JP7470894B2 - Rubber foam, its manufacturing method and uses - Google Patents

Description

The present invention relates to a foamed article made of a vulcanizate of a rubber composition which is lighter in weight and has high hardness, and to a method for producing the same.

Chloroprene rubber has a good balance of physical properties among various synthetic rubbers and is therefore used in a wide range of applications, such as belts, hoses, air springs, and adhesives.
It is also widely used as a foam, for example as a cushioning material, soundproofing material, heat insulating material, door packing, etc.

One of the major features of foams is their light weight. The specific gravity of vulcanized rubber without foaming is approximately 1.0 to 1.3, but vulcanized rubber foam can have a specific gravity of less than 1.0.
The specific gravity can be adjusted to any desired value by increasing the amount of foaming material added or by changing the vulcanization conditions. However, weight reduction by increasing the amount of foaming material results in a higher proportion of air bubbles, while changing the vulcanization conditions ends the vulcanization in an insufficient state, so both methods impair the strength of the vulcanized rubber foam and limit weight reduction. Other weight reduction methods include reducing the amount of reinforcing agents (carbon black, silica, etc.) and inorganic fillers (talc, clay, etc.) that have a higher specific gravity than the rubber composition. However, reducing the amount of reinforcing agents reduces the strength of the rubber composition, and reducing the amount of inorganic fillers increases the compounding cost, making it difficult to achieve significant weight reduction.

In response to this, a method has been proposed in which a supercritical fluid is introduced into unvulcanized rubber to foam it and reduce its weight without reducing its strength (Patent Document 1). However, the supercritical method is difficult to commercialize,
The cost is very high and the practical use is poor. In addition, a method for reducing the weight by micro-dispersing polyolefin resin in a molten state has been proposed (Patent Documents 2 and 3). However,
Because it contains polyolefin resin, it has poor setting properties and is unsuitable for use as a foam.

Meanwhile, cellulose nanofibers, which are made by pulverizing wood pulp, have been developed as a new material, and industrial applications are being explored in various fields. One of these applications is compounding with rubber. For example, a rubber composition in which cellulose nanofibers have been modified to enhance durability has been disclosed (Patent Document 4:
). A tire with low energy loss made by modifying cellulose nanofibers has also been proposed (Patent Document 5). A rubber composition has also been proposed in which a silane coupling agent is blended with cellulose nanofiber to improve compatibility with polymers (Patent Document 6). A vulcanized rubber foam for shoe soles containing cellulose nanofibers has also been proposed (Patent Document 7).

JP 2002-322308 A JP 2011-16978 A JP 2008-111137 A JP 2017-554109 A JP 2015-056788 A JP 2009-191198 A JP 2015-072833 A

However, the above Patent Documents 4 to 6 do not target highly expanded, low specific gravity vulcanized rubber foams, and do not address the compatibility of lightweight and high strength. Patent Document 7 targets vulcanized rubber foams, but the rubber components are natural rubber, SBR, EPDM, and EVA for shoe sole applications,
Not suitable for chloroprene rubber.

The present invention has been made in view of the above problems, and an object of the present invention is to provide a foamed article made of a chloroprene rubber composition which is lightweight and has high hardness.

The present inventors conducted intensive research to solve the above problems against such a background, and found that a foam made of a vulcanizate of a rubber composition containing chloroprene rubber and cellulose nanofibers can be used to achieve both light weight and high hardness.
[1] A foam made of a vulcanizate of a rubber composition containing 1 to 7 parts by weight of cellulose nanofibers having an average fiber diameter of 10 to 300 nm, an average fiber length of 0.5 to 200 μm, a lignin content of 20% by weight or less, and a hydroxymethyl group of cellulose not modified with a carboxylic acid or a carboxylate, per 100 parts by weight of chloroprene rubber.
[2] The foam according to the above [1], characterized in that the chloroprene rubber contains 3 to 7 parts by weight of a carboxylic acid or an alkali metal salt of a carboxylic acid per 100 parts by weight of the chloroprene rubber.
[3] The relationship between the hardness (Hs) described in JIS-K-6253 and the density (ρ) described in JIS-K-6268 is 100ρ (1.2 - ρ) in the density range of 0.2 to 0.6.
) + 14 ≦ Hs.
[4] A method for producing the foam according to any one of the above [1] to [3], comprising the following steps 1 to 3:

