JP2005200473A - Manufacturing method of crosslinked polyolefin based resin foamed sheet - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a manufacturing method of a crosslinked polyolefin based resin foamed sheet excellent in heat resistance and flexibility. <P>SOLUTION: The manufacturing method of the crosslinked polyolefin based resin foamed sheet comprises irradiating both surfaces of a foamable resin sheet, comprising 100 pts.wt. of a polyolefin based resin containing at least 30 wt% of a polypropylene based resin which is obtained using a metallocene catalyst and has an (Mw/Mn) of at most 3.2, at least 2.0 pts.wt. of an auxiliary crosslinking agent and a thermodecomposable blowing agent, with a first ionizing radiation at an accelerated voltage attenuating to zero before it reaches a depth of 2/3 of the sheet thickness with a dose not more than the standard dose for the polypropylene based resin, subsequently irradiating the foamable resin sheet with a second ionizing radiation at an accelerated voltage higher than that and with a dose not higher than that of the first ionizing radiation, and further heating the foamable resin sheet to make it expand. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

The present invention relates to a method for producing a crosslinked polyolefin resin foam sheet having excellent heat resistance and flexibility.

Conventionally, a crosslinked polyolefin resin foam sheet has been widely used as a material for heat insulation and miscellaneous goods because it has excellent heat resistance and heat insulation properties. Recently, it is used as an interior material for vehicles. Is increasing.

As a shaping method for producing vehicle interior materials using the above-mentioned crosslinked polyolefin resin foam sheet, a crosslinked polyolefin resin foam sheet is disposed in a cavity formed between a male and a male mold and melted in the cavity. There is a so-called stamping molding in which a cross-linked polyolefin resin foam sheet and a thermoplastic resin are laminated and integrated into a desired shape by supplying a thermoplastic resin in a state.

However, since the above-mentioned crosslinked polyolefin resin foam sheet is in direct contact with the molten thermoplastic resin, if the crosslinked polyolefin resin foam sheet has insufficient heat resistance, the foamed crosslinked polyolefin resin during stamping molding There has been a problem that the sheet is torn or wrinkles are generated on the surface of the crosslinked polyolefin resin foam sheet.

Furthermore, the male and female molds used in the stamping molding method are often used without being heated for reasons such as improving production efficiency. In such a case, the molten thermoplasticity in the crosslinked polyolefin resin foam sheet is often used. The surface that comes into contact with the resin is required to have heat resistance, while the surface that does not come into contact with the molten thermoplastic resin in the cross-linked polyolefin resin foamed sheet is at a temperature close to normal temperature, and if the elongation near normal temperature is insufficient, the cross-linked polyolefin There was a problem that the resin-based resin foam sheet was torn during molding.

Japanese Patent Application Laid-Open No. 10-45975 proposes a crosslinked polyolefin resin foam obtained by adding a predetermined amount of highly crystalline homopolypropylene.

However, when highly crystalline homopolypropylene is added to the crosslinked polyolefin resin foam, the elongation and flexibility at room temperature of the resulting crosslinked polyolefin resin foam are reduced.
When the cross-linked polyolefin resin foam was wound, another problem such as cracking occurred in the cross-linked polyolefin resin foam.

Japanese Patent Laid-Open No. 10-45975

The present invention provides a method for producing a crosslinked polyolefin resin foam sheet excellent in heat resistance and flexibility, particularly a crosslinked polyolefin resin foam sheet that can be suitably used for stamping molding.

The method for producing a crosslinked polyolefin-based resin foam sheet of the present invention is obtained using a metallocene catalyst, and the ratio of the weight average molecular weight (Mw) to the number average molecular weight (Mn) (Mw / M).
n) on both sides of a foamable resin sheet comprising 100 parts by weight of a polyolefin resin containing 30% by weight or more of a polypropylene resin having a content of 3.2 or less, 2.0 parts by weight or more of a crosslinking aid, and a pyrolytic foaming agent When the foamable resin sheet is irradiated in the thickness direction, the accelerating voltage attenuates to zero until reaching the depth of 2/3 of the sheet thickness from the surface of the foamable resin sheet. After irradiating the first ionizing radiation with a dose less than the dose, the second ionizing property with an acceleration voltage higher than the acceleration voltage of the first ionizing radiation and a dose less than the first ionizing radiation dose. The foaming resin sheet is irradiated with radiation and then heated and foamed.

