Airbag storage cover and manufacturing method thereof, thermoplastic elastomer composition, molded body, and composite molded body

Abstract

An airbag-housing cover obtained from a thermoplastic elastomer composition comprising a propylene-based random copolymer and a styrene-based elastomer having a glass transition temperature in the range of -65°C to -45°C, the total content of the propylene-based random copolymer and the styrene-based elastomer in the thermoplastic elastomer composition, which is taken as 100 mass%, being higher than 40 mass% but 100 mass% or less.

Description

[Technical Field]

[0001] The present invention relates to an airbag storage cover having excellent low-temperature impact resistance, high-temperature strength, and light transmittance, a method for producing the same, and a thermoplastic elastomer composition capable of providing such an airbag storage cover. The present invention also relates to a molded article and a composite molded article using the thermoplastic elastomer composition. [Background technology]

[0002] Automotive airbag systems protect drivers and passengers in the event of a collision, and consist of a device that detects the impact of a collision and an airbag device. Airbag devices are installed in the steering wheel, the instrument panel in front of the passenger seat, the driver's and passenger's seats, the front and side pillars, etc.

[0003] When the airbag is inflated, there is a risk that the cover housing the airbag may break, causing fragments to fly off, or that the cover's attachment portion may break, causing the cover to fly off. For this reason, various proposals have been made regarding the structure and materials of the cover to prevent abnormal breakage and flying off of the cover.

[0004] With regard to airbag storage covers, there is concern that they may break at low and high temperatures due to increased airbag deployment force. Therefore, from the viewpoints of enhanced safety and design freedom, there is a need for the development of thermoplastic elastomer compositions as materials with improved low-temperature impact resistance and high-temperature strength.

[0005] Known thermoplastic elastomer compositions for airbags use propylene-based resins as hard segments and olefin-based rubbers such as ethylene-propylene elastomers (EPDM, EPR, etc.) or styrene-based elastomers as soft segments.

[0006] For example, as shown in Patent Document 1, a thermoplastic elastomer composition for use in airbags is known, which contains a propylene-based resin, a styrene-butadiene-styrene block copolymer (SBS), and an ethylene-α-olefin copolymer.

[0007] On the other hand, techniques for increasing the transparency of thermoplastic elastomer compositions containing a propylene-based resin and a styrene-based elastomer are known (Patent Documents 2 and 3).

[0008] Patent Document 4 proposes a polypropylene resin composition that is excellent in moldability as well as rigidity (flexural modulus), impact resistance (impact strength), and transparency, and is made by blending a propylene resin, a styrene elastomer, and an ethylene-α-olefin copolymer.

[0009] Regarding thermoplastic elastomer compositions containing a propylene-based random copolymer and a styrene-based elastomer, Patent Document 5 proposes a thermoplastic elastomer composition for powder molding that can be easily produced into fine powder at room temperature and can give molded articles that are excellent in scratch resistance, heat resistance, molded appearance, etc.

[0010] Patent Document 6 proposes a thermoplastic elastomer composition for airbag storage covers that is excellent in low-temperature impact resistance, molded appearance, and injection moldability, and is made by blending a propylene-based polymer, an olefin-based elastomer, a styrene-based elastomer, and an acrylic resin.

[0011] [Patent Document 1] Japanese Patent Application Publication No. 2019-38925 [Patent Document 2] Japanese Patent Application Laid-Open No. 2005-23148 [Patent Document 3] Special Publication No. 2012-512277 [Patent Document 4] Japanese Patent Application Laid-Open No. 2017-75209 [Patent Document 5] Japanese Patent Application Laid-Open No. 2004-137306 [Patent Document 6] Japanese Patent Publication No. 2020-158615

[0012] In recent years, some airbag storage covers have been designed to function as touch panel displays. This type of airbag storage cover is composed of a surface layer made of a film that serves as the interface for the touch panel display and an airbag storage cover layer that is provided in contact with the surface layer, and an illuminated switch is embedded inside the airbag storage cover. In this configuration, the airbag storage cover must be made of a material that transmits light so that the user can easily see the illuminated switch. Therefore, the thermoplastic elastomer composition used in the airbag storage cover layer is now required to have optical transparency sufficient to allow the user to see the illuminated switch, in addition to the low-temperature impact resistance and high-temperature strength that have traditionally been required.

[0013] According to detailed studies by the present inventors, the thermoplastic elastomer composition used in the airbags described in Patent Document 1 has good low-temperature impact resistance and high-temperature strength, but is not intended to provide the above-mentioned optical transparency. Furthermore, in thermoplastic elastomers using a propylene-based resin as the hard segment and an olefin-based rubber, such as an ethylene-propylene elastomer (EPDM, EPR, etc.), as the soft segment, as described in Patent Document 1, the refractive index of the olefin-based rubber must be close to that of the propylene-based resin in order to improve optical transparency. Therefore, it was found that selecting an olefin-based rubber with a low refractive index would impair low-temperature impact resistance and high-temperature strength.

[0014] The thermoplastic elastomer composition described in Patent Document 2 has good transparency, but does not satisfy the low-temperature impact resistance and high-temperature strength required for airbag applications.

[0015] The thermoplastic elastomer compositions of Patent Documents 3 and 4 have good transparency and high-temperature strength, but do not have satisfactory low-temperature impact resistance.

[0016] The thermoplastic elastomer compositions of Patent Documents 5 and 6 do not take into consideration low-temperature impact resistance, high-temperature strength, transparency or light transmittance.

[0017] Furthermore, in none of the above conventional technologies has there been any consideration made of combining a resin film incorporating the operation function and design of a touch panel with an airbag storage cover for a touch panel display. Summary of the Invention

[0018] The present invention has been made in view of the above-mentioned problems of the prior art.

[0019] The object of the present invention is to provide an airbag storage cover and a thermoplastic elastomer composition which have excellent low-temperature impact resistance, high-temperature strength, and light transmittance and are also suitable for compounding with a film for a touch panel, as well as a molded article and a composite molded article made from the thermoplastic elastomer composition. [Means for solving the problem]

[0020] The present inventors have discovered that a thermoplastic elastomer composition containing a propylene-based random copolymer and a specific styrene-based elastomer can provide an airbag storage cover and a molded article that are excellent in low-temperature impact resistance, high-temperature strength, and light transmittance, and are also suitable for being combined with a film for a touch panel, and have arrived at the present invention.

[0021] The gist of the present invention lies in the following [1] to

[20] .

[0022] [1] An airbag storage cover made of a thermoplastic elastomer composition containing a propylene-based random copolymer and a styrene-based elastomer having a glass transition temperature in the range of -65°C or higher and -45°C or lower, wherein the total content of the propylene-based random copolymer and the styrene-based elastomer in 100% by mass of the thermoplastic elastomer composition is more than 40% by mass and 100% by mass or lower.

[0023] [2] The airbag storage cover described in [1], arranged in a visible position.

[0024] [3] The airbag storage cover according to [1] or [2], wherein the styrene-based elastomer is linear.

[0025] [4] The airbag storage cover according to any one of [1] to [3], wherein the melt flow rate of the thermoplastic elastomer composition at a temperature of 230°C and a measuring load of 21.18 N according to JIS K7210 (1999) is 1.0 g / 10 min or more and 50 g / 10 min or less.

[0026] [5] An airbag storage cover according to any one of [1] to [4], wherein the content of the styrene-based elastomer in the thermoplastic elastomer composition relative to 100 parts by mass of the propylene-based random copolymer is 45 parts by mass or more and 250 parts by mass or less.

[0027] [6] The airbag storage cover according to any one of [1] to [5], wherein the thermoplastic elastomer composition further contains 10 parts by mass or more and 100 parts by mass or less of an ethylene-α-olefin copolymer per 100 parts by mass of the propylene-based random copolymer.

[0028] [7] The airbag storage cover according to any one of [1] to [6], wherein the styrene unit content of the styrene-based elastomer is 20% by mass or more and 25% by mass or less.

[0029] [8] The airbag storage cover according to [7], wherein the styrene-based elastomer comprises a hydrogenated styrene-conjugated diene block copolymer (b1) having a styrene unit content of 10% by mass or more and 15% by mass or less, and a hydrogenated styrene-conjugated diene block copolymer (b2) having a styrene unit content of 25% by mass or more and 35% by mass or less.

[0030] [9] The airbag storage cover according to any one of [1] to [8], wherein a sheet-like test piece having a thickness of 2 mm, which is injection-molded from the thermoplastic elastomer composition under conditions of a resin temperature of 200°C and a mold temperature of 40°C, has a total light transmittance of 65% or more, measured in accordance with JIS K7136-1.

