Properties of Cellulose Esters of CARL J. MALM, CHARLES R. FORDYCE, AND HOWARD A. TANNER Eastman Kodak Company, Rochester, N. Y. S A RESULT of extensive commercial use of cellulose acetate in a wide variety of products, such as films, textiles, plastics, etc., the physical properties of this material are quite commonly known. There are now available an increasing number of cellulose mixed esters, the properties of which differ from those of cellulose acetate in a number of ways. The purpose of this paper is to present a correlation of some of the physical properties of this group of cellulose derivatives by graphic representations which include the individual and mixed esters of acetic, propionic, and butyric acids. Cellulose triacetate, the only acetate known before the introduction of partial hydrolysis to acetone solubility in 1905, is a fully acetylated cellulose. The solubilities of this material have not been attractive for commercial use, and it has achieved little success. With the introduction of acetonesoluble products there became available a range of materials varying in acetyl content and thereby in physical properties.
Cellulose esters containing, in addition to acetyl, either propionyl or butyryl groups, offer an additional variable to be considered and thus a further variation in physical properties by virtue of the higher acyl group. It is convenient to represent these cellulose esters graphically on trilinear charts, the three variables of which are the proportions of acetyl, higher acyl, and cellulosic residue. Thus in the accompanying figures, point C represents cellulose, the point a t 44.8per cent along the line CA represents triacetate, and that at 51.8 per cent along the line CP represents cellulose tripropionate; hydrolyzed cellulose acetates and cellulose propionates fall above these points along lines CA and CP, respectively. A line connecting the triacetate and tripropionate points identifies all fully esterified mixed esters of these acids. Hydrolyzed mixed esters fall within the area above that line. In a similar way in graphs representing butyric acid esters, the point a t 57.3 per cent along line CB represents cellulose tributyrate, esters are represented as and fully esterified and hvdrolvzed " " described for the propionic esters. All values are thus plotted directly according to weight composition of C acetyl, propionyl, a n d butyryl as obtained by analysis. Data on these charts were limited to compositions between the di- and triesters, inasmuch as these compounds cover the range of products in standard use. Cellulose acetate propionates and acetate butyrates, in which the properties of h 0 P A =€TICPROPIONIC ESTERS PICETIC-BUTYRIC ESTERS the esters can be changed MELTING POINTS within certain limits to fit special purposes, have found many commercial uses. These uses occur largely where cellulose acetate has proved to some degree unsatisfactory, owing t o its comparatively limited solubility in cheap solvents and limited compatibility with resins and high-boiling plasticizers, its poor flexibility a t low relative humidities and low temperatures, and its low moisture resistance. The factors entering into selection of the " V " V " " V v PCETIC-PROPIONIC ESTER5 ACETIC-BUTYRIC ESTERS proper cellulose esters for SPECIFIC ORb,VlTY specSc uses will become evident from consideration OF COMPOSITION OF CELLULOSE ESTERSOF ACETIC,PROPIONIC, AND FIGURE 1. RELATION of their physical properties. BUTYRIC ACIDBTO MELTINQ POINT (above) AND TO SPECIFIC GRAVITY (below)
A
"
430
,
Acetic, Propionic, and Butyric Acids Physical properties of both the simple and mixed cellulose esters of acetic, propionic, and butyric acids are presented. Variation in properties with changes in acyl group and degree of esterification is shown. The importance of these properties, including melting point, specific gravity, sorption of moisture, and solubility in solvents and plasticizers, in indicating the usefulness of the cellulose esters is considered.
