WO2003047842A1

WO2003047842A1 - Energy absorption unit

Info

Publication number: WO2003047842A1
Application number: PCT/GB2002/005442
Authority: WO
Inventors: Peter James Cate; Padraig Joseph Naughton
Original assignee: Dow Global Technologies Inc
Priority date: 2001-12-03
Filing date: 2002-12-02
Publication date: 2003-06-12
Also published as: AU2002349150A1; GB0128954D0

Abstract

An energy absorbing unit, for example a bonnet which has an outer and inner skin on a thermoplastic core, the core having greater compression strength in the direction perpendicular to the skins than in the direction parallel to them. One of the skins is bonded by heating the core and the other by heating and/or adhesive. Protection against hard points underlying the vehicle bonnet is provided.

Description

ENERGY ABSORPTION UNIT

This invention relates to an energy absorbing unit. In particular the invention relates to an article made of a thermoplastic foam sandwiched between two layers of material which is capable of absorbing energy transferred to the article during an impact and to a method of producing the article. The invention relates especially to energy absorbing units for use in automobile applications. An impact energy-absorbing unit (EAU) typically forms part of or is attached to an article to provide a means of reducing damage to the article during impact with a body and also to reduce damage to the body by absorbing and dissipating energy in an efficient manner.

Energy absorption units comprising a foam core having a layer on the core, for example a bumper fascia moulding, are known. The foam core material provides a significant part of the energy absorption function and should exhibit satisfactory dimensional recovery. Plastic materials which have been proposed for the foam core material include polyurethane, polyethylene bead, polystyrene and polypropylene bead foams. A conventional automobile EAU core of foamed olefinic polymer is typically prepared by feeding expandable beads of a polypropylene-type resin in a mould capable of enclosing the particles but allowing escape of gases therefrom, and heating the expandable beads at a temperature at which the beads expand and soften and melt-adhere to each other into a mass, whereby a foamed moulded article expanded to the same shape and size as the cavity of the mould is formed.

A critical consideration in designing automobiles and developing materials for use in the construction of automobiles, for example EAUs is the ability for a component or part of the automobile to minimise damage to a body involved in a collision with the automobile, in particular minimising personal injury or incidence of death of pedestrians, cyclists and the like resulting from collisions with automobiles.

An aspect of automobile design of particular interest in reducing serious injury and the incidence of death relates to those parts of automobiles where there are short distances between the likely point of impact with the human body and the location of hard points within the automobile, for example the engine unit, suspension components, radiator and external metal or metal framed body parts, such as the edge of a bonnet. Such short distances or clearances requires the energy absorbing unit to absorb energy of the impact efficiently so as to reduce the likelihood of the body part coming into contact with the hard points with consequent injury to the body especially if impact is with the head or other vital organs thereby reducing the severity of any injury. Energy is suitably absorbed by the EAU being compressed or crushed and also by the EAU bending or flexing. In at least some countries regimes for testing automobile impact with the human body are or may be required in legislation. As such there is legislative stimulus for improving safety in addition to the general desirability of this aim. Accordingly there remains a need to improve further the energy absorption characteristics of automobile parts likely to be involved in collisions, especially in collisions with the human body in the interests of public safety. In addition to safety considerations, external automobile body parts also provide structural rigidity or stiffness and suitably provide other functional and aesthetic characteristics.

US-A-3906137 discloses laminates comprising two cover layers bonded to a core of a thermoplastic foam made by bonding together several compressed foam sheets and cutting the adhered sheets in a direction perpendicular to the plane of the sheet. The cover layers may be formed in situ or pre-formed. The example in this patent discloses adhering aluminium plates to a core using an epoxy resin adhesive.

