Crosslinked polyolefin resin foam

Abstract

To provide a crosslinked polyolefin resin foam having both antistatic properties and good moldability.SOLUTION: The crosslinked polyolefin resin foam contains a crosslinked polyolefin resin and carbon black. The carbon black has a BET specific surface area of 10m2 / g or more and 100m2 / g or less, and a content of the carbon black is 22.0 parts by mass or more and 32.0 parts by mass or less. The crosslinked polyolefin foam preferably has a cell number of 80 cells / 25cm or more and 110 cells / 25cm or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a crosslinked polyolefin resin foam, and more particularly to a crosslinked polyolefin resin foam that combines antistatic properties with good moldability. [Background technology]

[0002] Cross-linked polyolefin resin foams are lightweight and have excellent heat insulation, water resistance, chemical resistance, and shock absorption properties, and are therefore widely used in automotive interior parts, building insulation materials, joint materials, pipe covers, household goods, health and sporting goods, and more. Although polyolefin resins are insulating, adding conductive particles to them and foaming and crosslinking them produces conductive crosslinked polyolefin resin foams. Because conductive foams are less likely to become charged, they are used in packaging, cushioning, trays, containers, and other applications for various electronic components (e.g., IC chips) where damage from static electricity is a problem.

[0003] When a crosslinked polyolefin resin foam is used as a packaging material for electronic components, the foam must have an appropriate electrical conductivity in order to prevent the foam from becoming charged and to suppress damage to the electronic components due to static electricity. On the other hand, if the conductivity of the crosslinked polyolefin resin foam becomes too high, external discharges are induced in the foam, which can result in current flowing through electronic components inside the foam, potentially damaging the electronic components. Therefore, in order to impart an appropriate antistatic property to the foam, it is necessary to optimize the type and amount of conductive particles added.

[0004] Various proposals have been made so far regarding such crosslinked polyolefin resin foams containing conductive particles. For example, Patent Document 1 describes a polymer that is not intended for antistatic purposes, but has a nitrogen adsorption specific surface area of ​​225 m for 100 parts by weight of a resin component (a mixture of EPDM polymer and polyethylene). 2 / g, and the DBP oil supply is 155 cm 3The paper discloses a radio wave absorber obtained by adding 67 parts by mass of carbon black, which is equivalent to 100g / 100g, and foaming and vulcanizing a raw material mixture containing these.

[0005] The same document states: (a) To increase the attenuation rate of the radio wave absorber, the nitrogen adsorption specific surface area of ​​carbon black should be increased to 130 to 300 m 2 / g, and (b) To achieve an attenuation of 8 dB or more in the frequency band of 8 to 12.5 GHz, the amount of carbon black to be mixed must be 55 to 160 parts by weight. is stated.

[0006] Patent Document 2 discloses a cross-linked polyolefin resin foam obtained by foaming and cross-linking a raw material mixture containing 90% by weight of low-density polyethylene and 10% by weight of carbon black having a DBP oil absorption of 480 mL / 100 g. The document states that if the carbon black content exceeds 20% by weight, the dispersion state of the carbon black deteriorates, and carbon black aggregates may deteriorate the mechanical properties and surface properties of the crosslinked polyolefin resin foam.

[0007] Carbon black with a small primary particle size and a well-developed structure (e.g., Ketjen Black (registered trademark)) has a large specific surface area and a high DBP oil absorption. When such carbon black with a high specific surface area is used as conductive particles, a foam exhibiting high conductivity can be obtained with just a small amount added.

[0008] However, carbon black with a high specific surface area has poor dispersibility and is prone to molding defects when producing foams. This is thought to be because the carbon black adsorbs resins, dispersants, solvents, etc., and the viscosity of the raw material mixture increases excessively when the raw material mixture is heated and kneaded. Furthermore, carbon black with a well-developed structure is prone to breaking during foaming, which can reduce the conductivity of the foam or cause horizontal cracks in the foam (a phenomenon in which cracks grow horizontally from the side wall of the mold toward the interior of the foam).

