A fully biodegradable urea coating agent and a method for preparing the same

Abstract

The application discloses a kind of full biodegradable urea coating agent and preparation method thereof, the polyurethane system prepared using carbon dioxide-based polyol and bio-based material in the application, the coating agent obtained can be completely biodegradable, and the degradation rate is more than 93%, solve the white pollution caused by the non-degradable of traditional slow-release urea coating agent, it is an environment-friendly coating agent, and according to the different inner crosslinking structure in the structure of coating agent, three different release periods of slow-release urea fertilizer, 60 days, 90 days and 120 days can be prepared.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of slow-release fertilizer, and particularly relates to a fully biodegradable urea coating agent and a preparation method thereof. BACKGROUND

[0002] According to the results of the investigation of the United Nations Food and Agriculture Organization, the average yield increase effect of chemical fertilizer is as high as 40-60%, and nitrogen, phosphorus and potassium fertilizer is the three basic fertilizers, among which the use amount of nitrogen fertilizer is the largest, because the nitrogen fertility in the soil is not high, and nitrogen is not easy to adhere in the soil. Urea is an important high-nitrogen chemical fertilizer, also known as carbamide, and its nitrogen content is as high as 46%, which is a neutral fertilizer suitable for any soil. After fertilization, the amide bond in its structure must be converted into ammonium nitrogen under the action of microorganisms in the soil before it can be absorbed by the root system of the fabric, which needs 3-5 days. However, urea has very strong hygroscopicity, and once the soil humidity is high or it rains, it will soon enter the deep soil and be out of the absorption range of the fabric root system, causing loss. Moreover, during transportation and storage, nitrogen loss and environmental pollution will also occur due to water absorption. In order to solve this problem, many slow-release urea technologies have been developed in recent years, which improve the utilization rate of urea by coating a layer of coating agent on the surface of urea. The coating agent includes inorganic coating agent and organic high molecular coating agent, the inorganic coating agent includes phosphorite powder, sulfur, talc powder, etc., and the organic high molecular coating agent includes thermosetting and thermoplastic resin two categories of film-coated slow-release materials.

[0003] Patent 200410012251.8 reported ammonium sulfate coated urea granular fertilizer and preparation method, the product is in granular form, which is composed of core and coating, the core is urea, the coating agent is ammonium sulfate, the weight ratio of ammonium sulfate to urea is 1-4:9-6. After coated with ammonium sulfate, the defect of moisture absorption and decomposition loss of nitrogen during storage and transportation of urea can be effectively prevented. The particle size of urea core is 0.8-2.5 mm, and the thickness of ammonium sulfate coating is not more than 0.5 mm. The urea granules containing coating agent are obtained by spraying ammonium sulfate solution on the surface of urea, cooling and drying, which improves the nutrient utilization rate of urea fertilizer by 2-4%. However, ammonium sulfate as a coating agent belongs to inorganic coating agent, which has the disadvantage of no strength and no way to protect urea particles, and ammonium sulfate is water-soluble, so there is still a situation that it dissolves in water quickly, leading to the decline of urea fertilizer efficiency. Sulfur-coated urea is the first industrialized product, which sends preheated urea into a horizontally rotating cylindrical drum, heats sulfur to 156℃ to make it into a molten state, and then sprays it onto the surface of urea particles to make it solidify quickly on the surface of urea, obtaining sulfur-coated urea. However, this method needs high energy consumption, and the sulfur coating is easy to oxidize, break, and has poor wear resistance, which is easy to break and fall off during storage and transportation, and cracks and holes are generated on the surface. The coating process needs to spray a certain amount of paraffin wax for sealing, so that waxing, coating film and other processes are needed, which increases the cost. Moreover, a large amount of sulfur is decomposed by microorganisms in the soil, which can cause soil acidification and destroy soil structure. The general problem of inorganic coating agent is that the crystallization leads to the formation of large area cracks and holes, which accelerates the release of fertilizer nutrients. High molecular coating agent is widely used as coating material due to its difficulty in being decomposed by microorganisms and good film forming property, including polyethylene, polystyrene and polyester materials. Li et al. (Wuhan Univ Technol, 2012, 27: 126-129) used isocyanate and polyol to form a dense polyurethane coating layer with a thickness of 10-15 microns on the surface of urea particles, and added plasticizer paraffin wax during the reaction to increase the flowability of the reactants, so as to form an ultra-thin coating layer. Nutrient experiments show that the slow-release time of urea is 40-50 days. The advantages of high molecular coating agent are good adhesion of coating layer to urea particles and formation of wear-resistant coating, and the release rate of fertilizer nutrients is controlled by the composition and film thickness of the polymer. The problems of this technology are complex coating process, need for organic solvent, solvent pollution of the environment and non-degradation. The common problem of the above technical solutions is that the coating agent cannot be degraded in the soil, although the utilization rate of urea is improved, however, these coating agents will bring "white pollution" problem to the soil.