[0031]

[10] The airbag storage cover according to any one of [1] to [9], wherein a sheet-like test piece having a thickness of 2 mm is injection-molded from the thermoplastic elastomer composition under conditions of a resin temperature of 200°C and a mold temperature of 40°C, and the haze measured in accordance with JIS K7136-1 is 70% or less.

[0032]

[11] A method for producing an airbag storage cover according to any one of [1] to

[10] , comprising a step of injection molding the thermoplastic elastomer composition.

[0033]

[12] A thermoplastic elastomer composition comprising a propylene-based random copolymer and a linear styrene-based elastomer having a styrene unit content of 20% by mass or more and 35% by mass or less and a glass transition temperature in the range of -65°C or more and -45°C or less, wherein the total content of the propylene-based random copolymer and the linear styrene-based elastomer in 100% by mass of the thermoplastic elastomer composition is more than 40% by mass and 100% by mass or less, the content of the linear styrene-based elastomer relative to 100 parts by mass of the propylene-based random copolymer is 45 parts by mass or more and 250 parts by mass or less, and the melt flow rate at 230°C and a measuring load of 21.18 N in accordance with JIS K7210 (1999) is 1.0 g / 10 min or more and 50 g / 10 min or less.

[0034]

[13] The thermoplastic elastomer composition according to

[12] , wherein a sheet-like test piece having a thickness of 2 mm, which is injection-molded from the thermoplastic elastomer composition under conditions of a resin temperature of 200°C and a mold temperature of 40°C, has a total light transmittance of 65% or more, measured in accordance with JIS K7136-1.

[0035]

[14] The thermoplastic elastomer composition according to

[12] or

[13] , wherein the haze of a 2 mm thick sheet-like test piece obtained by injection molding the thermoplastic elastomer composition under conditions of a resin temperature of 200°C and a mold temperature of 40°C is 70% or less, as measured in accordance with JIS K7136-1.

[0036]

[15] The thermoplastic elastomer composition according to any one of

[12] to

[14] , further comprising 10 parts by mass or more and 100 parts by mass or less of an ethylene-α-olefin copolymer per 100 parts by mass of the propylene-based random copolymer.

[0037]

[16] The thermoplastic elastomer composition according to any one of

[12] to

[15] , wherein the linear styrene elastomer has a styrene unit content of 20% by mass or more and 25% by mass or less.

[0038]

[17] The thermoplastic elastomer composition according to

[16] , wherein the linear styrene-based elastomer comprises a hydrogenated styrene-conjugated diene block copolymer (b1) having a styrene unit content of 10% by mass or more and 15% by mass or less, and a hydrogenated styrene-conjugated diene block copolymer (b2) having a styrene unit content of 25% by mass or more and 35% by mass or less.

[0039]

[18] A molded article made of the thermoplastic elastomer composition according to any one of

[12] to

[17] .

[0040]

[19] A composite molded article having a first layer containing the thermoplastic elastomer composition according to any one of

[12] to

[17] and a second layer made of a resin film.

[0041]

[20] A composite molded product having a first layer made of a thermoplastic elastomer composition containing a propylene-based random copolymer and a styrene-based elastomer having a glass transition temperature in the range of -65°C to -45°C, and a second layer made of a resin film.

[0042] The present invention also provides the following <1> ~ <11> It includes:

[0043] <1> A thermoplastic elastomer composition for airbags comprising the following components (A) and (B): Component (A): Propylene polymer Component (B): A hydrogenated styrene-conjugated diene block copolymer having a styrene unit content of 20% by mass or more and 35% by mass or less and a glass transition temperature of -65°C or more and -45°C or less.

[0044] <2> the thermoplastic elastomer composition for airbags is injection molded under conditions of a resin temperature of 200°C and a mold temperature of 40°C, and a sheet-like test piece having a thickness of 2 mm is subjected to the injection molding, and the haze measured in accordance with JIS K7136-1 is 70% or less; <1> The thermoplastic elastomer composition for an airbag according to claim 1.

[0045] <3> the total content of the component (A) and the component (B) is 70% by mass or more and 100% by mass or less in 100% by mass of the thermoplastic elastomer composition for airbags; <1> or <2> The thermoplastic elastomer composition for an airbag according to claim 1.

[0046] <4> The content of the component (B) relative to 100 parts by mass of the component (A) is 20 parts by mass or more and 200 parts by mass or less. <1> ~ <3> 1. The thermoplastic elastomer composition for an airbag according to claim 1,

[0047] <5> The composition further contains 10 parts by mass or more and 100 parts by mass or less of the following component (C) relative to 100 parts by mass of the component (A), <1> ~ <4> 1. The thermoplastic elastomer composition for an airbag according to claim 1, Component (C): Ethylene-α-olefin copolymer

[0048] <6> The styrene unit content of the component (B) is 20% by mass or more and 25% by mass or less. <1> ~ <5> 1. The thermoplastic elastomer composition for an airbag according to claim 1,

[0049] <7> the component (B) comprises a hydrogenated styrene-conjugated diene block copolymer (b1) having a styrene unit content of 10% by mass or more and 15% by mass or less, and a hydrogenated styrene-conjugated diene block copolymer (b2) having a styrene unit content of 25% by mass or more and 35% by mass or less, <6> The thermoplastic elastomer composition for an airbag according to claim 1.

[0050] <8> <1> ~ <7> 1. A molded article comprising the thermoplastic elastomer composition for airbags according to any one of claims 1 to 9.

[0051] <9> <1> ~ <7> 1. A composite molded article having a first layer containing the thermoplastic elastomer composition for an airbag according to any one of claims 1 to 9, and a second layer made of a resin film.

[0052] <10> <1> ~ <7> 1. An airbag storage cover comprising the thermoplastic elastomer composition for an airbag according to any one of claims 1 to 9.

[0053] <11> A thermoplastic elastomer composition for airbags, comprising a propylene-based polymer and a styrene-based elastomer having a glass transition temperature of -65°C or higher and -45°C or lower, wherein the thermoplastic elastomer composition for airbags is injection-molded under conditions of a resin temperature of 200°C and a mold temperature of 40°C to obtain a sheet-like test piece with a thickness of 2 mm, and the haze of the sheet-like test piece is measured in accordance with JIS K7136-1 and is 70% or lower. [Effects of the Invention]

[0054] The thermoplastic elastomer composition of the present invention is excellent in low-temperature impact resistance, high-temperature strength, and light transmittance, and is also suitable for being combined with a film for a touch panel. Therefore, the thermoplastic elastomer composition of the present invention provides a molded article, a composite molded article, and an airbag storage cover that are excellent in low-temperature impact resistance, high-temperature strength, and light transmittance, and are also suitable for being combined with a film for a touch panel.

[0055] The airbag storage cover of the present invention can be suitably used as a driver's seat airbag storage cover, a passenger seat airbag storage cover, a pedestrian airbag storage cover, a knee airbag storage cover, a side airbag storage cover, a curtain airbag storage cover, etc.

[0056] In particular, the airbag storage cover of the present invention is suitable as an airbag storage cover incorporating a touch panel display due to its excellent transparency and suitability for being combined with a touch panel film. DETAILED DESCRIPTION OF THE INVENTION

[0057] The present invention will be described in detail below. The present invention is not limited to the following description, and can be practiced in any modified form without departing from the gist of the present invention.

[0058] [Thermoplastic elastomer composition] First, the thermoplastic elastomer composition according to the present invention will be described.

[0059] The thermoplastic elastomer composition constituting the airbag storage cover of the present invention is a thermoplastic elastomer composition containing a propylene-based random copolymer and a styrene-based elastomer having a glass transition temperature in the range of -65°C or higher and -45°C or lower, wherein the total content of the propylene-based random copolymer and the styrene-based elastomer in 100% by mass of the thermoplastic elastomer composition is more than 40% by mass and 100% by mass or lower (hereinafter, this thermoplastic elastomer composition may be referred to as "thermoplastic elastomer composition I"). The thermoplastic elastomer composition of the present invention is a thermoplastic elastomer composition containing a propylene-based random copolymer and a linear styrene-based elastomer having a styrene unit content of 20% by mass or more and 35% by mass or less and a glass transition temperature in the range of -65°C or more and -45°C or less, wherein the total content of the propylene-based random copolymer and the linear styrene-based elastomer in 100% by mass of the thermoplastic elastomer composition is more than 40% by mass and 100% by mass or less, the content of the linear styrene-based elastomer per 100 parts by mass of the propylene-based random copolymer is 45 parts by mass or more and 250 parts by mass or less, and the melt flow rate at a temperature of 230°C and a measuring load of 21.18 N in accordance with JIS K7210 (1999) is 1.0 g / 10 min or more and 50 g / 10 min or less (hereinafter, this thermoplastic elastomer composition may be referred to as "thermoplastic elastomer composition II"). As described above, the thermoplastic elastomer composition II is a preferred embodiment in which a specific styrene-based elastomer selected from the styrene-based elastomers in the thermoplastic elastomer composition I is used in a specific ratio. Hereinafter, thermoplastic elastomer composition I and thermoplastic elastomer composition II will be referred to as "thermoplastic elastomer compositions of the present invention."

