Evaluation of Ethylcellulose by
Load-Elongation Curves
CELLULOSE AND PLASTICS PRODUCTS LABORATORY COMPANY THE Dow CHEMICAL
SHAILER L. BASS A N D T. A. KAUPPI The Dow Chemical Company, Midland, Mich.
upon them. Subsequent papers will deal with the effect of resins and solvents on film properties and with the correlation of the physical properties of molded ethylcellulose plastics with load-elongation behavior.
E
THYLCELLULOSE, though known for a quarter of a century (8), is a comparatively recent addition to the family of cellulose derivatives manufactured in this country (19). The advent of a standardized ethylcellulose of domestic manufacture (3)a t a price made reasonable by largescale production is arousing increasing interest in the field of coating compositions and plastics. The properties of complete solubility in a wide variety of low-cost solvents, of high flexibility and extensibility, of low flammability, of compatibility with a wide variety of plasticizers, resins, and waxes, and of stability to heat, light, alkalies, and dilute acids are combined in ethylcellulose to a degree not found in any other commercially available ceUulose derivative.
Meaning of Load-Elongation Curves When plastic materials are subjected to deformation, curves resembling that shown in Figure 1 are obtained by plotting the deforming load per unit cross section of the test piece against the per cent deformation. The curves for test pieces of a given composition, whether obtained by a tension or a compression test, generally have the same form, although they may express different values. The initial linear rise in curve OA is generally recognized (1,10, 15, 17) to indicate the region of purely elastic behavior in which the material obeys Hooke’s law. As the load increases above A, the type of deformation changes more or less rapidly from that of a purely elastic body to the permanent deformation of a plastic one. This transformation is complete a t point B which, for all practical purposes, can be taken as the lower limit of plasticity (17). The load a t this point is called the “yield point” in subsequent discussions. Further small increments of load are almost entirely absorbed by stretching, indicating a purely plastic behavior of the material. In this region, BC,a realignment of the cellulose derivative from a random arrangement to one of partial orientation in the direction of the force applied begins to take place. The forces of molecular cohesion are thus brought more strongly into play, and an increase in the resistance to deformation, as shown by the portion of the curve from C to D, may result. When the cohesion and slippage ability of the chains are overcome, the test piece breaks (1). The coordinates of D,the end of the curve, are the tensile strength and elongation of the material. The strengthening effect indicated by the portion of the curve from C to D is characteristic of linear polymers. That
Need for Method of Evaluation The utility of ethylcellulose in lacquers, varnishes, films, and plastics (2,-3, 8, 19) seemed t o demand a rapid method for evaluating certain mechanical properties. Individual tests, such as those for impact strength, hardness, abrasion resistance, flexibility, tensile strength, and elongation, are now made with some particular use requirement in view. I t is difficult to determine from any one of these tests the general mechanical behavior of a cellulose derivative, although the behavior of the material in these tests must be a function of a small number of fundamental properties. It was found that curves showing the effect of load upon elongation of ethylcellulose films could be used not merely to define the ultimate tensile strength and elongation of the material but to define its toughness, flexibility, and hardness as well. The present paper is an attempt to correlate these properties and to evaluate the effects that plasticizers have 678
JUNE, 1937
INDUSTRIAL AND ENGINEERING CHEMISTRY
it is due to orientation of the colloidal units in the film has been proved by rotating the stretched film between crossed Nicols (9) or between Polaroid glasses. Highly plasticized films may not show this strengthening effect but may continue to stretch without increase in resistance to D’.
$3 1
J&
n
679
films can be considered relatively brittle since they break a t low deformations.
Flexibility The flexibility of a film coating is usually tested by bending sheets of metal coated with the film over mandrels of varying diameters and observing the formation of breaks. This is actually a measure of the ability of the coating to stretch, but is complicated by the factor of adhesion (14). Ordinarily the flexibility of a stripped film is determined by the number of folds it will undergo in the folding-endurance testers in common use. In comparing folding-endurance tests on films with widely varying yield points and elongations, however, anomalous results were obtained.
CELLULOSE DERIVATIVES
!
