Conductive polyolefin foam and method for producing the same

Abstract

To provide a conductive polyolefin foam capable of producing a conductive polyolefin foam having fine cells with high production efficiency, and to provide a method for producing the same.SOLUTION: A method for producing a conductive polyolefin foam includes: a first step of preparing a raw material mixture containing a polyolefin, a conductive carbon, a crosslinking agent, a foaming agent, and a foaming aid; a second step of primary-foaming the raw material mixture to obtain a primary foam; and a third step of secondary-foaming the primary foam to obtain a secondary foam. The first step contains ZnO and urea as the foaming aid and includes a step of preparing the raw material mixture whose ZnO / urea ratio (mass ratio) is more than 1.20 and less than 2.00. The conductive polyolefin foam is obtained by such a method and a cell diameter is 100 cells / 25 mm or more and 160 cells / 25 mm or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a conductive polyolefin foam and a method for producing the same, and more particularly to a method for producing a conductive polyolefin foam having fine cells with high production efficiency, and to a conductive polyolefin foam obtained by such a method. [Background technology]

[0002] Polyolefin foam is lightweight and has excellent heat insulation, water resistance, chemical resistance, and shock absorption properties, making it widely used in automotive interior parts, building insulation, joint fillers, pipe covers, household goods, and health and sports equipment. Furthermore, while polyolefin foam is an insulator, adding conductive particles to it yields a conductive polyolefin foam. Because conductive polyolefin foam is less prone to static charge, it is used in packaging materials, cushioning materials, trays, and containers for various electronic components (e.g., IC chips) where electrostatic discharge damage is a concern.

[0003] Two methods are known for producing conductive polyolefin foam: the one-stage foaming method and the two-stage foaming method. The one-stage foaming method involves heating and foaming the raw material mixture in a single step. The two-stage foaming method is a method in which the heating and foaming of the raw material mixture are carried out in two stages. Compared to the single-stage foaming method, the two-stage foaming method is less likely to cause the foam to crack during foaming, and is therefore used as a method for producing foams with a high foaming ratio.

[0004] Various proposals have been made regarding the production method of polyolefin foam using such a two-stage foaming method. For example, Patent Document 1 contains: (a) When a raw material mixture containing low-density polyethylene, Ketjenblack (registered trademark), azodicarbonamide, urea-based foaming agent, dicumyl peroxide, and calcium bicarbonate is foamed in two stages, (b) The height (H2) of the molding space of the mold used for the second stage of foaming (secondary mold) shall be 100 mm or more, and H2 shall be 3.1 times or more and 3.8 times or less the height (H1) of the molding space of the mold used for the first stage of foaming (primary mold). A method for producing conductive polyolefin foam is disclosed.

[0005] The document states: (A) When the foaming ratio is fixed, simply increasing H2 to 100 mm or more in order to obtain a thick conductive polyolefin foam may result in cracks or defects on the surface of the conductive polyolefin foam, or holes in the interior of the conductive polyolefin foam, and (B) When performing two-stage foaming, if H2 is set to 100 mm or more and H2 / H1 is set to 3.1 or more and 3.8 or less, the occurrence of cracks and defects on the surface of the conductive polyolefin foam, and the occurrence of holes inside the conductive polyolefin foam can be suppressed. It is stated.

[0006] Patent Document 2 contains: (a) When a raw material mixture containing low-density polyethylene, Ketjenblack (registered trademark), calcium bicarbonate, azodicarbonamide, dicumyl peroxide, zinc stearate, and a urea-based foaming agent is foamed in two stages, (b) The rate of increase in the coefficient of thermal expansion during the second stage of foaming (m) shall be between 0.5 and 2.0 times the rate of increase in the coefficient of thermal expansion during the first stage of foaming (n). A method for producing conductive polyolefin foam is disclosed. The same document states that, using this method, the volume resistivity is 4.8 × 10⁻⁶. 5 ~2.6×10 7 It is stated that a conductive polyolefin foam with a conductivity of Ω·cm can be obtained.

[0007] Patent Document 3 contains: (a) When a raw material mixture containing ethylene-vinyl acetate copolymer, furnace-type carbon black, azodicarbonamide, and dicumyl peroxide is subjected to two-stage foaming, (b) Fill a mold heated to 158°C with the raw material mixture and heat under pressure for 25 minutes to form a foamed crosslinked sheet. (c) Heat the foamed crosslinked sheet in a salt bath at 175°C for 40 minutes. A method for producing conductive crosslinked polyolefin foam is disclosed.

[0008] The document states: (A) Using this method, the rate of volume increase during foaming is slowed, and the chain of carbon particles is less likely to break, so the volume resistivity becomes 4 × 10 5 The advantage is that a conductive foam with a density of Ωcm can be obtained, and (B) When zinc oxide is further added to the raw material mixture as a foaming aid, and the conditions for primary foaming are 120°C × 5 minutes, and the conditions for secondary foaming are 160°C × 25 minutes + 170°C × 30 minutes, the volume resistivity of the foam becomes infinite. It is stated.

