JP6993155B2 - Cross-linked polyolefin resin foam and its molded product - Google Patents

Description

The present invention relates to a crosslinked polyolefin resin foam and a molded product thereof.

The crosslinked polyolefin foam is excellent in mechanical strength, flexibility, light weight, heat insulating property, etc., and is widely used in various fields as a heat insulating material, a cushioning material, and the like. For example, in the automobile field, it is used for automobile interior materials such as ceiling materials, doors and instrument panels. In the crosslinked polyolefin foam, a polyolefin-based resin such as a polypropylene-based resin is used as the resin.
As a crosslinked polyolefin resin foam used for such an automobile interior material, for example, the crosslinked polyolefin resin foam described in Patent Document 1 is known as a prior art. The crosslinked polyolefin resin foam described in Patent Document 1 can be molded into a complicated shape without causing molding defects, has excellent heat resistance, and has good molding processability at high temperatures.

Japanese Unexamined Patent Publication No. 2008-266589

However, today, the shape of the molded product is further complicated, and a crosslinked polyolefin resin foam having better moldability than the crosslinked polyolefin resin foam described in Patent Document 1 is desired.
Therefore, an object of the present invention is to provide a crosslinked olefin resin foam having excellent moldability and a molded product using the same.

As a result of diligent research to achieve the above object, the present inventors have found that a crosslinked polyolefin resin foam having a small friction coefficient can solve the above-mentioned problems, and have completed the present invention.
That is, the present invention relates to the following [1] to [10].
[1] A crosslinked polyolefin resin foam having a friction coefficient of 0.35 to 0.80.
[2] A foamed main body portion obtained by cross-linking and foaming a polyolefin-based foamable resin composition and a surface layer made of a polyolefin-based resin covering the surface of the foamed main body portion are included, and the thickness of the surface layer is 10. The crosslinked polyolefin resin foam according to the above [1], which is ~ 140 μm.
[3] The crosslinked polyolefin resin foam according to the above [2], wherein the polyolefin-based resin in the surface layer is a resin selected from the group consisting of polypropylene-based resins and polyethylene-based resins.
[4] The crosslinked polyolefin resin foam according to the above [2] or [3], wherein the surface layer is formed by extruding and laminating a polyolefin resin constituting the polyolefin resin.
[5] The crosslinked polyolefin resin foam according to any one of the above [2] to [4], wherein the surface layer of the polyolefin resin has a tensile elastic modulus of 1000 MPa or more.
[6] The crosslinked polyolefin resin foam according to any one of the above [2] to [5], wherein the polyolefin-based foamable resin composition contains a polypropylene-based resin.
[7] The crosslinked polyolefin resin foam according to the above [6], wherein the content of the polypropylene-based resin in the polyolefin-based resin composition is 45 to 85% by mass based on the total amount of resin components.
[8] The crosslinked polyolefin resin foam according to the above [6] or [7], wherein the polyolefin-based effervescent resin composition further contains an elastomer.
[9] The crosslinked polyolefin resin foam according to the above [8], wherein the content of the elastomer in the polyolefin-based resin composition is 15 to 55% by mass based on the total amount of resin components.
[10] The molded body of the crosslinked polyolefin resin foam according to any one of the above [1] to [9].

According to the present invention, it is possible to provide a crosslinked olefin resin foam having excellent moldability and a molded product using the same.

[Cross-linked polyolefin resin foam]
The crosslinked polyolefin resin foam of the present invention (hereinafter, may be simply referred to as “foam”) has a coefficient of friction of 0.35 to 0.80. If the coefficient of friction is larger than 0.80, the crosslinked polyolefin resin foam may not slide smoothly on the mold, and the foam may be torn. On the other hand, if the coefficient of friction is smaller than 0.35, it may be difficult to attach the adhesive required for attaching the foam molded body to the molded body. The friction coefficient of the crosslinked polyolefin resin foam of the present invention is preferably 0.75 from the viewpoint of improving the slip of the crosslinked polyolefin resin foam with respect to the mold and making it difficult for the foam to break during molding. It is less than or equal to, more preferably 0.70 or less, still more preferably 0.65 or less, still more preferably 0.50 or less.
The coefficient of friction is a value measured according to JIS K 7125 using an aluminum test table.