8 1
I
I
I
1
i
FIGURE 1
The point on curve OABCD a t which a film of a cellulose derivative will break depends upon the particular derivative tested, its viscosity ( I ) , its degree of substitution (4), and its conditions of manufacture. Low-viscosity nitrocellulose films from the */4- and I/rsecond R. S. types and films from lowviscosity cellulose acetate and acetobutyrate mere found t o break between A and B. The load-elongation curves of unplasticized, medium-viscosity nitrocellulose, cellulose acetate, cellulose acetobutyrate, and ethylcellulose are shown in Figure 2. It is apparent that the cellulose nitrate and acetate films have a higher tensile strength than ethylcellulose but have little ability to stretch. The curve for ethylcellulose indicates its ability to undergo a large amount of plastic deformation and to increase its resistance to the deforming force. This behavior characterizes it as a very tough material (IS, 17). Toughness is best measured by the total work required to break the film, as indicated by the area under the load-elongation curve. Therefore the toughness of a material is influenced by the height of the yield point, by the total elongation, and by the strengthening effect. The small areas under the curves for the cellulose esters indicate that they are inherently much less tough than ethylcellulose. Hence, the cellulose ester
The mechanical properties of ethylcellulose films are evaluated by their load-elongation curves. Yield point is recommended as a means of measuring film hardness which is unaffected by the complications in existing methods of hardness measurement. The hardness of ethylcellulose films is unaffected by the viscosity grade used ; flexibility and toughness, however, increase with the viscosity. The behavior of six representative plasticizers for ethylcellulose was determined from load-elongation curves on films with varied plasticizer contents, and several types of plasticizer
OF CELLULOSE DERIVATIVES
FIGURE 2 The relation between the behavior of the film in thefoldingendurance test and its performance in the tensile-elongation test was pointed out by Sheppard and Carver (15). This is illustrated as follows: When a test strip of ethylcellulose film is stretched beyond the elongation a t its yield point to some point on the secondary rise of the curve, the load removed, and the load-elongation curve to the ultimate breaking point then determined, it will be found that the second curve has
behavior have been established. From data on yield points, total elongation, and ultimate tensile strength on ethylcellulose films a t 15 and 40 per cent of plasticizer, the influence of eighteen commercial plasticizers on film properties is compared. Volatility of the plasticizer from the heated film is estimated by means of the change in yield point of the load-elongation curves on the films after heating. Plasticizers are indicated for various specific uses with ethylcellulose, and conceptions of the probable function of a plasticizer in an ethylcellulose film are correlated.
INDUSTRIAL AND ENGINEERING CHEMISTRY
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a higher yield point and less total elongation than a curve determined on the unstretched film. In other words, during the f i s t load application the ethylcellulose chains were oriented by stretching, and the film was made slightly less tough. If
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used directly to compare their hardness, since they occur a t a relatively low and almost constant deformation. The relation expressed by Equation 1 then becomes : hardness = K X yield point (2) The yield point on the load-elongation curve is recommended as a convenient method of measuring hardness. Yieldpoint hardness is the load a t which a section of the plastic material begins to undergo permanent deformation most rapidly. Like most hardness tests, it is influenced by the manner in which the test piece is prepared and the conditions under which the test is made. With a little care in preparing films of the same thickness and in measuring the thickness, duplicate determinations will check within 2 to 3 per cent. Yield-point hardness is believed to measure actual hardness of the plastic. It is not complicated by the toughness of the film or plastic as in the scratch or indentation hardness tests, or by the hardness of the substrate as in the Pfund hardness test. With a little experience in the feel of the resistance of films of varying yield point to indentation by the thumb nail, one can make comparisons of the hardness or softness of a series of compositions; these will be found to agree closely with the comparison made by yield-point hardness.
Comparison of Cellulose Derivatives the 6lm is stretched repeatedly beyond its yield point, it will become more and more brittle and finally will break with little elongation. I n agreement with the common flexibility test for lacquers, and with the definition of flexibility as the ability to undergo deformation, the ultimate elongation will be considered the measure of flexibility.
