WO2021187326A1

WO2021187326A1 - Method for producing water-absorbent resin particles

Info

Publication number: WO2021187326A1
Application number: PCT/JP2021/009871
Authority: WO
Inventors: 志保岡澤
Original assignee: 住友精化株式会社
Priority date: 2020-03-18
Filing date: 2021-03-11
Publication date: 2021-09-23
Also published as: JPWO2021187326A1

Abstract

Disclosed is a method for producing water-absorbent resin particles, said method comprising heating a mixture of a powder of polymer particles containing a polymer with a crosslinking solution containing a surface crosslinking agent to thereby crosslink the polymer by the surface crosslinking agent. In this method, when the centrifuge retention capacity (CRC) of the polymer particles before the polymer is crosslinked by the surface crosslinking agent is referred to as CRC (before surface crosslinking) and the CRC after the polymer is crosslinked by the surface crosslinking agent is referred to as CRC (after surface crosslinking), the CRC (before surface crosslinking) is 65 g/g or more and the polymer is crosslinked by the surface crosslinking agent so as to adjust the change amount of CRC, which is represented by ΔCRC and calculated by the formula: ΔCRC=CRC (before surface crosslinking) - CRC (after surface crosslinking), to 10-30 g/g.

Description

Method for producing water-absorbent resin particles

The present invention relates to a method for producing water-absorbent resin particles.

Patent Documents 1 and 2 disclose a method for producing surface-crosslinked water-absorbent resin particles that can be used for absorbent articles such as sanitary products.

Japanese Unexamined Patent Publication No. 2004-359943 Special Table 2018-511672

In general, it is desirable that the absorbent resin particles not only have a large amount of water that can be absorbed and retained under no pressure, but also have high absorption performance under pressure. Further, depending on the usage pattern of the absorber containing the water-absorbent resin particles, it may be desirable that the difference between the amount of water that can be absorbed and retained under no pressure and the amount of water that can be absorbed under pressure is large. Such absorption characteristics are advantageous when, for example, the water-absorbent resin particles are used in a portion of the absorbent body of a disposable diaper where the pressure fluctuates greatly depending on the posture of the wearer or the like. In that case, the pulp or the like in the absorber quickly absorbs water under temporary pressure, and when the pressure is released thereafter, the water-absorbent resin particles can efficiently absorb the water retained by the pulp. can.

Therefore, one aspect of the present invention is that a sufficient amount of water can be absorbed and retained under no pressurization, and the difference between the amount of water that can be absorbed and retained under no pressurization and the amount of water absorption under pressurization is large. Provided is a method capable of producing resin particles.

One aspect of the present invention relates to a method for producing water-absorbent resin particles containing polymer particles. The method comprises cross-linking the polymer with the surface cross-linking agent by heating a mixture of a powder of polymer particles containing the polymer and a cross-linking agent solution containing a surface cross-linking agent. The centrifuge retention capacity (CRC) of the polymer particles is CRC (before surface cross-linking) before the polymer is cross-linked by the surface cross-linking agent, and after the polymer is cross-linked by the surface cross-linking agent. When CRC (after surface cross-linking), CRC (before surface cross-linking) is 65 g / g or more, and the polymer is calculated by the formula: ΔCRC = CRC (before surface cross-linking) -CRC (after surface cross-linking). It is crosslinked by the surface cross-linking agent so that the amount of change ΔCRC of CRC is 10 to 30 g / g.

According to one aspect of the present invention, a sufficient amount of water can be absorbed and retained under no pressurization, and the difference between the amount of water that can be absorbed and retained under no pressurization and the amount of water absorption under pressurization is large. Resin particles can be produced.

It is sectional drawing which shows one Embodiment of an absorbent article. It is a schematic diagram which shows the measuring method of the absorption ratio under pressure (AAP).

Hereinafter, some embodiments of the present invention will be described in detail. However, the present invention is not limited to the following embodiments.

In the present specification, "(meth) acrylic" means both acrylic and methacryl. Similarly, "acrylate" and "methacrylate" are also referred to as "(meth) acrylate". The same is true for other similar terms. "(Poly)" means both with and without the "poly" prefix. In the numerical range described stepwise in the present specification, the upper limit value or the lower limit value of the numerical range of one step can be arbitrarily combined with the upper limit value or the lower limit value of the numerical range of another step. In the numerical range described in the present specification, the upper limit value or the lower limit value of the numerical range may be replaced with the value shown in the examples. "Water-soluble" means that it exhibits a solubility in water of 5% by mass or more at 25 ° C. The materials exemplified in the present specification may be used alone or in combination of two or more. "Saline" refers to a 0.9% by mass sodium chloride aqueous solution. "Room temperature" means 25 ° C.

One embodiment of the method for producing water-absorbent resin particles is to heat a mixture of a powder of polymer particles containing a polymer and a cross-linking agent solution containing a surface cross-linking agent to form a surface cross-linking agent for the polymer. Including cross-linking by.

The polymer forming the polymer particles may be any as long as it can impart water absorption to the polymer particles. The monomer constituting the polymer may be an ethylenically unsaturated monomer. The polymer may be a crosslinked polymer.

The ethylenically unsaturated monomer constituting the polymer is, for example, (meth) acrylic acid and a salt thereof, 2- (meth) acrylamide-2-methylpropanesulfonic acid and a salt thereof, (meth) acrylamide, N, N. -Dimethyl (meth) acrylamide, 2-hydroxyethyl (meth) acrylate, N-methylol (meth) acrylamide, polyethylene glycol mono (meth) acrylate, N, N-diethylaminoethyl (meth) acrylate, N, N-diethylaminopropyl ( It may contain at least one compound selected from the group consisting of meta) acrylate and diethylaminopropyl (meth) acrylamide. When the ethylenically unsaturated monomer contains an amino group, the amino group may be quaternized. The ethylenically unsaturated monomer constituting the polymer is acrylic acid, at least one compound selected from the group consisting of acrylic acid and its salt, methacrylic acid and its salt, acrylamide, methacrylamide, and N, N-dimethylacrylamide. At least one compound selected from the group consisting of acid and its salt, methacrylic acid and its salt, and acrylamide, or at least one compound selected from the group consisting of acrylic acid and its salt, and methacrylic acid and its salt. May include.

The polymer forming the polymer particles may have a functional group that reacts with the cross-linking agent. This functional group may be, for example, a carboxyl group, an amino group, or a combination thereof. The carboxyl group can be, for example, a group derived from acrylic acid, methacrylic acid or salts thereof.

