JP7759985B2 - Rubber foam, insulation for flexible tubes or hoses of industrial robots - Google Patents

Description

This disclosure relates to a rubber foam and a method for manufacturing a rubber foam.

As cylindrical rubber molded bodies, for example, hoses are disclosed in Patent Documents 1 and 2.
Patent Document 1 discloses a hose using the following rubber composition: The rubber composition contains a rubber component and a chlorine-based flame retardant, and contains less than 70 parts by mass of chloroprene rubber per 100 parts by mass of the rubber component, and contains 5 parts by mass or more of the chlorine-based flame retardant per 100 parts by mass of the rubber component.
Patent Document 2 discloses a hose using the following rubber composition: The rubber composition contains chloroprene rubber, butadiene rubber, and styrene-butadiene rubber as rubber components, with 60 to 80 parts by mass of chloroprene rubber per 100 parts by mass of the rubber component, and 5 to 25 parts by mass of silica blended with respect to 100 parts by mass of the rubber component.
Meanwhile, a cylindrical rubber foam is often used to surround a hose and provide thermal insulation. For example, a rubber foam is used as a so-called hose heater around a hose used in an industrial robot or the like.
For example, Patent Document 3 discloses a heat insulating foam made of chloroprene rubber (CR) and polyvinyl chloride (PVC).

Japanese Patent Application Laid-Open No. 2017-2173 JP 2013-129684 A JP 2016-74883 A

However, conventional heat insulating foams do not necessarily have sufficient durability or heat insulating properties, and improvements in these physical properties have been desired.
The present disclosure has been made in view of the above-described circumstances, and has an object to provide a rubber foam having good durability and high thermal insulation properties. The present disclosure can be realized in the following aspects.

[1] A rubber foam containing acrylonitrile-butadiene rubber and polyvinyl chloride,
When the total amount of the acrylonitrile-butadiene rubber and the polyvinyl chloride is 100 parts by mass, the amount of the acrylonitrile-butadiene rubber is 50 parts by mass or more and 90 parts by mass or less,
A rubber foam having a specific gravity of 0.05 or more and 0.15 or less.

The rubber foam disclosed herein has good durability and high thermal insulation properties.

FIG. 1 is a perspective view schematically illustrating an example of a rubber foam. 1 is a photograph showing a rubber foam after an abrasion resistance test.

Here, a preferred example of the present disclosure will be described.
[2] The rubber foam according to [1], which contains at least one selected from the group consisting of halogen-based flame retardants and antimony-based flame retardants.
This configuration improves the flame retardancy of the rubber foam.

[3] The rubber foam according to [1] or [2], which is cylindrical.
The cylindrical shape makes it useful as a hose heater.

[4] A method for producing a rubber foam using a mixture containing acrylonitrile-butadiene rubber, polyvinyl chloride, and azodicarboxylic acid amide particles as a raw material, comprising:
As the azodicarboxylic acid amide particles, two or more types of particles having different median diameters are used,
the median diameters of the two or more types of particles differ from one another by 1 μm or more and 15 μm or less,
the median diameter of any of the two or more types of particles is 5 μm or more and 40 μm or less.
This manufacturing method provides a rubber foam having good durability and high thermal insulation properties.

This disclosure is explained in detail below. Note that when a range is indicated as "x to y," it is assumed that x and y are included in the range unless otherwise specified.

1. Rubber foam 1
The rubber foam 1 contains acrylonitrile-butadiene rubber (NBR) and polyvinyl chloride (PVC). When the total amount of the acrylonitrile-butadiene rubber and polyvinyl chloride is 100 parts by mass, the amount of the acrylonitrile-butadiene rubber is 50 parts by mass or more and 90 parts by mass or less from the viewpoint of maintaining low thermal conductivity of the rubber foam 1 while exhibiting excellent abrasion resistance.

(1) Specific Gravity The specific gravity of the rubber foam 1 is 0.05 or more and 0.15 or less, preferably 0.07 or more and 0.13 or less, and more preferably 0.08 or more and 0.10 or less, from the viewpoint of imparting high durability and high thermal insulation to the rubber foam 1. The specific gravity of the rubber foam 1 refers to a value measured at a temperature of 23°C by the "underwater displacement method" according to Method A of JIS K7112.