The crosslinked polyolefin-based resin foam of the present invention includes a foamed main body portion obtained by cross-linking and foaming a polyolefin-based foamable resin composition, and a surface layer made of a polyolefin-based resin that covers the surface of the foamed main body portion. Is preferable. Since the crosslinked polyolefin resin foam contains a surface layer, the coefficient of friction of the crosslinked polyolefin resin foam can be reduced as compared with the case where the crosslinked polyolefin resin foam comprises only the foamed main body portion. As a result, it is possible to prevent the foam from being torn during molding, and it is possible to improve the formability of the foam. In addition, it is possible to prevent the foam from being torn during molding while maintaining the inherent flexibility of the foam body.

<Thickness of surface layer>
The thickness of the surface layer of the crosslinked polyolefin resin foam of the present invention is such that the coefficient of friction of the crosslinked polyolefin resin foam can be reduced as compared with the case where the crosslinked polyolefin resin foam is composed of only the foamed main body. Not particularly limited. For example, from the viewpoint of setting the friction coefficient of the crosslinked polyolefin resin foam to 0.35 to 0.80, the thickness of the surface layer of the crosslinked polyolefin resin foam is preferably 10 to 140 μm, more preferably 15. It is ~ 140 μm, more preferably 20 to 140 μm, still more preferably 70 to 140 μm, still more preferably 70 to 130 μm. Further, by setting the range of the surface layer to the above-mentioned range, it is possible to improve the formability of the foam while maintaining the inherent flexibility of the foam.

<Tension elastic modulus of polyolefin resin in the surface layer>
The tensile elastic modulus of the polyolefin resin in the surface layer of the crosslinked polyolefin resin foam of the present invention increases the mechanical strength of the surface layer and makes it difficult for the foam to be torn during molding. It is preferably 1000 MPa or more, more preferably 1500 MPa or more, and even more preferably 2000 MPa or more. The upper limit of the tensile elastic modulus is not particularly limited, but is, for example, 4000 MPa.

<Polyolefin-based resin composition>
The foamed main body portion of the crosslinked polyofine-based resin foam is obtained by cross-linking and foaming a polyolefin-based resin foamable composition containing a polyolefin-based resin. The polyolefin-based resin effervescent composition preferably contains at least a foaming agent, and if necessary, it also contains an effervescent agent.

(Polypropylene resin)
The polypropylene-based resin used for the foam is not particularly limited, and examples thereof include a propylene homopolymer (homopolypropylene) and a copolymer of propylene and another olefin. The copolymer of propylene and other olefins may be a block copolymer, a random copolymer, or a random block copolymer, but a random copolymer (random polypropylene) is preferable.
Examples of other olefins copolymerized with propylene include α such as ethylene, 1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene, 1-octene, 1-nonene and 1-decene. -Olefins are mentioned, of which ethylene is preferred. That is, the polypropylene resin is preferably an ethylene-propylene random copolymer.
The copolymer of propylene and other olefins usually contains 90 to 99.5% by weight of propylene and 0.5 to 10% by weight of α-olefins other than propylene, but 95 to 99% by weight of propylene. %, The α-olefin other than propylene is preferably 1 to 5% by mass.

The polypropylene-based resin preferably has a melt flow rate (hereinafter, also referred to as “MFR”) of 0.4 to 4.0 g / 10 minutes, and more preferably 0.5 to 2.5 g / 10 minutes. .. By using the polypropylene-based resin having the above-mentioned MFR, it becomes easy to improve the moldability when the resin composition is processed into a foam and the moldability when the foam is secondarily processed.
The polypropylene-based resin can be used alone or in combination of two or more.

The polypropylene-based resin is contained in the resin composition in an amount of 45 to 85% by mass based on the total amount of the resin component. If the polypropylene-based resin is less than 45% by mass, the mechanical strength and heat resistance of the foam are lowered, and it is difficult to improve the moldability, for example, the moldability when the foam is secondarily processed. On the other hand, if it exceeds 85% by mass, it becomes difficult to improve the flexibility of the foam, and it becomes difficult to soften the tactile sensation.
The content of the polypropylene-based resin is preferably 50% by mass or more, more preferably 60% by mass or more, based on the total amount of the resin components, from the viewpoint of improving the mechanical strength and moldability. Further, the content of the polypropylene-based resin is preferably 80% by mass or less, more preferably 70% by mass or less, based on the total amount of the resin components, in order to improve the flexibility and soften the tactile sensation. The resin component is a concept including an elastomer described later.

