Cellulose / urea-formaldehyde water-retaining slow-release fertilizer granules prepared by means of polycondensation-grafting-crosslinking integrated reaction extrusion process and application method thereof

Abstract

Cellulose / urea-formaldehyde water-retaining slow-release fertilizer granules prepared by means of a polycondensation-grafting-crosslinking integrated reaction extrusion process and an application method thereof. In a double-screw reaction extruder, low-polymerization-degree urea-formaldehyde undergoes a polycondensation reaction to generate urea-formaldehyde having a relatively high molecular weight, and under the strong shearing action of the extruder, cellulose having a large number of exposed active hydroxyl groups undergoes a nucleophilic substitution or condensation reaction, so that urea-formaldehyde macromolecules generated in situ are grafted onto macromolecular chains thereof; and -COOH of citric acid and -OH of cellulose simultaneously undergo a cross-linking reaction. By means of the polycondensation-grafting-crosslinking three-in-one reaction extrusion process, a cellulose / urea-formaldehyde compound is generated, thereby obtaining the cellulose / urea-formaldehyde water-retaining slow-release fertilizer granules in one step. By modifying urea-formaldehyde with hydroxyethyl cellulose, the strong intermolecular and intramolecular hydrogen bonding can enhance the mechanical properties of the urea-formaldehyde fertilizer, and the long molecular chains and abundant hydrophilic groups can also significantly improve the water absorption and water retention performance of the fertilizer.

Description

Preparation of cellulose / urea-formaldehyde water-retaining slow-release fertilizer granules by polycondensation-grafting-crosslinking reaction extrusion process and application method thereof

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to Chinese patent application number 202410127570.0, filed with the Patent Office of China on January 30, 2024, entitled "Preparation of Cellulose / Urea-Formaldehyde Water-Retaining Slow-Release Fertilizer Granules by Condensation-Grafting-Crosslinking Integrated Reaction Extrusion Process and Its Application Method", the entire contents of which are incorporated herein by reference. Technical Field

[0003] The present invention relates to the field of urea-formaldehyde slow / controlled-release fertilizers, and in particular to a polycondensation-grafting-crosslinking integrated reaction extrusion process for preparing cellulose / urea-formaldehyde water-retaining slow-release fertilizer particles and an application method thereof. Background Art

[0004] The use of water-retaining fertilizers can improve the soil's ability to absorb and retain water, releasing the absorbed water when water is insufficient to ensure plant growth. However, currently, most water-retaining fertilizers are synthesized using difficult-to-degrade or toxic monomers such as glyoxal, acrylamide, and polyvinyl alcohol. Furthermore, complex production processes, tedious procedures, and severe pollution have limited their widespread use.

[0005] Cellulose is the oldest and most abundant natural polymer on Earth, an inexhaustible and most valuable natural renewable resource. Cellulose ethers are a diverse, highly produced, research-worthy, and widely applicable cellulose derivative. Hydroxyethyl cellulose (HEC) is a non-ionic cellulose ether rich in hydroxyl groups on its macromolecular chain, making it easier to modify than natural cellulose. It is currently widely used in the petroleum, textile, coatings, construction, food, and pharmaceutical industries.

[0006] Cellulose is formed by the condensation of β-glucose and boasts a high degree of polymerization, good molecular orientation, and strong hydrogen bonding. Furthermore, the hydrophobic pyranyl ring and the hydrophilic exocyclic hydroxyl groups give cellulose high stability. Cellulose is inherently insoluble in common solvents and requires a strong protic, polar, alkaline, urea, or chemical modification to dissolve. Therefore, industrial processing of cellulose involves first converting it into cellulose derivatives before further utilization. For example, regenerated cellulose fiber utilizes a highly polar environment to dissolve and then condense cellulose, resulting in highly crystalline cellulose that can be processed. Nitrocellulose is modified with nitric acid and then nitrates are removed from the cellulose derivative using NH₄HSO₄, resulting in celluloid. Currently, the industrial production of HEC primarily utilizes the properties of the hydroxyl groups on the glucosyl ring. Using an alkaline urea environment, cellulose undergoes multiple reactions, first yielding alkali cellulose, which is then reacted with ethylene oxide, vinyl chloride, and other agents to produce hydroxyethyl cellulose. The introduction of more hydroxyl groups enhances the hydrophilicity of the resulting HEC. Furthermore, due to cellulose's inherent hydrophilicity, HEC possesses greater water absorption and retention capabilities than materials derived from other monomers.

[0007] Reactive extrusion has become a novel polymer material molding technology, combining the polymerization and processing processes into one, allowing chemical reactions and continuous production to occur simultaneously within the processing machinery. Reactive extrusion utilizes an extruder as the reaction vessel and a plasticizing and extrusion system consisting of a screw and barrel as the continuous reactor. Pre-reacted raw materials, such as monomers, initiators, polymers, and additives, are added in batches or in succession through the same or different feed ports. The screw's rotation achieves mixing, conveying, plasticizing, reaction, and extrusion through the die. This process offers promising advantages, including continuous large-scale production, low investment and cost, minimal or no use of solvents harmful to humans and the environment, a wide range of product and raw material options, simplified polymer de-volatiles, granulation, and molding processes, high reaction efficiency, and uniform product performance.

[0008] Summary of the Invention

[0009] Currently, most water-retaining and slow-release fertilizers are synthesized using difficult-to-degrade or toxic monomers as raw materials. In addition, there are problems such as complex production processes, cumbersome steps, and serious pollution. The present invention provides a condensation-grafting-crosslinking integrated reaction extrusion process for preparing cellulose / urea-formaldehyde water-retaining and slow-release fertilizer particles and an application method thereof.

[0010] The present invention is achieved through the following technical solution: a polycondensation-grafting-crosslinking integrated reaction extrusion process for preparing cellulose / urea-formaldehyde water-retaining slow-release fertilizer particles, comprising the following steps:

[0011] (1) adding a calculated amount of urea, paraformaldehyde, and water into a reactor, adjusting the pH of the system, and reacting at a certain temperature for a certain time to obtain a hydroxymethylurea solution;

[0012] (2) adding a calculated amount of citric acid and water to the hydroxymethyl urea solution obtained in step (1), and prepolymerizing at a certain temperature for a certain time to obtain a mixed aqueous solution of urea-formaldehyde with a low degree of polymerization;

[0013] (3) sealing the die between the reaction unit and the extrusion unit of the reaction extrusion integrated machine; adding a certain amount of cellulose and water to the mixed aqueous solution obtained in step (2), mixing them evenly, and then injecting them into the twin-screw reaction extruder of the reaction unit of the reaction extrusion integrated machine;

[0014] (4) The screws of the twin-screw reaction extruder of the reaction unit of the reaction extruder are turned on, and the vacuum devolatilization device is started at the same time to remove moisture from the reaction system. The reaction is carried out at a set temperature and a set screw speed for a certain time, and then the low-polymerization degree urea-formaldehyde is converted into relatively high molecular weight urea-formaldehyde through polycondensation. At the same time, the cellulose with a large number of active hydroxyl groups exposed under the strong shearing action of the extruder undergoes nucleophilic substitution reaction or dehydration condensation reaction with the hydroxymethyl or amide groups contained in the in-situ generated urea-formaldehyde macromolecular chain, so that the urea-formaldehyde macromolecule is grafted onto the cellulose macromolecular chain. In addition, the -COOH of citric acid and the -OH of cellulose also undergo cross-linking reaction at the same time. The three-in-one reaction extrusion process of polycondensation-grafting-cross-linking generates a viscous cellulose / urea-formaldehyde compound.