The polymer may contain a monomer unit derived from a monomer other than the ethylenically unsaturated monomer. The proportion of the monomer unit derived from the ethylenically unsaturated monomer (particularly (meth) acrylic acid and a salt thereof) in the polymer may be 70 to 100 mol% with respect to the total amount of the monomer.

The polymer particles can be obtained by, for example, a polymerization method selected from a reverse phase suspension polymerization method, an aqueous solution polymerization method, a bulk polymerization method, and a precipitation polymerization method. Polymer particles containing a crosslinked polymer can be obtained by self-crosslinking during polymerization, reaction with an internal crosslinking agent, or a combination thereof.

An example of a method for obtaining polymer particles by an aqueous solution polymerization method is to polymerize an ethylenically unsaturated monomer in a monomer aqueous solution containing an ethylenically unsaturated monomer, a radical polymerization initiator, and water to obtain a heavy weight. Forming a massive hydrogel polymer containing coalescence and water, crushing the hydrogel polymer to form a crushed product, and drying the crushed product to obtain a dried product. Includes crushing the dried product to obtain polymer particles. The powder of the polymer particles after pulverization may be classified by a sieve or the like.

The radical polymerization initiator may contain a persulfate, an azo compound, a peroxide or a combination thereof. When an azo compound is used as the radical polymerization initiator, polymer particles showing a large CRC tend to be easily obtained. The azo compound may be combined with the peroxide.

Examples of azo compounds used as radical polymerization initiators are 2,2'-azobis (2-amidinopropane) dihydrochloride, 2,2'-azobis {2- [1- (2-hydroxyethyl) -2. -Imidazoline-2-yl] propane} dihydrochloride, 2,2'-azobis [N- (2-carboxyethyl) -2-methylpropion amidine] tetrahydrate, 2,2'-azobis [2-( 2-Imidazoline-2-yl) propane] dihydrochloride, 2,2'-azobis [2- (N-phenylamidino) propane] dihydrochloride, 2,2'-azobis [2- (N-allylamidino)) Propane] dihydrochloride, 2,2'-azobis {2-methyl-N- [1,1-bis (hydroxymethyl) -2-hydroxyethyl] propionamide}, 2,2'-azobis [2-methyl- N- (2-Hydroxyethyl) -propionamide], and 4,4'-azobis (4-cyanovaleric acid). From the viewpoint of forming large CRC polymer particles, the radical polymerization initiators are 2,2'-azobis (2-amidinopropane) dihydrochloride and 2,2'-azobis {2- [1- (2-hydroxy). Ethyl) -2-imidazolin-2-yl] propane} dihydrochloride, 2,2'-azobis [N- (2-carboxyethyl) -2-methylpropionamidine] tetrahydrate, and 2,2'- It may contain at least one azo compound selected from azobis [2- (2-imidazolin-2-yl) propane] dihydrochloride.

Examples of persulfates used as radical polymerization initiators include potassium persulfate, ammonium persulfate, and sodium persulfate.

Examples of peroxides used as radical polymerization initiators are methyl ethyl ketone peroxide, methyl isobutyl ketone peroxide, di-t-butyl peroxide, t-butyl cumyl peroxide, t-butyl peroxyacetate, t-butyl. Peroxyisobutyrate, t-butylperoxypivalate, and hydrogen peroxide.

If the amount of the radical polymerization initiator in the monomer aqueous solution is small, the CRC of the polymer particles tends to increase. From this point of view, the amount of the radical polymerization initiator may be 0.01 to 15 mmol per 1 mol of the monomer unit constituting the polymer in the polymer particles.

The monomer aqueous solution may further contain a chain transfer agent. The use of chain transfer agents can also contribute to increasing the CRC of the polymer particles. The chain transfer agent may include, for example, hypophosphorous acid, phosphorous acid or a combination thereof.

The monomer aqueous solution may contain an internal cross-linking agent, in which case polymer particles containing a cross-linked polymer cross-linked by the internal cross-linking agent can be obtained. The internal cross-linking agent may be a compound having two or more reactive functional groups (for example, polymerizable unsaturated groups).

The internal cross-linking agent may contain a compound having a (meth) acrylic group, an allyl group, an epoxy group, or an amino group. When a compound having these reactive functional groups is used as an internal cross-linking agent, polymer particles exhibiting a large CRC tend to be easily obtained. Examples of compounds having a (meth) acrylic group include (poly) ethylene glycol di (meth) acrylate, trimethylolpropane tri (meth) acrylate, and N, N'-methylenebis (meth) acrylamide. Examples of compounds having an allyl group include triallylamine. Examples of compounds having an epoxy group include (poly) ethylene glycol diglycidyl ether, (poly) propylene glycol diglycidyl ether, (poly) glycerin diglycidyl ether, and epichlorohydrin. Examples of compounds having an amino group include triethylenetetramine, ethylenediamine, and hexamethylenediamine.

If the amount of the internal cross-linking agent in the monomer aqueous solution is small, the CRC of the polymer particles tends to increase. From this point of view, the amount of the internal cross-linking agent may be 0.02 to 0.4 mmol per 1 mol of the ethylenically unsaturated monomer.

The coarsely crushed product obtained by coarsely crushing the hydrogel polymer may be in the form of particles, or may have an elongated shape in which a plurality of particles are connected. The minimum width of the coarse crushed product may be, for example, about 0.1 to 15 mm or 1.0 to 10 mm. The maximum width of the coarse crushed product may be about 0.1 to 200 mm or 1.0 to 150 mm. Examples of devices for crushing include kneaders (eg, pressurized kneaders, double-armed kneaders), meat choppers, cutter mills, and pharma mills.

Most of the water in the crushed product is removed by drying the crushed product. The moisture content of the dried product obtained by drying may be, for example, 20% by mass or less, 10% by mass or less, or 5% by mass or less. The water content of the dried product here means the ratio of the water content in the dried product based on the total mass of the dried product including water. Usually, when a dried product containing water is heated at 200 ° C. for 2 hours, the difference in mass of the dried product before and after heating can be regarded as the amount of water in the dried product. The drying method may be a general method such as natural drying, heat drying, spray drying, freeze drying or a combination thereof. The coarsely crushed product may be dried under normal pressure or reduced pressure. The heating temperature for drying under normal pressure may be 70 to 250 ° C. or 80 to 200 ° C.

