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

Abstract

To provide an open cell structured crosslinked ethylene-vinyl acetate copolymer foam excellent in heat resistance suitable as a seal material used under compression under a heated state.SOLUTION: A crosslinked ethylene-vinyl acetate copolymer foam having an open-cell structure has a gel fraction (JIS K 6796:1998) of 65% or more, and a 50% compressive strain (JIS K 6767) of 45% or less for 24 hours at 60°C, making it suitable as a sealing material used under compression under a heated state.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a cross-linked ethylene-vinyl acetate copolymer foam having an open-cell structure, a method for producing the same, and a sealing material. [Background technology]

[0002] 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 due to their open-cell structure. Crosslinked polyolefin resin foams with open-cell structure include those that use open-cell crosslinked ethylene-vinyl acetate copolymer (EVA) foam. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2012-107161 Summary of the Invention [Problem to be solved by the invention]

[0004] 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.

[0005] One way to improve heat resistance is to change the resin used as the main raw material to highly crystalline low-density polyethylene (LDPE) or high-density polyethylene (HDPE). However, the foam obtained by this method 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 to conform to the unevenness of the sealing surface, making it prone to gaps and resulting in poor sealing performance.

[0006] The present invention has been made in view of the above points, and an object of the present invention is to provide a cross-linked ethylene-vinyl acetate copolymer foam having an open-cell structure that exhibits small compression strain at high temperatures (high heat resistance). [Means for solving the problem]

[0007] The first aspect is characterized in that the open-cell cross-linked ethylene-vinyl acetate copolymer foam has a gel fraction (based on JIS K 6796:1998) of 65% or more and a 50% compression set (based on 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.

[0008] The third aspect is a method for producing a cross-linked open-cell ethylene-vinyl acetate copolymer foam, the method 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-cell-breaking step of compressing the closed-cell foam and breaking the cells in the foam to obtain an open-cell foam; and a silane cross-linking step of cross-linking the open-cell foam.

[0009] A fourth aspect is the third aspect, characterized in that the amount of the silane coupling agent blended is 0.7 to 2.3 parts by weight with respect to 100 parts by weight of the ethylene-vinyl acetate copolymer resin.

[0010] The fifth aspect is the third or fourth aspect, characterized in that the gel fraction (based on JIS K 6796:1998) is 65% or more and the 50% compression strain (based on JIS K 6767) at 60°C for 24 hours is 45% or less.

[0011] 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 for a sealing material.

[0012] A seventh aspect is a sealing material comprising the open-cell cross-linked ethylene-vinyl acetate copolymer resin foam of the first or second aspect. [Effects of the Invention]

[0013] According to the present invention, a cross-linked ethylene-vinyl acetate copolymer foam having an open-cell structure and improved low compression set at high temperatures can be obtained. [Brief explanation of the drawings]

[0014] [Figure 1] 1 is a table showing the formulations and measurement results of each comparative example and each example. [Figure 2] FIG. 1 is a diagram of an apparatus for measuring heat sealability. DETAILED DESCRIPTION OF THE INVENTION

[0015] The present invention provides a cross-linked ethylene-vinyl acetate copolymer foam having an open-cell structure, which has excellent heat resistance and is suitable as a waterproof sealing material that is compressed between two objects.

[0016] The open-cell crosslinked ethylene-vinyl acetate copolymer foam of the present invention is a crosslinked ethylene-vinyl acetate copolymer foam with a closed-cell structure in which the cells have been broken and become interconnected, and has a gel fraction (JIS K 6796:1998) of 65% or more, preferably 70% or more, and a 50% compression set at 60°C for 24 hours (in accordance with JIS K 6767) of 45% or less.

[0017] When the gel fraction of an open-cell cross-linked ethylene-vinyl acetate copolymer foam is less than 65%, the heat resistance decreases, distortion due to heating increases, and the sealing property under heating decreases.

[0018] When a cross-linked ethylene-vinyl acetate copolymer foam with an open cell structure experiences 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 ability 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 (according to JIS K 6767) under normal conditions (at room temperature) of 10% or less, more preferably 5% or less.

[0019] The open-cell crosslinked ethylene-vinyl acetate copolymer foam of the present invention can be produced by carrying out 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, a compression-decompression step, and a silane crosslinking step.

[0020] The 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 blending amount of the ethylene-vinyl acetate copolymer resin included in the crystalline olefin resin is preferably 50 to 100% by weight. If the blending amount of the ethylene-vinyl acetate copolymer resin is too small, flexibility is reduced, adhesion to the mating surface is poor, and sealing properties are reduced.

[0021] 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 thereof 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.

[0022] 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, pt-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.

[0023] The organic peroxide acts as a crosslinking agent, and examples thereof include dicumyl peroxide (DCP), 2,5-dimethyl-2,5-bis-tert-butylperoxyhexane, 1,3-bis-tert-peroxy-isopropylbenzene, etc. The amount of the organic peroxide is about 0.5 to 1.5 parts by weight per 100 parts by weight of the crystalline olefin resin.

[0024] The silane coupling agent can be one used in the silane crosslinking method. Examples include vinyltrimethoxysilane, vinyltriethoxysilane, vinyltributoxysilane, vinyldimethoxyethoxysilane, vinyldimethoxybutoxysilane, allyltrimethoxysilane, and methacryloxypropyltrimethoxysilane, with vinyltrimethoxysilane or vinyltriethoxysilane being particularly preferred. The silane coupling agent may be used alone or in combination with two or more. The amount of the 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.