(Elastomer)
The polyolefin-based resin effervescent composition preferably further contains an elastomer.
The resin composition contains an elastomer, and the elastomer is preferably an olefin rubber. In the present invention, by using an elastomer such as an olefin rubber, it becomes easy to increase the flexibility of the foam.
As the olefin-based rubber, one having a Mooney viscosity (ML _{1 + 4} , 100 ° C.) of 15 to 85 is preferably used. By setting the Mooney viscosity within the above range, it becomes possible to improve flexibility and moldability in a well-balanced manner. Further, in order to improve the flexibility and moldability, the Mooney viscosity of the olefin rubber is more preferably 25 to 75, and further preferably 30 to 70.

As the olefin-based rubber, an amorphous or low-crystalline rubber-like substance in which two or more kinds of olefin-based monomers are copolymerized substantially at random is preferable, and more specifically, formability and flexibility are well-balanced. From the viewpoint of improvement, ethylene-α-olefin copolymer rubber is preferable.
The α-olefins used in the ethylene-α-olefin copolymer rubber include propylene, 1-butene, 2-methylpropylene, 3-methyl-1-butene, 1-pentene, 1-hexene and 4-methyl-. Examples thereof include one or more α-olefins having 3 to 15 carbon atoms, preferably 3 to 10 carbon atoms such as 1-pentene and 1-octene. Among these, propylene and 1-butene are preferable, and propylene is more preferable.

The ethylene-α-olefin copolymer rubber may have other monomer units in addition to the ethylene unit and the α-olefin unit.
Examples of the monomer forming the other monomer unit include 1,3-butadiene, 2-methyl-1,3-butadiene (isoprene), 1,3-pentadiene, 2,3-dimethyl-1,3-butadiene and the like. Conjugate diene with 4-8 carbon atoms; dicyclopentadiene, 5-ethylidene-2-norbornene, 1,4-hexadiene, 1,5-dicyclooctadiene, 7-methyl-1,6-octadien, 5-vinyl- Non-conjugated diene having 5 to 15 carbon atoms such as 2-norbornene; vinyl ester compound such as vinyl acetate; unsaturated carboxylic acid ester such as methyl acrylate, ethyl acrylate, butyl acrylate, methyl methacrylate, ethyl methacrylate; Examples thereof include unsaturated carboxylic acids such as acrylic acid and methacrylic acid. These monomers can be used alone or in combination of two or more. Among these, non-conjugated diene having 5 to 15 carbon atoms is preferable, 5-ethylidene-2-norbornene, 1,4-hexadiene and dicyclopentadiene (DCPD) are more preferable, and DCPD is the most preferable from the viewpoint of availability. preferable.

The ethylene unit content of the ethylene-α-olefin copolymer rubber is usually 30 to 85% by mass, preferably 40 to 80% by mass, more preferably 45 to 75% by mass, and has 3 to 3 carbon atoms of propylene or the like. The content of 15, preferably 3 to 10 α-olefin units is usually 10 to 60% by weight, preferably 15 to 50% by weight, and the content of other monomeric units such as non-conjugated diene is. It is usually 0 to 20% by weight, preferably 1 to 10% by weight.

Further, as the olefin-based rubber, an olefin-based thermoplastic elastomer (TPO) can also be used. The olefin-based thermoplastic elastomer (TPO) generally has a polyolefin such as polyethylene or polypropylene as a hard segment and a rubber component such as EPM or EPDM as a soft segment. As the olefin-based thermoplastic elastomer (TPO), any of a blend type, a dynamic cross-linking type, and a polymerization type can be used.

Preferable specific examples of the olefin-based rubber include ethylene-propylene copolymer rubber (EPR) and ethylene-propylene-diene copolymer rubber (EPDM), with EPDM being preferred. Examples of EPDM include ethylene-propylene-5-ethylidene-2-norbornene copolymer rubber and ethylene-propylene-dicyclopentadiene copolymer rubber, and among these, ethylene-propylene-dicyclopentadiene copolymer rubber. Is preferable.

When EPDM or EPR is used as the olefin rubber, only the elastomer selected from EPDM and EPR may be used as the elastomer, but it may be used in combination with the olefin thermoplastic elastomer (TPO). When TPO is used in combination, for example, it is preferable that the elastomer selected from EPDM and EPR is 50% by mass or more and less than 100% by mass, and the TPO is 50% by mass or less based on the total amount of olefin rubber. In this case, the elastomer selected from EPDM and EPR is more preferably EPDM as described above.

