Process for producing cross-linked cellulose ethers

Abstract

A method for producing a crosslinked cellulose ether comprises contacting an activated cellulosic material with (i) an aqueous crosslinking agent emulsion and (ii) at least one etherifying agent; wherein the aqueous crosslinker emulsion is (ia) at least one crosslinker; (ib) water; and (ic) a mixture of any other optional components as desired; wherein the aqueous crosslinking agent emulsion (i) and the at least one etherifying agent (ii) react with the activated cellulosic material to form the crosslinked cellulose ether; and a crosslinked cellulose ether produced by the above process.

Description

[0001] This application is a divisional application of Chinese Patent Application No. 202080025831.8 (filing date: April 17, 2020, invention title: Method for producing crosslinked cellulose ethers). Technical Field

[0002] The present invention relates to a method for producing crosslinked cellulose ethers and crosslinked cellulose ethers prepared by such methods. Background Art

[0003] Cellulose derivatives, such as cellulose ethers, are widely used as, for example, thickeners, binders, adhesives, and dispersants, water retention agents, protective colloids, stabilizers, and suspending agents, emulsifiers, and film formers due to their excellent properties and physiological safety. In addition, cellulose ethers are known to be used in dry-mixed mortars for various construction applications to improve the rheology of the mortar. In addition, cellulose ethers are used in mortars to impart water retention properties, thereby restricting the loss of water from the mortar to the absorbent substrate. This ensures that there is sufficient water available for the hydraulic binder (cement or gypsum) during the setting reaction, thereby resulting in a high mechanical strength of the final product. Lack of water will lead to incomplete setting, poor mechanical strength, crack formation, and low abrasion resistance.

[0004] To date, cellulose ethers have been produced by well-known conventional methods, which include subjecting a cellulose starting material, such as cellulose pulp, to two process operations (steps or stages): (1) an alkalization operation and (2) an etherification operation. For example, the well-known prior art conventional procedures for manufacturing cellulose ethers are described and illustrated in U.S. Patent No. 6,235,893 B1. The conventional method includes the following steps: (1) alkalizing the cellulose pulp; and (2) etherifying the alkalized cellulose pulp to form a cellulose ether. For example, in the conventional method, the cellulose pulp is alkalized with sodium hydroxide and etherified with chloromethane and an alkylene oxide (ethylene oxide or propylene oxide). Also, each of the above alkalization and etherification operations can be carried out stepwise, i.e., in one step or in two or more different separate steps, and each step can be carried out for a predetermined period of time under specified pressure and temperature process conditions. After manufacturing the cellulose ether, the cellulose ether can undergo additional desired process steps, such as: (3) washing the cellulose ether; (4) drying the cellulose ether; and (5) grinding the cellulose ether into a particulate form. Typically, the alkalization and etherification operations can be carried out in a single reactor or two or more reactors.

[0005] Recently, the conventional method of manufacturing cellulose ethers has been improved by using crosslinking techniques to manufacture crosslinked cellulose ethers. Crosslinking techniques involve cellulose ethers that can be chain extended or crosslinked using bifunctional crosslinking agents such as dichloromethane, epichlorohydrin, or various diglycidyl ethers. For example, U.S. Patent No. 6,958,393 B2 (equivalent to EP1384727 B9) discloses a method of manufacturing crosslinked cellulose ethers containing polyether groups using crosslinking techniques. The crosslinked cellulose ethers are produced by crosslinking the cellulose ethers at a temperature of 90 degrees Celsius (°C) or lower, in an inert atmosphere, such as nitrogen (N 2 ), in the presence of a crosslinking agent containing a polyether group and in the presence of a caustic or base. The crosslinking of the cellulose ethers is carried out in the reactor in which the cellulose ethers themselves are manufactured and in the presence of a caustic or base.

[0006] One advantage of using crosslinked cellulose ethers, for example in dry-mix mortar formulations, compared to pure non-crosslinked cellulose ethers is that by crosslinking the cellulose ether with a crosslinking agent, the viscosity of the aqueous solution of the cellulose ether can be increased and the resulting crosslinked cellulose ether having an increased or high viscosity can still remain water-soluble in the aqueous solution. Also, crosslinked cellulose ethers having an increased or high viscosity produced using crosslinking techniques can be used at reduced dosage levels, for example in mortar applications, without compromising the product and application properties. Cellulose ethers are a relatively expensive component used in mortar formulations, and any reduction in the dosage of cellulose ether in the formulation can result in cost savings for the formulation.

[0007] In previously known methods, the dosage of the crosslinking agent used to crosslink the cellulose ether was kept very low in order to prevent "overcrosslinking", which in turn provided a polymeric product that remained water-insoluble and did not contribute to the viscosity of the aqueous solution. However, it was very difficult to achieve a uniform distribution of the small amount of crosslinking agent added to the contents of a large reactor using previously known methods.

[0008] As used herein, "overcrosslinking" with reference to the crosslinking of cellulose ethers means that the reaction of the cellulose ether with the crosslinking agent is too extensive and results in a three-dimensional network of covalently linked cellulose ether chains; and such chains are insoluble in water and appear as gel particles in the aqueous phase. Overcrosslinking results in a decrease in the water-solubility of the cellulose ether in the aqueous solution. When overcrosslinking occurs in the cellulose ether, the amount of the resulting crosslinked cellulose ether that is soluble in the aqueous solution is reduced; and an appropriate viscosity of the crosslinked cellulose ether cannot be achieved in the aqueous solution. In other words, overcrosslinking results in an undesired decrease in viscosity. Compared to non-overcrosslinked cellulose ethers, overcrosslinked cellulose ethers exhibit a degree of crosslinking that results in a decrease in water-solubility.

