JP7737808B2 - Method for producing resin composition - Google Patents

Description

The present invention relates to a method for producing a resin composition containing polyamide and cellulose nanofibers.

Resin materials are lightweight and have excellent processability, making them widely used in a variety of applications, including automotive components, electrical and electronic components, office equipment housings, and precision components. However, resins alone often lack sufficient mechanical properties and dimensional stability, so composites of resins and various fillers are commonly used. In recent years, the use of natural fibers such as cellulose nanofibers (CNF) as such fillers has been explored. CNF has the advantages of being lightweight, posing a low environmental burden when disposed of, and also of significantly improving the physical properties of resin compositions. In particular, various resin compositions have been proposed that combine such CNF with polyamides, which have excellent heat resistance, dimensional stability, and other properties.

Patent Document 1 describes a polyamide resin composition containing (A) a polyamide in which the amino group terminal concentration [—NH ₂ ] and the carboxyl group terminal concentration [—COOH] satisfy the relationship [—NH ₂ ] > [—COOH], and (B) cellulose fibers having an average fiber diameter of 500 nm or less.

Patent Document 2 describes a polyamide resin composition containing (A) a polyamide in which the amino group terminal concentration [—NH ₂ ] and the carboxyl group terminal concentration [—COOH] satisfy the relationship [—NH ₂ ] < [—COOH], and (B) cellulose fibers having an average fiber diameter of 500 nm or less.

Japanese Patent Application Laid-Open No. 2020-7494 Japanese Patent Application Laid-Open No. 2020-7495

The technologies described in Patent Documents 1 and 2 focus on the degradation of cellulose during the production of a resin composition containing polyamide and cellulose fibers. Specifically, the technology described in Patent Document 1 seeks to provide a resin composition that enables stable molding by suppressing the generation of gas components due to cellulose decomposition, while the technology described in Patent Document 2 seeks to provide a resin composition with reduced discoloration and odor due to the release of acid residues from cellulose. The technologies described in these documents can improve the properties of a resin composition by reducing undesired reactions of cellulose that occur during the production of the resin composition. However, the inventors' investigations have revealed that in resin compositions containing polyamide and cellulose nanofibers, the polyamide may deteriorate due to the thermal history during production, preventing the resin composition from exhibiting the desired physical properties.

The present invention aims to solve the above problems and provide a method for producing a resin composition that contains polyamide and cellulose nanofibers and exhibits good physical properties regardless of the thermal history during production.

The present invention includes the following aspects.
[1] A method for producing a resin composition containing polyamide and cellulose nanofibers,
The method includes a mixing step of mixing a first polyamide (A1) satisfying the relationship of amino terminal group concentration [NH _{2 ]} > carboxyl terminal group concentration [COOH], a second polyamide (A2) satisfying the relationship of amino terminal group concentration [NH 2 ] < carboxyl terminal group concentration [COOH], and cellulose nanofibers (B) to obtain a resin composition;
Based on the number average molecular weight (M _A1 ) of the first polyamide (A1), the mass concentration (C _A1 ) of the first polyamide (A1) in the mixed component, the number average molecular weight (M A2 ) of the second polyamide ( _A2 ), the mass concentration (C A2 ) of the second polyamide (A2) in the mixed component, and the total mass concentration (C T ) of the mass concentration (C _A1 ) of the first polyamide ( _A1 ) and the mass concentration (C _A2 ) of the second polyamide ( _A2 ) in the mixed component, the following formula is calculated:
M _T = (M _A1 × C _A1 + M _A2 × C _A2 )/C _T
and the number average molecular weight ( _M ) of the polyamide in the resin composition is calculated according to the following formula (1a):
M/M _T >1.1 (1a)
A method for producing a resin composition that satisfies the above relationship.
[2] The amino terminal group concentration [NH ₂ ] _A1 and the carboxyl terminal group concentration [COOH] _A1 of the first polyamide (A1), the mass concentration (C _A1 ) of the first polyamide (A1) in the mixed component, the amino terminal group concentration [NH ₂ ] _A2 and the carboxyl terminal group concentration [COOH] _A2 of the second polyamide (A2), the mass concentration (C _A2 ) of the second polyamide (A2) in the mixed component, and the total mass concentration (C T ) of the mass concentration (C _A1 ) of the first polyamide (A1) and the mass concentration (C _A2 ) of the second polyamide ( _A2 ) in the mixed component are calculated using the following formulas (1b) to (1d):
[NH ₂ ] _T = ([NH ₂ ] _A1 × C _A1 + [NH ₂ ] _A2 × C _A2 )/C _T (1b)
[COOH] _T = ([COOH] _A1 × C _A1 + [COOH] _A2 × C _A2 )/C _T (1c)
([NH ₂ ] + [COOH]) _T = [NH ₂ ] _T + [COOH] _T (1d)
The method according to aspect 1, wherein the total concentration ([NH ₂ ]+[COOH]) _T calculated according to the following formula is 10 μequivalents/g or more and 500 μequivalents/g or less.
[3] The method according to aspect 2, wherein the ratio of the [NH ₂ ] _T to the [COOH] _T ([NH ₂ ] _T /[COOH] _T ) is 1 or more and 5 or less.
[4] The method according to any one of the above aspects 1 to 3, wherein the total concentration ([NH ₂ ] + [COOH]) of the amino terminal group concentration [NH ₂ ] and the carboxyl terminal group concentration [COOH] of each of the first polyamide (A1) and the second polyamide (A2) is 10 μequivalents/g or more and 500 μequivalents/g or less.
[5] A method for producing a resin composition containing polyamide and cellulose nanofibers, comprising:
The method includes a mixing step of obtaining a resin composition by mixing a mixed component containing polyamide (A) and cellulose nanofibers (B), the polyamide (A) having an amino terminal group concentration and a carboxyl terminal group concentration each of which is 20 μequivalents/g or more and at least one of which is 70 μequivalents/g or more, or the polyamide (A) having an amino terminal group concentration and a carboxyl terminal group concentration each of which is 50 μequivalents/g or more and at least one of which is 60 μequivalents/g or more,
The number average molecular weight (M) of the polyamide in the resin composition and the number average molecular weight (M _A ) of the polyamide (A) to be mixed satisfy the following formula (2a):
M/M _A >1.1 (2a)
Fulfilling the relationship,
The method for producing a resin composition, wherein the moisture content of the resin composition is higher than the moisture content of the polyamide (A) to be mixed.
[6] The method according to the above aspect 5, wherein the total concentration ([NH ₂ ] + [COOH]) of the amino terminal group concentration [NH ₂ ] and the carboxyl terminal group concentration [COOH] of the polyamide (A) is 90 μequivalents/g or more and 500 μequivalents/g or less.
[7] The method according to the above aspect 5 or 6, wherein one of the amino terminal group concentration [NH ₂ ] and the carboxyl terminal group concentration [COOH] of the polyamide (A) is 20 μequivalents/g or more and less than 70 μequivalents/g, and the other is 70 μequivalents/g or more and 480 μequivalents/g or less.
[8] The method according to any one of aspects 5 to 7, wherein the moisture content of the resin composition is 100 ppm by mass or more and 5000 ppm by mass or less, and the moisture content of the polyamide (A) to be mixed is 1 ppm by mass or more and 1000 ppm by mass or less.
[9] The method according to any one of the above aspects 1 to 8, wherein a part of the resin composition is used as a part of the mixed component.
[10] The method according to any one of Aspects 1 to 9, wherein the cellulose nanofibers (B) have a number average fiber diameter of 2 nm or more and 1000 nm or less.
[11] The method according to any one of Aspects 1 to 10, wherein the cellulose nanofibers (B) have an acyl substitution degree (DS) of 0 or more and 1.5 or less.
[12] The method according to any one of Aspects 1 to 11, wherein the cellulose nanofibers (B) have a crystallinity of 60% or more.
[13] The method according to any one of Aspects 1 to 12, wherein the cellulose nanofibers (B) have an average acid-insoluble component content of 10% by mass or less.

One aspect of the present invention provides a method for producing a resin composition that contains polyamide and cellulose nanofibers and exhibits good physical properties regardless of the thermal history during production.

Illustrative embodiments of the present invention (hereinafter referred to as "present embodiments") are described in detail below, but the present invention is not limited to these embodiments.

This embodiment provides a method for producing a resin composition containing polyamide and cellulose nanofibers. The inventors noticed an unexpected decrease in the molecular weight of polyamide when producing a resin composition by mixing (specifically, by heating and mixing) components containing polyamide and cellulose nanofibers, and conducted various investigations into the cause. As a result, they found that the moisture absorption of polyamide, which is inherently hygroscopic, may be more pronounced in the presence of cellulose nanofibers, which are inherently hydrophilic. In other words, heating polyamide in the presence of cellulose nanofibers may accelerate hydrolysis of the polyamide. After investigating various methods for producing resin compositions with good physical properties despite such polyamide hydrolysis, they found that controlling the structure of the polyamide itself allows for the formation of intermolecular amide bonds through the reaction between the amino and carboxyl terminal groups during the mixing process, i.e., polymerization, to proceed, thereby increasing the molecular weight, thereby enabling the production of resin compositions with excellent physical properties.

Furthermore, after producing a resin composition containing polyamide and cellulose nanofibers and a molded article obtained by molding the same, the resin composition or the molded article may be reused as part of the resin composition. In such cases, the recycled material partially contributes to the thermal history of the recycled resin, which is greater than that of the non-recycled resin. Therefore, it is generally known that the recycled resin has inferior physical properties compared to resin compositions composed solely of non-recycled resins. Therefore, in this embodiment, increasing the molecular weight of the polyamide during mixing with cellulose nanofibers is advantageous for reusing such resin compositions or molded articles. In other words, in the production of resin compositions and molded articles, the molecular weight of the polyamide in the resin composition and molded article is often lower than the molecular weight of the polyamide used for mixing with cellulose nanofibers due to the thermal history during the mixing process to produce the resin composition and the thermal history during the molding process to produce the molded article from the resin composition. Furthermore, a decrease in molecular weight due to moisture absorption by the polyamide can also occur during storage of the resin composition and molded article. Reusing a resin composition or molded article containing such degraded polyamide as a resin composition material results in a resin composition with inferior physical properties. According to this embodiment, when polyamide containing a portion derived from the resin composition or molded article is mixed with cellulose nanofibers containing a portion derived from the resin composition or molded article in the mixing step, the molecular weight of the polyamide increases. In other words, the reduced molecular weight of the polyamide in the resin composition or molded article is restored in the mixing step, resulting in a resin composition with good physical properties.

More specifically, this embodiment includes the following first and second aspects.

The method according to the first aspect includes a mixing step of mixing components including a first polyamide (A1) satisfying the condition of amino terminal group concentration [NH ₂ ] > carboxyl terminal group concentration [COOH], a second polyamide (A2) satisfying the condition of _amino terminal group concentration [NH 2 ] < carboxyl terminal group concentration [COOH], and cellulose nanofibers (B) to obtain a resin composition.
In one embodiment, the number average molecular weight (M A1 ) of the first polyamide ( _A1 ), the mass concentration (C _A1 ) of the first polyamide (A1) in the mixed component, the number average molecular weight (M _A2 ) of the second polyamide (A2), the mass concentration (C A2 ) of the second polyamide ( _A2 ) in the mixed component, and the total mass concentration (C _T ) of the mass concentration (C _A1 ) of the first polyamide (A1) and the mass concentration (C _A2 ) of the second polyamide in the mixed component are calculated using the following formula:
M _T = (M _A1 × C _A1 + M _A2 × C _A2 )/C _T
and the number average molecular weight ( _M ) of the polyamide in the resin composition are calculated according to the following formula (1a):
M/M _T >1.1 (1a)
Satisfy the relationship.

The method according to the second aspect includes a mixing step of mixing components containing polyamide (A) and cellulose nanofibers (B) to obtain a resin composition, the polyamide (A) having an amino terminal group concentration and a carboxyl terminal group concentration each of which is 20 μequivalents/g or more and at least one of which is 70 μequivalents/g or more, or an amino terminal group concentration and a carboxyl terminal group concentration each of which is 50 μequivalents/g or more and at least one of which is 60 μequivalents/g or more.
In one embodiment, the number average molecular weight (M) of the polyamide in the resin composition and the number average molecular weight (M _A ) of the polyamide (A) to be mixed satisfy the following formula (2a):
M/M _A >1.1 (2a)
Satisfy the relationship.
In one embodiment, the moisture content of the resin composition is higher than the moisture content of the polyamide (A) to be mixed.

