JP7777945B2 - Cross-linked ethylene-vinyl acetate copolymer foam with open cell structure, its manufacturing method and sealing material - Google Patents

Description

The present invention relates to a cross-linked ethylene-vinyl acetate copolymer foam with an open-cell structure, a method for producing the same, and a sealing material.

As a sealing material, there is one that uses a cross-linked polyolefin resin foam with an open-cell structure (Patent Document 1).
Crosslinked polyolefin resin foams are suitable for use as sealing materials because they are resistant to hydrolysis and have good compression recovery properties due to their open-cell structure. Examples of open-cell crosslinked polyolefin resin foams include those that use open-cell crosslinked ethylene-vinyl acetate copolymer (EVA) foam.

JP 2012-107161 A

However, conventional open-cell cross-linked ethylene-vinyl acetate copolymer foams have problems such as large compression strain at high temperatures (low heat resistance) and poor sealing properties at high temperatures.

One way to improve heat resistance is to change the resin used as the main raw material to highly crystallized low-density polyethylene (LDPE) or high-density polyethylene (HDPE). However, the foam obtained in this way loses flexibility, has high resistance when compressed, and is difficult to open, making it unsuitable as a sealing material. Furthermore, because of the loss of flexibility, it becomes difficult for the foam to conform to the unevenness of the sealing surface, making it more susceptible to gaps and resulting in poor sealing performance.

The present invention has been made in consideration of the above points, and aims to provide a cross-linked ethylene-vinyl acetate copolymer foam with an open-cell structure that exhibits low compression strain at high temperatures (high heat resistance).

The first aspect is characterized in that the cross-linked ethylene-vinyl acetate copolymer foam having an open-cell structure has a gel fraction (in accordance with JIS K 6796:1998) of 65% or more and a 50% compression set (in accordance with JIS K 6767) of 45% or less for 24 hours at 60°C.
The second aspect is characterized in that the composition of the first aspect is used for a sealing material.

The third aspect is a method for producing an open-cell cross-linked ethylene-vinyl acetate copolymer foam, characterized by comprising: a mixing step of mixing a resin composition containing an ethylene-vinyl acetate copolymer resin, a blowing agent, an organic peroxide, and a silane coupling agent; a foaming step of foaming the resin composition and cross-linking with the organic peroxide to form a closed-cell foam; a compression-decompression step of compressing the closed-cell foam and destroying the cells in the foam to obtain an open-cell foam; and a silane cross-linking step of the open-cell foam.

The fourth aspect is the third aspect, characterized in that the amount of the silane coupling agent is 0.7 to 2.3 parts by weight per 100 parts by weight of the ethylene-vinyl acetate copolymer resin.

The fifth aspect is the third or fourth aspect, characterized in that the gel fraction (in accordance with JIS K 6796:1998) is 65% or more and the 50% compression set (in accordance with JIS K 6767) at 60°C for 24 hours is 45% or less.

A sixth aspect is any one of the third to fifth aspects, characterized in that the open-cell cross-linked ethylene-vinyl acetate copolymer foam is used as a sealing material.

The seventh aspect is a sealing material comprising the open-cell cross-linked ethylene-vinyl acetate copolymer resin foam of the first or second aspect.

The present invention provides a cross-linked ethylene-vinyl acetate copolymer foam with an open-cell structure that exhibits improved low compression set at high temperatures.

1 is a table showing the formulations and measurement results of each comparative example and each example. FIG. 1 is a diagram of an apparatus for measuring heat sealability.

Embodiments of the present invention are described below. The open-cell cross-linked ethylene-vinyl acetate copolymer foam of the present invention has good heat resistance and is suitable as a waterproof sealing material that is compressed between two objects.

The open-cell cross-linked ethylene-vinyl acetate copolymer foam of the present invention is a closed-cell cross-linked ethylene-vinyl acetate copolymer foam in which the cells have broken down to form interconnected cells. It has a gel fraction (JIS K 6796:1998) of 65% or more, preferably 70% or more, and a 50% compression set (JIS K 6767-compliant) at 60°C for 24 hours of 45% or less.

