JP7636195B2 - Dithioesterified cellulose nanofiber - Google Patents

Description

This invention relates to modified xanthated cellulose nanofibers.

It has long been known that cellulose has a reinforcing effect on resins and rubbers (Patent Document 1). However, because typical finely powdered cellulose fibers are in the form of cut particles, it is difficult to expect a high reinforcing effect that utilizes the network of fine cellulose fibers.

A new material called nanocellulose, which is made by breaking down plant-derived cellulose material to the nano level, is attracting attention. Of these, materials in which the fibers in cellulose have been finely disintegrated to a fiber diameter of about 3 to 100 nm have been actively researched and developed because of their large specific surface area and excellent properties as reinforcing fibers. Various names have been proposed for this material, but in this application, this material is called cellulose nanofiber. To produce this cellulose nanofiber, it is necessary to finely defibrate cellulose materials such as pulp, but even if you try to defibrate the cellulose material itself, it requires a large amount of energy because the fibers are bound together by strong hydrogen bonds. For this reason, a technology has been proposed to make it easier to obtain cellulose nanofibers by chemically denaturing the cellulose material to make it easier to defibrate it.

Examples of cellulose nanofibers obtained by such chemical modification include the following: TEMPO-oxidized cellulose nanofibers, which are defibrated by introducing carboxyl groups into cellulose using 2,2,6,6-tetramethyl-1-piperidine-N-oxy radical (hereinafter referred to as "TEMPO") as a catalyst, xanthated cellulose nanofibers, which are obtained by adding carbon disulfide to alkali-treated cellulose to introduce xanthate groups (-OCSS ^- M ⁺ ), and phosphated cellulose nanofibers, which are obtained by introducing phosphate groups.

It has been reported in Patent Document 2 that xanthate cellulose nanofibers, when introduced into resins or rubbers, provide a high reinforcing effect. In particular, in rubber, it is believed that in addition to the reinforcing effect of the nanofibers, the sulfur contained in the xanthate group also exerts a vulcanization-accelerating effect.

JP 2005-75856 A International Publication WO2018/235912A1

However, xanthate-modified cellulose nanofibers have poor thermal stability, and some of the xanthate groups tend to detach even at room temperature, making them difficult to store. In addition, although cellulose itself is hydrophilic, the introduction of xanthate groups makes xanthate-modified cellulose nanofibers more hydrophilic, making them difficult to mix with hydrophobic resins and rubbers when kneaded together, resulting in an uneven dispersion and preventing the reinforcing effect of the cellulose nanofibers from being fully realized.

Therefore, the purpose of this invention is to obtain nanofibers with improved thermal stability and dispersibility in resins and rubbers in order to take advantage of the useful properties of xanthated cellulose nanofibers.

This invention solves the above problems by providing a cellulose nanofiber that has a cellulose skeleton in its molecular structure, and is a dithioesterified cellulose nanofiber in which the hydroxyl groups of cellulose are replaced with dithioester groups represented by the following formula (1):

-OCSSR (1)
(R is a hydrocarbon group having 1 to 12 carbon atoms)

Specifically, an embodiment can employ a dithioesterified cellulose nanofiber having a degree of substitution, which is the average number of hydroxyl groups substituted with the dithioester group per glucose unit constituting the cellulose skeleton, of 0.1 to 0.4 and an average fiber diameter of 3 nm to 250 nm.

The dithioesterified cellulose nanofibers of this invention can be produced by adding a halogenated hydrocarbon to an aqueous suspension of xanthated cellulose nanofibers and reacting it with the xanthate groups.

A resin composition containing the dithioesterified cellulose nanofibers of this invention in a resin or rubber is useful as a resin composition with improved strength.

The dithioesterified cellulose nanofibers obtained by this invention have improved thermal stability compared to xanthated cellulose nanofibers, and can be usefully used even when they are stored after production and then added to resins or rubbers after a period of time. In addition, compared to xanthate groups whose terminals are -OCSS ^- M ⁺ ions, dithioester groups which are -OCSSR groups have high hydrophobicity, and therefore have high affinity when added to resins or rubbers and are easy to knead, so that they can be uniformly dispersed in the resin composition and improve quality. Furthermore, their stability in acidic solutions is also improved compared to xanthated cellulose nanofibers, and they are widely used as additives for resins, rubbers, etc.

Graph comparing stress-strain curves of Experimental Examples 1-1 and 1-2 and Comparative Experimental Examples 1-1 and 1-2 Graph comparing stress-strain curves of Experimental Examples 1-1 and 1-3 and Comparative Experimental Examples 1-1 and 1-2 A graph comparing stress-strain curves of Experimental Examples 1-1, 1-4, 1-5, and Comparative Experimental Example 1-1 Graph comparing stress-strain curves of Experimental Example 1-6 and Experimental Comparative Examples 1-3 and 1-4 A graph comparing stress-strain curves of Experimental Examples 1-1 and 1-7 and Comparative Experimental Examples 1-1, 1-5, and 1-6. Graph comparing stress-strain curves of Experimental Examples 2-1 and 2-2 and Comparative Experimental Examples 2-1 and 2-2 Graph comparing stress-strain curves of Experimental Example 3-1 and Comparative Experimental Examples 3-1 and 3-2

The present invention is described in detail below. The present invention relates to a dithioesterified cellulose nanofiber having dithioester groups, a method for producing the same, and a resin composition containing the dithioesterified cellulose nanofiber.

The dithioesterified cellulose nanofibers of this invention are cellulose nanofibers that have a cellulose skeleton in which glucose units are linked together as the basic structure. If the basic structure is a cellulose skeleton, not all of the constituent units are glucose, and some of the end groups may be modified, or some of the structure may be changed.

