Patent Description:
Airbags are integrated in motor vehicles at various points, for example in steering wheels, dashboards, doors, seats and roof linings. For the airbag to function reliably, it is necessary that the cover has material weaknesses as predetermined breaking points in its layers. For optical reasons or because of the design, however, vehicle manufacturers are increasingly demanding that the passenger airbags are invisible.

Film materials can be used as an alternative to airbag cover coatings with material weakening. These film materials must show the required tear behaviour, with the deployment of the airbag taking place within defined timeframes, particle flight being avoided and passenger protection being ensured.

The use of foam laminates for airbag covers is described in <CIT>. The foam film laminate described therein comprises a compact top layer and a foam layer having a density of at least <NUM>/m<NUM> on the underside of the top layer, the top layer comprising an outer layer and an inner layer.

The use of film laminates for airbag covers is also described in <CIT>. The film laminate described there comprises a compact top layer and a foam layer with a thickness in the range of <NUM> to <NUM> and a density of <NUM> to <NUM>/m<NUM> on the underside of the top layer, whereby the top layer can be a two-ply layer. Additives such as polar polymers or microspheres are incorporated into the compact top layer to reduce the tensile strength. Such additives may have adverse effects on other properties such as softness and haptics.

The prior art does not disclose any composite structures having good properties as airbag covers such as a low tensile strength on the one hand and being soft and flexible on the other hand.

Therefore, the problem underlying the present invention is to provide a composite structure having an improved combination of properties such as tearing properties, flexibility, and haptics for the use as an airbag cover.

The problem was solved by providing a composite structure according to the attached patent claims. The composite structure is defined as follows:
A composite structure containing a foam layer, an inner layer and an outer layer of a thermoplastic compact cover layer, and a lacquer layer in this order, and having a tensile strength according to DIN EN ISO <NUM>-<NUM> at <NUM>/min and <NUM> of less than <NUM> MPa in both a first direction and a second direction perpendicular to the first direction, wherein the foam layer consists of or contains a polyolefin foam, the density of the foam layer is <NUM> to <NUM>/m<NUM>, the inner layer contains particles of an elastomer (polymer (F)) and at least <NUM> wt% of polypropylene (polymer (E)) and is directly bonded to the outer layer, wherein the following polymers (D) to (F) make up at least <NUM> wt% of the total polymer content in the inner layer and the inner layer contains the polymers (D) to (F) in a total amount of <NUM> parts by weight:.

wherein the outer layer differs from the inner layer in its composition, the following polymers (A) to (C) make up at least <NUM> wt% of the total polymer content in the outer layer and the outer layer contains the polymers (A) to (C) in a total amount of <NUM> parts by weight:.

wherein the weight content of particles of an elastomer in the outer layer is higher than in the inner layer, and wherein the Shore A hardness of the outer layer is lower than that of the inner layer, which has a Shore A hardness of <NUM> to <NUM>.

The composite structure according to the invention has a tearing behaviour that meets the requirements for airbags without the composite structure having to be weakened by using perforation lines or adding weakening components such as microspheres. The composite structure can even be used for airbags having more demanding geometries, such as U-shape.

The composite structure represents a cost-effective alternative to the high-priced PVC or PUR materials with spacer materials.

In particular, the sewability of the composite structure for the use in airbags having H-shape geometry allows it to be used as an alternative to prior art materials.

The composite structure shows a low wrinkle formation, which makes it easier to handle and, in particular, makes sewing easier.

In preferred embodiments, the composite structure can be processed by thermoforming.

A further advantage is the easy recyclability of the composite structure since polyolefins that can be used in the foam layer and the cover layer belong to the same family of compounds.

The composite structure according to the invention is preferably suitable as a tearable cover for an airbag cover. This means that the composite structure is located in an area of the airbag cover where the predetermined breaking point of the cover is located. When the airbag is triggered, i.e. when an airbag is fired, this predetermined breaking point breaks and causes the composite structure to tear.

To be suitable as an airbag cover, the composite structure has a tensile strength according to DIN EN ISO <NUM>-<NUM> at <NUM>/min and <NUM> of less than <NUM> MPa in both a first direction and a second direction perpendicular to the first direction.

Furthermore, the composite structure preferably has a resistance to tear according to ISO-<NUM>-<NUM> of less than <NUM> N/mm, more preferably less than <NUM> N/mm in both a first direction and a second direction perpendicular to the first direction.

