Jae-Rock Lee

Cure behavior of diglycidylether of bisphenol A/trimethylolpropane triglycidylether epoxy blends initiated by thermal latent catalyst

Cure behaviors of diglycidylether of bisphenol A (DGEBA)/trimethylolpropane triglycidylether (TMP) epoxy blends initiated by 1 wt % N-benzylpyrazinium hexafluoroantimonate (BPH) as a cationic latent catalyst were investigated using DSC and rheometer. This system showed more than one type of reaction and BPH could be excellent thermal latent catalyst without any co-initiator. The cure activation energy (Ea) obtained from Kissinger method using dynamic DSC data was higher in DGEBA/TMP mixtures than in pure DGEBA. Rheological properties of the blend system were investigated under isothermal condition using a rheometer. The gel time was obtained from the analysis of storage modulus (G′), loss modulus (G″) and damping factor (tanδ). The crosslinking activation energy (Ec) was also determined from the Arrhenius equation based on the gel time and curing temperature. As a result, the crosslinking activation energy showed a similar behavior with that obtained from Kissinger method. And the gel time decreased with increasing TMP content, which could be resulted from increasing the activated sites by trifunctional epoxide groups and decreasing the viscosity of DGEBA/TMP epoxy blend in the presence of TMP.

Isothermal cure kinetics of epoxy/phenol‐novolac resin blend system initiated by cationic latent thermal catalyst

The investigation of the cure kinetics of a diglycidyl ether of bisphenol A (DGEBA)/phenol-novolac blend system with different phenolic contents initiated by a cationic latent thermal catalyst [N-benzylpyrazinium hexafluoroantimonate (BPH)] was performed by means of the analysis of isothermal experiments using a differential scanning calorimetry (DSC). Latent properties were investigated by measuring the conversion as a function of curing temperature using a dynamic DSC method. The results indicated that the BPH in this system for cure is a significant thermal latent initiator and has good latent thermal properties. The cure reaction of the blend system using BPH as a curing agent was strongly dependent on the cure temperature and proceeded through an autocatalytic kinetic mechanism that was accelerated by the hydroxyl group produced through the reaction between DGEBA and BPH. At a specific conversion region, once vitrification took place, the cure reaction of the epoxy/phenol-novolac/BPH blend system was controlled by a diffusion-control cure reaction rather than by an autocatalytic reaction. The kinetic constants k1 and k2 and the cure activation energies E1 and E2 obtained by the Arrhenius temperature dependence equation of the epoxy/phenol-novolac/BPH blend system were mainly discussed as increasing the content of the phenol-novolac resin to the epoxy neat resin.

Influence of silane coupling agents on the surface energetics of glass fibers and mechanical interfacial properties of glass fiber-reinforced composites

The effect of various silane coupling agents on glass fiber surfaces has been studied in terms of the surface energetics of fibers and the mechanical interfacial properties of composites. γ-Methacryloxypropyltrimethoxysilane (MPS), γ-aminopropyltriethoxysilane (APS), and γ-glycidoxypropyltrimethoxysilane (GPS) were used for the surface treatment of glass fibers. From contact angle measurements based on the wicking rate of a test liquid, it was observed that silane treatment of glass fiber led to an increase in the surface free energy, mainly due to the increase of its specific (or polar) component. Also, for the glass fiber-reinforced unsaturated polyester matrix system, a constant linear relationship was observed in both the interlaminar shear strength (ILSS) and the critical stress intensity factor (KIC) with the specific component, γS SP, of the surface free energy. This shows that the hydrogen bonding, which is one of the specific components of the surface free energy, between the glass fibers and coupling agents plays an important role in improving the degree of adhesion at the interfaces of composites.

Studies on epoxy resins cured by cationic latent thermal catalysts: The effect of the catalysts on the thermal, rheological, and mechanical properties

To investigate the effect of catalysts on the thermal, rheological, and mechanical properties of an epoxy system, a resin based on diglycidyl ether of bisphenol-A (DGEBA) was cured by two cationic latent thermal catalysts, N-benzylpyrazinium hexafluoroantimonate (BPH) and N-benzylquinoxalinium hexafluoroantimonate (BQH). Differential scanning calorimetry was used for the thermal characterization of the epoxy systems. Near-infrared spectroscopy was employed to examine the cure reaction between the DGEBA and the latent thermal catalysts used. The rheological properties of the blend systems were investigated under an isothermal condition with a rheometer. To characterize the mechanical properties of the systems, flexure, fracture toughness (KIC), and impact tests were performed. The phase morphology was studied with scanning electron microscopy of the fractured surfaces of mechanical test samples. The conversion and cure activation energy of the DGEBA/BQH system were higher than those of the DGEBA/BPH system. The crosslinking activation energy showed a result similar to that obtained from the cure kinetics of the blend systems. The flexure strength, KIC, and impact properties of the DGEBA/BQH system were also superior to those of the DGEBA/BPH system. This was a result of the substituted benzene group of the BQH catalyst, which increased the crosslink density and structural stability of the epoxy system studied.