[0009] When producing conductive polyolefin foam using a two-stage foaming method, rapid secondary foaming can cause the polyolefin to fail to adapt to the change in shape, resulting in the foam cracking. Furthermore, the chain of carbon particles within the foam may be interrupted, increasing the volume resistivity (see Patent Document 3). Therefore, when producing conductive polyolefin foam using a two-stage foaming method, as described in Patent Document 1, only urea, which has a slow reaction rate, was used as a foaming aid, and secondary foaming was carried out over a long period (for example, about 4 hours).

[0010] However, conventional methods have very poor production efficiency because secondary foaming takes a long time. Also, if urea is used alone as a foaming aid, the foaming ratio of the primary foam becomes low, which can result in fewer foaming nuclei being formed within the primary foam. If secondary foaming is performed in this state, a large amount of gas flows into a small number of cells, which can result in coarse cells. [[PRIOR ART DOCUMENT] [[PATENT DOCUMENTS]

[0011] [[Patent Document 1] Japanese Unexamined Patent Application Publication No. 2004-244559 [[Patent Document 2] Japanese Patent No. 4159830 [[Patent Document 3] Japanese Examined Patent Publication No. 02-029095 [[Summary of the Invention] [[Problem to be Solved by the Invention]

[0012] An object of the present invention is to provide a method for producing a conductive polyolefin foam, which can produce a conductive polyolefin foam having fine cells with high production efficiency. Another object of the present invention is to provide a conductive polyolefin foam obtained by such a method. [[Means for Solving the Problem]

[0013] In order to solve the above problem, the method for producing a conductive polyolefin foam according to the present invention comprises: a first step of preparing a raw material mixture containing a polyolefin, conductive carbon, a crosslinking agent, a foaming agent, and a foaming aid; a second step of subjecting the raw material mixture to primary foaming to obtain a primary foam; and a third step of subjecting the primary foam to secondary foaming to obtain a secondary foam , wherein the first step comprises a step of preparing the raw material mixture that contains ZnO and urea as the foaming aid, and has a ZnO / urea ratio (mass ratio) of more than 1.20 and less than 2.00 .

[0014] The conductive polyolefin foam according to the present invention is It is obtained by foaming a raw material mixture containing polyolefin, conductive carbon, a crosslinking agent, a foaming agent, and a foaming aid. The foaming agent comprises ZnO and urea. The cell diameter is between 100 cells / 25 mm and 160 cells / 25 mm. Conductive polyolefin foam is Surface resistivity ρs is 1.0 × 10⁻⁶ 5 Ω / sq. or more 4.0×10 5 It is less than or equal to Ω / sq. Apparent density is 40 kg / m³ 3 More than 70kg / m 3 The following is I prefer that. [Effects of the Invention]

[0015] ZnO has the effect of rapidly decomposing the blowing agent at high temperatures. On the other hand, urea has the effect of gradually promoting the decomposition of the blowing agent from low temperatures. Therefore, by using ZnO and urea in combination as blowing aids and optimizing the amounts of ZnO and urea added and the primary blowing conditions, a primary foam containing a relatively large amount of blowing nuclei can be obtained. Subsequently, when the obtained primary foam is subjected to secondary blowing, optimizing the secondary blowing conditions can reduce the time required for secondary blowing to about half of the conventional time (about 2 hours) without significantly increasing the surface resistivity ρs. [Modes for carrying out the invention]

[0016] One embodiment of the present invention will be described in detail below. [1. Method for producing conductive polyolefin foam] The method for producing a conductive polyolefin foam according to the present invention is: A first step involves preparing a raw material mixture containing polyolefin, conductive carbon, a crosslinking agent, a foaming agent, and a foaming aid. A second step involves primary foaming the aforementioned raw material mixture to obtain a primary foam, A third step involves secondary foaming of the primary foam to obtain a secondary foam, It is equipped with.

[0017] [1.1. 1st step] First, a raw material mixture containing polyolefin, conductive carbon, a crosslinking agent, a foaming agent, and a foaming aid is prepared (Step 1). The raw material mixture may consist only of polyolefin, conductive carbon, crosslinking agent, blowing agent, and blowing aid, or it may also contain other additives.

[0018] In the present invention, the above first step is The foaming agent includes ZnO and urea. The ZnO / urea ratio (by mass) is greater than 1.20 and less than 2.00. This method includes a step of preparing the aforementioned raw material mixture. This differs from conventional methods.

[0019] Also, the previous step 1 is, The ZnO content is greater than 0.10 phr and less than 0.18 phr, The urea content is greater than 0.07 phr and less than 0.20 phr. Preferably, the process includes a step of preparing the aforementioned raw material mixture.

[0020] [1.1.1. Main components] [A. Polyolefin] In the present invention, the type of polyolefin is not particularly limited, and the most suitable material can be used depending on the purpose.

[0021] Examples of polyolefins 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 (ester content: 45 mol% or less), (c) Chlorinated products as described in (a) or (b) above (chlorine content: 60 mass% or less) These are some examples. The raw material mixture may contain one of these polyolefins, or it may contain two or more of them.

