전기차의 수요 및 보급이 확대됨에 따라 차량 내 이음(buzz, squeak, rattle, BSR) 개선에 대한 요구가 커지고 있다. 이에 풍절음, 도어 글라스 및 차량 진동을 차단하는 인너벨트 웨더스트립(innerbelt weatherstrip)의 댐핑(damping) 특성 향상을 통해 BSR을 저감하는 기술 개발이 필수적이다. 기존 열경화성(thermoset) 탄성체 대비 가볍고 재활용이 가능한 열가소성(thermoplastic) 탄성체가 주목을 받고 있지만 낮은 소재 댐핑과 영구압축줄음률(compression set)로 인해 도어 글라스와 웨더스트립 간 마찰 소음을 발생하는 문제가 있다. 고분자 댐핑 특성은 점탄성(viscoelastic)에 좌우되므로, 본 연구에서는 인너벨트 웨더스트립과 도어 글라스 간 마찰 소음을 개선하기 위해 EPDM (ethylene-propylene-diene monomer)/PP (polypropylene) thermoplastic vulcanizates (TPV)의 재료설계인자(EPDM/PP 비율, EPDM 내 ENB 함량)에 따른 점탄성 분석을 통해 소재 댐핑 특성을 평가하였다. EPDM/PP 비율에 따른 분석을 통해 PP 비율이 낮을수록 소재가 연화되고, 탄성회복력(resilience)이 증가하여 저장탄성률(storage modulus)은 10.8% 감소하고 댐핑 특성을 의미하는 감쇠계수(tanδ)는 88.2% 증가함을 확인하였다. 또한 EPDM 내 ENB 함량이 높을수록 소재의 가교밀도(crosslink density)가 증가하지만, 동적가교(dynamic vulcanizate) 과정 중 PP에 분산된 EPDM particle의 크기가 감소한다. 이로 인해 증가된 EPDM/PP 계면 간 면적 증가로 인해 계면 미끄러짐에서 기인한 손실탄성률(loss modulus)이 24.7% 증가하여 댐핑 특성이 향상되었다. 재료설계인자에 따른 물성분석을 바탕으로 최적 소재(낮은 PP 비율(14 wt%), 높은 ENB 함량 (8.9 wt%))를 배합한 결과 소재 댐핑 특성(tanδ peak)은 기존 소재(PP27, EPDM/PP 30/27, ENB content 5.7 wt%) 대비 140% 증가하여 재료설계인자에 따라 댐핑 특성을 제어할 수 있음을 확인하였다. 설계된 소재의 글라스 마찰 소음 개선 효과를 확인하기 위해 stick-slip 시험을 통해 마찰 소음을 평가하였다. 소재 댐핑 특성이 향상됨에 따라 마찰 진동의 가속도 peak가 약 57.9% 감소하였다. 이러한 결과로부터 재료설계인자에 따른 소재 댐핑 특성 향상을 통해 인너벨트 웨더스트립의 글라스 마찰 소음을 개선할 수 있음을 확인하였으며, 향후 소재 재료설계인자에 따른 물성 제어를 통해 부품의 요구 성능에 맞는 다양한 재료설계에 활용할 수 있을 것으로 기대된다.
Due to enormous market growing of electric vehicles without combustion engine, reducing unwanted BSR (buzz, squeak, and rattle) noise is highly demanded for vehicle quality and performance. Particularly, innerbelt weatherstrips which not only block wind noise, rain, and dust from outside, but also reduce noise and vibration of door glass and vehicle are required to exhibit high damping properties for improved BSR performance of the vehicle. Thermoplastic elastomers (TPEs), which can be recycled and have lighter weight than thermoset elastomers, are receiving much attention for weatherstrip material, but TPEs exhibit low material damping and compression set causing frictional noise and vibration between the door glass and the weatherstrip. In this study, high damping EPDM (ethylene-propylene-diene monomer)/PP (polypropylene) thermoplastic vulcanizates (TPV) were investigated by varying EPDM/PP ratio and ENB (ethylidene norbornene) fraction in EPDM. Viscoelastic properties of TPV materials were characterized by assuming that the material damping is directly related to the viscoelasticity. The optimum material damping factor (tanδ peak 0.611) was achieved with low PP ratio (14 wt%) and high ENB fraction (8.9 wt%), which was increased by 140% compared to the reference (tanδ 0.254). The improved damping is believed due to high fraction of flexible EPDM chains and higher interfacial slippage area of EPDM particles generated by increasing ENB fraction in EPDM. The stick-slip test was conducted to characterize frictional noise and vibration of the TPV weatherstrip. With improved TPV material damping, the acceleration peak of frictional vibration decreased by about 57.9%. This finding can not only improve BSR performance of electric vehicles by designing material damping of weatherstrips but also contribute to various structural applications such as urban air mobility or aircrafts, which require lightweight and high damping properties.
본 연구는 현대자동차가 지원하는 연구과제로 수행된 것이며, 지원에 대해 진심으로 감사드립니다.
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