Triesters of Corn Starch,
Amylose, and Amylopectin PROPERTIES IVAN A. WOLFF, DAVID W. OLDS,
IND G. E. HILBERT Northern Regional Research Luboratory, Peoria, I l l .
A
preferred approach to the discovery of industrial uses for triesters of corn starch, corn amylose, and amylopectin is the detailed study of the properties of such esters and of products derived therefrom. Applications can then be sought where the need exists for materials having such properties. In the present w-ork on these triesters the melting ranges, plastic properties, viscosities, x-raj diffraction patterns, film characteristics, and possible uses as lacquertype protective coatings were investigated and are reported in some detail. The amylose esters resemble the esters of cellulose in inany respects and appear to merit consideration if economic availability of the linear starch fraction can be achieved.
l~alniitateswere soft at rooiii tempei'ature arid began t o flow if kept a t 100" C. for estended periods of time. There was lew difference betrveen amylose and amylopectin caproates and bet,ween their palmitates than between the lower esters of the fractions, the carbohydrate portion apparently being subordinate t o the long hydrocarbon chains. Melting ranges of the whole starch eaters prepared b y different method9 \\.ere practically the same. PLASTIC PROPERTIES OF THE ESTERS
-1qualitative comparison of the molding properties of the unphsticized esters was made. Two- to three-gram samples were heated under 5000 pounds per square inch pressure in a small cylinder mold. Aniyloee acetate formed a stronger plastic t h i n did a-hole starch or ainylopectixi acetate, but the difference was not EO striking as with the propionates, butyrates, and benzoates. I n ~ h e s elast-named ester classes, the esters formed from aniyloje gave strong, tough, transparent plastics, whereas the aniylopectin antl \\.hole starch esters gave weak products, too brittle t o remove in one piece from the mold. Little difference was notictd between amylose, amylopectin, and whole starch caproates, or between their palmitates. S o n e of the products was brittle. In general, the longer chain fatty esters gave darker colored awl softer plastics than did the lower esters. Optimum molding temperatures were found to be 150" C. for the acetates and benzoates; 130" C. for the propionates; 75' ( ' , for the butyrates; 50" C. for the caproates; and 35" C. for tlic palmitates.
c
RITICAL examination of the properties of starch derivatives in order t o evaluate their possibilities for industrial use is receiving a n increasing amount of attention ( 1 , 8,4, 5, 7 ) . Most of t h e work has been done on !\-hole starch esters, although itin)-lose and aniylopectin have been acetylated and soiiie of t h e properties of t,heir acetyl derivatives reported (3, 9). I n another piper the authors ( 1I ) have described the preparation under mild ronditions of the triacetates, tripropionates, tribut'yrates, tricaaproates, tripalmitates, and tribenzoates of corn starch, amylose, and amylopectin and have presented optical rotation and soluhility d a t a on these derivatives. Solubility of the esters seemed &pendent t o a great extent on t h e history of the polysaccharide prior to esterificat,ion, although none of the pretreatments would t x expected t o cause extensive depolymerization of the material. .Idditional comparative d a t a on the progressive changes in 1Jr.operties of these polysaccharide esters as the molecular weight of the acyl group was increased are presented here. Among the ixoperties studied were melting ranges, plastic properties, and molding characteristics, viscosities, film characteristics, antl x-ray diffraction patterns.
VISCOSITY STUDIES
.I11 viscosities iTere taken at, 25.00" * 0.02" C. in 1,1,2-trichloroethane solution in Ostwald-Cannon-Fenske capillary tulw viscometers. Kinetic energy corrections were applied. T h e esters were dissolved in the solvent by allowing them to stand at room temperature. T h e solutions were freed from lint by p : i . ~ sage through a coarse fritted glass furinel before their viscosities were measured. Three concentrations, 0.5, 0.26, and 0.12570, xvere used. A comparison of the viscosities of amylose acetate, propionak, and butyrate is shorrn graphically in Figure 1. Amylose esters i n
MELTING RANGES
Melting ranges of the esters m r e determined in capillary tubes. T h e esters melted over a broad range, with preliminary sintering, and sometimes underwent partial decomposition while melting. T h e values obtained were dependent, in part, on personal judgment and the rate of temperature rise. Certain useful comparisons, however, may be made. T h e amylose esters (Table I ) usually had a narrower and higher melting range than did the corresponding amylopectin esters. Whole starch esters rvere intermediate in these respects. T h e sharper melting of the amylose esters may indicate a greater homogeneity of molecular species. T h e higher range probably results from greater associative forces between the linear molecules than are possible with the branched amylopectin ester molecules. T h e melting range in each series became loner as the chain length of the acyl group vas increased. T h e caproates and
TABLEI.
1 I E L T I S G R A S G h S O F STARCH, AUYLOSE, 4 " D
AVYLOPCCTIN TRIESTERS
Ester Amylose acetate Amylopectin acetate Starch acetate Amylose propionate Amylopectin propionate Starch propionate Amylose butyrate Amylopectin butyrate Starch butyrate
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Melting Range, 0
c.