[0060] <Mechanism> The thermoplastic elastomer composition of the present invention has excellent low-temperature impact resistance, high-temperature strength and light transmittance, and is also suitable for being combined with a film for a touch panel. Although the details of why the thermoplastic elastomer composition of the present invention exhibits such effects are not clear, the reason why such effects are exhibited is presumed to be as follows.

[0061] Propylene-based polymers include propylene homopolymers and propylene-based copolymers, and propylene-based copolymers include propylene-based random copolymers and propylene-based block copolymers. Of these, propylene-based random copolymers do not contain ethylene-propylene copolymers that contribute to light scattering and have lower crystallinity, and therefore provide a thermoplastic elastomer composition with particularly excellent light transmittance.

[0062] It is also known that the refractive index of a styrene-based elastomer is correlated with the styrene unit content, and that the refractive index increases as the styrene unit content increases. Therefore, by setting the styrene unit content of the styrene-based elastomer within a specific range, the refractive index of the propylene-based random copolymer and the refractive index of the styrene-based elastomer can be made closer to each other, thereby improving light transmittance.

[0063] In the present invention, the styrene-based elastomer may be specifically a hydrogenated product of a styrene-conjugated diene block copolymer. Here, the styrene unit content refers to the content of styrene units derived from the raw material styrene and introduced into the hydrogenated styrene-conjugated diene block copolymer.

[0064] Furthermore, by using a styrene-based elastomer having a glass transition temperature in the range of −65° C. or more and −45° C. or less, more preferably a linear styrene-based elastomer having a specific styrene unit content, as the styrene-based elastomer, it is possible to achieve a higher level of both low-temperature impact resistance and high-temperature strength, as well as optical transparency. Here, when the styrene-based elastomer has two or more glass transition temperatures, it is sufficient that at least one of the glass transition temperatures is in the range of -65°C or more and -45°C or less.

[0065] <Propylene-based random copolymer> The propylene-based random copolymer used in the thermoplastic elastomer composition of the present invention is a random copolymer of propylene units and other monomer units such as ethylene units, α-olefin units other than propylene, and monomer units other than ethylene and α-olefins.

[0066] The propylene unit content of the propylene random copolymer is usually 50% by mass or more, preferably 85% by mass or more, more preferably 90% by mass or more, and even more preferably 94% by mass or more, with the upper limit being preferably 99% by mass or less. When the propylene unit content of the propylene random copolymer is equal to or greater than the lower limit, the heat resistance and rigidity tend to be good. When the propylene unit content of the propylene random copolymer is equal to or less than the upper limit, the crystallinity is low, thereby improving the light transmittance of the resulting thermoplastic elastomer composition. The content of propylene units in the propylene random copolymer can be determined by infrared spectroscopy.

[0067] Examples of the α-olefin units other than propylene contained in the propylene-based random copolymer include α-olefins having 4 to 20 carbon atoms. Examples of α-olefins having 4 to 20 carbon atoms include 1-butene, 1-pentene, 1-hexene, 1-heptene, 1-octene, 1-nonene, 1-decene, 1-undecene, 1-dodecene, 1-tridecene, 1-tetradecene, 1-pentadecene, 1-hexadecene, 1-heptadecene, 1-octadecene, 1-nonadecene, 1-eicosene, 3-methyl-1-butene, 3-methyl-1-pentene, 4-methyl-1-pentene, 2-ethyl-1-hexene, and 2,2,4-trimethyl-1-pentene. α-olefins having 4 to 10 carbon atoms are preferred, and ethylene, 1-butene, 1-hexene, and 1-octene are more preferred. The propylene random copolymer may contain only one of these α-olefin units and ethylene units, or may contain two or more of them.

[0068] Specific examples of propylene-based random copolymers include propylene-ethylene random copolymers, propylene-1-butene random copolymers, propylene-1-hexene random copolymers, propylene-1-octene random copolymers, propylene-ethylene-1-butene random copolymers, propylene-ethylene-1-hexene random copolymers, and propylene-ethylene-1-octene random copolymers. A random copolymer of propylene and at least one monomer selected from ethylene and an α-olefin having from 4 to 10 carbon atoms is preferred.

[0069] Among these, from the viewpoint of achieving both low-temperature impact resistance and high-temperature strength and transparency, a propylene-based random copolymer containing one or more monomer units selected from ethylene units, butene units, hexene units, and octene units in an amount of 6% by mass or less, for example, 1% by mass or more and 6% by mass or less.

[0070] The melt flow rate (MFR) of the propylene random copolymer is not limited, but is usually 0.1 g / min or more, and from the viewpoint of the appearance of the molded article, it is preferably 10 g / 10 min or more, more preferably 20 g / 10 min or more, even more preferably 25 g / 10 min or more, and is usually 200 g / 10 min or less, and from the viewpoint of tensile strength, it is preferably 150 g / 10 min or less, more preferably 100 g / 10 min or less. The melt flow rate of the propylene random copolymer is measured according to JIS K7210 (1999) under the conditions of a measurement temperature of 230 ° C and a measurement load of 21.18 N.

[0071] The propylene random copolymer can be produced by a known polymerization method using a known olefin polymerization catalyst. For example, a multi-stage polymerization method using a Ziegler-Natta catalyst can be used. The multi-stage polymerization method can be a slurry polymerization method, a solution polymerization method, a bulk polymerization method, a gas phase polymerization method, or the like, or a combination of two or more of these methods.

[0072] The propylene-based random copolymer used in the thermoplastic elastomer composition of the present invention may be a commercially available product. The propylene-based random copolymer can be procured from the following manufacturers and can be appropriately selected. Commercially available products include, for example, Prim Polypro® from Prime Polymer, Sumitomo Noblen® from Sumitomo Chemical, Polypropylene Random Copolymer from SunAllomer, Novatec® PP from Japan Polypropylene, Moplen® from LyondellBasell, ExxonMobil PP from ExxonMobil, Formolene® from Formosa Plastics, Borealis PP from Borealis, Seetec PP from LG Chemical, ASI POLYPROPYLENE from A. Schulman, INEOS PP from INEOS Olefins & Polymers, Braskem PP from Braskem, Hanwha Total Petrochemical Random PP from Hanwha Total, Sabic® PP from Sabic, TOTAL PETROCHEMICALS Polypropylene from TOTAL PETROCHEMICALS, and YUPLENE® from SK.

[0073] The thermoplastic elastomer composition of the present invention may contain only one type of propylene random copolymer, or may contain two or more types with different monomer compositions, physical properties, etc.

[0074] <Styrene-based elastomer> The styrene-based elastomer used in the thermoplastic elastomer composition of the present invention has a glass transition temperature in the range of −65° C. to −45° C. When the styrene-based elastomer has a glass transition temperature in the range of −65° C. to −45° C., it can be combined with a propylene-based random copolymer to form a thermoplastic elastomer composition having excellent light transmittance, low-temperature impact resistance, and high-temperature strength. The glass transition temperature of the styrene elastomer is preferably in the range of -65°C or higher and -50°C or lower from the viewpoint of low-temperature impact resistance. The glass transition temperature of the styrene elastomer is measured by the DSC method.

[0075] When two or more types of styrene-based elastomers are used, the glass transition temperature of each styrene-based elastomer may be within or outside the above-mentioned glass transition temperature range, and it is sufficient that at least one glass transition temperature of the styrene-based elastomer is in the range of −65° C. or higher and −45° C. or lower, preferably −65° C. or higher and −50° C. or lower.

[0076] The styrene elastomer used in the present invention preferably has a styrene unit content of 20% by mass or more and 35% by mass or less from the viewpoints of low-temperature impact resistance, high-temperature strength, and light transmittance. The styrene unit content of the styrene-based elastomer is more preferably 21% by mass or more from the viewpoints of strength and heat resistance, and more preferably 30% by mass or less, even more preferably 28% by mass or less, and particularly preferably 25% by mass or less from the viewpoints of flexibility and impact resistance. The styrene unit content of the styrene-based elastomer can be calculated by carrying out proton NMR measurement using a nuclear magnetic resonance apparatus and quantifying the characteristic groups of styrene.

