GB2254618A

GB2254618A - Air bag cover

Info

Publication number: GB2254618A
Application number: GB9206536A
Authority: GB
Inventors: Takayasu Zushi; Mitsuo Yoshiyasu
Original assignee: Takata Corp
Current assignee: Takata Corp
Priority date: 1991-04-11
Filing date: 1992-03-26
Publication date: 1992-10-14
Also published as: JPH04314648A; KR920019582A; DE4211994A1; GB9206536D0; CA2064276A1; FR2675097A1

Description

2 2-)461,

TITLE OF THE INVENTION

COVER FOR CONTAINING EMERGENCY AIR BAG BACKGROUND OF THE INVENTION FIELD OF THE INVENTION

This invention relates to a cover for containing therein an emergency air bag installed in a vehicle and adapted to be set to abrupt inflation on sensing a shock or deformation exerted thereon at the instant of collision of the vehicle against another vehicle or other hard object.

RELATED ART STATEMENT

The covers of this kind are fractured at the instant an air bag contained therein is set to abrupt inflation. Since the fractured segments of such covers are hurled by the inflating air bag and, therefore, have the possibility of inflicting injuries on the faces, eyes, etc. of the vehicle's occupant.

The conventional covers generally have been rendered shatter proof by insertion therein of a reinforcing member (such as, for example, a net) (Japanese Unexamined Patent Publication No.

234,764/1987 and No. 127,336/1975 and Japanese Unexamined Utility model Publications No. 80,928/1977, No. 43,454/1975, and No. 25(342/1976).

The cover incorporating a reticular insert therein is generally produced by molding a cover proper with such a material as foam polyurethane which is strong at relatively low temperatures and embedding a reinforcing net in the cover proper throughout the entire volume thereof. In this case, this cover is generally provided with a line along which the insertion of the reinforcing net is omitted so that the air bag, during the inflation thereof, may be easily expanded and the cover itself may be consequently fractured easily.

Japanese Unexamined utility Model Publication No. 76,042/1977 proposes a cover for containing therein an emergency air bag composed of two layers, i.e. a surface skin layer formed of hard urethane foam intended to form a neatly finished surface and a core laver of low-density (high expansion ratio) urethane foam intended to impart overall flexibility to the cover, and provided with a slit (a line for enabling the cover to expand smoothly during the inflation of the air bag) extending from the core layer through part of the surface skin.

In the aforementioned conventional covers for containing an emergency bag, the cover reinforced with a reticular insert, because of the insertion of the reticular reinforcing material, has the disadvantage that the manufacture thereof involves a large number of man-hours and proves troublesome, proceeds accurately only with difficulty in spite of the use of a shaped of urethane foam (RIM), and suffers from poor yield and high cost.

Te cover disclosed in Japanese Unexamined Utility Model No. 76,042/1977 has a surface skin of hard urethane foam (exhibiting a Shore hardness in the range of from 30 to 40 as measured in accordance with ASTM D-2240) and is not particularly provided with a reinforcing material and, therefore, has a problem of dubious safety of use because the possibility of the fractured fragments of the cover being hurled during the abrupt inflation of the air bag particularly at -400C cannot be completely prevented.

Inventions directed to overcoming this problem have been proposed by Japanese Unexamined Patent Publications No. 202,550/ 1989, and No. 171, 362/1990.

The covers taught by these inventions are composed of a surface skin layer producing a soft sensation and a hard core layer exhibiting a shaperetaining property and furnished in the core layer with a part such as, for example, a perforated slit capable of being readily torn apart during the inflation of the air bag. The surface skin layer and the core layer are described as formed of a thermoplastic material and produced by the insert molding method or the two-color injection molding method.

Though the cover of this nature for containing an emergency air bag indeed excels in the sensation of softness and the ability to expand at a varying temperature between a low temperature (-401C) and a high temperature (+80'C), it has the following two drawbacks. The first drawback resides in the fact that the manufacture of one shaped article (finished product) requires two metal molds, one for the molding of a core layer and the other or molding a surface skin layer. This fact entails an economic problem in respect that the manufacture necessitates a special molding device and the molding cycle is long. The second drawback resides in the fact that, when materials differing in mold shrinkage are subjected to multilayer molding, the product inevitably sustains deformation and produces a fold along a cleaved part (such as, for example, a perforated slit) imparted in the core member. This fold is discerned through the surface skin member even by unaided eyes. This prominence of the fold disrupts the appearance of the cover as an interior decorative item because the emergency air bag is set in a conspicuous place such as the center of a steering wheel or the front of a dashboard.