(2) Acrylonitrile-Butadiene Rubber The acrylonitrile-butadiene rubber (NBR) is not particularly limited as long as a known acrylonitrile-butadiene rubber is appropriately selected and used. The amount of acrylonitrile (AN content) contained in the acrylonitrile-butadiene rubber (NBR) is not particularly limited. The amount of acrylonitrile (AN content) is, for example, preferably 23% by mass or more and 43% by mass or less, and more preferably 28% by mass or more and 38% by mass or less.
There are no particular limitations on the Mooney viscosity ML(1+4) of acrylonitrile-butadiene rubber (NBR) at 100°C measured in accordance with JIS K6300. From the viewpoints of reducing the specific gravity and maintaining the shape of the sponge, this Mooney viscosity ML(1+4) is preferably 30 or more and 70 or less, and more preferably 40 or more and 60 or less.

(3) Flame Retardant The rubber foam 1 preferably contains a flame retardant. Any flame retardant can be used as the flame retardant. The type of flame retardant is not particularly limited, but from the viewpoint of achieving a high flame retardant effect with a small blending amount without reducing the properties of the foam, such as the tensile modulus of elasticity, it is preferable to use a combination of a halogen-based flame retardant and an antimony-based flame retardant.

Examples of halogen-based flame retardants include chlorine compounds and bromine compounds, and two or more of these may be used in combination.
Suitable examples of the chlorine compound include chlorinated paraffin and perchloropentacyclodecane.
Suitable examples of the bromine compound include tetrabromobisphenol A, ethylenebistetrabromophthalimide, pentabromodiphenyl ether, octabromodiphenyl ether, decabromodiphenyl ether, hexabromocyclododecane, and brominated polystyrene.

Examples of antimony-based flame retardants include antimony trioxide, antimony pentoxide, antimony trichloride, and antimony pentachloride.

When a halogen-based flame retardant and an antimony-based flame retardant are used in combination, the content (addition amount) of the halogen-based flame retardant per 100 parts by mass of the rubber component (100 parts by mass of the total of the acrylonitrile-butadiene rubber and polyvinyl chloride) is preferably 30 parts by mass or more and 80 parts by mass or less, and more preferably 45 parts by mass or more and 65 parts by mass or less, from the viewpoint of imparting sufficient flame retardancy while maintaining high durability.
The content (addition amount) of the antimony-based flame retardant is preferably 5 parts by mass or more and 30 parts by mass or less, and more preferably 10 parts by mass or more and 20 parts by mass or less, per 100 parts by mass of the rubber component (100 parts by mass of the total of the acrylonitrile-butadiene rubber and polyvinyl chloride), from the viewpoint of imparting sufficient flame retardancy while maintaining high durability.

(4) Filler The rubber foam 1 may contain a filler. Any filler can be used as the filler. Examples of the filler include inorganic fillers such as calcium carbonate, talc, magnesium hydroxide, mica, clay, aluminum hydroxide, magnesium hydroxide, barium sulfate, silicic acid, titanium oxide, bentonite, carbon black, mica, glass fiber, and wood flour. From the viewpoints of processability, dimensional stability of the product, and strength of the product, the amount of the filler is preferably 40 parts by mass or more and 140 parts by mass or less per 100 parts by mass of the rubber component.

(5) Other Components Examples of other components include softeners, crosslinking agents, and foaming agents.
The softener may be any of vegetable oils such as lauric acid, ricinoleic acid, baltimic acid, cottonseed oil, soybean oil, castor oil, palm oil, etc., or mineral oils such as paraffinic, naphthenic, aromatic, etc. The amount of softener is preferably 30 to 110 parts by weight per 100 parts by weight of the rubber component from the viewpoints of kneadability, product strength, etc.
The total amount of the filler and softener added is preferably 70 to 250 parts by weight per 100 parts by weight of the rubber component. The compounding ratio of the filler to the softener is preferably 65:35 to 50:40. By setting the total amount and ratio of the filler and softener to be added within the above ranges, compatibility is improved and moldability, such as rubber kneading, is excellent.

Sulfur and organic peroxides are used as crosslinking agents. Sulfur is used in sulfur crosslinking, while organic peroxides are used in peroxide crosslinking. Examples of organic peroxides include dicumyl peroxide, di-t-butyl peroxide, 2,5-dimethyl-2,5-di-(t-butylperoxy)hexane, and t-butylcumyl peroxide. The amount of crosslinking agent is preferably 0.1 to 10 parts by mass per 100 parts by mass of the rubber component. It is preferable to use a crosslinking accelerator or crosslinking aid in combination with the crosslinking agent.

