콜레스테릭 액정 마이크로캡슐 및 코팅방법을 이용한 플렉서블 디스플레이 연구

Abstract

플렉서블 액정디스플레이를 구현하는 방법으로는 지금까지 고분자분사형 액정 (PDLC), 고분자네트웍 액정 (PNLC) 및 화소고립형 액정 (PILC) 등 다양한 기술들이 개발되어왔다. 각각의 기술들은 장점뿐만 아니라 단점도 가지고 있기 때문에 본 논문연구에서는 기존의 개발된 기술의 장단점을 분석하고 여기에 아래의 선택기준을 추가하여 플렉서블 디스플레이 개발에 가장 최적의 기술을 개발하는 것으로 목표를 세웠다. 이러한 선택기준으로는 첫째 특허문제가 없을 것, 둘째 롤투롤 2R) 공정에 적합할 것, 셋째 이페이퍼 (e-paper) 디스플레이에 적용이 가능할 것, 넷째 컬러 (Color) 표현이 가능할 것, 다섯째 대용량 정보표시가 가능할 것 들이었다. 이러한 선택 기준에 근거하여 철저한 비교분석 결과 고분자분산형 액정이 이에 가장 부합하는 기술로 판단되었다. 이러한 고분자분산형 기술은 또한 크게 이멀젼, 캡슐레이션, 및 상분리형의 세 가지 유형으로 나뉘어 지는데, 본 논문연구에서는 이중에서도 판단기준 부합 정도가 가장 높게 나온 두 번째 캡슐레이션 기술을 최종적인 연구기술 항목으로 결정했다. 마이크로캡슐형성이란 개개의 입자나 고체 혹은 액체 코어를 갖는 드롭넷이 연속적인 필름형태의 고분자 물질로 이루어진 벽 재료로 둘러싸인 형태로 그 입자의 크기가 보통 마이크로미터 범위에 존재하기 때문에 마이크로캡슐이라고 한다. 이와같이 마이크로캡슐형성 기술을 적용하는 이유는 다양하지만, 그 중에서도 용해도 개선, 분산도 개선 및 유동성 개선과 같은 제조 공정성 향상이 이를 플렉서블 액정디스플레이에 적용하는 가장 큰 이유다. 액정을 마이크로캡슐로 형성하게 되면 플렉서블 액정디스플레이를 제조하는 데 있어서 유연성과 휘거나 구부렸을 경우에도 디스플레이 화질이 변하지 않는 장점이 있다. 본 논문에서는, 먼저 일반적으로 식품이나 화장품 업계에서 상업용으로 가장 널리 사용되고 있는 코아서베이션이라는 마이크로캡슐형성 방법을 적용하여 콜레스테릭 액정마이크로캡슐 제조기술을 개발하였다. 이때 최신 기술인 멤브레인 유화방식을 적용하여 마이크로액정캡슐의 입도편차가 20% 이내로 아주 균일한 특성을 나타내도록 하였다. 그 다음은 이렇게 만든 콜레스테릭 마이크로액정캡슐을 이용하여 이를 균일하게 코팅할 수 있는 코팅기술을 개발하였다. 이때 여러 가지 코팅방식 중에 코팅성능 및 비용측면에서 가장 적절한 것으로 판단된 바코팅 방식을 적용하여 마이크로액정캡슐을 플라스틱 기판에 균일하게 코팅하는 기술을 개발하였다. 마이크로액정캡슐을 플라스틱 기판 위에 균일하게 코팅하는 데 있어서 가장 중요한 인자는 코팅용액 제조방법, 코팅속도 및 코팅높이 등으로 압축되었는데 본 실험에서 이들 중요 인자들에 대해 최적화 실험을 통해 최적의 공정조건을 확보하였다. 그 다음으로는 콜레스테릭 마이크로액정캡슐 디스플레이의 구동전압을 낮추기 위해 구동전압에 영향을 주는 인자들에 대한 연구를 진행하였다. 이를 통해, 콜레스테릭 마이크로액정캡슐 디스플레이의 구동전압에 크게 영향을 주는 인자로는 액정캡슐 사이즈, 마이크로액정캡슐 코팅층 두께, 마이크로캡슐 내부의 콜레스테릭액정과 벽물질과의 앵커링 에너지 및 투명전극 기판에 대항하는 맞은 편 전극형성 방법 등의 네 가지로 압축되었다. 이에 각각의 인자들에 대한 최적화 실험 진행결과, 액정캡슐 사이즈는 8 마이크로가 적당한 것으로 나타났고 마이크로액정캡슐층 두께는 15에서 20 마이크로미터 정도가 적당하였으며 투명전극에 대항하는 맞은 편 전극은 스크린 인쇄방법을 적용하여 마이크로액정캡슐층 상부에 직접 형성하는 방법을 선택하였다. 또한 마이크로캡슐 내부의 콜레스테릭 액정과 벽물질 간의 앵커링 에너지를 최대한 낮추기 위해 비이온성 계면활성제를 첨가하여 표면장력을 최소화하였다. 이러한 마이크로액정캡슐 코팅기술 개발과 더불어 이를 적용한 디스플레이 디바이스의 구동기술도 함께 개발하였는데, 기존의 콜레스테릭 액정디스플레이 구동에 널리 적용되고 있던 쓰리레벨 (3-Level) 패시브 메트릭스 구동방식을 개선하여 포레벨 (4-Level) 패시브 매트릭스 구동방식을 새롭게 개발하였다. 이를 통해, 기존의 구동방식보다 두 배정도 향상된 콘트라스트 특성을 확보하였으며 10x10 해상도의 콜레스테릭 마이크로액정캡슐 디스플레이에 실제 적용하여 성능을 확인하였다. 끝으로 빨강색 (R), 초록색 (G), 파랑색 (B)의 단색 콜레스테릭 마이크로액정캡슐을 적용하고 포레벨 (4-Level) 패시브 구동방식을 적용하여 10x10 해상도의 콜레스테릭 마이크로액정캡슐방식 플렉서블 디스플레이 시제품을 성공적으로 개발하였다. 이 시제품의 전기광학 포시특성으로는 반사율이 14% (녹색파장, 550nm)이고 세그멘트 구동시의 콘트라스트가 약 15:1, 패시브 메트릭스 구동시의 콘트라스트가 5:1 정도로 나왔으며 구동전압은 약 65V 정도로 비교적 낮게 나왔다. |A variety of technologies have been developed for fabricating flexible LCDs like polymer-dispersed liquid crystals (PDLCs), polymer-network liquid crystals (PNLCs), and pixel-isolated liquid crystals (PILCs), etc. Since each technology has its own merits and demerits, in this work I finally select one with thorough comparison between the pros and cons based on the following criteria: (1) free of patent issues, (2) good compatibility with R2R process (3) applicability to e-paper, (4) color capability, and (5) high information content. As a result of this contemplated comparison based on the above selection criteria, PDLC is in priority shown to satisfy them as best as possible. PDLC technology can generally be categorized into three types consisting of emulsion, encapsulation, and phase separation. Among them, encapsulation-type has been finally chosen for a suitable technology for the development of a single-substrate flexible LCD since it fulfills the requirements best. Microencapsulation is a process by which individual particles or droplets of solid or liquid material (the core) are surrounded or coated with a continuous film of polymeric material (the shell) to produce capsules in the micrometer range, known as microcapsules. The reasons for microencapsulation are countless. Among them, better processability like improving solubility, dispersibility, and flowability might be the highest priority in the viewpoint of those who try to adopt microencapsulation technology for the manufacture of flexible LCDs. LC Microencapsulation system offers potential advantages over the conventional method of fabricating flexible LCDs, especially good flexibility and superior bending capability without losing display performance. In this work, we first build on the Ch-LC microencapsulation