WO2025161274A1 - Urea-formaldehyde microcapsule, and preparation method therefor and use thereof - Google Patents

Abstract

A preparation method for a urea-formaldehyde microcapsule, comprising: uniformly mixing an emulsifier, urea, ammonium chloride, resorcinol and deionized water, adjusting the pH value, defoaming, adding dicyclopentadiene to form an emulsion, after stabilization, adding a formaldehyde solution and paraffin oil, covering and heating the emulsion for polymerization, and cooling and washing to obtain a urea-formaldehyde microcapsule.

Description

Urea-formaldehyde microcapsule and its preparation method and application Technical Field

The invention belongs to the field of material science and engineering technology, and particularly relates to a urea-formaldehyde microcapsule and a preparation method and application thereof.

Background Art

Microencapsulation technology is an innovation in materials science and engineering whose primary goal is to endow insulating materials with self-healing capabilities. The core concept involves embedding tiny capsules within the material. These capsules contain liquid healing agents and catalysts that are released upon damage to initiate the repair process. This approach offers the potential to extend the material's lifespan, reduce maintenance costs, and improve reliability.

Existing implementations involve complex capsule preparation techniques and combinations of specific materials to ensure that the microcapsules can release healing agents and effectively repair damage when it occurs. For example, microcapsules containing endo-dicyclopentadiene (endo-DCPD) in a urea-formaldehyde shell have been used. Although these microcapsules have shown good healing ability in monotonic fracture and fatigue, they suffer from wall roughness and nanoparticle aggregation, which may affect the adhesion and stability of the capsule to the matrix.

In addition to DCPD, research has also involved microcapsules using different oil-phase core materials, such as urea-formaldehyde (U/F) and melamine-formaldehyde (M/F) shells. However, these variations may require significant changes to the capsule preparation process to ensure the stability and repair properties of the microcapsules.

Paraffin wax is a material with a high heat of fusion and a low melting temperature. Previous studies have encapsulated paraffin using M/F as a shell. Using an in situ polymerization process, spherical microcapsules with rough surfaces were obtained. Paraffin oil was also encapsulated with gelatin and gum arabic through complex coacervation. It was found that adding a small amount of oppositely charged surfactant to the polyelectrolyte increased the encapsulation efficiency from 40% to 76%. Furthermore, the use of cationic surfactants during the coacervation of gelatin and gum arabic can also increase the yield of paraffin oil microcapsules. Currently, no research has examined the influence of paraffin oil microencapsulation parameters on microcapsule performance.

Summary of the Invention

The purpose of this section is to summarize some aspects of the embodiments of the present invention and briefly introduce some preferred embodiments. Some simplifications or omissions may be made in this section and the abstract and title of the invention to avoid blurring the purpose of this section, the abstract and the title of the invention. Such simplifications or omissions It is not intended to limit the scope of the invention.

In view of the above problems and/or the problems existing in the prior art, the present invention is proposed.

Therefore, the object of the present invention is to overcome the deficiencies in the prior art and provide a method for preparing urea-formaldehyde microcapsules.

In order to solve the above technical problems, the present invention provides the following technical solutions: a method for preparing urea-formaldehyde microcapsules, comprising:

The emulsifier, urea, ammonium chloride, resorcinol, and deionized water were mixed evenly, the pH was adjusted, 1-octanol was added to eliminate bubbles, and then dicyclopentadiene DCPD was added slowly to form an emulsion and stabilized for 20 minutes;

After stabilization, formaldehyde solution and paraffin oil are added, the emulsion is covered and heated, and heating is stopped after continuous stirring for a period of time. The emulsion is cooled to room temperature, filtered, rinsed, dried, and sieved to obtain the urea-formaldehyde microcapsules.

As a preferred embodiment of the preparation method of the present invention, the emulsifier is one of ethylene-maleic anhydride copolymer EMA, polyvinyl alcohol PVA, and polyvinyl alcohol PVOH.

As a preferred embodiment of the preparation method of the present invention, the mass ratio of urea, ammonium chloride and resorcinol is 10:1:1.

As a preferred embodiment of the preparation method of the present invention, the molar ratio of the formaldehyde solution to urea is 1:1.9.

As a preferred embodiment of the preparation method of the present invention, the emulsion is covered and heated, wherein the heating temperature is 60-80°C.