If the gel fraction of open-cell cross-linked ethylene-vinyl acetate copolymer foam is less than 65%, its heat resistance will be reduced, distortion due to heating will increase, and its sealing ability under heated conditions will decrease.

When a cross-linked ethylene-vinyl acetate copolymer foam having an open cell structure has a 50% compression strain of more than 45% at 60°C for 24 hours, the plastic deformation due to compression is large, and the sealing property under heated conditions decreases.
The open-cell crosslinked ethylene-vinyl acetate copolymer foam of the present invention preferably has a 24-hour 50% compression set (in accordance with JIS K 6767) under normal conditions (at room temperature) of 10% or less, more preferably 5% or less.

The open-cell crosslinked ethylene-vinyl acetate copolymer foam of the present invention can be produced by carrying out a mixing process in which a resin composition containing an ethylene-vinyl acetate copolymer resin, a blowing agent, an organic peroxide, and a silane coupling agent is mixed; a foaming process; a compression-decompression process; and a silane crosslinking process.

Ethylene-vinyl acetate copolymer resin is included in the resin composition as a crystalline olefin resin and may be used in combination with other crystalline olefin resins. The amount of ethylene-vinyl acetate copolymer resin included in the crystalline olefin resin is preferably 50 to 100% by weight. If the amount of ethylene-vinyl acetate copolymer resin included is too low, flexibility will be reduced, adhesion to the mating surface will be poor, and sealing properties will be reduced.

Other crystalline olefin resins that can be used together with the ethylene-vinyl acetate copolymer resin include low-density polyethylene, high-density polyethylene, ethylene-propylene copolymer resin, ethylene-butene copolymer resin, copolymer resin of ethylene with methyl, ethyl, propyl or butyl acrylate ester, chlorinated products thereof, mixtures thereof, and mixtures of these with isotactic polypropylene or atactic polypropylene.
In particular, low-density polyethylene is preferably used in combination with an ethylene-vinyl acetate copolymer resin because its use can improve heat resistance. The amount of low-density polyethylene contained in the crystalline olefin resin is preferably 5 to 50% by weight, more preferably 5 to 40% by weight, and most preferably 5 to 20%, with the remainder being the ethylene-vinyl acetate copolymer resin.

The blowing agent used is one that decomposes upon heating to generate gas. Examples of blowing agents include azodicarbonamide (ADCA), 2,2'-azobisisobutyronitrile, diazoaminobenzene, benzenesulfonylhydrazide, benzene-1,3-sulfonylhydrazide, diphenyloxide-4,4'-disulfonylhydrazide, 4,4'-oxybisbenzenesulfonylhydrazide, paratoluenesulfonylhydrazide, N,N'-dinitrosopentamethylenetetramine, N,N'-dinitroso-N,N'-dimethylphthalamide, terephthalazide, p-t-butylbenzazide, sodium bicarbonate, and ammonium bicarbonate. Azodicarbonamide and 4,4'-oxybisbenzenesulfonylhydrazide are preferred. The amount of blowing agent is approximately 10 to 30 parts by weight per 100 parts by weight of the crystalline olefin resin.

The organic peroxide acts as a crosslinking agent, and examples include dicumyl peroxide (DCP), 2,5-dimethyl-2,5-bis-tertiarybutylperoxyhexane, and 1,3-bis-tertiaryperoxy-isopropylbenzene. The amount of organic peroxide is approximately 0.5 to 1.5 parts by weight per 100 parts by weight of the crystalline olefin resin.

