CN114751789A

CN114751789A - Slow-release fertilizer and preparation method thereof

Info

Publication number: CN114751789A
Application number: CN202210170657.7A
Authority: CN
Inventors: 江东; 谭皓元; 孙立先; 孙野; 杨延华
Original assignee: Jilin University
Current assignee: Jilin University
Priority date: 2022-02-24
Filing date: 2022-02-24
Publication date: 2022-07-15

Abstract

The invention is suitable for the technical field of slow release fertilizers, and provides a preparation method of a slow release fertilizer, which comprises the following steps: taking polylactic acid, polycaprolactone, trichloromethane, deionized water and urea as raw materials for later use; dissolving polylactic acid and polycaprolactone by using trichloromethane as a solvent to prepare an electrostatic spinning solution; step three, dissolving urea by using deionized water as a solvent to prepare a urea solution; step four, respectively filling the electrostatic spinning solution and the urea solution into 5ml injectors, wherein the electrostatic spinning solution and the urea solution are respectively used as a shell and an inner core; connecting a high-voltage power supply anode to the needle of the injector by adopting a coaxial electrostatic spinning method, connecting a high-voltage power supply cathode to a receiving plate, grounding a collecting device and then spinning a fiber membrane; and step six, placing the spun fiber membrane in an oven for vacuum drying for 12 hours. The polylactic acid and the polycaprolactone belong to natural degradable nontoxic materials, and no toxic substance is generated to the environment in the slow release process.

Description

Slow-release fertilizer and preparation method thereof

Technical Field

The invention belongs to the technical field of slow release fertilizers, and particularly relates to a slow release fertilizer and a preparation method thereof.

Background

Currently, with the development of global agriculture, the use amount of chemical fertilizers is rapidly increased, and the reasonable use of the chemical fertilizers becomes the key for improving the yield and quality of crops. In 2019, the yield of urea is 21821 ten thousand tons, but the effective utilization rate is only about 35%, the fertility duration is short, in order to ensure the normal growth of crops, extra fertilization is needed when the fertilizing amount is too small, and the accumulation of nitrogen in the soil surface layer and the water body is easily caused when the fertilizing amount is excessive, so that very serious environmental pollution is caused.

Therefore, the development of the slow-release urea has important significance, the utilization rate of nitrogen can be improved, nutrients required by plants in each growth stage are met, and the harm to the environment is reduced. Can improve the compressive strength of urea and avoid damage in the transportation process. The slow-release urea researched in the market mostly adopts polymer physical blending, chemical modification and a urea coating method. The physical blending method mainly comprises the step of combining urea and a certain amount of dissolution inhibitor to form the slow-release urea with certain strength. The chemical modification method mainly comprises the step of carrying out chemical reaction on urea and other substances, the slow release capability of the urea is good, but the preparation process is complex, and toxic substances can be generated in the slow release process. The coated urea method is to coat a layer of substance with resistance-capacitance property on the outer surface of urea particles to prevent the external fast contact with the internal urea, thereby playing a certain slow release capacity.

The slow release fertilizer is the focus of development at present, the research of the slow release fertilizer mainly focuses on the research of the degradability of the coating, urea is coated by degradable resin, hydrogel, polyacrylamide and other materials to achieve a certain slow release effect, but the slow release fertilizer also has the problems of too high slow release rate, uneven coating thickness and the like, the coating is broken due to long-term transportation or storage, the slow release performance is lost, and some problems of toxicity and the like exist in the degradation process. Commonly used slow release coatings include molten sulfur, polystyrene, and others. However, these substances are not degraded in the slow release process, and can cause pollution to the environment.

Disclosure of Invention

The embodiment of the invention aims to provide a slow-release fertilizer and a preparation method thereof, and aims to solve the problems that the slow-release rate of the existing slow-release fertilizer is too high, the coating thickness is not uniform, the slow-release fertilizer cannot be degraded in the slow-release process, the environment is polluted and the like.