As a preferred embodiment of the preparation method of the present invention, the continuous stirring for a period of time is performed at a stirring rate of 200 r/min and a stirring time of 30 min.

As a preferred embodiment of the preparation method of the present invention, the core material is at least one of paraffin oil organic solvent, fragrance, and medicine.

As a preferred solution of the preparation method of the present invention, the volume fraction of the core material is 19.5%.

Another object of the present invention is to overcome the deficiencies in the prior art and provide a urea-formaldehyde microcapsule.

Another object of the present invention is to overcome the deficiencies in the prior art and provide an application of urea-formaldehyde microcapsules.

Beneficial effects of the present invention:

The present invention provides a urea-formaldehyde microcapsule and its preparation method and application, the preparation process is simple and the raw materials are It is easy to obtain, and the average diameter of the microcapsules can be adjusted by changing the type of emulsifier and the volume percentage of the core material, which can provide controllability for microcapsules of different diameters that may be required in different application scenarios.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for describing the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. Those skilled in the art can also derive other drawings based on these drawings without inventive effort. Among them:

FIG1 is a SEM image of EMA microcapsules of the present invention.

FIG2 is a particle size distribution diagram of the microcapsules of the present invention.

DETAILED DESCRIPTION

In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the specific implementation methods of the present invention are described in detail below in conjunction with the embodiments of the specification.

In the following description, many specific details are set forth to facilitate a full understanding of the present invention. However, the present invention may also be implemented in other ways different from those described herein. Those skilled in the art may make similar generalizations without violating the connotation of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

Secondly, the term "one embodiment" or "embodiment" herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The phrase "in one embodiment" appearing in various places throughout this specification does not necessarily refer to the same embodiment, nor does it refer to a separate or selective embodiment that is mutually exclusive of other embodiments.

Unless otherwise specified, the raw materials in the embodiments of the present invention can be purchased from the market.

Preparation of EMA copolymer (ethylene-methacrylate copolymer) aqueous solution: EMA is added to methanol solvent and stirred until EMA is completely dissolved; a surfactant can be added to the solution to improve the stability of the solution; the solution is added to deionized water and continued to stir until EMA is evenly dispersed in the water to form an aqueous solution.

Example 1

This embodiment provides a method for preparing EMA-based urea-formaldehyde microcapsules containing 19.5% paraffin oil, comprising the following steps:

(1) Preparation of urea-formaldehyde polymer shell:

At room temperature, 200 ml of deionized water and 50 ml of 2.5 wt% EMA copolymer aqueous solution were mixed in 1000 ml The mixture was mixed in a beaker suspended in a temperature-controlled water bath on a programmable hot plate with an external temperature probe. The water bath temperature was set at 60°C and the solution was stirred with a digital mixer.

Under stirring, 6 g of urea, 0.6 g of ammonium chloride, and 0.6 g of resorcinol were dissolved in the solution. The pH was raised from approximately 2.5 to 3.6 by dropwise addition of sodium hydroxide and hydrochloric acid.

Add 3-4 drops of 1-octanol to eliminate surface bubbles, add a slow stream of 50 ml of DCPD to form an emulsion, and allow it to stabilize for 20 minutes;

After stabilization, 15.2 g of 37 wt % formaldehyde solution was added to obtain a 1:1.9 molar ratio of formaldehyde to urea, and 10 g of paraffin oil was added, and the emulsion was covered and heated at a rate of 1.5 °C/min to a target temperature of 60 °C;

After continuous stirring at 200 rpm for 5 h, the stirrer and hot plate were turned off, the mixture was cooled to room temperature, and the microcapsule suspension was separated under vacuum using a coarse glass filter. The microcapsules were rinsed with deionized water and air-dried for 36 h, using a 200-mesh screen to help separate the microcapsules.

(2) Packaging core materials:

During the encapsulation process, urea and formaldehyde react to form hydroxymethylurea, which then condenses upon acidification to form a cross-linked polymer shell that encapsulates the core material.

The core material 19.5% paraffin oil is suspended in a water bath in the form of droplets. At the beginning of the polymerization process, the formed polymer is rich in polar groups, resulting in high hydrophilicity, which gradually decreases during the polymerization process. Finally, a hydrophobic polymer is formed, which is deposited on the emulsified oil droplets (core material) to form spherical capsules.