OBJECT AND SUMMARY OF THE INVENTION

One object of this invention is to solve the problems of the aforementioned cover for containing an emergency air bag and provide a cover for containing an emergency air bag which possesses moderate flexibility and strength enough to withstand external pressure, avoids being shattered during the inflation of the air bag proper, and breaks open easily.

A further object of this invention is to provide a cover for containing an emergency air bag producible by a simple method of molding such as injection molding, which method avoids yielding reject products and enjoys high yield, high accuracy, high productivity, and economy of manufacture.

The cover of this invention for containing an emergency air bag is provided on the inner surface thereof with a part of small thickness to be easily torn open when the air bag begins to inflate, which cover is characterized by being formed of an olefin type and/or styrene type thermoplastic elastomer (TPE) having a JIS (Japanese Industrial Standard) A hardness of not less than 70 as determined in accordance with JIS K6301 and a modulus of elasticity in bending of not more than 5,000 kg/cm2 as determined in accorrdance with JIS K-7203.

The cover of this invention for containing an emergency air bag is preferably characterized by being a shaped article produced by one-color injection molding of the material mentioned above and coated on the surface thereof.

The present inventors have continued a diligent study for the purpose of fulfilling the objects described above, to find that these objects are accomplished by a cover formed of a specific thermoplastic elastomer and provided on the inner surface thereof with a part of small thickness intended for fracture. This invention has been perfected as a result.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a schematic partially sectioned front view illustrat_4ng a cover as one embodiment of this invention for containing an emergency air bag as attached to the emergency air bag.

Fig. 2 is a rear view of the cover for containing an emergency air bag, illustrating one example of the part of small thickness intended for easy fracture.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In the diagrams, 1 stands for a cover for containing an emergency air bag, 2 for an air bag, 3 for an inflator, 4 for a ring retainer, 5 for a base retainer 6 for a metallic plate, and 7 for a rivet.

The cover 1 of this invention is formed of a single layer of a styrene type and/or olefin type TPE having a JIS A hardness of not less than 70 as determined in accordance with JIS K-6301 and a modulus of elasticity in bending of not more than 5,000 kg/C-2 as determined in accordance with JIS K-7203. This cover is provided on the inner surface thereof with a part 1C of small thickness adapted to be easily fractured at the instant the air bag begins to inflate.

The styrene type TPE to be used in this invention is a compound material having a styrene-conjugate diene block polymer and a hydrogenated version of the block polymer as a matrix material and incorporating in the matrix material a component selected from the group consisting of thermoplastic resins such as oleffin type resins and styrene type resins, various rubber softening agents such as petroleum hydrocarbon type rubber softening agents (such as, for example, oils), and synthetic softening agents (such as, for example, polybutene), inorganic fillers, waxes, and other various additives.

The conjugate dienes which are desirably usable in the styrene-conjugate diene block polymer mentioned above include butadiene and isoprene. For this invention, a hydrogenated version of the block polymer is used advantageously from the viewpoints of lasting durability (heatproofness and weatherability) and stability in processing.

The olefin type TPE which can be used in this invention is a compound material using a copolymer rubber of ethylene with an 0-olef._;n of 3 to 6 carbon atoms and a soft polymer as a matrix material and incorporating in the matrix material a component selected from the group consisting of the same resins, softening agents, inorganic fillers, waxes, and additives as those enumerated above with respect to the styrene type TPE.

The ethylene- O-olefin type conjugate diene rubber is allowed to incorporate therein a moderate amount of a diene component such as dicyclopentadiene or ethylidene norbornene in a copolymerized form. The '_-olefins which are usable advantageously herein in point of performance and cost include widely marketed products using C 3: propylene and C 4: butene-1.

The olefin type TPE which has been melted and mixed with an organic peroxide, a rubber grade vulcanizer, and a var,,ing crosslinking auxiliary in a Banbury mixer or an extruding device and, while the melting and mixing is in progress, dynamically crosslinkL.ng mainly the ethvlene-,Y_-olefin copolymer rubber partly or substantially completely is also usable. The olefin type TPE thus processed proves particularly desirable when it has a JIS A hardness in the range of from 70 to 95.