Hardness Since hardness is measured by the resistance offered to deformation, a comparison of the hardness of cellulose derivative films may be made by means of their load-elongation curves. At low deforming forces, hardness is characterized by the slope of the elastic portion of curve OA, Figure 1 (12): hardness
=
load K deformation
At greater deforming loads, hardness is dependent on the yield point, Y , since this is the load a t which deformation becomes appreciable. The yield points of cellulose derivatives can be
FIGURE 4
The properties of the unplasticized cellulose derivatives, as interpreted from their load-elongation curves in Figure 2 may be compared as follows: The cellulose esters form hard, brittle films. The cellulose ethers form softer, tougher, and more flexible films. The ultimate tensile strength of ethylcellulose does not differ from that. of cellulose acetate or nitrate as much as would be expected from their difference in yield points, since the strengthening effect of the highly plastic ethylcellulose increases its tensile strength considerably. If the tensile strengths had been calculated on the actual cross section of the film a t the time of the break, the tensile strength of the ethylcellulose would be approximately equal to that of the esters. The observed differences in yield point and ultimate elongation between the cellulose derivatives are in agreement with the conception of Houwink (6) that the condition for high elongation is the presence of chains bound internally by strong primary bonds but to each other by weaker secondary valences. The weaker the secondary valences, the lower will be the yield point and the greater the ultimate elongation. The secondary bonds of the cellulose ethers are weaker than those of the esters. Thig accounts for the lower resistance of ethyl-
FIGURE 5
JUiiE. 193i
INDUSTRIAL AND ENGINEEKIXC; CIIEMISTK Y
cellulose to plastic deformation and also for its greater ability to withstand deformation without rupture since greater relat.ive slippage of the particles is possible.
681
tion of viscosity to elongation and tensile strength has been noted for nitrocellulose by Blom (f), Jones and Miles (S),and others.
Preparation and Testing of Films The use of load-elongation curves for comparing the properties of cellulose derivative films presumes that the test films be all of the same uniform dimension, free from dirt and residual solvent, and cast and conditioned in the =me manner. The films u*ed were prepared hy casting a uniform thickneis of a solution conbaining about 15 per cent by weight of thecellulose derivative in a suitable mixture of low-hoiling solvents on a clean pieoe of plate glass. The solvent for ethylcellulose was oomuosed of 80 nwts toluene and 20 Darts abso1ut.e ethanol bv voluine. Tho firm deposited was 10 c;ii. wide and 0.04 * 0.002 mm. thick. It was allowed to dry on the plate overuight., rcmoved from the plat,e, heated one hour at 70" C., and conditioned a t 20" C. and 50 ner cent relative humidity far at least 2 days. At least five test &rips were cut from each film with a razor blade, using a rectangular template 12.85 mm. vide. The test strip wag clamped in a Scott tensile strength tester' YO that the length of the film between the jaws N&S 100 mm. With the 5-kg. weight on the load arm, the lower jaw was pulled downward uniformly at the rate of 25.4 mm. per minute. As the fiLn waa pulled down, it lifted the weight on the load arm, and an evw increasing load vas applied ju.t 5s in the SchopEr tensile tester. The elongation was read direct from a scale graduated in millimeters, and the load a t a given elongation was computed to kilograms p a square centimeter of original cross section. Sheppard (16) showed that, with a given weight on the load am, the yield point of a nitrocellulose film depends upon the rate a t which the load is applied. At high loading rates the elastic portion of curve OA (Figure 1) is longer and the yield point Y is higher, but the film breaks before it reaches the secondary rise of curve CD. At a constant rate of pull, the loading rate per unit of cross section depends upon the fiLm thickness. The film thickness of 0.04 mm. arid the slow rate of pull chosen for the present work permitted a complete loadelongation curve of a test strip to be obtained iii 1 to 3 minutes. Another important factor in the choice of the 0.04-mm. thickness was the ease of ridding the 6lms of residual solvent (18. Jones and Miles (6) iised a "reduced" tensile strength value based on the actual cross section of the breuking point of the film and corrected for the amount of plasticizer present. This did not seem to be required in the present work since the mechanical behavior of the films seemed to be sufficiently characterized by the ordinary method of expressing the load.
Effect of Viscosity on Film Properties The effect of theviscosity gradeof theethylcelluloseusedon the yield point of the unplasticized film was found to be very slight. The effect of viscosity upon the elongation, however, is quite pronounced, as Figure 3 shows. The curve represents approximately the elongat.ions and ultimate tensile strengths of 6lms from the various viscosity grades of domestic ethylcellulose, An ethylcellulose of 20 centipoises or lesslwill have a tensile strength of 450 kg. per sq. em. and an elongation of 10 per cent or less. A medium-viscosity gr'ade will have a tensile strength of 500 to 600 kg. and an elongation of 16 to 25 per cent. High-viscosity grades will sliow a tensile strength of 600 to 700 kg. and an elongation of 25 to 40 per cent. This leads to the conclusion that the tensile strengths of samples of a given cellulose derivative are dotermined largely by their ability to stretch. The greater tlip ability to stretch, the higher is the degree of orientation developed and the greater the tensile strength. In general, greater ability to stretch is a property of thematerials of higher viscosity. A similar rela2
Model x-6. Henry L. Scott Company. Providence, R. I.
