Chul Rim Choe

Preparation and characterization of epoxy composites filled with functionalized nanosilica particles obtained via sol–gel process

Abstract To investigate the interfacial effect on properties of epoxy composites, uniform sized silica particles (S) were synthesized by sol–gel reaction and then modified either by substituting surface silanol groups into epoxide ring (S–epoxide), amine (S–NH 2 ) or isocyanate (S–NCO) groups or by calcinating them to remove surface silanol groups (CS). The modified particles are identified by infrared and raman spectroscopy, differential scanning calorimetry (DSC), and particle size analyzer. It has been found that surface modified particles can be chemically reacted with epoxy matrix, which is confirmed by exothermic peaks in DSC thermograms. In scanning electron micrographs of fractured composites, it is observed that the particle dispersion and interface are considerably affected by functional groups of fillers. Weak interfaces and aggregation of particles are observed for composites filled with CS or S–NCO. However, the aggregation of fillers is highly suppressed in composites filled with S–epoxide and S–NH 2 particles. Generally, the coefficients of thermal expansion (CTE) of composites are reduced with an increase of filler contents. Moreover, composites with strong interface exhibit an additional reduction of CTEs. Composites with weak interface show essentially no change in glass transition temperature ( T g ) and damping with filler contents, while composites with strong interface show an increase of T g and a decrease of damping with filler content.

Characterization of functionally gradient epoxy/carbon fibre composite prepared under centrifugal force

Centrifugal force was employed in order to induce a spatial gradient of fibre distribution in the epoxy/carbon fibre system. The gradient structure of the epoxy/carbon fibre composite can be controlled by varying the rotation time and the material parameters, such as fibre length, fibre content and matrix viscosity. The spatial gradient distribution of carbon fibres in an epoxy matrix was achieved by the combined mechanism of packing and settling. The mechanical properties of the functionally gradient epoxy/carbon fibre composite were also investigated. At the same content of carbon fibre, the flexural strength of the functionally gradient composite was higher than that of conventional isotropic composite.

Effect of molecular weight between crosslinks on the fracture behavior of rubber-toughened epoxy adhesives

The effect of molecular weight between crosslinks, Mc, on the fracture behavior of rubber-toughened epoxy adhesives was investigated and compared with the behavior of the bulk resins. In the liquid rubber-toughened bulk system, fracture energy increased with increasing Mc. However, in the liquid rubber-toughened adhesive system, with increasing Mc, the locus of joint fracture had a transition from cohesive failure, break in the bond layer, to interfacial failure, rupture of the bond layer from the surface of the substrate. Specimens fractured by cohesive failure exhibited larger fracture energies than those by interfacial failure. The occurrence of transition from cohesive to interfacial failure seemed to be caused by the increase in the ductility of matrix, the mismatch of elastic constant, and the agglomeration of rubber particles at the metal/epoxy interface. When core-shell rubber, which did not agglomerate at the interface, was used as a toughening agent, fracture energy increased with Mc.

Effects of mixing temperatures on the morphology and toughness of epoxy/polyamide blends

A new mixing process was explored to increase further the fracture toughness and to investigate the toughening mechanisms of epoxy/nylon blend. In this process, without mechanical mixing, the mixtures of epoxy and premade nylon 6 powder were heated without the curing agent to specific temperatures, referred to as the “mixing temperature.” For epoxy/nylon blends, at sufficiently high temperatures, a semi-interpenetrating network-like structure can be developed at the interphase via the reaction between the amine end group and the epoxide group. The depth of interphase and the extent of reaction depends on the mixing temperature. The strong dependency of the fracture energy on mixing temperature reveals the positive effect of the newly developed structure at the interphase. The increase of fracture toughness is possibly due to the enhanced crack fingering bifurcation/deflection mechanism resulting from the lamellae developed in the interphase and the enhanced plastic deformation of epoxy as a result of preyielding of the interphase.

Effect of processing parameters on the mechanical properties of carbonized phenolic resin

Abstract Phenolic resin is carbonized in flowing helium up to 1100C°. The mechanical properties of carbonized products are closely related to the microstructure formed during carbonization. The microstructure of carbonized products is very porous. This porous structure is determined by the rate of gas evolution and the rate of diffusion of evolved gases, which are dependent on heating rate and sample dimension. At lower heating rates, the pore structure is uniform and mainly consists of narrow pores with small radius. With increasing heating rates, the mechanism of pore formation changes significantly. When the heating rate is greater than a critical level, some of the evolved gases create irregular pores and diffuse explosively from the bulk. Therefore, if the heating rate is over a critical level, the flexural modulus of carbonized products suddenly drops due to irregular pore formation in the bulk. The critical heating rate is highly affected by the sample dimension and falls off exponentially with increasing sample thickness.

