Yoshitaka Takezawa

Influences of inorganic fillers on curing reactions of epoxy resins initiated with a boron trifluoride amine complex

Abstract Inorganic fillers are widely used for epoxy resins to improve mechanical and thermal properties. However, inorganic fillers may unexpectedly affect the curing reactions of such epoxy resins. In our present study, influences of inorganic fillers on the curing reactions of phenol novolac epoxy resin (EPN) initiated with boron trifluoride ethylamine complex (BF3MEA) are detailed. The gel times of epoxy resins containing alumina (Al2O3) fillers were longer than those of corresponding unfilled epoxy resins, indicating that Al2O3 fillers delayed the curing reactions of epoxy resins. Mechanisms were proposed and influences of aluminum hydroxide (Al(OH)3), silica (SiO2), and aluminum fluoride (AlF3) fillers were also described.

Low excess losses in a Y-branching plastic optical waveguide formed through injection molding

We have demonstrated low excess losses (1.9 dB at 660-nm wavelength) in a Y-branching plastic optical waveguide (POWG) that was fabricated using an injection-molding method. The waveguide had an amorphous vinyl polymer as the core and transparent polyolefin as the cladding. We then studied a method for isolating the excess loss in the Y-branching POWG, and with that method we estimated the lower limit of the loss to be 1.41 dB at 660 nm. The sample had a heat-resistant plastic optical fiber (POF) with a core composed of crosslinked poly (methyl methacrylate) (PMMA) copolymer, and a cladding composed of poly (tetrafluoroethylene-co-hexafluoropropylene). The POWG has sufficient reliability for ordinary uses below 100 °C. A model for a bidirectional wavelength-division multiplexing opticalcommunication system with the developed Y-branching POWG and the POF was also demonstrated.

Thermal and mechanical properties of liquid crystalline epoxy resins as a function of mesogen concentration

Liquid crystalline thermosets are known to exhibit a number of improved characteristics in comparison with traditional plastics. We now describe quantitatively how the thermal and mechanical properties of a number of liquid crystalline epoxy resins vary with polymer mesogen concentration, using biphenyl- and biphenol-based diepoxides with varying spacer lengths and a range of aromatic diamine curing agents. Although macroscopically isotropic, the degree of order at the microdomain level can be related to the network physical properties, and thus increasing mesogen content leads to increased intermolecular interactions between the polymer chains and hence reduced micro-Brownian motion and reduced free volume. These properties are manifested as increased elastic moduli at high temperatures, reduced thermal coefficients of expansion, increased decomposition onset temperatures and reduced solvent absorption.

Preparation and characterization of thermoconductive polymer nanocomposite with branched alumina nanofiber

A highly thermoconductive resin with branched filler is reported. The filler was fabricated by calcination of aluminium isopropoxide adsorbed onto paper filter (cellulose fibers). Scanning electron microscopy and x-ray diffraction measurement demonstrated that the filler consists of α-alumina nanofiber with a branched structure. The thermal conductivity of the filler-resin nanocomposite was two to five times larger than that predicted by the well-known Bruggeman equation, which postulates the composite with spherical filler in the matrix. The branched shape of the α-alumina nanofiber increased the probability of formation of phonon paths with lower thermal resistance, leading to the high thermal conductivity of the nanocomposite.

Highly Thermoconductive Polymer Nanocomposite with a Nanoporous α-Alumina Sheet

A highly thermoconductive insulative polymer nanocomposite with a nanoporous alpha-alumina sheet was reported. The thermal conductivity of the nanocomposite along the surface normal was 12 W m(-1) K(-1) (41 vol % alumina), a value as high as that predicted theoretically for a nanocomposite with thermoconductive fillers that form a perfectly connected thermoconductive network. The high thermal conductivity is probably due to the continuous alpha-alumina phase that functions as an efficient phonon path in the nanocomposite. The results suggest that the structure of the filler is important for the design of highly thermoconductive materials.

High thermal conductive epoxy resins with controlled high order structure

Present electrical devices have large calorific power, and improvement of heat dissipation has been a very important subject. In this paper, we developed the novel epoxy resins which increased the thermal conductivity that has been a barrier to heat dissipation. The medium of thermal conduction for insulating resins is phonons. Phonon conduction depends on the crystallinity, since it is a lattice vibration. The scattering of phonons happens on the interface of an amorphous structure. If there is a macroscopic amorphous structure although crystal structure exists on the microscopic level, we expected that high thermal conduction could be attained by reduced scattering of phonons through controlling the nano scale structure. Using an epoxy resin which has the mesogen structure would solve this problem because its easy to carry out an orientation with this structure. As a result, we confirmed that thermal conductivities become larger when the amounts of mesogens were increased. The epoxy resin (A) contains biphenyl mesogen, and the epoxy resin (B) contains even bigger mesogenic units. The epoxy resin (A) was about two times higher than the conventional epoxy resin, and the epoxy resin (B) was able to attain five times as much thermal conductivity as the conventional one.

