WO2000061639A1

WO2000061639A1 - Oxidized starch and the use thereof, notably as superabsorbent

Info

Publication number: WO2000061639A1
Application number: PCT/EP2000/002456
Authority: WO
Inventors: Richard Fisher; Wolfgang A. Herrmann; P. Jochen Zoller
Original assignee: Celanese Chemicals Europe Gmbh
Priority date: 1999-03-27
Filing date: 2000-03-21
Publication date: 2000-10-19
Also published as: DE19914067C1

Abstract

The invention relates to oxidized starch which can be obtained by oxidizing native starch in an acid solvent. According to the invention, a) the oxidant is introduced in to the acid starch solution at quantities of between 0.1 and 1.9 equivalents, in relation to the anhydroglucose units present in the native starch; b) oxidation is carried out in the presence of a catalyst containing: i) methyl trioxorhenium (MTO) or an alkylrhenium oxide or ReO3 or Re2O7, ii) a ditertiary alkyl nitroxyl, and iii) hydrogen halide dissolved in a carboxylic acid; c) oxidation is carried out in water, a carboxylic acid, an organic solvent or a mixture containing at least two of these ingredients, as solvent; and d) oxidation is carried out at a temperature of between 0 and 50 DEG C. The invention also relates to a method for producing oxidized starch and to the use thereof as superabsorbent, seed and/or fertilizer carrier or soil improving agents, as material for galenicals or as adhesives and binders.

Description

Oxidized starch and its use, especially as

Superabsorbent

The present invention relates to oxidized starch and its use, in particular as a superabsorbent.

Starch is a biopolymer (approx. 30% amylose and approx. 70% amylopectin), which is to be understood as the condensation product of α-D-glucose units. The condensation of the α-D-glucose units with one another mainly occurs as a 1,4 linkage, as a result of which a linear chain molecule is formed. A 1, 6 linkage of the α-D-glucose units causes branches along the chain. However, since the main type of linkage is the 1, 4 linkage along the chain, strength can be expressed in a simplified manner using the following formula,

where n describes the number of anhydroglucose units linked together glucosidically.

This simplified structural formula for starch is intended to be the basis for further derivatives listed below, but there are also branches. conditions, ie α-1, 6-links, even if these are not explicitly shown in the above and in the following formulas. For example, native starch, ie chemically non-derivatized, natural starch, which is described according to the present invention with the structural formula mentioned above, is actually a polycondensate of α-D-glucose, which is mainly 1, 4 linkage and also 1, 6 - Has link.

Native starch is made from corn, cereals, potatoes, peas, cassava, rice, millet, etc. won. The number n of anhydroglucose units linked together glucosidically with native starch is generally between 20 and 15,000.

In the industrial field, starch is used primarily in a modified form. For example, starch graft polymers based on acrylate are used on a large scale as absorption materials, which in particular bind and store water. Water-binding and storing absorption materials are known under the term "superabsorbents" and are used particularly in the manufacture of incontinence articles.

Their water-binding effect is based on the accumulation of carboxyiat groups. These carboxylate groups are present, for example, as sodium or potassium salts in addition to free carboxylic acid groups, the degree of neutralization being between 60 and 90 mol%.

It is also known to oxidize starch in order to arrive at application-oriented derivatives. A basic distinction is made between two oxidation rates: First, a selective oxidation of the starch with breakage of the C2-C3 bond can occur, whereby dialdehyde starch or dicarboxyl starch is formed (cf. van Bekkum, H., Besemer, AC Carbohydrates in Europe 1995, 13 , 16). On the other hand, a selective oxidation on the C6 carbon from the primary alcohol to the carboxylate (or to the carboxylic acid) can be carried out the, whereby α-polyglucuronates (or α-polyglucuronic acids) are formed. These α-polyglucuronates (or α-polyglucuronic acids) can be described in a structurally simplified manner using the following formula,

where n describes the number of anhydroglucose units in the original strength.

