CN112852001A

CN112852001A - Preparation method of nano-cellulose ammonia gas induction aerogel

Info

Publication number: CN112852001A
Application number: CN202110023041.2A
Authority: CN
Inventors: 张正健; 张国华; 张金珊; 陈凤宝; 刘光发; 陈蕴智
Original assignee: Tianjin University of Science and Technology
Current assignee: Tianjin University of Science and Technology
Priority date: 2021-01-08
Filing date: 2021-01-08
Publication date: 2021-05-28
Anticipated expiration: 2041-01-08
Also published as: CN112852001B

Abstract

The invention provides a nano-cellulose ammonia induction aerogel with a high specific surface area and a preparation method thereof, wherein the nano-cellulose ammonia induction aerogel comprises the following steps of (1) pretreating eucalyptus sulfate bleaching pulp to prepare nano-cellulose hydrogel; (2) adding a polyvinyl alcohol solution into the prepared nano-cellulose hydrogel to prepare polyvinyl alcohol/nano-cellulose composite hydrogel; (3) fixing the radish red pigment on the fiber of the polyvinyl alcohol/nano-cellulose composite hydrogel in situ to prepare the polyvinyl alcohol/radish red pigment/nano-cellulose composite hydrogel; (4) the polyvinyl alcohol/radish red pigment/nano-cellulose composite aerogel is prepared by adopting a vacuum freeze-drying method. The invention greatly enhances the adsorption capacity to ammonia gas and shortens the response time. The pH color responsiveness of the radish red pigment is utilized to play a role in color indication on the existence of ammonia gas, the effective monitoring on the ammonia gas at the ppm level can be realized, and the method has good practicability in the fields of food freshness indication and the like.

Description

Preparation method of nano-cellulose ammonia gas induction aerogel

Technical Field

The invention belongs to the field of packaging materials, relates to the technical field of gas indication and gas response, and particularly relates to a nano-cellulose ammonia gas induction aerogel with a high specific surface area and a preparation method thereof.

Background

In the past, many examples of the gas sensing in food packaging have been made by using a film made of a material such as polyethylene, polylactic acid, starch, chitosan, etc. as a matrix and a pigment. Although materials such as polyethylene and polylactic acid are easily available, they are not easily degraded, easily cause environmental pollution, and have low sensitivity to gas induction. Although the film-forming base materials such as starch, chitosan and the like are green and easy to degrade, the porosity after film forming is extremely low and the cost is high. Many of the pigments used for gas sensing are chemical dyes that are harmful to the human body. Aiming at various defects of gas induction base materials and pigments, a green and environment-friendly material with wide sources is searched, and the adsorption effect of gas is determined by changing the color of the material by adsorbing specific gas after the structure of the material is designed, so that the defect of the existing film structure in gas adsorption is overcome.

The natural plant fiber is a renewable material, is low in price, non-toxic, wide in source and easy to degrade, and can improve the performance of an induction device, so that the fiber becomes a material with great development and use values. Microfibrillated cellulose (MFC) or nano-Cellulose (CNF) prepared by treating fibers by physical or chemical methods and the like is a novel nano-material with high light weight, high strength, high specific surface area and adjustable surface chemical property, and a nano-cellulose material prepared by using the material has high porosity and high specific surface area which are not possessed by a thin film material, and has a good adsorption effect on gas. The anthocyanin has strong oxidation resistance, can change the chemical structure and color of the anthocyanin under different pH values, and is suitable for detecting alkaline gas.

Therefore, the nano-cellulose is used as a base material to prepare the sensing material with excellent performance, the color indication property of the natural pigment is combined with the rigidity and the thermal stability of the cellulose, and the obtained composite material not only has excellent mechanical property, but also has biodegradability.

Disclosure of Invention

The invention aims to overcome the defects of the prior art and provides the nano-cellulose ammonia gas sensing aerogel with high specific surface area and the preparation method thereof.

