Hydrophobic aerogel containing carboxyl nano cellulose and xanthan gum as well as preparation method and application of hydrophobic aerogel

Abstract

The invention discloses hydrophobic aerogel containing carboxyl nanocellulose and xanthan gum as well as a preparation method and application of the hydrophobic aerogel. The preparation method comprises the following steps: mixing cellulose and an acidic deep-eutectic solvent, performing water bath treatment, and stirring in distilled water through a wall breaking machine; mixing the nanocellulose suspension and the xanthan gum solution in equal mass, and uniformly stirring to obtain composite hydrogel; pre-freezing the composite hydrogel in an ultralow-temperature refrigerator, and performing vacuum freeze drying through a freeze dryer to obtain composite aerogel; spraying wax emulsion on the obtained composite aerogel, and drying to obtain the hydrophobic aerogel. The invention provides the hydrophobic aerogel containing the carboxyl nano cellulose and the xanthan gum, which is safe and environment-friendly, has good hydrophobic performance, adsorption performance and mechanical performance, can be directly used for food, and can be used for preparing an oil absorption pad and an oil absorption film.

Description

Technical Field

[0001] The present invention relates to a preparation method of a hydrophobic and lipophilic adsorption material, and particularly to a hydrophobic aerogel of carboxyl-containing nanocellulose and xanthan gum, a preparation method thereof and an application thereof. Background Art

[0002] Nanocellulose (CNC) has attracted extensive attention in the field of materials science in recent years due to its characteristics such as renewable source, environmental friendliness, excellent mechanical properties, and large specific surface area. As a new type of green material, it not only has good biodegradability but also shows great application potential in many fields such as environmental protection, energy storage, and medical materials. Among them, nanocellulose aerogel has become a research hotspot of multifunctional materials due to its ultra-light, high porosity, and strong adsorption properties.

[0003] Chinese invention patent CN107722338B discloses a preparation method of nanocellulose aerogel and nanocellulose aerogel. This technology uses the TEMPO oxidation method to prepare a nanocellulose suspension, controls the oxidation process through a buffer solution, and forms a nanocellulose aerogel after adding a freeze-drying protectant, high-pressure homogenization treatment, and freeze-drying post-treatment. However, the aerogel of this technology requires TEMPO oxidation, high-pressure homogenization, and freeze-drying, with expensive equipment, a long preparation cycle, and relatively high industrialization costs; although the obtained aerogel has high strength, it is brittle and not suitable as a material for bending or bearing large forces; although the obtained aerogel has strong adsorption force, it is mainly applied to oil-water separation and environmental treatment, and requires additional safety assessment for food contact.

[0004] Chinese invention patent application CN114805920A discloses a preparation method of a superhydrophobic cellulose nanofiber aerogel and the aerogel. The preparation method includes ultrasonic dispersion treatment of a cellulose nanofiber solution, gelation treatment with a dilute aqueous acid solution, standing to remove the unfrozen part to obtain a cellulose nanofiber hydrogel, then directional freezing with liquid nitrogen, and freeze-drying to obtain a cellulose nanofiber aerogel; after soaking in a composite organic solution and then heating and curing and cooling in sequence, a superhydrophobic cellulose nanofiber aerogel is obtained. The obtained aerogel has a brittle structure and is easy to break, which limits its service life; the superhydrophobic aerogel of this technology requires liquid nitrogen freeze-drying and silanization treatment, with relatively high costs; although the aerogel of this technology is environmentally friendly and suitable for oil-water separation, the silanized material is limited in application in the food field. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a hydrophobic aerogel of carboxyl-containing nanocellulose and xanthan gum that is safe and environmentally friendly, has hydrophobic properties, good adsorption properties and mechanical properties, and can be directly used in food, and a preparation method thereof by combining deep eutectic solvent (DES) pretreatment and mechanical treatment.

[0006] Another object of the present invention is to provide the application of the hydrophobic aerogel of carboxyl-containing nanocellulose and xanthan gum in the preparation of oil-absorbing pads and oil-absorbing films.

[0007] The technical solution adopted by the present invention to solve the technical problem is as follows:

[0008] A preparation method of a hydrophobic aerogel of carboxyl-containing nanocellulose and xanthan gum, comprising the following steps:

[0009] 1) Mix cellulose with acidic deep eutectic solvent (DES) and perform water bath treatment at 80°C - 85°C. Place the obtained powder in distilled water and stir with a blender to obtain a nanocellulose suspension;

[0010] 2) Mix the nanocellulose suspension and xanthan gum solution in equal mass and stir evenly to obtain a composite hydrogel. Control the mass concentration of the xanthan gum solution to be 0.5% - 1%; the mass concentration of the nanocellulose suspension to be 1% - 1.2%;

[0011] 3) Place the composite hydrogel in an ultra-low temperature refrigerator for pre-freezing, and then perform vacuum freeze-drying with a freeze dryer to obtain a composite aerogel;

[0012] 4) Spray wax emulsion on the obtained composite aerogel and dry it to obtain a hydrophobic aerogel.

[0013] To further achieve the object of the present invention, preferably, in step 1); the cellulose is derived from the sponge layer of pomelo or citrus fiber; the acidic deep eutectic solvent is oxalic acid / choline chloride or lactic acid / choline chloride; the mass ratio of the acidic deep eutectic solvent to cellulose is 30:1 - 20:1.