The effect of crystalline morphology of poly (butylene terephthalate) phases on toughening of poly(butylene terephthalate)/epoxy blends

In an effort to investigate the effect of the crystalline morphology of a poly(butylene terephthalate) (PBT) phase on the toughening of PBT/epoxy blends, the blends, having different degrees of perfectness of the PBT crystalline phase, were prepared by blending PBT and epoxy at various temperatures ranging from 200 to 240 °C. As the blending temperature decreases, the degree of perfectness of the PBT crystalline phase increases as a result of the increase of crystal growth rate. For PBT/epoxy blends, the change in crystalline morphology induced by processing may be the most important cause for the dependency of the fracture energy on blending temperatures. It has been found that PBT phases with a well-developed Maltese cross are most effective for epoxy toughening. This dependency reveals the occurrence of a phase transformation toughening mechanism. Also, the higher relative enhancement of fracture energy of a higher molecular weight epoxy system is further indirect evidence for a phase transformation toughening mechanism. Some other toughening mechanisms observed from the fracture surfaces, such as crack bifurcation, crack bridging, and ductile fracture of PBT phases, have been found to also be affected by the blending temperatures.

Mechanical properties and failure mechanism of the polymer composite with 3-dimensionally stitched woven fabric

The mechanical properties and failure mechanisms of through-the-thickness stitched plain weave glass fabric/polyurethane foam/epoxy composites were studied. Hybrid composites were fabricated using resin infusion process (RIP). Stitched sandwich composite increased drastically the flexural properties as compared with the unstitched fabrics. The breaking of stitching yarns was observed during the flexural test and this failure mode yielded relatively high flexural properties. Composites with stitched sandwich structure improved the mechanical properties with increasing the number of stitching yarns. From this study, it was concluded that proper combination of stitching density and types of stitching fiber is important factor for through-the-thickness stitched composite panels.

Multi-phase toughened epoxy with poly(ether sulphone) and carboxyl-terminated butadiene-acrylonitrile rubber

The structure and properties of ternary blends of epoxy with poly(ether sulphone) (PES) and carboxyl-terminated butadiene-acrylonitrile rubber (CTBN) have been investigated. In these blends, the phase separation occurs in two stages: a macrophase separation during mixing and a microphase separation during curing. At low PES compositions, the PES-rich spherical domains are dispersed. With increasing PES composition, a co-continuous structure develops and, eventually, the phases are inverted. Regardless of structure change, the modulus and yield stress changes with composition just follow the simple rule of mixtures. However, the fracture toughness of these blends exhibits a synergistic effect. Among the various compositions, 5∶5 weight ratio of CTBN to PES exhibited the maximum toughness, which was 140% larger than that calculated from the rule of mixtures. The synergism is believed to be due to the bridging by the PES-rich phase followed by a lowering of the yield stress. The lowering of the yield stress can enlarge the process zone size and the amount of plastic dilatation of the matrix.

Functionally graded polymer composites: Simulation of fiber distribution

Centrifugation is a method to create functionally graded materials (FGM) with a thermosetting matrix. In this study the movement of short carbon fibers in an epoxy resin during the centrifugation process was modeled to determine the fiber distribution in the final product. For this purpose a form factorK was introduced to modify a set of equations that was previously shown to be valid for the motion of spheres. It was shown that the results of the simulation were in good agreement with the experimental data, when an empiricalK factor of four was chosen.

The phase transformation toughening and synergism in poly(butylene terephthalate)/poly(tetramethyleneglycol) copolymer-modified epoxies

The toughening of epoxy modified with poly(butylene terephthalate)/poly(tetra-methylene glycol) (PBT–PTMG) copolymers of various chemical composition was investigated. The fracture toughness of the brittle epoxy was highly enhanced by the inclusion of PBT–PTMG copolymer without loss of other intrinsic mechanical properties, such as modulus and yield stress. These modified epoxies also exhibited synergism in toughening. The remarkable enhancement and the synergism in fracture toughness of PBT/PTMG-modified epoxies is possibly due to the enhancement of the degree of phase transformation toughening, which is a result of the enhancement of the degree of perfectness of PBT spherulites in the presence of PTMG segments. The changes in micro-morphology of PBT/PTMG phases induced by the different chemical composition of copolymer is the most important cause of the dependency of the fracture energy on the processing variables, such as the relative PBT/PTMG composition and total amount of modifiers. Other toughening mechanisms, such as crack bifurcation, ductile fracture of PBT/PTMG phases, main crack-path alteration, and crack bridging, also contributed to toughness enhancement of the modified epoxies.