KR970008432B1 - Air bag containing cover - Google Patents

Abstract

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Description

Airbag Storage Cover

1 is a front view of a schematic partial cross-sectional view showing a state in which an airbag receiving cover according to an embodiment of the present invention is mounted on an airbag.

2 is a rear view of the airbag housing cover.

* Explanation of symbols for main parts of the drawings

1: Cover for airbag storage 1A: Skin layer

1B: core layer 2: air bag

3: inflator 4, 5: retainer

The present invention relates to a cover for accommodating an airbag that expands and expands by detecting impact, deformation, and the like during a vehicle crash.

As this kind of cover, it was common to insert a reinforcing material (net, etc.) (Patent No. 62-234764, Pat. No. 50-127336, Pat. 52-80928, Pat. 50-43454, Pat. 51-25342. Publications, etc.).

Covers with such net-shaped inserts are usually manufactured by making the entire cover from a relatively low-strength material such as foamed polyurethane and inserting the reinforcement net through the entire cover. It is common to install a line that does not insert the reinforcement net so that it can be deployed smoothly and the cover is easily broken.

On the other hand, Japanese Patent Application Laid-Open No. 52-76042 includes two layers of a skin layer made of a rigid urethane foam for finishing the surface cleanly and a core layer of a low density (high magnification foam) urethane foam to soften the entire cover. Covers for airbag storage have been proposed, which have slits (lines for smoothly deploying during airbag inflation) reaching a portion of the epidermis from the core layer.

Among the conventional airbag storage covers described above, the airbag storage cover in which the net-shaped insert is inserted has a lot of workmanship and is complicated to insert the net-shaped reinforcement material, for example, using urethane foam molding (RIM). It was difficult to manufacture with high precision, and there existed a problem that a yield was low and a cost was high.

In addition, since the cover specified in Japanese Patent Application Laid-Open No. 52-76042 has a hard urethane foam (ASTM-D2240, Shore hardness 30 to 40), when a part of the body hits the vehicle while driving, the user does not feel soft and feels uncomfortable.

Moreover, the urethane-based material had a low impact strength and had no reinforcement inserts, so that fragments could be scattered during airbag inflation.

The present invention solves the problems of the conventional airbag housing cover, the skin layer gives a soft and soft feeling, the core layer maintains a shape that can withstand external pressure, and in the temperature range of -40 ℃ to +80 ℃ In this case, the airbag is easily broken during inflation, but no debris is scattered, and it is actually considered to be mounted on the car, and even after being left at a high temperature of 110 ° C for 1000 hours, the same good breaking performance is achieved in the above temperature range. It is an airbag storage cover that displays quality and has almost no appearance change, and has a high yield, high productivity, and good economical efficiency by a high yield, a method that does not come out of defective products by a simple molding method such as injection molding. An object of the present invention is to provide an airbag storage cover that can be supplied.

Cover for airbag storage of the present invention is the following components A, B, C and D, that is,

Component A: Hydrogenated derivative of block copolymer consisting of styrene and conjugated zen

Component B: Olefin Resin

Component C: Polyisobutylene having a viscosity average molecular weight of 70000 or less

Component D: The kinematic viscosity at 40 ° C. contains a softener for hydrocarbon rubbers of 500 cst or less and / or polybutene of 2500 or less of a number average molecular weight, and the blending ratio thereof is

Component A = 40-80 wt%

Component B = 5-30 wt%

Component C = 2-30 wt%

Component D = 0-20 wt%

And a core layer made of a thermoplastic polymer having a JIS-A hardness of JIS-K6301 of 20-90 and a core layer made of an olefin resin having a bending elastic modulus of JIS-K7203 of 1000-7000 kg / cm ² . The core layer has a higher hardness than the skin layer described above, and the core layer has a portion for easily opening at the start of the airbag operation.

That is, the present inventor and the like have repeatedly studied to achieve the above-mentioned object, and as a result, the above-described object is adopted by employing a specific skin layer and a specific core layer and performing a portion that is easy to cleave in the core layer, for example, a weak shape. It has begun to see that this is accomplished and the present invention has been completed.

The present invention is described in detail below with reference to the drawings.

1 is a front view of a schematic partial cross-sectional view showing a state in which an airbag housing cover is mounted on an airbag, and FIG. 2 is a rear view of the airbag housing cover.