The chemical structure of the microcapsules was analyzed using Fourier transform infrared spectrophotometer (FTIR) in the range of 400 cm ^-1 to 4000 cm ^-1 , and its spectrum contained peaks at 3330 cm ^-1 (OH and NH stretching), 1620 cm ^-1 (C＝O stretching) and 1540 ^cm-1 (NH bending), indicating the formation of urea-formaldehyde polymers, which closely matched the spectrum of the polymerization reaction between urea and formaldehyde.

The spectrum of the filled microcapsules contained peaks in a single spectrum of both the shell and the core materials, indicating that the core material paraffin oil was successfully encapsulated in the urea-formaldehyde shell.

Example 2

This embodiment provides a method for preparing PVA-based urea-formaldehyde microcapsules containing 19.5% paraffin oil, comprising the following steps:

At room temperature, 200 ml of deionized water and 20 g of PVA were mixed in a 1000 ml beaker. The beaker was suspended in a 60 °C temperature-controlled water bath on a programmable hot plate with an external temperature probe, and the solution was stirred with a digital mixer.

Dissolve 6 g of urea, 0.6 g of ammonium chloride, and 0.6 g of resorcinol in a solvent to form a solution;

The PVA and urea-formaldehyde resin solutions were mixed, 10 g of paraffin oil and 15.2 g of 37 wt% formaldehyde solution were added, and stirred at 250 r/min for 30 min;

Harden in a 60°C water bath for 12 hours to form the outer layer of the microcapsules;

After cooling to room temperature, the microcapsule suspension was separated using a coarse frit filter under vacuum, and the microcapsules were rinsed with deionized water and air-dried for 36 hours using an approximately 200 mesh screen to aid in separation of the microcapsules.

A particle size analyzer and scanning electron microscope (SEM) were used to measure the diameters of the microcapsules containing EMA and PVA emulsifiers in Examples 1 and 2. An SEM image of the EMA microcapsules is shown in Figure 1. SEM observation of the microcapsule morphology reveals a relatively rough outer surface. Compared to the EMA-based microcapsules, the microcapsules prepared with PVA resulted in approximately 30% microcapsule rupture. This difference may be attributed to the thinner outer shell of the PVA-based microcapsules compared to the EMA-based microcapsules.

Figure 2 shows the particle size distribution of EMA- and PVA-based microcapsules containing 19.5% paraffin oil. When EMA was used as the emulsifier, the microcapsule diameters ranged from 155 μm to 553 μm, with an average of 196 μm to 277 μm. This diameter variation is due to turbulence around the agitator blades. The fluid near the blades has higher turbulence and produces smaller microcapsules, while areas farther from the blades produce larger microcapsules. When PVA was used as the emulsifier, the maximum diameter increased to 598 μm, with an average diameter ranging from 241 μm to 292 μm. Particle diameter analysis was consistent with SEM observations. Some small particles ranging from 56 μm to 92 μm were also formed during the manufacturing process. These particles comprised less than 2% of the total mass and were urea-formaldehyde polymers, not paraffin oil.

Example 3

The difference between this embodiment and embodiment 1 is that the volume percentages of paraffin oil in the water bath are 8.5% and 13.5% respectively, and the remaining steps are the same as those in embodiment 1.

Example 4

The difference between this embodiment and embodiment 2 is that the volume percentages of paraffin oil in the water bath are 8.5% and 13.5% respectively, and the remaining steps are the same as those in embodiment 2.

The diameter of the microcapsules was measured using a particle size analyzer and a scanning electron microscope. The surface morphology and shell thickness of the microcapsules were characterized using a scanning electron microscope. The shell thickness was determined by immersing the microcapsules in liquid nitrogen and evaluating the ruptured microcapsules. The results are shown in Table 1.

Table 1

It can be seen that for EMA-based microcapsules, as the volume percentage of the core material in the water bath increases, the average diameter of the final microcapsules increases, while the yield decreases. Changing the emulsifier to PVA has no significant effect on the average diameter of the microcapsules, but the yield is reduced by about 13%. However, compared with PVA-based microcapsules, EMA-based microcapsules have a smaller average diameter. This may be due to the higher viscosity of EMA than PVA (12%). The high viscosity of the emulsifier reduces the fluidity of the dispersed material (core material) and increases its uniformity in the bath solution, which leads to additional dispersion of the oil droplets in the shear field caused by the stirring action. These smaller oil droplets then form smaller microcapsules.

