A method for preparing a high-air-content plastic hydroxypropyl methylcellulose ether

Abstract

This invention belongs to the field of cellulose ether technology, specifically disclosing a high-air-content plastic hydroxypropyl methylcellulose ether and its preparation method. This technology introduces betaine and imidazole bifunctional groups into a hyperbranched backbone through free radical graft polymerization to form a zwitterionic-terminated epoxy copolymer; then grafts it onto the HPMC backbone to improve the surface air-entraining capacity and bubble electrostatic stability of HPMC, fundamentally solving the problems of low air-entraining capacity, large and uneven bubble size distribution, and poor stability of conventional HPMC, and significantly improving the plasticity and workability of HPMC in building materials such as mortar and putty.

Description

Technical Field

[0001] This invention belongs to the field of cellulose ether technology, specifically relating to a method for preparing a high-air-content, plastic hydroxypropyl methylcellulose ether. Background Technology

[0002] Hydroxypropyl methylcellulose ether (HPMC), as a non-ionic cellulose ether, is an indispensable thickening and water-retaining component in the building materials industry, especially in products such as dry-mixed mortar, putty powder, tile adhesive, and thermal insulation mortar. However, in mortar or putty systems, in addition to thickening and water retention, plasticity (also known as workability, thixotropy, or smoothness) is a key indicator determining the user experience and construction efficiency. HPMC itself has a certain degree of surface activity, and can introduce a small amount of air bubbles during mixing, which contributes to improving plasticity. However, HPMC produced by conventional processes has low air entrainment (the content of air bubbles introduced by physical means is usually no more than 5%), large bubble size (100-500μm), uneven distribution, and poor stability (it collapses within minutes to tens of minutes), resulting in limited effect on improving the plasticity of mortar or putty.

[0003] To improve the plasticity of HPMC in building materials, existing technologies mainly involve introducing air-entraining components through physical mixing after HPMC synthesis. For example, CN105646950A discloses a method for preparing modified hydroxypropyl methylcellulose (HPMC) for industrial-grade thermal insulation mortars. After HPMC is etherified, washed, dried, and pulverized, synthetic organic polymers (polyvinyl alcohol, polyacrylamide, polyquaternium salts, etc.) and air-entraining agents (soluble resin salts, sodium dodecyl sulfonate, sodium dodecylbenzene sulfonate, etc.) are added to HPMC through physical mixing to produce a modified product. Another example is CN105505253B, which obtains modified HPMC by physically mixing water-soluble synthetic organic polymers and special resins after the etherification reaction. The above-mentioned physical mixing modification methods lack chemical bonding between the HPMC and the air-entraining components, making them prone to migration, loss, or uneven distribution during use, resulting in unstable product performance after long-term storage.