[0014] Preferably, the mass ratio of oxalic acid to choline chloride is 10:9 - 10:11; the mass ratio of lactic acid to choline chloride is 10:9 - 10:11.

[0015] Preferably, the time of the water bath treatment is 3h - 3.5h; the rotation speed of the blender stirring is 35000 - 40000r / min, and the stirring time is 5min - 7min.

[0016] Preferably, in step 2), the fiber length in the composite hydrogel is 290nm - 330nm.

[0017] Preferably, in step 3), the freezing temperature is -75°C to -85°C, and the freezing time is 12h - 24h; the vacuum freeze-drying time is 24h - 48h.

[0018] Preferably, in step 4), the drying temperature is 72°C - 79°C, and the drying time is 20min - 30min;

[0019] In step 4), the wax emulsion is a single wax emulsion and / or a mixed wax emulsion; the wax emulsion is prepared by melting wax substances and fully stirring and mixing them with an emulsifier and water at 90°C - 95°C to obtain an emulsion; in the raw materials, by mass percentage, the wax substances account for 20% - 30%, the emulsifier accounts for 8% - 15%, and the balance is water.

[0020] Preferably, the single wax emulsion is a single palm wax emulsion or a single candelilla wax emulsion; the mixed wax emulsion is a mixture of a palm wax emulsion and a candelilla wax emulsion; the melting temperature of the single palm wax is 90°C - 95°C, the melting temperature of the single candelilla wax is 80°C - 85°C, and the melting temperature of the mixture of palm wax and candelilla wax is 90°C - 95°C;

[0021] The emulsifier is Tween 80, Span 80, or cetyltrimethylammonium bromide (CTAB); the stirring method for fully stirring at 90°C - 95°C is magnetic stirring, and the stirring time is 10min - 15min.

[0022] A hydrophobic aerogel of carboxyl-containing nanocellulose and xanthan gum is prepared by the above preparation method.

[0023] The application of the carboxyl-containing nanocellulose and xanthan gum hydrophobic aerogel in the preparation of oil-absorbing pads and oil-absorbing films.

[0024] Compared with the prior art, the present invention has the following advantages and beneficial effects.

[0025] (1) The CNC preparation process of the present invention is green and environmentally friendly. The deep eutectic solvent used can be recycled, the equipment used is simple, and the prepared CNC suspension is more stable than the carboxymethyl cellulose solution.

[0026] (2) The nanocellulose and xanthan gum composite aerogel prepared by the present invention has a double-network skeleton structure. The addition of an appropriate amount of xanthan gum is beneficial to improving the network structure and pore structure of the aerogel. The preparation method is simple, the prepared aerogel has good mechanical properties and good adsorption performance for oils and fats.

[0027] (3) The wax spraying method used for the hydrophobic method of the present invention is a green and environmentally friendly method and will not cause the collapse of the pore structure of the aerogel. Brief Description of the Drawings

[0028] Figure 1 Infrared spectra of nanocellulose treated with the acidic deep eutectic solvent obtained in Example 1, nanocellulose treated with the basic deep eutectic solvent and the neutral deep eutectic solvent, and untreated cellulose.

[0029] Figure 2 SEM image of the aerogel obtained in Comparative Example 3;

[0030] Figure 3 SEM image of the hydrophobic composite aerogel obtained in Example 1;

[0031] Figure 4 SEM image of the aerogel obtained in Comparative Example 2;

[0032] Figure 5 Stress-deformation curves of the aerogels obtained in Example 1, Example 2 and Comparative Example 4. Detailed implementation manners

[0033] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the implementation manners of the present invention are not limited to this. Therefore, the protection scope of the present invention should be defined by the claims.

[0034] Similar aerogels to the present invention in the prior art mainly include silica aerogels, graphene aerogels, polyimide aerogels, etc. Since the raw materials are not suitable for food contact, they are rarely used in food contact materials; and in the hydrophobic modification process of cellulose aerogels, silane modification is mostly used, which is highly harmful to the environment and the human body and is also not suitable for use in food contact materials.

[0035] In the present invention, cellulose is treated with an acidic deep eutectic solvent, and the obtained nanocellulose suspension has carboxyl groups. The carboxyl group (-COOH) dissociates in water to form carboxylate (-COO-), making the cellulose molecules negatively charged. The negatively charged cellulose molecules or particles repel each other, which can prevent particle aggregation or sedimentation; moreover, an electric double layer is formed on the surface of the cellulose particles, making the particles evenly distributed in the aqueous phase and maintaining stable suspension; a stable suspension is formed.

[0036] The present invention also utilizes the combination of xanthan gum and nano-cellulose suspension. As a natural polysaccharide, xanthan gum has been widely used in industries such as food, medicine, and oil extraction due to its good thickening, stability, gelling properties, etc. Xanthan gum has excellent water solubility and viscoelasticity, and its unique rheological properties enable it to exhibit extremely strong thickening and stabilizing effects in aqueous systems. The present invention discovers that the composite preparation of xanthan gum and carboxyl-containing nano-cellulose to form an aerogel can enhance the mechanical properties and adsorption properties of the aerogel, and it is particularly suitable as an adsorbent. Because this combination forms a double skeleton with nano-cellulose and xanthan gum, and the present invention discovers that the addition of an appropriate amount of xanthan gum is beneficial to the network structure and pore structure of nano-cellulose, and the hydrogen bond interaction between the carboxyl nano-cellulose and water molecules and the coordination interaction with other polar substances may be relatively significant, which leads to a strong combination and the prepared aerogel has good mechanical properties.

