JP7801924B2 - Peel strength improver - Google Patents

Description

The present invention relates to a peel strength improver. Furthermore, the present invention relates to a method for improving peel strength using such a peel strength improver.

In recent years, environmentally friendly technologies have been gaining attention, and against this technological backdrop, materials made from cellulose fibers, a naturally occurring biomass material, are attracting attention.

For example, Patent Document 1 proposes using a resin composition containing cellulose nanofibers as an adhesive.

JP 2016-138220 A

The performance required of adhesive compositions varies depending on the application. In recent years, there has been an increasing demand for adhesive compositions with higher peel strength.

Therefore, an object of the present invention is to provide a peel strength improver that can improve peel strength.

The present invention relates to the following [1] and [2].
[1] A peel strength improver containing modified cellulose fibers having an unsaturated hydrocarbon group as a modifying group.
[2] A method for improving peel strength using the peel strength improver according to [1] above.

By using the peel strength improver provided by this invention, peel strength can be improved.

As a result of extensive research conducted by the inventors to solve the above problems, they discovered that the peel strength of adhesives can be improved by incorporating modified cellulose fibers having unsaturated hydrocarbon groups as modifying groups into the adhesive, leading to the completion of the present invention.

1. Peel Strength Improver The peel strength improver of the present invention contains modified cellulose fibers.
The mechanism by which the peel strength improver of the present invention exerts the above-mentioned effect is not clear, but it is presumed that the unsaturated hydrocarbon groups of the modified cellulose fiber interact with the adherend, such as a metal, thereby strengthening the adhesion between the adhesive and the adherend.

[Modified cellulose fiber]
The modified cellulose fiber of the present invention is a cellulose fiber having an unsaturated hydrocarbon group as a modifying group. The modifying group is preferably bonded to the glucose constituting the cellulose fiber via an ionic bond and/or an amide bond, more preferably to some or all of the hydroxy groups of the glucose, or to carboxy groups obtained by converting the hydroxy groups of the glucose units to carboxy groups, and even more preferably to carboxy groups obtained by converting the group (—CH ₂ OH) at the C6 position of the glucose unit to a carboxy group.

[Modifying group]
The essential modifying group of the modified cellulose fiber is (a) an unsaturated hydrocarbon group. From the viewpoint of improving peel strength, the modified cellulose fiber preferably further contains a modifying group other than the unsaturated hydrocarbon group. Examples of the modifying group other than the unsaturated hydrocarbon group include one or more types selected from the group consisting of (b) a saturated hydrocarbon group and (c) a polymer group.

(a) Unsaturated Hydrocarbon Groups Examples of unsaturated hydrocarbon groups include monovalent unsaturated hydrocarbon groups, such as chain unsaturated hydrocarbon groups and cyclic unsaturated hydrocarbon groups.
From the viewpoint of improving peel strength, the number of carbon atoms in the unsaturated hydrocarbon group is 2 or more, preferably 3 or more, and from the viewpoint of easy availability of raw materials, it is preferably 30 or less, more preferably 22 or less, even more preferably 18 or less, even more preferably 12 or less, and still more preferably 8 or less. The unsaturated hydrocarbon group may have a substituent described later, and a portion of the unsaturated hydrocarbon group may be substituted with a hydrogen nitride group.
When unsaturated hydrocarbon groups are introduced into cellulose fibers using an amine compound, the number of unsaturated hydrocarbon groups per nitrogen atom is 1 to 3, and from the viewpoint of improving peel strength, 2 is preferred.

From the viewpoint of increasing peel strength, the chain unsaturated hydrocarbon group is preferably a straight-chain chain unsaturated hydrocarbon group. Specific examples of the chain unsaturated hydrocarbon group include propenyl, butenyl, isobutenyl, isoprenyl, pentenyl, hexenyl, heptenyl, octenyl, nonenyl, decenyl, dodecenyl, tridecenyl, tetradecenyl, and octadecenyl groups.

Examples of the cyclic unsaturated hydrocarbon group include an aryl group, an aralkyl group, and a heterocyclic aromatic hydrocarbon group.
Examples of the aryl group include a phenyl group, a naphthyl group, an anthryl group, a phenanthryl group, a biphenyl group, a triphenyl group, a terphenyl group, and groups in which these groups are substituted with the substituents described below.
Examples of the aralkyl group include a benzyl group, a phenethyl group, a phenylpropyl group, a phenylpentyl group, a phenylhexyl group, a phenylheptyl group, a phenyloctyl group, and groups in which these groups are substituted with the substituents described below.
Examples of the heterocyclic aromatic hydrocarbon group include an imidazole group, a methylimidazole group, an ethylimidazole group, a propylimidazole group, a 2-phenylimidazole group, a benzimidazole group, and groups in which these groups are substituted with a substituent.
(b) Saturated Hydrocarbon Group From the viewpoint of increasing adhesive strength, the saturated hydrocarbon group is preferably a linear chain saturated hydrocarbon group. Examples of the chain saturated hydrocarbon group include propyl, isopropyl, butyl, sec-butyl, tert-butyl, isobutyl, pentyl, tert-pentyl, isopentyl, hexyl, isohexyl, heptyl, octyl, 2-ethylhexyl, nonyl, decyl, dodecyl, tridecyl, tetradecyl, octadecyl, docosyl, and octacosanyl groups.