In a preferred embodiment, the composite structure has a tensile strength of less than <NUM> MPa and a resistance to tear of less than <NUM> N/mm.

In the present invention, the term "a first direction and a second direction perpendicular to the first direction" means that the first direction may be selected arbitrarily on a given sample. If the extrusion direction is known or can be identified on a given sample, the first direction is the lengthwise direction, i.e. the direction in extrusion direction. Accordingly, the second direction is the direction crosswise to the extrusion direction.

In order to be suitable for sewing, it is important that the composite structure according to the invention has a suitable stitch tear-out force. This is preferably at least <NUM> N according to DIN EN ISO <NUM>.

The foam layer, the cover layer, and the lacquer layer of the composite structure are thermoplastic. The composite structure is preferably thermoformable. In that case, it can be processed by thermoforming such as positive of negative vacuum thermoforming.

In the present invention, the term "thermoplastic" denotes polymers or polymer compositions that show thermoplastic properties, in particular thermoreversibility.

The cover layer of the composite structure preferably has a grain, i.e. a three-dimensionally structured surface on the top side covered with lacquer. The grain may be present in the cover layer only or both in the cover layer and the lacquer layer.

The compositions of the foam layer and/or compact layer may contain additives such as stabilizers (light or aging), antioxidants, metal deactivators, processing aids, waxes, fillers (silica, TiO<NUM>, CaCO<NUM>, Mg(OH)<NUM>, carbon black, mica, kaolin, clay, coal dust, lignin, talc, BaSO<NUM>, Al(OH)<NUM>, ZnO, and MgO), and/or colorants.

The foam layer consists of or contains polyolefin foam. The foam layer consists of or contains polypropylene foam (PP foam) in a preferred embodiment. Polypropylene (PP) is defined here as polymers or copolymers whose proportion by weight of propylene is greater than <NUM> % by weight.

The polyolefin of the foam layer may contain common additives such as lubricants, stabilizers, fillers such as inorganic fillers, and/or pigment.

The preferred polypropylene may be selected from the group consisting of polypropylene, polypropylene-ethylene copolymer, metallocene polypropylene, metallocene polypropylene-ethylene copolymer, polypropylene-based polyolefin plastomer, polypropylene-based polyolefin elastomer, polypropylene-based polyolefin elastomer, polypropylene-based polyolefin elastomer, polypropylene-based thermoplastic polyolefin blend and polypropylene-based thermoplastic elastomer blend. Polypropylene-based thermoplastic polyolefin blend is homopolypropylene and/or polypropylene-ethylene copolymer and/or metallocene homopolypropylene.

The foam layer may have a thickness of <NUM> to <NUM>. Preferably, the thickness of the foam layer is more than <NUM>, in particular <NUM> to <NUM>. The density of the foam layer is <NUM> to <NUM>/m<NUM>, preferably <NUM> to <NUM>/m<NUM> and more preferably <NUM> to <NUM>/m<NUM>. The higher the density of the foam, the higher is its strength. To achieve the properties desired for the invention, the preferred values of density and thickness of the foam layer have to correlate. Overall, however, the foam layer has a very low density. This is also reflected in the product of the density (in kg/m<NUM>) and the thickness (in mm) of the foam layer. The foam density [g/m<NUM>] multiplied by the foam thickness [mm] is preferably <NUM> to <NUM>, more preferably <NUM> to <NUM>.

In one embodiment, a foam density of more than <NUM> to <NUM>/m<NUM>, preferably <NUM> to <NUM>/m<NUM> is particularly suitable for the use of the composite structure in airbags having U-shape geometry.

A measure of the softness of the foam layer is the gel content. The gel content is an indication of the degree of crosslinking of the polymer. The lower the gel content, the softer the foam layer. The foam layer used in the composite structure according to the invention preferably has a gel content of <NUM> to <NUM> %, more preferably <NUM> to <NUM> % and most preferably <NUM> to <NUM> %.

The foamed layer is preferably formed by foam extrusion. The layer is produced by blowing a blowing agent, in particular an inert gas, into the molten plastic during the extrusion process above the melting temperature and loading the molten material with a blowing agent under overpressure, and then depressurizing this gas-containing molten material on leaving the extrusion plant and cooling it below the melting temperature. The layer of foamed plastic is thus produced by blowing a blowing agent under overpressure into a polymer melt during the extrusion process and then releasing the pressure of the blowing agent under overpressure. For example, inert gases can be used as blowing agents, possibly in combination with each other. The foam material can then be bonded, e.g. thermally, to the compact two-layer top layer in the form of a flat material, so that a multi-layer film with a foamed layer is formed. It is also possible to first join the foam layer with the inner layer and then apply the outer layer to the inner layer.