Various methods for the selective oxidation of starch to α-poly-glucuronate and / or α-polyglucuronic acids are known from the literature.

For example, the use of nitrogen oxides as an oxidizing agent for starch has been known for a long time. However, the use of nitrogen oxides is ecologically very questionable. In addition, the starch molecule is broken down in this process (Arjan de Nooy: Dissertation Technische Universität Delft 1997).

Furthermore, the selective oxidation of native starch to α-polyglucuronate or α-poly-glucuronic acids by means of chlorine bleach is known, a mixture of a di-tertiary alkyl nitroxyl and NaBr being used as the catalyst. This reaction is always carried out in aqueous solution at a pH of at least 9.0. For example, Arjan de Nooy, dissertation Technische Universität Delft 1997 (see also: de Nooy, A., Besemer, AC, van Bekkum, H. Recl. Trav. Chim. Pays-Bas 1994, 113, 165; de Nooy, A ., Besemer, AC, van Bekkum, H. Carbohydr. Res. 1995, 269, 89; and WO 95/07303) the oxidation of starch dissolved or suspended in water using chlorine bleach in the presence of a di-tertiary alkyl nitroxyl \ NaBr catalyst pH between 9 and 11.

According to this process, a modified starch is obtained in which at least 90 mol% of the primary alcohol groups present in the original starch are oxidized to carboxyl groups, ie a substance with the following structure

where n and m are integers and the quotient n / m is greater than 0.9.

US Pat. No. 5,504,246 describes an analogous process for the preparation of alkyl polyglucoside carboxylates by means of chlorine bleaching lye, the starting product used being not native starch but special polyglucosides protected by alkyl groups.

WO 91/04988 describes the selective oxidation of starch to α-polyglucuronic acid or α-polyglucuronate using chlorine bleach and Additives are known with which the starch interacts with or interacts with the addition (eg carboxylic acids). According to this method, at least 67% of the primary alcohol groups present in the original starch are oxidized to carboxyl groups.

Common to all known processes for the oxidation of starch using chlorine bleach is that the starch modified in this way is insufficiently suitable for the production of absorption materials, even if a corresponding starch derivative with a very high degree of oxidation of 90%, i.e. with many carboxyl groups. This effect may be due to a denaturation of the starch structure from a skein to an elongated macromolecule that occurs under the conditions used.

The object of the present invention is to provide an oxidized starch which is particularly suitable for the production of absorption materials.

This object is achieved by an oxidized starch, which is obtainable by oxidation of native starch in an acidic solvent, wherein

a) the oxidizing agent is introduced into the acidic starch solution in an amount of 0.1-1.9 equivalents, based on the anhydroglucose units present in the native starch;

b) the oxidation is carried out in the presence of a catalyst comprising:

i) methyltrioxorhenium (MTO) or an alkylrhenium oxide, which is an alkyl group of the general formula -C _n H ₂ n ₊ ι, in which n is an integer and is 2 to 20, or one Cycloalkyl group of the general formula -C _n H _2n -ι, in which n is an integer and is 3 to 10, contains, or Re0 ₃ or Re ₂ 0 ₇ , in a molar ratio of the anhydroglucose units present in the native starch to the rhenium compound used between 1: 1 and 1000: 1; and

ii) a ditertiary alkyl nitroxyl in a molar ratio of the anhydroglucose units present in the native starch to the ditertiary alkyl nitroxyl between 1: 1 and 1000: 1, and

iii) hydrogen halide dissolved in a carboxylic acid, the molar ratio of the anhydroglucose units present in the native starch to the halide being between 1: 1 and 100: 1, and

c) the oxidation is carried out in water or in a carboxylic acid or in an organic solvent or in a mixture which contains at least two of these constituents, and

d) the oxidation is carried out at a temperature between 0 and 50 ° C.