[0015] (5) opening the die between the reaction unit and the extrusion unit of the reaction extruder, starting the twin-screw extruder of the extrusion unit of the reaction extruder, and then the twin-screw reaction extruder conveys the viscous cellulose / urea-formaldehyde compound obtained in step (4) to the twin-screw extruder, and the twin-screw extruder extrude the cellulose / urea-formaldehyde compound at a set temperature and a set speed to obtain a strip-shaped cellulose / urea-formaldehyde compound;

[0016] (6) The strip-shaped cellulose / urea-formaldehyde compound obtained in step (5) is dried at a set temperature and then pelletized to obtain cellulose / urea-formaldehyde water-retaining slow-release fertilizer particles with good particle shape.

[0017] As a further improvement of the technical solution of the present invention, in step (1), the mass ratio of urea to paraformaldehyde is 2-3.5:1, and the amount of water added is 5-20% of the total mass of urea and paraformaldehyde; the pH of the system is adjusted to 8-12, and the reaction is carried out at 50-90°C for 0.5-4h.

[0018] As a further improvement of the technical solution of the present invention, in step (2), the amount of water added is 50-600% of the total mass of urea and paraformaldehyde; the amount of citric acid added is 5-25% of the total mass of urea and paraformaldehyde; the reaction temperature is 50-80°C, and the reaction time is 5-40 minutes; the degree of polymerization of the obtained low-polymerization urea-formaldehyde is ≤4.

[0019] As a further improvement of the technical solution of the present invention, in step (3), the cellulose added is hydroxyethyl cellulose, and the added amount is 5-60% of the total mass of urea and paraformaldehyde; the added amount of water is 300-5000% of the mass of the hydroxyethyl cellulose.

[0020] As a further improvement of the technical solution of the present invention, in step (4), the extrusion temperature of the twin-screw reaction extruder of the reaction unit of the reaction extruder integrated machine is 50-90°C, the screw speed is 50-300rpm, and the reaction time is 10-30min.

[0021] As a further improvement of the technical solution of the present invention, in step (4), the viscous cellulose / urea-formaldehyde compound has the following molecular structure:

[0022] Where: n = 10~100.

[0023] As a further improvement of the technical solution of the present invention, in step (5), the extrusion temperature of the twin-screw extruder of the extrusion unit of the reaction extruder is 30-80° C., and the screw speed is 50-150 rpm.

[0024] As a further improvement of the technical solution of the present invention, in step (6), the drying temperature is 50-80°C.

[0025] The present invention further provides the use of the cellulose / urea-formaldehyde water-retaining slow-release fertilizer particles prepared by the above-mentioned polycondensation-grafting-crosslinking integrated reaction extrusion process as a water-retaining slow-release fertilizer.

[0026] As a further improvement of the application technical solution of the present invention, the cellulose / urea-formaldehyde compound is applied at the same time as sowing using a seed and fertilizer sowing machine, and the application amount of the cellulose / urea-formaldehyde water-retaining slow-release fertilizer particles is 8 to 15 kg nitrogen per mu.

[0027] Compared with the prior art, the present invention has the following advantages:

[0028] 1) This invention utilizes urea-formaldehyde, a non-toxic, widely distributed and abundant natural material, to modify it. This effectively addresses the problem that current water-retaining, slow-release fertilizers are often synthesized using difficult-to-degrade or toxic monomers. Furthermore, it enhances the strength and slow-release properties of urea-formaldehyde fertilizer granules, significantly increasing the soil's water absorption and retention capacity, thus expanding the application of fertilizer preparation in a wider range of environments.

[0029] 2) In natural environments, hydroxyethyl cellulose degrades faster than unmodified cellulose, meeting plant nitrogen needs while also improving soil fertility by increasing soil carbon content. Furthermore, the degradation process does not produce any soil-harmful substances, making it environmentally friendly.

[0030] 3) The process of the present invention can simultaneously carry out the three-in-one reaction of polycondensation, grafting and cross-linking in a twin-screw reaction extruder, effectively solving the problems of complex production process, cumbersome steps and serious pollution in the current water-retaining fertilizer.

[0031] 4) Compared with traditional water-retaining fertilizers, the present invention introduces hydroxyethyl cellulose, which has a wide source and a simple production process. Its pyran ring has rich groups. At the same time, the strong hydrogen bonding between molecules and within molecules can also improve the mechanical properties of urea-formaldehyde fertilizers. The long molecular chain and rich hydrophilic groups can also greatly improve the water absorption and water retention properties of the fertilizer.

[0032] 5) The present invention introduces HEC and uniformly disperses it in a low-polymerization-degree urea-formaldehyde aqueous solution. During the reaction extrusion process, HEC can hinder the formation of high-molecular-weight urea-formaldehyde components and narrow the molecular weight distribution of the generated urea-formaldehyde. Therefore, it can reduce the initial release rate of nitrogen nutrients from the urea-formaldehyde fertilizer while increasing the release rate of nitrogen nutrients in the middle and later stages. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments consistent with the invention and, together with the description, serve to explain the principles of the invention.

[0034] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0035] FIG1 is an FT-IR spectrum of hydroxyethyl cellulose (HEC), the hydroxyethyl cellulose / urea-formaldehyde water-retaining slow-release fertilizer prepared in Examples 1 and 2, the urea-formaldehyde compound (UF) prepared in Comparative Example 1, and urea.

[0036] Figure 2 shows the UF at 1625cm2 of the hydroxyethyl cellulose / urea formaldehyde water-retaining slow-release fertilizer prepared in Example 1 and the comparative example 1.-1 and 1552cm -1 1 is the enlarged FT-IR spectrum of FIG.

[0037] Figure 3 shows the hydroxyethyl cellulose / urea formaldehyde water-retaining slow-release fertilizer prepared in Example 1 and the UF and HEC prepared in Comparative Example 1 at 1034 cm -1 and 1134cm -1 1 is the enlarged FT-IR spectrum of FIG.

[0038] FIG4 is an XRD spectrum of the hydroxyethyl cellulose / urea-formaldehyde water-retaining slow-release fertilizer prepared in Examples 1 and 2, and the UF and HEC prepared in Comparative Example 1.

[0039] FIG5 is a thermogravimetric curve of the hydroxyethyl cellulose / urea-formaldehyde water-retaining slow-release fertilizer prepared in Example 1 and Example 2, and the UF and HEC prepared in Comparative Example 1.

[0040] FIG6 is a differential curve of the thermogravimetric curves of the hydroxyethyl cellulose / urea-formaldehyde water-retaining slow-release fertilizer prepared in Examples 1 and 2, the UF prepared in Comparative Example 1, and the HEC.

[0041] FIG7 is a compression strength test curve of the hydroxyethyl cellulose / urea formaldehyde water-retaining slow-release fertilizer granules prepared in Example 1 and the UF granules prepared in Comparative Example 1.

[0042] FIG8 is a static water nutrient release curve of the hydroxyethyl cellulose / urea formaldehyde water-retaining slow-release fertilizer granules prepared in Example 1 and the UF granules prepared in Comparative Example 1.

[0043] FIG9 is a test curve of soil water retention rate of the hydroxyethyl cellulose / urea formaldehyde water-retaining slow-release fertilizer granules prepared in Example 1 and Example 2 and the UF granules prepared in Comparative Example 1. DETAILED DESCRIPTION

[0044] In order to more clearly understand the above-mentioned objectives, features and advantages of the present invention, the scheme of the present invention will be further described below. It should be noted that, in the absence of conflict, the embodiments of the present invention and the features therein can be combined with each other.