WO99/00236 discloses articles formed of thermoplastic foams. An energy absorbing article having a surface in which impact resistance is required containing an extruded thermoplastic foam having a greater impact resistance in a first direction than any other. Desirably the first direction is aligned with the direction in which impact resistance is anticipated is disclosed. WO99/00236 further discloses a monocoque-like structural reinforcement made by laminating a separate structural skin to the outer surface of the thermoplastic foam, the skin being made of a polymer material. Layers such as decorative and functional fabrics may be bonded to the EAU using known adhesives. WO99/00236 discloses that a layer for example a woven fabric of thermoplastic fibres may be welded thermally to the EAU so as to avoid the use of an adhesive. We have now found that an energy absorbing unit having a foam core with anisotropic properties and with a surface skin bonded to opposed surfaces of the core provides excellent impact protection, especially against head impact, for the human or animal body for example pedestrians, and also provides enhanced stiffness relative to isotropic foams so the unit may itself impart structural rigidity and reduce the need to employ separate support structures to impart rigidity.

Accordingly, in a first aspect, the invention provides an energy absorbing unit comprising a core material and a first and second surface layer located on opposing faces of the core wherein the core comprises a thermoplastic material having anisotropic strength properties such that its compression strength in the direction perpendicular to the surface layers is greater than in the direction parallel to the surface layers, the first surface layer being bonded to the core by heating the core and/or the first surface layer and contacting the core and the layer together and the second surface layer being bonded to the core i) by heating the core and/or the second surface layer and contacting the core and the layer together and/or ii) by means of an adhesive.

Suitably, the first surface layer is bonded to the core in the absence of an adhesive.

The invention provides in a second aspect, a method of preparing an energy absorbing unit according to the first aspect of the invention comprising; I) bonding a first surface layer to a first face of a core material by heating the face of the core material and/or the first surface layer and contacting the said layer with the said face thereby to bond the core and the layer together; and II) bonding a second surface layer comprising a metallic or plastics material to a second face of the core material opposing the first face, by applying an adhesive to or heating the said second face and/or the said second surface layer and contacting the second layer with the second face thereby to bond the layer to the core material; wherein the core comprises a thermoplastic material having anisotropic strength properties such that its compression strength in the direction perpendicular to the surface layers is greater than in the direction parallel to the surface layers.

EAU's according to the invention provide a desirable combination of energy absorption characteristics and stiffness which is provided at least in part by the anisotropic structure of the core. In addition, the thermoplastic core may be shaped during manufacture of the EAU to provide a desired profile and so allowing flexibility in the design of the automobile.

The EAU according to the present invention is especially suitable for absorbing energy effectively within in a short distance in a direction perpendicular to the surface layers. Suitably the EAU according to the invention absorbs energy through compression or crushing of the core in a direction perpendicular to the surface layers of the EAU especially when in close proximity to a hard area underlying the EAU. Preferably the EAU absorbs energy so that the level of load on the EAU rises rapidly during impact and remains at a high level in order to absorb the energy in as short a distance as possible providing excellent energy absorption per unit area. This characteristic provides the practical benefit of reducing the incidence and severity of injury due to impact with hard areas.

The EAU suitably absorbs energy by bending and energy being absorbed in the surface layer on the impact side of the EAU until the level of force reaches a critical limit at which the core is compressed or crushed against the surface layer opposite the side of impact which is in tension. Accordingly, the invention provides advantageous impact protection both in close proximity to and away from hard areas under the EAU. 5 Additionally, the EAU according to the invention exhibits low rebound of the impact object so as to reduce the severity of injury.

Suitably the energy absorbing unit exhibits sufficient stiffness to provide structural support for the vehicle part so reducing reliance on employing other support means for example metal framework which typically may be relatively hard o and so detrimental to the aim of reducing the incidence and severity of injury to the human body as a result of impact with the vehicle.

The advantages of an EAU according to this invention, in particular absorption of energy within in a short distance, accrue especially when the EAU is employed in a vehicle in an area of the vehicle in which there is a higher 5 probability of impact with a pedestrian than in other areas of the vehicle.

The EAU may be tailored to be able to absorb a certain level of force per unit area according to the location in which the EAU is to be used by selecting a core of appropriate density.

