solution cool, an irreversible gel is formed which will not redisperse, no matter how long it is heated or stirred. Materials that won't redissolve once they have been solubilized are said to retrograde. In the case of starch, the linear amylose molecules align themselves, and through hydrogen bonding, form 3-dimensional arrays that are difficult to break apart. This peculiarity in starch is the reason specific processing conditions are needed to economically remove it during desizing D. Modified Starch The working properties of starch solutions(viscosity, retrograding point, penetration into yarns etc. )are influenced by the source of starch. The reader is referred to any of a number of well written books and reviews extolling the virtues of the various natural starches. In addition to naturally occurring variations, there are chemical modifications where some of the natural starch properties are altered to make them more useful 1. Thin Boiling Starch Thin boiling starches are made by adding a small amount of acid to a starch suspension that is held just below its gel point. The acid cleaves the polymer at the glucosidic linkage thereby lowering the viscosity of a solution made from it Hydrolysis occurs within the granule without breaking the granule. Mostly corn starch is used to make thin boiling starches. their solutions still retrograde 2. Dextrin Dextrine are made by heating dry starch with a mineral acid. White dextrin is made by heating at moderate temperatures and yellow dextrine is made by heating at higher temperatures with less acid. The degree of hydrolysis is higher than for thin boiling starch so dextrine solutions have lower viscosities 3. British gum British gums are made by dry heating starch granules at 180C without acid Some of the 1-4 acetal linkages are broken and 1-6 are formed. A more highly branched, lower molecular weight product is formed having more reducing end groups. British gums are more water soluble and produce higher solution viscosities They are mainly used as print paste thickeners
solution cool, an irreversible gel is formed which will not redisperse, no matter how long it is heated or stirred. Materials that won't redissolve once they have been solubilized are said to retrograde. In the case of starch, the linear amylose molecules align themselves, and through hydrogen bonding, form 3-dimensional arrays that are difficult to break apart. This peculiarity in starch is the reason specific processing conditions are needed to economically remove it during desizing. D. Modified Starch The working properties of starch solutions (viscosity, retrograding point, penetration into yarns etc.) are influenced by the source of starch. The reader is referred to any of a number of well written books and reviews extolling the virtues of the various natural starches. In addition to naturally occurring variations, there are chemical modifications where some of the natural starch properties are altered to make them more useful. 1. Thin Boiling Starch Thin boiling starches are made by adding a small amount of acid to a starch suspension that is held just below its gel point. The acid cleaves the polymer at the glucosidic linkage thereby lowering the viscosity of a solution made from it. Hydrolysis occurs within the granule without breaking the granule. Mostly corn starch is used to make thin boiling starches. Their solutions still retrograde. 2. Dextrin Dextrine are made by heating dry starch with a mineral acid. White dextrin is made byheating at moderate temperatures and yellow dextrine is made by heating at higher temperatures with less acid. The degree of hydrolysis is higher than for thin boiling starch so dextrine solutions have lower viscosities. 3. British Gum British gums are made by dry heating starch granules at 180 0C without acid. Some of the 1-4 acetal linkages are broken and 1-6 are formed. A more highly branched, lower molecular weight product is formed having more reducing end groups. British gums are more water soluble and produce higher solution viscosities. They are mainly used as print paste thickeners. 17
Figure 12. Hydrolysis ofStarch H CHOH CH,OH OH H HO O CHOH H CH,OH HO 4. Oxidized starch Starch granules are oxidized with sodium hypochlorite which converts the 2-3 hydroxyl into- COoH groups breaking the ring at that point. Five to seven-COOH groups per 100 anhydroglucose are introduced. Sodium bisulfite is added to destroy excess hypochlorite. The granular structure is retained and films from oxidized starch are better than those formed from thin boiling starch Figure 13. Oxidation of Starch CE.OH CB,OB C OH O HOC
Figure 12. Hydrolysis of Starch 4. Oxidized Starch Starch granules are oxidized with sodium hypochlorite which converts the 2-3 hydroxyl into -COOH groups breaking the ring at that point. Five to seven -COOH groups per 100 anhydroglucose are introduced. Sodium bisulfite is added to destroy excess hypochlorite. The granular structure is retained and films from oxidized starch are better than those formed from thin boiling starch. Figure 13. Oxidation of Starch 18
