Process to prepare crosslinked cellulose ethers, crosslinked cellulose ethers obtainable by such process and the use thereof

Abstract

The invention relates to a dry or slurry process to prepare phosphate-crosslinked cellulose ethers from a cellulose starting material comprising the steps of adding an alkalizing agent to the cellulose starting material to achieve mercerization, adding an ethehfying agent to the reaction mixture to achieve etherification of the cellulose, and adding a crosslinking agent to the reaction mixture to achieve crosslinking of the cellulose, wherein at least part of the alkalizing agent is added to the cellulose starting material before the etherification and/or crosslinking reactions take place to obtain alkalized cellulose; the crosslinking agent and the etherifying agent are added one after the other in random order or simultaneously; the crosslinking agent is an alkali metal thmetaphosphate; and the crosslinking and etherification steps are performed at an elevated temperature. Additionally, the invention relates to crosslinked cellulose ethers obtainable by the above process having a viscosity and/or water absorption capacity higher than their non-crosslinked equivalents and the use thereof.

Claims

1. A dry or slurry process to prepare phosphate-crosslinked cellulose ethers from a cellulose starting material comprising the steps of adding an alkalizing agent to the cellulose starting material to achieve mercerization, adding an etherifying agent to the reaction mixture to achieve etherification of the cellulose, and adding a crosslinking agent to the reaction mixture to achieve crosslinking of the cellulose, wherein at least part of the alkalizing agent is added to the cellulose starting material before the etherification and/or crosslinking reactions take place to obtain alkalized cellulose; the crosslinking agent and the etherifying agent are added one after the other in random order or simultaneously; the crosslinking agent is an alkali metal trimetaphosphate; and the crosslinking and etherification steps are performed at an elevated temperature of between 20 and 95 C.; wherein the crosslinking step is carried out at an alkaline pH of higher than 7; and wherein the amount of crosslinking agent added to the reaction mixture is sufficiently high that the phosphate-crosslinked cellulose ethers prepared by said dry or slurry process are insoluble in water.

2. The process of claim 1 wherein the crosslinking step is carried out subsequent to the etherification step.

3. The process of claim 2 wherein the crosslinking step follows the etherification step without an intermediate neutralization step.

4. The process of claim 3 wherein the process is a slurry process that takes place in a reaction medium that is a liquid comprising water and an alcohol and the cellulose ether is a carboxyalkyl cellulose ether.

5. The process of claim 1 wherein the crosslinking step is carried out at an alkaline pH of higher than 8.

6. The process of claim 3 wherein the alkali metal trimetaphosphate is sodium trimetaphosphate.

7. The process of claim 2 wherein the process is a slurry process that takes place in a reaction medium that is a liquid comprising water and an alcohol and the cellulose ether is a carboxyalkyl cellulose ether.

8. The process of claim 2 wherein the alkali metal trimetaphosphate is sodium trimetaphosphate.

9. The process of claim 1 wherein the process is a slurry process that takes place in a reaction medium that is a liquid comprising water and an alcohol and the product of the etherification step is a carboxyalkyl cellulose ether.

10. The process of claim 9 wherein the carboxyalkyl cellulose ether is carboxymethyl cellulose ether.

11. The process of claim 9 wherein the alkali metal trimetaphosphate is sodium trimetaphosphate.

12. The process of claim 1 wherein the alkali metal trimetaphosphate is sodium trimetaphosphate.

Description

EXAMPLES

(1) In the Examples the following test methods are employed:

(2) The Settling Volume Measurement

(3) 1.5 grams of the dried (crosslinked) cellulose ether are thoroughly mixed with 100 ml water in a 100 ml measuring cylinder. After standing for 4 hours a sediment may be formed, giving a certain settling volume, expressed in ml.

(4) The settling volume is used as a measure for the crosslink density of the product. A settling volume of >90 ml in the above test indicates that the product contains no or few insolubles (no or only slight crosslinking). A settling volume of <25 ml (for a purified material) or <35 ml (for a crude material) indicates a lot of sedimentation and is a measure for a dense crosslink network.

