WO2014155521A1

WO2014155521A1 - Polyolefin resin foam sheet, sound absorbing material, and automotive parts, and method for producing polyolefin resin foam sheet

Info

Publication number: WO2014155521A1
Application number: PCT/JP2013/058764
Authority: WO
Inventors: 中山　弘哲
Original assignee: 古河電気工業株式会社
Priority date: 2013-03-26
Filing date: 2013-03-26
Publication date: 2014-10-02

Abstract

A polyolefin resin foam sheet having open cells and a portion in which the cell diameter changes along a thickness direction.

Description

POLYOLEFIN RESIN FOAM SHEET, SOUND ABSORBING MATERIAL, AUTOMOTIVE PARTS, AND METHOD FOR PRODUCING POLYOLEFIN RESIN FOAM SHEET

The present invention relates to a polyolefin resin foam sheet, a sound absorbing material using the foamed sheet, and an automobile part using the sound absorbing material. Moreover, this invention relates to the manufacturing method of the said foam sheet.

Foam is widely used as a sound absorbing material in the automobile field such as vehicles. Examples of such foam include urethane foam made of urethane resin, rubber foam made of synthetic rubber or natural rubber, and polyolefin. Examples include thermoplastic resin foams.

In recent years, these sound-absorbing materials are strongly required to be recyclable due to demands for environmental considerations. Especially in the automotive field, polyolefin resins such as polypropylene resin, low density polyethylene resin, high density polyethylene resin, and linear low density polyethylene resin are excellent in heat resistance, mechanical strength and moldability in addition to recyclability. Widely used as a material.

However, many of such sound-absorbing materials have a narrow bubble diameter distribution and are adjusted to a substantially uniform bubble diameter, and the sound absorbing property is not sufficient. For example, a sound-absorbing material having a thickness of 4 mm containing polypropylene resin cannot obtain a sufficient sound-absorbing property against high-frequency noise of an electric vehicle if the bubble diameter is made uniform.
Under such circumstances, a laminated structure in which sound absorbing materials are laminated is widely used in order to enhance sound absorption at a wider frequency, but it is difficult to avoid an increase in the weight and thickness of the sound absorbing material.

Patent Document 1 describes a foamed molded article containing at least three types of layers, a skin layer, a low foam layer, and a high foam layer, in this order. The problem is to improve the rigidity in the direction, and not to improve the sound absorption.

In recent years, the use of resinous foam sheets as exterior members for reducing the air resistance during driving of automobiles has been studied, and improvement in cold shock resistance is also required.

JP 2005-59224 A

The present invention uses a polyolefin resin foam sheet having improved sound absorption for noise in a wide region including a high frequency region while suppressing an increase in weight and thickness, a sound absorbing material using the foam sheet, and the sound absorbing material. It is an object to provide automotive parts.
Moreover, this invention makes it a subject to provide the manufacturing method of the said foam sheet.

The above object of the present invention has been achieved by the following means.
<1> A polyolefin resin foam sheet having continuous air bubbles,
A polyolefin-based resin foam sheet characterized by having a portion in which the bubble diameter changes along the thickness direction.
<2> The polyolefin resin foam sheet according to <1>, wherein the following (A) to (D) are satisfied:
(A) The thickness of the foam sheet is 1.0 mm or more,
(B) The open cell ratio of the entire foam sheet is 50% or more,
(C) A skin layer is formed on one surface of the foam sheet without exposing bubbles, and bubbles are exposed on the other surface,
(D) At the position of 0.5 mm from the skin layer side surface of the foam sheet toward the thickness direction of the foam sheet, the bubble diameter on the slice surface cut perpendicularly to the thickness direction is D1,
When the bubble diameter of the slice surface cut perpendicularly to the thickness direction is D2 at a position exceeding 0.5 mm from the skin layer side surface of the foam sheet toward the thickness direction of the foam sheet,
D2-D1 ≧ 0.05mm
The part which becomes.
<3> The polyolefin resin foam sheet according to <2>, wherein D1 is 0.25 mm or less.
<4> The polyolefin-based resin foam sheet according to any one of <1> to <3>, comprising a single layer.
<5> The polyolefin resin foam sheet according to any one of <1> to <4>, wherein the normal incident sound absorption coefficient at a measurement frequency of 5 to 7 kHz is 0.6 or more.
<6> The polyolefin-based resin foamed sheet according to any one of <1> to <5>, comprising high-density polyethylene.
<7> The polyolefin-based resin foam sheet according to any one of <1> to <6>, comprising a fibrous material.
<8> The polyolefin resin foam sheet according to any one of <1> to <7>, wherein the apparent density of the polyolefin resin foam sheet is from 180 kg / m ^{3 to} 600 kg / m ³ .
<9> A sound absorbing material comprising the polyolefin resin foam sheet according to any one of <1> to <8>.
<10> An automobile part using the sound absorbing material according to <9>.
<11> A method for producing a polyolefin resin foam sheet having continuous air bubbles,
A foamed sheet precursor having an increased bubble diameter is formed in an inclined manner from the sheet surface toward the center along the thickness direction, and the sheet precursor is cut perpendicularly to the thickness direction of the sheet precursor. A method for producing a polyolefin-based resin foam sheet.