The method of crushing the dried product is not particularly limited. For example, the dried product can be crushed by using a crusher such as a centrifugal crusher, a roller mill, a stamp mill, a jet mill, a high-speed rotary crusher, and a container-driven mill.

The powder of the polymer particles obtained by pulverization may be classified. Classification means an operation of dividing a particle group (powder) into two or more particle groups having different particle size distributions. A part of the powder of the polymer particles after the classification may be pulverized and classified again.

The classification method is not particularly limited, but may be, for example, screen classification or wind power classification. Screen classification is a method of classifying particles on a screen into particles that pass through the mesh of the screen and particles that do not pass through the screen by vibrating the screen. Screen classification can be performed using, for example, a vibrating sieve, a rotary shifter, a cylindrical stirring sieve, a blower shifter, or a low-tap shaker. Wind power classification is a method of classifying particles using the flow of air.

The medium particle size of the polymer particles before being mixed with the cross-linking agent solution, which is obtained through pulverization and, if necessary, classification, may be, for example, 200 to 500 μm.

By heating the mixture of the polymer particle powder and the cross-linking agent solution, the polymer mainly near the surface of the polymer particles is cross-linked by the surface cross-linking agent. Usually, the centrifuge holding capacity (CRC) of the polymer particles tends to decrease with this surface cross-linking. When the CRC of the polymer particles is CRC (before surface cross-linking) before the polymer is cross-linked by the surface cross-linking agent and CRC (after surface cross-linking) after the polymer is cross-linked by the surface cross-linking agent. (Before surface cross-linking) is 65 g / g or more. Further, the polymer in the polymer particles is a surface cross-linking agent so that the amount of change in CRC calculated by the formula: ΔCRC = CRC (before surface cross-linking) -CRC (after surface cross-linking) is 10 to 30 g / g. Is bridged by. By surface-crosslinking polymer particles exhibiting a relatively large CRC (before surface cross-linking) under conditions such that ΔCRC is within a specific range, a large CRC is exhibited, and the CRC and the absorption ratio under pressure (AAP) are combined. Absorbent resin particles having a large difference can be obtained.

CRC (before surface cross-linking) is 66 g / g or more, 67 g / g or more, 68 g / g or more, 69 g / g or more, 70 g / g or more, 71 g / g or more, 72 g / g or more, 73 g / g or more, 74 g / g. It may be g or more, or 75 g / g or more, 80 g / g or less, 79 g / g or less, 78 g / g or less, 77 g / g or less, 76 g / g or less, 75 g / g or less, 74 g / g or less, or It may be 73 g / g or less. CRC (before surface cross-linking) is 66 g / g or more and 80 g / g or less, 79 g / g or less, 78 g / g or less, 77 g / g or less, 76 g / g or less, 75 g / g or less, 74 g / g or less, or 73 g. It may be 67 g / g or more, 80 g / g or less, 79 g / g or less, 78 g / g or less, 77 g / g or less, 76 g / g or less, 75 g / g or less, 74 g / g or less, or It may be 73 g / g or less, 68 g / g or more and 80 g / g or less, 79 g / g or less, 78 g / g or less, 77 g / g or less, 76 g / g or less, 75 g / g or less, 74 g / g or less, Alternatively, it may be 73 g / g or less, and 69 g / g or more and 80 g / g or less, 79 g / g or less, 78 g / g or less, 77 g / g or less, 76 g / g or less, 75 g / g or less, 74 g / g or less. , 73 g / g or less, 70 g / g or more and 80 g / g or less, 79 g / g or less, 78 g / g or less, 77 g / g or less, 76 g / g or less, 75 g / g or less, 74 g / g Below, or 73 g / g or less, 71 g / g or more and 80 g / g or less, 79 g / g or less, 78 g / g or less, 77 g / g or less, 76 g / g or less, 75 g / g or less, 74 g / g It may be g or less, or 73 g / g or less, and 72 g / g or more and 80 g / g or less, 79 g / g or less, 78 g / g or less, 77 g / g or less, 76 g / g or less, 75 g / g or less, 74 g. / G or less, or 73 g / g or less, 73 g / g or more and 80 g / g or less, 79 g / g or less, 78 g / g or less, 77 g / g or less, 76 g / g or less, 75 g / g or less, Alternatively, it may be 74 g / g or less.

ΔCRC is 11 g / g or more, 12 g / g or more, 13 g / g or more, 14 g / g or more, 15 g / g or more, 16 g / g or more, 17 g / g or more, 18 g / g or more, 19 g / g or more, or 20 g. It may be 29 g / g or less, 28 g / g or less, 27 g / g or less, 26 g / g or less, 25 g / g or less, 24 g / g or less, 23 g / g or less, 22 g / g or less, 21 g. It may be / g or less, 20 g / g or less, 19 g / g or less, or 18 g / g or less. ΔCRC is 11 g / g or more and 29 g / g or less, 28 g / g or less, 27 g / g or less, 26 g / g or less, 25 g / g or less, 24 g / g or less, 23 g / g or less, 22 g / g or less, 21 g / g. It may be g or less, 20 g / g or less, 19 g / g or less, or 18 g / g or less, and 12 g / g or more and 29 g / g or less, 28 g / g or less, 27 g / g or less, 26 g / g or less, 25 g. / G or less, 24 g / g or less, 23 g / g or less, 22 g / g or less, 21 g / g or less, 20 g / g or less, 19 g / g or less, or 18 g / g or less, 13 / g or more 29 g / g or less, 28 g / g or less, 27 g / g or less, 26 g / g or less, 25 g / g or less, 24 g / g or less, 23 g / g or less, 22 g / g or less, 21 g / g or less, 20 g / g or less, It may be 19 g / g or less, or 18 g / g or less, and 14 g / g or more and 29 g / g or less, 28 g / g or less, 27 g / g or less, 26 g / g or less, 25 g / g or less, 24 g / g or less. , 23 g / g or less, 22 g / g or less, 21 g / g or less, 20 g / g or less, 19 g / g or less, or 18 g / g or less, and 15 g / g or more and 29 g / g or less, 28 g / g Below, 27 g / g or less, 26 g / g or less, 25 g / g or less, 24 g / g or less, 23 g / g or less, 22 g / g or less, 21 g / g or less, 20 g / g or less, 19 g / g or less, or 18 g / g or less. It may be 16 g / g or more and 29 g / g or less, 28 g / g or less, 27 g / g or less, 26 g / g or less, 25 g / g or less, 24 g / g or less, 23 g / g or less, 22 g / g or less. It may be g or less, 21 g / g or less, 20 g / g or less, 19 g / g or less, or 18 g / g or less, and 17 g / g or more and 29 g / g or less, 28 g / g or less, 27 g / g or less, 26 g. It may be / g or less, 25 g / g or less, 24 g / g or less, 23 g / g or less, 22 g / g or less, 21 g / g or less, 20 g / g or less, 19 g / g or less, or 18 g / g or less. 18 g / g or more and 29 g / g or less, 28 g / g or less, 27 g / g or less, 26 g / g or less, 25 g / g or less, 24 g / g or less, 23 g / g or less, 22 g / g or less, 21 g / g or less, It may be 20 g / g or less, or 19 g / g or less, and 19 g / g or more and 29 g / g or less, 28 g / g or less, 27 g / g or less, 26 g / g or less, 25 g / g or less, 24 g / g or less. , 23 g / g or less, 22 g / g or less, 21 g / g or less, or 20 g / g or less, 20 G / g or more and 29 g / g or less, 28 g / g or less, 27 g / g or less, 26 g / g or less, 25 g / g or less, 24 g / g or less, 23 g / g or less, 22 g / g or less, or 21 g / g or less It may be.