(5) Turbidity Measurement

(6) The turbidity of a solution of (crosslinked) cellulose ether in water was measured using a Monitek Model 251 turbidimeter (forward scatter design, Monitek GmbH). The turbidity value is related to the amount of insoluble material in a solution. Light crosslinking, which enhances the viscosity, will not give a remarkable increase in the amount of insoluble material and thus a lower value for turbidity. The turbidity will however go up when the amount of crosslinks increases, which gives rise to considerable amounts of insoluble material.

(7) Viscosity Measurement

(8) The viscosity of a solution of (crosslinked) cellulose ether in water was measured with a TA Instruments AR 1000 Rheometer at 0.1 s.sup.1 using a 40 mm, 4 steel cone and plate geometry. For the viscosity measurement a solution of the cellulose ether of 1, 1.5, 2, 3 or 4 wt % in water is taken. The concentration at which the viscosity of the crosslinked cellulose product is determined is the concentration at which the non-crosslinked equivalent cellulose ether has a viscosity of between about 1,000 and 5,000 mPas.

(9) Water Absorption Capacity

(10) The water absorption capacity of the crosslinked cellulose ethers was determined using a tea bag test. 50 mg (crosslinked) cellulose ether were weighed into a sealable teabag of approx 7.5 cm7.5 cm. After being sealed the teabag was soaked in a container with water for 20 minutes. The teabag was removed from the container and drained for one minute on a paper towel. The weight of the tea bag with (crosslinked) cellulose ether was determined before and after water absorption, as was the weight of a reference without (crosslinked) cellulose ether. The number of grams of water absorbed per gram of (crosslinked) cellulose ether was calculated.

(11) The following materials were used

(12) Sodium trimetaphosphate (STMP): Sigma T5508

(13) Sodium tripolyphosphate (STPP): Acros Organics No. 218675000

(14) Sodium hexametaphosphate (SHMP): Aldrich No. 305553

(15) Cellulose was used from both a cotton linters and a wood source, as further specified below.

Example 1

Preparation of Crosslinked CMC in Ethanol

(16) 16.6 kg cellulose (cotton linters) were mixed with 10.0 kg ethanol and 16.1 kg of a 50 wt % aqueous solution of caustic soda. After mixing for 24 minutes, 1.22 kg of 75 wt % solution of monochloroacetic acid in ethanol were added. The mixture was heated to 80 C. in about 30 minutes and after stirring for 50 minutes at this temperature the mixture was cooled to room temperature. The pH of a 2% solution in water was 11.5.

(17) 350 g of the crude reaction mixture, having a CMC content of about 42 wt %, were taken and 100 g ethanol/water (60/40 w/w) were added thereto. To this mixture were added 0.34 g sodium hydroxide in 10 ml water and 7.97 g sodium trimetaphosphate (STMP) in 80 ml water (4 mole % relative to the anhydroglucose units). The mixture was mixed for 30 minutes at room temperature, heated to 80 C. in 30 minutes, and mixed for another hour at 80 C. After cooling down the mixture was dried for 30 minutes at 65 C. in a fluid bed dryer. The resulting product was subjected to a settling volume test, giving a 30 ml settling volume.

Example 2

Preparation of Crosslinked CMC in Ethanol

(18) 116 g of the crude reaction mixture of Example 1, having a CMC content of about 42 wt %, were suspended in 300 ml ethanol water (40/60 w/w). 0.166 g of sodium trimetaphosphate (STMP) dissolved in 40 ml of water was added, corresponding to 0.25 mole % STMP relative to the amount of anhydroglucose units of the CMC. The mixture was mixed for 2 hours at room temperature, heated to 80 C. in about 45 minutes, and mixed for an additional 15 minutes. After cooling down, 100 ml of ethanol were added and the mixture was neutralized with acetic acid. The cellulose product was filtered off and washed with ethanol/water (65/35 w/w), twice with ethanol/water (80/20 w/w), and once with 96% ethanol. The purified product was dried in vacuum at 30 C., after which it was analyzed using the methods below. Using the above method, the viscosity was measured in a 1% solution in water. The turbidity of a 1 wt % solution in water was measured using the above method.

(19) Example 2 was repeated with different amounts of STMP which correspond to 0.00 (Comparative Example 2a), 0.05 (Example 2b), 0.50 (Example 2d), 0.75 (Example 2e), and 1.00 (Example 2f) mole % STMP per anydroglucose unit of the CMC.

