Film-Forming Plastics Effect of Solvents, Diluents, and Plasticizers FRANK W. REINHART AND GORDON M. KLINE National Bureau of Standards, Washington, D. C.
HE Bureau of Aeronautics of the United States Navy Department has requested the National Bureau of Standards to develop a satisfactory airplane dope based on comparatively nonflammable film-forming materials. A previous report on this project (4) showed that the most important single factor involved in the tautening property of a dope is the solvent composition. It was also indicated that before dopes could be formulated which would give the requisite tautness and flexibility, it would be necessary to secure detailed information on the effect of various solvents, diluents, and plasticizers on the properties of the film-forming plastics. The results obtained in a study of the films formed from various mixtures of such substances are presented in this paper. Clear airplane dope is made u p of a film-forming plastic and a plasticizer, dissolved in a mixture of volatile organic solvents and diluents. I n order t o be suitable as an airplane dope, compositions of this type should yield clear, flexible, stable films which undergo decided shrinkage. The volatile materials of a dope consist of low-boiling solvents to secure rapid drying, medium- and high-boiling solvents to prevent blushing or precipitation, and diluents to reduce cost and control the solubility (6). I n a well-formulated dope there may be cosolvent effects which give the desired results. The use of highly toxic solvents in dopes is undesirable. Such solvents have been avoided in the dope formulas selected as a result of this study. Plasticizers are employed in dopes to increase the flexibility and moisture resistance of the film. I n clear dopes the amount of plasticizer ordinarily used is about 10 per cent of the total solids. In pigmented dopes this is increased to 20 per cent. Large quantities are not desirable because of their proneness to retard shrinkage and to yield films which have excessive elasticity or a tendency to flow under load. These latter characteristics cause the doped fabric to deform to too great an extent during flight and ultimately to become slack. The shrinkage of the film causes the tautening effect characteristic of satisfactory dopes. The individual effect of each solvent can be determined from an examination of the film made from a solution of the plastic material in the solvent in question. The effect of diluents can be determined only by preparing films from solutions consisting of a mixture of one or more solvents with the diluent. This introduces complications caused by a possible cosolvent effect as well as the effect of the solvents used. A solvent must also be used in studying the effects of small proportions of plasticizer in order to produce a solution and to secure an intimate mixture of the plasticizer with the plastic.
The properties of films formed from airplane dopes are affected by the four variables involved in their formulationnamely, plastic, plasticizer, solvent, and diluent. Data are presented in this paper on the solubility of various plastics in twenty-one common organic solvents and mixtures of solvents and diluents, and on their compatibility with thirty-nine plasticizers. The degree of flexibility, clarity, and Shrinkage obtained with films prepared from plasticized and nonplasticized compositions is reported. Formulations of volatile components which yielded films having optimum flexibility and shrinkage, in so far as their use in airplane dopes is concerned, are listed for each material. These will serve as a basis for experimental dopes to be applied to fabric-covered test panels in order to determine their tautening and weathering properties.
T
Materials and Methods
PROPERTIES OF PLASTICS. The materials tested were furnished by various manufacturers whose interest and cooperation in this investigation have been very helpful. They
also supplied data regarding the viscosity and chemical composition of the compounds (Table I). In the manufacturers’ reports the chemical compositions were expressed as percentages. The number of equivalents of each substituent group in the compounds and of free hydroxyl groups were calculated from these percentage figures. The latter values are also listed in Table I and indicate immediately the degree of hydrolysis of the cellulose derivatives. DETERMINATION OF SOLUBILITY AND FILM PROPERTIES. Seven grams of film-forming material were dispersed in 100 grams of solvent by shaking overnight on a mechanical shaker. When the film-forming material was insoluble in the liquid, an active solvent was added together with a further amount of solid to keep the proportion by weight between solid and liquid at 7 to 100. A 25-gram portion of the solution was poured into each of two glass Petri dishes, one of which had a cellophane lining. The cellophane allowed the full amount of shrinkage t o take place. When the film was formed directly on glass, the adhesion to the glass reduced the normal shrinkage of the film. The films were examined for flexibility, clarity, and shrinkage after drying approximately a week and again after about a yoar. The data presented in this paper represent the condition of the films after storage for one year in such a way that escape of residual solvent was possible. To evaluate the flexibility, the films were folded on a diameter, unfolded, and folded in the opposite direction on the same line. This procedure was then repeated on a diameter at right angles to the first fold. If a break occurred, the film was rated nonflexible. Clarity was judged by visual observation only. Shrinkage was measured by a qualitative comparison of the final films with one another and with the inside dimensions of the dishes in which they were formed.
