STARCH STUDIES Possible Industrial Utilization of Starch Esters’ JAMES W. MULLEN IIaAND EUGENE PACSU Princeton University, Princeton, N. J.
The properties of starch triesters which might make them industrially useful are discussed. Because of the molecular shape of starch, its esters will probably never produce molded articles of great strength. In molding their use will be confined to that of a diluent and plasticizer for other thermoplastic materials.
H E large quantity of starch available and the low price at which it can be obtained have resulted in considerable research for derivatives with industrially utilizable properties. Reference is made only to some of the more recent work (1, 2, 6, 6, 8-11). Many uses have been suggested for these starch derivatives, including application as thermoplastic moldings, coatings, films, adhesives, sizing and stiffening agents for paper and textiles, and even as flotation agents in ore and mineral recovery processes. The purpose of this paper is to discuss the properties of the starch esters prepared by the process already described (6), and to see if these properties are favorable for commercial utilization of the esters. These derivatives are the full triesters; they are characterized by high molecular weight (from about 500,000 for the acetate to well over 1,000,000 for those containing larger acyl groups) which is due to the freedom from degradation of the native starch under the conditions of the process. I n consequence of these high molecular weights the esters form only colloidal solutions, the ease of dispersion being different for different esters. The viscosities of these ester “solutions” are considerably higher than those for the corresponding cellulose esters, especially a t higher concentrations. The viscosity relations were fully discussed in the previous paper (6). Melting takes place over a range of about 100’ C. Within this range the esters are plastic and workable. How the properties of the esters vary from species to species was shown previously (6),and this point will not be stressed further.
T
Properties of Starch Esters FATTYACID ESTBRS.The triacetate, tripropionate, tributyrate, and tricaproate of starch are all white powders (Figure 1). On application of heat they soften a t progressively lower temperatures with increasing size of the acyl group. The caproate is puttylike at room temperatures, whereas the acetate softens just under 200” C. The melting range
* For the first two papers in this series, Bee literature citations 6 and 6. 2
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Definite possibilities do exist, however, for these esters in the coating, sizing, and adhesive industries. The starch esters will perhaps find their greatest usefulness in two relatively new but important fieldsthe preparation of aqueous emulsions or suspensions of high polymers and the compounding of soft rubberlike plastics.
is wide for all these esters, covering nearly 100’ from the softening point to the formation of a clear melt. They are all insoluble in water. The acetate is insoluble in the lower alcohols and ether, but the other members of the series tend to become gummy in these solvents. Practically all of the ordinary organic solvents swell or colloidize these esters. Dispersion is perhaps best in the aromatic or the halogenated hydrocarbons and in the tertiary organic bases. Ease of dispersion in these solvents decreases with increasing molecular weight (i. e., increasing size of the acyl group) as well as with the natural increase in molecular weight which was observed (6) to parallel increase in granule size from species to species These “solutions” are very viscous, a 10 per cent solution of acetate in acetone showing practically no tendency to flow. However, i t must be noted that the viscosity of these “solutions” depends somewhat upon the length of time they have been allowed to stand after dispersion. For example, immediately after removal from the colloid mill, the 10 per cent solution is viscous but still fluid. After 48 hours the“‘so1ution” has set to an immobile gel. This gel may then again be redispersed. Such behavior is due entirely to aggregation phenomena. ESTERSOF DIBASIC ACIDS. When the anhydride of a dibasic acid (e. g., phthalic anhydride) is reacted with an alcohol, one carboxyl group is set free. The preparation of such esters involving cellulose as the alcohol was discussed by Malm and Fordyce (a). Such esters prepared from starch cannot be precipitated from their pyridine reaction jelly by addition of excess water, since their pyridine salts, formed between the pyridine and the three free carboxyl groups present on each six-carbon unit, are water soluble. The precipitation must be carried out by the addition of the calculated amount of dilute acid. The triphthalate and the trisuccinate of starch are both white powders, insoluble in water of pH 7 or less but dissolving in dilute alkalies. They are soluble in methyl and ethyl alcohol. I n the molten state they have strong adhesive properties. If the succinate is dissolved in absolute methyl alcohol and a few drops of sulfuric acid added, a white precipitate 381
INDUSTRIAL AND ENGINEERING CHEMISTRY
382
scarcely revealed the starch which they contained. Such high molecular weight “rosins” might find application in the varnish industry.