Thermal stability and mechanical behavior of cycloaliphatic–DGEBA epoxy blend system initiated by cationic latent catalyst

The effect of the composition of an epoxy blend based on a cycloaliphatic (CAE) and diglycidyl ether of bisphenol A (DGEBA) epoxides containing N-benzylpyrazinium hexafluoroantimonate (BPH) as a thermal or UV latent initiator was investigated in the context of their thermal stability and mechanical properties. The compositions of a CAE–DGEBA blend were varied within 100:0, 80:20, 60:40, 20:80, and 0:100 by mole percent. Latent properties were measured by the degree of conversion. As a result, the thermal stability characterized from the initial decomposition temperature (IDT), the temperature of maximum weight loss (Tmax), the integral procedural decomposition temperature (IPDT), and the decomposition activation energies by TGA increased when the DGEBA composition was increased. According to the mechanical measurements, the flexural and tensile strengths increased with an increase of the DGEBA composition because of the compact hydrogen bond, long repeat unit, and bulky side groups of the DGEBA, while both the elastic and tensile moduli decreased. This latter result was attributed to the DGEBA intermolecular interaction, resulting in a toughened three-dimensional network, which dispersed the internal stress.

Relationship between surface characteristics and interlaminar shear strength of oxyfluorinated carbon fibers in a composite system

In this work, a direct oxyfluorination method was used to study the effect of oxygen content on surface and mechanical interfacial characteristics of oxyfluorinated carbon fibers in an epoxy matrix system. The changes of surface functional groups, chemical compositions, and structures of the carbon fibers were characterized by Fourier transform infrared spectrometer, X-ray photoelectron spectroscopy, and X-ray diffraction measurements. Also, the mechanical interfacial properties of the composites were evaluated by means of interlaminar shear strength tests. The results indicated that graphitic carbon was the major carbon functional component on the carbon fiber surfaces and other functional groups were also present, such as Cz.sbnd;O, Cz.dbnd;O, HOz.sbnd;Cz.dbnd;O, Cz.sbnd;F(x), after oxyfluorination of carbon fibers. No large changes of structure were found with the content of oxygen. Consequently, these introductions of oxygen functional groups onto the carbon fiber surfaces led to an improvement of the ILSS of the composites.

Bending fracture and acoustic emission studies on carbon–carbon composites: effect of sizing treatment on carbon fibres

A comparative study using mechanical flexural tests and acoustic emission was carried out to determine the effect of carbon fibres with and without sizing treatment in carbon–carbon composites during the carbonization process. The composites had been fabricated in the form of two-directional polyacrylonitrile based carbon fibres impregnated with phenolic resin. It was found that the composites made with unsized polymer fibres result in better mechanical properties. Also the data obtained from acoustic emission appeared to show that the composites made with unsized fibres were more ductile. These results may be the result of the degree of adhesion at the interface between the fibre and the matrix. The bulk density and apparent porosity were measured as functions of the number of densification cycles and the results were correlated with the mechanical test results.

Effect of oxidation inhibitor on the low energy tribological behavior of carbon–carbon composites

Abstract In this study, the tribological performance of carbon–carbon composites impregnated with different amounts of MoSi2 as an oxidation inhibitor were investigated. The results of the friction tests indicated that the carbon–carbon composites underwent an abrupt transition of the coefficient of friction at the frictional temperature range of 150–180°C. And the composites made with MoSi2, exhibited lower frictional coefficient and wear rate in comparison with the composites made without MoSi2. The composites made with 4 wt% MoSi2 showed an improvement in activation energies for wear resistance when compared with the other composites under the present condition. These results were probably due to the consequence that the friction and wear properties of carbon–carbon composites are sensitive to the friction temperature and can be largely dependent on the adhering force between fibers and matrix–MoSi2, the reduction of porosity, and the formation of a lubricative, powdery, debris film, formed on the friction surfaces of the carbon samples.

Synthesis of crystalline polyimide film by ionized cluster-beam deposition

Polyimide thin films were deposited by the ionized cluster-beam deposition technique. Imidization and crystallization of polyimide films were investigated using transmission electron microscopy and Fourier transform infrared spectroscopy. Polyimide films deposited under optimum conditions showed a maximum imidization and good crystal structure, which is superior to that of films fabricated by other techniques.

Characterization of the impact properties of three-dimensional glass fabric-reinforced vinyl ester matrix composites

Three-dimensional glass woven fabric-reinforced composites (3-D composites) were fabricated by impregnating vinyl ester resin in a hand lay-up procedure for the honeycomb sandwich structure. Three kinds of 3-D had been investigated in terms of the impact properties as a function of the z-direction fiber length, i.e., 3, 4.5 and 8 mm in core structures. In this work, more precise impact behaviors were studied in the context of differentiating between initiation and propagation energies, and ductility index (DI) along with maximum force and total energy as a useful measure. As a result, the highest impact properties were reached in the case of the specimens with 3 mm length of z-direction fibers, but the lowest DI was observed in the same specimens with 3 mm length of ones, probably due to the effect of buckling. The damage photo analyses after impact tests were also discussed in terms of a hollowing on the front face against the load applied, and a whitening spread in the impact point on the back face of the fabric composites studied.