[0025] 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.

[0026] In the mixing step, the resin composition is kneaded in an extruder and extruded through a die to produce a pre-foamed sheet. The kneading is performed at a temperature lower than the decomposition temperature of the organic peroxide and the foaming agent contained in the resin composition. The thickness of the pre-foamed sheet is preferably 1.0 to 5.0 mm. Note that the kneading in the mixing step is not limited to kneading using an extruder, and may be kneading using other devices, such as a roll.

[0027] 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, and an example is a method in which the pre-foamed sheet is placed in a heating furnace. The heating temperature is set to be 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 cooled by natural cooling or the like to obtain a foam with a closed-cell structure.

[0028] In the compression cell breaking step, the foam with a closed cell structure obtained in the foaming step is pressurized and compressed using a press, pressure roll, etc. to break the cells of the foam, resulting in a foam with an open cell structure. In the compression cell breaking step, the foam with a closed cell structure is preferably compressed to a thickness of 1 / 2 to 1 / 20 of its original thickness.

[0029] 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. Specifically, the silane crosslinking step involves maintaining the open-cell foam obtained in the compression-decompression step at room temperature for 24 hours or more, maintaining the foam at a temperature of 40 to 60°C and a humidity of 60 to 80% for 24 hours or more, or maintaining the foam in warm water at about 50 to 80°C for 4 hours or more to perform silane crosslinking, thereby obtaining the desired open-cell crosslinked ethylene-vinyl acetate copolymer foam. The open-cell crosslinked ethylene-vinyl acetate copolymer foam obtained through the silane crosslinking process undergoes two crosslinking processes: pre-crosslinking with organic peroxide during the foaming process and post-crosslinking with silane crosslinking after compression and cell collapse. This improves shape retention after cell collapse and reduces compression strain.

[0030] The open-cell cross-linked 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. [Example]

[0031] 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 number: N8038, manufactured by TPI POLENE PUBLIC COMPAN LIMITED LDPE (low-density polyethylene): Part number F2225.4, manufactured by Asahi Kasei Chemicals Corporation ADCA (azodicarbonamide): Part number: 1L-K3, manufactured by Eiwa Chemical Industry Co., Ltd. Urea: Cellpaste 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 number: TMPT, manufactured by Shin-Nakamura Chemical Co., Ltd. Silane coupling agent: Part number: KBM-1003, manufactured by Shin-Etsu Chemical Co., Ltd.

[0032] The foam was prepared as follows. The resin compositions of the comparative example and the example having the formulation shown in FIG. 1 were kneaded at 110° C. using an open roll to prepare a pre-foamed sheet having a thickness of 3.0 mm. The obtained pre-foamed sheet was placed in a thermostatic chamber at 225°C and heated for 12 minutes to carry out the crosslinking and foaming reaction. After crosslinking and foaming, the foam was continuously cooled with a cooling roll to obtain a foam with a closed-cell structure and a thickness of 8 to 12 mm. The obtained foam with a closed cell structure was passed through a compression roll to compress it to 1 / 2 to 1 / 20 of its original thickness, breaking the cells to obtain a foam with an open cell structure. 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.

[0033] 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.

[0034] The 50% compressive stress-strain (%) was measured in accordance with JIS K 6767, with the compressive strain measured at normal temperature for 24 hours and the compressive strain measured at 60°C for 24 hours. For the 24-hour compressive 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 during heating at 60°C for 24 hours was measured by compressing the sample by 50% for 24 hours while heated to 60°C, then releasing the compression and measuring the strain (% of deformation relative to the original thickness) 24 hours later.

[0035] As shown in Figure 2, the heated sealability was measured by punching out 10 mm thick foams 50 of the comparative example and the example into U-shapes with a width of 15 mm and an opening width of 100 mm, sandwiching them 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 the plate 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.

[0036] Comparative Example 1 is an example that does not contain either EVA (ethylene-vinyl acetate copolymer resin) or a silane coupling agent, and uses 100 parts by weight of LDPE (low-density polyethylene) 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.

[0037] 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 at normal conditions and 50.0% 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.

[0038] 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 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% over 24 hours at room temperature and 44.3% over 24 hours at 60°C when heated, and a heat sealability of 24 hours or more. The compression strain was small both at room temperature and when heated, and the heat sealability was high.

[0039] 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 at room temperature and 42.5% over 24 hours at 60°C, and a heat sealability of 24 hours or more. The compression strain was small both at room temperature and when heated, and the heat sealability was high.

[0040] 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% over 24 hours at room temperature and 41.3% over 24 hours at 60°C when heated, and a heat sealability of 24 hours or more. The compression strain was small both at room temperature and when heated, and the heat sealability was high.

[0041] As described above, 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 sealing material, particularly a waterproof sealing material, 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

1. A composition having a gel fraction (in accordance with JIS K 6796:1998) of 65% or more and a 50% compression strain (in accordance with JIS K 6767) of 45% or less for 24 hours at 60°C, A cross-linked ethylene-vinyl acetate copolymer foam having an open-cell structure containing a chemical structure derived from a silane coupling agent.

2. 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; The open-cell foam is subjected 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. 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. 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.

5. A sealing material comprising the open-cell structure cross-linked ethylene-vinyl acetate copolymer foam described in claim 1.

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