by the percentMethyl isobutyl ketone S 44 i3 5 50 29 6 92 78 38 48 age change in diameter of the Methyl aaetate 6 S S S S S S S S S None Ethyl acetate (tech.) 9 S S S S S S 8 6 S S 47 film upon drying is given by S Ethyl acetate (c.P.) S S 8 5 5 S S 8 5 S S 48 = 100 (1 - .\s/G). This relaButyl acetate S 29 9 5 54 23 9 89 81 29 50 Cellosolre acetate S S S 8 2 3 s 8 8 8 6 4 s 55 tion follows from the fact t h a t Ethyl lactate 17 S S S 2 3 6 9 8 8 5 9 48 essentially the Rhape of the Ethyl alcohol 50 44 63 52 33 89 85 54 59 38 44 final shrunken mass is geomet38 50 38 50 Isoprop 1 alcohol 64 50 38 94 82 50 57 rically similar to the shape of Butyl acohol 38 50 38 50 64 50 38 93 82 54 51 Diacetone alcohol 17 S S 9 3 8 s 9 8 8 7 0 5 52 the gel at the instant of formation. Thus, the larger the 23 23 23 17 50 33 17 87 70 29 50 Cellosolve Toluene 17 17 17 17 38 29 29 92 64 38 46. volume fraction of residual . . , . 45 ,. 65 61 62 . . 84 . . Aromatic diluentd volatiles ?t the gel stage, the Aliphatic diluenta .. .. 65 .. 75 64 71 ,. 91 .. greater IS the subsequent a Acetone is the active solvent used, except with CAP-a-24 which required methyl acetate also. Values were shrinkage. Good shrinkage is determined to the nearest 5-gram increment of solvent added to an initial 50 grams of mixture of ester and diluent. Cellulose acetate propionate concentration was maintained at 8% by weight throughout incremental addition essential for the tautening process. 6 indicates cellulose ester was soluble in original solvent. property required in satisfacb For composition of cellulose ester, see 'cable I. C Values in this column are for total active solvent-acetone plus methyl acetate. The percentage of methyl tory airplane dopes. I n this acetate in the final solvent tnixture is indicated in the column at the extreme right. case the gel formed must be d 50: 50 heptane-toluene mixture. e 90: 10 heptane-toluene mixture. strong enough to withstand, without yielding, the stressea I
7 -
2094
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 43, No. 9
parent rather than a n inherent one, since the increased thickness accompanying such shrinkage would increase brittleness as measured by a fold test. CELLULOSE TRIPROPIONATE FILMS.The low molecular weight, practically unhydrolyzed cellulose tripropionate ester yielded films having excellent shrinkage but they were generally cloudy and extremely brittle. Flexible films were obtained only with diacetone alcohol and ethyl lactate, and a clear film was obtained only in combination with toluene. FILMS FROM SELECTED SOLVENT-DILUENT MIXTURES
O O O d
++
YCOdd
anav
The wide variations in the film properties resulting from differences in chemical composition among the various cellulose acetate propionate esters required that the solvent formula be "tailored" to each particular material on the basis of the data available. Satisfactory airplane dopes should contain low boiling active solvents to secure rapid drying, high boiling active solvents t o prevent blushing or precipitation, and medium boiling nonsolvents t o control the solubility in such a way as t o set up a gel structure a t a suitable stage in the drying cycle, so t h a t subsequent escape of solvent will result in decided shrinkage. Thi. shrinkage must be achieved without impairing flexibility or clarity of the film. Solvent-diluent mixtures containing 8% cellulose ester were prepared in 30-gram portions. I n most instances the cellulose ester was dispersed in the active solvent component before adding the diluent component in order to obtain a better solvating action (2). Flexibility, shrinkage, and clarity of the films prepared from thesc solutions are reported in Table VIII. Flexible films were obtained in the great majority of cases, but good shrinkage was difficult t o achieve. I n some instances, films with good flexibility and shrinkage were unsatisfactory in clarity. Frequently, good shrinkage could be achieved only through a sacrifice in flexibility. Of the 304 formulations tested, nine possessed the desired flexibility, shrinkage, and clarity-via., CAP-a-20 in formula 19; CAP-a-18, 19, 20, 22, and 23 in formula 48; CAP-a-I9 and 20 in formula 49; and CAP-a-22 iri formula 72. The most generally satisfactory film properties are obtained with solvent mixtures containing butyl alcohol as the nonsolvent diluent in combination with Cellosolve acetate or diacetone alcohol as the high boiling solvent and with methyl ethyl ketone and ethyl acetate as the low boiling