The content of the elastomer such as the olefin rubber is preferably 15 to 55% by mass based on the total amount of the resin component in the resin composition. When it is 15% by mass or more, the flexibility of the foam is improved and the tactile sensation is easily softened. Further, when it is 55% by mass or less, it becomes easy to improve the formability, mechanical strength and the like of the foam. From the viewpoint of improving the flexibility of the foam, the content of the elastomer is more preferably 20% by mass or more, further preferably 30% by mass or more. Further, from the viewpoint of improving the mechanical strength and formability, 50% by mass or less is more preferable, and 40% by mass or less is further preferable.

When the polyolefin-based resin effervescent composition further contains an elastomer, the slip of the crosslinked polyolefin-based resin foam on the mold becomes worse and the foam is easily torn. Therefore, when the polyolefin-based resin effervescent composition contains an elastomer. , The effect of the present invention becomes more remarkable.

(Other resin components)
The resin composition may be composed of only an elastomer such as a polypropylene-based resin and an olefin-based rubber, but may contain resin components other than these as long as it does not impair the object of the present invention.
As such a resin component, a polyethylene resin, an ethylene-vinyl acetate copolymer, an ethylene-acrylic acid copolymer, an ethylene- (meth) alkylacrylate copolymer, or a copolymer of maleic anhydride thereof is copolymerized with these. Examples thereof include modified copolymers.
When the resin composition contains another resin such as a polyethylene resin, it is preferably 30 parts by mass or less, more preferably 20% by mass or less, based on 100 parts by mass of the polypropylene resin.

The polyethylene-based resin is not particularly limited, and examples thereof include low-density polyethylene, medium-density polyethylene, high-density polyethylene, linear low-density polyethylene, and an ethylene-α-olefin copolymer containing ethylene as a main component. , These may be used alone or in combination of two or more. Among the above-mentioned polyethylene-based resins, linear low-density polyethylene is preferable.
The MFR of the polyethylene resin is preferably 0.5 to 70 g / 10 minutes, more preferably 1.5 to 50 g / 10 minutes, and even more preferably 2 to 30 g / 10 minutes.
The above MFR is a value measured under the conditions of a temperature of 190 ° C. and a load of 21.2 N in accordance with JIS K 7210.

(Effervescent agent)
As a method for foaming the resin composition, there are a chemical foaming method and a physical foaming method. The chemical foaming method is a method of forming bubbles by the gas generated by the thermal decomposition of the compound added to the resin composition, and the physical foaming method is a method of impregnating the resin composition with a low boiling point liquid (foaming agent). After that, it is a method of volatilizing the foaming agent to form a cell. The foaming method is not particularly limited, but a chemical foaming method is preferable. By using the chemical foaming method, the foam can be made into a closed cell foam. It is also possible to make the bubbles uniform.
As the foaming agent, a thermal decomposition type foaming agent is used, and for example, an organic or inorganic chemical foaming agent having a decomposition temperature of about 140 to 270 ° C. can be used.
Examples of the organic foaming agent include azodicarbonamides, azodicarboxylic acid metal salts (azodicarboxylic acid barium and the like), azo compounds such as azobisisobutyronitrile, and nitroso compounds such as N, N'-dinitrosopentamethylenetetramine. Examples thereof include hydrazodicarbonamides, 4,4'-oxybis (benzenesulfonyl hydrazide), hydrazine derivatives such as toluenesulfonyl hydrazide, and semicarbazide compounds such as toluenesulfonyl semicarbazide.

Examples of the inorganic foaming agent include ammonium acid, sodium carbonate, ammonium hydrogencarbonate, sodium hydrogencarbonate, ammonium nitrite, sodium boron hydride, anhydrous monosoda citrate and the like.
Among these, azo compounds and nitroso compounds are preferable from the viewpoint of obtaining fine bubbles, and from the viewpoint of economy and safety, azodicarbonamide, azobisisobutyronitrile, N, N'-dinitrosopentamethylene. Tetramine is more preferred, and azodicarbonamide is particularly preferred.
The foaming agent can be used alone or in combination of two or more.
The amount of the pyrolysis foaming agent added to the resin composition is preferably 1 to 30 parts by mass with respect to 100 parts by mass of the resin component, from the viewpoint of appropriately foaming the foam bubbles without bursting, and 2 to 15 parts by mass. Parts by mass are more preferred, and 5 to 12 parts by mass are even more preferred.