The silane coupling agent can be one used in the silane crosslinking method. Examples include vinyltrimethoxysilane, vinyltriethoxysilane, vinyltributoxysilane, vinyldimethoxyethoxysilane, vinyldimethoxybutoxysilane, allyltrimethoxysilane, and methacryloxypropyltrimethoxysilane. Vinyltrimethoxysilane or vinyltriethoxysilane is particularly preferred. Silane coupling agents may be used alone or in combination. The amount of silane coupling agent is preferably 0.5 to 3 parts by weight, more preferably 1.0 to 2.5 parts by weight, and most preferably 1.0 to 2.0 parts by weight, per 100 parts by weight of the crystalline olefin resin.

The foamable resin composition may contain an auxiliary agent, such as a foaming auxiliary, a crosslinking accelerator, or a colorant.
Examples of the foaming aid include metal oxides such as zinc oxide and lead oxide, metal carbonates such as zinc carbonate, metal chlorides such as zinc chloride, urea, and zinc stearate.
As the crosslinking accelerator, trimethylolpropane trimethacrylate (TMPTMA) is preferred.

In the mixing process, the resin composition is kneaded in an extruder and extruded through a die to produce a pre-foamed sheet. Kneading is carried out at a temperature lower than the decomposition temperature of the organic peroxide and blowing agent contained in the resin composition. The thickness of the pre-foamed sheet is preferably 1.0 to 5.0 mm. Note that kneading in the mixing process is not limited to kneading using an extruder, and kneading using other devices, such as rolls, may also be used.

In the foaming process, the pre-foamed sheet obtained in the mixing process is heated to cause a crosslinking and foaming reaction. There are no particular restrictions on the heating method; for example, the pre-foamed sheet may be placed in a heating oven. The heating temperature is set to a temperature equal to or higher than the decomposition temperature of the organic peroxide and foaming agent contained in the resin composition. The crosslinked and foamed foam is then cooled by natural cooling or other methods to obtain a foam with a closed-cell structure.

In the compression cell-breaking process, the closed-cell foam obtained in the foaming process is compressed using a press or pressure roll to break the cells in the foam, resulting in an open-cell foam. In the compression cell-breaking process, the closed-cell foam is preferably compressed to a thickness of 1/2 to 1/20 of its original thickness.

In the silane crosslinking step, the open-cell foam obtained in the compression cell breaking step is silane crosslinked via a silane coupling agent by the action of moisture. The moisture in the silane crosslinking may be moisture in the air.
A specific method for the silane crosslinking step is to carry out silane crosslinking by maintaining the open-cell foam obtained in the compression cell-breaking step at room temperature for 24 hours or more, or maintaining it at a temperature of 40 to 60°C and a humidity of 60 to 80% for 24 hours or more, or maintaining it in warm water at about 50 to 80°C for 4 hours or more, thereby obtaining a crosslinked ethylene-vinyl acetate copolymer foam having a desired open-cell structure.
The crosslinked ethylene-vinyl acetate copolymer foam with an open-cell structure obtained by the silane crosslinking step is crosslinked twice, first by pre-crosslinking with an organic peroxide in the foaming step and then by post-crosslinking with silane crosslinking after compression and cell collapse, thereby improving shape retention after cell collapse and reducing compression strain.

The open-cell crosslinked ethylene-vinyl acetate copolymer foam of the present invention is suitable for use as a sealant, particularly a waterproof sealant, after being punched into a ring or other shape or cut into a predetermined shape.
The open-cell cross-linked ethylene-vinyl acetate copolymer foam of the present invention is also suitable for use as a sealing material at least partially comprising the open-cell cross-linked ethylene-vinyl acetate copolymer foam of the present invention.

