JP7645445B2 - Heat-resistant crosslinked polyolefin foam and its manufacturing method - Google Patents

Description

The present invention relates to a heat-resistant crosslinked polyolefin foam and a method for producing the same.

It is common to add a blowing agent and a crosslinking agent to a polyolefin resin, knead the mixture to obtain a crosslinkable foamable composition, and then heat the composition to foam it to produce a foam.

Foams whose main ingredient is a resin with a low melting point and low heat resistance, such as low-density polyethylene (LDPE), shrink significantly when heat is applied.

One way to prevent the foam from shrinking at high temperatures is to use a more heat-resistant resin, such as high-density polyethylene (HDPE), as the raw material. However, this would change the texture of the foam and would require changes to the manufacturing process.

Therefore, there is a demand for a method for producing a crosslinked polyolefin foam that uses a resin that is not highly heat resistant as the main raw material and that has little shrinkage at high temperatures. One method is known in which pellets are obtained by fibrillating polybutylene terephthalate in polyethylene resin, and a foaming agent and a crosslinking agent are added to and kneaded with the pellets to cause foaming (see, for example, Patent Document 1).

However, the method described in Patent Document 1 requires kneading at a high temperature of 200°C to obtain pellets in which polybutylene terephthalate is fibrillated in polyethylene resin, which makes it difficult to use the same production line for products whose main raw material is a resin with a low melting point and low heat resistance.

Another method that can be considered to impart heat resistance is to knead a resin with a high melting point and high heat resistance with a resin with a low melting point and low heat resistance. However, if a resin with a high melting point is melted and kneaded, foaming reactions and crosslinking reactions will proceed during the kneading process, making it difficult to form a foam with a good shape.

Patent No. 4605364

The present invention aims to make it possible to manufacture heat-resistant cross-linked polyolefin foam by utilizing the same production line for products whose main raw material is a resin with a low melting point and low heat resistance.

The method for producing a heat-resistant cross-linked polyolefin foam according to the invention of claim 1 includes the steps of: generating a pre-foaming mixture by adding a powder of an additive, the powder being made of a resin having a melting point of 120°C or higher and having an average particle size of 10 μm to 20 μm, to a polyolefin resin, which is a main raw material, having a melting point of less than 120°C; adding a foaming agent and a cross-linking agent to the pre-foaming mixture; kneading the mixture at a temperature equal to or higher than the melting point of the polyolefin resin having a melting point of less than 120°C and lower than the melting point of the additive to disperse the powder of the additive in the pre-foaming mixture; and heating the pre-foaming mixture to a temperature at which the additive can melt to foam it.

According to this manufacturing method, the additive powder is dispersed by kneading at a temperature higher than the melting point of the main raw material polyolefin resin and lower than the melting point of the additive, and then the additive is heated to a temperature at which the additive can melt and foamed, whereby the additive powder melts and fibrillates during the foaming process, imparting heat resistance to the crosslinked polyolefin foam. Therefore, since the kneading process can be performed at a relatively low temperature corresponding to the melting point of the main raw material polyolefin resin, a heat-resistant crosslinked polyolefin foam can be manufactured using the same equipment that is used for the main raw material polyolefin resin.

In the present invention, the "temperature at which the additive can melt" is a concept that refers to a temperature equal to or higher than the melting point of the additive, or a temperature that is lower than the melting point but is 10°C or less different from the melting point. During the foaming process, heat is generated when the foaming agent reacts with the main raw material, so the temperature of the raw material during the foaming process is about 5 to 10°C higher than the heating temperature. Therefore, even if the heating temperature is lower than the melting point, it is possible to melt the additive powder during the foaming process as long as the difference from the melting point is 10°C or less.

Examples of polyolefin resins with low melting points that can be used in this manufacturing process include low-density polyethylene and ethylene-vinyl acetate copolymers.

Examples of powdered additive components used in this manufacturing process include high density polyethylene or polypropylene.

The heat-resistant cross-linked polyolefin foam of the present invention according to claim 4 contains a polyolefin resin having a melting point of less than 120°C as a main raw material, and an additive having an average particle size of 10 μm to 20 μm formed from a resin having a melting point of 120°C or higher, which has been melted and fibrillated.

According to the present invention, the kneading process prior to foaming can be carried out at a relatively low temperature, so a heat-resistant crosslinked polyolefin foam can be produced using the same production line for products whose main raw material is a resin with a low melting point and low heat resistance.

