MECHANICAL PRQPERTIES OF FILMS FROM
Amylose, Amylopectin, and Whole Starch Triacetates ROY L. WHISTLER AND 6. E. HILBERT Northern Regional Research Laboratory, U. S. Department of Agriculture, Peoria, 111. TARCH is an abundant Amylose and aniylopectin triacetates differ greatlj in triacetate was reported as and low-cost high polyfilm-forming ability. Amylose triacetate readily forms being soluble in acetone. All specimens of amylose triacemer, but has not been high-quality films having good tensile strength and pliautilized industrially as a raw bility ; amylopectin triacetate resembles whole starch tate prepared in this laboramaterial for the production triacetate in forming only weak, brittle films. Adequate tory, however, have been of films, fibers, or plastics. plasticization of the amylose triacetate films can be found acetone insoluble.) T o evaluate the factors inFilms havingtensilestrengths accomplished by the addition of 10-20% of plasticizer of of 4-6 kg. per sq. mm. can be the types employed with cellulose acetates. In general, fluencing film formation of formed from gelatinized the properties of the amylose triacetate films are similar starch acetate, a thorough study of the properties of films to those of cellulose triacetate films. Because of their starch (9),but such films are produced from its individual unsuitable for most industrial high quality and low plasticizer requirements, amylose applications. Although the triacetate films appear well suited to industrial uses. comnonents seems desirable. This article deals with the flms are pliable when their production and properties of moisture content is high, at films prepared from the acetates oi amylose and amylopectin fracrelatively low humidities they become quite brittle. Starch films, furthermore, are strongly hydrophilic. This latter distions of cornstarch. The fractions were separated according t o the procedure of Schoch (II), the ratio of amylose t o amylopectin advantage is, in general, easily overcome by transforming starch being about 1 t o 3. The most important properties of these t o its esters. Films or plastics produced from whole starch trifilms and the effects produced by various plasticizers were deteracetate or analogous derivatives, hoT1 ever, are very brittle and mined with particular emphasis on stress-strain measurement s. weak, even when large amounts of plasticizing agent are inFilms of high tensile strengths and considerable pliability are obcorporated (8, 8 ) . Thus, whole starch, as well as its substitution products, seems t o be basically unsuitable for film or fiber protained from acetates of the amylose fraction, whereas only very duction and also for plastics production, unless substitution inbrittle nonself-supporting films with low tensile strengths can be obtained from acetates of the amylopectin fraction. The volves a n increase in polymerization. latter films are even more brittle than those obtained from whole As a result of the recent advances in the field of high-polymer starch acetate. Because of the high qualities of films produced chemistry it is known that poor film- or fiber-forming characteristics of polymeric materials may be attributed to: ( a ) relatively from amylose acetate, there seems t o be little doubt that the low molecular weight, ( b ) tangled or branched molecular conamylose fraction will assume considerable commercial importanre if an economical method of isolation can be developed. figuration, or ( c ) the presence of a component having poor filmforming properties with one that is capable of forming films. METHODS I n the case of starch, its poor film-forming properties cannot be FRACTIONATION. Prior t o fractionation, cornstarch was ascribed t o too low a molecular weight since this appears t o be well above ( 1 , 4, 13) the limiting value of 10,000-20,000 required freed from as much fat as possible by methanol extraction ( I O ) . Both laboratory-prepared and common commercial starches were for film formation (6, l a ) . Within the last few years it has been subjected to fractionation by the use of butanol (11) with subdefinitely shown t h a t starch consists of a t least two compostantially equal results. nents (6, I O ) , one presumed t o have a linear configuration and the other a branched structure (6). This suggests that the brittleI n order t o obtain the amylose in a finely powdered form, the ness of starch acetate films is due t o the third cause mentioned centrifuged precipitate (1 part) was strongly stirred a i t h ethanol (5parts), filtered, and above, the presence thoroughly washed of t h e b r a n c h e d
S
from amylopectin triacetate. (Amylose
Amylose triacetate
Amylopectin triacetate
Figure 1. Appearance of Acetylated Starch Fractions
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the amylopectin was separated by pouring
September, 1944
INDUSTRIAL AND ENGINEERING CHEMISTRY