(c) Polymer Group The polymer group in the present invention is a functional group containing a polymer structure.
From the viewpoint of increasing peel strength, the formula weight (molecular weight) of the polymer group is preferably 100 or more, more preferably 200 or more, even more preferably 300 or more, even more preferably 500 or more, even more preferably 1,000 or more, and even more preferably 1,500 or more. From the same viewpoint, it is preferably 1,000,000 or less, more preferably 100,000 or less, even more preferably 10,000 or less, even more preferably 7,000 or less, even more preferably 5,000 or less, even more preferably 4,000 or less, even more preferably 3,500 or less, and even more preferably 2,500 or less.

From the viewpoint of increasing peel strength, the polymer group is preferably a functional group having a repeating structure linked by a structure containing oxygen atoms, more preferably a functional group having a repeating structure linked by oxygen atoms, such as a polyoxyalkylene structure or a polysiloxane structure, more preferably a functional group having a polyoxyalkylene structure, and even more preferably an alkoxypolyoxyalkylene group.

From the viewpoint of increasing peel strength, the polyoxyalkylene structure is preferably a (co)polymer structure of one or more oxyalkylenes selected from oxyalkylenes having from 2 to 8 carbon atoms, more preferably a (co)polymer structure of one or more oxyalkylenes selected from oxyalkylenes having from 2 to 4 carbon atoms, even more preferably a (co)polymer structure of one or two oxyalkylenes selected from ethylene oxide (EO) and propylene oxide (PO), and even more preferably a copolymer structure in which ethylene oxide and propylene oxide are polymerized randomly or in a block form (EO/PO copolymer structure).

Examples of copolymer structures in which ethylene oxide and propylene oxide are polymerized randomly or in blocks include those represented by the following formula:

(wherein R ¹ represents a hydrogen atom, a hydrocarbon group having from 1 to 6 carbon atoms, or a -CH ₂ CH(CH ₃ )NH ₂ group; EO and PO are present randomly or in a block form; a is a positive number indicating the average number of moles of EO added; and b is a positive number indicating the average number of moles of PO added).

From the viewpoint of increasing peel strength, R ¹ is preferably a linear or branched alkyl group having from 1 to 6 carbon atoms, more preferably a methyl group.
From the viewpoint of increasing the peel strength, a is preferably 1 or more, more preferably 3 or more, even more preferably 6 or more, even more preferably 11 or more, even more preferably 15 or more, even more preferably 20 or more, even more preferably 25 or more, and even more preferably 30 or more. From the same viewpoint, a is preferably 100 or less, more preferably 70 or less, even more preferably 60 or less, even more preferably 50 or less, and even more preferably 40 or less.

From the viewpoint of improving the releasability of the composition, b is preferably 1 or more, more preferably 3 or more, and even more preferably 5 or more. From the same viewpoint, b is preferably 50 or less, more preferably 40 or less, even more preferably 30 or less, even more preferably 25 or less, even more preferably 20 or less, even more preferably 15 or less, and even more preferably 10 or less.
In the above formula, a+b represents the average total number of moles of EO and PO added, and from the viewpoint of improving the peelability of the composition, it is preferably 4 or more, more preferably 6 or more, and even more preferably 8 or more, and from the same viewpoint, it is preferably 100 or less, more preferably 70 or less.

The PO content (mol %) in the EO/PO copolymer structure can be calculated based on the above a and b, specifically, by b × 100/(a + b). From the viewpoint of improving the release properties of the composition, the PO content is preferably 1 mol % or more, more preferably 5 mol % or more, even more preferably 7 mol % or more, even more preferably 10 mol % or more, and even more preferably 20 mol % or more. From the same viewpoint, the PO content is preferably 100 mol % or less, more preferably 90 mol % or less, even more preferably 85 mol % or less, even more preferably 75 mol % or less, even more preferably 60 mol % or less, even more preferably 50 mol % or less, even more preferably 40 mol % or less, and even more preferably 30 mol % or less.

(d) Further Substituents The modifying group may further have a substituent. Examples of the substituent include alkoxy groups having 1 to 6 carbon atoms, such as methoxy, ethoxy, propoxy, isopropoxy, butoxy, isobutoxy, sec-butoxy, tert-butoxy, pentyloxy, isopentyloxy, and hexyloxy; methoxycarbonyl, ethoxycarbonyl, propoxycarbonyl, isopropoxycarbonyl, butoxycarbonyl, isobutoxycarbonyl, sec-butoxycarbonyl, and tert-butoxycarbonyl; Examples thereof include alkoxycarbonyl groups having from 1 to 6 carbon atoms in the alkoxy group, such as a t-butoxycarbonyl group, a pentyloxycarbonyl group, and an isopentyloxycarbonyl group; halogen atoms such as a fluorine atom, a chlorine atom, a bromine atom, and an iodine atom; acyl groups having from 1 to 6 carbon atoms, such as an acetyl group and a propionyl group; aralkyl groups; aralkyloxy groups; alkylamino groups having from 1 to 6 carbon atoms; dialkylamino groups having from 1 to 6 carbon atoms in the alkyl group; and a hydroxy group.