(20) The results are given in Table 1 below.

(21) TABLE-US-00001 TABLE 1 Characteristics of the crosslinked cellulose ethers of Example 2 Viscosity of Turbidity of STMP 1% solution 1% solution Example (mole %) STMP (gram) (mPas) (ppm) 2a 0.00 2840 57 (comparative) 2b 0.05 0.033 n.d. 79 2c 0.25 0.166 5880 65 2d 0.50 0.332 12090 75 2e 0.75 0.497 4430 496 2f 1.00 0.663 n.d. >1000 n.d. means not determined

(22) It is demonstrated that using relatively low amounts of crosslinking agent gives products that have a higher viscosity than their non-crosslinked equivalents. Also it is demonstrated that a higher degree of crosslinking gives an increased turbidity, i.e. an increased amount of insolubles.

Example 3

Preparation of Crosslinked CMC in Isopropyl Alcohol

(23) 687 g of cellulose (wood cellulose) were mixed with 906 g of isopropyl alcohol and 701 g of an aqueous solution of 49 wt % of caustic soda. After mixing for 20 minutes 453 g of isopropyl alcohol and 473 g of a 80 wt % solution of monochloro-acetic acid in water were added. The mixture was heated to 80 C. in about 30 minutes and after stirring for 90 minutes at this temperature the mixture was cooled down to 30 C. An amount of 73 grams of sodium trimetaphosphate (6 mole % relative to the anhydroglucose units) in 654 ml of water was added to the reaction mixture. The mixture was heated to 65 C. in 30 minutes and kept at this temperature for 30 minutes. The reaction mixture was cooled down to room temperature and dried in a fluid bed dryer for 30 minutes at 65 C.

(24) The settling volume was determined using the above method. The settling volume was found to be 21 ml. The viscosity of a 2% solution of the product was measured using above method and found to be less than 100 mPas.

Example 4

Preparation of Crosslinked CMC in Isopropyl Alcohol

(25) The procedure of Example 3 was followed with the following deviations.

(26) Before the addition of monochloroacetic acid, an amount of caustic soda was added to the mixture as specified in Table 2 below.

(27) The amount of STMP added was 0.00 (Comparative Example 4a), 0.50 (Example 4b), 0.75 (Example 4c), 1.00 (Example 4d), 1.25 (Example 4e), and 3.00 (Example 4f) mole %, respectively, relative to the anhydroglucose units in the cellulosic starting material. The amounts are specified in Table 2 below.

(28) TABLE-US-00002 TABLE 2 Caustic soda and STMP used in Example 4 Caustic soda STMP (mole % STMP (g in 650 ml Example (50 wt %) (g) re AGU) water) 4a (comparative) 675 0 0 4b 678 0.50 6.1 4c 679 0.75 9.2 4d 681 1.00 12.2 4e 683 1.25 15.3 4f 694 3.00 36.6

(29) The viscosity and settling volume of the products obtained were determined. The viscosity was measured using a 2% solution of the non-purified product using the above method. Additionally, the water absorption capacity of the crosslinked cellulose ethers was determined using the above tea bag test.

(30) TABLE-US-00003 TABLE 3 Settling volume and viscosity of crosslinked cellulose ether products of Example 4 STMP Settling Viscosity Water (mole volume (2% solution) absorption Example % on AGU) (ml) (mPas) capacity (g/g) 4a 0.00 >90 3907 30 (comparative) 4b 0.50 >90 28120 62 4c 0.75 >90 20890 52 4d 1.00 >90 10280 45 4e 1.25 >90 4033 39 4f 3.00 41 n.d. 23

(31) It is demonstrated that using relatively low amounts of crosslinking agent gives products that have a higher viscosity and water absorption capacity than their non-crosslinked equivalents.

Comparative Examples 5 and 6

Preparation of Crosslinked CMC Using other Phosphate Crosslinking Agents

(32) Example 3 was repeated with the following deviation. Instead of sodium trimeta-phosphate (STMP) an equivalent amount of sodium tripolyphosphate (STPP) was used (Comparative Example 5) or an equivalent amount of sodium hexameta-phosphate (SHMP) was used (Comparative Example 6). The settling volume of the products obtained was measured. The viscosity was also measured for the product of Example 6 in a 2% solution using a Brookfield viscositymeter, type LVF, at 30 rpm and 25 C.