[0004] Several existing technologies involve grafting different functional monomers onto the backbone of HPMC or other cellulose ethers. CN111454401B describes the preparation of a temperature-sensitive thickening graft copolymer using HPMC as a matrix, copolymerized with acrylamide and diacetone acrylamide, primarily for use in petroleum enhanced oil recovery and drilling fluids. CN116970126A discloses the graft polymerization of HPMC with acrylamide and methyl allyl polyoxyethylene ether, also applied in oil extraction. Based on the increased requirements for the plasticity, air-entraining efficiency, and bubble stability of HPMC in building materials, this invention aims to provide a chemically modified hydroxypropyl methylcellulose ether, overcoming the problems of existing physically compounded modified ethers where the air-entraining components lack chemical bonding with HPMC, are prone to migration and loss, and exhibit poor batch stability and durability. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a high-air-content plastic hydroxypropyl methylcellulose ether and its preparation method. First, betaine and imidazole groups are introduced onto a hyperbranched framework via free radical graft polymerization to form a terminal epoxy-based bifunctional copolymer. Then, this copolymer is added during the later stages of HPMC etherification. The ternary epoxy ring at the end of the terminal epoxy-based copolymer opens, preferentially attacking the less sterically hindered terminal carbon atoms of HPMC, forming ether bonds. This allows the hyperbranched zwitterionic copolymer to be grafted onto the HPMC backbone via ether bonds, thereby improving surface air-entraining capacity and bubble electrostatic stability. The residual epoxy groups are hydrolyzed into terminal hydroxyl groups during subsequent pH adjustment and washing stages. This fundamentally solves the problems of low air-entraining capacity, large and unevenly distributed bubble size, and poor stability of conventional HPMC, significantly improving the plasticity and workability of HPMC in building materials such as mortar and putty.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows: This invention provides a method for preparing a high-air-content plastic hydroxypropyl methylcellulose ether, comprising the following steps: (1) Spray alkaline solution onto refined cotton, draw a vacuum, stir at 100-500 r / min for 2-6 h to alkalize and obtain activated alkali cellulose; (2) Add an etherifying agent to activated alkali cellulose. The first etherification is carried out at 0.5-1.5 MPa and 50-60℃ for 50-70 min. The second etherification is carried out at 1.5-2.0 MPa and 70-90℃ for 30-60 min. Then, a DMF solution containing zwitterionic terminal epoxy copolymer is added at 0.2-0.5 MPa and 50-60℃ and reacted for 3-5 h. The temperature is raised to 60-80℃ and kept for 1-3 h. The pH is adjusted, washed, dried and pulverized to obtain plastic hydroxypropyl methylcellulose ether with high gas content. In step (2), the zwitterionic terminal epoxy copolymer accounts for 10-20% of the mass of refined cotton.

[0007] While conventional hydroxypropyl methylcellulose ether (HPMC) possesses some gas-entraining capabilities, its gas-entraining primarily relies on physical stirring and the surface activity of HPMC itself, resulting in low bubble content, uneven particle size, and poor stability. This invention, based on the conventional HPMC etherification process, introduces a zwitterionic composite modification system: on the one hand, sulfonic acid groups provide high surface activity and rapid gas-liquid interface adsorption capacity; on the other hand, quaternary ammonium salts provide cationic charge stabilization and bubble wall structural support. The synergistic effect of these two components imbues the cellulose ether molecular chain with active groups possessing both foaming and foam-stabilizing functions, thereby improving the gas content of HPMC.

[0008] In some embodiments, the concentration of the alkali solution in step (1) is 40wt% to 50wt%; the mass ratio of refined cotton to alkali solution is 1:(0.9 to 1.2); and the alkalization temperature is controlled at ≤35℃.

[0009] In some embodiments, the zwitterionic-terminated epoxy copolymer in step (2) has a hyperbranched structure.

[0010] The multi-arm framework of hyperbranched structures can significantly increase the grafting density of subsequent monomers, enabling individual molecules to carry more functional groups, thereby enhancing the multivalent synergistic adsorption capacity at the gas-liquid interface.

[0011] In some embodiments, the preparation steps of the zwitterionic terminal epoxy copolymer are as follows: Under nitrogen protection, hyperbranched polyglycidyl ether, mixed monomers, initiator, chain transfer agent and solvent are added to the reactor; the first step is to heat the reaction; after the reaction is completed, the temperature is lowered, succinic anhydride and catalyst are added, and the second step is to heat the reaction; finally, epichlorohydrin and catalyst are added, and the third step is to heat the reaction to obtain the reaction solution; after the reaction solution is distilled under reduced pressure, the residue is poured into cold water to precipitate, filtered, dialyzed and vacuum dried to obtain zwitterionic terminal epoxy copolymer.

[0012] In some embodiments, the temperature of the first heating reaction is 60-70°C; the temperature of the second heating reaction is 55-65°C; and the temperature of the third heating reaction is 45-55°C.

[0013] In some embodiments, the mixed monomers comprise methacryloylethyl sulfobetaine and N-vinylimidazole.

