• Carbon letters
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• 제4권3호
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• pp.111-116
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• 2003

Carbon/carbon composites were developed using PAN based carbon fibres and phenolic resin as matrix in different volume fractions and heat treated to temperatures between $1000^{\circ}C$ to $2500^{\circ}C$. Although both the starting precursors are nongraphitizing hard carbons individually, their composites lead to very interesting properties e.g. x-ray diffractograms show the development of graphitic phase for composites having fibre volume fractions of 30~40%. Consequently the electrical resistivity of such composites reaches a value of $0.8\;m{\Omega}cm$, very close to highly graphitic material. However, it was found that by increasing the fibre volume fraction to 50~60%, the trend is reversed. Optical microscopy of the composites also reveals the development of strong columnar type microstructure at the fibre (matrix interface due to stress graphitization of the matrix. The study forcasts a unique possibility of producing high thermal conductivity carbon/carbon composites starting with carbon fibres in the chopped form only.

• Journal of the Korean Wood Science and Technology
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• 제39권2호
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• pp.179-186
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• 2011

In an effort to understand the hydrolytic degradation process of cured urea-formaldehyde (UF) resins responsible for the formaldehyde emission of wood-based composite panels, this study analyzed the influence of acid hydrolysis on the morphology of cured UF resins with different formaldehyde/urea (F/U) mole ratios such as 1.6, 1.4, 1.2 and 1.0. Field emission-scanning electron microscopy (FE-SEM) was employed to observe both exterior and fracture surfaces on thin films of cured UF resins before and after the etching with hydrochloric acid as a simulation of the hydrolytic degradation process. FE-SEM images showed that the exterior surface of cured UF resin with the F/U mole ratio of 1.0 had spherical structures after the acid hydrolysis while the other cured UF resins were not the case. However, the fracture surface observation showed that all the samples possessed spherical structures in the cured state of UF resins although their occurrence and size decreased as the F/U mole ratio increased. For the first time, we found the spherical structures in cured UF resins of higher F/U mole ratio of 1.4. After the acid hydrolysis, the spherical structures became a much predominant at the fracture surface. These results indicated that the spherical structures in cured UF resinswere much more resistant to the hydrolytic degradation by the acid than amorphous region.

• 한국포장학회지
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• 제16권2_3호
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• pp.59-65
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• 2010

Starch was modified with epichlorohydrin(ECH) to improve the miscibility with LDPE and LLDPE. Native starch or epichlorohydrin treated starch was mixed with grycerol and LDPE/LLDPE resin using a kneader and extruded using a single screw extruder to make pallets. The pallets were compression-molded at 145 into composite boards to evaluate their color, oxygen permeation, mechanical and thermal properties, and degradability under UV irradiation. Sheets with epichlorohydrin treated starch generally showed higher L-value than that of native starch blend sheets. The hunter b-values in both native starch blends and epichlorohydrin treated starch blends increased with Increasing starch contents. Tensile strength and percent elongation of sheets decreased with increasing starch contents. Tensile strength and percent elongation of sheets decreased with increasing starch contents. The degradability of blends under UV radiation increased with increasing starch contents in both blend types. The results represents that crosslinking of starch with epichlorohydrin may be a good method to improve miscibility of starch with petroleum-based materials.

• 한국전기전자재료학회:학술대회논문집
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• 한국전기전자재료학회 1999년도 추계학술대회 논문집
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• pp.479-482
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• 1999

Cured epoxy resins are extensively used for the electrical insulation in high-voltage equipments. The bisphenol A-based epoxy resins lured with azine show, especially, good thermal properties and mechanical resistances. For the technical and economic reasons, varing amount of inorganic fillers are added to endow the required special properties. In the large generators and motors of power plants, epoxy insulation is disclosed to the harsh conditions like the superheated steam and abrupt temperature variation. Hygrothermal aging at elevated temperatures tends to induce degradation in epoxy resins. To predict the effect of this degradation in DGEBA/MDA/SN/zeolite system, we proceeded the forced moisture absorption experiment using the autoclave. The thermal properties of the untreated and treated specimens were analyzed by DSC and TGA under the nitrogen flowing condition. The moisture absorption results showed a weight increase during hygrothermal aging at 1207. At the initial aging period, the system leaded to more or less postcuring but more prolonged environmental aging leaded the discoloration of specimen and lowering the T$_{g}$./.