[0045] In the following description, many specific details are set forth to facilitate a full understanding of the present invention, but the present invention may also be implemented in other ways different from those described herein; it is obvious that the embodiments in the specification are only part of the embodiments of the present invention, rather than all the embodiments.

[0046] The present invention provides a specific embodiment of preparing cellulose / urea-formaldehyde water-retaining slow-release fertilizer particles by a polycondensation-grafting-crosslinking integrated reaction extrusion process, comprising the following steps:

[0047] (1) adding a calculated amount of urea, paraformaldehyde, and water into a reactor, adjusting the pH of the system, and reacting at a certain temperature for a certain time to obtain a hydroxymethylurea solution;

[0048] (2) adding a calculated amount of citric acid and water to the hydroxymethyl urea solution obtained in step (1), and prepolymerizing at a certain temperature for a certain time to obtain a mixed aqueous solution of urea-formaldehyde with a low degree of polymerization;

[0049] (3) sealing the die between the reaction unit and the extrusion unit of the reaction extrusion integrated machine; adding a certain amount of cellulose and water to the mixed aqueous solution obtained in step (2), mixing them evenly, and then injecting them into the twin-screw reaction extruder of the reaction unit of the reaction extrusion integrated machine;

[0050] (4) The screws of the twin-screw reaction extruder of the reaction unit of the reaction extruder are turned on, and the vacuum devolatilization device is started at the same time to remove moisture from the reaction system. The reaction is carried out at a set temperature and a set screw speed for a certain time, and then the low-polymerization degree urea-formaldehyde is converted into relatively high molecular weight urea-formaldehyde through polycondensation. At the same time, the cellulose with a large number of active hydroxyl groups exposed under the strong shearing action of the extruder undergoes nucleophilic substitution reaction or dehydration condensation reaction with the hydroxymethyl or amide groups contained in the in-situ generated urea-formaldehyde macromolecular chain, so that the urea-formaldehyde macromolecule is grafted onto the cellulose macromolecular chain. In addition, the -COOH of citric acid and the -OH of cellulose also undergo cross-linking reaction at the same time. The three-in-one reaction extrusion process of polycondensation-grafting-cross-linking generates a viscous cellulose / urea-formaldehyde compound.

[0051] (5) opening the die between the reaction unit and the extrusion unit of the reaction extruder, starting the twin-screw extruder of the extrusion unit of the reaction extruder, and then the twin-screw reaction extruder conveys the viscous cellulose / urea-formaldehyde compound obtained in step (4) to the twin-screw extruder, and the twin-screw extruder extrude the cellulose / urea-formaldehyde compound at a set temperature and a set speed to obtain a strip-shaped cellulose / urea-formaldehyde compound;

[0052] (6) The strip-shaped cellulose / urea-formaldehyde compound obtained in step (5) is dried at a set temperature and then pelletized to obtain cellulose / urea-formaldehyde water-retaining slow-release fertilizer particles with good particle shape.

[0053] The main mechanism of the polycondensation-grafting-crosslinking reaction extrusion process of the present invention for preparing cellulose / urea-formaldehyde compounds is as follows:

[0054] In step (1):

[0055] In an alkaline environment, paraformaldehyde depolymerizes into small formaldehyde molecules, which then react with urea in the system to generate monomethylol urea and dimethylol urea.

[0056] In step (2):

[0057] Among them, m≦4.

[0058] In the acidic environment created by the added citric acid and water, the monomethylol urea, dimethylol urea and urea obtained in step (1) undergo a pre-condensation reaction to obtain a mixed aqueous solution of urea-formaldehyde with a low degree of polymerization.

[0059] In step (4):

[0060] Where: n = 10~100.

[0061] In the twin-screw extruder, four reactions occur simultaneously: 1. Low-polymerization urea-formaldehyde undergoes polycondensation to produce relatively high-molecular-weight urea-formaldehyde; 2. Cellulose, exposed to a large number of active hydroxyl groups under the strong shearing action of the extruder, undergoes nucleophilic substitution or dehydration condensation with the hydroxymethyl or amide groups on the in-situ generated urea-formaldehyde macromolecular chains, grafting the urea-formaldehyde macromolecular chains onto the cellulose macromolecular chains; and 3. The -COOH groups of citric acid and the -OH groups of cellulose undergo cross-linking in an acidic environment. Thus, a three-in-one polycondensation-grafting-cross-linking reaction extrusion process produces a cellulose / urea-formaldehyde compound.

[0062] Those skilled in the art can also prepare hydroxyethyl cellulose / urea-formaldehyde compounds with various nitrogen contents by controlling the addition amounts of the reaction raw materials urea, paraformaldehyde, and hydroxyethyl cellulose according to different needs.

[0063] The reaction extruder of the present invention includes a reaction unit and an extrusion unit. The twin-screw reaction extruder of the reaction unit and the twin-screw extruder of the extrusion unit are both well-known devices in the art, wherein the discharge die of the twin-screw reaction extruder is connected to the feed die of the twin-screw extruder; in addition, the vacuum devolatilization device is a component on the twin-screw reaction extruder, and the connection method between it and the twin-screw reaction extruder is well known in the art. The screw diameter of the twin-screw reaction extruder of the reaction extruder used in each embodiment of the present invention is 28 mm, and the aspect ratio is 34: 1; the screw diameter of the twin-screw extruder of the extrusion unit is 21 mm, and the aspect ratio is 36: 1.

[0064] In one embodiment provided by the present invention, in step (1), the mass ratio of urea to paraformaldehyde is 2-3.5:1, and the amount of water added is 5-20% of the total mass of urea and paraformaldehyde; the pH of the system is adjusted to 8-12, and the reaction is carried out at 50-90°C for 0.5-4h.

[0065] In another embodiment provided by the present invention, in step (2), the amount of water added is 50-600% of the total mass of urea and paraformaldehyde; the amount of citric acid added is 5-25% of the total mass of urea and paraformaldehyde; the reaction temperature is 50-80°C, and the reaction time is 5-40 min; and the degree of polymerization of the obtained low-polymerization urea-formaldehyde is ≤4.

[0066] In one embodiment provided by the present invention, in step (3), the cellulose added is hydroxyethyl cellulose, and the added amount is 5-60% of the total mass of urea and paraformaldehyde; the added amount of water is 300-5000% of the mass of the hydroxyethyl cellulose.

[0067] In another embodiment provided by the present invention, in step (4), the extrusion temperature of the twin-screw reaction extruder of the reaction unit of the reaction extruder integrated machine is 50-90° C., the screw speed is 50-300 rpm, and the reaction time is 10-30 min.

[0068] In one embodiment provided by the present invention, in step (4), the viscous cellulose / urea-formaldehyde compound has the following molecular structure:

[0069] Where: n = 10~100.

[0070] In another embodiment provided by the present invention, in step (5), the extrusion temperature of the twin-screw extruder of the extrusion unit of the reaction extruder is 30-80° C., and the screw speed is 50-150 rpm.

[0071] In one embodiment provided by the present invention, in step (6), the drying temperature is 50-80°C.

[0072] The present invention further provides the use of the cellulose / urea-formaldehyde water-retaining slow-release fertilizer particles prepared by the above-mentioned polycondensation-grafting-crosslinking integrated reaction extrusion process as a water-retaining slow-release fertilizer.

[0073] In one embodiment provided by the present invention, the cellulose / urea-formaldehyde compound is applied at the same time as sowing using a seed-fertilizer-seeding machine, and the application amount of the cellulose / urea-formaldehyde water-retaining slow-release fertilizer granules is 8 to 15 kg nitrogen per mu.

[0074] The specific embodiments of the present invention are described in detail below.