[Method for producing modified cellulose fibers]
The modified cellulose fiber can be produced, for example, by introducing anionic groups into raw cellulose fiber to produce anionically modified cellulose fiber (Step 1), and then bonding modifying groups to the anionic groups of the anionically modified cellulose fiber (Step 2). The "anionically modified cellulose fiber" used in the present invention is a cellulose fiber that has been anionically modified so as to contain anionic groups.

(Step 1)
Cellulose Fiber as Raw Material As the cellulose fiber as the raw material for the anion-modified cellulose fiber, natural cellulose is preferred from an environmental perspective, and examples thereof include wood pulp such as softwood pulp and hardwood pulp; cotton pulp such as cotton linter and cotton lint; non-wood pulp such as straw pulp and bagasse pulp; bacterial cellulose, etc., and these can be used alone or in combination of two or more.

The average fiber diameter of the raw cellulose fibers is not particularly limited, but from the standpoints of handleability and cost, it is preferably 5 μm or more, more preferably 7 μm or more, and from the same standpoints, it is preferably 500 μm or less, more preferably 300 μm or less. The average fiber diameter of the raw cellulose fibers can be determined by the method described in the Examples below.

The average fiber length of the raw cellulose fibers is not particularly limited, but from the standpoints of availability and cost, it is preferably 5 μm or more, more preferably 25 μm or more, and from the same standpoints, it is preferably 5,000 μm or less, more preferably 3,000 μm or less. The average fiber length of the raw cellulose fibers can be measured according to the method described in the Examples below.

Method for Introducing Anionic Groups Examples of methods for introducing carboxy groups as anionic groups into cellulose fibers include a method of oxidizing hydroxy groups in the cellulose fibers to convert them into carboxy groups, and a method of reacting the hydroxy groups in the cellulose fibers with at least one selected from the group consisting of compounds having carboxy groups, acid anhydrides of compounds having carboxy groups, and derivatives thereof.

Examples of methods for oxidizing the hydroxy groups of cellulose fibers include those described in JP 2015-143336 A and JP 2015-143337 A, which involve reacting raw cellulose fibers with an oxidizing agent such as sodium hypochlorite and a bromide such as sodium bromide using 2,2,6,6-tetramethyl-1-piperidine-N-oxyl (TEMPO) as a catalyst. By oxidizing cellulose fibers using TEMPO as a catalyst, the group (—CH ₂ OH) at the C6 position of glucose in the cellulose fiber structural unit is selectively converted to a carboxy group, thereby producing the oxidized cellulose fibers described above.
As a method for introducing sulfonic acid groups as anionic groups into cellulose fibers, a method of adding sulfuric acid to cellulose fibers and heating the fibers can be given.
Methods for introducing (phosphorous) groups as anionic groups into cellulose fibers include a method for mixing a powder or aqueous solution of (phosphorous) phosphorous or a (phosphorous) derivative with dry or wet cellulose fibers, a method for adding an aqueous solution of (phosphorous) phosphorous or a (phosphorous) derivative to a dispersion of cellulose fibers, etc. When these methods are employed, dehydration treatment, heat treatment, etc. are generally carried out after mixing or adding a powder or aqueous solution of (phosphorous) phosphorous or a (phosphorous) derivative.

The anionic group content in anion-modified cellulose fibers is preferably 0.1 mmol/g or more, more preferably 0.4 mmol/g or more, even more preferably 0.6 mmol/g or more, even more preferably 0.7 mmol/g or more, and even more preferably 0.8 mmol/g or more, from the viewpoint of introducing stable modifying groups and increasing peel strength through the introduction of modifying groups. Furthermore, from the viewpoint of improving handleability, the content is preferably 3 mmol/g or less, more preferably 2.5 mmol/g or less, more preferably 2 mmol/g or less, more preferably 1.9 mmol/g or less, even more preferably 1.8 mmol/g or less, more preferably 1.7 mmol/g or less, and even more preferably 1.5 mmol/g or less. The term "anionic group content" refers to the total amount of anionic groups in the glucose constituting the cellulose fibers, and is specifically measured by the method described in the Examples below.

(Step 2)
Introduction of modifying groups into the anionic groups of anion-modified cellulose fibers is achieved by reacting the anion-modified cellulose fibers with a compound for introducing modifying groups into the anionic groups (referred to as a "modifying compound"). As a method for introducing the modifying group, (1) when introducing the modifying group via an ionic bond, reference can be made to JP 2015-143336 A, and (2) when introducing the modifying group via an amide bond, reference can be made to JP 2015-143337 A.
After completion of step 2, post-treatment may be carried out as appropriate to remove unreacted compounds, etc. Examples of post-treatment methods that can be used include filtration, centrifugation, dialysis, etc.