The polyolefin-based resin foam sheet of the present invention has a portion where the bubble diameter changes along the thickness direction. That is, it has a portion where the apparent bubble diameter that continues as the communicating bubble changes. Thereby, the sound wave incident from one surface can be efficiently diffused inside the sheet and converted into thermal energy, and good sound absorption is exhibited in a wider frequency range.
The sound absorbing material of the present invention exhibits good sound absorbing properties in a wider frequency range.
The automobile part of the present invention can effectively absorb high-frequency noise from the vehicle, and can further suppress noise during engine operation.
According to the production method of the present invention, it is possible to obtain a polyolefin-based resin foam sheet having excellent sound absorption characteristics over a wider frequency range.

The above and other features and advantages of the present invention will become more apparent from the following description with reference to the accompanying drawings as appropriate.

It is a figure which shows the cut surface of the foam sheet of Example 1. FIG. It is a figure which shows the result of having measured the sound absorption data with the foam sheet of Example 1. FIG. It is a figure which shows the cut surface of the foam sheet of the comparative example 1. It is a figure which shows the result of having measured the sound absorption data with the foam sheet of the comparative example 1. FIG.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings as appropriate.
The polyolefin-based resin foam sheet of the present invention (hereinafter, also simply referred to as “the foam sheet of the present invention”) is characterized by having a portion where the bubble diameter changes along the thickness direction.
In the present specification, “having a portion where the bubble diameter changes” means that the apparent bubble diameter connected as continuous bubbles increases or decreases along the thickness direction of the foam sheet. Say that you have a part to do. More specifically, when the foam sheet is cut perpendicularly to the thickness direction to form a plurality of divided sheets having a thickness of 0.5 mm, the cell diameter of the slice intermediate surface, which is the cut surface of the divided sheet, is one side. It means that there is a divided sheet that is different between the surface and the other surface (preferably the bubble diameter is 0.02 mm or more, more preferably the bubble diameter is 0.03 mm or more). Although it does not specifically limit as a bubble diameter, Specifically, it is preferable to change within the range of about 100 micrometers-600 micrometers.
In this specification, the “bubble diameter” is calculated by the following formula using the bubble area average value S obtained by dividing the total area of the bubble portions in the slice intermediate plane by the number of bubbles.
Bubble diameter = 2 × √ (S / π)

More preferably, the polyolefin resin foam sheet of the present invention has a small or large bubble diameter in an inclined manner along the thickness direction of the sheet.
In the present specification, the form “in which the bubble diameter is small or large in an inclined manner” is as follows: (i) The bubble diameter gradually decreases or increases from one surface to the opposite surface. Or (ii) the bubble diameter gradually increases from both sides of the sheet toward the central portion so that the bubble diameter becomes maximum near the central portion along the thickness direction of the sheet. Are preferred.

In the case of the above (i), the foamed sheet of the present invention has a thickness of 1.0 mm or more, the bubble diameter on one surface is 0.01 mm or less, and from the one surface toward the sheet thickness direction. The cell diameter of the slice intermediate surface cut perpendicularly to the thickness direction at a position of 0.5 mm ± 0.05 mm is 0.01 mm or more, and the other surface (the surface opposite to the one surface) ) Is preferable that the bubble diameter is 0.2 mm or more. The sheet surface with the bubble diameter of 0.01 mm or less preferably has no skin and is a skin layer (that is, the bubble diameter is preferably 0 mm).