Step 1: A chloroprene latex having a pH of 10 to 14 is mixed with a fiber having an average diameter of 10 to 300 nm.
m, the average fiber length is 0.5 to 200 μm, and the lignin content is 20% by weight or less;
A process for producing a rubber composition by mixing an aqueous dispersion of cellulose nanofibers in which the hydroxymethyl groups of the cellulose have not been modified with a carboxylic acid or carboxylate to produce a cellulose nanofiber-dispersed rubber latex mixture, removing water from the cellulose nanofiber-dispersed rubber latex mixture to precipitate a rubber composition, and then drying with hot air. Step 2: A process for producing a compound by blending and kneading 2 to 15 parts by weight of a foaming agent per 100 parts by weight of the rubber composition produced in step 1. Step 3: A process for heat-molding the compound produced in step 2 at 120 to 180°C for 5 to 90 minutes to vulcanize and foam. [5] A method for producing a foam as described in [4] above, characterized in that in step 1, water is removed by freeze coagulation.
[6] The viscosity of the cellulose nanofiber-dispersed rubber latex mixture in step 1 is 1,500
The method for producing a foam according to the above [5], characterized in that the viscosity of the foam is 0.01 mPa·s or less.
[7] The method for producing a foam according to any one of [4] to [6] above, wherein the solid content of the cellulose nanofiber-dispersed rubber latex mixture in step 1 is 20% by weight or more.
[8] A sponge rubber made of the foam according to any one of [1] to [3] above.

The present invention will be described in detail below.

The foam of the present invention has an average fiber diameter of 10 to 300 mm based on 100 parts by weight of chloroprene rubber.
The cellulose nanofibers have a fiber length of 0.5 to 200 μm, a lignin content of 20% by weight or less, and a vulcanizate of a rubber composition containing 1 to 7 parts by weight of cellulose nanofibers in which the hydroxymethyl groups of the cellulose are not modified with a carboxylic acid or a carboxylate.

Chloroprene rubber can be obtained by emulsion polymerization of chloroprene or chloroprene and a monomer copolymerizable therewith.

Examples of monomers that can be copolymerized with chloroprene include sulfur, 2,3-dichloro-1,
3-butadiene, 2-cyano-1,3-butadiene, 1-chloro-1,3-butadiene,
Examples of the monomer include 1,3-butadiene, styrene, acrylonitrile, methyl methacrylate, methacrylic acid, acrylic acid, etc., and one or more of these can be used in combination. However, for example, copolymerization of sulfur, which serves as a crosslinking point, can improve mechanical properties such as modulus, but heat resistance decreases, so this is not always necessary and is used appropriately depending on the required properties. The amount of copolymerizable monomer is not particularly limited, but 30 parts by weight or less is generally used per 100 parts by weight of chloroprene rubber so as not to impair the properties of the chloroprene polymer. In particular, sulfur decreases heat resistance, so 3 parts by weight or less is preferable, and 1 part by weight or less is more preferable, per 100 parts by weight of chloroprene monomer.

The chloroprene rubber preferably contains 3 to 7% by weight of carboxylic acid or an alkali metal salt of a carboxylic acid. When the content is 3% by weight or more, excellent emulsion stability is achieved during chloroprene polymerization, and when the content is 7% by weight or less, freezing defects do not occur in the rubber precipitation step by freezing, enabling stable production of rubber products.

In the emulsion polymerization of chloroprene rubber, for example, the above-mentioned monomers are mixed with an emulsifier, water, a polymerization initiator,
For example, a chain transfer agent and other stabilizers are mixed, polymerization is carried out at a predetermined temperature, and a polymerization terminator is added at a predetermined polymerization conversion rate to terminate the polymerization.

