Super Absorbent Polymer

A super absorbent polymer, where a TD-NMR analysis result after swelling the super absorbent polymer with D2O satisfies Equation 1: Nt=0.1(%)≤50(%), where Nt=0.1 represents a normalized intensity measured at a signal acquisition time of 0.1 ms.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of Korean Patent Application No. 10-2024-0058659, filed on May 2, 2024, the disclosure of which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present disclosure relates to a super absorbent polymer exhibiting an improved absorption rate and improved absorption performance.

BACKGROUND

A super absorbent polymer (SAP) is a synthetic polymer material that can absorb 500 to 1,000 times its own weight in moisture, and is named differently by each developer, such as super absorbent material (SAM) and absorbent gel material (AGM). The super absorbent polymers described above began to be put to practical use as sanitary products, and are now widely used as soil water retainers for horticulture, water-retaining materials for civil engineering and construction, sheets for raising seedlings, and freshness-preserving agents and materials for steaming in the food distribution industry.

The super absorbent polymer has a crosslinked structure within the polymer that greatly affects its physical properties. The crosslinked structure is formed through chemical crosslinking and physical crosslinking. That is, although super absorbent polymers have a chemical crosslinking component, they also have physical crosslinking, such as chain entanglement, dangling chains, and interpenetrating networks, in which the polymer chains are physically entangled.

Among these, chemical crosslinking can determine the degree of crosslinking reaction of super absorbent polymers, and understanding the degree of chemical crosslinking is necessary for understanding the initial properties of super absorbent polymers.

In particular, since super absorbent polymers are widely used in sanitary products such as diapers and sanitary pads, they need to exhibit not only high absorption performance but also a fast absorption rate. That is, the initial properties of super absorbent polymers are important properties in the relevant field.

In addition, the development of so-called pulpless products, in which the pulp content is reduced or even no pulp is used at all, is being actively pursued in order to provide thinner products.

As a result, the product includes a relatively high proportion of super absorbent polymer, and super absorbent polymer particles are inevitably included in multiple layers within the product. In addition, in order for the super absorbent polymer to absorb a large amount of liquid such as water, saline, or urine, the absorption properties need to be improved so that it has not only high absorption performance but also a fast absorption rate.

To this end, a method is commonly used in which a foaming agent is included in the monomer composition to form a porous structure within the base resin powder as crosslinking polymerization progresses, thereby increasing the surface area of the super absorbent polymer.

However, a problem occurred in which various properties of the super absorbent polymer, such as surface tension, liquid permeability, or bulk density, were reduced depending on the use of the foaming agent. That is, a problem occurred in which it was difficult to maintain the shape in a swollen state. In addition, it had the disadvantage of increasing the amount of fine particles.

Conversely, when the crosslinking density of the super absorbent polymer is controlled to be high in order to improve the liquid permeability of the super absorbent polymer, there is a problem in that the centrifuge retention capacity, which is a basic property of the super absorbent polymer, is reduced because it is difficult to absorb moisture between the dense crosslinking structures.

Accordingly, there is a continuous demand for the development of super absorbent polymers to improve the initial properties of super absorbent polymers and fundamentally solve problems such as liquid permeability.

SUMMARY

The present disclosure is directed to providing a super absorbent polymer (SAP) having excellent chemical crosslinking and an excellent property balance by controlling the normalized intensity of a super absorbent polymer to be within a certain range. In particular, the present disclosure is directed to providing a super absorbent polymer having excellent absorbency under pressure and liquid permeability.

The present disclosure relates to a polyacrylic acid (salt)-based super absorbent polymer,

in Equation 1,

DETAILED DESCRIPTION

Unless otherwise defined herein, all technical and scientific terms used herein are used for the purpose of describing exemplary aspects only and are not intended to be limiting of the disclosure. Singular expressions include plural expressions unless the context clearly dictates otherwise. In this specification, the terms “comprises,” “includes,” or “has” and the like are intended to designate the presence of the features, numbers, steps, components, or combinations thereof that are realized, and are not to be understood as precluding the possibility of the presence or addition of one or more other features or numbers, steps, components or combinations thereof.

While aspects of the disclosure are susceptible to various modifications and forms, specific aspects thereof will be exemplified and described in detail below. However, this is not intended to limit the present disclosure to specific aspects, and it should be understood to include all modifications, equivalents, and substitutes included in the spirit and scope of the present disclosure.

The terminology used herein is for the purpose of describing particular aspects only and is not intended to limit the disclosure. Also, the singular forms used here include the plural forms unless the phrases clearly indicate a contrary meaning.

The term “polymer” or “high molecular weight molecule” as used here refers to a polymerized state of a water-soluble ethylenically unsaturated monomer, and can encompass any moisture content range or particle diameter range.

In addition, the term “super absorbent polymer” is used to mean, depending on the context, a base resin in powder form consisting of a crosslinked polymer or super absorbent polymer particles obtained by pulverizing the crosslinked polymer, or to encompass all products made suitable for commercialization by additional processes, such as drying, pulverizing, classifying, and surface crosslinking, for the crosslinked polymer or the base resin.

In addition, the term “chopping” is used to refer to cutting the hydrogel polymer into small pieces in millimeter units to increase drying efficiency, as opposed to pulverizing to a micrometer or normal particle size.

In addition, the term “micronizing” or “micronization” is used to differentiate it from “chopping”, as it refers to pulverizing a hydrogel polymer into particle sizes of tens to hundreds of micrometers.

In this specification, the element symbols used are those described in the periodic table.

Hereinafter, a super absorbent polymer and a method of preparing the same according to specific aspects of the disclosure will be described in more detail.

A super absorbent polymer of the present disclosure is a polyacrylic acid (salt)-based super absorbent polymer, and is characterized in that the TD-NMR analysis result after swelling the super absorbent polymer with D2O satisfies Equation 1.

Nt=0.1 in Equation 1 represents a normalized intensity measured at a signal acquisition time of 0.1 ms. Here, the term “normalized intensity” refers to the result obtained when the peak intensity is normalized by the method described below after performing TD-NMR analysis on the super absorbent polymer under specific conditions.

Specifically, the method for deriving the normalized intensity is as follows.

Step 1) Drying (Pretreatment) of Super Absorbent Polymer

The super absorbent polymer is dried at about 100° C. for about 12 hours.

Step 2) Swelling (Sampling) of Dried Super Absorbent Polymer

The analysis results derived from Step 3 refer to the peak intensity that appears as a result of observing free induction decay (FID), which appears as the decay of the 1H signal through TD-NMR analysis. The intensity is observed sequentially over time. At this time, the time at which the intensity is observed corresponds to the signal acquisition time.

The remaining intensities are normalized based on the third intensity (I3). Specifically, the relative sizes of the other intensities are calculated when the third intensity (I3) is set as 100.

For example, the nth intensity is normalized by calculating it using the following Equation A.

in Equation A,

In the process, the pretreatment of drying the super absorbent polymer at about 100° C. for about 12 hours is performed to measure the mobility of the super absorbent polymer without being affected by the moisture content. In TD-NMR analysis, the water component included in the super absorbent polymer is a material that greatly affects the result value. For example, the T1 relaxation time, which is one of the results derived from TD-NMR measurements, is affected by surrounding materials, and water is known to be a material that also takes a long time for T2 relaxation. Therefore, the pretreatment process is performed to understand the inherent characteristics of the super absorbent polymer.

The expression “making the super absorbent polymer have a centrifuge retention capacity (CRC) of 100%” means leaving the super absorbent polymer to swell as much as possible for a long period of time so that the particles absorb all the water.

D2O refers to heavy water (deuterium oxide, 99 atom %), and the reason for using D2O instead of H2O in the process of swelling super absorbent polymer is to evaluate the physical properties of only the super absorbent polymer. As described above, when using H2O, the results derived from the TD-NMR measurement are results that include H2O in addition to the super absorbent polymer. On the other hand, when using D2O, the properties of only the super absorbent polymer may be evaluated while excluding the 1H signal of H2O.

In addition, the physical crosslinking of the super absorbent polymer may be removed by swelling the super absorbent polymer with D2O of 0.9 wt % NaCl.

Specifically, when the super absorbent polymer is exposed to the solvent D2O, the solvent molecules penetrate between the chain gaps of the super absorbent polymer, causing the polymer volume to increase and the polymer to swell, resulting in a swelling phenomenon, which causes the polymers to untwist and thus the physical crosslinking is removed.

Therefore, when the super absorbent polymer is swelled with D2O, the physical crosslinking of the super absorbent polymer may be effectively removed, and the characteristics of only the chemical crosslinking of the super absorbent polymer may be accurately confirmed through TD-NMR analysis.

A super absorbent polymer (SAP) is prepared by crosslinking a polymer backbone consisting of carbon atoms to form a network structure and attaching water-loving ionic molecules to the network structure, or by polymerizing the ionic molecules themselves into the polymer backbone to form crosslinks. This process may be defined as chemical crosslinking.

In addition, super absorbent polymers may have physical crosslinking, such as chain entanglement and dangling chains, in addition to chemical crosslinking. The physical crosslinking also affects the network structure of the super absorbent polymer. That is, chemical crosslinking and physical crosslinking affect the absorption performance and liquid permeability, such as the centrifuge retention capacity and absorbency under pressure of the super absorbent polymer.

At this time, a problem of a decrease in absorption performance may occur in the process of increasing the liquid permeability of the super absorbent polymer. That is, liquid permeability and absorption performance are in a trade-off relationship, and thus optimization of liquid permeability and absorption performance is necessary to provide an excellent super absorbent polymer.

However, although the absorption performance may be improved by controlling the physical crosslinking, the liquid permeability is greatly affected by chemical crosslinking such as the rigidity of the molecules within the super absorbent polymer.

Therefore, in order to optimize the liquid permeability and absorption performance, which are in a trade-off relationship, it is important to control physical crosslinking and chemical crosslinking, and particularly, it is important to control chemical crosslinking within a certain range.

The super absorbent polymer according to the present disclosure is characterized in that the normalized intensity measured at a signal acquisition time of 0.1 ms satisfies 50% or less among the normalized intensities.

In addition, the super absorbent polymer according to the present disclosure satisfies the normalized intensities measured at a signal acquisition time of 0.1 ms among the normalized intensities of 45% or more, 46% or more, 47% or more, 48% or more, and 49% or more. That is, the TD-NMR analysis result after swelling the super absorbent polymer with D2O can satisfy Nt=0.1(%) of 45% or more, 46% or more, 47% or more, 48% or more, or 49% or more.

That is, the super absorbent polymer according to the present disclosure may satisfy the following Equation 1-1.

in Equation 1-1,

That is, in one aspect of the present disclosure, the Nt=0.1(%) can satisfy 45% to 50%, 46% to 50%, 47% to 50%, 48% to 50%, or 49% to 50(%).

Nt=0.1 in Equation 1 means the normalized intensity measured at a signal acquisition time of 0.1 ms among a number of normalized intensities.

The normalized intensity at a signal acquisition time of 0.1 ms in the present disclosure is measured is because that time period is the point where the peak intensity of the FID reaches a plateau. That is, the signal acquisition time of 0.1 ms in Equation 1 has technical significance in that it is a time period in which the characteristics of only the super absorbent polymer may be identified with various variables occurring during analysis eliminated.

T2 relaxation, which may be confirmed during TD-NMR analysis, refers to the phenomenon in which spins within atoms dephase when radio frequency is applied to a molecule and the spins within the atoms are transversely magnetized and in phase, and then the radio frequency is cut off. At this time, the signal measured when the radio frequency is cut off is called FID, and the size of the measured FID decreases over time. The magnitude of transverse magnetization may be determined using the measured FID signal. At this time, here, the size of the transverse magnetization magnitude means the intensity. The T2 relaxation time is the time taken for the magnitude of the transverse magnetization to decrease to 37% of the maximum transverse magnetization. Since the T2 relaxation time is not affected by the strength of the external magnetic field, it may be used to determine the inherent properties of the material.