[0075] Example 1

[0076] A polycondensation-grafting-crosslinking integrated reaction extrusion process for preparing hydroxyethyl cellulose / urea-formaldehyde water-retaining slow-release fertilizer granules comprises the following steps:

[0077] (1) Adding calculated amounts of urea (U), paraformaldehyde (Pf), and water (H2O) to a reactor, wherein the mass ratio of urea to paraformaldehyde is 2.5:1 and the amount of water added is 15% of the total mass of urea and paraformaldehyde, adjusting the pH of the system to 8, and reacting at 90°C for 0.5h to obtain a hydroxymethylurea (MU) solution. (2) Adding calculated amounts of citric acid and water to the hydroxymethylurea solution obtained in step (1), wherein the amount of citric acid added is 10% of the total mass of urea and paraformaldehyde; the amount of water added is 200% of the total mass of urea and paraformaldehyde; and reacting at 60°C for 10min to obtain a urea-formaldehyde mixed aqueous solution with a degree of polymerization ≤4. (3) sealing the die between the reaction unit and the extrusion unit of the reaction extrusion integrated machine; adding a certain amount of hydroxyethyl cellulose and water to the mixed aqueous solution obtained in step (2), mixing them evenly and then injecting them into the twin-screw reaction extruder of the reaction unit of the reaction extrusion integrated machine; wherein the amount of hydroxyethyl cellulose added is 30% of the total mass of urea and paraformaldehyde, and the amount of water added is 800% of the mass of the hydroxyethyl cellulose.

[0078] (4) The screws of the twin-screw reaction extruder of the reaction unit of the reaction extruder are turned on, and the vacuum devolatilization device is started at the same time to remove moisture from the reaction system. When the temperature is set to 60°C and the screw speed is set to 70 rpm, the reaction is carried out for 30 minutes, and the low-polymerization degree urea-formaldehyde is converted into relatively high molecular weight urea-formaldehyde through polycondensation. At the same time, the hydroxyethyl cellulose, which exposes a large number of active hydroxyl groups under the strong shearing action of the extruder, undergoes nucleophilic substitution reaction or dehydration condensation reaction with the hydroxymethyl or amide groups contained in the in-situ generated urea-formaldehyde macromolecular chain, so that the urea-formaldehyde macromolecular chain is grafted onto the hydroxyethyl cellulose macromolecular chain. In addition, the -COOH of citric acid and the -OH of hydroxyethyl cellulose also undergo cross-linking reaction in an acidic environment at the same time. The three-in-one reaction extrusion process of polycondensation-grafting-cross-linking generates a viscous hydroxyethyl cellulose / urea-formaldehyde compound. (5) Opening the die between the reaction unit and the extrusion unit of the reaction extruder, starting the twin-screw extruder of the extrusion unit of the reaction extruder, and then the twin-screw reaction extruder conveys the viscous hydroxyethyl cellulose / urea-formaldehyde compound obtained in step (4) to the twin-screw extruder, and extruding the hydroxyethyl cellulose / urea-formaldehyde compound through the twin-screw extruder at 60° C. and 70 rpm to obtain a strip of hydroxyethyl cellulose / urea-formaldehyde compound. (6) Drying the strip of hydroxyethyl cellulose / urea-formaldehyde compound obtained in step (5) at 60° C. and then pelletizing it to obtain cylindrical hydroxyethyl cellulose / urea-formaldehyde water-retaining slow-release fertilizer particles (HEC / UF) with good particle shape.

[0079] The prepared HEC / UF had a compressive strength of 15.52 MPa, a nitrogen content of 18.56 wt%, a water absorption rate of 500%, and an initial nitrogen release rate of 13.13%. During corn planting, a fertilizer application rate of 80.81 kg / mu (15 kg nitrogen / mu) was applied using a seed and fertilizer machine.

[0080] Example 2

[0081] A polycondensation-grafting-crosslinking integrated reaction extrusion process for preparing hydroxyethyl cellulose / urea-formaldehyde water-retaining slow-release fertilizer granules comprises the following steps:

[0082] (1) Calculated amounts of urea (U), paraformaldehyde (Pf), and water (H2O) were added to a reactor, wherein the mass ratio of urea to paraformaldehyde was 2.6:1 and the amount of water added was 15% of the total mass of urea and paraformaldehyde. The pH of the system was adjusted to 8, and the reaction was carried out at 70°C for 1.5 hours to obtain a hydroxymethylurea (MU) solution.

[0083] (2) adding calculated amounts of citric acid and water to the hydroxymethyl urea solution obtained in step (1), wherein the amount of citric acid added is 15% of the total mass of urea and paraformaldehyde; the amount of water added is 300% of the total mass of urea and paraformaldehyde; reacting at 60° C. for 10 minutes to obtain a urea-formaldehyde mixed aqueous solution with a degree of polymerization ≤4.

[0084] (3) sealing the die between the reaction unit and the extrusion unit of the reaction extrusion integrated machine; adding a certain amount of hydroxyethyl cellulose and water to the mixed aqueous solution obtained in step (2), mixing them evenly and then injecting them into the twin-screw reaction extruder of the reaction unit of the reaction extrusion integrated machine; wherein the amount of hydroxyethyl cellulose added is 30% of the total mass of urea and paraformaldehyde, and the amount of water added is 1500% of the mass of the hydroxyethyl cellulose.

[0085] (4) The screws of the twin-screw reaction extruder of the reaction unit of the reaction extruder are turned on, and the vacuum devolatilization device is started at the same time to remove the moisture in the reaction system. When the temperature is set to 60°C and the screw speed is set to 60 rpm, the reaction is carried out for 30 minutes, and the low-polymerization degree urea-formaldehyde is converted into relatively high molecular weight urea-formaldehyde through polycondensation. At the same time, the hydroxyethyl cellulose, which exposes a large number of active hydroxyl groups under the strong shearing action of the extruder, undergoes nucleophilic substitution reaction or dehydration condensation reaction with the hydroxymethyl or amide groups contained in the in-situ generated urea-formaldehyde macromolecular chain, so that the urea-formaldehyde macromolecular chain is grafted onto the hydroxyethyl cellulose macromolecular chain. In addition, the -COOH of citric acid and the -OH of hydroxyethyl cellulose also undergo cross-linking reaction in an acidic environment. The three-in-one reaction extrusion process of polycondensation-grafting-cross-linking generates a viscous hydroxyethyl cellulose / urea-formaldehyde compound.

[0086] (5) opening the die between the reaction unit and the extrusion unit of the reaction extruder, starting the twin-screw extruder of the extrusion unit of the reaction extruder, and then the twin-screw reaction extruder conveys the viscous hydroxyethyl cellulose / urea-formaldehyde compound obtained in step (4) to the twin-screw extruder, and extrudes the viscous hydroxyethyl cellulose / urea-formaldehyde compound at 50° C. and 50 rpm through the twin-screw extruder to obtain a strip-shaped hydroxyethyl cellulose / urea-formaldehyde compound.

[0087] (6) The strip-shaped hydroxyethyl cellulose / urea-formaldehyde compound obtained in step (5) is dried at 60° C. and then granulated to obtain cylindrical hydroxyethyl cellulose / urea-formaldehyde water-retaining slow-release fertilizer granules (HEC / UF) with good particle shape.

[0088] The prepared HEC / UF had a compressive strength of 16.14 MPa, a nitrogen content of 19.86 wt%, a water absorption rate of 900%, and an initial nitrogen release rate of 19.92%. When planting corn, a fertilizer-seeding machine was used to apply 75.52 kg of fertilizer per mu (i.e., 15 kg of nitrogen per mu).

