JP2000169826A - Vibration-proof material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a vibration-proof material high in durability, having desired performances such as excellent rubber elasticity, good oil retainability, low hardness and small compression permanent set, and having an excellent vibration-proof property for lightweight articles, especially in a low-frequency region. SOLUTION: This vibration-proof material comprises (a) 100 pts.wt. of a hydrogenated block copolymer having an average mol.wt. of >=60,000 and obtained by hydrogenating a block copolymer comprising at least one of polymer blocks consisting mainly of a vinyl aromatic compound and at least one of polymer blocks consisting mainly of a conjugated diene compound, (b) 1-3,000 pts.wt. of a non-aromatic rubber-softening agent having a dynamic viscosity of >=100 cSt at 40 deg.C, and (c) 0-50 pts.wt. of a polyolefinic resin consisting mainly of polypropylene.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a vibration damping material, and more particularly to a rubber material having excellent rubber elasticity, good oil retention, low hardness, and low compression set.
CD (Compact Disc), CD with desired performance
The present invention relates to an anti-vibration material suitably used for anti-vibration of small and light-weight objects such as ROMs and optical disks.

[0002]

2. Description of the Related Art In recent years, discs on which information such as a CD-ROM is written have been widely used in electronic devices such as computers, and these are mechanically vulnerable to external and internal vibrations. For this reason, an insulator made of synthetic rubber or the like is mounted for the purpose of vibration isolation. Also, the problem of vibration isolation is becoming important for small and lightweight devices, for example, optical disks such as mini disks and DVDs, and small performance microphones for small videos and micro tape recorders. Conventionally, synthetic rubbers such as silicone rubber, general vulcanized rubbers such as butyl rubber, gels, oil dampers and the like have been used as light-weight vibration damping materials such as small microphones and CD-ROMs. Among these, the rubber material has a hardness of about 20 to 30 ° in JIS A hardness, and even if it is the lowest hardness of about 20 °, the resonance frequency is limited to a high frequency, so that it is intended for light-weight equipment. It is difficult to set the frequency to be the vibration isolation region.

For this reason, rubber, which is a vibration isolating material, is formed into a complicated shape by a method such as thin molding, and the shape of the vibration isolating material is devised for vibration isolation in a low frequency range that cannot be achieved by the characteristics of the material. Has been achieved. However, when a rubber material is molded into such a shape, if the thickness is reduced, the strength is reduced, the rubber material is easily damaged during molding, and it is difficult to increase productivity. Further, in the vulcanized rubber, there is a concern that sulfur as a vulcanizing agent may remain and affect electronic components. In the case of a silicone rubber, there is a problem that a low molecular weight silicone may cause contact failure. In addition, in order to achieve vibration isolation in the low frequency range, it is conceivable to use a more flexible material, for example, a gel.However, a material having a low hardness is easily deformed by compression, and the vibration isolation property is reduced with time. There is a concern that the molecular weight may decrease or a low molecular weight component in the material may bleed, which may adversely affect a disk or the like that requires vibration isolation.

[0004]

Under such circumstances, the present invention uses a material having excellent rubber elasticity, good oil retention, low hardness, and low compression set. Accordingly, it is an object of the present invention to provide a vibration-proof material which is excellent in vibration-proof properties of a light-weight object, particularly in a low-frequency range, and has excellent durability.

[0005]

Means for Solving the Problems The inventors of the present invention have conducted intensive studies to achieve the above object, and as a result, have found that a vibration isolator made of a material containing a thermoplastic elastomer made of a specific block copolymer is used. Has found that the object can be achieved. The present invention has been completed based on such findings. That is, the present invention provides (a) at least one polymer block mainly composed of a vinyl aromatic compound,
A hydrogenated block copolymer obtained by hydrogenating a block copolymer consisting of at least one polymer block mainly composed of a conjugated diene compound and having a number average molecular weight of 60
100 parts by weight of a hydrogenated block copolymer having a molecular weight of 000 or more,
(B) 1 to 3000 parts by weight of a non-aromatic rubber softener having a kinematic viscosity at 40 ° C of 100 cSt or more; and (c)
Polyolefin resin containing polypropylene as a main component 0
An object of the present invention is to provide an anti-vibration material characterized by being composed of a material consisting of up to 50 parts by weight.