In the drawings, reference numeral 1 denotes an airbag housing cover, 2 denotes an airbag, 3 denotes an inflator, 4 denotes a ring retainer, 5 denotes a base retainer, 6 denotes a plate, and 7 denotes a rivet. (1A) is a skin layer, (1B) is a core layer, and (1C) is a sewing machine-shaped slit.

As shown, the airbag storage cover 1 of the present invention is made of a skin layer 1A containing the following component AD in the mixing ratio described below, and the JIS-A hardness of JIS-K6301 is 20-. A skin layer made of 90 thermoplastic polymers (thermoplastic elastomers) is used.

Component A: Hydrogenated derivative of block copolymer consisting of styrene and conjugated zen = 40-80 wt%

Component B: olefin resin = 5-30 weight%

Component C: Polyisobutylene = 2-30 weight% of viscosity average molecular weight 70000 or less

Component D: softener for hydrocarbon-based rubber having a kinematic viscosity at 40 ° C. or less of 500 cst and / or polybutene having a number average molecular weight of 2500 or less = 0-20% by weight

On the other hand, as core layer 1B, the core layer which consists of olefin resin of 1000-7000 kg / cm < ² > of bending elastic modulus of JIS-K7203 is used.

The core layer 1B has a higher hardness than the skin layer 1A, and the core layer 1B is a portion for easily opening when the airbag starts to operate, and has a thin shape such as a weak shape 1C, for example, a notch shape. And a slit shape such as a sewing scale.

In this invention, as a component A which comprises the thermoplastic elastomer used as a structural material of 1 A of skin layers, it is a general formula.

X-Y

X YX) _n -Y

X YX _n -Y

Examples thereof include hydrogenated derivatives of styrene block copolymers represented by [wherein X is a polymer block of styrene, Y is an elastomeric polymer block of conjugated zen, and n is an integer of 1-5].

As the monomer of the conjugated zen of the polymer block Y in each of the above formulas, butene or isoprene are suitable, and they may be a mixture.

Moreover, the ratio which occupies for the whole block copolymer of the component A of the polymer block X of styrene is 5-40 weight% normally, Preferably the thing of the range of 10-35 weight% is suitable.

When this ratio is less than 5% by weight, the mechanical strength of the skin layer is inferior, and it is easy to break during air bag inflation, and it is easy to cause discomfort such as deformation and traces left due to pressure from the outside.

Moreover, when it exceeds 40 weight%, a soft feeling of a skin layer will be inferior and it will become impractical.

Moreover, it is preferable that the number average molecular weight of said polymer block X is 5000-125000, and the number average molecular weight of polymer block Y exists in the range of 15000-250000.

As a method for producing a hydrogenated derivative of a styrene-based block copolymer represented by the aforementioned general formula, a number of methods have been proposed, but as a representative method, for example, the methods described in Japanese Patent Application Laid-Open No. 42-8704, 43-6636, etc. There is this.

In the present invention, at least 50%, preferably at least 80%, of hydrogen in the olefinic double bond in the polymer block Y is added at the time of the hydrogenation at the time of its preparation, and at most 25% of hydrogen in the polymer block X is added. What was added is suitable as a component.

As such a block polymer, KRATON-G (Shell Chemical Co., Ltd.), KL (Kuraray Co., Ltd.), Tapftech (Takeup Co., Ltd.) etc. which are commercially available polymers can be used.

As the component B, homopolymers or copolymers of α-olefins such as ethylene, propylene and butene-1 are suitable.

Component C was a high molecular weight polyisobutylene having almost no double bonds in the molecular chain.

C (CH ₃ ) ₂ -CH _{2 n}

It is desirable to have a bond such as

The polyisobutylene of component C has a viscosity average molecular weight of 70000 or less, preferably 50000 or less, but when the viscosity average molecular weight exceeds 70000, it is inconvenient that the softness and moldability of the skin layer obtained are inferior. Not preferred.

As polyisobutylene suitable for component C, commercially available polymers, Tetrax (Japan Petrochemical Co., Ltd.), Bisternex (Exon Chemical Co., Ltd.), etc. can be used.

Component D is a softener for hydrocarbon rubbers having a kinematic viscosity of 500 cSt or less at 40 ° C., specifically, paraffinic, naphthenic ester oils, process oils, and / or a number average molecular weight of 2500 or less, preferably Is copolymerized with polybutene of up to 2000, preferably isobutylene, and partly n-butene.

