Model Epoxy System Research Articles

Tack of epoxy resin films for aerospace-grade prepregs: Influence of resin formulation, B-staging and toughening

Aerospace-grade prepreg resin films based on multifunctional tetraglycidyl-4,4′-methylenedianiline (TGMDA), triglycidyl p-aminophenol (TGAP), Bisphenol A diglycidyl ether (DGEBA) and curing agent 4,4'diaminodiphenyl sulfone (DDS) are investigated in terms of tackiness by probe testing. The model epoxy systems are modified regarding the thermoplastic toughener content (polyethersulfone, PES) and the B-stage level, which is adjusted by cure prediction based on a model-free isoconversional method (Flynn-Wall-Ozawa). Additional DSC and rheological analysis are performed to study the thermal and viscoelastic material behavior in conjunction to its impact on temperature-dependent tack. Maximum achievable tack is found to decrease as a function of both degree of conversion and toughener content. Meanwhile, both influencing factors shift the tack maximum towards higher temperatures corresponding to increased flow characteristics attributed to evolving network formation and the incorporation of high molecular weight PES. In terms of absolute tack level and corresponding temperature, probe tack values similar to commercial prepreg systems (∼100 μJ mm−2) are recorded for TGMDA-based formulations containing 10 wt% PES at 20% pre-cure. Model formulations, which have neither been exposed to B-staging nor toughened, show exceptionally high tack below room temperature for all investigated epoxy prepolymers and are therefore not considered processable by automated fiber placement.

Interphase effect on the effective moisture diffusion in epoxy–SiO2 composites

Epoxy Mold Compounds (EMC) are used to protect integrated circuits (IC) from environmental influences, with one of these influences being moisture ingress, causing corrosion. To obtain the needed thermal and mechanical properties EMCs require a high loading of (silica) fillers, introducing a large amount of interface. While silane coupling agents promote good binding, they have shown to introduce an interphase volume that exhibits a faster moisture transport between epoxy and SiO2 in glass fiber filled epoxy. In this work, we investigate if such an interphase volume is also introduced by the filler particles in EMC and if it influences the moisture diffusion coefficient of the composite. We compare moisture uptake measurements performed by dynamic vapor sorption (DVS) with predictions from effective medium theory, as well as with numerical simulations based on micro-CT scans of our samples for a model epoxy system containing different filler levels and commercial EMC samples with two different filler levels. From the measured DVS data, we observe an effective diffusion coefficient, that is higher than predicted for an absence of any interphase for both the EMC and the model system. This suggests that an interphase layer should be present.

Chemical Interactions at the Al/Poly-Epoxy Interface Rationalized by DFT Calculations and a Comparative XPS Analysis.

A metal-polymer interface is pertinent to numerous technological applications, especially in spatial sectors. The focus of this work is to elaborate on the metallization process of the poly-epoxy surface with aluminum thin films, using atomistic details. To this end, X-ray photoelectron spectroscopy (XPS) under ultrahigh vacuum and density functional theory calculations are employed. The interfacial bonding between Al atoms and the poly-epoxide surface, represented by a dimer model, is studied by determining adsorption energies and by simulating XPS spectra. The latter simulations are mainly performed using the ΔKS method, taking into account the initial and the final state effects. Simulated atom-by-atom metal deposition on model epoxy systems is attempted to further elucidate energetics of metallization and preferential arrangement of metal atoms at the interface. A fair agreement obtained between XPS experiments and computations rationalizes the interaction mechanism at the atomic scale explaining the formation of the Al/poly-epoxy interface. Electronic structure properties highlight the charge transfer from the Al atom(s) to dehydrogenated model epoxy system.