active solvents. Butyl alcohol is outstanding among the nonsolvents in producing good shrinkage. Good results were obtained also with a few formulations containing toluene as the nonsolvent. Methyl isobutyl ketone and butyl acetate are less effective diluents for promoting good shrinkage because of their active or partial solvent effect on most of the cellulose acetate propionates investigated. For optimum shrinkage properties the amount of high boiling solvent should not be excessive; the data show that shrinkage was impaired by increasing the Cellosolve acetate content of a number of formulations from 10 t o 15'%. Formulations containing Cellosolve as the high boiling liquid gave generally poor films; those which were flexible did not undergo enough shrinkage, and those with adequate shrinkage were usually very brittle due t o phase separation during the drying cycle, resulting in milky or opaque films. Formulations containing ethyl lactate as the high boiler gave a n occasional film with good shrinkage, but this solvent tended t o be very strongly retained by the cellulose esters, as evidenced by the presence of residual soIvent odor even after 3 weeks of drying. Good shrinkage is more easily achieved, in general, with esters of low hydroxyl content and low propionyl. This is related to the lesser solubility of such cellulose esters and its effect in producing an early gel stage.
September 1951 COMPATIBILITY
2099
INDUSTRIAL AND ENGINEERING CHEMISTRY OF CELLULOSE ACETATE PROPIONATES WITH PLASTICIZERS
The compatibility and flexibility of cellulose ester films prrpared with various plasticizers in single-solvent solutions are reported in Table 111. The data permit classification of the plasticizers into several categories on the basis, of thew compatibilities at three concentrations with each cellulose ester, as follows: Plasticizers compatible in concentrations as high as 40 t o 50% of the film solids are considered to be highly compatible; those giving clear films in concentrations of 25% but not a t 40 or 50% are described as moderately compatible; those producing clear films a t a concentration of 10% but not a t 25% are classed as only slightly compatible; those not compatible a t 10% of the nonvolatiles are deemed incompatible. Although there is considerable variation in the compatibility of the different plasticizers with t h e various cellulose acetate propionates, no systematic correlation with composition is apparent. The cellulose ester of medium propionyl, low hydroxyl content (CAP-a-21) is generally less compatible than the others. Among the plasticizers, Flexol DOP, Flexol TOF, and dibutyl sebacate exhibit the lowest degrees of compatibility. Triphenyl phosphate, the plasticizer used in Navy specification AN-D-1 cellulose acetate butyrate dope, is among those exhibiting relatively low compatibility with most of the cellulose acetate propionate compositions. In interpreting the flexibility data, the propertles of the films containing 10% plasticizer are considered most indicative of the plasticizing efficiency, because at higher plasticizer concentrations differences in flexibility tend t o be erased by all the films becoming flexible. The observed flexibilities of the plasticized films may be a function of the solvent employed as well as of the cellulose acetate propionate composition, since it was necessary to use a different low boiling solvent in different instances in order to achieve good solutions. Thus, the films cast from methyl ethyl ketone solution are consistently good in flexibility, while those cast from methyl acetate are consistently poor in this property. T h e films cast from acetone solution are also rather poor in flexibility. Here, again, is definite evidence of the important influence which the solvent has on the ultimate film properties through its effect on the physical structure laid down during drying. I n view of the foregoing, interpretation of the flexibility data is considered reliable only when films cast from the same solvent medium are compared. From such a more critical viewpoint significantly superior flexibility is contributed only by tributyl phosphate and dibutyl sebacate, and inferior flexibility is indicated for triphenyl phosphate, acetyl tributyl citrate, Santicizer I-H, and Santicizer 140. Slightly better-than-average flexibility is shown by p-toluenesulfonamide, Santicizers 141 and 160, Flexol DOP, and dibutoxytstraglycol. The remaining plasticizers exhibit only average flexibility behavior. Neither Flexol TOF nor Flex01 D O P leads to excessive film brittleness, despite their comparatively low compatibility with cellulose acetate propionate. FILMS FROM SOLY ENT-DI LUENT-P LA STICI ZER MIXTURES
