BODY ELEMENT WITH A TRIM PART GLUED TO A COMPONENT

Description

[0001] The invention relates to a vehicle bodywork element comprising an appearance part and a structural part made of plastic material.

[0002] Bodywork elements of this type, particularly tailgates, consist of an inner structural panel made of thermosetting material and an outer panel, or outer skin, made of thermoplastic material, bonded to the intermediate piece and contributing to the exterior appearance of the vehicle.

[0003] The bodywork elements are connected to the vehicle chassis by the structural panel.

[0004] As is known, the structural component is obtained by molding a thermosetting material using a Sheet Moulding Compound (SMC) process. US patent document 8,211,268 B1 discloses a bonded assembly between thermoplastic and thermosetting parts.

[0005] The skin forming the outer face is also a molded part with a thickness of less than 3mm, or even on the order of 1mm, and considered small in relation to the other dimensions of the part, dyed in mass or painted, and resulting from the injection molding of a thermoplastic material such as Polypropylene (PP).

[0006] These two panels are joined by a bead of glue adapted to adhere to each of the panels, whose thickness varies from 0.5 mm to 3 mm.

[0007] The two plastic panels exhibit different thermomechanical behaviors due to their shape, rigidity dimensions, the potentially different molding materials used in their production, and also very different linear coefficients of thermal expansion (LCTE). For example, the skin can have a LCTE of around 40 x 10⁻⁶ mm / °C, such as talc-filled polypropylene, which has a LCTE of around 35 x 10⁻⁶ mm / °C, while the structural component can have a LCTE three to four times lower, such as a glass fiber-reinforced SMC PE, which has a LCTE of around 13 x 10⁻⁶ mm / °C.

[0008] These differences in thermomechanical behavior cause differential expansion between the panels when the temperature rises, particularly when exposed to sunlight. This results in significant stress within the adhesive bead. The deformation of the adhesive bead may then be insufficient to absorb this differential expansion, leading to visible marks when hot on the less rigid part, generally the outer skin. Similarly, after the two panels have cooled and returned to their original shape, the adhesive bead does not always return to its original position. This also creates residual stress that is transmitted to the skin, considered the more flexible part, also causing marks.This residual stress results from a characteristic of the adhesive known as permanent deformation, arising from the adhesive's inelastic nature when stretched. This permanent deformation causes irreversible plastic deformation of the adhesive during mechanical stress, particularly at high temperatures, such as when the vehicle is exposed to sunlight at temperatures often exceeding 80°C or even 100°C.

[0009] Another phenomenon also occurs during the assembly stage of the skin and the intermediate structural piece. Indeed, during the polymerization of the adhesive, and in the absence of thermal deformation of the panels, a shrinkage of the adhesive is observed, which can produce marks on the skin at the adhesive beads.

[0010] These surface defects, visible from the outside, are deemed unacceptable by motor vehicle manufacturers.

[0011] The invention aims to determine the most appropriate descriptive characteristics for selecting an adhesive for assembling the two panels of the vehicle bodywork element without the defects mentioned above.

[0012] The motor vehicle bodywork element according to the invention comprises a thermoplastic appearance panel forming an outer skin and an intermediate structural piece made of thermosetting material having a coefficient of thermal expansion lower than that of the appearance panel, the two pieces being joined by a bead of glue disposed on one face of one of the two pieces opposite the other piece.

[0013] This bodywork component is characterized by the fact that the adhesive possesses both: shrinkage of less than 0.5% during the polymerization phase of the glue, residual deformation of less than 3% for an imposed deformation of 15% at 110°C for a period of 15 minutes.

[0014] Selecting an adhesive with these characteristics thus offers the best performance in the majority of cases where an outer skin is assembled onto a structural part with a lower coefficient of expansion, and is commonly used in the automotive industry.

[0015] The appearance of deformations at the glue beads can thus be avoided, and makes it possible to meet the demanding specifications of car manufacturers.

