I
IVAN A. WOLFF, DAVID W. OLDS, and
G. E.
HILBERT
Northern Utilization Research Branch, Agricultural Research Service, U. S. Department of Agriculture, Peoria, 111.
Mixed Esters of Amylose Mixed aliphatic esters of amylose can be converted to good quality films and molded products; availability depends upon continued progress in development of special corn hybrids having high-amylose starch T H E PREPARATION of amylose triesters of single organic acids has been discussed (6) and their general similarity to cellulose esters pointed out (5). In considering the favorable properties of mixed esters of cellulose, and the prospect of greater availability of amylose in the form of starches from new corn varieties ( d ) , some mixed esters of amylose were prepared and their properties evaluated.
Materials and Methods Corn amylose was prepared by slight modification of the 1-butanol precipitation procedure of Schoch (3)and purified by recrystallization. It absorbed about 190 mg. of iodine per gram of amylose ( I ) , corresponding to 95% purity. Esterifying reagents used were commercial products purified when necessary by distillation. Per cent apparent acyl content was determined by saponification with 0.25N potassium hydroxide in aldehyde-free absolute ethanol ( 6 ) . Assuming complete acylation of all hydroxyl groups ( 6 ) ,the content of each acyl radical in the mixed esters was calculated algebraically. When one acyl group was introduced first and the partial ester isolated, the amount of acyl substituent found by analysis agreed with the value after analysis of the mixed ester obtained upon complete further esterification with a different acid. Solubilities were taken at 2% concentration. Fusion ranges were determined in capillary tubes. Molding tests were made in a small cylinder mold using a laboratory-size hydraulic press with electrically heated platens. Films were cast on glass plates from chloroform solution, and tested on a Scott IP-2 inclined plane serigraph. Preparation of Esters Whenever one of the ester components was formyl, this group was introduced first, using formic acid. Amylose was formylated at room temperature (2) ; the mono- or diformate could be prepared by controlling time of reaction and formic acid concentration. The resulting formate was soluble in the watercentaining medium used and was isolated before further esterification in a substantially anhydrous solvent system-
pyridine plus the other acid anhydride or acid chloride. A 70% excess over amount of acylating reagent theoretically required to esterify the remaining free hydroxyl groups was used. Benzoylation was carried out for 4 hours a t 80' C. and the other esterifications for 4 t o 6 hoursat110'C. If neither ester component was formyl, the appropriate acid anhydrides were used, successively or in combination, as the acylating reagent. For example, to 24.6 grams (dry weight) of air-dried amylose in 140 ml. of dry pyridine was added 45 grams of butyric anhydride. The mixture was heated for 3 hours at 100" C., then 53 grams of acetic anhydride was added. Reaction was continued for 3 hours at the same temperature. The ester was precipitated in ethanol, washed three times with ethanol, and dried. The fibrous ester weighed 49.1 grams and had 39.4y0 apparent acetyl, corresponding to 1.4 butyryl and 1.6 acetyl groups per 6-carbon unit. If the anhydrides were introduced simultaneously at the beginning of the reaction, the ester isolated after 6 hours weighed 44.4 grams and had 42.9y0 apparent acetyl, corresponding to 0.45 butyryl group and 2.55 acetyl groups per 6-carbon .unit. The only available information concerning the distribution of acyl groups in the mixed esters is that the first formyl group introduced into amylaceous materials was almost exclusively in the six position ( 2 ) . All of the mixed esters were white powders or fibrous solids. Properties of Esters Solubility. Whereas corn amylose triacetate is soluble only in chloroform, pyridine, and acetic acid (5), amylose mixed esters have a greater solubility range (Table I). Replacement of only 0.45 acetyl by a propionyl or butyryl radical renders the amylose ester soluble in acetone, dioxane, nitropropane, and ethyl acetate. The other mixed esters are generally soluble in the ester, aromatic, and nitroparaffin solvents. This increased solubility may make mixed esters tetter film- and fiber-forming materials. Fusion Ranges. The mixed esters
are softened at 145 'to 220 ' C. (Table I). At higher temperatures they were fused over a narrow range to give a clear melt in the capillary tube. The fusion ranges were intermediate between those of the triesters of the single acids. Amylose triformate was unavailable to verify this statement for mixed esters containing formyl. Molding Characteristics. Amylose acetate-propionate and acetate-butyrate are apparently good raw materials for thermoplastic molding, as clear disks prepared over a wide temperature range were easily released from the mold, and were strong and tough. The formatebenzoates discolored excessively on molding. Formate-acetate and formatebutyrate esters have a narrow optimum temperature range for molding, but formed clear, strong disks; without closely controlled temperature the outer portion of the disk will be imperfectly fused while the interior reaches a temperature sufficiently high to initiate decomposition of the esters. Film Characteristics. The amylose acetate-propionate and acetate-butyrate films, although somewhat weaker (Table I) than amylose triacetate film, have more plasticity and do not curl as much. Per cent elongation at break is slightly higher. Strengths are intermediate between those of the amylose triesters of the single acids. The higher plasticity may be useful in applications requiring a flexible nonbrittle film-combined with more favorable solubility characteristics it might make these materials preferable for lacquer-type protective coating materials. Mixed esters of amylose containing the formyl radical form films which are too brittle to be useful. These formyl esters also age rapidly in the solid state. Films prepared from an ester, stored in a screwcap bottle for 6 months, were weaker and more brittle than the film cast from the same freshly prepared ester. The formate-benzoates, diformatemonoacetate, and diformate-monobutyrate formed films too brittle to test. Discussion The amylose acetate-propionate and acetate-butyrate esters seem well suited VOL. 49, NO. 8
AUGUST 1957
1247
Table 1.