The foams of the comparative examples and examples were prepared using the resin compositions having the formulations shown in FIG. 1 using the following raw materials.
EVA (ethylene-vinyl acetate copolymer resin): Product code: N8038, manufactured by TPI POLENE PUBLIC COMPAN LIMITED. LDPE (low-density polyethylene): Product code: F2225.4, manufactured by Asahi Kasei Chemicals Corporation. ADCA (azodicarbonamide): Product code: 1L-K3, manufactured by Eiwa Chemical Industry Co., Ltd. Urea: Product code: Celpaste 101, manufactured by Eiwa Chemical Industry Co., Ltd. DCP (dicumyl peroxide): Product code: Perkadox BC-FF, manufactured by Kayaku Akzo Co., Ltd. TMPTMA (trimethylolpropane trimethacrylate): Product code: TMPT, manufactured by Shin-Nakamura Chemical Co., Ltd. Silane coupling agent: Product code: KBM-1003, manufactured by Shin-Etsu Chemical Co., Ltd.

The foam was prepared as follows.
The resin compositions of the comparative examples and examples having the formulations shown in FIG. 1 were kneaded at 110° C. using an open roll to prepare pre-foamed sheets having a thickness of 3.0 mm.
The obtained pre-foamed sheet was placed in a thermostatic oven at 225°C and heated for 12 minutes to carry out a crosslinking and foaming reaction. After crosslinking and foaming, the foam was continuously cooled with a cooling roll to obtain a foam having a closed-cell structure and a thickness of 8 to 12 mm.
The resulting closed-cell foam was passed through a compression roll to compress it to a thickness of 1/2 to 1/20 of its original thickness, breaking the cells to obtain an open-cell foam. For Comparative Examples 1 and 2, the process was terminated at this point to obtain the foams of Comparative Examples 1 and 2.
On the other hand, for the examples, the obtained open-cell foam was placed in warm water at a temperature of 50 to 80° C. for about 4 hours to carry out post-crosslinking by silane crosslinking, thereby obtaining the foams of the examples.

The foams of the comparative examples and examples were measured for foam moldability, gel fraction, 50% compression stress, 50% compression stress-strain, and heat sealability.
The foam moldability was evaluated by visually checking whether or not the obtained foam had any cracks or tears. If no cracks or tears were present, the evaluation was "Good", and if either cracks or tears were present, the evaluation was "Poor".
The gel fraction (%) was measured in accordance with JIS K 6796:1998 after aging at room temperature for 24 hours or more.
The 50% compressive stress (kPa) was measured in accordance with JIS K 6767.

The 50% compressive stress-strain (%) was measured in accordance with JIS K 6767, with the compressive strain measured at room temperature for 24 hours and the compressive strain measured at 60° C. for 24 hours.
For the 24-hour compression strain under normal conditions, the test specimen was compressed by 50% for 24 hours at room temperature, and then released from the compression, and the strain (% of the amount of deformation relative to the original thickness) was measured 24 hours later.
The compression strain at 60°C for 24 hours when heated was measured by compressing the sample by 50% for 24 hours while heated to 60°C, and then releasing the compression to measure the strain (the amount of deformation relative to the original thickness) 24 hours later.

As shown in Figure 2, the heated sealing property was measured by punching out a 10 mm thick foam 50 from each of the comparative example and the example into a U-shape with a width of 15 mm and an opening width of 100 mm, sandwiching it between two acrylic resin plates 71, 71 in a 50% compressed state, pouring water into the U-shape to a depth of 100 mm, maintaining it at a heated state of 60°C, and measuring the water leakage time. The water leakage time was the time until water began to leak out of the U-shape.

Comparative Example 1 is an example in which neither EVA (ethylene-vinyl acetate copolymer resin) nor a silane coupling agent was included, and 100 parts by weight of LDPE (low density polyethylene) was used as the resin.
In Comparative Example 1, the foam moldability was "good," the gel fraction was 51%, the 50% compression stress was 8.0 kPa, the 50% compression stress-strain was 2.5% after 24 hours at normal conditions and 47.0% after 24 hours at 60°C when heated, and the sealability when heated was within 1 hour, indicating that the compression strain was large both at normal conditions and when heated, and the sealability when heated was poor.