In a preferred embodiment, the force per unit area which the EAU is able to 0 absorb is 0.1 to 1.5 MPa.

A third aspect of the invention provides a vehicle part for use in an area of a vehicle susceptible to impact with a human or animal body comprising an energy absorbing unit according to the first aspect of the invention.

Preferably, the vehicle part is a bonnet, a bumper or another assembly 5 used in an area susceptible to impact with a human or animal body, for example reinforcing bars employed in doors and the like, comprising an energy absorbing unit according to the first aspect of the invention.

Conventionally, bumpers and other assemblies used in impact zones and bonnets are made from metal such as steel and aluminium and are relatively o heavy, especially in relation the impact resistance they provide. The structure of a conventional bonnet often includes a rigid framework to provide structural rigidity. Further, conventional bonnets are typically relatively thin in the dimension perpendicular to the surface of the bonnet and hard areas such as the rigid framework, suspension towers, engine unit and radiator may be located such that 5 the clearance between the bonnet and the hard points is short.

The invention further provides a vehicle bonnet comprising an EAU according to the first aspect of the invention in which the surface layers provide the upper, outer surface and underside surface of the bonnet and the anisotropic core of the EAU is aligned generally perpendicular to the surface layers, the bonnet 0 being able to absorb a force per unit area of 0.1 to 1.5 MPa from an impact in the direction perpendicular to the surface layers.

An EAU according to the first aspect of the invention is especially useful when used in the construction of a vehicle bonnet because the stiffness of the EAU allows a reduction in the use of supporting framework and excellent energy 5 absorbency in short distances reduces the incidence and severity of injury due to impact with hard points under the bonnet.

In use, a bonnet has areas where there is close proximity to underlying hard areas (a first area) and also areas where there is greater clearance between the EAU and an underlying hard area (a second area). When employed in a bonnet, o the core of the EAU in the second area may comprise a thermoplastic material which is anisotropic in a direction perpendicular to the surface layers if desired but, more preferably, comprises a thermoplastic material which is anisotropic in a direction parallel to the surface layers.

In a preferred embodiment, the core of the EAU is a composite structure 5 comprising, in a first area, a thermoplastic material having anisotropic strength properties such that its compression strength in the direction perpendicular to the surface layers is greater than in the direction parallel to the surface layers and, in a second area, a thermoplastic material having anisotropic strength properties such that its compression strength in the direction parallel to the surface layers is o greater than in the direction perpendicular to the surface layers. In the second area, the thermoplastic material which is anisotropic in a direction parallel to the surface layers, may be aligned in a single direction or more than one direction and still be parallel to the surface layers. In a preferred embodiment, the thermoplastic material in the second layer comprises material aligned in orthogonal directions within a plane parallel to the surface layers.

The first area of the core is, in use, intended to be located in close proximity to a hard point or area. The first area of the core provides excellent energy absorption within a short distance. The second area, in use, is intended to be located at a point away from an underlying hard point or area and energy absorption within a short distance is less critical than in the first area.

References herein to the properties or alignment of the thermoplastic material or core material in use are, in the case of an EAU having a composite core and unless otherwise stated, references to that part of the thermoplastic material having anisotropic properties perpendicular to the surface layers. Provision of a core having a composite structure provides a desirable combination of energy absorption through compression or crushing in the first area and flexing or bending in the second area where there is less need to ensure energy absorption within a short distance. Further, manufacture of an EAU having a composite core structure may be advantageous as regards lower costs of production.

Conventional bonnets often comprise material on the underside of the bonnet to provide acoustic damping, for example cotton felt, polyurethane foam and formaldehyde foam. Such materials may absorb moisture during use which may cause a reduction in the damping effect. The thermoplastic core of a bonnet according to this invention, may advantageously provide sufficient acoustic damping without the need for separate, conventional damping material. A reduction in damping properties which may be observed with conventional bonnets may accordingly be ameliorated, the cost associated with providing separate damping material may be reduced or avoided. The relatively high level of impact resistance and stiffness in combination with the relatively low weight of the EAU's of the present invention enable a lower weight EAU component to be employed than in conventional parts. In practice, this advantageously provides additional flexibility in designing the automobile and 5 enhanced fuel economy due to the lower weight required to provide a given or superior impact resistance and stiffness.