The polypropylene resin is obtained using a metallocene catalyst, and the ratio (Mw / Mn) of the weight average molecular weight (Mw) to the number average molecular weight (Mn) is 3.2 or less. The metallocene catalyst generally refers to a compound having a structure in which a transition metal is sandwiched between π-electron unsaturated compounds, and a bis (cyclopentadienyl) metal complex can be given as a representative example.

Specific examples of the metallocene catalyst used in the present invention include tetravalent transition metals such as titanium, zirconium, nickel, palladium, hafnium, platinum, and one or more cyclopentadienyl rings or analogs thereof. The compound which exists as a ligand (ligand) is mentioned.

Specific examples of the ligand include, for example, a cyclopentadienyl ring; a cyclopentadienyl ring substituted with a hydrocarbon group, a substituted hydrocarbon group, or a hydrocarbon-substituted metalloid group; a cyclopentadienyl oligomer ring; An indenyl ring; an indenyl ring substituted by a hydrocarbon group, a substituted hydrocarbon group, or a hydrocarbon-substituted metalloid group.

Examples of the hydrocarbon group substituted on the cyclopentadienyl ring or indenyl ring include, for example, a methyl group, an ethyl group, a propyl group, a butyl group, an amyl group, an isoamyl group, a hexyl group, an isobutyl group, a heptyl group, and an octyl group. , Nonyl group, decyl group, cetyl group, 2-ethylhexyl group, phenyl group and the like.

Further, transition metals include monovalent anion ligands such as chlorine and bromine, divalent anion chelate ligands, hydrocarbons, alkoxides, arylamides, aryloxides, amides, in addition to the above π-electron unsaturated compounds. Aryl amide, phosphide, aryl phosphide and the like may be coordinated.

Examples of such metallocene catalysts include cyclopentadienyl titanium tris (dimethylamide) and methylcyclopentadienyl titanium tris (dimethylamide).
Bis (cyclopentadienyl) titanium dichloride, dimethylsilyltetramethylcyclopentadienyl-tert-butylamidozirconium dichloride, dimethylsilyltetramethylcyclopentadienyl-tert-butylamidohafnium dichloride, dimethylsilyltetramethylcyclopentadi Enyl-pn-butylphenylamidozirconium chloride, methylphenylsilyltetramethylcyclopentadienyl-tert-
Butyramide hafnium dichloride, indenyl titanium tris (dimethylamide),
Indenyl titanium tris (diethylamide), indenyl titanium tris (di-n
-Propylamide), indenyl titanium bis (di-n-butylamide) (di-n-propylamide) and the like.

The metallocene catalyst changes the type of metal and the structure of the ligand,
In combination with a cocatalyst, the catalyst acts as a catalyst during olefin polymerization.

Examples of the cocatalyst include methylaluminoxane (MAO) and boron compounds. The amount of the cocatalyst added is 10 to 10,000 with respect to the metallocene catalyst.
00 molar times are preferable and 50-5000 molar times are more preferable.

Examples of the olefin polymerization method using the metallocene catalyst include a solution polymerization method using an inert medium, a bulk polymerization method substantially free of an inert medium, and a gas phase polymerization method. Is preferably −100 to 300 ° C., and the polymerization pressure is from normal pressure to 1 × 1
0 ⁷ Pa is preferred.

The metallocene catalyst has the characteristic that the properties of the active sites are uniform, and each active site has the same activity. Therefore, the polymer polymerized using the metallocene catalyst has its molecular weight and molecular weight distribution. , The uniformity of composition and composition distribution is improved.