5. Starch ethers Starch ethers are made by reacting the hydroxyl groups in the anhydroglucose ring with appropriate reagents. These reactions add to the hydrophilic nature of the starch and decrease the ability to form hydrogen bonds between polymers, modifying solution and dry film properties Figure 14. Starch Ethers Carboxylated starch Starch -OE ClCB2COOH Starch - CO Bydroxyethyl or Hydroxypropyl Starch Starch-O+R-cB-C且2 →> starch-O-cB-cH2OB R where r=且-oxc且3 E. Desizing Starch Once a cooked starch solution dries, the resulting film will not readily re dissolve in water; therefore, to completely remove starch from a fabric, the polymer must be chemically degraded to make it water soluble. Three chemical methods can be used to degrade starch into water soluble compounds namely, Enzymes, Acid Hydrolysis and Oxidation. Each method has its own peculiar advantages and disadvantages 1. Enzyme Desizing Enzyme desizing is the most widely practiced method of desizing starch Enzymes are high molecular weight protein biocatalyst that are very specific in their action. Enzymes are named after the compound they break down, for example Amylase breaks down amylose and amylopectin, Maltase breaks down maltose and Cellulase breaks down cellulose. For desizing starch, amylase and maltase are used Cellulase, on the other hand, is used for finishing cotton fabrics. This will be discussed in later chapters. Amylase will degrade starch into maltose, a water soluble disaccharide and Maltase will convert maltose into glucose, a simple sugar 19
5. Starch Ethers Starch ethers are made by reacting the hydroxyl groups in the anhydroglucose ring with appropriate reagents. These reactions add to the hydrophilic nature of the starch and decrease the ability to form hydrogen bonds between polymers, modifying solution and dry film properties. Figure 14. Starch Ethers E. Desizing Starch Once a cooked starch solution dries, the resulting film will not readily redissolve in water; therefore, to completely remove starch from a fabric, the polymer must be chemically degraded to make it water soluble. Three chemical methods can be used to degrade starch into water soluble compounds namely, Enzymes, Acid Hydrolysis and Oxidation. Each method has its own peculiar advantages and disadvantages. 1. Enzyme Desizing Enzyme desizing is the most widely practiced method of desizing starch. Enzymes are high molecular weight protein biocatalyst that are very specific in their action. Enzymes are named after the compound they break down, for example, Amylase breaks down amylose and amylopectin, Maltase breaks down maltose and Cellulase breaks down cellulose. For desizing starch, amylase and maltase are used. Cellulase, on the other hand, is used for finishing cotton fabrics. This will be discussed in later chapters. Amylase will degrade starch into maltose, a water soluble disaccharide and Maltase will convert maltose into glucose, a simple sugar. 19
a. Alpha and Beta Amylase There are two types of amy lase enzymes, Alpha and beta. Both alpha and beta amylases hydrolyse glucosidic linkages in starch; however, the point along the polymer chain at which the reaction occurs differs between the two Alpha amylase attacks the chain at random points. The molecular weight of the starch is rapidly reduced facilitating complete removal. Beta amylase, on the other hand, starts at a chain end removing one maltose unit at a time. The molecular weight is gradually educed thereby taking a longer time to complete breakdown. In addition, the action of beta amylase is stopped at the 1, 6 branch glucosidic linkage found in amy lopectin leaving relatively high molecular fractions. Alpha amylase is the predominate enzyme used in desizing starch. A major advantage favoring enzymes is that they not damage cellulosic fibers. On the other hand, cellulase enzyme will destroy cellulose and while it is not used in desizing, it has found a specialty application in the production of stone-washed denim look There are three major sources for amylase enzymes. Malt Enzymes are extracted from the fermentation of barley grain which produces a mixture of alpha and beta amylase. Pancreatic Enzymes are extracted from the pancreas of prepared from the bacteria "bacillus subtilin". This too is mainly alpha amylase o slaughtered cattle which is mainly alpha amylase and Bacterial Enzymes al b. Effect ofTemperature, pH and Electrolytes on Enzymatic Desizing Activity of enzymes increase with temperature, however, above a critical temperature, enzymes are deactivated. The effectiveness of enzymes exhibit a maximum at certain temperatures, usually 40-75C. Bacterial enzymes are the most thermally stable and can be used up to 100 oC under special stabilizing conditions Certain salts increase the activity of specific enzymes. Pancreatic amylase is ineffective without the addition of salt a com bination of sodium chloride and calcium chloride increases the stability of bacterial amy lase above 160F. activity of amylase enzymes are also optimum at specific at specific pH. Table I summarizes the optimum conditions for the various sources of enzymes 20