[0037] The present invention forms a hydrophobic surface by spraying a wax emulsion on the surface of the aerogel, which can maintain the pore structure of the aerogel and also ensure the greenness of preparation and use.

[0038] Based on the above findings, the present invention provides a method for preparing a hydrophobic aerogel of carboxyl-containing nano-cellulose and xanthan gum, which includes the following steps:

[0039] 1) Mix cellulose with an acidic deep eutectic solvent and then perform a water bath treatment at 80°C - 85°C. Place the obtained powder in distilled water and stir it with a blender to obtain a nano-cellulose suspension;

[0040] 2) Mix the nano-cellulose suspension and the xanthan gum solution in equal mass and stir evenly to obtain a composite hydrogel. Control the mass concentration of the xanthan gum solution to be 0.5% - 1%; the mass concentration of the nano-cellulose suspension to be 1% - 1.2%;

[0041] 3) Place the composite hydrogel in an ultra-low temperature refrigerator for pre-freezing, and then perform vacuum freeze-drying with a freeze dryer to obtain a composite aerogel;

[0042] 4) Spray the obtained composite aerogel with a wax emulsion and dry it to obtain a hydrophobic aerogel.

[0043] In the above steps, the concept of treating cellulose with acidic deep eutectic solvents, as well as the combination of nano-cellulose suspension and xanthan gum solution, are the basis for the preparation of the hydrophobic aerogel of nano-cellulose and xanthan gum containing carboxyl groups in the present invention. Reasonable control of the mass concentration of the xanthan gum solution is a key measure. For the convenience of controlling the mass concentration, the nano-cellulose suspension and the xanthan gum solution are mixed in equal mass in the present invention, and the mass concentration of the xanthan gum solution is controlled to be 0.5%-1%. Through comparative tests, this concentration control has achieved remarkable results. As for the source of cellulose, the selection of acidic deep eutectic solvents, and the specific process measures for treating cellulose with acidic deep eutectic solvents can achieve the invention purpose under the water bath treatment at 80°C - 85°C and stirring with a blender. Process parameters such as the water bath treatment time and stirring can be obtained through experiments under the invention purpose. There are many references in the prior art for the process of preparing composite aerogels from composite hydrogels and the specific process of spraying wax emulsions.

[0044] The hydrophobic aerogel prepared by the present invention has good mechanical properties, adsorption properties, and hydrophobic properties. For the first time, the present invention uses the hydrophobic aerogel in the field of food oil-water separation, and proposes the application of the hydrophobic aerogel of nano-cellulose and xanthan gum containing carboxyl groups in the preparation of oil-absorbing pads and oil-absorbing films.

[0045] Example 1

[0046] Hot water treatment of pomelo peel residue: Using Sanyou pomelo peel as raw material, extract pomelo peel cellulose according to the following steps. After washing, screening, drying, and pulverizing the pomelo peel residue, add pure water for water bath treatment (material-liquid ratio 1:20, g / mL, water bath temperature 90°C, stirring for 30 min), and dry to remove soluble impurities in the pomelo peel residue to obtain the pomelo peel residue after hot water treatment;

[0047] Acid treatment of pomelo peel residue: Perform acid treatment on the pomelo peel residue after hot water treatment (0.1 mol / L hydrochloric acid, material-liquid ratio 1:20, W / V, water bath temperature 85°C, stirring for 30 min, repeat twice) to remove pectin, wash with water until neutral and dry to obtain pectin-removed pomelo peel residue;

[0048] Alkali treatment of pomelo peel residue: Perform alkali treatment on the sample after acid treatment to remove hemicellulose (9.4 g / 100 mL sodium hydroxide, material-liquid ratio 1:20, water bath temperature 84°C, stirring for 77 min), wash with water and dry to obtain hemicellulose-removed pomelo peel residue;

[0049] Bleaching treatment of pomelo peel residue: Perform bleaching treatment on the sample after alkali treatment according to the material-liquid ratio 1:20 (g / mL) (10% hydrogen peroxide, water bath temperature 80°C, stirring for 30 min), wash with water and then dry, grind through an 80-mesh sieve to obtain pomelo peel cellulose.

[0050] It should be noted that the preparation method of the cellulose samples involved in the subsequent examples and comparative examples is the same as the above steps of this example. Due to the common properties of cellulose, the pomelo peel cellulose of the present invention is only used as a representative, and other types of cellulose are not substantially different from pomelo peel cellulose and can all be used in the present invention.

[0051] The pomelo peel cellulose was pretreated with an acidic deep eutectic solvent (DES), where the DES was prepared from oxalic acid dihydrate and choline chloride at a mass ratio of 10:9, and the mass ratio of DES to cellulose was controlled to be 20:1. The pretreatment was carried out in a water bath at 80 °C for 3 h; after pretreatment, it was passed through a 450 nm microporous filter membrane, and the nanocellulose was collected, dissolved in pure water, placed in a blender and stirred at 30000 r / min for 5 min to obtain a nanocellulose suspension, which was concentrated to 1 wt% and stored for later use.