When the kinematic viscosity of the softener for hydrocarbon-based rubber of component D exceeds 500 cSt, inconvenience is inferior to the molding processability of the skin layer material.

Moreover, also when the molecular weight of a polybutene exceeds 2500, the inconvenience that it is inferior to moldability arises similarly.

The component D may be a mixture of the above-described softener for hydrocarbon rubber and polybutene.

In the present invention, the blending ratio of these components A-D is

Component A = 40-80% by weight, preferably 60-75% by weight

Component B = 5-30% by weight, preferably 10-20% by weight

Component C = 2-30% by weight, preferably 5-20% by weight

Component D = 0-20% by weight, preferably 0-15% by weight.

When the styrene-based block polymer of component A is less than 40% by weight, the softness of the obtained skin layer is inferior, and when it exceeds 80% by weight, the molding processability of the thermoplastic elastomer obtained is inferior.

If the polyisobutylene of the component C is less than 2% by weight, the softness and molding processability are inferior, and if it is more than 30% by weight, the adhesion of the surface and the strength properties are inferior.

When the softener and / or polybutene for hydrocarbon rubber of component D exceeds 20% by weight, it causes an external appearance amount (deformation) and the like after elapse of 110 ° C and 1000 hours, and at the time of inflating the airbag at -40 ° C. There is a problem that failure of failure, that is, destruction or scattering of debris, etc., occurs at a place other than a predetermined weak part.

In the present invention, the thermoplastic polymer constituting the skin layer 1A is selected to have a JIS-A hardness of 20-90. However, the hardness of the thermoplastic polymer below the above-mentioned range is rich in flexibility, but moldability (for example, injection molding) The disadvantage is that it is inferior to mechanical strength and easy to break.

In addition, the hardness exceeding the above-mentioned range is not a problem in moldability, but has a disadvantage of being inferior in softness and not practical.

Although the hardness selection within the range of hardness 20-90 changes also with the thickness of the skin layer, Preferably it is the range of hardness 30-70, More preferably, it is set as the range of hardness 40-65.

Moreover, in this invention, the method of manufacturing the thermoplastic elastomer which consists of components A-D used for 1 A of skin layers can depend on mechanical melt kneading.

Specifically, general melt kneaders such as banbury mixers, various kneaders, and extruders can be used.

In addition, in the present invention, the constituent material of the skin layer is other resins, elastomeric glass fibers, talc, calcium carbonate, and silica, as long as it is in the range of hardness, in addition to the component AD. And various fillers such as a mica, a colorant, a stabilizer and the like may be filled.

The thickness of the epidermal layer 1A usually depends on the part of the cover 1 and is not completely uniform throughout, but in the present invention, at least 70% of the surface area of the epidermal layer 1A, preferably at least 80%. It is suitable for the part to be 0.5 mm or more in thickness.

Although there is no particular upper limit on the thickness, it is appropriate to set it to about 10 mm or less in terms of design, economy, softness, operation performance of the airbag, and molding processing surface.

Moreover, 1 A of skin layers may foam about 3.0 times or less for the purpose of improving a soft feeling and weight reduction.

In the present invention, an olefin system having a bending modulus of JIS-K7203 used as a constituent material of the core layer 1B is 1000-7000 kg / cm ² and having a higher hardness than the thermoplastic polymer of the skin layer 1A described above. As the resin, homopolymers or copolymers of? -Olefins such as ethylene and propylene are suitable.

These may be used independently, respectively and may mix and use 2 or more types.

In addition, an elastomer such as an olefin rubber composed of ethylene and α-olefin may be added to the above olefin resin for the purpose of improving flexibility and low temperature impact resistance, as long as it is within the range of the above elastic modulus and hardness.

Moreover, as long as it is within the range of the above-mentioned elastic modulus and hardness, various fillers, coloring agents, such as other various resins, elastomer, glass fiber, talc, calcium carbonate, a silica, a microphone, etc. by the method widely practiced, It may be filled with a stabilizer or the like.

When the elastomer is contained in the material of the core layer 1B, the content thereof is preferably 60% by weight or less. However, in the formulation composition of the olefin resin and the elastomer, when the olefin rubber exceeds 60% by weight, the molding is performed. Problems occur in workability and moldability.