[0009] For example, the known method for crosslinking cellulose ethers described in U.S. Patent No. 6,958,393 B2 includes the following steps, such as: (1) alkalizing cellulose with an aqueous alkali metal hydroxide solution in the presence of a suspension medium; (2) reacting the alkalized cellulose with one or more alkylene oxides; (3) reacting the alkalized cellulose with an alkyl halide present in the suspension medium; (4) subsequently or simultaneously, reacting the alkalized cellulose with a crosslinking agent using a specified amount of the crosslinking agent; (5) additionally adding an alkali metal hydroxide and / or an alkalizing agent to the reaction mixture of step (4); and (6) purifying and drying the resulting crosslinked cellulose ether.

[0010] The method described in U.S. Patent No. 6,958,393 B2 requires: (1) adding a crosslinking agent under high pressure during the etherification step of the method; (2) using epichlorohydrin (ECH) as a crosslinking agent in some cases; (3) adding a low level of the crosslinking agent in a pure state or dispersed in an organic solvent; (4) the crosslinking agent being uniformly distributed at a limited low pure crosslinking agent level; and (5) the resulting product of the method exhibiting good performance. In addition to the above requirements, in some cases, using the above-known method results in poor distribution of the crosslinking agent in the reactor; and overcrosslinking of the crosslinked cellulose product and an undesired reduction in viscosity have occurred. Also, the efficiency of the known method of U.S. Patent No. 6,958,393 B2 is not easily transferred from the laboratory scale to the pilot plant scale, and / or ultimately to the full-scale industrial plant scale. Therefore, a greater degree of expertise, knowledge, and technical effort is required to implement the above-known method on a plant scale; and to function under the conditions of the known method.

[0011] It has been found that, in order to carry out a successful crosslinking process, when the crosslinking agent is introduced into the process in a low amount, a uniform distribution of the crosslinking agent must be achieved. A uniform distribution must be achieved to produce the desired branched polymer and to prevent local overdose that leads to overcrosslinking. Overcrosslinking (i.e., a high level of crosslinking) results in an increase in the level of local crosslinked networks and local insoluble materials. It has also been found that overcrosslinking can depend on when and at which stage, step, or operation in the crosslinked cellulose ether production method the crosslinking agent is introduced into the process. For example, the crosslinking agent can be introduced or added before, during, or after the etherification operation. However, since the alkalization and etherification reactions are exothermic, there are many factors that can affect whether overcrosslinking occurs in the process. The factors can include, for example, the reaction time, the type of crosslinking agent introduced into the process, the amount of the crosslinking agent introduced into the process, how the crosslinking agent is introduced into the process, the process conditions when the crosslinking agent is introduced into the process, and the time period over which the crosslinking agent is introduced into the process.

[0012] Accordingly, there is a desire to provide a new and improved method for producing crosslinked cellulose ethers, which can be used on an industrial plant scale and implemented with higher efficiency than known conventional methods; and at the same time, wherein the properties of the resulting product of such improved methods remain the same or better than those of the known products of conventional methods. Summary of the Invention

[0013] The problems of the prior art methods can be solved by using the method of the present invention. The present invention relates to a novel method for preparing crosslinked cellulose ethers (referred to herein as "XCE") with a suitably increased viscosity.

[0014] The present invention relates to a method for producing XCE, which comprises contacting an activated cellulose material with (i) an aqueous crosslinking agent emulsion; and (ii) at least one etherifying reagent; wherein the aqueous crosslinking agent emulsion and the at least one etherifying reagent form a reaction mixture, which reacts with the activated cellulose material to form XCE. The formed XCE includes, for example, hydroxyethyl methylcellulose derivatives. XCE is formed in the presence of at least one crosslinking agent present in the aqueous crosslinking agent emulsion. In a preferred embodiment, the aqueous crosslinking agent emulsion comprises: (ia) at least one crosslinking agent; (ib) water; and (ic) any other optional components desired.

[0015] In another preferred embodiment, the present invention relates to a method for producing XCE, which comprises the steps of: (A) mixing at least one crosslinking agent with water to form an aqueous crosslinking agent emulsion; (B) contacting the activated cellulose material with (Bi) the crosslinking agent emulsion of step (A) and (Bii) at least one etherifying reagent to form a reaction mixture, which reacts with the activated cellulose material to form XCE; and (C) optionally, performing one or more steps of purification, washing, drying, granulating, and grinding on the XCE from step (C).

[0016] In yet another preferred embodiment, the aqueous crosslinking agent emulsion is introduced or added to the process during the etherification operation.

[0017] In still another embodiment, the present invention relates to an XCE produced by the above method. When compared with non-crosslinked cellulose ethers, the XCE produced by the above method of the present invention advantageously has high water solubility, resulting in an increase in viscosity in an aqueous solution.

[0018] Yet another embodiment of the present invention relates to a dry-mixed mortar forming composition, which comprises the above XCE.

[0019] The use of the method of the present invention provides an effective production method for manufacturing XCE. Surprisingly, it has been found that crosslinking agents such as diglycidyl ether can be easily dispersed in water; and the resulting aqueous dispersion can be easily distributed throughout the contents of a large reactor, thereby providing a more uniform distribution of the crosslinking agent in the contents of the reactor. Thus, the formation of excessive crosslinked particles that are insoluble in water can be avoided. Since excessive crosslinked particles are not produced, the use of crosslinking agents in aqueous dispersions is more effective than in organic solvents; and the total dose of the crosslinking agent can be reduced while maintaining excellent effectiveness of the crosslinking agent.