[0004] The invention patent 201611022848.X reports a degradable fertilizer slow-release coating, which is a double component of A and B. The A component is a solvent-free alkyd resin, and the relative molecular weight is 600-1200. The B component is a diisocyanate curing agent, which can prolong the release time of the fertilizer by about 40 times. The invention patent 200710013099.9 discloses a kind of biodegradable self-control slow-release fertilizer and its preparation method, which is a solution type slow-release material mainly composed of cellulose, epoxy resin and polyvinyl alcohol. However, the degradation performance of the above two coating materials is not good, especially the degradation performance of epoxy resin, polyvinyl alcohol and alkyd resin is very poor. SUMMARY

[0005] This section is intended to summarize some aspects of the embodiments of the present application and briefly introduce some preferred embodiments. Some simplifications or omissions may be made in this section and the abstract and title of the specification to avoid obscuring the purpose of this section, the abstract and the title, and such simplifications or omissions cannot be used to limit the scope of the present application.

[0006] In view of the above and / or problems existing in the prior art, the present application is proposed.

[0007] Therefore, the purpose of the present application is to overcome the deficiencies in the prior art and provide a preparation method of a fully biodegradable coating agent.

[0008] To solve the above technical problems, the present application provides the following technical scheme: a preparation method of a fully biodegradable coating agent, characterized in that it comprises,

[0009] Under the protection of nitrogen, the water-removed carbon dioxide-based diol, diisocyanate, catalyst, small molecule polyol and bio-based diol are mixed uniformly, heated for 1.5-2h, and then cooled to room temperature to obtain a coating agent resin;

[0010] The temperature of the coating resin is controlled at 30-45℃, then pigments, wetting agents, leveling agents, dispersants and waxes are added, stirred for 30-45min, and cooled to room temperature to obtain the fully biodegradable coating agent.

[0011] As a preferred scheme of the preparation method of the present application, the water content of the carbon dioxide-based diol is treated to be within 300ppm, the molecular weight is 500-3000, and the carbonate content is 15-50%.

[0012] As a preferred scheme of the preparation method of the present application, wherein: the diisocyanate is a mixture of one or more of isophorone diisocyanate, 1,6-hexane diisocyanate, 4,4'-dicyclohexylmethane diisocyanate, xylylene diisocyanate, 1,4-cyclohexane diisocyanate, tetramethyl-m-xylylene diisocyanate, trimethyl-1,6-hexamethylene diisocyanate, 1,4-cyclohexane dimethyl diisocyanate or norbornane diisocyanate.

[0013] As a preferred scheme of the preparation method of the present application, wherein: the small molecule polyol is a polyol with a hydroxyl functionality of not less than 3, preferably a mixture of one or more of glycerol, trimethylolpropane, pentaerythritol, dipentaerythritol; the bio-based diol is a mixture of one or more of isosorbide, castor oil, cellulose, lignin, rosin, palm oil, modified cashew nut shell oil polyol, rapeseed oil and soybean oil.

[0014] As a preferred scheme of the preparation method of the present application, wherein: the mass ratio of the carbon dioxide-based diol, diisocyanate, catalyst, small molecule polyol and bio-based diol is 2103.5-6250.2:336.4-998.5:0.75-3.9:40.5-164.3:59.3-726.5.