The above formulas (1a) and (2a) are indicators of the degree of increase in the molecular weight of the polyamide in the mixing step of this embodiment. The M/M _T ratio in the above formula (1a) and the M/M _A ratio in the above formula (2a) are each greater than 1.1 in one aspect, from the viewpoint of significantly increasing the molecular weight of the polyamide in the mixing step to obtain a resin composition with good physical properties, and are preferably 1.15 or more, or 1.2 or more, or 1.25 or more, or 1.3 or more, or 1.4 or more. From the viewpoint of accurately controlling the physical properties of the resin composition, they are preferably 2.5 or less, or 2 or less, or 1.5 or less. A means for controlling the M/M _T ratio and the M/M _A ratio within the ranges of this embodiment includes controlling the end group concentration of the polyamide to be mixed as exemplified in this embodiment, and then adjusting the mixing conditions when adding cellulose nanofibers and mixing them at or above the melting temperature of the resin in the mixing step (for example, setting the mixing time within a predetermined range, for example, 2 minutes or more).

<Components of Resin Composition>
<Polyamides (A1), (A2), and (A)>
As the polyamides (A1) and (A2) used in the first embodiment and the polyamide (A) used in the second embodiment, various polyamides having an aliphatic structure, an aromatic structure, or a combination thereof can be used. Examples of preferred polyamides are:
Polyamides obtained by polycondensation reaction of lactams, such as polyamide 6, polyamide 11, and polyamide 12;
Diamines such as 1,6-hexanediamine, 2-methyl-1,5-pentanediamine, 1,7-heptanediamine, 2-methyl-1-6-hexanediamine, 1,8-octanediamine, 2-methyl-1,7-heptanediamine, 1,9-nonanediamine, 2-methyl-1,8-octanediamine, 1,10-decanediamine, 1,11-undecanediamine, 1,12-dodecanediamine, and m-xylylenediamine; and butanedioic acid, pentanedioic acid, hexanedioic acid, heptanedioic acid, octanedioic acid, nonanedioic acid, decanedioic acid, benzyl alcohol, methyl ... Polyamides obtained as copolymers with dicarboxylic acids such as benzene-1,2-dicarboxylic acid, benzene-1,3-dicarboxylic acid, benzene-1,4-dicarboxylic acid, cyclohexane-1,3-dicarboxylic acid, and cyclohexane-1,4-dicarboxylic acid, for example, polyamide 6,6, polyamide 6,10, polyamide 6,11, polyamide 6,12, polyamide 6,T, polyamide 6,I, polyamide 9,T, polyamide 10,T, polyamide 2M5,T, polyamide MXD,6, polyamide 6,C, and polyamide 2M5,C; and copolymers in which these are copolymerized, for example, polyamide 6,T/6,I.

Among these polyamides, aliphatic polyamides such as polyamide 6, polyamide 11, polyamide 12, polyamide 6,6, polyamide 6,10, polyamide 6,11, and polyamide 6,12; and alicyclic polyamides such as polyamide 6,C and polyamide 2M5,C are more preferred.

The end group concentration of polyamide can be adjusted by known methods. For example, a terminal adjuster that reacts with end groups, such as a diamine compound, monoamine compound, dicarboxylic acid compound, monocarboxylic acid compound, acid anhydride, monoisocyanate, monoacid halide, monoester, or monoalcohol, can be added to the polymerization solution to achieve a predetermined end group concentration during polyamide polymerization.

Examples of the terminal modifier that reacts with the terminal amino group include:
aliphatic monocarboxylic acids such as acetic acid, propionic acid, butyric acid, valeric acid, caproic acid, caprylic acid, lauric acid, tridecanoic acid, myristic acid, palmitic acid, stearic acid, pivalic acid, and isobutyric acid;
alicyclic monocarboxylic acids such as cyclohexanecarboxylic acid;
aromatic monocarboxylic acids such as benzoic acid, toluic acid, α-naphthalenecarboxylic acid, β-naphthalenecarboxylic acid, methylnaphthalenecarboxylic acid, and phenylacetic acid;
Dicarboxylic acids such as adipic acid, suberic acid, azelaic acid, sebacic acid, dodecanedioic acid, terephthalic acid, isophthalic acid, naphthalenedicarboxylic acid, 2-chloroterephthalic acid, 2-methylterephthalic acid, 5-methylisophthalic acid, 5-sodiumsulfoisophthalic acid, hexahydroterephthalic acid, hexahydroisophthalic acid, and diglycolic acid; and mixtures of a plurality of compounds arbitrarily selected from these.

Among these, one or more terminal modifiers selected from the group consisting of acetic acid, propionic acid, butyric acid, valeric acid, caproic acid, caprylic acid, lauric acid, tridecanoic acid, myristic acid, palmitic acid, stearic acid, benzoic acid, isophthalic acid, terephthalic acid, and adipic acid are preferred in terms of reactivity, stability of the blocked terminals, cost, etc.

Examples of the terminal modifier that reacts with the terminal carboxyl group include:
aliphatic monoamines such as methylamine, ethylamine, propylamine, butylamine, hexylamine, octylamine, decylamine, stearylamine, dimethylamine, diethylamine, dipropylamine, and dibutylamine;
alicyclic monoamines such as cyclohexylamine and dicyclohexylamine;
aromatic monoamines such as aniline, toluidine, diphenylamine, and naphthylamine;
Diamines such as tetramethylenediamine, hexamethylenediamine, nonamethylenediamine, decamethylenediamine, undecamethylenediamine, dodecamethylenediamine, 2,2,4-/2,4,4-trimethylhexamethylenediamine, 5-methylnonamethylenediamine, 2,4-dimethyloctamethylenediamine, metaxylylenediamine, paraxylylenediamine, 1,3-bis(aminomethyl)cyclohexane, 1-amino-3-aminomethyl-3,5,5-trimethylcyclohexane, 3,8-bis(aminomethyl)tricyclodecane, bis(4-aminocyclohexyl)methane, bis(3-methyl-4-aminocyclohexyl)methane, 2,2-bis(4-aminocyclohexyl)propane, bis(aminopropyl)piperazine, and aminoethylpiperazine; and any mixture thereof.

Among these, one or more terminal modifiers selected from the group consisting of butylamine, hexylamine, octylamine, decylamine, stearylamine, cyclohexylamine, aniline, and hexamethylenediamine are preferred in terms of reactivity, boiling point, stability of the blocked terminals, cost, etc.

The method for adding a terminal modifier to a polyamide and reacting it with the terminal functional groups is not particularly limited, but one embodiment is a method in which a terminal modifier is added in any proportion during polymerization and reacted.

The concentrations of amino and carboxyl end groups are preferably determined from the integrated values of the characteristic signals corresponding to each end group by ^1H -NMR in terms of accuracy and simplicity. The method described in JP-A-7-228775 is specifically recommended as a method for determining the concentrations of these end groups. When using this method, deuterated trifluoroacetic acid is useful as the measurement solvent. Furthermore, the number of ^1H -NMR scans required is at least 300, even when measuring with an instrument with sufficient resolution.

The intrinsic viscosity [η] of each of polyamides (A1), (A2), and (A), measured in concentrated sulfuric acid at 30°C, is preferably 0.6 dL/g or more, or 0.7 dL/g or more, from the viewpoint of obtaining good physical properties of the resin composition; and is preferably 2.0 dL/g or less, or 1.4 dL/g or less, or 1.2 dL/g or less, or 1.0 dL/g or less, from the viewpoint of ease of production and processing of the resin composition and the appearance of the molded product.

In this disclosure, intrinsic viscosity is synonymous with the viscosity commonly referred to as intrinsic viscosity. A specific method for determining intrinsic viscosity is to measure the ηsp/c of several test solutions with different polymer concentrations in 96% concentrated sulfuric acid at a temperature of 30°C, derive the relationship between each ηsp/c and the concentration (c), and extrapolate the concentration to zero. This value extrapolated to zero is the intrinsic viscosity. Details of how to determine intrinsic viscosity are described, for example, on pages 291-294 of Polymer Process Engineering (Prentice-Hall, Inc. 1994). From the perspective of accuracy, it is desirable to use at least four test solutions with different polymer concentrations. The recommended polymer concentrations for the viscosity measurement solutions are preferably 0.05 g/dL, 0.1 g/dL, 0.2 g/dL, and 0.4 g/dL.

The number average molecular weight of each of polyamides (A1), (A2), and (A) is preferably 5,000 or more, or 10,000 or more, or 15,000 or more, or 20,000 or more, from the viewpoint of obtaining good physical properties of the resin composition; and is preferably 100,000 or less, or 80,000 or less, or 60,000 or less, or 50,000 or less, from the viewpoint of promoting amide bond formation in the polyamide during the mixing process.

The weight average molecular weight of each of polyamides (A1), (A2), and (A) is preferably 5,000 or more, or 10,000 or more, or 15,000 or more, or 20,000 or more, from the viewpoint of obtaining good physical properties of the resin composition; and is preferably 100,000 or less, or 80,000 or less, or 60,000 or less, or 50,000 or less, from the viewpoint of promoting amide bond formation in the polyamide during the mixing process.

The Mw/Mn of each of the polyamides (A1), (A2), and (A) is preferably 0.2 or more, or 0.5 or more, or 1 or more, from the viewpoint of promoting amide bond formation in the polyamides during the mixing step; and is preferably 5 or less, or 4 or less, or 2 or less, from the viewpoint of accurately controlling the physical properties of the resin composition.

The above number average molecular weight and weight average molecular weight are values determined using gel permeation chromatography in terms of standard polymethyl methacrylate.

The melting point of each of polyamides (A1), (A2), and (A) is preferably 220°C or higher, or 230°C or higher, in order to provide good heat resistance to the resin composition, and is preferably 350°C or lower, or 320°C or lower, or 300°C or lower, in order to facilitate the production and processing of the resin composition. The melting point refers to the peak-top temperature of the endothermic peak that appears when the temperature is increased from 23°C at a rate of 10°C/min using a differential scanning calorimeter (DSC). When two or more endothermic peaks appear, the melting point refers to the peak-top temperature of the endothermic peak with the highest temperature.

The glass transition temperature of each of polyamides (A1), (A2), and (A) is preferably 0°C or higher, or 10°C or higher, or 30°C or higher, in order to provide a resin composition with good mechanical strength; and is preferably 100°C or lower, or 90°C or lower, or 70°C or lower, in order to provide a resin composition with good toughness and easy production and processing. The glass transition temperature refers to the peak-top temperature at which the storage modulus drops significantly and the loss modulus reaches its maximum when measured using a dynamic viscoelasticity measuring device at an applied frequency of 10 Hz while increasing the temperature from 23°C at a heating rate of 2°C/min. When two or more loss modulus peaks appear, the glass transition temperature refers to the peak-top temperature of the lowest-temperature peak.

[Polyamide (A1) and Polyamide (A2)]
The polyamide (A1) satisfies the relationship of amino terminal group concentration [NH ₂ ] > carboxyl terminal group concentration [COOH]. In one aspect, the amino terminal group ratio, which is the ratio of the amino terminal group concentration [NH ₂ ] to the carboxyl terminal group concentration [COOH], is greater than 1, preferably 1.01 or more, or 1.05 or more, or 1.10 or more, and preferably 10,000 or less, or 1,000 or less, or 100 or less, or 10 or less.

The amino end group concentration of polyamide (A1) is preferably 20 μequivalents/g or more, or 30 μequivalents/g or more, and preferably 150 μequivalents/g or less, or 100 μequivalents/g or less, or 80 μequivalents/g or less.

The carboxyl end group concentration of polyamide (A1) is preferably 20 μequivalents/g or more, or 30 μequivalents/g or more, and preferably 150 μequivalents/g or less, or 100 μequivalents/g or less, or 80 μequivalents/g or less.

The total concentration of the amino terminal group concentration [ _NH2 ] and carboxyl terminal group concentration [COOH] of polyamide (A1) is preferably 10 μequivalents/g or more, or 50 μequivalents/g or more, or 100 μequivalents/g or more, from the viewpoint of promoting amide bond formation in the polyamide during the mixing process, and is preferably 500 μequivalents/g or less, or 300 μequivalents/g or less, or 135 μequivalents/g or less, from the viewpoint of maintaining good physical properties of the resin composition by preventing the molecular weight of the polyamide from becoming too small.