Therefore, the polypropylene resin obtained by polymerization using the above metallocene catalyst has a narrow molecular weight distribution, and in the case of a copolymer, the copolymer component is introduced into each molecular weight component at a substantially equal ratio. In addition, as a polypropylene-type resin obtained by superposing | polymerizing using a metallocene catalyst as a polymerization catalyst, it is marketed with the brand name "WFX6" from Nippon Polychem, for example.

When the ratio (Mw / Mn) of the number average molecular weight (Mn) and the weight average molecular weight (Mw) in the polypropylene resin polymerized using the metallocene catalyst is large, the foaming resin sheet is irradiated with ionizing radiation. Since the elongation at normal temperature after the process is reduced, the elongation is limited to 3.2 or less, preferably 2.8 or less, more preferably 1.5 to 2.8, and particularly preferably 2.0 to 2.8.

In addition, when the melt index of the polypropylene resin polymerized using the metallocene catalyst and the ratio (Mw / Mn) of the number average molecular weight (Mn) to the weight average molecular weight (Mw) is 3.2 or less is high, Since the foamable resin sheet may not be sufficiently crosslinked, it is preferably 15 g / 10 min or less, and more preferably 10 g / 10 min or less. In addition, the melt index of a polypropylene resin means what was measured on condition of temperature 230 degreeC and load 2.12N based on JISK7210.

Furthermore, if the density of the polypropylene resin is high, the degree of elongation of the crosslinked polyolefin resin sheet at room temperature may decrease, whereas if it is low, the heat resistance of the crosslinked polyolefin resin foam sheet may decrease. 0.87 to 0.92 g / cm ³ is preferable.

A polyolefin-based resin of a polypropylene-based resin obtained using the metallocene catalyst and having a ratio (Mw / Mn) of weight average molecular weight (Mw) to number average molecular weight (Mn) of 3.2 or less If the content in the content is small, the heat resistance of the resulting crosslinked polyolefin resin foamed sheet is lowered, so it is limited to 30% by weight or more, preferably 40% by weight or more, more preferably 45% by weight or more, and 50% by weight. % Or more is particularly preferable.

On the other hand, a polyolefin-based resin of a polypropylene-based resin obtained using the above metallocene catalyst and having a ratio (Mw / Mn) of weight average molecular weight (Mw) to number average molecular weight (Mn) of 3.2 or less If the content in the inside is too large, the flexibility of the crosslinked polyolefin resin foamed sheet may be lowered, so that it is preferably 90% by weight or less, more preferably 80% by weight or less.

The polyolefin resin is obtained using the metallocene catalyst described above, and the ratio (Mw / Mn) of the weight average molecular weight (Mw) to the number average molecular weight (Mn) is 3.2 or less. A polypropylene resin other than a certain polypropylene resin or a polyethylene resin may be contained.

Examples of such polypropylene-based resins include propylene homopolymers, copolymers of propylene and other olefins, and the like may be used alone or in combination of two or more. Further, the copolymer of propylene and other olefins may be any of a block copolymer, a random copolymer, and a random block copolymer. Examples of the olefin copolymerized with propylene include ethylene, 1-butene, 1-pentene, 4-methyl-
Examples include α-olefins such as 1-pentene, 1-hexene, 1-octene, 1-nonene, and 1-decene.

The polyethylene resin is not particularly limited, and examples thereof include high-density polyethylene, medium-density polyethylene, low-density polyethylene, and linear low-density polyethylene. May be.

And when there is little content of the polyethylene-type resin in the said polyolefin-type resin, since the softness | flexibility of a crosslinked polyolefin-type resin foam sheet will fall, it is limited to 10 weight% or more, and 20 weight% or more is preferable.

On the other hand, if the content of the polyethylene resin in the polyolefin resin is too large, the heat resistance of the cross-linked polyolefin resin foamed sheet may be lowered. Therefore, the content is preferably 70% by weight or less, more preferably 60% by weight or less. preferable.