[0052] The prepared nanocellulose suspension (1 wt%) and 0.5 wt% xanthan gum solution were mixed in equal mass and stirred evenly to obtain a uniform composite hydrogel;

[0053] The composite hydrogel prepared in the previous step was placed in an ultra-low temperature refrigerator (-75 °C) and pre-frozen for 12 h, and then a composite aerogel was prepared by vacuum freeze-drying for 28 h.

[0054] 2 g of palm wax was melted at 95 °C, mixed with 0.8 g of Tween 80, 7.2 g of boiling water was added, and the reaction was carried out with magnetic stirring at 800 r / min for 10 min to obtain an emulsion. The emulsion was evenly sprayed onto the obtained composite aerogel and dried in an oven at 72 °C to obtain a hydrophobic composite aerogel.

[0055] Example 2

[0056] The pomelo peel cellulose was pretreated with an acidic deep eutectic solvent (DES was prepared from oxalic acid dihydrate / choline chloride at a mass ratio of 1:1, DES:cellulose mass ratio of 25:1, water bath time of 3.2 h, water bath temperature of 82 °C). After pretreatment, it was passed through a 450 nm microporous filter membrane and the nanocellulose was collected, dissolved in pure water, placed in a blender and stirred at 35000 r / min for 6 min to obtain a nanocellulose suspension, which was concentrated to 1% and stored for later use.

[0057] The above-prepared nanocellulose suspension (1 wt%) and 1 wt% xanthan gum solution were mixed in equal mass and stirred evenly to obtain a uniform composite hydrogel;

[0058] The composite hydrogel prepared in the previous step was placed in an ultra-low temperature refrigerator (-80 °C) and pre-frozen for 18 h, and then a composite aerogel was prepared by vacuum freeze-drying for 36 h.

[0059] After melting 2 g of candelilla wax at 85 °C, it was mixed with 1 g of Span 80. 7 g of boiling water was added and the mixture was reacted under magnetic stirring at 900 r / min for 13 min to obtain an emulsion. The emulsion was evenly sprayed onto the obtained aerogel and dried in an oven at 75 °C to obtain a hydrophobic composite aerogel.

[0060] Example 3

[0061] Naringin cellulose was pretreated with an acidic deep eutectic solvent (DES was prepared from oxalic acid dihydrate / choline chloride with a mass ratio of 10:11, the mass ratio of DES:cellulose was 30:1, the water bath time was 3.5 h, and the water bath temperature was 85 °C). After pretreatment, the nanocellulose was collected by passing through a 450 nm microporous filter membrane, dissolved in a certain amount of pure water, and stirred in a blender at 40,000 r / min for 7 min to obtain a nanocellulose suspension, which was concentrated to 1% and stored for later use.

[0062] The above-prepared nanocellulose suspension (1 wt%) and 0.75 wt% xanthan gum solution were mixed in equal mass and stirred evenly to obtain a uniform composite hydrogel.

[0063] The composite hydrogel prepared in the previous step was pre-frozen in an ultra-low temperature refrigerator (-85 °C) for 24 h and then prepared into a composite aerogel by vacuum freeze-drying for 48 h.

[0064] 1 g of candelilla wax and 1 g of palm wax were melted at 95 °C, mixed with 1 g of Tween 80, 7 g of boiling water was added, and the mixture was reacted under magnetic stirring at 1000 r / min for 15 min to obtain an emulsion. The emulsion was evenly sprayed onto the obtained aerogel and dried in an oven at 75 °C to obtain a hydrophobic composite aerogel.

[0065] Example 4

[0066] Naringin cellulose was pretreated with an acidic deep eutectic solvent (DES was prepared from lactic acid / choline chloride with a mass ratio of 1:1, the mass ratio of DES:cellulose was 30:1, the water bath time was 3 h, and the water bath temperature was 80 °C). After pretreatment, the nanocellulose was collected by passing through a 450 nm microporous filter membrane, dissolved in pure water, and stirred in a blender at 40,000 r / min for 5 min to obtain a nanocellulose suspension, which was concentrated to 1% and stored for later use.

[0067] The above-prepared nanocellulose suspension (1 wt%) and 1 wt% xanthan gum solution were mixed in equal mass and stirred evenly to obtain a uniform hydrogel.

[0068] The composite hydrogel prepared in the previous step was pre-frozen in an ultra-low temperature refrigerator (-80 °C) for 12 h and then prepared into a composite aerogel by vacuum freeze-drying for 48 h.

[0069] After melting 1 g of candelilla wax and 1 g of palm wax at 95 °C, mix them with 1 g of CTAB, add 7 g of boiling water, and react under magnetic stirring at 1000 r / min for 15 min to obtain an emulsion. Spray the emulsion evenly onto the obtained aerogel, and dry it in an oven at 75 °C to obtain a hydrophobic composite aerogel.