The invention is realized in such a way, and the slow release fertilizer and the preparation method thereof comprise the following steps:

taking polylactic acid, polycaprolactone, trichloromethane, deionized water and urea as raw materials for later use;

dissolving polylactic acid and polycaprolactone by using trichloromethane as a solvent to prepare an electrostatic spinning solution;

step three, dissolving urea by using deionized water as a solvent to prepare a urea solution;

step four, respectively filling the electrostatic spinning solution and the urea solution into 5ml injectors, wherein the electrostatic spinning solution and the urea solution are respectively used as a shell and an inner core;

connecting a high-voltage power supply anode on a syringe needle by adopting a coaxial electrostatic spinning method, connecting a high-voltage power supply cathode on a receiving plate, grounding a collecting device and then spinning a fiber membrane;

and step six, placing the spun fiber membrane in an oven at the temperature of 30 ℃ for vacuum drying for 12 hours.

According to a further technical scheme, the mass ratio of the polylactic acid to the polycaprolactone in the step two is 4: 6. 5: 5 or 6: 4.

according to a further technical scheme, in the second step, the polylactic acid and the polycaprolactone account for 10% of the electrostatic spinning solution.

And in the second step, the electrostatic spinning solution is placed at room temperature and stirred for 12 hours by using a magnetic stirrer, and the stirred electrostatic spinning solution is kept stand until bubbles are discharged.

According to a further technical scheme, the urea accounts for 10%, 20%, 30% or 50% of the urea solution.

In the further technical scheme, the urea solution in the third step is placed at room temperature and stirred for 12 hours by using a magnetic stirrer, and the stirred electrostatic spinning solution is kept stand until bubbles are discharged.

According to a further technical scheme, the spinning parameters in the step five are set as follows: voltage 20kv, wherein the shell injection speed is 1ml/h, the core speed is 0.1ml/h, the spinning distance is 10cm, the ambient temperature is 20 ℃, and the relative humidity is lower than 50%.

A slow release fertilizer prepared by the preparation method.

According to the slow release fertilizer and the preparation method thereof provided by the embodiment of the invention, the optimal preparation conditions of the core/shell coaxial electrostatic spinning fiber membrane are determined by changing different polymer ratios and urea with different concentrations, so that the nano slow release fertilizer is successfully prepared. The fiber morphological characteristics show that the fiber can be used as a good release system, and a urea release test shows that the film has potential application prospect in agriculture. FTIR showed that there was no interaction between them other than hydrogen bonding, ensuring release of urea. The release tests of TGA and urea in acid-base environment show that the fiber membrane has certain acid-base resistance and high temperature resistance. Therefore, the prepared polylactic acid/polycaprolactone-coated urea coaxial electrostatic spinning fiber membrane has a potential application prospect in future slow-release fertilizer production, and provides a new idea for development of electrostatic spinning in agriculture.

Drawings

FIG. 1 is an SEM image and a fiber diameter distribution diagram of an in-line electrospun fiber under different reaction ratios in an example of the present invention;

FIG. 2 is a FTIR plot of co-axial electrospun fibers under different reaction ratios in examples of the present invention;

FIG. 3 is a WCA graph of electrospun fibers coaxially under conditions of different reaction ratios in an example of the invention;

FIG. 4 is a TGA graph of co-axial electrospun fibers under different reaction ratio conditions in an example of the invention;

FIG. 5 is a slow release diagram of coaxial electrospun fibers under different reaction ratio conditions under neutral conditions in an embodiment of the invention;

FIG. 6 is a slow release diagram of coaxial electrospun fibers under different reaction ratio conditions under acidic conditions in an embodiment of the invention;

FIG. 7 is a graph showing the sustained release of a coaxial electrospun fiber under different reaction ratio conditions under alkaline conditions in an example of the present invention;

FIG. 8 is a slow release diagram of coaxial electrospun fibers under different reaction ratio conditions under high temperature conditions in an embodiment of the invention;

fig. 9 is a schematic diagram of the operation of coaxial electrospinning fibers according to an embodiment of the present invention.

Detailed Description

In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

Specific implementations of the present invention are described in detail below with reference to specific embodiments.