FIG. 2 is a diagram showing the FT-IR spectrum intensity of a heat-resistant crosslinked polyolefin foam according to one embodiment of the present invention before a foaming process. FIG. 4 is a diagram showing the FT-IR spectrum intensity of the heat-resistant crosslinked polyolefin foam according to the embodiment after the foaming process.

The following describes an embodiment of the present invention. In this embodiment, a polyolefin-based foam having high heat resistance is manufactured by adding an additive formed of a resin having a melting point of 120°C or higher, specifically polypropylene or high-density polyethylene powder, to the main raw material polyolefin resin having a melting point of less than 120°C. This foam is manufactured by adding a blowing agent, a crosslinking agent, and polypropylene or high-density polyethylene powder to the polyolefin resin and kneading them to obtain a crosslinkable foamable composition, and then heating the composition to foam it.

Specific examples of polyolefin resins that are the main raw material for foams include low-density polyethylene (LDPE) and ethylene-vinyl acetate copolymer (EVA), which have melting points below 120°C.

Specific examples of foaming materials include organic chemical foams such as azo compounds such as azodicarbonamide and barium azodicarboxylate, nitroso compounds such as dinitrosopentamethylenetetramine and trinitrotrimethyltriamine, hydrazide compounds such as p,p'-oxybisbenzenesulfonylhydrazide, and sulfonylsemicarbazide compounds such as p,p'-oxybisbenzenesulfonylsemicarbazide and toluenesulfonylsemicarbazide. However, inorganic foaming agents such as baking soda can also be used.

The crosslinking agent is an organic peroxide that has a decomposition temperature in the polyolefin resin that is at least equal to or higher than the flow initiation temperature of the polyolefin resin, and is a radical generator that is decomposed by heating to generate free radicals and cause crosslinking between or within the molecules. Specific examples include dicumyl peroxide, n-butyl 4,4-bis(t-butylperoxy)valerate, 1,1-ditertiary butyl peroxide, 1,1-ditertiary butylperoxy-3,3,5-trimethylcyclohexane, 2,5-dimethyl-2,5-ditertiary butylperoxyhexane, 2,5-dimethyl-2,5-ditertiary butylperoxyhexyne, α,α-ditertiary butylperoxyisopropylbenzene, tertiary butyl peroxyketone, and tertiary butyl peroxybenzoate. However, it is necessary to select an optimal organic peroxide depending on the raw material resin.

In this embodiment, the additive, polypropylene or high-density polyethylene powder, has an average particle size of 10 μm to 100 μm.

In addition, in order to improve the physical properties of the crosslinkable foamable composition or to reduce the cost, compounding agents (fillers) that do not have a significant adverse effect on the crosslinking bonds, such as metal oxides such as zinc oxide, titanium oxide, calcium oxide, magnesium oxide, and silicon oxide, carbonates such as magnesium carbonate and calcium carbonate, various dyes, pigments, and fluorescent substances, and other commonly used rubber and plastic compounding agents, can be added as necessary.

In addition, a foaming assistant may be added according to the type of foaming agent. Specific examples of foaming assistants include compounds whose main component is urea, metal oxides such as zinc oxide and lead oxide, compounds whose main components are salicylic acid and stearic acid, in other words, higher fatty acids or metal compounds of higher fatty acids.

The procedure for producing the foam in this embodiment is basically the same as that for producing existing foams, except that polypropylene or high-density polyethylene powder is added to the polyolefin resin, which is the main raw material. That is, a foaming agent, a crosslinking agent, and polypropylene or high-density polyethylene powder are added to the polyolefin resin and kneaded, and then the mixture is heated to foam and obtain a foam.

The kneading process is carried out at a temperature equal to or higher than the melting point of the first polyolefin resin, which is the main raw material, and lower than the melting point of the additive polypropylene or high-density polyethylene, preferably at 90 to 120°C. If the temperature of the kneading process exceeds 120°C, problems may occur in which the foaming agent or crosslinking agent reacts with the main raw material during the kneading process.

On the other hand, the foaming process is carried out at a temperature at which the polypropylene or high-density polyethylene powder can melt during the foaming process, for example 160°C. It is desirable that the heating temperature during the foaming process is higher than the melting point of the additive, but even if it is lower than the melting point of the additive, as long as the difference with the additive's melting point is within 10°C, the actual temperature will rise above the additive's melting point due to the heat generated when the foaming agent reacts with the main raw material, making it possible to melt the additive.