Examples of the crosslinking accelerator include benzothiazyl disulfide, 2-mertocaptobenzothiazole, dipentamethylene thiuraum tetrasulfide, tetramethylthiuraum disulfide, and zinc oxide.
Examples of the crosslinking aid include divinylbenzene, ethylene dimethacrylate, trimethylolpropane trimethacrylate, ethylene glycol dimethacrylate, triallyl cyanurate, and triallyl isocyanurate.

The foaming agent may be one or more selected from organic foaming agents and inorganic foaming agents.
The organic foaming agent is not particularly limited, but azo compounds are preferred examples thereof, including, but not limited to, one or more of azodicarboxylic acid amide (ADCA), barium azodicarboxylate, azobisisobutyronitrile (AIBN), azocyclohexylnitrile, and azodiaminobenzene.
The inorganic foaming agent is not particularly limited, and examples of the inorganic foaming agent include hydrogen carbonates such as sodium hydrogen carbonate (baking soda) and ammonium hydrogen carbonate, carbonates such as sodium carbonate and ammonium carbonate, nitrites such as sodium nitrite and ammonium nitrite, borohydrides such as sodium borohydride, azides, and other known inorganic foaming agents.
The amount of the foaming agent is preferably 5 parts by mass or more and 100 parts by mass or less per 100 parts by mass of the rubber component.
It is also preferable to use a foaming assistant together with the foaming agent, and examples of the foaming assistant include urea compounds, organic compounds, and inorganic salts.
Other ingredients may also be added, such as processing aids, colorants, antioxidants, etc. Examples of processing aids include stearic acid and palmitic acid.

(6) Shape of Rubber Foam 1 There are no particular limitations on the shape of the rubber foam 1. The shape of the rubber foam 1 can be changed appropriately depending on the application. For example, a cylindrical shape can be suitably adopted as the shape of the rubber foam 1 (see FIG. 1 ). The cylindrical rubber foam 1 can be used as a heat insulating tube. Note that the heat insulating tube is used for insulating flexible tubes and flexible hoses for industrial robots, etc.

(7) Effects of the Rubber Foam 1 of the Embodiment The rubber foam 1 has good durability because it contains acrylonitrile-butadiene rubber and polyvinyl chloride in a specified ratio. The rubber foam 1 has a more slippery surface than EPDM sponge, and therefore has excellent abrasion resistance.
The rubber foam 1 has high thermal insulation properties because it contains acrylonitrile-butadiene rubber, which has low thermal conductivity, and polyvinyl chloride, which also has low thermal conductivity. The thermal conductivities of the polymers alone are 0.25 W/m K for acrylonitrile-butadiene rubber, 0.13 W/m K for polyvinyl chloride, and 0.36 W/m K for EPDM.
Moreover, the rubber foam 1 of this embodiment has a specific gravity of 0.05 or more and 0.15 or less, and has particularly low thermal conductivity.

2. Method for Manufacturing Rubber Foam 1 (1) Method for Manufacturing Rubber Foam 1 In the method for manufacturing rubber foam 1, a mixture containing acrylonitrile-butadiene rubber, polyvinyl chloride, and azodicarboxylic acid amide particles is used as a raw material. Two or more types of azodicarboxylic acid amide particles are used. The median diameters of the two or more types of particles differ from each other by 1 μm or more and 15 μm or less. The median diameter of each of the two or more types of particles is 5 μm or more and 40 μm or less.
The reason why it is believed that the use of two or more types of azodicarboxylic acid amide particles having different median diameters results in a rubber foam 1 having good durability and high heat insulating properties will be explained below.
In this manufacturing method, it is believed that vulcanization first proceeds moderately while the particles having a small median diameter expand. During this process, the foam retains gas. Then, when the expansion of the particles having a small median diameter is nearly completed, the particles having a large median diameter expand, which is believed to result in a decrease in the specific gravity of rubber foam 1.

When, among two or more types of particles having different median diameters, the particles having the smallest median diameter are designated as first particles and the particles having the largest median diameter are designated as second particles, the mass ratio of the first particles to the second particles is preferably 95:5 to 20:80, more preferably 92:8 to 25:75, and even more preferably 60:40 to 25:75, from the viewpoint of slowing down the vulcanization of the rubber during foaming, thereby improving the balance between foaming and vulcanization, suppressing gas escape, and reducing the specific gravity of the rubber foam.
When there are two types of particles with different median diameters, the particles with the smaller median diameter are the first particles, and the particles with the larger median diameter are the second particles.

