A naturally degradable cotton straw biological matrix and its preparation method

Abstract

The present invention provides a naturally degradable cotton straw biological matrix and a preparation method thereof. In the present invention, crushed and screened cotton straw powder is added with water and tannic acid and stirred, and then the prepared polyurethane prepolymer is added and rapidly and uniformly mixed. The mixture is poured into a mold sprayed with a polyurethane release agent, the excess water is discharged, and it is placed in an oven at 60 °C for foaming and drying for 24 h. This material has good adhesiveness, strength and degradability, and can be used in the fields of moisture retention and water storage of soil in arid areas, vegetation restoration, cultivation substrates for modern agriculture, etc.

Description

Technical Field

[0001] The present invention relates to the field of utilization of cotton straw and polyurethane, and particularly relates to a method for preparing a naturally degradable cotton straw biological matrix. Technical Background

[0002] Polyurethane (PU) is short for polycarbamate, which is a kind of macromolecular compound and an indispensable material in people's production and life. Different PUs have different properties. Among them, PU elastomer has properties such as wear resistance and low temperature resistance, and can be degraded under biological action.

[0003] In 2022, the national cotton sown area was about 3 million hectares, and the annual output of cotton straw was about 17.93 million tons. Among them, the cotton planting area in Xinjiang was about 2.5 million hectares, and the annual output of cotton straw was about 16.17 million tons or more. The main component contents in cotton straw are 44% cellulose, 10.7% hemicellulose, 15.4% lignin, 6.5% crude protein and 0.03% gossypol. It is an excellent renewable and degradable biological matrix material. If a large amount of straw is not properly disposed of, it will not only waste resources, but also cause certain pollution to the environment. At present, the main treatment methods of cotton straw are incineration, burial and feed utilization. Among them, straw incineration in the northwest region is likely to exacerbate the formation of "haze" weather; due to the arid climate in Xinjiang for burial, and the high content of lignin in cotton straw makes the straw rigid and not easy to rot, it cannot be turned into fertilizer in time but becomes a burden on the soil, resulting in a decline in soil fertility; the cellulose content in cotton straw is relatively high, the hemicellulose content is low, and it contains toxic gossypol, so the direct use as feed has low utilization rate and poor effect. The traditional treatment methods of straw have certain harms, resulting in difficulties in the utilization of cotton straw. In recent years, the moisture retention and water storage of soil, vegetation restoration, and cultivation substrates for modern agriculture in arid regions of the northwest have attracted much attention. The industry that uses straw resources as raw materials to replace non-renewable peat soil as biological substrates has good prospects. Summary of the Invention

[0004] In order to solve the problem of difficult utilization of cotton straw, the present invention provides a method for preparing a naturally degradable cotton straw biological matrix, and the product is used in fields such as moisture retention and water storage of soil in arid regions, vegetation restoration, and cultivation substrates for modern agriculture.

[0005] To achieve the above object, the present invention provides the following technical solutions:

[0006] In the first aspect, the present invention provides a naturally degradable cotton straw biological matrix, which is made of the following raw materials in parts by mass: 40 parts of cotton straw powder with a particle size less than 0.25 mm, 24 - 30 parts of polyurethane prepolymer, 160 - 240 parts of deionized water, and 0.4 - 0.8 parts of tannic acid.

[0007] The addition amount of deionized water affects the drying time and the foaming of PU, thereby affecting the sample strength.

[0008] Preferably, the biodegradable cotton straw bio-substrate is made from the following raw materials in parts by mass: 40 parts of cotton straw powder with a particle size less than 0.25 mm, 26.7 parts of polyurethane prepolymer, 200 parts of deionized water, and 0.4 part of tannic acid.

[0009] Furthermore, the polyurethane prepolymer is prepared by the following method:

[0010] Based on the polyurethane prepolymer, take 6 - 7 parts by mass of toluene diisocyanate and 21 - 24 parts by mass of HK-330E polyether polyol, stir and react at 60 - 100 °C for 1 - 3 h, add 0.8 part by mass of chain extender, and continue stirring for 5 - 15 min (10 min in the examples of the present invention) to obtain the polyurethane prepolymer.