[0022] [B. Conductive carbon] Conductive carbon is an additive used to impart desired conductivity to polyolefin foam. In this invention, the type of conductive carbon is not particularly limited, as long as it performs the functions described above. Examples of conductive carbon include conductive carbon blacks such as acetylene black, furnace black, and Ketjenblack (registered trademark). The raw material mixture may contain one of these conductive carbons, or it may contain two or more of them.

[0023] [C. Crosslinking agent] A crosslinking agent is an additive that increases the viscosity of the raw material mixture by crosslinking polyolefins during foaming, thereby retaining bubbles within the raw material mixture. In this invention, the type of crosslinking agent is not particularly limited, as long as it performs this function. Examples of crosslinking agents include dicumyl peroxide, 2,5-dimethyl-2,5-bis-(t-butylperoxy)hexane, 1,3-Bis-(t-butylperoxyisopropyl)benzene Examples of organic peroxides include the following. The raw material mixture may contain one of these crosslinking agents, or it may contain two or more of them.

[0024] [D. Foaming agent] A foaming agent is an additive that decomposes upon heating to generate gas. In this invention, the type of foaming agent is not particularly limited, as long as it performs the function described above.

[0025] Examples of foaming agents include azodicarbonamide, 2,2'-Azobisisobutyronitrile, diazoaminobenzene, Benzenesulfonyl hydrazide, Benzene-1,3-sulfonyl hydrazide, Diphenyl oxide-4,4'-disulfonyl hydrazide, 4,4'-Oxybis(benzenesulfonylhydrazide), p-toluenesulfonyl hydrazide, N,N'-dinitrosopentamethylenetetramine, N,N'-dinitroso-N,N'-dimethylphthalamide, terephthalazide, pt-butylbenzide, sodium bicarbonate, ammonium bicarbonate These are some examples. The raw material mixture may contain one of these blowing agents, or it may contain two or more of them.

[0026] [E. Foaming agent] A foaming agent is an additive used to promote the decomposition of the foaming agent. In this invention, a mixture of ZnO and urea is used as the foaming agent. This is a difference from conventional methods. The foaming agent may contain only ZnO and urea, or it may contain foaming agents other than ZnO and urea. Examples of foaming agents other than ZnO and urea include fatty acid metal salts such as zinc stearate, metal oxides such as magnesium oxide, and alcohols with 1 to 4 carbon atoms.

[0027] [1.1.2. Minor Components] The raw material mixture may contain only polyolefin, conductive carbon, crosslinking agent, blowing agent, and blowing aid, or it may contain other components. Specific examples of other components include the following: The raw material mixture may contain one of the following minor components, or it may contain two or more of the following:

[0028] [A. Nucleoforming agents] A nucleating agent is an additive that functions as a starting point for foam nuclei during the foam formation process. Adding a nucleating agent to the raw material mixture facilitates the formation of foam nuclei at the interface between the polyolefin and the nucleating agent, allowing for the formation of a relatively large number of foam nuclei within the primary foam. Furthermore, this enables the miniaturization of cells within the secondary foam. In the present invention, the type of nucleating agent is not particularly limited, as long as it performs the functions described above. Examples of nucleating agents include talc, light calcium carbonate, heavy calcium carbonate, and calcium carbonate.

[0029] [B. Lubricants] A lubricant is an additive that improves the sliding motion between the mixing equipment and the raw material mixture, or between the particles of the raw material, during the mixing of the raw material mixture. Adding a lubricant to the raw material mixture improves the sliding motion between the mixing equipment and the raw material mixture, thereby suppressing the sticking of the raw material mixture to the mixing equipment. In the present invention, the type of lubricant is not particularly limited, as long as it performs the functions described above. Examples of lubricants include sorbitan stearate, fatty acid-based lubricants, and fatty acid amide-based lubricants.

[0030] [1.1.3. Content of main components] [A. Content of the i-th polyolefin] When a raw material mixture contains n types (n≧2) of polyolefins, "content of the i-th polyolefin (1≦i≦n)" refers to the ratio of the mass of the i-th polyolefin to the total mass of polyolefins.

[0031] When the raw material mixture contains two or more polyolefins, the content of the i-th polyolefin is not particularly limited, and an optimal value can be selected depending on the purpose. For example, when extruding a raw material mixture, if the mixture contains only one type of polyolefin (first polyolefin), the viscosity of the raw material mixture may increase excessively, making extrusion difficult. In such cases, adding another polyolefin (second polyolefin) that has the effect of reducing the viscosity of the raw material mixture can facilitate extrusion.

[0032] In this case, it is preferable to select an optimal content of the second polyolefin depending on the types of the first and second polyolefins. For example, if the first polyolefin is low-density polyethylene (LDPE) and the second polyolefin is ethylene-vinyl acetate copolymer (EVA), the EVA content is preferably 5.0 mass% to 25.0 mass%. More preferably, the EVA content is 10.0 mass% to 15.0 mass%.

[0033] [B. Content of conductive carbon] "Conductive carbon content" refers to the mass of conductive carbon (phr) relative to the mass of polyolefin, which is set to 100.