The polyamide (A2) satisfies the condition that the amino terminal group concentration [ _NH2 ] is less than the carboxyl terminal group concentration [COOH]. In one aspect, the carboxyl terminal group ratio, which is the ratio of the carboxyl terminal group concentration [COOH] to the amino terminal group concentration [ _NH2 ], is greater than 1, preferably 1.01 or more, or 1.05 or more, or 1.10 or more, and is preferably 10,000 or less, or 1,000 or less, or 100 or less, or 10 or less.

The amino end group concentration of polyamide (A2) is preferably 20 μequivalents/g or more, or 30 μequivalents/g or more, and preferably 150 μequivalents/g or less, or 100 μequivalents/g or less, or 80 μequivalents/g or less.

The carboxyl end group concentration of polyamide (A2) is preferably 20 μequivalents/g or more, or 30 μequivalents/g or more, and preferably 150 μequivalents/g or less, or 100 μequivalents/g or less, or 80 μequivalents/g or less.

The total concentration of the amino terminal group concentration [ _NH2 ] and carboxyl terminal group concentration [COOH] of polyamide (A2) is preferably 10 μequivalents/g or more, or 50 μequivalents/g or more, or 100 μequivalents/g or more, from the viewpoint of promoting amide bond formation in the polyamide during the mixing process, and is preferably 500 μequivalents/g or less, or 300 μequivalents/g or less, or 135 μequivalents/g or less, from the viewpoint of maintaining good physical properties of the resin composition by preventing the molecular weight of the polyamide from becoming too small.
In a particularly preferred embodiment, the total concentration ([NH ₂ ]+[COOH]) of each of the polyamides (A1) and (A2) is within the above-mentioned range.

In one aspect, the ratio (C _A1 /C A2 ) of the mass concentration (C _A1 ) of the first polyamide (A1) to the mass concentration (C _A2 ) of the second polyamide ( _A2 ) in the mixed components may be, for example, 10/90 or more, or 20/80 or more, or 30/70 or more, or 40/60 or more, from the viewpoint of promoting amide bond formation in the polyamides during the mixing process, and may be, for example, 90/10 or less, or 80/20 or less, or 70/30 or less, or 60/40 or less.

In one embodiment, the following formulas (1b) to (1d) are calculated based on the amino terminal group concentration ([NH ₂ ] _A1 ) and carboxyl terminal group concentration ([COOH] _A1 ) of the first polyamide (A1), the mass concentration (C _A1 ) of the first polyamide (A1) in the mixed component, the amino terminal group concentration ([NH ₂ ] _A2 ) and carboxyl terminal group concentration ([COOH] _A2 ) of the second polyamide (A2), the mass concentration (C _A2 ) of the second polyamide (A2) in the mixed component, and the total mass concentration (C _T ) of the mass concentration (C _A1 ) of the first polyamide (A1) and the mass concentration (C _A2 ) of the second polyamide (A2) in the mixed component:
[NH ₂ ] _T = ([NH ₂ ] _A1 × C _A1 + [NH ₂ ] _A2 × C _A2 )/C _T (1b)
[COOH] _T = ([COOH] _A1 × C _A1 + [COOH] _A2 × C _A2 )/C _T (1c)
([NH ₂ ] + [COOH]) _T = [NH ₂ ] _T + [COOH] _T (1d)
The total concentration ([NH ₂ ] + [COOH]) _T calculated according to the above formula is preferably 10 μequivalents/g or more, or 50 μequivalents/g or more, or 100 μequivalents/g or more from the viewpoint of promoting amide bond formation in the polyamide in the mixing step, and is preferably 500 μequivalents/g or less, or 300 μequivalents/g or less, or 200 μequivalents/g or less from the viewpoint of maintaining good physical properties of the resin composition by preventing the molecular weight of the polyamide from becoming too small.

In one embodiment, the ratio of the above [NH ₂ ] _T to the above [COOH] _T ([NH ₂ ] _T /[COOH] _T ) may be, for example, 1 or more, or 1.5 or more, or 2 or more, and may be, for example, 5 or less, or 4 or less, from the viewpoint of smoothly proceeding the amide bond formation reaction of the polyamide in the mixing step.

[Polyamide A]
The polyamide (A) is a polyamide having an amino terminal group concentration and a carboxyl terminal group concentration each of which is 20 μequivalents/g or more and at least one of which is 70 μequivalents/g or more, or an amino terminal group concentration and a carboxyl terminal group concentration each of which is 50 μequivalents/g or more and at least one of which is 60 μequivalents/g or more.

In one embodiment, the amino terminal group concentration of polyamide (A) is 20 μequivalents/g or more, or 30 μequivalents/g or more, or 40 μequivalents/g or more, or 50 μequivalents/g or more, from the viewpoint of promoting amide bond formation in the polyamide during the mixing step; and from the viewpoint of maintaining good physical properties of the resin composition by preventing the molecular weight of the polyamide from becoming too small, the amino terminal group concentration is preferably 150 μequivalents/g or less, or 100 μequivalents/g or less, or 80 μequivalents/g or less.

In one embodiment, the carboxyl end group concentration of polyamide (A) is 20 μequivalents/g or more, or 30 μequivalents/g or more, or 40 μequivalents/g or more, or 50 μequivalents/g or more, from the viewpoint of promoting amide bond formation in the polyamide during the mixing step; and from the viewpoint of maintaining good physical properties of the resin composition by preventing the molecular weight of the polyamide from becoming too small, the concentration is preferably 150 μequivalents/g or less, or 100 μequivalents/g or less, or 80 μequivalents/g or less.

From the viewpoint of promoting amide bond formation in the polyamide during the mixing step, in one embodiment, the concentration of at least one of the amino terminal groups and the carboxyl terminal groups in the polyamide (A) is 50 μequivalents/g or more, or 60 μequivalents/g or more, or 70 μequivalents/g or more, or 80 μequivalents/g or more. In one embodiment, the polyamide (A) is rich in at least one of the amino terminal groups and the carboxyl terminal groups, and the reaction between the amino groups and the carboxyl groups in the polyamide molecules proceeds smoothly during the mixing step, resulting in a significant increase in molecular weight.

The total concentration of the amino terminal group concentration [ _NH2 ] and carboxyl terminal group concentration [COOH] of polyamide (A) is, in one embodiment, 90 μequivalents/g or more, preferably 100 μequivalents/g or more, or 110 μequivalents/g or more, from the viewpoint of promoting amide bond formation in the polyamide during the mixing process, and is preferably 500 μequivalents/g or less, or 300 μequivalents/g or less, or 200 μequivalents/g or less, from the viewpoint of maintaining good physical properties of the resin composition by preventing the molecular weight of the polyamide from becoming too small.

In one embodiment, one of the amino end group concentration and the carboxyl end group concentration of polyamide (A) may be 70 μequivalents/g or more, and the other may be less than 70 μequivalents/g. Alternatively, one of the amino end group concentration and the carboxyl end group concentration may be 60 μequivalents/g or more, and the other may be less than 60 μequivalents/g. In this case, the benefits of increasing the molecular weight during the mixing process are effectively obtained, and the molecular weight of the polyamide in the resin composition does not become too small, resulting in good physical properties of the resin composition. In a preferred embodiment, one of the amino end group concentration and the carboxyl end group concentration of polyamide (A) is 20 μequivalents/g or more, or 30 μequivalents/g or more and less than 70 μequivalents/g, or 60 μequivalents/g or less, and the other is 70 μequivalents/g or more, or 100 μequivalents/g or more and 480 μequivalents/g or less, or 300 μequivalents/g or less. In another preferred embodiment, one of the amino terminal group concentration and the carboxyl terminal group concentration of the polyamide (A) is 50 μequivalents/g or more and less than 60 μequivalents/g, and the other is 60 μequivalents/g or more, or 100 μequivalents/g or more and 480 μequivalents/g or less, or 300 μequivalents/g or less.

The amino terminal group ratio of polyamide (A), which is the ratio of the amino terminal group concentration [ _NH2 ] to the carboxyl terminal group concentration [COOH], is preferably 0.5 or more, or 1 or more, or 1.5 or more, and preferably 10 or less, or 5 or less, or 3 or less.

In the first embodiment, the molecular weight of the polyamide can be increased in the mixing step by using the polyamide (A1) and the polyamide (A2) in combination, and in the second embodiment, by using the polyamide (A). While not wishing to be bound by theory, it is speculated that the cellulose nanofibers (B) promote amide bonds between the polyamide ends in the mixing step, thereby increasing the molecular weight of the polyamide.

<Cellulose nanofiber (B)>
The raw materials for cellulose nanofibers can include natural cellulose and regenerated cellulose. Examples of natural cellulose include wood pulp obtained from wood species (broadleaf or coniferous trees), non-wood pulp obtained from non-wood species (cotton, bamboo, hemp, bagasse, kenaf, cotton linter, sisal, straw, etc.), and cellulose aggregates produced by animals (e.g., sea squirts), algae, and microorganisms (e.g., acetic acid bacteria). Examples of regenerated cellulose include regenerated cellulose fibers (e.g., viscose, cupra, Tencel), cellulose derivative fibers, and ultrafine threads of regenerated cellulose or cellulose derivatives obtained by electrospinning.

Cellulose nanofibers refer to fine cellulose fibers that are obtained by treating pulp or the like with hot water at 100°C or higher to hydrolyze and weaken the hemicellulose, and then defibrating the fibers using a pulverizing method such as a high-pressure homogenizer, microfluidizer, ball mill, disc mill, or mixer (e.g., a homomixer). In one embodiment, the cellulose nanofibers have a number-average fiber diameter of 1 nm or more and 1,000 nm or less. The cellulose nanofibers may also be chemically modified as described below.

The slurry may be prepared by dispersing the cellulose fibers in a liquid medium. Dispersion may be carried out using a high-pressure homogenizer, microfluidizer, ball mill, disc mill, mixer (e.g., homomixer), or the like. In one embodiment, the liquid medium in the slurry may contain water and, optionally, one or more organic solvents. Examples of organic solvents that can be used include commonly used water-miscible organic solvents, such as alcohols having a boiling point of 50°C to 170°C (e.g., methanol, ethanol, n-propanol, i-propanol, n-butanol, i-butanol, s-butanol, t-butanol, etc.); ethers (e.g., propylene glycol monomethyl ether, 1,2-dimethoxyethane, diisopropyl ether, tetrahydrofuran, 1,4-dioxane, etc.); carboxylic acids (e.g., formic acid, acetic acid, lactic acid, etc.); esters (e.g., ethyl acetate, vinyl acetate, etc.); ketones (e.g., acetone, methyl ethyl ketone, methyl isobutyl ketone, cyclopentanone, cyclohexanone, etc.); and nitrogen-containing solvents (dimethylformamide, dimethylacetamide, acetonitrile, etc.). In a typical embodiment, the liquid medium in the slurry is substantially water alone.

Since cellulose raw materials contain alkali-soluble and sulfuric acid-insoluble components (such as lignin), they can be reduced in alkali-soluble and sulfuric acid-insoluble components through purification processes such as delignification by cooking and bleaching. However, purification processes such as delignification by cooking and bleaching sever the molecular chains of cellulose, changing the weight-average molecular weight and number-average molecular weight. Therefore, it is desirable to control the purification and bleaching processes of cellulose raw materials so that the weight-average molecular weight of the cellulose nanofibers and the ratio of the weight-average molecular weight to the number-average molecular weight are within appropriate ranges.

In addition, refining processes such as delignification by cooking and bleaching processes reduce the molecular weight of cellulose molecules, so there are concerns that these processes will result in lower molecular weight cellulose nanofibers and alter the cellulose raw material, increasing the proportion of alkali-soluble matter. Because alkali-soluble matter has poor heat resistance, it is desirable that the refining and bleaching processes for cellulose raw materials be controlled so that the amount of alkali-soluble matter contained in the cellulose raw material remains below a certain value.

In one aspect, the number average fiber diameter of the cellulose nanofibers is preferably 2 to 1,000 nm, from the viewpoint of obtaining a favorable effect of improving physical properties due to the cellulose nanofibers. The number average fiber diameter of the cellulose nanofibers is more preferably 4 nm or more, or 5 nm or more, or 10 nm or more, or 15 nm or more, or 20 nm or more, and more preferably 500 nm or less, or 450 nm or less, or 400 nm or less, or 350 nm or less, or 300 nm or less, or 250 nm or less.

The average fiber length (L)/fiber diameter (D) ratio of cellulose nanofibers is preferably 30 or more, or 50 or more, or 80 or more, or 100 or more, or 120 or more, or 150 or more, from the viewpoint of effectively improving the mechanical properties of a resin composition containing cellulose nanofibers with a small amount of cellulose nanofibers. There is no particular upper limit, but from the viewpoint of handleability, it is preferably 5,000 or less.