lose derivatives of high melting point are also able to carry comparatively large amounts of plasticizer without serious loss of strength or rigidity. In studying the effect of composition upon the melting point of a cellulose ester, it should be noted that other factors affect this property to a certain degree; for instance, with large fluctuations in viscosity there is a change in melting point, and lower values occur with decreasing viscosity. The amount and nature of the ash content of the ester also influence the melting point. An increase in ash causes a n increase in melting point, especially if the ash consists mainly of alkaline earth metal salts. The melting point is also de8
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Melting Point Variations of melting point with composition are given in Figure 1 by means of lines connecting points of equal melting point value. These variations are somewhat irregular; cellulose acetate shows a lowering of melting point with hydrolysis while esters containing appreciable amounts of propionyl or butyryl have a range of minimum melting point with moderate hydrolysis. Cellulose triacetate has the highest melting point of any of these esters, while very high propionyl or butyryl esters give low values and reach a minimum point with slight hydrolysis. For a great many uses all of these cellulose esters are high enough in melting point to give satisfactory behavior. Unplasticized compositions show very little cold flow except at temperatures approaching the melting points, while addition of plasticizer usually widens the range of cold flow, proportional, in general, to the melting point of the cellulose ester and to the amount of plasticizer employed. For uses which require good plasticity at elevated temperatures, such as molding or heat forming, cellulose esters of comparatively low melting point offer the possibility of obtaining good behavior with comparatively low concentrations of plasticizer. Products within a high melting point range are particularly useful for applications which must withstand high temperatures during use, such as those encountered in ironing of cloth made from cellulose esters, or in cellulose ester insulation for electrical purposes where melting of the insulation would permit breakdown. Cellu-
C
A
A
LI
A
P
E
FIQURE2. PER CENTSORPTION OF MOISTURE BY CELLULOSE ESTERSOF ACETIC, PROPIONIC, AND BUTYRIC ACIDSAT DIFFERENT RELATIVE HUMIDITIES 431
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432 C
C
C
P
B SOLUBILITY IN HALOGENATED HYDROCARBONS
pendent upon stability; aless stable product decreases more readily in viscosity during heating and gives a melting point corresponding to that of a lower v i s c o s i t y ester. The data in this report were collected by determining the melting points of esters which are of medium viscosity, have char points above 280" C., and have been washed with water having a total hardness of 100150 p a r t s per million.
Specific Gravity
A
ACETONE
HYL ISO-BUTYL KETONE
SOL.
so A
0
ACETONE
A
METHYL ETHYL KETONE SOLUBILITY IN KETONES
METHYL \SO-BUTYL KETONE
C
C
C
"
A
"
"
V
"
"
METHYL ACETATE
"
"
40
90L
0
w 0 5 0
50
METWL PCETkTE
0
A
E T M ACETATE SOLUBILITY
3.
Sorption of Moisture
40
SOL.
5
FIGURE
V
ETHYL ACETATE
40
A
"
'
T h e specific gravity of ,cellulose esters varies directly in proportion to changes in composition (Figure 1). This property is important in some uses and is of general interest in that it influences the cost of an article made from the ester. Slight variations in observed values for specific gravities are obtained, depending upon the method of measurement employed. The method used in this work is believed to be sufficiently accurate for general purposes.
sol. BUTYL ACETATE
so B
IN ESTERS
SOLUBILITY O F C E L L U L O S E ESTERS O F ACETIC,P R O P I O N I C , AND BUTYRIC ACIDS I N CHLORIKATED SOLVENTS, IN KETONES, AXD IN ACETICACIDESTERS
One of the most important properties of products made from cellulose esters is their susceptibility t o sorption of mois-
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433
C C C ture and consequent dimensional change when exposed to atmospheres of different relative humidities. In plastiP cized compositions this property is determined by the combined effects of P cellulose ester and plasticizer. Molded articles warp or buckle under such dimensional change, moisture tends to drive plasticizer from the composition, and further p-WTOXYETHYL ALCOCIOL distortion results. SOLUBILITY \N ALKOXY ALCOHOLS Photographic films with low moisture resistance curl at FIQTJRE 4. SOLUBILITY high humidity and OF CELLULOSE ESTERS OF ACETIC, PROPIONIC, thus cause difficulty AND BUTYRIC ACIDS IN during photographic ALROXYETHYL ALCOprocessing, in the HOLS AND IN 1: 1 MIXoperation of amaTURES OF AROMATIC HYDROCARBONS WITH teur moving picALCOHOLS ture cameras, and in flatness of cut sheet film. Textile products with low *( BENZENE-MWOL 01) p ‘ A v p moisture resistance have poor wet strength. Coated fabrics, particularly for uses such as aircraft construction, require low dimensional changes over a wide range of humidity to maintain tautness. Figure 2 shows the influence of ester composition upon mois4o 40 40 ture sorption. Cellulose triacetate is the most moisture ream Lo1 sistant of the acetates. The propionic and butyric esters 60 SO so are superior to acetate in this property when fully esterified, 0 OENZWG-HCTHANOL (I I) and mixed esters containing moderate amounts of these acids A A TOLUENE’ METHANOL (II) SOLU6ILITY IN KYZWCAUEON-ALCOHOL MIXTURES may be hydrolyzed t o a considerable extent before becoming as moisture susceptible as the acetates. Advantages in . toughness of molded articles are to be gained by some degree of hydrolysis. Mixed esters, therefore, may be chosen which have at the same time superior moisture resistance to acetate vent, the