(Refining process)
By micronizing the cellulose fibers at any stage of the production method of the modified cellulose fibers (for example, before step 1, before step 2, and after step 2), it is possible to reduce the micrometer-scale cellulose fibers to nanometer-scale. Reducing the average fiber diameter to nanometer size is preferable because it improves dispersibility.

The micronization process can be carried out using known micronization methods. For example, to obtain modified cellulose fibers with an average fiber diameter of nanometer size, a processing method using a grinder such as a mass colloider, or a processing method using a high-pressure homogenizer in a medium, can be carried out.

Examples of the medium include alcohols having 1 to 6 carbon atoms, preferably 1 to 4 carbon atoms, such as water, methanol, ethanol, propanol, and 1-methoxy-2-propanol (PGME); ketones having 3 to 6 carbon atoms, such as acetone, methyl ethyl ketone, and methyl isobutyl ketone; ketones having 2 to 4 carbon atoms, such as ethyl acetate and butyl acetate; saturated or unsaturated hydrocarbons having 1 to 6 carbon atoms; aromatic hydrocarbons such as benzene and toluene; halogenated hydrocarbons such as methylene chloride and chloroform; lower alkyl ethers having 2 to 5 carbon atoms; and polar solvents such as N,N-dimethylformamide (DMF), N,N-dimethylacetamide, and dimethyl sulfoxide. These solvents can be used alone or in combination. The amount of medium used should be an effective amount capable of dispersing the modified cellulose fibers. It is preferable to use an amount that is at least 1 times, more preferably at least 2 times, and preferably no more than 500 times, more preferably no more than 200 times by weight of the modified cellulose fibers.

In addition to high-pressure homogenizers, known dispersing machines are also suitable for use in the micronization process. For example, disintegrators, beaters, low-pressure homogenizers, grinders, mass colloiders, cutter mills, ball mills, jet mills, single-screw extruders, twin-screw extruders, ultrasonic agitators, and household juicer mixers can be used. Furthermore, the solids content of the modified cellulose fiber during the micronization process is preferably 50% by mass or less.

(Shortening treatment)
At any stage in the process for producing modified cellulose fibers, the cellulose fibers may be subjected to a fiber shortening treatment.
The fiber shortening treatment can be carried out by subjecting the target cellulose fibers to one or more treatment methods selected from the group consisting of (i) alkali treatment, (ii) acid treatment, (iii) heat treatment, ultraviolet treatment, electron beam treatment, mechanical treatment, and enzyme treatment.

[Properties of modified cellulose fibers]
The main properties of the modified cellulose fiber of the present invention are as follows:

(Crystal structure)
The modified cellulose fiber preferably has a cellulose type I crystal structure from the viewpoint of improving peel strength. From the viewpoint of increasing peel strength, the crystallinity of the modified cellulose fiber is preferably 10% or more, more preferably 15% or more, and even more preferably 20% or more. Furthermore, from the viewpoint of raw material availability, it is preferably 90% or less, more preferably 85% or less, even more preferably 80% or less, and even more preferably 75% or less. In this specification, the crystallinity of cellulose fiber refers to the cellulose type I crystallinity calculated from the diffraction intensity value by X-ray diffraction, and can be measured according to the method described in the Examples below. Cellulose type I refers to the crystalline form of native cellulose, and cellulose type I crystallinity refers to the proportion of crystalline regions in the entire cellulose fiber. The presence or absence of cellulose type I crystal structure can be determined by the presence of a peak at 2θ = 22.6° in X-ray diffraction measurement.

(Average fiber diameter)
The modified cellulose fibers are preferably those that have been subjected to a micronization treatment to have nanometer-sized fibers. Therefore, the average fiber diameter of the modified cellulose fibers is preferably 1 nm or more, more preferably 2 nm or more, from the viewpoints of handleability, availability, and cost, and is preferably 300 nm or less, more preferably 200 nm or less, even more preferably 150 nm or less, even more preferably 120 nm or less, and still more preferably 20 nm or less, from the viewpoints of improving handleability, dispersibility, and peel strength.

(average fiber length)
The average fiber length of the modified cellulose fiber is preferably 10 nm or more, more preferably 30 nm or more, and even more preferably 50 nm or more from the viewpoint of improving peel strength, and from the same viewpoint, is preferably 1000 nm or less, more preferably 500 nm or less, even more preferably 300 nm or less, and even more preferably less than 150 nm.

(average aspect ratio)
The average aspect ratio of the modified cellulose fiber is preferably 5 or more, more preferably 10 or more, and even more preferably 20 or more from the viewpoint of increasing peel strength, while it is preferably 300 or less, more preferably 200 or less, even more preferably 150 or less, even more preferably 100 or less, and even more preferably 70 or less from the viewpoint of increasing handleability and peel strength.
The average fiber diameter, average fiber length and average aspect ratio of the modified cellulose fiber can be determined by the method described in the Examples below.