[0011] In an embodiment of the present invention, the polyurethane prepolymer is prepared by the following method:

[0012] 6 parts by mass of toluene diisocyanate and 21 parts by mass of HK-330E polyether polyol are stirred and reacted at 80 °C for 2 h, add 0.8 part by mass of chain extender, and continue stirring for 10 min to obtain the polyurethane prepolymer.

[0013] Preferably, the chain extender is diethylene glycol or 1,4-butanediol, preferably diethylene glycol. Different chain extenders have different groups, which affect the water absorption performance of polyurethane, and the chain extender, as the hard segment of polyurethane, affects the strength of polyurethane, thereby affecting the sample strength. The chain extender can effectively improve the water absorption performance, otherwise the final product is prone to dispersion and obvious chipping.

[0014] Those skilled in the art know that the polyether polyol can be placed in a drying oven at 80 °C for dehydration for more than 2 h before use, aiming to avoid the reaction and foaming of the water in the polyether polyol with isocyanate.

[0015] The prepolymer is preferably cooled to room temperature when preparing the biodegradable cotton straw bio-substrate, aiming to ensure the fluidity of the prepolymer and slow down the foaming reaction of the prepolymer with water.

[0016] In the second aspect, the present invention provides a preparation method of the above-mentioned biodegradable cotton straw bio-substrate, and the method includes the following steps:

[0017] Uniformly mix cotton straw powder with a particle size less than 0.25 mm, tannic acid, and deionized water in the formulated amounts, pour in the polyurethane prepolymer in the formulated amount, and mix uniformly. The resulting mixture is shaped using a mold, drained, and left to foam at 20 - 60 °C for 12 - 36 h (preferably left to foam at 60 °C for 24 h) to obtain the biodegradable cotton straw biological matrix.

[0018] The bottom of the mold is provided with drainage holes. Those skilled in the art know that before putting the mixture into the mold, a mold release agent can be sprayed on the inner surface of the mold. Well-known PU synthesis methods include the one-step method, the semi-prepolymer method, and the prepolymer method. The present invention uses the prepolymer method to prepare polyurethane, that is, the hydroxyl group of the polyol reacts with the isocyanate group of the isocyanate to form a prepolymer with an isocyanate group as the end-capping group. The isocyanate group in the prepolymer reacts with water, first generating unstable carbamic acid, and then decomposing into amine and carbon dioxide. The generation of carbon dioxide increases the small pores in the module, which is beneficial to improving the air permeability of the module. The PU prepolymer has a certain adhesiveness. After being compounded with the straw powder, the combination of the isocyanate group and the polyhydroxyl molecules in the straw further improves the adhesion effect, avoids the loosening of the sample, and facilitates the transportation of the sample.

[0019] The cotton straw can be selected with a length of less than 40 mm to increase the amount of cotton straw powder obtained after crushing. The cotton straw powder should be selected with a particle size less than 0.25 mm. If the powder particle size is too large, the fluidity during mixing with water is poor, which affects the molding of the sample.

[0020] Preferably, the prepolymerization reaction temperature is 60 - 100 °C, such as 60 °C, 70 °C, 80 °C, 90 °C, 100 °C, and the reaction time is 1 - 3 h, such as 1 h, 1.5 h, 2 h, 2.5 h, 3 h. Timing starts from when the heating temperature reaches the target temperature range. Within this temperature range, the amount of isocyanate group end-capping in the prepolymer can be controlled, mainly to make the generated prepolymer more stable and ensure the subsequent foaming effect.

[0021] The ratio of polyol to toluene diisocyanate directly affects the strength and performance of the prepolymer. The product of mixing and stirring the PU prepolymer, deionized water, and tannic acid is foamed PU. The reaction accelerates with the increase in temperature, and stirring at room temperature can reduce the foaming speed.

