It is required for plasma display panels (PDPs) industries to make every effort to improve price competitiveness and performance in order to survive in flat panel display markets against the strategic expansion of liquid crystal display (LCD) industries.
For PDPs, low brightness and low efficiency have been considered as most critical problems. Many researchers make an effort to accomplish high brightness and efficiency of PDPs. For example they have developed high brightness phosphor, highly efficient discharge mode, and optimized discharge cell. Among them, phosphor materials play an important role to achieve high brightness and efficiency of PDPs. Hence damages to phosphor should be minimized during drying and baking process after inserting phosphor paste into barrier ribs. One of fragment damages to phosphor is caused by high temperature treatment during baking process. Conventional phosphor pastes with cellulose type binder need to be baked at about 500℃ in order to decompose cellulose binder but unfortunately leaves relatively large amount of carbon residue during baking process. Especially, it is well known that $BaMgAl_{10}O_{17}$:$Eu^{2+}$ (BAM) phosphor for PDPs can be easily degraded after the baking process. Moreover, adsorption of the carbon residue is also regarded as a reason for low efficiency and low brightness of phosphors. Therefore, it is desirable to develop low temperature burnable binder instead of the commercial cellulose type binder, which needs high temperature baking process. To develop low temperature burnable phosphor paste, selecting the polymer binder is very important. Because polymer binder determines baking conditions including baking temperature, holding time, temperature increasing rate, and baking atmosphere. And also it serve many functions, including rheological behavior and dispersion properties of phosphor paste, an increment of packing density between phosphor particles by binding individual phosphor particles together, and enabling the lamination of phosphor layer.
Therefore, in this study we synthesized and adopted P(MMA-co-BMA) random copolymer as low temperature burnable binder for PDPs phosphor paste. The results of thermogravimetric analysis (TGA) showed that the prepared P(MMA-co-BMA) binder was completely burned out at just 300°C in air atmosphere. However, unexpectedly, after baking process of phosphor paste with P(MMA-co-BMA) at 300°C in bulk system, body color of the phosphor became brown. In addition, photoluminescence (PL) intensity of the phosphor decreased by about 40 % compared to a phosphor of the commercial paste after baking process at 500°C.
Generally in paste systems mixed with phosphor, polymer binder, and solvent, non volatile polymer binder is decomposed to volatile low molecular weight materials such as monomer, hydrocarbon, $H_2O$, $CO_2$, and olefin during baking process. These thermally decomposed materials are eliminated from phosphor paste system by diffusion or capillary force. Unless they can be removed rapidly in paste system, unexpected polyaromatic carbon residue can be generated by dehydrogenation and condensation reaction. Formation of the carbon residue depends on residence time (or removal rate) of thermally decomposed materials in phosphor paste system. Residence time (or removal rate) is influenced by diffusion length.
Therefore, we converted the bulk system into phosphor layer system in order to decrease diffusion length. After baking process of phosphor layer with P(MMA-co-BMA) at 300°C, PL intensity of the phosphor layer was about 3% higher than that of phosphor layer with the commercial paste after baking process at 500°C. In addition, the results of the carbon element analyzer show that residual carbon weight percent (%) of the baked paste was 0.0732 % in bulk system, on the other hand, 0.0365 % in phosphor layer system. From these experimental results, formation of the carbon residue depends on diffusion length proportional to phosphor layer thickness. Therefore we measured relative photoluminescence (PL) intensity as increasing BAM phosphor layer thickness after burning out P(MMA-co-BMA) binder in order to examine the effect of carbon residue. Based on the PL intensity results, it is found that critical thickness where the PL intensity is not affected by carbon residue was about 60μm. The critical thickness has sufficient stability compared to commercial thickness (20μm) which was inserted between barriers.
We optimized printability of phosphor paste for screen printing by adjusting ratio of phosphor : polymer binder : solvent. By using the optimized ratio, we obtained the phosphor layer with the thickness of 17μm, which was similar to a thickness of phosphor layer in commercial PDPs.
Through this study, we have verified that P(MMA-co-BMA) binder could be a good candidate material as a low temperature burnable binder for PDPs phosphor paste. If the phosphor paste with P(MMA-co-BMA) binder applies to PDPs, we could expect more bright and efficient PDPs.
PDP의 최종 품질은 형광체의 도포기술이 좌우한다고 말할 정도로 형광체의 후막형성 기술은 상당히 중요하며, 최적화된 형광막을 형성하기 위해서는 형광체 페이스트의 레올로지 (rheology) 특성 및 분산성에 대한 제어가 필요하다. 또한 후막형성 이후 소성공정 과정에서 형광체의 열화나 잔류 유기물 및 탄소침적물 점착에 의한 형광체의 손상 없이 형광체 본연의 성능을 최대한 발휘 할 수 있도록 하는 것이 중요하다. 그러나 기존 형광체 페이스트의 바인더로 사용되는 셀룰로오즈 타입은 소성온도가 높아 형광체의 열화현상을 일으킬 뿐만 아니라 잔류탄소침적물을 상대적으로 많이 남겨 소성공정 이후 발광강도의 감소를 야기한다. 이는 PDP가 않고 있는 저효율, 저휘도 문제에 심각한 영향을 미친다. 이러한 열화현상 및 잔류 탄소침적물에 대한 영향을 최대한 줄이고 형광체 본연의 성능을 최대한 발휘할 수 있도록 하기 위해서는 저온소성 가능한 형광체 페이스트의 개발이 필요하다. 저온소성형 형광체 페이스트를 개발하기 위한 가장 중요한 요소는 바인더로 사용될 고분자 수지(polymer resin)를 선정하는 일이며, 이는 고분자 수지의 특성에 따라 형광체 페이스트의 소성공정이 결정되며, 인쇄특성 및 분산성에도 상당한 영향력을 미치기 때문이다. 따라서 본 연구에서는 저온소성 가능한 고분자 수지로서 저온 소성특성이 좋고 상대적으로 탄소침적물을 적게 남기며 형광체와의 분산성이 좋은 아크릴 계열의 고분자 수지 P(MMA-co-BMA) random copolymer를 선정하였다. 상기 물질은 본 연구실에서 특허로 등록한 물질로 실제 공정에 적용되기 위해서는 인쇄특성에 대한 조절과, 저온소성 공정에서 탄소침적물이 일부 형성되는 문제점에 대한 해결이 필요하다. 이를 위해 합성조건의 최적화를 통한 분자량 조절 및 형광체 페이스트의 조성비율을 조절함으로써 인쇄특성에 대한 부분을 개선하여 최종적으로 상용 형광체 페이스트와 유사한 형광막을 얻을 수 있었으며, 소성공정 과정에서 고분자 수지 열분해 시 발생하는 탄소침적물의 형성 메커니즘에 대한 분석과 이에 대한 이해를 바탕으로 물리적인 소성과정 (확산거리, 확산속도)조절을 통한 탄소침적물 형성 제어에 관한 연구 진행하였다.
