고온 수침 환경에서 UPE 겔코트 코팅된 지중 매설 파이프용 GFRP의 열화 및 크랙 발생 특성에 관한 연구

Study on the Crack and Thermal Degradation of GFRP for UPE Gelcoat Coated Underground Pipes Under the High Temperature Water-Immersion Environment

Abstract

유리섬유강화폴리에스테르 복합소재는 지중 매설 파이프, 탱크용 구조재, 선체 등 가혹한 환경에서 구조재로 널리 사용되고 있으며, 장기 내수성을 필요로 하는 소재이다. 특히, 물에 잠겨 있을 때 삼투압으로 인하여 겔코트와 복합소재의 박리 등 열화가 진행된다. 본 연구에서는 지중 매설 파이프로 활용되는 GFRP 복합소재의 내구성 향상을 위해 인퓨전(진공성형) 공정으로 UPE (unsaturated polyester) 겔코트 표면 처리한 복합소재를 제작하여, 고온 수침 환경 ($65^{\circ}C$, $75^{\circ}C$, $85^{\circ}C$)에서의 표면 결함 및 크랙 발생과 경도 변화 특성을 확인하였다. 마이크로 CT 단층 촬영을 통하여 수침 온도에 따른 크랙의 침투 깊이를 조사하였으며, $75^{\circ}C$와 $85^{\circ}C$ 조건에서 크랙이 복합소재까지 침투하여 내구성을 저하시키는 것으로 확인되었다. 최초 크랙이 발생하는 지점을 고장시간으로 정의하고 아레니우스식을 활용하여 $23^{\circ}C$ 상온에서의 수명 예측을 실시하였다. 본 연구로 토목, 건축, 해양산업분야 등 겔코트가 적용되는 다양한 산업분야의 신뢰성 평가에 응용될 수 있을 것으로 기대된다.

Glass fiber reinforced polyester (GFRP) composites are widely used as structural materials in harsh environment such as underground pipes, tanks and boat hulls, which requires long-term water resistance. Especially, these materials might be damaged due to delamination between gelcoat and composites through an osmotic process when they are immersed in water. In this study, GFRP laminates were prepared by surface treatment of UPE (unsaturated polyester) gelcoat by vacuum infusion process to improve the durability of composite materials used in underground pipes. The composite surface coated with gelcoat was examined for surface defects, cracking, and hardness change characteristics in water-immersion environments (different temperatures of $60^{\circ}C$, $75^{\circ}C$, and $85^{\circ}C$). The penetration depth of cracks was investigated by micro CT imaging according to water immersion temperature. It was confirmed that cracks developed into the composites material at $75^{\circ}C$ and $85^{\circ}C$ causing loss of durability of the materials. The point at which the initial crack initiated was defined as the failure time and the life expectancy at $23^{\circ}C$ was measured using the Arrhenius equation. The results from this study is expected to be applied to reliability evaluation of various industrial fields where gelcoat is applied such as civil engineering, construction, and marine industry.

Fig. 1. Application example of GFRP in water environment

Fig. 2. Fiber reinforced material

Fig. 3. Infusion manufacturing method

Fig. 4. Preparation of water-immersion test (laboratory aging)

Fig. 5. Photo images of gelcoat surface after distilled water aging at various immersion temperatures

Fig. 6. Number of gelcoat surface cracks as a function of aging time at various immersion temperatures

Fig. 7. The number of gelcoat cracks on the surface and the depth of crack after immersion test at various temperatures

Fig. 8. Micro CT scan images of GFRP laminates after water immersion for 30 days

Fig. 9. FTIR results from sample aged for 30 days at various immersion temperatures

Fig. 10. Arrhenius plots, initial crack occurrence time of Gelcoat after immersion test at high temperature (65℃, 75℃, 85℃)

Fig. 11. Life time of the gelcoat as a function of temperature

Table 1. Experimental values of Polyester Gelcoat measured

Table 2. Aging test conditions

Table 3. Shore D type hardness of Gelcoat resin immersed in distilled water at various temperatures for 24 hours

Table 4. First crack (h) in 65℃, 75℃, and 85℃ Ortho type gelcoat specimens