• Journal of the Korean Ceramic Society
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• v.41 no.7
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• pp.508-512
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• 2004

For abutment of dental implants, (Y, Nb)-TZP/Alumina composites were prepared by addition of 10-90 vol% alumina at an interval of 10 vol％ into tetragonal zirconia solid solution which consists of 90.24 mol% Zr $O_2$, 5.31 mol% Y$_2$ $O_3$, and 4.45 mol% Nb$_2$O$\_$5/. Biaxial flexure strength and fracture toughness of composite were optimized by adding 10 vol% alumina, which resulted in 900 MPa and 8.9 MPam$\^$1/2/, respectively. The composite did not undergo low temperature degradation even after autoclave treatment at 200$^{\circ}C$ for 10 h. 65 of (Y, Nb)-TZP/Alumina composite abutments were employed into 40 patients and any adverse reaction, screw loosing, or fracture of abutments was not observed for the span of 2 years, indicating that the ceramic abutments can be safely used for restorations.

• Polymer(Korea)
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• v.26 no.1
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• pp.53-60
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• 2002

Polypropylene(PP : continuous phase)/poly (styrene-co-acrylonitrile)(SAN : dispersed phase) blends, and PP/poly(acrylonitrile-butadiene-styrene) (ABS : dispersed phase)blends, containing various amounts of compatibilizer(PP-SAN graft copolymer), were prepared at various shear rates by using twin-screw extruder. In the PP/SAN blend, the average size of the dispersed particles(SAN) was increased with SAN content, while the flexural strength and tensile strength were decreased with SAN content. When the screw rpm was increased from 10 to 60 rpm, the size of the dispersed phase was decreased while the flexural strength and the tensile strength were increased. Maximum mechanical strength and minimum droplet size were observed when the 5 phr compatibilizer was added to the PP/SAN blends. The mechanical strength of PP/ABS blends such as flexural strength and tensile strength increased by adding compatibilizer was reached maximum when blends contained 5 phr compatibilizer.

• Polymer(Korea)
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• v.35 no.1
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• pp.17-22
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• 2011

Polypropylene/multi-walled carbon nanotube(PP/MWCNT) composites along with various MWCNT contents up to 20 wt% were prepared by a twin screw extruder. Nanocomposites having 20 wt% MWCNT as a master batch(M/B) were diluted with PP by way of melt compounding. The electrical/thermal conductivity, morphology, thermal/viscoelastic/mechanical properties were investigated with the variation of MWCNT contents. Also, we compared some properties between 1-step PP/MWCNT and the diluted PP/MWCNT composites. The percolation threshold of electrical and thermal conductivity was measured at about 3 wt% MWCNT. And conductivity of diluted PP/MWCNT composites were superior to those of PP/MWCNT composites. The non-isothermal crystallization temperature and thermal decomposition temperature appeared at higher temperatures with increasing MWCNT contents. Morphology showed that length of MWCNT in diluted PP/MWCNT composites was shortened by twice melt blending, which contributed to improve the tensile strength of PP/MWCNT composites.

• Clean Technology
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• v.12 no.4
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• pp.198-204
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• 2006

The modification of biodegradable polycaprolactone was accomplished by reactive extrusion with various contents of free radical initiator. Reaction conditions were in the temperature range of $130^{\circ}C$ to $180^{\circ}C$ with initiator contents of 0.1, 0.3, 0.5 and 1.0 wt%. To characterize the modified polycaprolactone (PCL), molecular weight was measured by gel permeation chromatography(GPC) and thermal, mechanical and rheological properties as well as biodegradability were measured. The modified PCL (MPCL) with 1% of initiator showed ca. 20% increase in crystallinity and ca. 50% increase in tensile modulus. Also, a large increase in rheological properties such as complex viscosity, storage and loss modulus was observed. The biodegradability of most MPCL was higher than that of virgin PCL.

• Elastomers and Composites
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• v.46 no.3
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• pp.223-230
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• 2011

Considering of utilizing renewable resources and recycled plastics, green composites consisted of recycled polyethylene (PE) as matrix and eco-friendly natural fibers as reinforcement were processed and characterized in the present study. First, the wood flour/recycled polyethylene pellets with different wood flour contents were prepared by twin-screw extrusion processing. Using the pellets, wood flour/recycled polyethylene green composites were fabricated and the effects of wood flour loading on their flexural, tensile, impact properties, heat deflection temperature and fracture behavior were investigated. It was concluded that the flexural strength, flexural modulus, tensile modulus and heat deflection temperature of wood flour/recycled polyethylene green composites were increased with wood flour, whereas the tensile strength and impact strength were decreased. The fracture behavior observed by means of scanning electron microscopy supported qualitatively the tendency of the impact strength with wood flour loading, compared with the ductile fracture pattern of recycled polyethylene.