The T2 relaxation time is a numerical value that may be used to determine how dense a molecule's structure is, and may be measured using a time domain (TD) NMR device. The more structurally dense and rigid the molecule, the smaller (faster) the T2 relaxation time value is measured, and the less structurally dense and flexible the molecule, the larger (slower) the T2 relaxation time value is measured. This allows the relative rigidity of the molecule to be determined.

In addition, a super absorbent polymer having excellent liquid permeability and excellent absorption performance may be provided when the normalized intensity satisfies the range of Equation 1.

To satisfy this, the manufacturing process conditions of the super absorbent polymer were adjusted. For example, the super absorbent polymer of the present disclosure satisfies Equation 1 by controlling process conditions, such as controlling the type or content of additives in the surface crosslinking process or controlling polymerization and pulverization process conditions.

That is, the size of the peak intensity through TD-NMR analysis after swelling with D2O was derived as a factor that can determine the degree of chemical crosslinking of a super absorbent polymer excluding physical crosslinking.

When the super absorbent polymer is in a powder state, in the case of the super absorbent polymer, chain entanglement, which is physical crosslinking, and crosslinking by a crosslinking agent, which is chemical crosslinking, affect the rigidity of the molecules, whereas when swelling occurs with D2O, the physical crosslinking that was entangled is released, and chemical crosslinking mostly affects the rigidity of the molecules. That is, the super absorbent polymer swells, and as a result, the effect of chain entanglement is reduced, so that only the effect of chemical crosslinking is present.

Therefore, when the TD-NMR analysis results measured as described above are confirmed, the degree of chemical crosslinking excluding physical crosslinking of the super absorbent polymer may be indirectly predicted.

In order to obtain a super absorbent polymer having excellent absorption performance and liquid permeability, chemical crosslinking needs to be achieved at an appropriate level, and in particular, when the ‘normalized intensity obtained by normalizing the TD-NMR analysis results after swelling with D2O’, which can confirm the degree of chemical crosslinking, is within a specific range, it was confirmed that a super absorbent polymer having excellent absorption performance and excellent liquid permeability may be provided.

In one aspect of the present disclosure, the super absorbent polymer may have an average convexity value of 0.94 or less for all particles, calculated using the following Equation 2.

in Equation 2,

The convexity is a parameter for measuring the particle outline and surface roughness of the particle with values from 0 to 1, and the closer the convexity is to 1, the more the particle may be considered to have a very smooth outline, and the closer the convexity is to 0, the more the particle may be considered to have a rough or uneven outline.

At this time, the average values of the convexity are measured after particles are randomly scattered over the stage by vacuum within the measuring device, and statistical results are derived by obtaining n of 200 or more and averaging them.

In one aspect of the present disclosure, the average value of the convexity of all particles of the super absorbent polymer may be 0.80 or more, 0.83 or more, 0.85 or more, 0.87 or more, and 0.94 or less, 0.93 or less, or 0.92 or less.

In one aspect of the present disclosure, the super absorbent polymer may have an average circle equivalent (CE) diameter of 220 μm to 400 μm.

The CE diameter refers to the diameter of a circle having the same area as a 2D image of the 3D particle, and the size of the particle may be expressed through the CE diameter. The average value of the CE diameter of the super absorbent polymer may be 220 μm or more, 230 μm or more, or 240 μm or more, and 400 μm or less, 350 μm or less, 330 μm or less, 320 μm or less, or 310 μm or less.

In addition, convexity and CE diameter may be measured using several commercial instruments that quantify and analyze particle morphology based on image analysis of the particles. For example, the parameters may be measured by Morphologi 4 from Malvern Panalytical, and specifically, may be measured by the following four steps, which are described in more detail in the experimental examples described below.

At this time, the particle diameter of the super absorbent polymer particles may be measured according to the European Disposables and Nonwovens Association (EDANA) standard EDANA WSP 220.3 method.

In one aspect of the present disclosure, the super absorbent polymer may have a gel strength of 0.7 N or more, 0.8 N or more, or 0.9 N or more after swelling the super absorbent polymer with 50 ml of saline containing 0.005% ASC. In addition, the gel strength may be 1.5 N or less, 1.4 N or less, 1.3 N or less, 1.2 N or less, 1.1 N or less, or 1.0 N or less. By having such high gel strength, it is possible to reduce surface crosslinking damage and prevent deterioration of physical properties. In addition, even when the super absorbent polymer absorbs water and its volume increases, it may maintain its shape well, and as a result, exhibit improved absorbency and liquid permeability. The method for measuring the gel strength of the super absorbent polymer will be described in more detail in the experimental examples section described below.

In one aspect of the present disclosure, the super absorbent polymer may have an extractable component content of 10 wt % or less, 9.5 wt % or less, or 9 wt % or less, based on the total weight of the super absorbent polymer as measured after being freely swollen in water having an electrical conductivity of 100 to 130 μS/cm for 30 minutes.

In addition, in one aspect of the present disclosure, the super absorbent polymer may have an extractable component content of 5 wt % or less, or 4.8 wt % or less, or 4.5 wt % or less, or 4.3 wt % or less, or 4 wt % or less, or 3.9 wt % or less, as measured after swelling for 1 hour according to the EDANA method WSP 270.3. The lower the value of the content of the extractable component, the better it is, and the lower limit is theoretically 0 wt %, but may be, for example, 0.1 wt % or more or 1 wt % or more.

In addition, in one aspect of the present disclosure, the super absorbent polymer may have an extractable component content of 17 wt % or less, 16 wt % or less, or 15 wt % or less, based on the total weight of the super absorbent polymer as measured after being freely swollen in water having an electrical conductivity of 100 to 130 μS/cm for 3 hours.

The term “extractable component” refers to a compound in the form of a polymer that is not crosslinked during the preparation process of a super absorbent polymer, and the components may be produced when crosslinking is incomplete during polymerization of the super absorbent polymer, resulting in non-crosslinking, or when the crosslinking agent is decomposed or the main polymer chain is broken during the chopping or drying process.

The extractable components may be eluted when the super absorbent polymer is exposed to a liquid, and most of the eluted extractable components remain on the surface of the super absorbent polymer. Due to this, the surface of the super absorbent polymer may become sticky and the liquid permeability may be reduced. This may cause discomfort when the super absorbent polymer is used in actual products.

That is, problems related to crosslinking of the super absorbent polymer may be identified by measuring the content of extractable components eluted from the super absorbent polymer solution. That is, since the content of extractable components is closely related to the inter-chain crosslinking structure within the super absorbent polymer, a large content of eluted extractable components means that the inter-chain crosslinking structure within the super absorbent polymer is incomplete.

Super absorbent polymers are widely used in sanitary products such as diapers, so the elution amount of eluted extractable components is evaluated using 0.9% saline, which has similar ion concentration and electrical conductivity to urine discharged from the body.

However, in addition to sanitary materials, the super absorbent polymers are now widely used as soil water retainers for horticulture, water-retaining materials for civil engineering and construction, sheets for raising seedlings, and freshness-preserving agents and materials for steaming in the food distribution industry. In this case, the absorption behavior of the extractable components in water having an electrical conductivity of 100 to 130 μS/cm needs be excellent.

That is, even when the same super absorbent polymer is used, the absorption behavior in water having an electrical conductivity of 100 to 130 μS/cm and the absorption behavior in 0.9% saline with an electrical conductivity of about 16,100 μS/cm are bound to be different.

The content of extractable components is closely related to the inter-chain crosslinking structure within the super absorbent polymer, where when using 0.9% saline, the volume of expansion of the super absorbent polymer is small, so the elution amount of eluted extractable components is small, but when using water with an electrical conductivity of 100 to 130 μS/cm, the super absorbent polymer expands more, so the elution amount of eluted extractable components increases due to the separation of the chains within the super absorbent polymer, and through this, the correlation between the inter-chain crosslinking structure within the super absorbent polymer and the absorption behavior can be more accurately identified.

For example, even when two different super absorbent polymers have the same content of extractable components in 0.9% saline, the content of extractable components in water having an electrical conductivity of 100 to 130 μS/cm may vary significantly depending on the crosslinking characteristics, because the degree of crosslinking within the super absorbent polymer affects the content of extractable components.

For this reason, the experimental results on the elution amount and absorption characteristics of extractable components after free swelling using 0.9% saline having an electrical conductivity of about 16,100 μS/cm cannot be directly compared with the experimental results after free swelling using water having an electrical conductivity of 100 to 130 μS/cm as in the present disclosure.

That is, when the same super absorbent polymer is used, the absorption behavior in water having an electrical conductivity of 100 to 130 μS/cm and the absorption behavior in 0.9% saline having an electrical conductivity of about 16,100 μS/cm are bound to be different, and thus the content of extractable components after free swelling for 1 hour in water having an electrical conductivity of 100 to 130 μS/cm cannot be used to predict the content of extractable components after free swelling in 0.9% saline having an electrical conductivity of about 16,100 μS/cm, and vice versa.

Therefore, in order to implement a super absorbent polymer having an excellent property balance by simultaneously improving absorption characteristics and liquid permeability, it may be said that determining the elution amount, absorption performance and absorption rate of extractable components in water having an electrical conductivity of 100 to 130 μS/cm is independent of using 0.9% saline having an electrical conductivity of about 16,100 μS/cm.

The method for measuring the content (wt %) of extractable components in water having an electrical conductivity value of 100 to 130 μS/cm will be described in more detail in the experimental examples section described below.

In one aspect of the present disclosure, the super absorbent polymer has a centrifuge retention capacity (CRC) of 33 g/g or more, 34 g/g or more, or 35 g/g or more, as measured according to the EDANA method WSP 241.3. In addition, the centrifuge retention capacity measured by the same method may be 50 g/g or less, 45 g/g or less, or 40 g/g or less.

In addition, in one aspect of the present disclosure, the super absorbent polymer has an absorbency under pressure (AUP) of 28 g/g or more, 29 g/g or more, or 30 g/g or more, as measured under 0.3 psi according to EDANA method WSP 242.3. In addition, the absorbency under pressure measured by the same method may be 45 g/g or less, 42 g/g or less, or 40 g/g or less.

In addition, in one aspect of the present disclosure, the super absorbent polymer may have an effective absorption capacity (EFFC) calculated by Equation 3 below of 30 g/g or more, or 31 g/g or more, or 32 g/g or more, or 33 g/g or more. In addition, the effective absorption capacity (EFFC) calculated by Equation 3 may be 40 g/g or less, 39 g/g or less, 38 g/g or less, 37 g/g or less, or 36 g/g or less.

in Equation 3,

In one aspect of the present disclosure, the super absorbent polymer may have an absorption rate (vortex time) of 40 seconds or less as measured by a vortex measurement method at 24.0° C.

More specifically, the absorption rate (vortex time) may be 40 seconds or less, 35 seconds or less, 33 seconds or less, or 30 seconds or less. In addition, the smaller the value of the absorption rate, the better it is, and the lower limit of the absorption rate is theoretically 0 seconds, but may be, for example, 10 seconds or more, 15 seconds or more, or 20 seconds or more.

The method for measuring the centrifuge retention capacity, absorbency under pressure, and absorption rate of the super absorbent polymer will be described in more detail in the experimental examples described below.

In addition, when the super absorbent polymer is swelled in water having an electrical conductivity value of 100 to 130 μS/cm for 1 minute, a maximum capacity of water that the super absorbent polymer may hold (free swelling capacity) may be 170 g or more, 175 g or more, 180 g or more, or 185 g or more, and 230 g or less, 225 g or less, or 220 g or less. This is a numerical value showing the absorption capacity of the super absorbent polymer.

As described above, even when the same super absorbent polymer is used, the absorption behavior in water having an electrical conductivity of 100 to 130 μS/cm and the absorption behavior in 0.9% saline with an electrical conductivity of about 16,100 μS/cm are bound to be different.