2
Five per cent solution in 80-20 toluene-ethanol.
~TEY1,CELLTlLOGE
Filins from ethylcellulose of widely varying viscosity showed the same yield point, as represented in Figure 3. A similar observation for nitrocellulose was made by Blom ( 1 ) . This indicates that the hardness of ethylcellulose compo& tions should be independent of the viscosity grade. Actually, hardness measured by the scratch or indentation tests will increase with tho viscosity grade used, because of the greater toughness of the ethylcellulose of higher viscosity. The property of elongation, however, is also niarkedly affected by the solvent used in casting the film, by t.he presence of other film constituents such as rosins and plasticizers, and by the conditions of preparatioii of the derivative.
Choice of Plasticizer The choice of a plasticizer is completely dependent upon the use to be made of the product. lZlany properties of the composition other than its mechanical properties influence the selection. For example, lucqnor films must possess flexibility, hardness, and toughness, but must also adhere to the surface. Fabric coatings require high flexibility without undue softI ~ Sbut , may also have to be resistant to the action of various agents such as water, fire, sunlight, gasoline, alcohol, or alkalies. Transparent films must he flexible and have high tensile strength, but high dielectric strength and impermeaLility to inoisture are often desired in addition. Plastics should have good flow under molding conditions, but the molded product should be hard and should have high impact strength and toughness and a varied amount of flexibility depending on tire use. The effectof plasticizers on the tensile strength, elongation, toughness, flexibility, and hardness of ethylcellulose compositions ma>' be evaluated from the load-elongation curves on films of varied plest.icizer content.
Behavior of Solvent Plasticizers One of the most commonly observed types of plasticizer behavior in ethyleellulose compositions is exhibited by dibutyl phthalate. Load-elongation curves of films containing in-
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INDUSTRIAL AND ENGINEERING CHEMISTRY
FIGURE6
used, but a t these concentrations the films were very weak, extremely soft, and somewhat greasy to the touch. The effect of n-butyl stearate on yield point and elongation is believed to be characteristic of the behavior of nonsolvent plasticizers for ethylcellulose. Solubilitv tests indicated that ethylcellulose was neither soluble in n-butyl stearate nor swelled by it. Insolubility is further indicated by the fact that it would "sweat out7' on heating. This was observed when films in which the n-butyl stearate content was 40 per cent of the ethylcellulose were heated to 70" C. I n this respect the behavior of n-butyl stearate in ethylcellulose films may be said to resemble that of castor oil in nitrocellulose films.
Castor Oil The behavior of castor oil in ethylcellulose films presents atmarked contrast to that of n-butyl stearate. The loadelongation curves of ethylcellulose plasticized with castor oil are shown in Figure 9. A progressive increase in elongation is observed with increased plasticizer content without a n unusual decrease in yield point. The elongation effect with increasing concentration resembles that observed with diphenyl phthalate and diphenyl mono-o-xenyl phosphate, but the actual elongations a t the higher plasticizer contents are greater. The yield points indicate a hardness of castor-oilplasticized ethylcellulose films approximating that obtained with dibutyl phthalate or tricresyl phosphate. The comparatively high tensile strengths shown in Figure 9 are a result of the ability of the castor-oil-plasticized films to stretch and, in stretching, to develop resistance to stretching. This is shown by the secondary rise in the curves for the films with the lower contents of castor oil. The contrasting behavior of castor oil and n-butyl stearate as plasticizers is also observed in their solvent power. Ethylcellulose will not completely dissolve in cold-pressed castor oil but is strongly swelled by it. Mutual solubility is also indicated by the fact that films in which the castor oil content was 100 per cent of the ethylcellulose did not "sweat" when heated to 70" C.
