Method for preparing ultra-light thermal insulation material from corn straw and ultra-light thermal insulation material

Abstract

The invention discloses a method for preparing an ultra-light thermal insulation material by using corn straws and the ultra-light thermal insulation material.The method comprises the steps that the corn straws are aired and then smashed, and corn straw powder is obtained; the method comprises the following steps: firstly, carrying out alkalization treatment on corn straw powder, and then carrying out oxidation bleaching treatment to obtain purified cellulose; the preparation method comprises the following steps: dispersing purified cellulose in deionized water under vigorous stirring, then adding a coupling agent, stirring in a constant-temperature water bath to form a uniform suspension, and freezing and freeze-drying the suspension to remove moisture so as to form cellulose aerogel; performing hydrophobic modification on the cellulose aerogel and a hydrophobic modifier to obtain the hydrophobic modified ultralight thermal insulation material. The ultralight thermal insulation material is prepared according to the method. According to the invention, the corn straw is used as the precursor to synthesize the ultra-light thermal insulation material, the raw materials are biodegradable, wide in source and low in cost, and the synthesized ultra-light thermal insulation material has the characteristics of low density, low thermal conductivity and the like.

Description

Technical Field

[0001] This invention belongs to the field of agricultural straw biomass resource utilization and materials technology, specifically relating to a method for preparing ultralight thermal insulation materials using corn straw. Background Technology

[0002] Thermal energy is a crucial clean energy source in both daily life and industrial production, and thermal insulation materials play a vital role in protection and energy conservation in many fields, thus attracting significant attention. While traditional petroleum-based plastic foams are widely used for insulation, their non-renewable and non-degradable nature poses potential hazards, necessitating the development of new insulation materials. Aerogels, as highly porous solid materials, possess a series of excellent properties such as low density, high specific surface area, and low thermal conductivity. Existing technologies include lightweight materials made from silica aerogels and expanded perlite, such as Chinese patents CN118373619A, CN117699808A, CN112321948A, and CN 115073122 A. However, their application and development are limited by their high brittleness, difficulty in recovering from bending, easy powdering, environmental unfriendliness, and stringent synthesis conditions.

[0003] Cellulose is a natural polymer material with wide availability and advantages such as low cost, biodegradability, renewability, non-toxicity, and ease of modification, making it one of the main candidates for synthetic aerogels. Corn stalks contain up to 47.38% cellulose. Considering the rate of regeneration, yield, cost-effectiveness, and environmental impact, corn stalks are very suitable as a raw material for synthetic aerogels. Patents with publication numbers CN109796634A, CN108752623A, and CN118388842A use agricultural waste as raw materials to synthesize cellulose aerogels through different processes. They explore how to improve the mechanical properties of the aerogels and their use as composite fiber aerogel adsorbents or nanocellulose-based aerogel-type food preservation mats, but do not involve modifying the synthesized cellulose aerogels to obtain ultralight thermal insulation materials for application in the construction industry. Therefore, it is essential to develop a method for preparing ultralight thermal insulation materials using corn stalks. Summary of the Invention

[0004] To solve the above-mentioned technical problems, the first objective of this invention is to provide a method for preparing ultralight thermal insulation materials using corn stalks.

[0005] The second objective of this invention is to provide an ultralight thermal insulation material prepared by a method using corn stalks. This ultralight thermal insulation material, as the core layer of a board, has the characteristics of being lightweight, formaldehyde-free, having low thermal conductivity, low cost, and being environmentally friendly.

