EP1070086A1

EP1070086A1 - Method for producing starch esters

Info

Publication number: EP1070086A1
Application number: EP99907304A
Authority: EP
Inventors: Rolf Kakuschke; Inno Rapthel; Hartmut Stoye; Dietmar Runkel
Original assignee: Buna Sow Leuna Olefinverbund GmbH
Current assignee: Dow Olefinverbund GmbH
Priority date: 1998-02-11
Filing date: 1999-02-02
Publication date: 2001-01-24
Also published as: WO1999041287A1; DE19805367A1

Abstract

The invention relates to a method for producing compostable starch esters which can be subsequently converted into thermoplastically deformable products, said method being carried out in a continuous or discontinuous way. The activation of starch with carboxylic acid anhydrides is carried out under intensive influence of mechanical energy, pressure or cavitation whereby the organophoby of starch particles is overcome. Consequently, the carboxylic acid anhydride penetrates quickly and quasi-thoroughly in the starch particles, without any change in the partially crystalline structure of said particles, at the time when the acid carboxylic anhydrides simultaneously react with residual moisture of the natural or modified starches, without any noticeable destructuring. According to the invention, activation is carried out at a temperature ranging from 10 to 140 DEG C and at a pressure comprised between 0.001 and 100 bars, preferably at a temperature ranging from 15 to 100 DEG C and at a pressure comprised between 0,01 and 1000 bars.

Description

Process for the production of starch esters

The invention relates to a process for the production of compostable starch esters which can be further processed into thermoplastically deformable products.

The production of cellulose and starch esters has long been known (P. Schuetzberger, CR Hebd. Seances Acad. Se. 61 (1865), 485-486 and P. Schützenberger, CR Hebd. Seances Acad. Se. 68 (1869), 814-818). A transfer of the process of mineral acid-catalyzed esterification in glacial acetic acid, which is customary for the production of cellulose esters, to the production of starch esters from native starches, as described, for example, in EP 0204 353 or in US 5205 863, is not suitable for the production of thermoplastically processable materials with sufficient strengths, since under strongly mineral acidic conditions there is always a considerable degradation of the starch to oligomers and sugars.

In contrast to this, the esterification of native starches in aminic solvents, such as pyridine according to US Pat. No. 2,627,516 or EP 0342 599, enables starch esters to be prepared without significant chain degradation even in the presence of strongly acidic reaction components. Here, however, the processing of the starch esters is extremely problematic, since small residues of amines can lead to health problems in further processing and to discoloration of the product. Furthermore, the solvent recovery is extremely complex.

Numerous attempts have therefore been made to esterify native starches without the use of nitrogenous catalysts or solvents, using mainly glacial acetic acid as a solvent, a variant which is particularly obvious in the production of starch acetyl esters (starch acetates) by acetylation with acetic anhydride. According to CA Burkhard (Rayon Text. Month., 23 (1942), 340 ff), the use of anhydrous acetic acid / acetic anhydride mixtures as an acetylene reagent without a further catalyst only led to the starch triacetate after 40 hours of reaction at reflux temperature, and reaction times of this magnitude are one Conversion to the technical scale.

A representation of starch acetates with average degrees of substitution with an approximately homogeneous distribution of substituents over the starch molecules is not possible in this way, since the attack of the esterification reagent takes place from the surface of the starch granules.

In the area of medium degrees of substitution, a very inhomogeneous, optically highly cloudy mixture of almost completely substituted starch molecules is created from the surface layers of the starch granules and hardly substituted starch molecules from the inner areas of the starch granules. The thermoplastic processability of such inhomogeneous products is difficult, the sensitivity to water is very high and the highly substituted starch molecules with degrees of substitution above about 2.7 have a very low rate of biodegradation in the compost.