The foam contained in the foam layer can be open-cell or closed-cell. Particularly in the production of PP foam as webs, the cell structure on the surfaces differs from the rest of the homogeneous structure. In particular, the surfaces have harder, closed-cell areas. This results in different physical properties, such as different elongation and flexibility, but also different haptics. By carrying out a process called skiving, the lower and/or upper surface layer of the sheet may be processed to obtain a sheet which contains the homogeneously cell-structured core area of the foam. The use of such a foam has the advantage that the material of another layer or an adhesive can penetrate into the open cells in the connection area with another layer, and the anchoring obtained in this way can achieve a stronger connection. Skiving in the seam area can be performed with a sharpening machine, e.g. FORTUNA NG6. This process also has a very positive effect on the avoidance of creases when handling the material during the sewing process. The properties of the composite structure are selected in such a way that sufficient seam strength according to DIN ISO <NUM> is achieved without the need for an extra sewing aid, e.g. a thin textile. The seam strength is determined among other things by the thickness and structure of the composite structure.

Examples of polyolefins and their production are disclosed in <CIT>.

These polyolefins comprise <NUM> to <NUM> parts by weight of an olefin block copolymer and <NUM> to <NUM> parts by weight of a propylene based polymer and have a degree of crosslinking of <NUM> to <NUM>%. These polyolefins preferably contain closed cells.

For example, the degree of crosslinking may be <NUM> to <NUM> %, with <NUM> to <NUM> parts by weight of divinylbenzene crosslinker per <NUM> parts by weight of resin.

These polyolefins can be produced according to <CIT> and have the following properties:.

The standards listed in the table above and the test methods are explained in <CIT>.

The cover layer is compact, i.e. not foamed. The density of the cover layer is preferably higher than <NUM>/m<NUM> and more preferably higher than <NUM>/m<NUM>. This applies to each sublayer independently.

This means that the inner layer and the outer layer each has a density of preferably higher than <NUM>/cm<NUM>.

The cover layer is thermoplastic. This property requires that the overall degree of crosslinking is not too high. The gel content is preferably less than <NUM> %, more preferably less than <NUM> %. The gel content may be <NUM> to <NUM> %.

The cover layer comprises at least two sublayers, i.e. an inner layer and an outer layer directly bonded to each other. The direct bonding excludes an additional layer or adhesive layer between them.

The Shore A hardness of the outer layer is lower than that of the inner layer having a Shore A hardness of <NUM> to <NUM>. More preferably, the Shore A hardness of the inner layer is <NUM> to <NUM> and the Shore A hardness of the outer layer is <NUM> to <NUM>.

The thickness of the cover layer is preferably <NUM> to <NUM>, more preferably <NUM> to <NUM> and even more preferably <NUM> to <NUM>. The sublayers can have the same or different thickness. The outer layer may be thicker, preferably <NUM> to <NUM> times thicker, than the inner layer. In a preferred embodiment, the cover layer has a total thickness of <NUM> to <NUM> and consists of the inner layer and the outer layer, which has a thickness of <NUM> to <NUM> and is two times thicker than the inner layer.

In a preferred embodiment, the compact cover layer covers all compact layers between the top lacquer layer and the foam layer. In that case, the cover layer is bonded directly to the foam layer.

In other embodiments, an adhesive layer and/or a textile layer may be arranged between the foam layer and the cover layer. In one embodiment, a textile layer is arranged between the foam layer and the cover layer. In that case, it is preferred that an adhesive layer is used to bond the textile layer to the foam layer and the cover layer. Hence, the composite structure according to the invention may contain a foam layer, an adhesive layer, a textile layer, an adhesive layer, a cover layer, and a lacquer layer in this order. The textile layer affects the properties of the composite structure, e.g. tensile strength, resistance to tear, elongation at break, wrinkle formation, and thermoformability. The composite structure according to this embodiment is suitable for being processed by cutting and sewing.

The inner layer contains at least <NUM> wt% of polypropylene as a thermoplastic polyolefin (polymer (E)) and particles of an elastomer (polymer (F)).