[0020] Some other benefits of the method of the present invention include, for example: (1) no excessive crosslinking in large-scale plant processes (only the desired viscosity increase); (2) low water solubility of the crosslinking agent and formation of an aqueous emulsion; (3) a small amount of crosslinking agent is required in the process, thereby providing a significant reduction in the crosslinking agent dose; and (4) in large-scale plant processes, a uniform distribution of a low amount of crosslinking agent is possible. Additionally, advantageously, the method of the present invention provides an XCE product having the same or better performance characteristics as known products prepared by known methods. Further, advantageously, the method of the present invention can be carried out on a large-scale plant scale, the crosslinking agent dosing occurs in the etherification step of the method of the present invention and there are no problems of excessive crosslinking. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 A schematic flow chart showing the method of the present invention. DETAILED DESCRIPTION

[0022] "Uniform distribution" in the present disclosure, and with respect to bringing cellulose fibers into contact with different components, herein means that the cellulose fibers and other components have the same distribution in all the smallest volume elements of a given volume. The smallest volume element is a cube having a side length ten times the average particle size of the cellulose fibers in the given volume.

[0023] "Aqueous crosslinking agent emulsion" in the present disclosure herein means a beneficial composition comprising a combination of at least one crosslinking agent and water, which exhibits the advantageous properties of being easy to prepare and being uniformly distributed in the activated cellulose material during etherification. The aqueous crosslinking agent emulsion of the present invention is also non-toxic; and provides an XCE with a desired viscosity increase such that the final XCE product is water-soluble in an aqueous solution. The use of the aqueous crosslinking agent emulsion of the present invention also advantageously prevents or at least minimizes excessive crosslinking during etherification.

[0024] Generally, the method for producing XCE includes an alkalization step and an etherification step. A grinding step can be carried out before the alkalization step and is typically desirable; and water (hot H 2(O) The washing / granulation step and / or the drying / milling step may be carried out after the etherification step. In the etherification step of the process, a crosslinking agent is added to the etherification operation to provide crosslinking of the cellulose ether during the etherification step.

[0025] In a broad embodiment, the present invention relates to the crosslinking agent dosage and the addition of the crosslinking agent to a process for producing XCE products. In a preferred embodiment, the crosslinking agent is added or dosed to the etherification step or operation of the process in the form of an emulsion.

[0026] The use of a small dosage of the crosslinking agent in the present invention results in an ultra-high viscosity product having the same rheological properties as known products (e.g., a high viscosity level measured in millipascal seconds [mPa.s]), but the crosslinking agent has higher efficiency. Advantageously, the result is a reduced level of undesired side reactions and a minimal impact on wastewater treatment. In addition, in the present invention, the dosage of the expensive crosslinking agent is reduced and over-crosslinking is prevented.

[0027] The benefit of the crosslinking agent dosage used in the present invention is the use of water as the suspension medium for the crosslinking agent, and thus the purpose of uniform distribution can be more easily achieved compared to conventional methods. In addition, the present invention using an aqueous crosslinking agent emulsion has no safety problems and environmental problems in an XCE manufacturing plant, as is the case with methods known in the art that use organic solvents as diluents for the crosslinking agent.

[0028] Additional benefits of the method of the present invention include, for example, (1) the method uses easily obtainable crosslinking agents based on diglycidyl ether chemistry, such as Epilox M 985 or Epilox P13-42; (2); the emulsion is non-toxic, has a very high boiling point, and has a high molecular weight (Mw) (e.g., higher than 600 grams per mole [g / mol]). In contrast, known methods use epichlorohydrin (ECH) as the crosslinking agent system; and such known methods have several disadvantages, including, for example, that epichlorohydrin is known to be toxic, carcinogenic, and has a low boiling point (116 °C) / low Mw (92.53 g / mol). Table I describes the comparison of the physical data of Epilox products with epichlorohydrin.

[0029] Table I - Physical Data of Crosslinking Agents

[0030]

[0031]

[0032] The method of the present invention provides an irreversible crosslinked cellulose ether which has beneficial rheological behavior and advantages in various application fields. The present invention uses a C10-C24 alkyl difunctional compound (ethylene oxide and / or halide) or polypropylene glycol diglycidyl ether as a crosslinking agent to prepare XCE. The method of the present invention uses an aqueous emulsion to introduce the crosslinking agent into the etherification step of the method. The prior art methods of using ECH as a crosslinking agent to prepare XCE usually produce insoluble crosslinked products; and, compared with ECH, the crosslinking agent is less involved in environmental problems.

[0033] Generally, the crosslinking agent dispersion or emulsion composition useful in the present invention comprises a mixture of the following: (i) a crosslinking agent and (ii) water; and (iii) any other optional components desired.

[0034] The crosslinking agent (or crosslinking agents) used to prepare the cellulose ethers of the present invention includes, for example, one or more crosslinking agents known in the art. However, in a preferred embodiment, the crosslinking agent useful in the present invention is generally a water-insoluble or poorly water-soluble crosslinking agent. Since the crosslinking agent in the preferred embodiment is added to the process together with water, it is desirable to use a water-insoluble or crosslinking agent having a solubility in water of less than (<) 10 percent (%) to prevent the occurrence of, for example, hydrolysis and other undesired side reactions. Referring to the "water-insoluble" of the crosslinking agent means a crosslinking agent having no water solubility or low (weak or poor) water solubility < 10%. In other embodiments, the water solubility of the crosslinking agent can be from 0% to < 9% and from 0% to < 8%. The water solubility of the crosslinking agent can be measured by turbidity analysis as known in the art.

[0035] For example, the water-insoluble property of the crosslinking agent allows the optimization of the occurrence of the desired side reactions with the hydroxyl (-OH) groups of the cellulose backbone and minimizes the undesired side reactions with the -OH groups of sodium hydroxide (NaOH) used in the alkalization operation.

[0036] Crosslinking agents suitable for the present invention include, for example, compounds having the following: a polyalkylene oxide or polyalkylene glycol group and two or more, preferably two crosslinking groups that form ether bonds with the cellulose ether when crosslinking the cellulose ether, such as a halogen group, a glycidyl group or an epoxy group, or an ethylenically unsaturated group, such as a vinyl group. Suitable difunctional compounds include, for example, 1,2-dichloro(poly)alkoxy ethers, dichloro polyoxyethylene, diglycidyl polyalkoxy ethers, diglycidyl phosphonate, divinyl polyalkylene oxide containing a sulfone group; and mixtures thereof. Compounds having two different functional groups can also be used. Examples of compounds having two different functional groups include; epichlorohydrin, glycidyl (poly)oxyalkyl ester of methacrylic acid; and mixtures thereof.