[0006] The first objective of this invention is achieved by the following steps: (1) After drying the corn stalks, crush them to obtain corn stalk powder; (2) First, the corn stalk powder is alkalized. After full alkalization, it is repeatedly filtered and washed with deionized water until the filtrate becomes clear. The main purpose of alkalization is to remove SiO2 and inorganic impurities, destroy the cellulose structure and thus improve the purity of cellulose. The product obtained by alkalization is subjected to oxidative bleaching treatment. Post-treatment: the final residue is filtered with qualitative filter paper, washed with distilled water until the filtrate is neutral, and dried in an oven at 80°C to constant weight to obtain purified cellulose. The purpose of oxidative bleaching treatment is to efficiently remove residual impurities, remove lignin and hemicellulose, and improve the purity of cellulose. The process of this invention is simple, suitable for industrial production, balances oxidation capacity and cellulose protection, and ensures subsequent processing. (3) The purified cellulose is dispersed in deionized water under vigorous stirring, and then a coupling agent is added. The mixture is stirred in a constant temperature water bath to form a uniform suspension. The suspension is then frozen and freeze-dried to remove water and form cellulose aerogel. Freeze-drying technology can accurately preserve the three-dimensional porous structure, giving the material ultra-lightweight and high specific surface area characteristics, and has the advantages of being green and low-cost. (4) The cellulose aerogel and hydrophobic modifier are hydrophobically modified. After the modification is completed, the aerogel is washed. The washing can be repeated several times with ethanol to remove unreacted silane, catalyst and reaction by-products remaining in the pores of the aerogel, so as to obtain the hydrophobically modified ultralight thermal insulation material.

[0007] Preferably, step (1) involves mechanically crushing the material at a speed of 2500 rpm for 1 minute.

[0008] Preferably, the alkalizing reagent used in step (2) is a 1M NaOH solution, the mass-volume ratio of corn stalk powder to alkalizing reagent NaOH solution is 1:50, the alkalization temperature is 90℃, and the alkalization time is 2.5h.

[0009] Preferably, the treatment solution used in step (2) oxidative bleaching is a mixture of acetic acid and NaClO2 solution, with a volume ratio of acetic acid to NaClO2 solution of 1:120, a NaClO2 solution concentration of 5wt%, and a mass-volume ratio of corn straw powder to oxidative bleaching treatment solution of 1:60.5. The oxidative bleaching treatment is carried out under constant temperature water bath conditions of 75℃ for 2 hours.

[0010] Preferably, in step (3), the concentration of the cellulose solution is 2 wt%, the coupling agent is a polyvinyl alcohol solution with a concentration of 1 wt%, and the volume ratio of the polyvinyl alcohol solution to the cellulose solution is 1:1.

[0011] Preferably, in step (3), the coupling agent and cellulose are mixed uniformly by stirring in a constant temperature water bath at 60°C for 1 hour.

[0012] Preferably, in step (4), the volume-to-mass ratio of the hydrophobic modifier to the dried cellulose aerogel is 20:1.

[0013] Preferably, in step (4), the hydrophobic modifier is a mixture of 10% methyltrimethoxymethylsilane (MTMS) solution, ethanol, and water. Acetic acid is used as a catalyst to adjust the pH of the mixture to 4-5 to accelerate the hydrolysis of MTMS. The mixture is then magnetically stirred at 40-50°C for 1 hour to allow methyltrimethoxymethylsilane to be fully hydrolyzed to generate methylsilanetriol. The solution changes from turbid to clear, thus obtaining the hydrophobic modifier. The volume ratio of methyltrimethoxymethylsilane solution, ethanol, and water is 1:5:5.

[0014] Preferably, in step (4), the cellulose aerogel is placed in a hydrophobic modifier, sealed, and kept in a vacuum drying oven at 60°C for 10 hours.

[0015] The second objective of this invention is achieved by preparing the product according to the described preparation method.