Examples of the emulsifier include alkali metal salts of carboxylic acids and alkali metal salts of sulfonic acids, such as alkali metal salts of rosin acid, alkali metal salts of alkylbenzenesulfonic acids, alkali metal salts of fatty acids, alkali metal salts of alkenylsuccinic acids, alkali metal salts of polycarboxylic acids, nonionic emulsifiers such as polyoxyethylene alkyl ethers, and water-soluble polymer compounds. Examples of the alkali metal salts include lithium, sodium, potassium, and cesium. These may be used alone or in combination of two or more, but from the viewpoints of polymerization stability, cohesiveness during drying, and rubber performance, it is preferable to use an alkali metal salt of a carboxylic acid, and it is particularly preferable to use an alkali metal salt of rosin acid, and more preferably a potassium salt of rosin acid.

The amount of the emulsifier is not particularly limited, but considering the stability of the chloroprene latex obtained after polymerization, it is preferably 3 to 10 parts by weight per 100 parts by weight of the chloroprene rubber. Among them, the amount of the alkali metal salt of the carboxylic acid is preferably 3 to 8 parts by weight, and more preferably 5 to 7 parts by weight.
It is preferable to include parts by weight.

In order to adjust the pH of the polymerization solution, it is preferable to adjust the pH to 11 or more using a pH adjuster. If the pH is lower than this, the alkali metal salt of the carboxylic acid becomes acidic, and the stability of the latex decreases. Examples of the pH adjuster include basic compounds such as sodium hydroxide, potassium hydroxide, sodium phosphate, potassium phosphate, triethylamine, diethylamine, triethanolamine, diethanolamine, ethanolamine, and ammonia.
More than one type may be used alone or in combination.

Emulsion polymerization initiators include known free radical substances such as potassium persulfate,
Peroxides such as ammonium persulfate, hydrogen peroxide, inorganic or organic peroxides such as tertiary butyl hydroperoxide, etc. can be used. These may be used alone or in combination with reducing substances such as thiosulfates, thiosulfites, hydrosulfites, organic amines, etc. in a redox system.

The polymerization temperature is not particularly limited, but is preferably in the range of 10 to 50°C.

The time when the polymerization is to be completed is not particularly limited. From the viewpoint of productivity, however, it is preferable that the conversion rate of the monomer is 60% or more.
It is preferable to carry out the polymerization to 95% or more. If it is less than 60%, the production amount is small and the solid content of the latex is low. On the other hand, if it is more than 95%, the polymerization time becomes very long.

The polymerization terminator is not particularly limited as long as it is a commonly used terminator, and examples thereof include phenothiazine, 2,6-t-butyl-4-methylphenol, and hydroxylamine.

In the case of sulfur-modified chloroprene copolymerized with sulfur, it is possible to add a deflocculating agent and a deflocculating aid to the polymerization-stopped latex and continue deflocculation until an appropriate Mooney viscosity is obtained.
The peptizing agent includes thiuram compounds such as tetraethylthiuram disulfide, tetrabutylthiuram disulfide, tetraoctylthiuram disulfide, etc., which can be added in a state emulsified using the above-mentioned emulsifiers, etc. The peptizing reaction initiator includes thiocarbamic acid compounds such as sodium dibutyldithiocarbamate, ammonium dimethyldithiocarbamate, etc.

Deflocculation can be carried out at a temperature in the range of 5 to 60° C., preferably in the range of 20 to 50° C. In general, the higher the deflocculation temperature, the faster the deflocculation reaction.

The Mooney viscosity at which the peptization reaction is terminated is not particularly limited as long as it satisfies the high elastic stress of the present invention, but is preferably 20 to 80 in consideration of the ease of kneading.