In the present invention, when the bending elastic modulus of the olefin resin constituting the core layer 1B is less than 1000 kg / cm ² or the hardness is lower than that of the thermoplastic elastomer of the skin layer 1A, sufficient shape retention is not achieved and external pressure It is easy to deform | transform and it is unsuitable as an airbag accommodation cover.

On the other hand, a bending elastic modulus exceeding 7000 kg / cm < ² > tends to generate | occur | produce the softness of the whole as a two-layer airbag cover, and the failure of destruction at the time of airbag operation at 40 degreeC.

Accordingly, the bending elastic modulus is preferred and this, of 7000kg / cm ² or less, particularly 2000-50000kg / cm ² than 1000kg / cm ² in total cover the soft feeling, operating characteristics, and the form-retaining points.

The thickness of the core layer 1B is not particularly limited, but is generally about 0.5-5 mm, preferably about 1-3 mm.

In the present invention, the core layer 1B made of such a thermoplastic resin is provided with a weak shape for easily opening the core layer 1B at the start of the airbag operation.

In addition to the slitting shape 1C of the sewing scale, the fragile shape can adopt a notch-shaped thin shape and various other things.

There is no restriction | limiting in particular in the formation place, the formation range, etc., It determines suitably according to the shape, intensity | strength, stress at the time of airbag operation | movement, etc. of the core layer 1B.

Next, the manufacturing method of the airbag accommodation cover of this invention is demonstrated.

As the molding processing method for manufacturing the airbag storage cover of the present invention, general molding processing methods used for thermoplastic resins are enumerated, but injection molding methods can be enumerated as a particularly preferable method in terms of design and the like.

In the injection molding method, a core layer is formed in advance, and the molded product is mounted on a mold leaving a space of the skin layer, and a so-called insert injection method for injection molding a skin material made of a thermoplastic polymer on the skin layer portion or a core layer is molded. Subsequently, the so-called two-color molding (double injection) method, which separates the cavity mold and injects the skin material by coating a new cavity mold which continuously holds the skin layer space without removing the core molded product from the mold, is enumerated as an effective molding method. There is a number.

At this time, the core layer 1B needs to have the above-described weak shape for easily opening the airbag at the start of operation.

The fragile shape includes a notch-shaped thin shape and a sewing slit shape as described above, but this shape may be performed by molding into a mold that retains the shape when the core molded product is molded, and the core molded product. After molding, the coating may be subjected to a process to give such a shape, and then the skin layer may be laminated.

In addition, the cover for the airbag of the present invention may be coated with the skin layer 1A as necessary in order to improve the glare in the vehicle cabin, color matching with other interior parts, peeling, and the like.

Coating of the outer skin layer 1A is also preferable in terms of upper resistance.

In the cover for airbag storage of this invention, since a comparatively hard core layer is covered by the soft skin layer, even if a body hits the cover, it does not feel soft and uncomfortable.

In addition, since the core layer is hard, the cover is not deformed or destroyed even if some external force is applied.

The cover of the present invention is in the range of -40 ° C to + 80 ° C, and when the airbag in the cover is inflated during a vehicle collision or the like, the cover is opened according to the weak shape for opening the cover in advance in the core layer. At the time of cover cleavage, since the hard core layer adheres closely to the soft skin layer, the cover cleavage fragments do not scatter.

Furthermore, the cover of the present invention exhibits good fracture performance in the above-described temperature range (-40 ° C to + 80 ° C) even after elapse of time for 110 hours and 500 hours in consideration of actual vehicle mounting, and almost no apparent change in appearance.

Next, although an Example and a comparative example are given and this invention is demonstrated further more concretely, this invention is not stabilized by the following example, unless the summary is exceeded.

Example 1-7, Comparative Example 1-6

Core layer shown in the first table using an in-line screw two-color injection molding machine with a clamping pressure of 350 tons and a core layer containing slits (29 pieces having a length of about 0.7 cm) for cutting. 1.0-3.0 mm thick), followed by injection molding the epidermal layer to a predetermined thickness with the skin material shown in the first table thereon, thereby obtaining a modulus cover 1 for an airbag shown in FIG.

In addition, in Example (4), coating of the outer skin layer was performed.