solubility decreasing with increasing viscosity. and sufficient hydrolysis for toughness in the resulting ProdFor tests used in determining the fields of solubility in these uct. On the other hand, as the weathering resistance of an graphs, esters of medium viscosity were employed. Also, an ester is not only dependent upon the moisture resistance, but increase in temperature always increases the solubility of these also upon ease of oxidation when exposed to ultraviolet light, esters. it should be pointed out that, of two esters which have the A relation will be noted between the chemical nature of same moisture sorption, the one with less free hydroxyl conthe solvents and the areas of composition of cellulose esters tent has a higher weathering resistance. in which they are active. Certain types of solvents are more effective with fully esterified than with hydrolyzed esters, while others more readily dissolve hydrolyzed products or Solubility in Solvents show maximum solvent power with an intermediate range of The solubility of cellulose esters in organic solvents suitable hydrolysis. I n a given chemical series similar characteristics for technical application is Of prime interest. Solvent cornin the shape of the solubility areas will be noted, solvents of binations designed for specific behavior are required for most increasing molecular weight exhibiting more restricted soluapplications. I n a great many cases cost is a determining bility areas. factor in their use, especially where solvent recovery systems Chlorinated solvents are, in general, more effective with cannot be employed. Figures 3 and 4 show solubility ranges fully esterified than with hydrolyzed cellulose esters. Methylof a variety of solvents for these esters and should aid in selectene chloride is an active solvent Over a wide range of composiing suitable solvent combinations. tion and, by addition of alcohols, forms good solvent mixtures I n using these graphs it should be remembered that the for cellulose triacetate. Ethylene and propylene chlorides boundary between solubility and insolubility is not sharp but will dissolve only esters which contain some propionyl or gradual, and that a change in the viscosity of a cellulose ester butyryl and which have not been greatly hydrolyzed. Addiof definite composition usually changes its solubility in a soltion of lower alcohols to these solvents widens their solu-
loAo
’
a
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C
C
C
A
OCTYL PHTHALATE
A
8 ETHYL PHTHALATE
A
e u w L PHTHALATE
0 OCTYL PHTHALATE
P
6
SOLUBILIT~ IN PHTHALRTE FIASTlClZERS C
C
A
M E T H Y L PHTHALYL E T H Y L GLYCOLATE
’
A
SOLUBILITY
C
E T H Y L PHTHALVL E T H Y L GLYCOLATE
IN P H T H L L Y L
GLYCOLATE
A
BUTYL PHTHALYL B U T Y L GLYCOLATE
PLASTICIZERS
C
C
P
0
SOLUBILITY IN GLYCEROL ESTER PLASTICIZERS
OF CELLULOSE ESTERSOF ACETIC,PROPIONIC, AND BUTYRIC ACIDSIN FIGURE5. SOLUBILITY PHTHALATE, PHTHALYL GLYCOLATE, AND GLYCEROL ESTERPLASTICIZERS
bility ranges to include cellulose acetate; ethylene chloride gives very active solvent mixtures, while propylene chloridealcohol mixtures give cellulose acet a t e solutions which often tend t o “blush” upon evaporation. Mixed esters of 10 per cent or more propionyl or butyryl show good behavior with this solvent mixture. Acetone is a good solvent for hydrolyzed cellulose acetates; it dissolves products within the range of about 36 t o 42 per cent acetyl. Higher ketones are more restricted in their solvent power but may be used as diluents, particularly with cellulose esters near the area in which they are soluble. Methyl acetate exhibits solubility behavior very similar to that of acetone. Ethyl and butyl acetates are less effective, butyl acetate being confined to products of high propionyl or butyryl content and of comparatively limited hydrolysis. Glycol monoethers exhibit interesting s o 1u b i 1 i t y b ehavior, in that they are considerably more effective with hydrolyzed esters than with those which are fully esterified. Ethylene glycol monomethyl ether is a particularly good solvent for hydrolyzed products. Benzene and toluene in mixtures with alcohols
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A
A
TRIPHENYL PHOSPHATE
filled to a height of about 1 inch (2.5 cm.) with cellulose esters to be tested, and were placed in a suitable hole drilled to a depth of 2 inches (5 cm.) along the axis of a cylindrical copper block, 3 inches (7.6 cm.) indiameter and 3 inches high. Other holes in the block were provided for a thermometer and for illumination of the sample. The copper block was heated a t a rate of 5" C. per minute, and the melting point taken as the temperature a t which the sample changed in physical form and became fused. SPECIFICGRAVITY. Films approximately 0.008 inch (0.203 mm.) thick were cast from solutions of the samples in methylene chloride-methanol mixtures. After complete evaporation of solvent, samples of 0.5 to 0.8 gram were cut in the form of strips about 8/le X inch (4.8 X 12.7 mm.). The samples were weighed in air and placed in a 5-ml. pycnometer, which was then filled with heptane and weighed a t 25" C. The density of the heptane was determined in the same pycnometer at the same temperature. The film density, D,,is calculated
C
C
P
A
B
A
P
TRICRESYL PHOSPHATE
SOLUBILITY IN PHOSPHATE PLASTICIZERS
FIGURE 6. SOLUBILITY OF CELLULOSE ESTERSOF ACETIC, PROPIONIC, AND BUTYRIC ACIDS IN PHOSPHATE PLASTICIZERS
offer very cheap solvent mixtures and diluents, and may be used with a wide range of cellulose esters, particularly those of a t least 10 per cent propionyl or butyryl content which have been moderately hydrolyzed. From these characteristics suitable active solvents for cellulose esters of different compositions may be selected, and the possible use of each solvent as a diluent in compositions outside its active solvent range may be estimated.
