Bio-based polyol for polyurethane, coating liquid and coated controlled-release fertilizer

Abstract

The present invention discloses a bio-based polyol, a coating liquid, a coated controlled-release fertilizer, and a preparation method. The bio-based polyol is prepared according to the following method: biomass, a liquefier, a co-liquefier, and an acidic catalyst are added to a reactor in a mass ratio of 100:100 to 500:1 to 10:2 to 15 for reaction. After the reaction is completed, the biomass is cooled to below 100°C in an ice-water bath, and the supernatant is collected to obtain the bio-based polyol. The coating liquid comprises the bio-based polyol, an alkaline catalyst, other polyols, and an additive. The coated controlled-release fertilizer consists of a core and a membrane shell; wherein the core is a fertilizer particle, and the membrane shell is formed by a solvent-free in-situ reaction of the coating liquid and a curing agent. The mass of the membrane shell is 2 to 8% of the mass of the coated controlled-release fertilizer.

Description

Technical Field

[0001] The present invention belongs to the field of polyurethane composite materials and controlled-release fertilizers, and particularly relates to a bio-based polyol and a preparation method thereof, a coating liquid and a preparation method of the coated controlled-release fertilizer. Background Art

[0002] As a vital agricultural support industry, the fertilizer industry is directly linked to national food security and ecological environmental protection. Slow-release fertilizers, produced by coating quick-acting fertilizers with various polymer materials, quantitatively control nutrient release through the film. In practice, this allows the fertilizer's nutrient release rate to synchronize with crop nutrient absorption, significantly improving fertilizer utilization, reducing labor costs, and increasing economic benefits. This lays the foundation for the implementation of precision fertilization technology and the promotion of sustainable agricultural development.

[0003] The composition and properties of the coating directly influence the pattern and timing of nutrient release. Therefore, the selection and optimization of coating materials is a key component in the development of coated controlled-release fertilizers. Depending on the medium, polymer coating materials can be divided into three main types: organic solvent-based, water-based, and solvent-free. In the past decade, a growing number of research institutions both domestically and internationally have focused on solvent-free in-situ reaction film formation. This involves directly reacting small molecule monomers on the fertilizer surface to form a film to produce coated controlled-release fertilizers. This approach uses simple equipment and is easily implemented in a continuous process. Polyurethane is a non-toxic and odorless block copolymer composed of soft and hard segments, produced by the reaction of polyols and isocyanates. By adjusting the ratio and composition of the two phases, the microstructure can be modified, improving the specific macroscopic properties of the material. Petrochemical raw materials are expensive and non-renewable resources. Bio-based polyurethane coatings have rapidly developed due to their low cost, renewable nature, environmental friendliness, and resource conservation. Plant oil-based polyurethanes, particularly castor oil-based polyurethanes, have long been the dominant membrane material, but fiber-based polyurethanes are gaining popularity, and the sources and types of coatings are becoming increasingly diverse. Patent US20120111077 discloses a method for preparing polyurethane-coated controlled-release fertilizers by directly reacting castor oil with isocyanate, or by first crosslinking with oxygen or sulfur followed by a reaction. Bayer's Markusch et al. described castor oil-polyether polyol-based polyurethane controlled-release fertilizers and oil polyol-based polyurethane controlled-release fertilizers in patents US6364925B1 and US6358296B1, respectively. US9090517 discloses a coated fertilizer made from a methyl ester derivative of a natural oil. CN201310017918.2, CN201510416904.7, and CN201410230859.1, among others, disclose methods for preparing polyurethane-coated or composite-coated controlled-release fertilizers by in-situ reaction of liquefied products of starch, straw, or waste paper with a curing agent. These coatings exhibit good soil degradability.