[0009] To solve this problem, it is conceivable to use carbon black with a low specific surface area as the conductive particles, but when carbon black with a low specific surface area is used as the conductive particles, the conductivity of the foam tends to decrease. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Japanese Patent Application Laid-Open No. 2004-311586 [Patent Document 2] Japanese Patent Application Laid-Open No. 2009-203256 Summary of the Invention [Problem to be solved by the invention]

[0011] An object of the present invention is to provide a crosslinked polyolefin resin foam that has both antistatic properties and good moldability. [Means for solving the problem]

[0012] In order to solve the above problems, the crosslinked polyolefin resin foam according to the present invention comprises: a cross-linked polyolefin resin; Carbon black and Including, The carbon black has a BET specific surface area of ​​10 m 2 / g or more 100m 2 / g or less, The content of the carbon black is 22.0 parts by mass or more and 32.0 parts by mass or less. [Effects of the Invention]

[0013] When carbon black with a well-developed structure and a high specific surface area is used as conductive particles, a foam exhibiting high conductivity can be obtained with a small amount of addition. However, high-specific surface area carbon black has poor dispersibility and is prone to molding defects. On the other hand, when carbon black with a low specific surface area is used as conductive particles, moldability is relatively good, but the conductivity of the foam is likely to decrease. In contrast, when carbon black with a low specific surface area is used as the conductive particles, if the amount of carbon black added is increased compared to that of conventional foams, a crosslinked polyolefin resin foam that combines antistatic properties and good moldability can be obtained. DETAILED DESCRIPTION OF THE INVENTION

[0014] An embodiment of the present invention will be described in detail below. [1. Raw material mixture] First, the raw material mixture for producing the crosslinked polyolefin resin foam (hereinafter also simply referred to as "foam") according to the present invention will be described. In the present invention, the raw material mixture contains a polyolefin resin, carbon black, a crosslinking agent, a foaming agent, and a foaming assistant.

[0015] [1.1. Raw materials] [1.1.1. Polyolefin resin] In the present invention, the type of polyolefin resin is not particularly limited, and an optimum material can be selected depending on the purpose.

[0016] Examples of polyolefin resins include: (a) Polyethylene, ethylene-propylene copolymer, ethylene-butene copolymer, ethylene-vinyl acetate copolymer, (b) ethylene-acrylic acid ester copolymers such as ethylene-methyl acrylate copolymer, ethylene-ethyl acrylate copolymer, ethylene-propyl acrylate copolymer, and ethylene-butyl acrylate copolymer; (c) Chlorinated products of (a) or (b) above etc. The raw material mixture may contain any one of these polyolefins, or may contain two or more of them.

[0017] Polyethylene is particularly preferred as a polyolefin resin. Among polyethylenes, low-density polyethylene (LDPE) is particularly preferred. LDPE has a low melting point, so the viscosity of the raw material mixture containing it tends to decrease when it is heated and kneaded. Therefore, a kneaded product in which the additives are uniformly dispersed can be easily obtained.

[0018] 1.1.2. Carbon black Carbon black is an additive for imparting desired conductivity to the foam. In the present invention, the carbon black has a BET specific surface area of ​​10 m 2 / g or more 100m 2 / g or less is used, which is different from conventional methods.

[0019] If the BET specific surface area of ​​the carbon book is too small, the conductivity of the foam may decrease. Therefore, the BET specific surface area should be less than 10 m 2 The BET specific surface area is preferably 20 m 2 / g or more, 30m 2 / g or more, 40m 2 / g or more, or 50m 2 / g or more. On the other hand, if the BET specific surface area of ​​the carbon black is too large, the carbon black may aggregate in the raw material mixture, which may reduce moldability. 2 The BET specific surface area is preferably 90 m 2 / g or less, 80m 2 / g or less, or 70m 2 / g or less.

[0020] 1.1.3. Crosslinking Agents The crosslinking agent is an additive that crosslinks the polyolefin resin during foaming, thereby increasing the viscosity of the raw material mixture and maintaining the bubbles inside the raw material mixture. In the present invention, the type of crosslinking agent is not particularly limited as long as it performs this function.

[0021] Examples of crosslinking agents include dicumyl peroxide, 2,5-dimethyl-2,5-bis-(t-butylperoxy)hexane, 1,3-bis-(t-butylperoxyisopropyl)benzene and other organic peroxides. The raw material mixture may contain any one of these crosslinking agents, or may contain two or more of them.

[0022] The crosslinking agent is particularly preferably dicumyl peroxide, because dicumyl peroxide has high reactivity with polyolefin resins and is widely used.

[0023] 1.1.4. Foaming agents The foaming agent is an additive that decomposes when heated to generate gas. In the present invention, the type of foaming agent is not particularly limited as long as it exhibits this function.

[0024] Examples of the foaming agent include azodicarbonamide, 2,2'-azobisisobutyronitrile, diazoaminobenzene, Benzene sulfonyl hydrazide, benzene-1,3-sulfonyl hydrazide, Diphenyloxide-4,4'-disulfonylhydrazide, 4,4'-oxybis(benzenesulfonylhydrazide), paratoluenesulfonylhydrazide, N,N'-dinitrosopentamethylenetetramine, N,N'-dinitroso-N,N'-dimethylphthalamide, terephthalazide, pt-butylbenzazide, sodium bicarbonate, ammonium bicarbonate Examples include: The raw material mixture may contain any one of these blowing agents, or may contain two or more of them.