[0016] Compared with the prior art, the present invention has the following technical effects: 1. The method of this invention uses an aqueous solution of polyvinyl alcohol (PVA) as a crosslinking agent to physically crosslink with cellulose molecules through a hydrogen bond network (without the need for toxic chemical crosslinking agents), forming a stable three-dimensional framework. Its advantages include non-toxicity and safety: avoiding the toxicity and residue problems of traditional crosslinking agents (such as glutaraldehyde); improved toughness: significantly improving the brittleness of cellulose aerogel, increasing compressive strength by more than 30%. Freeze-drying technology is used: the sol-gel is frozen at low temperature (-20℃), and the ice crystal template forms a hierarchical porous structure (micropores-macropores), retaining high porosity (>95%) after sublimation, with drying costs only 1 / 5 of supercritical drying; one-step MTMS modification: the aerogel is soaked in a mixture of MTMS / ethanol / water (volume ratio 1:5:5), and after MTMS hydrolysis, it condenses with surface hydroxyl groups, grafting hydrophobic methyl groups (-CH3), achieving atmospheric pressure drying without the need for supercritical drying equipment; 2. This invention uses corn stalks as raw materials, which are widely available and inexpensive. Utilizing corn stalks to extract cellulose can realize the reuse of agricultural waste, reducing resource waste and environmental pollution. The corn stalk cellulose prepared by this invention is biocompatible, biodegradable, and renewable. 3. The method of the present invention uses polyvinyl alcohol as a coupling agent, which can enhance the interfacial bonding force between cellulose and other materials, making the aerogel structure more stable. This helps to extend the life of the insulation board core material and makes it less prone to damage. 4. The thermal insulation material of this invention has excellent thermal insulation performance. The aerogel has extremely low thermal conductivity, and its internal nanoporous structure can effectively prevent heat transfer, significantly improving the thermal insulation effect of the double-layer insulation board and meeting the needs of building energy conservation. The thermal insulation material of this invention is lightweight and has low density. When used as a core material of insulation board, it has a small impact on the environment, which is in line with the development trend of green building materials. It will not add extra weight to the building, which helps to reduce the load on the building structure. It is especially suitable for building designs with weight requirements. Attached Figure Description

[0017] Figure 1 This is a schematic flowchart of the method of the present invention; Figure 2 This is an appearance image of an ultralight thermal insulation material sample prepared according to the method of the present invention. Detailed Implementation

[0018] The present invention will be further described below with reference to the embodiments and accompanying drawings, but this does not limit the present invention in any way. Any changes or substitutions made based on the teachings of the present invention shall fall within the protection scope of the present invention.

[0019] Example 1 As attached Figure 1 As shown, this embodiment describes a method for preparing ultralight thermal insulation materials using corn stalks, which specifically includes the following steps: (1) Collected agricultural corn stalks are cleaned, air-dried, and then mechanically crushed at 2500 rpm for 1 minute before use; (2) Dissolve 10g of corn stalk powder in 500mL of NaOH (1M) solution and alkalize it at 90℃ for 2.5h to remove inorganic impurities, SiO2 and other substances. Then, repeatedly filter and wash until the filtrate becomes clear. Take 600mL of NaClO2 solution and 5mL of acetic acid mixed oxidative bleaching treatment solution with NaClO2 solution concentration of 5wt% and use the oxidative bleaching treatment solution to oxidize and bleach the alkalized corn stalk powder in a constant temperature water bath at 75℃ for 2h. Then, filter the final residue with qualitative filter paper, wash with distilled water until neutral, and dry in an oven at 80℃ to constant weight to obtain purified cellulose. (3) The purified cellulose was dispersed in deionized water under vigorous stirring to obtain a 2wt% cellulose solution. Then, a 1wt% polyvinyl alcohol solution was added. The volume ratio of the polyvinyl alcohol solution to the cellulose solution was 1:1. The mixture was stirred in a constant temperature water bath at 60℃ for 1h to form a uniform suspension. The suspension was then frozen and freeze-dried at 70℃ for 72h to remove moisture, forming a cellulose aerogel. (4) Place 5g of cellulose aerogel in 100mL of hydrophobic modifier (the hydrophobic modifier is a mixture of 10% methyltrimethoxymethylsilane solution, ethanol and water, with a volume ratio of methyltrimethoxymethylsilane solution, ethanol and water of 1:5:5, and use acetic acid as a catalyst to adjust the pH of the mixture to 4~5, and stir magnetically at 40℃~50℃ for 1h to fully hydrolyze methyltrimethoxymethylsilane to generate methylsilanetriol, and the solution changes from turbid to clear, thus obtaining the hydrophobic modifier), and keep it in a vacuum drying oven at 60℃ for 10h to carry out hydrophobic modification. After the modification is completed, open the sealed glass container in a fume hood, take out the modified aerogel, and then soak it in ethanol and wash it repeatedly 3~5 times to remove unreacted silane, catalyst and reaction by-products remaining in the pores of the aerogel. Dry it under normal pressure to obtain the hydrophobic modified ultralight thermal insulation material. The ultralight thermal insulation material prepared according to the method in Example 1 is named CA-2P.