More preferably, the foamed sheet of the present invention satisfies the following (A) to (D).
(A) The thickness of the foam sheet is 1.0 mm or more,
(B) The open cell ratio of the entire foam sheet is 50% or more,
(C) One surface of the foam sheet is formed with a skin layer without exposing bubbles, and the other surface (the surface facing the surface on the skin layer side) is exposed with bubbles,
(D) At the position of 0.5 mm from the skin layer side surface of the foam sheet toward the thickness direction of the foam sheet, the bubble diameter on the slice surface cut perpendicularly to the thickness direction is D1,
When the bubble diameter of the slice surface cut perpendicularly to the thickness direction is D2 at a position exceeding 0.5 mm from the skin layer side surface of the foam sheet toward the thickness direction of the foam sheet,
D2-D1 ≧ 0.05mm
The part which becomes.

The “position of 0.5 mm from the skin layer side surface of the foam sheet toward the thickness direction of the foam sheet” in the above (D) is preferably not a skin layer. The thickness of the skin layer of the foam sheet satisfying the above (A) to (D) is preferably 0.4 mm or less, and more preferably 0.2 mm or less.
In addition, the “position exceeding 0.5 mm from the skin layer side surface of the foam sheet in the thickness direction of the foam sheet” depends on the thickness of the foam sheet, for example, from the skin layer side surface of the foam sheet. The position of 1.0 mm toward the thickness direction of the foamed sheet may be sufficient, the position of 1.5 mm may be sufficient, and the position of 2.0 mm may be sufficient.
The D1 is preferably 0.25 mm or less. Further, the D1 is preferably 0.1 mm or more.

When the foamed sheet of the present invention is used for an automobile part, for example, the foamed sheet of the present invention has a continuous bubble and has a portion in which the bubble diameter changes along the sheet thickness direction. The noise generated from the engine or the like can be drawn into the seat from the incident surface (one surface), and the low frequency sound region can be appropriately diffused in the bubbles, and from the incident surface to the other surface (outgoing surface) Since the sound transmission path is formed long like a labyrinth, the high frequency sound range can also be converted into thermal energy while efficiently diffusing inside the seat. As a result, the sound absorption efficiency in a wide frequency range can be increased.
By changing the bubble diameter in an inclined manner from the incident surface toward the other surface (outgoing surface), it is possible to further improve the sound absorption effect in the high frequency sound range.

Preferably, the foam sheet of the present invention is composed of a single layer made of the same material. With this configuration, the weight and cost of the sound-absorbing material can be reduced as compared with the conventional laminated sheet, and the moldability is significantly improved. The foamed sheet of the present invention exhibits excellent sound absorption even with a single layer structure.
As described above, it is preferable that at least one of the surface layers of the foamed sheet of the present invention is a skin layer in which bubbles are not exposed, from the viewpoint of sound diffusion and blocking inside the material. It is more preferable that the surface (outgoing surface) opposite to the sound incident surface is a skin layer.

The foamed sheet of the present invention preferably has a normal incidence method sound absorption coefficient of 0.5 or more at a measurement frequency of 5 to 7 kHz, more preferably 0.6 or more, and even more preferably 0.75 or more. .
For example, an electric vehicle has a motor sound or an inverter noise around 6 kHz. If the foam sheet of the present invention is molded into a desired shape and applied to a dash insulator for an automobile, the sound absorbing material has high sound absorption and rigidity. It can be.
Here, the normal incidence method sound absorption coefficient is a numerical value measured based on JIS A 1405.

In the present invention, “having communicating bubbles” means having so-called open cells communicating with each other at the joint. That is, the bubbles in the foamed sheet of the present invention form a long cavity like a labyrinth that is connected to adjacent bubbles without closing. In the whole foam sheet of the present invention, the ratio of the open cells (open cell ratio) is preferably 50% or more. The ratio of the open cells is more preferably 50 to 90%, further preferably 53 to 85%, and further preferably 55 to 80%. The open cell ratio is measured by the method described in Examples described later.
If the open cell ratio is too low, the sound absorption of the polyolefin resin foam sheet may be reduced, and if it is too high, the moldability and rigidity of the polyolefin resin foam sheet may be reduced during compression.