Cellulose nanofibers are cellulose fibers contained in wood that have been defibrated to an average fiber diameter of several nanometers to several tens of nanometers. Wood and other materials that are the raw materials for cellulose nanofibers contain lignin, but cellulose nanofibers that contain a large amount of lignin impair the stability of chloroprene latex, so the amount of lignin contained is preferably 20% by weight or less. It is preferably 10% by weight or less, and more preferably 5% by weight or less. The defibration process can be performed mainly by mechanical treatment, or by chemical treatment to impart various functional groups and then combined with mechanical treatment to perform the process to a thinner single nanometer level. In the present invention, cellulose nanofibers obtained mainly by mechanical treatment are used, which have an average fiber diameter of 10 to 300 nm, an average fiber length of 0.5 to 200 μm, and in which the hydroxymethyl groups of cellulose are not modified with carboxylic acid or carboxylate. Cellulose nanofibers with an average fiber diameter of less than 10 nm increase the viscosity of the cellulose nanofiber-dispersed rubber latex, causing problems in the rubber precipitation process by freezing. Cellulose nanofibers with an average fiber diameter of more than 300 nm increase the viscosity of the cellulose nanofiber-containing chloroprene rubber composition, causing problems in the molding processability of the foam. The diameter is preferably 10 to 100 nm. On the other hand, cellulose nanofibers with an average fiber length of less than 0.5 μm decrease the hardness of the foam made of the cellulose nanofiber-containing rubber vulcanizate. If the average fiber length exceeds 200 μm, the viscosity of the cellulose nanofiber-containing rubber composition increases, resulting in poor molding processability of the foam. The diameter is preferably 0.5 to 100 μm. In addition, if the cellulose nanofiber contains a carboxylate or carboxylic acid, the dispersion state of the cellulose nanofiber in the rubber deteriorates, resulting in a decrease in the hardness of the foam made of the obtained vulcanized rubber and a decrease in handleability, so that cellulose that is not modified with a carboxylate or carboxylic acid is used.

In the present invention, the content of cellulose nanofiber is 1 to 7 parts by weight based on 100 parts by weight of chloroprene rubber. If the content of cellulose nanofiber is less than 1 part by weight, the hardness of the foam decreases. If the content of cellulose nanofiber exceeds 7 parts by weight, the viscosity of the cellulose nanofiber-dispersed rubber latex mixture increases, causing problems in the rubber precipitation step by freezing, and is not appropriate from the viewpoint of economical superiority. The content is preferably 1 to 5 parts by weight, and more preferably 1 to 3 parts by weight.

The foam of the present invention is made of a vulcanizate of the above rubber composition.

The foam of the present invention is produced by the following steps 1 to 3.

Step 1: A chloroprene latex having a pH of 10 to 14 is mixed with a fiber having an average diameter of 10 to 300 nm.
m, the average fiber length is 0.5 to 200 μm, and the lignin content is 20% by weight or less;
a step of mixing an aqueous dispersion of cellulose nanofibers in which the hydroxymethyl groups of the cellulose have not been modified with a carboxylic acid or a carboxylate to prepare a cellulose nanofiber-dispersed rubber latex mixture, removing water from the cellulose nanofiber-dispersed rubber latex mixture to precipitate a rubber composition, and then drying with hot air to prepare a rubber composition; step 2: a step of compounding 2 to 15 parts by weight of a foaming agent with respect to 100 parts by weight of the rubber composition prepared in step 1, and kneading the mixture; step 3: a step of heat-molding the compound prepared in step 2 at 120 to 180°C for 5 to 90 minutes to vulcanize and foam the compound; and a step of mixing an aqueous dispersion of cellulose nanofibers with chloroprene latex to prepare a cellulose nanofiber-dispersed rubber latex mixture, and removing water from the cellulose nanofiber-dispersed rubber latex mixture.

In step 1, the chloroprene latex is not particularly limited as long as it contains a chloroprene monomer and an unsaturated monomer copolymerizable with the chloroprene monomer and is produced by an emulsion polymerization method, and from the viewpoint of emulsion stability of the chloroprene rubber latex, it is preferable that the pH is 10 to 14.

The aqueous dispersion of cellulose nanofibers is prepared by dispersing in water cellulose nanofibers that have been defibrated to an average fiber diameter of 10 to 300 nm and an average fiber length of 0.5 to 200 μm and have a lignin content of 20% by weight or less.

The method for mixing the chloroprene rubber latex and the aqueous dispersion of cellulose nanofibers is not particularly limited, and the mixture can be obtained by using a propeller-type stirring device, a homomixer, a high-pressure homogenizer, or the like, and mixing the chloroprene latex and the aqueous dispersion of cellulose nanofibers until they appear uniform (no lumps, etc.).