In this disclosure, the fiber length, fiber diameter, and L/D ratio of cellulose nanofibers are determined by dispersing an aqueous dispersion of cellulose nanofibers using a high-shear homogenizer (e.g., Nippon Seiki Co., Ltd., product name "Excel Auto Homogenizer ED-7") at a rotation speed of 15,000 rpm for 5 minutes. The aqueous dispersion is then diluted with pure water to 0.1 to 0.5 mass%, cast on mica, and air-dried to obtain a measurement sample. This sample is then measured using a high-resolution scanning electron microscope (SEM) or atomic force microscope (AFM). Specifically, the length (L) and diameter (D) of 100 randomly selected cellulose nanofibers are measured in an observation field with a magnification adjusted so that at least 100 cellulose nanofibers are observed, and the ratio (L/D) is calculated. The number-average fiber length (L), fiber diameter (D), and ratio (L/D) of the cellulose nanofibers are calculated.

Alternatively, the fiber length, fiber diameter, and L/D ratio of the cellulose nanofibers in the resin composition can be confirmed by measuring the solid resin composition as a measurement sample using the above-mentioned measurement method.

Alternatively, the fiber length, fiber diameter, and L/D ratio of the cellulose nanofibers in a resin composition can be confirmed by dissolving the resin components in the resin composition in an organic or inorganic solvent that can dissolve the resin components of the resin composition, separating the cellulose nanofibers, thoroughly washing them with the solvent, and then replacing the solvent with pure water to prepare an aqueous dispersion. The cellulose nanofiber concentration is then diluted with pure water to 0.1 to 0.5% by mass, cast onto mica, and air-dried. This is then used as a measurement sample and measured using the measurement method described above. At this time, the measurement is performed on at least 100 randomly selected cellulose nanofibers.

The degree of crystallinity of cellulose nanofibers is preferably 55% or higher. When the degree of crystallinity is within this range, the mechanical properties (strength, dimensional stability) of the cellulose itself are high, and when the cellulose nanofibers are dispersed in a resin, the strength and dimensional stability of the resin composition tend to be high. A more preferable lower limit for the degree of crystallinity is 60%, even more preferably 70%, and most preferably 80%. There is no particular upper limit for the degree of crystallinity of cellulose nanofibers, and the higher the better, but from a production standpoint, a preferable upper limit is 99%.

Alkali-soluble polysaccharides such as hemicellulose and acid-insoluble components such as lignin are present between the microfibrils and between the microfibril bundles of plant-derived cellulose nanofibers. Hemicellulose is a polysaccharide composed of sugars such as mannan and xylan, and forms hydrogen bonds with cellulose to connect the microfibrils. Lignin is a compound with an aromatic ring, and is known to be covalently bonded to hemicellulose in plant cell walls. If there are large amounts of impurities such as lignin remaining in the cellulose nanofibers, discoloration may occur due to heat during processing. Therefore, from the perspective of suppressing discoloration of the resin composition during extrusion and molding, it is desirable to keep the crystallinity of the cellulose nanofibers within the above-mentioned range.

When the cellulose is cellulose type I crystals (derived from natural cellulose), the degree of crystallinity referred to here is determined by the Segal method from the diffraction pattern (2θ/deg. is 10 to 30) obtained by measuring a sample by wide-angle X-ray diffraction, using the following formula:
Crystallinity (%)=([diffraction intensity due to the (200) plane at 2θ/deg.=22.5]−[diffraction intensity due to amorphous phase at 2θ/deg.=18])/[diffraction intensity due to the (200) plane at 2θ/deg.=22.5]×100

When the cellulose is cellulose type II crystal (derived from regenerated cellulose), the degree of crystallinity can be calculated by the following formula from the absolute peak intensity h0 at 2θ=12.6° assigned to the (110) plane peak of cellulose type II crystal in wide-angle X-ray diffraction and the peak intensity h1 from the baseline at this interplanar spacing.
Crystallinity (%) = h1 /h0 ×100

Known crystalline forms of cellulose include types I, II, III, and IV, with types I and II being particularly widely used, while types III and IV have been obtained on a laboratory scale but are not widely used on an industrial scale. The cellulose nanofibers disclosed herein have relatively high structural mobility, and dispersing these cellulose nanofibers in a resin results in a resin composition with a lower linear expansion coefficient and superior strength and elongation during tensile and bending deformation. Therefore, cellulose nanofibers containing cellulose type I crystals or cellulose type II crystals are preferred, and cellulose nanofibers containing cellulose type I crystals and having a crystallinity of 55% or more are more preferred.

In addition, from the viewpoint of exhibiting good mechanical properties, the degree of polymerization of the cellulose nanofiber is preferably 100 or more, or 150 or more, or 200 or more, or 300 or more, or 400 or more, or 450 or more, and from the viewpoint of processability, it is preferably 3500 or less, or 3300 or less, or 3200 or less, or 3100 or less, or 3000 or less.

The degree of polymerization of cellulose nanofibers refers to the average degree of polymerization measured according to the reduced specific viscosity method using a copper ethylenediamine solution, as described in Verification Test (3) of the "15th Edition Japanese Pharmacopoeia Commentary (published by Hirokawa Shoten)."

In one embodiment, the weight-average molecular weight (Mw) of the cellulose nanofiber is 100,000 or more, more preferably 200,000 or more. The ratio of the weight-average molecular weight to the number-average molecular weight (Mn) (Mw/Mn) is 6 or less, preferably 5.4 or less. A higher weight-average molecular weight indicates a lower number of terminal groups in the cellulose molecule. Furthermore, since the ratio of the weight-average molecular weight to the number-average molecular weight (Mw/Mn) represents the width of the molecular weight distribution, a lower Mw/Mn indicates a lower number of terminal groups in the cellulose molecule. Since the terminals of cellulose molecules are the starting points for thermal decomposition, cellulose nanofibers with particularly high heat resistance can be obtained when the cellulose molecules in the cellulose nanofiber have not only a high weight-average molecular weight but also a narrow molecular weight distribution. From the perspective of easy availability of cellulose raw materials, the weight-average molecular weight (Mw) of the cellulose nanofiber may be, for example, 600,000 or less, or 500,000 or less. From the viewpoint of ease of production of cellulose nanofibers, the ratio of weight-average molecular weight to number-average molecular weight (Mn) (Mw/Mn) may be, for example, 1.5 or greater, or 2 or greater. Mw can be controlled within the above range by selecting a cellulose raw material with an Mw appropriate for the purpose, or by subjecting the cellulose raw material to appropriate physical and/or chemical treatments within an appropriate range. Mw/Mn can also be controlled within the above range by selecting a cellulose raw material with an Mw/Mn appropriate for the purpose, or by subjecting the cellulose raw material to appropriate physical and/or chemical treatments within an appropriate range. In both Mw and Mw/Mn control, examples of the physical treatment include dry or wet grinding using a microfluidizer, ball mill, or disk mill, or physical treatments that apply mechanical forces such as impact, shear, shear, or friction using a crusher, homomixer, high-pressure homogenizer, or ultrasonic device. Examples of the chemical treatment include digestion, bleaching, acid treatment, and conversion to regenerated cellulose.

The weight-average molecular weight and number-average molecular weight of the cellulose nanofibers referred to here are values determined by dissolving the cellulose nanofibers in N,N-dimethylacetamide containing lithium chloride and then performing gel permeation chromatography using N,N-dimethylacetamide as the solvent.

Methods for controlling the degree of polymerization (i.e., average degree of polymerization) or molecular weight of cellulose nanofibers include hydrolysis. Hydrolysis promotes depolymerization of the amorphous cellulose inside the cellulose nanofibers, reducing the average degree of polymerization. At the same time, hydrolysis removes impurities such as hemicellulose and lignin in addition to the amorphous cellulose mentioned above, making the interior of the fibers more porous.

The hydrolysis method is not particularly limited, and examples include acid hydrolysis, alkaline hydrolysis, hydrothermal decomposition, steam explosion, and microwave decomposition. These methods may be used alone or in combination. In acid hydrolysis, for example, α-cellulose obtained as pulp from fibrous plants is used as the cellulose raw material. This is dispersed in an aqueous medium, and an appropriate amount of a protonic acid, carboxylic acid, Lewis acid, heteropolyacid, etc. is added. The mixture is then heated with stirring, allowing for easy control of the average degree of polymerization. The reaction conditions, such as temperature, pressure, and time, vary depending on the cellulose species, cellulose concentration, acid species, and acid concentration, but are appropriately adjusted to achieve the desired average degree of polymerization. For example, cellulose nanofibers may be treated with a mineral acid solution of 2% by mass or less at 100°C or higher under pressure for 10 minutes or longer. Under these conditions, the acid or other catalyst component penetrates deep into the cellulose nanofibers, promoting hydrolysis. This reduces the amount of catalyst component required and facilitates subsequent purification.

Alkali-soluble polysaccharides that cellulose nanofibers may contain include hemicellulose, as well as β-cellulose and γ-cellulose. Alkali-soluble polysaccharides are understood by those skilled in the art as components obtained as the alkali-soluble portion of holocellulose (i.e., the component remaining after α-cellulose has been removed from holocellulose), which is obtained by solvent extraction and chlorine treatment of plants (e.g., wood). Alkali-soluble polysaccharides are polysaccharides containing hydroxyl groups and have poor heat resistance, which can lead to problems such as decomposition when exposed to heat, yellowing during thermal aging, and a decrease in the strength of cellulose nanofibers. Therefore, it is preferable that the alkali-soluble polysaccharide content in cellulose nanofibers be low.

In one aspect, the average alkali-soluble polysaccharide content in the cellulose nanofibers is preferably 20% by mass or less, 18% by mass or less, 15% by mass or less, or 12% by mass or less, relative to 100% by mass of the cellulose nanofibers, from the viewpoint of obtaining good dispersibility of the cellulose nanofibers. From the viewpoint of ease of production of the cellulose nanofibers, the above content may be 1% by mass or more, 2% by mass or more, or 3% by mass or more.

The average alkali-soluble polysaccharide content can be determined by the method described in the non-patent document (Wood Science Experiment Manual, edited by the Japan Wood Research Society, pp. 92-97, 2000), by subtracting the alpha-cellulose content from the holocellulose content (Wise method). This method is understood in the industry as a method for measuring hemicellulose content. The alkali-soluble polysaccharide content is calculated three times for each sample, and the number average of the calculated alkali-soluble polysaccharide contents is taken as the average alkali-soluble polysaccharide content.

In one aspect, the average content of acid-insoluble components in the cellulose nanofibers is preferably 10% by mass or less, 5% by mass or less, or 3% by mass or less, relative to 100% by mass of the cellulose nanofibers, from the viewpoint of avoiding a decrease in the heat resistance of the cellulose nanofibers and the resulting discoloration. From the viewpoint of ease of production of the cellulose nanofibers, the above content may be 0.1% by mass or more, 0.2% by mass or more, or 0.3% by mass or more.

The average acid-insoluble component content is determined by quantifying the acid-insoluble component using the Klason method described in the non-patent document (Wood Science Experiment Manual, edited by the Japan Wood Research Society, pp. 92-97, 2000). This method is recognized in the industry as a method for measuring lignin content. The sample is stirred in a sulfuric acid solution to dissolve cellulose, hemicellulose, etc., and then filtered through glass fiber filter paper. The resulting residue corresponds to the acid-insoluble component. The acid-insoluble component content is calculated from the weight of this acid-insoluble component, and the number average of the acid-insoluble component contents calculated for three samples is used as the average acid-insoluble component content.

From the viewpoint of being able to exhibit the heat resistance and mechanical strength desired for in-vehicle applications, etc., the thermal decomposition onset temperature (T _D ) of the cellulose nanofiber is, in one embodiment, 270°C or higher, preferably 275°C or higher, more preferably 280°C or higher, and even more preferably 285°C or higher. A higher thermal decomposition onset temperature is preferable, but from the viewpoint of ease of production of the cellulose nanofiber, it may be, for example, 320°C or lower, or 300°C or lower.

In this disclosure, _TD is a value obtained from a graph in thermogravimetric (TG) analysis, where the horizontal axis represents temperature and the vertical axis represents the weight retention rate (%). Starting from the weight of the cellulose nanofiber at 150°C (a state in which moisture is almost completely removed) (weight loss of 0 wt%), the temperature is further increased, and a straight line is obtained that passes through the temperature at which the weight loss is 1 wt% (T1 _% ) and the temperature at which the weight loss is 2 _{wt% (T2%} ). _TD is defined as the temperature at the point where this straight line intersects with a horizontal line (baseline) that passes through the starting point of the weight loss of 0 wt%.