[0077] When two or more types of styrene elastomers are used, the styrene unit content can also be calculated by proportional calculation using the styrene unit content of each styrene elastomer and the blending mass ratio of each styrene elastomer. In this case, the styrene unit content of each styrene elastomer may be within or outside the above-mentioned preferred range of the styrene unit content, as long as the styrene unit content of the total styrene elastomer calculated by proportional calculation is within the above-mentioned preferred range of the styrene unit content.

[0078] For example, by combining a hydrogenated styrene-conjugated diene block copolymer (b1) having a styrene unit content of 10% to 15% by mass with a hydrogenated styrene-conjugated diene block copolymer (b2) having a styrene unit content of 25% to 35% by mass as the styrene elastomer, so that the styrene unit content is 20% to 35% by mass, particularly 20% to 25% by mass, it becomes easier to achieve both high-temperature strength, low-temperature impact resistance, and optical transparency. It is preferable that the hydrogenated styrene-conjugated diene block copolymer (b1) and the hydrogenated styrene-conjugated diene block copolymer (b2) are each hydrogenated styrene-conjugated diene-styrene block copolymers.

[0079] Suitable conjugated dienes for hydrogenated styrene-conjugated diene block copolymers as styrene-based elastomers are butadiene, isoprene, or a mixture thereof. Examples of hydrogenated styrene-conjugated diene block copolymers include hydrogenated styrene-butadiene block copolymers (hereinafter sometimes abbreviated as "SEBS").

[0080] By using a hydrogenated styrene-conjugated diene block copolymer as a styrene-based elastomer, it is possible to maintain good light resistance in a thermoplastic elastomer composition that has high transmittance to visible light and ultraviolet light, compared to when a polymer with double bonds in the molecular structure, such as styrene-butadiene-styrene block copolymer (SBS), is used. Furthermore, by combining the aforementioned propylene-based random copolymer with a hydrogenated styrene-conjugated diene block copolymer having a styrene unit content of 20% by mass or more and 35% by mass or less, particularly 20% by mass or more and 25% by mass or less, and having a glass transition temperature in the range of -65°C or more and -45°C or less, a thermoplastic elastomer composition having excellent low-temperature impact resistance, high-temperature strength, and optical transparency can be obtained.

[0081] The styrene elastomer used in the present invention is preferably a linear styrene elastomer, since the linear styrene elastomer is easily dispersed in the propylene random copolymer and can further improve light transmittance. Here, the linear styrene-based elastomer is, for example, a hydrogenated product of a styrene-conjugated diene block copolymer in which the conjugated diene portion is butadiene, isoprene, or a mixture thereof.Specific examples include styrene-ethylene-butylene-styrene copolymers, which are hydrogenated products of styrene-butadiene block copolymers.

[0082] The hydrogenated styrene-conjugated diene block copolymer as a styrene-based elastomer preferably has a 1,2-microstructure of 60 mol% or less, more preferably 45 mol% or less, as analyzed by NMR. A 1,2-microstructure of 60 mol% or less is preferred from the viewpoint of moldability and flexibility. Furthermore, from the viewpoint of weather resistance, the hydrogenation rate of the hydrogenated styrene-conjugated diene block copolymer as a styrene-based elastomer is preferably 90 mol% or more, more preferably 95 mol% or more.

[0083] From the viewpoint of mold releasability, the mass average molecular weight (Mw) of the styrene elastomer is preferably 40,000 or more, more preferably 45,000 or more, and even more preferably 48,000 or more. From the viewpoint of transparency, the mass average molecular weight (Mw) of the styrene elastomer is preferably 120,000 or less, more preferably 110,000 or less, and even more preferably 105,000 or less.

[0084] On the other hand, a material with high light transmittance but low transparency can produce a molded article with a frosted glass-like appearance. From the viewpoint of such appearance, the lower limit of the mass average molecular weight (Mw) of the styrene-based elastomer is preferably 125,000 or more, more preferably 130,000 or more, even more preferably 135,000 or more, and particularly preferably 140,000 or more. In this case, from the viewpoint of fluidity, the upper limit of the mass average molecular weight (Mw) is preferably 350,000 or less, more preferably 325,000 or less, and even more preferably 300,000 or less. By setting the molecular weight of the styrene-based elastomer within the above range, light that reaches the interior of the molded article is appropriately scattered, enabling a frosted glass-like appearance.

[0085] When two or more types of styrene elastomers are used, the mass average molecular weight of the styrene elastomer can be determined by measuring a mixture of each component of the styrene elastomer by the GPC method described below. In this case, the mass average molecular weight of each styrene elastomer may be within or outside the preferred range of the mass average molecular weight of the styrene elastomer as described above, as long as it is within the preferred range of the mass average molecular weight of the styrene elastomer as a mixture of styrene elastomers.

[0086] The mass average molecular weight (Mw) of the styrene elastomer is measured by gel permeation chromatography (GPC), and can be measured, for example, under the following conditions. Equipment: Tosoh Corporation "HLC-8220 GPC(R)" Column: Tosoh Corporation "TSKgel Super HM-M (6.0 mm ID x 15 cm x 2+G)" Detector: Differential refractive index detector (RI / built-in) Solvent: Chloroform Temperature: 40℃ Flow rate: 0.25mL / min Injection volume: 0.1 mass% x 20 μL Calibration sample: monodisperse polystyrene Calibration method: Polystyrene equivalent Calibration curve approximation: cubic (hyperbolic): Exclusion limit setting time: 12 minutes

[0087] Examples of methods for producing styrene-based elastomers include the method described in Japanese Patent Publication No. 40-23798, in which a styrene-conjugated diene block copolymer is synthesized in an inert solvent using a lithium catalyst, and then hydrogenated in an inert solvent in the presence of a hydrogenation catalyst, as described in Japanese Patent Publication Nos. 42-8704, 43-6636, 59-133203, and 60-79005.

[0088] The styrene-based elastomer can be obtained as a commercially available product, such as "Kraton (registered trademark) G" manufactured by Kraton Polymers, "Septon (registered trademark)" manufactured by Kuraray Co., Ltd., or "Tuftec (registered trademark)" manufactured by Asahi Kasei Corporation.

[0089] The thermoplastic elastomer composition of the present invention may contain only one type of styrene-based elastomer, or may contain two or more types that differ in monomer composition, physical properties, etc.

[0090] <Ethylene-α-olefin copolymer> The thermoplastic elastomer composition of the present invention may further contain an ethylene-α-olefin copolymer from the viewpoint of low-temperature impact resistance.

[0091] Examples of ethylene-α-olefin copolymers include those containing propylene, 1-butene, 2-methylpropylene, 1-pentene, 3-methyl-1-butene, 1-hexene, 4-methyl-1-pentene, and 1-octene as α-olefin units. The α-olefins are preferably α-olefins having 4 to 8 carbon atoms and a carbon-carbon double bond at the terminal carbon atom, such as propylene, 1-butene, 1-hexene, and 1-octene. The ethylene-α-olefin copolymers may be those in which only one α-olefin is copolymerized with ethylene, or in which two or more α-olefins are copolymerized with ethylene.

[0092] The ethylene unit content of the ethylene-α-olefin copolymer is preferably high to prevent fusion due to blocking of the ethylene-α-olefin copolymer, but low to improve low-temperature impact resistance when the thermoplastic elastomer composition of the present invention is molded. Specifically, the lower limit of the ethylene unit content of the ethylene-α-olefin copolymer, relative to 100% by mass of the total of ethylene units and α-olefin units, is usually 50% by mass or more, preferably 55% by mass or more, more preferably 60% by mass or more, and even more preferably 65% ​​by mass or more. On the other hand, the upper limit is usually 99% by mass or less, preferably 80% by mass or less.

[0093] The content of ethylene units, the content of α-olefin units, and the content of non-conjugated diene units (described below) in an ethylene-α-olefin copolymer can each be determined by infrared spectroscopy.

[0094] In addition to ethylene units and α-olefin units, the ethylene-α-olefin copolymer may contain other monomer units, such as monomer units based on non-conjugated dienes (non-conjugated diene units). Examples of non-conjugated dienes include linear non-conjugated dienes such as 1,4-hexadiene, 1,6-octadiene, 2-methyl-1,5-hexadiene, 6-methyl-1,5-heptadiene, and 7-methyl-1,6-octadiene; and cyclic non-conjugated dienes such as cyclohexadiene, dicyclopentadiene, methyltetrahydroindene, 5-vinylnorbornene, 5-ethylidene-2-norbornene, 5-methylene-2-norbornene, 5-isopropylidene-2-norbornene, and 6-chloromethyl-5-isopropenyl-2-norbornene. 5-ethylidene-2-norbornene and dicyclopentadiene are preferred.