The styrene type TPE and the olefin type TPE mentioned above' can be used either singly or as combined in a suitable ratio.

Specifically, the styrene type TPE products which are available today on the market include "Clayton" produced by Shell Chemical, "Rubberon" produced by Mitsubishi Petrochemical, "Elastomer-AR" produced by Aron Kasei, "Sumitomo TPE-SB" produced by Sumitomo Chemical, and "Tuftex" produced by Asahi Chemical, for excmple.

The olefin type TPE products available today on the market include "Ther. molan," "SPX," and "Yukaron Cell" produced by Mitsubishi Petrochemical, "Sumitomo TPE" produced by Sumitomo Chemical, "Milastomer" and "Goodmer" produced by Mitsui Petrochemical, "Santoprene" produced by MitsubishiMonsanto, "Softlex" produced by Nihon Sekiyu Kagaku, "Oleflex" produced by Showa Denko, and "Hi-fax" produced by Himont, for example.

The olefin type and/or styrene type TPE to be used in this invention is selected by hardness as a criterion such that the JIS A hardness is not less than 70 as determined in accordance with JIS K-6301 and the modulus of elasticity in bending is not more than 5,000 kg/cm2 as determined in accordance with JIS K-7203 because this TPE because it is required to produce a good sensation of softness, a high shape-retaining property, and a stable ability to allow smooth expansion of an air bag.

Desirably, the JIS A hardness is not less than 75 and the modulus of elasticity in bending is not more than 3,000 kg/cm2. More desirably, the JIS A hardness is not less than 80 and the modulus of elasticity is not more than 2,000 kg/cm2.

If the JIS A hardness is less than 70, though the TPE is rich in sensation of softness, it is deficient not only in the shape-retaining property and the ability to allow firm attachment of a rivet but also in the ability to allow expansion of the emergency air back at elevated tempertures (+800C). If the modulus of elasticity in bending exceeds 5, 000 kg/CM2, the TPE is imDracticable because it is deficient in the sensation of softness and the ability to allow expansion of the air bag at low temperatures (-400C).

The average wall thickness of the cover of this invention (the averaae of the wall thickness of the part of the cover account4 ng for not less than 60% of the surface area) for containing an emergency air bag generally is approximately in the 8 - range of from 0.5 to 5 mm. It needs not be uniform throughout the entire area. Properly, the cover is designed so that parts destined to seat a rivet will have a rather large thickness and the parts destined to produce a fracture a small thickness less than about 50% of the average thickness.

Though the conventional molding methods available for hermoplastic resins may be employed for the production of a cover of this invention for containing an emergency air bag, the one-color molding method may be cited as a particularly desirable choice in point of design.

The cover of this invention for containing an emergency air bag is desired to be coated for the purpose of adding to the gorgeousness of car interior, ensuring good color matching with other interior decorative items, and enhancing the feeling. The coating is also desirable in point of freedom from defilement with dust and resistance to scratches. For this coating, various coating materials heretofore employed for covers of this class can be used.

Since the cover of this invention for containing an emergency air bag emits an overall sensation of softness, it avoids inflicting any objectionable sensation on the car occupant on contact therewith and it escapes being deformed or broken under external force.

The cover of this invention is fractured along the part of small thickness furnished on the inner surface thereof at the instant the emergency air bag contained in the cover is expanded on collision of the car against a hard object. During the fracture of the cover, this cover will not be shattered and sent flying in all directions.

The coating applied to the cover improves the gorgeousness of the car interior, the color match with other interior decorative items, and the feeling and promotes the resistance of the cover to deposition of dust and infliction of scratches.

Now, this invention will be described more specifically below with reference to working examples and comparative experiments. Of course, this invention is not limited to the examples cited hereinbelow but may be practised otherwise without departure from the spirit of the invention. Examples 1 to 3 and Comparative Experiments 1 and 2:

A varying core material indicated in Table I was processed with an inline screw type injection molding device having a die pressure of 350 tons to form a product containing an H-shaped part of small thickness (1 mm in thickness) for fracture and measuring 3 mm in overall average thickness and 5 mm in thickness in the parts destined to allow attachment of a rivet. The products of Examples 1 and 2 and Comparative Experiments 1 and 2 were coated.