That is, the polyolefin resin is a polypropylene resin obtained using a metallocene catalyst and having a weight average molecular weight (Mw) to number average molecular weight (Mn) ratio (Mw / Mn) of 3.2 or less. It is preferably composed of 30 to 90% by weight and 10 to 70% by weight of a polyethylene resin, and is obtained by using a metallocene catalyst, and the ratio (Mw) of the weight average molecular weight (Mw) to the number average molecular weight (Mn) / Mn) is a polypropylene resin 40 having a value of 3.2 or less.
More preferably, it consists of ˜80 wt% and polyethylene resin 20˜60 wt%.

If the melt index of the polyolefin resin is small, it may be difficult to produce a foamable resin sheet. On the other hand, if the melt index is large, the heat resistance of the crosslinked polyolefin resin foam sheet may be reduced. 0.5 to 70 g / 10 min is preferable.
5 to 50 g / 10 min is more preferable, and 2 to 40 g / 10 min is particularly preferable. The melt index of the polyolefin resin is 230 ° C. according to JIS K7210.
This is measured under the condition of load 2.12N.

The crosslinking aid is not particularly limited as long as it is used in the production of foams. For example, divinylbenzene, trimethylolpropane trimethacrylate, 1,9
Nonanediol dimethacrylate, 1,10-decanediol dimethacrylate, trimellitic acid triallyl ester, triallyl isocyanurate, ethyl vinyl benzene,
Neopentyl glycol dimethacrylate, 1,2,4-benzenetricarboxylic acid triallyl ester, 1,6-hexanediol dimethacrylate, lauryl methacrylate,
Examples include stearyl methacrylate, diallyl phthalate, diallyl terephthalate, diallyl isophthalate, and the like. These may be used alone or in combination of two or more. In addition, since some crosslinking assistants become unstable and homopolymerize when the purity is too high, low purity ones may be used.

And if content of the crosslinking adjuvant in the said foamable resin sheet is small, since a desired crosslinking degree cannot be provided to a foamable resin sheet, it is 2. with respect to 100 weight part of said polyolefin resin. It is limited to 0 parts by weight or more, preferably 2 to 30 parts by weight,
-20 parts by weight is more preferable, and 2.5-10 parts by weight is particularly preferable.

Furthermore, the pyrolytic foaming agent is not particularly limited as long as it has been conventionally used in the production of foams. For example, azodicarbonamide, benzenesulfonylhydrazide, dinitrosopentamethylenetetramine, toluenesulfonyl Examples thereof include hydrazide and 4,4-oxybis (benzenesulfonylhydrazide), and these may be used alone or in combination of two or more.

If the amount of the pyrolytic foaming agent added is small, the foamable resin sheet may not foam. On the other hand, if the amount added is large, the foamable resin sheet may break during foaming. 1 to 50 parts by weight is preferable with respect to 100 parts by weight, and 4 to 25 parts by weight is more preferable.

Further, the foamable resin sheet has a cell forming agent such as calcium carbonate, talc, clay, magnesium oxide, 2,6-di-t-butyl-p within the range that does not impair the physical properties of the crosslinked polyolefin resin foam sheet. -Phenols such as cresol, phosphorus, amines,
Sulfur-based antioxidants such as dilauryl thiopropionate, metal hazard inhibitors such as methylbenzotriazole, halogen-based flame retardants such as hexabromodiphenyl ether, phosphorus-based flame retardants such as ammonium polyphosphate and trimethyl phosphate, Additives such as fillers, antistatic agents, stabilizers and pigments may be contained.

The foamable resin sheet is molded using a general-purpose method, and as a method for producing the foamable resin sheet, for example, in a general-purpose kneading apparatus such as a single-screw extruder, a twin-screw extruder, a Banbury mixer, a roll, Examples thereof include a polyolefin-based resin, a crosslinking aid, a thermal decomposition-type foaming agent, and, if necessary, the above additives are supplied and melt-kneaded to form a sheet.