The esterification of activated starches provides better results, the intra- and intermolecular hydrogen bonds of the starch molecules being largely destroyed by swelling processes in the aqueous phase and subsequent extraction of the water (J. Muetgeert et al., Strength 12 (10) 1958, 303-308). The considerable effort involved in such activation processes, in particular in the case of extraction and the separation of large amounts of water / extractant mixtures, does not allow such processes to be implemented inexpensively.

The process of alkaline activation of the native is more favorable

Strengthen.

So using sodium hydroxide solution as an activator according to US 3795670,

DE 4 114 185 and DE 4223471 the presentation of high quality starch Acetylesters with degrees of substitution> 1.8 with almost homogeneous distribution of substituents along the polymer chain.

A disadvantage of such an alkaline activation is the increased consumption of esterification reagent (acetic anhydride) by reaction with the alkali and the need to remove all alkali residues from the end product by complex washing processes, since traces of alkali can cause discoloration and decomposition reactions during further processing with thermoplastic. The method according to DE 19633474 is more advantageous, but the activation proposed there with an acetic acid / acetic anhydride mixture after the esterification reaction has been carried out requires the removal of a relatively large amount of acetic acid during product drying, which requires increased effort, particularly in the case of continuous process design.

The object of the invention is to avoid the disadvantages of the prior art and to propose an inexpensive process for the production of starch esters with a defined adjustable degree of substitution with an approximately homogeneous distribution of substituents, which process can be carried out both continuously and discontinuously.

Surprisingly, it was found that activation of native or modified starches with carboxylic acid anhydrides of chain lengths C ₂ to C ₆ , with mixtures of the carboxylic acid anhydrides mentioned or with mixtures of the carboxylic acid anhydrides mentioned with the cyclic anhydrides of maleic acid, malonic acid, succinic acid, glutaric acid, phthalic acid, pyromellitic acid, trimellitic acid and their derivatives is possible by overcoming the known organophobia of the starch particles by intensive action of mechanical energy or pressure or cavitation in such a way that the carboxylic acid anhydride or carboxylic acid anhydride mixture rapidly obtains the partially crystalline structures of the starch particles and penetrates almost completely when the carboxylic anhydrides start reacting with the residual moisture of the native or modified starches without any recognizable destructuring taking place. The slurry used in the activation step consists of starch and 60% - 120%, preferably 95% - 105%, acid anhydride based on the stoichiometric conversion of the moisture content of the starch plus the desired degree of substitution.

The activation is carried out either continuously or discontinuously in a device which is able to press the acid anhydride rapidly into the starch granules by mechanical energy or pressure or cavitation, for example in an ultrasound cell, a mill, a disperser, a high-pressure disintegrator, a kneader, a Jet cooker or a combination of the devices mentioned, preferably in a ball mill or in a disperser. According to the invention, the activation is carried out in the temperature range between 10 ° C - 140 ° C and at pressures between 0.001 - 1400 bar, preferably in the temperature range between 15 ° - 100 ° C and at pressures between 0.01 - 1000 bar. After activation, the reaction of the residual moisture is completed by increasing the temperature or temperature and pressure and the esterification reaction is carried out, the desired degree of substitution being set by the amount of carboxylic anhydride.

In this way, degrees of conversion of the carboxylic acid anhydride component between 96-99.8% can be achieved, and the proportion of carboxylic acid which has to be removed after the reaction in energy-intensive work-up steps corresponds solely to the amount caused by the stoichiometry of the esterification. The esterification can be carried out continuously or batchwise in a stirred or kneading reactor in the temperature range from 60 to 200 ° C. at pressures from 0.5 to 400 bar, preferably in the temperature range from 110 to 180 ° C. and at pressures from 1 to 300 bar become.

As starch components, native starches, such as corn, wheat, potato, rice, pea, tapioca, waxy maize or high amylose-containing corn, potato or pea starches, as well as their derivatives, such as hydroxyethylated, hydroxypropylated, acetalized , phosphorylated, sulfated, oxidized, carboxyalkylated, ni- starches, allyl starches, xanthate starches with degrees of substitution of 0.001-2.5, preferably 0.05-1.8, can be used in a naturally moist or predried state.