The dithioesterified cellulose nanofiber according to the present invention is a modified compound having a dithioester group in which a hydroxyl group is substituted for one of the hydroxyl groups of the glucose unit of the cellulose skeleton. Cellulose has three hydroxyl groups per glucose unit, and it is preferable that an average of 0.1 or more hydroxyl groups per glucose unit are substituted with a dithioester group. Hereinafter, the average number of hydroxyl groups substituted with a dithioester group per glucose unit is referred to as the degree of substitution. If the degree of substitution is less than 0.1, the hydrophobicity improving effect and vulcanization accelerating effect of the dithioester group are insufficient, and the advantages compared to unsubstituted cellulose nanofibers are not sufficient. Basically, the higher the degree of substitution, the better from the viewpoint of reinforcing effect, but in terms of production, it is not preferable for the degree of substitution to exceed 0.4. If the degree of substitution exceeds 0.4, there is a risk that the xanthate cellulose nanofiber, which is the raw material for the dithioesterified cellulose nanofiber, will dissolve in water as a dispersion medium during the defibration process when producing the xanthate cellulose nanofiber.

The dithioester group has the structure of the following formula (1). R is a hydrocarbon group having 1 to 12 carbon atoms. Specific examples of R include linear alkyl groups such as butyl and octyl groups, aromatic alkyl groups such as benzyl groups, and alkenyl groups such as vinyl and allyl groups. R may be a saturated or unsaturated hydrocarbon group. R is not limited to linear groups. From the viewpoint of ease of dissolving in water, it is preferable that R has 8 or less carbon atoms, and from the viewpoint of ease of reaction, it is more preferable that R has 4 or less carbon atoms.

-OCSSR (1)

The dithioesterified cellulose nanofibers of the present invention preferably have an average fiber diameter of 3 nm or more, and more preferably 5 nm or more. Since the fiber diameter of microfibrils, the structural unit of natural cellulose, is 2 to 3 nm in higher plants, a large amount of energy is required for defibration to make it less than 3 nm, which may increase the production cost. On the other hand, a diameter of 250 nm or less is preferable, and a diameter of 200 nm or less is more preferable. If the nanofibers are too thick due to insufficient defibration, it is difficult to form a network structure of nanofibers when they are included in a resin composition, and the reinforcing effect may not be fully exerted.

The dithioesterified cellulose nanofibers of the present invention can be produced, for example, by adding a halogenated hydrocarbon to an aqueous suspension of xanthated cellulose nanofibers and reacting with the xanthate group. The dispersion medium of this aqueous suspension may be water alone, or a liquid in which a polar solvent other than water is mixed with water to facilitate dissolving the halogenated hydrocarbon during the reaction may be used. Examples of such polar solvents include dimethyl sulfoxide (DMSO) and N,N-dimethylformamide (DMF). In addition, an emulsifying aid may be added to uniformly disperse the halogenated hydrocarbon in the aqueous dispersion medium. This is because halogenated hydrocarbons are hydrophobic and difficult to disperse in an aqueous suspension as is.

The aqueous suspension of xanthated cellulose nanofibers may be the dispersion liquid obtained after defibrating the xanthated cellulose, or may be the one obtained by dispersing the xanthated cellulose nanofibers once dried in a dispersion medium again. The xanthated cellulose used for defibration is usually one having Na ⁺ as a cation, but may be one having another cation by ion exchange.

In the above halogenated hydrocarbons, Cl, Br, and I can be suitably used as the halogen. The hydrocarbon group of the above halogenated hydrocarbons is a functional group corresponding to R in the above dithioester group. That is, examples of such groups include linear alkyl groups such as butyl and octyl groups, aromatic alkyl groups such as benzyl groups, and alkenyl groups such as vinyl and allyl groups.

However, as long as dithioesterified cellulose nanofibers having dithioester groups can be obtained, the production method is not limited to the method using the above-mentioned esterification reaction.

The dithioesterified cellulose nanofibers according to the present invention can be incorporated into resins or rubbers to form resin compositions, and can effectively improve the strength of the resin compositions. In particular, when dithioesterified cellulose nanofibers are incorporated into resin compositions that use rubbers such as natural rubber and synthetic rubber as the resin, not only can the nanofibers improve the strength of the nanofibers, but the sulfur contained in the dithioester groups can also promote vulcanization. There are no particular limitations on the method of adding the resin composition. For example, it is advisable to add dithioesterified cellulose nanofibers during kneading of the resin composition, knead the composition, and then perform operations such as curing and vulcanization.

The above resin composition may contain additives other than the dithioesterified cellulose nanofiber, such as vulcanization accelerators, vulcanizing agents, antioxidants, fillers, waxes, reinforcing agents, softeners, bulking agents, colorants, flame retardants, lubricants, plasticizers, and other additives, depending on the application. Examples of fillers include carbon black, silica, and calcium carbonate. Examples of vulcanizing agents for rubber include sulfur, which is added separately.

The following describes specific examples of the present invention.
First, as a preliminary step to obtain dithioesterified cellulose nanofibers, the manufacturing procedure for obtaining the raw material xanthated cellulose nanofibers and the method for measuring the physical properties of the xanthated cellulose nanofibers will be described. Note that cellulose nanofibers are sometimes abbreviated as "CNF". The following materials were used.
Kraft pulp (manufactured by Nippon Paper Industries Co., Ltd.: NBKP, α-cellulose content: 90% by mass, average degree of polymerization of α-cellulose: 1000) hereinafter referred to as "NBKP".

(Example 1: Dithioesterified CNF Allyl)
<Alkaline treatment: Preparation of dithioesterified CNF 1>
NBKP was weighed so that the pulp solid content (pulp solid content is the content excluding water in pulp; the same applies below) was 1 kg. 25 kg of an 8.5% by mass aqueous sodium hydroxide solution was added to this NBKP, and the mixture was stirred at room temperature for 3 hours to perform an alkali treatment. The pulp after this alkali treatment was subjected to solid-liquid separation using a centrifugal dehydrator (Kokusan Co., Ltd., H-130C, filter cloth 400 mesh) to obtain a dehydrated alkali cellulose. The sodium hydroxide content in this dehydrated alkali cellulose was 7.5% by mass, and the pulp solid content was 27.4% by mass.

<Xanthate treatment: Preparation of dithioesterified CNF 2>
The dehydrated alkali cellulose prepared above was weighed to have a pulp solid content of 300 g. 105 g of carbon disulfide (35% by mass relative to the pulp solid content) was added to the dehydrated alkali cellulose, and the sulfurization reaction was carried out at room temperature for 4.5 hours to perform a xanthate treatment, thereby obtaining xanthated cellulose.