[0025] Azodicarbonamide is particularly preferred as the blowing agent, because it generates a large amount of gas, is inexpensive, and does not generate harmful substances when decomposed.

[0026] 1.1.5. Foaming aids The foaming aid is an additive for promoting the decomposition of the foaming agent. In the present invention, the type of foaming aid is not particularly limited, and an optimum material can be selected depending on the purpose.

[0027] Examples of foaming aids include urea, zinc oxide (ZnO), aliphatic metal salts such as zinc stearate, metal oxides such as magnesium oxide, and alcohols having 1 to 4 carbon atoms. The raw material mixture may contain any one of these foaming aids, or may contain two or more of them.

[0028] The foaming aid is preferably a mixture of ZnO and urea for the following reasons. Known methods for producing foams include a one-stage expansion method and a two-stage expansion method. In the one-stage expansion method, the raw material mixture is heated and expanded in a single stage. The two-stage expansion method is a method in which the raw material mixture is heated and expanded in two stages. The two-stage expansion method is suitable for producing foams with a high expansion ratio because the foam is less likely to crack during expansion than the one-stage expansion method.

[0029] When manufacturing foams using the two-stage foaming method, if the second expansion occurs too quickly, the polyolefin resin cannot keep up with the shape change, which can cause the foam to crack. Also, the carbon black chains within the foam can be broken, which can increase the surface resistance. On the other hand, if the foaming speed of the secondary foaming is slowed down, cracking of the foam can be suppressed, but the secondary foaming takes a long time, resulting in a decrease in production efficiency.

[0030] Here, urea has the effect of promoting the decomposition of the blowing agent from a low temperature range, but the decomposition rate of the blowing agent is relatively slow even at a high temperature range. Therefore, if urea alone is used as a foaming aid, a highly conductive and crack-free foam is easily obtained, but foaming takes a long time. On the other hand, ZnO is highly effective in accelerating the decomposition rate of the foaming agent at high temperatures. Therefore, using ZnO alone as a foaming aid can shorten the foaming time, but the foam is prone to cracking due to the fast foaming rate. In addition, the carbon black chain may be broken, resulting in an increase in surface resistance.

[0031] In contrast, when ZnO and urea are used together as foaming aids and the primary foaming is carried out at a relatively low temperature, urea mainly functions as the foaming aid, resulting in a primary foam containing a relatively large amount of foam nuclei. The resulting primary foam is then subjected to secondary foaming at a relatively high temperature, where ZnO acts primarily as a foaming aid, shortening the time required for secondary foaming without significantly increasing the surface resistance of the foam.

[0032] 1.1.6. Other additives The raw material mixture for producing a foam may contain additives other than the polyolefin resin, carbon black, crosslinking agent, foaming agent, and foaming aid. Specific examples of other additives include the following: The raw material mixture may contain any one of the following additives, or may contain two or more of them.

[0033] [A. Lubricant] A lubricant is an additive that improves the sliding between the kneading tool and the raw material mixture, or between raw material particles, when the raw material mixture is kneaded. Adding a lubricant to the raw material mixture improves the sliding between the kneading tool and the raw material mixture, thereby preventing the raw material mixture from sticking to the kneading tool. In the present invention, the type of lubricant is not particularly limited as long as it exhibits the above-mentioned functions. Examples of lubricants include sorbitan stearate, fatty acid lubricants, and fatty acid amide lubricants.

[0034] [B. Nucleating Agents] Nucleating agents are additives that function as the starting point for foam nuclei during the foam formation process. When a nucleating agent is added to a raw material mixture, foam nuclei are more likely to form at the interface between the polyolefin resin and the nucleating agent, allowing a relatively large number of foam nuclei to form within the primary foam. This also allows the cells contained in the secondary foam to be finer. In the present invention, the type of nucleating agent is not particularly limited as long as it exhibits the above-mentioned functions. Examples of nucleating agents include talc, light calcium carbonate, heavy calcium carbonate, and calcium carbonate.

[0035] [1.2. Content] 1.2.1. Carbon black content The "carbon black content" refers to the mass of carbon black when the total mass of the polyolefin resin before crosslinking is taken as 100.

[0036] If the carbon black content is too low, the surface resistance of the foam may increase. Therefore, the carbon black content is preferably 22.0 parts by mass or more, more preferably 24.0 parts by mass or more, or even 24.5 parts by mass or more. On the other hand, if the carbon black content is excessive, the fluidity of the raw material mixture during foaming decreases, which may cause the foam to crack during foaming. Therefore, the carbon black content is preferably 32.0 parts by mass or less. The content is more preferably 31.0 parts by mass or less, or 30.0 parts by mass or less.