Thermal deterioration diagnosis by optical fiber sensors for mica-epoxy insulation of HV induction motors

Optical diagnosis for aged mica-epoxy insulation of HV (3.0 to 6.6 kV), enclosed induction motors used in compressors has been studied. The developed optical fiber sensor consists of two optical fiber cables (one for light transmission, the other for light reception; each /spl sim/3 m long), two types of laser diodes as light sources, and an optical power meter as a detector. It was used to measure the change /spl Delta/A/sub R/ in the reflective absorbance of insulation at two wavelengths for model pieces and samples obtained from three aged induction motors. This optical sensor could detect only deterioration on the surface layer as a /spl Delta/A/sub R/ change, and thus deterioration of the inner layer was evaluated by the peak temperature of tan /spl delta/ (T/sub g,DMA/) in dynamic mechanical analysis (DMA). The T/sub g,DMA/ of insulation is one of the most important parameters which can be used to evaluate material lifetimes, because it relates to elasticity. The /spl Delta/A/sub R/ correlated with the T/sub g,DMA/ of DMA, and the correlation coefficient of an accelerated experiment using model pieces was /spl sim/0.79. Thus, this optical diagnosis can be applied to evaluate the deterioration of mica-epoxy insulation of HV induction motors. The extent of insulation deterioration can be calculated quantitatively using the diagnostic master curve, which is obtained from accelerated heating experiments based on chemical kinetics.

Development of a portable diagnostic apparatus for coil insulators in low-voltage induction motors

When most organic insulators are exposed to thermal stress, they turn darker in color. The increase of electronic transition absorption due to thermal oxidation of the resin is responsible for this darker coloration. A correlation between the degree of aging of these insulators and their reflective absorbances in the near IR (infrared) wavelength range has been established previously. In this paper, a novel non-destructive diagnostic apparatus using an optical fiber sensor developed to evaluate the aging of thermally deteriorated insulators is reported. The key point of this technique is measuring the change of reflective absorbance ratio at two different wavelengths. The sensor consists of two plastic optical fibers (for light transmitting and receiving), a plastic optical coupler, two kinds of near-IR light emitting diodes as light sources, and an optical power meter as a detector, and hence is very compact and lightweight (<2 kg). The aging of an insulator can be estimated using the diagnostic curve, which is obtained by an accelerated heating experiment using model pieces. We applied this measurement technique to predict the residual life of low-voltage induction motors used in elevators.

High thermal conductive epoxy resins with controlled high-order structure [electrical insulation applications]

The thermal conductivities of resins can be improved by introducing a high-order structure having microscopic anisotropy while maintaining their macroscopic isotropy. We studied four kinds of diepoxy monomers with a biphenyl group or two phenyl benzoate groups as mesogens, and cured them thermally with an aromatic diamine curing agent. Their thermal conductivities were up to 5/spl times/ higher than those of conventional epoxy resins, because the molecular groups, mesogens, form highly ordered crystal-like structures which suppress phonon scattering. We confirmed the existence of crystal-like structures in the epoxy resins directly using TEM observation. We also observed mezoscopic structures in the resins using an AFM. The results suggest a novel method to improve the thermal conductivities by controlling the high-order structures. Furthermore, the laminates were prepared with the high thermal conductive epoxy resin containing a biphenyl group and ceramic fillers as a part of a feasibility study. Thermal conductivities more than 10/spl times/ higher than ordinary epoxy resin laminates were obtained for test pieces of the laminates.

Delamination mechanism of high-voltage coil insulators made from mica flakes and thermosetting epoxy resin

To clarify the delamination mechanism of high-voltage coil insulators made from mica flakes and epoxy resin due to static mechanical stress, the relationships between the shear strength of the insulator and the physical properties of the component materials were studied. The mechanism of their delamination was thought to be either a lack of epoxy resin between the mica flakes, interface failure between the mica flakes and the epoxy resin, or cleavage of the mica flakes. The first two mechanisms were discounted because the shear strength of the insulator was found to be independent of both the contact angle of the corresponding liquid epoxy resin on the mica flakes and the critical surface tension of the epoxy resin. Furthermore, the shear strength of the model insulator was improved by using an epoxy resin with a higher bending elastic modulus, implying that the delamination mechanism in this system is the cleavage of mica flakes. Therefore, the epoxy resin should have a high elastic modulus to ensure high delamination resistance, that is, the temperature to which the insulators are exposed should be lower than the glass transition temperature of the corresponding epoxy resin. Optical microscope studies also supported these results.