• Elastomers and Composites
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• v.45 no.2
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• pp.129-136
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• 2010

In this work, PLLA/EGMA blends were prepared by melt blending of biodegradable Poly-L-lactic acid(PLLA) with Poly(ethylene-co-glycidyl methacrylate)(EGMA) and Engage as impact modifiers by twin screw extruder. Blend compositions of PLLA/Impact modifier blends were 100/0, 75/25, 50/50, 25/75 and 0/100, respectively. Also, Talc was added to 3 PLLA rich phases on PLLA/EGMA blends. The morphology, viscoelastic/mechanical properties were characterized by FESEM, DMA, UTM and Izod impact tester. DMA and Izod impact test data showed that storage modulus at room temperature with increasing EGMA and Engage contents decreased, and impact strength increased. However, storage modulus at room temperature increased by adding talc. From FESEM image, we observed that domain phase was well dispersed into matrix. Although the tensile strength and flexural modulus were decreased with increasing the content of EGMA and Engage in them, they could be supplemented by adding talc.

• Journal of the Korea Concrete Institute
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• v.15 no.3
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• pp.433-442
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• 2003

This paper presents a hypoplastic model for three-dimensional analysis of concrete structures under monotonic, cyclic, proportional and non-proportional loading. The constitutive model is based on the concept of equivalent uniaxial strains that allows the assumed orthotropic model to be described via three equivalent uniaxial stress-strain curves. The characteristics of these curves are obtained from the ultimate strength surface in the principal stress space based on the Willam-Warnke curve. A cap model is added to consider loading along or near the hydrostatic axis. The equivalent uniaxial curve is based on the Popovics and Saenz models. The post-peak behavior is adjusted to account for the effects of confinement and to describe the change in response from brittle to ductile as the lateral confinement increases. Correlation studies with available experimental tests are presented to demonstrate the model performance. Tests with monotonic loading on specimens under constant lateral confinement are considered first, followed by biaxial and triaxial tests with cyclic loads. The triaxial test example considers non-proportional loading.

• Proceedings of the Korea Concrete Institute Conference
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• 2008.04a
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• pp.693-696
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• 2008

The biaxial tensile strength of concretes was measured by the Biaxial Flexure Test(BFT) which was recently developed to measure the biaxial tensile strength of concretes. From the test result, The circular specimen is generally fractured after 1${\sim}$3 of the initial crack were formed on the top of specimen. The direction and number of the initial crack was completely arbitrary. As the specimen was larger, the number of the crack increased. And, the strengths of the different radii and thickness of specimens were calculated by the commercial finite element program to study the size effect of the biaxial tensile strength like the uniaxial tensile strength. The parameters such as radii to the support and to the load point, were studied using the program. The results of the FE analysis were entirely consistent with the predictive solution, when b/a>0.4, and the thickness of the specimens were increased. On the other hands, those with lesser free length showed good results.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.20 no.6
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• pp.719-730
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• 2007

This study presents a thin-walled space frame element based on the classical Timoshenko beam theory. The element is derived according to the assumed strain field in order to resolve the shear-locking phenomenon. The shape function is developed in accordance with the strain field which is assumed to be constant at a 2-noded straight frame element. In this study, the geometrically nonlinear analysis applies the Corotational procedure in order to evaluate unbalanced loads. The bowing effect is also considered faithfully. Two numerical examples are given; monosymmetric curved and nonsymmetric straight cantilever. When these example structures behave lateral-torsional bucking, the critical loads are obtained by this study and ABAQUS shell elements. Also, the post-buckling behavior is examined. The results give good agreement between this study and ABAQUS shell.

• Elastomers and Composites
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• v.41 no.3
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• pp.172-181
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• 2006

Flame retardant synergism of phyllite was studied in ABS resins containing brominated flame retardant(tetrabromobisphenol A(TBBA) or brominated epoxy oligomer(BEO)) and antimony trioxide($Sb_2O_3)$. Talc was used for the comparison purpose. ABS compounds were manufactured by a twin-screw co-rotating extruder and subsequently injection molded into several specimen for mechanical and thermal properties. Flame retardancy of ABS compounds measured by UL 94 vertical test with 1.6 mm thick bar specimen was enhanced by the replacement of antimony trioxide into phyllite or talc in the range of 12.5%(0.5 wt%) to 37.5%(1.5 wt%). Phyllite showed better synergistic effect comparing with talc especially for BEO. Only phyllite enhanced the flowability of ABS compounds. Notched Izod impact strength decreased with the proportion of phyllite or talc content. Phyllite could replace the antimony trioxide up to the content of 25%(1 wt%) to give better flame retardancy and flowability without darkening problem.