The technical scheme adopted by the invention for solving the technical problem is as follows:

in order to achieve the purpose, the invention provides a nano-cellulose ammonia-induced aerogel with a high specific area and a preparation method thereof. The radish red pigment is in-situ fixed on the fiber of the nano-cellulose hydrogel to obtain composite hydrogel, the composite porous aerogel is prepared after freeze drying, ammonia gas can penetrate through pores, and hydroxyl ions generated after hydrolysis enable yellow molten salt ions in the radish red pigment to be converted into methanopseudobase and quinoid base, so that the radish red pigment is changed in color, the color of the aerogel is changed, and an indicating effect is achieved.

The composite aerogel is a natural plant fiber material with a dense network structure and a high specific surface area.

The composite hydrogel is prepared by a physical crosslinking and blending method, the nano-cellulose aerogel has a net-shaped pore structure, and the density and the size of pore grids can be controlled by regulating and controlling the proportioning concentration and the pre-freezing temperature.

The composite hydrogel preparation material is selected from nano-cellulose, radish red pigment, polyvinyl alcohol or methyl cellulose.

The invention also provides a preparation method of the nano-cellulose ammonia-induced aerogel with high specific surface area, which comprises the following steps:

(1) pretreating eucalyptus sulfate bleaching pulp, adding a freeze-drying protective agent, uniformly mixing, and carrying out high-pressure homogenization to prepare nano-cellulose hydrogel;

(2) adding a polyvinyl alcohol solution into the prepared nano-cellulose hydrogel, and uniformly mixing to prepare polyvinyl alcohol/nano-cellulose composite hydrogel;

(3) fixing the radish red pigment on the fiber of the polyvinyl alcohol/nano-cellulose composite hydrogel in situ to prepare the polyvinyl alcohol/radish red pigment/nano-cellulose composite hydrogel;

(4) the polyvinyl alcohol/radish red pigment/nano-cellulose composite aerogel is prepared by adopting a vacuum freeze-drying method.

And the nano-cellulose hydrogel is prepared by a TEMPO mediated oxidation method.

Moreover, the freeze-drying protective agent is glycerol and accounts for 1 percent of the mass of the oven dry paper pulp.

The polyvinyl alcohol accounts for 10% by mass of the mixed system.

Moreover, the mass ratio of the radish red pigment to the oven-dried paper pulp is 1: 1.5.

And, the in-situ fixing in the step (3) adopts a method of stirring, shearing and mixing the system for a proper time by using a high-speed shearing machine.

Further, the specific conditions of the vacuum freeze-drying method are: freezing in a freeze dryer for 5-7h to freeze completely, and vacuum freeze drying for 36-48 h.

And placing the polyvinyl alcohol/radish red pigment/nano cellulose composite aerogel which is freeze-dried in an oven at 80 ℃ for annealing treatment for 30 min.

The invention has the advantages and beneficial effects that:

1. according to the invention, the nanocellulose and the polyvinyl alcohol are crosslinked, so that the fiber framework structure is enhanced, and the three-dimensional porous light aerogel with high strength, low density and high porosity is formed.

2. According to the invention, the radish red pigment is dispersed in the hydrogel by a high-speed shearing method, so that the defects of nonuniform dispersion of the radish red pigment and damage to an aerogel structure caused by addition of water in the process of dissolving the radish red pigment are overcome.

3. The nano-cellulose ammonia gas induction aerogel prepared by the invention has the advantages of wide raw material source, environmental protection, simple preparation method, mass production, effective detection of ammonia gas, effective indication of food freshness and application in the field of food freshness detection.