[0037] [1.2.2. Crosslinker content] The "content of crosslinking agent" refers to the mass of the crosslinking agent when the mass of the polyolefin resin before crosslinking is taken as 100.

[0038] If the content of the crosslinking agent is too low, the viscosity of the raw material mixture does not increase during foaming, and air bubbles tend to escape from the raw material mixture. Therefore, the content of the crosslinking agent is preferably 0.5 parts by mass or more. The content is more preferably 0.7 parts by mass or more. On the other hand, if the content of the crosslinking agent is excessive, the viscosity of the raw material mixture increases excessively during foaming, which may cause the foam to crack. Therefore, the content of the crosslinking agent is preferably 1.5 parts by mass or less, and more preferably 1.3 parts by mass or less.

[0039] 1.2.3. Foaming agent content The "content of foaming agent" refers to the mass of the foaming agent when the mass of the polyolefin resin before crosslinking is taken as 100.

[0040] If the content of the blowing agent is too low, the amount of bubbles generated will be small, and the desired expansion ratio may not be obtained. Therefore, the content of the blowing agent is preferably 5.0 parts by mass or more. The content is more preferably 6.0 parts by mass or more. On the other hand, if the content of the foaming agent is excessive, the amount of bubbles generated may be excessive, and the expansion ratio may increase excessively. As a result, cracks may occur in the foam, the carbon black chain may be broken, and the surface resistance may increase. Therefore, the content of the foaming agent is preferably 15.0 parts by mass or less. The content is more preferably 12.0 parts by mass or less.

[0041] [1.2.4. Content of foaming aid] [A. Total content of foaming aids] The "total content of foaming aids" refers to the total mass of the foaming aids when the mass of the polyolefin resin before crosslinking is taken as 100.

[0042] If the total content of the foaming aid is too low, the following problems may occur. (a) When two-stage foaming is performed, the second foaming may take a long time. (b) When two-stage foaming is performed, the primary foaming ratio may become excessively small, causing cell roughness in the foam during the secondary foaming. Therefore, the total content of the foaming aid is preferably 0.2 parts by mass or more, and more preferably 0.3 parts by mass or more.

[0043] On the other hand, if the total content of the foaming aid is excessive, the following problems may occur. (a) When two-stage expansion is performed, the secondary expansion ratio and / or the volume increase rate during the secondary expansion may become excessively large, which may cause the foam to crack during the secondary expansion. (b) When two-stage foaming is performed, the primary foaming ratio may become excessively large, causing the peripheral portion of the foam to bulge in an arc shape during the secondary foaming, a phenomenon known as "rolling-in." Therefore, the total content of the foaming aid is preferably 1.0 part by mass or less, and more preferably 0.8 part by mass or less.

[0044] [B. ZnO / urea ratio] "ZnO / urea ratio (M2 / M1)" refers to the ratio of the mass of ZnO (M2) to the mass of urea (M1). The ZnO / urea ratio is a parameter that is desirably optimized when ZnO and urea are used in combination as foaming aids and a foam is produced using a two-stage foaming method.

[0045] Although urea has the effect of promoting the decomposition of the blowing agent from a low temperature range, the decomposition rate of the blowing agent is relatively slow even at a high temperature range. Therefore, when urea alone is used as a foaming aid, it is easy to obtain a foam that is highly conductive and free of cracks, but it takes a long time for foaming. On the other hand, if ZnO alone is used as a foaming aid, the foaming time can be shortened, but the foaming speed is so fast that cracks may occur in the foam, breaking the carbon black chain and increasing the surface resistance. In contrast, when ZnO and urea are used in combination as a foaming aid, a highly conductive, crack-free foam can be produced in a short foaming time.

[0046] If the ZnO / urea ratio is too small, the secondary expansion may take a long time. Furthermore, if the ZnO / urea ratio is small and the primary expansion conditions are inappropriate, the primary expansion ratio may become excessively large. As a result, the foam may crack during the primary or secondary expansion. Therefore, the ZnO / urea ratio is preferably greater than 1.20. The ZnO / urea ratio is more preferably 1.30 or greater.

[0047] On the other hand, if the ZnO / urea ratio is too high, the expansion rate during secondary foaming becomes excessively high, which may cause the foam to crack. Furthermore, the carbon black chain may be broken, resulting in an increase in surface resistance. Therefore, the ZnO / urea ratio is preferably less than 2.00. The ZnO / urea ratio is more preferably 1.80 or less.

[0048] [1.2.5. Content of other additives] The contents of other ingredients are not particularly limited, and the optimum contents can be selected depending on the purpose.