The invention also comprises a vehicle comprising a vehicle part according to the third aspect of the invention, the part being aligned so that the surface layers are generally perpendicular to the anticipated direction of impact. 0 Suitably, the core materials and the first surface layer and desirably the second surface layer are comprised of recyclable materials to environmental advantage.

The core material comprises a foamable thermoplastic material and may comprise a homopolymer or a copolymer. Suitable thermoplastic materials include 5 polyethylene, including low density polyethylene and high density polyethylene (HOPE), polypropylene, and copolymers of ethylene or propylene and a monoethylenically unsaturated monomer copolymerizable therewith. Examples include copolymers of ethylene and acrylic acid or methylacrylic acid and C_1-4 alkyl esters or ionomeric derivatives thereof; ethylene vinyl-acetate copolymers; o ethylene/carbon monoxide copolymers; anhydride containing olefin copolymers of a diene; copolymers of ethylene and an alpha-olefin having ultra low molecular weight (i.e., densities less than 0.92 g/cc); blends of all of the above materials; blends thereof with polyethylene (high, intermediate or low density). Preferred polyolefins include polypropylene homopolymers and copolymers of polypropylene 5 which are comprised of at least 50% propylene monomeric unit by weight. Other suitable polyolefins include branched polypropylene homopolymer and branched copolymers of polypropylene.

The polymers of ethylene or propylene and a polar co-monomer may be prepared by known addition polymerization techniques, or by grafting reaction of o the reactive co-monomer with a preformed polymer of ethylene or propylene. Additional elastomeric components such as polyisobutylene, polybutadiene, ethylene/propylene copolymers, and ethylene/propylene/diene interpolymers may be included in the blend if desired. Moreover, additional components such as cross linking agents designed to provide latent cross linking of the ethylenic or propylenic polymer, such as silane functional cross linking agents, or covalent or ionic cross linking agents, may be included if desired.

Other suitable foamable thermoplastic compositions which may be used as the core material in the practice of this invention include polyesters, polyamides, polyvinylchloride, polyvinylidene chloride, polycarbonates and polystyrene resins. Suitably, the thermoplastic polymer material or blend of materials is melt processed in a conventional manner by feeding, melting, metering it into a conventional melt processing apparatus such as an extruder. A volatile blowing agent and an optional cross-linking agent are preferably mixed with the polymer or blend of polymers under a pressure suitable to form a flowable gel or admixture. A cross linking agent may be added in an amount which is sufficient to initiate cross linking and raise the pressure of the gel or admixture to less than that pressure which causes melt fracture of the polymer to occur. The term "melt fracture" is used in the art to describe a melt flow instability of a polymer as it is extruded through a die, which flow instability causes voids and/or other irregularities in the final product.

As desired, additives known in the art, for example nucleating agents, inorganic fillers, pigments, anti-oxidants, acid scavengers, ultra-violet absorbers, flame retardants, surfactants, processing aids, and extrusion aids, may be included in the thermoplastic core material. In a preferred embodiment the thermoplastic core comprises a material disclosed in WO99/00236 or is produced by a process described in it.

By extruding the thermoplastic material, anisotropic properties are imparted to it. The strength in the extrusion direction is greater than the strength in directions perpendicular to the extrusion direction. Strength properties as used herein include, but are not necessarily limited to, compressive strength, compressive modulus, and impact resistance. For example, depending upon the various parameters, material and apparatus used during the extrusion process, compressive strength in the extrusion direction may be 25%, 50%, 100% or even 200% or more greater than in a direction perpendicular to the extrusion direction. 5 For example, stranded or coalesced polypropylene extrudates have exhibited compressive strength in the extruded direction which is more than four times that in a direction perpendicular to the extrusion direction.