In one embodiment of the composite structure, the total of <NUM> parts by weight of polymers (A) to (C) consists of <NUM> to <NUM> parts by weight of polymer (A), <NUM> to <NUM> parts by weight of polymer (B), and <NUM> to <NUM> parts by weight of polymer (C). This composite structure is particularly suitable for being processed by cutting and sewing.

In another embodiment of the composite structure, the total of <NUM> parts by weight of polymers (A) to (C) consists of <NUM> to <NUM> parts by weight of polymer (A), <NUM> to <NUM> parts by weight of polymer (B), and <NUM> to <NUM> parts by weight of polymer (C). This composite structure is particularly suitable for being processed by thermoforming.

The melting point and the Shore A hardness of the outer layer are mainly determined by the properties of the TPV. The melting point of the TPV is preferably in the range of <NUM> to <NUM>, more preferably <NUM> to <NUM>. In a preferred embodiment, more than <NUM> wt% of the TPV used for the preparation of the outer have a melting point of higher than <NUM>, more preferably higher than <NUM>. Hence, the melting point of the outer layer is preferably higher than <NUM>, more preferably higher than <NUM>. The Shore A hardness of the TPV is preferably in the range of <NUM> to <NUM>, more preferably <NUM> to <NUM>. In a preferred embodiment, more than <NUM> wt% of the TPV used for the preparation of the outer layer have a Shore A hardness of less than <NUM>, more preferably less than <NUM>. Hence, the hardness Shore of the outer layer is preferably less than <NUM>, more preferably less than <NUM>.

In the composite structure of the present invention, the total of <NUM> parts by weight of polymers (A) to (C) preferably.

In a specific embodiment, the composite structure of the present invention contains an outer layer.

In a more preferred embodiment, the composite structure has a combination of above features (i) and (iv), (i) and (v), (i) and (vi), (ii) and (iv), (ii) and (v), (ii) and (vi), (iii) and (iv), (iii) and (v), or (iii) and (vi). The composite structures having features (i) or (ii) are particularly suitable for being processed by cutting and sewing.

Polymers (A) to (F) contained in the compact layer will be described in the following.

Polymers (A) and (F) are elastomers having the shape of particles. The particles are preferably embedded in a matrix of the other polymers present in the layer. The particle size is preferably below <NUM>, more preferably below <NUM>. Preferably, at least <NUM> % of the particles have a size of less than <NUM>. The size can be determined by scanning electron microscopy (SEM) of a cross section of the layer. The longest diameter of a particle image in an SEM micrograph is defined as its size.

Polymers (A) and (F) are selected independently from each other and may be the same or different.

In the present invention, the terms "elastomer" and "rubber" are used synonymously. They denote polymers that show elastic properties and are crosslinked. Their gel content is preferably between <NUM> % and <NUM> %, more preferably between <NUM> % and <NUM> %, even more preferably between <NUM> % and <NUM> %.

The rubber component contained in the composite structure according to the present invention is preferably contained in a thermoplastic vulcanizate (TPV), which can be used as a raw material for producing the composite structure, as discussed below.

The rubber comprises ethylene propylene diene monomer rubber (EPDM) and may contain rubbers selected from the group consisting of natural rubber (NR), nitrile butadiene rubber (NBR), hydrogenated nitrile butadiene rubber (HNBR), carboxylated nitrile rubber (XNBR), butyl rubber (IIR), chlorobutyl rubber (CIIR), bromobutyl rubber (BIIR), polychloroprene (CR), styrene-butadiene rubber (SBR), polybutadiene (BR), ethylene-propylene rubber (EPR or EPM), silicone rubber, acrylic rubber (ACM), ethylene-vinylacetate copolymer rubber (EVM), polyurethane rubber (PU), and any combination of the above. The rubber can also be a styrene based thermoplastic elastomer (STPE).