[0070] Example 5

[0071] Pretreat naringin cellulose with an acidic deep eutectic solvent (DES is prepared with a mass ratio of oxalic acid dihydrate / choline chloride of 10:11, the mass ratio of DES:cellulose is 30:1, the water bath time is 3.5 h, and the water bath temperature is 85 °C). After pretreatment, collect the nanocellulose through a 450 nm microporous membrane, dissolve it in a certain amount of pure water, and stir it in a blender at 40000 r / min for 7 min to obtain a nanocellulose suspension. Concentrate it to 1.2% and store it for later use.

[0072] Mix the above-prepared nanocellulose suspension (1.2 wt%) and 1 wt% xanthan gum solution in equal mass, and stir evenly to prepare a uniform composite hydrogel;

[0073] Place the composite hydrogel prepared in the previous step in an ultra-low temperature refrigerator (-85 °C) and pre-freeze it for 24 h, and then prepare a composite aerogel by vacuum freeze-drying for 48 h.

[0074] After melting 1 g of candelilla wax and 1 g of palm wax at 95 °C, mix them with 1 g of Tween 80, add 7 g of boiling water, and react under magnetic stirring at 1000 r / min for 15 min to obtain an emulsion. Spray the emulsion evenly onto the obtained aerogel, and dry it in an oven at 75 °C to obtain a hydrophobic composite aerogel.

[0075] Comparative Example 1

[0076] Pretreat naringin cellulose with an acidic deep eutectic solvent (DES is prepared with a mass ratio of oxalic acid dihydrate / choline chloride of 1:1, the mass ratio of DES:cellulose is 30:1, the water bath time is 3 h, and the water bath temperature is 80 °C). After pretreatment, collect the nanocellulose through a 450 nm microporous membrane, dissolve it in pure water, and stir it in a blender at 40000 r / min for 5 min to obtain a nanocellulose suspension. Concentrate it to 1% and store it for later use.

[0077] Mix the above-prepared nanocellulose suspension (1 wt%) and 1.5 wt% xanthan gum solution in equal mass, and stir evenly to prepare a uniform composite hydrogel;

[0078] Place the composite hydrogel prepared in the previous step in an ultra-low temperature refrigerator (-80 °C) and pre-freeze it for 12 h, and then prepare a composite aerogel by vacuum freeze-drying for 48 h.

[0079] After melting 1 g of candelilla wax and 1 g of palm wax at 95 °C, mix them with 1 g of CTAB, add 7 g of boiling water, and react under magnetic stirring at 1000 r / min for 15 min to obtain an emulsion. Spray the emulsion evenly onto the obtained aerogel, and dry it in an oven at 75 °C to obtain a hydrophobic composite aerogel.

[0080] Comparative Example 2

[0081] Pretreat the pomelo peel cellulose with an acidic deep eutectic solvent (DES is prepared from oxalic acid dihydrate / choline chloride with a mass ratio of 1:1, the mass ratio of DES:cellulose is 30:1, the water bath time is 3 h, and the water bath temperature is 80 °C). After pretreatment, collect the nanocellulose through a 450 nm microporous filter membrane, dissolve it in pure water, and stir it in a blender at 40000 r / min for 5 min to obtain a nanocellulose suspension. Concentrate it to 1% and store it for later use.

[0082] Mix the above-prepared nanocellulose suspension (1 wt%) and a 2 wt% xanthan gum solution in equal mass, and stir evenly to prepare a uniform composite hydrogel;

[0083] Place the composite hydrogel prepared in the previous step in a ultra-low temperature refrigerator (-80 °C) and pre-freeze it for 12 h, then prepare a composite aerogel by vacuum freeze-drying for 48 h.

[0084] After melting 1 g of candelilla wax and 1 g of palm wax at 95 °C, mix them with 1 g of CTAB, add 7 g of boiling water, and react under magnetic stirring at 1000 r / min for 15 min to obtain an emulsion. Spray the obtained emulsion evenly onto the obtained aerogel, and dry it in an oven at 75 °C to obtain a hydrophobic composite aerogel.

[0085] Comparative Example 3

[0086] Pretreat the pomelo peel cellulose with an acidic deep eutectic solvent (DES is prepared from oxalic acid dihydrate / choline chloride with a mass ratio of 1:1, the mass ratio of DES:cellulose is 30:1, the water bath time is 3 h, and the water bath temperature is 80 °C). After pretreatment, collect the nanocellulose through a 450 nm microporous filter membrane, dissolve it in pure water, and stir it in a blender at 40000 r / min for 5 min to obtain a nanocellulose suspension. Concentrate it to 1% and store it for later use.

[0087] Stir the above-prepared nanocellulose suspension (1 wt%) evenly to prepare a uniform hydrogel;

[0088] Place the hydrogel prepared in the previous step in a ultra-low temperature refrigerator (-80 °C) and pre-freeze it for 12 h, then prepare a composite aerogel by vacuum freeze-drying for 48 h.

[0089] After melting 1 g of candelilla wax and 1 g of palm wax at 95°C, mix them with 1 g of CTAB, add 7 g of boiling water, and react under magnetic stirring at 1000 r / min for 15 min to obtain an emulsion. Spray the obtained emulsion evenly onto the obtained aerogel, and dry it in an oven at 75°C to obtain a hydrophobic composite aerogel.