Further, when the open cell ratio is in the above preferred range, the polyolefin resin foam sheet becomes porous, so that the maze degree of the solid propagation sound of the incident sound wave and the propagation path of the air propagation sound is increased, and the sound absorption is improved. it can.

The apparent density of the foamed sheet of the present invention is preferably 600 kg / m ³ or less, more preferably 180 to 450 kg / m ³ . The apparent density is a value measured according to JIS K 7222: 2005.
If the apparent density is too high, sound absorption may be reduced. On the other hand, if it is too low, foam moldability by extrusion may be reduced. That is, by making the apparent density within the above preferable range, it is possible to achieve both moldability and sound absorption at a higher level.

Examples of the polyolefin resin constituting the foamed sheet of the present invention include low density polyethylene, medium density polyethylene, high density polyethylene, linear low density polyethylene, linear ultra-low density polyethylene, and ethylene-propylene block copolymer. , Ethylene-propylene random copolymer, ethylene-butene block copolymer, ethylene-butene random copolymer, ethylene-vinyl acetate copolymer, ethylene-methyl methacrylate copolymer, ethylene-methacrylic acid copolymer molecule Ionomer resin crosslinked with metal ions, propylene homopolymer, propylene-ethylene random copolymer, propylene-butene random copolymer, propylene-ethylene block copolymer, propylene-butene block copolymer, polybutene, Polypente , Propylene - ethylene - butene terpolymer, a propylene - acrylic acid copolymer, propylene - maleic anhydride copolymer are exemplified, and these may be used alone or in combination. Among these, the polyolefin resin constituting the foamed sheet of the present invention is preferably composed of at least one selected from high-density polyethylene, propylene homopolymer, and propylene-ethylene block copolymer.
The polyolefin resin constituting the foamed sheet of the present invention preferably contains a polypropylene resin from the viewpoints of recyclability, moldability, and heat resistance. The content of the polypropylene resin in the foamed sheet may be 100% by mass, but is preferably 75 to 99.9% by mass, and may be 95 to 99% by mass. Further, from the viewpoint of obtaining good cold shock resistance, it is preferable to include high-density polyethylene. When the foamed sheet of the present invention contains high-density polyethylene, the content thereof may be 100% by mass, but is preferably 75 to 99.9% by mass, and more preferably 95 to 99% by mass.

The foamed sheet of the present invention preferably contains a propylene-ethylene block copolymer. The foamability of the foam sheet obtained by including the propylene-ethylene block copolymer is improved. That is, since the melting temperature at the time of extrusion is relatively low, viscoelasticity suitable for foaming of the resin composition appears over a wide temperature range, and as a result, a good foamed structure can be easily formed.
The propylene-ethylene block copolymer preferably has a melt index of 1.9 or less.
In the foamed sheet of the present invention, the content of the propylene-ethylene block copolymer is preferably 15 to 70% by mass.

The foamed sheet of the present invention may contain a thermoplastic resin other than the polyolefin resin and the thermoplastic elastomer described later as long as the effects of the present invention are not significantly impaired. As used herein, “other thermoplastic resin” means a resin that does not contain halogen, such as polystyrene, polymethyl methacrylate, acrylic resin such as styrene-acrylic acid copolymer, styrene-butadiene copolymer, polyvinyl acetate, polyvinyl Alcohol, polyvinyl acetal, polyvinyl pyrrolidone, petroleum resin, cellulose, cellulose acetate, cellulose nitrate, methyl cellulose, hydroxymethyl cellulose, hydroxymethyl cellulose, hydroxypropyl cellulose and other cellulose derivatives, saturated alkyl polyester resins, polyethylene terephthalate, polybutylene terephthalate, polyarytate Group polyester resin, polyamide resin, polyacetal resin, polycarbonate resin, polyester sulfone resin, poly E double sulfide resin, polyether ketone resin, and a copolymer having a vinyl polymerizable monomer and nitrogen-containing vinyl monomers. One type of thermoplastic resin other than these polyolefin-based resins may be used, or a plurality of types may be included. The kind and amount can be selected according to the desired physical properties.