And if the thickness of a foamable resin sheet is thin, variation may generate | occur | produce in the bubble of a crosslinked polyolefin resin foamed sheet, Therefore 0.5 mm or more is preferable and 0.7 mm or more is more preferable.

On the other hand, even if the thickness of the foamable resin sheet is large, there may be variations in the air bubbles in the crosslinked polyolefin resin foamed sheet. Therefore, it is preferably 3 mm or less, more preferably 2.5 mm or less, and 2.0 mm or less. Particularly preferred is 1.5 mm or less.

In the present invention, the “degree of crosslinking” means that measured by the following method. That is, the resin sheet is weighed, immersed in xylene at 120 ° C. for 24 hours, the insoluble matter is filtered through a 200 mesh wire mesh, the residue on the wire mesh is vacuum dried, and the weight of the dry residue is measured. (Bg),
It is calculated by the following formula.
Crosslinking degree (% by weight) = (B / A) × 100

Next, both surfaces of the foamable resin sheet obtained as described above are irradiated with ionizing radiation twice under different conditions. Examples of the ionizing radiation include α rays, β rays, γ rays, electron beams, and the like, and electron beams are preferable.

The ionizing radiation irradiated on the both surfaces of the foamable resin sheet for the first time needs to satisfy the following two conditions. The first condition is that when the foaming resin sheet is irradiated with ionizing radiation in the thickness direction of the sheet, the surface of the foaming resin sheet reaches a depth of 2/3 of the thickness of the foaming resin sheet. It is necessary to irradiate both surfaces of the foamable resin sheet with ionizing radiation at an acceleration voltage that attenuates to zero. In addition, the thickness direction of a foamable resin sheet means the direction orthogonal to the surface of a foamable resin sheet.

When the ionizing radiation at the first time is an accelerating voltage that does not decay to zero even if it exceeds the depth of 2/3 of the thickness of the foamable resin sheet from the surface of the foamable resin sheet, This is because the entire foamable resin sheet is largely cross-linked by the irradiation of the active radiation, and the flexibility of the resulting cross-linked polyolefin resin foam sheet is lowered. The acceleration voltage of the first ionizing radiation is preferably an acceleration voltage that attenuates to zero from the surface of the foamable resin sheet until it reaches half the thickness of the foamable resin sheet.

On the other hand, if the acceleration voltage of the first ionizing radiation is small, the degree of cross-linking is insufficient in the surface portion of the foamable resin sheet, and the heat resistance of the cross-linked polyolefin resin foam sheet may be lowered. It is preferable that the acceleration voltage attenuates to zero at a depth exceeding 1/5 of the thickness of the foamable resin sheet from the surface of the sheet, and the thickness of the foamable resin sheet from the surface of the foamable resin sheet. More preferably, the acceleration voltage attenuates to zero at a depth exceeding a depth of ¼.

Here, when ionizing radiation is irradiated to the foamable resin sheet in the thickness direction, the depth of the ionizing radiation from the surface of the foamable resin sheet is attenuated to zero.
It can be calculated based on the Dose curve.

This Depth-Dose curve can be obtained as follows. That is, a polyethylene terephthalate film having a predetermined thickness is placed on a cellulose triacetate film, and the polyethylene terephthalate film is irradiated with ionizing radiation. Next, the absorbance at 280 nm in the cellulose triacetate film is measured, and the degree of attenuation of the electron beam is measured from this absorbance, for example, a Depth-Dose curve as shown in FIG. 1 can be obtained.

The degree of attenuation of the electron beam does not depend on the type of the synthetic resin, depends on the specific gravity of the synthetic resin, and is proportional to the specific gravity of the synthetic resin (proportional constant: 1). By calculating proportionally the experimental values corresponding to the specific gravity of the foamable resin sheet, it is possible to determine at what depth the ionizing radiation attenuates to zero from the surface of the foamable resin sheet.