(Amount of Modification Group Bonded and Introduction Rate)
The amount of the modifying group bonded to the modified cellulose fiber is preferably 0.01 mmol/g or more from the viewpoint of increasing the peel strength, and from the same viewpoint, is preferably 3.0 mmol/g or less. When any two or more types of modifying groups are simultaneously introduced into the modified cellulose fiber, the amount of the modifying group bonded is preferably within the above range.

From the viewpoint of dispersibility, the introduction rate of modifying groups in modified cellulose fibers is preferably 10 mol% or more. When any two or more types of modifying groups are simultaneously introduced as modifying groups, it is preferable that the total introduction rate be within the above range, provided that it does not exceed the upper limit of 100 mol%.

The amount and introduction rate of the modifying group bonded can be adjusted by adjusting the type and amount of modifying compound added, reaction temperature, reaction time, type of solvent, etc. The amount (mmol/g) and introduction rate (mol%) of the modifying group bonded refer to the amount and rate of the modifying group introduced (bonded) to the anionic group in the modified cellulose fiber. For example, when the anionic group is a carboxy group, the amount and introduction rate of the modifying group bonded to the modified cellulose fiber can be calculated using the method described in the Examples below.

The amount of modified cellulose fiber in the composition of the present invention, calculated as the blend amount and calculated as cellulose (not including modifying groups, etc.), is preferably 0.01% by mass or more, more preferably 0.05% by mass or more, more preferably 0.1% by mass or more, more preferably 0.3% by mass or more, and even more preferably 0.5% by mass or more from the viewpoint of increasing peel strength. On the other hand, from the viewpoint of handleability, it is preferably 30% by mass or less, more preferably 20% by mass or less, more preferably 15% by mass or less, more preferably 10% by mass or less, more preferably 7% by mass or less, even more preferably 5% by mass or less, and even more preferably 3% by mass or less.

〔resin〕
The peel strength improver of the present invention may further contain a resin.
In view of the use of the resin in various structures, the resin in the present invention is preferably a water-insoluble resin that does not dissolve in water or has extremely low solubility in water. Specifically, a resin whose solubility in water at 25°C is 1 mg or less per 100 g of water is called a water-insoluble resin.

The solubility is measured as follows.
100 mg of resin is added to 1 L of water (25°C) and stirred for 24 hours using a stirring device such as a stirrer. The solution (or suspension) is then centrifuged at 25°C and 3000 x g for 30 minutes to collect the insoluble residue. This residue is dried at 105°C for 3 days, and the mass after drying (dry mass) is measured. Resins with a dry mass of less than 99 mg are considered water-soluble, and those with a dry mass of 99 mg or more are considered water-insoluble.

The resins used in the present invention are preferably those that have adhesive properties on their own or those that exhibit adhesive properties when used in combination with a curing agent. Resins may be used alone or as a mixture of two or more types of resins.

Specific examples of resins include epoxy resin, urethane resin, acrylic resin, vinyl chloride resin, phenoxy resin, phenolic resin, urea resin, melamine resin, polyimide resin, unsaturated polyester resin, diallyl phthalate resin, and rubber-based resin.

Among resins, curable resins (e.g., epoxy resin, urethane resin, acrylic resin, phenoxy resin, phenolic resin, urea resin, and melamine resin) are preferred from the viewpoint of increasing peel strength. Depending on the type of resin, photocuring and/or heat curing treatment can be performed. Thermosetting resins are preferred as the resins used in the present invention.

[Other ingredients]
The peel strength improver of the present invention may contain, as necessary, components known in the adhesive field, such as a polymerization initiator, a plasticizer, a stabilizer, a lubricant, a surfactant, an inorganic or organic filler, etc. The amounts of such components are not particularly limited, and appropriate amounts may be used as appropriate.

From the viewpoint of improving peel strength, the resin content in the peel strength improver is preferably 50% by mass or more, more preferably 70% by mass or more, and even more preferably 80% by mass or more. From the same viewpoint, the resin content is preferably 99.9% by mass or less, more preferably 99% by mass or less, and even more preferably 95% by mass or less.

On the other hand, when the peel strength improver contains a resin, the content of modified cellulose fiber in the peel strength improver is preferably 0.1% by mass or more, more preferably 1% by mass or more, and even more preferably 2% by mass or more, from the viewpoint of improving peel strength; from the same viewpoint, it is preferably 10% by mass or less, more preferably 5% by mass or less, and even more preferably 4% by mass or less.

The mass ratio of modified cellulose fiber to resin in the peel strength improver of the present invention, calculated in terms of cellulose (excluding modifying groups, etc.) (however, if the cellulose has anionic groups, calculated in terms of anion-modified cellulose), is preferably 0.01/100 or more, more preferably 0.05/100 or more, more preferably 0.1/100 or more, more preferably 0.3/100 or more, and even more preferably 0.5/100 or more, from the viewpoint of improving peel strength. On the other hand, from the viewpoint of handleability, it is preferably 30/100 or less, more preferably 20/100 or less, more preferably 15/100 or less, more preferably 10/100 or less, more preferably 7/100 or less, even more preferably 5/100 or less, and even more preferably 3/100 or less.