[0016] The invention also includes, individually or in combination, the following elements: The exterior panel is made of a material selected from one of the following: ∘ Polypropylene (PP) or , ∘ Polycarbonate (PC) or, ∘ Acrylonitrile Butadiene Styrene (ABS) or, ∘ Acrylonitrile Butadiene Styrene / Polycarbonate (ABS / PC) or, ∘ Polycarbonate / Polypropylene Terephthalate (PC / PBT) or, ∘ Polycarbonate / Polyethylene Terephthalate (PC / PET) or, ∘ Polyethylene (PE). The intermediate structural component is made of a material selected from one of the following: ∘ Polypropylene (PP), optionally reinforced with glass fibers (PPGFL type) or talc-filled or, ∘ Polyester matrix SMC or, ∘ Vinylester or, ∘ Epoxy reinforced with glass or carbon fibers. The adhesive bead has a width of less than 25 mm and preferably less than 15 mm. The adhesive bead has a thickness of between 0.8 mm and 3 mm, and preferably between 1 mm and 2 mm.The Young's modulus of the adhesive is between 0.5 MPa and 10 MPa, and preferably between 0.5 and 3 MPa. The adhesive exhibits an elongation at break of between 20% and 800%. The ratio between the coefficient of thermal expansion of the outer skin panel and the coefficient of thermal expansion of the intermediate structural component is between 1 and 5. The outer skin panel has a thickness of between 1.5 mm and 5 mm. The vehicle body component includes, in particular, a tailgate or rear trunk lid, a roof or tailgate spoiler, a hood, or a side door. Brief description of the figures

[0017] The invention will be better understood upon reading the following description and the accompanying figures, which are provided by way of example and are not intended to be limiting, in which: [ Fig. 1 ] There figure 1is a schematic perspective representation of a vehicle's rear tailgate. Fig. 2 ] There figure 2 is a schematic cross-sectional view of the panels at room temperature. Fig. 3 ] There figure 3 is a schematic cross-sectional view of the panels subjected to strong sunlight. Detailed description

[0018] The rear part of the vehicle body 1 as shown in the figure 1 This allows visualization of the various parts that make up the tailgate. The molded intermediate structural part 10 supports the hinges and the tailgate closing elements. An exterior skin panel 20, shown here in two parts, is intended to be bonded to the intermediate structural part 10.

[0019] The structural piece is preferably formed from a polypropylene panel 3 to 4 mm thick, reinforced with 40% glass fibers, whose Young's modulus is around 8000 MPa and whose coefficient of thermal expansion is about 30 10 -6< °K -1< depending on the material chosen.

[0020] If necessary, the intermediate structural part can be produced by an SMC type process.

[0021] The exterior skin panel 20 is made of one or more pieces molded to the external shape of the tailgate, with a thickness ranging from 1 mm to 3 mm. The material chosen for this exterior skin panel is usually polypropylene, which may contain talc. The exterior skin panel may be colored throughout or painted on the side facing outwards from the vehicle.

[0022] The Young's modulus of the filled polypropylene skin is around 1800 MPa and its coefficient of thermal expansion can also vary from 30 to 40 10 -6< °K -1< depending on the material selected.

[0023] If necessary, this appearance panel can be made of Acrylonitrile Butadiene Styrene / Polycarbonate (ABS / PC), Polycarbonate / Polypropylene Terephthalate (PC / PBT), Polycarbonate / Polyethylene Terephthalate (PC / PET) or Polyethylene (PE).

[0024] Depending on the materials chosen for the production of the intermediate structural part 10 and the appearance panel forming the outer skin 20, it is observed that the ratio between the coefficient of thermal expansion of the intermediate structural part and the coefficient of thermal expansion of the outer skin 20 is much greater than 1 and can commonly reach a value of 5.

[0025] There figure 2This is a cross-sectional view of the intermediate structural component 10 and the appearance panel 20 joined by a bead of adhesive 30. Both panels are shown at an ambient temperature of 20°C. No differential expansion is observed. The distance between the two adhesive beads 30, corresponding to the free and deformable length of the panel between these two beads, is equal to L1. For the remainder of this description, and as an example, the distance L1 is equal to 400 mm. The adhesive beads 30 are not subjected to any particular tension.

[0026] When the structural part 10 and the appearance panel 20 are subjected to intense sunlight, the temperature of the materials can rise rapidly and reach, for example, 80°C or even more in some regions.