<Xanthate Substitution Degree Measurement>
The degree of xanthate substitution of the xanthated cellulose was measured by the Breedee method and found to be 0.312. The degree of xanthate substitution is the average number of hydroxyl groups substituted with xanthate groups per glucose unit of cellulose. The measurement of the degree of xanthate substitution by the Breedee method was carried out as follows. 1.5 g of xanthated cellulose was weighed as a solid content, and 40 mL of saturated ammonium chloride aqueous solution cooled to 5 ° C. was added. The xanthated cellulose sample was crushed with a glass rod and mixed well, and then left for 15 minutes, filtered with GFP filter paper (GS-25 manufactured by ADVANTEC Co., Ltd.), and thoroughly washed with saturated ammonium chloride aqueous solution. 50 mL of 0.5 mol / L sodium hydroxide aqueous solution cooled to 5 ° C. was added to the sample remaining on the GFP filter paper, and the GFP filter paper was stirred. The stirred liquid was left for 15 minutes and then neutralized with 1.5 mol / L acetic acid aqueous solution. Phenolphthalein indicator was used as the indicator. After neutralization, 250 mL of distilled water was added and thoroughly stirred, and 10 mL of 1.5 mol/L acetic acid aqueous solution and 10 mL of 0.05 mol/L iodine aqueous solution were added. This solution was titrated with 0.05 mol/L sodium thiosulfate aqueous solution. A 1% by mass starch aqueous solution was used as the indicator. The degree of xanthate substitution was calculated from the following formula (1) based on the titration amount of the sodium thiosulfate aqueous solution and the cellulose content in the sample. The cellulose content in the xanthated cellulose sample was determined by adding water to the xanthated cellulose to disperse it, adding hydrochloric acid to perform a regeneration treatment, filtering the cellulose after the regeneration treatment, thoroughly washing it, and then drying it to measure the mass of only the cellulose.

Xanthate substitution degree = (0.05 x 10 x 2 - 0.05 x sodium thiosulfate titration (mL)) ÷ 1000 ÷ (cellulose content in sample (g) / 162.1) ... (1)

<Defibrillation treatment: Part 3 of preparation of dithioesterified CNF>
The xanthated cellulose produced by the above xanthation treatment was weighed to a solid content of 300 g, and distilled water was added to a solid content concentration of 5% by mass, and the mixture was stirred and dispersed with a stirrer. The dispersed dispersion was centrifuged using the centrifugal dehydrator, and distilled water was added to thoroughly wash the dispersion to remove impurities, alkali, unreacted carbon disulfide, etc. All the xanthated cellulose after washing was collected, and distilled water was added to obtain a 60 kg aqueous suspension having a cellulose content concentration (hereinafter referred to as "cellulose concentration") of 0.5% by mass. This aqueous suspension was defibrated by passing it five times at a flow rate of 2.5 L/min and a pressure of 40 MPa using a high-pressure homogenizer (manufactured by Sanwa Engineering Co., Ltd., H20 type) to obtain xanthated CNF. The degree of xanthate substitution of the obtained xanthated CNF was 0.294.

<Degree of defibration of xanthate-conjugated CNF>
Distilled water was added to the aqueous suspension of the xanthated CNF (cellulose concentration 0.5% by mass) to dilute the cellulose concentration to 0.1% by mass. This diluted aqueous suspension was centrifuged at 12,000 G for 10 minutes using a centrifuge (Avanti J-25I, manufactured by Beckman Coulter) to settle the undefibrated material. The supernatant was separated, distilled water was added to the settled undefibrated material, and centrifugation was performed again to wash the undefibrated material. The undefibrated material was transferred to a crucible and completely dried, and the mass of the undefibrated material was measured. The production rate of nanofibers produced was calculated from the mass of the undefibrated material and the cellulose content in the xanthated cellulose using the following formula (2), which was 99.0% by mass.

Nanofiber production rate (mass%) = (Cellulose content in xanthate cellulose - mass of undefibrated material) ÷ (Cellulose content in xanthate cellulose) x 100... (2)

<Measurement of fiber diameter of xanthate-conjugated CNF>
A water suspension of xanthated CNF diluted with water to a cellulose concentration of 0.1% by mass was placed in a centrifuge tube, and centrifuged at 12,000 G for 10 minutes using the centrifuge to recover the centrifugal supernatant. This centrifugal supernatant was further diluted with distilled water, applied to a support film, stained with uranyl acetate, and dried on the support film to obtain a dried specimen. The dried specimen was observed at an accelerating voltage of 120 kV using a transmission electron microscope (TEM: JEM-1400, manufactured by JEOL Ltd.). 50 nanofibers were selected from the 50,000x image thus obtained, and the fiber diameter was measured. The fiber diameter was in the range of 3.0 nm to 7.4 nm, and the average fiber diameter, which is the average of these measured values, was 6.1 nm.

<Measurement of sulfur content of xanthate-converted CNF>
The aqueous suspension of xanthated CNF (cellulose content 0.5% by mass) obtained by the same procedure as the above defibration process was freeze-dried to obtain a freeze-dried product of xanthated CNF. The sulfur content of this freeze-dried product was measured using a chlorine/sulfur analyzer (TOX-2100H, manufactured by Mitsubishi Chemical Analytech Co., Ltd.). The sulfur content in the xanthated CNF was 8.7% by mass.

<Dithioesterification treatment and redispersion treatment: Part 4 of preparation of dithioesterified CNF>
Distilled water was added to 5 kg of an aqueous suspension of xanthate-containing CNF (cellulose content 0.5% by mass) obtained by the same process as the above defibration process, and the concentration was diluted to 0.25% by mass, and neutralized with 1.5 mol / L of acetic acid. After neutralization, the mixture was stirred in a water bath at 0 ° C. for 1 hour, and when the liquid temperature reached 5 ° C. or less, 55 g of allyl bromide (manufactured by Tokyo Chemical Industry Co., Ltd.) was added, and the mixture was stirred for 12 hours while maintaining the liquid temperature at 5 ° C. or less to perform a dithioesterification process, thereby producing dithioesterified CNF having an allyl group (hereinafter, sometimes referred to as "allyl dithioesterified CNF").