[Method for producing peel strength improver]
The peel strength improver of the present invention can be produced, for example, by mixing the modified cellulose fiber with the resin, etc. Furthermore, if necessary, a solvent, a curing agent, or other components described above can be mixed.
The method for mixing the components constituting the peel strength improver is not particularly limited, and includes common methods such as those using a stirrer, an ultrasonic homogenizer, or a high-pressure homogenizer.

Examples of solvents that can be used during production include dimethylformamide, ethyl acetate, methyl methacrylate, ethanol, isopropanol, N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), N,N-dimethylacetamide, tetrahydrofuran (THF), a diester of succinic acid and triethylene glycol monomethyl ether, acetone, methyl ethyl ketone (MEK), acetonitrile, dichloromethane, chloroform, toluene, 1-methoxy-2-propanol (PGME), and acetic acid, and these can be used alone or in combination of two or more.
When a solvent is used, the amount thereof is, for example, preferably 50 parts by mass or more, more preferably 100 parts by mass or more, per 100 parts by mass of the resin, while preferably 5,000 parts by mass or less, more preferably 2,000 parts by mass or less.

The peel strength improver of the present invention and the adhesive composition containing the peel strength improver are in liquid or solid form (e.g., pellet or powder form) at room temperature (25°C). If they are solid, they can be made into a paste, solution, or dispersion by adding an appropriate medium. They can also be heated as needed at the time of use to make them into a fluid form.

(Applicable subjects)
The peel strength improver of the present invention is applicable to adhesives for bonding various adherends. Examples of the material of the adherend include metals such as iron, aluminum, and copper, alloys such as steel, stainless steel, and aluminum alloys, plastics such as polypropylene, polyamide, and polyacetal, fiber-reinforced plastics such as carbon fiber-reinforced plastics and glass fiber-reinforced plastics, rubber, carbon fiber, glass, and ceramics, with metals, especially iron, steel, and stainless steel, being preferred.

2. Method for Improving Peel Strength Since peel strength can be improved by using the peel strength improver of the present invention, the present invention provides a method for improving peel strength using the peel strength improver.
A specific embodiment of the peel strength improving method of the present invention is to add the above-mentioned peel strength improving agent of the present invention to various known adhesives.

In this specification, the "peel strength of the adhesive" refers to the degree of resistance to peeling between the adhesive and the adherend; specifically, the greater the value of the peel strength (N/25 mm) measured by the "T-peel test" described in the Examples below, the higher the peel strength.
Furthermore, it can be said that an adhesive that shows cohesive failure rather than interfacial failure after measurement in the "T-peel test" has superior properties.

The present invention will be explained in more detail below with reference to examples. Note that the following examples are merely illustrative of the present invention and are not intended to be limiting in any way. Note that "normal pressure" refers to 101.3 kPa, and "normal temperature" refers to 25°C.

[Average fiber diameter and average fiber length of cellulose fibers, anionically modified cellulose fibers, and modified cellulose fibers]
Depending on the size of the cellulose fibers to be measured, one of the following two measurement methods was selected for measurement.
(1) Deionized water or N,N-dimethylformamide (DMF) was added to the cellulose fibers to be measured to prepare a dispersion with a content of 0.0001% by mass. The dispersion was dropped onto mica and dried to prepare an observation sample. The fiber height (the difference in height between the area with and without fibers) of the cellulose fibers in the observation sample was measured using an atomic force microscope (AFM) (Nanoscope II Tapping mode AFM manufactured by Digital Instrument; the probe used was Point Probe (NCH) manufactured by Nanosensors). In this case, 100 cellulose fibers were extracted from a microscopic image in which the cellulose fibers could be observed, and the average fiber diameter was calculated from their fiber height. The average fiber length was calculated from the distance in the fiber direction.

(2) Deionized water was added to the cellulose fibers to be measured to prepare a dispersion with a cellulose content of 0.01% by mass. This dispersion was measured using a wet dispersion type image analysis particle size distribution analyzer (manufactured by Jusco International, product name: IF-3200) under the following conditions: front lens: 2x, telecentric zoom lens: 1x, image resolution: 0.835 μm/pixel, syringe inner diameter: 6515 μm, spacer thickness: 500 μm, image recognition mode: ghost, threshold: 8, analysis sample volume: 1 mL, and sampling: 15%. The cellulose fibers were then approximated as a rectangle, with the length of the minor axis being the fiber diameter and the length of the major axis being the fiber length. Each value was measured for 100 cellulose fibers, and the average was calculated.