The performance of the plasticizers was evaluated with films formed from selected solventrdiluent mixtures. The shrinkage obtainable with films formed from unplasticized solutions was markedly reduced by the addition of plasticizer, even in a concentration as low as 10% of the nonvolatiles. It was necessary to modify the various formulations with respect to solvent balance and type and amount of plasticizer in order to obtain satisfactory shrinkage. The process of selective formulation was continued until it resulted in suitable mixtures yielding good shrinkage and flexibility for all the cellulose esters under investigation except CAP-a-24 and CP-e-1. CAP-a-24 (low propionyl, low hydroxyl) could not be satisfactorily formulated because of its very poor solubility. CP-c-1 (the low molecular weight tripro-
pionatr) n’as unsuitable for dopes because it yielded films which were usually weak, brittle, and of poor clarity. The properties of the films obtained in the course of the selective formulation procedure are summarized in Table IV. The optimum experimental dope formulations selected by t.his process for each of the cellulose acetate propionates found to be suitable are :is follows: Cellulo~c Acetate Propionate Designation CAP-a-18 CAP-a-I9 CAP-a-20 CAP-a-21 CAP-a-22 CAP-a-23 CAP-a-25 CAP-s-26
Plasticizer Solvent, Formula No. 48
Nanie Flesol I’OF San ticiser 14 1 Dibutyl sebacate Triphenyl phosphate Flexol TOI‘ Dibutyl sebacate Santiciaer Dibutyl sebacate 141
19
51 50 21 51-a
74 20
% of Nonvolatiles R 10 R
11 10
s
12’
10
Evolved from solvent formula 53 as a result of efforts to improve brush> viscosity and blush resistance during doping.
Thebe dopes have been applied to fabric-covered test panels for a fmal evaluation of their suitability as airplane dopes through outdoor weathering tests. T h e result8 of these teats will be given in a subsequent report when sufficient weathering data have been accumulated to permit a reliable evaluation. Further information concerning plasticizer efficiency was sought by testing a slightly compatible plasticizer (trioctyl phosphate) and a highly compatible plasticizer (triethyl citrate) in a typical airplane dope formulation (CAP-a-18 in formula 19) as 4, 7, 10, 13, and IS%, respectively, of the nonvolatiles content,. The following data were obtained: Plasticizer Trioctyl phosphate
Ani oun t ,
age,
yo
Flexibility
4
10
Yery poor Very poor Very poor Very poor Very poor
30 30 30
4
Very poor
25
7
Triethyl citrate
Shrink-
C, 1o
7 10
13 16
Poor Poor Poor I’air
30 40
25 25 25 20
Appearanw IlaaJHazy Hasv FIaz Hazy Hazy Hazy HaEy Hazy
With the slightly compatible plast,ioizer, film shrinkagc and flexibility remain practically unchanged over the concentration range examined; shrinkage is good but flexibility poor, rxcept for excellent shrinkage (40%) with the specimen containing 7% plasticizer. With the highly compatible plasticizer, shrinkagc is between moderate and good (about 25%) and is unchanged over the concentration range examined except for a slight decrease at the highest (16%) plasticizer concentration; however, flexibility gradually improves as the plasticizer cont,eqt is increased from 4 t o l6yO. It appears, therefore, t h a t a highly compatible plasticizer may be more effective than a slightly compatible one in promoting flexibility but is likely to have an adverse effect on shrinkage. An explanation for this behavior is found in the intermolecular (van der Waals) forces of attraction between active centen on cellulose ester ckiain and plasticizer. A highly compatible p1:tsticizer owes its action to the relatively large attractive forces (1serted between it and its polymer host. Conversely, the incompatibility of some plasticizer-polymer systems is due to the fact that active centers either are not present,, or else actually r q w l rather than attract each other. As has already been indicnttd, t’hedegree of shrinkage of a gel on drying depends on the volume fraction of residual volat,iles in the gel :it the instant it is formid. Since the plasticizer does not evaporLttt’, its prcsence reduces the final shrinkage by an amount, depcnding on the volume it occupies in the ahrunken macromolecu1:ir network. Afore significant than this bulk el’fect, howevcr, is the influence whic~hthe plasticizcr cxrrts on t,he gcl st:tgv c.oriiliosition ; to nn r s t r n t di.pendent on its abilit,y to disrupt poljxiclr-polymer bonds, it cnn