Another object of the invention relates to a method for producing oxidized starch by oxidation of native starch in an acid solvent, which is characterized in that

a) the oxidizing agent is introduced into the acidic starch solution in an amount of 0.1-1.9 equivalents, based on the anhydroglucose units present in the native starch; b) the oxidation is carried out in the presence of a catalyst comprising:

i) methyltrioxorhenium (MTO) or an alkylrhenium oxide which is an alkyl group of the general formula -CnH _{2n +} ι, in which n is an integer and is 2 to 20, or a cycloalkyl group of the general formula -CnH ₂ n-ι, in which n is an integer and is 3 to 10, contains, or ReO ₃ or Re ₂ 0 ₇ , in a molar ratio of the anhydroglucose units present in the native starch to the used

Rhenium compound between 1: 1 and 1000: 1; and

iii) hydrogen halide dissolved in a carboxylic acid, the molar ratio of the anhydroglucose units present in the native starch to the haiogenide being between 1: 1 and 100: 1, and

d) the oxidation is carried out at a temperature between 0 and 50 ° C.

Hydrogen peroxide, alkali or alkaline earth metal percarbonates, alkali or alkaline earth metal perborates and others are used as the oxidizing agent Peroxo compounds, aliphatic and aromatic peroxo acids, such as peracetic acid or perbenzoic acid. The preferred oxidizing agent is hydrogen peroxide.

The carboxylic acid used in step a), in step c) and that used in b) (iii) can be both different and identical. The same carboxylic acid is preferably used to prepare the starch solution (step a), to carry out the reaction after step c) and to dissolve the hydrogen halide according to b) (iii). Suitable carboxylic acids are aliphatic or aromatic mono- and dicarboxylic acids with 1-12 carbon atoms in the molecule. Aliphatic monocarboxylic acids having 2-6 carbon atoms in the molecule, in particular acetic acid, propionic acid, n- and i-butyric acid and the isomeric valeric acids, are preferably used. The use of acetic acid is particularly preferred. Mixtures of carboxylic acids can also be used, but a carboxylic acid is preferably used.

HBr and HCl, in particular HBr, are particularly suitable as hydrogen halide.

As an organic solvent, use is made of polar organic solvents, such as, for example, aliphatic or cycloaliphatic mono- or polyalcohols having 1 to 6 carbon atoms, such as, for example, methanol, ethanol, glycerol or neopentyl glycol, and also mono-, di-, triethyiene glycol or polyethylene glycol with an average molecular weight of 320 to 650, aliphatic or cycloaliphatic ethers with 2 to 10 carbon atoms, such as diethyl ether or 1,4-dioxane, aliphatic or cycloaliphatic ketones with 3 to 8 carbon atoms, such as acetone, methyl isobutyl ketone, polyhalogenated hydrocarbons with 1 to 8 carbon atoms, such as methylene chloride or chloroform and alkylbenzenes with 7 to 10 carbon atoms, such as toluene or ethylbenzene. Suitable di-tertiary alkyl nitroxyls are nitroxyl compounds of the general formula:

in which R ¹ and R ^{2 are the} same or different and represent an alkyl group with 4-12 carbon atoms, at least the carbon atoms bonded directly to the nitrogen atom each carrying two identical or different alkyl groups with 1-4 carbon atoms, or R ¹ and R ² together form an alkyl-substituted five- or six-membered heterocycle with the nitrogen atom. At least in such heterocycles the carbon atoms bonded directly to the nitrogen atom each carry two identical or different alkyl groups with 1-4 carbon atoms. In particular, two methyl groups are bonded to the carbon atoms adjacent to the nitrogen atom. If there is a six-membered heterocycle, the carbon atom in the 4-position to the nitrogen atom can carry further substituents. Such substituents are the tertiary-butyl group, the amino group NR ³ R ⁴ in which R ³ and R ^{4 are} identical or different and hydrogen or represent a straight-chain or branched alkyl group with 1-4 carbon atoms, or the hydroxy or an alkoxy group with 1-4 carbon atoms, in particular the methoxy group. 2,2,6,6-Tetramethylpiperidin-1-yloxyl, 2,2,6,6-tetramethylpiperidin-4-methoxy-1-yloxyl, 2,2,6,6-tetramethylpiperidine-4-hydroxy-1 are particularly preferred -yloxyl or 2,2,6,6-tetra-methylpiperidine-4-amino-1 -yloxyl.