Drawings

FIG. 1 is a flow chart of the preparation of nano cellulose ammonia gas induction aerogel according to the present invention;

FIG. 2 is a schematic diagram of the color change of the nanocellulose ammonia gas response aerogel according to the invention;

FIG. 3 is a graph of the appearance of comparative examples 1-6 aerogels with different polyvinyl alcohol contents;

FIG. 4 is a graph of the appearance of comparative examples 7-12 aerogels with different methylcellulose contents;

FIG. 5 is a graph of the appearance of comparative examples 13-21 aerogels with different polyvinyl alcohol and methylcellulose contents;

FIG. 6 is a graph of the appearance of comparative examples 22-30 aerogels with different polyvinyl alcohol and methylcellulose contents;

FIG. 7 is a graph of the static compression of well formed aerogels in three experimental protocols;

FIG. 8 is an SEM topography of a nano-cellulose/radish red pigment aerogel prepared by comparative example 31 of the invention;

fig. 9 is an SEM topography of the nanocellulose/polyvinyl alcohol/radish red pigment composite aerogel prepared in example 1 of the present invention;

FIG. 10 is a graph comparing the compressive stress and compressive load experienced by comparative example 31 and example 1 of the present invention;

FIG. 11 is a graph of stress strain curves under cyclic compression for comparative example 31 and example 1 in accordance with the present invention;

FIG. 12 is a graph comparing the specific surface area, average pore diameter and pore volume of comparative example 31 of the present invention with example 1;

FIG. 13 (a) and (b) are graphs showing the comparison of the color difference in response to ammonia gas and the comparison of the sensitivity to ammonia gas in comparative example 31 and example 1, respectively, according to the present invention;

FIG. 14 is a graph showing the effect of color response of comparative example 31 of the present invention to ammonia gas;

FIG. 15 is a graph showing the effect of color response to ammonia gas in example 1 of the present invention.

Detailed Description

The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings, and it is to be understood that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Other embodiments, which can be derived by one of ordinary skill in the art from the embodiments given herein without making any creative effort, shall fall within the scope of the present invention.

The embodiment provides a nano-cellulose ammonia-induced composite aerogel with a high specific surface area.

Example 1

The embodiment provides a preparation method of a nanocellulose ammonia-induced aerogel with a high specific surface area, which comprises the following steps:

(1) uses eucalyptus bleached sulfate pulp as raw material and uses TEMPO mediated oxidation method to prepare nano cellulose

1) Weighing a certain amount of eucalyptus bleached sulfate pulp, soaking in deionized water, stirring for 2-3h with a stirrer to completely disperse the eucalyptus bleached sulfate pulp, washing the fiber with deionized water for 2-3 times, and placing the fiber in a certain proportion of 0.5mol/L Na after suction filtration₂CO₃/NaHCO₃A slurry with the pulp concentration of 1% is prepared from a mixed solution of the buffer solution and deionized water, and is placed into a three-neck flask to be stirred.

2) TEMPO accounting for 1 percent of the mass of the oven-dried pulp sample and NaBr accounting for 10 percent of the mass of the oven-dried pulp sample are respectively weighed, placed in a beaker, added with a proper amount of deionized water, and then heated in a water bath at 40 ℃ until the pulp is completely dissolved.

3) Measuring the NaClO with the effective chlorine being larger than or equal to 8 percent in a beaker according to the dosage of 8mmol/g (corresponding to 8mmol of pure NaClO per gram of pulp sample), diluting the NaClO to 10 percent by using deionized water, and dropwise adding 0.1mol/L HCl solution to adjust the pH value of the NaClO to about 10.5.

4) And dropwise adding the dissolved TEMPO and NaBr mixed solution into a three-neck flask under the stirring state.

5) Dropwise adding NaClO solution into the reaction system by using a peristaltic pump to perform oxidation reaction, and monitoring the pH value of the reaction system in real time; and when the NaClO solution is added dropwise and the pH value of the reaction system is not changed any more, adding a proper amount of absolute ethyl alcohol to terminate the reaction.

6) Filtering the reaction solution to leave slurry, repeatedly washing the slurry with deionized water until the slurry is neutral, and preparing the slurry into a fiber suspension with the mass percentage of 2%; simultaneously adding glycerol with the mass of 1 percent of that of the oven dry paper pulp as a freeze-drying protective agent, stirring for 2 hours by magnetic force to mix uniformly, and homogenizing for 6-8 times under the pressure condition of 80-100MPa after ultrasonic treatment to obtain the nano-cellulose.