[0004] Biomass is rich in hydroxyl groups, and low-temperature, atmospheric-pressure catalytic liquefaction can transform it from a solid substance into liquid bio-based polyols, increasing the activity of the functional groups. To ensure a complete biomass liquefaction reaction, the amount of liquefier used is often more than three times the amount of biomass used, resulting in a low biomass content in the liquefied product, with a maximum of only 25%. In addition, the liquefied product contains a large number of hydrophilic groups that easily absorb water, and since the biomass liquefaction product is not dehydrated, the reaction of isocyanate with water will produce gas. As a result, the resulting polyurethane coating contains micropores and has poor nutrient release properties, thus limiting the amount used to replace petroleum-based polyols. To reduce production costs and improve the biodegradability of polyurethane materials, it is necessary to increase the content of biomass components involved in the liquefaction reaction and, at the same time, increase the amount of petroleum-based polyols used to replace them.

[0005] Polyurethane nanocomposites have seen rapid development in recent years. Zhang Chaoqun et al. used nanobentonite to modify soybean oil-based polyurethane membranes. When the polyethylene glycol-intercalated modified bentonite was added at a 5% concentration, the nutrient release period of the controlled-release fertilizer was 74 days. While domestic and international researchers have conducted extensive research on bio-based polyurethane nanocomposite-coated controlled-release fertilizers, the difficulty in dispersing nanoparticles within polyurethane nanocomposites remains a key factor affecting their performance. Summary of the Invention

[0006] The technical problems to be solved by the present invention are: first, solving the problem of dispersing nanoparticles in polyurethane nanocomposites; and second, increasing the biomass content in the coating of bio-based coated controlled-release fertilizers. Accordingly, the present invention provides a bio-based polyol for polyurethane, a coating solution, and a coated controlled-release fertilizer.

[0007] The bio-based polyol for polyurethane provided by the present invention is obtained by liquefying solid biomass; the bio-based polyol is an organic-inorganic nanocomposite. Multiple reactions occur during the synthesis of the bio-based polyol, primarily alcoholysis, hydroxyalkylation, and repolymerization. The specific liquefaction mechanism involves the depolymerization of macromolecules of the solid biomass into liquid small molecules under the action of a catalyst, a liquefying agent, and a co-liquefying agent. These small molecules have good thermal fluidity and high reactivity, reacting with each other or with the solvent to produce the bio-based polyol.

[0008] The bio-based polyol can be specifically prepared according to a method comprising the following steps: adding biomass, a liquefier, a co-liquefier, and an acidic catalyst into a reactor in a mass ratio of 100:100 to 500:1 to 10:2 to 15 for reaction, and cooling the reaction mixture to below 100° C. in an ice-water bath after the reaction is completed to obtain the bio-based polyol.

[0009] Furthermore, the mass ratio of the biomass, liquefier, co-liquefier and acid catalyst is 100:100-500:6-10:2-15; specifically 100:290:10:8 or 100:100:8:2 or 100:400:6:15 or 100:290:10:8 or 100:500:10:15.

[0010] wherein the biomass is selected from at least one of corn starch, potato starch, wheat starch, sweet potato starch, mung bean starch, cellulose, lignin, corn stalks, corn cobs, rice straw, wheat straw, peanut shells and cotton straw;

[0011] The liquefier is selected from any one of polyethylene glycol, ethylene glycol, ethylene carbonate, glycerol and diethylene glycol. The relative molecular weight of the polyethylene glycol can be specifically 200 to 600, such as polyethylene glycol 200 and polyethylene glycol 400;

[0012] The co-liquefaction agent is selected from at least one of cage-type polysilsesquioxane, nanocellulose, and hydroxyapatite; preferably, the co-liquefaction agent is selected from cage-type polysilsesquioxane; more preferably, the cage-type polysilsesquioxane is selected from trisilanol cage-type polysilsesquioxane, such as trisilanol isobutyl cage-type polysilsesquioxane POSS-OH (purchased from Hybrid Plastics, USA, with the trademark SO1450) and trisilanol phenyl cage-type polysilsesquioxane POSS-OH (purchased from Hybrid Plastics, USA, with the trademark SO1458);

[0013] The acidic catalyst is selected from at least one of an inorganic acid, an organic acid and a heteropoly acid.