[0015] As a preferred scheme of the preparation method of the present application, wherein: the pigment is one of iron oxide red S130, iron oxide green 5605, iron oxide yellow 313 and iron oxide blue YHT-01; the wetting agent is one or more of BYK-331, BYK-333, BYK-378, BYK-310, BYK-337, BYK-320, BYK-306; the leveling agent is one or more of BYK-356, BYK-358N, BYK-355, BYK-340, BYK-322, BYK-392, BYK-323; the dispersing agent is one or more of BYK-9076, DISPERBYK-161, DISPERBYK-163, DISPERBYK-2000, DISPERBYK-2020, DISPERBYK-2025; the wax is one or more of No. 56 semi-refined paraffin wax, No. 58 semi-refined paraffin wax and No. 60 semi-refined paraffin wax.

[0016] As a preferred scheme of the preparation method of the present application, wherein: the mass ratio of the pigment, wetting agent, leveling agent, dispersing agent and wax is 35.1-226.4:10.5-56.2:16.2-40.1:18.4-75.8:20.1-135.8.

[0017] As a preferred scheme of the preparation method, the heating is performed for 1.5-2 hours, and the heating temperature is 85-95 DEG C.

[0018] Another object of the present application is to provide an application of the fully biodegradable coating agent in the preparation of the fully biodegradable slow-release urea.

[0019] As a preferred scheme of the application, the preparation method of the fully biodegradable slow-release urea comprises the following steps: heating urea particles with an average particle size of 3.0-5 mm to 80 DEG C for 15 minutes, then adding the coating agent, heating to 90-100 DEG C, adjusting the rotating speed to 50 r / min, adding 20 g of crosslinking agent after 15 minutes, continuing to stir for 20 minutes, reducing the temperature to room temperature, discharging, and obtaining the slow-release urea.

[0020] The present application has the following advantages:

[0021] The polyurethane system prepared from the carbon dioxide-based polyols and the bio-based materials is used in the present application, the obtained coating agent can be completely biodegraded, and the degradation rate is more than 93 %, the white pollution caused by the non-degradable traditional slow-release urea coating agent is solved, the coating agent is environment-friendly, and according to the different internal crosslinking structures in the coating agent structure, three kinds of slow-release urea fertilizers with different release periods (60 days, 90 days and 120 days) can be prepared. DETAILED DESCRIPTION

[0022] In order to make the above objects, features and advantages of the present application more apparent and comprehensible, the specific embodiments of the present application will be described in detail in combination with the description examples.

[0023] In the following description, a large number of specific details are set forth in order to facilitate a thorough understanding of the present application, but the present application can also be implemented in other manners different from those described herein, and those skilled in the art can make similar generalizations without departing from the connotation of the present application, therefore the present application is not limited to the specific embodiments disclosed below.

[0024] Secondly, the "one embodiment" or "embodiment" referred to herein means that the specific features, structures or characteristics can be included in at least one implementation of the present application. The "in one embodiment" appearing in different places in the specification does not mean the same embodiment, nor is the embodiment independent or alternative to other embodiments.

[0025] The raw materials used in the present application are as follows:

[0026] Isophorone diisocyanate, diphenylmethane-4,4'-diisocyanate, 4,4'-dicyclohexylmethane diisocyanate, 1,4-cyclohexane diisocyanate, glycerol, isosorbide, palm oil, pentaerythritol, rapeseed oil, dipentaerythritol and soybean oil were from Macklin Reagent Co. Ltd. BYK-310, BYK-331, BYK-337, BYK-340, BYK-355, BYK-356, BYK-9076, BYK-306, BYK-392, DISPERBYK-163, DISPERBYK-2000 and DISPERBYK-2025 were from BYK Co. Ltd. Polymethylene polyphenyl isocyanate PM200 was from Yantai Wanhua Group. Semi-refined paraffin wax No. 56, semi-refined paraffin wax No. 60 and semi-refined paraffin wax No. 58 were from Daqing Petrochemical Branch Co. Ltd. of China Petroleum and Natural Gas Corporation. Iron oxide red S130, iron oxide blue S463, iron oxide yellow S313 and iron oxide green S5605 were from Yipin Pigment. Urea was from Shandong Hualu Hengsheng Group Co. Ltd.