[0037] In a preferred embodiment, the crosslinking agent useful in the present invention may be based on diglycidyl ether chemistry. For example, the crosslinking agent may be a diglycidyl ether type crosslinking agent as illustrated by the following chemical structure (I):

[0038]

[0039] Wherein, in the above structure (I), "n" may be 3 to 25 in one embodiment, 7 to 20 in another embodiment, and 9 to 15 in yet another embodiment.

[0040] Examples of some of the commercial cross-linking agents that can be used in the present invention, for example, cross-linking agents based on diglycidyl ether chemistry, include Epilox P13-42 and Epilox M 985 (both available from Leuna-Harze GmbH). Epilox M 985 poly(propylene glycol) diglycidyl ether cross-linking agent is a linear poly(propylene glycol) diglycidyl ether made from polypropylene glycol (PPG).

[0041] Using a crosslinking agent, such as Epilox M 985, true covalent bonding is established. This establishment of covalent bonding advantageously strengthens the cellulose backbone of the cellulose ether. The cellulose ethers of the present invention having a strengthened cellulose backbone provide a means of achieving beneficial properties such as increased viscosity of aqueous solutions. For example, crosslinking agents such as Epilox M 985 and Epilox P13-42 are preferred embodiments because such crosslinking agents provide beneficial properties. For example, preferred crosslinking agents: (1) are non-toxic; and (2) have a high boiling point (e.g., greater than (>) 200°C) and a high Mw (e.g., greater than 600 g / mol). This is in contrast to some of the known crosslinking agent systems used in the prior art, such as epichlorohydrin, which: (1) are toxic and / or carcinogenic; (2) have a low boiling point (e.g., 116°C) and a low Mw (e.g., 92.53 g / mol); and (3) show no desired viscosity increase.

[0042] Furthermore, when using the XCE of the present invention, the amount of XCE required to obtain desired properties is reduced compared to non-crosslinked cellulose ethers, for example when used in mortar formulations. Such reductions in XCE dosage in turn reduce the cost of use, which is associated with the reduced amount of cellulose ether required to obtain desired properties, for example, in cement tile adhesive applications.

[0043] Generally, the amount of the crosslinker emulsion used in the method of the present invention can be in the range of 0.0001 equivalents to 0.05 equivalents, where the unit "eq" represents the molar ratio of the number of moles of the crosslinker to the number of moles of anhydroglucose units (AGU) of the cellulose ether. In other embodiments, the amount of the crosslinker used in the method is, for example, 0.0005 equivalents to 0.03 equivalents in one embodiment and 0.001 equivalents to 0.005 equivalents in another embodiment. When the amount of the crosslinker used is higher than 0.05 equivalents, overcrosslinking may occur, and thus the produced XCE becomes insoluble. When the amount of the crosslinker used is lower than 0.0001 equivalents, for example, the increase in viscosity in a 1% solution is undetectable.

[0044] The water used to form the crosslinker emulsion can be obtained from any source. Different types of water include, for example, tap water, drinking water, and deionized water.

[0045] The crosslinker emulsion is formed by mixing the crosslinker and water by conventional mixing means before or just before the etherification step of the method for manufacturing the XCE of the present invention. In a preferred embodiment, the crosslinker is dosed, for example, as a 50% water emulsion. For example, the amount of the crosslinker in water is 10 weight percent (wt%) to 90 wt% in one embodiment, 20 wt% to 80 wt% in another embodiment, and 30 wt% to 70 wt% in yet another embodiment.

[0046] In a general embodiment, the method for producing a crosslinker emulsion that can be used in the present invention includes: mixing (a) at least one crosslinker with (b) water to form a crosslinker emulsion. The crosslinker and water are mixed to form an emulsion by any conventional mixing means such as any horizontal or vertical mixing device; or mixing can be carried out by pumping the crosslinker into water; or a combination of the feed lines of water and the crosslinker.

[0047] An example of one of the advantageous properties exhibited by the crosslinker emulsion includes that the emulsion is easily and evenly distributed on the activated cellulose material during etherification.

[0048] In a broad embodiment, the method for producing XCE from a cellulose starting material includes the following steps: (A) mixing at least one crosslinker with water to form an aqueous crosslinker emulsion as described above; and (B) contacting the activated cellulose material with (Bi) the crosslinker emulsion of step (A) and (Bii) at least one etherifying agent to form a reaction mixture, which reaction mixture then reacts to form XCE such that XCE is formed in the presence of at least one crosslinker.

[0049] Reference Figure 1, showing the method of the present invention, generally indicated by reference numeral 10, having various general process operations or steps for enhancing the viscosity of an aqueous solution and being useful, for example, in XCE for dry-mix mortar formulations. As Figure 1 shown, the method includes, for example: a cellulose starting material 11, such as pulp, and an alkalizing reagent 12, which respectively travel to an alkalizing step 13 as indicated by arrows 11a and 12a. In the alkalizing step 13, the alkalizing reagent 12 is mixed with the cellulose material 11 to form activated cellulose 14 as indicated by arrow 13a. As indicated by arrow 14a, the formed activated cellulose 14 travels to an etherification step 15. In the etherification step 15, an etherifying reagent 16 is introduced into the etherification step 15 as indicated by arrow 16a to form a cellulose ether. Also introduced into the etherification step 15 is an aqueous crosslinker emulsion 17, as indicated by arrow 17a, which is added to the etherification step 15, where the crosslinker emulsion 17 contacts the activated cellulose ether 14 to form XCE 18 that travels from the etherification step 15 as indicated by arrow 15a.