[0006]

BEST MODE FOR CARRYING OUT THE INVENTION In the vibration damping material of the present invention, (a) at least one of a polymer block mainly composed of a vinyl aromatic compound and (a) a polymer block mainly composed of a conjugated diene compound are used. 100 parts by weight of a hydrogenated block copolymer obtained by hydrogenating at least one block copolymer and having a number average molecular weight of 60,000 or more; (b) dynamics at 40 ° C. A composition comprising 1 to 3000 parts by weight of a softener for a non-aromatic rubber having a viscosity of 100 cSt or more and 0 to 50 parts by weight of a polyolefin resin (c) containing polypropylene as a main component (hereinafter referred to as a thermoplastic material). Is used. Examples of the hydrogenated block copolymer (a) include, for example, a block copolymer of polybutadiene and polystyrene, a block copolymer of polyisoprene and polystyrene, or a polybutadiene or ethylene-butadiene random copolymer. Obtained by hydrogenating a block copolymer with polystyrene, for example, a diblock copolymer of crystalline polyethylene and polystyrene, styrene-
Ethylene / butylene-styrene triblock copolymer (SEBS), styrene-ethylene / propylene-styrene triblock copolymer (SEPS), etc.
Examples thereof include a styrene-ethylene / butylene-styrene block copolymer and a styrene-ethylene / propylene-styrene block copolymer. As the component (a), a block copolymer having at least two polymer blocks mainly composed of a vinyl aromatic compound and at least one polymer block mainly composed of a conjugated diene compound (for example, styrene-butadiene- A hydrogenated block copolymer obtained by hydrogenating a styrene block copolymer or a styrene-isoprene-styrene block copolymer is more preferred. Although the number average molecular weight of these hydrogenated block copolymers is 60,000 or more, if this number average molecular weight is less than 60,000, the bleeding of the softener is remarkable, the compression set is large, and it cannot be put to practical use. Inconvenience occurs. The upper limit of the number average molecular weight is not particularly limited, but is usually about 400,000.

The content of the amorphous styrene block in the hydrogenated block copolymer is desirably in the range of 10 to 70% by weight, preferably 15 to 60% by weight. The glass transition temperature (Tg) of the amorphous styrene block portion is 6
Those having a temperature of 0 ° C. or higher, preferably 80 ° C. or higher are desirable. As the polymer of the portion connecting the amorphous styrene blocks at both ends, an amorphous polymer is also preferable, and examples thereof include an ethylene-butylene copolymer, a butadiene polymer, and an isoprene polymer. And a block copolymer or a random copolymer thereof. In addition, these hydrogenated block copolymers are mainly used alone, but two or more kinds may be blended and used.

The thermoplastic material according to the present invention has a kinematic viscosity at 40 ° C. of 100 cS at 40 ° C. for the purpose of lowering the hardness of the thermoplastic elastomer (a).
A softener for a non-aromatic rubber having a value of t or more is blended. When the kinematic viscosity at 40 ° C. of the softener is less than 100 cSt, the weight loss and bleed of the composition due to volatilization are remarkable,
The inconvenience of not being able to withstand actual use occurs. This kinematic viscosity is 10 ° C. at 40 ° C. from a practical and manufacturing point of view.
It is preferably 0 to 10000 cSt, particularly preferably 20 cSt.
0 to 5000 cSt is preferred. Further, from the viewpoint of molecular weight, the number average molecular weight is less than 20,000, particularly 10,000.
Below, especially what is 5000 or less is preferable. As such a softening agent, usually, a liquid or liquid at room temperature is suitably used. Further, both hydrophilic and hydrophobic softeners can be used. The softener having such properties can be appropriately selected from various softeners for non-aromatic rubbers such as mineral oils, vegetable oils, and synthetic oils. Here, mineral oils include naphthenes,
Process oils such as paraffinic oils and vegetable oils include castor oil, cottonseed oil, linseed oil, rapeseed oil, soybean oil, palm oil, locust oil, peanut oil, wood wax, pine oil and olive oil. Among them, one or more selected from mineral oil-based paraffinic oil, naphthenic oil or synthetic polyisobutylene-based oil, the number average molecular weight of which is 450 to 500
Those which are 0 are preferred.