[0014] Methacryloxyethyl sulfobetaine contains both quaternary ammonium cations and sulfonic acid anions, exhibiting both strong hydrophilicity and surface activity. This significantly reduces surface tension at the gas-liquid interface, promoting rapid microbubble generation and refinement. The introduction of N-vinylimidazolium into the imidazolium ring structure enhances the mechanical strength and elasticity of the bubble film through hydrogen bonding and π-π stacking interactions, synergistically improving bubble stability with methacryloxyethyl sulfobetaine.

[0015] In some embodiments, the mass ratio of the methacryloyl ethyl sulfobetaine to N-vinylimidazole is (30-50):(5-15).

[0016] In some embodiments, the mass ratio of the mixed monomers to succinic anhydride is 1:(0.2 to 0.4).

[0017] In some embodiments, the etherifying agent in step (2) comprises chloromethane and propylene oxide in a mass ratio of (80-120):(30-50).

[0018] In some embodiments, the etherifying agent in step (2) accounts for 120-150% of the mass of refined cotton.

[0019] Compared with the prior art, the present invention has the following beneficial effects: 1. This invention anchors zwitterionic terminal epoxy copolymers onto the main chain of HPMC via covalent bonds, achieving permanent and irreversible modification. This overcomes the defects of existing physical compounding technologies, such as easy migration, loss, and uneven distribution of air-entraining components, improving batch-to-batch consistency and long-term storage stability. The resulting HPMC can generate and stabilize a large number of uniform microbubbles in mortar or putty systems, imparting excellent smoothness, spreadability, and anti-sagging properties to the material during construction, thereby improving work efficiency and construction quality.

[0020] 2. The hyperbranched structure of the zwitterionic-terminated epoxy copolymer core framework enables simultaneous adsorption at multiple sites on the gas-liquid interface through functional groups at the ends of each arm. This reduces the desorption energy of individual bubbles, making the bubbles more firmly anchored at the interface and effectively resisting bubble compression and coalescence. The methacryloxyethyl sulfobetaine molecule in the mixed monomers contains both quaternary ammonium cations and sulfonic acid anions. The quaternary ammonium cations provide surface activity, reducing the surface tension of water and promoting the generation and uniform distribution of numerous bubbles. The permanent positive charge of the quaternary ammonium cations forms an electrostatic repulsion layer on the bubble wall, preventing bubble coalescence and improving bubble stability. N-vinylimidazolium forms additional physical crosslinking and toughening effects at the bubble wall interface through hydrogen bonding and π-π stacking interactions, enhancing the mechanical strength and elasticity of the bubble film. This allows the bubbles to remain intact under strong shear conditions such as stirring and pumping, extending the bubble stabilization time. Detailed Implementation

[0021] The present invention will be described below with reference to specific implementation schemes. It should be noted that the following embodiments and comparative examples are examples of the present invention and are used only to illustrate the invention, not to limit it. Other combinations and various modifications within the scope of the present invention can be made without departing from its spirit or scope. It is worth noting that, unless otherwise specified, the raw materials used in the following preparation examples and embodiments are all from any commercially available manufacturer.

[0022] Preparation Example 1 The preparation steps of zwitterionic terminal epoxy copolymer A are as follows: Under nitrogen protection, 200 g of hyperbranched polyglycidyl ether (hPG-OH, Mw=5000, terminal hydroxyl number 32), mixed monomers (80 g methacryloylethyl sulfobetaine, 20 g N-vinylimidazolium), 1.5 g AIBN, 2.5 g 2-mercaptoethanol, and 1000 g anhydrous DMF were added to a reactor. The reaction was first heated to 65±3℃ for 7 h. After the reaction, the temperature was lowered to 50℃, and 30 g succinic anhydride and 0.8 g DMAP were added. The reaction was then heated to 60±3℃ for 5 h, and the carboxyl content was determined by acid-base titration. Finally, 25 g epichlorohydrin and 1.0 g TBAB were added, and the reaction was heated to 50±3℃ for 6 h. The reaction was then measured by FT-IR at 910 cm⁻¹. -1 The characteristic peak of epoxy appeared, and the reaction solution was obtained. After the reaction solution was distilled under reduced pressure at 60°C, the residue was poured into cold water to precipitate, filtered, dialyzed with deionized water for 72 hours (molecular weight cutoff 3500 Da), and freeze-dried to obtain zwitterionic terminal epoxy copolymer A.