[Anionic group content of anion-modified cellulose fibers and modified cellulose fibers]
A dry mass of 0.5 g of the cellulose fiber to be measured was placed in a beaker, and deionized water or a 2:1 (volume ratio) methanol/deionized water mixture was added to a total volume of 55 mL. 5 mL of 0.01 M aqueous sodium chloride solution was then added to prepare a dispersion. The dispersion was stirred until the cellulose fiber to be measured was sufficiently dispersed. 0.1 M hydrochloric acid was added to the dispersion to adjust the pH to 2.5-3. Using an automatic titrator (DKK-TOA Corporation, AUT-701), 0.05 M aqueous sodium hydroxide solution was added dropwise to the dispersion with a waiting time of 60 seconds, and the conductivity and pH were measured every minute. Measurements were continued until the pH reached approximately 11, and a conductivity curve was obtained. The sodium hydroxide titration amount was determined from this conductivity curve, and the anionic group content of the cellulose fiber to be measured was calculated using the following formula:
Anionic group content (mmol/g) = [titration amount of aqueous sodium hydroxide solution (mL) × concentration of aqueous sodium hydroxide solution (0.05 M)] / [mass of cellulose fiber to be measured (0.5 g)]

[Content of each ingredient]
The content of modified cellulose fiber was calculated from the blended amounts of each component. The content of modified cellulose fiber was calculated assuming that all of the blended shortened anion-modified cellulose fiber and amine were ionically bonded. The glucose units in the modified cellulose fiber were calculated from the content of the modified cellulose fiber and the blended amount of modified cellulose fiber.

[Confirmation of crystalline structure in various cellulose fibers]
The crystal structures of various cellulose fibers, such as cellulose fibers, anion-modified cellulose fibers, and modified cellulose fibers, were confirmed by measurement under the following conditions using a diffractometer (MiniFlex II, manufactured by Rigaku Corporation).
Measurement pellet preparation conditions: A pressure in the range of 10 to 20 MPa was applied to the target cellulose fiber using a tablet press to prepare smooth pellets with an area of 320 mm2 and a thickness of 1 mm.
X-ray diffraction analysis conditions: step angle 0.01°, scan speed 10°/min, measurement range: diffraction angle 2θ = 5 to 40°
X-ray source: Cu/Kα-radiation, tube voltage: 15kv, tube current: 30mA
Peak division conditions: After removing background noise, the peaks were fitted with a Gaussian function so that the error between 2θ=13° and 23° was within 5%.
The degree of crystallinity of the cellulose type I crystal structure was calculated based on the following formula (A) using the area of the X-ray diffraction peak obtained by the above-mentioned peak division.
Cellulose type I crystallinity (%) = [I _cr / (I _cr + I _am )] × 100 (A)
(In the formula, I _cr represents the area of the diffraction peak of the lattice plane (002 plane) (diffraction angle 2θ=22-23°) in X-ray diffraction, and I _am represents the area of the diffraction peak of the amorphous portion (diffraction angle 2θ=18.5°).)

[Anion-modified cellulose fiber]
Anion-modified cellulose fibers having the physical properties shown in Table 1 were used as raw materials.

Such anionically modified cellulose fibers can be prepared, for example, by the TEMPO oxidation treatment described below.

[TEMPO oxidation treatment]
10 g of bleached coniferous kraft pulp fiber (as raw natural cellulose fiber) and 990 g of deionized water were weighed into a 2 L PP beaker equipped with a mechanical stirrer and stirring blades and stirred at 25 ° C and 100 rpm for 30 minutes. Next, 0.13 g of TEMPO, 1.3 g of sodium bromide, and 35.5 g of a 10.5 wt% aqueous sodium hypochlorite solution were added to the 10 g of pulp fiber in this order. Next, pH stat titration was performed using an automatic titrator, and 0.5 M aqueous sodium hydroxide solution was added dropwise to maintain the pH at 10.5. The reaction was carried out at 25 ° C for 120 minutes at a stirring speed of 100 rpm. Next, 0.01 M hydrochloric acid was added to the suspension while stirring to adjust the pH to 2. The solids were then filtered off by suction filtration. The solids are dispersed in deionized water and filtered off by suction. This procedure is repeated until the conductivity of the filtrate reaches 200 μs/cm or less. The resulting solids are then dehydrated to obtain anion-modified cellulose fibers.

[Preparation of aqueous suspension of shortened anion-modified cellulose fibers]
Anion-modified cellulose fibers having the physical properties shown in Table 1 were subjected to alkaline hydrolysis treatment to prepare aqueous suspensions of shortened anion-modified cellulose fibers (solid concentration: 5% by mass) having the physical properties shown in Table 2.

[Preparation of Resin Composition]
(Examples and Comparative Examples)
Shortened anion-modified cellulose fibers were added to 1-methoxy-2-propanol (PGME) to obtain a dispersion with a solids concentration of 2.0% by mass. The amines listed in Table 3 were added to the resulting dispersion and stirred at 25°C for 1 hour to obtain dispersions of two types of modified cellulose fibers. An epoxy resin was further added to the resulting modified cellulose fiber dispersion, and after stirring at 25°C for 1 hour, the mixture was dispersed five times at 150 MPa using a high-pressure homogenizer (NanoVita L-ES, manufactured by Yoshida Kikai Kogyo Co., Ltd.). PGME was then removed from the modified cellulose fiber dispersion using an evaporator.