These softeners may be used alone or as a mixture of two or more as long as they have good compatibility with each other. The compounding amount of these softeners is 1 to 3000 parts by weight, preferably 50 to 1000 parts by weight, particularly preferably 100 to 300 parts by weight based on 100 parts by weight of the component (a). If the amount is less than 1 part by weight, it is not possible to achieve a sufficiently low hardness, and the flexibility of the thermoplastic material may be insufficient. If the amount exceeds 3000 parts by weight, the softener tends to bleed, and This causes the mechanical strength of the plastic material to decrease. The amount of the softener is appropriately selected from the above range according to the molecular weight of the hydrogenated block copolymer of the component (a) and the type of other components added to the hydrogenated block copolymer. Is preferred.

In order to improve the processability and heat resistance of the thermoplastic material according to the present invention, a polyolefin-based hydrocarbon resin containing polypropylene as a main component can be added as the component (c). Examples of the resin include isotactic polypropylene, a copolymer of propylene and a small amount of other α-olefin (for example, a propylene-ethylene copolymer, a propylene / 4-methyl-1-pentene copolymer), and the like. be able to. When a copolymer of isotactic polypropylene is used as the polyolefin resin, its MFR (JIS K7210)
Can be suitably used in the range of 0.1 to 100 g / 10 min, particularly 0.5 to 50 g / 10 min. The amount of the component (c) is from 0 to 50 parts by weight, preferably from 0 to 30 parts by weight, particularly preferably from 5 to 20 parts by weight, per 100 parts by weight of the component (a). If the amount is more than 50 parts by weight, the hardness of the obtained thermoplastic material becomes too high, which is not preferable.

In the thermoplastic material according to the present invention, in order to improve the processability and heat resistance of the material, (c)
A polystyrene resin can be used in combination as a component. As the polystyrene resin, any one obtained by a radical polymerization method or an ionic polymerization method can be suitably used as long as it is obtained by a known production method. The number average molecular weight of the polystyrene resin can be preferably selected from the range of 5,000 to 500,000, more preferably 10,000 to 200,000, and the molecular weight distribution [ratio of weight average molecular weight (Mw) to number average molecular weight (Mn) (Mw / Mn)] ] Is preferably 5 or less. Examples of the polystyrene resin include polystyrene, a styrene-butadiene block copolymer having a styrene unit content of 60% by weight or more, rubber-reinforced polystyrene, poly α-methyl styrene, poly pt-butyl styrene, and the like. May be used alone or in combination of two or more. Further, a copolymer obtained by polymerizing a mixture of monomers constituting these polymers can also be used. In the thermoplastic material according to the present invention,
When the polystyrene resin is used in combination with the component (c), the hardness of the obtained material tends to be higher than when the polystyrene resin is not used together. Therefore, the hardness of the obtained thermoplastic material can be adjusted by selecting these compounding ratios. In this case, the ratio of polyolefin resin / polystyrene resin is 95/5 to 5/95.
It is preferable to select from the range of (weight ratio).

In the thermoplastic material according to the present invention, it is preferable that the high-molecular organic material constituting the thermoplastic material has a three-dimensional continuous network skeleton structure. Has an average diameter of 50 μm or less, preferably 30 μm or less, and the average diameter of the cells (mesh) is 50 μm or less.
0 μm or less, preferably 300 μm or less, when the volume fraction of the high-molecular organic material is defined as [volume of high-molecular organic material / (volume of high-molecular organic material + volume of softener)] × 100 (%) The volume fraction of the high molecular weight organic material is preferably 50% or less, particularly preferably 33% or less.

In order to obtain a thermoplastic material containing a large amount of a softening agent and a smaller amount of a high molecular weight organic material, the solubility parameter value δ of each of the softening agent and the high molecular weight organic material is used.
= (ΔE / V) ^1/2 (ΔE = molar evaporation energy, V =
It is preferable to select both materials so that the difference in molar volume) is 3.0 or less, preferably 2.5 or less. This difference is 3.
If it exceeds 0, it is difficult to maintain a large amount of the softening agent from the viewpoint of the compatibility of the two materials, which is an obstacle to lowering the hardness of the obtained thermoplastic material, and bleeding of the softening agent is liable to occur.