INDUSTRIAL AND ENGINEERING CHEMISTRY
2096
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2097
delay the time of gel formation, thereby lowering the concentration of residual volatiles at the gel stage and reducing the final shrinkage. On the basis of such reasoning, a slightly compatible plasticizer, whose active centers interact only weakly with those of the polymer, is unable to disrupt many of the bonds forming between polymer chains, and BO has little effect on the gel stage composition or the final shrinkage. Even a substantial increase in the amount of a slightly compatible plasticizer does not disrupt many more polymer-to-polymer bonds, so t h a t the degree of shrinkage is relatively insensitive to the plasticizer concentration. If, however, the attractive forces between plasticizer and polymer are very strong, even stronger than polymer-topolymer attraction, many of the bonds between neighboring polymer chains are broken b y the penetration of plasticizer molecules seeking to solvate the chains. In this case more solvent must be evaporated before a sufficient number of polymer-polymer junctions are formed to produce a gel structure. The lowered amount of residual volatilw in such a gel is reflected in a lower degree of shrinkage. The effects of plasticizer concentration on flexibility are explained by a line of reasoning similar t o t h a t just given for shrinkage. Since the molecules of the slightly compatible plasticizer are unable to disrupt polymer-to-polymer bonds, adjacent cellulose ester chains remain linked and relatively immobilized. The restriction placed on their movement is manifest as a lack of film flcxibility. Moreover, the flexibility is not greatly affected by the plasticizer concentration because of the ineffectiveness of the Iatter in penetrating polymer chains. The highly compatible plasticizer, on the other hand, by virtue of the strong attractive forces between it and the polymer, is able t o disrupt many of the polymer-to-polymer bonds, increasing the freedom of adjacent chains t o stretch or slip past each other, with consequent improvement in flexibility. I n this instance, the number of polymer-to-polymer bonds broken depends on the number of plasticizer molecules present, 60 that flexibility, like shrinkage, becomes a function of the plasticizer concentration. SUMMARY
A systematic investigation was carried out t o determine the suitability of cellulose acetate propionate as the filmforming base in airplane dopes. This report describes the relationships which were found t o exist between the chemical constitution of cellulose acetate propionates of various acyl and hydroxyl contents and such properties as solubility, dilution tolerance, plas-
2098
INDUSTRIAL AND E N G I N E E R I N G C H E M I S T R Y
ticizer compatibility, and the properties of films formed from solutions in various solvent-diluent-plasticizer mixtures. Solubility of cellulose acetate propionates in organic liquids improves with increasing propionyl and wit,h increasing hydroxyl content. T h e dilution tolerance of the solutions for toluene and for butyl alcohol is greater with the higher propionyl materials. The flexibility of cellulose acetate propionate films cast from single solvents and from solvenbdiluent mixtures improves with increasing hydroxyl content and, usually, with increasing propionyl content. Films cast from solvent-diluent mixtures are characteiized by much greater shrinkage than can be obtained from aingle solvents. This is a consequence of the gel structure set u p in such mixtures a t a critical stage in the drying process. There is no systematic correlation between cellulose ester composition and compatibility with the plasticizers examined. The most satisfactory solvent mixtures for cellulose acetate propionate dopes contain methyl ethyl ketone and ethyl acetate as the low boiling active solvent constituents, Cellosolve acetate or diacetone alcohol as the high boiling solvent, and butyl alcohol as the nonsolvent diluent, together with butyl acetate in some instances. Promising ex erimental dopes formulated on the basis of the data obtained Rave been applied to fabric-covered test panels and are currently undergoing outdoor weathering tests to rovide a final evaluation of their suitability as airplane dopes. &he results of these tests will be given in a subsequent report.