(2) Preparation of polyvinyl alcohol/nanocellulose hydrogel

1) Dissolving polyvinyl alcohol powder in deionized water under the condition of heating in a water bath at 95 ℃ for 2-3h by magnetic stirring to obtain a polyvinyl alcohol solution with the polyvinyl alcohol mass fraction of 10%.

2) Filtering the prepared polyvinyl alcohol solution to remove insoluble impurities, then weighing the polyvinyl alcohol solution with the mass fraction of 10% of the mixed system, adding the polyvinyl alcohol solution into the prepared nano-cellulose hydrogel, and stirring for 30min by a stirrer to uniformly mix the polyvinyl alcohol solution and the nano-cellulose hydrogel to prepare the polyvinyl alcohol/nano-cellulose composite hydrogel.

(3) Preparation of polyvinyl alcohol/radish red pigment/nano-cellulose composite aerogel

1) Weighing the radish red pigment according to the mass ratio of the radish red pigment to the oven-dried paper pulp of 1:1.5, putting the radish red pigment into the polyvinyl alcohol/nano-cellulose hydrogel with corresponding mass, and stirring and shearing the mixed system for a proper time by using a high-speed shearing machine to prepare the polyvinyl alcohol/radish red pigment/nano-cellulose composite hydrogel.

2) Carrying out ultrasonic dispersion (800W, 60min) on the polyvinyl alcohol/radish red pigment/nano-cellulose composite hydrogel in an ultrasonic cleaner, placing the hydrogel in a pore plate die (a cylinder with the diameter of a single pore groove being 15mm and the height being 18 mm), freezing the placed sample in a freeze dryer for 5-7h to completely freeze and freeze, and then carrying out vacuum freeze drying for 36-48h to completely sublimate the water in the sample, thus obtaining the polyvinyl alcohol/radish red pigment/nano-cellulose composite aerogel.

3) And (3) putting the freeze-dried polyvinyl alcohol/radish red pigment/nano-cellulose composite aerogel into an oven at the temperature of 80 ℃ for annealing treatment for 30 min.

4) The treated polyvinyl alcohol/radish red pigment/nanocellulose composite aerogel is suspended in a conical flask into which 50ml of 0.8mol/L ammonium hydroxide aqueous solution is added, the color change of the aerogel is recorded every 4min, and the research shows that the aerogel has real-time colorimetric reaction in an ammonia atmosphere environment, as shown in fig. 10.

Comparative example

The whole preferred process of polyvinyl alcohol/radish red pigment/nanocellulose composite aerogel synthesis is discussed, including comparative example 1-comparative example 30.

Because the nanocellulose with the solid content of 2% is difficult to manufacture and difficult to produce, the nanocellulose with the solid content of 1% is used as a raw material to synthesize the aerogel in the exploration stage on the basis of not adding the radish red pigment. The appearance and the compressive stress of the aerogel are used as judgment bases, the composite aerogel with a complete structure and certain strength is preferably selected, and the formula of the composite aerogel is used as the basis of the next-stage experiment. As shown in fig. 3, 4, 5, 6.

FIG. 3 is a graph of the appearance of the aerogels of comparative examples 1-6 having different polyvinyl alcohol contents. As can be seen from FIG. 3, as the PVA content decreases, the surface of the aerogel becomes rough from smooth and irregular in shape. After the addition of glycerol, the shape of the aerogel becomes irregular and the overall fibrous framework collapses severely. The aerogels of comparative example 1, comparative example 2, comparative example 3 and comparative example 6 have regular shapes and have no collapse or shrinkage. For the aerogels of the two formulas of comparative example 4 and comparative example 5, the deformation is serious, and the normal state cannot be maintained, so the two formulas are abandoned, and the subsequent experiments are not considered.

FIG. 4 is a graph of the appearance of comparative examples 7-12 aerogels with different methylcellulose contents. From fig. 4, it can be seen that the overall shape structure of the aerogel does not change significantly with the decrease of the amount of methylcellulose added, and the addition of glycerin has some influence on the morphology of the aerogel, but is within an acceptable range.