The coating was produced by wiping the surface of the injection molded cover with isopropyl alcohol, then applying a primer (MEX5440 and methylethyl ketone = 1: 1) produced by Sakai Kagaku K.K. in a thickness of 7 to 10 pm on the cleaned surface by spraying, drying the applied layer of the primer at normal room temperature for 10 minutes, applying an urethane top coat (MEX6047: curing agent F-3: thinner 58 p = 100: 10: 50) produced by Sakai Kagaku K.K. in a thickness of 20 to 25 pm on the primer undercoat by spraying, drying the applied top coat at normal room temperature for 10 minutes, and firing the dry top coat at 800C for 30 to 45 minutes.

Comparative Experiment 3 represents a case in which a cover furnished with a reinforcing net was produced. The net-containing urethane rim molding was carried out by placing a polyester net in a urethane dope except for the part to be given a small thickness for fracture, setting fitting metals (hooks) attaching the hooks to a RIM metallic die, and molding the urethane dope. In this molding, Freon gas was used as a foaming agent. The mold was removed from the die and left curing at 100C for 30 minutes. By this method, it was difficult to obtain a cover endowed with stable quality owing to the insertion of the net. The molding required a lengthy cycle and entailed heavy loss.

The used materials indicated by symbols in Table I were as follows.

TPS-1: Styrene type TPE based on hydrogenated SBS, produced by Mitsubishi Petrochemical and marketed under trademark designation of "Rubberon MJ8300C," JIS A hardness 85 and density 0.90 g/CM3.

TP.S-2: Styrene type TPE based on hydrogenated SBS, produced by Mitsubishi Petrochemical and marketed under trademark designation of "rubberon SJ5400B," JIS A hardness 55, and density 1.1 g/CM3.

TPO-1: Olefin type TPE (partially cross-linked type) based on ethylenepropylene rubber, produced by Mitsubishi Petrochemical and marketed under trademark designation of "Thernnolan 3980B," JIS A hardness 93, and density 0. 89 g/cm3.

TPO-2: Olefin type TPE (not cross-linked) based on ethylenepropylleme rubber/pol,,pror,,.,--ene/polyethylene, produced by Mitsubishi Petrochemical and marketed under trademark designation oil: "Thermolan 2920B," modulus of elasticity in bending 2,000 kg/ CM2, and density 0.90 g/CM3.

-PO-3: A mixture of polypropylene BC8D produced by Mitsubishi Petrochemical, MFR 1.8 g/10 min. (230C, 2.16 kq), and spec if4C gravity 0. 90 g/CM3 with 10% of EPO 2P produced by Nippon Synthetic Rubber, Mooney viscosity 24 ( ML 1 4 100), and propylene content 26% by the use of a melt-mixing device, the mixture with modulus of elasticity in bending 7, 000 kg/cm2 and density 0.9 g/CM3. [Method of testinal (1) Sensation of softness A module cover or cover 1 unit composed of an air bag 2, a cover 1 for containing an air bag, retainers (fitting metals) 4 and 5, and an inflator 3 was left standing in a varying ambient air temperature in the range of from -1-O'C to 60C until the temperature of the module cover reached the ambient air temperature. Then, a Panel of 10 persons tested the module cover after the standing for softness by touch of hand. A sample rated as possess'ng satisfactory sensat-on of softness by at least seven pane- mnembers was designated as o and a sample rated so by not more t.han six panel members was designated as x.

(2) Shape-retaining property An air bag module case assembled in the same manner as described above was heated at 11VC for 1,000 hours. Separately, a similar module case was subjected to ten cycles of temperature fluctuation between -400C and 1000C. A sample showing virtually no sign of deformation was reported as o and a sample showing a marked deformation as x. (3) Test for expansibility An air bag module case assembled in the same manner as in (1) was attached to a steering wheel and subjected to a test for expansibility at -400C and +800C. A sample which was shattered and set flying in all directions, fractured so as to form sharp edges and consequently prevent the bag from normally operating, or torn off at the rivetted parts securedon the retainers was rated as unfit and reported as x. A sample which was free from the drawbacks mentioned above and was allowed to operate normally was reported as o. [Appearance of product] A cover fresh from injection molding, either in its unmodified form or after being coated, was examined as to flaw mark, delamination, uneven gloss, and deformation in the part of small %Pall thickness. A sample showing a conspicuous flaw was reported as x and a sample showing absolutely no flaw was reported as o. [Productivity] A total of 20 consecutively formed products were examined as to appearance and size. A group of products which wholly passed acceptable levels were reported as o and a group of products which contained a sample ratable as rejectable were reported as x.