[0049] [2. Method for producing crosslinked polyolefin resin foam] The method for producing a crosslinked polyolefin resin foam according to the present invention comprises the steps of: A first step of preparing a raw material mixture containing a polyolefin resin, carbon black, a crosslinking agent, a foaming agent, and a foaming assistant; a second step of subjecting the raw material mixture to primary foaming to obtain a primary foam; a third step of subjecting the primary foam to secondary foaming to obtain a secondary foam; It is equipped with:

[0050] [2.1. 1st step] First, a raw material mixture containing a polyolefin resin, carbon black, a crosslinking agent, a foaming agent, and a foaming assistant is prepared (first step). After blending the raw materials in a predetermined ratio, the raw materials are kneaded. The method for kneading the raw materials is not particularly limited, and an optimal method can be selected depending on the purpose. Other points regarding the raw material mixture are as described above, so further explanation will be omitted.

[0051] [2.2. 2nd process] Next, the raw material mixture is subjected to primary foaming to obtain a primary foam (second step). Specifically, the second step includes: filling the raw material mixture into a primary mold; heating the raw material mixture in the primary mold at a temperature of 135°C or higher and 145°C or lower for 50 minutes or longer and 70 minutes or shorter while suppressing expansion of the raw material mixture; removing the heated raw material mixture from the primary mold to obtain the primary foam; Preferably, it contains:

[0052] [2.2.1. Primary expansion ratio] The "primary expansion ratio" refers to the ratio (=V1 / V0) of the volume of the primary foam (V1) to the volume (V0) of the raw material mixture before expansion (i.e., the volume of the molding space in the primary mold). "Primary mold" refers to a mold that can enclose the raw material mixture in a molding space and that can suppress the expansion of the raw material mixture that occurs when the raw material mixture is heated by pressurizing the raw material mixture.

[0053] The raw material mixture is filled into the molding space of the primary mold and heated under pressure, causing the polyolefin to be crosslinked by the crosslinking agent, while at the same time causing a portion of the foaming agent to decompose and generate gas. After heating at a specified temperature for a specified time, the raw material mixture is removed from the primary mold, causing the gas within the raw material mixture to expand and form a primary foam.

[0054] In this case, if the content of the foaming aid and / or the primary foaming conditions are inappropriate, the primary expansion ratio will be small. A small primary expansion ratio means that the amount of foam nuclei generated in the primary foam is small. If secondary expansion is performed in this state, the cells in the secondary foam tend to become coarse. Therefore, the primary expansion ratio is preferably 3.0 times or more. The primary expansion ratio is more preferably 3.2 times or more, 3.4 times or more, or 3.6 times or more.

[0055] On the other hand, if the primary expansion ratio is too large, it may be difficult to remove the primary foam from the primary mold. Furthermore, the foam may become "entangled" during secondary expansion. Therefore, the primary expansion ratio is preferably 5.0 times or less. The primary expansion ratio is more preferably 4.8 times or less, 4.6 times or less, or 4.4 times or less.

[0056] [2.2.2. Conditions for primary foaming] It is preferable to select the optimum conditions for the primary expansion depending on the content of the foaming aid and the desired primary expansion ratio. Generally, if the heating temperature during primary expansion (primary heating temperature) is too low, the primary expansion ratio may become excessively small. Therefore, the primary heating temperature is preferably 135°C or higher. On the other hand, if the primary heating temperature is too high, the primary expansion ratio may become excessively large. Therefore, the primary heating temperature is preferably 145°C or less.

[0057] Similarly, if the heating time during primary expansion (primary heating time) is too short, the primary expansion ratio may become excessively small. Therefore, the primary heating time is preferably 50 minutes or more. On the other hand, if the primary heating time is too long, the primary expansion ratio may become excessively large, so the primary heating time is preferably 70 minutes or less.

[0058] [2.3. Third step] Next, the primary foam is subjected to secondary foaming to obtain a secondary foam (third step). Specifically, the third step includes the following steps: placing the primary foam in a secondary mold; a step of heating the primary foam in the secondary mold at a temperature of 150°C or higher and 160°C or lower for 110 minutes or longer and 130 minutes or shorter without suppressing expansion of the primary foam; and removing the secondary foam from the secondary mold; Preferably, it contains:

[0059] [2.3.1. Secondary expansion ratio] The "secondary expansion ratio" refers to the ratio (=V2 / V1) of the volume of the secondary foam (V2) to the volume of the primary foam (V1). The term "secondary mold" refers to a mold that has a molding space larger than that of the primary foam and that allows the primary foam to expand freely within the molding space when heated.