[0034] If the conductive carbon content is too low, the surface resistivity of the conductive polyolefin foam may increase. Therefore, a conductive carbon content of 10.5 phr or more is preferred. More preferably, the content is 11.0 phr or more. On the other hand, if the conductive carbon content is excessive, the fluidity of the raw material mixture decreases, which may cause the foam to crack during foaming. Therefore, the conductive carbon content is preferably 14.0 phr or less. More preferably, the content is 13.5 phr or less, or 13.0 phr or less.

[0035] [C. Content of crosslinking agent] "Crosslinking agent content" refers to the mass of the crosslinking agent (phr) relative to the mass of the polyolefin, which is set to 100.

[0036] If the crosslinking agent content is too low, the viscosity of the raw material mixture will not increase during foaming, and air bubbles will easily escape from the raw material mixture. Therefore, the crosslinking agent content is preferably 0.60 phr or more. More preferably, the content is 0.70 phr or more. On the other hand, if the crosslinking agent content is excessive, the viscosity of the raw material mixture may increase excessively during foaming, potentially causing the foam to crack. Therefore, the crosslinking agent content is preferably 0.90 phr or less. More preferably, the content is 0.85 phr or less, or 0.80 phr or less.

[0037] [D. Foaming agent content] "Foaming agent content" refers to the mass of the foaming agent (phr) relative to the mass of the polyolefin, which is set to 100.

[0038] If the amount of blowing agent is too low, the amount of bubbles generated will be small, and the desired foaming ratio may not be achieved. Therefore, the amount of blowing agent is preferably 5.0 phr or more. More preferably, the amount is 5.5 phr or more, or 6.0 phr or more. On the other hand, if the foaming agent content is excessive, the amount of bubbles generated will be excessive, and the foaming ratio may increase excessively. As a result, cracks may occur in the foam, the chain of conductive carbon may be broken, and the surface resistivity may increase. Therefore, the foaming agent content is preferably 15.0 phr or less. More preferably, the content is 13.0 phr or less, or 12.0 phr or less.

[0039] [E. Content of foaming agent] [E.1. ZnO / urea ratio] The "ZnO / urea ratio" refers to the ratio of the mass of ZnO contained in a raw material mixture to the mass of urea contained in that mixture.

[0040] Urea promotes the decomposition of foaming agents even at low temperatures, but its decomposition rate is relatively slow even at high temperatures. Therefore, using urea alone as a foaming aid makes it easy to obtain highly conductive and crack-free polyolefin foam, but it requires a long time to foam. On the other hand, using ZnO alone as a foaming aid can shorten the foaming time, but the high foaming rate can cause cracking in the polyolefin foam, disrupting the conductive carbon chain and increasing the surface resistivity. In contrast, using ZnO and urea together as foaming aids allows for the production of highly conductive and crack-free polyolefin foam in a short foaming time.

[0041] If the ZnO / urea ratio becomes too low, secondary foaming may take a long time. Also, if the primary foaming conditions are inappropriate when the ZnO / urea ratio is low, the primary foaming ratio may become excessively high. As a result, the foam may crack during primary or secondary foaming. Therefore, the ZnO / urea ratio needs to be greater than 1.20. Preferably, the ZnO / urea ratio is 1.25 or higher, or 1.30 or higher.

[0042] On the other hand, if the ZnO / urea ratio becomes too high, the expansion rate during secondary foaming will become excessively high, which may cause the foam to crack. Also, the conductive carbon chain may be interrupted, and the surface resistivity may increase. Therefore, the ZnO / urea ratio needs to be less than 2.00. Preferably, the ZnO / urea ratio is 1.90 or less, 1.80 or less, or 1.70 or less.

[0043] [E.2. ZnO content] "ZnO content" refers to the mass of ZnO (phr) relative to the mass of polyolefin, which is set to 100.

[0044] ZnO is highly effective in accelerating the decomposition rate of the foaming agent at high temperatures. Therefore, if the ZnO content is too low, secondary foaming may take a long time. Also, if the ZnO content is too low, the primary foaming ratio may become excessively small. As a result, the number of foaming nuclei generated decreases, and the cells contained in the foam after secondary foaming may become coarse. For this reason, a ZnO content of more than 0.10 phr is preferable. More preferably, the content is 0.11 phr or more, or 0.12 phr or more.

[0045] On the other hand, if the ZnO content is excessive, the secondary foaming ratio may become excessively large, or the volume increase rate during secondary foaming may become excessively large. As a result, the secondary foam may crack during secondary foaming. Therefore, the ZnO content is preferably less than 0.18 phr. More preferably, the content is 0.17 phr or less, or 0.16 phr or less.

[0046] [E.3. Urea content] "Urea content" refers to the mass of urea (phr) relative to the mass of polyolefin, which is set to 100.

[0047] If the urea content is too low, the amount of foam nuclei generated during primary foaming will be reduced, and the primary foaming ratio may become excessively small. As a result, the cells contained in the foam may become coarse after secondary foaming. Therefore, a urea content of more than 0.07 phr is preferable. More preferably, the content is 0.08 phr or higher, or 0.09 phr or higher.