Method for removing water from cellulose nanofiber-dispersed rubber latex mixture (drying method)
Methods for removing rubber include heat drying, coagulation with acids or salts, and freeze drying. However, coagulation leaves emulsifiers, coagulating liquid, and water inside the rubber, so the rubber is precipitated by freezing (freeze coagulation).
The most efficient and easy method is to freeze and solidify the cellulose nanofiber-dispersed rubber latex mixture at a pH of 10 or less, after which the excess emulsifier and the like are washed away and then dried with hot air. Since freezing makes the rubber more likely to precipitate, it is even more preferable to freeze and solidify the cellulose nanofiber-dispersed rubber latex mixture at a pH of 10 or less.

The rubber composition thus obtained is dried by evaporating the water content by hot air drying. Specifically, the frozen and solidified sheet-like rubber is placed on a belt conveyer, and air heated to 100° C. or higher is blown into the sheet to evaporate the water content and obtain a dried rubber composition.

Specifically, the frozen and solidified sheet-like rubber is placed on a belt conveyor, and air heated to 100° C. or higher is pumped into it to evaporate the water and obtain dried rubber.

In the method of freezing and coagulating and drying, the viscosity of the cellulose nanofiber-dispersed rubber latex mixture is preferably 1500 mPa·s or less, more preferably 1000 mPa·s or less and 30 mPa·s or more. The solid content of the cellulose nanofiber-dispersed rubber latex mixture is preferably 20% by weight or more, more preferably 25% by weight or more and 35% by weight or less. When the viscosity is 1500 mPa·s or less, freezing and coagulation are good, which is preferable. When the solid content is 20% by weight or more, the frozen rubber film is less likely to crack and the strength of the rubber film when thawed is increased, which is suitable for continuous production of rubber products and is preferable.

In step 2, 2 to 15 parts by weight of a foaming agent is added to 100 parts by weight of the rubber composition prepared in step 1.
In step 3, the compound prepared in step 2 is heated and molded at 120 to 180° C. for 5 to 90 minutes to be vulcanized and foamed.

In step 2, the rubber composition prepared in step 1 is mixed with various compounding agents in the same manner as with normal chloroprene rubber. In step 2, the foaming agent is not particularly limited, but from the viewpoint of processability and production efficiency, for example, OBSH (4,4'-oxybis(benzenesulfonylhydrazide)) can be used. The number of parts of the foaming agent mixed is, relative to 100 parts by weight of chloroprene rubber,
The amount is preferably from 2 to 15 parts by weight, more preferably from 2 to 10 parts by weight, and most preferably from 2 to 8 parts by weight.

In step 3, the compound containing the foaming agent produced in step 2 is heated in a rubber mold at 120 to 180°C for 5 to 90 minutes, preferably at 130 to 170°C for 10 to 60 minutes, to generate gas from the foaming agent, causing vulcanization and foaming to proceed simultaneously, thereby producing a rubber foam made of a vulcanized product (hereinafter sometimes referred to as vulcanized rubber foam).

From the viewpoint of weight reduction, the foam of the present invention preferably has a density of 0.6 kg/cm ³ or less, more preferably 0.5 kg/cm ³ or less, and particularly preferably 0.45 kg/cm ³ or less.

The vulcanized rubber foam thus obtained is lightweight and has high hardness, and can be used for a variety of applications, with its application to wet suits and cushioning materials being particularly favorable.

In general, as the vulcanized rubber foam becomes more highly foamed and the density (ρ) becomes smaller, the hardness (Hs) also decreases.