Next, the curing agent and curing accelerator were added, and the mixture was stirred for 3 minutes at 2000 rpm at 25°C using an automatic revolving mixer (Thinky Corporation, Awatori Rentaro), and then degassed for 2 minutes at 2200 rpm at 25°C, to obtain the composition shown in Table 4.

(Reference example)
The resin, curing agent, and curing accelerator were added, and the mixture was stirred at 2000 rpm at 25°C for 3 minutes using an automatic revolving mixer (Thinky Corporation, Awatori Rentaro), and then degassed at 2200 rpm at 25°C for 2 minutes to obtain the composition shown in Table 4.

[Creating test specimens]
Two SPCC steel plates (25 mm wide x 200 mm long x 0.5 mm thick) were bent into an L-shape to create a 150 mm x 25 mm flat surface. A resin composition was then applied to the 150 mm x 25 mm flat surface of one of the L-shaped SPCC steel plates, and the 150 mm x 25 mm flat surface of the other L-shaped SPCC steel plate was bonded to form a T-shape, sandwiching a spacer with a diameter of 150 μm. The resulting specimen was then left to stand at 130°C for 2 hours, yielding a test piece in which the SPCC steel plates were bonded together via a 150 mm x 25 mm x 0.15 mm adhesive layer.

[Evaluation of Peel Strength]
The evaluation was carried out in accordance with JIS K6854-3. The test piece was left standing overnight in an environment of 23°C and 50% RH, and then subjected to evaluation.
A bench-top precision universal testing machine (AGS-X, manufactured by Shimadzu Corporation) was used, and the test was carried out at a load cell of 1000 N, a chuck distance of 50 mm, room temperature (23° C.), and a tensile speed of 100 mm/min.
The peel strength value was calculated by deriving an approximate straight line from the obtained test force/stroke diagram, excluding the initial peak, from the part where the test force was almost constant, and substituting half the value of the fracture stroke. The results are shown in Table 4. In the table, CF means cohesive failure, and AF means interfacial failure.

From Table 4, it can be seen that the peel strength was improved in the Examples compared to the Comparative Examples and Reference Examples.
Comparing the Comparative Examples with the Reference Examples, it was found that the use of Modified Cellulose Fiber 2 actually reduced the peel strength and changed the failure mode from cohesive failure to interfacial failure. In contrast, comparing the Examples with the Comparative Examples, it was found that the use of Modified Cellulose Fiber 1 not only improved the peel strength but also maintained the cohesive failure mode.
These results indicate that differences in the structure of modified cellulose fibers, i.e., whether or not the modified cellulose fibers have unsaturated hydrocarbon groups, contribute to the peel strength and fracture state.

[reagent]
The following reagents and raw materials were used without further purification.
Resin: Epoxy resin (manufactured by Mitsubishi Chemical Corporation, jER828, epoxy equivalent = 184 to 194, weight average molecular weight = 370)
Curing agent: Dicyandiamide (DICY, manufactured by Mitsubishi Chemical Corporation) was crushed using a mini blender (manufactured by Osaka Chemical Co., Ltd.) and used.
Curing accelerator: 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) (manufactured by Thermo Scientific Co., Ltd.) was ground using a mini blender (manufactured by Osaka Chemical Co., Ltd.) and used.
PGME: 1-methoxy-2-propanol (manufactured by Daicel Corporation)
EO/PO amine: methoxypoly(oxyethylene/oxypropylene)-2-propylamine (JEFFAMINE M2070, Mw=2000, EO:PO=4:1, manufactured by HUNTSMAN)
Diallylamine (Fujifilm Wako Pure Chemical Industries, Ltd.)

The peel strength improver of the present invention can improve the peel strength of adhesives and can therefore be used as an additive to a variety of adhesives.

Claims (6)

A peel strength improver containing modified cellulose fibers having a chain unsaturated hydrocarbon group and an alkoxy polyoxyalkylene group as modifying groups ,
the chain unsaturated hydrocarbon group is at least one selected from the group consisting of an allyl group, a propenyl group, a butenyl group, an isobutenyl group, an isoprenyl group, a pentenyl group, a hexenyl group, a heptenyl group, an octenyl group, a nonenyl group, a decenyl group, a dodecenyl group, a tridecenyl group, a tetradecenyl group, and an octadecenyl group;
Peel strength improver . The peel strength improver according to claim 1 , further comprising an epoxy resin. The peel strength improver according to claim 1 or 2 , wherein the modifying group of the modified cellulose fiber is bonded to glucose constituting the cellulose via an ionic bond and/or an amide bond. The peel strength improver according to claim 2 or 3 , wherein the ratio of the modified cellulose fiber to the resin ("modified cellulose fiber" / "resin") is 0.01/100 or more and 30/100 or less in mass ratio. The peel strength improver according to any one of claims 2 to 4 , wherein the content of the modified cellulose fiber is 0.1% by mass or more and 10% by mass or less. A method for improving peel strength, comprising using the peel strength improver according to any one of claims 1 to 5 .