technology using a coacervation method, which is widely used for the mass production of foods and cosmetics. With adopting state of the art technology of membrane emulsification, we can fabricate Ch-LC microcapsules having good size uniformity with narrow distribution whose degree of size deviation in mean diameter is less than 20%. Then taking advantage of these LC microcapsules, we further explore the coating technology of Ch-LC microcapsules using a bar coater, in which the microcapsule-printed layer uniformity is found to be much dependent on the formulation of coating solution as well as coating process conditions like coating speed and coating depth. Through optimization of those critical parameters, coating solution via adjusting the mix ratio between microcapsule and binder, coating process conditions via the variation of both coating speed and coating depth, we can get good Ch-LC microencapsulated layer on a plastic substrate with very uniform thickness. In order to reduce the driving voltage (Vreset) of the Ch-LC microencapsulated displays, we first investigate which kind of parameters may give stronger influence on the driving voltage. According to the experimental investigations, the most influential parameters like the size of Ch-LC microcapsules, the thickness of the LC microencapsulated layer, anchoring energy between the Ch-LC and wall material (PVA) and the way of how to build the opposite electrodes, are picked out. Through optimization of these parameters for the purpose of reducing the driving voltage, each parameter is finally decided as follows: the optimum mean diameter of the LC microcapsules is about 8 um, the thickness of the LC microencapsulated layer is 15 to 20um, and the best way to construct the opposite electrode is to print the carbon black directly onto the LC microencapsulated layer. Additionally, in order to decrease the anchoring energy between the Ch-LC and wall material (PVA), we have applied a variety of surfactants during the manufacture of LC microcapsules, thereby achieving the characteristics of much lower driving voltage for the Ch-LC microencapsulated displays. Along with this development on the microcapsule coating techniques, we also studied the passive matrix drive scheme for better performing the Ch-LC microencapsulated displays. Starting from a 3-level drive scheme, a conventional one for addressing the Ch-LCs, we have eventually upgraded its level up to a 4-level drive scheme, which is able to perform Ch-LC microcapsule display with about 2 times better contrast ratio compared to that of 3-level drive scheme in the passive matrix drive system. Finally we have successfully developed a single-substrate flexible LCD using Ch-LC microencapsulation technology and printing method, which can be driven with 10x10 passive matrix drive scheme. Owing to the unique characteristics of Ch-LC which can reflect its own color designated by the chiral pitch of the liquid crystal mixture, P, we have fabricated three different monochromatic Ch-LC microencapsulated displays having Red, Green, Blue colors. EO performances of the monochromatic demo samples are eventually achieved that the reflectance is about 14% (@ green, 550nm), contrast ratio is 15:1 for the segment-type and 5:1 for the passive matrix-type, and the driving voltage (Vreset) is about 65V.; A variety of technologies have been developed for fabricating flexible LCDs like polymer-dispersed liquid crystals (PDLCs), polymer-network liquid crystals (PNLCs), and pixel-isolated liquid crystals (PILCs), etc. Since each technology has its own merits and demerits, in this work I finally select one with thorough comparison between the pros and cons based on the following criteria: (1) free of patent issues, (2) good compatibility with R2R process (3) applicability to e-paper, (4) color capability, and (5) high information content. As a result of this contemplated comparison based on the above selection criteria, PDLC is in priority shown to satisfy them as best as possible. PDLC technology can generally be categorized into three types consisting of emulsion, encapsulation, and phase separation. Among them, encapsulation-type has been finally chosen for a suitable technology for the development