(Other additives)
The resin composition may contain an additive in addition to the above-mentioned foaming agent. Examples of the additive include a cross-linking aid and an antioxidant. These may contain one or both.
As the cross-linking aid, a polyfunctional monomer can be used. For example, trifunctional (meth) acrylate compounds such as trimethylolpropanetrimethacrylate and trimethylolpropanetriacrylate; trimellitic acid triallyl ester, 1,2,4-benzenetricarboxylic acid triallyl ester, triallyl isocyanurate and the like. Compounds having three functional groups in one molecule; bifunctional (1,6-hexanediol dimethacrylate, 1,9-nonanediol dimethacrylate, 1,10-decanediol dimethacrylate, neopentylglycoldimethacrylate, etc.) Meta) Compounds having two functional groups in one molecule such as acrylate compounds and divinylbenzene; examples thereof include diallyl phthalate, diallyl terephthalate, diallyl isophthalate, ethyl vinylbenzene, lauryl methacrylate, and stearyl methacrylate. Of these, trifunctional (meth) acrylate compounds are more preferred.
The cross-linking aid can be used alone or in combination of two or more.
By adding a cross-linking aid to the resin composition, it becomes possible to cross-link the resin composition with a small amount of ionizing radiation. Therefore, it is possible to prevent cutting and deterioration of each resin molecule due to irradiation with ionizing radiation.
The content of the cross-linking aid is 0.2 to 20 parts by mass with respect to 100 parts by mass of the resin component in the resin composition from the viewpoint of easy adjustment and control of the degree of cross-linking when foaming the resin composition. It is preferably 0.5 to 15 parts by mass, more preferably 0.5 to 15 parts by mass.

Examples of the antioxidant include a phenol-based antioxidant, a sulfur-based antioxidant, a phosphorus-based antioxidant, an amine-based antioxidant, and the like. Among these, phenol-based antioxidants and sulfur-based antioxidants are preferable, and it is more preferable to use a phenol-based antioxidant and a sulfur-based antioxidant in combination.
Examples of the phenolic antioxidant include 2,6-di-tert-butyl-p-cresol, n-octadecyl-3- (3,5-di-tert-butyl-4-hydroxyphenyl) propionate, and 2-tert-. Butyl-6- (3-tert-butyl-2-hydroxy-5-methylbenzyl) -4-methylphenylacrylate, tetrakis [methylene-3- (3,5-di-tert-butyl-4-hydroxyphenyl) propionate ] Methyl and the like can be mentioned.
Examples of the sulfur-based antioxidant include dilaurylthiodipropionate, dimyristylthiodipropionate, disstearylthiodipropionate, and pentaerythrityltetrakis (3-laurylthiopropionate).
These antioxidants can be used alone or in combination of two or more.
The content of the antioxidant is preferably 0.1 to 10 parts by mass, more preferably 0.2 to 5 parts by mass with respect to 100 parts by mass of the resin component in the resin composition.
Further, the resin composition may be a decomposition temperature adjusting agent such as zinc oxide, zinc stearate, urea, etc., a flame retardant, a metal damage inhibitor, an antistatic agent, a stabilizer, a filler, a pigment, etc. other than the above, if necessary. May contain the additive of. It is preferable to use it.

<Polyolefin-based resin for the surface layer>
The polyolefin-based resin used as the surface layer of the polyolefin-based resin foam of the present invention is not particularly limited. For example, the polyolefin-based resin is preferably a resin selected from the group consisting of polypropylene-based resins and polyethylene-based resins.

Examples of the polypropylene-based resin constituting the polypropylene-based resin include homopolymers of propylene, block copolymers of propylene and other resins, random copolymers, and the like alone or in combination of two or more. Used as a mixture.

Examples of resins other than propylene include ethylene and olefins such as α-olefins having 4 to 10 carbon atoms (1-butene, 1-pentene, 1-hexene, 4-methyl-1-pentene, etc.). One kind or two or more kinds are mentioned. Particularly suitable polypropylene-based resins include, for example, a propylene homopolymer, a propylene-ethylene random copolymer, a propylene-ethylene-α-olefin random copolymer, and a propylene component and a propylene-ethylene random copolymer component. Examples thereof include block copolymers and the like.

Further, the polypropylene-based resin may be mixed with other resins as long as the effects of the present invention are not impaired. Examples of the other resin include homopolymers or copolymers such as ethylene and α-olefin, olefin resins such as polyolefin waxes and polyolefin elastomers, and hydrocarbon resins such as petroleum resins and terpene resins. However, one type is used alone, or two or more types are mixed and used.