[0089] Example 3

[0090] A polycondensation-grafting-crosslinking integrated reaction extrusion process for preparing hydroxyethyl cellulose / urea-formaldehyde water-retaining slow-release fertilizer granules comprises the following steps:

[0091] (1) Calculated amounts of urea (U), paraformaldehyde (Pf), and water (H2O) were added to a reactor, wherein the mass ratio of urea to paraformaldehyde was 3.5:1 and the amount of water added was 15% of the total mass of urea and paraformaldehyde. The pH of the system was adjusted to 8, and the reaction was carried out at 70°C for 3 h to obtain a hydroxymethylurea (MU) solution.

[0092] (2) adding calculated amounts of citric acid and water to the hydroxymethyl urea solution obtained in step (1), wherein the amount of citric acid added is 10% of the total mass of urea and paraformaldehyde; the amount of water added is 250% of the total mass of urea and paraformaldehyde; reacting at 60° C. for 10 minutes to obtain a urea-formaldehyde mixed aqueous solution with a degree of polymerization ≤4.

[0093] (3) sealing the die between the reaction unit and the extrusion unit of the reaction extrusion integrated machine; adding a certain amount of hydroxyethyl cellulose and water to the mixed aqueous solution obtained in step (2), mixing them evenly and then injecting them into the twin-screw reaction extruder of the reaction unit of the reaction extrusion integrated machine; wherein the amount of hydroxyethyl cellulose added is 30% of the total weight of urea and paraformaldehyde, and the amount of water added is 3000% of the mass of the hydroxyethyl cellulose.

[0094] (4) The screws of the twin-screw reaction extruder of the reaction unit of the reaction extruder are turned on, and the vacuum devolatilization device is started at the same time to remove moisture from the reaction system. When the temperature is set to 60°C and the screw speed is set to 60 rpm, the reaction is carried out for 30 minutes, and the low-polymerization degree urea-formaldehyde is converted into relatively high molecular weight urea-formaldehyde through polycondensation. At the same time, the hydroxyethyl cellulose, which exposes a large number of active hydroxyl groups under the strong shearing action of the extruder, undergoes nucleophilic substitution reaction or dehydration condensation reaction with the hydroxymethyl or amide groups contained in the in-situ generated urea-formaldehyde macromolecular chain, so that the urea-formaldehyde macromolecular chain is grafted onto the hydroxyethyl cellulose macromolecular chain. In addition, the -COOH of citric acid and the -OH of hydroxyethyl cellulose also undergo cross-linking reaction in an acidic environment at the same time. The three-in-one reaction extrusion process of polycondensation-grafting-cross-linking generates a viscous hydroxyethyl cellulose / urea-formaldehyde compound.

[0095] (5) opening the die between the reaction unit and the extrusion unit of the reaction extruder, starting the twin-screw extruder of the extrusion unit of the reaction extruder, and then the twin-screw reaction extruder conveys the viscous hydroxyethyl cellulose / urea-formaldehyde compound obtained in step (4) to the twin-screw extruder, and extrudes the viscous hydroxyethyl cellulose / urea-formaldehyde compound through the twin-screw extruder at 60° C. and 100 rpm to obtain a strip-shaped hydroxyethyl cellulose / urea-formaldehyde compound.

[0096] (6) The strip-shaped hydroxyethyl cellulose / urea-formaldehyde compound obtained in step (5) is dried at 60° C. and then granulated to obtain cylindrical hydroxyethyl cellulose / urea-formaldehyde water-retaining slow-release fertilizer granules (HEC / UF) with good particle shape.

[0097] The prepared HEC / UF had a compressive strength of 14.77 MPa, a nitrogen content of 21.66 wt%, a water absorption rate of 300%, and an initial nitrogen release rate of 35.82%. When planting corn, a seed and fertilizer machine was used to apply 69.25 kg of fertilizer per mu (i.e., 15 kg of nitrogen per mu).

[0098] Example 4

[0099] A polycondensation-grafting-crosslinking integrated reaction extrusion process for preparing hydroxyethyl cellulose / urea-formaldehyde water-retaining slow-release fertilizer granules comprises the following steps:

[0100] (1) Calculated amounts of urea (U), paraformaldehyde (Pf), and water (H2O) were added to a reactor, wherein the mass ratio of urea to paraformaldehyde was 2:1 and the amount of water added was 5% of the total mass of urea and paraformaldehyde. The pH of the system was adjusted to 8, and the reaction was carried out at 50°C for 4 h to obtain a hydroxymethylurea (MU) solution.

[0101] (2) adding calculated amounts of citric acid and water to the hydroxymethyl urea solution obtained in step (1), wherein the amount of citric acid added is 25% of the total mass of urea and paraformaldehyde; the amount of water added is 600% of the total mass of urea and paraformaldehyde; reacting at 80° C. for 40 minutes to obtain a urea-formaldehyde mixed aqueous solution with a degree of polymerization ≤4.

[0102] (3) sealing the die between the reaction unit and the extrusion unit of the reaction extrusion integrated machine; adding a certain amount of hydroxyethyl cellulose and water to the mixed aqueous solution obtained in step (2), mixing them evenly and then injecting them into the twin-screw reaction extruder of the reaction unit of the reaction extrusion integrated machine; wherein the amount of hydroxyethyl cellulose added is 5% of the total weight of urea and paraformaldehyde, and the amount of water added is 5000% of the mass of the hydroxyethyl cellulose.

[0103] (4) The screws of the twin-screw reaction extruder of the reaction unit of the reaction extruder are turned on, and the vacuum devolatilization device is started at the same time to remove the moisture in the reaction system. When the temperature is set to 50°C and the screw speed is 50 rpm, the reaction is carried out for 10 minutes, and the low-polymerization degree urea-formaldehyde is converted into relatively high molecular weight urea-formaldehyde through polycondensation. At the same time, the hydroxyethyl cellulose, which exposes a large number of active hydroxyl groups under the strong shearing action of the extruder, undergoes nucleophilic substitution reaction or dehydration condensation reaction with the hydroxymethyl or amide groups contained in the in-situ generated urea-formaldehyde macromolecular chain, so that the urea-formaldehyde macromolecular chain is grafted onto the hydroxyethyl cellulose macromolecular chain. In addition, the -COOH of citric acid and the -OH of hydroxyethyl cellulose also undergo cross-linking reaction in an acidic environment at the same time. The three-in-one reaction extrusion process of polycondensation-grafting-cross-linking generates a viscous hydroxyethyl cellulose / urea-formaldehyde compound.

[0104] (5) opening the die between the reaction unit and the extrusion unit of the reaction extruder, starting the twin-screw extruder of the extrusion unit of the reaction extruder, and then the twin-screw reaction extruder conveys the viscous hydroxyethyl cellulose / urea-formaldehyde compound obtained in step (4) to the twin-screw extruder, and extrudes the viscous hydroxyethyl cellulose / urea-formaldehyde compound through the twin-screw extruder at 30° C. and 150 rpm to obtain a strip-shaped hydroxyethyl cellulose / urea-formaldehyde compound.

[0105] (6) The strip-shaped hydroxyethyl cellulose / urea-formaldehyde compound obtained in step (5) is dried at 60° C. and then granulated to obtain cylindrical hydroxyethyl cellulose / urea-formaldehyde water-retaining slow-release fertilizer granules (HEC / UF) with good particle shape.

[0106] The prepared HEC / UF had a compressive strength of 7.62 MPa, a nitrogen content of 27.56 wt%, a water absorption rate of 50%, and an initial nitrogen release rate of 8.13%. When planting corn, a fertilizer-seeding machine was used to apply 54.43 kg of fertilizer per mu (i.e., 15 kg of nitrogen per mu).