When the CRC (before surface cross-linking) is 65 g / g or more and less than 74 g / g, or 70 g / g or more and less than 74 g / g, the Δ CRC may be 10 to 20 g / g. When the CRC (before surface cross-linking) is 74 g / g or more and 80 g / g or less, the ΔCRC may be 10 to 30 g / g.

The CRC (after surface cross-linking) may be, for example, 30 to 70 g / g, 40 to 70 g / g, or 50 to 70 g / g.

The CRC here is a value measured in an environment where the temperature is 25 ± 2 ° C. and the humidity is 50 ± 10%. The method for measuring CRC is to use a non-woven bag having a rectangular main surface of 60 mm × 85 mm containing polymer particles (or water-absorbent resin particles) having a mass of Mc (g) and physiological salt having a temperature of 25 ± 2 ° C. The polymer particles are swollen by immersing in water for 30 minutes to form a gel, and the gel in the non-woven bag is dehydrated by applying a centrifugal force of 250 G for 3 minutes with a centrifugal separator, and after dehydration. The mass Ma (g) of the gel and the non-woven fabric bag was measured, and the non-woven fabric bag containing no polymer particles was immersed in physiological saline for 30 minutes, and then a centrifugal force of 250 G was applied by a centrifugal separator. Dehydrate by adding for a minute, and measure the mass Mb (g) of the non-woven bag after dewatering, and the following formula:
CRC [g / g] = {(Ma-Mb) -Mc} / Mc
Includes calculating CRC by. Mc is 0.2 ± 0.002 g. While the non-woven fabric bag containing the polymer particles is immersed in the saline solution, the saline solution may be agitated if necessary so that the polymer particles absorb water to the limit amount. The method of measuring CRC will be described in more detail in Examples described later. The CRC (after surface cross-linking) may be a value measured for the fraction of the powder of the surface-cross-linked polymer particles that has passed through a sieve having an opening of 850 μm.

The ΔCRC associated with surface cross-linking is adjusted based on, for example, the ratio of the amount of the surface cross-linking agent to the amount of polymer particles, the ratio of the amount of the surface cross-linking agent to the amount of water in the cross-linking agent solution, or a combination thereof. be able to. For example, the amount of surface cross-linking agent (particularly alkylene carbonate) is 0.01 to 0.15 mmol per gram of polymer particles, and the molar ratio of the amount of surface cross-linking agent (particularly alkylene carbonate) to the amount of water in the cross-linking agent solution is When it is 0.01 to 0.10, the polymer in the polymer particles is easily crosslinked by the surface cross-linking agent so that the ΔCRC is 10 to 30 g / g.

The absorption ratio (AAP) of the polymer particles under pressure at a pressure of 2.07 kPa after the polymer is crosslinked with a surface cross-linking agent may be 9.0 g / g or more. The upper limit of AAP is not particularly limited, but is usually about 40 g / g. The method for measuring AAP will be described in detail in Examples described later.

The cross-linking agent solution can be water and a solution containing a surface cross-linking agent dissolved in water. The surface cross-linking agent may further contain a solvent other than water, but the solvent contained in the cross-linking agent solution may be substantially only water. The proportion of the solvent other than water may be 10% by mass or less, 5% by mass or less, or 1% by mass or less based on the mass of the cross-linking agent solution.

Examples of surface cross-linking agents contained in the cross-linking agent solution are alkylene carbonate compounds such as ethylene carbonate; ethylene glycol, propylene glycol, 1,4-butanediol, trimethylolpropane, glycerin, polyoxyethylene glycol, polyoxypropylene glycol. , And polyol compounds such as polyglycerin; (poly) ethylene glycol diglycidyl ether, (poly) glycerin diglycidyl ether, (poly) glycerin triglycidyl ether, trimethylpropan triglycidyl ether (poly) propylene glycol polyglycidyl ether, and Polyglycidyl compounds such as (poly) glycerol polyglycidyl ethers; haloepoxy compounds such as epichlorohydrin, epibromhydrin, and α-methyl epichlorohydrin; isocyanate compounds such as 2,4-tolylene diisocyanate and hexamethylene diisocyanate; 3-methyl -3-oxetane methanol, 3-ethyl-3-oxetane methanol, 3-butyl-3-oxetane methanol, 3-methyl-3-oxetane ethanol, 3-ethyl-3-oxetane ethanol, and 3-butyl-3-oxetane Examples thereof include oxetane compounds such as ethanol; oxazoline compounds such as 1,2-ethylene bisoxazoline; and hydroxyalkylamide compounds such as bis [N, N-di (β-hydroxyethyl)] adipamide. These surface cross-linking agents may be used alone or in combination of two or more. The surface cross-linking agent may contain an alkylene carbonate compound, a polyol compound, or a combination thereof. The ratio of the alkylene carbonate compound in the surface cross-linking agent is 50 to 100% by mass, 60 to 100% by mass, 70 to 100% by mass, 80 to 100% by mass, or 90 to 100% by mass based on the total mass of the surface cross-linking agent. It may be.