1522
DECEMBER, 1939
INDUSTRIAL AND ENGINEERING CHEMISTRY
OF TESTMATERIALS TABLE I. VISCOSITYAND COMPOSITION
Material Cellulose acetate
Sample Designstion Viscosity" CA-a4 28 sec.
Cellulose triacetate Hydroxypropylcellulose triacetate Cellulose acetopropionate
CA-a10 HC-f 1 CAP-b4
465 cp.
Cellulose acetobutyrate
CAB-h4
344 cp.
Cellulose nitrate
CN-czb
10.5 sec.
Ethylcellulose
EC-b2
Benzvlcellulose ChloFinated rubber Methyl methacrylate resin Xsobutyl methacrylate resin
BC-el CR-b2 MM-cl BM-cl
.... ....
22 sec. High 1166 cp. Medium Medium
--CompositionPer cent basis 40.4 acetyl
Equivalents basis 2 . 5 1 a c e t y 1, 0.49 hydroxyl About 3 acetyl (7) 12.1 acetyl, 0.80 acetyl, 1.58 3!.7 prop r o p i onyl, pionyl 0.62 hydroxyl 32.5 acetyl, 2.31 acetyl, 15.5 butyryl 0.67 butyryl 0.02 hydroxyl About 2.4 nitrate 47.7 ethoxy 2.44 ethoxy, 0.56 hydroxyl
...... ......
......
...... ......
...... ...... ...... ......
...... ......
a The viscosity values were obtained in the manufacturers' laboratories b y the following variety of methods (all parts by weight): cellidose acetate, A. S. T. M. fal!ing-ball method using 20% solutions in acetone (I): cellulose acetobutyrate and acetopropionate, capillary viscometer using 10% solution in acetone a t 25' C.:cellulose nitrate, A. S. T. M. falling-ball method; ethylcellulose, A. S. T . M. falling-ball method using a 207' solution in a mixture of 80 parts of toluene and 20 parts of 95% ethyl alcohol at 25' C.: ckorinated rubber, capillary viscometer using a 20% solution in toluene at 25' C. b The cellulo~enitrate contained approximately 30% by weight ,of ethyl alcohol: a sufficient quantity of this material was taken so that the resulting solution contained 7 grams of cellulose nitrate.
TABLE11. TOLERANCE OF SOLUTIONS OF CELLULOSE ACETOPROPIONATER FOR TOLUENE
-yo
Sample CAP-a2 CAP43 CAP-a3 CAP-a7 CAP-a8 CAP-a9 C A P-a4 CAP-a1 CAP-b4 CAP-a5 CAP-all CAP-a12
-
BasisProAcetyl pionyl 16.1 13.1 15.6 12.8 13.2 13.3 14.3 17.1 12.1 13.6 30.0 28.2
32.9 35.3 32.2 35.1 34.1 33.3 30.4 26 9 31.7 28.6 16.7 17.0
Composition -Equivalents Acetyl
BasisHydroxyl
1.17 0.94 1.10 0.91 0.93 0.92 0.96 1 13 0.80 0.87
Part I 1.80 0.03 1.91 0 15 1.71 0 19 1.88 0 21 181 0.26 174 0 34 154 0 50 0 63 1.34 0 62 1.58 1.38 0.75
2.06 1.91
Part I1 0.86 0.08 0.87 0.22
8 soluble: PS = partially soluble; b Determined within 0.5.
(1
.
Propionyl
-Toluene Dilution Ratio in:Ethyl Acetate Butyl Acetate Sols. of Soly. of cellucellulose lose acetoacetopropropionate pionate in Toluene in Toluene ethyl dilution butyl dilution acetatea ratio acetate= ratio
S S S 8
S
S 8 S
S S PS S
4.06 7.0b 5.06 5 Ob 4 Ob
3 Ob 1 80 1 4c 1.2c
0.8C
1:id
I = insoluhle.