For example, as shown in FIG. 1, when the surface of a polyethylene terephthalate film having a specific gravity of 1 and a thickness of 1 mm is irradiated with an electron beam in the thickness direction, the surface of the polyethylene terephthalate film is exposed from the surface of the polyethylene terephthalate film at an acceleration voltage of about 300 kV. 2/3 of sheet thickness in the thickness direction
It can be seen that the electron beam is attenuated to zero at a depth of. In other words, even in the case of a foamable resin sheet having a specific gravity of 1 and a thickness of 1 mm, when the surface of the foamable resin sheet is irradiated with an electron beam in the thickness direction, the foamable resin sheet has an acceleration voltage of about 300 kV. It can be seen that the electron beam attenuates to zero at a depth of 2/3 of the sheet thickness in the thickness direction from the surface.

Next, as the second condition, the irradiation dose of the ionizing radiation to the foamable resin sheet is a dose not more than the reference dose of the polypropylene resin, preferably not more than 90% of the reference dose, more preferably the reference dose. Performed at a dose of 80% or less. This is because, when the irradiation dose of ionizing radiation is large, the degree of elongation at room temperature of the resulting crosslinked polyolefin resin foamed sheet is lowered, and the stamping moldability is lowered.

On the other hand, if the irradiation dose of the ionizing radiation to the foamable resin sheet is too small, the crosslinking degree of the resulting crosslinked polyolefin resin foamed sheet may be insufficient and the heat resistance may be lowered. 20% or more of the dose is preferable, 30% of the standard dose
More preferably, the above dose is used.

Here, the reference dose of the polypropylene resin is that the degree of cross-linking of the polypropylene resin sheet gradually increases as the dose of ionizing radiation applied to the polypropylene resin sheet is gradually increased. When the dose of ionizing radiation reaches a certain dose, the degree of cross-linking of the polypropylene resin sheet does not increase even if the dose of ionizing radiation is increased further. The irradiation dose of ionizing radiation at the time when the degree of crosslinking of the polypropylene resin sheet is no longer high is referred to as the reference dose of the polypropylene resin.

Specifically, on the one side of a polypropylene resin sheet made of the same polypropylene resin as the polypropylene resin constituting the foamable resin sheet and having a thickness of 1 mm, various acceleration voltages are applied in the thickness direction at 800 kV. Irradiate with ionizing radiation at a dose.

And measure the degree of crosslinking of the polypropylene resin sheet after irradiating ionizing radiation,
Even if the irradiation dose of ionizing radiation is increased, the irradiation dose of ionizing radiation when the degree of crosslinking of the polypropylene resin sheet no longer increases is set as the reference dose of the polypropylene resin (in FIG. 2, 6Mrad is the reference dose for polypropylene resins).

Next, the second ionizing radiation is irradiated to the foamable resin sheet. The second irradiation with ionizing radiation to the foamable resin sheet is performed under the following two conditions. First, as the first condition, the acceleration voltage of the second ionizing radiation is adjusted to be higher than the acceleration voltage of the first ionizing radiation. In addition, it is preferable to adjust the acceleration voltage of the second ionizing radiation so that a substantially uniform dose is irradiated in the thickness direction of the foamable resin sheet.

This is because when the acceleration voltage of the second ionizing radiation is equal to or less than the acceleration voltage of the first ionizing radiation, the degree of cross-linking in the central portion in the thickness direction of the foamable resin sheet is insufficient, and the resulting cross-linked polyolefin resin This is because the flexibility of the foam sheet is lowered.

Next, the second condition is that the dose is less than or equal to the first dose of ionizing radiation, preferably 80% or less of the dose of the first ionizing radiation, more preferably the first dose. The foaming resin sheet is irradiated with the second ionizing radiation at a dose of 70% or less of the ionizing radiation dose.