From the viewpoint of water absorption performance under pressure of the water-absorbent resin particles, the amount of the surface cross-linking agent is 0.001 to 0.10 mol, 0 per mol of the monomer unit constituting the polymer in the polymer particles. It may be .005 to 0.05 mol or 0.01 to 0.02 mol. The amount of the surface cross-linking agent per 1 g of the polymer particles was 0.01 to 1.30 mmol, 0.05 to 0.65 mmol, 0.08 to 0.25 mmol, or 0.10 to 0.20 mmol. You may.

The heating temperature and heating time for surface cross-linking are adjusted so that the cross-linking reaction proceeds appropriately in consideration of the type of surface cross-linking agent and the like. For example, the heating temperature for surface cross-linking may be 80 ° C. or higher, 100 ° C. or higher, 120 ° C. or higher, 150 ° C. or higher, or 180 ° C. or higher, or 190 ° C. or higher. The heating temperature for surface cross-linking may be 250 ° C. or lower. The heating time for surface cross-linking may be, for example, 5 to 90 minutes.

The surface-crosslinked polymer particles may be further dried or classified if necessary. Inorganic particles may be attached to the surface of the polymer particles. Examples of inorganic particles include silica particles such as amorphous silica.

According to the method according to the present embodiment, water-absorbent resin particles (or surface-crosslinked polymer particles) having a large difference (CRC-AAP) between CRC and AAP can be easily obtained. The CRC-AAP value indicated by the water-absorbent resin particles may be, for example, 30 g / g or more, 34 g / g or more, 38 g / g or more, or 40 g / g or more, 55 g / g or less, or 50 g / g or less. It may be. The CRC-AAP value indicated by the water-absorbent resin particles may be 55 g / g or less at 30 g / g or more, or 50 g / g or less, and 55 g / g or less at 34 g / g or more, or 50 g / g or less. It may be 38 g / g or more and 55 g / g or less, or 50 g / g or less, and 40 g / g or more and 55 g / g or less, or 50 g / g or less.

The produced water-absorbent resin particles are used to form an absorbent body that constitutes an absorbent article such as a diaper, for example. FIG. 1 is a cross-sectional view showing an example of an absorbent article. The absorbent article 100 shown in FIG. 1 includes a sheet-shaped absorber 10, core wraps 20a and 20b, a liquid permeable sheet 30, and a liquid permeable sheet 40. In the absorbent article 100, the liquid permeable sheet 40, the core wrap 20b, the absorbent body 10, the core wrap 20a, and the liquid permeable sheet 30 are laminated in this order. The absorber 10 has water-absorbent resin particles 10a produced by the method according to the above-described embodiment, and a fiber layer 10b containing a fibrous material. The water-absorbent resin particles 10a are dispersed in the fiber layer 10b.

Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited to these examples.

1. 1. Measurement method The centrifuge holding capacity (CRC) and the absorption ratio under pressure (AAP) were measured by the following procedure.

1-1. Centrifuge retention capacity (CRC)
The CRC was measured in the environment of a temperature of 25 ° C. ± 2 ° C. and a humidity of 50 ± 10% by the following procedure.
A non-woven fabric having a size of 60 mm × 170 mm (product name: Heat Pack MWA-18, manufactured by Nippon Paper Papylia Co., Ltd.) is folded in half in the longitudinal direction to adjust the size to 60 mm × 85 mm. Nonwoven fabrics are heat-sealed against each other on both sides extending in the longitudinal direction. This prepares a non-woven fabric bag having a size of 60 mm in width and 85 mm in length and having an opening at one end in the longitudinal direction. On both sides along the longitudinal direction of the non-woven fabric bag, a crimping portion having a width of 5 mm is formed in which a heat seal is interposed between the non-woven fabrics. The non-woven fabric bag contains 0.2 ± 0.002 g of particles to be measured. Next, the non-woven fabric bag is closed by crimping the opening of the non-woven fabric bag with a heat seal.

The entire non-woven fabric bag is completely moistened by floating the non-woven fabric bag on 1000 g of physiological saline contained in a stainless steel vat (240 mm × 320 mm × 45 mm) without folding the non-woven fabric bag. Immerse the non-woven fabric bag in the saline solution with a spatula within 1 minute after putting the non-woven fabric bag in the saline solution, and then stir the saline solution at 300 rpm using a stirrer tip (8 mmφ × 30 mm, no ring). By doing so, the particles to be measured are sufficiently swelled.

Thirty minutes after the non-woven fabric bag is put into the physiological saline solution, the non-woven fabric bag is taken out from the physiological saline solution. Then, the non-woven fabric bag is put into a centrifuge (manufactured by Kokusan Co., Ltd., model number: H-122). After the centrifugal force in the centrifuge reaches 250 G, the non-woven fabric bag is dehydrated for 3 minutes. After dehydration, the mass Ma of the non-woven fabric bag containing the mass of the gel is weighed. The non-woven fabric bag is subjected to the same operation as described above without accommodating the particles to be measured, and the mass Mb of the non-woven fabric bag is measured. The CRC is calculated based on the following formula. Mc is a precise value of 0.2 g of the mass of the particles to be measured used in the measurement.
CRC [g / g] = {(Ma-Mb) -Mc} / Mc

1-2. Absorption rate under pressure (AAP)
Using the measuring device 110 shown in FIG. 2, the absorption ratio (AAP) under pressure at a pressure of 2.07 kPa (0.3 psi) is measured. The measuring device 110 is composed of a weight 112, a plastic cylinder 114 having an inner diameter of 60 mm, and a wire mesh 116 having a 400 mesh (opening 38 μm). The wire mesh 116 closes one opening of the cylinder 114, and the cylinder 114 and the wire mesh 116 are arranged so that the wire mesh 116 is horizontal. The weight 112 has a disc portion 112a, a rod-shaped portion 112b extending from the center of the disc portion 112a in a direction perpendicular to the disc portion 112a, and a cylindrical portion 112c having a through hole in the center. The rod-shaped portion 112b is inserted into the through hole of the cylindrical portion 112c. The disk portion 112a has a diameter substantially equal to the inner diameter of the cylinder 114 so that it can be moved in the longitudinal direction of the cylinder 114 inside the cylinder 114. The diameter of the cylindrical portion 112c is smaller than the diameter of the disc portion 112a. The weight of the weight 112 is adjusted so that a pressure of 2.07 kPa is applied to the particles to be measured. Inside the cylinder 114, 0.90 g of particles 120 to be measured are uniformly scattered on the wire mesh 116. The weight 112 inserted in the cylinder 114 is placed on the sprayed particles. In this state, the total mass Wa [g] of the measuring device 110 and the particles 120 before absorbing the liquid is measured.