The foamed sheet of the present invention may contain a cell nucleating agent for refining the cells as necessary. As the cell nucleating agent, inorganic fine particles such as talc, calcium carbonate, titanium oxide, and barium sulfate, heat-decomposable organic cell nucleating agent, thermoplastic elastomer, and melt-type crystallization nucleating agent can be used. It is not limited to.
In the foam sheet of the present invention, the content of the cell nucleating agent is preferably 0.1 to 30 parts by mass with respect to 100 parts by mass of the polyolefin resin.

In the foam sheet of the present invention, in addition to the cell nucleating agent, the foaming agent, the crystallization accelerator, the antioxidant, the antistatic agent, the ultraviolet ray preventing agent, as long as the purpose of the present invention is not impaired. Light stabilizers, fluorescent brighteners, pigments, dyes, compatibilizers, lubricants, reinforcing agents, flame retardants, crosslinking aids, plasticizers, thickeners, thickeners, thermal stabilizers, processing aids, impact modifiers Various additives such as a quality agent and a filler can be blended.

The foaming agent is classified into a solid compound that decomposes to generate a gas, a liquid that vaporizes when heated, and an inert gas that can be dissolved in a resin under pressure, and any of these can be used. Among these, as solid compounds, for example, azodicarbonamide, dinitrosopentamethylenetetramine, hydrazole dicarbonamide, sodium bicarbonate, p, p′-oxybisbenzenesulfonylhydrazide (OBSH), N, N′-dinitrosopentamethylene Examples thereof include tetramine (DPT), p-toluenesulfonyl hydrazide, benzenesulfonyl hydrazide, diazoaminobenzene, N, N′-dimethyl-N, N′-dinitroterephthalamide, azobisisobutyronitrile and the like. Examples of the liquid to be vaporized include saturated aliphatic hydrocarbons such as propane, n-butane, isobutane, n-pentane, isopentane, and hexane, aromatic hydrocarbons such as benzene, xylene, and toluene, methyl chloride, and chlorofluorocarbon. And halogenated hydrocarbons such as dimethyl ether and ether compounds such as methyl tert-butyl ether. Further, examples of the inert gas include inert gases such as carbon dioxide, nitrogen, helium, and argon, and alternative chlorofluorocarbons. Carbon dioxide (carbon dioxide gas) is more preferable in consideration of the solubility in the resin and the ease of miniaturization of the bubbles.

The foamed sheet of the present invention preferably contains a fibrous material. By blending the fibrous material, weight reduction and cold shock resistance can be improved. Moreover, the fibrous material does not affect the foamability.
Examples of the fibrous material include polyamide fiber, polyvinyl alcohol fiber, cellulose semi-synthetic fiber, cellulose regenerated fiber, acrylic fiber, polyester fiber, polyurethane fiber, polyvinyl chloride fiber, polyvinylidene chloride fiber, aramid fiber, poly Allylate fiber, polyimide fiber, polyparaphenylene benzobisoxazole fiber, polyphenylene sulfide fiber, polyethylene fiber, polypropylene fiber, fluorine fiber, carbon fiber, polycarbonate fiber, glass fiber, epoxy fiber, phenol fiber and melamine fiber, etc. 1 type, or 2 or more types of these can be used. Among these, glass fiber is particularly preferable in order to maintain a high foaming rate and high rigidity. Furthermore, in order to maintain a higher foaming rate, the content of the fibrous material in the foamed sheet is preferably 15% by mass or less, more preferably 1 to 14.9% by mass, and more preferably 5 to 14%. It is more preferably 8% by mass, and further preferably 10 to 14.5% by mass.