[0050] Referring again to Figure 1 , an optional grinding step 21 as shown by the dashed line in Figure 1 can be carried out to provide a ground granular cellulose material, which travels to the cellulose material 11 as indicated by arrow 21a and then to the alkalizing step 13 as indicated by arrow 11a. Although the grinding step 21 is optional, the grinding step 21 is typically used in the preferred embodiment such that the cellulose starting material 11 (e.g., wood pulp) can be ground to form ground cellulose fluff, which can easily flow and mix in the reactor. The ground fluff also easily contacts the alkalizing reagent, such as by spraying the alkalizing reagent onto the mixed cellulose fluff in the reactor using conventional spraying means.

[0051] In another embodiment, the XCE 18 from the etherification step 15 can travel to Figure 1 one or more additional optional operations or steps 22 as shown by the dashed line in 2 . Although the steps 22 are optional, one or more of the steps 22 are typically desired and used in the method of the present invention. The optional steps 22 can be selected from, for example: (1) a water (hot H

[0052] For example, the XCE 18 after the etherification step 15 typically has unwanted volatile by-products and salts (NaCl); and thus, in a preferred embodiment, the XCE 18 after the etherification step 15 is processed through a washing step 22 as indicated by arrow 18a to wash out the unwanted volatile by-products, salts, and other impurities from the XCE, and then a drying step 22 is carried out.

[0053] The starting material for manufacturing the cellulose ethers of the present invention is cellulose. Cellulose pulp is typically obtained from, for example, wood pulp or cotton linter pulp. The pulp is typically ground using conventional grinding means to provide cellulose in the form of powder or fluff. In a preferred embodiment, suitable cellulose starting materials for use in the present invention include ground wood pulp, ground linter cellulose, and mixtures thereof. In another preferred embodiment, wood pulp is used in the process; and the wood pulp is ground into ground fluff of cellulose as a means to make the cellulose feed more flowable when fed into the alkalization process step. Generally, the pulp is ground into particles of an optimal size, for example, in one embodiment from 1,000 micrometers (μm) to 10 μm, and in another embodiment from 900 μm to 20 μm. Coarser particle sizes can reduce the efficiency of the alkalization step, while finer particle sizes can take too much time to grind. Before the alkalization step, the reactor is typically filled with pulp.

[0054] The alkalized or "activated" cellulose material for forming XCE is produced by admixing at least one alkalizing reagent (or alkalizing agent) with at least one cellulose material under conditions for forming the activated cellulose material. Alkalizing reagents available during the alkalization step of the method of the present invention include, for example, one or more alkalizing reagents known in the art. However, for economic reasons, in a preferred embodiment, when carrying out the method of the present invention, an alkali metal hydroxide such as an aqueous solution of sodium hydroxide (NaOH) is used as the alkalizing reagent for native cellulose or cellulose hydrate. In other embodiments, other alkaline aqueous solutions, such as an aqueous solution of potassium hydroxide (KOH) or lithium hydroxide (LiOH), are also suitable for use as alkalizing reagents. In a preferred embodiment, the alkali metal hydroxide used in the present invention is 50% caustic soda, which is available from The Dow Chemical Company.

[0055] Generally, the alkalizing reagent is in the form of a mixture of the alkalizing reagent in water; and the concentration of the aqueous solution can vary within a wide range. In some embodiments, suitably, the aqueous solution is in the range of about 30 wt% to 70 wt% in one embodiment, in the range of 40 wt% to 60 wt% in another embodiment, and in the range of 45 wt% to 55 wt% in yet another embodiment. In a preferred embodiment, the aqueous solution is used as a 50% reagent in water.

[0056] The amount of the basifying reagent added to the XCE production method is 2 mol / mol AGU to 4 mol / mol AGU in one embodiment, 2.5 mol / mol AGU to 3.5 mol / mol AGU in another embodiment, and 2.7 mol / mol AGU to 3.2 mol / mol AGU in yet another embodiment.

[0057] The basifying step of the method is carried out under low pressure and low temperature conditions. For example, the pressure of the basifying step is in the range of 0 kPa to 500 kPa in one embodiment, in the range of 100 kPa to 400 kPa in another embodiment, and in the range of 200 kPa to 300 kPa in yet another embodiment. The above pressure ranges are the typical pressure level ranges in the reactor during the basifying step. And, for example, the temperature of the basifying step of the method is in the range of 10 °C to 50 °C in one embodiment, in the range of 15 °C to 45 °C in another embodiment, and in the range of 20 °C to 40 °C in yet another embodiment. The above temperature ranges are the typical temperature ranges in the reactor during the basifying step. A higher temperature used in the method of the present invention will result in an undesired pressure increase.

[0058] To produce a cellulose ether according to the method of the present invention, an etherifying reagent is mixed with the activated cellulose material prepared in the above basifying step. The etherifying reagents for preparing the cellulose ether include, for example, one or more etherifying reagents known in the art. For example, the etherifying reagents include ethylene oxide (EO), propylene oxide (PO), butylene oxide (BO), methyl chloride (MCl), chloroethane, chloroacetic acid, and mixtures thereof. In a preferred embodiment, the etherifying reagents that can be used in the present invention are, for example, MCl, PO, EO, and mixtures thereof.

[0059] The amount of the etherifying reagent added to the etherification operation of the XCE production method includes, for example, 4 mol / AGU to 6 mol / AGU in one embodiment, 4.5 mol / AGU to 5.5 mol / AGU in another embodiment, and 4.7 mol / AGU to 5.3 mol / AGU in yet another embodiment. If the amount of the etherifying reagent used < 4 mol / AGU, then the cellulose ether may be under-substituted and not completely soluble in water. If the amount of the etherifying reagent used > 6 mol / AGU, then this higher dose of the etherifying reagent may cause the cellulose ether to become hydrophobic and its water solubility to decrease.