The 1% weight loss temperature (T _1% ) is the temperature at which a 1% weight loss occurs, starting from a weight of 150° C., when the temperature is continued to be increased by the above-mentioned _TD method.

The 250°C weight loss rate (T _250°C ) of the cellulose nanofiber is the weight loss rate when the cellulose nanofiber is held at 250°C under a nitrogen flow for 2 hours in TG analysis.

(chemical modification)
The cellulose nanofibers may be chemically modified cellulose nanofibers, which may be chemically modified in advance, for example, at the stage of raw pulp or linter, during or after the defibration treatment, or during or after the slurry preparation step, or during or after the drying (granulation) step.

As a modifying agent for cellulose nanofibers, compounds that react with the hydroxyl groups of cellulose can be used, including esterifying agents, etherifying agents, and silylating agents. In a preferred embodiment, the chemical modification is acylation using an esterifying agent, with acetylation being particularly preferred. Preferred esterifying agents are acid halides, acid anhydrides, vinyl carboxylic acid esters, and carboxylic acids.

The acid halide may be at least one selected from the group consisting of compounds represented by the following formula:
R ¹ -C(=O)-X
(In the formula, ^R1 represents an alkyl group having 1 to 24 carbon atoms, an alkenyl group having 2 to 24 carbon atoms, a cycloalkyl group having 3 to 24 carbon atoms, or an aryl group having 6 to 24 carbon atoms, and X represents Cl, Br, or I.)
Specific examples of acid halides include, but are not limited to, acetyl chloride, acetyl bromide, acetyl iodide, propionyl chloride, propionyl bromide, propionyl iodide, butyryl chloride, butyryl bromide, butyryl iodide, benzoyl chloride, benzoyl bromide, and benzoyl iodide. Among these, acid chlorides are preferably used in terms of reactivity and ease of handling. In the reaction of the acid halide, one or more alkaline compounds may be added to act as a catalyst and neutralize the by-product acidic substances. Specific examples of alkaline compounds include, but are not limited to, tertiary amine compounds such as triethylamine and trimethylamine; and nitrogen-containing aromatic compounds such as pyridine and dimethylaminopyridine.

As the acid anhydride, any appropriate acid anhydride can be used. For example,
Saturated aliphatic monocarboxylic acid anhydrides such as acetic acid, propionic acid, (iso)butyric acid, and valeric acid; unsaturated aliphatic monocarboxylic acid anhydrides such as (meth)acrylic acid and oleic acid;
Alicyclic monocarboxylic acid anhydrides such as cyclohexanecarboxylic acid and tetrahydrobenzoic acid;
Aromatic monocarboxylic acid anhydrides such as benzoic acid and 4-methylbenzoic acid;
Examples of dibasic carboxylic acid anhydrides include saturated aliphatic dicarboxylic acid anhydrides such as succinic anhydride and adipic acid, unsaturated aliphatic dicarboxylic acid anhydrides such as maleic anhydride and itaconic anhydride, alicyclic dicarboxylic acid anhydrides such as 1-cyclohexene-1,2-dicarboxylic acid anhydride, hexahydrophthalic anhydride and methyltetrahydrophthalic anhydride, and aromatic dicarboxylic acid anhydrides such as phthalic anhydride and naphthalic anhydride;
Examples of the tri- or higher basic carboxylic acid anhydrides include polycarboxylic acid (anhydrides) such as trimellitic anhydride and pyromellitic anhydride.
In the reaction of the acid anhydride, one or more of the following may be added as a catalyst: an acidic compound such as sulfuric acid, hydrochloric acid, or phosphoric acid; a Lewis acid (for example, a Lewis acid compound represented by MYn, where M represents a semimetallic element such as B, As, or Ge; a base metal element such as Al, Bi, or In; a transition metal element such as Ti, Zn, or Cu; or a lanthanoid element; n is an integer corresponding to the valence of M and represents 2 or 3; and Y represents a halogen atom, OAc, _OCOCF3 , _ClO4 , _SbF6 , _PF6 , or _OSO2CF3 ( _OTf )); or an alkaline compound such as triethylamine or pyridine.

The vinyl carboxylic acid ester may be a vinyl carboxylic acid ester represented by the following formula:
R-COO-CH=CH ₂
{wherein R is any one of an alkyl group having 1 to 24 carbon atoms, an alkenyl group having 2 to 24 carbon atoms, a cycloalkyl group having 3 to 16 carbon atoms, and an aryl group having 6 to 24 carbon atoms.} is preferred. The vinyl carboxylate is more preferably at least one selected from the group consisting of vinyl acetate, vinyl propionate, vinyl butyrate, vinyl caproate, vinyl cyclohexanecarboxylate, vinyl caprylate, vinyl caprate, vinyl laurate, vinyl myristate, vinyl palmitate, vinyl stearate, vinyl pivalate, vinyl octylate, divinyl adipate, vinyl methacrylate, vinyl crotonate, vinyl pivalate, vinyl octylate, vinyl benzoate, and vinyl cinnamate. In the esterification reaction with a vinyl carboxylate, one or more catalysts selected from the group consisting of alkali metal hydroxides, alkaline earth metal hydroxides, alkali metal carbonates, alkaline earth metal carbonates, alkali metal hydrogencarbonates, primary to tertiary amines, quaternary ammonium salts, imidazole and derivatives thereof, pyridine and derivatives thereof, and alkoxides may be added.

Examples of alkali metal hydroxides and alkaline earth metal hydroxides include sodium hydroxide, potassium hydroxide, lithium hydroxide, calcium hydroxide, and barium hydroxide. Examples of alkali metal carbonates, alkaline earth metal carbonates, and alkali metal bicarbonates include lithium carbonate, sodium carbonate, potassium carbonate, cesium carbonate, magnesium carbonate, calcium carbonate, barium carbonate, lithium bicarbonate, sodium bicarbonate, potassium bicarbonate, and cesium bicarbonate.

Primary, secondary, and tertiary amines refer to primary, secondary, and tertiary amines, and specific examples include ethylenediamine, diethylamine, proline, N,N,N',N'-tetramethylethylenediamine, N,N,N',N'-tetramethyl-1,3-propanediamine, N,N,N',N'-tetramethyl-1,6-hexanediamine, tris(3-dimethylaminopropyl)amine, N,N-dimethylcyclohexylamine, and triethylamine.

Examples of imidazole and its derivatives include 1-methylimidazole, 3-aminopropylimidazole, and carbonyldiimidazole.

Examples of pyridine and its derivatives include N,N-dimethyl-4-aminopyridine and picoline.

Examples of alkoxides include sodium methoxide, sodium ethoxide, and potassium t-butoxide.

The carboxylic acid may be at least one selected from the group consisting of compounds represented by the following formula:
R-COOH
(In the formula, R represents an alkyl group having 1 to 16 carbon atoms, an alkenyl group having 2 to 16 carbon atoms, a cycloalkyl group having 3 to 16 carbon atoms, or an aryl group having 6 to 16 carbon atoms.)

Specific examples of carboxylic acids include at least one selected from the group consisting of acetic acid, propionic acid, butyric acid, caproic acid, cyclohexanecarboxylic acid, caprylic acid, capric acid, lauric acid, myristic acid, palmitic acid, stearic acid, pivalic acid, methacrylic acid, crotonic acid, pivalic acid, octylic acid, benzoic acid, and cinnamic acid.

Among these carboxylic acids, at least one selected from the group consisting of acetic acid, propionic acid, and butyric acid, and particularly acetic acid, is preferred from the viewpoint of reaction efficiency.
In the reaction of carboxylic acid, one or more of the following may be added as a catalyst: an acidic compound such as sulfuric acid, hydrochloric acid, or phosphoric acid; a Lewis acid (for example, a Lewis acid compound represented by MYn, where M represents a semimetallic element such as B, As, or Ge; a base metal element such as Al, Bi, or In; a transition metal element such as Ti, Zn, or Cu; or a lanthanoid element; n is an integer corresponding to the valence of M and represents 2 or 3; and Y represents a halogen atom, OAc, _OCOCF3 , _ClO4 , _SbF6 , _PF6 , or _OSO2CF3 ( _OTf )); or an alkaline compound such as triethylamine or pyridine.

Among these esterification reactants, at least one selected from the group consisting of acetic anhydride, propionic anhydride, butyric anhydride, vinyl acetate, vinyl propionate, vinyl butyrate, and acetic acid, with acetic anhydride and vinyl acetate being particularly preferred from the standpoint of reaction efficiency.

In one embodiment, the chemical modification is acylation. From the viewpoint of affinity with the matrix resin, the degree of acyl substitution (DS) may be 0 or more, or 0.2 or more, or 0.25 or more, or 0.3 or more, or 0.5 or more. Furthermore, since the remaining unmodified cellulose backbone allows for the production of esterified cellulose nanofibers that combine the high tensile strength and dimensional stability inherent to cellulose with the high dispersibility in the matrix resin inherent to the chemical modification, the degree of acyl substitution (DS) is preferably 1.5 or less, or 1.2 or less, or 1.0 or less, or 0.8 or less, or 0.7 or less, or 0.6 or less, or 0.5 or less.

The degree of acyl substitution (DS) can be calculated from the reflection infrared absorption spectrum of the esterified cellulose nanofiber based on the peak intensity ratio between the peak derived from the acyl group and the peak derived from the cellulose skeleton. The peak of the absorption band of C═O derived from the acyl group appears at 1730 cm ⁻¹ , and the peak of the absorption band of C—O derived from the cellulose skeleton appears at 1030 cm ⁻¹ . The DS of the esterified cellulose nanofiber can be determined by preparing a correlation graph between the DS obtained from the solid-state NMR measurement of the esterified cellulose nanofiber described below and the modification rate (IR index 1030), which is defined as the ratio of the peak intensity of the absorption band of C═O derived from the acyl group to the peak intensity of the absorption band of C—O derived from the cellulose skeleton, and calculating the calibration curve of the degree of substitution DS = 4.13 × IR index (1030) from the correlation graph.
It can be found by using

The DS of esterified cellulose nanofibers can be calculated by solid-state NMR by performing ^13C solid-state NMR measurement on freeze-pulverized esterified cellulose nanofibers, and calculating the DS using the following formula from the total area intensity (Inp) of signals assigned to carbon atoms C1-C6 derived from the pyranose ring of cellulose that appear in the range from 50 ppm to 110 ppm, relative to the area intensity (Inf) of a signal assigned to one carbon atom derived from the modifying group.
DS=(Inf)×6/(Inp)
For example, when the modifying group is an acetyl group, the signal at 23 ppm assigned to —CH ₃ can be used.
The conditions for ^{the 13} C solid state NMR measurement are as follows, for example:
Equipment: Bruker Biospin Avance500WB
Frequency: 125.77MHz
Measurement method: DD/MAS method Waiting time: 75 seconds
NMR sample tube: 4 mm diameter
Total number of times: 640 (approx. 14 hours)
MAS: 14,500Hz
Chemical shift reference: glycine (external reference: 176.03 ppm)

The DS heterogeneity ratio (DSs/DSt), defined as the ratio of the degree of modification (DSs) of the fiber surface to the degree of modification (DSt) of the entire fiber of the esterified cellulose nanofiber (which is synonymous with the above-mentioned degree of acyl substitution (DS)), is preferably 1.05 or higher. The higher the DS heterogeneity ratio, the more pronounced the sheath-core-like heterogeneous structure (i.e., a structure in which the fiber surface is highly chemically modified while the fiber center retains a cellulose structure close to the original, unmodified structure). This allows for the high tensile strength and dimensional stability inherent in cellulose, while improving the affinity with resins when composited with resins and the dimensional stability of resin compositions. The DS heterogeneity ratio is more preferably 1.1 or higher, or 1.2 or higher, or 1.3 or higher, or 1.5 or higher, or 2.0 or higher. From the viewpoint of ease of production of chemically modified cellulose nanofibers, it is preferably 30 or lower, or 20 or lower, or 10 or lower, or 6 or lower, or 4 or lower, or 3 or lower.
The DSs value varies depending on the degree of modification of the esterified cellulose nanofiber, but as an example, it is preferably 0.1 or more, more preferably 0.2 or more, even more preferably 0.3 or more, and even more preferably 0.5 or more, and is preferably 3.0 or less, more preferably 2.5 or less, particularly preferably 2.0 or less, even more preferably 1.5 or less, particularly preferably 1.2 or less, and most preferably 1.0 or less. The preferred range of DSt is as described above for the acyl substituent (DS).