• Restorative Dentistry and Endodontics
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• 제6권1호
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• pp.123-132
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• 1980

The author conducted a dental survey of a college girl students and compared with that of E college students 10 years ago which based on the same criteria. The items of this survey included the prevalence of dental caries, DMFT and the kinds of restored material. All college students 1,414 were examined in May 1978, with the recommended criteria and method of W.H.O.. Among them, the data of 1,393 were analyzed for this study. The following results were obtained: 1. The average number of present teeth were 28.97${\pm}$1.64. 2. The prevalence of dental caries was 84.92% and D.M.F. index was 15.51. Average D.M.F. teeth were 4.49${\pm}$3.69. 3. The average number of decayed (D) teeth were 2.09${\pm}$2.16. 4.30% of the present decayed teeth was secondary decay, and 2.44% of the decayed teeth was indicated for extraction. 4. The average number of missing(M) teeth were 0.32${\pm}$0.79. It included 8.56% of root fragements. 5. Average number of filled(F) teeth were 2.09${\pm}$3.17. F-ratio of this group was 46.45%. The analysis revealed 50.88% of amalgam alloy, 31.41% of gold inlays, and 1.20%, the least among the filling materials, silicate cement or composite resin. 6. While the prosthodontic treatment, such as crown and bridge has reduced than that of 10 years ago, the fillings and inlays as the conservative means has increased.

• Design & Manufacturing
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• 제11권3호
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• pp.25-28
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• 2017

The production of carbon fiber reinforced thermoplastics(CFRTP) parts using an injection/compression molding process that differs from the conventionally used fabrication methods was investigated Before the application of composite molding in the injection/compression molding process, a simple compression molding experiment was performed using a hydraulic press machine to determine the characteristics of resin impregnation and to obtain a basic physical property data for the CFRTP. Based on these results, injection/compression molded specimens were manufactured and an additional insert/over molding process was applied to improve the impregnation rate of the molded specimens. The results demonstrated that the tensile strength of the molded parts using the faster injection/compression process was similar to that of a hydraulic press molded product.

• 한국표면공학회지
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• 제46권5호
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• pp.223-228
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• 2013

In this work, the grow of carbon nanotube (CNT) on carbon fiber was introduced on PAN-based carbon fibers for the enhancement of mechanical interfacial strength of carbon fibers-reinforced composites. The surface properties of carbon fibers were determined by scanning electron microscopy (SEM) and mechanical interfacial properties of the composites were studied by interlaminar shear strength (ILSS). From the results, it was found that the mechanical interfacial properties of CNT-carbon fibers-reinforced composites (CNT-CFRPs) enhanced with decreasing the CNT content. The excessive CNT content can lead the failure due to the interfacial separation between fibers and matrices in this system. In conclusion, the optimum CNT content on carbon fiber surfaces can be a key factor to determine the mechanical interfacial properties of the CNT-CFRPs.

• Journal of Korean Dental Science
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• 제17권2호
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• pp.92-104
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• 2024

Managing multiple non-carious cervical lesions (NCCLs) with gingival recession and dentin hypersensitivity can be challenging. Herein, we present two cases of successful treatment procedure for multiple NCCLs with gingival recession and dentin hypersensitivity using an envelope coronally advanced flap with CTG and composite resin restoration. Through the combined approach of restorative and periodontal procedure, both patients showed adequate extent of gingival coverage and esthetic outcome based on the Modified Root Coverage Esthetic Score (MRES) at 6 months postoperatively. Also, dentin hypersensitivity was reduced effectively during the follow up period. Although the pocket depth slightly increased in patient 1, possibly due to the amount of restoration located sub-gingivally, pocket depth remained within 3 mm. This suggest that re-establishing the clinical CEJ and performing partial restoration is advantageous for periodontal tissue and is expected to contribute to maintain gingival height in the long term. These case reports emphasize the efficacy of the combined approach for treating multiple NCCLs with gingival recession and dentin hypersensitivity, highlighting the importance of careful restoration planning for optimal clinical and aesthetic outcomes.

• 대한치과보철학회지
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• 제52권2호
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• pp.113-120
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• 2014