of a single-substrate flexible LCD since it fulfills the requirements best. Microencapsulation is a process by which individual particles or droplets of solid or liquid material (the core) are surrounded or coated with a continuous film of polymeric material (the shell) to produce capsules in the micrometer range, known as microcapsules. The reasons for microencapsulation are countless. Among them, better processability like improving solubility, dispersibility, and flowability might be the highest priority in the viewpoint of those who try to adopt microencapsulation technology for the manufacture of flexible LCDs. LC Microencapsulation system offers potential advantages over the conventional method of fabricating flexible LCDs, especially good flexibility and superior bending capability without losing display performance. In this work, we first build on the Ch-LC microencapsulation technology using a coacervation method, which is widely used for the mass production of foods and cosmetics. With adopting state of the art technology of membrane emulsification, we can fabricate Ch-LC microcapsules having good size uniformity with narrow distribution whose degree of size deviation in mean diameter is less than 20%. Then taking advantage of these LC microcapsules, we further explore the coating technology of Ch-LC microcapsules using a bar coater, in which the microcapsule-printed layer uniformity is found to be much dependent on the formulation of coating solution as well as coating process conditions like coating speed and coating depth. Through optimization of those critical parameters, coating solution via adjusting the mix ratio between microcapsule and binder, coating process conditions via the variation of both coating speed and coating depth, we can get good Ch-LC microencapsulated layer on a plastic substrate with very uniform thickness. In order to reduce the driving voltage (Vreset) of the Ch-LC microencapsulated displays, we first investigate which kind of parameters may give stronger influence on the driving voltage. According to the experimental investigations, the most influential parameters like the size of Ch-LC microcapsules, the thickness of the LC microencapsulated layer, anchoring energy between the Ch-LC and wall material (PVA) and the way of how to build the opposite electrodes, are picked out. Through optimization of these parameters for the purpose of reducing the driving voltage, each parameter is finally decided as follows: the optimum mean diameter of the LC microcapsules is about 8 um, the thickness of the LC microencapsulated layer is 15 to 20um, and the best way to construct the opposite electrode is to print the carbon black directly onto the LC microencapsulated layer. Additionally, in order to decrease the anchoring energy between the Ch-LC and wall material (PVA), we have applied a variety of surfactants during the manufacture of LC microcapsules, thereby achieving the characteristics of much lower driving voltage for the Ch-LC microencapsulated displays. Along with this development on the microcapsule coating techniques, we also studied the passive matrix drive scheme for better performing the Ch-LC microencapsulated displays. Starting from a 3-level drive scheme, a conventional one for addressing the Ch-LCs, we have eventually upgraded its level up to a 4-level drive scheme, which is able to perform Ch-LC microcapsule display with about 2 times better contrast ratio compared to that of 3-level drive scheme in the passive matrix drive system. Finally we have successfully developed a single-substrate flexible LCD using Ch-LC microencapsulation technology and printing method, which can be driven with 10x10 passive matrix drive scheme. Owing to the unique characteristics of Ch-LC which can reflect its own color designated by the chiral pitch of the liquid crystal mixture, P, we have fabricated three different monochromatic Ch-LC microencapsulated displays having Red, Green, Blue colors. EO performances of the monochromatic demo samples are eventually achieved that the reflectance is about 14% (@ green, 550nm), contrast ratio is 15:1 for the segment-type and 5:1 for the passive matrix-type, and the driving voltage (Vreset) is about 65V.