the centrifugate into strongly stirred absolute ethanol (1 part of solution t o 5 parts of ethanol). The precipitate was then washed and dried as described for amylose. ACETYLATION.The crude starch fractions were acetylated, and the products were worked according t o the method of Whistler, Jeanes, and Hilbert (14), using a 3-hour acetylation period. Acetates prepared b y this procedure contain 44.644.870acetyl. Acetates of the two starch fractions differ considerably from each other in appearance. As Figure 1 shows, amylose triacetate is highly fibrous, while amylopectin triacetate is a fine white powder. PREPARATION OF FILMS.The following procedure was used for preparing films: A definite weight of the amylose, amylopectin, or whole starch triacetate (usually 2 grams) was dissolved in 50 ml. of chloroform, and the solution freed from lint and dust particles by passage through a coarse fritted glass funnel. The solution was concentrated a t about 70" C. in a glycerol bath, and a definite weight of plasticizer was carefully mixed into the thickened solution when required. After concentrating t o a 10% solution, the mixture was allowed t o stand several minutes t o become free of bubbles and was cast on a clean glass plate with a casting knife 6 inches long and a blade set 0.020 inch above the glass plate. After about 4 hours, the film was loosened by moistening the edges with water, carefully stripped from the plate, immediately blotted between filter paper, and placed in a conditioning room at 21.1' C. (70' F.) and 50% relative humidity for 12 days. The dried film formed was clear and 0.030-0.040 mm. (0.0012-0.0016 inch) thick. During the conditioningperiod, films rapidly lose weight, mainly as a result of volatilization of chloroform. Equilibrium is attained in about 10 days. Since chloroform exerts strong plasticizing action, the films were subjected t o 12-day conditioning t o ensure their testing under comparable conditions. The slow rate at which the solvent evaporates from the film is comparable to the slow rate of solvent evaporation from cellulose acetate films ( 3 ) . STRESS-STRAIN MEASUREMENT.A Scott IP2 graphically recording, inclined-plane serigraph was used which had a load capacity of 0-2000 grams and a constant loading rate of 36 grams per second. The distance between film clamps was set at 50 mm. for zero load. All tensile strength values reported are corrected for the reduction in area of cross section brought about by stretching, the assumption being made that no appreciable volume change occurred in the film during stretching. The tensile strength values, therefore, pertain t o the actual film cross section at the time of rupture. Numerous measurements on a large number of films free of mechanical defects show that the reproducibility of results is good. The tensile strength and elongation values recorded are averages of a t least ten tests conducted on a t least two different films. FILM, CHARACTERISTICS
Amylose, amylopectin, and whole starch triacetates yield colorless, transparent, lustrous films differing greatly in strength and flexibility. Films of amylopectin and whole starch triacetate are weak and brittle, even after plasticization. The film of amylose triacetate, on the other hand, possesses great strength and flexibility; i t is optically isotropic and exhibits the same tensile strength in all directions. It swells and can be dispersed in pyridine, acetic acid, chloroform, and tetrachloroethane, and is insoluble in acetone, alcohol, ether, and water. Films were characterized mainly by determination of their stress-strain relations. This provides a convenient means both for comparing and evaluating films, and for rapidly determining the important mechanical effects produced in the films upon incorporation of plasticizing agents. Films from whole starch triacetate and amylopectin triacetate are so weak and brittle that no measurements could be made on them. A typical stress-strain curve ( A ) for unplasticized film prepared from amylose triacetate is shown in Figure 2a. Application of a n increasing stress t o the unplasticized film produces a considerable
79'1
elastic deformation which obeys Hooke's law. When the yield point is reached, plastic flow c o m m e n c e s . During the period of plastic flow, the curve continually bends away from the stress coordinate, indicating an increase in rate of flow as stress is applied. Plastic flow c o n t i n u e s t o an elongation of about 22%, at which point the film ruptures. The relatively long r e g i o n of plastic 0 50 100 0 50 100 flow is evidence that the film possesses Percent Elongation considerable inherFigure 2. Stress-Strain Curves ent plasticity. of Amylose and Cellulose TriL i k e f i l m s of acetate Films Containing Varimany other high ous Amounts of Dibutyl Phthalate polymers, the film a. Amylose triacetate cross-secof amylose triacetional area 0.311 sq. mm'. b . Cellulose triacetate, croas-sectate decreases in tional area 0.222 s mm. tensile strength on A , E , c,. D , E, an8 F represent films containing 0, 10, 20, 30, 40 and exposure to water. 50% dibutyl phthalate, respectively. When soaked i n water for 24 hours the film loses approximately one third of its original strength. Very little change is noted in the ultimate tensile strength on a r celerated aging of the film by maintenance at 65" c. for 12 days. Plasticizers modify the stress-strain properties of amylose triacetate film. Stress-strain curves of films containing varying amounts of dibutyl phthalate are shown in Figure 2a. On incorporation of the plasticizer, the elastic deformation of the film is decreased, the yield value is lowered, the pliability is increased, and the character of plastic flow is changed. The last effect is most pronounced in film containing about 20% dibutyl phthalate. After the yield value is attained in such film, plastic flow commences and proceeds at 'an increasing rate as stress is applied until the film has stretched 20-30%; thereupon the stress-strain curve rises sharply and becomes concave t o the stress axis. This is evidence of increasing resistance t o the applied stress and the development of greater strength within the film. This property is
.