Table 1 shows specific examples of this embodiment. It goes without saying that the present invention is not limited to the following examples. Table 2 shows comparative examples of this embodiment.

Example 1: Ethylene-vinyl acetate copolymer (EVA, product name: Novatec EVA L
V-440, melting point 89°C, density 0.938g/ ^cm3 , manufactured by Japan Polyethylene Co., Ltd.) 1
A composition consisting of 0.00 parts by weight of high density polyethylene (additive) (product name: Flow Beads HE-3040, melting point 130°C, particle size 11 μm, viscosity 303.6 Pa·s, manufactured by Sumitomo Seika Chemicals Co., Ltd.), 4 parts by weight of azodicarbonamide (ADCA, foaming agent), 2 parts by weight of zinc oxide (ZnO, foaming assistant), 0.5 parts by weight of zinc stearate (Zn-St, foaming assistant), and 0.6 parts by weight of dicumyl peroxide (DCP, crosslinking agent) was heated to 100°C and kneaded, and the kneaded mixture was filled into a mold in a press heated to 160°C and heated under pressure for 50 minutes, thereby decomposing the foaming agent and crosslinking agent and foaming. The apparent density of the resulting foam was 89 kg/ ^m3 .

Example 2: Ethylene-vinyl acetate copolymer (EVA, product name: Novatec EVA L
V-440, melting point 89°C, density 0.938g/ ^cm3 , manufactured by Japan Polyethylene Co., Ltd.) 1
A composition consisting of 0.00 parts by weight of polypropylene (additive) (product name: PPW-5J, melting point 143°C, particle size less than 10 μm, viscosity 0.456 Pa·s, manufactured by Seishin Enterprise Co., Ltd.), 4 parts by weight of azodicarbonamide (ADCA, foaming agent), 2 parts by weight of zinc oxide (ZnO, foaming assistant), 0.5 parts by weight of zinc stearate (Zn-St, foaming assistant), and 0.6 parts by weight of dicumyl peroxide (DCP, crosslinking agent) was heated to 100°C and kneaded, and the kneaded mixture was filled into a mold in a press heated to 160°C and heated under pressure for 50 minutes, thereby decomposing the foaming agent and crosslinking agent and foaming. The apparent density of the resulting foam was 79 kg/ ^m3 .

Example 3: Ethylene-vinyl acetate copolymer (EVA, product name: Novatec EVA L
V-440, melting point 89°C, density 0.938g/ ^cm3 , manufactured by Japan Polyethylene Co., Ltd.) 1
A composition consisting of 0.00 parts by weight of polypropylene (additive) (product name: Flow Beads RP-2050, melting point 146°C, particle size 10 μm, viscosity 629.4 Pa·s, manufactured by Sumitomo Seika Chemicals Co., Ltd.), 4 parts by weight of azodicarbonamide (ADCA, foaming agent), 2 parts by weight of zinc oxide (ZnO, foaming assistant), 0.5 parts by weight of zinc stearate (Zn-St, foaming assistant), and 0.6 parts by weight of dicumyl peroxide (DCP, crosslinking agent) was heated to 100°C and kneaded, and the kneaded mixture was filled into a mold in a press heated to 160°C and heated under pressure for 50 minutes, thereby decomposing the foaming agent and crosslinking agent and foaming. The apparent density of the resulting foam was 88 kg/ ^m3 .

Example 4: Ethylene-vinyl acetate copolymer (EVA, product name: Novatec EVA L
V-440, melting point 89°C, density 0.938g/ ^cm3 , manufactured by Japan Polyethylene Co., Ltd.) 1
A composition consisting of 0.00 parts by weight, polypropylene (additive) (product name: Toyotac H-1000P, melting point 150°C, particle size 100μm or more, viscosity 140.3 Pa·s, manufactured by Toyobo Co., Ltd.) 30 parts by weight, azodicarbonamide (ADCA, foaming agent) 4 parts by weight, zinc oxide (ZnO, foaming assistant) 2 parts by weight, zinc stearate (Zn-St, foaming assistant) 0.5 parts by weight, and dicumyl peroxide (DCP, crosslinking agent) 0.6 parts by weight was heated to 100°C and kneaded, and the kneaded mixture was filled into a mold in a press heated to 160°C and heated under pressure for 50 minutes, thereby decomposing the foaming agent and crosslinking agent and foaming. The apparent density of the resulting foam was 82 kg/ ^m3 .