Methyltrioxorhenium has proven particularly effective among the rhenium compounds. The starch oxidized according to the invention shows three essential differences in its application properties compared to known oxidized starches:

Firstly, it has a much higher absorption capacity. This difference is particularly clear in the case of starch derivatives with a low degree of oxidation, for example of only 10%, based on the ratio of anhydroglucoronic acid units to anhydroglucose units originally present.

On the other hand, the starch oxidized according to the invention forms a gel in the acidic solution, while the known oxidized starches dissolve without gel formation. Furthermore, the method according to the invention for producing the starch oxidized according to the invention is distinguished by the use of environmentally friendly and inexpensive oxidizing agents, such as Hydrogen peroxide.

In addition, the starch oxidized according to the invention can be produced much more simply and cost-effectively. On the one hand, the amount of oxidizing agent can be reduced in order to provide an absorption material with comparable properties. On the other hand, significantly fewer salts have to be removed after the reaction than in the known processes.

The macromolecules produced according to the invention also show little or no degradation of the 1,4 linkages of the α-D-glucose units. In contrast, the oxidized starch of the prior art shows a comparatively greater breakdown of the macromolecule compared to the starting product, which is due to the β-elimination in the more alkaline environment. Studies of the starch oxidized according to the invention have shown that, in addition to primary alcohol and carboxyl groups, it also has aldehyde groups, some of which are present as free groups and some are hydrates.

The following formula offers one possibility for describing the oxidized starch according to the invention:

Herein the sum n + m + o means the number of anhydroglucose units of the native starch used.

According to a preferred embodiment of the present invention, the hydrogen peroxide is introduced into the starch solution in an amount of 0.3 to 1.7 equivalents, based on the anhydroglucose units present in the native starch.

In a further, particularly preferred embodiment, HBr is used in solution in acetic acid and as the di-tertiary alkyl nitroxyl 2,2,6,6-tetramethyipiperid-1-yloxyl or 2,2,6,6-tetramethylpiperidine-4-methoxy-1-yloxyl .

However, the addition of ditertiaralkylnitroxyl can also be dispensed with. Surprisingly, it turns out that in the absence of the ditertiaralkylnitroxyl, the oxidation reaction is selective to the Polyglucuronic acids expire. The procedure is then as usual, with the only exception that a rhenium-containing catalyst is used without the addition of di-tertiary alkyl nitroxyl. An oxidation system consisting of MTO, hydrogen peroxide and bromide is then preferably used to produce oxidized starch.

To further increase the absorption properties, the starch oxidized according to the invention can be crosslinked with crosslinking agents such as borax, diepoxides, dialdehydes, titanates.

Methods for crosslinking polymers are known per se and are described, for example, in "Modified Starches: Properties and Uses" (ed .: O.B. Wurzburg; CRC Press, Boca Raton Florida 1986, p. 41).

In addition to the use of the starch oxidized according to the invention as an absorption material, it is particularly suitable for the production of seed and / or fertilizer carriers, as a material in galenics, as a thickener, soil improver, adhesive or binder.

The following example serves to explain the invention.

example

In a 250 ml flask equipped with a Teflon KPG stirrer, native starch (7.0 g, water content approx. 10%, approx. 39 mmol) is suspended in 150 ml glacial acetic acid (100%). 97.3 mg of MTO (0.39 mmol of methyltrioxorhenium), 60.1 mg of TEMPO (0.39 mmol of 2,2,6,6-tetramethylpiperidin-1-yloxyl) and 1 ml of HBr (30% by weight dissolved in glacial acetic acid) are added. While stirring (300 rpm), 20 ml of a 30% H ₂ 0 ₂ solution in water are slowly injected. After the reaction (about 2 h), about 200 ml of methanol are added to the solution while stirring, the oxidized starch precipitating in the form of a gel.