Determined within 0.2. d Determined within 0.1.
0
Relation of Solubility to Composition of Plastics The solubility of a given type of plastic depends upon its degree of acylation, etherification, or polymerization (9, 3). This is shown by data for the toluene-dilution ratio of cellulose acetopropionate which were obtained in the following manner: Several samples of cellulose acetopropionate of varying degrees of hydrolysis were dissolved in ethyl acetate and butyl acetate, respectively, to yield solutions having a concentration of 8 per cent of cellulose acetopropionate by weight. After the toluene was added, the solutions were shaken for one hour on a mechanical shaker and allowed to stand overnight. Examination was then made for separation I 0 When the sample was soluble, the properties of the film are shown. The symbols used for indicating the degree of insolubility and the pro er ties of the films are as follows! I = insoluble: PS, = artially solube: F = flexible. N F = nonflexible. C = clear. SH = sltght gaze. H = haze: SB = slight blush; B = blush: INS = no shhnkage; SS sligdt shrinkage; MS = moderate shrinkage; GS = good shrinkage. b These films did not adhere to glass and ceilophane. (The films not marked by b or c adhered to the glass but could be readily removed without soaking in water.) C These films had t o be soaked in warm water t o loosen them from the glass or cellophane.
-
PS S S S S 6 S €4
2:ib 2 40 2 Ob 1 5b 1.5b 0.80 0 3d
I
..
s I I
O.ld
.. ..
1523
of the cellulose derivative. The values given in Table I1 are the number of parts by weight of toluene added to one part of the original solution without causing precipitation of the cellulose derivative or gelling of the solution. The data in Table I1 show the variation in solubility of cellulose acetopropionates containing different amounts of acetyl, propionyl, and hydroxyl groups. All the samples in part I have acetyl equivalents between 0.8 and 1.2 and propionyl equivalents between 1.3 and 1.9. I n this series the dilution ratio increases a t first in passing from the practically unhydrolyzed sample to one containing 0.15 equivalent of hydroxyl, and thereafter the dilution ratio decreases with increasing degree of hydrolysis. Samples in part I1 do not behave similarly to those in part I, probably because of their higher acetyl content. As regards acetyl and propionyl equivalen&, those in part If are practically the reverse of those near the top of the list in part I. The cellulose acetopropionates in part I1 behave more like cellulose triacetate with respect to solubility in ethyl ace. tate and butyl acetate. The effect of the composition of ethylcellulose on the degree of solubility and the properties of the films formed from solutions are shown in Table 111. These data serve to emphasize the fact that the properties of the films are greatly affected by the degree of solubility of the plastic in the solvent or solvent mixture employed in their preparation, and that the degree of solubility is closely related t; the composition of the plastic. It is therefore apparent that the results given in this paper on the types of
TABLE111. SOLUBILITY OF ETHYLCELLULOSES IN VARIOUS SOLVENTS, AND PROPERTIES OF FILMS FORMED FROM THE SOLD TIONSO
Sample number EC-b2 Ethoxyl equivalents per CeHloO: 2.44 Hydroxyl equivalents per CnHioOr 0.56 Viscosity rating, cp. 30 Solvent Methyl alcohol NF. B, SSb Ethvl alcohol F. C, SS But$l alcohol F. C, NS Cyclohexanol F, C, MS Acetone F, C, SS Methyl ethyl ketone F, C, SS Diacetone alcohol F, C, SSC Mesityl oxide F, C. NS Methyl acetate NF. SB. SS Ethvl acetate F. C, SS ButGI acetate F, C, NS Cellosolve acetate F, C, NSC Ethyl lactate F, C. NS Cellosolve F , C, NS Methyl Cellosolve N F C GS Dioxane NF: C: NS Benzene F, C ,88 Toluene F. c. ss Petroleum naphtha I Chloroform NF, B, NSO Ethylene chloride NF, c, NS
EC-d3 2 52 0.48 35 NF. B. M S , NF, C. h S F, C, N S F, C, SS F, C, N s c F, C, h S F, C, NS F, C, NS N F , SB, MS F, C, NS F. C, h S F, H, SS F, H , SS F, C. SS N F , B, NSC N F , C, NSC F C NS F: H', NS
I
EC-d2 2 77 0.23 30
I
PS
N F , C, NSo I PS
PS PS PS
NF, PS R, NS PS
PS II
NF. B, M S b N F H SSO NF: H: ss=
I
N F B SS NF. H, NSO NF: c: NSC NF, H,ss=
INDUSTRIAL AND ENGINEERING CHEMISTRY
15 24
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VOL. 31, NO. 12
DECEMBER, 1939
INDUSTRIAL AND ENGINEERING CHEMISTRY
films formed from various solvents apply only to materials very similar in composition to the ones used in this experimental work.
