DE102022117375A1 - Process for producing thermoplastic polysaccharide esters - Google Patents

Abstract

The invention relates to a process for producing thermoplastic polysaccharide esters, in which (i) at least one polysaccharide is mixed with a carboxylic acid anhydride and, if necessary with heating, is suspended therein without using a solvent, (ii) at least one carboxylic acid for the suspension from step ( i) is added, and (iii) the mixture from step (ii), optionally with heating, is reacted to obtain a polysaccharide ester.

Description

The invention relates to a process for producing thermoplastic polysaccharide esters.

Thermoplastics are plastics that can be deformed within a certain temperature range. This process is reversible and can be repeated as often as desired by reheating the cooled material, at least as long as no thermal decomposition of the material has occurred, which can happen, for example, due to overheating. Thermoplastics are used in many areas of industry and in everyday life as films, molded parts, packaging containers and as a basis for hot melt adhesives. They consist of synthetic polymers such as polyethylene, polypropylene, polystyrene and polyethylene terephthalate and a variety of additives, whereby both the polymers and the additives are not biodegradable due to the chemical structure. In addition, they are made from non-renewable raw materials such as petroleum and therefore cannot be produced sustainably. There is still an increasing need for thermoplastics. Due to the extreme exposure to plastics, including microplastics, there is an urgent need for thermoplastic and biodegradable materials. In addition, new thermoplastics must be able to be produced sustainably from renewable, biogenic raw materials.

From the DE 10 2008 024 089 A1 a process for producing hot melt adhesives is known, using polysaccharides as the starting material. The process is based on the homogeneous conversion of starch and cellulose as well as other polysaccharides such as scleroglucan, curdlan and xylan, dissolved in melted imidazole with fatty acid halides, fatty acid anhydrides and lactones. The key component imidazole is naturally present in large excess as a solvent and has to be recycled at great expense. The recovery/recycling of imidazole requires high temperatures (melting point approx. 95°C). Removing the imidazole from the products is only possible through complicated precipitation and intensive washing with, for example, alcohols. The reaction must be carried out at high temperature to keep the highly viscous system liquid. It is impossible to increase the concentration of starch above a content in the reaction system of over 5-10% by mass, because at higher starch concentrations a viscous system is created that is no longer miscible, and products with an uneven distribution of the ester units are created, which are no longer miscible have controlled melting. The preferred use of fatty acid halides also leads to the formation of corrosive chloride ions and imidazolium chloride as a further by-product in an equimolar amount, which also has to be recycled. Overall, the process is not efficient.

Also known is a process for producing starch esters that can be used as adhesives. The starch esters are obtained by reacting the biopolymer with a carboxylic acid imidazolide in a mixture of water and an organic solvent.

From the EP 0603768 A1 It is known that starch esters in the range of 10-80% by weight with a degree of substitution of 0.7 - 2.4 can be produced in a diluent consisting of an organic material and alcohols or compounds containing OH groups and Compounds that may contain other functional groups exist. The starch esters are produced according to typical processes by reacting starch in toxic media such as pyridine with carboxylic anhydrides or carboxylic acid chlorides. The reactions with carboxylic acid anhydrides are inefficient and produce the carboxylic acid in an equimolar amount as a byproduct. If carboxylic acid chlorides are used, equimolar corrosive hydrogen chloride is formed, which also binds to pyridine, also a corrosive compound, pyridinium hydrochloride.

Another process is the reaction of the starch with carboxylic anhydride in an aqueous-alkaline medium, whereby at least 50% of the carboxylic anhydride used cannot react with the starch due to the reaction mechanism. In addition, in the aqueous-alkaline medium, direct hydrolysis of the carboxylic anhydride results in a further loss of reagent, which is not available for the reaction with starch and further worsens the inherently poor efficiency of the reaction.

The DE 10 2011 005 849 A1 discloses a process for the homogeneous esterification of cellulose to produce a thermoplastic polymer that can serve as a base for a hot melt adhesive. The cellulose is reacted homogeneously with the carboxylic acid after activation with a mixture of reactive and toxic oxalyl chloride and N,N-dimethylformamide under anhydrous conditions in a molten salt such as 1-butyl-3-methylimidazolium chloride at high temperatures. Oxalyl chloride and its derivatives are corrosive.

The activation of carboxylic acids with a mixture of oxalyl chloride and N,N-dimethylformamide or p-toluenesulfonic acid chloride or 1,1-carbonyldiimidzole and the subsequent or simultaneous (in situ) esterification of polysaccharides is from the Literature well known (Esterification of Polysaccharides, Th. Heinze, T. Liebert, A. Koschella, Springer Verlag Berlin 2006, pp. 75). In addition, reagents such as 4-(1-pyrrolidonyl)pyridine have been used to activate the carboxylic acids.