[0014] The inorganic acid may be selected from sulfuric acid, sulfamic acid, hydrochloric acid, and phosphoric acid. Preferably, the inorganic acid is selected from sulfuric acid or sulfamic acid. More preferably, the inorganic acid is selected from sulfamic acid. The organic acid may be benzenesulfonic acid. The heteropolyacid may be tungsten trioxide.

[0015] The reaction conditions of the reaction may be: reaction at 130-180° C. (eg, 130° C., 150° C.) for 30-120 min (eg, 30 min, 60 min).

[0016] According to one embodiment of the present invention, when preparing the bio-based polyol, the mass ratio of biomass (corn starch), liquefying agent (polyethylene glycol 400), co-liquefying agent (trisilanol isobutyl cage-type polysilsesquioxane POSS-OH) and acidic catalyst (sulfuric acid) is 100:290:10:8;

[0017] According to one embodiment of the present invention, when preparing the bio-based polyol, the mass ratio of biomass (corn starch), liquefying agent (polyethylene glycol 200), co-liquefying agent (trisilanol isobutyl cage-type polysilsesquioxane POSS-OH) and acidic catalyst (aminosulfonic acid) is 100:100:8:2;

[0018] According to one embodiment of the present invention, when preparing the bio-based polyol, the mass ratio of biomass (corn straw), liquefier (polyethylene glycol 200), co-liquefier (trisilanol phenyl cage polysilsesquioxane POSS-OH) and acid catalyst (sulfuric acid) is 100:400:6:15;

[0019] According to one embodiment of the present invention, when preparing the bio-based polyol, the mass ratio of biomass (corn starch), liquefying agent (polyethylene glycol 600), co-liquefying agent (trisilanol phenyl cage polysilsesquioxane POSS-OH) and acidic catalyst (aminosulfonic acid) is 100:290:10:8;

[0020] According to one embodiment of the present invention, when preparing the bio-based polyol, the mass ratio of biomass (corn straw), liquefier (polyethylene glycol 400), co-liquefier (hydroxyapatite) and acid catalyst (sulfuric acid) is 100:500:10:15.

[0021] The invention also provides a coating solution.

[0022] The raw materials of the coating liquid include: the above-mentioned bio-based polyol for polyurethane.

[0023] Furthermore, the raw material composition of the coating solution includes: the bio-based polyol, alkaline catalyst, other polyols and additives.

[0024] Furthermore, the mass ratio of the bio-based polyol, the alkaline catalyst, other polyols and the auxiliary agent in the coating solution can be 100:0.5-5:50-300:3-30.

[0025] The method for preparing the coating liquid comprises the following steps: measuring the polyurethane bio-based polyol, alkaline catalyst, other polyols and additives and stirring them in a mixing tank at 60-80° C. to form a uniform coating liquid.

[0026] wherein the alkaline catalyst is selected from at least one of triethylenediamine, ethylenediamine, triethylenediamine, triethylamine, triethanolamine, dimethylcyclohexylamine, dimethylhexadecylamine, dimethylethanolamine and tetramethyliminodipropylamine;

[0027] The auxiliary agent is selected from at least one of paraffin wax, microcrystalline wax, chlorinated paraffin, petroleum resin, α-olefin, polyethylene wax, EVA wax, asphalt and silicone wax;

[0028] The other polyols are selected from at least one of polyether polyols, polyester polyols, and vegetable oil polyols.

[0029] The polyether polyol can specifically be polyether triol, polyether diol, or polyether tetraol; the polyester polyol can specifically be phthalic anhydride polyester polyol, polycaprolactone diol, dimer polyester diol, or polycarbonate diol; and the vegetable oil polyol can specifically be castor oil, soybean oil polyol, or palm oil polyol.