The smaller the coefficient of variation (CV) of the DS heterogeneity ratio of chemically modified cellulose nanofibers, the smaller the variability in the various physical properties of the resin composition, and therefore the better. The coefficient of variation is preferably 50% or less, or 40% or less, or 30% or less, or 20% or less. For example, the coefficient of variation can be further reduced in a method in which chemically modified cellulose nanofibers are obtained by defibrating a cellulose raw material and then chemically modifying it (i.e., a sequential method), whereas it can be increased in a method in which defibration and chemical modification of the cellulose raw material are performed simultaneously (i.e., a simultaneous method). While the mechanism behind this is unclear, it is thought that in the simultaneous method, chemical modification is more likely to proceed in the fine fibers produced in the early stages of defibration. Furthermore, when chemical modification reduces the hydrogen bonding between cellulose microfibrils, defibration proceeds further, resulting in an increase in the coefficient of variation of the DS heterogeneity ratio.

The coefficient of variation (CV) of the DS heterogeneity ratio can be calculated using the following formula: 100 g of an aqueous dispersion of chemically modified cellulose nanofiber (solid content of 10 mass% or more) is collected, 10 g of each is freeze-pulverized, and the DS heterogeneity ratio is calculated from the DSt and DSs of 10 samples. The standard deviation (σ) and arithmetic mean (μ) of the DS heterogeneity ratio among the obtained 10 samples are then used to calculate the coefficient of variation (CV) of the DS heterogeneity ratio using the following formula:
DS heterogeneity ratio = DSs/DSt
Coefficient of variation (%) = standard deviation σ / arithmetic mean μ × 100

The DSs is calculated as follows. Specifically, esterified cellulose nanofibers powdered by freeze-pulverization are placed on a 2.5 mm diameter dish-shaped sample stage, the surface is pressed down to flatten it, and then measurement is performed using X-ray photoelectron spectroscopy (XPS). The XPS spectrum reflects the constituent elements and chemical bonding state of only the surface layer of the sample (typically about a few nm). Peak separation is performed on the obtained C1s spectrum, and the DSs can be calculated using the following formula, which is the integrated intensity (Ixf) of the peak assigned to one carbon atom derived from the modifying group relative to the integrated intensity (Ixp) of the peak (289 eV, C-C bond) assigned to carbons C2-C6 derived from the pyranose ring of cellulose.
DSs=(Ixf)×5/(Ixp)
For example, when the modifying group is an acetyl group, the C1s spectrum is subjected to peak separation at 285 eV, 286 eV, 288 eV, and 289 eV, and then the peak at 289 eV is used as Ixp, and the peak (286 eV) derived from the O—C═O bond of the acetyl group is used as Ixf.
The conditions for the XPS measurement used are, for example, as follows.
Equipment used: ULVAC-Phi VersaProbe II
Excitation source: mono. AlKα 15 kV x 3.33 mA
Analysis size: Approximately 200 μmφ
Photoelectron extraction angle: 45°
Capture area Narrow scan: C 1s, O 1s
Pass Energy: 23.5eV

<Additional ingredients>
The resin composition of this embodiment may further contain additional components as necessary to improve its performance. Examples of additional components include dispersants; organic or inorganic filler components other than cellulose; compatibilizers; plasticizers; colorants; fragrances; flow control agents; leveling agents; conductive agents; antioxidants; antistatic agents; UV absorbers; UV dispersants; and deodorizers. The content of any additional components in the resin composition is appropriately selected within a range that does not impair the desired effects of the present invention, and may be, for example, 0.01% by mass to 50% by mass, or 0.1% by mass to 30% by mass.

Preferred dispersants are compounds capable of reacting with or hydrogen bonding to the hydroxyl groups of cellulose. Suitable examples of dispersants include one or more selected from the group consisting of cellulose derivatives, polyalkylene oxides, amides, and amines. Cellulose derivatives are cellulose-based substances and therefore have a high affinity with cellulose. However, they are also thermoplastic resins, and are therefore highly effective in improving the dispersion stability of cellulose in resin compositions, making them preferable. Dispersants preferably have a boiling point higher than that of water. Note that a boiling point higher than that of water refers to a boiling point higher than the boiling point of water at each pressure on its vapor pressure curve (for example, 100°C at 1 atmosphere).

Examples of cellulose derivatives include cellulose esters and cellulose ethers, with cellulose esters being preferred due to their excellent heat resistance.

The polyalkylene oxide may be composed of one or more types of oxyalkylene units, and the arrangement of the two or more types of oxyalkylene units may be random or block. A suitable example from the standpoint of good affinity with cellulose is polyethylene oxide. The number of alkylene oxide repeat units in the polyalkylene oxide may be, for example, 500 to 100,000, or 1,000 to 80,000, or 2,000 to 50,000.

An amide compound is a compound that has one or more amide bonds (-C(=O)NH- bonds) in the molecule. The amide compound may be an aliphatic or aromatic amide, or a combination thereof. It is preferable that the amide compound has an amide bond in the molecular backbone (i.e., a portion that is not a side chain) because it has a good effect of improving the dispersibility of cellulose. Examples of amine compounds include primary amines, secondary amines, and tertiary amines, such as amine hexanoic acid.

Surfactants include copolymers (e.g., block copolymers of propylene oxide and ethylene oxide, block copolymers of tetrahydrofuran and ethylene oxide) obtained using one or more compounds that provide hydrophilic segments (e.g., polyethylene glycol) and one or more compounds that provide hydrophobic segments (e.g., polypropylene glycol, poly(tetramethylene ether) glycol (PTMEG), polybutadiene diol, etc.), as well as modified products thereof (e.g., acid-modified products).

In the resin composition, the amount of dispersant per 100 parts by mass of cellulose nanofibers is preferably 1 part by mass or more, or 5 parts by mass or more, or 10 parts by mass or more, or 20 parts by mass or more, from the viewpoint of good dispersion of cellulose and network formation; and preferably 500 parts by mass or less, or 300 parts by mass or less, or 200 parts by mass or less, from the viewpoint of reducing variation in the performance of the resin composition.

<Production of resin composition>
The method of this embodiment includes a mixing step of mixing components containing the above-described components to obtain a resin composition. In the mixing step, a mixed component containing polyamide (A1), polyamide (A2), and cellulose nanofibers (B) (in a first embodiment) or a mixed component containing polyamide (A) and cellulose nanofibers (B) (in a second embodiment) is mixed.

In one aspect, the amount of cellulose nanofibers per 100 parts by mass of polyamide in each of the mixed component and the resin composition is preferably 0.1 parts by mass or more, or 0.5 parts by mass or more, or 1 part by mass or more, or 2 parts by mass or more, from the viewpoint of obtaining a good property-improving effect, and is preferably 100 parts by mass or less, or 80 parts by mass or less, or 70 parts by mass or less, or 60 parts by mass or less, from the viewpoint of processability.

In one aspect, the amount of cellulose nanofibers, based on 100% by total mass of the mixed components and 100% by total mass of the resin composition, is preferably 0.1% by mass or more, or 0.5% by mass or more, or 1% by mass or more, or 2% by mass or more, or 3% by mass or more, or 5% by mass or more, from the viewpoint of obtaining a good effect of improving physical properties, and is preferably 50% by mass or less, or 40% by mass or less, or 30% by mass or less, or 20% by mass or less, from the viewpoint of processability.

Mixing is typically performed by melt-kneading. Cellulose nanofibers may be subjected to the mixing process in the form of a dried product or a slurry (e.g., an aqueous dispersion). The heating temperature during melt-kneading of the mixed components may be adjusted depending on the polyamide used. A temperature that is equal to or higher than the melting point of the polyamide but not significantly higher than the melting point is preferred. The heating temperature is preferably equal to or higher than the melting point of the polyamide, or equal to or higher than the melting point + 20°C, or equal to or higher than the melting point + 30°C, or equal to or higher than the melting point + 40°C. From the viewpoint of suppressing degradation of the mixed components, the heating temperature is preferably equal to or lower than the melting point + 90°C, or equal to or lower than the melting point + 80°C, or equal to or lower than the melting point + 70°C. The melting point is the peak-top temperature of the highest endothermic peak that appears when the temperature is increased from 23°C at a rate of 10°C/min using a differential scanning calorimeter (DSC). In embodiments where the mixed components contain multiple polyamides, this refers to the highest melting point.

From the viewpoint of suppressing deterioration of the mixed components (particularly the polyamide and cellulose nanofibers), the moisture content of the mixed components is preferably 5,000 ppm by mass or less, or 2,000 ppm by mass or less, or 1,500 ppm by mass or less, or 1,000 ppm by mass or less. On the other hand, from the viewpoint of ease of process control, the moisture content may be, for example, 1 ppm by mass or more, or 100 ppm by mass or more, or 500 ppm by mass or more. In a preferred embodiment, the moisture content of the polyamides to be mixed, more specifically, the polyamides (A1) and (A2) or the polyamide (A), is within the above range.

For melt kneading, extruders such as single-screw and twin-screw extruders can be used, but twin-screw extruders are preferred for controlling the dispersibility of cellulose. The ratio L/D, calculated by dividing the cylinder length (L) of the extruder by the screw diameter (D), is preferably 40 or greater, and particularly preferably 50 or greater. Furthermore, the screw rotation speed during kneading is preferably in the range of 100 to 800 rpm, more preferably 150 to 600 rpm. These values vary depending on the screw design. From the perspective of suppressing thermal degradation of the resin composition, weak kneading (i.e., kneading under low shear force) is preferred. Weak kneading can be achieved, for example, by providing many conveying zones in the extruder's screw configuration or by reducing the screw rotation speed.

Each screw inside the extruder's cylinder is optimized by combining a full-flight screw with an elliptical two-wing screw shape, a kneading element called a kneading disc, and more.

In one embodiment, an addition port is installed midway through the extruder cylinder, and raw materials fed into the addition port are guided to a screw inside the cylinder. There are no particular restrictions on the location of the addition port, but in one embodiment, the addition port is located downstream of the melt-kneading zone. In normal kneading using an extruder, the initial resin melting zone is the area where shear is strongest, so by adding a filler component to the unmolten resin moving through the conveying zone, the filler is finely dispersed by the shear force generated when the resin is subsequently heated and melted.

When finely dispersing cellulose nanofibers in polyamide, adding them before the resin melting zone can cause cellulose degradation due to the strong shear forces in the resin melting zone, which can lead to problems such as loss of the cellulose's inherently strong crystalline structure, reduced mechanical properties as a reinforced resin, and coloration and odor. In particular, when using a high-melting-point resin such as polyamide, the kneading temperature is high, which tends to subject the cellulose nanofibers to a harsh thermal history. From this perspective, adding cellulose nanofibers to pre-melted polyamide is advantageous, as the cellulose is dispersed in the polyamide with minimal degradation.

In order to reduce the thermal history experienced by the mixed components as they pass through the cylinder, it is preferable to design the cellulose nanofiber addition port downstream of the melt-kneading zone of the extruder. Specifically, it is preferable to design the length (L2) from the cylinder outlet to the addition port to be less than half the total length of the cylinder (L1). Note that the total length of the cylinder also includes parts not involved in kneading (e.g., the conveying zone). Cellulose is introduced through the addition port and mixed into the polyamide that has been melt-kneaded inside the extruder.

When the distance that cellulose nanofibers are transported within the extruder is shorter than that of polyamide, a uniform dispersion can be achieved by devising a screw configuration within the cylinder after the cellulose nanofibers are mixed in. Specifically, and not limited to this, a high degree of dispersion of cellulose nanofibers can be more reliably achieved by installing one or more counterclockwise screws that create a feed in the opposite direction to the forward direction within the cylinder downstream of the addition port.

In the mixing step, it is preferable to reuse a portion of the resin composition and/or molded article as part of the mixed components. Such a resin composition and/or molded article may be the resin composition and/or molded article produced in this embodiment, or may be one produced by another process (for example, a recycled polyamide produced by another process). By returning a resin composition in which the molecular weight of the polyamide has been reduced by the thermal history in the mixing step to produce the resin composition, and/or a molded article in which the molecular weight of the polyamide has been reduced by the thermal history in the mixing step and the thermal history in the molding step to produce a molded article from the resin composition, to the mixing step, the reduced molecular weight of the polyamide can be restored, and a resin composition or molded article with excellent physical properties can be obtained.

