• Composites Research
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• v.12 no.2
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• pp.91-100
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• 1999

The manufacture of high strength polyethylene(HSPE) tape yarns has been accomplished by a solid state processing(SSP) method as the compaction of ultra-high molecular weight polyethylene(UHMWPE) powders and drawing of the compacted film under the melting point without any organic solvents. In this study, the characteristics of HSPE tape yarns produced by SSP which is desirable for production cost and environmental aspect were analyzed. As the results, tensile strengths of HSPE tape yarns increased with increasing the draw ratio and the fracture morphology of highly drawn HSPE tape yarns showed more fibrillar shape than the low drawn one. Interfacial shear strengths of HSPE tape yarns with vinylester resin increased by $O_2$ plasma treatment and maximum interfacial shear strength was obtained in the plasma treatment condition of 100W and 5min. In addition, mechanical properties of HSPE tape yarn reinforced composites were investigated and compared with those of the gel spun HSPE fiber reinforced composites.

• Composites Research
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• v.34 no.5
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• pp.277-282
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• 2021

Graphene oxide can be reduced to graphene under supercritical fluid condition even without using a specific reducing agent or applying a high thermal process. In this study, a process for converting graphene oxide into graphene was studied under supercritical fluid conditions in methanol and ethanol solvents. When the structure of asprepared graphene was analyzed by using FE-SEM and XRD, the reduction of graphene oxide in supercritical fluid condition was more affected by the change of solvent than other variables such as concentration of graphene oxide and reaction time. The use of ethanol showed better results for the reduction than the use of methanol. The graphene prepared in this study was mixed with epoxy resin up to 20 wt.% to make composites, and the thermal conductivity of the composites were analyzed. Thermal conductivity of the composite increased proportionally with graphene loadings. The graphene prepared in supercritical ethanol condition was more effective on the thermal conductivity of the composite.

• Composites Research
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• v.33 no.6
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• pp.341-345
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• 2020

We propose a measurement method for evaluating constituent contents of carbon fiber/epoxy composites through a thermogravimetric analyzer (TGA). The sample used in the test was taken from a strand specimen made of carbon fiber/epoxy tow prepreg, and the change in weight of the sample over time was measured in real time. Using a field emission scanning electron microscope (FE-SEM), we examine the thermal damage condition of the carbon fiber depending on whether resin was removed or not. We find that it was possible to test even a small amount of sample when using TGA vis-à-vis using a conventional muffle furnace. In addition, TGA enables the temperature and exposure time to be controlled, allowing the constituent contents of composite materials to be efficiently and quantitatively evaluated.

• Applied Chemistry for Engineering
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• v.22 no.3
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• pp.266-271
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• 2011

Single-walled carbon nanotube (SWCNT)/Epoxy composites were prepared for improving thermal conductivities and dispersion of SWCNTs in the epoxy matrix. Composites obtained different types of SWCNTs which are pristine and functionalized of the SWCNTs by acid and amine treatments. Three types of SWCNTs were dispersed in diglycidyl ether of bisphenol A (DGEBA) and bisphenol F (DGEBF). Enhanced interaction between functional groups on SWCNT and epoxy resins was evidenced by an improvement in the dispersion of the SWCNTs in the epoxy matrix. Thermal conductivity of composites containing acid SWCNTs were found to be much better than those containing pristine and amine treated SWCNTs.

• Restorative Dentistry and Endodontics
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• v.46 no.4
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• pp.51.1-51.13
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• 2021

Objectives: This study aimed to evaluate the effect of improper positioning single-peak and multi-peak lights on color change, microhardness of bottom and top, and surface topography of bulk fill and incremental composites after artificial aging for 1 year. Materials and Methods: Bulk fill and incremental composites were cured using multi-peak and single-peak light-emitting diode (LED) following 4 clinical conditions: (1) optimal condition (no angulation or tip displacement), (2) tip-displacement (2 mm), (3) slight tip angulation (α = 20°) and (4) moderate tip angulation (α = 35°). After 1-year of water aging, the specimens were analyzed for color changes (ΔE), Vickers hardness, surface topography (Ra, Rt, and Rv), and scanning electron microscopy. Results: For samples cured by single-peak LED, the improper positioning significantly increases the color change compared to the optimal position regardless of the type of composite (p < 0.001). For multi-peak LED, the type of resin composite and the curing condition displayed a significant effect on ΔE (p < 0.001). For both LEDs, the Vickers hardness and bottom/top ratio of Vickers hardness were affected by the type of composite and the curing condition (p < 0.01). Conclusions: The bulk fill composite presented greater resistance to wear, higher color stability, and better microhardness than the incremental composite when subjected to improper curing. The multi-peak LED improves curing under improper conditions compared to single-peak LED. Prevention of errors when curing composites requires the attention of all personnel involved in the patient's care once the clinical relevance of the appropriate polymerization reflects on reliable long-term outcomes.