Orientation to produce the anisotropic thermoplastic may be produced by any known technique for example, by extruding the foamable thermoplastic. 0 Suitably, the thermoplastic core is formed and the process parameters for forming it, for example temperatures and extrusion rate, are selected so that the thermoplastic foam has a density of less than 320 kg/m³ preferably from 16 to 128kg/m³. The resulting extrudates exhibit anisotropic compressive strength properties, with the highest compressive strengths suitably being observed in the 5 extrusion direction of at least about 170 kPa at about 25% compression as measured in accordance with ASTM D3575-93 (Suffix D).

The thermoplastic material may be foamed or formed into the desired shape of the core energy absorbing unit or, more preferably, is cut into the desired shape. In an especially preferred embodiment, planks of the extruded anisotropic o foam are produced having the desired density and are then cut to the desired size and shape according to the design of the core, corresponding to a cross-section perpendicular to the surface layers which are to be subsequently applied to the core. The cut planks are suitably bonded together in a thermoforming step to form the core of the EAU. 5 Accordingly, the core of the energy absorbing units of this invention exhibit anisotropic compressive strength properties, with the direction of maximum strength of the extruded thermoplastic foam being approximately aligned with the direction in which impact is expected and impact resistance is desired. In particular, the direction of maximum strength of the thermoplastic foam core and o the direction in which impact resistance is desired in the energy absorbing article should differ by an angle of less than about 45°, more preferably less than 10°, and most preferably less than 5°. Approximate alignment or the direction of maximum strength of the core with the direction in which impact is expected or occurs or in which impact resistance is desired means that the direction of 5 maximum strength of the extrudate is more nearly parallel to the direction in which impact resistance is anticipated than perpendicular to the direction in which impact resistance is anticipated.

The surface layers may comprise the same or different materials and suitably comprise a thermoplastic polymer, a thermoset polymer or a metal, for 0 example steel and aluminium. The surface layers may be reinforced or non- reinforced as desired. Preferably, the first surface layer comprises a thermoplastic polymer and the second surface layer comprises a thermoplastic polymer or a metal, preferably steel or aluminium. Suitable polymers for the surface layer(s) include polypropylene, especially homopolymers and copolymers of propylene 5 which are comprised of at least 50% propylene monomeric unit by weight polypropylene, polyurethane (PU) RIM, PA, SPS, TPU and GMT.

The surface layers may be produced by conventional processes including injection moulding, compression moulding, thermoforming, stamping and the like.

As desired one or both surface layers may be contoured, for example o ribbed to provide improved stiffness and to contribute to the absorption of energy. The surface layer may be shaped or contoured for design or aesthetic purposes also, for example to include fittings as an integral part of the design of the EAU.

Where the second layer is bonded to the core by an adhesive, known adhesives suitable for bonding the materials selected for the core and the layer 5 may be employed.

In a preferred embodiment of the invention the first surface layer comprises a thermoplastic polymer and the second surface layer comprises a metal bonded to the core by means of an adhesive. Preferably, the first surface layer and the core are bonded together by heating the layer and/or the core and contacting o them, desirably with the application of pressure. In a further preferred embodiment, the second layer comprises a metal and the core comprises a low energy thermoplastic material, for example polypropylene, and the second layer is bonded to the core by means of an adhesive capable of bonding to a low energy thermoplastic material for example as described in US-A-5795657 or US-A-5961065.

If desired, either surface layer may be treated prior to assembly to produce the EAU or the at least one surface layer may be treated after assembly by subjected the EAU to the desired treatment.

Depending on the materials, the core and one or both of the surface layers may be aligned and then heated to bond the surface layer or layers to the core and, as desired, formed to the desired shape or profile required for the end use of the EAU. If a first layer is applied in this manner, the second layer may suitably be bonded to the opposing face of the core using adhesive applied to the opposing face of the core and/or the second layer. This latter method of production is especially suitable in producing a bonnet according to the invention in which the first layer comprises a thermoplastic and will form the underside of the bonnet and the second layer comprises a metal and will form the outer or external layer of the bonnet.