The total proportion of the resin composition-derived components and the molded product-derived components in the mixed components is preferably 1% by mass or more, or 2% by mass or more, or 5% by mass or more, from the viewpoint of achieving a good recovery effect for the molecular weight reduction described above; and is preferably 99% by mass or less, or 80% by mass or less, or 50% by mass or less, from the viewpoint of accurately controlling the physical properties of the resin composition.

<Shape of resin composition>
The resin composition of this embodiment can be provided in various shapes. Specific examples include resin pellets, sheets, fibers, plates, rods, etc., with the resin pellet shape being preferred due to ease of post-processing and transportation. Preferred resin pellet shapes include round, oval, and cylindrical shapes, and the shape may vary depending on the cutting method used during extrusion. For example, pellets cut using a cutting method known as underwater cutting are often round, pellets cut using a cutting method known as hot cutting are often round or oval, and pellets cut using a cutting method known as strand cutting are often cylindrical. The preferred pellet diameter for round pellets is 1 mm or more and 3 mm or less. The preferred diameter for cylindrical pellets is 1 mm or more and 3 mm or less, and the preferred length is 2 mm or more and 10 mm or less. The above diameter and length are preferably set to be equal to or greater than the lower limit from the viewpoint of operational stability during extrusion, and are preferably set to be equal to or less than the upper limit from the viewpoint of fitment into a molding machine during post-processing.

<Characteristics of resin composition>
<Moisture percentage>
From the viewpoint of the stability over time of the resin composition, the moisture content of the resin composition is preferably 5000 ppm by mass or less, or 2000 ppm by mass or less, or 1500 ppm by mass or less. On the other hand, from the viewpoint of ease of process control, the moisture content may be 1 ppm by mass or more, or 100 ppm by mass or more, or 500 ppm by mass or more in one embodiment.

In one aspect, the moisture content of the resin composition is higher than that of the polyamide to be mixed (more specifically, in the first aspect, the moisture content of the mixture of polyamide (A1) and polyamide (A2), and in the second aspect, the moisture content of polyamide (A)). In this embodiment, a moisture content of the resin composition higher than that of the polyamide to be mixed is an indicator that water was generated during the mixing process through the formation of intermolecular amide bonds in the polyamide. According to the method of this embodiment, the polyamide is polymerized through amide bond formation, thereby increasing the molecular weight of the polyamide to a level that satisfies, for example, the aforementioned formula (1a) or (2a). In one aspect, the ratio of the moisture content of the polyamide in the resin composition to that of the polyamide to be mixed is greater than 1, preferably 1.01 or greater, 1.03 or greater, or 1.05 or greater. The higher the ratio, the more significant the benefits of the method of this embodiment. On the other hand, from the viewpoint of maintaining good physical properties of the resin composition by keeping the moisture content of the polyamide in the resin composition low, the above ratio is preferably 1.50 or less, 1.30 or less, or 1.25 or less.

<Tensile yield strength>
In the resin composition of this embodiment, the tensile yield strength tends to be dramatically improved compared to polyamide alone due to the contribution of cellulose nanofibers. The ratio of the tensile yield strength of the resin composition to the tensile yield strength of polyamide alone, taken as 1.0, is preferably 1.1 times or more, more preferably 1.11 times or more, even more preferably 1.12 times or more, and most preferably 1.13 times or more. There is no particular upper limit to this ratio, but from the viewpoint of ease of production, it is preferably 1.5 times, more preferably 1.2 times. The tensile yield strength is a value measured in accordance with JIS K6920-2 using a multipurpose test piece molded in accordance with ISO294-3.

<Linear expansion coefficient>
Because the resin composition of this embodiment contains cellulose, it is possible to exhibit low thermal expansion without increasing the specific gravity. Specifically, the thermal expansion coefficient of the resin composition in the temperature range of 20°C to 100°C is preferably 70 ppm/K or less, more preferably 60 ppm/K or less, more preferably 50 ppm/K or less, more preferably 45 ppm/K or less, even more preferably 40 ppm/K or less, and most preferably 35 ppm/K or less. The lower limit of the thermal expansion coefficient is not particularly limited, but from the viewpoint of ease of production, it is preferably, for example, 5 ppm/K, more preferably 10 ppm/K. The thermal expansion coefficient is a value determined by thermomechanical measurement in accordance with ISO 11359-2.

<Tensile breaking strain>
The tensile break strain of the resin composition of this embodiment is preferably 10% or more, or 20% or more, or 30% or more, or 40% or more, or 50% or more. Such a resin composition is preferable because it has excellent toughness. Although a higher tensile break strain of the resin composition is preferable, from the viewpoint of ease of production of the resin composition, it may be, for example, 500% or less, or 200% or less, or 100% or less. The tensile break strain is a value obtained as the arithmetic average of five data points of strain at break when a tensile test is performed on an ISO 37 type 3 test piece using a tensile tester at a temperature of 23°C and a relative humidity of 50% at a tensile speed of 5 mm/min.

<Production of Molded Body>
Molded articles of various shapes can be produced from the resin composition of the present embodiment.
The mixing step of this embodiment (i.e., a step of mixing components containing polyamide and cellulose nanofibers to obtain a resin composition),
a molding step of molding the resin composition;
Examples of molding methods include injection molding (e.g., injection compression molding, injection press molding, gas-assisted injection molding, and ultra-high speed injection molding), various extrusion moldings (cold runner method or hot runner method), foam molding (including supercritical fluid injection), insert molding, in-mold coating molding, heat-insulating mold molding, rapid heating and cooling mold molding, and various profile extrusion moldings (e.g., two-color molding and sandwich molding). For example, various extrusion moldings are suitable for molding sheets, films, fibers, etc. Inflation methods, calendaring methods, casting methods, etc. can also be used to mold sheets or films. Furthermore, by subjecting the material to a specific stretching operation, it can be molded into heat-shrinkable tubing. It can also be made into hollow molded products by rotational molding, blow molding, etc. In terms of design and cost, injection molding is the preferred molding method.

The molding temperature can be selected appropriately depending on the composition of the resin composition, but may be, for example, above the melting point of the polyamide used, or above that melting point + 20°C, or above the melting point + 30°C, or below the melting point + 90°C, or below the melting point + 80°C, or below the melting point + 70°C.

<Uses of resin composition and molded article>
The resin composition and molded article of this embodiment are useful as substitutes for steel plates, fiber-reinforced plastics (e.g., carbon fiber-reinforced plastics, glass fiber-reinforced plastics, etc.), resin composites containing inorganic fillers, etc. Suitable applications of the resin composition and molded article include industrial machine parts, general machine parts, automobile, railway, vehicle, ship, and aerospace-related parts, electronic and electrical parts, construction and civil engineering materials, daily necessities, sports and leisure goods, housing members for wind power generation, containers and packaging members, etc.

The following provides a more detailed explanation of exemplary embodiments of the present invention, but the present invention is not limited to these examples.

Evaluation Method
<Evaluation of Polyamide and Resin Composition>
[Concentration of amino end groups of polyamide [NH ₂ ]]
The weighed sample was dissolved in a 90% by mass aqueous phenol solution, and the solution was subjected to potentiometric titration with 1/50N hydrochloric acid at 25°C to calculate the value.

[Polyamide carboxyl end group concentration [COOH]]
The weighed sample was dissolved in benzyl alcohol at 160°C, and the content was calculated by potentiometric titration at 25°C with 1/50N hydrochloric acid as an indicator.

[Weight average molecular weight (Mw), number average molecular weight (Mn) and Mw/Mn ratio of polyamide]
The polymer pellets were dissolved in hexafluoroisopropanol, and the content was measured by gel permeation chromatography, and calculated in terms of standard polymethyl methacrylate.
The pellet was cut using a microtome (Leica, model number: RM2245) and approximately 5.0 mg of the sample was taken. Hexafluoroisopropyl alcohol containing 0.0848% sodium trifluoroacetate was added at a ratio of 1 mg:1 mL relative to the calculated resin content, and the mixture was allowed to stand for 1 hour to dissolve. The solution was filtered through a membrane filter with a pore size of 0.22 μm, and the filtrate was used as a sample for gel permeation chromatography. The equipment and measurement conditions used were as follows:
Apparatus: Tosoh HLC-8320GPC
Column: Tosoh Super HM-M
Guard column: TSK-GEL Guard manufactured by Tosoh Corporation Eluent: hexafluoroisopropyl alcohol (sodium trifluoroacetate 0.0848% by mass)
Flow rate: 0.3 mL/min Detector: RI detector Calibration curve sample: EasiVialTMPM (PMMA) manufactured by AMR (molecular weights used were 2,210,000, 1,020,000, 538,500, 260,900, 146,500, 72,800, 30,780, 13,900, 7,290, 1,810, 1,090, and 540)
Analysis software: EcoSEC-WorkStation

[Melting point of polyamide]
The polymer pellets were measured for their endothermic peak top temperatures when heated from 23° C. at a heating rate of 10° C./min using a differential scanning calorimeter (DSC8500 manufactured by Perkinelmer). When two or more endothermic peaks appeared, the peak top temperature of the highest temperature peak was taken as the melting point.

[Glass transition temperature of polyamide]
The central part of a plate-shaped multipurpose test piece obtained by molding the polymer in accordance with ISO 294-1 was measured using a dynamic viscoelasticity measuring device (ARES G2 manufactured by TA Instruments) while raising the temperature from -100°C at a heating rate of 2°C/min at an applied frequency of 10 Hz, and the peak-top temperature at which the storage modulus significantly decreased and the loss modulus reached a maximum was measured. When two or more peaks of the loss modulus appeared, the peak-top temperature of the lowest-temperature peak was taken as the glass transition point.

[Moisture content of polyamide and resin composition]
The moisture content (ppm) was measured using a Karl Fischer moisture meter (Coulometric titration trace moisture analyzer CA-200 manufactured by Mitsubishi Chemical Analytech Co., Ltd.) according to a method in accordance with ISO 15512.

<Evaluation of cellulose nanofibers>
[Preparation of porous sheet]
First, the wet cake was added to tert-butanol, and further dispersed using a mixer or the like until no aggregates were present. The concentration was adjusted to 0.5% by mass per 0.5 g of cellulose solids. 100 g of the resulting tert-butanol dispersion was filtered on filter paper, dried at 150°C, and then the filter paper was peeled off to obtain a sheet. This sheet had an air resistance of 100 sec/100 ml or less per 10 g/ ^m² of sheet basis weight, and was used as a porous sheet and as a measurement sample.
After measuring the basis weight W (g/ ^m2 ) of the sample left standing for 1 day in an environment of 23°C and 50% RH, the air resistance R (sec/100 ml) was measured using an Oken type air resistance tester (manufactured by Asahi Seiko Co., Ltd., model EG01). At this time, the value per 10 g/ ^m2 basis weight was calculated according to the following formula.
Air resistance per ^unit area of 10 g/m² (sec/100 ml) = R/W x 10

[Degree of acyl substitution (DS)]
The infrared spectrum of the porous sheet was measured by ATR-IR at five points using a Fourier transform infrared spectrophotometer (FT/IR-6200 manufactured by JASCO Corporation) under the following conditions:
Number of times accumulated: 64 times,
Wavenumber resolution: 4cm ^-1 ,
Measurement wave number range: 4000 to 600 cm ⁻¹ ,
ATR crystal: diamond,
Incident angle: 45°
From the obtained IR spectrum, the IR index was calculated using the following formula:
IR index = H1730/H1030
In the formula, H1730 and H1030 are the absorbances at 1730 cm ⁻¹ and 1030 cm ⁻¹ (absorption bands of the C—O stretching vibration of the cellulose backbone chain). Note that the lines connecting 1900 cm ⁻¹ and 1500 cm ⁻¹ and 800 cm ⁻¹ and 1500 cm ⁻¹ are taken as baselines, and these mean the absorbances when these baselines are taken as absorbance 0.
The average degree of substitution at each measurement point was calculated from the IR index according to the following formula, and the average value was taken as DS.
DS = 4.13 x IR index

[Crystallization degree]
The porous sheet was subjected to X-ray diffraction measurement, and the crystallinity was calculated using the following formula.
Crystallinity (%) = [I ₍₂₀₀₎ - I _(amorphous) ] / I ₍₂₀₀₎ × 100
I ₍₂₀₀₎ : Diffraction peak intensity due to the 200 plane (2θ = 22.5°) in cellulose type I crystals. I _(amorphous) : Halo peak intensity due to amorphous in cellulose type I crystals, peak intensity at an angle 4.5° lower than the diffraction angle of the 200 plane (2θ = 18.0°) (X-ray diffraction measurement conditions).
Device: MiniFlex (manufactured by Rigaku Corporation)
Operation axis 2θ/θ
Radiation source CuKα
Measurement method: Continuous Voltage: 40 kV
Current 15mA
Starting angle 2θ=5°
End angle 2θ = 30°
Sampling width 0.020°
Scan speed: 2.0°/min
Sample: A porous sheet is attached to the sample holder

[Average fiber diameter, L/D]
The wet cake was diluted to 0.01% by mass with tert-butanol and dispersed using a high-shear homogenizer (manufactured by IKA, trade name "Ultra Turrax T18") under processing conditions of 25,000 rpm for 5 minutes. The resulting dispersion was cast onto mica, air-dried, and measured with a high-resolution scanning electron microscope. The measurement was performed by adjusting the magnification so that at least 100 cellulose particles could be observed. The length (L), major axis (D), and ratio of these for 100 randomly selected cellulose particles were determined, and the arithmetic average of the 100 cellulose particles was calculated.