[0090] Comparative Example 4

[0091] Prepare a 1 wt% concentration suspension of sodium carboxymethyl cellulose with distilled water, and stir it on a magnetic stirrer for 5 h;

[0092] Mix the prepared sodium carboxymethyl cellulose solution and 1 wt% xanthan gum solution in equal mass, and stir evenly to obtain a uniform composite hydrogel;

[0093] Place the composite hydrogel obtained in the previous step in a ultra-low temperature freezer (-80°C) and pre-freeze it for 12 h, then prepare the composite aerogel by vacuum freeze-drying for 48 h.

[0094] After melting 1 g of candelilla wax and 1 g of palm wax at 95°C, mix them with 1 g of CTAB, add 7 g of boiling water, and react under magnetic stirring at 1000 r / min for 15 min to obtain an emulsion. Spray the obtained emulsion evenly onto the obtained aerogel, and dry it in an oven at 75°C to obtain a hydrophobic composite aerogel.

[0095] Comparative Example 5

[0096] Pretreat naringin cellulose with an acidic deep eutectic solvent (DES is prepared by mixing oxalic acid dihydrate / choline chloride with a mass ratio of 1:1, the mass ratio of DES:cellulose is 30:1, the water bath time is 3 h, and the water bath temperature is 80°C). After pretreatment, collect the nanocellulose through a 450 nm microporous filter membrane, dissolve it in pure water, and stir it in a blender at 40000 r / min for 5 min to obtain a nanocellulose suspension. Concentrate it to 1% and store it for later use.

[0097] Mix the above-prepared nanocellulose suspension (1 wt%) and 0.25 wt% xanthan gum solution in equal mass, and stir evenly to obtain a uniform composite hydrogel; place the composite hydrogel obtained in the previous step in a ultra-low temperature freezer (-80°C) and pre-freeze it for 12 h, then prepare the composite aerogel by vacuum freeze-drying for 48 h.

[0098] After melting 1 g of candelilla wax and 1 g of palm wax at 95°C, mix them with 1 g of CTAB, add 7 g of boiling water, and react under magnetic stirring at 1000 r / min for 15 min to obtain an emulsion. Spray the obtained emulsion evenly onto the obtained aerogel, and dry it in an oven at 75°C to obtain a hydrophobic composite aerogel.

[0099] Table 1 Comparison of parameters between examples and comparative examples

[0100]

[0101] The nanofibrillated cellulose suspension and xanthan gum solution are mixed in equal mass and stirred evenly to obtain a composite hydrogel, and the mass concentration of the xanthan gum solution is controlled to be 0.5%-1%; the mass concentration of the nanofibrillated cellulose suspension is 1%-1.2%

[0102] It was found from Table 1 of the present invention that under the condition of mixing the nanofibrillated cellulose suspension and xanthan gum solution in equal mass, the mass concentration of xanthan gum will cause changes in the density of the aerogel, directly affecting the porosity. When the xanthan gum concentration is lower than 0.5 wt% (Comparative Example 5), the overall concentration of the hydrogel will decrease, making the aerogel in a loose state, and the oil absorption rate of the composite aerogel will be lower. Without adding xanthan gum (Comparative Example 3), due to the irregular pore structure, the absorbed oil is easily released, resulting in a low oil absorption rate; while adding xanthan gum with a concentration greater than 1 wt% (Comparative Examples 1 and 2) leads to a tight pore structure and a decrease in porosity, and the oil absorption rate also decreases accordingly. By controlling the mass concentration of xanthan gum to be 0.5%-1% (Examples 1-5), the prepared composite aerogels all have a significantly lower density than the comparative examples and have significantly better oil absorption effects. The lowest oil absorption rate in the examples is 69.88 g / g, which is also greatly improved compared to the highest 52.02 g / g in the comparative examples. The aerogel density of the present invention is all lower than 15 mg / cm3, making its porosity greater than 99%. The extremely high porosity ensures that the oil absorption rate of the aerogel is as low as 69 g / g at least. It can also be seen from Table 1 that when the carboxyl nanofibrillated cellulose is replaced with sodium carboxymethylcellulose (Comparative Example 4), the oil absorption rate of the prepared composite aerogel also decreases significantly.

[0103] In Example 1 of the present invention, the obtained pomelo peel cellulose was treated to obtain a nanofibrillated cellulose suspension with a concentration of 1 wt%. The specific method is as follows: The pomelo peel cellulose was pretreated with an acidic deep eutectic solvent (DES), where DES was prepared by mixing oxalic acid dihydrate and choline chloride in a mass ratio of 10:9, and the mass ratio of DES to cellulose was controlled to be 20:1. The water bath pretreatment temperature was controlled at 80 °C and the pretreatment time was 3 h; after pretreatment, it was passed through a 450 nm microporous filter membrane, and the nanofibrillated cellulose was collected, dissolved in pure water, placed in a blender and stirred at 30,000 r / min for 5 min to obtain a nanofibrillated cellulose suspension, which was concentrated to 1 wt% and stored for later use.