The shell wall thickness of the microcapsules depends on the type of emulsifier and the amount of core material used in the microcapsule manufacturing process. Microcapsules prepared with EMA or PVA as emulsifiers showed that the shell wall thickness decreased as the volume percentage of core material increased in the process. In addition, microcapsules prepared with PVA had thinner shell walls than EMA microcapsules. The higher viscosity of EMA facilitated the deposition of urea-formaldehyde particles on the core material droplets, thereby forming a thicker shell. Therefore, the lower yield of PVA-based microcapsules can be attributed to the rupture of the thinner PVA-based microcapsule shells. In addition, as the volume percentage of core material used in the PVA microcapsule manufacturing process increased, the viscosity of the microcapsules also increased. Large microcapsule diameters and thinner shell walls facilitated the diffusion of core material through the shell wall. This caused the microcapsules to clump together and become difficult to separate.

Example 5

The difference between this embodiment and embodiment 1 is that the stirring rates are 400 rpm, 500 rpm, 800 rpm, and 1200 rpm, respectively, and the remaining steps are the same as those in embodiment 1.

Example 6

The difference between this embodiment and embodiment 3 is that the stirring rates are 400 rpm, 500 rpm, 800 rpm, and 1200 rpm, respectively, and the remaining steps are the same as those in embodiment 3.

The average diameters of the prepared microcapsules are shown in Table 2.

Table 2

It can be seen that changes in stirring rate significantly affect the diameter of the microcapsules. Lower stirring rates produce larger microcapsules, and under the conditions used in this study, increasing the stirring rate can cause the diameter of the microcapsules to change by up to 61%. Microcapsules prepared using EMA at a low stirring rate (500prm) resulted in an average microcapsule diameter of 248μm. As the stirring rate increases, the interfacial area increases and leads to improved uniformity of the reaction medium, resulting in microcapsules with more uniform diameters. In addition, the average diameter of the microcapsules is strongly controlled by the stirring rate and is relatively unaffected by the amount of core material. Changes in the amount of core material (paraffin oil) only result in a 15% change in the diameter of the microcapsules.

Example 7

This example tests the thermal stability of the microcapsules prepared in the example using a thermogravimetric analyzer in a nitrogen environment, with a sample weight of 5 mg and a heating rate of 10°C/min between 20°C and 400°C.

The test results are shown in Table 3.

Table 3

It can be seen that the weight loss of the microcapsules is very small at temperatures between 20°C and 200°C. This small weight loss is attributed to the evaporation of residual water and the elimination of free formaldehyde. The type of emulsifier and the amount of paraffin oil used in the microcapsule manufacturing process have no significant effect on the thermal stability of the microcapsules at temperatures below 200°C.

At elevated temperatures between 200 and 400°C, different emulsifiers and core materials become important. Within this range, the shell wall degrades and the core material evaporates. Weight loss at higher temperatures depends on the hardness of the microcapsule shell. EMA-based microcapsules have higher thermal stability than PVA-based microcapsules. This can be attributed to the higher average molecular weight of EMA and the crosslinking density of EMA-based microcapsules. EMA-based microcapsules undergo extensive decomposition above 400°C. On the other hand, microcapsules prepared with PVA have a decomposition temperature of 372°C.

Example 8

This example tests the mechanical stability of EMA-based microcapsules. A physical property tester measures the force required to deform the microcapsules, characterizing their stiffness. A probe moving vertically at a constant speed is used to place a single layer of microcapsules on a measuring plate. The probe is lowered at a speed of 0.5 μm/s, compressing the microcapsules until they rupture.

The test results are shown in Table 4.

Table 4 Force at 80% deformation/N

The table shows the relationship between microcapsule diameter and applied force at 80% deformation. Microcapsule stiffness increases with decreasing diameter. For both EMA and PVA microcapsules, smaller diameters increase stiffness. EMA microcapsules are stiffer than PVA microcapsules because EMA microcapsules have a higher crosslink density. However, stiffness is not only related to crosslink density but also to shell flexibility and the volume fraction of the core. For example, a 270-μm-diameter PVA microcapsule has a core volume fraction of approximately 13.5%. For a corresponding EMA microcapsule, the core volume fraction is approximately 19.5%. These differences in core volume fractions contribute to the differences in flexibility between microcapsules.