[0027] Example 1

[0028] This example is a preparation of a fully biodegradable coating agent, comprising the following steps:

[0029] (1) Under nitrogen protection, 6250.2 g of carbon dioxide-based diol (prepared in Example 8 of the invention patent 201210086834.X, molecular weight 2300 g / mol, carbonate content 41%), 998.5 g of isophorone diisocyanate, 3.5 g of dibutyltin dilaurate, 55.2 g of glycerol and 175.3 g of isosorbide were added to a three-necked flask, the temperature was controlled at 85°C, and the reaction was carried out for 2 hours. After cooling to room temperature, a coating agent resin was obtained.

[0030] (2) The obtained coating resin was temperature controlled at 30°C, then 83.5 g of iron oxide red S130, 16.8 g of BYK-331, 21.5 g of BYK-356, 30.4 g of BYK-9076 and 40.1 g of semi-refined paraffin wax No. 56 were added, stirred for 30 min, and cooled to room temperature to obtain a coating agent.

[0031] Example 2

[0032] This example is a preparation of a fully biodegradable coating agent, comprising the following steps:

[0033] (1) 3620.5 g of carbon dioxide-based diol (prepared in Example 7 of the invention patent 201210086834.X, molecular weight of 3000 g / mol, carbonate content of 40.6%), 336.4 g of 1,6-hexane diisocyanate, 1.8 g of stannous octoate, 40.5 g of trimethylolpropane, and 726.5 g of castor oil were added to a three-necked flask under nitrogen protection, and the temperature was controlled at 95°C, and the reaction was carried out for 1.5 hours. After cooling to room temperature, a coating agent resin was obtained.

[0034] (2) The obtained coating resin was controlled at a temperature of 45°C, and then 351 g of iron oxide green 5605, 56.2 g of BYK-333, 40.1 g of BYK-358N, 75.8 g of DISPERBYK-161, and 135.8 g of No. 58 semi-refined paraffin wax were added, and stirred for 45 min, and then cooled to room temperature to obtain a coating agent.

[0035] Example 3

[0036] This example is a preparation of a fully biodegradable coating agent, which comprises the following steps:

[0037] (1) 2485.2 g of carbon dioxide-based diol (prepared in Example 9 of the invention patent 201210086834.X, molecular weight of 2000 g / mol, carbonate content of 43.8%), 839.4 g of 4,4'-dicyclohexylmethane diisocyanate, 2.6 g of bismuth neodecanoate, 164.3 g of glycerol, and 324.1 g of palm oil were added to a three-necked flask under nitrogen protection, and the temperature was controlled at 90°C, and the reaction was carried out for 1 hour. After cooling to room temperature, a coating agent resin was obtained.

[0038] (2) The obtained coating resin was controlled at a temperature of 35°C, and then 81.5 g of iron oxide blue YHT-01, 26.5 g of BYK-310, 18.2 g of BYK-355, 30.2 g of DISPERBYK-163, and 48.1 g of No. 60 semi-refined paraffin wax were added, and stirred for 32 min, and then cooled to room temperature to obtain a coating agent.

[0039] Example 4

[0040] This example is a preparation of a fully biodegradable coating agent, which comprises the following steps:

[0041] (1) Under nitrogen protection, 2240.5 g of carbon dioxide-based diol (prepared in Example 12 of the invention patent 201210086834.X, molecular weight of 1600 g / mol, carbonate content of 63.2%), 398.5 g of 1,4-cyclohexane diisocyanate, 3.9 g of stannous octoate, 68.5 g of pentaerythritol, and 148.2 g of rapeseed oil were added into a three-necked flask, and the temperature was controlled at 88°C, and the reaction was carried out for 2 hours. After cooling to room temperature, a coating agent resin was obtained.