In particular, the rubber may be selected from the group comprising ethylene/α-olefin copolymer rubber (EAM) as well as ethylene/α-olefin/diene terpolymer rubber (EADM). Preferably the diene in the ethylene-α-olefin-diene rubber is preferably a nonconjugated diene. Suitable non-conjugated dienes include dicyclopentadiene, alkyldicyclopentadiene, <NUM>,<NUM>-pentadiene, <NUM>,<NUM>-hexadiene, <NUM>,<NUM>-hexadiene, <NUM>,<NUM>- heptadiene, <NUM>-methyl-<NUM>,<NUM>-hexadiene, cyclooctadiene, <NUM>,<NUM>-octadiene, <NUM>,<NUM>-octadiene, <NUM>-ethylidene-<NUM>-norbornene, <NUM>-n-propylidene-<NUM>-norbornene, and <NUM>-(<NUM>-methyl-<NUM>-butenyl)-<NUM>-norbornene. In preferred embodiments, the rubber component comprises an ethylene-α-olefin-diene rubber. The ethylene-α-olefin-diene rubber may comprise an α-olefin having <NUM> to <NUM> carbon atoms. If the α-olefin in an EAM or EADM rubber is propylene, the rubber is referred to as EP(D)M. Here, EPM may also be referred to as EPR. It is also possible to use a mixture of the rubbers mentioned above.

In a preferred embodiment of the present invention, the rubber in the outer layer consists of EPDM and EPR. In that case, EPDM preferably constitutes at least <NUM> wt% of the total rubber content in the outer layer. It is preferred that EPDM is contained in a TPV raw material and EPR is contained in a TPO raw material.

EPDM is made from ethylene, propylene and a diene comonomer that enables crosslinking via sulphur vulcanization systems. EPDM contains crosslinks and is preferably fully cured, i.e. crosslinked to an extent of at least <NUM>%, most preferably to an extent of <NUM>%. The content of propylene is preferably <NUM>% to <NUM>% by weight. Preferred dienes used in the manufacture of EPDM rubbers are ethylidene norbornene (ENB), dicyclopentadiene (DCPD), and vinyl norbornene (VNB). EPDM is preferably crosslinked via vulcanization with sulphur or by using peroxides. Propylene reduces the formation of the typical polyethylene crystallinity. EPDM is a semi-crystalline material with ethylene-type crystal structures at higher ethylene contents, becoming essentially amorphous at ethylene contents that approach <NUM> wt%. EPDM may be compounded with fillers such as carbon black and calcium carbonate, and with plasticizers such as paraffinic oils.

EPR (also referred to as "EPM") is a random copolymer of ethylene and propylene. EPR is similar to EPDM but contains no diene units. It is crosslinked using radical methods such as peroxides. EPR of different properties can be obtained by varying the monomer ratios. These properties are e.g. viscosity or crystallinity. EPR having from amorphous to semi-crystalline structure can be obtained. EPR preferably comprises <NUM> to <NUM> wt%, more preferably <NUM> to <NUM> wt% of ethylene units. EPR copolymers containing less than about <NUM> wt% ethylene generally are known to have poor elasticity at low temperatures, and thus may provide compositions that are too rigid and lack the balance of mechanical properties over a wide temperature range needed for most automotive applications. At high levels of ethylene, generally above about <NUM> wt% ethylene units, separate crystalline ethylene domains may form within the rubber component, and interphase adhesion and miscibility are reduced. EPR may be contained in a thermoplastic resin composition in an amount of <NUM> to <NUM> wt%, preferably <NUM> to <NUM> wt%. Resins with less than <NUM> wt% of EPR are more rigid and less flexible. The presence of the rubber component at high levels above about <NUM> wt% decrease stiffness and tensile strength.

The thermoplastic polyolefin (TPO) of the inner layer and the TPO of the outer layer, i.e. polymers (C) and (E), are polypropylenes (PP) and are selected independently from each other and may be identical or different from each other.

The degree of crosslinking in thermoplastic polyolefins is low. Their gel content is preferably less than <NUM> %, more preferably less than <NUM> %.

Polypropylene (PP) is defined here as polymers or copolymers whose proportion by weight of propylene is more than <NUM> % by weight. The properties of TPO can be influenced by the addition of elastomers or other substances such as talcum.

Thermoplastic vulcanizate (TPV) is part of the thermoplastic elastomer (TPE) family of polymers. A thermoplastic elastomer is defined as an elastomer comprising a thermoreversible network. TPE is a copolymer or a physical mix of polymers, e.g. a plastic and a rubber, that consists of materials with both thermoplastic and elastomeric properties. TPV as one class of TPE combines the characteristics of vulcanized rubber with the processing properties of thermoplastics. TPV contains rubber particles cured by vulcanization. The rubber particles encapsulated in a thermoplastic matrix.