[0030] According to one embodiment of the present invention, the raw materials of the coating solution are composed of bio-based polyol (starch-based polyol), alkaline catalyst (ethylenediamine), other polyol (polyether tetraol), and auxiliary agent (paraffin) in the mass ratio of 100:0.8:100:5.

[0031] According to one embodiment of the present invention, the raw materials of the coating solution are composed of bio-based polyol (starch-based polyol), alkaline catalyst (ethylenediamine), other polyol (castor oil), and auxiliary agent (paraffin) in a mass ratio of 100:0.8:200:10.

[0032] According to one embodiment of the present invention, the raw materials of the coating solution are composed of bio-based polyol (straw-based polyol), alkaline catalyst (triethanolamine), other polyols (polyether tetraol), and additives (paraffin) in a mass ratio of 100:0.5:50:3;

[0033] According to one embodiment of the present invention, the raw materials of the coating solution are composed of bio-based polyol (starch-based polyol), alkaline catalyst (ethylenediamine), other polyol (polyether tetraol), and additive (microcrystalline wax) in a mass ratio of 100:0.5:300:30.

[0034] According to one embodiment of the present invention, the raw materials of the coating liquid are composed of bio-based polyol (straw-based polyol), alkaline catalyst (ethylenediamine), other polyols (castor oil), and additives (chlorinated paraffin) in a mass ratio of 100:5:50:3.

[0035] The present invention also provides a coated controlled-release fertilizer.

[0036] The coated controlled-release fertilizer consists of a core and a membrane shell; wherein the core is a fertilizer particle, and the membrane shell is formed by the coating liquid and a curing agent through a solvent-free in-situ reaction, and the mass of the membrane shell is 2 to 8% of the mass of the coated controlled-release fertilizer.

[0037] In the coated controlled-release fertilizer, the fertilizer particles may be water-soluble elemental fertilizers or compound fertilizers obtained by mixing at least two of the water-soluble elemental fertilizers. Specifically, the water-soluble elemental fertilizers may be selected from any one of the following: urea, ammonium sulfate, ammonium chloride, ammonium nitrate, monoammonium phosphate, diammonium phosphate, potassium chloride, potassium sulfate, potassium nitrate, magnesium sulfate, magnesium nitrate, zinc sulfate, copper sulfate, and zinc chloride.

[0038] In the above-mentioned coated controlled-release fertilizer, the average particle size of the fertilizer particles may be 2 to 6 mm.

[0039] In the above-mentioned coated controlled-release fertilizer, the curing agent is selected from at least one of polymethylene polyphenyl polyisocyanate, toluene diisocyanate, hexamethylene diisocyanate, diphenylmethane diisocyanate (MDI), liquefied MDI, isophorone diisocyanate, 1,6-hexamethylene diisocyanate (HDI), trimer of HDI, trimethylhexamethylene diisocyanate, xylylene diisocyanate and dimethyldiphenyl diisocyanate.

[0040] In the above-mentioned coated controlled-release fertilizer, the mass ratio of the coating liquid to the curing agent is 1:0.6-2, specifically 1:0.6, 1:0.68, 1:1, 1:1.11 or 1:1.56.

[0041] The method for preparing the coated controlled-release fertilizer comprises the following steps:

[0042] 1) Preheat the fertilizer granules;

[0043] 2) The coating liquid and the curing agent are mixed and then atomized onto the surface of the fertilizer particles obtained in step 1) to perform solvent-free in-situ reaction to form a film, thereby obtaining the coated controlled-release fertilizer.

[0044] In step 1) of the above method, the preheating device is a film coating machine with a copy plate; the preheating temperature is 50-70°C.

[0045] In step 2) of the above method, the reaction temperature of the solvent-free in-situ reaction is 50-70° C., specifically 60° C. or 65° C.; the reaction time is 3-6 minutes, specifically 3 minutes or 6 minutes.