Fig. 5 and 6 are graphs showing the appearance of aerogels of comparative examples 13 to 30 having different polyvinyl alcohol contents and methylcellulose contents. From FIG. 6, it can be seen that the aerogel with glycerol added has a much poorer overall shape structure than the aerogel without glycerol added, and the collapse and shrinkage are serious. Wherein comparative example 14, comparative example 17, comparative example 18, comparative example 19 and comparative example 20 remained relatively intact in shape and the fibrous skeleton was not severely deformed. The aerogel for gas induction still needs to have certain strength under the condition of ensuring the structural integrity of the aerogel, and can resist the impact of external force. The aerogels with better forms can only determine whether the structures are complete or not from the appearance, and the strength of the aerogels cannot be determined, so that the aerogels with relatively complete structures need to be subjected to compression experiments, the compression stress and the compression load are considered to eliminate the influence of other factors, and the optimal experimental scheme is determined.

In FIG. 7, (a), (b), (c), and (d) are aerogel compression tests, respectively. It can be seen from the figure that comparative example 1 has the highest compression resistance and the highest strength, which is the optimal experimental formulation. Example 1 was prepared on the basis of comparative example 1.

Comparative example 31

According to the comparative example, the nanocellulose with the solid content of 2% is used as the raw material, the step (2) is omitted compared with the step (1), the radish red pigment is directly added into the nanocellulose hydrogel to be subjected to high-speed shearing dispersion, and the obtained nanocellulose ammonia gas response aerogel does not have high specific surface area and compressive strength. The nanocellulose ammonia-responsive aerogel prepared in example 1 was subjected to the relevant tests for comparison:

1. SEM topography

Scanning Electron Microscope (SEM) images of the aerogels obtained in example 1 and comparative example 31 are shown in fig. 8 and 9, respectively. As is apparent from the image, example 1 shows a uniform and dense network structure as a whole, and uniform and dense pores have higher adsorptivity to gas; the whole comparative example is of a laminated structure and is in a disordered stacking state, and the pore diameter is larger and uneven. It is demonstrated that example 1 is superior to the comparative example in structural state.

2. Compression testing

Including static compression and cyclic compression testing.

The test method comprises the following steps:

(1) static compression: the aerogels prepared in the comparative example 31 and the example 1 are sequentially placed in a test platform of a 3369 type electronic universal test machine, a certain pressure is applied to the aerogels, the height of the aerogels is compressed by 60%, and the magnitude of the compressive stress and the magnitude of the compressive load borne by the two types of aerogels are compared.

(2) And (3) cyclic compression: the aerogels prepared in the comparative example 31 and the example 1 are sequentially placed in a test platform of a 3369 type electronic universal test machine, a certain pressure is applied to the aerogels, the height of the aerogels is compressed by 20%, the cycle number is 5, and the stress-strain curves of the two types of aerogels are compared.

And (3) testing results:

(1) FIG. 10 is a bar graph of compressive stress versus compressive load experienced under static compression conditions for example 1 and comparative example 31. It can be seen that example 1 (polyvinyl alcohol/radish red pigment/nanocellulose composite aerogel) can withstand higher compressive stress and compressive load, i.e. example 1 has higher structural strength than comparative example 31.

(2) In FIG. 11, (a) and (b) are stress-strain curves under cyclic compression conditions for comparative example 31 and example 1. It can be seen that example 1 withstands higher compression stress under 20% cyclic compression than comparative example 31 and possesses a higher compression resistance.

3. BET test

The test method comprises the following steps: weighing about 100mg of the aerogel prepared in the comparative example 31 and the aerogel prepared in the example 1 respectively, putting the weighed samples into a degassing tube with the thickness of 90mm, and degassing in vacuum for 210min at the temperature of 105 ℃; after degassing is finished, nitrogen adsorption-desorption experiments are carried out by utilizing good reversible adsorption characteristics of nitrogen, the specific surface area is calculated according to a BET (Brunaner-Emmett-Teller) formula and adsorption data with the relative pressure (P/P0) within the range of 0.05-0.30, and the pore diameter and pore volume parameters of the aerogel are calculated through analysis data.