Comparative Example 2 is an example in which 92.3 parts by weight of EVA (ethylene-vinyl acetate copolymer resin) and 7.7 parts by weight of LDPE (low density polyethylene) are used in combination as resins, and no silane coupling agent is included.
In Comparative Example 2, the foam moldability was "good," the gel fraction was 55%, the 50% compression stress was 5.5 kPa, the 50% compression stress-strain was 2.0% after 24 hours under normal conditions and 50.0% after 24 hours under heating at 60°C, and the sealability when heated was within 1 hour, indicating that the compression strain was large under normal conditions and when heated, and the sealability when heated was poor.

Example 1 is an example in which 92.3 parts by weight of EVA (ethylene-vinyl acetate copolymer resin) and 7.7 parts by weight of LDPE (low density polyethylene) are used in combination as resins, and 1.0 part by weight of a silane coupling agent is also included.
Example 1 had foam moldability of "good," a gel fraction of 72%, a 50% compression stress of 5.7 kPa, a 50% compression stress-strain of 1.3% for 24 hours under normal conditions and 44.3% for 24 hours under heating at 60°C, and a heat sealability of 24 hours or more. The compression strain under normal conditions and under heating was small, and the heat sealability was high.

Example 2 is an example in which the amount of silane coupling agent was increased to 1.5 parts by weight, and the other conditions were the same as Example 1.
Example 2 had foam moldability of "good", a gel fraction of 74%, a 50% compression stress of 5.8 kPa, a 50% compression stress-strain of 1.5% over 24 hours under normal conditions and 42.5% over 24 hours under heating at 60°C, and a heat sealability of 24 hours or more. The compression strain under normal conditions and under heating was small, and the heat sealability was high.

Example 3 is an example in which the amount of silane coupling agent was increased to 2.0 parts by weight, and the other conditions were the same as those of Example 1.
Example 3 had foam moldability of "good," a gel fraction of 78%, a 50% compression stress of 6.1 kPa, a 50% compression stress-strain of 1.4% for 24 hours under normal conditions and 41.3% for 24 hours under heating at 60°C, and a heat sealability of 24 hours or more. The compression strain under normal conditions and under heating was small, and the heat sealability was high.

Thus, the present invention provides a cross-linked ethylene-vinyl acetate copolymer foam having an open-cell structure that exhibits small compression strain at high temperatures and is suitable as a sealant, particularly a waterproof sealant, to be used at high temperatures.
The present invention is not limited to the examples, and can be modified within the scope of the invention.

Claims (5)

a gel fraction (in accordance with JIS K 6796:1998) of 65% or more, and a 50% compression set at 60°C for 24 hours (in accordance with JIS K 6767) of 45% or less;
A cross-linked ethylene-vinyl acetate copolymer foam having an open-cell structure containing a chemical structure derived from a silane coupling agent. A method for producing an open-cell cross-linked ethylene-vinyl acetate copolymer foam, comprising:
a mixing step of mixing a resin composition containing an ethylene-vinyl acetate copolymer resin, a foaming agent, an organic peroxide, and a silane coupling agent;
a foaming step of foaming the resin composition and crosslinking the resin composition with the organic peroxide to form a foam with a closed cell structure;
a compression cell-breaking step of compressing the foam with the closed-cell structure to break the cells of the foam and obtain a foam with an open-cell structure;
subjecting the open-cell foam to a silane crosslinking step;
A method for producing a cross-linked ethylene-vinyl acetate copolymer foam having an open-cell structure and a gel fraction (in accordance with JIS K 6796:1998) of 65% or more. 3. The method for producing a cross-linked ethylene-vinyl acetate copolymer foam having an open-cell structure according to claim 2, wherein the foam has a 50% compression set (in accordance with JIS K 6767) of 45% or less after 24 hours at 60°C. 4. The method for producing an open-cell cross-linked ethylene-vinyl acetate copolymer foam according to claim 2, wherein the open-cell cross-linked ethylene-vinyl acetate copolymer foam is used for a sealing material. A sealing material comprising the open-cell cross-linked ethylene-vinyl acetate copolymer foam according to claim 1 .