[0095] When the ethylene-α-olefin copolymer contains other monomer units such as non-conjugated diene units, the content thereof is usually 10% by mass or less, preferably 5% by mass or less, based on 100% by mass of the entire ethylene-α-olefin copolymer.

[0096] The melt flow rate (MFR) of the ethylene-α-olefin copolymer is not limited, but is usually 10 g / 10 min or less, and from the viewpoint of strength, it is preferably 8.0 g / 10 min or less, more preferably 5.0 g / 10 min or less, and even more preferably 3.0 g / 10 min or less. The melt flow rate of the ethylene-α-olefin copolymer is usually 0.01 g / 10 min or more, and from the viewpoint of flowability, it is preferably 0.05 g / 10 min or more, more preferably 0.10 g / 10 min or more. The melt flow rate of ethylene-α-olefin copolymers is measured in accordance with ASTM D1238 at a measurement temperature of 190°C and a measurement load of 21.18N.

[0097] The density of the ethylene-α-olefin copolymer is preferably 0.88 g / cm from the viewpoint of low-temperature impact resistance. 3 On the other hand, from the viewpoint of transparency, i.e., refractive index, it is preferably 0.86 g / cm 3That's all. The density of ethylene-α-olefin copolymers can be measured by ISO 1183-A method (measurement temperature: 23°C).

[0098] Ethylene-α-olefin copolymers are also available as commercial products, such as Engage (registered trademark) and INFUSE (registered trademark) manufactured by The Dow Chemical Company.

[0099] The thermoplastic elastomer composition of the present invention may contain only one type of ethylene-α-olefin copolymer, or may contain two or more types that differ in monomer composition, physical properties, etc.

[0100] <Nucleating agent> The thermoplastic elastomer composition of the present invention may contain a nucleating agent. A diacetal nucleating agent is preferred as the nucleating agent. Adding a diacetal nucleating agent is expected to promote crystallization of the propylene random copolymer, thereby improving light transmittance.

[0101] Known diacetal crystal nucleating agents can be used, such as Gelall (registered trademark) D, Gelall (registered trademark) MD, and Gelall (registered trademark) DXR from New Japan Chemical Co., Ltd.; Milad 3988, Milad NX8000, Milad NX8000J, Milad NX8000K, and Milad NX8000ECO from Milliken (USA); and SIPAXNA-2 from GCHTECHNOLORY Co., Ltd. (China). These may be used alone or in combination of two or more.

[0102] <Mixing ratio> From the viewpoint of achieving excellent low-temperature impact resistance, high-temperature strength, and optical transparency, the total content of the propylene-based random copolymer and the styrene-based elastomer in the thermoplastic elastomer composition of the present invention is usually more than 40% by mass, preferably 50% by mass or more, more preferably 70% by mass or more, even more preferably 75% by mass or more, particularly preferably 80% by mass or more, and especially preferably 85% by mass or more, relative to 100% by mass of the thermoplastic elastomer composition, with the upper limit being 100% by mass.

[0103] In the thermoplastic elastomer composition of the present invention, the content of the styrene elastomer per 100 parts by mass of the propylene random copolymer is typically 20 parts by mass or more, and preferably 45 to 250 parts by mass. From the viewpoints of low-temperature impact resistance and light transmittance, the lower limit of the content of the styrene elastomer per 100 parts by mass of the propylene random copolymer is more preferably 60 parts by mass or more, even more preferably 70 parts by mass or more, and particularly preferably 80 parts by mass or more. From the viewpoint of high-temperature strength, the upper limit of the content of the styrene elastomer per 100 parts by mass of the propylene random copolymer is more preferably 200 parts by mass or less, even more preferably 150 parts by mass or less, particularly preferably 140 parts by mass or less, and particularly preferably 130 parts by mass or less.

[0104] When the thermoplastic elastomer composition of the present invention contains an ethylene-α-olefin copolymer, it preferably contains 10 to 100 parts by mass of the ethylene-α-olefin copolymer per 100 parts by mass of the propylene-based random copolymer. By blending the ethylene-α-olefin copolymer so as to satisfy the above relationship, it is possible to obtain a molded article having low-temperature impact resistance while controlling the flexural modulus within a suitable range. From the viewpoint of low-temperature impact resistance, the lower limit of the content of the ethylene-α-olefin copolymer per 100 parts by mass of the propylene-based random copolymer is more preferably 20 parts by mass or more, even more preferably 30 parts by mass or more, and particularly preferably 40 parts by mass or more. From the viewpoint of high-temperature strength, the upper limit of the content of the ethylene-α-olefin copolymer per 100 parts by mass of the propylene-based random copolymer is more preferably 70 parts by mass or less, even more preferably 60 parts by mass or less, and particularly preferably 55 parts by mass or less.

[0105] When the thermoplastic elastomer composition of the present invention contains a nucleating agent, preferably a diacetal-based nucleating agent, the content of the nucleating agent in the thermoplastic elastomer composition of the present invention is preferably 0.05 parts by mass or more and 5.0 parts by mass or less, more preferably 0.05 parts by mass or more and 1.0 part by mass or less, per 100 parts by mass of the total of the propylene-based random copolymer, the styrene-based elastomer, and the ethylene-α-olefin copolymer (or the total of the propylene-based random copolymer and the styrene-based elastomer when the ethylene-α-olefin copolymer is not contained).

[0106] <Other ingredients> In addition to the above-mentioned propylene-based random copolymer, styrene-based elastomer, ethylene-α-olefin copolymer, and nucleating agent, the thermoplastic elastomer composition of the present invention may contain optional components such as the following additives, inorganic fillers, organic fillers, and resins other than the propylene-based random copolymer, styrene-based elastomer, and ethylene-α-olefin copolymer (hereinafter referred to as "other resins") depending on various purposes, within the range that does not significantly impair the effects of the present invention.

[0107] Examples of additives include colorants, antioxidants, heat stabilizers, light stabilizers, UV absorbers, neutralizing agents, lubricants, antifogging agents, antiblocking agents, slip agents, flame retardants, dispersants, antistatic agents, conductivity imparting agents, metal deactivators, molecular weight modifiers, antibacterial agents, and fluorescent brighteners. These additives can be typically used in an amount of 0.01 to 2 parts by mass per 100 parts by mass of the total of the propylene random copolymer, styrene elastomer, and the ethylene-α-olefin copolymer used as needed.

[0108] Examples of other resins that may be contained in the thermoplastic elastomer composition of the present invention include low-density polyethylene, polyester elastomer, urethane elastomer, polyester resin, polyamide resin, polyurethane resin, styrene resin, polycarbonate resin, polyvinyl chloride resin, various elastomers other than those mentioned above (excluding those corresponding to styrene-based elastomers and ethylene-α-olefin copolymers), etc. The other resins listed above may be contained alone or in combination of two or more.

[0109] <Method of producing thermoplastic elastomer composition> The thermoplastic elastomer composition of the present invention can be produced by kneading the above-mentioned propylene-based random copolymer and styrene-based elastomer, and optionally the ethylene-α-olefin copolymer and other components in a conventional manner using a conventional extruder, Banbury mixer, roll, Brabender Plastograph, Kneader Brabender, etc. Among these production methods, the use of an extruder, particularly a twin-screw extruder, is preferred.

[0110] When the thermoplastic elastomer composition of the present invention is produced by kneading it in an extruder or the like, it is usually melt-kneaded in a heated state at 155° C. or higher and 240° C. or lower, preferably 180° C. or higher and 220° C. or lower. At this time, partial crosslinking can be achieved by adding a known crosslinking agent or crosslinking aid and subjecting the mixture to dynamic heat treatment, but from the viewpoint of light transmittance, it is preferable not to crosslink it.

[0111] <Physical properties> The thermoplastic elastomer of the present invention contains the propylene-based random copolymer, a styrene-based elastomer, and optionally an ethylene-α-olefin copolymer, and further contains a crystal nucleating agent, and therefore has excellent high-temperature strength, low-temperature impact resistance, and light transmittance.

[0112] The thermoplastic elastomer composition of the present invention is preferably used for an airbag storage cover. In the present invention, the term "airbag storage cover" refers to any container that stores an airbag. For example, in a container that stores an airbag, it refers to the opening through which the airbag deploys, or the entire container that is integrated with this opening. In such applications, the thermoplastic elastomer composition of the present invention preferably has the following physical properties.

[0113] (Izod impact strength) In the present invention, the Izod impact strength at -35°C, -40°C and -45°C according to ISO 180 is used as an index of low-temperature impact resistance. The Izod impact strength of a molded article of the thermoplastic elastomer composition of the present invention is 50 kJ / m 2 Preferably, it is 70 kJ / m or more. 2 The upper limit of the Izod impact strength of the thermoplastic elastomer composition of the present invention is not particularly limited, but is usually 150 kJ / m 2 The following is the result.