This precipitate is decanted, sucked off through a suction filter and pressed out. The product thus obtained is then dried in vacuo at room temperature.

To determine the absorption capacity, the TA value (total absorption) and the CRET value (absorption capacity after spinning) are determined. For this, approx. 340 mg (mass m1) of the ground and sieved product (0.5 - 0.315 mm grain size) are weighed into a tea bag. The bag is closed with three clips and weighed again (mass m2). The bag is then allowed to swell in 50 ml of a NaCl solution (0.9 g / l NaCl) for 20 minutes. Then the bag is allowed to drain for three minutes and the mass of the moist tea bag (mass m3) is determined.

The TA value is determined according to the following equation:

(m3 - m2)

TA = ml

The bag is then centrifuged for 3 minutes in a spin dryer (1400 rpm, diameter 330 mm) and weighed again (mass m4):

The CRET value is determined according to the following equation:

_CRET = ^ ΓJ? ml

The results are shown in Table 1 below. Table 1:

Claims

claims

1.) Oxidized starch, which is obtainable by oxidation of native starch in an acidic solvent, whereby

b) the oxidation is carried out in the presence of a catalyst comprising:

i) methyltrioxorhenium (MTO) or an alkylrhenium oxide, which is an alkyl group of the general formula -C _n H ₂ n ₊ ι, in which n is an integer and is 2 to 20, or one

Cycloalkyl group of the general formula -C _n H2n-ι, in which n is an integer and is 3 to 10, contains, or Re0 ₃ or Re ₂ 0 ₇ , in a molar ratio of the anhydroglucose units present in the native starch to the rhenium compound used between 1 : 1 and 1000: 1; and

iii) hydrogen halide dissolved in a carboxylic acid, the molar ratio of those present in the native starch Anhydroglucose units to the halide is between 1: 1 and 100: 1, and

d) the oxidation is carried out at a temperature between 0 and 50 ° C.

2.) Oxidized starch according to claim 1, characterized in that the oxidizing agent is introduced into the acidic starch solution in an amount of 0.3 to 1.7 equivalents, based on the anhydroglucose units present in the native starch.

3.) Oxidized starch according to one of claims 1 or 2, characterized in that the oxidizing agent is hydrogen peroxide, alkali or alkaline earth metal percarbonates, alkali or alkaline earth metal perborates and other peroxo compounds, aliphatic and aromatic

Peroxy acids can be used.

4.) Oxidized starch according to one of claims 1 to 3, characterized in that hydrogen peroxide is used as the oxidizing agent.

5.) Oxidized starch according to one of claims 1 to 4, characterized in that aliphatic or aromatic mono- and dicarboxylic acids having 1-12 carbon atoms, preferably 2-6 carbon atoms in the molecule are used as carboxylic acids.

6.) Oxidized starch according to one of claims 1 to 5, characterized in that acetic acid, propionic acid, n- and i-butyric acid and the isomeric valeric acids, especially acetic acid, are used.

7.) Oxidized starch according to one of claims 1 to 6, characterized in that HBr or HCl, in particular HBr, is used as the hydrogen halide.

8.) Oxidized starch according to one of claims 1 to 7, characterized in that ditertiary alkyl nitroxyls of the general formula:

R ¹ ® _. e

N - θ]

_R 2 /.

are used, in which R ¹ and R ^{2 are} identical or different and represent an alkyl group with 4-12 carbon atoms, at least the carbon atoms bonded directly to the nitrogen atom each carrying two identical or different alkyl groups with 1 -4 carbon atoms, or R ¹ and R ² together with the nitrogen atom form an alkyl-substituted five- or six-membered heterocycle, at least the carbon atoms bonded directly to the nitrogen atom each carrying two identical or different alkyl groups with 1 -4 carbon atoms.