Another disadvantage of reacting cellulose and starch with the activated carboxylic acid derivatives is the need to carry out a homogeneous reaction in order to achieve an average degree of substitution required to obtain a meltable and thermoplastic material. The polysaccharide must be laboriously dissolved in a solvent before the actual chemical reaction. A mixture of N,N-dimethylacetamide and lithium chloride or ionic liquids such as 1-butyl-3-methylimidazolium chloride have proven to be particularly suitable for cellulose and starch. To dissolve, the polysaccharide must be preactivated, which is time-consuming and costly, for example by thermal treatment at 130°C. In addition, the salt lithium chloride must be recycled. The chloride ions have a corrosive effect.

It is known that starch in N,N-dimethylacetamide or in N,N-dimethylacetamide in the presence of LiCl can be reacted with fatty acids in the presence of an equimolar amount of an activating reagent such as p-toluenesulfonic acid chloride or a carboxylic acid chloride and an equimolar amount of a base such as pyridine (S. Blohm, T. Heinze, Starch 2019, 71, 1900053; S. Blohm, T. Heinze, Carbohydrate Research 2019, 486, 107833). The starch fatty acid ester dissolves in the reaction medium during the reaction and must be worked up by precipitation with a 10-20-fold excess of a precipitant such as an alcohol. In addition, HCl, which can be bound as pyridinium hydrochloride, and p-toluenesulfonic acid are formed, which as a strong acid leads to an undesirable and uncontrolled decrease in the molecular weight and to a higher molecular weight distribution of the starch or starch ester.

It has become known from Carbohydrate Polymers 83 (2011) 1631-1635 that starch esters can be synthesized by the homogeneous reaction of starch dissolved in the ionic liquid 1-butyl-3-methylimidazolium chloride. However, ionic liquids are very expensive and can only be recycled with great effort.

Due to the corrosive properties of the chloride ions, all reaction media that contain chloride ions can only be used in special reactors that are unsuitable for the technical process or in very expensive reactors, for example coated with enamel.

The homogeneous reaction media must be processed by precipitation with a 5-20-fold excess of a precipitant in order to isolate the solid, meltable products. This essential procedure is not efficient. The high volumes of liquids have to be separated from the solids in a time-consuming process, which is often done using filters that become clogged with the often sticky polymer. The precipitants have to be recycled, for example by distillation, which requires additional energy. High-boiling solvents such as N.N-dimethylacetamide can only be distilled without decomposition in a vacuum. In addition, by-products are created during the reactions that are harmful to the environment and have to be disposed of properly, which is costly and expensive, including pyridinium hydrochloride and p-toluenesulfonic acid.

The known processes for the esterification of polysaccharides including starch are not efficient, not sustainable and have a variety of serious disadvantages.

The invention is therefore based on the object of providing a novel process for the production of thermoplastic polysaccharide esters. The present invention pursues the goal of moving away from petroleum-based raw materials towards renewable and natural resources and efficiently generating biodegradable, non-toxic thermoplastics. This can also eliminate the plastic and microplastic problem. Despite the already known use of polysaccharides as starting materials for esterification with carboxylic acids, there is still a strong need for a synthesis of thermoplastic polysaccharide esters that are technically simple, with high reagent yield and high efficiency under simple conditions in conventional reactors in salt-free reaction media and without using a solvent.

The object is achieved according to the invention by a method with the features of claim 1.

Advantageous embodiments of the invention are the subject of the subclaims.

In the process according to the invention, polysaccharide is suspended in a carboxylic anhydride and a carboxylic acid and, if necessary, reacted in the presence of a catalyst, so that a polysaccharide ester is formed.

The process synthesizes polysaccharide esters inexpensively with up to 100% chemical yield, which are obtained without the usual use of a solvent and through simple processing can do that. After the reaction, the liquid phase of the reaction mixture surprisingly separates and can be easily removed by simple decantation or suction. No byproducts are formed because the carboxylic acids are bound to the polysaccharide molecule. After separation, excess carboxylic acid can be used again for the reaction. The selected carboxylic acids such as acetic acid, propionic acid, stearic acid, palmitic acid, lauric acid or their anhydrides are non-toxic. The reaction is complete in a few hours.