That is, not only the content of extractable components, but also the absorption capacity, such as the maximum capacity of water, has an independent meaning when using water having an electrical conductivity of 100 to 130 μS/cm and when using 0.9% saline having an electrical conductivity of about 16,100 μS/cm.

The maximum capacity of water that a super absorbent polymer can hold was determined by using water having an electrical conductivity of 100 to 130 μS/cm at 24° C., which has a lower ion concentration than 0.9% saline and about 1/100 of the electrical conductivity of about 16,100 μS/cm at 24° C. of 0.9% saline, and in one embodiment, water having an electrical conductivity of 110 μS/cm at 24° C. was used. For water having an electrical conductivity in the range of 100 to 130 μS/cm, there is no significant difference in absorption characteristics according to electrical conductivity.

Accordingly, the present inventors attempted to develop a super absorbent polymer having an excellent absorption rate and excellent absorption capacity for water having an electrical conductivity of 100 to 130 μS/cm, which has a lower ion concentration and electrical conductivity than 0.9% saline, that is, has about 1/100 of an electrical conductivity of 0.9% saline, and this was implemented by manufacturing the super absorbent polymer so as to satisfy the normalized intensity.

The method of measuring absorption capacity in water having an electrical conductivity value of 100 to 130 μS/cm will be described in more detail in the experimental examples section described below.

In one aspect of the present disclosure, the super absorbent polymer may have a liquid permeability of 65 ml or more, calculated by the following Equation 4. In some examples, the liquid permeability may be 70 ml or more, 75 ml or more, or 80 ml or more.

in Equation 4,

Meanwhile, the term “liquid permeability” refers to the mobility of the solution in the super absorbent polymer, and a higher value means better permeability. That is, there is no theoretical upper limit to the liquid permeability, but it may be, for example, 400 ml or less, 240 ml or less, or 180 ml or less.

That is, the super absorbent polymer according to the present disclosure may have a T1 of 18 seconds or less. In some examples, T1 may be 17 seconds or less, 16 seconds or less, or 15 seconds or less. The smaller the value, the better it is. The theoretical lower limit of T1 is 0 seconds. For example, it may be 3 seconds or more, 5 seconds or more, or 10 seconds or more.

Meanwhile, the super absorbent polymer according to the present disclosure may be implemented by appropriately controlling preparation process conditions such as the components/content of the super absorbent polymer, polymerization process conditions of the super absorbent polymer, or pulverizing process conditions. That is, by controlling these process conditions, it is possible to prepare a super absorbent polymer that satisfies the normalized intensity.

For example, the super absorbent polymer may be controlled to satisfy the normalized intensity by adjusting the type and content of the monomer composition, and the type and content of an internal crosslinking agent in a polymerization process; and the type, input amount, and input timing of a surfactant, the type, input amount, and input timing of a neutralizing agent, the type, rotation speed, and hole size of a micronizing device, the number of times of micronization, and the like in the neutralization and micronization steps.

Hereinafter, each component that makes up the super absorbent polymer will be described in more detail.

A polyacrylic acid (salt)-based super absorbent polymer according to one aspect of the disclosure includes a base resin including a crosslinked polymer of a water-soluble ethylenically unsaturated monomer having an acidic group and an internal crosslinking agent. The crosslinked polymer may be formed by polymerizing a monomer composition including components such as a monomer, an internal crosslinking agent, and a polymerization initiator.

Here, the water-soluble ethylenically unsaturated monomer may be any monomer commonly used in the preparation of a super absorbent polymer. As a non-limiting example, the water-soluble ethylenically unsaturated monomer may be a compound represented by the following Chemical Formula 1:

in Chemical Formula 1,

The monomer may be one or more selected from the group consisting of a (meth)acrylic acid, and a monovalent (alkali) metal salt, a divalent metal salt, an ammonium salt, and an organic amine salt of this acid.

The water-soluble ethylenically unsaturated monomer has an acidic group. Meanwhile, in the preparation of a super absorbent polymer, a polymer is formed by crosslinking and polymerizing a monomer in which some of the acidic groups are neutralized by a neutralizing agent, but in the present disclosure, the acidic groups are not neutralized during polymerization, but may be neutralized after the polymer is formed. More specific details on this will be described in the section on the preparation method for the super absorbent polymer.

The concentration of the water-soluble ethylenically unsaturated monomer in the monomer composition may be appropriately adjusted in consideration of the polymerization time, reaction conditions, and the like, and may be about 20 to about 60 wt % or about 20 to about 40 wt %.

The term “internal crosslinking agent” used in this specification is a term used to distinguish it from a surface crosslinking agent for crosslinking the surface of super absorbent polymer particles described below, and it serves to form a polymer including a crosslinked structure by introducing crosslinking bonds between the unsaturated bonds of the water-soluble ethylenically unsaturated monomers described above.

The crosslinking in the above step is performed without distinction between the surface and the interior, but when the surface crosslinking process of the super absorbent polymer particles described below is performed, the surface of the finally prepared super absorbent polymer particles may include a structure newly crosslinked by the surface crosslinking agent, and the interior of the super absorbent polymer particles may maintain the structure crosslinked by the internal crosslinking agent.

According to one aspect of the present disclosure, the internal crosslinking agent may include one or more of a polyfunctional acrylate compound, a polyfunctional allylic compound, or a polyfunctional vinyl compound.

The polyfunctional allylic compound or polyfunctional vinyl compound may form a crosslinked structure during the polymerization process by allowing two or more unsaturated groups included in the molecule to bond with unsaturated bonds of water-soluble ethylenically unsaturated monomers or unsaturated bonds of other internal crosslinking agents, and unlike acrylate compounds including an ester bond (—(C═O)O—) in the molecule, the crosslinked bond may be more stably maintained even during the neutralization process after the polymerization reaction described below.

Accordingly, the gel strength of the prepared super absorbent polymer may be increased, process stability may be improved during the discharge process after polymerization, and the amount of extractable components may be minimized.

As described above, the extractable components are mainly eluted when the super absorbent polymer absorbs liquid and swells, and a large content of the eluted extractable components may be said to mean that the crosslinking characteristics within the super absorbent polymer are not excellent. In addition, since most of the eluted extractable components remain on the surface of the super absorbent polymer, when applied to actual products, the liquid permeability may decrease, causing discomfort.

Therefore, as described above, it is necessary to minimize the amount of extractable components of the super absorbent polymer.

Crosslinking polymerization of the water-soluble ethylenically unsaturated monomer in the presence of such an internal crosslinking agent may be performed in the presence of a polymerization initiator and, as needed, a thickener, a plasticizer, a preservation stabilizer, an antioxidant, and the like.

In the monomer composition, such an internal crosslinking agent may be used in an amount of 0.01 to 5 parts by weight based on 100 parts by weight of the water-soluble ethylenically unsaturated monomer. For example, the internal crosslinking agent may be used in an amount of 0.01 parts by weight or more, 0.05 parts by weight or more, or 0.1 parts by weight or more, and 5 parts by weight or less, 3 parts by weight or less, or 2 parts by weight or less, 1 part by weight or less, or 0.7 parts by weight or less, based on 100 parts by weight of the water-soluble ethylenically unsaturated monomer. When the content of the internal crosslinking agent is too low, crosslinking may not occur sufficiently, making it difficult to achieve strength above an appropriate level, and when the content of the internal crosslinking agent is too high, the internal crosslinking density may increase, making it difficult to achieve the desired centrifuge retention capacity.

Meanwhile, when the content of the internal crosslinking agent is small to ensure that the base resin has high centrifuge retention capacity (CRC), the gel strength of the formed polymer may be low, and the operation of a chopper, and the like may be difficult when cutting the hydrogel polymer due to the low gel strength. In this case, by mixing and using two or more internal crosslinking agents for operation of a high-speed rotary chopper, and the like, the gel strength may be increased, thereby improving the operating stability of the chopper, and the like.

The formed hydrogel polymer may change the shape of the particles depending on the degree of internal crosslinking, and the polymer formed using such an internal crosslinking agent can have a three-dimensional network structure in which main chains formed by polymerizing the water-soluble ethylenically unsaturated monomer are crosslinked by the internal crosslinking agent.

As such, when the polymer has a three-dimensional network structure, the overall physical properties of the super absorbent polymer, such as centrifuge retention capacity and absorbency under pressure, may be remarkably improved compared to when the polymer has a two-dimensional linear structure that is not further crosslinked by an internal crosslinking agent.

The polymer is formed by polymerizing a monomer and an internal crosslinking agent in the presence of a polymerization initiator, and the type of polymerization initiator is not particularly limited, but in some examples, the polymerization may be performed using a thermal polymerization method in a batch-type reactor, and thus a thermal polymerization initiator may be used as the polymerization initiator.

As the thermal polymerization initiator, one or more selected from the group of initiators consisting of a persulfate-based initiator, an azo-based initiator, hydrogen peroxide, and ascorbic acid may be used. Specifically, examples of persulfate-based initiators include sodium persulfate (Na2S2O8), potassium persulfate (K2S2O8), and ammonium persulfate ((NH4)2S2O8), and examples of azo-based initiators include 2,2-azobis(2-amidinopropane) dihydrochloride, 2,2-azobis-(N,N-dimethylene)isobutyramidine dihydrochloride, 2-(carbamoylazo)isobutylonitrile, 2,2-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride, and 4,4-azobis-(4-cyanovaleric acid). More diverse thermal polymerization initiators are provided in Odian's book, ‘Principle of Polymerization (Wiley, 1981), p203, and are not limited to the examples described above.

Such polymerization initiators may be used in an amount of 2 parts by weight or less based on 100 parts by weight of the water-soluble ethylenically unsaturated monomer. That is, when the concentration of the polymerization initiator is too low, the polymerization rate may be slow and a large amount of residual monomer may be extracted in the final product, which is not desirable. Conversely, when the concentration of the polymerization initiator is higher than the above range, the polymer chains forming the network become shorter, which is undesirable because the properties of the resin may deteriorate, such as increasing the content of extractable components and decreasing the absorbency under pressure.

Meanwhile, in one aspect of the present disclosure, polymerization may be initiated by inputting a reducing agent forming a redox couple with the polymerization initiator to the monomer composition.

Specifically, the initiator and reducing agent react with each other to form radicals when input to a polymer solution.

The formed radicals react with the monomer, and since the oxidation-reduction reaction between the initiator and the reducing agent is highly reactive, polymerization is initiated even when only a small amount of the initiator and reducing agent is input, so there is no need to increase the process temperature, enabling low-temperature polymerization and minimizing changes in the physical properties of the polymer solution.

The polymerization reaction using the oxidation-reduction reaction can occur smoothly even at a temperature near room temperature (25° C.) or lower. For example, the polymerization reaction may be performed at a temperature of 5° C. or higher and 25° C. or lower, or 5° C. or higher and 20° C. or lower.

In one aspect of the present disclosure, when a persulfate-based initiator is used as the initiator, the reducing agent may be one or more selected from the group consisting of sodium metabisulfite (Na2S2O5); tetramethyl ethylenediamine (TMEDA); a mixture of iron (II) sulfate and EDTA (FeSO4/EDTA); sodium formaldehyde sulfoxylate; and disodium 2-hydroxy-2-sulfinoacetate.

For example, potassium persulfate may be used as the initiator and disodium 2-hydroxy-2-sulfinoacetate may be used as the reducing agent; ammonium persulfate may be used as the initiator and tetramethylethylenediamine may be used as the reducing agent; or sodium persulfate may be used as the initiator and sodium formaldehyde sulfoxylate may be used as the reducing agent.