1 14 - 1 1 1 Table 1

Example Comparative Experiment 2 3 1 2 3 Material TPS-1 TPO-1 TPO-2 TPS-2 TPO-3 Kind of TPE JIS A hardness 85 93 96 55 99 Urethane Modulus of elasticity in 640 1300 2000 200 max. 7000 (reinforcing bending (kg/cm2) net) Quality 0 0 0 0 x 0 Sensation of softness Shape-retaining property 0 0 0 x 0 0 Test for +800C 0 0 0 x 0 0 expansibility (Torn at rivet) -40"C 0 0 0 0 x 0 (Shattered) Appearance of product 0 0 0 0 0 0 Productivity 0 0 0 0 0 x Remark Coating Coating - Coating As clearly noted from the working examples cited above, the covers of this invention for containing an emergency air bag were highly agreeable to touch because imparted comfortable sensation of softness to human bodies on contact therewith. They possessed moderate rigidity and, consequently, excelled in the shaperetaining property. Because of the part of weakened strength intended for fracture, the cover was quickly fractured at the instant the air bag was set to inflation. The cover had a good appearance in spite of the part of large thickness formed on the inner surface thereof.

The cover of this invention for containing an emergency air bag can be manufactured by a rationalized method of injection molding which enjoys simplicity of procedure, high accuracy, very rare occurrence of rejectables, high yield, and reliable and efficient operation. These merits allow a generous cut of cost.

The coating formed on the cover not merely enhances the gorgeousness of the car interior, the color match with other interior decorative items, and the feeling but also promotes the prevention of the cover from defilemnt with dust and infliction of injuries.

J -

Claims

1. A cover for containing an emergency air bag provided on the inner surface thereof with a part of small thickness adapted to fracture easily at the instant said emergency air bag begins to operate, which cover is characterized by the fact that the material of said cover is an olefin type and/or styrene type thermoplastic elastomer having a JIS A hardness of not less than 70 as determined in accordance with JIS K6301 and a modulus of elasticity in bending of not more than 5,000 kg/cm2 as determined in accordance with JIS K7203.

2. A cover according to claim 1, wherein said cover is a shaped article obtained by subjecting said material to one-color injection molding and is coated on the surface thereof.

3. A cover according to claim 1, wherein said styrene type TPE is a compound material using a styrene-conjugate diene block polymer and a hydrogenated version of said block polymer as a matrix material and incorporating in said matrix material a component selected from among olefin type and/or styrene type thermoplastic resins, rubber grade softening agents, inorganic fillers, waxes, and additives.

4. A cover according to claim 3, wherein said conjugate diene is butadiene and/or isoprene.

5. A cover according to claim 1, wherein said olefin type TPE is a compound material using a copolymer rubber of ethylene with an CC-olefin of three to six carbon atoms and a soft polymer as a matrix material and incorporating in said matrix material a component selected from among olefin type and/or styrene type thermozlastic resins, softening agents, inorganic fillers, waxes, and additIves.

6. A cover accor-ding to claim 5, wherein said ethylene- Xole..E:-. conugate dlene rubber has a small amount of a diene component such as dicyclopentadiene or ethylidene norbornene incorzorated therein in a copolymerized form.

I. A cover according to claim 5, wherein said QC-olefin is c 3: cronvlene and/or C 4: butene-l.

8. A cover according to claim 1, wherein said alefin type TPEE is nelted and mixed with an organic peroxide, a rubber grade vulcanizer, or a varving cross-linking auxiliary by the use of a Banbury mixer or an extrusion molding device and, while the ml ing is in progress, the ethylene- e-,t-n-j and mix--- t-olefin copolymer rubber is rnainly cross-linked dynamically partly or substantially COMZD-, ete".,.7.

9..1% cover according to claim 1, wherein the JIS A hardness is not less than 75 and the modulus of elasticity in bending is not more than 3,000 kg/c-n2.

10. A cover according to claim 1, wherein the JIS A hardness is not less than 80 and the modulus of elasticity in bending is not more than 2,000 kailcm2.

11.. A cover subszantiallv as hereinbefore described with to the accor,, zanv4na description and drawings.

1