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Epoxy-based composites of ferrite nanoparticles (50 nm) with 3-glycidoxypropyl- (GPTMS), aminopropyl-(APTMS), or methyl-silsesquioxane (MTMS) coatings are reported. The GPTMS coatings (30-nm thick) allowed uniform particle dispersion in the epoxy and prevented sedimentation of the nanoparticles, whereas the APTMS-coated particles formed agglomerates, leading to particle sedimentation. The particles with the thinnest coating (MTMS - 3 nm) agglomerated in the composites without sedimentation. The composites based on GPTMS-coated particles showed higher fracture toughness than the composites based on MTMS-coated particles. The uniformity and thickness of the coatings were related to alcohol composition of the coating media. Coating removal by a novel ultrasonic etching allowed precise determination of the effective ferrite content in the coated nanoparticles. A markedly lower coercivity for nanoparticles without coatings as compared with the nanoparticles with thicker coatings was observed. The saturation magnetization and the coercivity of the composites were independent of coating and casting procedures. POLYM. ENG. SCI., 51:862-874, 2011. [C]2011 Society of Plastics Engineers, INTRODUCTION Attractive optical, electrical and magnetic properties (1-7) can be explored by polymer-based magnetic nanocomposites, provided that efficient and reliable strategies for the nanoparticlc synthesis, and the surface modification and [...]

The dc breakdown strengths of 4-15 [micro]m thick capacitor grade polypropylene (PP) films were determined. The measurements were performed with an automatic measurement system using five electrodes with areas between 0.045 and 9.3 [cm.sup.2]. Some samples were studied with larger electrodes. Samples were not aged but were measured as received. The measurements were performed in air at room temperature and typically 40-80 breakdowns per sample per electrode area were collected. The Weibull distribution parameters a and p were fitted to the data. Depending on the electrode area and the film grade, the a-parameter values obtained were between 450 and 850 V/[micro]m. The breakdown strength values showed decreasing area dependence with decreasing electrode area and the Weibull scaling law was not valid for the smallest electrode areas. The [alpha]-values for the large sample areas were extrapolated from the small area data by area and Weibull extrapolation. The area extrapolation overestimated the breakdown strength at 4 [m.sup.2] by 40-50% whereas the Weibull extrapolation gave an accuracy of [+ or -]15%. The extrapolated breakdown strengths for the full-scale power capacitors deviated from the capacitor manufacturers test data. This indicated that different defect distributions were responsible for the breakdown strengths in the oil impregnated power capacitors than in the small dry film samples. Index Terms--Polypropylene (PP), biaxially oriented PP (BOPP), breakdown strength, Weibull distribution, area dependence, extrapolation, capacitor, capacitor film, capacitor dielectric.

An automatic method for determining the breakdown strength of thin polymer films was evaluated. The equipment consists of a scanning electrode arm with which the breakdown voltage over the film sample is measured. The measurement electrodes were of solid brass and the ground electrode was of thin aluminum foil. Five different electrode areas between 0.045 and 9.3 [cm.sup.2] were used and 40-100 individual measurements per electrode area per sample were performed. All measurements were performed in air. The Weibull function parameters a and p were fitted to the obtained data. Different features concerning the measurement system and conditions, e.g. the criteria for the automatic detection of the breakdowns, the effect of the electrode edge design, the DC ramp speed, partial discharges and humidity were analyzed. In order to control the reproducibility of the results, also another type of measurement setup was tested. It was concluded that the measured [alpha]-parameter values were stable and repeatable over a period of several years. The average [beta]-parameter values varied [+ or -]1030%, more for the large than the small electrodes, and were also sensitive to changes in the sample, in the measurement conditions and the choice of the electrode system. Index Terms--Breakdown strength, polymer film, polypropylene film, dielectric film, capacitor insulation, measurement method, area dependence, Weibull distribution function.

Blends of an extrusion-grade high-density polyethylene and two liquid crystalline copolyesters (LCP; Vectra A950 and Vectra RD501) were prepared by melt mixing and injection molding, and the morphologies and oxygen permeabilities of the blends were assessed. Scanning electron microscopy revealed that the LCP was present in the blends as mixed oriented bands and small spheres at low LOP contents (4-9 vol%), whereas blends with more than 18 vol% LCP showed LCP lamellae of macroscopic lateral size (mm). Scanning electron microscopy revealed a two-dimensional continuity of the LCP domains in the disc plane due to radial shear deformation and circumferential stretching of the melt leaving the central gate of the disc-shaped cavity. The oxygen permeability, diffusivity and solubility decreased with increasing LOP content of the blends. The decrease in permeability with respect to polyethylene was significant (46%- 55%) already at 9 vol% LCP. At 27 vol% LCP, the decrease with respect to polyethylene, was 92% for the Vectra A950 blend and 98% for the Vectra RD501 blend. These blends showed a greater decrease in diffusivity (86%-92%) than in solubility (39%-76%) with respect to polyethylene, which showed the very pronounced effect of the LOP lamellae on the geometrical impedance factor. Microvoids were present in all the blends despite the use of a very high injection pressure (180 MPa) but their impact on the oxygen permeability was negligible for the Vectra RD501 blends and relatively small for the Vectra A950 blends., 1. INTRODUCTION Thermotropic liquid crystalline polymers (LCPs) are extremely good barriers against permanent gases and larger penetrant molecules. This is the reason for the emerging interest in these materials in [...]