In the present invention, the rubber particles are made of the rubber described above, and the thermoplastic matrix is made of TPO described above. The resin components of TPV may consist of <NUM>-<NUM> parts by weight of the polyolefin resin and correspondingly <NUM>-<NUM> parts by weight of the rubber, preferably <NUM>-<NUM>/<NUM>-<NUM> parts by weight, and more preferably <NUM>-<NUM>/<NUM>-<NUM> parts by weight. In the present invention, a TPV based on polypropylene (PP) and EPDM rubber is preferred.

The polyolefin contained in the TPV is polymer (A) or polymer (F), each which can be identical to or different from the TPO otherwise contained in a layer, e.g. polymer (B) or polymer (C). For instance, PP contained in a layer may be derived from TPO added as polymer (C) and, in addition, from the matrix polymer of TPV, in which PP and rubber is contained e.g. in equal amounts. Hence, in a preferred composite structure of the invention the polymer (A) is ethylene propylene diene monomer rubber and at least part of polymer (C) is polypropylene, which is contained in an amount equal to or higher than the amount of polymer (A).

The TPV can either be prepared by mixing the polyolefin with a particulate form of the vulcanized rubber or via a process known as dynamic vulcanization. Dynamic vulcanization consists of intimately mixing a blend of compatible polymers, then introducing a crosslinking system in the mixture while the mixing process is continued. The mechanical performance of TPV improves with the degree of crosslinking of the rubbery phase and with the inverse of the particle size of rubbery domains. The particle size of the vulcanized rubber is preferably below <NUM>, more preferably below <NUM>. TPV can be processed using conventional thermoplastic processes such as injection molding, blow molding and extrusion.

TPV may be compounded with additives described below. In particular, the TPV may contain different ingredients such as reinforcing fillers (carbon black, mineral fillers), stabilizers, plasticizing oils, and curing systems.

LDPE is a thermoplastic c made from the monomer ethylene. It has a density in the range of <NUM>-<NUM>/cm<NUM> and preferably a melting point of about <NUM> to about <NUM>. LDPE has a high degree of short- and long-chain branching, which means that the chains do not form crystal structures and LDPE confers isotropic properties. This results in a lower tensile strength. The high degree of branching with long chains gives molten LDPE desirable flow properties.

LLDPE is defined by a density range of <NUM>-<NUM>/cm<NUM>. It is a substantially linear polymer and is made by copolymerization of ethylene with longer-chain olefins. Examples of such olefins are <NUM>-butene, <NUM>-hexene, and <NUM>-octene, which result in short branches in the polymer chain.

LLDPE contains branches derived from comonomers of not longer than <NUM>-octene, whereas LDPE also contains branches derived from longer comonomers. LLDPE has higher tensile strength than LDPE, and it exhibits higher impact and puncture resistance than LDPE. LLDPE shows toughness, flexibility, and relative transparency. LLDPE is less shear sensitive than LDPE because of its narrower molecular weight distribution and shorter chain branching. LLDPE confers melt strength. The linearity of LLDPE results from the different manufacturing processes of LLDPE and LDPE. In general, LLDPE is produced at lower temperatures and pressures by copolymerization of ethylene and alpha-olefins. LLDPE can be advantageous for securing excellent moldability on the basis of an appropriate elongation and securing rigidity to be broken at a certain level of strength or more.

The lacquer layer is applied directly on top of the cover layer as a finish of the composite structure and serves to protect artificial leather from chemical agents, physical damage, e.g. scratches or abrasion, and UV radiation. The surface lacquering can further reduce the surface adhesion of the artificial leather. Conventional lacquers used for artificial leather for the interior of vehicles can be employed. For example, solvent-based or water-based polyurethanes that are crosslinked with isocyanates or have properties for UV curing are suitable.

The surface coating is preferably applied over the entire composite structure and completely covers it. The surface coating is formed from at least one layer of lacquer, but can be formed from several layers of lacquer, preferably from one to four layers of lacquer.

The surface coating to protect the surface usually consists of one or more, preferably up to four, transparent layers of lacquer. In one embodiment, the surface coating is coloured, e.g. by adding colour pigments to the coating.

The surface coating is formed by applying a coating in one or preferably several coats. The varnish is preferably applied by gravure printing, but can also be applied by other methods, such as roller application, spray application or in an embossing step for surface embossing.

In one version, the surface coating can be applied in the form of a lacquer layer in a two-stage roller printing process. The surface coating has a thickness in the range of a few micrometers, preferably <NUM> to <NUM>, more preferably <NUM> to <NUM> and even more preferably <NUM> to <NUM>.