For coating, after injection molding of the cover, wipe the epidermal layer with isopropyl alcohol, and then apply primer (MEX 5440 and methyl ethyl ketone 1 to 1) with 7-10 μm spray and dry at room temperature for 10 minutes. Apply urethane topcoat (MEX-6047: hardener F-3: thinner-58u = 100: 10:50) by spraying 20-25μm, dry at room temperature for 10 minutes, and melt and melt at 80 ℃ for 30-45 minutes Was carried out.

The obtained product was evaluated by the method described later, and the results are shown in the second table.

In addition, the used material and manufacturing method shown in a 1st table | surface are as follows.

(Core material)

P0-1: Propylene-ethylene block copolymer resin (Mitsubishi emulsifier SPX 9800); MFR (230 ℃, 2.16kg) = 1g / 10min Flexural Modulus = 4500kg / cm ²

P0-2: propylene-ethylene block copolymer resin (Mitsubishi emulsifier BC-8); MFR (230 ℃, 2.16kg) = 1.8g / 10min Flexural Modulus = 10800kg / cm ²

P0-3: ethylene-propylene rubber (EpO7p made from Japan Synthetic Rubber); ML (100 ° C) = 70

Propylene Content = 27%

P0-4: low pressure polyethylene resin (Mitsubishi emulsifier JX-20); MFR (190 ℃, 2.16kg) = 5.5g / 10min Flexural Modulus = 9000kg / cm ²

(Core material manufacturing method)

0.2 weight part of Chibagagi group Irganox 1010 and 0.5 weight part of carbon black were added as a stabilizer to 100 weight part which made the said core material the compounding composition shown in the 1st table, and to 200 degreeC in a twin screw extruder. Kneaded.

(Epidermal material)

Component A

A-1: hydrogenated derivative of styrene-butene block copolymer; Styrene Content = 33% by weight Number Average Molecular Weight = 180000

A-2: hydrogenated derivative of styrene-isoblene block copolymer; Styrene Content = 13% by weight Number Average Molecular Weight = 60000

Component B

B-1: same thing as P0-1 used for core material

B-2: propylene-ethylene block copolymer (Mitsubishi emulsifier BC 1); MFR (230 ℃, 2.16kg) = 30g / 10min Flexural Modulus = 10000kg / cm ²

Component C

C-1: polyisobutylene (Japanese petrochemical Tetrax 3T); Viscosity Average Molecular Weight = 30000

C-2: polyisobutylene (Japanese petrochemical Tetrax 5T); Viscosity Average Molecular Weight = 50000

C-3: polyisobutylene (VISTANEX MML-100 by Exxon Chemical); Viscosity Average Molecular Weight = 90000

D-1: Softener (OIL) for paraffinic rubber (light redness pW 380); 40 ℃ kinematic viscosity = 380cSt

D-2: Polybutene (Japanese Poly Chemicals PolyVene HV-100); Number average molecular weight = 970

(Skin material manufacturing method)

The above-mentioned ingredients were formulated as shown in Table 1. To 100 parts by weight of Irganox 1010 used as a stabilizer as a stabilizer was added 0.2 parts by weight and 0.5 parts by weight of carbon black, and a 180 kneader with a Banbury kneader. It knead | mixed at 20 degreeC for 20 minutes, it was made into the sheet form, and each pellet was formed with the sheet cutter.

(Comparative Example 7)

For the comparative example, an airbag storage cover composed of two layers of a skin layer of a high density urethane foam and a low density urethane core layer was produced and evaluated.

Molding was carried out by an insert molding method in which the core layer was previously cured (cured) by a rim urethane molding method, cured (cured), and then attached to a mold to form a skin layer, thereby forming a skin layer in the same manner.

Frongas was used as a foam for shaping the skin layer.

This method required the curing (curing) step twice, resulting in poor productivity.

The obtained airbag storage cover was evaluated in the same manner as in Example 1, and the results are shown in the second table.

(Comparative Example 8)

Furthermore, in order to compare, the airbag storage cover which installed the reinforcing net was manufactured, and the evaluation was performed.

In the rim urethane molding into which this net enters, it carried out two-layer shaping similarly to the comparative example 7, and woven the polyester fiber into net shape, melted it into the shape according to the cleavage of a cover, and attached the bracket (hook).

This was attached to a rim mold and rim-molded with urethane.

At this time, proton gas was used as a foaming material, and after demolding, it was hardened at 100 degreeC and 30 minutes.