Example 1

A preparation method of a slow release fertilizer comprises the following steps:

step two, dissolving polylactic acid and polycaprolactone by using trichloromethane as a solvent to prepare an electrostatic spinning solution, wherein the mass ratio of the polylactic acid to the polycaprolactone is 4: 6, the polylactic acid and the polycaprolactone account for 10% of the electrostatic spinning solution, the electrostatic spinning solution is placed at room temperature and stirred for 12 hours by using a magnetic stirrer, and the stirred electrostatic spinning solution is kept stand until bubbles are discharged;

dissolving urea by using deionized water as a solvent to prepare a urea solution, wherein the urea accounts for 10% of the urea solution, stirring the urea solution at room temperature for 12 hours by using a magnetic stirrer, and standing the stirred electrostatic spinning solution until bubbles are discharged;

connecting the syringe needle with the anode of a high-voltage power supply by adopting a coaxial electrostatic spinning method, connecting the receiving plate with the cathode of the high-voltage power supply, and setting the spinning parameters as follows: the voltage is 20kv, wherein the injection speed of the shell is 1ml/h, the speed of the core is 0.1ml/h, the spinning distance is 10cm, the ambient temperature is 20 ℃, and the relative humidity is lower than 50%, and the collecting device is grounded to spin the fiber membrane;

Example 2

dissolving urea by using deionized water as a solvent to prepare a urea solution, wherein the urea accounts for 20% of the urea solution, stirring the urea solution at room temperature for 12 hours by using a magnetic stirrer, and standing the stirred electrostatic spinning solution until bubbles are discharged;

sixthly, placing the spun fiber membrane in an oven at the temperature of 30 ℃ for vacuum drying for 12h

Example 3

step two, using trichloromethane as a solvent to dissolve polylactic acid and polycaprolactone to prepare an electrostatic spinning solution, wherein the mass ratio of the polylactic acid to the polycaprolactone is 4: 6, the polylactic acid and the polycaprolactone account for 10% of the electrostatic spinning solution, the electrostatic spinning solution is placed at room temperature and stirred for 12 hours by using a magnetic stirrer, and the stirred electrostatic spinning solution is kept stand until bubbles are discharged;

dissolving urea by using deionized water as a solvent to prepare a urea solution, wherein the urea accounts for 30% of the urea solution, stirring the urea solution at room temperature for 12 hours by using a magnetic stirrer, and standing the stirred electrostatic spinning solution until bubbles are discharged;

connecting a high-voltage power supply anode on a syringe needle and connecting a high-voltage power supply cathode on a receiving plate by adopting a coaxial electrostatic spinning method, wherein the spinning parameters are as follows: the voltage is 20kv, wherein the injection speed of the shell is 1ml/h, the speed of the core is 0.1ml/h, the spinning distance is 10cm, the ambient temperature is 20 ℃, and the relative humidity is lower than 50%, and the collecting device is grounded to spin the fiber membrane;

Example 4

dissolving urea by using deionized water as a solvent to prepare a urea solution, wherein the urea accounts for 50% of the urea solution, stirring the urea solution at room temperature for 12 hours by using a magnetic stirrer, and standing the stirred electrostatic spinning solution until bubbles are discharged;

Fig. 1 shows that when polylactic acid and polycaprolactone account for 10% of the electrospinning solution, polylactic acid and polycaprolactone are polymers, the polymer concentration is fixed at 10%, and the urea concentration is respectively adjusted at 10% (a, b), 20% (c, d), 30% (e, f), 50% (g, h), it can be seen that the fiber diameter distribution is more extensive and uneven along with the increase of the urea concentration, because along with the increase of the urea concentration, the urea filled in the polymer fibers is gradually increased, the urea can not be guaranteed to be completely filled in the polymer fibers, and the fibers are enabled to present unevenness. In addition, when the urea concentration is increased to a certain degree, and the fiber is filled with urea, a part of urea on the surface of the fiber is crystallized, so that the coating of the urea by the polymer is influenced, and the coating rate is reduced. However, when the urea concentration is too low, the viscosity of the spinning solution is lowered, and the spinnability of the fiber is lowered. Thus, both polymer concentration and urea concentration affect the morphology of the fibers. It can also be seen from FIG. 1 that when the polymer concentration is 10% and the urea concentration is 10% or 20%, the fibers produced are relatively uniform in diameter and have better coverage.