[0107] Example 5

[0108] A polycondensation-grafting-crosslinking integrated reaction extrusion process for preparing hydroxyethyl cellulose / urea-formaldehyde water-retaining slow-release fertilizer granules comprises the following steps:

[0109] (1) Calculated amounts of urea (U), paraformaldehyde (Pf), and water (H2O) were added to a reactor, wherein the mass ratio of urea to paraformaldehyde was 3:1 and the amount of water added was 10% of the total mass of urea and paraformaldehyde. The pH of the system was adjusted to 8, and the reaction was carried out at 90°C for 0.5h to obtain a hydroxymethylurea (MU) solution.

[0110] (2) adding calculated amounts of citric acid and water to the hydroxymethyl urea solution obtained in step (1), wherein the amount of citric acid added is 5% of the total mass of urea and paraformaldehyde; the amount of water added is 200% of the total mass of urea and paraformaldehyde; reacting at 60° C. for 10 minutes to obtain a urea-formaldehyde mixed aqueous solution with a degree of polymerization ≤4.

[0111] (3) sealing the die between the reaction unit and the extrusion unit of the reaction extrusion integrated machine; adding a certain amount of hydroxyethyl cellulose and water to the mixed aqueous solution obtained in step (2), mixing them evenly and then injecting them into the twin-screw reaction extruder of the reaction unit of the reaction extrusion integrated machine; wherein the amount of hydroxyethyl cellulose added is 30% of the total weight of urea and paraformaldehyde, and the amount of water added is 300% of the mass of the hydroxyethyl cellulose.

[0112] (4) The screws of the twin-screw reaction extruder of the reaction unit of the reaction extruder are turned on, and the vacuum devolatilization device is started at the same time to remove moisture from the reaction system. When the temperature is set to 50°C and the screw speed is set to 150 rpm, the reaction is carried out for 30 minutes, and the low-polymerization degree urea-formaldehyde is converted into relatively high molecular weight urea-formaldehyde through polycondensation. At the same time, the hydroxyethyl cellulose, which exposes a large number of active hydroxyl groups under the strong shearing action of the extruder, undergoes nucleophilic substitution reaction or dehydration condensation reaction with the hydroxymethyl or amide groups contained in the in-situ generated urea-formaldehyde macromolecular chain, so that the urea-formaldehyde macromolecular chain is grafted onto the hydroxyethyl cellulose macromolecular chain. In addition, the -COOH of citric acid and the -OH of hydroxyethyl cellulose also undergo cross-linking reaction in an acidic environment at the same time. The three-in-one reaction extrusion process of polycondensation-grafting-cross-linking generates a viscous hydroxyethyl cellulose / urea-formaldehyde compound.

[0113] (5) opening the die between the reaction unit and the extrusion unit of the reaction extruder, starting the twin-screw extruder of the extrusion unit of the reaction extruder, and then the twin-screw reaction extruder conveys the viscous hydroxyethyl cellulose / urea-formaldehyde compound obtained in step (4) to the twin-screw extruder, and extrudes the viscous hydroxyethyl cellulose / urea-formaldehyde compound through the twin-screw extruder at 80° C. and 50 rpm to obtain a strip-shaped hydroxyethyl cellulose / urea-formaldehyde compound.

[0114] (6) The strip-shaped hydroxyethyl cellulose / urea-formaldehyde compound obtained in step (5) is dried at 60° C. and then granulated to obtain cylindrical hydroxyethyl cellulose / urea-formaldehyde water-retaining slow-release fertilizer granules (HEC / UF) with good particle shape.

[0115] The prepared HEC / UF had a compressive strength of 15.52 MPa, a nitrogen content of 21.28 wt%, a water absorption rate of 300%, and an initial nitrogen release rate of 13.13%. During corn planting, a fertilizer application rate of 70.49 kg / mu (15 kg nitrogen / mu) was applied using a seed and fertilizer machine.

[0116] Comparative Example 1

[0117] A reactive extrusion preparation process for urea-formaldehyde fertilizer granules comprises the following steps:

[0118] (1) Calculated amounts of urea (U), paraformaldehyde (Pf), and water (H2O) were added to a reactor, wherein the mass ratio of urea to paraformaldehyde was 2.5:1 and the amount of water added was 15% of the total mass of urea and paraformaldehyde. The pH of the system was adjusted to 8, and the reaction was carried out at 90°C for 0.5h to obtain a hydroxymethylurea (MU) solution.

[0119] (2) The die between the reaction unit and the extrusion unit of the reaction extrusion integrated machine is sealed, and then the pH of the MU solution obtained in step (1) is adjusted to 5 and then injected into the twin-screw reaction extruder of the reaction unit of the reaction extrusion integrated machine.

[0120] (3) The screw of the twin-screw reaction extruder of the reaction unit of the reaction extruder is turned on, and the vacuum devolatilization device is started at the same time to remove moisture from the reaction system. The temperature is set to 60°C and the screw speed is set to 70 rpm. The reaction is carried out for 30 minutes, and MU generates a viscous urea-formaldehyde compound through a condensation reaction.

[0121] (4) Open the die between the reaction unit and the extrusion unit of the reaction extrusion integrated machine, start the twin-screw extruder of the extrusion unit of the reaction extrusion integrated machine, and then the twin-screw reaction extruder of the reaction extrusion integrated machine transports the viscous urea-formaldehyde compound obtained in step (3) to the twin-screw extruder, and extrude the urea-formaldehyde compound in a strip shape at 60°C and 70rpm through the twin-screw extruder.

[0122] (5) The strip-shaped urea-formaldehyde compound obtained in step (4) is dried at 60° C. and then granulated to obtain cylindrical urea-formaldehyde compound particles (UF) with good particle shape.

[0123] The compressive strength of the prepared UF is 2.48 MPa, the content of nutrient element N is 31.83 wt%, and the initial nitrogen nutrient release rate is 12.36%.

[0124] The performance tests and characterizations of the present invention all adopt the following standards:

[0125] 1) Grind the cylindrical particles and pass them through a 0.25 mm sieve to obtain the test sample powder. Take a small amount of dry powder KBr and press it into a pellet. Use a Nicolet IS50 infrared spectrometer to measure the infrared spectrum at room temperature with a scanning range of 500 to 4000 cm -1 X-ray diffractometer (HAOYUANDX-2700B) was used to analyze the samples with a scanning range of 5-80°. The thermal stability of the samples was measured using a thermogravimetric analyzer (TAQ50) under a nitrogen atmosphere with a set temperature range of 30-800°C, a temperature increase of 10°C / min, and a nitrogen flow rate of 40 mL / min.

[0126] 2) Compression Strength Test: The test was conducted using a universal testing machine MTSCMT5105. The cylindrical particles were placed on the machine and compressed in the longitudinal direction at a pressure of 20,000 kgf and a compression rate of 1 mm / min.

[0127] 3) Slow-release performance test: The slow-release performance and initial release rate of nitrogen nutrients were characterized by a static water release test. Weigh 5.00g of fertilizer granules, put them into a 100-mesh nylon mesh bag and seal it, then place it in a bottle filled with 100mL of deionized water. Incubate it in a constant temperature water bath at 25°C, and take samples at 1, 3, 5, 7, 10, 14, and 28 days. When sampling, use tweezers to place the nylon mesh bag at the bottle mouth to drain the water. After no water drips, place it in a new culture bottle filled with 100ml of deionized water and continue culturing. Turn the original culture bottle upside down to ensure that the internal solution concentration is consistent. Take 20ml of the solution, use the sulfuric acid-hydrogen peroxide digestion method and the Kjeldahl nitrogen method to determine the nitrogen content and calculate the cumulative nutrient release rate. The test result on the first day is the initial release rate.