From the perspective of achieving a good balance between foaming and vulcanization, suppressing gas leakage, and lowering the specific gravity of the rubber foam, the total amount of azodicarboxylic acid amide particles is preferably 5 parts by mass or more and 100 parts by mass or less, and more preferably 10 parts by mass or more and 50 parts by mass or less, per 100 parts by mass of the rubber component (100 parts by mass of the total of acrylonitrile-butadiene rubber and polyvinyl chloride).

Here is an example of a suitable method for producing a rubber foam. First, the rubber components, compatibilizer, filler, etc. are primarily mixed using a Banbury mixer, kneader, etc. (excluding crosslinking agents, crosslinking accelerators, crosslinking aids, foaming agents, and foaming aids). The primary mixing conditions are preferably a temperature of 50°C to 180°C and a mixing time of 3 to 30 minutes. The resulting primary mixture is then mixed with the crosslinking agent, crosslinking accelerator, crosslinking aid, foaming agent, and foaming aid, and the mixture is then mixed for the secondary mixing. The secondary mixing conditions are preferably a temperature of 20°C to 80°C and a mixing time of 3 to 30 minutes. The resulting secondary mixture is then formed into the desired shape using a calendar molding machine, extrusion molding machine, etc. The mixture is then placed in a crosslinking/foaming mold and subjected to primary crosslinking/foaming and secondary crosslinking/foaming, in that order, to obtain a rubber foam. The primary crosslinking/foaming is preferably performed at 100°C to 160°C for 3 to 20 minutes. Secondary crosslinking and foaming is preferably carried out at 160°C to 220°C for 5 to 20 minutes.

(2) Effects of the Method for Producing Rubber Foam 1 of the Embodiment In the method for producing rubber foam 1 of the embodiment, two or more types of azodicarboxylic acid amide particles having different median diameters are foamed in a rubber component containing acrylonitrile-butadiene rubber and polyvinyl chloride so as to reduce the density of the rubber foam 1. Therefore, this production method can produce a rubber foam 1 that contains acrylonitrile-butadiene rubber, which has low thermal conductivity as an inherent material property, and polyvinyl chloride, which has low thermal conductivity as an inherent material property, and also has a low specific gravity, and as a result has particularly low thermal conductivity. Furthermore, this production method can produce a rubber foam 1 that contains acrylonitrile-butadiene rubber and polyvinyl chloride, and has a slippery surface and excellent abrasion resistance.

The present invention will be explained in more detail below with reference to the following examples.

1. Preparation of Rubber Foam (Examples 1 to 8 and Comparative Examples 1 to 3)
Various rubber foams were produced according to the formulations shown in Table 1. In this case, the raw materials shown in Table 3 were commonly blended in Examples 1 to 8 and Comparative Examples 1 to 3.
Table 2 shows the names of the raw materials and the names of the manufacturers listed in Table 1.
In Table 1, "DPT" means "N,N'-dinitrosopentamethylenetetramine."

The primary kneading was carried out using a kneader at a temperature of 160° C. for a kneading time of 5 minutes, and the secondary kneading was carried out using a roll at a temperature of 50° C. for a kneading time of 10 minutes.
The primary crosslinking and foaming was carried out by placing 70 g of the secondary kneaded product in a mold having an internal volume of 57 cm ³ at 130° C. for 10 minutes.
Secondary crosslinking and foaming were carried out at 200° C. for 5 minutes using the same mold as used for primary crosslinking and foaming.

2. Evaluation Methods (1) Specific Gravity Measured at a temperature of 23°C according to JIS K7112 Method A "Water Substitution Method".

(2) Thermal Conductivity The thermal conductivity is a value measured in accordance with JIS-A1412-2. The thermal conductivity was measured by the probe method using a rapid thermal conductivity meter (QTM-500) manufactured by Kyoto Electronics Manufacturing Co., Ltd.

(3) Flame Retardancy Flame retardancy was measured in accordance with the 20 mm vertical flame test (IEC 60695-11-10 Method B, ASTM D3801). Test specimens were attached vertically to a clamp and exposed to a 20 mm flame for 10 seconds twice. The flame behavior was evaluated as V-0, V-1, V-2, or Not.
If each of the criteria was as follows, the flammability classification was determined to be "V-0".

Burning time of each test piece: 10 seconds or less Total burning time of 5 pieces: 50 seconds or less Burning + glowing time of each test piece: 30 seconds or less Burning up to clamp: None Ignition of cotton by dripping: None

(4) Abrasion Resistance The abrasion resistance test method (abrasion test method) was performed as follows: After 2000 rubs, if there was no abrasion on the surface, it was rated as "good", and if there was abrasion, it was rated as "poor".
Testing equipment name: Gakushin-type friction testing machine Testing method: JIS K 6404-4 Testing method for rubber-coated and plastic-coated fabrics Part 4: Durability test 8.3 Gakushin-type friction test Load (weight of friction element): 200 g
・Friction speed: 30 times/min ・Friction distance: 100mm
- Cotton cloth was wrapped around the friction element.

