Wulff Possart

Network structure in epoxy aluminium bonds after mechanical treatment

Abstract Mechanical pretreatment is the oldest and most common technique for improving the performance of adhesive metal bonds. But it is poorly understood, how mechanical pretreatment affects the surface state of the metal adherend and how in turn the pretreated surface influences the properties of the applied adhesive or the whole joint, respectively. This paper describes how mechanical pretreatment by blasting with alumina grit or with glass beads changes the topography and the chemical state of an aluminium adherend. For an epoxy adhesive different network structures evolve in joints of the pretreated aluminium adherend. These network structures are referred to the chemical states of the pretreated adherend.

GRADIENT OF THE MECHANICAL MODULUS IN GLASS–EPOXY–METAL JOINTS AS MEASURED BY BRILLOUIN MICROSCOPY

The newly developed Brillouin microscopy is used for the first time to measure in situ the longitudinal elastic stiffness coefficient in the GHz-range inside of glass–epoxy–metal joints as a function of distance from the substrates. Interphases with a local variation of mechanical properties are quantitatively characterized. These interphases possess unexpected widths of tens to hundreds of microns. Inside the interphases, the spatial variation of the longitudinal stiffness coefficient depends on the type of substrate, on the curing conditions for the epoxy and probably on the distribution of internal stresses. The obtained spatial mechanical profiles provide valuable insight into the morphology-driven mechanics of the interphase, but additional information is needed for a full understanding of their physical and chemical origin. The presented results prove the sensitivity of the Brillouin microscopy; the elastic stiffness coefficients are detected with an accuracy in the subpercentage range. The spatial resolution is better than 10 µm.

Epoxy and Polyurethane Networks in Thin Films On Metals—Formation, Structure, Properties

Thin adhesive films on metals are used as an appropriate model for the characterization of the interphase in polymer‐metal compounds. This interphase is a result of the specific interactions between the adhesive partners. It is particularly relevant for the performance, and the long‐term stability of adhesively bonded structures and coatings. The influence of the metal substrate affects the formation of the polymer network during the polymerization, as well as its resulting structure and properties significantly as compared to the bulk. As examples, a thermosetting epoxy (EP) and a polyurethane (PU) elastomer are chosen for the investigation of the morphology and the chemical structure of the thin films (thickness between 20 nm and 3 μm) on native surfaces of gold, aluminium, and copper. The results for the two types of polymers reveal characteristic differences with respect to reaction rate and final degree of cure as a function of the film thickness on the different metals. The final structure turns out to be a consequence of competing processes such as polyaddition and side reactions including their kinetics, catalysis or inhibition, complexation and dissolution of the complexes, preferential adsorption and phase separation, and the changes in molecular mobility.

Characterization of triazine derivatives on silicon wafers studied by photoelectron spectroscopy (XPS, UPS) and metastable impact electron spectroscopy (MIES)

Abstract Thin films of a diandicyanato bisphenol A (DCBA) prepolymer and thin layers of the trimer of the 2,4,6-tris-(p-cumylphenylcyanate)-1,3,5-triazine (p-CPC trimer), both deposited on silicon wafers convered by their native oxide, have been investigated by X-ray photoelectron spectroscopy (XPS), Ultraviolet photoelectron spectroscopy (UPS) and metastable impact electron spectroscopy (MIES). MIES as well as angle dependent XPS indicate a preferential orientation of the molecules of the first adlayer on the surface. The adsorption of the first layer is governed by the interaction of the trioxytriazine rings with the substrate surface. This adsorption model is supported by the calculated conformation of the DCBA and p-CPC trimer.

Adhesion mechanisms in a cyanurate prepolymer on silicon and on aluminium

Abstract XPS, UPS, MIES, IR reflection spectroscopy and AFM are employed in a study of interaction mechanisms in thin films of a cyanurate prepolymer or various organic model compounds on different surface states of Si and oxidised Al. The trioxytriazine entity is identified as the only chemical part of the substances which can establish an adhesive interaction with the substrates. On the silicon surfaces, the mechanism is identified as a donor–acceptor interaction where the 2p electron lone pairs of N and O in the trioxytriazine act as donors for Si cations. On the Al oxide, the Lewis acidic OH-groups act as electron acceptors which withdraw electron density from the lone pairs at O and / or at N of the trioxytriazine. Moreover, a backdonation of electron density takes place from the metal Al beneath the oxide into the organic layer. The dicyanate monomer does not adhere at all, but desorbs quickly out of the interphase region on the substrates. Hence the thermosetting reaction of the prepolymer will be hampered and the resulting polymer network will be less dense near the substrate than in the bulk.