[0048] On the other hand, if the urea content is excessive, the primary foaming ratio may become excessively large. When the dimensions of the secondary mold are predetermined, if the primary foaming ratio becomes excessively large, the foam that expands horizontally during secondary foaming will collide with the side wall of the secondary mold, and the foam, having nowhere else to go, will expand vertically along the side wall of the secondary mold. As a result, so-called "entrapment" may occur, in which only the periphery of the foam bulges in an arc shape. Therefore, the urea content is preferably less than 0.20 phr. More preferably, the content is 0.18 phr or less, or 0.16 phr or less.

[0049] [1.1.4. Content of minor components] The amount of auxiliary components is not particularly limited, and the optimal amount can be selected according to the purpose.

[0050] [1.1.5. Preparation of Raw Material Mixture] After mixing each ingredient in the predetermined ratio, the ingredients are kneaded. The method of kneading the ingredients is not particularly limited, and the most suitable method can be selected according to the purpose.

[0051] [1.2. 2nd process] Next, the raw material mixture is subjected to primary foaming to obtain a primary foam (second step). The second step preferably includes a step of primary foaming the raw material mixture so that the primary foaming ratio is 3.0 times or more and 5.0 times or less.

[0052] The second step is, The process of filling the aforementioned raw material mixture into a primary mold, A step of heating the raw material mixture at a temperature of 135°C to 145°C for 50 minutes to 70 minutes while suppressing the expansion of the raw material mixture in the primary mold, The process involves removing the heated raw material mixture from the primary mold to obtain the primary foam, and It is preferable that it contains [the specified ingredient].

[0053] [1.2.1. Primary foaming ratio] "Primary foaming ratio" refers to the ratio (=V1 / V0) of the volume of the primary foam to the volume of the raw material mixture before foaming (V0) (i.e., the volume of the molding space in the primary mold). A "primary mold" refers to a mold that can seal the raw material mixture within the molding space and suppress the expansion of the raw material mixture that occurs when the raw material mixture is heated by pressurizing the mixture.

[0054] When the raw material mixture is filled into the molding space of the primary mold and heated under pressure, the polyolefin is crosslinked by the crosslinking agent, and at the same time, some of the foaming agent decomposes, generating gas. After heating at a predetermined temperature for a predetermined time, when the raw material mixture is removed from the primary mold, the gas inside the raw material mixture expands, forming a primary foam.

[0055] In this case, if the foaming agent content and / or primary foaming conditions are inappropriate, the primary foaming ratio will be small. A small primary foaming ratio means that the amount of foaming nuclei generated in the primary foam is small. If secondary foaming occurs under these conditions, the cells in the secondary foam tend to become coarser. Therefore, a primary foaming ratio of 3.0 times or higher is preferable. A primary foaming ratio of 3.2 times or higher, 3.4 times or higher, or 3.6 times or higher is even more preferable.

[0056] On the other hand, if the primary foaming ratio becomes too high, it may become difficult to remove the primary foam from the primary mold. Also, "entrapment" of the foam may occur during secondary foaming. Therefore, a primary foaming ratio of 5.0 times or less is preferable. A primary foaming ratio of 4.8 times or less, 4.6 times or less, or 4.4 times or less is even more preferable.

[0057] [1.2.2. Conditions for Primary Foaming] The conditions for primary foaming should preferably be selected according to the content of the foaming agent and the desired primary foaming ratio. Generally, if the heating temperature during primary foaming (primary heating temperature) is too low, the primary foaming ratio may become excessively small. Therefore, a primary heating temperature of 135°C or higher is preferable. On the other hand, if the primary heating temperature becomes too high, the primary foaming ratio may become excessively large. Therefore, the primary heating temperature is preferably 145°C or lower.

[0058] Similarly, if the heating time during primary foaming (primary heating time) is too short, the primary foaming ratio may become excessively small. Therefore, a primary heating time of 50 minutes or more is preferable. On the other hand, if the primary heating time is too long, the primary foaming ratio may become excessively large. Therefore, the primary heating time is preferably 70 minutes or less.

[0059] [1.3. Third step] Next, the primary foam is subjected to secondary foaming to obtain a secondary foam (third step). The third step preferably includes a step of causing the primary foam to undergo secondary foaming such that the secondary foaming ratio is 3.0 times or more and 5.0 times or less, and the total foaming ratio (= primary foaming ratio × secondary foaming ratio) is 10.0 times or more and 20.0 times or less.

[0060] The third step is, The process of placing the primary foam into the secondary mold, The process involves heating the primary foam in the secondary mold without suppressing its expansion, at a temperature of 150°C to 160°C for 110 to 130 minutes, to obtain the secondary foam. The process of removing the secondary foam from the secondary mold and It is preferable that it contains [the specified ingredient].

[0061] [1.3.1. Secondary foaming ratio] "Secondary foaming ratio" refers to the ratio (=V2 / V1) of the volume of the secondary foam (V2) to the volume of the primary foam (V1). A "secondary mold" is a mold that has a larger molding space than the primary foam and allows the primary foam to expand freely within the molding space when heated.