The aqueous suspension of xanthate-treated CNF (the dithioesterified CNF according to the present invention) after dithioesterification was thoroughly washed by adding ethanol and distilled water while centrifuging using the centrifugal dehydrator. All the washed dithioesterified CNF was collected, and distilled water was added to prepare 10 kg of aqueous suspension with a dithioesterified CNF content of 1% by mass. This aqueous suspension was redispersed by passing it twice using a high-pressure homogenizer (H20 type, manufactured by Sanwa Engineering Co., Ltd.) at a flow rate of 2.5 L/min and a pressure of 40 MPa. After the redispersion process, a micrograph of the redispersed dithioesterified CNF was taken to measure the fiber diameter, which was in the range of 3.2 nm to 7.4 nm and the average fiber diameter was 6.1 nm. The sulfur content was also measured and found to be 8.6% by mass. As mentioned above, the sulfur content of the xanthate-modified CNF before modification was 8.7% by mass, so it was confirmed that the dithioesterification treatment modified the xanthate groups but did not result in much elimination, and the degree of substitution was maintained.

(Example 2: Dithioesterified CNF·allyl with varying degrees of substitution)
In order to obtain allyl dithioesterified CNF with different degrees of substitution, in the "xanthation treatment: dithioesterified CNF preparation 2" of the precursor xanthated CNF production process, the dehydrated alkali cellulose was changed to 100 g of pulp solids, and the amount of carbon disulfide added was changed to 24 g (24% by mass relative to the pulp solids), and the same operation was performed to obtain xanthated CNF with different degrees of substitution. The fiber diameter of the obtained xanthated CNF was 3.2 to 14.5 nm, the average fiber diameter was 10.3 nm, the degree of substitution was 0.204, and the sulfur content was 6.3% by mass.

Using this aqueous suspension of xanthate-modified CNF with different degrees of substitution, the same procedure as in "Dithioesterification and redispersion treatment: Preparation of dithioesterified CNF 4" was carried out to obtain allyldithioesterified CNF with different degrees of substitution. The fiber diameter of the obtained allyldithioesterified CNF was 3.1 to 14.9 nm, the average fiber diameter was 11.1 nm, and the sulfur content was 6.3%.

(Example 3: Ester group change: butyl)
In the dithioesterification treatment of Example 1, allyl bromide was changed to 62.5 g of butyl bromide (manufactured by Tokyo Chemical Industry Co., Ltd.), and allyl dithioester was changed to butyl dithioester. Except for the above, butyl dithioesterified CNF was produced by the same procedure as in Example 1. The fiber diameter of the obtained butyl dithioesterified CNF was 3.1 to 7.2 nm, the average fiber diameter was 6.0 nm, and the sulfur content was 8.3%.

(Example 4: Ester group change: Octyl)
In the dithioesterification treatment of Example 1, allyl bromide was changed to 87.5 g of octyl bromide (manufactured by Tokyo Chemical Industry Co., Ltd.), and allyl dithioester was changed to octyl dithioester. Except for the above, octyl dithioesterified CNF was produced by the same procedure as in Example 1. The fiber diameter of the obtained octyl dithioesterified CNF was 3.2 to 7.7 nm, the average fiber diameter was 6.2 nm, and the sulfur content was 8.1%.

<Evaluation of physical properties based on difference in degree of substitution>
(Comparative Example 1: Increase in Substitution Degree)
Among the steps of producing xanthated CNF, which is the precursor of Example 1, in <Xanthation treatment: Preparation of dithioesterified CNF 2>, the dehydrated alkali cellulose was changed to 100 g of pulp solids, and the amount of carbon disulfide added was changed to 70 g (70% by mass relative to the pulp solids), and the same operation was performed to obtain xanthated cellulose with different degrees of xanthate substitution. The degree of xanthate substitution of the obtained xanthated cellulose was 0.412. When the same operation as in <Defibrillation treatment: Preparation of dithioesterified CNF 3> was performed using 100 g of this xanthated cellulose, the amount of xanthated CNF recovered after washing was 19.2 g, and most of it was dissolved during washing. For this reason, it was found that if dithioesterified CNF is produced using xanthated CNF with a high degree of xanthate substitution, the production itself is possible, but the production efficiency is significantly deteriorated.

<Confirmation of thermal stability of dithioesterified CNF>
Distilled water was added to the allyl dithioesterified CNF aqueous suspension (dithioesterified CNF content 1% by mass, sulfur content 8.6% by mass) prepared in Example 1 to dilute the dithioesterified CNF content to 0.5% by mass. 100 g of this diluted aqueous suspension was taken out and placed in a plastic container and stored in a box-type dryer at 30 ° C. The sulfur content was measured before the start of storage (0 hours), and after 18 hours, 24 hours, 48 hours, 72 hours, and 144 hours from the start of storage. In addition, the same operation was performed on a xanthated CNF aqueous suspension (CNF concentration 1% by mass, sulfur content 8.7% by mass) that was not subjected to dithioesterification treatment for comparison. The results are shown in Table 1.

(Discussion)
In the case of xanthate-treated CNF that had not been subjected to dithioesterification treatment, the sulfur content tended to decrease over time, but in the case of allyl dithioesterified CNF, the sulfur content remained almost constant. From this, it was found that by subjecting xanthate-treated CNF to dithioesterification treatment, the -CSS- group was less likely to be eliminated even when heated, improving the thermal stability.

<Confirmation of stability of dithioesterified CNF in acid solution>
Distilled water was added to the allyl dithioesterified CNF aqueous suspension (dithioesterified CNF content 1% by mass, sulfur content 8.6% by mass) prepared in Example 1 to dilute the dithioesterified CNF content to 0.5% by mass. Formic acid was added to this diluted aqueous suspension to adjust the pH of the aqueous suspension to 4, and the suspension was allowed to stand at room temperature. The sulfur content was measured in the same manner as in the case of the xanthated CNF described above immediately after the start of standing (0 hours), and after 1 hour, 2 hours, 4 hours, and 6 hours from the start of standing. In addition, the same operation was performed on a xanthated CNF aqueous suspension (CNF concentration 1% by mass, sulfur content 8.7% by mass) that was not subjected to dithioesterification treatment for comparison. The results are shown in Table 2.

