Starch triacetate

The properties displayed by a starch triacetate, or by other starch esters are dependent upon the type and molecular weight of the starch or starch fraction used for the esterification. The properties may also be influenced by trace constituents or impurities such as compounds of nitrogen and phosphorus which are often observed in starch esters. While the nature of the nitrogenous constituent is not known, it has been demonstrated that much of the phosphorus of potato starch exists as a natural phosphoric ester of the amylopectin fraction. It is not surprising, therefore, that the phosphorus of potato starch is carried through an esterification and may influence the properties of the starch ester. The exact modifying effects of these components on the properties of starch esters, however, are still to be explained. 

In general, starch triacetates are soluble in acetic acid and, except perhaps for potato starch triacetate, are soluble in chloroform, 1,1,2-tri-chloroethane, tetrachloroethane, and other halogenated hydrocarbons. High grade starch triacetates do not appear to be soluble in ethyl acetate or alcohol, while some controversy exists as to the extent of their solubility in pyridine and acetone. Waxy com starch triacetate, perhaps due to its smaller molecular weight, is readily soluble in a wide variety of organic solvents. 

In some cases the solutions have exhibited structural effects, the nature or extent of which have not been fully established. Staudinger and Eilers° found that the viscosity of their starch triacetate solutions displayed little dependence on temperature and considered the acetate to be mono-dispersed. Higginbotham and Richardson, on the other hand, found a noticeable temperature dependence for viscosities of carefully prepared potato starch triacetate in tetrachloroethane solutions. For example, a 0.25% solution was thbcotropic at 25°, but not at 60°. The existence of structural effects in starch acetate solutions was likewise noted by Mullen and Pacsu for potato starch triacetate dissolved in tetrachloroethane. 

As a rule, the viscosity of potato starch triacetate solution appears to be higher than the viscosity of other starch acetates while the viscosity of waxy corn starch triacetate is lower than the viscosity of other starch acetates in keeping with the relative molecular weights of these starches. 

Starch propionates, with various degrees of propionation up to the tripropionate, have been prepared by refluxing corn starch with propionic acid and propionic anhydride. The lower propionates are water-soluble, but the tripropionate is soluble only in organic solvents. Starch tripropionate and starch tributyrate are easily prepared by treating swollen starch with pyridine and the acid anhydride. These triesters are more soluble in organic solvents than the starch triacetates. 

Both starch tripropionate and tributyrate form clear plastics which, although brittle, are somewhat softer than those of typical whole-starch triacetates. This fact is in keeping with similar results noted for cellulose esters and sustains the view that the mutual attraction of the starch chains is continually rendered less effective as the aliphatic chain increases in length. 

Starch esters of either mono-, di-, or trichloroacetic acid can be pre-pared by heating starch with the appropriate acid or by treatment with the acid anhydride and sulfuric acid. Starch triacetates can be chlorinated by phosphorus pentachloride at 135-140 to produce a starch trichloroacetic ester. ... 

The acetate of potato amylose is more flexible than cellulose acetate, but the persistent length of the latter is significantly higher.2026 Starch triacetate, amylose triacetate, and amylopectin triacetate are reported to be suitable for... 

Using molecular weight values from osmotic pressure data, Stau-dinger evaluated the K , constant for partially degraded potato starch triacetate as 1.02 X 10 in chloroform, 0.69 X 10 in acetone, and... 

X 10 in m-cresol. Wheat starch triacetate in chloroform gave a value agreeing with that of potato starch triacetate. Since these values are 5 to 10 times smaller than the K value for cellulose triacetate, Staudinger has concluded that the starch molecules are shorter, and, in addition, are probably branched. By plotting the value of Kraemer s intrinsic viscosity, [ij], against c, Mullen and Pacsu obtained for potato... 

Standard C-NMR spectra of the tosyl starch acetates and propionates are appropriate to gain information about the distribution of the functional groups by using the carbonyl signals. In contrast to a starch triacetate which possesses three separate signals at 170.5, 170.1 and 169.2 ppm in agreement to a C=0 moiety at position 2,6 and 3, the C-NMR spectrum of tosyl starch acetate (DStos = 1-02) shows only two carbonyl peaks at 169.9 and 169.4 ppm indicating that the position 6 and 3 are acetylated. Comparable results were obtained for tosyl starch propionates. ... 

Appropriately formulated starch esters with plasticizers and other additives provide resin compositions that can be used to make injection molded products and for direct lamination onto Kraft paper. Starch acetates up to ds=2.S undergo complete and rapid biodegradation. In the case of starch triacetates, 70% of the carbon is converted to CO2 at 58°C in 45 days (25). 

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