The thermoplastic material according to the present invention may optionally contain a polyphenylene ether resin for the purpose of improving the compression set of the material.
The polyphenylene ether resin used here is a homopolymer composed of a repeating unit represented by the following general formula or a copolymer containing the repeating unit.

[0015]

Embedded image

(In the formula, R ¹ , R ² , R ³ and R ⁴ each independently represent a hydrogen atom, a halogen atom or a hydrocarbon group.) As the polyphenylene ether resin, known ones can be used. Specifically, poly (2,6-dimethyl-1,4-phenylene ether), poly (2-methyl-6-ethyl-1,4-phenylene ether), poly (2,6-diphenyl-1, 4-phenylene ether), poly (2-methyl-6-phenyl-1,4-phenylene ether), poly (2,6-dichloro-1,4-
Phenylene ether), and 2,6-
Dimethylphenol and monovalent phenols (for example,
2,3,6-trimethylphenol or 2-methyl-6
A polyphenylene ether copolymer such as a copolymer with (butylphenol) can also be used. Above all, poly (2,6-dimethyl-1,4-phenylene ether)
And a copolymer of 2,6-dimethylphenol and 2,3,6-trimethylphenol, and more preferably poly (2,6-dimethyl-1,4-phenylene ether).

The compounding amount of the polyphenylene ether resin is as follows:
It can be suitably selected in the range of 10 to 250 parts by weight based on 100 parts by weight of the thermoplastic material. If this blending amount is 2
If the amount is more than 50 parts by weight, the hardness of the thermoplastic material may be increased. If the amount is less than 10 parts by weight, the effect of improving the compression set obtained by blending is not sufficient. The thermoplastic material according to the present invention includes flaky inorganic additives such as clay, diatomaceous earth, silica, talc, barium sulfate, calcium carbonate, magnesium carbonate, metal oxides, mica, graphite, and aluminum hydroxide. Various metal powders, wood chips, glass powders, ceramic powders, granular or powdery solid fillers such as granular or powdered polymers, other various natural or artificial short fibers,
Long fibers (for example, straw, wool, glass fiber, metal fiber, various other polymer fibers, and the like) can be blended.

The weight can be reduced by blending a hollow filler, for example, an inorganic hollow filler such as a glass balloon or a silica balloon, or an organic hollow filler made of polyvinylidene fluoride, a polyvinylidene fluoride copolymer or the like. . Further, in order to improve various physical properties such as weight reduction, it is possible to mix various foaming agents, and it is also possible to mechanically mix gas during mixing or the like.

As other additives, if necessary,
Flame retardants, antibacterial agents, hindered amine light stabilizers, ultraviolet absorbers, antioxidants, coloring agents, silicone oils, silicone polymers, cumarone resins, cumarone-indene resins, phenol terpene resins, petroleum hydrocarbons, rosin derivatives, etc. Various adhesive elastomers such as various tackifiers (Tackifire), Rheostomer B (trade name: manufactured by Riken Vinyl), and High Blur (trade name: manufactured by Kuraray Co., Ltd.) A polystyrene block is connected to both ends of a vinyl-polyisoprene block. Block copolymer) or other thermoplastic elastomers or resins such as Norex (trade name: polynorbornene obtained by ring-opening polymerization of norbornene manufactured by Zeon Corporation).

The above silicone polymer desirably has a weight average molecular weight of 10,000 or more, preferably 100,000 or more. The silicone polymer improves the surface tackiness of a molded article using a thermoplastic material. As the silicone polymer, a general-purpose thermoplastic polymer such as polyethylene, polypropylene, or polystyrene, which is blended at a high concentration, can be used in order to improve handleability. In particular, the blended product with polypropylene has good workability and physical properties. As such a material, for example, a commercially available silicone concentrate BY27 series general-purpose type commercially available from Toray Dow Corning Silicone Co., Ltd. may be used. By blending the silicone polymer, the surface condition of the molded article can be improved.However, the miscibility of the silicone polymer and the copolymer composed of the vinyl aromatic compound block and the conjugated diene compound block,
Not always good. This can easily be imagined from the fact that the chemical composition of each polymer is significantly different. Therefore, the silicone polymer may be separated depending on the content of the blend and the molding conditions of the molded article. At this time, the condition can be improved by using a graft polymer obtained by chemically bonding a silicone polymer to a polymer having relatively good miscibility with the component (a), for example, a polyolefin resin. As such a material, for example, a material commercially available as a BY27 series graft type from Toray Dow Corning Silicone Co., Ltd. may be used.