[0042] (2) The obtained coating resin was controlled at a temperature of 30-45°C, and then 40.8 g of iron oxide yellow 313, 10.5 g of BYK-337, 16.2 g of BYK-340, 18.4 g of DISPERBYK-2000, and 20.1 g of No. 58 semi-refined paraffin wax were added, and stirred for 45 min, and then cooled to room temperature to obtain a coating agent.

[0043] Example 5

[0044] This example is a preparation of a fully biodegradable coating agent, which comprises the following steps:

[0045] (1) Under nitrogen protection, 2103.5 g of carbon dioxide-based diol (prepared in Example 11 of the invention patent 201210086834.X, molecular weight of 1500 g / mol, carbonate content of 78.6%), 420.1 g of isophorone diisocyanate, 1.3 g of dibutyltin dilaurate, 50.8 g of dipentaerythritol, and 130.5 g of soybean oil were added into a three-necked flask, and the temperature was controlled at 92°C, and the reaction was carried out for 1.5 hours. After cooling to room temperature, a coating agent resin was obtained.

[0046] (2) The obtained coating resin was controlled at a temperature of 30-45°C, and then 130.5 g of iron oxide green 5605, 40.5 g of BYK-310, 20.6 g of BYK-355, 24.1 g of DISPERBYK-2020, and 48.6 g of No. 60 semi-refined paraffin wax were added, and stirred for 35 min, and then cooled to room temperature to obtain a coating agent.

[0047] Example 6

[0048] This example is a preparation of a fully biodegradable coating agent, which comprises the following steps:

[0049] (1) Under nitrogen protection, 2858.4 g of pre-water-removed carbon dioxide-based diol (prepared in Example 10 of the invention patent 201210086834.X, molecular weight is 2100 g / mol, and carbonate content is 50%), 446.3 g of 1,4-cyclohexane dimethyl diisocyanate, 0.75 g of bismuth neodecanoate, 101.3 g of glycerol, and 59.3 g of palm oil were added into a three-necked flask, and the temperature was controlled at 85°C, and the reaction was carried out for 1.5 hours. After cooling to room temperature, a coating agent resin was obtained.

[0050] (2) The obtained coating resin was controlled at a temperature of 40°C, and then 226.4 g of iron oxide red S130, 24.7 g of BYK-306, 18.5 g of BYK-392, 29.6 g of DISPERBYK-2025, and 85.3 g of No. 56 semi-refined paraffin wax were added, and stirred for 45 min, and then cooled to room temperature to obtain a coating agent.

[0051] Example 7

[0052] The difference between this example and Example 5 is that the dipentaerythritol in step (1) is replaced by glycerol, and the remaining steps are the same as those in Example 5.

[0053] Example 8

[0054] The difference between this example and Example 5 is that the isophorone diisocyanate in step (1) is replaced by diphenylmethane-4,4'-diisocyanate, and the remaining steps are the same as those in Example 5.

[0055] Comparative Example 1

[0056] The difference between this comparative example and Example 5 is that the carbon dioxide-based diol (Example 11, molecular weight is 1500 g / mol, and carbonate content is 78.6%) in step (1) is replaced by adipic acid butanediol (molecular weight is 1500 g / mol), and the remaining steps are the same as those in Example 5.

[0057] Comparative Example 2

[0058] The difference between this comparative example and Example 5 is that the carbon dioxide-based diol (Example 11, molecular weight is 1500 g / mol, and carbonate content is 78.6%) in step (1) is replaced by polypropylene glycol (molecular weight is 1500 g / mol), and the remaining steps are the same as those in Example 5.

[0059] Example 9

[0060] This example is a preparation of a fully biodegradable slow and controlled release urea, which comprises the following steps:

[0061] 1000 g urea particles with average particle size of 3.0-5.0 mm were added into a 3 L rotating drum, preheated to 80 °C for 15 min, then 15 g of the coating agent of Examples 1-8 and Comparative Examples 1-2 was added into the urea particles, while the fertilizer in the rotating drum was heated to 90-100 °C, the rotating speed was adjusted to 50 rpm, after 15 min, 10 g of polyisocyanate PAPI was added, and stirring was continued for 20 min, the temperature was reduced to room temperature, and the product was discharged, to obtain slow-release controlled urea, which was labeled as N1-N8, D1-D2, respectively.