• Journal of the korean academy of Pediatric Dentistry
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• v.28 no.1
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• pp.146-153
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• 2001

The difference of composition of composite resin may affect the mechanical properties of composite resin and the environment is important for the properties of materials. The composite resin restoration is always exposed to fluid in oral cavity and the composite resin matrix is able to absorb water, which is accompanied by some swelling of the composite The uptake of water by composites has been correlated with decreases in surface hardness and wear resistance. The purpose of this study was to investigate the effects of water storage in $37^{\circ}C$ distilled water after 7days, 30days, 60days, 120days on compressive strength and flexural strength of dental composite resin, Z-100(group 1) Spectrum(group 2), Clearfil AP-X(group 3), Pyramid(group 4), Heliomolar(group 5). The compressive and flexural strength were measured by instron machine. The following results were obtained: 1. There were significant reduction of compressive strength as water storage time increased, 7days, 30days, 60 days, 120days(p<0.05). 2. There were significant reduction of flexural strength as water storage time increased, 7days, 30days, 60days, 120days(p<0.05). 3. Group 1, 2, 3 -hybrid type showed higher compressive and flexural strength than group 5-microfine type which had lower filler contents.

• Journal of Adhesion and Interface
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• v.11 no.4
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• pp.149-154
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• 2010

Interfacial evaluation was investigated for single-carbon fiber/phenolic and carbon nanotube (CNT)-phenolic composites by micromechanical technique and electrical resistance measurement combined with wettability test. Compressive strength of pure phenol and CNT-phenolic composites were compared using Broutman specimen. The contact resistance of CNT-phenolic composites was obtained using a gradient specimen by two and four-point methods. Surface energies and wettability by dynamic contact angle measurement were measured using Wilhelmy plate technique. Since hydrophobic domains are formed as heterogeneous microstructure of CNT in the surface, the dynamic contact angle exhibited more than $90^{\circ}$. CNT-phenolic composites exhibited a higher apparent modulus than neat phenolic case due to better stress transferring effect. Work of adhesion, $W_a$ between single-carbon fiber and CNT-phenolic composites exhibited higher than neat phenolic resin due to the enhanced viscosity by CNT addition. It was consistent with micro-failure patterns in microdroplet test.

• Composites Research
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• v.30 no.6
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• pp.365-370
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• 2017

Mechanical properties of carbon fiber reinforced thermoplastic composites (CFRTPs) are affected by various factors. One of the them are poor compatibility of the epoxy sizing layer on the carbon fiber surface with thermoplastic matrix, which causes the inferior interfacial strength between fibers and matrix. In addition, the high molten-viscosity of thermoplastics attributes to the poor impregnation state. Consequently, many voids in the composite materials were generated, which leads to poor mechanical properties of the thermoplastic composites. In this study, the epoxy sizing on the carbon fiber surface was removed and the polyamide 6,6 solution was coated on the de-sized carbon fiber surface to improve the impregnation state and mechanical properties. Interlaminar shear strength (ILSS) of CFRPTs was estimated by implementing short beam shear tests. In addition, flexural strength was measured and the impregnation state of the composites was evaluated by calculating void content.

• Composites Research
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• v.36 no.2
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• pp.126-131
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• 2023

Growing environmental concerns regarding pollution caused by conventional plastics have increased interest in biodegradable polymers as alternative materials. The purpose of this study is to develop a 100% biodegradable nanocomposite material by introducing organic nucleating agents into the biodegradable and thermoplastic resin, poly(lactic acid), to improve its properties. Accordingly, cellulose nanofibers, an eco-friendly material, were adopted as a substitute for inorganic nucleating agents. To achieve a uniform dispersion of cellulose nanofibers (CNFs) within PLA, the aqueous solution of nanofibers was lyophilized to maintain their fibrous shape. Then, they were subjected to primary mixing using a twin-screw extruder. Test specimens with double mixing were then produced by injection molding. Differential scanning calorimetry was employed to confirm the reinforced physical properties, and it was found that the addition of 1 wt% CNFs acted as a reinforcing material and nucleating agent, reducing the cold crystallization temperature by approximately 14℃ and increasing the degree of crystallization. This study provides an environmentally friendly alternative for developing plastic materials with enhanced properties, which can contribute to a sustainable future without consuming inorganic nucleating agents. It serves as a basis for developing 100% biodegradable green nanocomposites.

• Applied Chemistry for Engineering
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• v.16 no.5
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• pp.672-676
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• 2005

To improve the application of polymeric dental restorative composite (PDRC) for the posterior and anterior restoration, silica bridged with siloxane unit was firstly prepared by heat-treating a silica filler at various temperatures. Degree of conversion (DC), depth of cure, and dynamic volumetric polymerization shrinkage values of PDRC filled with silica bridged with siloxane unit were investigated to study the effect of heat-treated silica on the polymerization behavior of PDRC. From the experimental result, it was found that depth of cure was decreased with an increase of heat treatment temperature. on the other hand, both DC and polymerization shrinkage values were uniformly enhanced with increasing the heat treatment temperature. This phenomenon can be explained from the study that showed decrease of average particle size of silica resulted in the increase of relative amount of resin matrix in PDRC.