(Discussion)
In the case of xanthate-modified CNF that was not subjected to dithioesterification treatment, the sulfur content tended to decrease with time, but in the case of allyl dithioesterified CNF (Example 1), the sulfur content remained almost constant over time. From this, it was found that by subjecting xanthate-modified CNF to dithioesterification treatment, the -CSS- group is less likely to be eliminated even when it comes into contact with formic acid, and stability is improved even when the pH is low.

Below is an experimental example in which the dithioesterified CNF prepared in the examples was added to a resin.
<1. Confirmation of the effect of increasing the strength of natural rubber sheets by mixing with natural rubber latex>
(Comparison by degree of substitution)
(Experimental Example 1-1)
Preparation of Masterbatch by Heat Coagulation The allyldithioesterified CNF aqueous suspension (dithioesterified CNF content 1% by mass) prepared in Example 1, natural rubber (hereinafter abbreviated as "NR") latex (HA NR LATEX manufactured by Resitex Co., Ltd., solid content 60% by mass, ammonia 0.7% by mass), and 14% by mass ammonia water were mixed. The content of dithioesterified CNF was 5 parts by mass per 100 parts by mass of NR solid content. The mixture of these was stirred and mixed at 8000 rpm for 5 minutes using a rotary homogenizer (manufactured by Nippon Seiki Seisakusho Co., Ltd., AM-7). The slurry after stirring and mixing was left to dry at 70 ° C for 2 days, and then dried under reduced pressure to prepare an NR masterbatch.

- Preparation of compound sheet and vulcanized rubber sheet 300 g of the NR master batch prepared above was masticated using a two-roll (φ200 mm × L500 mm mixing roll machine manufactured by Nippon Roll Manufacturing Co., Ltd.) heated to 50 ° C. Next, 1.5 g of stearic acid (manufactured by Nacalai Tesque Co., Ltd.), 18 g of zinc oxide (manufactured by Nacalai Tesque Co., Ltd.), 10.5 g of sulfur (manufactured by Nacalai Tesque Co., Ltd.), and 2.1 g of vulcanization accelerator (Suncerer NS-G manufactured by Sanshin Chemical Industry Co., Ltd.) were added and mixed to prepare a compound sheet having a thickness of 2 mm or more.

The 90% vulcanization time (Tc90) was measured using the obtained compound sheet with a vulcanization judging device (Olintec Co., Ltd.: Curastometer V type, measurement temperature 150°C, measurement time 20 minutes). At this time, Tc90 was 6.6 minutes.

The resulting compound sheet was placed in a mold and compression molded at 150°C for 6.6 minutes based on the measured Tc90 value to produce a vulcanized rubber sheet with a thickness of 2 mm.

Measurement of strength properties Test pieces were punched out from the obtained vulcanized rubber sheet into a JIS No. 3 dumbbell shape (n=5), and the thickness of each test piece was measured using a thickness measuring machine dial gauge 0.001 mm manufactured by Ozaki Seisakusho Co., Ltd. (n=3). The test pieces were subjected to a tensile test (gripping width: 50 mm, speed: 500 mm/min JIS K6251 compliant: corresponding to ISO37) using a tensile tester (Shimadzu Corporation Precision Universal Tester AG-1000D) to determine the tensile stress and elongation. The results are shown in Table 3. Note that phr indicates the added mass parts (addition amount) of CNF relative to 100 mass parts of rubber, M100 indicates the tensile stress at 100% elongation, M300 indicates the tensile stress at 300% elongation, and M500 indicates the tensile stress at 500% elongation.

(Experimental Example 1-2)
Using the allyldithioesterified CNF with different degrees of substitution prepared in Example 2, the "Preparation of master batch by heat solidification", "Preparation of compound sheet and vulcanized rubber sheet", and "Measurement of strength properties" of Experimental Example 1-1 were carried out in the same manner. The results are shown in Table 3.

(Experimental Comparative Example 1-1)
A vulcanized rubber sheet was prepared in the same manner as in "Preparation of a master batch by heat solidification" in Experimental Example 1-1, except that dithioesterified CNF was not added, and strength properties were measured. The results are shown in Table 3. Tc90 was 8.0 minutes.

(Experimental Comparative Example 1-2)
In the "Preparation of Masterbatch by Heat Coagulation" of Experimental Example 1-1, a rubber sheet was prepared in the same manner as above, except that xanthated CNF was used instead of dithioesterified CNF, and strength properties were measured. The results are shown in Table 3. Tc90 was 5.4 minutes.

(Discussion)
For Experimental Examples 1-1 and 1-2 and Experimental Comparative Examples 1-1 and 1-2, a graph showing a stress-strain curve with strain (=elongation) on the horizontal axis and stress on the vertical axis is shown in FIG. 1. The stress at the same strain was improved by the addition of either xanthated CNF or dithioesterified CNF, but the dithioesterified CNF showed a higher value than the xanthated CNF, and it was confirmed that the strength improvement effect of the dithioesterified CNF was higher. In addition, in a comparison between dithioesterified CNFs with different degrees of substitution, it was shown that Experimental Example 1-1, which has a high degree of substitution of about 0.3, has a higher strength improvement effect than Experimental Example 1-2, which has a degree of substitution of about 0.2. Furthermore, it was confirmed that the Tc90 of the dithioesterified CNF (Experimental Example 1-1) was not changed much compared to the xanthated CNF (Experimental Comparative Example 1-2), and the vulcanization acceleration effect was maintained.

(Evaluation of physical properties regarding differences in added amounts)
(Experimental Example 1-3: Increased Addition Amount)
In the "Preparation of master batch by heat solidification" stage of Experimental Example 1-1, the amount of dithioesterified CNF was increased so that the content of dithioesterified CNF of Example 1 was 5 to 10 parts by mass (10 phr) per 100 parts by mass of NR solid content. A vulcanized rubber sheet was prepared in the same manner as in Experimental Example 1-1 except for the above. Tc90 was 7.3 minutes.