[0062] When the primary foam is placed in the molding space of the secondary mold and heated again, the polyolefin is further crosslinked by the remaining crosslinking agent, and at the same time, the remaining blowing agent decomposes, generating gas. As a result, the primary foam expands freely within the molding space of the secondary mold, and a secondary foam with a shape almost identical to the molding space of the secondary mold is obtained. After being removed from the secondary mold, the obtained secondary foam is processed to a predetermined size and used for various applications.

[0063] In this case, if the secondary foaming conditions are inappropriate, the secondary foaming ratio will be small. If the secondary foaming ratio is too small, the density of the secondary foam may become excessively high. Also, the hardness of the secondary foam may become excessively high, and its elongation may decrease. Therefore, a secondary foaming ratio of 3.0 times or higher is preferable. More preferably, the secondary foaming ratio is 3.2 times or higher, 3.4 times or higher, or 3.6 times or higher. On the other hand, if the secondary foaming ratio becomes too high, the foam may crack during secondary foaming. Therefore, a secondary foaming ratio of 5.0 times or less is preferable. More preferably, the secondary foaming ratio is 4.8 times or less, 4.6 times or less, or 4.4 times or less.

[0064] [1.3.2. Total foaming ratio] "Total foaming ratio" refers to the product of the primary foaming ratio (V1 / V0) and the secondary foaming ratio (V2 / V1) (=V2 / V0).

[0065] If the total expansion ratio becomes too low, the density of the foam becomes excessively high, which can result in excessive hardness and reduced elongation. Therefore, a total expansion ratio of 10.0 times or higher is preferable. More preferably, the total expansion ratio is 11.0 times or higher, 12.0 times or higher, or 13.0 times or higher. On the other hand, if the total foaming ratio becomes too high, cracks may occur in the foam, the chain of conductive carbon may be interrupted, and the surface resistivity may become excessively high. Therefore, the total foaming ratio is preferably 20.0 times or less. More preferably, the total foaming ratio is 19.0 times or less, 18.0 times or less, or 17.0 times or less.

[0066] [1.3.3. Conditions for secondary foaming] It is preferable to select optimal conditions for secondary foaming according to the content of the foaming aid and the target secondary expansion ratio. In general, if the heating temperature during secondary foaming (secondary heating temperature) is too low, the secondary expansion ratio may become excessively low. Therefore, the secondary heating temperature is preferably 150°C or higher. On the other hand, if the secondary heating temperature is excessively high, the secondary expansion ratio may become excessively high. Therefore, the secondary heating temperature is preferably 160°C or lower.

[0067] Similarly, if the heating time during secondary foaming (secondary heating time) is too short, the secondary expansion ratio may become excessively low. Therefore, the secondary heating time is preferably 110 minutes or longer. On the other hand, if the secondary heating time is excessively long, the secondary expansion ratio may become excessively high. Therefore, the secondary heating time is preferably 130 minutes or shorter.

[0068] [2. Conductive polyolefin foam] The conductive polyolefin foam according to the present invention is obtained by foaming a raw material mixture containing a polyolefin, conductive carbon, a crosslinking agent, a foaming agent, and a foaming aid, wherein the foaming aid contains ZnO and urea, and has a cell count of 100 cells / 25 mm to 160 cells / 25 mm.

[0069] The conductive polyolefin foam is wherein the foaming aid contains ZnO and urea, has a cell count of 100 cells / 25 mm to 160 cells / 25 mm, has a surface resistivity ρs of 1.0×10 5 Ω / sq. or more and 4.0×10 5 Ω / sq. or less, has an apparent density of 40 kg / m 3 or more and 70 kg / m 3 or less , which is preferable.

[0070] [2.1. Method for producing conductive polyolefin foam] The conductive polyolefin foam according to the present invention is obtained by foaming a raw material mixture containing polyolefin, conductive carbon, a crosslinking agent, a foaming agent, and a foaming aid. The foaming aid also contains ZnO and urea. Details of the method for producing the polyolefin foam according to the present invention are as described above and will therefore be omitted.

[0071] [2.2. Characteristics] [2.2.1. Cell diameter] "Cell diameter (cells / 25mm)" refers to the number of cells that intersect a straight line of 25mm in length when a straight line is drawn in any direction on the cross-section of a conductive polyolefin foam.

[0072] The conductive polyolefin foam according to the present invention has fine cells because the type and content of the foaming aid have been optimized. When the manufacturing conditions are optimized, the cell diameter is between 100 cells / 25 mm and 160 cells / 25 mm.

[0073] [2.2.2. Surface resistivity] "Surface resistivity" refers to the value measured in accordance with JIS K 6911:1995.

[0074] The conductive polyolefin foam according to the present invention exhibits a low surface resistivity despite a shorter secondary foaming time than conventional foams. This is thought to be because the primary and secondary foaming ratios are optimized, thereby suppressing an excessive increase in the foaming rate. When the manufacturing conditions are optimized, the surface resistivity is 1.0 × 10⁻⁶. 5 Ω / sq. or more 4.0×10 5 It will be less than or equal to Ω / sq.

[0075] [2.2.3. Apparent Density] "Apparent density" refers to the value measured in accordance with JIS K 6767:1999.