The lacquer layer is preferably a polyurethane lacquer layer. An example of a surface coating is a coating based on a silicone-containing aliphatic polyurethane. This varnish can be applied in a thickness of <NUM>.

This invention is further illustrated by the following examples.

In this invention, the following measurement methods were used to determine the parameters of the composite structure:
(The norms and standards stated in this application refer to the latest versions at the filing date of this application unless otherwise indicated).

Thickness: ISO <NUM>; Weight: ISO <NUM>-<NUM>:<NUM>; Melting points (DSC): ASTM D <NUM>-<NUM>; elongation at break: ISO <NUM>-<NUM>:<NUM>; density: ISO <NUM>; Hardness (Shore A, ShA): DIN <NUM>; resistance to tear: ISO-<NUM>-<NUM>; tensile strength: DIN EN ISO <NUM>-<NUM> at <NUM>/min and <NUM>.

Gel content: Gel content measurement is based on ASTM D2765-<NUM>. The composite material is weighed (initial weight) and placed in xylene for <NUM> hours at <NUM>, the dissolved material is separated and the weight of the remaining material is determined (final weight); gel content [%] = [(final weight)/initial weight)] × <NUM>.

The tear properties at airbag deployment were tested at -<NUM>, <NUM> and <NUM>. Evaluation:.

The composite structure of example <NUM> has the following configuration:.

The composite structures having configurations shown in Table <NUM> were prepared as described in Example <NUM>.

Table <NUM> also shows the results of Examples <NUM> to <NUM> and Comparative Examples <NUM> and <NUM> as well as the results of composite structures <NUM>(V) and <NUM>(V) of <CIT> (DE'<NUM>).

The composite structures of Comparative Examples <NUM> and <NUM> do not meet the feature that the inner layer contains at least <NUM> wt% of a thermoplastic polyolefin (polymer (E)) other than low-density polyethylene.

The results show that the composite structures according to the present invention have advantageous properties such as low tensile strength and low resistance to tear. Therefore, the composite structures are suitable for the use as airbag covers. All composite structures are suitable for the use in airbags having H-shape geometry. Airbags having U-shape are more demanding and require a composite structure having isotropic properties resulting in an elongation at break that is similar in lengthwise and crosswise direction. LDPE confers isotropic properties. In addition, the foam density should be higher for the use in U-shaped airbags. Therefore, the composite structures of Examples <NUM>, <NUM>, and <NUM> are particularly suitable for the use in airbags having U-shape geometry.

The composite structures of Examples <NUM> to <NUM> are thermoplastic and can be used in a thermoforming process.

Claim 1:
A composite structure containing a foam layer, an inner layer and an outer layer of a thermoplastic compact cover layer, and a lacquer layer in this order, and having a tensile strength according to DIN EN ISO <NUM>-<NUM> at <NUM>/min and <NUM> of less than <NUM> MPa in both a first direction and a second direction perpendicular to the first direction, wherein the foam layer consists of or contains a polyolefin foam, the density of the foam layer is <NUM> to <NUM>/m<NUM>, the inner layer contains particles of an elastomer (polymer (F)) and at least <NUM> wt% of polypropylene (polymer (E)) and is directly bonded to the outer layer, wherein the following polymers (D) to (F) make up at least <NUM> wt% of the total polymer content in the inner layer and the inner layer contains the polymers (D) to (F) in a total amount of <NUM> parts by weight:
<NUM> to <NUM> parts by weight of particles of an elastomer (polymer (F)) comprising ethylene propylene diene monomer rubber,
<NUM> to <NUM> parts by weight of a linear low-density polyethylene (polymer (D)), and
<NUM> to <NUM> parts by weight of polypropylene (polymer (E));
wherein the outer layer differs from the inner layer in its composition, the following polymers (A) to (C) make up at least <NUM> wt% of the total polymer content in the outer layer and the outer layer contains the polymers (A) to (C) in a total amount of <NUM> parts by weight:
<NUM> to <NUM> parts by weight of particles of an elastomer (polymer (A)) comprising ethylene propylene diene monomer rubber,
<NUM> to <NUM> parts by weight of a low-density polyethylene (polymer (B)), and
<NUM> to <NUM> parts by weight of polypropylene (polymer (C));
wherein the weight content of particles of an elastomer in the outer layer is higher than in the inner layer, and wherein the Shore A hardness of the outer layer is lower than that of the inner layer, which has a Shore A hardness of <NUM> to <NUM>, measured according to DIN <NUM>.