Solubility in Plasticizers A determining factor in use of plasticizers with a cellulose ester is often its solubility relation, either hot or cold. Some applications depend upon active solvent behavior of the plastiaieer a t room temperature. More frequently, as in molding, in extrusion of sheets, rods, or tubes, or in heatforming operations, plasticizers must be chosen which are active solvents a t elevated temperatures, their solvent power cold being unimportant. Figures 5 and 6 show regions of solubility of the cellulose esters in several related groups of plasticizers at 25" and 180" C. This property is of direct application only to the workability of plasticized compositions and tells nothing as to their durability or aging properties (1). In general, plasticizer retention is better with higher molecular weight members of a group of plasticizers, as compared with the more volatile lower compounds. It is often advantageous, therefore, to use the cellulose esters which are soluble in the less volatile plasticizers; at the same time, advantage can be taken of the lower melting points of the propionic or butyric esters which permit the use of lower concentrations of plasticizer for suitable flow.
Experimental Procedure CELLULOSE ESTERSAMPLES.The cellulose esters were made by commercial esterification methods, precipitated in flake form, and washed with water having a total hardness of 100-150 p, p. m. All products were of average viscosity and good heat stability. MELTINGPOINT. Pyrex test tubes of 2.5-mm. internal diameter, 4-mm. external diameter, and 75-mm. length were
435
from the formula: S X Dh
Dj
=
s + Wl
- w*
where Dg = density of heptane S = sample weight Wl = weight of pycnometer tilled with heptane Wz = weight of pycnometer containing the film sample and filled with heptane All samples were run in duplicate, and the variation between the two determinations was within 0.02 gram per cc. SORPTIONOF MOISTURE. Samples of the cellulose esters were thoroughly dried and weighed in wide-mouthed weighing bottles and placed uncovered in desiccators above suitable aqueous sulfuric acid solutions, adjusted according to data of Wilson (2) to provide the proper relative humidity for test. The desiccators were kept in a water bath and held at 25" C. Weighings were made a t intervals over a period of 7 days, and values which by that time had become constant weights were used to calculate per cent sorption. SOLUBILITY IN SOLVENTS.One-gram samples of cellulose esters were placed in test tubes and covered with 10-cc. portions of solvents. After standing 2 or 3 hours with occasional stirring, solubility was determined by inspection; only those solutions completely uniform and free from graininess were recorded as soluble. SOLUBILITY IN PLASTICIZERS. Plasticizer solubility was determined in test tubes using ten parts of plasticizer to one of cellulose ester. The mixtures were kept at 25' C. for 3 hours and at 180" C. for 1 hour and the solubilities were observed a t each temperature.
Acknowledgment The authors wish to acknowledge the helpful assistance of M. Salo, K. T. Barkey, and B. C. McKussick in preparing the graphs used in this paper.
Literature Cited (1) Fordyoe and Meyer, IND.ENB.CHEM.,32, 1053 (1940). (2) Wilson, R.E.,Zbid., 13,326 (1921). PR~IISNTSD before the Division of Cellulose Chemistry at the 102nd Meeting of the AMBESCANCHDMIICAG SocxmY, Atlantia City, N. J.