In another aspect of the present disclosure, when a hydrogen peroxide-based initiator is used as the initiator, the reducing agent may be one or more selected from the group consisting of ascorbic acid; sucrose; sodium sulfite (Na2SO3); sodium metabisulfite (Na2S2O5); tetramethyl ethylenediamine (TMEDA); a mixture of iron (II) sulfate and EDTA (FeSO4/EDTA); sodium formaldehyde sulfoxylate; disodium 2-hydroxy-2-sulfinoacteate; and disodium 2-hydroxy-2-sulfoacteate.

The monomer composition may additionally include additives such as a thickener, a plasticizer, a preservation stabilizer, and an antioxidant, as needed.

In addition, the monomer composition including the monomer may be, for example, in a solution state dissolved in a solvent such as water, and the solid content in the monomer composition in the solution state, i.e., the concentration of a monomer, an internal crosslinking agent and a polymerization initiator, may be appropriately controlled in consideration of the polymerization time and reaction conditions. For example, the solid content in the monomer composition may be 10 to 80 wt %, 15 to 60 wt %, or 30 to 50 wt %.

The solvent that may be used at this time is not limited in its composition as long as it can dissolve the components, and for example, one or more selected from water, ethanol, ethylene glycol, diethylene glycol, triethylene glycol, 1,4-butanediol, propylene glycol, ethylene glycol monobutyl ether, propylene glycol monomethyl ether, propylene glycol monomethyl ether acetate, methyl ethyl ketone, acetone, methyl amyl ketone, cyclohexanone, cyclopentanone, diethylene glycol monomethyl ether, diethylene glycol ethyl ether, toluene, xylene, butyrolactone, carbitol, methyl cellosolve acetate, or N,N-dimethylacetamide may be used in combination.

The polymer obtained in this way may form a polymer having a high molecular weight and a uniform molecular weight distribution by polymerizing using an ethylenically unsaturated monomer in an unneutralized state.

In addition, the polymer may have a moisture content of 30 to 80 wt %. For example, the moisture content of the polymer may be 30 wt % or more, 45 wt % or more, or 50 wt % or more, and 80 wt % or less, 70 wt % or less, or 60 wt % or less.

When the moisture content of the polymer is too low, it may be difficult to secure an appropriate surface area in the subsequent pulverizing step, and thus the polymer may not be pulverized effectively, and when the moisture content of the polymer is too high, the pressure applied in the subsequent pulverization step may increase, making it difficult to pulverize to the desired particle size.

Meanwhile, throughout this specification, the term “moisture content” means the content of moisture relative to the total polymer weight, which is the value obtained by subtracting the weight of the polymer in a dry state from the weight of the polymer. Specifically, it is defined as a value calculated by measuring the weight reduction due to moisture evaporation in the polymer during the drying process by raising the temperature of the polymer in a crumb state through infrared heating. At this time, the drying conditions are such that the temperature is increased from room temperature to about 180° C. and then maintained at 180° C., and the total drying time is set to 40 minutes including a 5-minute temperature increase step, and the moisture content is measured.

The super absorbent polymer according to one aspect of the disclosure includes a base resin powder including a crosslinked polymer of a water-soluble ethylenically unsaturated monomer having an acidic group and an internal crosslinking agent; and a surface crosslinked layer formed on the base resin powder by further crosslinking the crosslinked polymer via a surface crosslinking agent.

The surface crosslinked layer is formed on at least a portion of the surface of the base resin powder, and may be formed by additional crosslinking of the crosslinking polymer included in the base resin powder via a surface crosslinking agent.

As the surface crosslinking agent, any surface crosslinking agent that has been used in the preparation of super absorbent polymers may be used without particular limitation. For example, the surface crosslinking agent may include one or more polyols selected from the group consisting of ethylene glycol, propylene glycol, 1,3-propanediol, 1,4-butanediol, 1,6-hexanediol, 1,2-hexanediol, 1,3-hexanediol, 2-methyl-1,3-propanediol, 2,5-hexanediol, 2-methyl-1,3-pentanediol, 2-methyl-2,4-pentanediol, tripropylene glycol, and glycerol; one or more carbonate compounds selected from the group consisting of ethylene carbonate, propylene carbonate, and glycerol carbonate; an epoxy compound such as ethylene glycol diglycidyl ether; an oxazoline compound such as oxazolidinone; a polyamine compound; an oxazoline compound; a mono-, di- or polyoxazolidinone compound; or a cyclic urea compound.

Specifically, one or more, two or more, or three or more of the surface crosslinking agents described above may be used as the surface crosslinking agent, for example, two of propylene glycol and ethylene glycol diglycidyl ether may be used, or ethylene carbonate-propylene carbonate (ECPC), propylene glycol, and/or glycerol carbonate may be used.

Such surface crosslinking agents may be used in an amount of 0.001 to 0.5 parts by weight based on 100 parts by weight of the super absorbent polymer particles. At this time, 100 parts by weight of the super absorbent polymer particles are based on a dried state. In addition, the content refers to the total amount of the surface crosslinking agent used.

For example, the surface crosslinking agent may be used in an amount of 0.005 parts by weight or more, 0.01 parts by weight or more, or 0.05 parts by weight or more, based on 100 parts by weight of the super absorbent polymer particles. In addition, the surface crosslinking agent may be used in an amount of 0.5 parts by weight or less, 0.4 parts by weight or less, or 0.2 parts by weight or less, based on 100 parts by weight of the super absorbent polymer particles. By adjusting the content range of the surface crosslinking agent within the above range, a super absorbent polymer exhibiting excellent overall physical properties may be prepared.

In addition, the surface crosslinked layer may be formed by adding an inorganic material to the surface crosslinking agent. That is, in the presence of the surface crosslinking agent and the inorganic material, the surface of the base resin powder may be additionally crosslinked to form a surface crosslinked layer.

As such inorganic materials, one or more inorganic materials selected from the group consisting of silica, clay, alumina, a silica-alumina composite material, titania, zinc oxide, and aluminum sulfate may be used. The inorganic material may be used in powder or liquid form. In addition, the inorganic material may be used in an amount of 0.001 to 1 part by weight based on 100 parts by weight of the super absorbent polymer particles. At this time, 100 parts by weight of the super absorbent polymer particles are based on a dried state. In addition, the content refers to the total amount of the inorganic material used.

For example, the inorganic material may be used in an amount of 0.005 parts by weight or more, 0.01 parts by weight or more, or 0.05 parts by weight or more, based on 100 parts by weight of the super absorbent polymer particles. In addition, the inorganic material may be used in an amount of 0.5 parts by weight or less, 0.4 parts by weight or less, or 0.2 parts by weight or less, based on 100 parts by weight of the super absorbent polymer particles. By adjusting the content range of the surface crosslinking agent within the range, a super absorbent polymer exhibiting excellent overall physical properties may be prepared.

When mixing the surface crosslinking agent and base resin powder, water and methanol may be additionally mixed and added. When adding water and methanol, there is an advantage in that the surface crosslinking agent may be evenly dispersed in the resin composition. At this time, the content of water and methanol added may be appropriately adjusted to induce even dispersion of the surface crosslinking agent, prevent clumping of the resin composition, and optimize the surface penetration depth of the crosslinking agent.

In addition, when mixing the surface crosslinking agent and the base resin powder, a surfactant may be additionally mixed and added, and examples of the surfactant include sucrose stearate. The surfactant may also perform the function of inducing even dispersion of the surface crosslinking agent and preventing clumping of the resin composition.

As described above, a super absorbent polymer including the base resin powder and the surface crosslinked layer formed on the base resin powder may absorb body fluid or water at a fast rate, and may also absorb a relatively large amount initially, thereby preventing problems such as body fluid or water not being absorbed but accumulating or leaking out.

II. Preparation Method of Super Absorbent Polymer

Conventional super absorbent polymers are prepared by crosslinking and polymerizing a water-soluble ethylenically unsaturated monomer having at least some neutralized acidic groups in the presence of an internal crosslinking agent and a polymerization initiator to form a hydrogel polymer, drying the hydrogel polymer thus formed, and then pulverizing it to a desired particle size, and at this time, a chopping process is usually performed to cut the hydrogel polymer into particles of several millimeters in size before the drying process to facilitate drying of the hydrogel polymer and increase the efficiency of the pulverization process. However, in this chopping process, due to the adhesiveness of the hydrogel polymer, the hydrogel polymer is not pulverized to a micro-sized particle level and becomes an aggregated gel. When such an aggregated gel-shaped hydrogel polymer is dried, a plate-shaped dried product is formed, and in order to pulverize this to a micro-sized particle level, it must go through a multi-step pulverizing process that reduces the adhesiveness of the polymer, and accordingly, there has been a problem that a lot of fine particles are generated during this process.

To solve this problem, a method was used in which the separated fine particles were mixed with an appropriate amount of water, reassembled into fine particles, and then reused in the chopping step or pre-drying step. However, in the process of reusing the fine particles in this way, problems such as increased device load and/or energy usage have occurred. In addition, the properties of the super absorbent polymer were deteriorated due to the fine particles remaining without being classified even after reuse.

As a result of repeated research to solve this problem, unlike the conventional method of preparing super absorbent polymers, in which polymerization is performed in a state where the acidic group of a water-soluble ethylenically unsaturated monomer is neutralized, it was confirmed that when polymerization is first performed in a state where the acidic group is not neutralized to form a polymer, and the hydrogel polymer is micronized in the presence of a surfactant and then the acidic group of the polymer is neutralized, or the acidic group of the polymer is neutralized to form a hydrogel polymer and then the hydrogel polymer is micronized in the presence of a surfactant, or the acidic group present in the polymer is neutralized at the same time as the micronization, the surfactant is present in a large amount on the surface of the polymer and can sufficiently play a role in lowering the high adhesiveness of the polymer, preventing the polymer from excessively agglomerating, and controlling the agglomeration state to a desired level.

Meanwhile, the super absorbent polymer according to the present disclosure may be implemented by controlling the resin components and content, polymerization conditions, or pulverizing process conditions. For example, the super absorbent polymer can be controlled to satisfy Equation 1 by adjusting the type and content of the monomer composition in a polymerization process; the type and content of an internal crosslinking agent; the type, input amount, and input timing of a surfactant in the neutralization and micronization steps; the type, input amount, and input timing of a neutralizing agent; the type, the rotational speed, the hole size, the number of micronizations; the component and content of the surface crosslinking solution, and the like.

In particular, the super absorbent polymer may be controlled to satisfy Equation 1 by controlling the amount of neutralizing agent input in the neutralization step, applying an ultrafine pulverization process as a micronization method, or controlling the components and content of the surface crosslinking solution in the surface crosslinking step. The ultrafine pulverization process and control of the components and contents of the surface crosslinking solution will be described below.

Hereinafter, each step of the method of preparing a super absorbent polymer according to an aspect of the present disclosure will be described in more detail.

Step 1: Polymerization Step

First, polymerization is performed on a monomer composition including a water-soluble ethylenically unsaturated monomer having an acidic group and an internal crosslinking agent to prepare a base resin powder including a polymer in which the water-soluble ethylenically unsaturated monomer having the acidic group and the internal crosslinking agent are crosslinked.

The step may consist of a step of preparing a monomer composition by mixing a water-soluble ethylenically unsaturated monomer having an acidic group, an internal crosslinking agent, and a polymerization initiator, and a step of polymerizing the monomer composition to form a polymer.

Here, the contents of each component may be equally applied to the contents described in the super absorbent polymer of the above-described item.

According to one aspect of the present disclosure, polymerization is first performed in a state where the acidic group of the water-soluble ethylenically unsaturated monomer is not neutralized to form a polymer.

The water-soluble ethylenically unsaturated monomer (e.g., acrylic acid) in which the acidic group is not neutralized is liquid at room temperature and has high miscibility with a solvent (water), so it is present in the state of a mixed solution in the monomer composition. However, a water-soluble ethylenically unsaturated monomer with neutralized acidic groups is solid at room temperature and has different solubility depending on the temperature of the solvent (water), and the solubility decreases at lower temperatures.

