Starch acetals, preparation

Low DS starch acetates ate manufactured by treatment of native starch with acetic acid or acetic anhydride, either alone or in pyridine or aqueous alkaline solution. Dimethyl sulfoxide may be used as a cosolvent with acetic anhydride to make low DS starch acetates ketene or vinyl acetate have also been employed. Commercially, acetic anhydride-aqueous alkaU is employed at pH 7—11 and room temperature to give a DS of 0.5. High DS starch acetates ate prepared by the methods previously detailed for low DS acetates, but with longer reaction time. 

Figure 16.2 Preparation of starch acetate microparticles by water-in-oil-in-water double emulsion technique.

Aldehyde starches are prepared by treatment with periodic acid/periodate ions, which selectively oxidize the adjacent hydroxyl groups on carbon atoms 2 and 3 to aldehyde groups. Dialdehyde starch can react with cellulose by forming covalent hemiacetal and acetal bonds.40 It is primarily used as a wet strength agent in the production of tissue and other sanitary grades. 

Starch acetates have been extensively investigated both as to manner of preparation and as to properties. This is in contrast to the other organic esters of starch of which only a few have been prepared, and, of these, only a few characterized. Among the inorganic esters, the starch nitrates have received the most attention and have risen to industrial importance because of their use in explosives. Partly because of this use and the secrecy attending their manufacture, a full disclosure of the preparation and properties of starch nitrates has not been published. 

Starch acetates are the most easUy prepared of the starch esters. They have served frequently for the characterization of starch fractions, for investigations of starch structure and as intermediates - in the preparation of methylated starch. Although most acetylation work has been done on white potato starch, considerable attention has been given to the acetylation of other starches such as those from com, - - - waxy com, wheat, - - - canna, rice, " horse chestnut and banana. ... 

Methods for the preparation of starch acetates may be classified as acetylation (1) in the presence of pyridine, (2) in the presence of specifically added catalysts other than pyridine, and (3) in the absence of a specifically added catalyst. Methods classified in (1) produce little or no degradation of the starch molecules. Those in (2) may or may not produce degradation, depending upon the reagents selected, while methods in (3) almost invariably cause a depolymerization of the starch. 

Starch has been acetylated by Barnett s mixture, W hich is composed of acetic acid, acetic anhydride, and a catalyst consisting of equal parts of sulfur dioxide and chlorine. This mixture, originally developed for cellulose esterification," readily acetylates starch at 55 , the acetyl content reaching the theoretical value of 44.8% in about four hours. A critical evaluation of the reaction by Hassid and Dore, and particularly by Higginbotham and Richardson, has shown that the acetates prepared have the viscosity, reducing power, and solubility characteristics indicative of a partially degraded starch. 

The preparation of the first starch acetate, as well as the first cellulose acetate, was announced by SchUtzenberger in 1865. These acetates were prepared by heating the carbohydrates in acetic anhydride to about 140-160 . Further examination of this reaction has been made by Traquair who found that on heating starch to 90° with acetic anhydride a derivative of low acetyl content (1-4%) is obtained which is capable of forming clear, somewhat elastic films. This starch acetate, termed Feculose, was produced commercially for a time, being sold for use as a thickening agent and as a size for textiles and paper. 

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. 

Optical rotations of starch acetates have, as a rule, been measured by means of the sodium-D-lines with chloroform or, less frequently, pyridine as the solvent. Although individual specific rotations have ranged from + 128 to +276 , the values reported for triacetates prepared by mild treatments seem to lie relatively close together, as indicated in Table I. 

Methyl violet is gradually turned blue green, and ultimately a pale salmon colour by nitric peroxide. It is much less sensitive than starch iodide. For the test, 2 5 g. of the dried sample, of explosive is placed in a glass tube 29 cm. long and i 5 cm. internal diameter. The sample is pressed down to occupy a depth of 5 cm. and a methyl violet paper is placed in the tube with its lower edge 2 5 cm. above the explosive. The tube is inserted in a bath at 154 to 135 so that about 6 to 7 mm. of the tube projects from the bath. After twenty minutes the tube is partially withdrawn to examine the paper, and this is repeated at intervals of five minutes until the paper becomes salmon-pink. The time of the test should be at least thirty minutes. The papers are prepared by dipping Schleicher and Schiill s filter paper No. S97 in a solution of pure rosaniline acetate (prepared from 0 25 g. basic rosaniline, 0168 g. methyl violet (crystal violet), 4 c.c. glycerine, 30 c.c. water, made up to too c.c. with pure 95 per cent, alcohol. The dried paper is cut into strips 2X7 cm. 

The reaction of polysaccharides ivith aldehydes to give cross-linked products has been known and exploited industrially for some time. Starch acetals may also be prepared by a method that does not give rise to cross-links and involves treatment of starch with a cyclic vinyl ether, 3,4-dihydro-2H-pyran, to give a tetrahydropyran-2-yl derivative (16). At low levels of substitution, the acetals are water-soluble, at... 

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