The Adhesion of Maleic Anhydride on Native Aluminum Oxide: An Approach by Infrared Spectroscopy and Quantum Mechanical Modeling

The bonding between maleic anhydride, a curing agent for epoxy resins, and natively oxidized aluminum is studied by a combination of infrared spectroscopy and quantum mechanical modeling. On native aluminum oxide, maleic anhydride is hydrolyzed to maleic acid in the early stages of adhesion. The acid prefers a bridged chelate bond of one of its acid groups to two Al atoms of the metal oxide. Furthermore, a monodentate bond between a carbonyl oxygen of an acid group and an Al atom is identified. The anhydride itself shows no tendency to adhere on the native aluminum oxide. Since maleic acid is less reactive in the polymerization than the anhydride, the results imply that the chemical curing reaction could be hampered in the vicinity of the Al substrate.

Influence of Composition, Humidity, and Temperature on Chemical Aging in Epoxies: A Local Study of the Interphase with Air

The developing chemical depth profile in an epoxy adhesive bulk (with varying amine content) is monitored during aging by FTIR microspectroscopy on sample cuts prepared with low angle microtomy. Three aging regimes are applied in order to separate the role of temperature and water: dried or moist air (90% rel. humidity) at 60°C and dried air at 120°C for up to 300 days. Quantitative evaluation of the IR spectra shows: thermo-oxidative aging (= dried air) is controlled by the diffusion of atmospheric oxygen. It affects a gradient region of more than 200 μm in depth. At given aging time, the depth profiles depend on temperature, humidity, and on the epoxy-amine ratio. Humidity mainly affects the IR band intensities. The plasticizing effect of water promotes the loss of small network fragments. At 120°C, autoxidation of α-CH2 at ether and amine groups and the oxidative attack on tertiary amines dominate aging. At 60°C in dried air, these processes proceed only very slowly. In the case of amine excess, aging is extended by the additional oxidation of primary and secondary amines to carbonyls. Carbonyls undergo consecutive reactions, especially in the presence of water. Hence, increasing temperature does not simply accelerate the aging mechanisms but it reduces their selectivity and changes their hierarchy. Thus, the long-term aging behavior at moderate temperatures cannot be predicted safely from accelerated aging tests.

The generalized Cauchy relation: a probe for local structure in materials with isotropic symmetry

The elastic properties of the isotropic state of condensed matter are given by the elastic constants ell and c44. In the liquid state the static shear stiffness c44 vanishes whereas at sufficient high probe frequencies a dynamic shear stiffness may appear. In that latter case the question about the existence of a Cauchy relation appears. It will be shown that a pure Cauchy relation can appear only under special conditions which are rarely fulfilled. For all investigated materials, including ceramics, liquids and glasses, a linear relation between ell and c44 called generalized Cauchy relation is observed, which, surprisingly, follows a linear transformation

Acoustic profilometry within polymers as performed by Brillouin microscopy

Using high performance Brillouin spectroscopy we present a new technique, which enables us to perform acoustic microscopy with a spatial resolution of about 1 µm. This technique, called Brillouin microscopy, is tested on several bulk- and film-like polymer samples.

The method of non-linear ultrasound as a tool for the non-destructive inspection of structural epoxy-metal bonds—a résumé

Abstract The method of non-linear ultrasound (NLUS) is frequently discussed as a promising non-destructive inspection tool to detect the deteriorating effect of operational loading on the performance of structural adhesive metal bonds. The non-linear component in the ultrasound pulse propagating across the joint arises in the weakest region of the adhesive. As the weakest region limits the mechanical strength of the bond a correlation between the ultrasonic-mechanical deformation behaviour and the bond strength is expected. This paper reports on efforts to verify such a correlation for epoxy–aluminium bonds undergoing practice relevant hydrothermal or mechanical loading. In conclusion, no corresponding NLUS signal is measured although the joints are aged significantly or strained up to fracture.