As such, a water-soluble ethylenically unsaturated monomer whose acidic group is not neutralized has a higher solubility or miscibility in a solvent (water) than a monomer with neutralized acidic groups, so it does not precipitate even at low temperatures, and thus is advantageous for long-term polymerization at low temperatures. Accordingly, a water-soluble ethylenically unsaturated monomer in which the acidic group is not neutralized may be used for long-term polymerization to stably form a polymer having a higher molecular weight and a uniform molecular weight distribution.

In addition, since the formation of a polymer having a longer chain is possible, the effect of reducing the content of extractable components that are present in a non-crosslinked state due to incomplete polymerization or crosslinking may be achieved, and thus the content of extractable components measured after free swelling in water having an electrical conductivity of 100 to 130 μS/cm for 1 hour according to the present disclosure is suitable for implementing the content of extractable components in the target range.

In addition, in this way, polymerization is first performed in a state where the acidic groups of the monomer are not neutralized to form a polymer, and after neutralization, the polymer is micronized in the presence of a surfactant, or after micronization in the presence of a surfactant, neutralization is performed, or when the acidic groups present in the polymer are neutralized at the same time as micronization, the surfactant may sufficiently play a role in reducing the adhesiveness of the polymer by being present in a large quantity on the surface of the polymer.

According to one aspect of the present disclosure, the step of performing polymerization on the monomer composition to form a polymer may be performed for 1 hour or more in a batch-type reactor.

In the conventional method of preparing a super absorbent polymer, the polymerization method is largely divided into thermal polymerization and photopolymerization depending on the polymerization energy source, and in the case of conventional thermal polymerization, it can be performed in a reactor with a stirring shaft such as a kneader, and in the case of photopolymerization, it can be performed in a flat-bottomed container.

Meanwhile, when the polymerization is performed as continuous polymerization, for example, when the polymerization is performed in a reactor having a reactor stirring shaft equipped with a conveyor belt, a new monomer composition is supplied to the reactor as the polymerization product moves, so that the polymerization is performed continuously, and thus polymers having different polymerization rates are mixed, and accordingly, it is difficult to achieve even polymerization throughout the monomer composition, which may result in the deterioration of the overall physical properties.

However, according to one aspect of the present disclosure, since polymerization is performed in a fixed-bed manner in a batch-type reactor, there is less concern that polymers with different polymerization rates will be mixed, and thus a polymer with consistent quality may be obtained.

In addition, the polymerization step is performed in a batch-type reactor having a predetermined volume, and the polymerization reaction is performed for a longer period of time, for example, 1 hour or more, 3 hours or more, or 6 hours or more, than when the polymerization is performed continuously in a reactor equipped with a conveyor belt. Despite the long polymerization reaction time as described above, since polymerization is performed on a water-soluble ethylenically unsaturated monomer in an unneutralized state, the monomer is not easily precipitated even when polymerization is performed for a long time, and thus it is advantageous for long-term polymerization.

Meanwhile, since polymerization in the batch-type reactor of the present disclosure uses a thermal polymerization method, the polymerization initiator uses a thermal polymerization initiator, and the description of the corresponding component is as described above.

Steps 2 and 3: Micronization and Neutralization Steps

Next, the method includes a step (step 2) of preparing a mixture including a micronized hydrogel polymer by micronizing the hydrogel polymer in the presence of a surfactant.

The micronization step is a step of micronizing the polymer in the presence of a surfactant, and is a step in which micronization and agglomeration into a size of tens to hundreds of micrometers occur simultaneously, rather than chopping the polymer into a size of millimeters.

That is, it is a step for manufacturing secondary aggregated particles in the form of aggregated primary particles that are micronized into a size of tens to hundreds of micrometers by imparting appropriate adhesiveness to the polymer. The hydophilic super absorbent polymer particles, which are secondary aggregated particles manufactured through this step, have a normal particle size distribution and a greatly increased surface area, so that the absorption rate may be remarkably improved.

Meanwhile, when a high mechanical shear force is applied in the micronization step to perform ultra-fine pulverizing at a rotational speed of 500 rpm to 4,000 rpm, agglomerated hydrogel particles having finer pores may be formed.

At this time, when ultra-fine pulverizing is performed at a rotational speed of 500 rpm to 4,000 rpm, a high mechanical shear force is applied, so that fine pores of 100 μm or less are easily formed in the polymer, thereby increasing the surface roughness, and the total surface area of the polymer is significantly increased by the pores formed inside and outside the polymer particles. Since the fine pores are formed in a stable form compared to pores formed using a foaming agent in the polymerization step, the degree of fine particle generation by the pores may be remarkably reduced in a subsequent process. The super absorbent polymer particles manufactured in this step can have a remarkably increased surface area, and thus the content of extractable components measured after free swelling in water having an electrical conductivity of 100 to 130 μS/cm for 1 hour according to the present disclosure is suitable for implementing the content of extractable components in the target range.

The ultra-fine pulverizing process is performed at a rotational speed of 500 rpm to 4,000 rpm, wherein when the rotational speed of the process is less than 500 rpm, since is difficult to form sufficient pores to the desired degree, it is difficult to expect a fast absorption rate and secure the desired level of productivity. In addition, when the rotational speed exceeds 4,000 rpm, the polymer chains may be damaged due to excessive shear force, and thus the extractable components may increase, and the overall physical properties of the prepared super absorbent polymer may be somewhat deteriorated. In some examples, the ultra-fine pulverizing process may be performed at 1,000 rpm to 3,500 rpm, or 2,000 rpm to 3,000 rpm. In this range, it is easy to form the desired fine pores without the above-described problems.

According to one aspect of the present disclosure, the micronization step is performed by a micronization device, the micronization device may include a body part including a transport space in which a polymer is transported therein; a screw member rotatably installed inside the transport space to allow the polymer to move; a driving motor providing a rotational driving force to the screw member; a cutter member installed in the body part to pulverize the polymer; and a porous plate having a plurality of holes formed therein, which discharges the polymer pulverized by the cutter member to the outside of the body part.

At this time, the hole size provided in the porous plate of the micronization device may be 1 mm to 25 mm, 5 mm to 20 mm, or 5 mm to 15 mm.

As such, when the polymer mixed with the surfactant is micronized while controlling agglomeration using a micronization device, a smaller particle size distribution is realized, so that the subsequent drying and pulverizing processes may be performed under milder conditions, thereby preventing fine particles from occurring and improving the properties of the super absorbent polymer, and when ultra-fine pulverizing is performed, the absorption rate may be improved by simultaneously forming appropriate fine pores on the surface of the polymer and increasing the surface area.

The micronization step may be performed one or more times, for example one to six times, or one to four times, or one to three times. This may be performed using a plurality of micronization devices, or a single micronization device including a plurality of porous plates and/or a plurality of cutter members, or among the plurality of micronization devices, some of the devices include a plurality of porous plates and/or a plurality of cutter members.

According to one aspect of the present disclosure, a surfactant may be additionally used in the micronization step, and thus agglomeration between polymer particles is effectively controlled, thereby reducing the load on the equipment used in the pulverization process and further improving productivity.

The surfactant may be selected from compounds represented by the following Chemical Formulas 2-1 to 2-14, but is not limited thereto:

According to one aspect of the present disclosure, the surfactant may be, but is not limited to, glycerol monolaurate (GML).

Meanwhile, the amount of the surfactant used is not particularly limited, but the surfactant may be used in an amount of 0.06 g to 0.48 g per 1,000 g of the hydrogel polymer depending on the productivity required or the device load condition.

When the surfactant is used in an excessively small amount, the surfactant may not be evenly adsorbed on the polymer surface, causing re-agglomeration of particles after pulverizing, or the absorption performance, such as centrifuge retention capacity and absorbency under pressure, may be reduced due to a large part of the surfactant being sharing with the polymer. Meanwhile, when the surfactant is used in an excessive amount, the overall properties of the finally prepared super absorbent polymer may deteriorate due to a decrease in surface tension.

Therefore, for example, the surfactant may be used in an amount of 0.06 g or more, or 0.1 g or more, or 0.2 g or more, and 0.48 g or less, or 0.45 g or less, or 0.4 g or less per 1,000 g of the hydrogel polymer, and thus it is easy to control the content of extractable components measured after free swelling in water having an electrical conductivity of 100 to 130 μS/cm for 1 hour to the target range according to the present disclosure.

The method for mixing the surfactant into the polymer is not particularly limited, and any method that may evenly mix it into the polymer may be appropriately adopted and used. Specifically, the surfactant may be mixed in a dry manner, dissolved in a solvent and then mixed in a solution state, or the surfactant may be melted and then mixed.

For example, the surfactant may be mixed in a solution state in which it is dissolved in a solvent. At this time, any type of solvent may be used without limitation, including inorganic and organic solvents, but water is most appropriate when considering the ease of the drying process and the cost of the solvent recovery system. In addition, the solution may be prepared by mixing the surfactant and the polymer in a reaction tank, by putting the polymer in a mixer and spraying the solution, or by continuously supplying the polymer and the solution to a continuously operating mixer.

Meanwhile, when the surfactant is mixed in a solution state in which it is dissolved in water, it may be used by adding it to an aqueous solution so that a concentration of about 0.01% to 10% is attained.

For example, when the surfactant is to be used in an amount of 0.1 g per 1,000 g of the hydrogel polymer, 100 g of an aqueous solution having a concentration of 0.1%, in which 0.1 g of the surfactant is dissolved in 99.9 g of water, can be used. Alternatively, 10 g of an aqueous solution having a concentration of 1%, in which 0.1 g of surfactant is dissolved in 9.9 g of water, may be used.

That is, when using the same amount of surfactant, the water content may be increased or decreased to use the surfactant in an aqueous solution having a desired concentration, and the concentration may be appropriately adjusted in consideration of the properties of the super absorbent polymer to be finally prepared.

Meanwhile, when the surfactant is hydrophobic and has very low solubility in water, the surfactant may be mixed into the polymer in a dry manner, or the surfactant may be used in a form in which the surfactant is dispersed in water by inputting the surfactant to water. For example, when the surfactant is dry mixed in powder form and dispersed in a polymer, the degree of dispersion is very weak, so it may be used by dispersing it in water and evenly applying it to the surface.

According to one aspect of the present disclosure, a step of neutralizing at least some of the acidic groups of the polymer (step 3) is performed, and the micronization step of step 2 and the neutralization step of step 3 described above may be performed sequentially, alternately, or simultaneously.

That is, a neutralizing agent may be input to the polymer to first neutralize the acidic group, and then a surfactant may be input to the neutralized polymer to micronize the polymer mixed with the surfactant (performed in the order of step 3->step 2), or a neutralizing agent and a surfactant may be input to the polymer at the same time to neutralize and micronize the polymer (perform steps 2 and 3 at the same time). Alternatively, the surfactant may be input first and then the neutralizing agent be input (performed in the order of step 2->step 3). Alternatively, the neutralizing agent and surfactant may be input alternately. Alternatively, the surfactant may be input first to form particles, then a neutralizing agent may be input to neutralize the particles, and then an additional surfactant may be input to the neutralized hydrogel polymer to further perform the micronization process.

In this case, when the neutralization step is performed independently from the micronization step of step 2, it may be performed in a manner in which the additive is input simultaneously with the pulverization of the polymer. More specifically, a screw-type extruder including a porous plate having a plurality of holes formed therein may be used. The screw-type extruder is a device that performs pulverization under mild conditions compared to the micronization device used in the micronization step, and the rotational speed may be about 150 rpm to 500 rpm, and the holes of the porous plate may have a size of about 3 mm to 25 mm, but are not limited thereto.

The rotational speed of the screw-type extruder and the size of the porous plate holes affect the discharge state of the super absorbent polymer discharged from the extruder, and the particle shape of the super absorbent polymer may change depending on the discharge state.

In particular, by controlling the rotational speed of the screw-type extruder to 150 rpm to 500 rpm, the content of extractable components measured after free swelling for 1 hour in water having an electrical conductivity of 100 to 130 μS/cm according to the present disclosure may be controlled within a desired range.