An extrusion foam molding method can be used as the method for producing the polyolefin resin foam sheet. For example, in the horizontal direction from the center of the tip of the extrusion foaming die, a molding roll die composed of a pair of (two) cooling rolls adjusted to a temperature not higher than the glass transition point (Tg) of the polyolefin resin is installed. The resin-based fluid is passed between the molding roll dies and continuously extruded and foamed in the longitudinal direction without being crushed by the lip of the roll die, and then the extruded product is cooled by two cooling rolls to obtain polyolefin. A sheet precursor of the resin-based resin foam sheet is generated. Since the sheet precursor is cooled without being crushed, the bubble diameter of the communicating bubbles can be increased in an inclined manner from one surface to the central portion along the thickness direction of the sheet precursor. By slicing the precursor vertically along the thickness direction along the longitudinal direction so that the precursor has a desired length and thickness, for example, 4 mm thickness of 1000 × 1000 along the longitudinal direction, the polyolefin resin foam Sheets can be manufactured.
Further, a sheet precursor may be generated by preparing an extruded sheet in advance and simultaneously performing crosslinking and foaming on the sheet.
The above Tg is a value measured with a differential scanning calorimeter (DSC-60, manufactured by Shimadzu Corporation) from −50 ° C. to 200 ° C. at a heating rate of 5 ° C./min. The extrapolated glass transition start temperature in “Transition temperature measurement method” was defined as Tg.

The cutting method of the foam sheet may be either a band knife method or a canna method. For example, the band knife method uses a band knife with a belt-shaped blade as a ring and sends the foam sheet to the rotated band knife. A process of slicing by cutting parallel to the sheet surface.

The thickness of the foamed sheet of the present invention is preferably 1.0 mm or more. The thickness of the foamed sheet of the present invention is usually 20 mm or less, and may be 10 mm or less.

The foam sheet of the present invention is lightweight and excellent in sound absorption, and is applied to a sound absorbing material of an automobile, particularly an electric automobile, for example, a dash insulator located on the side of a cab of a dash panel after being molded into a desired shape, Can be prevented from propagating to the cab.

Hereinafter, the present invention will be described in more detail based on examples, but the present invention is not limited thereto.

(Example 1)
Pellets of polypropylene resin (Nippon Polypro Novatec PP FY6H: Melt Index 1.9) are supplied to the hopper, a 1360 mm wide sheet die is used, and a 40 mm outer diameter forming roll die is used as the lip tip and cooling roll. When the foam foam sheet precursor having a thickness of 5 mm and a width of 1375 mm was obtained by arranging the distance to the contacting line to 6.0 mm and performing extrusion foam molding. The obtained precursor had an expansion ratio of 3 times. A slice of 1 mm thickness was sliced from one end of the precursor to remove the skin, and a polyolefin resin foam sheet (size 1000 × 1000 × thickness 4 mm) was produced.
As shown in FIG. 1, a skin layer having a thickness of 150 μm or less is formed along one surface (the left surface in FIG. 1) of this foam sheet (hereinafter, this surface is referred to as a skin surface). Moreover, it can be seen that the other surface (the right surface in FIG. 1) is a slice surface, and bubbles are exposed. The area partitioned by the white resin is bubbles, the bubble diameter is small on the left side of the sheet shown in FIG. 1, and the bubble diameter increases as it moves to the right side. The foamed sheet was a flat sheet having no wavy shape.
When the normal incidence sound absorption coefficient on the slice surface (right side of FIG. 1) of this foam sheet was measured, the sound absorption coefficient shown in FIG. 2 was obtained, and the sound absorption coefficient exceeded 0.6 particularly in a high sound range including 5 to 7 kHz. showed that. Moreover, this foam sheet had an open cell ratio of 63% and an apparent density of 300 kg / m ³ .
No bubbles were observed on the skin surface of this foam sheet, and therefore the bubble diameter was 0 mm.
The cell diameter of the slice intermediate surface 0.5 cut perpendicularly to the thickness direction at a distance of 0.5 mm ± 0.05 mm from the skin surface toward the sheet thickness direction was 0.19 mm.
The cell diameter of the slice intermediate surface 1.0 cut perpendicularly to the thickness direction at a distance of 1.0 mm ± 0.05 mm from the skin surface toward the sheet thickness direction was 0.21 mm.
Similarly, the slice intermediate surface 1 cut every 0.5 mm perpendicularly to the thickness direction at a distance of 1.5 mm ± 0.05 mm to 3.5 mm ± 0.05 mm from the skin surface toward the sheet thickness direction. .5, slice intermediate plane 2.0, slice intermediate plane 2.5, slice intermediate plane 3.0, and slice intermediate plane 3.5 have bubble diameters of 0.28 mm, 0.29 mm, 0.28 mm,. They were 28 mm and 0.28 mm.
Furthermore, the bubble diameter of the porous slice surface which is the bubble exposed surface on the opposite side of the skin surface was 0.22 mm.