[0060] When the primary foam is placed in the molding space of the secondary mold and heated again, the residual crosslinking agent further crosslinks the polyolefin resin, while the residual blowing agent decomposes, generating gas. As a result, the primary foam expands freely within the molding space of the secondary mold, resulting in a secondary foam with a shape roughly equivalent to the molding space of the secondary mold. The resulting secondary foam is then removed from the secondary mold and processed to the desired size for various uses.

[0061] In this case, if the secondary expansion conditions are inappropriate, the secondary expansion ratio will be small. If the secondary expansion ratio is too small, the density of the secondary foam may become excessively high. Furthermore, the hardness of the secondary foam may become excessively high, and the elongation may decrease. Therefore, the secondary expansion ratio is preferably 3.0 times or more. The secondary expansion ratio is more preferably 3.2 times or more, 3.4 times or more, or 3.6 times or more. On the other hand, if the secondary expansion ratio is too large, the foam may crack during secondary expansion. Therefore, the secondary expansion ratio is preferably 5.0 times or less. The secondary expansion ratio is more preferably 4.8 times or less, 4.6 times or less, or 4.4 times or less.

[0062] [2.3.2. Total expansion ratio] The "total expansion ratio" refers to the product (=V2 / V0) of the primary expansion ratio (V1 / V0) and the secondary expansion ratio (V2 / V1).

[0063] If the total expansion ratio is too small, the density of the foam becomes excessively high, which may result in excessively high hardness and reduced elongation. Therefore, the total expansion ratio is preferably 10.0 times or more. The total expansion ratio is more preferably 11.0 times or more, 12.0 times or more, or 13.0 times or more. On the other hand, if the total expansion ratio is too large, cracks may occur in the foam, the carbon black chain may be broken, and the surface resistivity may become excessively high. Therefore, the total expansion ratio is preferably 20.0 times or less. The total expansion ratio is more preferably 19.0 times or less, 18.0 times or less, or 17.0 times or less.

[0064] [2.3.3. Secondary foaming conditions] It is preferable to select the optimum conditions for the secondary expansion depending on the content of the foaming aid and the desired secondary expansion ratio. Generally, if the heating temperature during secondary expansion (secondary heating temperature) is too low, the secondary expansion ratio may become excessively small. Therefore, the secondary heating temperature is preferably 150°C or higher. On the other hand, if the secondary heating temperature is too high, the secondary expansion ratio may become excessively large. Therefore, the secondary heating temperature is preferably 160°C or less.

[0065] Similarly, if the heating time during secondary expansion (secondary heating time) is too short, the secondary expansion ratio may become excessively small. Therefore, the secondary heating time is preferably 110 minutes or more. On the other hand, if the secondary heating time is too long, the secondary expansion ratio may become excessively large, so the secondary heating time is preferably 130 minutes or less.

[0066] [3. Cross-linked polyolefin resin foam] The crosslinked polyolefin resin foam according to the present invention includes a foam made of a crosslinked polyolefin resin and carbon black dispersed within the foam. In other words, the crosslinked polyolefin resin foam according to the present invention is obtained by reacting and foaming a raw material mixture containing a polyolefin resin, carbon black, a crosslinking agent, a foaming agent, and a foaming aid.

[0067] 3.1. Method for producing crosslinked polyolefin resin foam The details of the method for producing the crosslinked polyolefin resin foam according to the present invention are as described above, and therefore will not be described again.

[0068] [3.2. Characteristics] [3.2.1. Number of cells] "Number of cells (cells / 25 mm)" refers to a value measured in accordance with JIS K6400-1. Specifically, "number of cells (cells / 25 cm)" refers to the number of cells that intersect with a line equivalent to a length of 25 mm when a line is drawn in any direction on the cross section of a cross-linked polyolefin resin foam.

[0069] When the crosslinked polyolefin resin foam according to the present invention is produced by the two-stage foaming method, the cells can be made finer (that is, the number of cells can be increased) by optimizing the type and content of the foaming aid.

[0070] If the number of cells is too small, the appearance may be poor. Therefore, the number of cells is preferably 80 cells / 25 cm or more. The number of cells is more preferably 85 cells / 25 cm or more, or 90 cells / 25 cm or more. On the other hand, if the number of cells is too large, it may result in poor appearance. Therefore, the number of cells is preferably 110 cells / 25 cm or less. The number of cells is more preferably 105 cells / 25 cm or less, or 100 cells / 25 cm or less.

[0071] [3.2.2. Surface Resistivity] "Surface resistance value" refers to a value measured in accordance with JIS K 6911. When the crosslinked polyolefin resin foam according to the present invention is produced by the two-stage foaming method, by optimizing the type and content of the foaming aid and the foaming conditions, a foam exhibiting low surface resistance can be obtained despite the short foaming time of the second foaming.