At this time, basic materials such as sodium hydroxide, potassium hydroxide, and ammonium hydroxide that can neutralize acidic groups may be used as neutralizing agents.

In addition, the degree of neutralization, which refers to the degree of neutralization of acidic groups included in the polymer by the neutralizing agent, may be 50 to 90 mol %, or 60 to 85 mol %, or 65 to 85 mol %, or 65 to 80 mol %. The range of the degree of neutralization may vary depending on the final properties, and the absorption rate and absorption performance may be controlled by adjusting the degree of neutralization.

At this time, when the degree of neutralization is too high, the absorption capacity of the super absorbent polymer may decrease, and when the concentration of carboxyl groups on the particle surface is too low, it may be difficult to properly perform surface cross-linking in a subsequent process, which may result in a decrease in absorbency under pressure characteristics or liquid permeability. On the other hand, when the degree of neutralization is too low, not only will the absorbency of the polymer greatly decrease, but it may also exhibit properties similar to elastic rubber that are difficult to handle.

Meanwhile, it may be desirable to have a certain time gap between the inputting of the neutralizing agent and the micronization process to ensure even neutralization of the entire polymer.

Step 4: Drying Step

Next, a step (step 4) is performed to dry the micronized and neutralized polymer to prepare a base resin powder.

The step is a step of drying the moisture in the base resin powder, which is a polymer obtained by neutralizing at least some of the acidic groups of the polymer and microionizing the polymer.

In a conventional method of preparing a super absorbent polymer, the drying step is performed so that the moisture content of the base resin powder becomes about 4 to 20 wt %, about 4 to about 15 wt %, or about 6 to about 13 wt %. However, the present disclosure is not limited thereto.

Step 4 may be performed by fixed-bed type drying, moving type drying, or a combination thereof.

According to one aspect of the disclosure, step 4 may be performed by fixed-bed type drying.

The fixed-bed type drying refers to a method in which the material to be dried is placed on a porous iron plate through which air can pass, and the material is dried by passing hot air from the bottom to the top.

Since the fixed-bed type drying dries in a plate-like shape without particle movement, it is difficult to achieve uniform drying simply by the flow of hot air. Therefore, fixed-bed type drying requires delicate control of hot air and temperature to obtain a dried product having a uniform high-moisture content. In the present disclosure, by changing the direction of hot air from the bottom to the top, the plate-shaped dried product is prevented from bending during drying, thereby preventing the hot air from leaking. In addition, the drying temperature was changed by section so that the upper, middle, and lower layers inside the dried product could be dried evenly with a moisture content difference of less than 5%.

Devices capable of drying using the fixed-bed type drying method include, but are not limited to, belt-type dryers.

For the fixed-bed type drying step, the drying process may be performed at a temperature of about 80° C. to 200° C., or 90° C. to 190° C., or 100° C. to 180° C. When the drying temperature is lower than 80° C., the drying time may become too long, and when the drying temperature is higher than 200° C. and too high, a super absorbent polymer having a moisture content lower than the desired moisture content may be obtained. Meanwhile, the term “drying temperature” may mean the temperature of the hot air used or the internal temperature of the instrument during the drying process.

According to one aspect of the disclosure, step 4 may be performed by moving type drying.

The term “moving type drying” refers to a method of drying in which the dried product is mechanically stirred during drying. At this time, the direction in which the hot air passes through the material may be the same as or different from the direction in which the material circulates. Alternatively, the material may be dried by circulating it inside the dryer and passing a heat-mediating fluid (heat-mediating oil) through a separate pipe outside the dryer.

Devices that may be used for drying using this moving type drying method may include a horizontal-type mixer, a rotary kiln, a paddle dryer, a steam tube dryer, or a commonly used moving type dryer.

For the moving type drying step, the drying process may be performed at a temperature of about 100° C. to 300° C., 120° C. to 280° C., or 150° C. to 250° C. When the drying temperature is too low, such as below 100° C., the drying time may become too long, and when the drying temperature is too high, such as above 300° C., the polymer chain of super absorbent polymer may be damaged, resulting in a decline in overall physical properties, and a super absorbent polymer having a lower moisture content than the desired moisture content may be obtained.

Step 5: Pulverizing Step

Next, a step of pulverizing the dried base resin powder is performed.

Specifically, the pulverizing step may be performed by pulverizing the dry base resin powder to have a particle size of a normal particle level, i.e., a particle size of 150 μm to 850 μm.

The pulverizer used for this purpose may be specifically a vertical grinder, a turbo cutter, a turbo grinder, a rotary cutter mill, a cutter mill, a disc mill, a shred crusher, a crusher, a chopper, or a disc cutter, but is not limited to these examples.

Alternatively, a pulverizer such as a pin mill, a hammer mill, a screw mill, a roll mill, a disc mill or a jog mill may be used, but is not limited to these examples.

Meanwhile, in the preparation method of the present disclosure, in the micronization step, super absorbent polymer particles having a smaller particle size distribution than in the conventional chopping step may be implemented, and since the moisture content after drying is maintained relatively high, even when the pulverizing is performed under mild conditions with a smaller pulverizing force, a super absorbent polymer having a very high content of a normal particle size of 150 μm to 850 μm can be formed, and the fine particle generation ratio may be greatly reduced.

The super absorbent polymer particles manufactured as described above may include super absorbent polymer particles having a particle diameter of 150 μm to 850 μm, i.e., normal particles in an amount of 80 wt % or more, 85 wt % or more, 89 wt % or more, 90 wt % or more, 92 wt % or more, 93 wt % or more, 94 wt % or more, or 95 wt % or more, based on the total weight. The particle diameter of these super absorbent polymer particles can be measured according to the European Disposables and Nonwovens Association (EDANA) standard EDANA WSP 220.3 method.

In addition, the super absorbent polymer particles may include fine particles having a particle diameter of less than 150 μm in an amount of about 20 wt % or less, about 18 wt % or less, about 15 wt % or less, about 13 wt % or less, about 12 wt % or less, about 11 wt % or less, about 10 wt % or less, about 9 wt % or less, about 8 wt % or less, or about 5 wt % or less, based on the total weight. This is in contrast to the case where the super absorbent polymer is prepared using a conventional manufacturing method, which has a fine particle content of about 20 wt % to about 30 wt %.

Additive Input Step

Meanwhile, according to one aspect of the disclosure, a step of inputting an additive to the micronized and neutralized polymer may be further included before the drying step (step 4).

The additive input process is a process for improving properties by using additional additives within a range that does not hinder the desired effect, and the types of additives are not particularly limited, and examples thereof include, but are not limited to, a polymerization initiator for removing residual monomers, a liquid permeability improver for improving absorption properties, fine particles for recycling the fine particles generated, an anti-caking agent, a fluidity improver, an antioxidant, a neutralizing agent, a surfactant, and the like.

The additive input step may be performed simultaneously with step 2, simultaneously with step 3, after steps 2 and 3, or in one or more of these steps. The additive input step may be performed multiple times as needed, and may also be performed more than once in each step.

When the additive input step is performed independently from steps 2 and 3, i.e., when performed after step 2 and step 3 but before step 4, it may be performed in a way that the additives are input simultaneously with the polymer being pulverized.

The pulverizing may be applied in the same manner as the pulverizing step of step 5 described above, and the additive may be input once or multiple times during the pulverizing step and mixed with the polymer.

Classification Step

Next, after the step of pulverizing the base resin powder (step 5), a step of classifying the pulverized super absorbent polymer particles according to particle diameter may be further included.

Step 6: Surface Crosslinking Step

In addition, the method may further include a step of forming a surface crosslinked layer on at least a portion of the surface of the base resin particles in the presence of a surface crosslinking agent after pulverizing (step 5) and/or classifying the base resin powder. By this step, the crosslinked polymer included in the base resin powder may be additionally crosslinked via a surface crosslinking agent, so that a surface crosslinked layer may be formed on at least a portion of the surface of the base resin powder.

The description of the surface crosslinking agent can all be applied as described above. In addition, the description of water, methanol, and surfactant added when mixing the surface crosslinking agent and base resin powder can all be applied as described above.

In addition, there is no limitation on the composition of the method of mixing the surface crosslinking agent with the base resin powder. For example, a method may be used in which a composition including a surface crosslinking agent and a base resin powder is placed in a reaction tank and mixed, a method may be used in which a surface crosslinking agent is sprayed onto the composition, and a method may be used in which the resin composition and the surface crosslinking agent are continuously supplied to a continuously operated mixer and mixed.

The surface crosslinking process may be performed at a temperature of about 80° C. to about 250° C. More specifically, the surface crosslinking process may be performed at a temperature of about 100° C. to about 220° C., or about 120° C. to about 200° C., for about 20 minutes to about 2 hours, or about 40 minutes to about 80 minutes. When the conditions for the surface crosslinking process described above are met, the surface of the super absorbent polymer particles may be sufficiently crosslinked, thereby increasing the absorbency under pressure.

The temperature-increasing means for the surface crosslinking reaction is not particularly limited.

Heating may be accomplished by supplying a heating medium or directly supplying a heat source. At this time, the types of heating medium that may be used include, but are not limited to, heated fluids such as steam, hot air, and hot oil, and the temperature of the supplied heating medium may be appropriately selected by considering the means of the heating medium, the heating rate, and the target temperature for heating. Meanwhile, as a directly supplied heat source, methods including heating through electricity and heating through gas may be included, but are not limited to these examples.

According to one aspect of the present disclosure, after forming a surface crosslinked layer on at least a portion of the surface of the base resin powder, the method may further include at least one of a cooling step for cooling the super absorbent polymer particles on which the surface crosslinked layer is formed, a watering step for inputting water to the super absorbent polymer particles on which the surface crosslinked layer is formed, and a post-treatment step for inputting an additive to the super absorbent polymer particles on which the surface crosslinked layer is formed. At this time, the cooling step, the watering step, and the post-treatment step may be performed sequentially or simultaneously.

Water or brine may be used in the watering step, thereby controlling the generation amount of riddlings, and the like. The amount of water used may be appropriately adjusted considering the moisture content of a desired final product, and the like, and, 0.1 to 10 wt %, 0.5 to 8 wt %, or 1 to 5 wt % of water relative to the absorbent resin may be used, but is not limited thereto.

In addition, a maturation step may be further performed after the watering step.

When using brine in the watering step, the solution absorption rate is relatively low due to the conductivity of the brine, so that the brine evenly spreads in the maturation step, enabling even absorption by the absorbent polymer. The maturation step may be performed without any special limitation by a commonly used method, for example, at a temperature of 100° C. or lower, 80° C. or lower, or 50° C. or lower for 10 minutes to 1 hour using a rotary stirring device.

The additives input in the post-treatment step may include surfactants, inorganic salts, liquid permeability improvers, anti-caking agents, fluidity improvers, and antioxidants, but the present disclosure is not limited thereto.

By selectively performing the cooling step, watering step, and post-treatment step, the moisture content of the final super absorbent polymer may be improved by controlling the occurrence of riddlings, and the like, and a higher quality super absorbent polymer product may be manufactured.

Hereinafter, the operation and effect of aspects of the disclosure will be described in more detail through specific examples of aspects of the disclosure. However, the following examples are only presented as examples of aspects of the disclosure, and the scope of the disclosure is not determined thereby.

Examples

In a 2 L glass container equipped with a stirrer and a thermometer, 1,000 g of acrylic acid, 2.5 g of pentaerythritol triallyl ether (PETTAE) as an internal crosslinking agent, and 2,260 g of water were mixed while stirring. At this time, a reaction temperature was maintained at 5° C. Nitrogen was input at a rate of 1,000 cc/min into the glass container including the mixture for 1 hour to replace the inside of the glass container with nitrogen. Thereafter, 13.0 g of a 0.3% hydrogen peroxide aqueous solution, 15.0 g of a 1% ascorbic acid aqueous solution, and 30.0 g of a 2% 2,2′-azobis amidinopropane dihydrochloride aqueous solution were input as polymerization initiators. At the same time, 15.0 g of a 0.01% iron sulfate aqueous solution as a reducing agent was added and mixed to initiate polymerization. A polymerization reaction was initiated in the mixture, and after the temperature of the polymer reached 85° C., the polymerization was performed in an oven at 90±2° C. for about 6 hours to prepare a hydrogel polymer.