A glass filter 140 (ISO4793 P-250) having a diameter of 90 mm and a thickness of 7 mm is placed in the center of the bottom surface (diameter 150 mm) in the recess of the stainless steel petri dish 130. Next, 0.90 mass% sodium chloride aqueous solution (25 ° C. ± 2 ° C.) is added to the stainless steel petri dish until the water surface is level with the upper surface of the glass filter 140. A sheet of filter paper 150 with a diameter of 90 mm (ADVANTEC Toyo Co., Ltd., product name: (No. 3), thickness 0.23 mm, reserved particle diameter 5 μm) is placed on the glass filter 140, and the entire surface of the filter paper 150 is sodium chloride. Wet with an aqueous solution to remove excess sodium chloride aqueous solution. Subsequently, the measuring device 110 in which the particles 120 before liquid absorption are charged is placed on the filter paper 150, and the particles 120 are made to absorb the sodium chloride aqueous solution while pressurizing the particles 120 at a pressure of 2.07 kPa. One hour after the start of absorption, the measuring device 110 is lifted and the total mass Wb [g] of the measuring device 110 and the particles 120 after absorbing the liquid is measured.

From Wa and Wb, the absorption ratio under pressure (AAP) [g / g] is calculated by the following formula.
AAP [g / g] = (Wb [g] -Wa [g]) /0.90 [g]

2. Polymer particles before surface cross-linking (Production Example 1)
340.0 g (4.72 mol) of acrylic acid was placed in a separable flask having an internal volume of 2 L. To the acrylic acid in the separable flask, 291.7 g of ion-exchanged water was added with stirring. Then, 297.8 g of 48% by mass sodium hydroxide was added dropwise under an ice bath to prepare a partially neutralized solution of acrylic acid having a monomer concentration of 45% by mass.
888.30 g of the prepared acrylic acid partial neutralizing solution, 150.49 g of ion-exchanged water, and 0.309 g of polyethylene glycol diacrylate (n = 9) as an internal cross-linking agent (Nichiyu Co., Ltd., Blemmer ADE-400A). Is placed in a fluororesin-coated stainless steel bat (outer dimensions: 297 mm x 232 mm x height 50 mm) and stirred with two stirrers (diameter 8 mm, length 45 mm) to make it uniform in the stainless steel bat. A mixture was formed. Then, the inside of the stainless steel vat was sealed by sealing the upper part of the stainless steel vat with a polyethylene film. After adjusting the temperature of the mixture in the stainless steel pad to 25 ° C., the amount of dissolved oxygen was adjusted to 0.1 ppm or less by substituting nitrogen in the mixture. Then, while stirring the mixture at 300 rpm, 6.46 g (manufactured by Wako Pure Chemical Industries, Ltd.) aqueous solution (2,2'-azobis (2-amidinopropane) dihydrochloride (V-50)) having a concentration of 5% by mass (manufactured by Wako Pure Chemical Industries, Ltd.) 1.191 mmol), 3.39 g of L-ascorbic acid aqueous solution with a concentration of 0.5 mass%, and 3.69 g of hydrogen peroxide solution with a concentration of 0.35 mass%. , Terumo Co., Ltd. injection needle) was used to drop the drops in order in a sealed state.
Immediately after the hydrogen peroxide solution was added dropwise, the polymerization reaction started. After the viscosity of the reaction solution increased as the polymerization reaction proceeded, the reaction solution gelled. The thermometer installed at 3 minutes after the completion of dropping the hydrogen peroxide solution showed 106 ° C., and then the temperature began to decrease. A stainless steel vat containing a hydrogel polymer containing water and a polymer formed by gelation of the reaction solution was immersed in a water bath at 75 ° C., and the hydrogel polymer was aged in that state for 20 minutes.
The entire amount of the hydrogel-like polymer after aging was taken out from a stainless steel vat and roughly crushed by cutting into approximately 0.8 cm squares. The obtained coarse crushed product was spread over a wire mesh having a mesh size of 0.8 cm × 0.8 cm and placed, and dried with hot air at 180 ° C. for 30 minutes to obtain a dried product.
Next, the dried product was crushed using a centrifugal crusher (manufactured by Retsch, ZM200, screen diameter 1 mm, 6000 rpm). The powder after pulverization was sieved by shaking for 1 minute using a sieve having an opening of 850 μm and a sieve having an opening of 180 μm. Amorphous crushed polymer particles that passed through a sieve having a mesh size of 850 μm and remained on the sieve having a mesh size of 180 μm were obtained. The CRC and AAP of the obtained polymer particles were measured.

(Manufacturing Example 2)
A partially neutralized solution of acrylic acid was prepared in the same manner as in Production Example 1. 888.30 g of the prepared acrylic acid partial neutralizing solution, 157.49 g of ion-exchanged water, and 0.077 g of polyethylene glycol diacrylate (n = 9) as an internal cross-linking agent (Nichiyu Co., Ltd., Blemmer ADE-400A). Is placed in a fluororesin-coated stainless steel bat (outer dimensions: 297 mm x 232 mm x height 50 mm) and stirred with two stirrers (diameter 8 mm, length 45 mm) to obtain a uniform mixture in the stainless steel bat. It was formed inside. Then, the inside of the stainless steel vat was sealed by sealing the upper part of the stainless steel vat with a polyethylene film. After adjusting the temperature of the mixture in the stainless steel pad to 25 ° C., the amount of dissolved oxygen was adjusted to 0.1 ppm or less by substituting nitrogen in the mixture. Then, while stirring the mixture at 300 rpm, 3.23 g (manufactured by Wako Pure Chemical Industries, Ltd.) aqueous solution (2,2'-azobis (2-amidinopropane) dihydrochloride (V-50)) having a concentration of 5% by mass (manufactured by Wako Pure Chemical Industries, Ltd.) 1.70 g of L-ascorbic acid aqueous solution having a concentration of 0.596 mmol) and 0.5 mass% of concentration, and 1.85 g of hydrogen peroxide solution having a concentration of 0.35 mass% were added to a syringe (10 mL disposable syringe manufactured by Terumo Co., Ltd.). , Terumo Co., Ltd. injection needle) was used to drop the drops in order in a sealed state.
Immediately after the hydrogen peroxide solution was added dropwise, the polymerization reaction started. After the viscosity of the reaction solution increased as the polymerization reaction proceeded, the reaction solution gelled. The thermometer installed at 4 minutes after the completion of dropping the hydrogen peroxide solution showed 80 ° C., and then the temperature began to decrease. A stainless steel vat containing a hydrogel polymer containing water and a polymer formed by gelation of the reaction solution was immersed in a water bath at 75 ° C., and the hydrogel polymer was aged in that state for 20 minutes.
The hydrogel polymer after aging was coarsely crushed in the same manner as in Production Example 1 to obtain a coarsely crushed product. This coarsely crushed product (subdivided particulate hydrogel polymer) is spread and placed on a wire mesh with a mesh size of 0.8 cm × 0.8 cm, and then dried with hot air at 180 ° C. for 30 minutes to obtain a dried product. rice field.
Next, the dried product was crushed using a centrifugal crusher (manufactured by Retsch, ZM200, screen diameter 1 mm, 6000 rpm). The powder after pulverization was sieved by shaking for 1 minute using a sieve having an opening of 850 μm and a sieve having an opening of 180 μm. Amorphous crushed polymer particles that passed through a sieve having a mesh size of 850 μm and remained on the sieve having a mesh size of 180 μm were obtained. The CRC and AAP of the obtained polymer particles were measured.