The above strength property measurements were also performed on Experimental Example 1-3 to determine the stress at break and elongation (= strain). The results were compared between Experimental Example 1-1 (allyl dithioesterified CNF added amount 5 phr) and Experimental Example 1-3 (allyl dithioesterified CNF added amount 10 phr), and are shown in Table 4 and Figure 2 along with Experimental Comparative Example 1-1 (no addition) and Experimental Comparative Example 1-2 (xanthate-treated CNF) for reference. Note that M30 indicates the tensile stress at 30% elongation, M100 indicates the tensile stress at 100% elongation, and M300 indicates the tensile stress at 300% elongation. When the amount of dithioesterified CNF added was changed, it was shown that Experimental Example 1-3, which added a larger amount, generally had a greater strength improvement effect than Experimental Example 1-1, which added a smaller amount.

(Evaluation of modification of ester group)
(Experimental Example 1-4: Ester group change: butyl)
In the "Preparation of a master batch by heat solidification" of Experimental Example 1-1, a rubber sheet was prepared in the same manner as in the previous example, except that the butyl dithioesterified CNF prepared in Example 3 was used instead of the allyl dithioesterified CNF, and strength properties were measured. Tc90 was 7.0 minutes.

(Experimental Example 1-5: Ester group change: Octyl)
In the "Preparation of a master batch by heat solidification" of Experimental Example 1-1, a rubber sheet was prepared in the same manner as in the previous example, except that the octyldithioesterified CNF prepared in Example 4 was used instead of the allyldithioesterified CNF, and strength properties were measured. Tc90 was 7.1 minutes.

The above strength property measurements were also performed on Experimental Examples 1-4 and 1-5 to determine the stress at break and elongation. The results are shown in Table 5 and Figure 3, comparing Experimental Example 1-1, in which the ester group is an allyl group, Experimental Example 1-4, in which the ester group is a butyl group, Experimental Example 1-5, in which the ester group is an octyl group, and Comparative Experimental Example 1-1, in which no CNF was added. Note that M30 indicates the tensile stress at an elongation of 30%, M100 indicates the tensile stress at an elongation of 100%, and M300 indicates the tensile stress at an elongation of 300%.

(Discussion)
All of the substituents showed values superior to those of Comparative Experiment 1-1 in which no substituent was added, and almost no difference was observed in the change in stress due to the substituent.

(Evaluation of changes in coagulation method)
(Experimental Example 1-6: Preparation of master batch by formic acid coagulation)
Using the allyl dithioesterified CNF prepared in Example 1, the dithioesterified CNF, NR latex, and 14% by mass ammonia water were mixed as shown in the configuration of "Preparation of master batch by heat coagulation" in Experimental Example 1-1. While stirring the mixed slurry, a 2% by mass formic acid aqueous solution was dropped until the pH became 4. Furthermore, the precipitated rubber particles were collected by vacuum filtration and washed until neutral. Then, the mixture was dried at 70 ° C for 2 days, and then dried under reduced pressure to prepare an NR master batch. The sulfur content in the NR master batch was measured by the same procedure as in Example 1 <Measurement of sulfur content of xanthate-treated CNF>, and the result was 306.6 ppm. The obtained NR master batch was used to prepare a rubber sheet by the same procedure as in Experimental Example 1-1, and strength properties were measured. Tc90 was 7.6 minutes.

(Experimental Comparative Examples 1-3 and 1-4)
In Experimental Example 1-6, except that dithioesterified CNF was not added, the NR master batch (Experimental Comparative Example 1-3) was prepared by the same procedure as in Experimental Example 1-6, and except that xanthated CNF was used instead of dithioesterified CNF, the NR master batch (Experimental Comparative Example 1-4) was prepared by the same procedure as in Experimental Example 1-6, and a rubber sheet was prepared by the same procedure and strength properties were measured. The sulfur content in the NR master batch of Experimental Comparative Example 1-3 was 45 ppm, Tc90 was 7.8 minutes, and the sulfur content in the NR master batch of Experimental Comparative Example 1-4 was 49 ppm, Tc90 was 8.1 minutes.

Table 6 shows the results of measuring the sulfur content in the NR master batch for Experimental Example 1-6 and Comparative Experimental Examples 1-3 and 1-4. The strength properties were measured as described above, and the results of determining the stress at break and elongation are also shown in Table 6, and a graph is shown in Figure 4. Note that M30 indicates the tensile stress at 30% elongation, M100 indicates the tensile stress at 100% elongation, and M300 indicates the tensile stress at 300% elongation.

(Discussion)
The master batch (Experimental Example 1-6) containing allyl dithioesterified CNF solidified by adding formic acid resulted in a high sulfur content. In comparison, the xanthated CNF solidified by adding formic acid (Experimental Comparative Example 1-4) showed a low sulfur content almost equivalent to that of the non-added case (Experimental Comparative Example 1-3). This is thought to be because the -SCC- group of xanthated CNF is eliminated by formic acid, but the functional group was not eliminated even when dithioesterified CNF was solidified by adding formic acid. In addition, from the stress-strain curve in FIG. 4, it was confirmed that the strength improvement effect was higher for dithioesterified CNF (Experimental Example 1-6) than for xanthated CNF (Experimental Comparative Example 1-4). This is thought to be because the hydrophobicity was maintained high because the functional group was not eliminated by formic acid, and the dispersibility in the rubber improved.

(Consideration of combined use with carbon black)
(Experimental Example 1-7)
In Experimental Example 1-1, when preparing a compound sheet from the NR master batch to which 5 phr of allyldithioesterified CNF was added, 20 phr of carbon black (manufactured by Tokai Carbon Co., Ltd.: SEAT 3: abbreviated as "CB" in the table) was also added, and a vulcanized rubber sheet was obtained in the same manner as in Experimental Example 1-1. Tc90 was 7.9 minutes. The measurement results of the stress at break and elongation are shown in Table 7 and FIG. 5. Note that M30 indicates the tensile stress at 30% elongation, M100 indicates the tensile stress at 100% elongation, and M300 indicates the tensile stress at 300% elongation.

(Experimental Comparative Examples 1-5 and 1-6)
A vulcanized rubber sheet was obtained in the same manner as in Experimental Example 1-7, except that dithioesterified CNF was not added and only 20 phr of carbon black was added when preparing the compound sheet (Experimental Comparative Example 1-5). Similarly, a vulcanized rubber sheet was obtained in the same manner as in Experimental Example 1-7, except that only 40 phr of carbon black was added (Experimental Comparative Example 1-6). The measurement results of the breaking stress and elongation are shown in Table 7 and Figure 5.