[0104] The acidic deep eutectic solvent (DES) in the examples was replaced with a basic deep eutectic solvent (glycerol / potassium carbonate) and a neutral deep eutectic solvent (urea / choline chloride) respectively to treat the pomelo peel cellulose obtained in Example 1. Among them, the mass ratio of glycerol to potassium carbonate in the basic deep eutectic solvent of glycerol / potassium carbonate was 5:1; the mass ratio of urea to choline chloride in the neutral deep eutectic solvent of urea / choline chloride was 1:2; other methods for treating pomelo peel cellulose were the same as those in Example 1.

[0105] The nanocellulose obtained by treating with the acidic deep eutectic solvent in Example 1, the nanocellulose obtained by treating with the basic deep eutectic solvent and the neutral deep eutectic solvent obtained in the above steps, and the untreated cellulose (the pomelo peel cellulose obtained in the example) were characterized by infrared spectroscopy. As Figure 1 shown, the nanocellulose suspension obtained by treating with the acidic deep eutectic solvent in Example 1 had a peak at 1737 cm -1 This indicated that the nanocellulose obtained by treating with the acidic deep eutectic solvent had carboxyl groups. The carboxyl group (-COOH) dissociated in water to form carboxylate (-COO-), making the cellulose molecules negatively charged. This charge effect brought the following effects: ① Charge repulsion: The negatively charged cellulose molecules or particles repelled each other, preventing particle aggregation or sedimentation. ② Electric double layer effect: An electric double layer was formed on the surface of the cellulose particles, making the particles evenly distributed in the aqueous phase and maintaining stable suspension. This provided great advantages for the subsequent preparation of the suspension, while the cellulose of the other three could not form a stable suspension and could not be used for preparing aerogels.

[0106] Figure 2 It can be seen that the nanocellulose aerogel obtained in Comparative Example 3 without adding xanthan gum had a loose structure and very irregular arrangement. This was because the nanocellulose alone served as a support, and the skeleton support was sparser than that of the aerogel skeleton with added xanthan gum, resulting in a loose structure of the obtained aerogel and poor subsequent mechanical properties, leading to an obvious gap in the mechanical properties of Comparative Example 3 compared with Example 1 and Example 2.

[0107] Figure 3 SEM image of the hydrophobic composite aerogel obtained in Example 1. Figure 3 It can be seen that when adding 0.5 wt% xanthan gum solution, the obtained hydrophobic composite aerogel formed a uniform network structure, with filamentous substances on the surface connecting and tightly connected. At the same time, in Example 2, 1 wt% xanthan gum solution was added, and in Example 3, 0.75 wt% xanthan gum solution was added. The SEM of the obtained hydrophobic composite aerogels was the same as Figure 3Basically the same. Perhaps because xanthan gum is a kind of high molecular polysaccharide with a large number of hydroxyl groups and ether bonds, which can form hydrogen bonds or electrostatic interactions with the hydroxyl groups on the surface of nanocellulose. This intermolecular interaction promotes the more uniform dispersion of nanocellulose and xanthan gum in the solution, preventing the aggregation of nanocellulose during the drying process, and thus forming a more uniform aerogel network structure. Moreover, due to the entanglement of filamentous substances on the surface, the aerogel has good mechanical properties.

[0108] Figure 4 Figure 4 is the SEM image of the aerogel obtained in Comparative Example 2. From Figure 4 It can be seen that adding xanthan gum with a mass concentration greater than 1 wt% will make the aerogel structure very compact, resulting in extremely small pores, which is not conducive to the entry of oil substances and reduces the oil absorption performance of the aerogel. The oil absorption rate of Comparative Example 2 is reduced by 28 g / g and 37 g / g compared with Example 1 and Example 2.

[0109] Figure 5 It can be seen that when comparing Example 1, Example 2 with Comparative Example 4, when the compression reaches 10%, the compression stress of the sodium carboxymethyl cellulose / xanthan gum aerogel is significantly less than that of the nanocellulose / xanthan gum. The prepared carboxyl-containing nanocellulose / xanthan gum composite aerogel has better mechanical properties and stronger elastic potential energy compared with the sodium carboxymethyl cellulose / xanthan gum composite aerogel. Compared with nanocellulose, sodium carboxymethyl cellulose is a linear or slightly cross-linked polymer with a relatively large molecular weight and has no fibrous structure, so it cannot provide rigid support or form a stable skeleton like nanocellulose. Although xanthan gum can provide a certain thickening effect, it mainly shows a network with relatively high flexibility and cannot significantly improve the rigidity and compressive properties of the composite material.

[0110] Existing silica aerogels, graphene aerogels, polyimide aerogels, etc. are rarely used in food contact materials because their raw materials are not suitable for food contact; while cellulose aerogels mostly use silane modification in their hydrophobic modification process, which has great harm to the environment and human body and is also not suitable for application in food contact materials. The hydrophobic aerogel of carboxyl-containing nanocellulose and xanthan gum prepared by the present invention is not only environmentally friendly and safe, but also has good mechanical properties and hydrophobic properties, and can be used as an oil absorption pad and an oil absorption film.

[0111] When used as an oil-absorbing pad, the hydrophobic aerogel of carboxyl-containing nanocellulose and xanthan gum obtained in the present invention is cut into the size of a pad and placed under fried foods such as fried chicken and French fries to effectively absorb the spilled oil and keep the food crispy. The porous structure of nanocellulose can quickly absorb oil, while xanthan gum plays a strengthening role in the structure to prevent the oil-absorbing pad from being overly softened or deformed. The wax spray coating prevents excessive oil penetration while maintaining the breathability of the material, making the oil-absorbing effect more lasting. It solves the problems that fried foods such as fried chicken and French fries will release excess oil during storage, resulting in the food becoming greasy and the taste deteriorating.