(Example 2)
In Example 1, a foam sheet having the same size as Example 1 was obtained in the same manner as Example 1 except that high-density polyethylene resin (trade name: Novatec HD HY540, manufactured by Nippon Polypropylene) was used instead of polypropylene resin. It was. The obtained foamed sheet had an open cell ratio of 56% and an apparent density of 320 kg / m ³ . The skin layer thickness on the skin surface was 150 μm or less. When the normal incidence sound absorption coefficient of the sliced surface of this foam sheet was measured, the same result as in Example 1 was obtained, and the sound absorption coefficient at 5 to 7 kHz exceeded 0.6. Further, in the cold shock resistance test, a test was performed in which a 1 kg mass of iron ball was dropped from a position of 50 cm vertically above a test piece at −30 ° C. to give an impact. As a result, no significant damage such as cracking was observed in the test piece even by the impact. On the other hand, in the foam sheet of Example 1, in the cold impact resistance test, the test piece was broken by the impact. That is, it was shown that the foamed sheet obtained in Example 2 has remarkably improved impact resistance in a low-temperature environment by using a high-density polyethylene resin as a main component.
In the foam sheet of Example 2, the cell diameter was examined in the same manner as in Example 1. As in the foam sheet of Example 1, the cell diameter was inclined from the skin surface toward the slice intermediate surface 1.5. It was getting bigger.

(Example 3)
In Example 1, instead of polypropylene resin, Example 1 was used except that 1 part by weight of glass fiber (trade name: Chopped Strand DS 2200-13P, manufactured by 3B Company) was blended with 100 parts by weight of polypropylene resin. In the same manner as in Example 1, a foamed sheet having the same size as Example 1 was obtained. The obtained foamed sheet had an open cell ratio of 56% and an apparent density of 300 kg / m ³ . The skin surface layer thickness was 150 μm or less. When the normal incidence sound absorption coefficient of the sliced surface of this foam sheet was measured, the same result as in Example 1 was obtained, and the sound absorption coefficient at 5 to 7 kHz exceeded 0.6. Further, in the cold shock resistance test, a test was performed in which a 1 kg mass of iron ball was dropped from a position of 50 cm vertically above a test piece at −30 ° C. to give an impact. As a result, no significant damage such as cracking was observed in the test piece even by the impact.
In the foam sheet of Example 3, the cell diameter was examined in the same manner as in Example 1. As in the foam sheet of Example 1, the cell diameter was inclined from the skin surface toward the slice intermediate surface 1.5. It was getting bigger.

(Comparative Example 1)
Pellets of polypropylene resin (Novatech PP FY6H manufactured by Nippon Polypro Co., Ltd .: Melt Index 1.9) are supplied to the hopper of the same apparatus as in Example 1, a sheet die having a width of 1360 mm is used, and a molding roll die having an outer diameter of 300 mm is formed on the lip. When the foam sheet was extrusion-foamed with the distance between the tip and the line where the cooling roll contacts the sheet being 100 mm, a foam sheet precursor having a thickness of 5 mm and a width of 1375 mm was obtained. The obtained precursor had an expansion ratio of 3 times. This precursor was sliced by 1 mm from one end and the skin was removed to produce a polyolefin-based resin foam sheet (size 1000 × 1000 × thickness 4 mm). The cut surface of this foam sheet is shown in FIG. As shown in FIG. 3, from the left end to the right end of the sheet, a uniform cell structure is obtained in which the cell diameters are substantially equal except for the skin surface layer at the left end. That is, it was a flat sheet having a continuous cell structure of uniform cells throughout and having no wavy shape. When the normal incidence sound absorption coefficient of the slice surface of this foam sheet was measured, the sound absorption coefficient shown in FIG. 4 was obtained, and the characteristic that the sound absorption property exceeded 0.4 was not shown. Further, this foamed sheet had an open cell ratio of 25% and an apparent density of 300 kg / m ³ .
In the foam sheet of Comparative Example 1, no bubbles were observed on the skin surface, and therefore the bubble diameter on the skin surface was 0 mm. In addition, the cell diameter of the slice intermediate surface 0.5 cut perpendicularly to the thickness direction at 0.5 mm ± 0.05 mm from the skin surface is 0.31 mm, and is a porous surface opposite to the skin surface. Up to the slice surface, the bubble diameter was 0.28 mm ± 0.04 mm, and no inclined bubble diameter change was observed.