Properties of Films Formed from Solutions of Plastics SoLurIoNs I N SINGLESOLVENTS.Data on the solubility in single solvents of the various plastics described in Table I and the properties of the films formed from these solutions are presented in Table IV. Commercial grades of solvents were used in these tests. Ethylcellulose has the widest range of solubility and cellulose triacetate the narrowest in the solvents studied. The range of solubility of methyl methacrylate resin is only slightly greater than that of cellulose triacetate. The ketones and esters appear to be the most generally effective types of solvents, methyl ethyl ketone and ethyl acetate being particularly active. The films of cellulose acetate prepared from solutions in esters and ketones were flexible and clear, and in general underwent moderate shrinkage. More pronounced shrinkage was obtained with cellulose acetobutyrate, the maximum being produced from solutions in diacetone alcohol and ethyl lactate. Cellulose acetopropionate was characterized by much less tendency to undergo shrinkage than either the acetate or the acetobutyrate. Solutions of the acetopropionate in methyl ethyl ketone and Cellosolve gave the maximum shrinkage for this material. Later experimental work has shown that the difference in the shrinkage characteristics of the cellulose acetobutyrate and cellulose acetopropionate samples used for these film studies can be largely attributed to the difference in hydroxyl content of the two materials. Cellulose nitrate and ethylcellulose showed little tendency to shrink when deposited as films from solutions in single solvents with no diluents present. Films of cellulose triacetate could be prepared only from solutions in the chlorinated solvents because of the insolubility of this material in other types of solvents. The properties of the film formed from solution in chloroform are shown in Table IV. In addition, it was found that a sym-tetrachloroethane solution of cellulose triacetate would yield a clear flexible film with good shrinkage. No appreciable shrinkage was obtained with benzylcellulose and isobutyl methacrylate resin. Tbe films formed from chlorinated rubber and methyl methacrylate resin without the use of plasticizer were nonflexible. SOLUTIONS IN MIXTURESOF DILUEXTSAND SOLVENTS. The data obtained for films formed from solutions of the plastics in mixtures of solvents and nonsolvents are also presented in Table IV. Low-boiling solvents were chosen for addition to the diluent in order to determine in so far as possible the effect of the latter on the film properties. Cloudy films occurred in most instances, owing to precipitation of the plastic as evaporation left an excess of diluent in the residual mass. However, an occasional clear film was produced under these circumstances, which indicated a cosolvent effect. The shrinkage obtained in the presence of some diluents, particularly those which have a swelling action on the plastic, was also greater than that observed for the solvent alone, which can be attributed to a gelling action of the diluent in the solvent-diluent-plastic mixture during drying. The gels formed under such conditions undergo marked shrinkage as the remaining volatile materials evaporate, in contrast to those films which are simply deposited from a very active solvent. For example, a solution of cellulose acetopropionate in ethyl acetate yielded a film which underwent only moderate shrinkage, ‘whereas when a mixture of equal parts of ethyl acetate and toluene was employed, the film obtained showed good shrinkage. In both instances the films were clear and flexible. It is necessary to avoid the premature
1523
appearance of the gel stage since this ordinarily leads to the formation of so-called cheesy gels which develop cracks spontaneously during the drying process and yield very brittle films. Cellulose acetate in cyclohexanol and ethylcellulose in methyl Cellosolve gave gels of this latter type. Data for the properties of films of cellulose triacetate are not given in Table IV because addition of as much as 80 per cent of chloroform to the various other organic liquids investigated failed to bring about solution of the cellulose triacetate.