[0062] Example 10

[0063] This example is a performance test of the fully biodegradable slow-release controlled urea prepared in Example 9, including the following steps:

[0064] (1) Initial nutrient release rate: the nitrogen release amount of the prepared urea slow-release fertilizer in 24 h of immersion in static water at 25 °C accounts for the mass fraction of the total nitrogen amount.

[0065] (2) Cumulative nutrient release rate: the cumulative nitrogen release amount of the urea slow-release fertilizer in static water at 25 °C within a certain period accounts for the mass fraction of the total nitrogen amount

[0066] (3) Nutrient release period: the time required for the urea slow-release fertilizer in static water at 25 °C to reach 80% of the cumulative nutrient release rate from the start of immersion.

[0067] (4) Biodegradation test: according to GB / T 19277.1-2011, the biodegradation performance of the coating agent is tested to study its ultimate aerobic biodegradation and disintegration ability under controlled composting conditions. In a 2 L test system, the coating agent is used as an organic carbon source, and air without carbon dioxide is used to aerate the test mixture at a controlled rate. The degradation rate is determined by measuring the amount of carbon dioxide produced. 240 g of culture soil was mixed with 40 g of the coating agent obtained by the present application and 40 g of microcrystalline cellulose, and 240 g of culture soil was used as a blank control. Distilled water was added to adjust the humidity of the mixture to about 50%. The composting container was placed in a test environment at (58±2) °C, and humidified air without CO2 was used to aerate the test system at a flow rate of 0.05 L / min. The test was carried out at (58±2) °C. The biodegradation rate of the test material was calculated based on the ratio of the actual amount of carbon dioxide produced by the test material during the test to the theoretical release amount of carbon dioxide of the test material.

[0068] The test results of N1-N8 and D1-D2 are shown in Table 1.

[0069] Table 1 Performance test results of N1-N8 and D1-D2

[0070]

[0071] From the results of the table, it can be seen that the cumulative nutrient release of the coated urea of Example 1, Example 2, Example 3 and Example 6 reached 100% at 90 days, and the release rate at 60 days was 72%, 68%, 74% and 75% respectively, all within 80%, indicating that the nutrient release period of the coated urea of the three examples was 60 days; the cumulative nutrient release of the coated slow-release urea of Example 4 reached 100% at 120 days, and the release rate at 90 days was 73%, within 80%, indicating that the release period of the coated urea of Example 4 was 90 days; the nutrient release of the slow-release urea of Example 5 was 76% at 120 days, lower than 80%, so the release period of the coated urea of this example was 120 days. The reason why the slow-release urea prepared by different coated urea has different release periods is that the small molecule polyols in the coated urea are different. The coated urea structure of 90 days release period uses trimethylolpropane with three functionalities and pentaerythritol with four functionalities, high crosslinking network structure, which improves the crosslinking density of the coated film, thereby improving the release period of the urea; 120 days uses double pentaerythritol with six functionalities, which forms a higher crosslinking density on the surface of the urea, thereby achieving a longer release period of 120 days. Compared with Example 5, Example 7 uses glycerol with 3 functionalities instead of double pentaerythritol with six functionalities, which reduces the crosslinking density of the coated urea, thereby reducing the compactness of the coated urea, resulting in faster nutrient release, thereby achieving a release of 90 days, which cannot meet the release of 120 days; compared with Example 5, Example 8 uses aromatic diisocyanate diphenylmethane-4,4'-diisocyanate to replace isophorone diisocyanate, which strengthens the rigidity of the coated film due to the addition of benzene ring, thereby improving the slow-release effect, so it can still achieve a release of 120 days, and the nutrient release rates at 120 days and 90 days are lower, however, due to the introduction of benzene ring in the coated resin, the biodegradability is reduced. Compared with Example 5, the initial release rate of Comparative Example 1 exceeds the standard requirement of 15%, as high as 18%, and the release rate at 60 days also exceeds the standard requirement of 85%, which is caused by the poor water resistance of polybutylene adipate, which is an aliphatic polyester, so it has certain biodegradability. Compared with Example 5, the initial nutrient release of Comparative Example 2 reached the standard requirement, and the release rates at 60 days and 90 days also reached the standard requirement, which is because polypropylene glycol has certain water resistance and hydrolysis resistance, so it has good slow-release effect, however, polypropylene glycol is a polyether, which has poor biodegradability.