[0046] Compared with the prior art, the present invention has the following beneficial effects:

[0047] First, the biomass content of bio-based polyols for polyurethane exceeds 25%;

[0048] Second, the nanoparticles are evenly dispersed in the polyurethane nanocomposite;

[0049] Third, the replacement rate of petroleum-based polyols in the coating of bio-based coated controlled-release fertilizers can reach 100%. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Figure 1 The present invention is a production flow chart of the bio-based polyol for polyurethane, coating liquid and coated controlled-release fertilizer thereof.

[0051] Figure 2 This is the SEM / EDX image of the film material after curing of the starch-based polyol prepared in Example 1;

[0052] Figure 3 The nutrient release curves of the coated controlled-release fertilizers in the examples and control examples are shown in FIG.

[0053] The following are marked in the figure:

[0054] 1. Add biomass, liquefier, co-liquefier and acid catalyst into the reactor to react and prepare bio-based polyol. The reaction temperature is 130-180℃. Collect the supernatant, which is the bio-based polyol. 2. Measure the bio-based polyol, alkaline catalyst, other polyols and additives and stir them in a mixing tank to form a uniform coating liquid. The mixing temperature is 60-80℃. 3. Input the granular fertilizer into a coating machine with a copy plate and preheat the temperature to 50-70℃. 4. Measure the coating liquid and curing agent, mix them and spray them onto the rotating granular fertilizer for solvent-free in-situ reaction to obtain coated controlled-release fertilizer. 5. Input the coated controlled-release fertilizer into the cooling equipment and cool it to below 50℃. 6. Weigh and package the cooled coated controlled-release fertilizer. DETAILED DESCRIPTION

[0055] The present invention is further described below with reference to specific examples, but the present invention is not limited to the following examples. The methods described are conventional methods unless otherwise specified. The raw materials described can be obtained from public commercial channels unless otherwise specified.

[0056] The controlled-release performance of coated controlled-release fertilizers was tested using the water immersion method. 10g of coated fertilizer was placed in a 100-mesh nylon mesh bag. The bag was sealed and placed in a plastic container containing 250mL of distilled water. The bag was then placed in a sealed, incubator at 25°C. Nutrient release was measured at 1, 3, 7, 14, 21, 28, 35, 42, 49, 56, and 63 days. Nitrogen release was measured spectrophotometrically.

[0057] The nutrient controlled-release period refers to the number of days required for the cumulative release rate of coated controlled-release fertilizer to reach 80% in 25°C water.

[0058] Initial dissolution rate (%) = cumulative amount of nutrients dissolved on the first day / content of the nutrient in the sample × 100%.

[0059] Example 1

[0060] Preparation of starch-based polyol: 100g corn starch, 290g polyethylene glycol 400, 10g trisilanol isobutyl cage-type polysilsesquioxane POSS-OH (purchased from Hybrid Plastics, USA, brand SO1450), and 8g sulfuric acid were mixed uniformly in a reaction kettle equipped with a mechanical stirrer and a condenser. The mixture was reacted at 130°C for 30 minutes and then cooled in an ice-water bath to below approximately 100°C. This produced the starch-based polyol. After the polyol was cured into a film, the nanoparticles in the film were characterized using SEM-EDX. The results were as follows: Figure 2 As shown, the existence of POSS was confirmed and its distribution was uniform.

[0061] Preparation of coating liquid: Add 20g paraffin with a melting point of 60℃, 3.2g ethylenediamine and 400g polyether tetraol (purchased from Tianjin Petrochemical, brand TAE-470, hydroxyl value of 470) to 400g starch-based polyol and stir in a mixing tank at 70℃ to form a uniform coating liquid.

