Dynamic Mechanical Properties of Thin Adhesive Joints - METTLER TOLEDO

Dynamic Mechanical Properties of Thin Adhesive Joints

The mechanical properties of polymer-metal adhesive joints were studied as a function of the thickness of the adhesive layer using DMA. The glass transition temperature and the effective crosslinking density were evaluated from the shear modulus measurement curves. The results show that both quantities are strongly dependent on the thickness of the polymer layer. This is due to the formation of an interphase in the contact region of polymer and metal. The properties of the interphase depend on the metal used.

 

Introduction

The mechanical properties of adhesive joints and composite materials are largely determined by the viscoelastic behavior of the polymer used. The viscoelastic properties of the polymers depend on the temperature and deformation conditions in a complex way.

With metal-polymer adhesive joints, the mechanical properties of the adhesive joint are strongly influenced by interactions in the region between the surface of the metal and the polymer.

An interphase is formed, whose influence on the adhesive joint will be discussed in this article. The interphase is responsible for the adhesion between the polymer and the substrate. In composite materials, the interphase determines the interactions between the matrix polymer and the filler.

In general, any adhesive bonding mechanism immobilizes the polymer molecules in contact with the metal surface. The adhesive interactions lead to preferential orientation of the adhesive molecules close to the metal contact surface and trigger a tendency for polymer components to separate. The influence of the metal surface on the polymer structure and dynamics acts over a relatively large range.

Several articles describe the formation of such interphases in adhesive joints and report concentration gradients in the chemical composition of the adhesive in the contact region with the metal substrate [1].

These primary effects can lead to changes in other interphase properties such as mechanical properties and the distribution of internal mechanical stresses. This means that the mechanical properties of an adhesive-substrate composite also depend on the thickness of the adhesive layer.

In a joint with a thin adhesive layer, the interphase plays a much greater role than in a thick adhesive joint. In practice, the mechanical behavior of adhesives and of composite materials is often characterized by tension or bending experiments. These measurements are however not sufficient to completely describe the mechanical properties of adhesive joints because they do not take the influence of the substrate into account.

In this article, we investigate the suitability of dynamic mechanical analysis (DMA) for characterizing the relationship between the thickness of the layer and the effective mechanical behavior using adhesive joints of different thickness.

Conclusions

Metal-polymer adhesive layers with a polymer thickness down to about 5 μm were measured in the shear mode using the METTLER TOLEDO DMA861e. The measurements were performed in the linear viscoelastic range using a displacement amplitude of 0.3% of the sample thickness.

The glass transition temperature, Tg, is defined as the peak temperature of the loss modulus, G". The effective crosslinking density is determined from the temperature dependence of the storage modulus, G', in the rubbery plateau. Neither of these two quantities provides information about local properties in the adhesive joint because they are determined as mean values averaged over the whole sample volume.

The relationship between the layer thickness and the effective mechanical properties of gold-polyurethane and aluminum-polyurethane adhesive joints points to the existence of a mechanical interphase in such adhesive joints. In the interphase, cooperative mobility is significantly reduced and the crosslinking density increased. In an aluminum adhesive joint, the influence of the interphase can be detected to an adhesive layer thickness of more than 600 μm. With gold adhesive joints, it is only about 400 μm.

The DMA861e is an excellent instrument for the characterization of adhesive joints and other composites due to its high stiffness, high sensitivity for the displacement measurement, and external sample preparation. A detailed description of the investigations performed on PU-gold adhesive joints is given in reference [1].

 

Dynamic Mechanical Properties of Thin Adhesive Joints | Thermal Analysis Application No. UC 435 | Application published in METTLER TOLEDO Thermal Analysis UserCom 43