Example 5: Ethylene-vinyl acetate copolymer (EVA, product name: Novatec EVA L
V-440, melting point 89°C, density 0.938g/ ^cm3 , manufactured by Japan Polyethylene Co., Ltd.) 1
A composition consisting of 0.00 parts by weight of polypropylene (additive) (product name: MA04A, melting point 165°C, particle size 20μm, viscosity 813.3 Pa·s, manufactured by Japan Polypropylene Corporation) 30 parts by weight of azodicarbonamide (ADCA, foaming agent) 4 parts by weight of zinc oxide (ZnO, foaming assistant) 2 parts by weight of zinc stearate (Zn-St, foaming assistant) 0.5 parts by weight of zinc stearate (Zn-St, foaming assistant), and 0.6 parts by weight of dicumyl peroxide (DCP, crosslinking agent) was heated to 100°C and kneaded, and the kneaded mixture was filled into a mold in a press heated to 160°C and heated under pressure for 50 minutes to decompose the foaming agent and crosslinking agent and foam. The apparent density of the resulting foam was 73 kg/ ^m3 .

Example 6: Ethylene-vinyl acetate copolymer (EVA, product name: Novatec EVA L
V-440, melting point 89°C, density 0.938g/ ^cm3 , manufactured by Japan Polyethylene Co., Ltd.) 1
A composition consisting of 0.00 parts by weight of polypropylene (additive) (product name: MA4AHB, melting point 165°C, particle size 20μm, viscosity 2686 Pa·s, manufactured by Japan Polypropylene Corporation) 30 parts by weight of azodicarbonamide (ADCA, foaming agent) 4 parts by weight of zinc oxide (ZnO, foaming assistant) 2 parts by weight of zinc stearate (Zn-St, foaming assistant) 0.5 parts by weight of zinc stearate (Zn-St, foaming assistant), and 0.6 parts by weight of dicumyl peroxide (DCP, crosslinking agent) was heated to 100°C and kneaded, and the kneaded mixture was filled into a mold in a press heated to 160°C and heated under pressure for 50 minutes, thereby decomposing the foaming agent and crosslinking agent and foaming. The apparent density of the resulting foam was 81 kg/ ^m3 .

Comparative Example 1: Ethylene-vinyl acetate copolymer (EVA, product name: Novatec EVA L
V-440, melting point 89°C, density 0.938g/ ^cm3 , manufactured by Japan Polyethylene Co., Ltd.) 1
A composition consisting of 0.00 parts by weight of propylene glycol, 4 parts by weight of azodicarbonamide (ADCA, foaming agent), 2 parts by weight of zinc oxide (ZnO, foaming assistant), 0.5 parts by weight of zinc stearate (Zn-St, foaming assistant), and 0.6 parts by weight of dicumyl peroxide (DCP, crosslinking agent) was heated to 100°C and kneaded, and the kneaded mixture was filled into a mold in a press heated to 160°C and heated under pressure for 50 minutes, thereby decomposing the foaming agent and crosslinking agent and foaming. The apparent density of the completed foam was 86 kg/ ^m3 .

Comparative Example 2: Ethylene-vinyl acetate copolymer (EVA, product name: Novatec EVA L
V-440, melting point 89°C, density 0.938g/ ^cm3 , manufactured by Japan Polyethylene Co., Ltd.) 1
A composition consisting of 0.00 parts by weight of PA6 (additive) (product name: A1020LP, melting point 225°C, particle size 100μm, viscosity 75.20 Pa·s, manufactured by Unitika Ltd.) 30 parts by weight of azodicarbonamide (ADCA, foaming agent) 4 parts by weight of zinc oxide (ZnO, foaming assistant) 2 parts by weight of zinc stearate (Zn-St, foaming assistant), and 0.5 parts by weight of dicumyl peroxide (DCP, crosslinking agent) 0.6 parts by weight was heated to 100°C and kneaded, and the kneaded mixture was filled into a mold in a press heated to 160°C and heated under pressure for 50 minutes to decompose the foaming agent and crosslinking agent and foam. The apparent density of the resulting foam was 97 kg/ ^m3 .