The method for producing a thermoplastic material according to the present invention comprises:
There is no particular limitation, and a known method can be applied.
For example, the above components and optionally used additive components are mixed with a heat kneader, for example, a single screw extruder, a twin screw extruder,
Melt and knead using a roll, Banbury mixer, pravender, kneader, high-shear mixer, etc., and further add a cross-linking agent such as an organic peroxide, a cross-linking assistant, etc., if necessary, or these necessary components. Can be easily manufactured by simultaneously mixing and kneading under heating and melting. Also, a thermoplastic material prepared by kneading a high-molecular organic material and a softening agent is prepared in advance, and this material is further mixed with one or more high-molecular organic materials of the same type or different types used here. It can also be manufactured. Furthermore, in the thermoplastic material according to the present invention,
Crosslinking can also be performed by adding a crosslinking agent such as an organic peroxide, a crosslinking aid, or the like.

The crosslinking agent that can be added for partial crosslinking is preferably an organic peroxide. Specifically, for example, 2,5-dimethyl-2,5-di (t
-Butylperoxy) -hexane; 2,5-dimethyl-
2,5-di (benzoylperoxy) -hexane; t-
Butylperoxybenzoate; dicumyl peroxide; t-butylcumyl peroxide; diisopropylbenzohydroperoxide; 1,3-bis- (t-
Butylperoxyisopropyl) -benzene; benzoyl peroxide; 1,1-di (t-butylperoxy) -3,3,5-trimethylcyclohexane and the like, and useful crosslinking aids include, for example, Divinylbenzene, trimethylolpropane triacrylate, ethylene dimethacrylate, diallyl phthalate,
Examples include quinone dioxime, phenylene bismaleimide, polyethylene glycol dimethacrylate, and unsaturated silane compounds. These organic peroxides and cross-linking assistants, when the total amount of the blended materials was 100 parts by weight, were 0.1%
The degree of crosslinking can be adjusted arbitrarily in the range of 1 to 5 parts by weight. These organic peroxides and crosslinking assistants may be used in combination of two or more, if necessary. When an unsaturated silane compound is used as a crosslinking aid, crosslinking can be further advanced by contact with moisture in the presence of a silanol condensation catalyst.

The thus obtained thermoplastic material according to the present invention can be molded into a desired shape by a known method, for example, injection molding or the like, and used as a vibration isolator. The anti-vibration material of the present invention can be suitably used for audio equipment, information-related equipment, information transmission equipment, game-related equipment, and the like.
Insulators for CDs mounted on players and automobiles; Microphone holders for anti-vibration such as home video, boombox, karaoke, and various handy microphones; cone edges of speakers; tape holders for boombox; holders for portable mini-discs; A holder and the like. Examples of information-related devices include insulators for hard disk drives; insulators for various motors such as hard disk spindle motors and stepping motors; insulators for floppy disk drives; and CD-ROs for various personal computer devices.
M insulator; MO and other optical disk holders; Examples of the information transmission device include small high-performance microphone holders and speaker holders for various types of mobile phones, pagers, PHSs and the like; microphone holders for wireless devices; and disk halters for portable electronic notebooks. Examples of game-related devices include insulators for CD-ROMs for home game devices; small high-performance microphone holders;
Other application fields include various word processors, printers for personal computers, small and medium-sized hand-held printing devices, vibration-proofing members for the print head of printers such as various name printing devices, and CD-ROMs used for various measuring instruments. And insulators.

