Sustainable UV-curable low refractive index resins with novel polymers for polymer cladding materials

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1 Sustainable UV-curable low refractive index resins with novel polymers for polymer cladding materials Hiroki Tokoro*, Takako Ishikawa, Nobuyuki Koike, Yohzoh Yamashina DIC Corporation, 12 Yawatakaigan-dori, Ichihara, Chiba JAPAN ABSTRACT Low refractive index polymers are used as cladding materials for high numerical aperture (NA) fibers. Since transparent fluoro polymers are ideal for this application, they have been used over many years. However, some fluoro chemicals face an issue related to perfluoro octanoic acid (PFOA) which is caused by its longtime persistence in the environment and human body. In this research, non-pfoa type UV curable fluoro resins suitable for cladding were developed with novel materials. The cured films showed high transparency, good adhesion to glass and low refractive index of and at 850 nm. Optical fibers prepared with those cladding showed almost equivalent attenuation to a fiber with commercially available material. Keywords: High NA, plastic clad silica fiber, polymer, low refractive index, UV cure, fluoro resin 1. INTRODUCTION High NA fibers are interesting for power transmission, remote sensing or for laser fibers with high pump efficiency. Especially, plastic clad silica core fibers are widely used in medical, industrial, scientific and military fields. For those fibers, fluoro polymers with low refractive index and high transparency are used for cladding 1-5. However, United States Environmental Protection Agency has been concerned perfluorooctanoic acid PFOA and its related materials for a decade. PFOA well known as C8 is very persistent in the environment, so once PFOA is released to the environment, it can hardly get into biodegradation process. Furthermore, PFOA tends to remain in human body for long period of time, probably in the scale of years. In 2006, EPA and major companies in fluorochemical industry launched the 2010/15 PFOA Stewardship Program, in which companies committed to reduce global facility emissions and product content of PFOA and related chemicals by 95% by 2010, and to work toward eliminating emissions and product content by In this study, we developed novel non-pfoa type fluoro polymers and the low refractive index coatings. Also evaluation results of the coatings, cured films, and optical fibers coated by the newly developed coatings as cladding are reported. Figure 1. Structure of PFOA *hiroki-tokoro@ma.dic.co.jp; phone ; fax ; Organic Photonic Materials and Devices XVI, edited by Christopher E. Tabor, François Kajzar, Toshikuni Kaino, Yasuhiro Koike, Proc. of SPIE Vol. 8983, 89831O 2014 SPIE CCC code: X/14/$18 doi: / Proc. of SPIE Vol O-1

2 2. METHODOLOGY Instruments used for this research are shown in Table 1. Cured films (76μm and 1mm thick) were made on glass substrate using GS Yuasa 160W/cm metal halide lamp under nitrogen atmosphere. Table 1. Measurement items and instruments Measurement items Company Model Conditions Refractive index ATAGO Abbe refractometer DR-M2 Temperature: 25 C Wavelength (nm): 486, 589, 656, 850 Viscosity TOKI SANGYO BII Viscometer Temperature: 25 C Transmittance HITACHI U4100 UV-Visible-NIR Spectrophotometer Elongation Young s modulus Adhesion A&D TENSILON RTC Temperature: 23 C TGA SII TG/DTA 6200 rt to 500 C, 10 C/min 3. RESULTS AND DISCUSSION 3.1 Synthesis of fluoro polymers The formulated fluoro coatings consisted of fluoro polymers, monomers, photoinitiators and additives. The four fluoro polymers were synthesized with non-pfoa type fluoro (meth)acrylates and other monomers as shown in Table 2. Polymerization was monitored by viscosity. Polymer 1 was designed for lower refractive index, while each formulation of Polymer 2 and 3 was more focused on higher modulus with different monomers from those of polymer 1. Since some fiber drawing processes require higher viscosity coatings, Polymer 4 consisted of the same monomer component as Polymer 2 but had higher viscosity. All polymer solutions were colorless and transparent. Table 2. Properties of synthesized fluoro polymers Polymer Fluorine content (wt%) Refractive index (n 25 D ) Viscosity (mpa s) Polymer ,050 Polymer ,250 Polymer ,230 Polymer , Formulation of low refractive index coatings Solvent free formulation was designed with Polymer 1 to 4, respectively. Basically F1 was designed so as to have refractive index of 1.38 as cured film at 589 nm of wavelength and the other formulations were for around Refractive index of cured films was obtained as a function of wavelength (Figure 2). The cured films of F1 showed refractive index of and NA of 0.51 (to silica), while both F2 and F3 showed and 0.43 at 850nm, respectively. Proc. of SPIE Vol O-2

$Table 3. Properties of formulations with fluoro polymers Formulation Containing polymer Refractive index (n 25 D ) Viscosity (mpa s) F 1 Polymer 1 1.355 2,000 F 2 Polymer 2 1.$ $Refractive index of cured films 3.$