[0023] Preparation Example 2 The preparation steps of zwitterionic-terminated epoxy copolymer B differ from those in Preparation Example 1 in that N-vinylimidazolium is replaced by an equal amount of methacryloylethyl sulfobetaine.

[0024] Preparation Example 3 The preparation steps of copolymer C differ from those in preparation example 1 in that 80 g of methacryloyl ethyl sulfobetaine (286.4 mmol) was replaced with the same molar amount of methyl methacrylate sulfonyl ester.

[0025] Preparation Example 4 The preparation steps of copolymer D differ from those in preparation example 1 in that 80 g of methacryloylethyl sulfobetaine (286.4 mmol) was replaced with the same molar amount of methacryloyloxyethyl trimethylammonium chloride.

[0026] Example 1 A method for preparing a high-air-content plastic hydroxypropyl methylcellulose ether includes the following preparation steps: (1) By weight, spray 100 parts of 45wt% sodium hydroxide solution onto 100 parts of refined cotton, vacuum, and stir at 300r / min below 35℃ for 3h to obtain activated alkali cellulose. (2) Add etherifying agent (100 parts chloromethane and 40 parts propylene oxide) to all activated alkali cellulose. The first etherification is carried out at 0.9 MPa and 55℃ for 50 min. The second etherification is carried out at 1.8 MPa and 80℃ for 40 min. Then, add 50 parts of DMF solution containing 15 parts of zwitterionic terminal epoxy copolymer A at 0.3 MPa and 60℃ and react for 4 h. Raise the temperature to 80℃ and keep it for 2 h. Adjust the pH to 7 with glacial acetic acid, wash with hot water above 95℃, dry with airflow at 200℃ until the moisture content is ≤5%, and pulverize to 90 mesh to obtain plastic hydroxypropyl methylcellulose ether with high air content.

[0027] Example 2 This embodiment provides a method for preparing a high-air-content plastic hydroxypropyl methylcellulose ether. The specific implementation method is the same as in Embodiment 1, except that zwitterionic-terminated epoxy copolymer A is replaced by an equal amount of zwitterionic-terminated epoxy copolymer B.

[0028] Example 3 This embodiment provides a method for preparing a high-gas-content plastic hydroxypropyl methylcellulose ether. The specific implementation method is the same as in Example 1, except that: the zwitterionic terminal epoxy copolymer A is replaced by an equal part of copolymer C.

[0029] Example 4 This embodiment provides a method for preparing a high-gas-content plastic hydroxypropyl methylcellulose ether. The specific implementation method is the same as in Embodiment 1, except that: zwitterionic terminal epoxy copolymer A is replaced by an equal amount of copolymer D.

[0030] Comparative Example 1 A method for preparing hydroxypropyl methylcellulose ether, comprising the following preparation steps: (1) By weight, spray 100 parts of 45wt% sodium hydroxide solution onto 100 parts of refined cotton, vacuum, and stir at 300r / min below 35℃ for 3h to obtain activated alkali cellulose. (2) Add etherifying agent (100 parts chloromethane and 40 parts propylene oxide) to all activated alkali cellulose. The first etherification is carried out at 0.9 MPa and 55℃ for 50 min; the second etherification is carried out at 1.8 MPa and 80℃ for 40 min; adjust the pH to 7 with glacial acetic acid, wash with hot water above 95℃, dry with airflow at 200℃ until the moisture content is ≤5%, and pulverize to 90 mesh to obtain hydroxypropyl methylcellulose ether.