1,000 g of the hydrogel polymer obtained in step 1 and 0.1 g of glycerol monolaurate (GML) were dissolved in water at 60° C. or higher and input into a cylindrical grinder in the form of an aqueous solution. Thereafter, a high-speed rotary chopper (F-150/Karl Schnell) equipped inside the cylindrical grinder was used to push the aqueous solution through a porous plate with multiple 10 mm holes at a rotational speed of 1,600 rpm. Subsequently, the polymer was further pushed through the porous plate with multiple 10 mm holes at a rotational speed of 2,000 rpm to obtain a pulverized gel-shaped hydrogel polymer. Thereafter, the pulverized gel-shaped hydrogel polymer was pushed three times through a porous plate with multiple 6 mm holes at a rotational speed of 250 rpm using a screw-type extruder equipped inside the cylindrical grinder to obtain hydrogel super absorbent polymer particles.

At this time, in the first pass through the porous plate, 460 g of a 32% NaOH aqueous solution was input, and in the second pass, 42.8 g of a 0.5% Na2S2O8 aqueous solution (SPS aqueous solution) was input and pushed through the porous plate. In the third pass, it was passed through a porous plate without adding any additives.

The hydrogel super absorbent polymer particles obtained as a result of the pulverizing were placed on a porous plate capable of vertically transferring air flow, and dried at 120° C. for 40 minutes using an air-flow oven.

200° C. hot air and 100° C. hot air were sequentially flowed from the top to the bottom for 5 minutes and 10 minutes, respectively, so that the moisture content of the dried super absorbent polymer became about 10%, and then 100° C. hot air was flowed from the bottom to the top for 15 minutes to uniformly dry the hydrogel super absorbent polymer particles to obtain a dried product.

The dried product was ground with a grinder (GRAN-U-LIZERM, MPE) and then classified through a standard mesh sieve of ASTM standards to obtain a base resin powder having a size of 150 to 850 μm.

Next, as described in Table 1 below, for 100 g of the base resin powder, a surface crosslinking agent aqueous solution including 4 g of water, 6 g of methanol, 0.08 g of ethylene glycol diglycidyl ether (EJ-1030S), 0.1 g of propylene glycol, 0.2 g of aluminum sulfate, and 0.1 g of silica particles (Aerosil 200) was input and mixed. At this time, the surface crosslinking agent aqueous solution was mixed so as to be evenly distributed on the super absorbent polymer powder.

Subsequently, the base resin powder mixed with the surface crosslinking solution was placed in a surface crosslinking reactor, and a surface crosslinking reaction was performed to obtain a surface-crosslinked super absorbent polymer.

Specifically, in the surface crosslinking reactor, the base resin powder was subjected to a surface crosslinking reaction at 140° C. for 50 minutes.

After the surface crosslinking step, a super absorbent polymer having a particle size of 150 μm to 850 μm was prepared by classifying through a standard mesh sieve according to ASTM standards.

A hydrogel polymer was prepared using the same method as in Example 1.

Hydrogel super absorbent polymer particles were obtained in the same manner as in Example 1, except that 405 g of a 32% NaOH aqueous solution was input instead of 460 g of a 32% NaOH aqueous solution in the first pass through the porous plate in Example 1.

The hydrogel super absorbent polymer particles were uniformly dried in the same manner as in Example 1 to obtain a dried product.

The dried product was ground with a grinder (GRAN-U-LIZERM, MPE) and then classified through a standard mesh sieve of ASTM standards to obtain a base resin powder having a size of 150 to 850 μm.

A super absorbent polymer of Example 2 was prepared by performing the surface crosslinking step in the same manner as in Example 1, except that the components and contents of the surface crosslinking agent aqueous solution were used as described in Table 1 below.

A hydrogel polymer was prepared using the same method as in Example 1.

1,000 g of the hydrogel polymer obtained in step 1 and 0.1 g of glycerol monolaurate (GML) were dissolved in water at 60° C. or higher and input into a cylindrical grinder in the form of an aqueous solution. Thereafter, a high-speed rotary chopper (F-150/Karl Schnell) equipped inside the cylindrical grinder was used to push the material through a porous plate with multiple 10 mm holes at a rotational speed of 1,500 rpm. Subsequently, the polymer was further pushed through a porous plate with multiple 12 mm holes at a rotational speed of 2,500 rpm to obtain a pulverized gel-shaped hydrogel polymer. Thereafter, the ground gel-shaped hydrogel polymer was pushed three times through a porous plate with multiple 6 mm holes at a rotational speed of 250 rpm using a screw-type extruder equipped inside the cylindrical grinder to obtain hydrogel super absorbent polymer particles.

At this time, in the first pass through the porous plate, 400 g of a 32% NaOH aqueous solution was input, and in the second pass, 42.8 g of a 0.5% Na2S2O8 aqueous solution (SPS aqueous solution) was input and pushed through the porous plate. In the third pass, it was passed through a porous plate without adding any additives.

The hydrogel super absorbent polymer particles were uniformly dried in the same manner as in Example 1 to obtain a dried product.

The dried product was ground with a grinder (GRAN-U-LIZERM, MPE) and then classified through a standard mesh sieve of ASTM standards to obtain a base resin powder having a size of 150 to 850 μm.

A super absorbent polymer of Example 3 was prepared by performing the surface crosslinking step in the same manner as in Example 1, except that the components and contents of the surface crosslinking agent aqueous solution were prepared as described in Table 1 below.

A hydrogel polymer was prepared using the same method as in Example 1.

1,000 g of the hydrogel polymer obtained in step 1 and 0.1 g of glycerol monolaurate (GML) were dissolved in water at 60° C. or higher and input into a cylindrical grinder in the form of an aqueous solution. Thereafter, a high-speed rotary chopper (F-150/Karl Schnell) equipped inside the cylindrical grinder was used to push the aqueous solution through a porous plate with multiple 8 mm holes at a rotational speed of 1,200 rpm. Subsequently, the polymer was further pushed through a porous plate with multiple 10 mm holes at a rotational speed of 2,800 rpm to obtain a pulverized gel-shaped hydrogel polymer. Thereafter, the ground gel-shaped hydrogel polymer was pushed three times through a porous plate with multiple 6 mm holes at a rotational speed of 250 rpm using a screw-type extruder equipped inside the cylindrical grinder to obtain hydrogel super absorbent polymer particles.

At this time, in the first pass through the porous plate, 365 g of a 32% NaOH aqueous solution was input, and in the second pass, 42.8 g of a 0.5% Na2S2O8 aqueous solution (SPS aqueous solution) was input and pushed through the porous plate. In the third pass, it was passed through a porous plate without adding any additives.

The hydrogel super absorbent polymer particles were uniformly dried in the same manner as in Example 1 to obtain a dried product.

The dried product was ground with a grinder (GRAN-U-LIZERM, MPE) and then classified through a standard mesh sieve of ASTM standards to obtain a base resin powder having a size of 150 to 850 μm.

A super absorbent polymer of Example 4 was prepared by performing the surface crosslinking step in the same manner as in Example 1, except that the components and contents of the surface crosslinking agent aqueous solution were used as described in Table 1 below.

A hydrogel polymer was prepared using the same method as in Example 1.

Hydrogel super absorbent polymer particles were obtained in the same manner as in Example 1, except that 300 g of a 32% NaOH aqueous solution was input instead of 475 g of a 32% NaOH aqueous solution in the first pass through the porous plate in Example 1.

The hydrogel super absorbent polymer particles were uniformly dried in the same manner as in Example 1 to obtain a dried product.

The dried product was ground with a grinder (GRAN-U-LIZERM, MPE) and then classified through a standard mesh sieve of ASTM standards to obtain a base resin powder having a size of 150 to 850 μm.

A super absorbent polymer of Comparative Example 1 was prepared by performing the surface crosslinking step in the same manner as in Example 1, except that the components and contents of the surface crosslinking agent aqueous solution were prepared as described in Table 1 below.

A hydrogel polymer was prepared using the same method as in Example 1.

1,000 g of the hydrogel polymer obtained in step 1 and 0.1 g of glycerol monolaurate (GML) were dissolved in water at 60° C. or higher and input into a cylindrical grinder in the form of an aqueous solution. Thereafter, a high-speed rotary chopper (F-150/Karl Schnell) equipped inside the cylindrical grinder was used to push the material through a porous plate with multiple 10 mm holes at a rotational speed of 1,400 rpm. Subsequently, the polymer was further pushed through a porous plate with multiple 8 mm holes at a rotational speed of 2,700 rpm to obtain a pulverized gel-shaped hydrogel polymer. Thereafter, the ground gel-shaped hydrogel polymer was pushed three times through a porous plate with multiple 6 mm holes at a rotational speed of 250 rpm using a screw-type extruder equipped inside a cylindrical grinder to obtain hydrogel super absorbent polymer particles.

At this time, in the first pass through the porous plate, 320 g of a 32% NaOH aqueous solution was input, and in the second pass, 42.8 g of a 0.5% Na2S2O8 aqueous solution (SPS aqueous solution) was input and pushed through the porous plate. In the third pass, it was passed through a porous plate without adding any additives.

The hydrogel super absorbent polymer particles were uniformly dried in the same manner as in Example 1 to obtain a dried product.

The dried product was ground with a grinder (GRAN-U-LIZERM, MPE) and then classified through a standard mesh sieve of ASTM standards to obtain a base resin powder having a size of 150 to 850 μm.

A super absorbent polymer of Comparative Example 2 was prepared by performing the surface crosslinking step in the same manner as in Example 1, except that the components and contents of the surface crosslinking agent aqueous solution were prepared as described in Table 1 below.

A hydrogel polymer was prepared using the same method as in Example 1.

1,000 g of the hydrogel polymer obtained in step 1 and 0.1 g of glycerol monolaurate (GML) were dissolved in water at 60° C. or higher and input into a cylindrical grinder in the form of an aqueous solution. Thereafter, a high-speed rotary chopper (F-150/Karl Schnell) equipped inside the cylindrical grinder was used to push the material through a porous plate with multiple 10 mm holes at a rotational speed of 1,750 rpm. Subsequently, the polymer was further pushed through a porous plate with multiple 8 mm holes at a rotational speed of 2,200 rpm to obtain a pulverized gel-shaped hydrogel polymer. Thereafter, the ground gel-shaped hydrogel polymer was pushed three times through a porous plate with multiple 6 mm holes at a rotational speed of 250 rpm using a screw-type extruder equipped inside a cylindrical grinder to obtain hydrogel super absorbent polymer particles.

At this time, in the first pass through the porous plate, 340 g of a 32% NaOH aqueous solution was input, and in the second pass, 42.8 g of a 0.5% Na2S2O8 aqueous solution (SPS aqueous solution) was input and pushed through the porous plate. In the third pass, it was passed through a porous plate without adding any additives.

The hydrogel super absorbent polymer particles were uniformly dried in the same manner as in Example 1 to obtain a dried product.

The dried product was ground with a grinder (GRAN-U-LIZERM, MPE) and then classified through a standard mesh sieve of ASTM standards to obtain a base resin powder having a size of 150 to 850 μm.

A super absorbent polymer of Comparative Example 3 was prepared by performing the surface crosslinking step in the same manner as in Example 1, except that the components and contents of the surface crosslinking agent aqueous solution were prepared as described in Table 1 below.