[Weight average molecular weight (Mw), number average molecular weight (Mn) and Mw/Mn ratio]
0.88 g of the porous sheet was weighed, chopped into small pieces with scissors, gently stirred, and then 20 mL of pure water was added and left for one day. Next, the water and solids were separated by centrifugation. Next, 20 mL of acetone was added, gently stirred, and left for one day. Next, the acetone and solids were separated by centrifugation. Next, 20 mL of N,N-dimethylacetamide was added, gently stirred, and left for one day. Again, the N,N-dimethylacetamide and solids were separated by centrifugation, and then 20 mL of N,N-dimethylacetamide was added, gently stirred, and left for one day. The N,N-dimethylacetamide and solids were separated by centrifugation, and 19.2 g of N,N-dimethylacetamide solution, prepared so that the lithium chloride concentration was 8% by mass, was added to the solids. The mixture was stirred with a stirrer, and dissolution was confirmed visually. The solution in which the cellulose was dissolved was filtered through a 0.45 μm filter, and the filtrate was used as a sample for gel permeation chromatography. The equipment and measurement conditions used are as follows.
Equipment: Tosoh HLC-8120
Column: TSKgel SuperAWM-H (6.0 mm ID x 15 cm) x 2 Detector: RI detector Eluent: N,N-dimethylacetamide (lithium chloride 0.2%)
Flow rate: 0.6 mL/min Calibration curve: Pullulan equivalent

[Average alkali-soluble polysaccharide content]
The alkali-soluble polysaccharide content was determined by subtracting the α-cellulose content from the holocellulose content (Wise method) using the method described in a non-patent document about cellulose (Wood Science Experiment Manual, edited by the Japan Wood Research Society, pp. 92-97, 2000). The alkali-soluble polysaccharide content was calculated three times for each sample, and the number average of the calculated alkali-soluble polysaccharide contents was taken as the average alkali-soluble polysaccharide content of cellulose.

[Average content of acid-insoluble components]
The quantity of acid-insoluble components was determined for cellulose fibers using the Clason method described in the non-patent literature (Wood Science Experiment Manual, edited by the Japan Wood Research Society, pp. 92-97, 2000). Bone-dried cellulose fibers were precisely weighed, placed in a designated container, and 72% by mass of concentrated sulfuric acid was added. The contents were appropriately pressed with a glass rod to homogenize them, and then autoclaved to dissolve the cellulose and hemicellulose in the acid solution. After allowing to cool, the contents were filtered through glass fiber filter paper, and the acid-insoluble components were obtained as a residue. The acid-insoluble component content was calculated from the weight of the acid-insoluble components, and the number average of the acid-insoluble component contents calculated for the three samples was used as the average acid-insoluble component content.

≪Materials used≫
<Polyamide>
First polyamide (A1): "UBE nylon 1013A" (manufactured by Ube Industries, Ltd.)
Second polyamide (A2): "UBE nylon 1013B" (manufactured by Ube Industries, Ltd.)
Recycled polyamide (R1):
It was prepared by the following procedure.
Polyamide A1 pellets were circulated and kneaded for 5 minutes at 250°C and 200 rpm using a small kneader (manufactured by Xplore Instruments, product name "Xplore"), then melted at 250°C in an attached injection molding machine to prepare dumbbell-shaped test specimens conforming to JIS K7127. These specimens were then pulverized using a Mini Speed Mill MS-05 manufactured by LabNect Co., Ltd. The powder obtained by pulverization was subjected to the same melt-kneading, injection molding, and pulverization processes four times to obtain recycled polyamide powder. The resulting resin had a number-average molecular weight (Mn) of 19,363, a weight-average molecular weight (Mw) of 32,647, and an Mw/Mn ratio of 1.69.
Recycled polyamide (R2):
It was prepared by the following procedure.
Polyamide A1 pellets were circulated and kneaded for 5 minutes at 250°C and 200 rpm using a small kneader (manufactured by Xplore Instruments, product name "Xplore"), then melted at 250°C using an attached injection molding machine to prepare dumbbell-shaped test specimens conforming to JIS K7127. These specimens were then pulverized using a Mini Speed Mill MS-05 manufactured by LabNect Co., Ltd. The powder obtained by pulverization was subjected to the same melt-kneading, injection molding, and pulverization processes four times to obtain recycled polyamide powder. The resulting resin had a number-average molecular weight (Mn) of 17,431, a weight-average molecular weight (Mw) of 32,668, and an Mw/Mn ratio of 1.87.

<Cellulose nanofiber>
[CNF-1] (unmodified cellulose nanofiber)
It was prepared by the following procedure.
After cutting the linter pulp, the pulp was subjected to a beating treatment to shorten and fibrillate the pulp to a high degree in pure water to a solid content of 1.5% by mass, thereby obtaining defibrated cellulose. Here, the beating treatment was carried out using a disc refiner, with a beating blade having a high cutting function (hereinafter referred to as a cutting blade) for 4 hours, followed by another beating for 1.5 hours using a beating blade having a high defibrating function (hereinafter referred to as a defibrating blade), thereby obtaining an unmodified cellulose nanofiber slurry.

[CNF-2] (acetylated cellulose nanofiber)
1 kg of defibrated cellulose and 19 kg of dimethyl sulfoxide were added to a KAPPA VITA (registered trademark) homogenizer (tank size 35 L) and stirred until homogenous. 2.1 kg of vinyl acetate and 0.321 kg of sodium bicarbonate were added thereto, and acetylation was carried out at 60°C for 60 minutes with stirring to obtain an acetylated cellulose nanofiber slurry.

<Dispersant>
Polyethylene glycol-polypropylene glycol copolymer Sannix GL-3000 (degree of polymerization 3000, available from Sanyo Chemical Industries, Ltd.)

<Production of resin composition>
Example 1
To 7 parts by mass of the cellulose nanofiber solids in the obtained unmodified cellulose nanofiber slurry, 3 parts by mass of Sannix GL-3000 was added, and the mixture was vacuum dried at 50 rpm and 50 ° C using a planetary mixer (Primix Corporation, Hibismix 2P-1) to obtain dried composite particles C1. 26 parts by mass of UBE Nylon 1013A and 60 parts by mass of UBE Nylon 1013B were added to 14 parts by mass of the obtained dried composite particles, and the mixture was circulated and kneaded for 5 minutes at 250 ° C and 200 rpm using a small kneader (Xplore Instruments, product name "Xplore"), and then melted at 250 ° C in an attached injection molding machine. Dumbbell-shaped test specimens according to JIS K7127 were prepared and used for evaluation. Each evaluation was performed using the resin composite 1 in the form of the obtained pellets and dumbbell-shaped test specimens.
Furthermore, 13 parts by mass of UBE nylon 1013A, 13 parts by mass of recycled polyamide (R1), 30 parts by mass of UBE nylon 1013B, and 30 parts by mass of recycled polyamide (R2) were added to 14 parts by mass of the dry composite particles, and the mixture was circulated and kneaded for 5 minutes at 250°C and 200 rpm using a small kneader (manufactured by Xplore instruments, product name "Xplore"), and then melted at 250°C in an attached injection molding machine to prepare dumbbell-shaped test pieces according to JIS K7127. These were evaluated according to the following criteria.
◯: The Mn of the composition to which the recycled polyamide was added was equal to _or greater than the Mn of the composition to which the recycled polyamide was not added. ×: The Mn of the composition to which the recycled polyamide was added was less than the Mn of the composition to which the recycled _polyamide was not added.

<Examples 2 to 5, Comparative Examples 1 to 4>
Resin compositions were produced and evaluated in the same manner as in Example 1, except that the formulations of polyamide and cellulose nanofibers were as shown in Table 4. The results are shown in Table 4.

The resin composition provided by one aspect of the present invention has excellent physical properties such as heat resistance and dimensional stability, and can be suitably used to produce molded articles for a wide range of applications.

Claims (9)

A method for producing a resin composition containing polyamide and cellulose nanofibers,
The method includes a mixing step of mixing a first polyamide (A1) satisfying the relationship of amino terminal group concentration [NH _{2 ]} > carboxyl terminal group concentration [COOH], a second polyamide (A2) satisfying the relationship of amino terminal group concentration [NH 2 ] < carboxyl terminal group concentration [COOH], and cellulose nanofibers (B) to obtain a resin composition;
Based on the number average molecular weight (M _A1 ) of the first polyamide (A1), the mass concentration (C _A1 ) of the first polyamide (A1) in the mixed component, the number average molecular weight (M A2 ) of the second polyamide ( _A2 ), the mass concentration (C A2 ) of the second polyamide (A2) in the mixed component, and the total mass concentration (C T ) of the mass concentration (C _A1 ) of the first polyamide ( _A1 ) and the mass concentration (C _A2 ) of the second polyamide ( _A2 ) in the mixed component, the following formula is calculated:
M _T = (M _A1 × C _A1 + M _A2 × C _A2 )/C _T
and the number average molecular weight ( _M ) of the polyamide in the resin composition is calculated according to the following formula (1a):
M/M _T >1.1 (1a)
Fulfilling the relationship,
A method for producing a resin composition, wherein the total concentration ([NH ₂ ] + [COOH]) of the amino terminal group concentration [NH ₂ ] and the carboxyl terminal group concentration [COOH] of each of the first polyamide (A1) and the second polyamide (A2) is 10 μequivalents/g or more and 500 μequivalents/g or less . Based on the amino terminal group concentration [NH ₂ ] _A1 and carboxyl terminal group concentration [COOH] _A1 of the first polyamide (A1), the mass concentration (C _A1 ) of the first polyamide (A1) in the mixed component, the amino terminal group concentration [NH ₂ ] _A2 and carboxyl terminal group concentration [COOH] _A2 of the second polyamide (A2), the mass concentration (C _A2 ) of the second polyamide (A2) in the mixed component, and the total mass concentration (C T ) of the mass concentration (C _A1 ) of the first polyamide (A1) and the mass concentration (C _A2 ) of the second polyamide ( _A2 ) in the mixed component, the following formulas (1b) and ( 1c ) are used:
[NH ₂ ] _T = ([NH ₂ ] _A1 × C _A1 + [NH ₂ ] _A2 × C _A2 )/C _T (1b)
[COOH] _T = ([COOH] _A1 × C _A1 + [COOH] _A2 × C _A2 )/C _T (1c)
2. _The method of claim 1 , wherein the ratio of [NH ₂ ] _T to [COOH ] _T ([NH _{2 ] T} /[COOH] _T ) calculated according to the following formula is 1 or more _and 5 or less. The method according to claim 1 or 2 , wherein the moisture content of the resin composition is higher than the moisture content of the mixture of the first polyamide (A1) and the second polyamide (A2) to be mixed. The method according to any one of claims 1 to 3, wherein the moisture content of the resin composition is 100 ppm by mass or more and 5000 ppm by mass or less, and the moisture content of the first polyamide (A1) and the second polyamide (A2) to be mixed is 1 ppm by mass or more and 1000 ppm by mass or less. The method according to any one of claims 1 to 4 , wherein a part of the resin composition is used as a part of the mixed component. The method according to any one of claims 1 to 5 , wherein the cellulose nanofibers (B) have a number average fiber diameter of 2 nm or more and 1000 nm or less. The method according to any one of claims 1 to 6 , wherein the cellulose nanofibers (B) have an acyl substitution degree (DS) of 0 or more and 1.5 or less. The method according to any one of claims 1 to 7 , wherein the cellulose nanofibers (B) have a crystallinity of 60% or more. The method according to any one of claims 1 to 8 , wherein the average acid-insoluble component content of the cellulose nanofibers (B) is 10 mass% or less.