The acid anhydride components are the anhydrides of aliphatic carboxylic acids with chain lengths C ₂ to C ₆ , preferably C ₂ to C ₅ , or mixtures of the carboxylic anhydrides mentioned or mixtures of the carboxylic anhydrides mentioned with the cyclic anhydrides of maleic acid, malonic acid, succinic acid, glutaric acid, phthalic acid , pyromellitic acid or trimellitic acid and their derivatives.

The starch esters produced by the process according to the invention are white to slightly yellowish powders or granules, which almost completely dissolve in solvents such as chloroform, ethyl methyl ketone or methyl acetate at degrees of substitution above DS 2,0 2.0.

With degrees of substitution between 1, 2 and 2.6, the starch esters according to the invention can be processed on their own or with suitable plasticizers to compostable, biodegradable moldings, films or thermoformed articles with good mechanical properties.

Example 1 :

A slurry of 1600 g wheat starch (13% moisture) and 3400 g acetic anhydride is activated in a temperature-controlled continuous ball mill with an average residence time of 5 min under normal pressure at 90 ° C. The activated slurry is transferred to a 5 l kneading reactor, heated to 180 ° C. and acetylated at this temperature and a pressure of approx. 5 bar for 30 minutes. The clear, highly viscous starch acetate solution is dried under vacuum at approx. 140 ° C, the condensed acetic acid contains approx. 1% acetic anhydride. Yield: 2200 g DS: 2.30 Example 2:

A slurry of 1600 g pre-dried corn starch (5.5% moisture) and 2900 g acetic anhydride is degassed in a temperature-controlled ball mill for about one minute at 15 ° C and 0.1 bar and then activated for 3 minutes at 10 bar and 60 ° C. The activated slurry is continuously heated to 100 ° C. by means of a pump and a preheater and acetylated in a subsequent continuous kneading reactor at 160 ° C. and 3.2 bar with an average residence time of 20 min. The crystal-clear, highly viscous starch acetate solution is dried under vacuum at approx. 120 ° C, the condensed acetic acid contains approx. 2% acetic anhydride. Yield: 2550 g DS: 2.60

Example 3:

A slurry of 1600 g hydroxyethylated (DS ~ 0.05), high amylose corn starch (approx. 70% amylose, 13% moisture) and 2200 g acetic anhydride is first in a continuous volume flow of 3.5 kg / h with an Ultraturrax at normal pressure and 80 ° C (average residence time approx. 2.5 min) activated, heated to 150 ° C via a preheater and then acetylated in a continuous high-viscosity stirred reactor at 150 ° C and 300 bar with an average residence time of approx. 15 min.

The slightly cloudy starch acetate solution that is discharged is dried under vacuum at approx. 130 ° C., the condensed acetic acid contains 0.1% acetic anhydride. Yield: 1830 g DS: 1.20

Example 4:

A slurry of 1600 g phosphorylated potato starch (DS ~ 0.07) with a moisture content of 16.5% and a mixture of 3000 g acetic anhydride and 700 g of butyric anhydride is activated at 5 bar and 90 ° C in an ultrasonic cell over 5 min with approx. 2.5 kW / 20 - 100 kHz. The activated slurry is esterified in a continuous kneading reactor at 170 ° C. and 4 bar with an average residence time of approx. 45 min.