Vol. 43, No. 9
ACKNOWLEDGMENT
This investigation was sponsored by the Bureau of Aeronautics, Department of the Navy, and the results are published b y permission of the chief of that bureau. The authors express their appreciation for the helpful cooperation of A. M. Malloy of the Bureau of Aeronautics during the course of this work. The interest and cooperation of J. W. Tamblyn of the Tennessee Eastman Corp. in making arrangements for the preparation of the cellulose acetate propionates for use in this investigation are greatly appreciated. The assistance of Honora M. Roberts and D. R. Burr in the experimental work is gratefully acknowledged. LITERATURE CITED
Doolittle, A. K., IND. ENG.CHEM.,36,239 (1944). Quarles, R. W., Ibid., 35, 1033 (1943). (3) Reinhart, F. W., and Kline, G. M., Zbid., 31, 1522 (1939). (4)Ibid., 32, 185 (1940). (5) Shell CLernical Corp., Technical Booklet SC: 46-1,22, 28 (1946). (6) Spence, J., J . Phys. Chem., 43, 865 (1939). (1) (2)
RECEIVED June 29, 1950.
Presented before the Division of Paint, Varnish,
and Plastics Chemistry at the 118th Meeting of the AMERICAN CnEsrIcaL SOCIETY, Chicago. Ill.
Dehydrogenation of Methylcyclopentane over Chromia-
Alumina Catalvsts J
HEINZ HEINEMANN, Houdry Laboratories, Houdry Process Corp., Linwood, Pa. T h e work was undertaken in an effort to produce cyclohexene from petroleum fractions, in order to open up an increased supply of this important chemical. The dehydrogenation at atmospheric pressure of methylcyclopentane over chromia-alumina catalysts in the presence of hydrogen and small amounts of benzene gives better yields of mono-olefins and lower coke deposits than either dehydrogenation at reduced pressure in the absence of hydrogen and benzene, or dehydrogenation in
the presence of hydrogen or benzene individually. Pretreating of the catalyst with benzene with the feed gives larger yields of desired products. More than enough hydroyen and benzene are produced in the process to make the process self-sufficient for these materials. The results obtained in this work show that product distribution in the dehydrogenation of methylcyclopentane can be varied by selectively treating or poisoning the dehydrogenation catalyst.
T
Methylcyclopentene produced can then be separated from the reaction mixture by such methods as extractive distillation and can be isomerized over aluminous or siliceous catalysts ( 1 , I , 7, 8). The process variables of the dehydrogenation step were studied using a chromia-alumina catalyst. The effect of external hydrogen and of added aromatics on the reaction was investigated.
HE preparation from petroleum hydrocarbons of cyclohexene has been described (7). Dehydrogenation of cyclohexane over conventional dehydrogenation catalysts leads primarily t o benzene and cannot be easily arrested at cyclohexene. It has been suggested t o dehydrogenate methylayclopentane (MCP), a hydrocarbon relatively abundant in nsphthenic crude oils (3-6) and isomerize methylcyclopentene t o cyclohexene (7). Dehydrogenation steps described in the literature have been carried out over molybdena-alumina or chromia-alumina catalysts in the absence or presence of external hydrogen. As most dehydrogenation catalysts a t dehydrogenation conditions are also active for the isomerization of olefins, mixtures of methylcyclopentene, cyclohexene, and benzene are usually obtained. Depending on operating conditions and catalysts used, other products, such as diolefins and coke, are formed. It was the purpose of this investigation t o find a method of dehydrogenating methylcyclopentane t o mono-olefin, either methylcyclopentene or cyclohexene and t o reduce the amount of by-products formed-namely, roke, aromatics, and diolefins.
EQUIPMENT, MATERIALS, EXPERIMENTAL PROCEDURE, AND ANALYSIS
The apparatus consisted of a n electrically heated lead bath furnace, holding a quartz reactor tube, 2.5 cm. in diameter and 100 cm. long. The catalyst in the reactor was preceded by a quartz chip preheater layer and flow was downward. Feed stock was charged t o the reactor from a pressure buret t o equalize pressure. The effluent from the reactor was cooled in a glass condenser, through the jacket of which a brine solution flowed at 0" to 5' C. Following a n icecooled receiver flask, vapors were passed