연구 목적: 본 연구는 근관치료된 치아에 레진 시멘트로 fiber reinforced composite post (FRC) post의 접착시 포스트의 다양한 표면처리를 비교하여, 새롭게 제안되고 있는 다용도 프라이머의 효용성에 대해 평가하고자 하였다. 연구 재료 및 방법: 근관치료된 소구치 중 치근길이 1.8 cm 이상 되는 치아만을 총 24개 선택하였다. 표면 처리하지 않은 대조군과 표면처리를 시행한 FRC posts (DT Light Post, Size3, Bisco Inc., Schaumburg, IL, US)를 다음과 같이 6개의 그룹으로 무작위 분류하였다: Group A: airborne-particle abrasion (Cojet sand, 3M ESPE), Group S: silanization (Bis-silane, Bisco Inc.), Group M: universal primer (Monobond-plus primer, Ivoclar Vivadent Inc.), Group AS: silanization after airborne-particle abrasion, Group AM: universal primer treatment after airborne-particle abrasion. 하였다. CEJ에서 1.5 cm 근관성형 후 레진 시멘트를 이용하여 표면처리된 총 24개의 FRC post를 접착하고 광중합하였다. 생리식염수에 24시간 보관한 후 포스트 장축에 수직으로 1 mm 두께로 절단하여 만능시험기로 push-out test를 시행하였다. 포스트가 탈락되는 접착 실패 강도를 측정하고 SEM을 관찰하여 접착 실패 양상을 분류하고, 접착 강도를 Kruskal-Wallis test와 Tukey HSD value of rank test를 이용하여 통계 분석하였다(${\alpha}=0.05$). 결과: Airborne-particle abrasion후 실란 처리한 실험군에서 유의할만하게 높은 접착 강도를 보였다. 실란 처리, airborne-particle abrasion후 다용도 프라이머를 처리한 실험군의 순서로 접착 강도가 높게 나타났다. FRC post의 표면에 특정 전처리를 하지 않은 대조군에서 가장 낮은 결합 강도를 보이고, 이어서 다용도 프라이머, airborne-particle abrasion 순으로 낮은 평균 접착 강도를 보였다. 결론: FRC post의 접착 전 표면 처리 과정으로서 실란 처리를 거친 실험군에서 높은 결합 강도를 보였으며, 다용도 프라이머를 이용한 표면 처리는 실란과 비교하여 유의성 있는 접착 강도의 변화를 보이지 않았다.

• Restorative Dentistry and Endodontics
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• 제32권4호
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• pp.377-384
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• 2007

작금의 치질 접착시스템은 도말층 처리 방법에 따라 전부식형과 자가부식형 접착시스템으로 대별된다. 이러한 두 가지 접착시스템의 효용성을 비교, 평가하고 열순환 횟수에 따른 미세누출도 변화를 측정하기 위해 각각의 접착시스템으로 수복된 우치 5급 수복물에, 수복 초기의 효용성를 의미하는 500회의 열순환 자극과 상대적으로 긴 내구성을 의미하는 5,000회의 열순환 자극을 부여한 다음, 전기화학적 방법으로 측정하였다. 건전한 40개의 단근관을 가진 우치를 이용하여 백악법랑 경계부를 중심으로 5급 와동을 형성하였으며, 치아를 각각 10개씩 4개의 실험군 (열순환 횟수 2종 $\times$ 복합레진 수복 2개 군)으로 분류하였다. 20개의 치아에는 전부식형 접착시스템인 Single bond와 Z250 (shade A4)을, 나머지 20개의 치아에는 자가부식형 접착시스템인 AQ bond와 Metafil (shade A4)로 각각 충전하고 광중합기 (XL2500, 3M ESPE, St. Paul, MN, USA)를 이용하여 $600\;mW/cm^2$의 광도로 40초간 광중합하였다. 모든 시편을 실온에서 24 시간동안 증류수에 보관한 다음, 연마하고 각 수복물의 반은 섭씨 5도와 55도의 수조에 30초씩 담궜으며 이동 시간 10초의 열순환 (thermocycling)을 500회 시행하였고, 나머지 반은 5,000회 실시하였다. 미세 전류 측정을 위해 직류 공급원인 TOE 8841 (TOELLNER electronic instrument GMBH, Germany)을 이용하여 10 V의 전압을 부여하였으며 6514 system Electrometer (Keithley Co., Cleveland, Ohio, USA)로 미세전류 (${\mu}A$)를 측정하였다. 전류를 흐르게 한 다음 5-10분까지 20초 간격으로 측정 한 15개 측정치의 평균값을 시편의 미세전류 측정치로 인정하였다. 각 군간의 미세전류 측정치에 대한 유의성은 수복방법 및 열순환 횟수의 변수에 대한 Two-way ANOVA test로 95% 유의 수준에서 검증하여 다음과 같은 결과를 얻었다. 수복방법과 열순환 횟수 사이의 상호작용은 없었으며 (p = 0.485), 열순환 횟수에 따른 미세누출의 차이도 없었다(p = 0.814). 그러나 자가부식형 접착시스템인 AQ Bond와 Metafil로 수복된 실험군이 전부식형 접착시스템인 Single Bond와 Z250으로 수복된 군에 비해 적은 미세누출도를 보였다 (p = 0.005).