(Discussion)
The stress was better in Experimental Example 1-7, which used 5 phr of dithioesterified CNF in combination, than in Experimental Example 1-5, which used only 20 phr of carbon black. In addition, Experimental Example 1-6, which used only 40 phr of carbon black, and Experimental Example 1-7, which used 5 phr of dithioesterified CNF and 20 phr of carbon black in combination, showed almost the same behavior in the stress-strain curve up to about 200% elongation. This shows that dithioesterified CNF exerts a synergistic effect with carbon black when added to a resin. This shows that it is possible to replace a part of carbon black with dithioesterified CNF and reduce the amount of carbon black added.

(Vulcanization acceleration effect)
The vulcanization times of the above experimental examples and comparative examples were measured, and the results are shown in Table 8.

(Discussion)
In the dithioesterified CNF (Experimental Examples 1-1, 1-3, 1-4, 1-5, 1-7) coagulated by heating, the Tc90 value was smaller than that in the case where no formic acid was added (Experimental Comparative Example 1-1). This is thought to be due to the effect of accelerating vulcanization due to the influence of dithioester groups. In addition, in the dithioesterified CNF (Experimental Example 1-6) coagulated by adding formic acid, the Tc90 value was smaller than that in the xanthated CNF (Experimental Comparative Example 1-4) coagulated by adding formic acid and the case where no formic acid was added (Experimental Comparative Example 1-3). This is thought to be due to the fact that although the -SCC- group of xanthated CNF is eliminated by formic acid, the functional group is not eliminated even when dithioesterified CNF is coagulated by adding formic acid, and thus the vulcanization accelerating effect was observed.

<2. Confirmation of the effect of enhancing the strength of TPE film by mixing with thermoplastic elastomer>
(Experimental Example 2-1)
Preparation of CNF composition To the allyl dithioesterified CNF aqueous suspension (dithioesterified CNF content 1 mass%, sulfur content 8.6 mass%) prepared in Example 1, 1,4-butanediol (manufactured by Tokyo Chemical Industry Co., Ltd.) was added as a redispersant after drying in a mass ratio of 10 times the dithioesterified CNF to prepare 30 g of a mixture with a CNF concentration of 0.7 mass% and a polyol concentration of 7.0 mass%. This mixture was dried for 15 hours at 60°C using a hot air dryer to obtain a dithioesterified CNF composition. The obtained dithioesterified CNF composition had a CNF solid content of 9.4 mass%, a water content of 5.8 mass%, and a 1,4-butanediol content of 84.8 mass%.

Preparation of DMF Dispersion of Dithioesterified CNF To the dithioesterified CNF composition obtained above, N,N'-dimethylformamide (DMF) (Kanto Chemical Co., Ltd.) was added so that the solid concentration of dithioesterified CNF was 0.2 mass%, and the mixture was stirred and mixed at 10,000 rpm for 10 minutes using a rotary homogenizer (AM-7, Nippon Seiki Seisakusho Co., Ltd.) to disperse the dithioesterified CNF in DMF. The composition of the DMF dispersion of allyldithioesterified CNF is shown in Table 9.

Mixing with TPE and film formation The DMF dispersion of allyldithioesterified CNF obtained above was mixed with a thermoplastic elastomer (TPE) solution (T-78F manufactured by Dainichiseika Color & Chemicals Mfg. Co., Ltd.) so that the solid content of dithioesterified CNF was 1 part by mass (=1 phr) per 100 parts by mass of TPE, and a stirring and mixing process was carried out for 15 minutes at 8000 rpm using a rotary homogenizer. This dithioesterified CNF mixed TPE solution was degassed for 3 minutes at 3000 rpm using a centrifuge (Avanti J-251 manufactured by Beckman Coulter, Inc.). The degassed solution was cast onto an OPP film (thickness 40 μm) to a thickness of 380 μm using a Baker-type applicator (manufactured by Tester Sangyo Co., Ltd.), and dried at 120°C for 5 minutes using a hot air dryer to produce a TPE film (250 mm x 200 mm) containing allyldithioesterified CNF.

Measurement of thickness of TPE film The obtained TPE film was measured at six random locations (120 locations per 1 m x 1 m) according to JIS Z1702 using a thickness measuring instrument dial gauge 0.001 mm manufactured by Ozaki Seisakusho Co., Ltd., and the average of the measured values was taken as the thickness. The results are shown in Table 10.

Measurement of tensile strength: Strip-shaped test pieces measuring 20 mm wide x 100 mm long were punched out from the obtained TPE film, and a tensile test (n=3) was performed using an autograph AG-500NX manufactured by Shimadzu Corporation under the conditions of a gripping width of 20 mm and a tensile speed of 300 mm/min, and the tensile stress and elongation (=strain) were obtained from the results. The results are shown in Table 10 and Figure 6. Note that phr indicates the added mass part (addition amount) of CNF relative to 100 parts by mass of TPE, M100 indicates the tensile stress at 100% elongation, and M300 indicates the tensile stress at 300% elongation.

(Evaluation of physical properties regarding differences in added amounts)
(Experimental Example 2-2: Increased Addition Amount)
In the preparation stage of the DMF dispersion of allyl dithioesterified CNF in Experimental Example 2-1, the solid content concentration of dithioesterified CNF was increased to 1 mass%, and the dithioesterified CNF was mixed with the DMF dispersion of dithioesterified CNF so that the solid content of dithioesterified CNF was 5 mass parts (=5 phr) per 100 mass parts of TPE when mixed with the TPE solution. A TPE film containing dithioesterified CNF was prepared by the same procedure as in Experimental Example 2-1. The composition of the DMF dispersion of allyl dithioesterified CNF is shown in Table 9, and the strength measurement results are shown in Table 10 and FIG. 6.

(Experimental Comparative Example 2-1: Dithioesterified CNF not added)
In the procedure of "mixing with TPE and film formation" in Experimental Example 2-1, a film was produced using only the TPE solution without adding the DMF dispersion of dithioesterified CNF. The film thickness and tensile strength of the produced film were measured in the same manner as in Experimental Example 2-1. The results are shown in Table 10 and Figure 6.