Optimum Solvent-Diluent Compositions On the basis of the experimental data obtained with the single solvents and solvent-diluent combinations, various mixtures were formulated and tested. This process of selective formulation was continued for each material until no further improvement in the desired film properties was noted. Flexibility, clarity (indicating freedom from material precipitated by lack of solvency or too rapid drying), and shrinkage of the films were used as the criteria in evaluating the formulas. The following mixtures were found to give the best results for the materials investigated. All figures indicated are for parts by weight. CELLULOSE ACETATE. Formula B
Formula A 10-20
Ethyl lactate Diacetone alcohol Toluene Methyl ethyl ketone Acetone Ethyl acetate
.....
10-20 Any two or all three t o total 100 parts in the formula
10-15 0-20
25-40 Either or both t o total 100 parts i n the formula
i’
Formula A apparently gives a little better result when some ethyl acetate is used. CELLULOSE TRIACETATE. Formula C 70 30
Chloroform s urn-Tetrachloroethane
The addition of nonchlorinated organic liquids to solutions of cellulose triacetate in chloroform and sym-tetrachloroethane quickly results in precipitation of the plastic. Cellulose triacetate in formula C would not be considered by the military services as a suitable composition for an airplane dope because of the presence of chlorinated solvents. CELLULOSE ACETOPROPIONATE.
Cellosolve Diacetone alcohol Butyl acetate Butyl alcohol Toluene Acetone Methyl ethyl ketone Ethyl acetate
...
. 10 ..
... ...
i’
Formula F
Formula E
Formula D 10-20
50-iO
Any one t o total 100 parts in t h e formula
...
30L40
3040
20-35
5
100 parts in the formula
3o
I n formula D the large amount of toluene is necessary to secure good shrinkage. CELLULOSE ACETOBUTYRATE. Formula G Diacetone alcohol Ethyl lactate Butyl acetate Toluene Acetone Methyl ethyl ketone Ethyl acetate
10-20
...
..
i
Formula H 20
Formula I 20
Formula J 10-.20
10-20 15-25 30 20-30 15-2 5 Any t w o or all three t o total 100 parts in the formula 30
Too much toluene or acetone tends to cause brittleness. When no diacetone alcohol or ethyl lactate is present, butyl acetate causes brittleness. The presence of methyl Cellosolve or Cellosolve also produces a nonflexible film. Formula H
1526
INDUSTRIAL AND ENGINEERING CHEMISTRY
is slow drying. All formulations of cellulose acetobutyrate containing ethyl lactate remain tacky much longer than formulations with diacetone alcohol. CELLULOSE NITRATE. Formula K 5-10 40- 50 15-20 5 20-25
Butyl acetate Toluene Butyl alcohol Acetone Ethyl acetate
Up to 15 parts of the toluene may be replaced by aliphatic hydrocarbons. All or part of the toluene may be replaced by commercial aromatic petroleum naphthas of high toluene content (6).
ETHYLCELLULOSE. Formula L 60 10 Either one, 30
Toluene Diacetone alcohol Acetone Methyl ethyl ketone
BENZYLCELLULOSE. Formula M Isopropyl alcohol Toluene Methyl acetate Methyl ethyl ketone
25
1
50 Either one, 25
This cellulose derivative gave very little shrinkage when deposited from any solvent mixture. CHLORINATED RUBBER. Formula N Toluene Butyl acetate Methyl acetate Ethyl acetate
50-60 10-20
Either one, 25-30
All of the films of chlorinated rubber prepared without the use of plasticizers were nonflexible. METHYL METHACRYLATE RESIN. Formula 0 Ethyl acetate Toluene Butyl acetate
50 25 25
All fdms of methyl methacrylate resin prepared without the use of plasticizers were nonflexible. No formulation was found for isobutyl methacrylate resin which would yield films characterized by appreciable shrinkage. The relative order of shrinkage for the ten film-forming materials in the best formulation for each was as follows: 1. Cellulose triacetate Cellulose nitrate 3. Cellulose acetobutyrate 4. Cellulose acetopropionate 6 . Hydrolyzed cellulose acetate 6. Ethylcellulose 7. Methyl methacrylate resin 8. Chlorinated rubber 9. Benzyloellulose 10. Isobutyl methacrylate resin 2.