Effect of Ethylcellulose Viscosity I n the discussion of Figure 3 it was noted that as the intrinsic viscosity of the ethylcellulose is increased a progressive increase in elongation of the unplasticized ethylcellulose films occurs, without appreciable change in the yield point. These effects were also found in plasticized ethylcellulose films. The load-elongation curves of Figures 10, 9, and 11
683
FIGURE 7
show the effect of castor oil on the properties of films made from low-, medium-, and high-viscosity grades of ethylcellulose, respectively. The increase in viscosity of the ethylcellulose used did not appreciably affect the yield point of films of the same plasticizer content. The hardness of plasticized films, like that of unplasticized films of the derivative, is therefore independent of the viscosity grade used. The curves show that the principal result of using ethylcellulose of higher viscosity is an increased elongation or flexibility a t the same plasticizer content. They also show that the higher the viscosity of the ethylcellulose the greater is the increase in tensile strength and toughness which accompanies the strengthLOAD
~ T E ~ A R A T' E
b~
EFFEICT AMOUNT' OF'N-BUTYL ON ETHYL CELLULOSE FILM PROPERTIES
% ' -!
0
I
I
1
1
I
I
l
16
I
I
l
I
I
I
24
/
1
/
I
l
I
l
~
1
I
~
32 40 48 PER CENT ELONGATION
I
5e
1
FIGURE 8
ening effect. This strengthening effect was observed with films containing large amounts of plasticizer only when highviscosity ethylcellulose was used. The medium-viscosity ethylcellulose used to obtain the curves of Figure 9 was also used in obtaining the curves for the other plasticizers, since it seemed to show plasticizer differences most clearly.
Comparison of Plasticizers Having evaluated the effects of several representative plasticizers upon the mechanical properties of ethylcellulose films, it did not seem necessary to run load-elongation curves
. VOL. 29, NO. 6
INDUSTRIAL AND ENGINEERIKG CHEMISTRY
684
shown the second indicates the effect on e l o n g a t i o n a t the h i g h e r COnCentration. A l t h o u g h toughness is measured exactly by the total area under the loadelongation curve, it may be i n d i c a t e d approximately by the product of the yield point and the total elongation. Plasticizers in Table I which yield t h e t o u g h e s t ethylcellulose c o m positions are characterized b y a high hardness index and a flexibility rating of A .
TABLEI. PROPERTIES OF PLASTICIZED ETHYLCELLULOSE FILMS Plasticizer None
-15% PlasticiseraYield Elonga- Tensile point tion strength Kg./sq. em. % Kg./sq. cm. 445 31 635
YieldPoint Hardness 100
-40% PlasticizeraYield Elonga- Tensile Flexibility point tion strength Rating Ko./sq. cm. % Kg./sq. om. 445 31 635 A
Triphenyl hos hate Tricresyl pgospxate Diphenyl mono-o-xenyl phosphate Monophenyl di-o-xenyl phosphate Tri-o-xenyl phosphate
310 345 360 410 430
32 37 40 40 30
420 500 550 560 540
81 92 97
110 120 145 220 275
Diethyl phthalate Dibutyl hthalate Diphenyyphthalate Dimethoxyethyl phthalate
325 335 375 295
34 40 40 35
460 495 560 420
73 75 84 66
(300)b 95 170 100
Methyl phthalyl ethyl glycollate Ethyl phthalyl ethyl glycollate Butyl phthalyl butyl glycollate o,p-Toluene sulfonanilide Ethyl o,p-toluene sulfonamide
280 285 335 410 355
30 40 25 32 35
365 420 420 575 530
63 64 75 92 80
25 550 470 Methyl abietate 30 540 Hydrogenated methyl abietate 425 Castor oil (AA cold-pressed) 335 40 500 n-Butyl stearate 265 25 320 a Plasticizer content in % of ethyl cellulose. b Values too high because of excessive plasticizer volatility.
