Such a part is in particular able to form an automotive sound-proofing assembly. Such an assembly is intended to resolve the acoustic problems that arise in a substantially closed space, such as the passenger compartment of a motor vehicle (mat, roof, door panel, etc.), near noise sources such as an engine (fire wall, etc.), pneumatic contact with a road (wheel passage, etc.), etc.
In general, in the low frequency domain, the acoustic waves created by the aforementioned noise sources undergo a “damping” by materials in the form of single or double sheets (pre-stressed sandwich) having a viscoelastic behavior or by acoustic attenuation of a porous and elastic mass-spring system.
Within the meaning of the present invention, a soundproofing assembly provides “insulation” when it prevents the entry of medium and high frequency acoustic waves into the soundproofed space, essentially by reflecting waves toward the noise sources or the outside of the soundproofed space.
A soundproofing assembly operates by “sound absorption” (in the medium and high frequency field) when the energy from the acoustic waves dissipates in an absorptive material.
A high-performance soundproofing assembly must work both by providing good insulation and absorption. To characterize the performance of such an assembly, the notion of noise reduction (NR) index is used, which takes into account the notions of insulation and absorption: this index can because related using the following equation:NR(dB)=TL−10log(S/A)
Where TL is the sound transmission loss index (hereinafter referred to as the loss index) reflecting the insulation. The higher this index is, the better the insulation is.
A is the equivalent absorption surface. The higher A is, the better the absorption is.
To produce good soundproofing, for example for a motor vehicle passenger compartment, it is desirable to implement a set of materials that will make it possible to use these two concepts wisely. This has been described in many articles, in particular in the article “Faurecia Acoustic Light-weight Concept” by A Duval from 2002 during the 2002 SIA/CTTM conference in Le Mans.
In particular, it is desirable to obtain light assemblies, recyclable if possible, having a sufficient absorption and retaining a high insulation performance level. To that end, WO03/069596 describes that complexes comprising a foam base layer, a tight layer (also called heavy mass) with a low surface density, and a porous layer, have been developed.
These complexes provide an excellent compromise between absorption and insulation, while retaining appropriate lightness.
To improve the recyclability, EP 2,170,576 describes an acoustic complex including a foam base layer, a porous upper layer, and a tight intermediate layer formed by penetration of the precursor material of the foam base layer in the porous layer.
In the method described in this patent, it is difficult to control the thickness, and therefore the mass, of the tight intermediate layer, which often leads to an overuse of raw materials and therefore an excess cost. Indeed, the composition of this layer depends on the porosity of the upper layer (which is adapted based on the application), which will allow more or less of the precursor material to penetrate.
Likewise, FR 2,979,308 describes an acoustic complex including a foam base layer, a tight intermediate layer, and a porous so-called stiffening foam layer because of its stiffness in bend. The choice of a high stiffness in bend for the stiffening layer provides optimized acoustic properties in terms of insulation.
The manufacture of these complexes includes arranging at least one porous layer in a foaming mold, then introducing a precursor material of the foam base layer, which expands.
However, in some cases, the manufacture of the aforementioned complexes may prove complicated. Indeed, the expansion of the precursor material causes a compression of the porous layer.
In some cases, in particular when the porous layer is particularly stiff in bend, it may be crushed abruptly under the effect of the pressure from the foaming material. This phenomenon causes a sudden increase in the volume in which the precursor material may foam, causing the foam of the base layer in formation to collapse.
This phenomenon is comparable to what one skilled in the art knows as “collapse”. Indeed, the sudden increase in volume occurs while the foam is in formation, at a moment when the walls of the cells are not yet cross-linked, and are therefore not very strong. The rapid increase in volume causes an expansion of the gases, which causes the destruction of the walls. The skeleton of the foam collapses and then forms a sort of skin.
The complex then has deteriorated mechanical and acoustic properties.
Another drawback of these methods lies in the fact that the cells of the base layer can remain partially closed, limiting the porosity, and therefore the acoustic performance, of the base foam. This closing of the cells also causes volatile organic compounds (VOCs) from the foaming reaction to be trapped, which will nevertheless be able to be gradually discharged by migration during the use of the product.