Good shrinkage Good shrinkage Good shrinkage Moderate shrinkage Moderate shrinkage Moderate shrinkage Slight shrinkage Slight shrinkage Slight shrinkage No shrinkage
Compatibility of Plasticizers with Plastics For the determination of compatibility of plastic-plasticizer compositions, films were prepared in which the ratios of film-forming materials to plasticizers were 1 to 1, 3 to 1, and 9 to 1. These were examined after drying for evidences of separation of plasticizer from the film as indicated by haziness, oiliness, or blooming. Very volatile solvents were used so that solvent effects caused by residual solvent were minimized. Chlorinated rubber was dissolved in ethyl acetate,
VOL. 31, NO. 12
benzylcellulose in benzene, and cellulose triacetate in chloroform; the other plastics were dissolved in methyl ethyl ketone. Plasticizers were added as solutions in methyl ethyl ketone to all the plastics except cellulose triacetate; in the latter case i t was found necessary to add the plasticizer in chloroform solution. Twenty-five gram portions of the mixtures were poured into Petri dishes of 95-mm. outside diameter and 15-mm. depth, lined with cellophane, and the films were inspected for plasticizer compatibility after approximately one week and one year. Table V presents the observations made after storage of the films for one year. The unusual feature of these data is the high degree of compatibility exhibited by cellulose triacetate, which was found to have the narrowest range of solubility in the common organic liquids. Methyl methacrylate resin also has a high order of compatibility, although it ranks next to cellulose triacetate in degree of solubility. The hydrolyzed cellulose acetate sample, on the other hand, is by far the most incompatible material of any included in this study; i t forms nonhomogeneous films with twenty-two of the thirty-nine plasticizers examined in concentrations as low as 10 per cent of plasticizer. Benzylcellulose ranks next in order of incompatibility, yielding nonhomogeneous films with eight of the thirty-nine plasticizers.
Properties of Films from Plastic-VolatilePlasticizer Mixtures The effects on the film properties of the addition of various plasticizers to the selected plastic solutions are shown by the data in Table VI. The fact that all of the films prepared with hydroxypropylcellulose triacetate were relatively brittle may be attributed to improper solvent formulation. Some evidence that this was the cause was obtained, but our investigation of this factor was incomplete owing to an insufficient supply of this material. In most cases the films containing 10 per cent (9 to 1) of plasticizer displayed more shrinkage than those containing 25 per cent (3 to 1) of plasticizer. There were a few exceptions. Cellulose acetobutyrate gave better shrinkage with di-ptert-butylphenyl mono-tert-butylcresyl phosphate and acetanilide a t 3 to 1 than at 9 to 1 ratio. Benzylcellulose gave better shrinkage with p-toluenesulfonamide at a ratio of 3 to 1 than a t 9 to 1. I n many instances the shrinkage a t 3 to 1 and 9 to 1 ratios was the same. Comparison of compatibility data from Tables V and VI shows some inconsistencies. This is probably due to the difference in solvents used. Where the plasticizer is compatible in a higher ratio in Table VI, this can be explained by the retention of some high-boiling liquid, which acts as a solvent for the plasticizer in the film. Where the plasticizer is less compatible in Table VI, it is probably due to the retention of some high-boiling liquid which is a nonsolvent for the plasticizer.