FIG. 2 pure PLA (a), pure PCL (b), PLA/PCL blended electrospun nanofiber (c), pure urea (d), 10% PLA/PCL coated 20% urea core/shell coaxial electrospun nanofiber (e). Giving a 3508 cm in PLA^-1Is a characteristic peak of O-H, 2997 cm ^-12943 cm for C-H asymmetric stretching vibration^-1Is C-H symmetric stretching vibration, 1757 cm^-1For C = O stretching vibration, 1384 cm^-1Is C-H symmetric bending vibration, 1282 cm^-1For C = O bending vibration, 1186 cm^-1，1110 cm^-1、1083 cm^-1Vibrating for C-O stretching, 1454 cm^-1is-CH₃Flexural vibration, 870 cm^-1C-C stretching vibration. In PCL, 1731 cm^-1C = O tensile vibration. In pure urea, 3444 cm^-1is-N-H absorption peak, 1681 cm ^-11153 cm as amide absorption peak^-1Is the C-N tensile vibration peak.

In the polylactic acid/urea coaxial electrostatic spinning fiber prepared by electrostatic spinning, the stretching vibration peaks of the carbonyl and hydroxyl groups of the polymer are shifted, which is probably because the N-H in the polymer and the urea form intermolecular hydrogen bonds, so that the infrared peak is shifted to a certain extent. From the analysis results, it can be seen that two characteristic group peaks of the polymer and urea are respectively retained, and meanwhile, a certain degree of intermolecular force exists in the system to move the characteristic peak of some hydrogen bond forming groups, so that the urea is not released too fast.

Fig. 3 shows the change in contact angle between different concentrations of polymer/urea co-axial electrospun fiber membranes, as reflected by the change in contact angle within 30 s. The polylactic acid/polycaprolactone composite material comprises (a) 10% polylactic acid/polycaprolactone coated 10% urea core/shell coaxial electrostatic spinning nanofiber, (b) 10% polylactic acid/polycaprolactone coated 20% urea core/shell coaxial electrostatic spinning nanofiber, (c) 10% polylactic acid/polycaprolactone coated 30% urea core/shell coaxial electrostatic spinning nanofiber and (d) 10% polylactic acid/polycaprolactone coated 50% urea core/shell coaxial electrostatic spinning nanofiber. The experimental results show that when t =0, good hydrophobicity is exhibited, and the hydrophobic state is maintained after 30s, showing good surface stability. However, with the addition of urea, part of the urea will crystallize on the fiber surface, resulting in a decrease in the contact angle. As the concentration of urea increases, the more urea on the fiber surface, the smaller the contact angle. But still exhibit overall hydrophobicity, ensuring that the urea of the inner core is not released too quickly.

Fig. 4 gives thermogravimetric analysis of different concentrations of polymer/urea co-axial electrospun fiber membranes. It can be clearly seen that the decomposition temperature of pure PLA is 329.5 deg.C, that of pure PCL is 320.7 deg.C, and that the decomposition of pure urea is divided into two stages. The first stage is the decomposition of urea to generate ammonia gas and cyanic acid at 161.6 ℃. The second stage is the decomposition of the by-products, at a temperature of 264.5 ℃. As is clear from the figure, in the coaxial electrospun fiber membrane, each substance had its own decomposition platform, indicating that no chemical reaction occurred during the preparation process, and each substance maintained its own properties. However, it can be seen that the polymer does not decompose below 160 ℃, and the thermal decomposition temperature of PLA and PCL is lower than that of urea, so that the polymer has good stability and is a good slow-release material for urea.

FIG. 5 shows the release of the prepared polymer/urea core/shell structured coaxial electrospun fibers in a neutral buffer solution. From the figure, it can be found that when t ≈ 0, the main reason that the release speed is fast is the influence of the dissolution of urea in water on the fiber surface, in the subsequent time, it can be seen through a curve that the polymer-coated urea has a slow release effect, the polymer is contacted with the PBS buffer solution, the urea in the polymer fiber is released in the slow dissolution process, the release rate in the early stage is relatively slow, and part of the relatively uniform coaxial electrospinning fibers releases urea. After 24 days, the release rate was slower because the residual urea was now located in the thicker polymer fibers. However, if the same amount of urea is directly placed under the same conditions without coating, the urea will dissolve quickly and completely in a few minutes.