In this method, it is difficult to make a safe skin layer by inserting a net, and there existed a fault, such as a molding cycle not rising.

About the obtained airbag storage cover, evaluation was performed by the method similar to Example 1, and the result was shown to the 2nd table | surface.

As is clear from Table 2, the airbag housing cover of the present invention shown in Example 1-7 is very good with softness, shape retention, -40 ° C development test, 110 ° C, 500 hours durability test, and productivity. Was.

(Assessment Methods)

①Soft feeling

As shown in FIG. 1, the module cover or cover 1, in which the airbag 2, the airbag storing cover 1, and the retainers (attachment) 4, 5 and the inflator 3 are bonded together is -20 ° C- The sample is left to stand at 80 ° C until the atmosphere temperature is reached, and the cover at that time is evaluated by the hand feeling by ten people.

(Circle) of 10 people who felt a soft feeling, (triangle | delta), and the following were made into 8 of 10 people.

② Shape retention

As shown in FIG. 1, a modulus cover in which the airbag 2, the airbag housing cover 1, the retainers (attachment) 4, 5, and the inflator 3 are bonded to each other, or a column in which the cover 1 is separately Cycle, that is, 100 ° C. × 4 hours -40 degrees Celsius * 4 hours 85 degrees Celsius * 98% (temperature) four hours 10 cycles were left at -40 ° C x 4 hours in a thermal cycle, and the state of deformation was observed during or after the test, and almost no deformation was used as an airbag housing cover, and a significant deformation was found in the airbag. It was set as unsuitable (x) as a cover for accommodation.

③ -40 ℃ development test

Attach the modulus glued by ① and ② to the steering and leave the time to reach the ambient temperature at -40 ℃ and conduct the development test within 1 minute, at which time the modulus cover is torn and scattered or The case where there was no normal operation as an air bag, such as a cover being torn to a sharp shape and a bag being torn, was made into unmarked (x), and the case where normal operation was performed was made into a family register ((circle)).

④ 110 ℃, 1000 hours durability test

After aging the modulus cover with the gear-type thermal aging tester under the above conditions, the development test was carried out at -40 ° C according to the test method of ③. Furthermore, the appearance of the modulus cover before and after aging was compared, and the warpage, deformation, and core material were compared. The thing which markedly changed, such as the bleed which is a softener of furnace, was made into (circle) and the thing without.

⑤ Productivity

20 products are manufactured in succession and the molding processability (releasability, short shot presence, etc.), length, weight, and skin layer are especially the appearance of the molded article (stickiness, delamination of the gate part, welder appearance, sink, ply arc). , Wrinkle appearance, uneven gloss, warpage, and the like) were checked, and all the numbers were 0 for the pass level, and if 90% or more,?

Table 1

2nd table

As mentioned above, since the outer skin layer is soft in the cover for airbag accommodation of this invention, even if a part of body hits, it is soft, and comfort is high.

Moreover, since a core layer is comparatively hard, it is excellent in shape retention and high in durability.

In operation of the airbag, the cover is quickly opened due to the weak shape for opening the cover.

In addition, the soft skin layer reliably prevents the core layer from dispersing during the cleavage of the cover, and exhibits excellent fracture performance and is excellent in durability.

Therefore, the airbag housing cover of the present invention can be manufactured easily and with high accuracy and almost no defective products by a rational molding method such as a two-color molding method, and can be manufactured reliably and efficiently with a high yield and greatly reduce the cost of the product. Can be planned.

Claims (1)

The following components A, B, C and D, i.e. Component A: Hydrogenated derivative of block copolymer consisting of styrene and conjugated zen Component B: Olefin Resin Component C: Polyisobutylene having a viscosity average molecular weight of 70000 or less Component D: The kinematic viscosity at 40 ° C. contains a softener for hydrocarbon rubbers of 500 cst or less and / or polybutene of 2500 or less of a number average molecular weight, and the blending ratio thereof is Component A = 40-80 wt% Component B = 5-30 wt% Component C = 2-30 wt% Component D = 0-20 wt% And a core layer made of a thermoplastic polymer having a JIS-A hardness of JIS-K6301 of 20-90 and a core layer made of an olefin resin having a bending elastic modulus of JIS-K7203 of 1000-7000 kg / cm ² , The core layer has a higher hardness than the skin layer described above, and the core layer has a portion for easily opening at the start of the airbag operation.