[0024]

Next, the present invention will be described in more detail with reference to examples, but the present invention is not limited to these examples. The evaluation of physical properties in the following Examples and Comparative Examples was performed by the following methods. (1) Hardness Based on JIS K6301 (A type). (2) Oil Retentivity A sheet-like sample of 10 mm × 10 mm and a thickness of 2 mm was sandwiched between graph papers, and a load of 1 kg / cm ² was applied to the sample at 60 ° C.
For 24 hours, and oil seepage was observed.
A sample without oil seepage was indicated by “○”, and a sample with oil seepage was indicated by “×”. (3) Compression set Based on JIS K6301. The number average molecular weight of the copolymer was measured by gel permeation chromatography [apparatus: HLC-8020 (manufactured by TOSO), column: GMHXL-L × 2 + G1000HXL + precolumn (manufactured by TOSO), flow rate: 1.0 ml / min. (Solvent: tetrahydrofuran)], and using a differential refractive index (RI) in terms of polystyrene using monodisperse polystyrene as a standard. The measurement of kinematic viscosity is based on JI
The measurement was performed using a B-type viscometer (manufactured by Tokimec Co., Ltd.) in accordance with SK2283.

Examples 1 and 2 and Comparative Examples 1 to 4 (1) Preparation of Thermoplastic Material Samples of the thermoplastic material were prepared by sufficiently kneading the components having the compositions shown in Table 1 to obtain physical properties. An evaluation was performed.
As a result, as is clear from Table 1, the thermoplastic materials of Examples 1 and 2 had good oil retention, but the thermoplastic materials of Comparative Examples 1 to 4 showed oil bleed.

[0026]

[Table 1]

Polymer A: SEPS having a number average molecular weight of 100,000 and a styrene content of 30% by weight Polymer B: SEBS having a number average molecular weight of 50,000 and a styrene content of 30% by weight Oil A: A paraffinic oil having a kinematic viscosity at 40 ° C. of 380 cSt. [Manufactured by Idemitsu Kosan Co., Ltd., trade name: Diana Process Oil PW380] (number average molecular weight: 750) Oil B: paraffinic oil having a kinematic viscosity at 40 ° C. of 30 cSt [manufactured by Idemitsu Kosan Co., Ltd., trade name: Diana Process Oil PW32] (number average molecular weight 400) Oil C: a paraffinic oil having a kinematic viscosity at 40 ° C. of 1300 cSt [Mitsui Chemicals Co., Ltd., trade name: Lucant HC100] (number average molecule 1450)

(2) Molding of Thermoplastic Material (Production of Vibration Isolator) The above material is molded at a temperature of 180 to 230 ° C. by an injection molding machine, and a portable CD player having the shape shown in FIG. As a rubber, a vibration isolator was obtained. The weight of the used floating part was 90 g. When the thus obtained anti-vibration material was used, it was found that the anti-vibration material made of the thermoplastic material of the example exhibited excellent performance, and the anti-vibration property was maintained after 30 days of use. confirmed. On the other hand, the vibration isolator made of the thermoplastic material of the comparative example has a thickness of 30%.
After use for days, the material was sagged, and it was confirmed that the material did not fulfill the function as a floating rubber.
In addition, when the floating rubber of the portable CD player having the shape shown in FIG. 2 was used in the same manner as described above, the above-mentioned results were obtained when the thermoplastic material of the example was used and when the thermoplastic material of the comparative example was used. A similar difference in performance was confirmed.

[0029]

The vibration damping material of the present invention is made of a material having excellent properties such as excellent rubber elasticity, good oil retention, low hardness, and low compression set. It is excellent in vibration proof, especially in a low frequency range, and has high durability.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view showing floating rubbers of an example and a comparative example.

FIG. 2 is a cross-sectional view showing another embodiment of the floating rubber of the example and the comparative example.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) // C08F 8/04 C08F 8/04 (C08L 53/02 23:10) F-term (Reference) 3J048 BD04 EA31 4J002 AE053 BB112 BP011 FD023 FD026 FD140 GM00 4J100 AA02Q AA03P AA17Q CA04 DA07 DA36 DA47 JA28

Claims (1)

[Claims] 1. A method comprising hydrogenating a block copolymer comprising (a) at least one polymer block mainly composed of a vinyl aromatic compound and at least one polymer block mainly composed of a conjugated diene compound. 100 parts by weight of a hydrogenated block copolymer having a number average molecular weight of 60,000 or more, and (b) a softener 1 for a non-aromatic rubber having a kinematic viscosity at 40 ° C. of 100 cSt or more. A vibration isolator characterized by being composed of a material consisting of up to 3000 parts by weight and (c) 0 to 50 parts by weight of a polyolefin resin containing polypropylene as a main component.