Scratch behavior of model epoxy resins with different crosslinking densities

The scratch behavior of model epoxy resins with varying degrees of crosslink density was investigated according to the ASTM D7027/ISO 19252 scratch test methodology. Three model epoxy systems with low, medium, and high crosslinking densities were prepared to systematically investigate how the scratch behavior might be influenced. The effect of crosslinking density on the onset of groove, crack and plowing formation during scratching was investigated. Coefficient of friction measurements, uniaxial tensile and compressive stress-strain curves, and dynamic mechanical analyses were conducted to correlate the intrinsic material properties to the observed scratch-induced deformation of the model epoxy systems. The findings indicate that the scratch behavior of epoxy resins can be directly correlated to their material properties. It is found that both the tensile strength and compressive yield stress determine the resistance against damage formation during scratch. Additionally, a comparison between pencil hardness and scratch test is presented to assess the suitability of both methods for evaluating scratch performance of polymers with significantly different mechanical properties.

Effect of pigment and temperature onto swelling and water uptake during organic coating ageing

Free films and coatings of a model epoxy system, with or without pigment (TiO2), were aged in saline solution at different temperatures (30°C, 40°C and 50°C). The water uptake was evaluated using gravimetry for free film and using electrochemical impedance spectroscopy (EIS) for coatings. In parallel, the swelling of coatings was in situ monitored using scanning electrochemical microscopy (SECM).It was shown that the temperature has no effect on the water uptake values for unpigmented systems and a slight effect onto swelling. For pigmented systems, the water uptake values slightly increase likely due to additional water absorption at the pigment/binder interphase. The swelling of pigmented coating was found to be lower than that of unpigmented coatings for all ageing temperatures. This result was explained by particular internal stresses that develop in pigmented system.Water uptake values obtained from gravimetry and EIS, using the Brasher and Kingsbury equation, were different for all the temperatures for unpigmented and pigmented coatings. Using a modified Brasher and Kingsbury equation that considers the experimental swelling obtained by SECM, the same water uptake values were obtained for all systems and all temperatures. Finally, it appears that the coating swelling is a key parameter to evaluate water uptake in organic coatings, especially when different coatings are compared.

Microstructural reorganization in model epoxy network during cyclic hygrothermal ageing

The water sorption characteristics and physico-chemical properties have been determined for a fully cured model epoxy systems based on the DGEBA resin and DAMP amine hardener, during cyclic hygrothermal ageing (absorption-desorption-reabsorption-redesorption). Gravimetric measurements on epoxy free films were carried out to follow the water sorption kinetics at different temperatures (30, 40, 50 and 60 °C). The water diffusion processes follow a pseudo-Fickian behaviour with two diffusive stages during the first sorption and a Fickian behaviour during the first desorption. During the first sorption, a plasticization phenomenon is observed leading to a decrease of rigidity and Tg but this phenomenon is reversible, showing that water reacts only with the polymeric network via low energy links. The water diffusion processes evolve during the second absorption-desorption cycle. They follow a Fickian behaviour which implies a disappearance of diffusion heterogeneity of the first sorption. This behavior was explained by a microstructure reorganization during cyclic ageing highlighted through the FTIR and DMA techniques: chain fractions became accessible to the water after being relaxed in the first absorption-desorption cycle. A thermodynamic approach applied to solubility and diffusion processes corroborates these results. This study allows to a better understanding of diffusion phenomena in epoxy model systems.

Influence of pigment on the degradation of anticorrosion polymer coatings using a thermodynamic analysis of electrochemical impedance spectroscopy data

As commercial paints have a complex composition, this work was carried out on a model epoxy system (DGEBA/DAMP) with or without pigment (TiO2). Firstly, physico-chemical properties of the composites free films were studied by using DSC and DMA and compared to those of the sole resin. Then, composites films were applied onto steel panels and were immersed in 3wt.% NaCl solution at different temperatures (30, 40, 50 and 60°C). The water uptake and diffusion processes were followed by Electrochemical Impedance Spectroscopy (EIS). Using a thermodynamic approach, it was shown that the diffusion process is accelerated in presence of pigments due to internal stresses that develop in composite systems. However, these stresses are relaxed by water sorption during ageing. Finally, it appears that the monitoring of water sorption by EIS allows to characterize the mechanical effects at the coating/substrate interface during immersion.