[0022] Preferably, the addition amount of the prepolymer in the prepolymer and powder mixture is 37.5% - 42.5%, specifically 37.5%, 40%, 42.5%. The addition amount of the prepolymer affects the adhesiveness, strength, and water absorption of the sample.

[0023] Tannic acid contains a polyhydroxy structure and reacts with the PU prepolymer to form a high-molecular elastic body with a spatial network structure, improving the adhesion between the powder and PU, thus ensuring the overall strength of the module, reducing the situation of chipping, and tannic acid is a good biodegradable material, improving the degradation efficiency of the product.

[0024] After discharging the moisture, the foaming reaction rate of the foaming PU accelerates with the increase of temperature, causing the PU to continuously expand and fill the voids of the cotton straw powder, improving the overall adhesion of the module and providing a certain porosity for the module.

[0025] Preferably, the drying methods selected are drying in an oven at 60 °C and natural drying in the sun. Different drying methods affect the foaming of PU. At high temperatures, the foaming reaction rate is fast, which is beneficial to the formation of large pores; at low temperatures, the foaming reaction rate is slow, which is beneficial to the formation of small pores. Large pores are beneficial for water absorption, and small pores are beneficial for increasing strength.

[0026] In a third aspect, the present invention provides an application of the above-mentioned naturally degradable cotton straw biological matrix in the preparation of a cultivation substrate.

[0027] In view of the problem that cotton straw is difficult to utilize, the present invention designs a preparation method of a naturally degradable cotton straw biological matrix. The cotton straw is pulverized and then reacts with the PU prepolymer. The stiffness of the sample is much lower than that of the straw, making it easier to degrade compared to the straw. And because the cotton straw has a high content of lignin, the sample can have good strength, reducing the phenomenon of fragmentation and chipping, and facilitating transportation. The cotton straw has a high content of cellulose, and there are many hydrophilic hydroxyl groups in the cellulose, improving the moisture absorption and water retention of the matrix. As a cultivation substrate, the straw and PU components in the sample release nutrients after degradation, which can promote plant growth. The present invention effectively utilizes the waste cotton straw and opens up a new technical route for the moisture retention and water storage of the soil in arid areas, vegetation restoration, and large-scale production of cultivation substrates in modern agriculture. Its significance lies in realizing the green cycle of crops.

[0028] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0029] 1. The PU is synthesized by the prepolymer method. During foaming, the internal heat generation is less, the temperature rise is low, which is beneficial to the formation of large bubbles, and the yield of the sample is high.

[0030] 2. The cotton straw has a relatively high content of lignin and cellulose, making the module have relatively high strength and water absorption and moisture retention.

[0031] 3. The tannin added to the sample is a good biodegradable biological material, which can effectively improve the degradability and strength of the sample.

[0032] 4. The cotton straw and PU release nutrients through biodegradation, which helps plant growth. Description of the Drawings

[0033] Figure 1 Infrared spectrum (IR) map of the biological matrix prepared for Example 1

[0034] Figure 2 Pore volume and pore size distribution curve of the biological matrix prepared for Example 1

[0035] Figure 3 Scanning electron microscope (SEM) image of the biological matrix prepared for Example 1

[0036] Figure 4 Scanning electron microscope (SEM) image of the biological matrix prepared for Example 1 after 105 days of degradation in grassland Detailed Description of the Invention

[0037] Example 1

[0038] The crushed cotton straw in this example was transported from Xinjiang after being broken locally. The cotton straw was put into a crusher and crushed for 90 s. The cotton straw powder was obtained and screened, and the cotton straw powder with a mesh number greater than 60 (particle size less than 0.25 mm) was selected for standby.

[0039] The HK-330E polyether polyol was placed in a vacuum drying oven at 80 °C for dehydration for more than 2 h for standby.

[0040] The Meishu 606 polyurethane release agent was evenly sprayed on the surface of the mold and left for standby.