[0027] There figure 3This allows us to visualize the phenomenon of differential expansion. The structural part 10, whose coefficient of thermal expansion α₁ is equal, for example, to 8 x 10⁻⁶ °K⁻¹, then expands by a value ΔL₁ = α₁ x ΔT x L₁ = α₁ x 60° x 400 = approximately 0.2 mm. And the appearance panel, whose coefficient of thermal expansion α₂ is equal, for example, to 40 x 10⁻⁶ °K⁻¹, then expands by a value ΔL₂ = α₂ x 60° x 400 = approximately 1 mm. This represents a difference (ΔL₂ - ΔL₁) in expansion between the two parts, viewed in the mid-plane and between the two adhesive beads, of approximately 0.8 mm.

[0028] Since the adhesive beads have a Young's modulus much lower than that of the facing panel or the intermediate structural member, they deform in the transverse direction and bulge in the direction perpendicular to the panels. For an adhesive bead with a width L0 of 15 mm, this represents a bead deformation equal to (ΔL2 - ΔL1) / 2 / L0, or approximately 2.6%. Naturally, the greater the temperature difference, the more the panels have different coefficients of thermal expansion, or the further apart the adhesive beads are, the higher this ratio will be.

[0029] This deformation is not purely elastic and causes creep in the material forming the adhesive bead. Upon returning to room temperature, this so-called residual deformation of the adhesive bead can generate parasitic stresses, known as residual stresses, causing a slight deformation at the adhesive joint. The wider the adhesive bead, or the thinner its thickness (e), the greater the residual stress.

[0030] To establish a threshold value for the deformation of the adhesive bead that must not be exceeded, and below which the marking phenomenon is no longer visible, it has been demonstrated experimentally that a residual deformation of less than 3% for exposure to a temperature of 110°C for a period of 15 minutes, made it possible to satisfy the specifications imposed by car manufacturers for most of the materials chosen for the production of vehicle body parts, when the ratio of the coefficients of thermal expansion of the intermediate structural part 10 and the appearance panel 20 remain between 1 and 5, as mentioned above for the materials usually used for the manufacture of vehicle body parts.The test is carried out using a standard duster specimen of glue by comparing, at room temperature, the length of a crosslinked or polymerized glue sample, and the length of the same sample returned to room temperature after having undergone an imposed deformation of 15% at a temperature of 110°C for a period of 15 minutes.

[0031] It has also been observed that the adhesive undergoes shrinkage during the body panel manufacturing process, specifically during the curing stage. This polymerization stage occurs at ambient temperature. This shrinkage also generates permanent stresses in the adhesive bead, which can locally deform the skin at the bead's location, even without the body panel being exposed to high temperatures.

[0032] When the shrinkage of the glue during the crosslinking phase is less than 0.5%, it has also been shown, again experimentally, that the defect related to residual stresses described above no longer appears visibly.

[0033] Respecting other characteristics also helps to reduce the risk of this marking phenomenon occurring.

[0034] Thus, to improve elastic deformation and reduce residual stresses, the glue bead has a width of less than 25 mm and preferably less than 15 mm, and a thickness between 0.8 mm and 3 mm and preferably between 1.5 mm and 3 mm.

[0035] To minimize residual stresses, the Young's modulus of the glue is between 0.5 MPa and 10 MPa, and preferably between 0.5 and 3 MPa.

[0036] Finally, to avoid the initiation of glue failure, we will choose a glue with an elongation at break between 20% and 800%.

[0037] Although many adhesives are likely to meet these requirements, an adhesive compatible with the materials forming the structural part and the appearance panel should be chosen.

[0038] Given the most commonly used materials mentioned previously, a polyurethane (PU) adhesive seems the most appropriate, and generally of the two-component polyurethane type, formed from the mixture of a polyol and an isocyanate, of the silane-modified polyurethane (SPUR) type, or of the silane-modified polyethers (SPE or SPTE) type.

[0039] Adhesives formed by combining polyoxypropylene diamine, in a weight proportion of less than 2.5%, and diphenyl methane diisocyanate or homologous isomers, in a weight proportion of between 2.5% and 10%, methylene diphenyl diisocyanate in a weight proportion of between 2.5% and 10%, 4,4'-methylene diphenylene diisocyanate in a weight proportion of less than 2.5%, and comprising dodecane-1-OL in a mass proportion of less than 25% and calcium carbonate, in a mass proportion of between 50% and 100%, exhibit the desired physical properties. This composition may also include an organosilane, in a mass proportion of between 10% and 25%.