[0128] 4) Determination of water absorption rate: At room temperature, 1g (denoted as W d ) water-retaining fertilizer was placed in 500 ml of deionized water and weighed every 2 hours (denoted as W S ) until the weight stops changing, which is the maximum water absorption rate of the fertilizer.

[0129] The water absorption rate calculation formula is as follows:

[0130] 5) Determination of soil water retention rate: Before testing, first mix the fertilizer with the same nitrogen content and the same weight (M t ) dry soil and mix evenly, put it into a PVC tube with 800 mesh filter paper at the bottom, add water until water seeps out from the bottom, record the maximum water holding rate, weigh the above mixture (recorded as M0) and place it in a flower pot for incubation at room temperature, weigh it every 3 days and record it as M1, and record it continuously for 24 days.

[0131] The calculation formula of soil water retention rate is as follows:

[0132] Figure 1 shows the HEC, hydroxyethyl cellulose / urea formaldehyde water-retaining slow-release fertilizer HEC / UF prepared in Examples 1 and 2, and UF prepared in Comparative Example 1 at 4000 cm -1 ~500cm -1 FT-IR spectrum of -1 -OH stretching vibration peak of HEC; 1626 cm -1 and 1603cm -1 The HEC / UF prepared in Examples 1 and 2 both have typical characteristic peaks of HEC and UF prepared in Comparative Example 1, indicating that a polycondensation reaction to form urea-formaldehyde occurs during the reactive extrusion process. The materials prepared in Examples 1 and 2 are both composed of hydroxyethyl cellulose and urea-formaldehyde.

[0133] Figure 2 is an enlarged view of the normalized infrared spectra of HEC / UF prepared in Example 1 and UF prepared in Comparative Example 1. As shown in Figure 2, the infrared spectra at 1549 cm -1 The absorption peak of the NH in-plane bending vibration and part of the CN stretching vibration of UF at 1625cm is blue-shifted in the HEC / UF spectrum. The reason is that after being grafted onto the HEC molecular chain, the groups on the HEC molecular chain cause the UF peak to blue-shift. In addition, the absorption peak at 1625cm -1 The carbonyl stretching vibration peak of UF at is also blue-shifted in the HEC / UF spectrum. This is also due to the fact that after grafting onto the HEC molecular chain, the groups on the HEC molecular chain cause the UF peak to blue-shift. This indicates that UF reacts with HEC during the reactive extrusion process to form the HEC / UF compound.

[0134] Figure 3 is an enlarged view of the normalized infrared spectra of the HEC / UF water-retaining slow-release fertilizer prepared in Example 1 and the UF and HEC prepared in Comparative Example 1. Since the amount of HEC added is relatively small, the 1118-1060 cm -1 The asymmetric bending vibration peak of HEC at 1033 cm is broadened due to the coverage of the corresponding characteristic peak of UF. -1 The -COC- ether bond stretching vibration peak intensity of UF at the position is significantly enhanced and red-shifted, indicating that during the reaction extrusion process of forming HEC / UF compounds, the in-situ generated UF reacted with HEC to form an ether bond, causing UF to be grafted onto the HEC macromolecular chain.

[0135] Figure 4 shows the XRD spectra of the HEC / UF water-retaining slow-release fertilizers prepared in Examples 1 and 2, and the UF and HEC prepared in Comparative Example 1. In the HEC / UF spectra, the 18.97° peak of pure HEC disappears, indicating that the crystallization of HEC uniformly dispersed in the low-polymerization UF mixed aqueous solution is hindered. The peak intensity at 22.56° of pure UF is significantly weakened and broadened, indicating that the UF structure is destroyed during the reaction with HEC. The UF grafted onto the HEC macromolecular chain through the reaction hinders the crystallization of both HEC and UF. Therefore, compared to the pure UF in Comparative Example 1, the crystallinity of the HEC / UF in the examples is reduced, which will facilitate the degradation rate of microorganisms and thus increase the nutrient release rate.

[0136] Figure 5 shows the thermogravimetric curves of the hydroxyethyl cellulose / urea-formaldehyde water-retaining slow-release fertilizer HEC / UF prepared in Examples 1 and 2, and the UF and HEC prepared in Comparative Example 1. It can be seen that at temperatures below 320°C, the overall thermal stability of the HEC / UF prepared in Examples 1 and 2 is reduced compared to the UF and HEC in Comparative Example 1. This is due to two factors: first, the grafting reaction between HEC and UF reduces the regularity of the molecular chains of the two polymers, lowering their crystallinity and, consequently, their thermal stability. Second, the introduction of HEC partially hinders the extrusion polycondensation reaction during UF formation, resulting in a lower degree of polymerization (DP) than that of pure UF obtained by the same process, thus reducing thermal stability. Due to the introduction of the thermally stable HEC, the thermal stability of HEC / UF is superior to that of pure UF prepared in Comparative Example 1 at temperatures above 320°C.

[0137] Figure 6 shows the thermogravimetric differential curves of the hydroxyethyl cellulose / urea-formaldehyde water-retaining slow-release fertilizer HEC / UF prepared in Examples 1 and 2, and the UF and HEC prepared in Comparative Example 1. The two pyrolysis peaks of HEC at 294°C and 773°C shift to 272°C and 762°C, respectively, in the HEC / UF of Example 1, and to 264°C and 751°C, respectively, in the HEC / UF of Example 2. This further demonstrates that the grafted UF in the HEC / UF disrupts the regular arrangement of the HEC macromolecular chains, hindering the crystallization of the HEC components and resulting in reduced thermal stability. The UF of Comparative Example 1 begins to decompose at 243°C, with two pyrolysis peaks at 289°C and 310°C, corresponding to its low- and high-molecular-weight components, respectively. The HEC / UF of Example 1 begins to decompose at 124°C, with the first pyrolysis peak appearing at 173°C and the second at 245°C, both significantly lower than those of the UF of Comparative Example 1. This is because, during the reactive extrusion process, HEC, uniformly dispersed in the low-polymerization UF aqueous solution, not only affects the polycondensation of MU to form UF, hindering the formation of high-molecular-weight UF components, but also hinders UF crystallization. Consequently, the degree of polymerization and crystallinity of the UF component in the HEC / UF mixture are lower than those of pure UF prepared by the same process, which helps to increase the N release rate. Furthermore, for the HEC / UF prepared in Examples 1 and 2, a new pyrolysis peak at 200°C, attributed to ether bonds, appears. This further demonstrates that the hydroxymethyl groups in the UF are grafted onto the HEC macromolecular chain via nucleophilic substitution with the hydroxyl groups in the HEC, forming ether bonds, and that the -COOH groups of citric acid undergo cross-linking reactions by forming ether bonds with the -OH groups of cellulose.

[0138] Figure 7 shows the compressive strength test curves of the hydroxyethyl cellulose / urea formaldehyde water-retaining slow-release fertilizer granules (HEC / UF) prepared in Example 1 and the UF granules prepared in Comparative Example 1. The compressive strength of the HEC / UF granules prepared in Example 1 was 15.21 MPa, significantly higher than the 2.48 MPa of the UF granules. This indicates that the introduction of HEC significantly improves the overall strength of the modified UF fertilizer granules. This not only prevents breakage of the fertilizer granules during storage, transportation, and application, but also allows simultaneous fertilization using a seeding and fertilizer sowing machine, saving labor, time, fertilizer, and effort.