a. Alpha and Beta Amylase There are two types of amylase enzymes, Alpha and Beta. Both alpha and beta amylases hydrolyse glucosidic linkages in starch; however, the point along the polymer chain at which the reaction occurs differs between the two. Alpha amylase attacks the chain at random points. The molecular weight of the starch is rapidly reduced facilitating complete removal. Beta amylase, on the other hand, starts at a chain end removing one maltose unit at a time. The molecular weight is gradually reduced thereby taking a longer time to complete breakdown. In addition, the action of beta amylase is stopped at the 1,6 branch glucosidic linkage found in amylopectin leaving relatively high molecular fractions. Alpha amylase is the predominate enzyme used in desizing starch. A major advantage favoring enzymes is that they not damage cellulosic fibers. On the other hand, cellulase enzyme will destroy cellulose and while it is not used in desizing, it has found a specialty application in the production of stone-washed denim look. There are three major sources for amylase enzymes. Malt Enzymes are extracted from the fermentation of barley grain which produces a mixture of alpha and beta amylase. Pancreatic Enzymes are extracted from the pancreas of slaughtered cattle which is mainly alpha amylase and Bacterial Enzymes are prepared from the bacteria "bacillus subtilin". This too is mainly alpha amylase. b. Effect of Temperature, pH and Electrolytes on Enzymatic Desizing Activity of enzymes increase with temperature; however, above a critical temperature, enzymes are deactivated. The effectiveness of enzymes exhibit a maximum at certain temperatures, usually 40 -75 0 C. Bacterial enzymes are the most thermally stable and can be used up to 100 0 C under special stabilizing conditions. Certain salts increase the activity of specific enzymes. Pancreatic amylase is ineffective without the addition of salt. A combination of sodium chloride and calcium chloride increases the stability of bacterial amylase above 160 0 F. Activity of amylase enzymes are also optimum at specific at specific pH. Table 1 summarizes the optimum conditions for the various sources of enzymes. 20
Table 1 Summary of optimum operational Conditions for Amylases Optimum Effect of Effect of Effect of Amylase Temp. C NaCl Ca cl2 Pancreas 6.8-7.0 40·55 Malt 4.6-52 5565 Bacteria 5.0-7.0 2. Desizing with Acids Mineral acids will hydrolyze starch by attacking glucosidic linkages. Acid hydrolysis lowers the molecular weight and eventually reduce starch to glucose Hydrochloric and sulfuric acids can be used. One problem with acid desizing is that cellulose fibers are also degraded which is why the method is not used much. One advantage with using acids is that cotton fibers can be demineralized more easily Insoluble salts are solubilized by acids making the removal of such troublesome metals such as iron more thorough 3. Desizing with Oxidizing agents Sodium hypochlorite, sodium hypobromite and sodium or potassium persulfate will also degrade starch. The reaction opens anhydroglucose rings at the- 2, 3 hydroxyls, converting them into carboxylic acid groups. This method is not used much either because it too degrades cellulose fibers. Hypochlorite is used for bleaching however 4. Test for Starch a drop of iodine solution placed on a test specimen resulting in a characteristic blue color is the universal test for identifying starch. It can be used as a qualitatively test to show whether all the starch was removed. Absence of the blue color signifies that all the starch has been removed. The intensity of the color is some what related to how much is left. Usually, if the color is faintly perceptible, the remaining starch will come out in the scouring and bleaching steps that follow
Table 1 Summary of Optimum Operational Conditions for Amylases 2. Desizing with Acids Mineral acids will hydrolyze starch by attacking glucosidic linkages. Acid hydrolysis lowers the molecular weight and eventually reduce starch to glucose. Hydrochloric and sulfuric acids can be used. One problem with acid desizing is that cellulose fibers are also degraded which is why the method is not used much. One advantage with using acids is that cotton fibers can be demineralized more easily. Insoluble salts are solubilized by acids making the removal of such troublesome metals such as iron more thorough. 3. Desizing with Oxidizing Agents Sodium hypochlorite, sodium hypobromite and sodium or potassium persulfate will also degrade starch. The reaction opens anhydroglucose rings at the - 2,3- hydroxyls, converting them into carboxylic acid groups. This method is not used much either because it too degrades cellulose fibers. Hypochlorite is used for bleaching however. 4. Test for Starch A drop of iodine solution placed on a test specimen resulting in a characteristic blue color is the universal test for identifying starch. It can be used as a qualitatively test to show whether all the starch was removed. Absence of the blue color signifies that all the starch has been removed. The intensity of the color is some what related to how much is left. Usually, if the color is faintly perceptible, the remaining starch will come out in the scouring and bleaching steps that follow. 21