If desired, mechanical fixing means, for example mechanical snaps may be used to secure one or both layers to the core.

The invention is described by way of reference to the drawings in which Figure 1 illustrates a cross-section of an EAU according to the invention, Figure 2 shows an exploded cross-section of a bonnet according to the invention and Figure 3 shows a cross-section of an assembled bonnet shown in Figure 2. Figure 1 shows an energy absorbing unit (1 ) according to the invention having a core of thermoplastic material (2) which is bonded to a first surface layer (3) and a second surface layer (4). The core (2) is made of thermoplastic material which has anisotropic properties and is oriented in a direction shown by the lines A and perpendicular to whichever layer (3) or (4) will form the outer surface with which impact may be anticipated to occur in use thereby utilising the high compression strength of the oriented material.

The surface layers (3) and (4) preferably and independently are made of thermoplastic or metal. 5 In the exploded illustration of the components of a bonnet according to the invention shown in Figure 2, a thermoplastic core (4) which is oriented in the direction B is bonded to a surface layer (3) which is made of a thermoplastic, thermoset or metallic material and which in use in a vehicle would be the underside of the bonnet. An adhesive layer (5) is to be applied to the upper face 0 of the core (2) and a second surface layer (4), suitably made of steel or aluminium and having been painted prior to assembly, is to be bonded to the core (2) by bringing the core (2) and the layer (4) into contact by relative movement in the direction B and application of pressure.

In Figure 3, the components of the bonnet shown in Figure 2 have been 5 assembled and comprise the core (2), the underside layer (3), the outer layer (4) and the layer of adhesive interposed between the core (2) and the outer layer (5). Suitably, the outer surface (4) extends over the edge of the core (2) and is shaped according to the design of the bonnet.

The surface layers (3) and (4) may be planar, curved as shown in Figures 1 o and 2 or may have other contours as desired. The layers (3) and (4) may be parallel but need not be so, the shape of the layers being selected according to the overall shape or configuration of the EAU. Suitably, the orientation of the material of core (2) is perpendicular to the plane of the surface layer (3) or (4) or, where the layer (3) or (4) is non-planar, may be perpendicular to a plane which is tangential 5 to the point on the surface of the layer (3) or (4) which the locus of the oriented material intersects.

Alternatively, the material of core (2) may be oriented in one direction, as shown in Figures 2 and 3 in the EAU and the surface layers (3) or (4) may be shaped such that the layers (3) or (4) in some areas are not perpendicular to the o direction of orientation of the material forming the core (2). In this case, it is generally preferred that the angle between the layers (3) or (4) and the direction B of orientation of the material of the core (2) is at least 45° and preferably as close as possible to perpendicular thereto.

Claims

1. An energy absorbing unit comprising a core material and a first and second surface layer located on opposing faces of the core wherein the core comprises a

5 thermoplastic material having anisotropic strength properties such that its compression strength in the direction perpendicular to the surface layers is greater than in the direction parallel to the surface layers, the first surface layer being bonded to the core by heating the core and/or the first surface layer and contacting the core and the layer together and the second surface layer being bonded to the 0 core i) by heating the core and/or the second surface layer and contacting the core and the layer together and/or ii) by means of an adhesive.

2. An energy absorbing unit according to claim 1 in which the first surface layer is bonded to the core in the absence of an adhesive. 5

3. An energy absorbing unit according to any one of the preceding claims in which the core material comprises a foamable thermoplastic material.

4. An energy absorbing unit according to any one of the preceding claims in o which the core material comprises a polyolefin selected from low density polyethylene, high density polyethylene, polypropylene, and copolymers of ethylene or propylene and a monoethylenically unsaturated monomer copolymerizable therewith.