The solid obtained can, for example, be cleaned in the usual way with water or alcohol or mixtures of water and alcohol and dried thermally. The efficient process produces polysaccharide esters with an average degree of substitution (DS) of up to 3. The polysaccharide ester thus obtained with a high degree of purity behaves thermoplastically without the addition of plasticizers or solvents or any other substance. The polysaccharide ester softens at temperatures below 160°C and produces a clear thermoplastic processable mass which, after cooling, produces a dimensionally stable solid. The process of melting and solidification is reversible several times.

A catalyst from the group of nitrogen bases such as triethylamine, pyridine or imidazole or salts such as the chlorides or acetates of alkali and alkaline earth metals can be used, organic bases such as pyridine or triethylamine and preferably imidazole, which can be easily removed due to its water solubility and is also non-toxic, are particularly suitable. The catalyst is usually used in 0.5-2 times the molar amount, based on the total of polysaccharide, carboxylic anhydride and carboxylic acid.

According to the invention, native starch or degraded starch of different origins, for example made from potatoes, corn and peas, or starch degraded enzymatically or acid hydrolytically or thermally or using other methods can be used. The content of the starch in terms of amylose and amylopectin can vary within wide limits of 100% amylose or 100% amylopectin, such as the commercial starch types Hylon VII, starch from corn with an amylose content of 70% (National Starch and Chemical Company, USA), starch from corn with an amylose content of 1% (Roquette, France) or tapioca starch with an amylose content of 17% (Ingredion, USA). With regard to the average degrees of polymerization (DP, number average), starches with DP values of 200 to 1500, preferably 300 to 600 and particularly preferably 350 to 450 can be used.

In addition to starch, other polysaccharides such as cellulose or xylan can also be used as starting materials.

The carboxylic acids can be saturated or unsaturated, unbranched or branched aliphatic or alicyclic or aromatic acids with 2 - 20 carbon atoms, preferably fatty acids, i.e. saturated aliphatic carboxylic acids, with 8 - 20 carbon atoms and particularly preferably 12 - 18 carbon atoms, such as palmitic acid, stearic acid and lauric acid. The carboxylic acid is usually used in 3-10 times the molar amount, based on the polysaccharide.

In principle, all of these carboxylic acids can also be used as a basis for the carboxylic anhydrides, although carboxylic anhydrides of carboxylic acids with 2-6 carbon atoms and in particular with 2-4 carbon atoms, such as acetic acid and propionic acid, are preferred. The carboxylic anhydride is usually used in 3-10 times the molar amount, based on the polysaccharide.

In a usual embodiment, the polysaccharide is added in a conventional reactor with a stirrer and heating jacket with 4 - 6 times the molar amount (based on the polysaccharide) of a short-chain carboxylic acid anhydride such as acetic anhydride or propionic anhydride and brought to 50 - 95 ° C, preferably to 70 - 95°C, particularly preferably heated to approx. 95°C. The system is stirred at this temperature for about 20 minutes and then 3-5 times the molar amount (based on the polysaccharide) of a carboxylic acid, preferably a fatty acid such as palmitic acid, stearic acid or lauric acid, is added. The system is further stirred and a basic compound such as imidazole is added as a catalyst in a molar ratio of approximately 1:1 (based on the total of polysaccharide, carboxylic anhydride and carboxylic acid). The mixture is allowed to react for 4-16 hours at 80 - 120 ° C, preferably at 100 ° C - 120 ° C, particularly preferably at approx. 120 ° C, with stirring.

The resulting precipitate is filtered off with suction and washed with an alcohol such as methanol, ethanol and isopropanol, separated, for example, by filtration and dried in vacuo at room temperature.

The invention will be explained in more detail below using exemplary embodiments, without limiting the protection claim to these.

Examples:

example 1

Corn starch (FLOJEL 60) with an amylose content of 70% was dried at 100 ° C in a vacuum for 24 h. 50 g (0.31 mol) were mixed with 202 g (1.55 mol) of propionic anhydride in a standard laboratory glass reactor and the mixture was heated to 95 ° C. After 20 minutes, 248.4 g (1.24 mol) of lauric acid were added. After the lauric acid had been distributed by stirring, 211.0 g (3.1 mol) of imidazole were added. The reaction mixture was heated to 120°C and then stirred at 120°C for 16 hours. The reaction was stopped by pouring into 3 l of methanol. The precipitate formed was filtered off with suction and washed once with 1 l of methanol and four times with 1 l of ethanol. The solid was dried at room temperature in vacuo for 3 days.

Yield: 112 g.

Melting range: 110°C to 125°C.

Average degree of substitution of lauryl groups: 1.29 and propionyl groups: 1.71.