[0020] Comparative Example 1 This comparative example is based on the method of preparing ultralight thermal insulation material using corn stalks in Example 1. Except for replacing the polyvinyl alcohol solution in step (3) with sodium alginate solution, the rest is the same as in Example 1. The thermal insulation material prepared according to the method of Comparative Example 1 is named CA-2S.

[0021] Comparative Example 2 This comparative example is based on the method of preparing ultralight thermal insulation material using corn stalks in Example 1. Except for step (3) where the coupling agent polyvinyl alcohol solution was not added, the rest is the same as in Example 1. The thermal insulation material prepared according to the method of Comparative Example 2 is named CA.

[0022] Comparative Example 3 This comparative example is based on the method of preparing ultralight thermal insulation material using corn stalks in Example 1. Except for step (3) using 1wt% cellulose solution, the rest is the same as in Example 1. The thermal insulation material prepared according to the method of Comparative Example 3 is named CA-P.

[0023] Comparative Example 4 The method for preparing ultralight thermal insulation material using corn stalks in this embodiment is based on Example 1, except that step (3) uses 1 wt% cellulose solution and 1 wt% sodium alginate solution as coupling agent, the rest is the same as in Example 1; the thermal insulation material prepared according to the method of Comparative Example 4 is named CA-S.

[0024] Performance testing The thermal insulation materials obtained in Example 1 and Comparative Examples 1-4 of the present invention were used as core materials to make boards. Performance tests were conducted on each board, and the test methods were carried out in accordance with conventional methods in the art. The results are shown in Table 1. Table 1 Performance Test Results As shown in Table 1, Comparative Example 2, without using a coupling agent, synthesized a cellulose aerogel with a lower density but exhibited poor compressive strength and high thermal conductivity. Comparative Example 1, using sodium alginate as a coupling agent, showed significantly improved compressive strength and thermal insulation properties, but the increased density led to a heavier material. Further, Comparative Example 4 used a 1 wt% sodium alginate solution as a coupling agent, reducing the concentration of the cellulose solution by 1 wt%. The resulting aerogel showed no improvement in density, thermal conductivity, or compressive strength; in fact, a significant decrease. In contrast, Example 1, using a 1 wt% polyvinyl alcohol solution as a coupling agent, synthesized a cellulose aerogel with lower density, stronger compressive strength, and better thermal insulation properties. Comparative Example 3, using a 1 wt% polyvinyl alcohol solution as a coupling agent, showed a slight decrease in thermal conductivity and density when the cellulose concentration was reduced to 1 wt%, but a decrease in compressive strength. Considering that cellulose aerogel used as the core material in insulation boards must not only possess low density and low thermal conductivity but also a certain degree of compressive strength to prevent deformation under pressure during use, polyvinyl alcohol (PVA) is used as a coupling agent to form a three-dimensional framework by binding with cellulose through a network of hydroxyl hydrogen bonds. Its long-chain, flexible molecular structure endows the aerogel with elastic deformation capabilities. Furthermore, PVA can inhibit ice crystal growth, reduce pore wall rupture, and result in a more uniform pore size distribution. In contrast, using sodium alginate as a coupling agent requires an additional calcium ion cross-linking step, increasing the processing steps and increasing the risk of metal ion residue. The resulting aerogel is more brittle, lacks elasticity, and has a lower thermal conductivity. Therefore, using PVA as a coupling agent combined with freeze-drying technology and hydrophobic modification to synthesize cellulose aerogels offers irreplaceable advantages.