107 95 75 60
70
77
20 30 45 50 35
A-B A A B
'p ' y y 55 28
275 125
110 105 100 270 105
30 20 32 45 40
130 105 140 450 155
205 195 110 80
35 52 40 32
280 310 155 105
FIGURE 9
for all concentrations of other plasticizers. From Figures 4 to 9, inclusive, it appeared that load-elongation curves on films in which the plasticizer content was 15 and 40 per cent of the ethylcellulose taken would characterize sufficiently the effect of the plasticizer. The data on yield point, total elongation, and ultimate tensile strength of the medium-viscosity (75centipoise) ethylcellulose films having these plasticizer contents is shown in Table I. The data for the films made with 15 per cent of the plasticizer may be used to estimate its value in common lacquers; the data for the films containing 40 per cent plasticizer may indicate its utility in more flexible coating compositions. Relative softness of the plasticized film is measured by the reduction in yield point from that of the unplasticized film. For purposes of comparing the softening effect of the plasticizers, a yield-point hardness index, calculated from the ratios of the yield point of plasticized to unplasticized film, is shown in the fifth column of Table I. Relative flexibility is indicated by comparing the total elongations a t 15 and 40 per cent plasticizer content. The plasticizers may be classified by their effect on elongation in the two concentrations. The flexibility rating given in the last column of Table I provides a means of indicating plasticizers of similar behavior. A rating of A indicates a n elongation greater than that of the unplasticized ethylcellulose. A rating of B indicates little or no change in elongation and C indicates a reduction in elongation. Where two ratings are
B-C
110 150 190 330 360
B
A- C A B
B
A-C C-B B-A A B
B-A A C-B
FIQURE 10 OF PLASTICIZED ETHYLCELLULOSE TABLE11. PROPERTIES FILMS -4FTER HEATING. 2 WEEKS AT 70" c.
Plasticizer Blank Triphenyl hos hate Trioresyl ptospxate Diphenyl mono-o-xenyl phosphate h4bnophenyl di-o-xenyl phosphate Tri-o-xenyl phosphate
15% Foil Heated 2 Weeks a t 70' C. APP??X. Yield ElongaTensile Plasticizer point tion strength Loss Ko./sa. cm. % Kg./sq. em. % 470 31 635 0 330 25 390 0 360 35 500 0 365
35
495
0
450 460
35 35
580 575
0 0
420 380
520 590 570 540
60 20 0 50
Diethyl phthalate Dibutyl phthalate Diphenyl phthalate Dimethoxy ethyl phthalate
400 390
30 45 35 35
Methyl phthalyl ethyl glycollate Ethyl phthalyl ethyl glycollate Butyl phthalyl butyl glycollate
430 400 355
20 30 25
480 480 400
60 0
o,p-Toluene sulfonanilide
440 470 550 510 326 345
30 25
550 525
10 90
4 4 38 25
550 510 480 405
.. ..
Ethyl-omtoluene
~;~b~w$~,e,"tt;ethyl abietate Castor oil (AA cold-pressed)
80
0
30
Accelerated Aging of Plasticized Films The effect of aging on the properties of plasticized cellulose derivatives is the result of a combination of influences. The
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JUNE, 1937
685
It is evident, therefore, that the solvent. power of a relatively volatile plasticizer has great influence on its volatility from the film.
plasticizer may evaporate and slowly cause the film to become harder, as a n increased yield point will show. The plasticizer may decompose and cause degradation of the film, as a sharp decrease in elongation without a n appreciable change in yield point will indicate.
Aromatic Phosphate Plasticizers The aromatic phosphates were characterized by their lack of volatility from the ethylcellulose films. As shown by the yield points and elongation data in Table I, the diphenyl mono-o-xenyl phosphate and monophenyl di-o-xenyl phosphate impart unusually high flexibility without unduly softening the films. Where tri-o-xenyl phosphate was used, the films possessed unusual hardness even a t high plasticizer concentrations. Since these aromatic phosphates are good fireproofing agents, they should find use in the preparation of completely f%eproof lacquers which possess a hardness and flexibility not obtainable with tricresyl phosphate.
Phthalate Plasticizers Dibutyl and diphenyl phthalates are among the best plasticizers for ethylcellulose. Dibutyl phthalate is to be recommended because it is a good solvent for ethylcellulose, is stable to light, and imparts high flexibility to ethylcellulose films a t low concentrations. Diphenyl phthalate exhibits better flexibility, particularly a t higher concentrations in the film. Since it is nonvolatile and moisture resistant, and produces harder films than dibutyl phthalate, it is considered to be more suitable for general lacquer use, especially when a very durable film is desired. The amount of diphenyl phthalate that can $e used, however, is limited to some extent by its tendency to crystallize out of the films at very high concentrations. Diethyl phthalate was found to be much too volatile for ordinary use. The low solvent power of dimethoxyethyl phthalate is believed to account for the softness and low flexibility of the films containing it.