(33) TABLE-US-00004 TABLE 4 Characteristics of crosslinked cellulose ether products of Comparative Examples 5 and 6 Comparative Amount of phosphate Settling volume Viscosity of 2% Example crosslinker (g) (ml) solution (mPas) 5 88 >90 6 76 >90 14,640

(34) The high settling volumes show that these alternative crosslinking agents give no or barely any crosslinks in a cellulose ether. The viscosity increase measured for the product of Comparative Example 6 compared to comparative Example 4a was found to be the result of a small degree of crosslinking having taken place when using SHMP.

Example 7

Preparation of Crosslinked CMC in Isopropyl Alcohol

(35) 687 g of cellulose (wood cellulose) were mixed with 906 g of isopropyl alcohol and 714 g of an aqueous solution of 49 wt % of caustic soda. After mixing for 20 minutes, 453 g isopropyl alcohol, 473 g of 80 wt % solution of monochloroacetic acid in water, and 73 g of sodium trimetaphosphate in about 250 ml water were added. The mixture was heated to 80 C. in about 30 minutes and mixed for 90 minutes at this temperature. The reaction mixture was cooled down to room temperature and dried in a fluid bed dryer for 30 minutes at 65 C. The settling volume was found to be 80 ml.

Example 8

Preparation of Crosslinked EHEC

(36) 8 kg wood cellulose were mixed with 14 kg ethyl chloride and 6 kg of a 50 wt % aqueous solution of caustic soda. After mixing 7 kg of ethylene oxide were added in portions. The temperature was gradually increased to 100 C. and the reactor content was mixed for an additional 25 minutes. After cooling down the excess ethyl chloride was evaporated.

(37) 50 g of the crude reaction mixture, having an EHEC content of about 55 wt %, were taken and 0.8 g sodium hydroxide in 10 ml water were added thereto. After mixing for 30 minutes 3.2 g sodium trimetaphosphate (STMP) in 20 ml water were added (10 mole % relative to the amount of anhydroglucose units). The mixture was mixed for 60 minutes at 80 C. After cooling down the mixture was washed three times with 2 wt % sodium sulfate solution at 95 C. The washed material was dried for 30 minutes at 70 C. in a fluid bed dryer. The resulting product was subjected to a settling volume test, giving a 21 ml settling volume. The viscosity of a 1% solution of the product was measured using the above method and found to be less than 120 mPas.

Example 9

Preparation of Crosslinked EHEC

(38) 50 g of the crude reaction mixture of Example 8, having an EHEC content of about 55 wt %, were mixed with 0.16 g of sodium trimetaphosphate (STMP) dissolved in 20 ml of water, corresponding to 0.50 mole % STMP relative to the amount of anhydroglucose units of the EHEC. The mixture was mixed 1 hour at 80 C.

(39) After cooling down the mixture was washed three times with 2 wt % sodium sulfate solution at 92 C. The washed material was dried for 30 minutes at 70 C. in a fluid bed dryer.

(40) Example 9 was repeated with different amounts of STMP which correspond to 0.00 (Comparative Example 9a) and 6.6 (Example 9c) mole % STMP per anhydro-glucose unit of the EHEC. In Example 9c an amount of 0.6 gram of sodium hydroxide in 10 ml water was added before STMP addition as described in Example 8.

(41) Using the above method, the viscosity was measured in a 1% solution in water. The results are given in Table 5 below.

(42) TABLE-US-00005 TABLE 5 Characteristics of the crosslinked cellulose ethers of Example 9 Viscosity of 1% Example STMP (mole %) STMP (gram) solution (mPas) A 0.00 0.00 1,155 (comparative) B 0.50 0.16 1,626 C 6.6 2.1 360

(43) It is demonstrated that using relatively low amounts of crosslinking agent gives products that have a higher viscosity than their non-crosslinked equivalents and that using relatively high amounts of crosslinking agent gives products with an even lower viscosity than the non-crosslinked cellulose ether.