This is because when the dose of ionizing radiation for the second time is larger than the dose of ionizing radiation for the first time, the degree of crosslinking at the center in the thickness direction of the foamable resin sheet becomes greater than the degree of crosslinking at the surface. This is because the flexibility of the obtained cross-linked polyolefin resin foam sheet is lowered.

Thus, after ionizing radiation is divided into two portions and irradiated to the foamable resin sheet with a predetermined acceleration voltage and irradiation dose, the foamable resin sheet is cross-linked, and then the foamable resin sheet is used in the same manner as before. A crosslinked polyolefin resin foam sheet can be obtained by heating and foaming in the manner described above.

The surface portion of the cross-linked polyolefin resin foam sheet obtained as described above has excellent elongation despite its high degree of cross-linking and excellent heat resistance. In the thickness direction of the cross-linked polyolefin resin foam sheet, the cross-linking polyolefin resin foam sheet has a low degree of cross-linking and has excellent flexibility and elongation at room temperature. It has excellent heat resistance that can withstand resin, and is excellent in flexibility and elongation at room temperature, and in particular, has excellent stamping moldability.

According to the method for producing a crosslinked polyolefin resin foam sheet of the present invention, the foamed resin sheet is irradiated with ionizing radiation twice under different conditions. The surface portion has high heat resistance and excellent elongation, and the central portion in the thickness direction has a lower degree of cross-linking than the surface portion, and excellent flexibility at normal temperature and Has elongation.

Therefore, the crosslinked polyolefin resin foam sheet produced by the method for producing a crosslinked polyolefin resin foam sheet of the present invention has excellent moldability and can be accurately and reliably molded into a complex shape. In particular, since the crosslinked polyolefin resin foam sheet is excellent in heat resistance as described above, it can be molded into a desired shape without being damaged even in stamping molding in which a synthetic resin in a molten state comes into contact.

(Example 1)
Polypropylene resin obtained using a metallocene catalyst (trade name “Nippon Polychem”
WFX6 ", melt index: 2.0 g / 10 min, Mw / Mn: 2.8, density: 0.00.
90 g / cm ³ , standard dose: 6 Mrad 60 parts by weight, linear low-density polyethylene (trade name “ZF231” manufactured by Tosoh Corporation), melt index: 2.0, density: 0.917 g / c
m ³ ) 40 parts by weight, 3 parts by weight of divinylbenzene, 13 parts by weight of azodicarbonamide, 2,
0.3 parts by weight of 6-di-t-butyl-p-cresol, dilauryl thiopropionate
3 parts by weight and 0.5 parts by weight of methylbenzotriazole were supplied to a single screw extruder, melted and kneaded at 185 ° C., and extruded into a sheet to obtain a foamable resin sheet having a thickness of 1 mm.

The first electron beam was applied to both sides of the obtained foamable resin sheet at an acceleration voltage of 200 kV and 2.4 Mr.
Irradiated with a dose of ad. The electron beam is 0.47 m from the surface of the foamable resin sheet.
It attenuated to zero at a depth of m. Furthermore, the electron beam of the 2nd time was irradiated to both surfaces of the foamable resin sheet with the dose of 1.3 Mrad with the acceleration voltage of 800V.

Thereafter, the foamable resin sheet is heated to 250 ° C. to be foamed, and the density is 0.056.
A cross-linked polyolefin resin foam sheet of g / cm ³ was obtained. The density of the cross-linked polyolefin resin foamed sheet is determined by an electronic hydrometer (trade name “Electron Hydrometer ED20T manufactured by Mirage”).
)).

(Comparative Example 1)
A crosslinked polyolefin resin foam sheet having a density of 0.048 g / cm ³ was obtained in the same manner as in Example 1 except that the polypropylene resin was 25 parts by weight and the linear low density polyethylene was 75 parts by weight. .

(Comparative Example 2)
As polypropylene resin, polypropylene resin (trade name “M” manufactured by Sun Allomer Co., Ltd.
822V ", melt index: 20 g / 10 min, Mw / Mn: 3.8), and a crosslinked polyolefin resin foam sheet having a density of 0.053 g / cm ³ was obtained in the same manner as in Example 1. It was.