For example, the density (ρ) of a vulcanized rubber foam obtained by vulcanizing and foaming a rubber compound containing 10 parts by weight of OBSH as a foaming agent was 0.21 and the hardness (Hs) was 32.
The density of the vulcanized rubber foam obtained by vulcanizing and foaming a rubber compound containing 6.5 parts by weight of SH (
The density (ρ) of the vulcanized rubber foam obtained by vulcanizing and foaming 5 parts by weight of the rubber compound was 0.3 and the hardness (Hs) was 38. The density (ρ) of the vulcanized rubber foam obtained by vulcanizing and foaming 2.1 parts by weight of the rubber compound was 0.5 and the hardness (Hs) was 43.
From these experimental results, the approximation curve of the four points is 100ρ(1.2-ρ)+
11=Hs. On the other hand, vulcanized rubber foam containing cellulose nanofibers exhibits a low density with the same hardness. This is desirable because it allows for the weight reduction of products that require hardness, but if the density of the vulcanized rubber foam is higher than the appropriate range, it will not be lightweight, which is an advantage of the foam, and it is difficult to achieve a low range by foaming. For example, when a chloroprene rubber composition containing 1 part by weight of cellulose per 100 parts by weight of rubber component is mixed with OBSH, a foaming agent,
The density (ρ) of the vulcanized rubber foam obtained by vulcanizing and foaming a rubber compound containing 5 parts by weight of
The vulcanized rubber foam obtained by vulcanizing and foaming a rubber compound in which 5 parts by weight of OBSH as a foaming agent is blended with a chloroprene rubber composition containing 2.5 parts by weight of cellulose per 100 parts by weight of the rubber component has a density (ρ) of 0.35 and a hardness (Hs) of 46, which satisfies 100ρ(1.2-ρ)+14≦Hs.
The vulcanized rubber foam obtained by vulcanizing and foaming a rubber compound in which 5 parts by weight of OBSH as a foaming agent is blended with a chloroprene rubber composition containing 5 parts by weight of cellulose per 100 parts by weight of the rubber component has a density (ρ) of 0.33 and a hardness (Hs) of 4.
9, which satisfies 100ρ(1.2-ρ)+18≦Hs. Therefore, in the range of 0.2 to 0.6, which is useful as a vulcanized rubber foam, it is preferable that 100ρ(1.2-ρ)+14≦Hs, more preferably 100ρ(1.2-ρ)+16≦Hs, and most preferably 100ρ(1.2-ρ)+18≦Hs.

By using a foam made of the cellulose nanofiber-containing chloroprene rubber composition of the present invention, a lightweight vulcanized rubber foam with high hardness can be obtained.

The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples.

<Preparation of Chloroprene Rubber Latex>
As a monomer mixture, 0.3 part by weight of sulfur was added to 100 parts by weight of chloroprene, 4.0 parts by weight of potassium salt of rosin acid, 0.5 part by weight of sodium salt of a condensate of naphthalenesulfonic acid and formaldehyde, 0.05 part by weight of sodium hydroxide, and 1.
The emulsion was mixed and stirred with an emulsifying aqueous solution containing 1.0 parts by weight and 100 parts by weight of water, and a polymerization catalyst consisting of 1.0 parts by weight of potassium persulfate, 0.01 parts by weight of sodium anthraquinone-β-sulfonate, and 30 parts by weight of water was added to the emulsion at a constant rate by a pump to polymerize. The polymerization was carried out by adding the polymerization catalyst until the polymerization conversion rate reached 70%, and the polymerization was terminated by adding a polymerization terminator consisting of 0.01 parts by weight of thiodiphenylamine, 4-t-butylcatechol, 0.05 parts by weight of 2,2'-methylenebis-4-methyl-6-t-butylphenol, 0.1 parts by weight of diethylhydroxylamine, 0.05 parts by weight of sodium lauryl sulfate, 5.0 parts by weight of chloroprene, and 1.0 parts by weight of water. Subsequently, 2 parts by weight of tetraethylthiuram disulfide was added to the emulsion, and the polymerization catalyst was added at a constant rate by a pump to polymerize. The polymerization was carried out by adding the polymerization terminator consisting of 0.01 parts by weight of thiodiphenylamine, 4-t-butylcatechol, 0.05 parts by weight of 2,2'-methylenebis-4-methyl-6-t-butylphenol, 0.1 parts by weight of diethylhydroxylamine, 0.05 parts by weight of sodium lauryl sulfate, 5.0 parts by weight of chloroprene, and 1.0 parts by weight of water.
Parts by weight of a toluene solution emulsified with potassium rosinate and 0.3 parts by weight of sodium dibutyldithiocarbamate were added, and peptization was carried out at 40°C until the Mooney viscosity reached 60. Unreacted chloroprene was removed and recovered by steam stripping under reduced pressure to obtain a chloroprene rubber latex. The pH of the obtained chloroprene rubber latex was 11.
. The answer was 3.

<Preparation of chloroprene rubber composition containing cellulose nanofibers>
A specified amount of the cellulose nanofiber aqueous dispersion was added to the chloroprene rubber latex, and the mixture was mixed in an auto homogenizer (PRIMIX, manufactured by Primix) at 2,000 rpm for 1 min.
Mix for 0 minutes.