[0139] Figure 8 shows the hydrostatic nitrogen release curves for the hydroxyethyl cellulose / urea-formaldehyde water-retaining slow-release fertilizer granules (HEC / UF) prepared in Example 1 and the UF granules prepared in Comparative Example 1. It can be seen that the initial nitrogen release rate of the HEC / UF prepared in Example 1 is significantly higher than that of the pure UF in Comparative Example 1 due to the relatively low degree of polymerization of its UF component. From days 14 to 42, the nitrogen release of the HEC / UF in Example 1 remains higher than that of the pure UF in Comparative Example 1, indicating that the nitrogen release rate of HEC / UF during the middle and late stages of crop growth is significantly higher than that of pure UF. This can better meet plant growth needs and help improve fertilizer nutrient utilization efficiency.

[0140] Figure 9 shows the water retention curves for soil treated with the hydroxyethyl cellulose / urea formaldehyde water-retaining slow-release fertilizer granules (HEC / UF) prepared in Examples 1 and 2 and the UF granules prepared in Comparative Example 1. It can be seen that the addition of HEC effectively improves the water retention capacity of the treated soil. At 12 days, the water retention rate of the UF-treated soil in Comparative Example 1 was 22.8%, while that of the HEC / UF-treated soils prepared in Examples 1 and 2 was 51.2% and 55.2%, respectively. At 21 days, the UF-treated soil in Comparative Example 1 had completely lost water, while the HEC / UF-treated soils prepared in Examples 1 and 2 still retained water retention rates of 11.2% and 18.4%, respectively. This demonstrates that the HEC / UF water-retaining fertilizer of the present invention effectively improves the soil's water absorption and retention capacity.

[0141] The above description is merely a specific embodiment of the present invention, which enables those skilled in the art to understand or implement the present invention. Although detailed descriptions have been made with reference to the aforementioned embodiments, those skilled in the art should understand that they may still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein with equivalents; and such modifications or replacements do not deviate from the essence of the corresponding technical solutions within the scope of the technical solutions of the embodiments, and they should all be included in the scope of protection of the claims.

Claims

1. A polycondensation-grafting-crosslinking integrated reaction extrusion process for preparing cellulose / urea-formaldehyde water-retaining slow-release fertilizer particles, characterized in that: The steps include: (1) adding a calculated amount of urea, paraformaldehyde, and water into a reactor, adjusting the pH of the system, and reacting at a certain temperature for a certain time to obtain a hydroxymethylurea solution; (2) adding a calculated amount of citric acid and water to the hydroxymethyl urea solution obtained in step (1), and prepolymerizing at a certain temperature for a certain time to obtain a mixed aqueous solution of urea-formaldehyde with a low degree of polymerization; (3) sealing the die between the reaction unit and the extrusion unit of the reaction extrusion integrated machine; adding a certain amount of cellulose and water to the mixed aqueous solution obtained in step (2), mixing them evenly, and then injecting them into the twin-screw reaction extruder of the reaction unit of the reaction extrusion integrated machine; (4) The screws of the twin-screw reaction extruder of the reaction unit of the reaction extruder are turned on, and the vacuum devolatilization device is started at the same time to remove moisture from the reaction system. The reaction is carried out at a set temperature and a set screw speed for a certain time, and then the low-polymerization degree urea-formaldehyde is converted into relatively high molecular weight urea-formaldehyde through polycondensation. At the same time, the cellulose with a large number of active hydroxyl groups exposed under the strong shearing action of the extruder undergoes nucleophilic substitution reaction or dehydration condensation reaction with the hydroxymethyl or amide groups contained in the in-situ generated urea-formaldehyde macromolecular chain, so that the urea-formaldehyde macromolecule is grafted onto the cellulose macromolecular chain. In addition, the -COOH of citric acid and the -OH of cellulose also undergo cross-linking reaction at the same time. The three-in-one reaction extrusion process of polycondensation-grafting-cross-linking generates a viscous cellulose / urea-formaldehyde compound. (5) opening the die between the reaction unit and the extrusion unit of the reaction extruder, starting the twin-screw extruder of the extrusion unit of the reaction extruder, and then the twin-screw reaction extruder conveys the viscous cellulose / urea-formaldehyde compound obtained in step (4) to the twin-screw extruder, and the twin-screw extruder extrude the cellulose / urea-formaldehyde compound at a set temperature and a set speed to obtain a strip-shaped cellulose / urea-formaldehyde compound; (6) The strip-shaped cellulose / urea-formaldehyde compound obtained in step (5) is dried at a set temperature and then pelletized to obtain cellulose / urea-formaldehyde water-retaining slow-release fertilizer particles with good particle shape.

2. The cellulose / urea-formaldehyde water-retaining slow-release fertilizer granules prepared by a polycondensation-grafting-crosslinking integrated reaction extrusion process according to claim 1, characterized in that: In step (1), the mass ratio of urea to paraformaldehyde is 2-3.5:1, and the amount of water added is 5-20% of the total mass of urea and paraformaldehyde; the pH of the system is adjusted to 8-12, and the reaction is carried out at 50-90° C. for 0.5-4 h.

3. The cellulose / urea-formaldehyde water-retaining slow-release fertilizer granules prepared by a polycondensation-grafting-crosslinking integrated reaction extrusion process according to claim 1, characterized in that: In step (2), the amount of water added is 50-600% of the total mass of urea and paraformaldehyde; the amount of citric acid added is 5-25% of the total mass of urea and paraformaldehyde; the reaction temperature is 50-80° C., and the reaction time is 5-40 min; and the degree of polymerization of the obtained low-polymerization urea-formaldehyde is ≤4.

4. The method of preparing cellulose / urea-formaldehyde water-retaining slow-release fertilizer particles by a polycondensation-grafting-crosslinking integrated reaction extrusion process according to claim 1, characterized in that: In step (3), the cellulose added is hydroxyethyl cellulose, and the added amount is 5-60% of the total mass of urea and paraformaldehyde; the added amount of water is 300-5000% of the mass of the hydroxyethyl cellulose.

5. The cellulose / urea-formaldehyde water-retaining slow-release fertilizer granules prepared by a polycondensation-grafting-crosslinking integrated reaction extrusion process according to claim 1, characterized in that: In step (4), the extrusion temperature of the twin-screw reaction extruder of the reaction unit of the reaction extruder is 50-90° C., the screw speed is 50-300 rpm, and the reaction time is 10-30 min.

6. The method of preparing cellulose / urea-formaldehyde water-retaining slow-release fertilizer particles by a polycondensation-grafting-crosslinking integrated reaction extrusion process according to claim 1, characterized in that: In step (4), the viscous cellulose / urea-formaldehyde compound has the following molecular structure: Where: n = 10~100.

7. The method of preparing cellulose / urea-formaldehyde water-retaining slow-release fertilizer particles by a polycondensation-grafting-crosslinking integrated reaction extrusion process according to claim 1, characterized in that: In step (5), the extrusion temperature of the twin-screw extruder of the extrusion unit of the reaction extruder is 30-80° C., and the screw speed is 50-150 rpm.

8. The cellulose / urea-formaldehyde water-retaining slow-release fertilizer granules prepared by a polycondensation-grafting-crosslinking integrated reaction extrusion process according to claim 1, characterized in that: In step (6), the drying temperature is 50-80°C.

9. The method for preparing cellulose / urea-formaldehyde water-retaining slow-release fertilizer granules by a polycondensation-grafting-crosslinking integrated reaction extrusion process according to any one of claims 1 to 8, characterized in that: The cellulose / urea-formaldehyde water-retaining slow-release fertilizer granules are applied at the same time as sowing using a seed-fertilizer-sowing machine. The application amount of the cellulose / urea-formaldehyde water-retaining slow-release fertilizer granules is 8-15 kg nitrogen per mu.

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