[0114] (MFR) In the present invention, the melt flow rate (MFR) at a temperature of 230°C and a measuring load of 21.18 N according to JIS K7210 (1999) is used as an index of the injection moldability of a thermoplastic elastomer composition. To ensure that the thermoplastic elastomer composition of the present invention has excellent injection moldability, the melt flow rate is preferably 1.0 g / 10 min or more and 50 g / 10 min or less. When the thermoplastic elastomer composition for airbags has an MFR of 1.0 g / 10 min or more, the flowability tends to be good, and when it is 50 g / 10 min or less, the generation of flash during molding is easily suppressed, which is preferable. From the viewpoint of flowability, the MFR of the thermoplastic elastomer composition for airbags is more preferably 2.0 g / 10 min or more, and even more preferably 2.5 g / 10 min or more. On the other hand, from the viewpoint of suppressing flash and the like during injection molding, the MFR of the thermoplastic elastomer composition is more preferably 40 g / 10 min or less, even more preferably 20 g / 10 min or less, particularly preferably 17 g / 10 min or less, and especially preferably 15 g / 10 min or less.

[0115] (room temperature tensile breaking elongation / tensile breaking strength) From the viewpoint of material strength, the thermoplastic elastomer composition of the present invention preferably has a tensile break elongation at 23°C according to JIS K6251 of 300% or more, more preferably 350% or more. If the tensile break elongation at 23°C is equal to or greater than the above lower limit, the material has good elongation, and therefore the airbag storage cover made of the thermoplastic elastomer composition of the present invention tends to have good unfolding properties. There is no particular upper limit to the tensile break elongation of the thermoplastic elastomer composition of the present invention, but it is usually 1500% or less. From the same viewpoint, the tensile breaking strength measured when measuring the tensile breaking elongation is preferably 10 MPa or more, more preferably 12 MPa or more. If the tensile breaking strength at 23°C is equal to or higher than the above lower limit, the material strength is high, and the airbag storage cover made of the thermoplastic elastomer composition of the present invention tends to have good deployability. The upper limit of the tensile breaking strength of the thermoplastic elastomer composition of the present invention is not particularly limited, but is usually 25 MPa.

[0116] (High temperature tensile strength) From the viewpoint of material strength, the thermoplastic elastomer composition of the present invention preferably has a tensile breaking strength at 85°C according to JIS K6251 of 4.5 MPa or more, more preferably 5.0 MPa or more, and even more preferably 5.5 MPa or more. If the tensile breaking strength at 85°C is equal to or greater than the above lower limit, the material strength is high, and therefore the unfolding properties of an airbag storage cover made of the thermoplastic elastomer composition of the present invention tend to be good. There is no particular upper limit to the tensile breaking strength of the thermoplastic elastomer composition of the present invention, but it is usually 8 MPa or less.

[0117] (flexural modulus) The thermoplastic elastomer composition of the present invention preferably has a flexural modulus of 600 MPa or less, particularly 200 MPa or more and 550 MPa or less, as measured in accordance with JIS K 7203. When the flexural modulus of the thermoplastic elastomer composition is equal to or less than the above upper limit, the low-temperature impact resistance tends to be good, and when it is equal to or more than the above lower limit, the rigidity tends to be excellent.

[0118] (Total light transmittance) From the viewpoint of light transmittance, the thermoplastic elastomer composition of the present invention preferably has a total light transmittance of 60% or more, more preferably 65% ​​or more, even more preferably 70% or more, and particularly preferably 75% or more, measured in accordance with JIS K7136-1 using a 2 mm thick sheet-like test piece obtained by injection molding the thermoplastic elastomer composition under conditions of a resin temperature of 200°C and a mold temperature of 40°C. By setting the value within such a range, when the composition is used as an airbag storage cover, the light transmittance is good and the illuminated switch can be easily seen.

[0119] (Haze) From the viewpoint of transparency, the thermoplastic elastomer composition of the present invention preferably has a haze of 70% or less, more preferably 60% or less, even more preferably 50% or less, and particularly preferably 40% or less, measured in accordance with JIS K7136-1 using a 2 mm thick sheet-like test piece obtained by injection molding the thermoplastic elastomer composition under conditions of a resin temperature of 200°C and a mold temperature of 40°C. From the viewpoint of switch visibility, it is essential that the total light transmittance described above is good, and by having the haze, which is an evaluation index of transparency, fall within the above numerical range, the lit-up switch can be more clearly and easily viewed when used as an airbag storage cover.

[0120] On the other hand, from the viewpoint of a design appearance like frosted glass, the haze is preferably more than 70%, more preferably 75% or more, and even more preferably 80% or more. In this case, the total light transmittance is preferably 60% or more, more preferably 65% ​​or more, even more preferably 70% or more, and particularly preferably 75% or more. By setting the haze and total light transmittance within the above numerical ranges, it is possible to achieve a frosted glass appearance that is highly cloudy while transmitting light.

[0121] <Molded products such as airbag storage covers> The thermoplastic elastomer composition of the present invention can generally be used to form molded articles such as airbag housing covers using a conventional injection molding method, or, as necessary, various molding methods such as gas injection molding, injection compression molding, and short-shot foam molding. It is particularly preferable to form molded articles such as airbag housing covers by injection molding the thermoplastic elastomer composition of the present invention. The molding conditions for injection molding are as follows:

[0122] The molding temperature when injection molding the thermoplastic elastomer composition is usually 150°C to 300°C, preferably 180°C to 280°C. The injection pressure is usually 5 MPa to 100 MPa, preferably 10 MPa to 80 MPa. The mold temperature is usually 0°C to 80°C, preferably 20°C to 60°C.

[0123] <Composite molded body> The composite molded product of the present invention, which has a first layer made of a thermoplastic elastomer composition containing a propylene-based random copolymer and a styrene-based elastomer having a glass transition temperature in the range of -65°C to -45°C, and a second layer made of a resin film, is suitable as an airbag storage cover. That is, the thermoplastic elastomer composition of the present invention can be integrated with a resin film to form a composite molded article, and a composite molded article of the present invention having a first layer containing the thermoplastic elastomer composition of the present invention and a second layer made of a resin film is suitable as an airbag storage cover.

[0124] By using such a composite molded body, it is possible to combine a resin film incorporating the operation function and design of a touch panel with an airbag storage cover. Examples of resin films include polypropylene films, polyethylene films, films made of polyolefin-based thermoplastic elastomers, and polyester films.

[0125] The composite molded article of the present invention can be molded by insert molding, in which a resin film is placed in a mold before molding, and then the mold is filled with the thermoplastic elastomer composition of the present invention, thereby obtaining a composite molded article of the present invention having a first layer made of the thermoplastic elastomer composition of the present invention and a second layer made of a resin film.

[0126] <Application> The thermoplastic elastomer composition of the present invention has excellent high-temperature strength, low-temperature impact resistance, and light transmittance, and is therefore suitable as a molding material for airbag storage covers. In particular, the composition is suitable as an airbag storage cover for an airbag system that activates upon detecting impact or deformation in the event of a collision or other accident involving a high-speed moving object such as an automobile, and inflates to protect the occupant. In particular, due to its excellent light transmittance, the airbag storage cover of the present invention is suitable as an airbag storage cover to be placed in a visible position such as a touch panel display.

[0127] < <1> ~ <11> Thermoplastic elastomer compositions for airbags> The aforementioned <1> ~ <11> In the thermoplastic elastomer composition for airbags, an example of the propylene polymer of component (A) is the propylene random copolymer contained in the thermoplastic elastomer composition of the present invention described above. Examples of propylene polymers other than the propylene random copolymer include propylene homopolymers and propylene block copolymers containing, in addition to propylene units, preferably 15% by mass or less, more preferably 10% by mass or less, and even more preferably 6% by mass or less of ethylene units, α-olefin units other than propylene, or monomer units other than ethylene and α-olefins. Suitable examples of the α-olefin units contained in the propylene block copolymer are the same as those of the α-olefin units of the propylene random copolymer described above. Examples of propylene-based polymers other than propylene-based random copolymers include propylene homopolymers, propylene-ethylene block copolymers, propylene-1-butene block copolymers, propylene-1-hexene block copolymers, propylene-1-octene block copolymers, propylene-ethylene-1-butene block copolymers, propylene-ethylene-1-hexene block copolymers, propylene-ethylene-1-octene block copolymers, and propylene-based block copolymers obtained by polymerizing a propylene homopolymer in a first step and subsequently polymerizing a propylene-ethylene copolymer in a second step. Preferred are propylene homopolymers, block copolymers of propylene and at least one monomer selected from ethylene and α-olefins having from 4 to 10 carbon atoms, and propylene-based block copolymers obtained by polymerizing a propylene homopolymer in a first step and subsequently polymerizing a propylene-ethylene copolymer in a second step. The physical properties and other aspects of the propylene polymer of component (A) are the same as those of the propylene random copolymer described above. <1> ~ <11> For the detailed description of the styrene-based elastomer of component (B) and other components in the thermoplastic elastomer composition for airbags, the descriptions of the styrene-based elastomer and other components in the thermoplastic elastomer composition of the present invention also apply. [Example]

[0128] The present invention will be described in more detail below using examples. The present invention is not limited to the following examples as long as it does not deviate from the gist of the invention. The values ​​of various production conditions and evaluation results in the following examples represent preferred upper or lower limit values ​​in the embodiments of the present invention, and preferred ranges may be defined by combining the above-mentioned upper or lower limit values ​​with the values ​​in the following examples or values ​​between the examples.