Comparative Example 1

The difference between this comparative example and Example 1 is that the EMA copolymer aqueous solution is replaced by a polyethylene glycol (PEG) aqueous solution, and the remaining steps are the same as in Example 1 to prepare microcapsules.

Comparative Example 2

The difference between this comparative example and Example 1 is that 6 g urea, 0.6 g ammonium chloride and 0.6 g resorcinol are replaced by 6 g urea, 0.6 g ammonium chloride and 0.6 g p-cresol, and the remaining steps are the same as in Example 1 to prepare microcapsules.

Comparative Example 3

The difference between this comparative example and Example 1 is that 6 g of urea, 0.6 g of ammonium chloride and 0.6 g of resorcinol are replaced by 4.8 g of urea, 1.2 g of ammonium chloride and 1.2 g of resorcinol. The remaining steps are the same as in Example 1 to prepare microcapsules.

Comparative Example 4

The difference between this comparative example and Example 1 is that paraffin oil is replaced by polydimethylsiloxane (PDMS), and the remaining steps are the same as those in Example 1 to prepare microcapsules.

Comparative Example 5

The difference between this comparative example and Example 1 is that 19.5% by volume of the core material is replaced with 26.5% by volume, and the remaining steps are the same as those in Example 1 to prepare microcapsules.

The performance of the microcapsules obtained in Comparative Examples 1 to 5 was measured, and the results are shown in Table 5.

Table 5

It can be seen that for Comparative Examples 1, 3, and 5, the microcapsules have smaller shell wall thickness, lower viscosity, and increased average diameter, but at the same time, the yield decreases. This is because the weakened shell wall stability makes the microcapsules more likely to rupture, thereby affecting the yield.

In Comparative Examples 2 and 4, the microcapsule viscosity was low, and the average microcapsule diameter increased. The larger diameter facilitated diffusion of the core material through the shell wall, causing the microcapsules to aggregate and become difficult to separate. The lower viscosity also resulted in lower microcapsule shell wall stability, negatively impacting yield.

Overall, these ratio changes highlight the complex effects of emulsifier type and core material amount on microcapsule properties and yield during microcapsule preparation. During microcapsule preparation, various factors need to be carefully weighed to select the most suitable preparation conditions for a specific application.

It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the spirit and scope of the technical solutions of the present invention, and all of these should be included in the scope of the present invention.

Claims (10)

A method for preparing urea-formaldehyde microcapsules, characterized by comprising: The emulsifier, urea, ammonium chloride, resorcinol, and deionized water were mixed evenly, the pH was adjusted, 1-octanol was added to eliminate bubbles, and then dicyclopentadiene DCPD was added slowly to form an emulsion and stabilized for 20 minutes; After stabilization, formaldehyde solution and paraffin oil are added, the emulsion is covered and heated, and heating is stopped after continuous stirring for a period of time. The emulsion is cooled to room temperature, filtered, rinsed, dried, and sieved to obtain the urea-formaldehyde microcapsules. The preparation method according to claim 1, characterized in that the emulsifier is one of ethylene-maleic anhydride copolymer EMA, polyvinyl alcohol PVA, and polyvinyl alcohol PVOH. The preparation method according to claim 1, characterized in that the mass ratio of urea, ammonium chloride and resorcinol is 10:1:1. The preparation method according to claim 1, characterized in that the molar ratio of the formaldehyde solution to urea is 1:1.9. The preparation method according to claim 4, characterized in that: the emulsion is covered and heated, wherein the heating temperature is 60 to 80°C. The preparation method according to claim 1, characterized in that: the continuous stirring for a period of time, wherein the stirring rate is 200 r/min and the stirring time is 30 min. The preparation method according to claim 1, characterized in that the core material is at least one of paraffin oil organic solvent, fragrance, and medicine. The preparation method according to claim 1, characterized in that the volume fraction of the core material is 19.5%. Urea-formaldehyde microcapsules prepared by the preparation method according to any one of claims 1 to 8. Use of the urea-formaldehyde microcapsules as claimed in claim 9.