The slightly cloudy starch ester solution which is discharged is dried at 160 ° C. under a fine vacuum, the distilled acetic acid-butyric acid mixture contains approx. 0.7% acetic anhydride and approx. 2% butyric anhydride. Yield: 2250 g DS: 2.32

Example 5:

A slurry of 1600 g pea starch (13% moisture, amylose content approx. 80%) and a mixture of 3000 g acetic anhydride and 600 g trimellitic anhydride is activated in two runs at 100 ° C and 1000 bar using a Gaulin high-pressure homogenizer. The activated starch paste is heated in a 5 l kneading reactor to 180 ° C. and esterified at 12 bar within 55 minutes. The slightly cloudy, highly viscous starch ester solution is dried under vacuum at 130 ° C., the condensed acetic acid contains approx. 0.7% acetic anhydride. Yield: 2650 g DS: 2.25

Example 6:

A slurry of 1600 g hydroxypropylated rice starch (13% moisture, DS ~ 1.8) and a mixture of 3000 g acetic anhydride and 300 g maleic anhydride is degassed in a disperser (Ultraturrax) for 2 min at 0.01 bar and 20 ° C and after rapid heating to 100 ° C at normal pressure activated for 10 min. The activated slurry is heated to 140 ° C. using a preheater and esterified in a continuous kneading reactor at 150 ° C. and 2.5 bar with an average residence time of approx. 35 min. The clear starch ester solution is dried at 150 ° C. under a fine vacuum, the condensed acetic acid contains <0.1% acetic anhydride. Yield: 2350 g DS (ester): 2.30

Example 7:

A slurry of 1600 g oxidized tapioca starch (0.65% carboxyl, 13% moisture) and 3300 g acetic anhydride is activated in a temperature-controlled ball mill at 90 ° C and 2.5 bar for 4 minutes. The activated slurry is acetylated in a 5 l kneading reactor at 160 ° C. and 15 bar within 30 min.

The slightly cloudy, highly viscous starch acetate solution is dried at 125 ° C. under vacuum, the condensed acetic acid contains <0.8% acetic anhydride. Yield: 2250 g DS: 2.35

Example 8:

A slurry of 1600 g pre-dried waxy maize starch (5.5% moisture) and 2700 g acetic anhydride is activated with an Ultraturrax at normal pressure and 90 ° C for 15 min. The activated slurry is acetylated in a 5 l kneading reactor at 170 ° C. and 4 bar within 55 min.

The clear, highly viscous starch acetate solution is dried under vacuum at 130 ° C, the condensed acetic acid contains approx. 1% acetic anhydride. Yield: 2400 g DS: 2.4

Claims

claims

1. Process for the production of compostable starch esters which can be processed into thermoplastically deformable products, characterized in that the starch

a) is activated by intensive continuous or discontinuous mechanical, pressure or cavitation treatment of a slurry of starch in acid anhydride in the temperature range between 10 - 140 ° C at pressures between 0.001 - 1400 bar while maintaining partially crystalline areas of the starch,

b) the starch activated in this way is subsequently destructured and esterified to the desired degree of substitution, the esterification being carried out continuously or batchwise in a stirred or kneading reactor in the temperature range between 60-200 ° C. at pressures between 0.5-400 bar.

2. The method according to claim 1, characterized in that the slurry used in step a) consists of starch and 60% - 120%, preferably 95% - 105% acid anhydride, based on the stoichiometric conversion of moisture content of the starch plus the desired degree of substitution.

3. Process according to claims 1 and 2, characterized in that the activation is preferably carried out in the temperature range between 15 ° C - 100 ° C and at pressures between 0.01 - 1000 bar.

4. Process according to claims 1 to 3, characterized in that the esterification is preferably carried out in the temperature range between 110 ° C - 180 ° C and at pressures between 1 - 300 bar. 10

5. Process according to Claims 1 to 4, characterized in that the anhydrides of aliphatic carboxylic acids of chain lengths C ₂ to C ₆ , preferably C ₂ to C ₅ or mixtures of said carboxylic acid anhydrides or mixtures of said carboxylic acid anhydrides with the cyclic anhydrides of maleic acid as acid anhydride components , malonic acid, succinic acid, glutaric acid, phthalic acid, pyromellitic acid or trimeuitic acid or their derivatives are used.