[0060] The etherifying reagent added to the etherification step to contact the activated cellulose can be carried out by any known addition means, for example, by pumping a liquid compound into the reactor.

[0061] The etherification step of the method is carried out, for example, in a reaction vessel (reactor) under an inert atmosphere. The inert materials used in the method include, for example, N 2 , argon, and mixtures thereof. During the etherification step of the method and as the reactants react and proceed to form the cellulose ether material, the pressure and temperature increase due to the exothermic reaction.

[0062] As the cellulose ether product is formed during the etherification step, the pressure of the etherification step of the method is, in one embodiment, in the range of, for example, 1,000 kPa to 3,500 kPa, in another embodiment, in the range of 1,500 kPa to 3,000 kPa, and in yet another embodiment, in the range of 2,000 kPa to 2,500 kPa. At pressures <1,500 kPa, the reaction rate is too slow; and at pressures >2,500 kPa, special high-pressure equipment is required.

[0063] The temperature of the etherification step of the method is, in one embodiment, in the range of, for example, 60 °C to 120 °C, in another embodiment, in the range of 70 °C to 110 °C, and in yet another embodiment, in the range of 80 °C to 100 °C. At temperatures <60 °C, the reaction rate becomes unacceptably slow. At temperatures >100 °C, unwanted side reactions can occur; and the pressure increases, and if the pressure increases beyond, for example, 2,500 kPa, then a different pressure reactor capable of handling high pressure and more expensive will be required.

[0064] When the crosslinker emulsion of the present invention is added to the etherification operation of the method and as the etherification process step of the method proceeds, the crosslinking of the activated cellulose ether of the XCE of the present invention begins to occur. The above crosslinker emulsion is used in the crosslinker addition step of the method of the present invention; and the amount of the crosslinker emulsion used in the method of the present invention is such that the crosslinker present in the etherification step is in the range of 0.0001 equivalent to 0.05 equivalent as described above.

[0065] As the cellulose ether product is formed during the etherification step, the crosslinker emulsion is added to the etherification step. The pressure of the crosslinker emulsion addition step of the method is, in one embodiment, in the range of, for example, 1,000 kPa to 3,500 kPa, in another embodiment, in the range of 1,500 kPa to 3,000 kPa, and in yet another embodiment, in the range of 2,000 kPa to 2,500 kPa.

[0066] The temperature of the crosslinking agent addition step of the method is in the range of, for example, 60°C to 120°C in one embodiment, in the range of 70°C to 110°C in another embodiment, and in the range of 80°C to 100°C in yet another embodiment. At temperatures <60°C, the reaction rate becomes unacceptably slow; and at temperatures >100°C, unwanted side reactions can occur and the pressure increases. If the pressure increases by more than, for example, 3,000 kPa, then a different pressure reactor capable of handling high pressure and more expensive will be required.

[0067] Generally, the crosslinking agent emulsion is added to the etherification step of the method and mixed with the activated cellulose material; such that the etherification reagent used with the activated cellulose material in the etherification step forms a homogeneous reaction mixture. The homogeneously distributed reaction mixture reacts to form the XCE product. The cellulose ether typically used in the crosslinking reaction with the crosslinking agent emulsion is a mixed cellulose ether containing hydroxyalkyl groups and alkyl ether groups. For example, in one embodiment of the mixed cellulose ether containing hydroxyalkyl groups and alkyl ether groups, it includes alkyl hydroxyethyl cellulose, such as hydroxyalkyl methyl cellulose.

[0068] In other embodiments, examples of cellulose ether compounds suitable for crosslinking include methyl cellulose (MC), ethyl cellulose, propyl cellulose, butyl cellulose, hydroxyethyl methyl cellulose (HEMC), hydroxypropyl methyl cellulose (HPMC), hydroxyethyl cellulose (HEC), methyl hydroxyethyl hydroxypropyl cellulose (MHEHPC), ethyl hydroxyethyl cellulose (EHEC), methyl ethyl hydroxyethyl cellulose (MEHEC), hydrophobically modified ethyl hydroxyethyl cellulose (HMEHEC), hydrophobically modified hydroxyethyl cellulose (HMHEC), sulfoethyl methyl hydroxyethyl cellulose (SEMHEC), sulfoethyl methyl-hydroxypropyl cellulose (SEMHPC), sulfoethyl hydroxyethyl cellulose (SEHEC), and mixtures thereof. In some preferred embodiments, the mixed cellulose ether may include, for example, HEMC, HPMC, and mixtures thereof.

[0069] For the cellulose ethers useful in the present invention, alkyl substitution is described in cellulose ether chemistry by the term "degree of substitution DS", as determined by the Zeisel method. The DS is the average number of substituted OH groups per anhydroglucose unit. Methyl substitution may be reported, for example, as DS(methyl) or DS(M). Hydroxyalkyl substitution is described by the term "molar substitution MS", as determined by the Zeisel method. The MS is the average number of moles of etherifying reagent bonded as ether per mol of anhydroglucose units. Etherification with the etherifying reagent EO is reported, for example, as MS(hydroxyethyl) or MS(HE). Etherification with the etherifying reagent PO is correspondingly reported as MS(hydroxypropyl) or MS(HP). Side groups are determined using the Zeisel method (reference: G. Bartelmus and R. Ketterer, Z. Anal. Chem. 286 (1977), 161 - 190).

[0070] The XCE produced by the method of the present invention includes, for example, any of the above cellulose ethers having an alkyl degree of substitution. For example, in one embodiment, the crosslinked HEC has a degree of substitution MS(HE) of 1.5 to 4.5, and in another embodiment has a degree of substitution MS(HE) of 2.0 to 3.0. In yet another embodiment, a mixed ether of methylcellulose to be crosslinked can be used in the method of the present invention. For example, in the case of HEMC, in one preferred embodiment, the DS(M) value ranges from 1.2 to 2.1; in another embodiment from 1.3 to 1.7; and in yet another embodiment from 1.35 to 1.6. In another preferred embodiment, the MS(HE) value can range from 0.05 to 0.75; in another embodiment from 0.15 to 0.45; and in yet another embodiment from 0.20 to 0.40. In the case of HPMC, in one preferred embodiment, the DS(M) value can range from 1.2 to 2.1; and in another embodiment from 1.3 to 2.0. In yet another preferred embodiment, the MS(HP) value can range from 0.1 to 1.5; and in another embodiment from 0.2 to 1.2.