3 Table 3. Properties of formulations with fluoro polymers Formulation Containing polymer Refractive index (n 25 D ) Viscosity (mpa s) F 1 Polymer ,000 F 2 Polymer ,000 F 3 Polymer ,000 F 4 Polymer , ái ai >,_ a, cc A 1 F1 C F L 7 F Wavelength (nm) Figure 2. Refractive index of cured films 3.3 Polymer clad fibers using F1, F2 and F3 as cladding To evaluate the coatings F1 to F3 as cladding materials, multicomponent glass fibers were drawn and it was coated by claddings according to the formulation of F1 to F3 as well as a reference material generally used. The core / cladding thickness was 220 / 240 μm. Transmission loss was measured by cut back method (Figure 3). In a range of 800 to 1600 nm, F3 and the benchmark material showed same loss. - - F 2 - -F 3 - Standard ""- µ '.. *arëi#'r"á.i? ßß Wavelength (nm) 1600 Figure 3. Light transmission of the polymer clad fibers using F1 to F3 Proc. of SPIE Vol O-3

4 3.4 Cured films Transmittance through the cured films of F2, F3 and F4 with thickness of 1mm was measured as shown in Figure 4. All samples showed excellent transparency in the visible light region. The data exhibited around 5% loss induced by reflection at the interface of the surface / air at both side of the cured samples. Comparison of the cured films of F3 and F4, with the identical monomer composition, showed that the viscosity could be adjusted for fiber drawing conditions without affecting transparency and refractive index. N0 pittance ( %T) m 0 c o L, ',,, 40 C - F Wavelength (nm) Figure 4. Transmittance of cured films of F2 to F4 3.5 Mechanical properties Mechanical properties of the cured films are shown in Table 4. Preparation of the films was carried out by using 160W/cm metal halide lamp with UV dose of 20kJ/m 2 under nitrogen atmosphere and the measured film thickness was 76 μm. Since F1 showed very low refractive index and low modulus, it can be used as primary coating with harder secondary coatings. Modulus and hardness of F3 was successfully improved by modifying the formulation of F2 (Table 4). Comparison of F3 and F4 suggested that coating viscosity did not affect mechanical properties as in transmittance test. Table 4. Mechanical properties of cured samples Formulation Refractive Index at 850nm Elongation (%) Young s modulus (MPa) Shore D Hardness F F F F Thermal properties Thermal degradation behavior of F1 to F3 is shown in Figure 5 and Table 6. All the samples showed 1% or less weight loss at 200 C. Especially in the formulation of F1 and F2, decomposition began at around 300 C, it seems to have enough durability required for thermal treatment of fibers, for example medical sterilization 7. Proc. of SPIE Vol O-4

-1_00 0 100 200 300 400 500 Temperature (SC) Figure 5. TG curve of the cured films Table 5. Weight loss (%) from TG analysis Formulation 200 C 300 C 400 C 500 C F 1-0.5-1.3-82.2-99.1 F 2-0.9-1.9-71.

5 -1_ Temperature (SC) Figure 5. TG curve of the cured films Table 5. Weight loss (%) from TG analysis Formulation 200 C 300 C 400 C 500 C F F F Adhesion to glass Adhesion to glass is the important requirement for the fiber claddings, adhesion promoter such as silane coupling agent is generally used. Adhesion of coatings F1 to F3 to glass was measured with tensilone meter as shown Figure 6. F1 showed excellent adhesion to glass without the promoter as shown Figure 8, the reason being considered its low modulus and elastic deformation N E\ Z 0.15 á F1 F2 F3 Figure 6. Adhesion of F1 to F3 to glass substrate Proc. of SPIE Vol O-5

6 4. CONCLUSION Non-PFOA type low refractive fluoro resins are developed for cladding of plastic clad fiber. The resins showed high transparency and low refractive index as and at 850nm. Thus these resins can be used for medical fibers and fiber lasers. REFERENCES [1] K. Schuster, J. Kobelke, J. Kirchhof, C. Aichele, K Mörl and A. B. Wojcik, High NA fibers - A Comparison of Optical, Thermal and Mechanical Properties of Ultra Low Index Coated Fibers and Air Clad MOFs, 54th International Wired and Cable Symposium, (2005). [2] Jungwoo Yoon, Kyoungbeom Min, Sanghwan Kim, Seungjo Kwak and Minjeong Kim, New Development of Low Refractive Index Polymer Cladding Resin, 56th International Wired and Cable Symposium, (2007). [3] Seungjo Kwak, Kyoungbeom Min, Jungwoo Yoon, Minjeong Kim, Sanghwan Kim and Ji-hye Lee, Development a New Polymer Clad Coating Having a High Modulus and Fast Cure Speed, 57th International Wire and Cable Symposium, (2008) [4] Alexis Mendez and T. Morse, [Specialty Optical Fibers Handbook], Academic Press, Burlington, (2007) [5] Anna B. Wojcik, M. John Matthewson, Lisa C. Klein, Paul R. Foy, Elias Snitzer and Ka Pak Wong, Mechanical behavior of silica optical fibers coated with low index, low surface energy perfluorinated polymer, Proc. SPIE 2611 Optical Network Engineering and Integrity, 110 (1996) [6] 2010/2015 PFOA Stewardship Program United States Environmental Protection Agency, 17 January 2014, < (27 January 2014) [7] Andrei A. Stolov, Brian E. Slyman, David T. Burgess, Adam S. Hokansson, Jie Li and R. Steve Allen, Effects of sterilization methods on key properties of specialty optical fibers used in medical devices, Proc. SPIE 8576 Optical Fibers and Sensors for Medical Diagnostics and Treatment Applications XIII, (2013) Proc. of SPIE Vol O-6

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