[0129] <Raw materials> [Propylene random copolymer] (A-1): Novatec (registered trademark) PP MG03E manufactured by Japan Polypropylene Corporation Propylene-ethylene random copolymer MFR (JIS K7210 (1999)): 30g / 10 minutes (measurement conditions: 230℃, load 21.18N (2.16kgf)) Propylene unit content: 96% by mass

[0130] [Styrene-based elastomer] (b-1): Kraton (registered trademark) G1652 manufactured by Kraton Polymers Hydrogenated styrene-butadiene block copolymer (styrene-ethylene-butylene-styrene copolymer (SEBS)) Mass average molecular weight (Mw): 70,000 Styrene unit content: 30% by mass Glass transition temperature: -55℃ (b-2): Kraton (registered trademark) G1657 manufactured by Kraton Polymers Hydrogenated styrene-butadiene block copolymer (styrene-ethylene-butylene-styrene copolymer (SEBS)) Mass average molecular weight (Mw): 110,000 Styrene unit content: 13% by mass Glass transition temperature: -55℃ (b-3): Kraton (registered trademark) G1654 manufactured by Kraton Polymers Hydrogenated styrene-butadiene block copolymer (styrene-ethylene-butylene-styrene copolymer (SEBS)) Mass average molecular weight (Mw): 180,000 Styrene unit content: 31% by mass Glass transition temperature: -55℃ (b-4): Kraton (registered trademark) G1651 manufactured by Kraton Polymers Hydrogenated styrene-butadiene block copolymer (styrene-ethylene-butylene-styrene copolymer (SEBS)) Mass average molecular weight (Mw): 260,000 Styrene unit content: 33% by mass Glass transition temperature: -55℃

[0131] [Other styrene elastomers (Tg outside the range of -65°C to -45°C)] (b'-1): Kraton (registered trademark) G1643 manufactured by Kraton Polymers Hydrogenated styrene-butadiene block copolymer (styrene-ethylene-butylene-styrene copolymer (SEBS)) Mass average molecular weight (Mw): 60,000 Styrene unit content: 20% by mass Glass transition temperature: -35℃

[0132] [Ethylene-α-olefin copolymer] (C-1): Engage (registered trademark) 8100 manufactured by The Dow Chemical Company Ethylene-octene copolymer MFR (ASTM D1238): 1g / 10min (measurement conditions: 190°C, load: 21.18N (2.16kgf)) Density (ISO 1183-A method): 0.87g / cm 3 (Measurement temperature: 23℃) (C-2): Engage (registered trademark) 8150 manufactured by The Dow Chemical Company Ethylene-octene copolymer MFR (ASTM D1238): 0.5 g / 10 min (measurement conditions: 190°C, load: 21.18 N (2.16 kgf)) Density (ISO 1183-A method): 0.87g / cm 3 (Measurement temperature: 23℃)

[0133] [Nucleating agent] (D-1): Gelall (registered trademark) MD manufactured by New Japan Chemical Co., Ltd. 1,3:2,4-Bis-O-(4-methylbenzylidene)-D-sorbitol Melting point: 255-267℃

[0134] [PMMA] (E-1): Mitsubishi Chemical Corporation, Acrypet (registered trademark) VH PMMA (Polymethyl methacrylate) MFR (JIS K7210 (1999)): 2.0g / 10 minutes (measurement temperature 230℃, measurement load 37.3N) Deflection temperature under load (ISO 75-2): 100°C (measurement load 1.8MPa)

[0135] <Evaluation method> 1) Injection moldability: Melt flow rate (MFR) Measurements were carried out in accordance with JIS K7210 (1999) at a temperature of 230°C and a load of 21.18N.

[0136] In the evaluations of 2) to 6) below, the thermoplastic elastomer composition pellets obtained in each example were injection molded into test sheets for each test using an inline screw type injection molding machine (Toshiba Machine Co., Ltd.'s "IS130") at an injection pressure of 50 MPa, a cylinder temperature setting of 190°C (resin temperature of 200°C), and a mold temperature of 40°C.

[0137] 2) Low temperature impact resistance: Izod impact strength Using the above injection molding method, notched test pieces for measuring Izod impact strength, 10 mm thick x 4 mm wide x 80 mm long, were molded and measured at temperatures of -35°C, -40°C, and -45°C in accordance with ISO 180 (2010). Test pieces that were not broken in the Izod impact test were rated "P," those that were on the verge of breaking but still had a skin remaining were rated "H," and those that were broken were rated "C." The higher the Izod impact strength value, and the more unbroken the "P" rating, the better the low-temperature impact resistance.

[0138] 3) Flexural modulus Measurement was carried out in accordance with JIS K7203 under conditions of a span of 64 mm and a bending speed of 1 mm / min.

[0139] 4) Room temperature tensile elongation and tensile strength (23°C): Tensile fracture test (JIS-3 dumbbell, tensile speed 500mm / min) Test pieces for tensile tests (sheets measuring 2 mm thick x 120 mm wide x 80 mm long) were punched out in accordance with JIS K6251 (JIS No. 3 dumbbell). The tensile elongation at break and tensile strength at break were measured for these punched test pieces in an atmosphere of 23°C in accordance with JIS K6251. The larger the values ​​of tensile elongation at break and tensile strength at break, the better the evaluation was.

[0140] 5) High temperature tensile strength (85℃): Tensile strength test (JIS No. 3 dumbbell, tensile speed 500mm / min) (unit: %) Test pieces for tensile tests (sheets measuring 2 mm thick x 120 mm wide x 80 mm long) were punched out in accordance with JIS K6251 (JIS No. 3 dumbbell). The tensile breaking strength of these punched test pieces was measured in an atmosphere of 85°C in accordance with JIS K6251. The higher the tensile breaking strength value, the better the evaluation. In Table 1 below, values ​​for tensile breaking strength at 85°C marked with a ">" indicate that the measured strength reached the upper limit of the instrument and did not break. The number indicates the stress when the upper limit of the instrument was reached, and in this evaluation, this stress can be considered the same as the breaking stress.

[0141] 6) Haze Haze was measured for an injection-molded sheet of 2 mm thick x 120 mm wide x 80 mm long in accordance with JIS K7136-1. A smaller haze value was evaluated as having better transparency.

[0142] 7) Total light transmittance The total light transmittance of an injection-molded sheet having a thickness of 2 mm, a width of 120 mm, and a length of 80 mm was measured in accordance with JIS K7136-1. A larger total light transmittance value was evaluated as having better light transmittance.

[0143] <Examples / Comparative Examples> [Example 1] As shown in Table 1, the following ingredients were added to 100 parts by mass of (A-1), 100 parts by mass of (b-1), and the total of 100 parts by mass of (A-1) and (b-1): 0.15 parts by mass of an antioxidant (trade name Irgastab (registered trademark) FS301FF, manufactured by BASF Japan Ltd.), 0.2 parts by mass of a light stabilizer (HALS (SABOSTAB (registered trademark) UV119, manufactured by SONGWON Co., Ltd.), 0.1 parts by mass of an ultraviolet absorber (TINUVIN (registered trademark) 326, manufactured by BASF Japan Ltd.), 0.1 parts by mass of a silicone oil (manufactured by Shin-Etsu Chemical Co., Ltd.), 0.15 parts by mass of an antioxidant (trade name Irgastab (registered trademark) FS301FF, manufactured by BASF Japan Ltd.), 0.2 parts by mass of a light stabilizer (HALS (SABOSTAB (registered trademark) UV119, manufactured by SONGWON Co., Ltd.), 0.1 parts by mass of an ultraviolet absorber (TINUVIN (registered trademark) 326, manufactured by BASF Japan Ltd.), 0.15 parts by mass of an antioxidant (manufactured by Shin-Etsu Chemical Co., ... 0.2 parts by mass of a mixture of 100% propylene glycol (KF96-100CS) and 100% propylene glycol (KF96-100CS) was blended in a Henschel mixer for 1 minute. The blend was fed into a co-rotating twin-screw extruder (Toshiba Machine Co., Ltd., "TEM-26SS", L / D = 48.5, number of cylinder blocks: 12) at a rate of 25 kg / h, and melt-kneaded at a temperature in the range of 160°C to 210°C to produce pellets of a thermoplastic elastomer composition. The obtained pellets of the thermoplastic elastomer composition were evaluated in the above items 1) to 7). The evaluation results are shown in Table 1.