[0041] Weigh 30 g of toluene diisocyanate and 105 g of HK-330E polyether polyol and add them to a beaker. Place the beaker in an electric heating mantle, set the temperature to 80 °C, place the stirring paddle above the liquid, adjust the rotation speed to 400 r / min, react for 2 h, then add 4 g of the chain extender diethylene glycol and stir for 10 min to obtain a prepolymer. Take out the beaker and cool the prepolymer to room temperature. Weigh 200 g of cotton straw powder, add 2 g of tannic acid and 1000 g of deionized water, and stir slowly to make all the powder moist and have a certain fluidity. Quickly pour 133.3 g of the PU prepolymer into the powder, and use a stirrer to stir quickly for 30 s to make the PU and the powder evenly mixed. After the stirring is completed, pour the reactant into the mold, use a press block to drain the excess water from the holes at the bottom of the mold, place it in an oven at 60 °C and let it stand for foaming and drying. After 24 h, demold to obtain a cotton straw biodegradable biological matrix. The water absorption rate and sample conditions of the example are shown in Table 1. As Figure 1 shown, the characteristic absorption peak of the hydroxyl group corresponding to 3342 cm -1 may come from lignin, cellulose, and hemicellulose. The absorption peak at 2970 cm -1 is mainly the stretching vibration of the methyl group, and the absorption peak at 1729 cm -1, 1649 cm -1 The corresponding absorption peak is the antisymmetric stretching vibration of C=O, 1604 cm -1 and the absorption peaks at 1452 cm -1 are the stretching and bending vibrations of N-H. The absorption peak at 1537 cm-1 corresponds to the characteristic absorption peak of the benzene ring, and 1373 cm -1 corresponds to the characteristic absorption peak of CH2, 1229 cm -1 corresponds to the characteristic absorption peak of C=O, and 1076 cm -1 is the characteristic absorption peak of the ester group. As shown in Table 2, through mercury intrusion testing of the pore structure, the porosity of Example 1 reached 68.51%, the total pore area reached 1.475 m 2 / g, the average pore diameter reached 5.061×10 3 nm, the total pore volume reached 1.866 mL / g, and the permeability reached 81.40×10 3 mD. As Figure 2 shown, the pore diameter of the sample is mainly distributed around 10 5 nm. Figure 3 , 4 are the SEM images of the undegraded and degraded samples buried in the grassland for 105 days respectively. By comparison, it is found that Figure 4 larger-sized pores appear, proving that microorganisms have an obvious degradation effect on the PU in the sample.

[0042] Example 2

[0043] The powder, polyether polyol and mold used in this example are the same as those in Example 1

[0044] Weigh 35 g of toluene diisocyanate and 105 g of HK-330E polyether polyol and add them to a beaker. Place the beaker in an electric heating mantle, set the temperature to 80 °C, place the stirring paddle above the liquid, adjust the rotation speed to 400 r / min, react for 2 h, then add 4 g of chain extender diethylene glycol and stir for 10 min to obtain a prepolymer. Take out the beaker and cool the prepolymer to room temperature. Weigh 200 g of cotton straw powder, add 2 g of tannic acid and 1000 g of deionized water, and stir slowly to make all the powder moist and have a certain fluidity. Quickly pour 133.3 g of PU prepolymer into the powder and use a blender to stir quickly for 30 s to make the PU and the powder evenly mixed. After stirring, pour the reactant into the mold, use a press block to drain the excess water from the holes at the bottom of the mold, place it in an oven at 60 °C and let it stand for foaming and drying. Demold after 24 h to obtain a cotton straw biodegradable biological matrix. The water absorption rate and sample conditions of the example are shown in Table 1. The increase in the proportion of toluene diisocyanate increases the hardness content of the PU, resulting in the sample being too hard and lacking elasticity.