FIGURE11
Investigators of plasticizer volatility have used the evaporation rate of the plasticizer a t elevated temperatures, the weight loss of the plasticized films on heating (7, Is), and the change in the load-elongation curves on heating (19). The latter method was chosen for this work, since the comparative evaporation rates of placticizers alone do not agree with their rate of evaporation from films containing them because of differences in solvent power of the ethylcellulose for the various plasticizers. To determine plasticizer volatility, ethylcellulose films with 15 per cent plasticizer were hung in a n oven a t 70" C. for 2 weeks. They were allowed to condition several days a t 20" C. and 50 per cent relative humidity before testing. The loadelongation curves on heated films containing dibutyl phthalate are shown in Figure 12. By comparison with the curves for different concentrations of dibutyl phthalate in Figure 4, the volatility of the plasticizer can be estimated b y the rise in yield point of the heated film. The rise in yield point is a definite criterion of plasticizer volatility, since the yield point is the property of the film most consistently affected by plasticizer content. The properties of the heated films are listed in Table 11. The approximate loss of the dasticizer from the film was calculated-from the yield-pointdata of Tables I and I I * b ythe ratio : (Ys
-
Yp )
-
Y B
(YHB Y P
-
-
YHP)
x
100 =
Ethylcellulose films plasticized with castor oil have been shown to possess unusual flexibility and toughness, particularly a t the higher plasticizer concentrations. Castor oil was retained in the films on heating, and the f3n-1properties were found to be unchanged. These characteristics indicate that castor oil is a desirable plasticizer for the preparation of ethylcellulose coating compositions where extreme flexibility is desired.
% volatility
yield point of unheated a m containing noplasticizer Yp = yield-point plasticized film, shown in Table I Y H B= yield point observed on heated film containing no plasticizer Yap = yield point of heated plasticized film which had an original yield point of Yp
where Yg
Castor Oil
=
Influence of Solvent Power on Plasticizer Volatility n-Butyl stearate was found to evaporate more rapidly from the ethylcellulose film than dibutyl phthalate, although the latter was recently reported to be the more volatile (18). Solubility tests show that dibutyl phthalate is a good solvent for ethylcellulose but that n-butyl stearate scarcely swells it.
'
z
7 5 CPS. VlSCOSlTY D UNPLASTICIZED FILM, NO I 5 15% DIBUTYL PHTHALATE E PLASTICIZED FILM. NU. 3
2 WEEKS I UOUR I WEEK
P
2 WEEKS
PLASTICIZED FILM, NO. 3
10
FIGURE 12
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Resin Plasticizers
Acknowledgment
When the films containing methyl abietate and hydrogenated methyl abietate mere heated over a long period, a rise in yield point and a sharp decrease in elongation were observed. These effects are not tlie result of plasticizer vo1at.ilityor degradation of the ethylcellulose, but appear to be due to heat polymerization or “setting“ of these rosin esters. In any ease, heating caused them to behave more like hard resins than plasticizers. The effect of resins on the inechanieal properties of ethylcellulose films will he the suhject of a later paper.
The authors wish to thank W. R. Veaaeyand W. R. Collings for their interest which led to permission to publish the results of this work.
Discussion From the theoretical coiisideratioris of fihn structure advanced by another investigator ( 5 ) , the behavior of a plasticizer depends upon its aetioir as a lubricant between tlie cellulose-derivative chain molecules and upon the relative strength of the attractive forces between the molecules of the plasticizer and the chains of the cellulose derivative. When a solvent plasticizer is used, the attractive forces between an ethylcellulose chain and a plasticizer molecule are greater than those between the chains themselves. E I only a small amount of plasticizer is present, it is thought to act as a lubricant between the particles without appreciably veakening the secondary bonds; thus the ability of the chains to slide past each other is increased slightly without decreasing the yield point of the fihn. As the amount of plasticizer is increased, the chains of ethylcellulose become more completely surrounded by the plastic semifluid miaterial. This serves to separate the chains and to decrease the strength’of the attractive forces between them. Thus the yield point and the elongation increasingly become functions of the attractive forces hetween the plasticizer molecules atid also of the rnolecular complexity and viscosity of the plasticizer. These conceptions of the function of a plasticizer in ethylcellulose films are corroborated hy the following observations from the experimental data:
Literature Cited (1) Blom, A.
V.. Kolloid-2.. 61, 235-9 (1932); 65, 223-.8 (1933).
