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Abstract

The thermal and hydro‐thermal aging of a hot‐cured epoxy system (diglycidylether of bisphenol A (DGEBA) + dicyandiamide (DDA)) in the glassy state is revisited using DSC and IR attenuated total reflection spectroscopy. Because of the diffusion of DDA from the solid particles into the liquid DGEBA matrix, curing produces a highly crosslinked amorphous matrix that contains low crosslinked amorphous regions. After full curing, the network possesses a relatively low molecular mobility and no residual reactive groups. Thermal and hydro‐thermal loading is performed at 60°C, well below the principal glass transition temperature (Tg1 = 171°C). Both aging regimes cause significant chemical and structural changes to the glassy epoxy. It undergoes a phase separation of relatively mobile segments inside the low mobile matrix, providing a second glass transition that shifts from Tg2 = 86–114°C within 108 days of aging. This phase separation is reversible on heating into the viscoelastic state. Hydro‐thermal aging leads to a reversible and a nonreversible plasticizing effect as well. On thermal aging, no chemical changes are observed but hydro‐thermal aging causes significant chemical modifications in the epoxy system. These modifications are identified as a partial degradation of crosslinks produced by the cyano groups of the DDA and correspond to the nonreversible plasticitation. These changes in the cured epoxy should exert an influence on the mechanical properties of an adhesive bond. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci, 2006

An amine cured epoxy is prepared in two different network states for hydrothermal aging. The “low-crosslinked” network has a considerable amount of residual reactive groups and a relatively high-molecular mobility. The low-crosslinked matrix contains high-crosslinked regions. In contrast, the “highly crosslinked” epoxy system has little reactive groups and a lower molecular mobility. Here, low-crosslinked regions are found in a high-crosslinked matrix. Hydrothermal loading for both networks is performed in demineralized water at temperatures below their glass transition. The water plasticizes both kinds of networks which remain in the glassy state, however. As a consequence, in the low-crosslinked epoxy, the increased molecular mobility promotes an ongoing curing reaction leading to the consumption of epoxy groups until an almost complete network has formed. As a new aging process, phase separation occurs in the highly crosslinked epoxy. The new phase is more mobile than the matrix because it has its own glass transition at a lower temperature. In addition, thermooxidative degradation is observed for both network states. Certainly, these chemical and structural changes in the epoxy networks should influence the performance of an adhesive joint, a coating, or a fiber-reinforced composite. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 369–377, 2004

The thermal aging of an amine-cured epoxy in the glassy state is studied for two network states by using DSC and attenuated total reflection-infrared (IR-ATR). The “low-crosslinked” network possesses a relatively high molecular mobility and a considerable amount of residual reactive groups. In the low crosslinked matrix, the presence of high crosslinked regions is revealed. In contrast, the “highly crosslinked” epoxy system has a reduced molecular mobility and only small reactive groups. The high crosslinked matrix contains low crosslinked regions. Thermal loading for both networks is performed below their glass transition. During thermal aging, an ongoing curing reaction takes place in the low-crosslinked epoxy. Thermooxidative degradation and the disintegration of short-range ordering are observed as well. The highly crosslinked epoxy system undergoes a phase separation of relatively mobile segments in the low mobile matrix, which is a reversible process on heating. Thermooxidative degradation is also detected for this kind of network. In summary, for the “low” and the “highly” crosslinked epoxy, significant chemical and structural changes take place during thermal aging even though the networks are vitrified. It is convincing that these changes in the cured epoxy should exert an influence on the mechanical properties of a bonded structure. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 361–368, 2004

The interphase between polymers and metal substrates can be elucidated by studying ultra‐thin films. In this paper, spin coating films (20 nm ‐ 2 μm) are prepared from a two‐part epoxy adhesive on Au, Al and Cu. After room temperature cure, the chemical structure of the epoxy films is studied by FTIR external reflection absorption spectroscopy (FTIR‐ERAS) and the results are compared with the state in the polymer bulk as measured by attenuated total reflection (FTIR‐ATR). Inevitably, this evaluation of the thin film spectra has to account for the optical situation which is significantly different from bulk measurements. Spectra calculation provides the essential tool for a reasonable comparison between bulk and thin film spectra, thus allowing for a detailed quantitative analysis. Thickness and substrate effects on the interphase can then be separated from the optical situation of the measurement.

The interphase between polymers and metal substrates can be elucidated by studying ultra-thin films. In this paper, spin coating films (20 nm - 2 μm) are prepared from a two-part epoxy adhesive on Au, Al and Cu. After room temperature cure, the chemical structure of the epoxy films is studied by FTIR external reflection absorption spectroscopy (FTIR-ERAS) and the results are compared with the state in the polymer bulk as measured by attenuated total reflection (FTIR-ATR). Inevitably, this evaluation of the thin film spectra has to account for the optical situation which is significantly different from bulk measurements. Spectra calculation provides the essential tool for a reasonable comparison between bulk and thin film spectra, thus allowing for a detailed quantitative analysis. Thickness and substrate effects on the interphase can then be separated from the optical situation of the measurement. The results reveal very specific features caused by adhesive interactions and different cure behaviour in the interphase on different metal surfaces.