[0062] Preparation of coated controlled-release fertilizer: Weigh 1 kg of urea granules with a particle size of 2-4 mm and place them in a coating machine equipped with a lifter. Preheat to 65°C. Then, spray 4.17 g of coating solution and 4.17 g of polymethylene polyphenyl polyisocyanate onto the rotating fertilizer granules at 65°C for a solvent-free in-situ reaction. Hold the mixture for 3 minutes. Repeat this spraying process three times, resulting in a coating efficiency of 2.5%. After spraying, transfer the coated controlled-release fertilizer to a cooling unit and cool it to below 50°C. The cooled coated controlled-release fertilizer is weighed and packaged. The controlled-release performance of the fertilizer is tested using the water immersion method, revealing an initial dissolution rate of 0.1% and a controlled-release period of 90 days.

[0063] Example 2

[0064] Preparation of starch-based polyol: 100g corn starch, 100g polyethylene glycol 200, 8g trisilanol isobutyl cage polysilsesquioxane POSS-OH (purchased from Hybrid Plastics, USA, brand SO1450), and 2g aminosulfonic acid were mixed uniformly in a reactor equipped with a mechanical stirrer and a condenser, reacted at 150°C for 60min, and cooled in an ice-water bath to below about 100°C to obtain starch-based polyol.

[0065] Preparation of coating liquid: Add 20g paraffin with a melting point of 60°C, 1.6g ethylenediamine and 400g castor oil (purchased from Shanghai Aladdin, brand C110663, hydroxyl number 163) to 200g starch-based polyol and stir in a mixing tank at 60°C to form a uniform coating liquid.

[0066] Preparation of coated controlled-release fertilizer: Weigh 1 kg of urea granules with a particle size of 2-4 mm and place them in a coating machine equipped with a lift plate. Preheat to 60°C. Then, spray 6.25 g of coating solution and 4.25 g of polymethylene polyphenyl polyisocyanate onto the rotating fertilizer granules at 60°C for a solvent-free in-situ reaction. Hold the mixture for 3 minutes. Repeat this spraying process five times, achieving a coating efficiency of 5%. After spraying, transfer the coated controlled-release fertilizer to a cooling device and cool it to below 50°C. The cooled coated controlled-release fertilizer is weighed and packaged. The controlled-release performance of the fertilizer is tested using the water immersion method, revealing an initial dissolution rate of 0.08% and a controlled-release period of 180 days.

[0067] Example 3

[0068] Preparation of straw-based polyol: 100g corn straw, 400g polyethylene glycol 200, 6g trisilanol phenyl cage polysilsesquioxane POSS-OH (purchased from Hybrid Plastics, USA, brand SO1458), and 15g sulfuric acid were mixed in a reactor equipped with a mechanical stirrer and a condenser. The mixture was reacted at 180°C for 90 minutes and then cooled in an ice-water bath to below approximately 100°C. The supernatant was collected to obtain the straw-based polyol.

[0069] Preparation of coating liquid: Add 12g paraffin with a melting point of 60℃, 2.0g triethanolamine and 200g polyether tetraol (purchased from Tianjin Petrochemical, brand TAE-470, hydroxyl value of 470) to 400g straw-based polyol and stir in a mixing tank at 80℃ to form a uniform coating liquid.

[0070] Preparation of coated controlled-release fertilizer: Weigh 1 kg of urea granules with a particle size of 2-4 mm and place them in a coating machine equipped with a lifter. Preheat to 65°C. Mix 4.3 g of coating solution with 6.7 g of MDI and spray them onto the rotating fertilizer granules at 65°C for a solvent-free in-situ reaction. Hold the mixture for 3 minutes. Repeat this spraying process eight times, achieving a coating efficiency of 8%. After spraying, transfer the coated controlled-release fertilizer to a cooling device and cool it to below 50°C. The cooled coated controlled-release fertilizer is weighed and packaged. The controlled-release performance of the fertilizer is tested using the water immersion method, revealing an initial dissolution rate of 0.5% and a controlled-release period of 270 days.