[0045] Example 3

[0046] The powder, polyether polyol, and mold used in this example are the same as those in Example 1

[0047] Weigh 30 g of toluene diisocyanate and 120 g of HK-330E polyether polyol and add them to a beaker. Place the beaker in an electric heating mantle, set the temperature to 80 °C, place the stirring paddle above the liquid, adjust the rotation speed to 400 r / min, react for 2 h, then add 4 g of chain extender diethylene glycol and stir for 10 min to obtain a prepolymer. Take out the beaker and cool the prepolymer to room temperature. Weigh 200 g of cotton straw powder, add 2 g of tannic acid, and 1000 g of deionized water, and stir slowly to make all the powder moist and have a certain fluidity. Quickly pour 133.3 g of the PU prepolymer into the powder, and use a blender to stir quickly for 30 s to make the PU and the powder evenly mixed. After the stirring is completed, pour the reactant into the mold, use a press block to drain the excess water from the holes at the bottom of the mold, place it in an oven at 60 °C and let it stand for foaming and drying. Demold after 24 h to obtain a natural degradable cotton straw biological matrix. The water absorption rate and sample conditions of the examples are shown in Table 1. As the proportion of HK-330E polyether polyol increases, the amount of isocyanate groups decreases, the foaming amount decreases, the number of holes decreases, and the water absorption rate decreases.

[0048] Example 4

[0049] The powder, polyether polyol, and mold used in this example are the same as those in Example 1

[0050] Weigh 30 g of toluene diisocyanate and 105 g of HK-330E polyether polyol and add them to a beaker. Place the beaker in an electric heating mantle, set the temperature to 80 °C, place the stirring paddle above the liquid, adjust the rotation speed to 400 r / min, react for 2 h, then add 4 g of chain extender 1,4-butanediol and stir for 10 min to obtain a prepolymer. Take out the beaker and cool the prepolymer to room temperature. Weigh 200 g of cotton straw powder, add 2 g of tannic acid, and 1000 g of deionized water, and stir slowly to make all the powder moist and have a certain fluidity. Quickly pour 133.3 g of the PU prepolymer into the powder, and use a blender to stir quickly for 30 s to make the PU and the powder evenly mixed. After the stirring is completed, pour the reactant into the mold, use a press block to drain the excess water from the holes at the bottom of the mold, place it in an oven at 60 °C and let it stand for foaming and drying. Demold after 24 h to obtain a natural degradable cotton straw biological matrix. The water absorption rate and sample conditions of the examples are shown in Table 1. Using 1,4-butanediol as the chain extender, due to the different chemical properties of the chain extender, the elasticity of the generated PU elastomer is poorer than that in Example 1.

[0051] Example 5 has no tannic acid

[0052] The powder, polyether polyol, and mold used in this example are the same as those in Example 1

[0053] Weigh 30 g of toluene diisocyanate and 105 g of HK-330E polyether polyol and add them to a beaker. Place the beaker in an electric heating mantle, set the temperature to 80 °C, place the stirring paddle on the upper part of the liquid, adjust the rotation speed to 400 r / min, react for 2 h, then add 4 g of chain extender diethylene glycol and stir for 10 min to obtain a prepolymer. Take out the beaker and cool the prepolymer to room temperature. Weigh 200 g of cotton straw powder, without adding tannic acid, and 1000 g of deionized water, and stir slowly to make all the powder moist and have a certain fluidity. Quickly pour 133.3 g of PU prepolymer into the powder, and use a blender to stir quickly for 30 s to make the PU and the powder evenly mixed. After stirring is completed, pour the reactant into a mold, use a pressing block to drain the excess water from the holes at the bottom of the mold, place it in an oven at 60 °C and let it stand for foaming and drying. Demold after 24 h to obtain a cotton straw biodegradable biological matrix. The water absorption rate and sample conditions of the examples are shown in Table 1. Since the straw powder directly reacts and bonds with the PU prepolymer without adsorbing tannic acid, the powder bonding is poor and chipping occurs. The amount of foaming due to the reaction of isocyanate groups with water increases, and the pores of the sample increase, but due to the change in the PU structure, the water absorption rate of the sample decreases, and the water absorption rate per unit volume decreases even more.

[0054] Example 6

[0055] The powder, polyether polyol and mold used in this example are the same as those in Example 1.