(Comparative Example 3)
A cross-linked polyolefin resin having a density of 0.066 g / cm ^{3 in} the same manner as in Example 1 except that both surfaces of the foamable resin sheet were irradiated with a first electron beam at an acceleration voltage of 400 kV and a dose of 2.4 Mrad. A foam sheet was obtained. The electron beam was attenuated to zero at a depth of 0.75 mm from the surface of the foamable resin sheet.

(Comparative Example 4)
A crosslinked polyolefin resin foam sheet having a density of 0.044 g / cm ^{3 in} the same manner as in Example 1 except that both surfaces of the foam resin sheet were irradiated with a first electron beam at an acceleration voltage of 200 kV and a dose of 8 Mrad. Got.

(Comparative Example 5)
An attempt was made to produce a crosslinked polyolefin resin foam sheet in the same manner as in Example 1 except that a second electron beam was applied to both sides of the foamable resin sheet at an acceleration voltage of 100 kV with a dose of 1.8 Mrad. The surface roughness was sometimes severe, and a crosslinked polyolefin resin foam sheet worthy of evaluation could not be obtained.

(Comparative Example 6)
An attempt was made to produce a crosslinked polyolefin resin foam sheet in the same manner as in Example 1 except that a second electron beam was applied to both surfaces of the foamable resin sheet at an acceleration voltage of 800 kV with a dose of 6 Mrad. The roughness was so severe that a crosslinked polyolefin resin foam sheet worthy of evaluation could not be obtained.

The stamping moldability of the crosslinked polyolefin resin foam sheets obtained in Example 1 and Comparative Examples 1 to 4 was measured in the manner shown below.

(Stamping formability)
A test piece was prepared by cutting out from a cross-linked polyolefin-based resin foam sheet into a planar square shape having a side of 150 mm. This test piece is disposed between flat plate-shaped upper and lower molds held at 20 ° C., and a molten polypropylene resin (melt index: 2) at a temperature of 200 ° C. is formed on the test piece.
0 g / 10 min), and then the upper and lower molds were clamped for 50 seconds with a pressure of 4.9 × 10 ⁶ Pa.

Subsequently, the upper and lower molds are clamped for 50 seconds with a pressure of 9.8 × 10 ⁵ Pa, and the upper and lower molds are cooled by passing water into the upper and lower molds. Opened to obtain a molded product. The surface of the obtained molded product was visually observed and judged according to the following criteria. In the molded product of Example 1, the foamed sheet was not torn and the appearance was good. In the molded articles of Comparative Examples 1 to 4, the foamed sheet was torn, and the polypropylene resin was exposed through this tearing, and there was a problem in appearance.

It is the graph which showed an example of the Depth-Dose curve. It is the graph which showed an example of the relationship between the irradiation dose of ionizing radiation, and the crosslinking degree of a polypropylene resin sheet.

Claims (1)

The weight average molecular weight (Mw) and the number average molecular weight (Mw) obtained using a metallocene catalyst
100 parts by weight of a polyolefin resin containing 30% by weight or more of a polypropylene resin having a ratio (Mw / Mn) of 3.2 or less to Mn), 2.0 parts by weight or more of a crosslinking aid, and a pyrolytic foaming agent Accelerating voltage that attenuates to zero on both sides of the foamable resin sheet to reach a depth of 2/3 of the sheet thickness from the surface of the foamable resin sheet when the foamable resin sheet is irradiated in the thickness direction. And after irradiating the first ionizing radiation at a dose below the reference dose of the polypropylene resin, the acceleration voltage is higher than the acceleration voltage of the first ionizing radiation and less than the first ionizing radiation dose. A method for producing a cross-linked polyolefin resin foam sheet, comprising heating and foaming the foamable resin sheet after irradiating the foamable resin sheet with a second dose of ionizing radiation.

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