Comparative Example 3: Ethylene-vinyl acetate copolymer (EVA, product name: Novatec EVA L
V-440, melting point 89°C, density 0.938g/ ^cm3 , manufactured by Japan Polyethylene Co., Ltd.) 1
A composition consisting of 0.00 parts by weight of PA11 (additive) (product name: Rilsan D30, melting point 186°C, particle size 24 μm, viscosity 316.0 Pa·s, manufactured by Arkema Co., Ltd.), 4 parts by weight of azodicarbonamide (ADCA, foaming agent), 2 parts by weight of zinc oxide (ZnO, foaming assistant), 0.5 parts by weight of zinc stearate (Zn-St, foaming assistant), and 0.6 parts by weight of dicumyl peroxide (DCP, crosslinking agent) was heated to 100°C and kneaded, and the kneaded mixture was filled into a mold in a press heated to 160°C and heated under pressure for 50 minutes to decompose the foaming agent and crosslinking agent and foam. The apparent density of the resulting foam was 108 kg/ ^m3 .

Comparative Example 4: Ethylene-vinyl acetate copolymer (EVA, product name: Novatec EVA L
V-440, melting point 89°C, density 0.938g/ ^cm3 , manufactured by Japan Polyethylene Co., Ltd.) 1
A composition consisting of 0.00 parts by weight of PA12 (additive) (product name: VESTOSINT 2159, melting point 184°C, particle size 10μm, viscosity 126.0 Pa·s, manufactured by Daicel-Evonik Co., Ltd.), 4 parts by weight of azodicarbonamide (ADCA, foaming agent), 2 parts by weight of zinc oxide (ZnO, foaming assistant), 0.5 parts by weight of zinc stearate (Zn-St, foaming assistant), and 0.6 parts by weight of dicumyl peroxide (DCP, crosslinking agent) was heated to 100°C and kneaded, and the kneaded mixture was filled into a mold in a press heated to 160°C and heated under pressure for 50 minutes to decompose the foaming agent and crosslinking agent and foam. The apparent density of the resulting foam was 101 kg/ ^m3 .

The above-mentioned viscosities of the resins used as additives were measured at 170°C and 1 Hz for high-density polyethylene and polypropylene, at 240°C and 1 Hz for PA6, and at 200°C and 1 Hz for PA11 and PA12.

The foam of Comparative Example 1, which does not contain any additives, shrinks in volume by 48.2% when left in a room at 100°C for 22 hours. In this way, conventional foams that use low-density polyethylene as the main raw material have low heat resistance and shrink significantly in temperature environments close to the melting point of low-density polyethylene.

In contrast, the foam of Example 1, which contains high-density polyethylene powder with a melting point of 130°C, a particle size of 11 μm, and a viscosity of 303.6 Pa·s, shrinks in volume by 7.0% when left in a room at 100°C for 22 hours. The foam of Example 2, which contains polypropylene powder with a melting point of 143°C, a particle size of less than 10 μm, and a viscosity of 0.456 Pa·s, shrinks in volume by 2.5% when left in a room at 100°C for 22 hours. The foam of Example 3, which contains polypropylene powder with a melting point of 146°C, a particle size of 10 μm, and a viscosity of 629.400 Pa·s, shrinks in volume by 20.0% when left in a room at 100°C for 22 hours. Similarly, the foam of Example 4, which contains polypropylene powder with a melting point of 150°C, a particle size of 100 μm or more, and a viscosity of 140.300 Pa·s, shrinks in volume by 8.5% when left in a room at 100°C for 22 hours. The foam of Comparative Example 5, which contains polypropylene powder with a melting point of 165°C, particle size of 20 μm, and viscosity of 813.300 Pa·s, shrinks in volume by 17.5% when left in a room at 100°C for 22 hours. The foam of Comparative Example 6, which contains polypropylene powder with a melting point of 165°C, particle size of 20 μm, and viscosity of 2686.000 Pa·s, shrinks in volume by 32.8% when left in a room at 100°C for 22 hours.