[0072] If the surface resistance is too small, discharge from the outside may be induced in the foam. Therefore, the surface resistance should be less than 1×10 2 The surface resistance is preferably more than 1×10 3 It is greater than or equal to Ω. On the other hand, if the surface resistance is too high, the antistatic properties may become insufficient. 12 The surface resistance is preferably less than 1×10 7 It is less than Ω.

[0073] 3.2.3. Apparent Density "Apparent density" refers to a value measured in accordance with JIS K 6767. When the crosslinked polyolefin resin foam according to the present invention is produced by the two-stage expansion method, a foam with a low density can be obtained by optimizing the total expansion ratio.

[0074] A small apparent density indicates a large expansion ratio. If the apparent density is too small, cracks may occur in the foam or the carbon black chain may be broken. Therefore, the apparent density should be less than 40 kg / m 3 The apparent density is preferably 45 kg / m or more. 3 or more, or 50 kg / m 3 That's all. On the other hand, if the apparent density is too high, the foam may become too hard and the elongation may become too low. 3 The apparent density is preferably 75 kg / m or less. 3 or less than 70 kg / m 3 The following is the result.

[0075] [4. Effect] When carbon black with a well-developed structure and a high specific surface area is used as conductive particles, a foam exhibiting high conductivity can be obtained with a small amount of addition. However, high-specific surface area carbon black has poor dispersibility and is prone to molding defects. On the other hand, when carbon black with a low specific surface area is used as conductive particles, moldability is relatively good, but the conductivity of the foam is likely to decrease. In contrast, when carbon black with a low specific surface area is used as the conductive particles, if the amount of carbon black added is increased compared to that of conventional foams, a crosslinked polyolefin resin foam that combines antistatic properties and good moldability can be obtained. [Example]

[0076] (Examples 1 to 3, Comparative Examples 1 to 4) 1. Sample Preparation The foam raw materials used were as follows: Polyolefin resin: Low-density polyethylene (LDPE), product number: Mirason (registered trademark) 403P, manufactured by Dow Mitsui Polychemicals Co., Ltd. Carbon Black 1: BET specific surface area: 65m 2 / g, Product Number: Ensaco (registered trademark) 250G, manufactured by Imerys GC Japan Co., Ltd.

[0077] Carbon Black 2: BET specific surface area: 800m 2 / g, Product code: Ketjenblack (registered trademark) EC300J, manufactured by Lion Corporation Foaming agent: Azodicarbonamide (ADCA), product number: Azodicarbonamide, manufactured by Eiwa Chemical Industry Co., Ltd. Crosslinking agent: dicumyl peroxide (DCP), product number: Perkadox (registered trademark) BC-FF, manufactured by Kayaku Akzo Co., Ltd.

[0078] Foaming aid 1: Urea, product number: Cellpaste (registered trademark) 101, manufactured by Eiwa Chemical Industry Co., Ltd. Foaming agent 2: Zinc oxide, product number: Zinc oxide type 1, manufactured by Hakusui Tech Co., Ltd. Lubricant: Sorbitan stearate, product number: Rikemal (registered trademark) S-300W, manufactured by Riken Vitamin Co., Ltd.

[0079] The above raw materials were blended in a predetermined ratio. The raw material blend was kneaded using a 1 L kneader at 100 to 130°C for 15 to 20 minutes. The resulting raw material mixture was then further kneaded using a 10-inch mixing roll at 100°C for 10 to 15 minutes.

[0080] The kneaded raw material mixture was filled into the molding space of the primary mold. The dimensions of the molding space were 210 mm x 210 mm x t29 mm. Next, the raw material mixture was subjected to primary heating under pressure. The primary heating temperature was 140°C, and the primary heating time was 60 minutes. After heating, the raw material mixture was removed from the primary mold (i.e., the pressure was released), and the raw material mixture was subjected to primary foaming.

[0081] Next, the primary foam was placed in the molding space of the secondary mold. The size of the molding space was 500mm x 500mm x 70mm (t). The primary foam was then subjected to secondary heating under normal pressure. The secondary heating temperature was 160°C, and the secondary heating time was 2 hours. After heating, the secondary foam was removed from the secondary mold.

[0082] 2. Test Method [2.1. Size of primary foam] The sizes of the primary foam in the horizontal direction (x-axis direction and y-axis direction) were measured, and the average values ​​were calculated.

[0083] [2.2. Number of cells] The number of cells (cells / 25 mm) was measured in accordance with JIS K6400-1. Regarding the number of cells, "○" indicates that the number of cells is 90 or more per 25 mm. "△" indicates that the number of cells is between 80 / 25mm and 90 / 25mm. "X" indicates that the number of cells is less than 80 / 25mm.