I
0
,
Tensiil Strength2;Film Tensile Strength Of Film Corrected For Plasticizer Present
I
1
yo 4
c 2
30 40 Percent Plasticizer (Dibutyl Phthalate)
Figure 3.
5
Effect of Plasticizer on Tensile Strength of Amylose Triacetate Films
INDUSTRIAL A N D ENGINEERING CHEMISTRY
798
desired in commercial films because, by proper stretching during formation, films can be produced with a considerably increased strength in one direction. Such an increase in strength is due to partial orientation of the film molecules and is typical of high polymers having extreme linearity. Variation of tensile strength of the films with increasing dibutyl phthalate content is shown in Figure 3. As the solid line indicates, a progressive weakening of the film occurs when increasing amounts of plasticizer are incorporated. The broken line represents tensile strength values corrected for the plasticizer present, and, hence, calculated only for the amylose acetate in the films. The initial slight rise in tensile strength is similar t o that produced by certain plasticizers in cellulose acetate films. Variation of elongation at rupture with increasing dibutyl phthalate content is shown in Figure 4. As expected, the addition of plasticizer causes the f ilm t o become softer, more plastic, and readily extensible. At plasticizer contents of 50% or more, the films are soft and weak. 1401
;i
I
I
I
I
m 100
Percent Plasticizer (Dibutyl Phthalate 1
Figure 4.
Effect of Plasticizer on Elorigatioii of Ar~iylosc Triacetate Films
Akhough most quantitative information was obtained with dibutyl phthalate, other common commercial or potentially available plasticizers are suitable for plasticizing amylose triacetate. Tensile strengths and elongation at rupture of films prepared from mixtures of amylose triacetate with various plasticizers are shown in Table I. Tricresyl phosphate, diethylene glycol butyl ether acetate, pentaerythritol tetraacetate, and the butyl esters give results comparable t o those obtained with dibutyl phthalate. These are all excellent plasticizers for amylose triacetate. Pilnis containing polyethylene glycol and dimerized methyl linoleate become slightly hazy after several months. Glycerol is not cornpatible with amylose triacetate. In addition t o tensile strength and elongation, a few other properties of amylose triacetate film are of interest. The film exhibits good fold resistance. Strips 15 mm. wide and 0.03 mm. thick, for example, undergo 400-650 double folds on a Schopper
AND TENSILE STRENGTH TABLE I. EXTEXSIBILITY TRIACET.4TE
CONTAINING VARIOUS
Plasticizer (20%) Polyethylene glycol (Carbowax 1500) Diethylene lycol butyl ether acetate Dibutyl pht%alate Dibutyl sebacate Dibutyl tartrate Dimethyl phthalate Dimethyl linoleate dimer Levoglucosan triacetate Pentaerythritol tetraacetate Sorbitol hexaacetate Tributyl citrate Trihutyl phosphate Tricresyl phosphate
Alvrrmsr:
PLASTICIZERS
Extensibility (*5%)
OF
Tensile Strength Kg./Sq. M m . (t0.3)
Vol. 36, No. 9
TABLE 11. PROPERTIES OF UKPLASTICIZED FILMS FROM ANYLOSE TRIACETATE AND FROX CELLULOSE TRIACETATE Amylose Cellulose Pioperty Triacetate Triacetate (6d) Tensile strength kg./mm.2 7 9 8.6 22 4 Elongation a t b;eak, % Refract,ive index 1.469 1.48 1.341 Density 1.4 Jl60-65 .... IIardness Rockwell l f o d u l u s bf elasticity,, kg./pmm.z ( X 10’) 2.2 Capillary melting point". C. 290-300 270-290 a Determined on powdered amylose triacetate.