[0071] After the above etherification step, the produced XCE product can be processed through various additional optional processing steps. For example, the resulting XCE can be washed or purified, granulated, dried, and / or ground into powder form using conventional methods commonly used for producing cellulose derivative products and the resulting powders. For example, before or after washing, volatile organic components present in the XCE can be reduced or removed from the XCE product by distillation or steam stripping. The optional steps are well known to those skilled in the art.

[0072] Some of the advantageous properties exhibited by the XCE product manufactured by the above method of the present invention may include, for example, an increase in the viscosity of the aqueous solution, enabling a reduction in the dosage in dry-mixed mortar formulations without compromising performance.

[0073] For example, when compared to an aqueous solution of a non-crosslinked cellulose ether based on the same pulp source, the increase in the viscosity of the aqueous solution of XCE produced by the method of the present invention includes an increase in viscosity of >15% in one embodiment, >20% in another embodiment, >30% in yet another embodiment, and >50% in still another embodiment.

[0074] In a broad embodiment, the XCE of the present invention can be used as an additive in dry-mixed mortar formulations, plastering, cement extrusion, etc. For example, in a method of manufacturing a dry-mixed mortar formulation, the method includes the step of mixing: (A) the above XCE used as a water retention agent; and (B) desired conventional dry-mixed mortar formulation components, such as a hydrolyzed binder, including, for example, cement or gypsum.

[0075] In a preferred embodiment, the mortar formulation can be prepared by mixing components (A) and (B) by conventional mixing means known in the art (e.g., physical blend of solid powders). Some of the advantageous properties exhibited by the mortar formulation may include, for example, the ability to use a lower dosage of XCE (>10%) thereby resulting in a reduction in formulation cost. Generally, in order to prepare the mortar formulation, the amount of XCE used as the water retention agent component (A) can be, for example, 0.01 wt% to 1.0 wt% in one embodiment, 0.05 wt% to 0.8 wt% in another embodiment, and 0.1 wt% to 0.5 wt% in yet another embodiment. Below these levels, the water retention of the mortar is insufficient, and above these limits, the cost of the formulation becomes too high.

[0076] Conventional dry-mixed mortar formulation components, i.e., component (B), may include components such as hydrolyzed binders such as cement, gypsum, fly ash, slag, etc., and aggregates (sand), fine fillers (calcium carbonate, fumed silica, dolomite, etc.), air-entraining agents, defoaming agents, redispersible polymer powders, water repellents; and mixtures thereof.

[0077] Examples

[0078] The following examples are provided to illustrate the present invention in further detail, but should not be construed as limiting the scope of the claims. Unless otherwise indicated, all parts and percentages are by weight.

[0079] The explanations of various terms and nomenclatures used in the Invention Examples (Inv.Ex.) and Comparative Examples (Comp.Ex.) are as follows:

[0080] "HEMC" represents hydroxyethyl methyl cellulose.

[0081] "AGU" represents the anhydroglucose unit of the cellulose ether.

[0082] "LVN" represents the limiting viscosity number of the pulp measured according to the procedure described in ISO 5351 (2010).

[0083] The various raw materials or components used in the examples are explained as follows:

[0084] Epilox M 985 is a poly(propylene glycol) diglycidyl ether available from Leuna Harze.

[0085] Examples of crosslinking compounds that can be used in the method of the present invention and are based on diglycidyl ethers are described in Eugene W. Jones, "Crosslinking of Cotton Cellulose with Diglycidyl Ether", Journal of Applied Polymer Science, Vol. V, No. 18, 714 - 720 (1961).

[0086] Test Methods

[0087] Insoluble Content

[0088] Dissolve the cellulose ether sample in water: 1.0 part by weight of HEMC and 99.0 parts by weight of water. HEMC (dry basis) is dispersed in water with stirring at room temperature (about 25 °C) to avoid forming lumps.

[0089] Centrifuge 50 g of the HEMC solution in a Thermo Scientific Sorvall Lynx 4000 centrifuge at 13535 UPM for 30 minutes (min). The insoluble matter ("insolubles") of the prepared solution will accumulate at the bottom of the centrifuge bottle.

[0090] Weigh a certain amount of the suspension solution and dry it overnight in an oven at 105 °C. After weighing the resulting residue, calculate the soluble matter content. Subtract the obtained value from the starting concentration to get the insoluble matter content.

[0091] Inventive Examples 1 and 2 and Comparative Examples A - C

[0092] Typically, HEMC is produced according to the principle of Williamson ether synthesis. After activating the ground cellulose with 50% caustic soda, alkalized cellulose is produced. Then it is etherified with MCl and EO. For example, useful guidelines for this method are found in R. Donges, “Non-Ionic cellulose Ethers”, British Polymer Journal, Vol. 23, pp. 315 - 326 (1990). The crosslinking compound used in the method of the present invention is based on diglycidyl ether; and an overview of the characteristics of the crosslinking agent used in the examples described in Table II.

[0093] The limiting viscosity number (LVN) of the pulp is measured according to the procedure described in ISO 5351 (2010). The ground cellulose fluff (400 mol; LVN greater than or equal to (≥) 1,450 mL / g) is added to a 1,000 liter (L) autoclave (reactor).