Claims

1. A method for preparing ultralight thermal insulation materials using corn stalks, characterized in that... Includes the following steps: (1) After drying the corn stalks, crush them to obtain corn stalk powder; (2) First, the corn stalk powder is alkalized and then oxidized and bleached. After post-treatment, purified cellulose is obtained. (3) The purified cellulose was dispersed in deionized water under vigorous stirring, and then a coupling agent was added. The mixture was stirred in a constant temperature water bath to form a uniform suspension. The suspension was then frozen and freeze-dried to remove water and form a cellulose aerogel. (4) The cellulose aerogel and hydrophobic modifier are hydrophobically modified, and the mixture is washed after modification to obtain the hydrophobically modified ultralight thermal insulation material.

2. The method for preparing ultralight thermal insulation material using corn stalks according to claim 1, characterized in that... Step (1) The crushing is mechanical crushing at a speed of 2500 rpm for 1 min.

3. The method for preparing ultralight thermal insulation material using corn stalks according to claim 1, characterized in that... Step (2) The alkalization reagent used is 1M NaOH solution. The mass-volume ratio of corn straw powder to alkalization reagent NaOH solution is 1:

50. The alkalization temperature is 90℃ and the alkalization time is 2.5h.

4. The method for preparing ultralight thermal insulation material using corn stalks according to claim 1, characterized in that... Step (2) The treatment solution used for oxidative bleaching is a mixture of acetic acid and NaClO2 solution. The volume ratio of acetic acid to NaClO2 solution is 1:120, the concentration of NaClO2 solution is 5wt%, and the mass-volume ratio of corn straw powder to oxidative bleaching treatment solution is 1:60.

5. The oxidative bleaching treatment is carried out under constant temperature water bath conditions of 75℃ for 2 hours.

5. The method for preparing ultralight thermal insulation material using corn stalks according to claim 1, characterized in that... In step (3), the concentration of the cellulose solution is 2 wt%, the coupling agent is a polyvinyl alcohol solution with a concentration of 1 wt%, and the volume ratio of the polyvinyl alcohol solution to the cellulose solution is 1:

1.

6. The method for preparing ultralight thermal insulation material using corn stalks according to claim 1 or 5, characterized in that... Step (3) The coupling agent and cellulose are mixed evenly by stirring in a constant temperature water bath at 60℃ for 1 hour.

7. The method for preparing ultralight thermal insulation material using corn stalks according to claim 1, characterized in that... Step (4) The volume-to-mass ratio of the hydrophobic modifier to the dried cellulose aerogel is 20:

1.

8. The method for preparing ultralight thermal insulation material using corn stalks according to claim 1 or 7, characterized in that... Step (4) The hydrophobic modifier is a mixture of 10% methyltrimethoxymethylsilane solution, ethanol and water. Acetic acid is used as a catalyst to adjust the pH of the mixture to 4-5. The mixture is then magnetically stirred at 40-50°C for 1 hour to allow methyltrimethoxymethylsilane to be fully hydrolyzed to generate methylsilanetriol. The solution changes from turbid to clear, and the hydrophobic modifier is obtained. The volume ratio of methyltrimethoxymethylsilane solution, ethanol and water is 1:5:

5.

9. The method for preparing ultralight thermal insulation material using corn stalks according to claim 1, characterized in that... Step (4) Place the cellulose aerogel in a hydrophobic modifier, seal it, and place it in a vacuum drying oven at 60°C for 10 hours.

10. An ultralight thermal insulation material prepared by the method for preparing ultralight thermal insulation material using corn stalks according to any one of claims 1 to 9.

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