[0056] Weigh 30 g of toluene diisocyanate and 105 g of HK-330E polyether polyol and add them to a beaker. Place the beaker in an electric heating mantle, set the temperature to 80 °C, place the stirring paddle on the upper part of the liquid, adjust the rotation speed to 400 r / min, react for 2 h, then add 4 g of chain extender diethylene glycol and stir for 10 min to obtain a prepolymer. Take out the beaker and cool the prepolymer to room temperature. Weigh 200 g of cotton straw powder, add 2 g of tannic acid, and 1200 g of deionized water, and stir slowly to make all the powder moist and have a certain fluidity. Quickly pour 133.3 g of PU prepolymer into the powder, and use a blender to stir quickly for 30 s to make the PU and the powder evenly mixed. After stirring is completed, pour the reactant into a mold, use a pressing block to drain the excess water from the holes at the bottom of the mold, place it in an oven at 60 °C and let it stand for foaming and drying. Demold after 24 h to obtain a cotton straw biodegradable biological matrix. The water absorption rate and sample conditions of the examples are shown in Table 1. By increasing the proportion of deionized water, the water content of the powder mixture increases and the drying time increases, resulting in a decrease in the bondability of the sample and it being prone to looseness.

[0057] Example 7

[0058] The powder, polyether polyol and mold used in this example are the same as those in Example 1.

[0059] Weigh 30 g of toluene diisocyanate and 105 g of HK-330E polyether polyol and add them to a beaker. Place the beaker in an electric heating mantle, set the temperature to 80 °C, place the stirring paddle on the upper part of the liquid, adjust the rotation speed to 400 r / min, react for 2 h, then add 4 g of chain extender diethylene glycol and stir for 10 min to obtain a prepolymer. Take out the beaker and cool the prepolymer to room temperature. Weigh 180 g of cotton straw powder, add 2 g of tannic acid and 720 g of deionized water, and stir slowly to make all the powder moist and have a certain fluidity. Quickly pour 133 g of PU prepolymer into the powder, and use a blender to stir quickly for 30 s to make the PU and the powder evenly mixed. After stirring is completed, pour the reactant into a mold, use a press block to drain the excess water from the holes at the bottom of the mold, place it in an oven at 60 °C and let it stand for foaming and drying. Demold after 24 h to obtain a cotton straw biodegradable biological matrix. The water absorption rate and sample conditions of the examples are shown in Table 1. Reducing the proportion of deionized water, the water content of the powder mixture is low, and the degree of PU reaction foaming is low, which increases the hardness of the sample.

[0060] Example 8

[0061] The powder, polyether polyol and mold used in this example are the same as those in Example 1

[0062] Weigh 30 g of toluene diisocyanate and 105 g of HK-330E polyether polyol and add them to a beaker. Place the beaker in an electric heating mantle, set the temperature to 80 °C, place the stirring paddle on the upper part of the liquid, adjust the rotation speed to 400 r / min, react for 2 h, then add 4 g of chain extender diethylene glycol and stir for 10 min to obtain a prepolymer. Take out the beaker and cool the prepolymer to room temperature. Weigh 200 g of cotton straw powder, add 2 g of tannic acid and 1000 g of deionized water, and stir slowly to make all the powder moist and have a certain fluidity. Quickly pour 120 g of PU prepolymer into the powder, and use a blender to stir quickly for 30 s to make the PU and the powder evenly mixed. After stirring is completed, pour the reactant into a mold, use a press block to drain the excess water from the holes at the bottom of the mold, place it in an oven at 60 °C and let it stand for foaming and drying. Demold after 24 h to obtain a cotton straw biodegradable biological matrix. The water absorption rate and sample conditions of the examples are shown in Table 1. Reducing the addition amount of PU prepolymer, the bonding component reacting with the powder decreases, and the insufficient bonding property causes the sample to be easily loose.