In each of the above examples and comparative examples, various physical properties of the foam sheet were measured as follows.
(Normal incidence method sound absorption coefficient)
The normal incident method sound absorption coefficient was measured at a normal incident sound absorption coefficient based on JIS A 1405. The sound absorption performance was prepared by preparing a cylindrical sample with a diameter of 29 mm using a dumbbell punching machine as the target sheet-like sample, and a multi-channel analysis system SR-4100 (analysis is DS-2100) B tube manufactured by Ono Sokki Co., Ltd. The sound absorption coefficient at 500 Hz to 6.4 kHz was measured for each sample by the 2-microphone method.

(Open cell ratio)
The open cell rate (communication rate) was specified by the following formula using a value measured with an air comparison type hydrometer 1000 type (manufactured by Tokyo Science Co., Ltd.) according to the method described in ASTM D-2856-87. .
Open cell rate (%)
= [(Apparent volume−volume measured with air comparison hydrometer) / apparent volume] × 100
[The apparent volume was calculated from the outer dimensions of the foam sheet. ]

(Cold impact resistance test)
A test was performed on a test piece of −30 ° C. by dropping an iron ball having a mass of 1 kg from a position 50 cm vertically above and giving an impact.

Claims

A polyolefin resin foam sheet having continuous air bubbles,
A polyolefin-based resin foam sheet characterized by having a portion in which the bubble diameter changes along the thickness direction.
The polyolefin resin foam sheet according to claim 1, wherein the following (A) to (D) are satisfied:
(A) The thickness of the foam sheet is 1.0 mm or more,
(B) The open cell ratio of the entire foam sheet is 50% or more,
(C) A skin layer is formed on one surface of the foam sheet without exposing bubbles, and bubbles are exposed on the other surface,
(D) At the position of 0.5 mm from the skin layer side surface of the foam sheet toward the thickness direction of the foam sheet, the bubble diameter on the slice surface cut perpendicularly to the thickness direction is D1,
When the bubble diameter of the slice surface cut perpendicularly to the thickness direction is D2 at a position exceeding 0.5 mm from the skin layer side surface of the foam sheet toward the thickness direction of the foam sheet,
D2-D1 ≧ 0.05mm
The part which becomes.
The polyolefin resin foam sheet according to claim 2, wherein the D1 is 0.25 mm or less.
The polyolefin-based resin foam sheet according to any one of claims 1 to 3, wherein the foamed polyolefin resin sheet comprises a single layer.
The polyolefin resin foam sheet according to any one of claims 1 to 4, wherein the normal incident sound absorption coefficient at a measurement frequency of 5 to 7 kHz is 0.6 or more.
The polyolefin resin foam sheet according to any one of claims 1 to 5, which comprises high-density polyethylene.
The polyolefin resin foam sheet according to any one of claims 1 to 6, wherein the polyolefin resin foam sheet comprises a fibrous material.
The polyolefin resin foam sheet according to any one of claims 1 to 7, wherein the apparent density of the polyolefin resin foam sheet is from 180 kg / m 3 to 600 kg / m 3 .
A sound absorbing material using the polyolefin resin foam sheet according to any one of claims 1 to 8.
An automobile part using the sound absorbing material according to claim 9.
A method for producing a polyolefin resin foam sheet having continuous air bubbles,
A foamed sheet precursor having an increased bubble diameter is formed in an inclined manner from the sheet surface toward the center along the thickness direction, and the sheet precursor is cut perpendicularly to the thickness direction of the sheet precursor. A method for producing a polyolefin-based resin foam sheet.