In Comparative Example 4, a super absorbent polymer was prepared by a method of preparing a super absorbent polymer, including: neutralizing at least some of the acidic groups of a water-soluble ethylenically unsaturated monomer (neutralization); crosslinking and polymerizing a water-soluble ethylenically unsaturated monomer having at least some of the acidic groups neutralized in the presence of an internal crosslinking agent and a polymerization initiator to form a hydrogel polymer (polymerization); chopping the hydrogel polymer (chopping); drying the chopped hydrogel polymer (drying); pulverizing the dried polymer and then classifying it into normal particles and fine particles (pulverization/classification); and forming a surface crosslinked layer on at least a portion of the surface of the normal particles in the presence of a surface crosslinking agent (surface crosslinking).

Specifically, the preparation process of Comparative Example 4 was as follows.

A monomer composition was prepared by mixing 495 g of acrylic acid, 0.4 g of ethylene glycol diglycidyl ether as an internal crosslinking agent, 19.2 g of 1% IGAGURE 819 as a photopolymerization initiator, 0.6 g of capsule-type foaming agent F-36D as a foaming agent, 0.1 g of sodium dodecyl sulfate as a foaming stabilizer, and 292.6 g of water in a 3 L glass container equipped with a stirrer and a thermometer. Subsequently, the monomer solution was continuously supplied using a quantitative pump, while 611.1 g of a 32% NaOH aqueous solution was continuously mixed through the line to prepare a monomer aqueous solution. At this time, after confirming that the temperature of the monomer aqueous solution had risen to approximately 72° C. or higher due to the heat of neutralization, waiting was performed for the temperature to cool to 40° C. When the temperature was cooled to 40° C., 41.2 g of a 2 wt % sodium persulfate aqueous solution and 21.2 g of a mixed solution of 0.6 g of sodium bicarbonate dissolved in a 1 wt % sodium dodecyl sulfate aqueous solution were added. The mixed solution was placed in a stainless steel vessel measuring 250 mm in width, 250 mm in length, and 30 mm in height, which was installed in a square polymerization reactor equipped with a light irradiation device on the top and preheated to 80° C. inside, and light irradiation was performed to initiate photopolymerization. It was confirmed that after about 15 seconds of light irradiation, a gel was generated from the surface, and after approximately 30 seconds, a polymerization reaction occurred simultaneously with foaming, and a sheet-shaped hydrogel polymer was obtained by additionally reacting for 3 minutes.

The hydrogel polymer was cut into 5 cm wide and 5 cm long pieces, and the hydrogel gel was pulverized using a screw-type chopper (meat chopper) equipped with a porous plate including numerous holes. At this time, the rotational speed of the screw-type chopper was 250 rpm, and the hole size of the porous plate was 10 mm.

1,000 g of the pulverized polymer was input to a ventilated belt-type dryer including a porous plate capable of vertically transferring air flow. 180° C. hot air at was flowed from the bottom to the top for 15 minutes so that the moisture content of the dried product became approximately 2%, and then was flowed from the top to the bottom for another 15 minutes to uniformly dry the polymer and prepare a dried base resin powder.

The dried base resin powder was ground with a grinder (GRAN-U-LIZERM, MPE) and then classified through a standard mesh sieve of ASTM standards to obtain a super absorbent polymer powder having a size of 150 to 850 μm.

A super absorbent polymer of Comparative Example 4 was prepared by performing the surface crosslinking step in the same manner as in Example 1 using the surface crosslinking agent aqueous solution of Example 1.

A
B
C
D
E
F
G

The materials A to G in Table 1 above are as follows, and in Table 1 above, ‘—’ means a component not included in the surface crosslinking agent aqueous solution, and the units of each number are g. That is, Table 1 shows the amount of material used per 100 g of the base resin powder.

<Experimental Example 1>—Time-Domain Nuclear Magnetic Resonance Spectroscopy (TD-NMR) Analysis

The normalized intensity of the super absorbent polymer of Example 1 was obtained according to steps 1 to 4 below.

Step 1) Drying (Pretreatment) of Super Absorbent Polymer

The super absorbent polymer of Example 1 was dried at about 100° C. for about 12 hours.

Step 2) Swelling (Sampling) of Dried Super Absorbent Polymer

The analysis results derived from step 3 refer to the peak intensity that appears as a result of observing free induction decay (FID), which appears as the decay of the 1H signal through TD-NMR analysis. The intensity is observed sequentially over time. At this time, the time at which the intensity is observed corresponds to the signal acquisition time.

The remaining intensities were normalized based on the third intensity (I3). Specifically, the relative sizes of the other intensities were calculated when the third intensity (I3) was set as 100.

For example, the nth intensity is normalized by calculating it using the following Equation A.

in Equation A,

Subsequently, the normalized intensity (Nt=0.1-) corresponding to a signal acquisition time of 0.1 ms was measured.

The results are shown in Table 2 below.

In addition, for Examples 2 to 4 and Comparative Examples 1 to 4, the normalized intensity (Nint, 0.1) corresponding to a signal acquisition time of 0.1 ms was measured in the same manner as in Example 1.

The results are also shown in Table 2 below.

<Experimental Example 2> Measurement of Convexity and CE Diameter

In addition, the convexity and CE diameter of the super absorbent polymer particles manufactured in the examples and comparative examples were measured using the following methods, and are shown in Table 3 below.

Unless otherwise specified, all of the following properties evaluations were performed under constant temperature and humidity (23±1° C., relative humidity 50±10%), and saline or brine refers to a 0.9 wt % sodium chloride (NaCl) aqueous solution.

After the sample to be measured was left under constant temperature and humidity conditions for 24 hours, each property was evaluated.

For the super absorbent polymers of the examples and comparative examples, the convexity and CE diameter were measured using Morphologi 4 from Malvern Panalytical by the following method.

Among these, the average value of convexity calculated by Equation 2 and CE diameter are shown in Table 3 below.

Average CE

Average convexity
diameter (μm)

<Experimental Example 3> Property Evaluation

In addition, the properties of the super absorbent polymers prepared in the examples and comparative examples were evaluated using the following methods and are shown in Tables 4 to 6 below.

Unless otherwise specified, all of the following properties evaluations were performed under constant temperature and humidity (23±1° C., relative humidity 50±10%), and saline or brine refers to a 0.9 wt % sodium chloride (NaCl) aqueous solution.

After the sample to be measured was left under constant temperature and humidity conditions for 24 hours, each property was evaluated.

(1) Gel Strength Measurement

The gel strength of the super absorbent polymers prepared in the examples and comparative examples was measured by the following method.

Brine with 0.005% ascorbic acid (ACS) was prepared using brine and ascorbic acid (ACS). Subsequently, 0.005% ASC brine and about 2.5 (2.5±0.01 g) of super absorbent polymer were input to a 100 ml beaker and mixed, the beaker was sealed with a transparent wrap, and then input to an oven and the super absorbent polymer was allowed to swell at a temperature of 40° C. for 24 hours. Thereafter, the transparent wrap was removed from the beaker taken out of the oven, and after being left at room temperature for 30 minutes, the gel strength of the super absorbent polymer was measured using a tensile compression tester (manufacturer: Nidec-Shimpo).

The results are shown in Table 4 below.

The centrifuge retention capacity of the super absorbent polymers of the examples and comparative examples by the absorption rate under no-load was measured according to the European Disposables and Nonwovens Association (EDANA) standard EDANA WSP 241.3.

Measurements were performed at a temperature of 23±2° C. and a relative humidity of 45±15% as described in EDANA WSP 241.0.

Specifically, W0 (g) (about 0.2 g) of the super absorbent polymer obtained through each of the examples and comparative examples was uniformly placed in a nonwoven bag, sealed, and then immersed in saline (0.9 wt %) at room temperature. After 30 minutes, water was removed from the bag using a centrifuge at 250 G for 3 minutes, and the mass W2 (g) of the bag was measured. In addition, the same operation was performed without using the resin, and the mass W1 (g) at that time was measured.

Using each mass obtained, CRC (g/g) was calculated according to the following Mathematical Formula 1.

The measurement was repeated five times, and the average and standard deviation were calculated.

The results are shown in Table 4 below.

The absorbency under pressure of the super absorbent polymers of the examples and comparative examples at 0.3 psi was measured according to the EDANA method WSP 242.3.

Measurements were performed at a temperature of 23±2° C. and a relative humidity of 45±15% as described in EDANA WSP 242.0.

Specifically, a 400 mesh stainless steel wire mesh was installed on the bottom of a plastic cylinder with an inner diameter of 25 mm. Under the conditions of room temperature and 50% humidity, W0 (g) (0.9 g) of the super absorbent polymer was uniformly sprayed on the wire mesh, and a piston capable of uniformly applying a load of 0.3 psi thereon was formed with an outer diameter slightly smaller than 25 mm, and there were no gap with the inner wall of the cylinder and no obstructed up-and-down movement. At this time, the weight W3 (g) of the device was measured.

A glass filter with a diameter of 90 mm and a thickness of 5 mm was placed on the inside of a petroleum dish with a diameter of 150 mm, and saline composed of 0.9 wt % sodium chloride was placed so that it was at the same level as the upper surface of the glass filter. A sheet of filter paper with a diameter of 90 mm was placed thereon. The measuring device was placed on a filter paper and the liquid was absorbed under a load for 1 hour. After 1 hour, the measuring device was lifted and its weight W4 (g) was measured.

Using each mass obtained, the absorbency under pressure (g/g) was calculated according to the following Mathematical Formula 2.

The measurement was repeated five times, and the average and standard deviation were calculated.

The results are shown in Table 4 below.

The measured retention capacity and absorbency under pressure were applied to Equation 3 below to calculate the effective absorption capacity (EFFC).

The results are shown in Table 4 below.

The absorption rate (vortex time) of the super absorbent polymer of the examples and comparative examples was measured by the following method.

The results are shown in Table 4 below.

(6) Free Swelling Capacity (FSC110) and 1-Minute absorption capacity (WFA110) in Water Having Electrical Conductivity Value of 110 μS/Cm

For the super absorbent polymer of Example 1, the free swelling capacity (FSC110) and 1-minute absorption capacity (WFA110) in water having an electrical conductivity value of 110 μS/cm at 24° C. were measured using the following method. The specific measurement process was as follows.

In addition, for the super absorbent polymers of Examples 2 to 4 and Comparative Examples 1 to 4, the same experiments as for the super absorbent polymer of Example 1 were additionally performed.

The results are shown in Tables 4 and 5 below.

(7) Measurement of Liquid Permeability of Super Absorbent Polymer

At room temperature (23° C. to 25° C.), 0.2 g of the super absorbent polymers of the examples and comparative examples were placed in a cylinder with an inner diameter of 20 mm, and saline (0.9 wt % sodium chloride aqueous solution) was poured so that the super absorbent polymer was completely submerged, and the super absorbent polymer was allowed to swell for 30 minutes. Thereafter, a pressure of 0.3 psi was applied using a piston with an outer diameter slightly smaller than 20 mm.

The time (T1) taken for 20 mL of saline to pass through the swollen super absorbent polymer was measured by measuring the decrease in height of the saline filled in the cylinder under a pressure of 0.3 psi.

Subsequently, the liquid permeability of the super absorbent polymer was calculated according to Equation 4 below.

The results are shown in Table 6 below.

Vortex

time
WFA110

Free swelling capacity (FSC110) over time in water with

Liquid permeability of

As can be seen in Tables 3 to 6 above, in the case of the super absorbent polymers of the examples satisfying Equation 1, as measured by TD-NMR of the super absorbent polymer and normalized by Equation A, it was confirmed that they can exhibit an excellent physical property balance of absorption performance and liquid permeability.

The super absorbent polymer of the present disclosure has excellent absorbency under pressure and liquid permeability, and when applied to a product, can quickly absorb body fluid and retain a large amount of body fluid without it leaking to the outside. That is, the super absorbent polymer of the present disclosure has an excellent balance between physical properties.