[0112] When used as an oil-absorbing film, the hydrophobic aerogel of carboxyl-containing nanocellulose and xanthan gum obtained in the present invention is cut into thin slices of appropriate size and directly placed in the soup or on the soup surface to adsorb the oil in the soup. The film made of the hydrophobic aerogel of carboxyl-containing nanocellulose and xanthan gum has good buoyancy and adsorption, and can directly float on the soup surface to adsorb the surface oil through the pores; the wax layer provides waterproofness, so that the film only adsorbs oil and does not absorb water. Floating the oil-absorbing film on the soup surface can reduce the surface oil and improve the taste of the soup. It can also be used to adsorb oil substances during the soup-making process by directly placing it in the soup for adsorption. The oil-absorbing film is especially suitable for the application of chicken soup and pork rib soup, solving the problem of excessive oil content in the soup.

Claims

1. A method for preparing a hydrophobic aerogel of carboxyl-containing nanocellulose and xanthan gum, characterized in that The following steps are involved: 1) mixing cellulose and an acidic low eutectic solvent and treating them in a water bath at 80° C.-85° C., placing the obtained powder in distilled water and stirring it in a wall breaking machine to obtain a nanocellulose suspension; 2) mixing the nanocellulose suspension and the xanthan gum solution in equal amounts, stirring evenly to obtain a composite hydrogel, wherein the mass concentration of the xanthan gum solution is controlled to be 0.5%-1%; the mass concentration of the nanocellulose suspension is controlled to be 1%-1.2%; 3) pre-freezing the composite hydrogel in an ultra-low temperature refrigerator, and then freeze-drying the composite hydrogel in a freeze dryer to obtain a composite aerogel; 4) Spraying the obtained composite aerogel with wax emulsion and drying it to obtain a hydrophobic aerogel.

2. The method for preparing a hydrophobic aerogel of carboxyl-containing nanocellulose and xanthan gum according to claim 1, characterized in that: In step 1), the cellulose is derived from the sponge layer of grapefruit or citrus fiber; the acidic low eutectic solvent is oxalic acid / choline chloride or lactic acid / choline chloride; the mass ratio of the acidic low eutectic solvent to cellulose is 30:1-20:

1.

3. The method for preparing a hydrophobic aerogel of carboxyl-containing nanocellulose and xanthan gum according to claim 2, characterized in that, The mass ratio of the oxalic acid to the choline chloride is 10:9-10:11; the mass ratio of the lactic acid to the choline chloride is 10:9-10:

11.

4. The method for preparing a hydrophobic aerogel of carboxyl-containing nanocellulose and xanthan gum according to claim 1, characterized in that, The water bath treatment time is 3h-3.5h; the stirring speed of the wall breaking machine is 35000-40000r / min, and the stirring time is 5min-7min.

5. The method for preparing a hydrophobic aerogel of carboxyl-containing nanocellulose and xanthan gum according to claim 1, characterized in that, In step 2), the fiber length in the composite hydrogel is 290nm-330nm.

6. The method for preparing the hydrophobic aerogel of carboxyl-containing nanocellulose and xanthan gum according to claim 1, characterized in that, In step 3), the freezing temperature is -75°C to -85°C, and the freezing time is 12h-24h; the vacuum freeze-drying time is 24h-48h.

7. The method for preparing the hydrophobic aerogel of carboxyl-containing nanocellulose and xanthan gum according to claim 1, characterized in that: In step 4), the drying temperature is 72°C-79°C, and the drying time is 20min-30min; In step 4), the wax emulsion is a single wax emulsion and / or a mixed wax emulsion; the wax emulsion is prepared by melting the wax substance, and then stirring and mixing it with an emulsifier and water at 90°C-95°C to obtain an emulsion; in the raw materials, by mass percentage, the wax substance accounts for 20%-30%, the emulsifier accounts for 8%-15%, and the rest is water.

8. The method for preparing the hydrophobic aerogel of carboxyl-containing nanocellulose and xanthan gum according to claim 7, characterized in that, The single wax emulsion is a single palm wax emulsion or a single candelilla wax emulsion; the mixed wax emulsion is a compound of a palm wax emulsion and a candelilla wax emulsion; the melting temperature of a single palm wax is 90°C-95°C, the melting temperature of a single candelilla wax is 80°C-85°C, and the melting temperature of a mixture of palm wax and candelilla wax is 90°C-95°C; The emulsifier is Tween 80, Span 80 or hexadecyltrimethylammonium bromide; the stirring method for fully stirring at 90° C.-95° C. is magnetic stirring, and the stirring time is 10 min-15 min.

9. A hydrophobic aerogel of carboxyl-containing nanocellulose and xanthan gum, characterized in that The invention is prepared by the preparation method according to any one of claims 1 to 8.

10. Use of the hydrophobic aerogel of carboxyl-containing nanocellulose and xanthan gum according to claim 9 in preparing oil absorption pads and oil absorption films.

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