Impact of polar groups concentration and free volume on water sorption in model epoxy free films and coatings

Abstract The water sorption characteristics have been determined for two model epoxy systems based on the DGEBA resin and two amine hardeners (TETA and DAMP). A particular attention was paid to the curing protocol to control the macromolecular architectures. Using FTIR, DSC and DMTA measurements, it was demonstrated that both network microstructures differ mainly in the polar groups density, the potential crosslinking nodes and the free volume. DGEBA/DAMP system presents a higher free volume fraction and a lower polar groups concentration compared to the DGEBA/TETA system. Gravimetric measurements on free films and EIS experiments onto coated steel Q-panels were carried out to follow the water sorption kinetics at different temperatures (30, 40, 50 and 60 °C). For both epoxy systems, the diffusion processes follow a pseudo-fickian behaviour with two diffusive stages. Solubility and diffusion processes are discussed in relation with the microstructural specificities of the polymer networks using a thermodynamic framework. The main result is that in DGEBA/DAMP and DGEBA/TETA systems, the water diffusion and solubility are mainly governed by a water polar group interaction mechanism.

Cure profiles, crosslink density, residual stresses, and adhesion in a model epoxy

The influence of network chemical composition and cure conditions in a model epoxy system was investigated. Addition of 1,4-butanediol to a bisphenol-F epoxy resin cured with 4-methyl-2-phenyl imidazole led to a decrease in the modulus and glass transition temperature, which resulted in lower residual stresses. Moisture uptake increased with the addition of 1,4-butanediol and is associated with an increase in the free volume of the epoxy. However, even with greater moisture uptake, the addition of 1,4-butanediol to the epoxy increased its adhesion to quartz, primarily through lower residual stress in the samples and increased energy dissipation on debonding. Differences in residual stress for different cure conditions were not measurable in these systems. However, increasing the cure time increased the adhesion of the epoxy to quartz.

Debonding Under Fatigue Loading at Polymer/Inorganic Interfaces

AbstractThe mechanisms associated with cycle-by-cycle damage accumulation resulting in fatigue crack propagation between a highly constrained polymer layer and an adjacent elastic substrate are explored. Specifically, cyclic fatigue-induced crack growth between a bisphenol F model epoxy system and a passivated silicon substrate under Mode I loading is reported. Preliminary findings regarding the effects of fatigue load ratio on interfacial crack growth rates are presented. While intermediate crack growth rates were significantly accelerated under cyclic loading, the near-threshold crack growth behavior under cyclic and monotonic loading was surprisingly similar.

Physical properties of a bisphenol-F epoxy containing a silica filler treated with silane coupling agents

A model epoxy system consisting of a diglycidyl ether of bisphenol-F epoxy resin, 1,4-butanediol, and cured with 4-methyl-2-phenylimidazole has been investigated. Thermal analysis indicated that 3 parts per hundred resin (phr) is the optimum amount of curing agent for this system. The influence of silane-treated amorphous fumed silica fillers on properties of the cured epoxy was also examined. Silica particles were treated with 3-aminopropyldiethoxymethylsilane (APDS) and3-aminopropyltriethoxysilane (APTS) coupling agents. No change in glass transition temperatures was observed with the addition of the filler (with or without coupling agents) to the epoxy. Addition of the filler led to a slight increase in the activation energy for the glass transition; however, no change in activation energy was observed when using the coupling agent. Addition of either coupling agent to the filler surface led to an increase in cooperativity. Fumed silica also did not significantly affect moisture diffusion properties, but a small decrease was observed in the moisture saturation mass with the addition of silica particles treated with APDS.