(Manufacturing Example 3)
500 g (6.94 mol) of 100% acrylic acid was placed in a round bottom cylindrical separable flask having an inner diameter of 11 cm and an internal volume of 2 L equipped with a stirrer. While stirring this acrylic acid, 428.27 g of ion-exchanged water was added into the separable flask. Subsequently, 439.41 g of 48 mass% sodium hydroxide was added dropwise under an ice bath to prepare 1367.68 g of a partially neutralized acrylic acid solution having a monomer concentration of 45 mass%. The same operation was repeated 3 times to obtain a required amount of acrylic acid partial neutralizing solution.
To 2784.19 g of the prepared acrylic acid partial neutralizing solution, 406.49 g of ion-exchanged water and 0.970 g of polyethylene glycol diacrylate (n = 9) (Nippon Oil Co., Ltd., Blemmer ADE-400A) were added to the reaction solution (single). (Medium aqueous solution) was obtained. This reaction solution was replaced with nitrogen gas for 30 minutes in a nitrogen gas atmosphere. Next, the reaction solution was supplied to a stainless steel 5L double-armed kneader with a jacket having two sigma-type blades with lids that can be opened and closed. The inside of the kneader was replaced with nitrogen gas while maintaining the temperature of the reaction solution at 30 ° C. Subsequently, while stirring the reaction solution, 92.52 g (7.771 mmol) of a 2.0 mass% sodium persulfate aqueous solution and 15.83 g of a 0.5 mass% L-ascorbic acid aqueous solution were added. Polymerization started after 1 minute. After 3 minutes, the maximum temperature during polymerization was 90 ° C., and then 60 minutes after the start of polymerization while keeping the internal temperature at 60 ° C., the polymer and the hydrogel polymer containing water were taken out from the kneader. rice field.
The obtained hydrogel polymer was sequentially charged into a meat chopper 12VR-750SDX manufactured by Kiren Royal Co., Ltd. and coarsely crushed. The diameter of the hole in the plate located at the outlet of the meat chopper was 6.4 mm. A coarsely crushed product of a hydrogel polymer formed by coarse crushing was spread over a wire mesh having a mesh size of 0.8 cm × 0.8 cm, and dried with hot air at 160 ° C. for 60 minutes to obtain a dried product.
The dried product was pulverized using a centrifugal pulverizer (Resch, ZM200, screen diameter 1 mm, 6000 rpm). The powder after pulverization was sieved by shaking for 1 minute using a sieve having an opening of 850 μm and a sieve having an opening of 180 μm. Amorphous crushed polymer particles that passed through a sieve having a mesh size of 850 μm and remained on the sieve having a mesh size of 180 μm were obtained. The CRC and AAP of the obtained polymer particles were measured.

(Manufacturing Example 4)
500 g (6.94 mol) of 100% acrylic acid was placed in a round bottom cylindrical separable flask having an inner diameter of 11 cm and an internal volume of 2 L equipped with a stirrer. While stirring this acrylic acid, 428.27 g of ion-exchanged water was added into the separable flask. Subsequently, 439.41 g of 48 mass% sodium hydroxide was added dropwise under an ice bath to prepare 1367.68 g of a partially neutralized acrylic acid solution having a monomer concentration of 45 mass%. The same operation was repeated twice to obtain a required amount of acrylic acid partial neutralizing solution.
To 2320.16 g of the prepared acrylic acid partial neutralizing solution, 337.13 g of ion-exchanged water and 2.420 g of polyethylene glycol diacrylate (n = 9) (Nippon Oil Co., Ltd., Blemmer ADE-400A) were added to the reaction solution (single). (Medium aqueous solution) was obtained. This reaction solution was replaced with nitrogen gas for 30 minutes in a nitrogen gas atmosphere. The reaction solution was supplied to a stainless steel 5L double-armed kneader with a jacket having two sigma-type blades with lids that can be opened and closed. The inside of the kneader was replaced with nitrogen gas while maintaining the temperature of the reaction solution at 30 ° C. Subsequently, while stirring the reaction solution, 77.10 g (6.476 mmol) of a 2.0 mass% sodium persulfate aqueous solution and 13.19 g of a 0.5 mass% L-ascorbic acid aqueous solution were added. Polymerization started after 1 minute. After 3 minutes, the maximum temperature during polymerization was 90 ° C., and then 60 minutes after the start of polymerization while keeping the internal temperature at 60 ° C., the polymer and the hydrogel polymer containing water were taken out from the kneader. rice field.
The obtained hydrogel polymer was sequentially charged into a meat chopper 12VR-750SDX manufactured by Kiren Royal Co., Ltd. and coarsely crushed. The diameter of the hole in the plate located at the outlet of the meat chopper was 6.4 mm. A coarsely crushed product of a hydrogel polymer formed by coarse crushing was spread over a wire mesh having a mesh size of 0.8 cm × 0.8 cm, and dried with hot air at 160 ° C. for 60 minutes to obtain a dried product.
The dried product was pulverized using a centrifugal pulverizer (Resch, ZM200, screen diameter 1 mm, 6000 rpm). The powder after pulverization was sieved by shaking for 1 minute using a sieve having an opening of 850 μm and a sieve having an opening of 180 μm. Amorphous crushed polymer particles that passed through a sieve having a mesh size of 850 μm and remained on the sieve having a mesh size of 180 μm were obtained. The CRC and AAP of the obtained polymer particles were measured.