Thereafter, the pH was adjusted to 6.5 using 15% by weight diluted acetic acid, and then the rubber composition was precipitated by freeze coagulation, washed with water, and then dried with hot air.

<Viscosity, solids content, pH>
The viscosity was measured using a Vismetron viscometer (Shibaura Semtec Corp.: VD2).

The solid content was calculated by weighing out about 3 g of the liquid, drying it on an aluminum evaporating dish at 170° C. for 15 minutes to remove moisture, and then calculating the solid content from the weights before and after removal.

The pH was measured by a pH meter (manufactured by Horiba, Ltd.) at 23°C.
was measured.

<Concentration of Carboxylic Acid or Alkali Metal Salt of Carboxylic Acid>
The concentration per solid content was calculated from the amount used.

<Preparation of compound>
Chloroprene rubber component 1 in chloroprene rubber composition containing cellulose nanofibers
00 parts by weight of magnesium oxide (Kyowa Mag 150, manufactured by Kyowa Chemical Industry Co., Ltd.)
parts by weight, stearic acid (NOF Corp.) 0.5 parts by weight, zinc oxide (Sakai Chemical Co., Ltd.) 5 parts by weight, SRF carbon black (Tokai Carbon Co., Ltd., Seast S) 30 parts by weight, ethylene thiourea (Sanceler 22-C) 0.35 parts by weight, OBS
H (Cellogen OT manufactured by Uniroyal Chemical Co., Ltd.) (5 parts by weight) was added using an open roll kneader to obtain a chloroprene rubber compound containing cellulose nanofibers.

<Preparation of vulcanized rubber foam>
The obtained cellulose nanofiber-containing chloroprene rubber compound was heated at 150°C for 1
The mixture was press-vulcanized for 5 minutes to prepare a vulcanized foam sheet.

<Measurement of mechanical properties of vulcanizates>
The hardness (Hs) of the obtained vulcanized foam sheet was evaluated by a type C durometer in accordance with JIS-K-6253.Furthermore, the density (ρ) was evaluated under the conditions of method A in accordance with JIS-K-6268.

Example 1
A water dispersion of cellulose nanofibers produced by mechanical fiberization in chloroprene rubber latex (solid content: 36.5%) (manufactured by Mori Machinery, grade: C-100)
The cellulose nanofibers were mixed and stirred for 10 minutes in the above manner to obtain a cellulose nanofiber-dispersed rubber latex mixture. The amount of cellulose nanofiber mixed was
The amount was set so that the cellulose content was 2.5 parts by weight per 100 parts by weight of the solid rubber component. The viscosity and solid content of the obtained cellulose nanofiber-dispersed rubber latex mixture are shown in Table 1. The viscosity of the cellulose nanofiber-dispersed rubber latex mixture was 520 mPa·s,
The solid content was 28.7%, and the freezing and drying steps were carried out without any problems, resulting in a chloroprene rubber composition containing cellulose nanofibers.

The chloroprene rubber composition containing the cellulose nanofibers was used to obtain a chloroprene rubber compound containing cellulose nanofibers and a vulcanized foam sheet according to the above-mentioned method, and the hardness and density were measured. The test results are shown in Table 1. From Table 1, the hardness was 46, the density was 0.3, and the hardness was 1.0.
5 had high hardness and low density.

Example 2
A cellulose nanofiber-containing chloroprene rubber compound and a vulcanized foam sheet were obtained in the same manner as in Example 1, except that the amount of cellulose nanofiber mixed was changed to an amount such that the content of cellulose nanofiber was 5 parts by weight per 100 parts by weight of the solid rubber component, and the hardness and density were measured. The test results are shown in Table 1. As shown in Table 1, the viscosity of the cellulose nanofiber-dispersed rubber latex mixture was 1200 mPa·s, the solid content was 23.8%, and ...1200 mPa·s, and the hardness of the cellulose nanofiber-
There were no problems with the drying process. The hardness was 49 and the density was 0.33 , which means high hardness and low density.