[0063] Example 9

[0064] The powder, polyether polyol and mold used in this example are the same as those in Example 1

[0065] Weigh 30 g of toluene diisocyanate and 105 g of HK-330E polyether polyol and add them to a beaker. Place the beaker in an electric heating mantle, set the temperature to 80 °C, place the stirring paddle on the upper part of the liquid, adjust the rotation speed to 400 r / min, react for 2 h, then add 4 g of chain extender diethylene glycol and stir for 10 min to obtain a prepolymer. Take out the beaker and cool the prepolymer to room temperature. Weigh 200 g of cotton straw powder, add 2 g of tannic acid and 1000 g of deionized water, and stir slowly to make all the powder moist and have a certain fluidity. Quickly pour 133.3 g of PU prepolymer into the powder, and use a blender to stir quickly for 30 s to make the PU and the powder evenly mixed. After stirring, pour the reactant into a mold, use a press block to drain the excess water from the holes at the bottom of the mold, and let it stand on the balcony to foam and dry naturally. Demold after one week to obtain a cotton straw biodegradable biological matrix. The water absorption rate and sample conditions of the examples are shown in Table 1. Changing the drying temperature and increasing the drying time, the PU is prone to hydrolysis, resulting in a decrease in the adhesiveness of the sample and slight chipping and other situations.

[0066] Example 10

[0067] The powder, polyether polyol and mold used in this example are the same as those in Example 1

[0068] Weigh 30 g of toluene diisocyanate and 105 g of HK-330E polyether polyol and add them to a beaker. Place the beaker in an electric heating mantle, set the temperature to 80 °C, place the stirring paddle on the upper part of the liquid, adjust the rotation speed to 400 r / min, react for 2 h, then add 4 g of chain extender diethylene glycol and stir for 10 min to obtain a prepolymer. Take out the beaker and cool the prepolymer to room temperature. Weigh 200 g of cotton straw powder, add 4 g of tannic acid and 1000 g of deionized water, and stir slowly to make all the powder moist and have a certain fluidity. Quickly pour 133.3 g of PU prepolymer into the powder, and use a blender to stir quickly for 30 s to make the PU and the powder evenly mixed. After stirring, pour the reactant into a mold, use a press block to drain the excess water from the holes at the bottom of the mold, place it in an oven at 60 °C and let it stand to foam and dry. Demold after 24 h to obtain a cotton straw biodegradable biological matrix. The water absorption rate and sample conditions of the examples are shown in Table 1. Increasing the tannin addition amount has basically no change in the sample conditions and water absorption rate, and too much tannin addition amount causes oxidation and makes the sample turn black.

[0069] Example 11

[0070] Take 5 samples prepared in Example 9 and weigh and record the mass of each sample. On November 1st, dig a soil pit in the grassland with a length and width of about 15 cm and a depth of about 20 cm. Bury the weighed samples in the soil pit, pile back the soil and cover it with turf to ensure that the soil environment remains unchanged. According to the degradation days of 15d, 30d, 60d, 105d, and 165d, take one sample from the soil pit each time (restore the original state after taking the sample), carefully clean the soil on the surface of the sample and put it into the oven for drying. Calculate the degradation rate of the sample in the soil by the weight loss method (i.e., degradation rate = 1 - mass of the sample after degradation / original mass of the sample). The degradation rates of the samples are shown in Table 3. It can be seen from Table 3 that the degradation rate of the sample in the grassland soil pit has reached 39.9% after 165 days, indicating that the sample has good biodegradability. The degradation rates at 30d and 60d are close because the temperature was low from December to January at that time, and the degradation activity of microorganisms was weak.