Further, various additives may be appropriately added to the polypropylene-based resin, if necessary, as long as the effects of the present invention are not impaired. Examples of the additive include an antistatic agent, an anti-fog agent, an anti-blocking agent, an antioxidant, a light stabilizer, a crystal nucleating agent, a lubricant, a surfactant for the purpose of imparting slipperiness and anti-blocking property, and the like. Be done.

Examples of the polyethylene-based resin constituting the polyethylene-based resin include low-density polyethylene, medium-density polyethylene, high-density polyethylene, ethylene-α-olefin copolymer, and ethylene-vinyl acetate copolymer.

Examples of the α-olefin constituting the ethylene-α-olefin copolymer include propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, 1-octene and the like. Examples of the α-olefin constituting the propylene-α-olefin copolymer include ethylene, 1-butene, 4-methyl-1-pentene, 1-hexene, 1-octene and the like.

Further, various additives may be appropriately added to the polyethylene-based resin, if necessary, as long as the effects of the present invention are not impaired. Examples of the additive include an antistatic agent, an anti-fog agent, an anti-blocking agent, an antioxidant, a light stabilizer, a crystal nucleating agent, a lubricant, a surfactant for the purpose of imparting slipperiness and anti-blocking property, and the like. Be done.

<Manufacturing method of crosslinked polyolefin resin foam>
The foam is produced, for example, by melt-kneading the resin composition to form a desired shape, irradiating the resin composition with ionizing radiation to crosslink the resin composition, heating and foaming, and laminating the surface layer. ..
Specifically, a production method having the following steps 1 to 4 is more preferable.
Step 1: After melt-kneading each component constituting the resin composition such as various resin components and a thermal decomposition foaming agent, a step of obtaining a resin composition having a predetermined shape such as a sheet shape Step 2: Resin obtained in Step 1. Step 3: A step of irradiating the composition with ionizing radiation to cross-link the resin composition Step 3: A step of heating the resin composition cross-linked in step 2 to a temperature higher than the decomposition temperature of the heat-decomposable foaming agent and foaming the composition to obtain a foamed main body. 4: Step 4 of forming a surface layer of the polyolefin resin on the surface of the foamed main body by laminating molding the polyolefin resin using the foamed main body obtained in step 3 as a base material.

In step 1, first, each component constituting the resin composition is supplied to a kneading device, melt-kneaded at a temperature lower than the decomposition temperature of the pyrolysis foaming agent, and then the melt-kneaded resin composition is preferably used. Is formed into a desired shape such as a sheet by the kneading device used in the melt kneading.
Examples of the kneading device used here include an injection molding machine, an extruder (single-screw extruder, twin-screw extruder, etc.), a Banbury mixer, a general-purpose kneading device such as a roll, and the like. An extruder is preferable, and if an extruder or an injection molding machine is used, the product can be manufactured with high productivity.
The resin temperature inside the injection molding machine or extruder is preferably 120 to 220 ° C, more preferably 140 to 200 ° C, and even more preferably 150 to 195 ° C.

In step 2, the resin composition formed into a desired shape is irradiated with ionizing radiation. Examples of the ionizing radiation include electron beam, α ray, β ray, γ ray, X-ray and the like. Among these, an electron beam is preferable from the viewpoint of productivity and uniform irradiation.
The acceleration voltage of the ionizing radiation depends on the thickness of the foamable resin composition to be irradiated, but is preferably 400 to 1200 kV, more preferably 500 to 1100 kV, and further preferably 600 to 1000 kV. preferable.
The irradiation dose of the ionizing radiation may be an amount that can obtain a desired degree of cross-linking without causing surface roughness or cracks in consideration of the thickness of the foamable resin composition to be irradiated. 1 to 10 Mrad is preferable, and 0.5 to 5 Mrad is more preferable.

In step 3, the crosslinked resin composition is heated to a temperature higher than the decomposition temperature of the foaming agent to foam it, and foaming and molding are performed at the same time to obtain a foam. Here, the temperature at which the resin composition is heated and foamed depends on the decomposition temperature of the thermally decomposable foaming agent used as the foaming agent, but is usually 140 to 300 ° C, preferably 150 to 280 ° C, more preferably 160 to 260 ° C. Is. Further, the foamed sheet may be stretched in either or both of the MD direction and the CD direction after foaming or while being foamed.