Fig. 6 and 7 show the release curves in the acid and base environments, and it can be found that the release curves are about the same as those in the neutral environment, but the release rates in the acid and base environments are faster than those in the neutral environment, and the release rate in the alkaline environment is faster than that in the acid environment. This is due to the influence of the structure of the polymer itself, and under weak acid or weak base conditions, the slow dissolution process is accelerated due to the presence of ester linkages in the polymer fibers. Under strong acid or alkaline conditions, the degradation of aliphatic polyesters will be accelerated. The acid is the only catalyst for acidic hydrolysis; in the alkaline hydrolysis reaction, the alkali not only accelerates the chemical reaction, but also is incorporated into the product, which may also be the reason why the alkali resistance of the polymer fiber is inferior to the acid resistance. In this test, the sustained release property was better in a weak acid or weak base environment.

FIG. 8 shows that under the condition of ensuring that other conditions are not changed, three coaxial electrospinning fiber membranes are placed in neutral PBS buffer solution at the constant temperature of 50 ℃ to test the slow release performance of the fiber membranes under the high-temperature condition. It can be found that the fertilizer release rate of the fiber membrane is faster than that of the fiber membrane under acid and alkali conditions, and about 90% of the total amount can be achieved within 28 days, which indicates that the temperature rise has a greater influence on the release rate of the fiber membrane. The fiber membrane still maintains good slow release capability and can be released as a slow release fertilizer even when the temperature is increased.

In conclusion, the polylactic acid/polycaprolactone and the urea are spun into the coaxial electrostatic spinning fiber membrane with the shell-core structure through coaxial electrostatic spinning, so that the fertilizer efficiency of the urea is improved, the polylactic acid/polycaprolactone and the urea have certain acid-base resistance and high temperature resistance, and the environment is not polluted.

The key point of the invention is that the electrostatic spinning voltage and distance, and the concentration of the polymer and the urea are well controlled, so that the fiber membrane can be continuous and uniform in the spinning process, the slow release rate can be controlled, and the environment is not damaged in the release process; compared with the common slow release fertilizer, the slow release fertilizer has the advantages that the slow release time is prolonged by about 40 days when the slow release fertilizer is actually applied to the slow release process, the full utilization of the fertilizer can be ensured, and the fertilizer has certain acid-base resistance and high temperature resistance. The slow-release fertilizer is a nano-scale slow-release fertilizer, the shape of the fiber can ensure that the fertilizer cannot deform and damage due to extrusion in the transportation process, and the requirements of different crops on the fertilizer can be met by changing the concentration of the polymer and the concentration of urea. Meanwhile, the polylactic acid and the polycaprolactone belong to natural degradable non-toxic materials, and do not generate toxic substances to the environment in the slow release process, so that the environment is prevented from being polluted.

The above description is only for the purpose of illustrating the preferred embodiments of the present invention and is not to be construed as limiting the invention, and any modifications, equivalents and improvements made within the spirit and principle of the present invention are intended to be included within the scope of the present invention.

Claims

1. The preparation method of the slow release fertilizer is characterized by comprising the following steps of:

step two, using trichloromethane as a solvent to dissolve polylactic acid and polycaprolactone to prepare an electrostatic spinning solution;

2. The preparation method of the slow release fertilizer as claimed in claim 1, wherein the mass ratio of polylactic acid to polycaprolactone in the second step is 4: 6. 5: 5 or 6: 4.

3. the method for preparing a slow release fertilizer according to claim 1, wherein the polylactic acid and the polycaprolactone account for 10% of the electrospinning solution in the second step.

4. The method for preparing a slow release fertilizer according to claim 1, wherein the electrospinning solution in the second step is stirred for 12 hours at room temperature by using a magnetic stirrer, and the stirred electrospinning solution is allowed to stand until air bubbles are discharged.

5. The method of claim 1, wherein the urea comprises 10%, 20%, 30% or 50% of the urea solution.

6. The method for preparing the slow release fertilizer as claimed in claim 1, wherein the urea solution in the third step is stirred for 12 hours at room temperature by using a magnetic stirrer, and the stirred electrostatic spinning solution is kept standing until air bubbles are discharged.

7. The method for preparing a slow release fertilizer according to claim 1, wherein the spinning parameters in the fifth step are set as follows: voltage 20kv, wherein the shell injection speed is 1ml/h, the core speed is 0.1ml/h, the spinning distance is 10cm, the ambient temperature is 20 ℃, and the relative humidity is lower than 50%.

8. A slow release fertilizer prepared by the preparation method as claimed in any one of claims 1 to 7.