1 and 2 show the peak (2947 cm ⁻¹ ) intensity of polypropylene in the FT-IR spectrum of the mixture to which polypropylene was added as an additive. FIG. 1 shows the spectrum before the foaming process, and FIG. 2 shows the spectrum after the foaming process. The locations where polypropylene is present are shown as dark areas within the area surrounded by the dashed line at the tip of the arrow in each figure. Before the foaming process, as shown in FIG. 1, the aspect ratio of the polypropylene peak is close to 1:1, and polypropylene exists in a powder state. In contrast, after the foaming process, as shown in FIG. 2, the aspect ratio of the polypropylene peak is significantly different from 1:1. From this, in the foam to which polypropylene was added as an additive, the additive is melted and fibrillated.

Here, there is a tendency that the smaller the particle size of the resin and the lower the viscosity of the resin, the less the volume shrinks, i.e., the better the heat resistance. However, when the difference in particle size and viscosity is not large, as in the above-mentioned Examples 3 and 5, the additive with a higher melting point melts and fibrillates during the foaming process, so that the foam using an additive with a larger particle size and viscosity may have higher heat resistance (lower thermal shrinkage).

On the other hand, the foam of Comparative Example 2, which contains PA6 resin powder with a melting point of 225°C and a particle size of 100 μm, shrinks in volume by 49.7% when left in a room at 100°C for 22 hours. The foam of Comparative Example 3, which contains PA11 resin powder with a melting point of 186°C and a particle size of 24 μm, shrinks in volume by 42.4% when left in a room at 100°C for 22 hours. And the foam of Comparative Example 4, which contains PA12 resin powder with a melting point of 184°C and a particle size of 10 μm, shrinks in volume by 41.4% when left in a room at 100°C for 22 hours.

In the above-mentioned Comparative Examples 2 to 4, unlike the Examples, the melting point of the additive is 10°C or more higher than the kneading temperature (160°C), so it remains in powder form in the foam.

In this way, by adding polyolefin powder, which is an additive with a melting point higher than that of ethylene-vinyl acetate copolymer and which can melt during the foaming process, and kneading and then foaming at a temperature lower than the melting point of the polyolefin powder, the polyolefin powder disperses and fibrillates during the foaming process, suppressing the shrinkage of the resin skeleton of the foam and reducing the shrinkage rate in a high-temperature environment. In other words, the heat resistance of the foam can be greatly improved. Furthermore, in this embodiment, kneading is performed at 100°C and the foaming process is performed at 160°C, so that a heat-resistant foam can be obtained by directly using equipment for producing foams containing conventional polyolefin resins with low melting points.

The present invention is not limited to the embodiments described above.

For example, not limited to the ethylene-vinyl acetate copolymers described in Examples 1 to 4 above, any resin with a melting point below 120°C, such as low-density polyethylene, can be used as the main raw material for the foam according to the present invention.

In addition, any resin that has a melting point of 120°C or higher, can be melted during the foaming process, and is processed into a powder form can be used as an additive to the foam of the present invention, not limited to those used in the above-mentioned Examples 1 to 4. In this case, it is more preferable that the particle size is small and the viscosity is low.

Furthermore, the foaming process may be carried out at a temperature above 160°C. In that case, the heat resistance of the resulting foam will not change significantly from that of the foaming process carried out at 160°C. However, if the foaming process is carried out at, for example, 200°C, a resin with a melting point of approximately 205 to 210°C can be used as an additive.

Various modifications are possible without departing from the spirit of the present invention.

Claims (4)

A step of adding a powder of an additive having an average particle size of 10 μm to 20 μm, which is formed of a resin having a melting point of 120° C. or more, to a polyolefin resin having a melting point of less than 120° C. as a main raw material, to produce a pre-foaming mixture;
adding a blowing agent and a crosslinking agent to the pre-foam mixture;
A step of kneading at a temperature equal to or higher than the melting point of the polyolefin resin having a melting point of less than 120° C. and lower than the melting point of the additive to disperse the powder of the additive in the pre-foaming mixture;
and heating the pre-foaming mixture to a temperature at which the additive can be melted to cause foaming. The method for producing a heat-resistant crosslinked polyolefin foam according to claim 1, wherein the polyolefin resin is low-density polyethylene or an ethylene-vinyl acetate copolymer. The method for producing a heat-resistant crosslinked polyolefin foam according to claim 1 or 2, wherein the additive is high-density polyethylene or polypropylene. A heat-resistant cross-linked polyolefin foam containing the main raw material polyolefin resin with a melting point of less than 120°C and an additive with an average particle size of 10μm to 20μm formed from a resin with a melting point of 120°C or higher that has been melted and fibrillated.