Viscoelastic properties of steel/polymer/steel sandwich structures

Polymer lamined-steel sandwiches are used for their vibrations and sound damping properties in the automotive industry and household applications. The studied sandwich assemblies are constituted by two steel sheets and one thin polymer layer (ethylene vinyl acetate copolymer EVA) inserted between them. In this work, the influence of the thickness of the polymer on the damping properties of the sandwiches has been analysed. Dynamic mechanical measurements have been performed on steel/polymer/steel sandwiches. The mechanical transition temperature Tmech at which tan delta goes through a maximum is related to the glass transition of the polymer, and reflects the mobility of the polymer chains. The results show that for high polymer thicknesses, Tmech remains constant and corresponds to the glass transition temperature of the bulk copolymer. However, when the polymer thickness decreases, Tmech increases greatly. This increase can be explained by the reduced mobility of the polymer chains near the interface. These observations suggest the presence of an interphase, localized in the vicinity of the interface with the substrate, for which the polymer properties differ from that in the bulk. Migration and orientation of the polar acetate groups of the EVA towards the polar steel surface and consequently crystallinity modifications are able to explain the reduction of the chains mobility.