(Experimental Comparative Example 2-2: Xanthate-treated CNF added)
In the same manner as in the method of Experimental Example 2-1, 1,4-butanediol was added as a redispersant after drying to the xanthated CNF aqueous suspension (sulfur content 8.7% by mass) prepared in Example 1 so that the amount was 10 times that of the xanthated CNF, and 30 g of a mixture with a CNF concentration of 0.7% by mass and a polyol concentration of 7.0% by mass was prepared. This mixture was dried for 15 hours at 60 ° C. using a hot air dryer to obtain a CNF composition. The CNF solid content of the obtained composition was 9.7% by mass, the water content was 6.8% by mass, and the 1,4-butanediol content was 83.5% by mass. DMF was added to this CNF composition and a redispersion process was performed, but many aggregates were confirmed in the DMF after the redispersion process. This DMF dispersion was mixed with a TPE solution to form a film, and the results of measuring the strength of the film are shown in Table 10 and FIG. 6.

The TPE film containing xanthated CNF (Experimental Comparative Example 2-2) has lower thermal stability than allyldithioesterified CNF (Experimental Examples 2-1 and 2-2), and it is believed that the xanthate groups were eliminated during drying when the CNF composition was prepared, causing the CNF to aggregate. The aggregated CNF was not dispersed by redispersion processing in DMF, and existed as aggregates when mixed with the TPE solution and formed into a film, which is believed to have resulted in less film elongation in Experimental Comparative Example 2-2 compared to Experimental Examples 2-1 and 2-2.

<3. Confirmation of the strength enhancement effect by mixing with polypropylene>
(Experimental Example 3-1)
Preparation of CNF composition To the allyl dithioesterified CNF aqueous suspension (dithioesterified CNF content 1% by mass, sulfur content 8.6% by mass) prepared in Example 1, dipropylene glycol (manufactured by Nacalai Tesque) was added as a redispersing agent after drying in an amount 5 times the amount of dithioesterified CNF, and 30 g of a mixture with a CNF concentration of 0.7% by mass and a polyol concentration of 3.5% by mass was prepared. This mixture was dried at 60 ° C. for 15 hours using a hot air dryer to obtain a dithioesterified CNF composition. The CNF solid content of the obtained dithioesterified CNF composition was 15.2% by mass, the water content was 5.8% by mass, and the dipropylene glycol content was 79.0% by mass.

-Kneading treatment into PP resin and preparation of test pieces 1.0 g of the dithioesterified CNF composition obtained above was mixed with 15 g of polypropylene resin (Wintec WFX4M manufactured by Japan Polypropylene Co., Ltd.) so that the solid content of allyl dithioesterified CNF was 1 part by mass (= 1 phr) per 100 parts by mass of PP, and kneaded at 180 ° C. for 5 minutes using a small tabletop kneader (DSM Xplore MC15M manufactured by Xplore Instruments) to obtain a kneaded resin. Then, a dumbbell test piece (JIS K 7162-1BA type) was prepared using an injection molding machine for preparing test pieces (DSM Xplore IM12M manufactured by Xplore Instruments).

Measurement of film thickness of dumbbell test piece The film thickness of the obtained dumbbell test piece was measured at three points on the neck part using a film thickness measuring instrument dial gauge 0.001 mm manufactured by Ozaki Seisakusho Co., Ltd., and the average value was calculated. The results are shown in Table 11.

Measurement of tensile strength Using an autograph AG-500NX manufactured by Shimadzu Corporation, a tensile test (n=3) was carried out under the conditions of a grip width of 55 mm and a tensile speed of 100 mm/min, and the tensile yield stress and the breaking strain ("strain" in the figure = elongation) were obtained from the results. The results are shown in Table 11 and Figure 7. Note that phr indicates the parts by mass (amount added) of CNF added to 100 parts by mass of PP.

(Experimental Comparative Example 3-1: Dithioesterified CNF not added)
In the method of "Kneading into PP resin and preparation of test pieces" in Experimental Example 3-1, a dumbbell test piece was prepared using only PP without adding the dithioesterified CNF composition. The film thickness and tensile strength of the prepared test piece were measured in the same manner as in Experimental Example 3-1. The results are shown in Table 11 and Figure 7.

(Experimental Comparative Example 3-2: Xanthated CNF)
In the same manner as in the method of Experimental Example 3-1, dipropylene glycol was added as a redispersant after drying to the xanthated CNF aqueous suspension (sulfur content 8.7% by mass) prepared in Example 1 so that the amount was 5 times that of the xanthated CNF, and 30 g of a mixture with a CNF concentration of 0.7% by mass and a polyol concentration of 3.5% by mass was prepared. This mixture was dried at 60 ° C. for 15 hours using a hot air dryer to obtain a CNF composition. The CNF solid content of the obtained composition was 16.1% by mass, the water content was 6.2% by mass, and the dipropylene glycol content was 77.7% by mass. This CNF composition was added to PP resin and kneaded, but many aggregates were confirmed in the dumbbell test piece. The results of measuring the strength of this dumbbell test piece are shown in Table 11 and FIG. 7.

(Discussion)
The xanthate-treated CNF (Experimental Comparative Example 3-2) has lower thermal stability than the dithioesterified CNF (Experimental Example 3-1), and it is believed that the xanthate group was eliminated during drying when preparing the CNF composition, causing the CNF to aggregate. The aggregated CNF was not dispersed by kneading with the PP resin, and existed as aggregates in the PP resin, which is believed to have reduced the stress and elongation.

Claims (4)

A cellulose nanofiber is a nanofiber having a cellulose skeleton, and has a dithioester group represented by the following formula (1) in place of any of the hydroxyl groups of cellulose.
-OCSSR (1)
(R is a hydrocarbon group having 1 to 12 carbon atoms) The dithioesterified cellulose nanofiber according to claim 1, in which the degree of substitution, which is the average number of hydroxyl groups substituted with the dithioester group per glucose unit constituting the cellulose skeleton, is 0.1 to 0.4, and the average fiber diameter is 3 nm to 250 nm. A method for producing dithioesterified cellulose nanofibers by adding a halogenated hydrocarbon to an aqueous suspension of xanthate-modified cellulose nanofibers to react with the xanthate groups. A resin composition containing the dithioesterified cellulose nanofiber according to claim 1 or 2.

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