(5) Overall Judgment Judgment was made as follows:
"Good": Meets all three of the following criteria.
"Fail": Does not meet one or more of the following three criteria.
<Item>
[1] Thermal conductivity: 0.043 W/m·K or less [2] Flame retardancy: V-0 equivalent [3] Abrasion resistance test result: Good

3. Results The results are shown in Table 4.

(1) Satisfaction of Each Requirement in Examples 1 to 8 The rubber foams of Examples 1 to 8 satisfy all of the following requirements (a) to (d).
Requirement (a): A rubber foam containing acrylonitrile-butadiene rubber and polyvinyl chloride.
Requirement (b): When the total amount of the acrylonitrile-butadiene rubber and the polyvinyl chloride is 100 parts by mass, the amount of the acrylonitrile-butadiene rubber is 50 parts by mass or more and 90 parts by mass or less.
Requirement (c): The specific gravity is 0.05 or more and 0.15 or less.
Requirement (d): Two types of azodicarboxylic acid amide particles having different median diameters are used.

(2) Satisfaction of the Requirements of Comparative Examples 1 to 3 In contrast, the rubber foams of Comparative Examples 1 to 3 do not satisfy the following requirements.
Comparative Example 1 does not satisfy the requirements (a), (b), (c), and (d).
Comparative Example 2 does not satisfy the requirements (a), (b), and (d).
Comparative Example 3 does not satisfy the requirements (c) and (d).

(3) Results and Discussion of Examples 1 to 8 The rubber foams of Examples 1 to 8 received good results in the overall assessment. The rubber foams of Examples 1 to 8 had low thermal conductivity of 0.043 W/m K or less, flame retardancy equivalent to V-0, and excellent abrasion resistance.
2 shows the state of Example 2 and Comparative Example 1 after the abrasion resistance test. Example 2 on the right side of FIG. 2 had no abrasion on the surface, while Comparative Example 1 on the left side had abrasion on the surface.

(4) Results and Discussion of Comparative Examples 1 to 3 The rubber foam of Comparative Example 1 used EPDM as the rubber component and had a high specific gravity. The rubber foam of Comparative Example 1 had high thermal conductivity and poor abrasion resistance.
The rubber foam of Comparative Example 2 uses only NBR as the rubber component. The rubber foam of Comparative Example 2 had poor abrasion resistance.
The rubber foam of Comparative Example 3 uses NBR and PVC as rubber components. However, the rubber foam of Comparative Example 3 is not produced using two or more types of azodicarboxylic acid amide particles with different median diameters, and has a high specific gravity. The rubber foam of Comparative Example 3 has a high thermal conductivity of 0.059 W/m K.
From the above results, it was confirmed that the specific gravity of the rubber foam was low when a rubber foam containing acrylonitrile-butadiene rubber and polyvinyl chloride, in which the acrylonitrile-butadiene rubber was 50 parts by mass or more and 90 parts by mass or less, was produced using two types of azodicarboxylic acid amide particles having different median sizes, as in Examples 1 to 8. It was confirmed that the rubber foams of Examples 1 to 8 had low thermal conductivity, high flame retardancy, and high abrasion resistance.

4. Effects of the Examples According to the above examples, it is possible to provide a rubber foam having low thermal conductivity, high flame retardancy, and high abrasion resistance.

The present invention is not limited to the embodiments detailed above, and various modifications and variations are possible within the scope of the claims of the present invention.

1...Rubber foam

Claims (2)

A rubber foam comprising acrylonitrile-butadiene rubber, polyvinyl chloride, a halogen-based flame retardant, and an antimony-based flame retardant,
When the total amount of the acrylonitrile-butadiene rubber and the polyvinyl chloride is 100 parts by mass, the amount of the acrylonitrile-butadiene rubber is 50 parts by mass or more and 90 parts by mass or less,
the content of the halogen-based flame retardant is 45 parts by mass or more and 65 parts by mass or less, and the content of the antimony-based flame retardant is 10 parts by mass or more and 20 parts by mass or less,
A rubber foam having a thermal conductivity of 0.032 W/m·K or more and 0.043 W/m·K or less. An insulating material for a flexible tube or hose of an industrial robot, comprising the rubber foam according to claim 1.