[0071] Example 4

[0072] Preparation of starch-based polyol: 100g corn starch, 290g polyethylene glycol 600, 10g trisilanol phenyl cage polysilsesquioxane POSS-OH (purchased from Hybrid Plastics, USA, brand SO1458), and 8g aminosulfonic acid were mixed uniformly in a reactor equipped with a mechanical stirrer and a condenser, reacted at 150°C for 40min, and cooled in an ice-water bath to below about 100°C to obtain starch-based polyol.

[0073] Preparation of coating solution: Add 120g of microcrystalline wax, 2g of ethylenediamine and 1200g of polyether tetraol (purchased from Tianjin Petrochemical, brand TAE-470, hydroxyl value 470) to 400g of starch-based polyol and stir in a mixing tank at 70°C to form a uniform coating solution.

[0074] Preparation of coated controlled-release fertilizer: Weigh 1 kg of urea granules with a particle size of 2-4 mm and place them in a coating machine equipped with a lifter. Preheat to 65°C. Mix 5 g of coating solution with 5 g of polymethylene polyphenyl polyisocyanate and spray onto the rotating fertilizer granules at 65°C for a solvent-free in-situ reaction. Hold the mixture for 3 minutes. Repeat this spraying process twice, achieving a coating efficiency of 2%. After spraying, transfer the coated controlled-release fertilizer to a cooling device and cool it to below 50°C. The cooled coated controlled-release fertilizer is weighed and packaged. The controlled-release performance of the fertilizer is tested using the water immersion method, revealing an initial dissolution rate of 0.8% and a controlled-release period of 60 days.

[0075] Example 5

[0076] Preparation of straw-based polyol: 100g corn straw, 500g polyethylene glycol 400, 10g hydroxyapatite (purchased from Shanghai Aladdin, brand H106378), and 15g sulfuric acid were mixed in a reactor equipped with a mechanical stirrer and a condenser. The mixture was reacted at 150°C for 120 minutes, and then cooled in an ice-water bath to below approximately 100°C. The supernatant was collected as the straw-based polyol.

[0077] Preparation of coating solution: 15 g auxiliary agent chlorinated paraffin, 25 g ethylenediamine and 250 g castor oil (purchased from Shanghai Aladdin, brand C110663, hydroxyl number 163) were added to 500 g straw-based polyol and stirred in a mixing tank at 70 ° C to form a uniform coating solution.

[0078] Preparation of coated controlled-release fertilizer: Weigh 1 kg of urea granules with a particle size of 2-4 mm and place them in a coating machine equipped with a lifter. Preheat to 65°C. Then, spray 4.5 g of the coating solution mixed with 5 g of polymethylene polyphenyl polyisocyanate onto the rotating fertilizer granules at 65°C for a solvent-free in-situ reaction. Hold the mixture for 6 minutes. Repeat this spraying process four times, achieving a coating efficiency of 4%. After spraying, transfer the coated controlled-release fertilizer to a cooling unit and cool it to below 50°C. The cooled coated controlled-release fertilizer is weighed and packaged. The controlled-release performance of the fertilizer is tested using the water immersion method, revealing an initial dissolution rate of 0.1% and a controlled-release period of 120 days.

[0079] Comparative Example 1

[0080] The method is basically the same as Example 1, except that trisilanol isobutyl cage-type polysilsesquioxane POSS-OH is not added during the preparation of starch-based polyol. The controlled release performance is determined by water immersion method, and the initial dissolution rate is 3.2%, and the controlled release period is 30 days.

[0081] Comparative Example 2

[0082] The method is basically the same as Example 1, except that glycerol is used as the co-liquefying agent during the preparation of the starch-based polyol. The controlled release performance is determined by the water immersion method, and the initial dissolution rate is 1.8%, and the controlled release period is 45 days.