Polypentapeptides (GVGVP)nwhich are designed in analogy to the connective tissue protein elastin are reported to transform various kinds of energy into mechanical work by the so-called ?Tt-mechanism in cross-linked macroscopic polypentapeptide (PPP) films. In the literature, the responsible element of conformation in such polypeptides is described as a ß- spiral and the ?Tteffect is explained as a sudden change of macroconformation of single polypeptide molecules from an extended but not regular state below a transition temperature Ttto the ß-spiral above Tt. We examined the secondary structure of the linear PPPC(GVGVP)6 in solution with DSC, CD, UV absorption, FTIR and NMR spectroscopy. The results suggest that the ß-spiral is not present in the conformational structure of the PPP molecules. The antiparallel ß-sheet is proposed to be the basic regular part of conformation because it agrees with all spectroscopic data. As a consequence, the elasticity of natural elastin must be considered from a new perspective.

The aluminium–magnesium alloy EN‐AW 5754 is studied with SEM, energy‐dispersive x‐ray imaging (EDX), AES and XPS after treatment by chemical etching and blasting with glass beads or alumina grit. Blasting proves to change more than the surface profile. The freshly treated surfaces are fully covered with a carbon contamination of characteristic oxygen content ‘etch ≪ glass ≪ grit’ and average thickness ‘grit ≪ glass ≪ etch < 10 nm’. Beneath the carbon, an oxide mix is found as a continuous layer on the Al matrix. The oxide Na2O is transferred from both blast materials whereas CaO is observed after glass bead treatment only. Etching almost removes Mg but blasting retains traces of MgO. On the etched surface, the oxide layer (average thickness: glass ≪ etch ≪ grit < 10 nm) is Al2O3. Just after blasting with glass or alumina Al2O2.9and Al2O1.3, respectively, are detected. After glass blasting, these Al atoms continue to oxidize whereas the incomplete Al oxide is almost stable after alumina blasting. These findings show that blasting produces particular chemical states that rule the interaction between the aluminium alloy and another material such as a coating or adhesive. Copyright © 2002 John Wiley & Sons, Ltd.

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.

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.

AbstractTwo major effects are observed during annealing. First, some cyanate groups hydrolyze into carbamate structures on Al and Au. The reaction is driven by the metal surface and both hydroxyl groups and adsorbed water can be involved. On Al, where the carbamate production is most prominent, a part of the cyanate groups in the prepolymer is expected to react with substrate hydroxyl groups. The resulting carbamate coupling to the oxide provides a new chemical adhesion mechanism.As a second effect, an unexpected strong loss of monomer molecules from the prepolymer layers is observed on both substrates at elevated temperatures. It is concluded that the monomer molecules are pushed out of the interphase layer with the substrates due to negative adsorption. Hence, the less mobile, bulky oligomers remain on the substrate and thermal curing of the adhesive must result in a polycyanurate network which is much weaker than in the bulk polymer. This could reduce the mechanical strength of an adhesive joint made with pure polycyanurates and could afford additional chemical modification for practical applications.

FTIR reflection spectroscopy was used to characterize thin films of a cyanurate prepolymer on evaporated aluminium and on silicon single crystal wafers. Both substrates are covered by their native oxides. The optical function of the prepolymer is derived from ATR measurements. The measured thin film reflectance spectra have to be interpreted in comparison with the corresponding bulk spectra obtained by simulation. All samples re-produce the bulk composition. The cyanate groups of the prepolymer are not involved in specific intermolecular interactions or in preferential orientation. This is also found for the triazine rings on Si. On Al, however, a considerable excess of triazine rings are oriented parallel to the interface. The vibration frequencies of the triazine groupings that are perpendicular to the Al substrate show a red shift to some 2-8 c-1. This special interaction effect does not occur on Si. Both the preferential orientation and the specific inter-molecular interaction act at least 100 nm into the prepolymer layer.

A program was developed for IBM-compatible personal computers to separate the peaks of XPS spectra using a Gauss/Lorentz product or a Gauss/Doniach-Sunic model function. The background is removed by common methods.

XPS was applied to PMMA layers of varying thickness in the nanometer range on gold and natural aluminium surfaces, respectively. The numerical decomposition of the core levels into the contributions from the various atomic bonding states reveals some peculiar features of the interphase region to the metals. First, for both metals special mechanisms of thermal destruction are deduced. On Au the splitting of complete methacrylate side groups and, even more intense, the stripping of (-O-CH3)-units is found while on Al the side group scission is verified only. Secondly, the metals reveal different effects on the electron density in the methacrylate groups. Au appears to be inert but the natural Al surface exerts a withdrawing effect on the electrons of both the carbon and the oxygen atoms in the side groups.