---f
FIGURE1.
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CORN-
Molding Starch Esters An attempt was made to use the starch esters as thermoplastic molding compounds. The work was carried o u t in POWDER a small cylinder mold, a small cup mold, and a X 5 x inch bar mold. The molding operation (compression molding) was performed a t about 200’ C. and 5000 pounds per square inch pressure. Of all the esters molded in the unplasticized state, the butyrate and caproate had the best properties in that they were not brittle. They pared but did not chip when cut cwith a knife. Their surface hardness was, however, low. FIGURE 2. BAR The results of molding the unplasticized esters are given AND SVALL CUP in Table I. MOLDEDFROM The plasticized esters showed more promise. The results CORNSTARCH of the plasticized cornstarch moldings are given in Table 11. TRIPROPION *TE Dibutyl phthalate was more effective than any of the other plasticizers tried. All the esters were molded with this TEXT DIBUTYL plasticizer. Results are given in Table 111. Little if any PHTHALATE difference was observed between the properties of the plasticized acetates of potato, tapioca, wheat, corn, and rice starch. is thrown down in a short time. This is the methyl ester Meyer (4) reported the triacetate of amylose to form much of the starch trisuccinate. A similar reaction is obtained stronger films and fibers than the triacetate of amylopectin. This is to be expected if the amylose is a straight-chain with the phthalate. STARCHTRIBENZOATE. This is one of the few esters compound, free from branching. It would resemble cellulose in behavior in that chains lying alongside of one another which may be prepared from the acid chloride via the could form the strength-giving micellar bonds. The molding pyridine method using no other diluent. The pure ester of amylose acetate prepared by a previously described is white, but as usually prepared it retains a trace of color from the deep orange reaction mixture. I t s properties method (7) was no different in properties from the molding resemble those of the lower fatty acid esters. Most notable made from total starch acetate. This observation is, however, based on only one experiment, involving an amylose is the strong adhesive bond which it forms with metals. sample of doubtful purity. If some of the molten benzoate is pressed between two blocks Two of the molded articles are shown in Figure 2. The of steel and allowed to cool, it becomes exceedingly difficult strength of the bars molded from starch acetate was conto break the steel blocks apart. This bond is even stronger siderably less than that of similar bars of cellulose acetate. after the ester has been suitably plasticized. ESTERS OF ROSINACIDS. The rosin acids (e. g., abietic) All in all, the molding of the starch esters was unsatisfactory. This fact is no doubt due to the roughly spherical shape of contain a conjugated double-bond system to which may be added maleic anhydride. This adduct, possessing an anthe major component (amylopectin) of starch. The evidence for this was previously discussed (6). Spherical molecules hydride grouping, may then be used to esterify starch. (highly branched “clumps”) would not form the large Actually they were obtained by partial esterification of starch number of secondary bonds, the existence of which is rewith the adducts of methyl abietate and diethylene glycol sponsible for the strength characteristic of strictly linear diabietate. Though only partially esterified, the products polymers. resembled the original rosin derivatives, and their properties STARCH PIONATE
TRIPRO-
IN THE FORMOF A WHITE
~
T -\RLE I.
PROPERTIES OF MOLDED STARCH ESTERS TR-\NS-
SGKFACE
Cornstarch acetate Cornstarch propionate Cornstarch butyrate
Yellow-gieen Amber Rosin
Clem Cleai Clear
TACKINESS Yone None Stuck to mold
Cornstarch caproate
Rosin
Clear
None
Glossy
Fairly hard
Cornstarch benzoate Cornstarch phthalate
Orange Tan
Clear Opaque
None Stuck to mold
Glossy Gummy (water sol.)
Hard Soft (hardens in air)
Hosin
Dark
None
Glossy
Hard
Very brittle
Rosin Rosin White
Dark Clear Opaque
None None None
Glossy Glossy Smooth
Hard Hard Soft
Very brittle Very brittle Brittle
ESTER
Cornstarch methyl abietatemaleic anhydride adduct estera Cornstarch diethylene glycol diabietate-maleic anhydride adduct ester“ Cornstarch @-amyloseacetate Tapioca starch acetate a
COLUH
PARENCI
HARDNESS Glossy Glossy Glossy
Hard Hard Fairly hard
These two esters contain only one or less than one hydroxyl substituted per Ce unit.