[0040] Similarly, two-component, low-modulus, high-flexibility polyurethane adhesives that cure at room temperature, supplied by AXSON (registered trademark) under the reference ADEKIT H6257, exhibit a residual deformation of 1.8% after being subjected to 15% tension for 15 minutes at 110°C, and a shrinkage of less than 0.5% during the curing phase. Likewise, the adhesive designated under the reference ADEKIT XP3767 / 7, from the same supplier, or under the reference H6258 from the supplier SIKA, demonstrates a residual deformation of 1.7% under the same conditions as those mentioned above, and a shrinkage also of less than 0.5% during the curing phase.

[0041] These two glues give good results and meet the technical characteristics mentioned above. NOMENCLATURE

[0042] 1. Vehicle body. 10. Intermediate structural component. 20. Exterior skin panel. 30. Adhesive bead. L₀ Width of the adhesive bead. L₁ Length between two adhesive beads. e Thickness of the adhesive bead. α₁ Coefficient of thermal expansion of the intermediate structural component. α₂ Coefficient of thermal expansion of the exterior skin panel. ΔL₁ Thermal expansion of the intermediate structural component. ΔL₂ Thermal expansion of the exterior skin panel.

Claims

1. A vehicle body part of a motor vehicle comprising a thermoplastic appearance panel forming an outer skin (20) and an intermediate structural part (10) made of a thermosetting material having a coefficient of thermal expansion lower than that of the appearance panel, the two parts being joined by an adhesive bead (30) disposed on a surface of one of the two parts facing the other part, characterised in that the adhesive is selected from two-component polyurethane adhesives formed by mixing a polyol and an isocyanate, and has both: - a shrinkage of less than 0.5 % during the polymerisation phase of the adhesive, and - a residual deformation of less than 3 % for an imposed deformation of 15 % at 110 °C for a duration of 15 minutes.

2. The vehicle body part of a motor vehicle according to claim 1, wherein the appearance panel (20) is made of a material selected from one of the following: - polypropylene (PP), or - polycarbonate (PC), or - acrylonitrile-butadiene-styrene (ABS), or - acrylonitrile-butadiene-styrene / polycarbonate (ABS / PC), or - polycarbonate / polybutylene terephthalate (PC / PBT), or - polycarbonate / polyethylene terephthalate (PC / PET), or - polyethylene (PE).

3. The vehicle body part of a motor vehicle according to claim 1 or 2, wherein the intermediate structural part (10) is made of a material selected from one of the following: - polypropylene (PP), optionally reinforced with glass fibres (PP GFL type) or filled with talc, or - polyester-matrix SMC, or - vinyl ester, or - epoxy reinforced with glass or carbon fibres.

4. The vehicle body part of a motor vehicle according to any one of the preceding claims, wherein the adhesive bead (30) has a width (L0) of less than 25 mm, and preferably less than 15 mm.

5. The vehicle body part of a motor vehicle according to any one of the preceding claims, wherein the adhesive bead (30) has a thickness (e) between 0.8 mm and 3 mm, and preferably between 1 mm and 2 mm.

6. The vehicle body part of a motor vehicle according to any one of the preceding claims, wherein the Young's modulus of the adhesive is between 0.5 MPa and 10 MPa, and preferably between 0.5 MPa and 3 MPa.

7. The vehicle body part of a motor vehicle according to any one of the preceding claims, wherein the adhesive has an elongation at break between 20 % and 800 %.

8. The vehicle body part of a motor vehicle according to any one of the preceding claims, wherein the ratio between the coefficient of thermal expansion of the appearance panel forming the outer skin (20) and the coefficient of thermal expansion of the intermediate structural part (10) is between 1 and 5.

9. The vehicle body part of a motor vehicle according to any one of the preceding claims, wherein the appearance panel forming the outer skin (20) has a thickness between 1.5 mm and 5 mm.

10. The vehicle body part of a motor vehicle according to any one of the preceding claims, in particular a tailgate or a rear trunk door, or a roof or tailgate spoiler, or a bonnet, or a side door.