Claims

1. A coating solution, the raw materials of which include: Bio-based polyols, alkaline catalysts, other polyols and additives; The mass ratio of the bio-based polyol, alkaline catalyst, other polyols and additives is 100:0.5-5:50-300:3-30; The alkaline catalyst is selected from at least one of triethylenediamine, ethylenediamine, triethylenediamine, triethylamine, triethanolamine, dimethylcyclohexylamine, dimethylhexadecylamine, dimethylethanolamine and tetramethyliminodipropylamine; The other polyols are selected from at least one of polyether polyols, polyester polyols, and vegetable oil polyols; The auxiliary agent is selected from at least one of paraffin wax, microcrystalline wax, petroleum resin, α-olefin, polyethylene wax, EVA wax, asphalt and silicone wax; The method for preparing the bio-based polyol comprises the following steps: adding biomass, a liquefier, a co-liquefier, and an acidic catalyst into a reactor at a mass ratio of 100:100-500:1-10:2-15 for reaction, and cooling the mixture to below 100° C. in an ice-water bath after the reaction is completed to obtain the bio-based polyol; the bio-based polyol is an organic-inorganic nanocomposite; The biomass is selected from at least one of corn starch, potato starch, wheat starch, sweet potato starch, mung bean starch, cellulose, lignin, corn stalks, corn cobs, rice straw, wheat straw, peanut shells and cotton straw; The liquefier is polyethylene glycol; the relative molecular mass of the polyethylene glycol is 200 to 600; The co-liquefier is trisilanol cage-type polysilsesquioxane; The acidic catalyst is selected from at least one of an inorganic acid, an organic acid and a heteropoly acid.

2. The coating solution according to claim 1, characterized in that: The reaction conditions are: reaction at 130-180° C. for 30-120 min.

3. The method for preparing the coating solution according to claim 1 or 2, comprising the steps of: metering the bio-based polyol, alkaline catalyst, other polyols, and additives and stirring them in a mixing tank at 60-80°C to form a uniform coating solution.

4. A coated controlled-release fertilizer, consisting of a core and a membrane shell; characterized in that: The core is a fertilizer particle, and the membrane shell is formed by solvent-free in-situ reaction of the coating liquid according to claim 1 or 2 and a curing agent. The mass of the membrane shell is 2 to 8% of the mass of the coated controlled-release fertilizer.

5. The coated controlled-release fertilizer according to claim 4, characterized in that: The mass ratio of the coating liquid to the curing agent is 1:0.6-2; The fertilizer particles are water-soluble single-element fertilizers or compound fertilizers obtained by mixing at least two of the water-soluble single-element fertilizers; wherein the water-soluble single-element fertilizer is specifically selected from at least one of urea, ammonium sulfate, ammonium chloride, ammonium nitrate, monoammonium phosphate, diammonium phosphate, potassium chloride, potassium sulfate, potassium nitrate, magnesium sulfate, magnesium nitrate, zinc sulfate, copper sulfate and zinc chloride; The average particle size of the fertilizer particles is 2 to 6 mm; The curing agent is selected from at least one of polymethylene polyphenyl polyisocyanate, toluene diisocyanate, hexamethylene diisocyanate, diphenylmethane diisocyanate, isophorone diisocyanate, 1,6-hexamethylene diisocyanate, HDI trimer, trimethylhexamethylene diisocyanate, xylylene diisocyanate and dimethyldiphenyl diisocyanate.

6. The method for preparing the coated controlled-release fertilizer according to claim 4 or 5, comprising the following steps: 1) Preheat the fertilizer granules; 2) The coating liquid and the curing agent are mixed and then atomized onto the surface of the fertilizer particles obtained in step 1) to perform solvent-free in-situ reaction to form a film, thereby obtaining the coated controlled-release fertilizer.

7. The preparation method according to claim 6, characterized in that: In the step 1) of preheating, the preheating temperature is 50-70°C; In the step 2), the mass ratio of the coating liquid to the curing agent is 1:0.6-2; In the step 2), the reaction temperature of the solvent-free in-situ reaction is 50-70° C., and the reaction time is 3-6 minutes.

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