9.) Oxidized starch according to claim 8, characterized in that in

In the case of a six-membered heterocycle the carbon atom in 4-

Position to the nitrogen atom optionally carries further substituents, selected from the group tertiary butyl group, amino group

NR ³ R ⁴ , in which R ³ and R ^{4 are the} same or different and are hydrogen or a straight-chain or branched alkyl group having 1-4 carbon atoms, or hydroxyl or alkoxy group having 1-4 carbon atoms.

10.) Oxidized starch according to one of claims 1-9, characterized in that the ditertiary alkyl nitroxyl 2,2,6,6-tetramethylpiperidin-1-yloxyl, 2,2,6,6-tetra-methylpiperidine-4-methoxy-1 -yloxyl, 2,2,6, 6-tetra-methylpiperidine-4-hydroxy-1-yloxyl or 2,2,6, 6-tetramethylpiperidine-4-amino-1-ylyloxy.

1 1.) Oxidized starch according to one of claims 1-10, characterized in that a polar organic solvent is used as the organic solvent.

12.) Oxidized starch according to claim 1 1, characterized in that the polar organic solvent is aliphatic or cycloaliphatic mono- or polyalcohols having 1 to 6 carbon atoms, furthermore mono-, di-, triethylene glycol or polyethylene glycol with an average molecular weight of 320 to 650 , aliphatic or cycloaliphatic ethers with 2 to 10

Carbon atoms, aliphatic or cycloaliphatic ketones with 3 to 8 carbon atoms, polyhalogenated hydrocarbons with 1 to 8 carbon atoms and alkylbenzenes with 7 to 10 carbon atoms and mixtures thereof.

13.) Oxidized starch according to claim 12, characterized in that the polar organic solvent is methanol, ethanol,

Glycerin, neopentyl glycol, diethyl ether, 1, 4-dioxane, acetone,

Methyl isobutyl ketone, methylene chloride, chloroform, toluene or ethylbenzene or mixtures thereof are used.

14.) Oxidized starch according to any one of claims 1-13, characterized in that methyltrioxorhenium is used.

15.) Oxidized starch according to one of claims 1-14, characterized in that the oxidized starch is cross-linked by borax and / or diepoxides and / or dialdehydes and / or titanates.

16.) Process for the production of oxidized starch by oxidation of native starch in an acid solvent, characterized in that

b) the oxidation is carried out in the presence of a catalyst comprising:

i) methyltrioxorhenium (MTO) or an alkylrhenium oxide which is an alkyl group of the general formula -C _n H _2n + ι, in which n is an integer and is 2 to 20, or a cycloalkyl group of the general formula -CnH ₂ n-ι, in which n is an integer and is 3 to 10, contains, or Re0 ₃ or Re ₂ 0, in a molar ratio of the anhydroglucose units present in the native starch

Rhenium compound between 1: 1 and 1000: 1; and

ii) a ditertiary alkyl nitroxyl in a molar ratio of the anhydroglucose units present in the native starch to the ditertiary alkyl nitroxyl between 1: 1 and 1000: 1, and iii) hydrogen halide dissolved in a carboxylic acid, the molar ratio of the anhydroglucose units present in the native starch to the halide being between 1: 1 and 100: 1, and

d) the oxidation is carried out at a temperature between 0 and 50 ° C.

17.) Process for producing oxidized starch by oxidation of native starch, characterized in that the oxidation is carried out in the presence of MTO, hydrogen peroxide and bromide.

18.) Use of an oxidized starch according to any one of claims 1-17 as a superabsorbent, as a seed and / or fertilizer carrier, as a soil conditioner, as a material in galenics or as an adhesive and

Binder.