New bonded assembly configuration for dynamic mechanical analysis of adhesives

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Abstract

A new sample configuration has been developed in order to study molecular mobility of an adhesive in a bonded assembly configuration by dynamic mechanical analysis. The torsional rectangular mode is used to provide a shear solicitation all along the adherend/adhesive interface. The initial mechanical properties of each assembly's constituent are first investigated as reference. The modulus of aluminum foils used as substrates exhibits a classic elastic component and a slight viscous part due to microstructural changes or stress relaxation. Four relaxation modes are highlighted and identified for epoxy adhesive tested as a bulk material. Its viscoelastic behavior is compared to the one of adhesive tested in assembly configuration. The relaxation modes of the adhesive remain visible in spite of the sample stiffening by aluminum foils. Relaxation modes comparison shows that the temperature of loss modulus associated with the mechanical manifestation of glass transition slightly increases for the assembly configuration. Energy losses during this relaxation are much higher in the assembly configuration. Influence of rigid aluminum substrates is discussed in terms of the adhesively bonded joint solicitation mode.

Introduction

Dynamic Mechanical Analysis is a technique commonly used for mechanical characterization of materials [1]. It consists in subjecting a sample to controlled mechanical oscillation and measuring its response. The data collected allow us to determine the viscoelastic properties of bulk polymeric material. Molecular mobility can also be analyzed through primary and secondary relaxations of polymer as temperature is scanned [2]. This technique is particularly suitable to provide information about changes in molecular mobility or in physical properties of a polymeric system due to introduction of fillers [3], ageing [4] or changes in chemical formulation or in manufacturing process [5].

In the past decade, dynamic mechanical analysis has found some applications in the field of adhesion. However, previously reported dynamic mechanical analysis results on adhesives have been focused mainly on the behavior of adhesive resins only as bulk material [6], [7], [8]. A few studies are dedicated to adhesive in a configuration of bonded assembly. Most of them use this technique for evaluating the curing of thermosets [8], [9], [10] as an alternative to differential scanning calorimetry. Influence of environmentally induced ageing [11], [12] or specific parameters to bonding process, like surface pretreatment of substrates [13], [14], is also investigated.

There has been controversy on whether the adhesive properties in the thin film form (adhesive joint) are the same as the corresponding bulk properties. However, in many applications, it is crucial to take into account the intrinsic properties of adhesive joint [15] because it conditions the assembly strength, stiffness and durability [16]. In several mechanical investigations, a good agreement between the two configurations has been found [17], [18], [19], [20], [21]. Based on other experiments, some authors highlight differences in the mechanical behavior of adhesive depending on the kind of sample configuration [22], [23], [24]. These properties can differ due to changes in chemistry resulting from specific interactions with the adherends during the curing reaction. Existence of a diffused interphase at the boundary substrate/adhesive is mentioned [25], [26], [27], [28]. Another explaination can be the complex state of stress in adhesive which affects measurements: test specimen used often presents non-uniform states of stress in the adhesive bond line.

The authors mainly compared static mechanical properties and comparing relevantly to other parameters which characterize macromolecular structure of the adhesive. Dynamic mechanical analysis is a relevant technique to examine molecular mobility of adhesive's chain sequences through its relaxation processes. Solid samples can be tested in a torsional analyzer as bars that are twisted about their long axis. Samples are inexpensive and easy to make. This test geometry is expected to provide a shear solicitation all along the interface.

Aim of this study [29] is to explore the feasibility of performing dynamic mechanical testing for adhesively bonded joint. A sample configuration is developed and optimized to be tested in torsion. This configuration is expected to be representative of a usual bonded assembly using aluminum substrate and a commercial epoxy adhesive. Data resulting from this new kind of sample are compared with the ones resulting from the experiment carried out with a bulk configuration.

Section snippets

Adhesive and substrates

The adhesive is a commercial (3 M) amine-epoxy bi-component adhesive. The two parts are prepared and a nozzle allows us to make and extrude the mix with an accurate repeatability. The hardener (part A) is a mix of several components where aliphatic amine is preponderant. The part B is based on diglycidyl ether of bisphenol-A epoxy resin mixed with other components (fillers, catalyst…). This adhesive is toughened by a blend of polybutadiene and thermoplastic copolymers. Parts A and B are mixed at

Preliminary experiments

Fig. 2 shows the storage modulus G′ as a function of the angular frequency for the three kinds of tested configuration. Measurements have been achieved at room temperature. All the values are not influenced by the frequency rate; the adhesive G′ increase (≈0.2 GPa) is not significant compared to the high aluminum modulus (≈30 GPa). Adhesive is tested as a bulk material. The value of the storage modulus is about 1 GPa. It is consistent with values frequently measured for thermoset polymer. Aluminum

Conclusion

A new configuration for dynamic mechanical analysis, in the torsional mode, has been developed and tested in order to study the mechanical behavior of an epoxy adhesive in a bonded assembly configuration i.e. in functional conditions. The anelastic behavior of these bonded joints, mainly governed by the viscoelasticity of adhesive, has been compared with the one of bulk adhesive. The mechanical response of the adhesive in an assembly configuration is different from the one of bulk adhesive.

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