Example 2

Corn starch (FLOJEL 60) with an amylose content of 70% was dried at 100 ° C in a vacuum for 24 h. 50 g (0.31 mol) were mixed with 202 g (1.55 mol) of propionic anhydride in a standard laboratory glass reactor and the mixture was heated to 95 ° C. After 20 minutes, 248.4 g (1.24 mol) of lauric acid were added. After the lauric acid had been distributed by stirring, 211.0 g (3.1 mol) of imidazole were added. The reaction mixture was heated to 120°C and then stirred at 120°C for only 6 hours. The reaction was stopped by pouring into 3 l of methanol. The precipitate formed was filtered off with suction and washed once with 1 l of methanol and four times with 1 l of ethanol. The solid was dried at room temperature in vacuo for 3 days.

Yield: 114 g.

Melting range: 115°C to 135°C.

Average degree of substitution of lauryl groups: 0.84 and propionyl groups: 2.15.

Example 3

Corn starch (FLOJEL 60) with an amylose content of 70% was dried at 100 ° C in a vacuum for 24 h. 7.96 g (61.73 mmol) of propionic anhydride were added to 2 g (12.35 mmol) in a standard laboratory glass reactor and the mixture was heated to 95 ° C. 12.35 g (61.73 mmol) of lauric acid were then added. After the lauric acid had been distributed by stirring, 8.4 g (123.4 mmol) of imidazole were added. The reaction mixture was heated to 120°C and then stirred at 120°C for 16 hours. The reaction was stopped by adding 80 ml of methanol. The mixture was poured into another 100 ml of methanol. The precipitate formed was filtered off with suction and washed three times with 100 ml of methanol and the solid was dried at room temperature in vacuo for 3 days.

Yield: 4.67 g.

Melting range: 90°C to 110°C.

Average degree of substitution of lauryl groups: 0.83 and propionyl groups: 2.17.

Example 4

Cellulose was dried at 100 ° C in vacuo for 24 h. 7.96 g (61.73 mmol) of propionic anhydride were added to 2 g (12.35 mmol) in a standard laboratory glass reactor and the mixture was heated to 95 ° C. 12.35 g (61.73 mmol) of lauric acid were then added. After the lauric acid had been distributed by stirring, 8.4 g (123.4 mmol) of imidazole were added. The reaction mixture was heated to 120°C and then stirred at 120°C for 16 hours. The reaction was stopped by adding 80 ml of ethanol. The mixture was poured into another 100 ml of ethanol. The precipitate formed was filtered off with suction and washed three times with 100 ml of ethanol and the solid was dried at room temperature in vacuo for 3 days.

Yield: 4.25 g.

Melting range: 135°C to 155°C.

Average degree of substitution of lauryl groups: 0.78 and propionyl groups: 1.98.

Example 5

Xylan was dried at 100 °C in vacuo for 24 h. 9.77 g (75.75 mmol) of propionic anhydride were added to 2 g (15.15 mmol) in a standard laboratory glass reactor and the mixture was heated to 95 ° C. 15.15 g (75.75 mmol) of lauric acid were then added. After the lauric acid had been distributed by stirring, 10.31 g (151.5 mmol) of imidazole were added. The reaction mixture was heated to 120°C and then stirred at 120°C for 16 hours. The reaction was stopped by adding 80 ml of ethanol. The Mixture poured into another 100 ml of ethanol. The precipitate formed was filtered off with suction and washed three times with 100 ml of ethanol and the solid was dried at room temperature in vacuo for 3 days.
Yield: 3.9 g.
Melting range: 190°C to 220°C.
Average degree of substitution of lauryl groups: 0.75 and propionyl groups: 1.71

Example 6

Corn starch (FLOJEL 60) with an amylose content of 70% was dried at 100 ° C in a vacuum for 24 h. 6.30 g (61.73 mmol) of acetic anhydride were added to 2 g (12.35 mmol) in a standard laboratory glass reactor and the mixture was heated to 95 ° C. 15.83 g (61.73 mmol) of palmitic acid were then added. After the palmitic acid had been distributed by stirring, 8.4 g (123.4 mmol) of imidazole were added. The reaction mixture was heated to 120°C and then stirred at 120°C for 16 hours. The reaction was stopped by adding 80 ml of methanol. The mixture was poured into 100 ml of methanol. The precipitate formed was filtered off with suction and washed 3 times with 100 ml of methanol. After drying at room temperature in vacuo (3 days), 3.69 g of product are obtained, which melts in the range from 215 ° C to 225 ° C.

Average degree of substitution palmityl groups: 1.31 and acetyl groups 1.69.