Cure monitoring of epoxy matrices for composites by ultrasonic wave propagation

Although ultrasonic wave propagation is a well-known technique for nondestructive analysis, it can be also applied for dynamic mechanical characterization (DMA) of polymers and composites. Most of DMA characterizations at ultrasonic frequencies are performed on thermoplastics and only a few articles are available on the characterization of the reactive properties of thermosetting resins. Therefore, in this work a complete characterization of the cure of a model epoxy system is presented, by comparing isothermal and nonisothermal data. The propagation of ultrasonic waves acting as a dynamic mechanical deformation at high frequencies can be used for the calculation of complex longitudinal bulk moduli during the cure of the epoxy resin. The evolution of attenuation and velocity during reaction is related to the strong physical changes occurring during the cure process. Furthermore, a comparison between the degree of reaction measured by Differential Scanning Calorimetry and ultrasonic data is proposed. The ultrasonic velocity (or the bulk longitudinal modulus) can be considered the most interesting parameter for cure monitoring because it follows the growth and evolution of the mechanical stiffness of the resin during cure. In particular, the obtained results suggest that the measurement of longitudinal velocity or L′ could be exploited for on-line measurements of post-gel properties. Finally, an immediate correlation is also proposed between the gel time and the end of cure and the ultrasonic data. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 1969–1977, 1999

Isothermal curing of an epoxy resin by alternating differential scanning calorimetry

The quasi-isothermal curing of a diepoxide resin with a triamine of polyoxypropylene was studied by alternating differential scanning calorimetry (ADSC), which is a temperature modulated DSC technique. The complex heat capacity measurements allows to analyse the vitrification process at curing temperatures ( T c) below the maximum glass transition of the fully cured epoxy ( T g ∞ =85.8°C). Initially, the modulus of the complex heat capacity, |C ∗ p| , increases until a maximum (conversion between 0.42 and 0.56) and then decreases. This step is followed by an abrupt decay of |C ∗ p| , due to the vitrification of the system, which allows the determination of the vitrification time. This value agrees well with that determined by the partial curing method. The phase angle and out-of-phase heat capacity show an asymmetric wide peak during the vitrification process. The change in |C ∗ p| at vitrification decreases with the increase of T c becoming zero at temperature T g ∞ . This epoxy-triamine system shows a delay of the vitrification process respect to other model epoxy systems probably due to the presence of polyoxypropylene chains in the network. The decay of |C ∗ p| during vitrification may be normalised between unity and zero by defining a mobility factor. This mobility factor has been used to simulate the reaction rate during the stage where the reaction is controlled by diffusion. The observed reaction rate is simulated by the product of the kinetic reaction rate, determined by the autocatalytic model, and the mobility factor.

The Influence of Curing Agent Content on the Mechanical and Adhesive Properties of Dicyandiamide Cured Epoxy Systems

Abstract Mechanical and adhesive properties were evaluated for an elastomer-modified model epoxy system cured with varying amounts of dicyandiamide (Dicy). The modulus and bulk fracture toughness values were found to be independent of Dicy content, whereas the adhesive performance was shown to be greatly influenced. For increases in Dicy content, single lap shear (SLS) failure values increased, while quasi-static double cantilever beam (DCB) performance was decreased. These adhesion test results are discussed in a comparative fashion. Preliminary x-ray photoelectron spectroscopy (XPS) surface evaluations are presented to discuss the drastic changes in DCB performance. It is postulated that in these model adhesive systems the Dicy variations produce significant chemical changes only in the epoxy/steel interphase region.

Activation energy determinations from dielectric thermal analysis of reacting polymeric systems

Methods were developed for calculating reaction activation energies from dielectric property measurements during isothermal and nonisothermal cure of thermosetting polymers. These methods were derived by assuming that features of the dielectric response occurred at unique extent of reaction values. Activation energy results were obtained through dielectric analysis for two thermosetting resin systems: a model epoxy system — tetraglycidyl-4, 4′-diaminodiphenylmethane cured with 25 parts per hundred of 4,4′-diaminiphenylsulfone hardener; and a model dicyanate blend resin, 55% (by weight) of bisphenol A dicyanate and 45% of tetrao-methyl bisphenol F dicyanate. Results were in excellent agreement with activation energies determined from differential scanning calorimetry measurements.