TOUGHNESS Very brittle Fairly brittle Slightly brittle Very slightly brittle Very brittle Tough, sticky
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March, 1943
Additions of cellulose acetate were found to improve the strength of the starch esters tremendously. In fact, when the starch esters were used as a diluent for cellulose acetate, even up t o 75 per cent of the total weight, the moldings turned out to be quite satisfactory. I n the above discussion the usual rigid type of plastic has been considered ; today, however, soft, rubberlike plastic compositions are becoming of increasing importance ( l a ) .
383
more than 5 per cent solid matter could not be sprayed. Freshly prepared solutions containing more material could be applied with a brush. It would, however, be necessary to lower the viscosity of the ester solutions by some degradative process in order to increase the concentration. When no plasticizer is added to the solution, the coatings formed from the acetates check on standing. Those formed
OF PLASTICIZERS (20 PERCDNT)ON MOLDEDCORNSTARCH ACETATE TABLE11. EFFECT
TRANSNo.
PLASTICIZER
6 6
Dibutyl phthalate Phthalic anhydride Equal parts of 1 & 2 Diethylene glycol diabietate (20% plasticizer incompatible) Equal parts of 1 & 4 Camphor
7
10% No. 1
1 2
3
4
+ 1% No. 6
COLOR
PARENCY
TACKINESS
SURF.4CE
HARDNESS
Yellow-green Yellow-green Yellow-green
Clear Opaque Murky
None Stuck to mold Stuck to mold
Glossy Gummy Smooth
Medium Soft Soft
Green-gray
Opaque
Slight
Smooth
Medium
Tan Olive green
Murky Clear
None None
Glossy Glossy
Hard Hard
Olive green
Clear
None
Glossy
Hard
TOUGHNESS Tough Fairly tough Fairly tough Very tough, a t first, brittle on standing Brittle Tough Fairly tough, cracks on standing
from the propionate and butyrate show less tendency to check. Plasticizers, however, improve them all. Dimethyl, die%hyl, and dibutyl phthalates were found to be most efficient. Tricresyl phosphate is apparently incompatible with the starch esters. The coatings of these esters resemble clear varnishes when applied to wood. Their surfaces are quite glossy. The hardest surface is formed by the acetate, the softest by the butyrate. Paper and cloth coated with the plasticized esters may be crumpled without cracking the coating. This resistance to cracking is impressive. These starch ester coatings adhere well to glass and metals, and seem subject to considerably less shrinkage on drying than similarly applied coatings of cellulose acetate.
Such compositions find use as aircraft sealing compounds, as adhesives, and as substitutes for natural rubber in certain specialty items. The starch esters, suitably plasticized, either as such or compounded with other plastic materials, may become of considerable value in this new development. I n support of this statement it is to be noted that starch butyrate, containing 25 per cent dibutyl phthalate, is quite soft and possesses a relatively high tack. Starch caproate, even without plasticizer, is puttylike a t room temperature.
Fibers Exactly the same shortcomings that exist for the starch ester moldings also limit the usefulness of the fibers formed from them. As long as the fiber is in a hot semimolten state, it is tough, flexible, and elastic; on cooling, however, it becomes very brittle, especially the acetate. Plasticizers iemedy this condition but little. Extrusion of the fibers under tension does not result in a great increase in tensile strength. This is to be expected since there can be no axial o r i e n t a t i o n of a “spherical” molecule, as is the case with fibers made up of longc h a i n molecules (e. g . , r a y o n fibers).