Example 3
A cellulose nanofiber-containing chloroprene rubber compound and a vulcanized foamed sheet were obtained in the same manner as in Example 1, except that the amount of cellulose nanofiber mixed was 1 part by weight per 100 parts by weight of the solid rubber component, and the hardness and density were measured. The test results are shown in Table 1. As shown in Table 1, the viscosity of the cellulose nanofiber-dispersed rubber latex mixture was 110 mPa·s, the solid content was 32.9%, and there were no problems in the freezing and drying processes. The hardness was 45 and the density was 0.37, indicating high hardness and low density.

Comparative Example 1
A cellulose nanofiber-dispersed rubber latex mixture was obtained in the same manner as in Example 1, except that the amount of cellulose nanofiber mixed was set to an amount such that the content of cellulose nanofiber was 10 parts by weight per 100 parts by weight of the solid rubber component. The viscosity of the obtained cellulose nanofiber-dispersed rubber latex mixture was 3,020 mPa s, the solid content was 18.1%, and
There were problems with the smoothness and strength of the film after freezing, and a chloroprene rubber composition containing cellulose nanofibers could not be obtained.

Comparative Example 2
A cellulose nanofiber-containing chloroprene rubber compound and a vulcanized foam sheet were obtained in the same manner as in Example 1, except that the amount of cellulose nanofiber mixed was set to an amount such that the content of cellulose nanofiber was 0.5 parts by weight per 100 parts by weight of the solid rubber component, and the hardness and density were measured. The test results are shown in Table 1. As shown in Table 1, the viscosity of the cellulose nanofiber-dispersed rubber latex mixture was 50 mPa·s, the solid content was 34.6%, and ...
There were no problems with the drying process. The hardness was 44 and the density was 0.38, so it was not possible to achieve both high hardness and low density.

Comparative Example 3
A chloroprene rubber compound and a vulcanized foam sheet were obtained in the same manner as in Example 1, except that the cellulose nanofibers were not mixed, and the hardness and density were measured. The test results are shown in Table 1.
The hardness was 43 and the density was 0.39, so it was not possible to achieve both high hardness and low density.

Comparative Example 4
A chloroprene rubber compound and a vulcanized foam sheet were obtained in the same manner as in Comparative Example 3, except that the amount of OBSH mixed was 2 parts by weight, and the hardness and density were measured. The test results are shown in Table 1.
The hardness was 48 and the density was 0.52, so it was not possible to achieve both high hardness and low density.

Comparative Example 5
A chloroprene rubber compound and a vulcanized foam sheet were obtained in the same manner as in Comparative Example 3, except that the amount of OBSH mixed was 8 parts by weight, and the hardness and density were measured. The test results are shown in Table 1.
The hardness was 33 and the density was 0.23, so it was not possible to achieve both high hardness and low density.

Comparative Example 6
Cellulose nanofibers were treated with lignin-containing lignocellulose nanofibers (manufactured by Mori Machinery, grade: L-45, fiber diameter: 50-300 nm, average fiber length: 45 μm).
A cellulose nanofiber-dispersed rubber latex mixture was prepared in the same manner as in Example 1, except that the solids concentration was 3% by weight and the lignin content was approximately 30% by weight. However, rubber precipitated during the preparation process, and chloroprene rubber containing cellulose nanofibers was not obtained.

Claims (3)

A foam comprising a vulcanizate of a rubber composition containing 1 to 7 parts by weight of cellulose nanofibers, the cellulose nanofibers having an average fiber diameter of 10 to 300 nm, an average fiber length of 0.5 to 200 μm, a lignin content of 20% by weight or less, and a hydroxymethyl group of the cellulose not modified with a carboxylic acid or a carboxylate, per 100 parts by weight of chloroprene rubber; a density as defined in JIS-K-6268 in the range of 0.33 or more and 0.6 or less; and a relationship between hardness (Hs) and density (ρ) as defined in JIS-K-6253 satisfying 100ρ(1.2-ρ)+14≦Hs. 2. The foam according to claim 1, wherein the chloroprene rubber contains 3 to 7 parts by weight of a carboxylic acid or an alkali metal salt of a carboxylic acid per 100 parts by weight of the chloroprene rubber. A sponge rubber made of the foam according to claim 1 or 2.