In step 4, a surface layer of the polyolefin resin is formed on the surface of the foamed main body by laminating molding the polyolefin resin using the foamed main body as a base material. The lamination molding performed in step 4 is, for example, extrusion laminating molding, dry laminating molding or solvent-free laminating molding, and more preferably extrusion laminating molding.
The extruded laminate molding is a molding method in which a polyolefin-based resin is heated and melted, a thin film is extruded, and the polyolefin-based resin is adhered to the main body.

However, the production method is not limited to the above, and a foam may be obtained by a method other than the above. For example, instead of irradiating with ionizing radiation, an organic peroxide may be mixed in advance with the resin composition, and the resin composition may be heated to decompose the organic peroxide, or the like may be used for crosslinking. Further, foaming may be performed using a foaming agent other than the pyrolysis foaming agent.

<Molded body>
The molded product of the present invention is obtained by molding the crosslinked polyolefin resin foam of the present invention by a known method. When manufacturing a molded product, it is also possible to laminate and bond other materials such as a base material and a skin material. That is, it is also possible to form a structure in which a skin material or the like is laminated on the crosslinked polyolefin foam and integrated.
The base material is the skeleton of the molded product, and a thermoplastic resin is usually used. As the thermoplastic resin for the base material, the above-mentioned polyolefin resin, a copolymer of ethylene and α-olefin, vinyl acetate, acrylic acid ester and the like, ABS resin, polystyrene resin and the like can be applied.
Examples of the skin material include polyvinyl chloride sheets, sheets made of a mixed resin of polyvinyl chloride and ABS resin, thermoplastic elastomer sheets, textiles using natural fibers and artificial fibers, knitted fabrics, non-woven fabrics, artificial leathers and synthetic leathers. Examples include leather. Further, a composite molded body may be formed in which a design such as a lenticel or a wood grain pattern is applied to the surface by using genuine leather, a silicone stamper having irregularities transferred from stone, wood, or the like.
Examples of the method of laminating the skin material include an extrusion laminating method, an adhesive laminating method of applying an adhesive and then laminating, a thermal laminating method (heat fusion method), a hot melt method, a high frequency welder method, and the like. Both may be adhered by any method.

Examples of the molding method of the molded body of the present invention include a stamping molding method, a vacuum forming method, a compression molding method, an injection molding method and the like. Among these, the stamping molding method and the vacuum forming method are preferable, and the stamping molding method is more preferable.
As a stamping molding method, for example, a composite sheet in which a skin material is laminated on one surface of the foam is placed between a convex mold and a concave mold, the mold is closed, and the convex mold and the concave mold are formed. There is a method of press forming by mold-clamping and.
As the vacuum forming method, either a male pulling vacuum forming method or a female pulling vacuum forming method can be adopted, but the male pulling vacuum forming method is more preferable.

In the polyolefin-based resin foam of the present invention, the surface layer is formed on the surface where the polyolefin-based resin foam is in contact with the mold. As a result, the slip of the crosslinked polyolefin resin foam on the mold can be improved, the foam can be less likely to be torn during molding, and the moldability of the foam can be improved. In particular, in deep drawing, the foam is deformed while the peripheral part of the foam moves, so that the cross-linked polyolefin resin foam is slippery to the mold and the foam is less likely to be torn during molding. The effect is further demonstrated. Therefore, as a molding method for the molded product of the present invention, a molding method accompanied by deep drawing is preferable. That is, as the molding method of the molded product of the present invention, a stamping molding method including deep drawing and a vacuum forming method including deep drawing are preferable, and a stamping molding method including deep drawing is more preferable.
The molded product of the present invention is fixed to another member or the like using, for example, an adhesive. For this reason, it is important that the molded product adheres to the adhesive.

The molded product obtained by molding the crosslinked polyolefin resin foam of the present invention can be used as a heat insulating material, a cushioning material, etc., but is used as a packaging material for vehicles, especially in the field of automobiles, as a ceiling material, a door, and an instrument. It can be suitably used as an automobile interior material such as a panel.

The present invention will be described in more detail by way of examples, but the present invention is not limited to these examples.

The crosslinked polyolefin resin foams of Examples and Comparative Examples were evaluated as follows.
(Density of foam body)
The density (apparent density) of the foam body of the crosslinked polyolefin resin foam was measured according to JIS K 7222.

(Effervescence magnification of the foam body)
Since the foaming ratio of the foaming body is the reciprocal of the density of the foaming body, the foaming ratio of the foaming body was calculated from the density of the foaming body described above.

(Thickness of foam body)
The thickness of the foam body was measured using a dial gauge.