(2) DeBelI, J . M . , Chem. & Met. Eng., 44,8 1 (lQd7). (3) Dow Chemical Co., “Ethooel,” 1937; Hercules
Powder Co.. “Ethyl Cellulose.” 1937. ( 4 ) Gloor, W.E., IXD. Ex*. CHESS., 27, 1163 (1935) (1935). (.5,) Houwink. 1 R X !193R). Houwink,. Kotloid-Z.. Kolloid-Z., 77, 183 (1936). ~~.77. i ~ ~ ~ ~ , (6) Jones. G. G. C.. and Miles. F. D.. J . SOC.Chem. Ind., 52, 25ST ilW.1, (1933). ~ ~”--,. (7) Kraus. Farbe U . Lack, 1936,243. (8) Lilienfeld. Leon, Ikitish Patont 12.854 (1912); U. S. Patent 1,188.376 (1916). MoNalIy. J. G..and Shepperd, S.E., J . Phys. Chem., 34, 170 (9) McNalIy. i~..--,. iwm (10) Miinzinger, W. M., “Tochnologie der \VoiohmeohunpsniitteI:’ pp. 9-15, 82-97, Munich. J. F. Lohmanns Verlag. 1935. (11) Rou. M.. and Eichinger, h..Zurich Congr. Intern. Assoc. Tesliny Afateiials, 11, 542 (1931). (12) Rundle, G.W . , and Norris. W.C.. Pioc. Am. Sac. Testing M a terials. 26. Pt. 11, 548 (1926). (15) Sehob, A., Zurich Ccongr. Intern. Bssoc. Testing Materiala. 11. 560, 572: Sschr. G.. Ibid., 11. 548; Seely. F. H . . Ibid.. 11,
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S.E., and Csrvcr, E. K., J . Phys. Chem.. 29, 1253 (1925); ClOment and Rivibre, “La cellulose et les Bthcrs eellulosiuuea.” Paris. Ch. 1Unmeer. 1930. (16) Sheppard, 8. E., Carver, E. K., and Sweet, S. S.,IND.ENS. (15) Shoppard,
CUPM 1 A 77 Ii9761
(17) Theodorides, Ph., Zurich Congr. Intern. dssoc. Testing Materiels, 11. 667 (1931). (18) Van Henckeroth. A. W., Natl. Paint, Varnish 1,ncquor h s o o . . Circ. 485 (July. 1955). (19) YTiggam, D. R., i o i i e r n Plastics, 14,31 (1936) Rncmueo l p r i l 20. 1037. Presented 8 8 part of the Syrnpusium on Organic PlSstieS beiore the Division of Paint nnd Varnislr Charnistrv st the 93rd Meeting of the American Chemical Society Chapel Will, N. C., April 12 to 13, 19337.
1. The better the solvent power oi the plasticizer, the higher the yield point of the plasticized film. This is illustrated by the fact that the yield points of curves from dibutyl phthalate, dimethoxyethyl phthalate, and n-butyl stearat,e decrease in the same order as their solvent power for ethylccllulose. 2. Increasing the molcculer complexity in a given plasticizer acries raises the yield point of the plasticized film. This is illustrated by the series of phosphates in which the phenyl groups of triphenyl phosphate are successively replaced by 0-xenyl groups. The yield points of the films increased directly with the number of more complex groups in the molecule. A similar effect is noticed when diethyl and dibutyl phthalates are compared with diphenyl pht,halate, when toluene ethyl sulfonamide is compared to toluene sulfonanilide, and in the series of the alkyl phthalyl glycollates. 3. Plasticizers of low solvent power produce films of low flexibility at relatively small concentrations of plasticizer. St high concentrations of 8. plssticizer, flexibility seem8 to depend more on molecular size or complexity than on solvent power. 4. The higher the viscosity of the ethyleellulose,the greater is the flexibility obtained with a given plasticizer, and the greater is the secondary strengthening effect on stretching. This wa.. illustiatad by the curves for eastor oil shown in Figures 9, IO. and 11.
The effect of the molecular complexity and solvent power of the plasticizer may be obscured if the chain 1engt.h of the ethylcellulose is increased. With high-viscosity ethylcellulosc (150 oentipoises or more) the variations in the amount of stretching obtained with the different plasticizers become less significant. These indicate that the greater the chain length, the more plasticizer is needed to separate the chains to such an extent that thrir cohesive forces arc prevented from acting.
Couileay. du Ponl Cornpony CONDENSER USED WITB FRACTIONATING STILLFOR SEPNCATINQ SOLYENTB AT TBE E. I. DU Porn DE NEMOURS Q COMPANY PIANT, DEEPWATER POINT. N. J.
STILL