And (3) testing results: FIG. 12 is a graph comparing the specific surface area, average pore diameter and pore volume of aerogel samples of example 1 and comparative example 31. It can be seen that example 1 (polyvinyl alcohol/radish red pigment/nanocellulose composite aerogel) has higher specific surface area, average pore diameter and pore volume. The larger the specific surface area, average pore diameter and pore volume, the more favorable the adsorption and release of the gas and the more favorable the gas response.

4. Ammonia sensitive Performance test

The test method comprises the following steps: samples of the aerogel of comparative example 31 and example 1 were suspended in a flask into which 80mL of ammonia solution (8mM) had been previously poured, the aerogel was exposed at a distance of 1cm from the liquid surface for 24 minutes, and the aerogel was tested using PR730 every 4 minutes, and the L, a, b, and R, G, B values of the aerogel were obtained. Calculating the color difference value delta E and sensitivity according to the formulas (1) and (2)Sensitivity value S_RGB。

Wherein L, a and b are measured values of the sample;

is the initial value of the aerogel.

In the formula R_a、G_a、B_aAre initial values for red, green and blue; r_b、G_b、B_bIs the stored gray value.

And (3) testing results: in FIG. 13, (a) and (b) are a color difference contrast chart and a sensitivity contrast chart of a comparative example and example 1, respectively; FIG. 14 is a graph showing color responses of comparative examples when exposed to an ammonia atmosphere for 0min, 4min, 8min, 12min, 16min, 20min, and 24 min; FIG. 15 is a graph showing color responses of example 1 when exposed to an ammonia gas atmosphere for 0min, 4min, 8min, 12min, 16min, 20min, and 24 min. Combining fig. 13 (a) with fig. 14 and fig. 15, it can be seen that the color response speed of comparative example 31 to ammonia gas is slower than that of example 1, the color difference change of example 1 is the largest at 4min, and comparative example 31 has a larger color difference change at 8min, which proves that the response ratio of example 1 to ammonia gas is better than that of comparative example 1. It can be seen from the combination of fig. 13 (b), fig. 14 and fig. 15 that the sensitivity of example 1 to ammonia gas is higher than that of comparative example 1, and the color of the reaction end point is already approached at 8min, while the response of comparative example 31 to ammonia gas still exists at 24min, indicating that the sensitivity is worse than that of example, which proves that the sensitivity of example 1 to ammonia gas is quicker and the response is quicker.

In summary, the above-mentioned embodiment is only one of the preferred embodiments of the present invention, the content of the present invention is not limited to the above-mentioned embodiment, and persons skilled in the same field can easily propose other embodiments within the technical teaching of the present invention, but such embodiments are included in the protection scope of the present invention.

Claims

1. A preparation method of nano-cellulose ammonia gas induction aerogel is characterized by comprising the following steps: the method comprises the following steps:

2. The method of claim 1, wherein: the nano-cellulose hydrogel is prepared by a TEMPO mediated oxidation method.

3. The method of claim 1, wherein: the freeze-drying protective agent is glycerol and accounts for 1 percent of the mass of oven-dried paper pulp.

4. The method of claim 1, wherein: the polyvinyl alcohol accounts for 10% of the mixed system by mass.

5. The method of claim 1, wherein: the mass ratio of the radish red pigment to the oven-dried paper pulp is 1: 1.5.

6. The method of claim 1, wherein: and (3) in-situ fixing by using a method of stirring, shearing and mixing the system for a proper time by using a high-speed shearing machine.

7. The method of claim 1, wherein: the specific conditions of the vacuum freeze-drying method are as follows: freezing in a freeze dryer for 5-7h to freeze completely, and vacuum freeze drying for 36-48 h.

8. The method of claim 1, wherein: and (3) putting the freeze-dried polyvinyl alcohol/radish red pigment/nano-cellulose composite aerogel into an oven at the temperature of 80 ℃ for annealing treatment for 30 min.

9. A nano cellulose ammonia-induced aerogel prepared by the method of any one of claims 1 to 8.