[0076] The conductive polyolefin foam according to the present invention has a low density because the total foaming ratio is optimized. When the manufacturing conditions are optimized, the apparent density is 40 kg / m³. 3 More than 70kg / m 3 The following applies.

[0077] [3. Effect] ZnO has the effect of rapidly decomposing the blowing agent at high temperatures. On the other hand, urea has the effect of gradually promoting the decomposition of the blowing agent from low temperatures. Therefore, by using ZnO and urea in combination as blowing aids and optimizing the amounts of ZnO and urea added and the primary blowing conditions, a primary foam containing a relatively large amount of blowing nuclei can be obtained. Subsequently, when the obtained primary foam is subjected to secondary blowing, optimizing the secondary blowing conditions can reduce the time required for secondary blowing to about half of the conventional time (about 2 hours) without significantly increasing the surface resistivity ρs. [Examples]

[0078] (Examples 1-5, Comparative Examples 1-9) [1. Sample Preparation] Polyolefins include, (A) Low-density polyethylene (LDPE) (Product code: Petrocene® 207R, manufactured by Tosoh Corporation), (B) Ethylene-vinyl acetate copolymer (EVA) (Product numbers: Ultrasene® 540, 630, manufactured by Tosoh Corporation) I used it.

[0079] For the conductive carbon, carbon black (product number: Ketjenblack® EC300J, manufactured by Lion Corporation) was used. Azodicarbonamide (ADCA) (product code: Azizocarbonamide, manufactured by Eiwa Chemical Industries, Ltd.) was used as the foaming agent. Calcium carbonate (product code: calcium bicarbonate, manufactured by Maruo Calcium Co., Ltd.) was used as the nucleating agent. Dicumyl peroxide (DCP) (Catalog No.: PERKADOX® BC-FF, manufactured by Kayaku Akzo Co., Ltd.) was used as the crosslinking agent.

[0080] Foaming aids include, (A) Urea (Product No.: Cellpaste (registered trademark) 101, manufactured by Eiwa Chemical Industries Co., Ltd.) (B) ZnO (Product code: Zinc oxide type 1, manufactured by Hakusui Tech Co., Ltd.) (C)MgO (Product No.: Kyowa Mag® 150, manufactured by Kyowa Chemical Industry Co., Ltd.) I used it.

[0081] Lubricants include, (A) Stearic acid (product code: Lunac® S-70V, manufactured by Kao Corporation), (B) Sorbitan stearate (Product code: Rikemar (registered trademark) S-300W, manufactured by Riken Vitamin Co., Ltd.) I used it.

[0082] The above raw materials were blended in predetermined ratios. Table 1 shows the raw material formulations for Comparative Example 1, Comparative Example 5, and Example 3. The raw material formulations for Comparative Examples 2-4 and 6-9 were the same as those for Comparative Example 5, except for the amount of foaming agent used. The raw material formulations for Examples 1-2 and 4-5 were the same as those for Example 3, except for the amount of foaming agent used.

[0083] [Table 1]

[0084] The raw material mixture was kneaded in a 1L kneader at 100-130°C for 15-20 minutes. Then, the resulting raw material mixture was further kneaded in a 10-inch mixing roll at 100°C for 10-15 minutes. The kneaded raw material mixture was filled into the molding space of the primary mold. The size of the molding space was 210 mm × 210 mm × 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., depressurized), and the raw material mixture was subjected to primary foaming.

[0085] Next, the primary foam was placed in the molding space of the secondary mold. The size of the molding space was 500 mm × 500 mm × t70 mm. Then, the primary foam was subjected to secondary heating under atmospheric pressure. The secondary heating temperature was 160°C, and the secondary heating time was 4 hours (Comparative Example 1) or 2 hours (Comparative Examples 2-9, Examples 1-5). After heating, the secondary foam was removed from the secondary mold.

[0086] [2. Test Method] [2.1. Size of the primary foam] The horizontal dimensions (x-axis and y-axis directions) of the primary foam were measured, and the average value was calculated.

[0087] [2.2. Formability] The presence or absence of secondary foam inclusion and cracks were visually inspected.

[0088] [2.3. Surface resistivity and volume resistivity] Surface resistivity and volume resistivity were measured in accordance with JIS K 6911:1995.

[0089] [2.4. Cell diameter] A test apparatus was prepared equipped with a magnifying glass with a magnification of 5x or more, which could be moved left and right, and had a scale to measure the distance moved. Next, the test specimen was placed on the stand of the test apparatus, and the number of cells n in a 10 mm interval was determined while moving the magnifying glass in a straight line. Furthermore, the number of cells N (cells / 25 mm) in a 25 mm interval was calculated using the following formula, and this was defined as the "cell diameter". N = n × 2.5

[0090] [2.5. Tensile Strength and Elongation] Tensile strength and elongation were measured in accordance with JIS K 6767:1999.