5 5. An energy absorbing unit according to claim 4 in which the core material is selected from polypropylene homopolymers and copolymers of polypropylene which are comprised of at least 50% propylene monomeric unit by weight.

6. An energy absorbing unit according to claim 5 in which the core material 0 comprises stranded or coalesced polypropylene extrudate having a compressive strength in the extrusion direction of more than four times that in a direction perpendicular to the extrusion direction.

7. An energy absorbing unit according to any one of the preceding claims in 5 which the surface layers, independently, comprise a thermoplastic polymer, a thermoset polymer, a metal or fibreglass.

8. An energy absorbing unit according to any one of the preceding claims in which the first surface layer comprises a thermoplastic polymer and the second o surface layer comprises a thermoplastic polymer or a metal, preferably steel or aluminium.

9. An energy absorbing unit according to claim 8 in which the thermoplastic polymer is selected from homopolymers and copolymers of polypropylene which 5 are comprised of at least 50% propylene monomeric unit by weight polypropylene.

10. An energy absorbing unit according to any one of the preceding claims in which the first surface layer comprises a thermoplastic polymer and the second surface layer comprises a metal bonded to the core by means of an adhesive. 0

11. An energy absorbing unit according to any one of the preceding claims in which the second layer comprises a metal and the core comprises a low energy thermoplastic material, for example polypropylene, and the second layer is bonded to the core by means of an adhesive capable of bonding a metal and a low surface 5 energy thermoplastic.

12.An energy absorbing unit according to any one of the preceding claims in which the core of the EAU is a composite structure comprising, in a first area, a thermoplastic material having anisotropic strength properties such that its o compression strength in the direction perpendicular to the surface layers is greater than in the direction parallel to the surface layers and, in a second area, a thermoplastic material having anisotropic strength properties such that its compression strength in the direction parallel to the surface layers is greater than in the direction perpendicular to the surface layers.

5

13. A method of preparing an energy absorbing unit as defined in any one of the preceding claims comprising:

I) bonding a first surface layer to a first face of a core material by heating the face of the core material and/or the first surface layer and l o contacting the said layer with the said face thereby to bond the core and the layer together; and

II) bonding a second surface layer comprising a metallic or plastics material to a second face of the core material opposing the first face, by applying an adhesive to or heating the said second face and/or

15 the said second surface layer and contacting the second layer with the second face thereby to bond the layer to the core material; wherein the core comprises a thermoplastic material having anisotropic strength properties such that its compression strength in the direction perpendicular to the surface layers is greater than in the direction parallel to the surface layers..

20

14. A vehicle part for use in an area of a vehicle susceptible to impact with a human or animal body comprising an energy absorbing unit as defined in any one of claims 1 to 12.

25 15. A vehicle part according to claim 14, the said part being a bonnet comprising an EAU as defined in any one of claims 1 to 12 in which the surface layers provide the upper, outer_.surface and underside surface of the bonnet and the anisotropic core of the EAU is aligned generally perpendicular to the surface layers, the bonnet being able to absorb a force per unit area of 0.1 to 1.5 MPa

3 o from an impact in the direction perpendicular to the surface layers.

16. A vehicle part according to claim 15 in which the first layer comprises a thermoplastic and, in use, is the underside of the bonnet and the second layer comprises a metal and, in use, is the outer or external layer of the bonnet.

17. A vehicle part according to any one of claims 14 to 16 in which the core of the EAU is a composite structure comprising, in a first area, a thermoplastic material having anisotropic strength properties such that its compression strength in the direction perpendicular to the surface layers is greater than in the direction parallel to the surface layers and, in a second area, a thermoplastic material having anisotropic strength properties such that its compression strength in the direction parallel to the surface layers is greater than in the direction perpendicular to the surface layers wherein, in use, the first area is in close proximity to an underlying hard area or point and the second area is located at a point away from an underlying hard point or area.

18. A vehicle comprising a vehicle part as defined in any one of claims 14 to 17, the part being aligned so that the surface layers are generally perpendicular to the anticipated direction of impact.