Properties of Mixed Triesters of Corn Amylose and Films Prepared from Them
Composition of Ester CS Unit 0.45 butyryl 2.55 acetyl 1.4 butyryl 1.6 acetyl (2 formyl
Amylose Ester 1. Acetate-butyrate
i
2. Formate-acetate
Acyl Analysis, % Acetyl 42.9
Chloroform S
Benzene S
Fibrous
39.4
S
{;$I ; ; : ; 3. Formate-butyrate
1.3 formyl 1.7 butyryl 2 formyl 1 butyryl 2 formyl 1 benzoyl 1.3 formyl 1.7 benzoyl 0.4 propionyl 2.6 acetyl 1.6 propionyl 1.4 acetyl 3 acetyl
4. Formate-benzoate
({ 5. Acetate-propionate
6. Triacetate (6)
Solubilities" -
Physical Appearance Fibrous
1.
2.
3.
4.
5.
e
Diethyl Cellosolve I
S
I
S
S
S
PS
PS
Powder
49.6
S
PS
s
S
PS
I
Fibrousd
47.9
S
PS
s
S
I
I
Fibrous
40.7
S
S
S
S
I
PS
Powder
44.4
S
PS
S
S
I
I
Powder
38.5
S
PS
S
S
I
I
Powder
34.5
S
S
s
S
PS
I
Fibrous
44.0
S
PS
S
s
I
I
Fibrous
41.5
S
S
S
S
1
I
Approx. Opt. Molding Temp., O C.,
Skelly%Nitro1-Butanol solve C Ethanol Acetone propane
Ethyl acetate
Acetic acid Fusion Rangeb, ' C.
6000 Lb./Sq. In.
Film Film Tensile Elongation Strengthc, a t Break, Kg./Sq. M m . 55
I
I
I
S
S
S
S
239-42 (150)
130
5.8
14
I
I
I
S
S
S
S
226-9 (212)
110-20
5.1
18
I
I
I
S
S
S
S
198-210 (166)
130
*..
...
I
I
I
S
PS
PS
S
207-12 (166)
130-5
6.4(4.8)e3/
12 (6)e
PS
I
PS
S
S
S
S
170-80 (160)
120-5
4.6
I
I
I
S
S
S
S
165-90 (145)
110
PS
I
I
S
PS
S
PS
208-15 (181)
140-5
I
I
I
S
S
S
PS
191-225 (165)
I
I
I
S
S
S
PS
280-2 (169)
for practical uses and were similar to analogous cellulose esters. Amylose esters containing formyl are less desirable. An increase in the length of the acyl group Of an amy1ose decreases strength (5)' Esters containing the formyl group should form strong films. The formyl grouping is so small that no plasticizing action occurs; the films containing formyl are brittle and inflexible. Starch and amylose formates are unstable to hydrolysis. Although further esterification with other acyl radicals improves somewhat the stability of the formyl group, the mixed esters containing formyl are not as indicated by the 25% decrease in film
strength of an aged acetate-formate (Table I ) , Acknowledgment The authors are grateful to B. J. Jirsa and to T. A. McGuire for their assistance, Literature Cited Bates, F. L., French, D.,
('1
R. E., J . Am. Chem. Sac. 65, 142-8 (1943); Wilson, E. J., Jr., Schoch, T. J., Hudson, C. S.:Ibid., 6 5 , 1380-3 (1943).
( 2 ) Gottlieb, D., Caldwell,
c. G., Hixon,
R.M., Ibid., 62, 3342-4 (1940).
(3) Hodge, J. E., Karjala, S. A , , Hilbert, G. E., Ibid., 73, 3312-16 (1951).
INDUSTRIAL AND ENGINEERING CHEMISTRY
. . $
12
*..
150
... ...
...
150
5.6
18
5.4 6.2
20
I I I S S S S 247-58 (220) 125 6. S = soluble; I = substantially insoluble; PS = partially soluble. Sintering pt. in parentheses. Referred t o original cross section of film before rupture. Fibrous when first precipitated. Less so after ethanol washes. First value for film cast from freshly prepared ester; in parentheses, value on film from ester aged 6 months. 1.2 formyl, 1.8 acetyl.
1248
__-_
S
Solubilities Amylose Ester
Dioxane
Diethyl Pyridine ether
...
13
( 4 ) LYolff, I. A. Hofreiter, B. T., Watson, P. R . , Deatherage, W. L., MacMasters, M. hi., Ibid., 77, 1654-9 (1955); Deatheraqe, Mi. L., hlacMasters, A I . M . , Vineyard, M. L., Bear, R . P., Cereal Chem. 31, 50-2
(1954).
( 5 ) Wolff, I. .A,: Olds, D. W.! Hilbert, G. E.: IND.ENG.CHEM.43, 911-14 (1951). ( 6 ) Wolff, I. A , , Olds, D . W.: Hilbert, G. E., J . Am. Chem. Sac. 73, 346-9 (1951). RECEIVEDfor review .August 8, 1956 ACCEPTED January 2, 1957 Mention of trade names or specific manufacturers in this paper does not necessarily imply their endorsement over others by the U. S. Department of Agriculture.