[0144] Furthermore, a composite molded article was prepared by insert molding using pellets of the obtained thermoplastic elastomer composition and a polypropylene film. Specifically, a film of Novatec (registered trademark) PP MG03E manufactured by Japan Polypropylene Corporation was pressed to a thickness of 0.5 mm and cut into a 100 mm square. This was set in a mold measuring 100 mm x 100 mm x 3 mm, and the pellets were injection molded. This method enabled us to obtain a composite molded article in which a first layer made of a thermoplastic elastomer composition and a second layer made of a polypropylene film were fused together.

[0145] [Examples 2 to 8 and Comparative Examples 1 to 3] Pellets of a thermoplastic elastomer composition were obtained in the same manner as in Example 1, except for the formulations shown in Tables 1 and 2. As in Example 1, 0.15 parts by mass of an antioxidant (product name Irgastab® FS301FF, manufactured by BASF Japan Ltd.), 0.2 parts by mass of a light stabilizer (HALS (SABOSTAB® UV119, manufactured by SONGWON Co., Ltd.), 0.1 parts by mass of an ultraviolet absorber (TINUVIN® 326, manufactured by BASF Japan Ltd.), and 0.2 parts by mass of silicone oil (KF96-100CS, manufactured by Shin-Etsu Chemical Co., Ltd.) were blended per 100 parts by mass of the total of the propylene random copolymer, the styrene elastomer or other styrene elastomer, and the ethylene-α-olefin copolymer used as needed. The blend amounts of these ingredients are omitted in Tables 1 and 2. The obtained pellets of the thermoplastic elastomer composition were evaluated in the above items 1) to 7). The evaluation results are shown in Tables 1 and 2.

[0146] [Table 1]

[0147] [Table 2]

[0148] [Evaluation results] Examples 1 to 8 were excellent in high-temperature strength, low-temperature impact resistance, and light transmittance. Comparative Example 1 is an example in which (b'-1), a hydrogenated styrene-butadiene block copolymer with a glass transition temperature exceeding -45°C, was blended, resulting in significantly poor low-temperature impact resistance. Comparative Example 2 is an example in which the composition equivalent to Example 2 of Patent Document 6 was compounded, but the high-temperature strength was poor. Comparative Example 3 is an example in which the total content of the propylene random copolymer and the styrene elastomer was blended to be 37.5% by mass, but the high-temperature strength was poor.

[0149] Although the present disclosure has been described in detail with reference to specific embodiments, it will be apparent to those skilled in the art that various modifications can be made without departing from the spirit and scope of the present disclosure. This application is based on Japanese Patent Application No. 2021-108344 filed on June 30, 2021, and Japanese Patent Application No. 2021-207288 filed on December 21, 2021, both of which are incorporated by reference in their entireties.

Claims

1. An airbag storage cover made of a thermoplastic elastomer composition containing a propylene-based random copolymer and a styrene-based elastomer having a glass transition temperature in the range of -65°C or higher and -45°C or lower, wherein the total content of the propylene-based random copolymer and the styrene-based elastomer in 100% by mass of the thermoplastic elastomer composition is more than 40% by mass and 100% by mass or lower.

2. The airbag storage cover according to claim 1, which is disposed in a visible position.

3. 3. The airbag storage cover according to claim 1, wherein the styrene-based elastomer is linear.

4. 4. The airbag storage cover according to claim 3, wherein the thermoplastic elastomer composition has a melt flow rate of 1.0 g / 10 min or more and 50 g / 10 min or less at a temperature of 230°C and a measurement load of 21.18 N in accordance with JIS K7210 (1999).

5. 5. The airbag storage cover according to claim 4, wherein the content of the styrene-based elastomer in the thermoplastic elastomer composition is 45 parts by mass or more and 250 parts by mass or less per 100 parts by mass of the propylene-based random copolymer.

6. The airbag storage cover according to claim 1 or 2, wherein the thermoplastic elastomer composition further contains 10 parts by mass or more and 100 parts by mass or less of an ethylene-α-olefin copolymer per 100 parts by mass of the propylene-based random copolymer.

7. The airbag storage cover according to claim 1 or 2, wherein the styrene-based elastomer has a styrene unit content of 20% by mass or more and 25% by mass or less.

8. 8. The airbag storage cover according to claim 7, wherein the styrene-based elastomer comprises a hydrogenated styrene-conjugated diene block copolymer (b1) having a styrene unit content of 10% by mass or more and 15% by mass or less, and a hydrogenated styrene-conjugated diene block copolymer (b2) having a styrene unit content of 25% by mass or more and 35% by mass or less.

9. The airbag storage cover according to claim 1 or 2, wherein a sheet-like test piece having a thickness of 2 mm is injection-molded from the thermoplastic elastomer composition under conditions of a resin temperature of 200°C and a mold temperature of 40°C, and the total light transmittance measured in accordance with JIS K7136-1 is 65% or more.

10. The airbag storage cover according to claim 1 or 2, wherein a sheet-like test piece having a thickness of 2 mm is injection-molded from the thermoplastic elastomer composition under conditions of a resin temperature of 200°C and a mold temperature of 40°C, and the haze measured in accordance with JIS K7136-1 is 70% or less.

11. 3. A method for manufacturing the airbag storage cover according to claim 1, comprising a step of injection molding the thermoplastic elastomer composition.

12. A thermoplastic elastomer composition comprising a propylene-based random copolymer and a linear styrene-based elastomer having a styrene unit content of 20% by mass or more and 35% by mass or less and a glass transition temperature in the range of -65°C or more and -45°C or less, wherein the total content of the propylene-based random copolymer and the linear styrene-based elastomer in 100% by mass of the thermoplastic elastomer composition is more than 40% by mass and 100% by mass or less, the content of the linear styrene-based elastomer relative to 100 parts by mass of the propylene-based random copolymer is 45 parts by mass or more and 250 parts by mass or less, and the melt flow rate at a temperature of 230°C and a measuring load of 21.18 N in accordance with JIS K7210 (1999) is 1.0 g / 10 min or more and 50 g / 10 min or less.

13. The thermoplastic elastomer composition according to claim 12, wherein a sheet-like test piece having a thickness of 2 mm is injection-molded from the thermoplastic elastomer composition under conditions of a resin temperature of 200°C and a mold temperature of 40°C, and the total light transmittance measured in accordance with JIS K7136-1 is 65% or more.

14. The thermoplastic elastomer composition according to claim 12 or 13, wherein a sheet-like test piece having a thickness of 2 mm is injection-molded from the thermoplastic elastomer composition under conditions of a resin temperature of 200°C and a mold temperature of 40°C, and the haze measured in accordance with JIS K7136-1 is 70% or less.

15. The thermoplastic elastomer composition according to claim 12 or 13, further comprising 10 parts by mass or more and 100 parts by mass or less of an ethylene-α-olefin copolymer relative to 100 parts by mass of the propylene-based random copolymer.

16. The thermoplastic elastomer composition according to claim 12 or 13, wherein the linear styrene-based elastomer has a styrene unit content of 20% by mass or more and 25% by mass or less.

17. 17. The thermoplastic elastomer composition according to claim 16, wherein the linear styrene-based elastomer comprises a hydrogenated styrene-conjugated diene block copolymer (b1) having a styrene unit content of 10% by mass or more and 15% by mass or less, and a hydrogenated styrene-conjugated diene block copolymer (b2) having a styrene unit content of 25% by mass or more and 35% by mass or less.

18. A molded article made of the thermoplastic elastomer composition according to claim 12 or 13.

19. A composite molded article having a first layer containing the thermoplastic elastomer composition according to claim 12 or 13 and a second layer made of a resin film.

20. A composite molded article having a first layer made of a thermoplastic elastomer composition containing a propylene-based random copolymer and a styrene-based elastomer having a glass transition temperature in the range of -65°C to -45°C, and a second layer made of a resin film.

Images