tester before breaking. N. C. Schieltz of this laboratory called our attention t o the possibility that the film might exhibit unusual electrical properties. Tests carried out by a commercial company on the insulating quality of plasticized and unplasticixed amylose triacetate film showed it to have an electrical breakdown value of 805-880 volts per mil. The plasticized film did not break down when submitted t o 200 volts of direct current for 3 weeks in a humidity cabinet kept a t about 120” F. The film, therefore, compares favorably with the usual polymers or electric tapes used for electrical insulation. Of special interest is a comparison of the propertiw of film from amylose triacetate with those of film from commercial cellulose trixetate. (Sample of high-grade commercial primary cellulose acetate containing 44.2% acetyl, obtained from Hercules Powder Company). The general similarity between the two films in such properties as tensile strength, refractive index, density, and melting point (Table 11) is striking. Comparable stress-strain curves obtained with films of amylose triacetate and commercial cellulose triacetate (Figure 2) reveal some differences. The curves show that the amylose triacetate film has considerably greater inherent plasticity- that is, can be stretched to a greater extent. I n general, plasticizers found applicable t o cellulose acetates are lilcemise applicable t o amylose triacetate, although the latter requires less plasticizer. Approximately 10-20% of Plasticizer is sufficient to give the amylose triacetate film a useful and lasting degree of plasticity, while commercial cellulose triacetates usually require 20-4070. The lower plasticizer requirement is an irnportant advantage in favor of amylose triacetate a t present due to the critical shortage of plasticizing agents. ACKNOWLEDGMENT
The writers wish to express their appreciation t o Robert A. Lleane and William D. Johnson of this laboratory for aid in carrying out the work. LITERATUHE CITED (1)
Becknian, C. O., and Landis, Q., J . pm. Chern. Soc., 61, 1495
(2)
Burlchard, C. A., and Degeririg, E. F., Rayon Taztile Monthlg,
( 1939).
23. 80 (1942).
(3) Durr’ans,‘T. €i.,and Davidson, D. G., J . SOC.Chem. I d . , 55, 162 (1936). (4) Foster, J. F., (1943).
and Hixon, R. Xi., J .
am.
Chem. SOC.,65, 618
( 5 ) Gloor, W. E., IXD.E m . CHEJI.,27, 1162 (1935). (5A) Hercules Powder Co., “Hercules Cellulose Acetate”. 1941. (6) Meyer, K. H., “Advances in Colloid Sciences”, pp. 143-65, New
York, Inteiscience Publishers, 1942.
(7) Meyer, K. H., Bernfeld, P., and Hohenemser, H., Helv. Chim. Acta, 23,885 (1940). (8) Mullen, J. W., and Pacsu, E., IND.ENG.CHEM.,35,381 (1943). (9) Neale, S. M., J . Teztile Inst., 15, 443T (1924). (10) Schoch, T. J., J . Am. Chem. SOC.,64, 2954 (1942). (11) Schoch, T. J., Ibid.,64, 2957 (1942). (12) Sookne, A. M., and Harris, M., J . Research Xatl. Bur. S t u d aTds, 30, 1 (1943). (13) (14)
Staudinger, H., Nutuiwissenschuften,25, 673 (1937). Whistler, R. L., Jeanes, A., and Hilbert, G. E., J . A m . Chern. Soc , to be published.
PHESEZNTED before the Division oi Sugar Cheniistig and Technology a t the 105th Meeting of the AMERICAS CHEWCAL ~ O C I E T Yin Detroit, Miah.