[0091] [3. Results] The results are shown in Table 2. From Table 2, the following can be seen:

[0092] (1) Comparative Example 1 required 4 hours for secondary foaming. This is thought to be because it did not contain ZnO as a foaming aid. (2) In Comparative Example 2, cracks occurred in the foam, and the cell diameter was 67 cells / 25 mm. This is thought to be due to an excessively low urea content and an excessively high ZnO / urea ratio. (3) In Comparative Example 3, cracks occurred in the foam. This is thought to be because the urea content was excessively low, the ZnO content was excessively high, and the ZnO / urea ratio was excessively high.

[0093] (4) In Comparative Example 4, cracks occurred in the foam, and the cell diameter was 64 cells / 25 mm. This is thought to be due to an excessively low urea content, an excessively low ZnO content, and an excessively low ZnO / urea ratio. (5) Comparative Example 5 showed cracking in the foam and had a cell diameter of 66 cells / 25 mm. This is thought to be due to an excessively high ZnO content and an excessively high ZnO / urea ratio. (6) Comparative Example 6 showed cracking in the foam and had a cell diameter of 82 cells / 25 mm. This is thought to be due to an excessively low ZnO / urea ratio.

[0094] (7) Comparative Example 7 showed cracking in the foam and had a cell diameter of 84 cells / 25 mm. This is thought to be due to an excessively high urea content and an excessively low ZnO / urea ratio. (8) In Comparative Example 8, cracks occurred in the foam. This is thought to be due to an excessively low ZnO / urea ratio. In addition, despite having the same raw material composition as Comparative Example 6, Comparative Example 8 had a smaller cell diameter than Comparative Example 6. This is thought to be because, due to unavoidable circumstances, the primary foaming temperature of Comparative Example 8 was higher than that of Comparative Example 6, resulting in an increase in the number of foaming nuclei.

[0095] (9) In Comparative Example 9, the size of the primary foam was 210 mm, which was almost the same as the original size (size of the molding space). In addition, the secondary foaming ratio of Comparative Example 9 was significantly reduced. This is thought to be because MgO was used instead of ZnO as the foaming aid. (10) In Example 1, no cracks occurred in the foam. However, in Example 1, the cell diameter was 64 cells / 25 mm, resulting in a coarse cell structure. This is thought to be due to a slightly low amount of urea.

[0096] (11) In Example 2, no cracks occurred in the foam. However, in Example 2, the cell diameter was 85 cells / 25 mm, which was slightly coarser than in Example 4. This is thought to be because the primary foaming temperature in Example 2 was lower than that of Example 4, resulting in a decrease in the number of foaming nuclei. (12) In Examples 3 and 4, no cracks occurred in the foam, and the cell diameter exceeded 110 cells / 25 mm. This is thought to be because the ZnO / urea ratio, ZnO content, and urea content were within an appropriate range. (14) Example 5 had a cell diameter of 116 cells / 25 mm. However, in Example 5, some entrapment occurred during secondary foaming. This is thought to be because the primary foam was large and expanded vertically along the wall early on, resulting in only the peripheral part of the foam rising in an arc shape.

[0097] [Table 2]

[0098] Although embodiments of the present invention have been described in detail above, the present invention is not limited in any way to the above embodiments, and various modifications are possible without departing from the spirit of the present invention. [Industrial applicability]

[0099] The conductive polyolefin foam according to the present invention can be used as packaging material, cushioning material, tray, container, etc., for various electronic components (e.g., IC chips) that are susceptible to damage due to static electricity.

Claims

1. A first step involves preparing a raw material mixture containing polyolefin, conductive carbon, a crosslinking agent, a foaming agent, and a foaming aid. A second step involves primary foaming the aforementioned raw material mixture to obtain a primary foam, A third step involves further foaming the primary foam to obtain a secondary foam. Equipped with, The first step is, The foaming agent includes ZnO and urea. The ZnO / urea ratio (mass ratio) is greater than 1.20 and less than 2.

00. The process includes preparing the aforementioned raw material mixture. A method for producing conductive polyolefin foam.

2. The first step is, The ZnO content is greater than 0.10 phr and less than 0.18 phr, The urea content is greater than 0.07 phr and less than 0.20 phr. The process includes preparing the aforementioned raw material mixture. A method for producing a conductive polyolefin foam according to claim 1.

3. The second step includes a step of primary foaming the raw material mixture so that the primary foaming ratio is 3.0 times or more and 5.0 times or less. The third step includes a step of causing the primary foam to undergo secondary foaming such that the secondary foaming ratio is 3.0 times or more and 5.0 times or less, and the total foaming ratio (= primary foaming ratio × secondary foaming ratio) is 10.0 times or more and 20.0 times or less. A method for producing a conductive polyolefin foam according to claim 1.

4. It is obtained by foaming a raw material mixture containing polyolefin, conductive carbon, a crosslinking agent, a foaming agent, and a foaming aid. The foaming agent comprises ZnO and urea. The cell diameter is 100 cells / 25 mm or more and 160 cells / 25 mm or less. Conductive polyolefin foam.

5. Surface resistivity ρs is 1.0 × 10⁻⁶ 5 Ω / sq. More than 4.0×10 5 It is less than or equal to Ω / sq. Apparent density is 40 kg / m³ 3 More than 70kg / m 3 The following is The conductive polyolefin foam according to claim 4.

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