[0031] Comparative Example 2 A method for preparing a physically mixed hydroxypropyl methylcellulose ether includes the following preparation steps: (1) By weight, spray 100 parts of 45wt% sodium hydroxide solution onto 100 parts of refined cotton, vacuum, and stir at 300r / min below 35℃ for 3h to obtain activated alkali cellulose. (2) Add etherifying agent (100 parts chloromethane and 40 parts propylene oxide) to all activated alkali cellulose. The first etherification is carried out at 0.9 MPa and 55℃ for 50 min. The second etherification is carried out at 1.8 MPa and 80℃ for 40 min. Adjust the pH to 7 with glacial acetic acid, wash with hot water above 95℃, dry with airflow at 200℃ until the moisture content is ≤5%, pulverize to 90 mesh, take 100 parts and mix with 12 parts sodium dodecylbenzenesulfonate at 800 rpm for 30 min to obtain the physically mixed hydroxypropyl methylcellulose ether.

[0032] Performance testing Hydroxypropyl methylcellulose ethers provided in Examples 1-4 and Comparative Examples 1-2 were mixed with standard dry mortar (cement:standard sand mass ratio = 1:3) at dosages of 0.2%, 0.3%, and 0.5%, respectively. Water was added at a water-cement ratio of 0.5, and the mixture was stirred for 5 minutes to obtain fresh mortar. The fresh mortar was tested as follows, and the results are shown in Table 1: 1. Air content (air content) of fresh mortar: The air content of fresh mortar is determined by the air pressure method in GB / T 50080-2016 "Standard for Test Methods of Performance of Ordinary Concrete Mixtures"; the instrument is a direct-reading mortar air content meter.

[0033] 2. Average particle size and particle size distribution of bubbles: Take 0.5% of fresh mortar, use a dropper to evenly spread it on a glass slide, cover it with a coverslip, observe and photograph it with an optical microscope (×200x); use ImageJ image analysis software to randomly count the diameter of at least 200 bubbles, calculate the arithmetic mean particle size in μm; and record the particle size (D50).

[0034] 3. Bubble stability (foam holding capacity): After the fresh mortar with an admixture of 0.5% has been left to stand for 30 minutes, the air content is tested again according to test 1. Calculate the air retention rate = (air content after standing for 30 minutes / initial air content) × 100%.

[0035] 4. Mortar fluidity: According to GB / T 2419-2005 "Method for Determination of Flowability of Cement Mortar", the initial fluidity (mm) of freshly mixed mortar with a dosage of 0.5% was tested using the jumping table method.

[0036] Table 1 Performance Test Results

[0037] As shown in Table 1, the gas content of all samples increased from 0.2% to 0.5%, but the chemically grafted zwitterionic copolymers in Examples 1 and 2 showed the largest increase, indicating that gas entrainment is more sensitive to the dosage. The fully zwitterionic terminal epoxy copolymer used in Example 1 exhibited significantly better bubble stability and particle size distribution than other examples and comparative examples. Although Example 2 had a gas content of 22.0%, its bubble stability and flowability decreased, indicating that the hydrogen bonding enhancement provided by N-vinylimidazolium is crucial for long-term bubble stabilization. Example 3, containing only sulfonic acid groups and no quaternary ammonium salt, had a acceptable gas content, but poor stability and particle size. This may be due to the lack of electrostatic repulsion stabilization from the quaternary ammonium salt, making bubbles prone to coalescence and collapse. Example 4 showed low gas entrainment efficiency, with large and unstable bubbles. This may be because while the quaternary ammonium salt provides electrostatic stabilization, its ability to reduce surface tension is far weaker than that of the sulfonic acid groups, making bubble formation difficult; furthermore, the surface activity of a single cationic group is limited, failing to effectively promote microbubble nucleation.

[0038] Compared to Example 1, Comparative Example 1 had an air content of only 4.2%, a particle size as high as 210 μm, a stability of only 41.2%, and extremely poor plasticity, with almost no air-entraining and plasticizing effect. This indicates that the small number of large bubbles introduced by conventional HPMC through physical stirring are very easy to collapse. In Comparative Example 2, SDBS was mixed only in a physical form and did not chemically bond with the HPMC backbone. Although it had a certain air-entraining effect, the bubble quality was poor and unstable. In the alkaline environment of mortar, it was easy to desorb, migrate, or precipitate with calcium ions, and could not achieve long-term stable bubbles.