[0094] After purging the autoclave three times with N 2 The autoclave is heated to 40 °C. Then, dimethyl ether (DME, 4.7 mol / mol AGU) and the first batch of chloromethane (“MCl 1”; 3.2 mol / mol AGU) are added to the autoclave. During a 2 - minute period at a temperature of 40 °C, the first batch of caustic soda (“NaOH 1”; (strength 50%, 1.9 mol NaOH / mol AGU) is added in 3 portions to the mixture in the autoclave. Then the mixture is maintained at 40 °C for 30 minutes. Then EO (0.45 mol / mol AGU) is added to the mixture in the autoclave; and the resulting reaction mixture is maintained at 40 °C for 10 minutes. Then, the crosslinking agent is sprayed into the autoclave reactor.

[0095] Before adding the emulsion to the reactor, the crosslinking agent of the method of the present invention is prepared as an emulsion with water via thorough mixing of the water and crosslinking agent phases. Then, the emulsion is sprayed into the reactor immediately after its production. In the comparative method, 100% pure crosslinking agent is sprayed into the reaction mixture in the reactor.

[0096] The material formed in the reactor is heated to 80 °C within 45 minutes. At 80 °C, a second batch of MCl (“MCl2”; 1.3 mol / mol AGU) is rapidly injected into the material. Then, a second batch of caustic soda (“NaOH 2”; 0.67 mol / mol AGU) is added in 7 portions over a 30-minute period, followed by a digestion time of 70 minutes at 80 °C. After the 70-minute digestion time, the resulting XCE product is formed. The resulting XCE product is then subjected to the following process steps using conventional procedures known in the art: hot water washing, neutralization with formic acid, granulation using a laboratory granulator (Bosch Mum), drying, and grinding.

[0097] Comparative Example A, Invention Example 1, and Invention Example 2 described in Table II are carried out using the method of the present invention, except that no crosslinking agent is used in Comparative Example A. Comparative Example B and Comparative Example C described in Table III are carried out using the above comparative method.

[0098] Table II

[0099]

[0100] The addition of an aqueous emulsion of 50% Epilox results in a significant increase in the viscosity of the 1% aqueous solution. At the same time, the amount of insoluble matter is even lower than the level of the uncrosslinked comparative example.

[0101] Table III

[0102]

[0103]

[0104] When the crosslinking agent is used as a pure additive, a significant decrease in the viscosity of the 1% aqueous solution is observed. At the same time, the increase in the amount of insoluble matter indicates uneven or non-uniform distribution of the crosslinking agent present in the reactor.

Claims

1. A method for producing a crosslinked cellulose ether, which comprises contacting an activated cellulose material with (i) an aqueous crosslinking agent emulsion comprising (ia) at least one crosslinking agent and (ib) water; and (ii) at least one etherifying agent; wherein the aqueous crosslinking agent emulsion and the at least one etherifying agent form a reaction mixture, and the reaction mixture reacts with the activated cellulose material to form the crosslinked cellulose ether.

2. The method according to claim 1, which comprises the following steps: (A) Mixing (a) at least one crosslinking agent with (b) water to form an aqueous crosslinking agent emulsion; and (B) Contacting the activated cellulose material with (Bi) the aqueous crosslinking agent emulsion of step (A) and (Bii) at least one etherifying agent; wherein the aqueous crosslinking agent emulsion and the at least one etherifying agent form a reaction mixture, and the reaction mixture reacts with the activated cellulose material to form the crosslinked cellulose ether.

3. The method according to claim 1 or claim 2, which further comprises one or more steps of purifying, washing, drying, granulating and grinding the crosslinked cellulose ether.

4. The method according to claim 1, wherein the crosslinked cellulose ether is formed at a pressure of greater than or equal to 1,000 kPa and a temperature above 70 °C.

5. The method according to claim 1, wherein the activated cellulose material is formed by contacting at least one cellulose material with at least one alkalizing agent.

6. The method according to claim 5, wherein the at least one cellulose material is selected from the group consisting of wood pulp, cotton linters and mixtures thereof; and wherein the at least one alkalizing agent is selected from the group consisting of solutions of sodium hydroxide, potassium hydroxide, lithium hydroxide and mixtures thereof.

7. The method according to claim 5, wherein the concentration of the at least one alkalizing agent is from 1 mole of sodium hydroxide per mole of anhydroglucose unit of the cellulose material to 3.5 moles of sodium hydroxide per mole of anhydroglucose unit of the cellulose material to form the activated cellulose material.

8. The method according to claim 5, which further comprises a step of grinding the at least one cellulose material to form a ground cellulose fluff material; and wherein the grinding step is carried out before mixing the cellulose material with the alkalizing agent.

9. The method according to claim 1, wherein the at least one crosslinking agent is a diglycidyl ether type crosslinking agent having the following chemical structure (I): wherein n is from 3 to 25.

10. The method according to claim 1, wherein the at least one etherifying agent is selected from the group consisting of ethylene oxide, propylene oxide, butylene oxide, methyl chloride, ethyl chloride and mixtures thereof.

11. The method according to claim 1, wherein the contacting is carried out in an inert atmosphere.

12. The method according to claim 1, wherein the aqueous crosslinking agent emulsion is an emulsion with a weight ratio of crosslinking agent to water of 1 / 99 to 99 / 1.

13. The method according to claim 1, wherein the aqueous crosslinking agent emulsion is present at a concentration of from 0.0001 moles of the crosslinking agent per mole of anhydroglucose units of the cellulose ether to 0.05 moles of the crosslinking agent per mole of anhydroglucose units of the cellulose ether; and wherein the etherifying agent is present at a concentration of from 4 moles of the etherifying agent per mole of anhydroglucose units of the cellulose ether to 6 moles of the etherifying agent per mole of anhydroglucose units of the cellulose ether.

14. A crosslinked cellulose ether produced by the method according to claim 1.

15. A dry mortar formulation comprising: (I) the crosslinked cellulose ether product according to claim 14; and (II) a hydrolytic binder; and wherein the hydrolytic binder is a cement or gypsum material.

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