Adhesion for structures made of polymers and metals – ASPOME

The plastic-metal hybrid technology was developed to bridge the gap between the two worlds of metal and plastics. The technology allows the combination of the advantages of metals (stiffness, strength, ductility, low cost) and plastics (function integration, low density). It is now extensively used in automotive but it is currently limited to semi structural applications such as front end carriers or other bolt on body in white parts.
The standard hybrid technology consists in placing a metal insert inside an injection mold and to over-mold it with plastics. The plastic goes through holes and wraps around the insert edges. The link between the metal and the plastic is mainly mechanical and the shear bond strength if limited to 2 MPa.In the case of structural parts, high strength steel is used and not a preferred solution to have holes in a load bearing structure. For these two reasons, the hybrid technology is not currently used for load bearing structures.

The objective of the ASPOME project is to extend the use of the hybrid technology to load bearing structural parts. To reach this target, the direct adhesion of injected thermoplastics on a metal or composite structural substrate will be developed. The substrates will be surface-treated by atmospheric pressure cold plasma process. Atmospheric plasma treatment is currently used mainly for cleaning surfaces, activation of surfaces, and surface coating. It is not yet used for the purpose of the study.
The surface treatment will be adapted by ENSCP to the substrate (steel with or without e-coat treatment, aluminum, continuous fiber reinforced polyamide composites) and the over-molded plastic to reach adhesion strength between the injected plastic and the insert of 20 MPa or more. To reach this target, the atmospheric plasma treatment will be coupled if needed with the deposition of a thin layer of a primer that can be adapted to the substrate. The effect of physical treatment of the insert (heating before injection, grit blasting) will also be assessed on the quality of the interface.
The effect of mechanical loading and ageing of the structure on the quality of the interface between the injected polymer and the surface-treated substrate will be evaluated by Mines de Paris. A mechanical model of the interface between the thermoplastic and the surface-treated substrate will be developed to enable an improved simulation of load bearing polymer metal structures
To define the conditions, two case studies will be developed by the industrial partners FAS and FAE.
The first case study is a seat structure element with multi-axial static, crash, and fatigue conditions. The structure and the interface will also be evaluated for long term ageing for interior conditions. The recyclability of the polymer-metal hybrid structures will be assessed, as well as the life cycle analysis of the component.
The second case study is a front end carrier with multi-axial static, fatigue and vibration conditions. The structure and the interface will be evaluated for long term ageing for under the bonnet conditions (temperature, humidity, and resistance to fluids).

In conclusion, ASPOME has the following objectives
1. Adhesion strength between polymer and metal or composite substrate of 20 MPa minimum by direct injection of the polymer
2. Development of surface treatments adapted to the substrate and the over-molded thermoplastics
3. Evaluation of the effect of mechanical loading and aging on the quality of the interface
4. Simulation of the structures in static and in crash by modeling the interface
5. Development of a cost effective process for direct adhesion of injected thermoplastics on a metal substrate and application to load bearing structures in large series
6. Replacement of seat structures made in metal by hybrid structures metal/plastics/composite)
7. Assessment of the recycling of hybrid structures and life cycle analysis