(Tension elastic modulus of polyolefin resin in the surface layer)
A plate obtained by extruding a polyolefin-based resin of a polyolefin-based resin in the surface layer was machined according to JIS K 7144 to prepare a test piece. The tensile modulus of elasticity of the test piece was measured according to JIS K 7161.

(Friction coefficient of foam)
The coefficient of friction of the crosslinked polyolefin resin foam was measured according to JIS K 7125 using aluminum as the material of the test table.

(Evaluation of adhesive adhesiveness and molding processability of foam)
The foams obtained in each Example and Comparative Example were formed by a vacuum forming machine under the condition of a surface temperature of 160 ° C., and formed into a cup-shaped molded body having a bottomed cylinder having a diameter of 80 mm and a height of 56 mm. The molded product was visually observed and its formability was evaluated in the following three stages.
×: There is tear, ○: There is no tear, but there is a scale, ◎: The entire surface is even. In addition, a urethane adhesive is applied to the molded body, and the adhesive is heated and cured at a heating temperature of 180 ° C, and then bonded. The adhesiveness of the agent was evaluated in the following two stages.
×: Adhesive did not adhere, ○: Adhesive adhered

Each of the resin components and additives shown in Table 1 was put into a single-screw extruder in the number of parts shown in Table 1, melt-kneaded at a resin temperature of 180 ° C., and extruded to obtain a sheet-shaped resin composition. The resin composition was crosslinked by irradiating both sides of the sheet-shaped resin composition with an electron beam. Then, the crosslinked resin composition was heated in a hot air oven at 250 ° C. for 5 minutes and foamed by the heating to obtain a foamed main body portion having a predetermined thickness. Then, a foam was produced by forming a surface layer of the resin shown in Table 1 on the foam body portion so as to have the thickness shown in Table 1 by the extrusion laminating molding method. However, the foam of Comparative Example 3 did not form a surface layer. The evaluation results of the foam are shown in Table 1.

Details of the resin components and additives shown in Table 1 are as follows.
PP: Ethylene-propylene random copolymer (random polypropylene), manufactured by Japan Polypropylene Corporation, product name: Novatec EG7F, MFR: 1.3 g / 10 minutes, ethylene content: 3% by mass
EPDM: Ethylene-propylene-diene copolymer, manufactured by Sumitomo Chemical Co., Ltd., product name: Esplen 301, Mooney viscosity (ML1 + 4,100 ° C) = 55, ethylene content: 62
Mass%, DCPD content: 3 mass%
Foaming agent: Azodicarbonamide Crosslinking aid: Trimethylolpropane Trimethacrylate Antioxidant 1: 2,6-di-tert-butyl-p-cresol Antioxidant 2: Dilaurylthiodipropionate

As is clear from the results in Table 1, it was found that the moldability of the foam can be improved by setting the friction coefficient of the foam to 0.35 to 0.80.

Claims (9)

The coefficient of friction is 0.35 to 0.80 ,
It contains a foamed main body portion obtained by cross-linking and foaming a polyolefin-based foamable resin composition, and a surface layer made of a polyolefin-based resin that covers the surface of the foamed main body portion.
A crosslinked polyolefin resin foam having a tensile elastic modulus of 1000 MPa or more for the polyolefin resin in the surface layer . The crosslinked polyolefin resin foam according to claim 1, wherein the surface layer has a thickness of 10 to 140 μm. The crosslinked polyolefin resin foam according to claim 1 or 2 , wherein the polyolefin-based resin in the surface layer is a resin selected from the group consisting of polypropylene-based resins and polyethylene-based resins. The crosslinked polyolefin resin foam according to any one of claims 1 to 3 , wherein the surface layer is formed by extruding and laminating a polyolefin resin constituting the polyolefin resin. The crosslinked polyolefin resin foam according to any one of claims 1 to 4 , wherein the polyolefin-based foamable resin composition contains a polypropylene-based resin. The crosslinked polyolefin resin foam according to claim 5 , wherein the content of the polypropylene-based resin in the polyolefin-based resin composition is 45 to 85% by mass based on the total amount of resin components. The crosslinked polyolefin resin foam according to claim 5 or 6 , wherein the polyolefin-based foamable resin composition further contains an elastomer. The crosslinked polyolefin resin foam according to claim 7 , wherein the content of the elastomer in the polyolefin resin composition is 15 to 55% by mass based on the total amount of the resin component. The molded product of the crosslinked polyolefin resin foam according to any one of claims 1 to 8 .