Table 1 shows the CRC and AAP of the polymer particles obtained in Production Examples 1 to 4.

3. 3. Surface-crosslinked polymer particles (water-absorbent resin particles)
(Example 1)
A cross-linking agent solution consisting of 0.0783 g of ethylene carbonate (EC), 0.125 g of propylene glycol (PG), and 0.5 g of deionized water was mixed with 25 g of the polymer particles obtained in Production Example 1. Surface cross-linking was allowed to proceed by heating the formed mixture at 200 ° C. for 35 minutes. By classifying the polymer particles after surface cross-linking with a sieve having a mesh size of 850 μm, fractions that passed through the sieve having a mesh size of 850 μm were obtained, and the surface-crosslinked polymer particles were obtained as water-absorbent resin particles.

(Example 2)
Surface-crosslinked polymer particles were obtained as water-absorbent resin particles in the same manner as in Example 1 except that the amount of deionized water contained in the cross-linking agent solution was changed to 0.1625 g.

(Example 3)
The surface was crosslinked in the same manner as in Example 1 except that the amount of EC contained in the cross-linking agent solution was changed to 0.2345 g and the amount of deionized water contained in the cross-linking agent solution was changed to 2.0 g. Polymer particles were obtained as water-absorbent resin particles.

(Example 4)
The surface was crosslinked in the same manner as in Example 1 except that the amount of EC contained in the cross-linking agent solution was changed to 0.2345 g and the amount of deionized water contained in the cross-linking agent solution was changed to 1.5 g. Polymer particles were obtained as water-absorbent resin particles.

(Example 5)
The surface was crosslinked in the same manner as in Example 1 except that the amount of EC contained in the cross-linking agent solution was changed to 0.2345 g and the amount of deionized water contained in the cross-linking agent solution was changed to 1.0 g. Polymer particles were obtained as water-absorbent resin particles.

(Example 6)
Surface-crosslinked polymer particles were obtained as water-absorbent resin particles in the same manner as in Example 1 except that the amount of EC contained in the cross-linking agent solution was changed to 0.2345 g.

(Example 7)
Surface-crosslinked polymer particles were obtained as water-absorbent resin particles in the same manner as in Example 3 except that the polymer particles obtained in Production Example 2 were used.

(Example 8)
The surface was crosslinked in the same manner as in Example 7 except that the amount of EC contained in the cross-linking agent solution was changed to 0.3126 g and the amount of deionized water contained in the cross-linking agent solution was changed to 2.75 g. Polymer particles were obtained as water-absorbent resin particles.

(Comparative Example 1)
The surface was crosslinked in the same manner as in Example 7 except that the amount of EC contained in the cross-linking agent solution was changed to 0.3908 g and the amount of deionized water contained in the cross-linking agent solution was changed to 3.5 g. Polymer particles were obtained as water-absorbent resin particles.

(Comparative Example 2)
Surface-crosslinked polymer particles were obtained as water-absorbent resin particles in the same manner as in Example 3 except that the polymer particles obtained in Production Example 3 were used.

(Comparative Example 3)
Surface-crosslinked polymer particles were obtained as water-absorbent resin particles in the same manner as in Comparative Example 2 except that the amount of deionized water contained in the cross-linking agent solution was changed to 0.5 g.

(Comparative Example 4)
The surface was crosslinked in the same manner as in Example 1 except that the polymer particles obtained in Production Example 4 were used and the amount of deionized water contained in the cross-linking agent solution was changed to 2.0 g. Polymer particles were obtained as water-absorbent resin particles.

(Comparative Example 5)
Surface-crosslinked polymer particles were obtained as water-absorbent resin particles in the same manner as in Example 1 except that the polymer particles obtained in Production Example 4 were used.

4. Evaluation The CRC and AAP of the obtained water-absorbent resin particles were measured. Difference in CRC before and after surface cross-linking ΔCRC (= CRC (before surface cross-linking) -CRC (after surface cross-linking), sum of CRC and AAP after surface cross-linking (CRC + AAP), and CRC and AAP after surface cross-linking The difference (CRC-AAP) was also determined.

Table 2 shows the results. The EC amount, PG amount, and water amount in the table are the amount of substances of ethylene carbonate, propylene glycol, and water per 1 g of the polymer particles, respectively. In each example, it was confirmed that a sufficiently large CRC was exhibited and that water-absorbent resin particles having a large difference between the CRC and AAP were obtained.

10 ... Absorbent, 10a ... Water-absorbent resin particles, 10b ... Fiber layer, 20a, 20b ... Core wrap, 30 ... Liquid permeable sheet, 40 ... Liquid permeable sheet, 100 ... Absorbent article, 110 ... Measuring device, 112 ... Weight, 112a ... Disc, 112b ... Rod, 112c ... Cylindrical, 114 ... Cylindrical, 116 ... Wire mesh, 120 ... Particles to be measured, 130 ... Stainless petri dish, 140 ... Glass filter, 150 ... Filter paper.

Claims

A method for producing water-absorbent resin particles containing polymer particles.
It comprises cross-linking the polymer with the surface cross-linking agent by heating a mixture of a powder of polymer particles containing the polymer and a cross-linking agent solution containing a surface cross-linking agent.
The centrifuge retention capacity of the polymer particles is CRC (before surface cross-linking) before the polymer is cross-linked by the surface cross-linking agent, and CRC (surface) after the polymer is cross-linked by the surface cross-linking agent. After cross-linking)
CRC (before surface cross-linking) is 65 g / g or more,
Formula: ΔCRC = CRC (before surface cross-linking) -The polymer is cross-linked by the surface cross-linking agent so that the amount of change ΔCRC of CRC calculated by CRC (after surface cross-linking) is 10 to 30 g / g. Method.
The method according to claim 1, wherein the absorption ratio of the polymer particles under pressure at a pressure of 2.07 kPa after the polymer is crosslinked with the surface cross-linking agent is 9.0 g / g or more.