[0071] The experimental factors of different examples are as follows: As can be seen from the comparison between Example 2 and Example 1, the increase in the isocyanate group ratio makes the hardness of the sample increase; as can be seen from the comparison between Example 3 and Example 1, the increase in the polyol ratio makes the water absorption performance of the sample decrease; as can be seen from the comparison between Example 4 and Example 1, different chain extenders result in different elasticities of the samples; as can be seen from the comparison between Example 5, 10 and Example 1, adding tannin can effectively increase the water absorption rate per unit volume, but too much tannin will cause the sample to oxidize and turn black, and obvious degradation traces can be clearly seen in the polyurethane added with tannin from the SEM image; as can be seen from the comparison between Example 6, 7 and Example 1, the higher the deionized water ratio, the looser the sample, but too low will make the sample too hard and lack elasticity; as can be seen from the comparison between Example 8 and Example 1, the decrease in the prepolymer ratio will make the sample loose and crumble significantly; as can be seen from the comparison between Example 9 and Example 1, the water absorption rate and the strength of the sample dried naturally both decrease slightly. Compared with other examples, Example 1 has relatively better water absorption performance and good sample conditions, but the experimental conditions should be changed according to specific requirements.

[0072] Table 1 Water Absorption Rates of Each Example

[0073]

[0074]

[0075] Table 2 Pore Size Data of the Samples

[0076]

[0077] Table 3 Degradation Rates of the Samples

[0078] Degradation time 15d 30d 60d 105d 165d Original mass (g) 5.53 4.22 4.78 4.56 4.74 Mass after degradation (g) 4.95 3.38 3.79 3.22 2.85 Degradation rate 10.5％ 19.9％ 20.7％ 29.3％ 39.9％

Claims

1. A naturally degradable cotton straw biological matrix, characterized in that The natural degradable cotton straw biological matrix is made from the following raw materials in parts by mass: 40 parts of cotton straw powder with a particle size less than 0.25 mm, 24 - 30 parts of polyurethane prepolymer, 160 - 240 parts of deionized water, and 0.4 - 0.8 parts of tannic acid.

2. The biodegradable cotton straw biological matrix according to claim 1, wherein The natural degradable cotton straw biological matrix is made from the following raw materials in parts by mass: 40 parts of cotton straw powder with a particle size less than 0.25 mm, 26.7 parts of polyurethane prepolymer, 200 parts of deionized water, and 0.4 parts of tannic acid.

3. The biodegradable cotton straw biological matrix according to claim 1, characterized in that The polyurethane prepolymer is prepared by the following method: Based on the polyurethane prepolymer, take 6 - 7 parts by mass of toluene diisocyanate and 21 - 24 parts by mass of HK - 330E polyether polyol, stir and react at 60 - 100 °C for 1 - 3 h, add 0.8 parts by mass of chain extender, and continue stirring for 5 - 15 min to obtain the polyurethane prepolymer.

4. The biodegradable cotton straw biological substrate according to claim 3, wherein The polyurethane prepolymer is prepared by the following method: 6 parts by mass of toluene diisocyanate and 21 parts by mass of HK - 330E polyether polyol are stirred and reacted at 80 °C for 2 h, add 0.8 parts by mass of chain extender, and continue stirring for 10 min to obtain the polyurethane prepolymer.

5. The biodegradable cotton straw biological matrix according to claim 3 or 4, characterized in that: The chain extender is diethylene glycol or 1,4 - butanediol.

6. The biodegradable cotton straw biological matrix according to claim 5, characterized in that: The chain extender is diethylene glycol.

7. The preparation method of the naturally degradable cotton straw biological matrix according to claim 1, characterized in that The method includes the following steps: Evenly mix the cotton straw powder with a particle size less than 0.25 mm, tannic acid, and deionized water in the above - mentioned parts by mass, pour into the polyurethane prepolymer in the above - mentioned parts by mass, mix evenly, shape the obtained mixture with a mold, drain water, and let it stand and foam at 20 - 60 °C for 12 - 36 h to obtain the natural degradable cotton straw biological matrix.

8. The preparation method of the naturally degradable cotton straw biological matrix according to claim 7, characterized in that: The temperature of the standing and foaming is 60 °C and the time is 24 h.

9. The preparation method of the naturally degradable cotton straw biological matrix according to claim 7, characterized in that: The bottom of the mold is provided with drainage holes.

10. Use of the natural degradable cotton straw biological matrix according to claim 1 in the preparation of a cultivation substrate.

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