300-301 205-265 270-292 241 195-230 230-250 168-200 110-155 130-160
Ester Amylose caproate .4mglopectiii caproate Starch caproate Amylose palmitate rimylopectm palmitate Starch palmitate Amylose benzoate Amylopectin benzoate Starch benzoate
Melting Range 0
c.
196-208 65-125
65-155 90-1-10 65-135 60-140 215-240 195-210 200-220
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Vol. 43, No. 4
INDUSTRIAL AND ENGINEERING CHEMISTRY
the coneelitration found Ixsst. . i l l filnir ivc.i'e c a s t fro111 an 8% solution except the palmitate for which a 4% dispersion was employed. Ber:1nge of 0 t o o.5yo I':iuse of their insolubilit,y, the amylose caproate and palmitate give :I straight l i n r l i u d to he beaten in a Waring Blendor for dispersal. Potato relation xvheii rr:imylosr acetate is very viscous in chloroform solution and wa,q -,oc AMYLOSE, ACETATE c4:ist a t 4% conceiitration, using the 0.040-inch blade. duced viscosity is zI1lfilins were conditioned for a t least 2 weeks a t i o " F. a n d plotted against con.joyorelative humiditv prior t o testing. Ko differencr in test centration. Theo\ ~ I U I Y\vas noticeable if the film was heated for 4 days a t 42' C., prior t o conditioning, t o ensure removal of traces of chloroforni. retically, the limitI t should be remembered that the tensile strrngth and elongation ing v:ilueP at ZITO o f filr~isiuuy tie dependent on their thickness and also O I I thti c o II c c n t r : r t i o n i o l v i ~ n tf r o i n tvhich t h y :it,(> vast ( 6 ) . should be the wine Stress-Strain Measurements. 12ilni .strips 50 nini. loug anti for the three ester.. 8 to 10 111111. wide were tested with a Scott rerording inclined siilce iiioI:irunit,~: n c Iilanr. scrigraph at a loading rate of 36 grams per second. Galc111ploycd. €10 \v cvlations Iimm the tensilgrani.~are reported on the basis of kg. ever, visuisity ( I t a I)rr q u a r e min. of original cross->cctional area, as is convenperids 011 nio1ecul:ir 45 t ioirxlly tlonc with crllulose wters shape, as well : i L 1 .21 I 1 I I .6 1 . 0 1 . 4 1.8 ct1:iiii length, and c' x IO' thehigher acyl radiFigure 1 . \'iscosit! of' i i t i y l o s c . cal; probably inEsters or oI' vroas-wxionsl :ire:i at the tinie of rupturc (9), as deduced froin crease the breadth C' = concentration i n 1,aw mole- prr liter of the molecule to it (6. prr l i t e r l w t . of r r p r a t i n p u n i t ) Tcnsile .+trrngth = sufficient, extent t o breaking strength X elongated length ( 2I caure the displaceo r i g i d cross-rection:il area X original length' rnent of the curves :is s h o ~ v i i t IIC. elongated length measured on the tensilgram. Calculations T h e amylopectin ester> ~ : L V P virc.orit.J. C U I ' V ~ S \vhicli ii:td :L :icrortiing to method 1 :ire Ion-er than b y method 2 and are more grcx:ite,r slope than those of the amylose esters. The tor111(~r\ v t ~ f s tlirectlJ- comparable n-ith recorded values for cellulose derivative t lines but are concave t o the wrtical a~ film,-. .Ilso, more consistent tensile strength values were often ociation of molrcules at higher coIicr,iitr ol)tainetl by inethod 1 from the same set of data than when callimiting viscosity values were of the s:tnie order of magnitude : I iwlntion as by method 2. This arise?; from the fact t h a t strip$ those determined for amylose est,ers. G r a t e r difficulty WLLSC Y r u t from the same film, cast untlrr lalioratory conditions, ofttln perienced in reproducing viscosity re.wlts for the nmylopt~cti i i ha11 r : i t h ( ~ r\vidi~l\. differing rloiigations, nnd this variahility is ester;. The whole starch ebters a l w presented sufficient solubility diificulties t h a t the results were not as exact as desired. % . coinparison of whole starch hutyratt. viecositics, the estrrs having brrn I prepared by the three mcthotir previously dePcribecl ( f f 1, i. shown in Figure 2. T h e curves are siniil:rr it1 sh:ipc t o those of the a n i ~ ~ l o p ~ ~ ~ t i i i esters. T h e lowered ority of the starch butyrate p r q x t r d from alcohol-precipitated n o i i g r : ~ n u l ~st:iwh ir i s consistent Ivith the high solubility of thi.< m:itc,rial in org:rnir soIv[,iit,- n-eaker than the corresponding cellulose films, whereas the percentage of elongation is of the same order of magnitude. T h e difference in tensile strength i:: undoubtedly related t o the difference in the nature of the glucosidic linkage in amylose and in cellulose with resulting differences in molecular configuration and intramolecular forces. Although chain length can be important, it probably does not account for the variance between aniylow and ccllulose. Thus,
iio
Acetate Propionate Caproate
_7 .I. 4:l 2.6:l
T e n d e Streiigtli (Based on Original