Coatings and Adhesives A1though the unsupported films formed both by evaporation of the solvent from a solution of the ester and by extrusion of the molten ester are brittle when unplasticized and weak when plasticized, the behavior of the starch esters as coatings was far more gratifying. Colloidal solutions of the esters were prepared in the usual commercial lacquer thinners. T h e s e s o l u t i o n s were q u i t e viscous so that those containing
acid conversion
I
I
FIGURE 3. FLOW SHEET FOR STARCH ESTERPREPARATION
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384
OF STARCH ESTERSPLASTICIZED WITH APPROXIMATELY 5 PERCENTMIXEDCA~WPHOR AND DIBUTYL TABLE 111. PROPERTIES PHTHALATE
ESTER Cornstarch acetate Cornstarch propionate Cornstarch butyrate Cornstarch caproate Cornstarch benzoate Tapioca starch acetate
TRANSCOLOR
PARlNCY
Clear Clear
Olive green Olive green Amber Dark amber Orange Lemon
Cloudy Dark Clear Clear
TACKINESS None None None Stuck to mold Stuck to mold None
SURFACE Glossy Glossy Glossy Smooth Glossy Glossy
TOUQHNESS Fairly brittle Fairly brittle Tough Tough
Soft Hard
Brittle
Hard
Very slightly
brittle
Conclusions
4 II I , , . IOUX , PYRIDINEI (25%of total)
Any hope for the industrial development of the starch esters seems to lie in their use:
I
i return
to
process
reactor
T
c
Hard Medium Medium
acid recovery procedures, based on distillation of the aqueous p y r i d i n e - a c i d solution treated with calcium carbonate and then with mineral acid, is illustrated in Figure 4.
still
\L [organic ~
HARDNESS
o
n
v
e
r ACID s ANHYDRIDE ~
1. 2. 3. 4.
As coatings. As adhesives. As diluents for other plastics. In the compounding of soft
rubberlike plastics.
5 . In the preparation of aqueous
emulsions or suspensions,
6. In some heretofore unsus-
pected use stemming from their possession of high molecular weights and shapes which are definitely nonlinear.
RECOVERY FIGURE 4. FLOWSHEETFOR PYRIDINE-ACID
Aclinowledgmen t The bond formed between certain of these esters and pieces of steel, wood, glass, or paper, is quite strong, Most noteworthy in this respect are the benzoate and the dibasic acid esters.
We gratefully acknowledge the financial assistance given by the Research Corporation, of New York, which made this investigation possible. We wish to thank Stein, Hall & Company, Inc., and the Corn Products Refining Company for their generous supply of starches.
Aqueous Emulsions or Suspensions One of the newer developments in plastics technology has been the incorporation of high polymeric materials in aqueous emulsions or suspensions. These “solutions” find widespread use (e. g., adhesives and sealing compounds, to name but two). The molecular structure of the starch esters which results in poor moldings and fibers should show to advantage in the preparation of emulsions and suspensions, the forces producing aggregation and its resultant colloid instability being lower than in long straight-chain polymers.
Preparation of Esters A detailed procedure for the preparation of the different starch esters by precipitation in water of the respective pyridine-starch ester jellies is shown in Figure 3. As to the recovery of the pyridine and the acid anhydrides employed in the procedure, it can be accomplished by several methodse. g., distillation, solvent extraction, ternary distillation of the pyridine azeotrope with benzene, etc. One of these pyridine-
Literature Cited Ellis, C., “Chemistry of Synthetic Resins”, p. 757 et seg., New York, Reinhold Pub. Corp., 1935. (2) Mack, D. E., and Shreve, R. N., IND. ENG. CHEM.,34, 304 (1)
(1942). (3) Malm, C. J., and Fordyce, C. R., Ibid., 32, 405 (1940). (4) Meyer, K. H., Bernfeld, P., and Hohenemser, W., Hdv. Chim. Acta, 23, 886 (1940). (5) Mullen, J. W., XI, and Pacsu, Eugene, IND.ENG.CHEM.,34, 807 (1942). (6) Ibid., 34, 1209 (1942). (7) Pacsu, Eugene, and Mullen, J. W., 11, J . A m . Chem. Soc., 63, 1168 (1941). (8) Radley, J. A , , “Starch and Its Derivatives”, p. 91 et seq., New York, D. Van Nostrand Co., 1940. (9) Seiberlich, J., Modern Plastics, p. 64, March, 1941. (10) Seiberlich, J . , Rayon Teztile Monthly, 22, 605, 686 (1941). (11) Stein, H., 2. Spiritusind., 57, 323, 339 (1934). (12) Wiggam, D. R., Modern Plastics, p. 60, Aug., 1942. A B B T R A C T from ~ D a thesis presented by James W. Mullen I1 to the faculty of Princeton University in partial fulfillment of the requirements for the degree of doctor of philosophy.