[0072] It should be noted that the above examples are only used to illustrate the technical solutions of the present application but not limit the present application. Although the present application is described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present application can be modified or equivalently replaced, without departing from the spirit and scope of the present application, which should be covered in the scope of the present application.

Claims

1. A method for preparing a fully biodegradable coating agent, characterized in that: include, Under nitrogen protection, pre-dehydrated carbon dioxide-based diol, diisocyanate, catalyst, small molecule polyol and bio-based diol are mixed evenly, heated for 1.5 to 2 hours, and then cooled to room temperature to obtain coating resin. The coating resin temperature is controlled at 30-45℃, then pigments, wetting agents, leveling agents, dispersants and waxes are added, stirred for 30-45 minutes, and cooled to room temperature to obtain the fully biodegradable coating agent. The diisocyanate is one or more of isophorone diisocyanate and 1,4-cyclohexane diisocyanate; The small molecule polyol is one or more of pentaerythritol and dipentaerythritol; The catalyst is one or more of stannous octoate and dibutyltin dilaurate; The mass ratio of the carbon dioxide-based diol, diisocyanate, catalyst, small molecule polyol, and bio-based diol is 2103.5–6250.2: 336.4–998.5: 0.75–3.9: 40.5–164.3: 59.3–726.

5.

2. The preparation method according to claim 1, characterized in that: The water content of the carbon dioxide-based diol is treated to be below 300 ppm, the molecular weight is 500-3000, and the carbonate content is 15-50%.

3. The preparation method according to claim 1, characterized in that: The bio-based diol is one or more of isosorbide, castor oil, cellulose, lignin, rosin, palm oil, modified cashew nut shell oil polyol, rapeseed oil, and soybean oil.

4. The preparation method according to claim 1, characterized in that: The pigment is one of iron oxide red S130, iron oxide green 5605, iron oxide yellow 313, and iron oxide blue YHT-01; the wetting agent is one or more of BYK-331, BYK-333, BYK-378, BYK-310, BYK-337, BYK-320, and BYK-306; the leveling agent is BYK-356, BYK-358N, BYK-355, BYK-340, and BYK-3 22. One or more of BYK-392 and BYK-323; the dispersant is one or more of BYK-9076, DISPERBYK-161, DISPERBYK-163, DISPERBYK-2000, DISPERBYK-2020, and DISPERBYK-2025; the paraffin is one or more of No. 56 semi-refined paraffin, No. 58 semi-refined paraffin, and No. 60 semi-refined paraffin.

5. The preparation method according to claim 4, characterized in that: The mass ratio of the pigment, wetting agent, leveling agent, dispersant, and wax is 35.1–226.4: 10.5–56.2: 16.2–40.1: 18.4–75.8: 20.1–135.

8.

6. The preparation method according to claim 1, characterized in that: The heating process lasts for 1.5 to 2 hours, with the heating temperature being 85 to 95°C.

7. The application of a fully biodegradable coating agent in the preparation of fully biodegradable sustained-release urea, characterized in that: The fully biodegradable coating agent is prepared by the method described in any one of claims 1 to 6.

8. The application as described in claim 7, characterized in that: The preparation method of the fully biodegradable controlled-release urea includes heating urea particles with an average particle size of 3.0-5 mm to 80°C for 15 min, then adding a coating agent, heating to 90-100°C, adjusting the speed to 50 r / min, and after 15 min, adding 20 g of crosslinking agent, continuing to stir for 20 min, lowering the temperature to room temperature, and discharging to obtain controlled-release urea.

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