[0084] [2.3. Moldability of secondary foam] The secondary foam was evaluated for cracks. Regarding moldability, "○" indicates that the scratch depth is 1mm or less. "△" indicates that the scratch depth is more than 1mm and less than 5mm. "X" indicates that the scratch depth is greater than 5 mm.

[0085] [2.4. Size of secondary foam] The sizes of the secondary foams in the horizontal directions (x-axis direction and y-axis direction) were measured, and the average values ​​were calculated. Regarding size, "○" indicates that the area of ​​the secondary foam is 100% of the area of ​​the secondary mold. "△" indicates that the area of ​​the secondary foam is 90% or more but less than 100% of the area of ​​the secondary mold. "X" indicates that the area of ​​the secondary foam is less than 90% of the area of ​​the secondary mold.

[0086] 2.5. Apparent Density The apparent density of the secondary foam was measured in accordance with JIS K 6767.

[0087] [2.6. Surface Resistivity] The surface resistance was measured in accordance with JIS K 6911. Regarding surface resistance, "○" indicates a surface resistance of 1×10 7 represents that the value is less than Ω. "△" indicates a surface resistance of 1×10 7 Ω or more 1×10 12 represents that the value is less than Ω. "×" indicates a surface resistance of 1×10 12 Indicates that the resistance is Ω or greater.

[0088] [3. Results] The results are shown in Table 1. Table 1 also shows the raw material composition of each sample. From Table 1, the following can be seen:

[0089] (1) Comparative Example 1 has a surface resistance of 2.1 × 10 8 Ω. This is thought to be due to the low content of carbon black 2. Furthermore, in Comparative Example 1, cracks occurred in the secondary foam. This is thought to be due to the fact that the specific surface area of ​​carbon black 2 was too high, resulting in insufficient dispersion of carbon black 2 in the raw material mixture. (2) Comparative Example 2 is an example in which the contents of carbon black 2 and the foaming agent are increased compared to Comparative Example 1. Comparative Example 2 has a surface resistance of 3.2 × 10 4 The hardness was Ω, and no cracks occurred in the secondary foam. However, the size of the secondary foam was slightly smaller in Comparative Example 2. This is thought to be due to the relatively excessive content of Carbon Black 2.

[0090] (3) Comparative Example 3 has a surface resistance of 9.0 × 10 14 This is thought to be due to the small specific surface area of ​​Carbon Black 1 and the low content of Carbon Black 1. (4) Comparative Example 4 has a surface resistance of 3.4 × 10 3 Ω. However, in Comparative Example 4, cracks occurred in the secondary foam and the size of the secondary foam became slightly smaller. This is thought to be because the content of Carbon Black 1 was relatively excessive.

[0091] (5) In Examples 1 to 3, the surface resistance was 1×10 7 The resistance was less than Ω. Furthermore, no cracks were observed in the secondary foam, and the size of the secondary foam was 100%. Table 1 shows that when carbon black 1, which has a low specific surface area, is used as the conductive particles, if the amount of carbon black 1 added is relatively large, the surface resistance can be maintained within an appropriate range without reducing moldability.

[0092] [Table 1]

[0093] Although the embodiments of the present invention have been described in detail above, the present invention is not limited to the above-described embodiments, and various modifications are possible within the scope of the gist of the present invention. [Industrial Applicability]

[0094] The crosslinked polyolefin resin foam according to the present invention can be used as packaging materials, cushioning materials, trays, containers, etc. for various electronic parts (for example, IC chips) where destruction by static electricity is a problem.

Claims

1. a cross-linked polyolefin resin; Carbon black and Including, The carbon black has a BET specific surface area of ​​10 m 2 / g or more 100m 2 / g or less, The content of the carbon black is 22.0 parts by mass or more and 32.0 parts by mass or less. Cross-linked polyolefin resin foam. Here, the "carbon black content" refers to the mass of the carbon black when the total mass of the polyolefin resin before crosslinking is taken as 100.

2. 2. The crosslinked polyolefin resin foam according to claim 1, wherein the number of cells is 80 cells / 25 cm or more and 110 cells / 25 cm or less.

3. Surface resistance is 1 x 10 2 Ω super 1×10 12 The crosslinked polyolefin resin foam according to claim 1, having a modulus of elasticity of less than Ω.

4. Apparent density is 40 kg / m 3 More than 80kg / m 3 The crosslinked polyolefin resin foam according to claim 1, wherein the crosslinked polyolefin resin foam is:

5. The crosslinked polyolefin resin foam according to claim 1 , wherein the polyolefin resin is polyethylene.

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