[0039] The embodiments and comparative examples described above do not limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A method for preparing a high-air-content plastic hydroxypropyl methylcellulose ether, characterized in that, Includes the following steps: (1) Spray alkaline solution onto refined cotton, draw a vacuum, stir at 100-500 r / min for 2-6 h to alkalize and obtain activated alkali cellulose; (2) Add an etherifying agent to activated alkali cellulose. The first etherification is carried out at 0.5-1.5 MPa and 50-60℃ for 50-70 min. The second etherification is carried out at 1.5-2.0 MPa and 70-90℃ for 30-60 min. Then, a DMF solution containing zwitterionic terminal epoxy copolymer is added at 0.2-0.5 MPa and 50-60℃ and reacted for 3-5 h. The temperature is raised to 60-80℃ and kept for 1-3 h. The pH is adjusted, washed, dried and pulverized to obtain plastic hydroxypropyl methylcellulose ether with high gas content. In step (2), the zwitterionic terminal epoxy copolymer accounts for 10-20% of the mass of refined cotton.

2. The method for preparing the high-air-content plastic hydroxypropyl methylcellulose ether according to claim 1, characterized in that, In step (1), the concentration of the alkali solution is 40wt% to 50wt%; the mass ratio of refined cotton to alkali solution is 1:(0.9 to 1.5); and the alkalization temperature is controlled at ≤35℃.

3. The method for preparing the high-air-content plastic hydroxypropyl methylcellulose ether according to claim 1, characterized in that, The zwitterionic-terminated epoxy copolymer in step (2) has a hyperbranched structure.

4. The method for preparing the high-air-content plastic hydroxypropyl methylcellulose ether according to claim 3, characterized in that, The preparation steps of the zwitterionic terminal epoxy copolymer are as follows: Under nitrogen protection, hyperbranched polyglycidyl ether, mixed monomers, initiator, chain transfer agent and solvent are added to the reactor; the first step is to heat the reaction; after the reaction is completed, the temperature is lowered, succinic anhydride and catalyst are added, and the second step is to heat the reaction; finally, epichlorohydrin and catalyst are added, and the third step is to heat the reaction to obtain the reaction solution; after the reaction solution is distilled under reduced pressure, the residue is poured into cold water to precipitate, filtered, dialyzed and vacuum dried to obtain zwitterionic terminal epoxy copolymer.

5. The method for preparing the high-air-content plastic hydroxypropyl methylcellulose ether according to claim 4, characterized in that, The temperature of the first heating reaction is 60-70℃; the temperature of the second heating reaction is 55-65℃; and the temperature of the third heating reaction is 45-55℃.

6. The method for preparing the high-air-content plastic hydroxypropyl methylcellulose ether according to claim 4, characterized in that, The mixed monomers comprise methacryloylethyl sulfobetaine and N-vinylimidazolium.

7. The method for preparing the high-air-content plastic hydroxypropyl methylcellulose ether according to claim 6, characterized in that, The mass ratio of methacryloylethyl sulfobetaine to N-vinylimidazole is (30-50):(5-15).

8. The method for preparing the high-air-content plastic hydroxypropyl methylcellulose ether according to claim 4, characterized in that, The mass ratio of the mixed monomer to succinic anhydride is 1:(0.2-0.4).

9. The method for preparing the high-air-content plastic hydroxypropyl methylcellulose ether according to claim 1, characterized in that, In step (2), the etherifying agent comprises chloromethane and propylene oxide in a mass ratio of (80-120):(30-50).

10. The method for preparing the high-air-content plastic hydroxypropyl methylcellulose ether according to claim 1, characterized in that, In step (2), the etherifying agent accounts for 120-150% of the mass of refined cotton.

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