Summary and Conclusions 1. Clear airplane dope is made up of a film-forming plastic and a plasticizer, dissolved in a mixture of volatile organic solvents and diluents. I n order to be suitable as an airplane dope, compositions of this type should yield clear, flexible, stable films which undergo decide$ shrinkage. This paper presents the results of an investigation of the effects of the four variables-plastic, plasticizer, solvent, and diluenton the properties of the films formed from such solutions. 2. The degree of film shrinkage, whtch causes the tautening effect characteristic of a satisfactory dope, varies greatly with the type and composition of the plastic. The order of decreasing shrinkage noted for the ten plastics studied in the
DECEMBER, 1939
INDUSTRIAL AND ENGINEERING CHEMISTRY
1527
optimum formulation for each was . as follows: cellulose triacetate, cellulose n i t r a t e , cellulose acetobutyrate, cellulose acetopropionate, hydrolyzed cellulose a c e t a t e , ethylcellulose, methyl methacrylate resin, chlorinated rubber, benzylcellulose, a n d isobutyl methacryl a t e resin. T h e latter four materials show almost negligible shrinkage and are not satisfactory for use in airplane dope. Cellulose triacetate is also unsatisfactory because its solubility is practically limited to chlorinated solvents a n d because t h e films produced with i t are relatively brittle. The most promising materials for use in place of cellulose nitrate in airplane dopes are cellulose acetobutyrate and cellulose acetopropionate. 3. A study of the c o m p a t i b i l i t y of thirty-nine plasticizers in concentrations of 10 to 50 per cent with the various plastics and the effects of some of these plasticizers on the properties of the films showed that, in general, an increase in the amount of plasticizer improves the flexibility of the film but reduces the degree of shrinkage. I n some instances, however, the shrinkage observed with 25 per cent of plasticizer exceeded that taking place with 10 per cent of plasticizer, and i n a greater number of cases the shrinkages observed for both concentrations were identical.
1528
4. Investigation of the solubility of the plastics in twentyone common organic solvents revealed that ethylcellulose has the widest range of solubility and cellulose triacetate the narrowest in the solvents studied. The ketones and esters appear to be the most generally effective types of solvents, methyl ethyl ketone and ethyl acetate being particularly active. The degree of solubility is greatly affected by variations in the composition of the plastic or its state of polymerization. The properties of the films are dependent to a marked extent both on the solvent employed and the composition of the plastic. 5. Data obtained concerning the effect of diluents on the properties of films formed from solutions of the plastics in mixtures of solvents and d i l u e n t s showed t h a t t h e shrinkage in the presence of some diluents, p a r t i c u l a r 1y those which have a swelling action, is greater than that observed for the solvent alone. This phenomenon can be attributed to a gelling action of the diluent on the plastic solution during drying. It is necessary to avoid solventd i l u e n t combinations which lead to an early appearance of a gel state, since brittle films ordinarily result under such conditions. 6 . Formulations were developed on the basis of the experimental data obtained with single solvents and solventdiluent combinations, which yielded optimum film properties for each plastic in so far as their use in airplane dopes is concerned. These solutions with the addition of selected plasticizers have been applied to panels covered with airplane cloth to determine their tautening properties and the weathering characteristics of the deposited films. Reports covering exposure and flammability tests on these experimental dopes are in preparation.
Acknowledgment T h i s i n v e s t i g a t i o n was sponsored by the Bureau of Aeronautics, U n i t e d S t a t e s
INDUSTRIAL AND ENGINEERING CHEMISTRY
VOL. 31, NO. 12 C
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1529
INDUSTRIAL AND ENGINEERING CHEMISTRY
DECEMBER, 1939
Navy D e p a r t ment, and the results are published by permission of the chief of that bureau. The a u t h o r s express their appreciation of the interest and suggestions during the course of this work of Lieutenant Commander C. F. Cotton and J. E. S u l l i v a n of t h e Bureau of Aeronautics.
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63 &g$ m
Literature Cited (1) Am. SOC. Testing
Materials, Standards, Pt. 11, 1936. (2) B u r k , R . E . , Thompson, H. E., Weith, A. J., and W i l l i a m s , I., "Polymer i z ation", Chap. V I I , New P o r k , Reinhold Pub. Corp., 1937. (3) J o r d a n , O t t o , "Technology of Solvents", tr. by A. D. Whitehead, p. 87, Chem. Pub. Co. of N. Y., 1938. (4) K l i n e , G . M., and Malmberg, C. G., IND. ENG. CHEM., 30, 542-9 (1938). ( 5 ) Ibid., 30, 545. (6) Navy Aeronau-
tical Specification for Aromatic Petroleum Naphtha RM106, Type I, June 1,1937; Specification Amendment No. I, April 1, 1938.
(7) Schorger, A. W., and Shoemaker, M. J., I N D . ENQ. CHEM.,29, 114 (1937).
PRESENTED before the Division of Paint and Varnish Chemistry at theQ7th Meeting of the A meri oan Chemioal Society, B a1 t i m o r e , Md.