Example 7

Corn starch (FLOJEL 60) with an amylose content of 70% was dried at 100 ° C in a vacuum for 24 h. 6.30 g (61.73 mmol) of acetic anhydride were added to 2 g (12.35 mmol) in a standard laboratory glass reactor and the mixture was heated to 95 ° C. 17.56 g (61.73 mmol) of stearic acid were then added. After distributing the stearic acid by stirring, 8.4 g (123.4 mmol) of imidazole were added. The reaction mixture was heated to 120 °C and then stirred at 120 °C for 16 hours. The reaction was stopped by adding 80 ml of methanol. And the mixture was poured into 100 ml of methanol. The precipitate formed was filtered off with suction and washed 3 times with 100 ml of methanol. After drying at room temperature in vacuo for 3 days, 4.05 g of solid product are obtained.

Melting range: 180°C to 195°C.

Average degree of substitution of stearyl groups: 1.75 and acetyl groups 1.25.

Example 8

Corn starch (FLOJEL 60) with an amylose content of 70% was dried at 100 ° C in a vacuum for 24 h. 6.30 g (61.73 mmol) of acetic anhydride were added to 2 g (12.35 mmol) in a standard laboratory glass reactor and the mixture was heated to 95 ° C. 12.35 g (61.73 mmol) of lauric acid were then added. After the lauric acid had been distributed by stirring, 8.4 g (123.4 mmol) of imidazole were added. The reaction mixture was heated to 120°C and then stirred at 120°C for 16 hours. The reaction was stopped by adding 80 ml of methanol. The mixture was poured into 100 ml of methanol. The precipitate formed was filtered off with suction, washed 3 times with 100 ml of methanol and the solid was dried at room temperature in vacuo for 3 days.

Weight: 3.9 g.

Melting range: 160°C to 180°C.

Average degree of substitution of lauryl groups: 2.90 and acetyl groups 0.10.

Example 9

Corn starch (FLOJEL 60) with an amylose content of 70% was dried at 100 ° C in a vacuum for 24 h. 6.30 g (61.73 mmol) of acetic anhydride were added to 2 g (12.35 mmol) in a standard laboratory glass reactor and the mixture was heated to 95 ° C. 12.35 g (61.73 mmol) of lauric acid were then added. After the lauric acid had been distributed by stirring, 9.78 g (123.4 mmol) of pyridine were added. The reaction mixture was heated to 120°C and then stirred at 120°C for 16 hours. The reaction was stopped by adding 80 ml of methanol and then the mixture was poured into 100 ml of methanol. The precipitate formed was filtered off with suction, washed 3 times with 100 ml of methanol and the solid obtained was dried at room temperature in vacuo for 3 days.

Weight: 3.55 g.

Melting range: 160°C to 180°C.

Average degree of substitution of lauryl groups: 1.85 and acetyl groups 1.15.

QUOTES INCLUDED IN THE DESCRIPTION

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Cited patent literature

• DE 102008024089 A1 [0003]
• EP 0603768 A1 [0005]
• DE 102011005849 A1 [0007]

Claims (9)

Process for the production of thermoplastic polysaccharide esters, characterized in that (i) at least one polysaccharide is mixed with a carboxylic acid anhydride and, if necessary with heating, is suspended therein without using a solvent, (ii) at least one carboxylic acid for the suspension from step (i) is added, and (iii) the mixture from step (ii), optionally with heating, is reacted to obtain a polysaccharide ester. Procedure according to Claim 1 , characterized in that native or degraded starch is used as the polysaccharide. Procedure according to one of the Claims 1 or 2 that a carboxylic anhydride is used as the carboxylic acid anhydride, which is based on a carboxylic acid with 2 to 8 carbon atoms, preferably 2 to 6 carbon atoms and particularly preferably 2 to 4 carbon atoms. Procedure according to Claim 3 , characterized in that acetic anhydride and/or propionic anhydride is used as the carboxylic anhydride. Method according to one of the preceding claims, characterized in that the carboxylic acid used is a carboxylic acid with 2 to 20 carbon atoms, preferably 8 to 20 carbon atoms, particularly preferably 12 to 18 carbon atoms. Procedure according to Claim 5 , characterized in that palmitic acid, stearic acid and/or lauric acid is used as the carboxylic acid. Method according to one of the preceding claims, characterized in that the method is carried out using a catalyst. Procedure according to Claim 7 , characterized in that a nitrogen-based compound such as triethylamine, pyridine or imidazole or a salt such as a chloride or acetate of an alkali or alkaline earth metal is used as the catalyst. Procedure according to Claim 8 , characterized in that imidazole is used as a catalyst.