Photostabilization and Cure Kinetics of UV-Curable Optical Resins Containing Photostabilizers

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1 Mcromoleculr Reserch, Vol. 15, No. 6, pp (2007) Photostbiliztion nd Cure Kinetics of UV-Curble Opticl Resins Contining Photostbilizers Jung-De Cho Institute of Photonics & Surfce Tretment, Q-Sys Co. Ltd., Gwngju , Kore Sung-Hw Kim nd In-Cheol Chng Opticl Solution Lb., Centrl R&D Institute, Smsung Electro-mechnics, Suwon , Kore Kwon-Seok Kim nd Jin-Who Hong* Deprtment of Polymer Science & Engineering, Chosun University, Gwngju , Kore Received My 23, 2007; Revised August 6, 2007 Abstrct: The photostbiliztion nd cure kinetics of UV-curble, opticl resins contining vrious formultions of photostbilizers were investigted to determine the system with the highest cure conversion nd durbility. Photo- DSC nlysis reveled tht incresing the concentrtion of UV bsorber (UVA) decresed both the cross-link density nd the cure rte due to competition for the incident photons between the photoinititor nd the UVA, wheres including hindered mine light stbilizer (HALS) hrdly ffected either the cure conversion or the cure rte due to its very low bsorption of 365 nm. This result ws confirmed by FTIR-ATR spectroscopy nd UV-visible spectroscopy nlyses. QUV geing experiments showed tht the cure conversion nd durbility were the highest for the UVA/ HALS formultion t rtio of 1 : 2, which is due to their synergistic ction. Keywords: photostbiliztion, cure kinetics, UV-curble opticl resin, photo-dsc, FTIR-ATR. Introduction UV-initited polymerized mterils hve recently received considerble ttention due to their rpid rectivity, environmentlly friendly (solvent-free) curing, nd the vriety of formultions vilble for pplictions such s electronics, photonics, cotings, dhesives, dentl-restortive mterils, vrnishes, nd inks. 1-3 In electronics-industry pplictions, UV-curble mterils hve been widely used in photolithogrphic fbriction, UV embossing repliction processes, nd UV nnoimprint lithogrphy processes. 4-9 Mny electronics compnies competing in the mobile phone mrket focus on the slimness of these devices so s to enhnce their portbility. A promising pproch is to employ cmer modules whose thickness is comprble to tht of disply pnel, fcilitting new rrngement of the cmer module t the rim of the disply pnel. A proposed wfer-scle cmer module ws fbricted using wfer-scle UV embossing repliction process in which UV-curble resins formed wfer-scle lenses, which lso gve lens designers significnt freedom in mteril selection. 9 However, the durbility of UV-cured *Corresponding Author. E-mil: jhhong@chosun.c.kr embossed lenses hs not been fully considered in outdoor pplictions, given tht UV solr rdition cn cuse the photodegrdtion, discolortion, nd serious decrese in the light trnsmittnce of UV-cured opticl resin. Introducing photostbilizer such s UV bsorber (UVA) into the resin formultion not only increses the durbility of the cured resin subject to photogeing but lso effectively screens UV solr rdition, thus gretly reducing the photodegrdtion of UV-cured orgnic mterils However, the ddition of UVA to the resin formultion is expected to reduce the cure speed due to its filtering the UV rdition. Other photostbilizers such s hindered mine light stbilizer (HALS) re known to hve little effect on the UV curing process whilst hving strong stbilizing effect on the photoinduced degrdtion. Moreover, the presence of HALS ws found to significntly increse the life of the UVA, nd thus prolong its screening effect upon wethering. 10 In order to improve the wethering resistnce of UV-cured opticl resin, it is importnt to understnd how photostbilizers influence the curing behvior nd kinetics within the orgnic mtrix. An ccurte kinetics nlysis of the systems would help in designing the most pproprite formultion nd controlling the curing process. The im of the present 560

2 Photostbiliztion nd Cure Kinetics of UV-Curble Opticl Resins Contining Photostbilizers study ws to quntify the enhncement of the wethering resistnce of UV-cured opticl resin obtined by the ddition of different types of photostbilizer. The effects of different photostbilizers on both the cure kinetics nd the photodegrdtion process were investigted using photodifferentil scnning clorimetry (photo-dsc), Fourier-trnsform infrred spectroscopy with ttenuted totl internl reflection (FTIR-ATR) spectroscopy, UV-visible spectroscopy, nd QUV wetherometer, from which the most efficient system ws determined with respect to the cure conversion nd durbility. Experimentl Mterils nd Curing Process. UV-curble fluorinted crylte (ChemOptics) contining photoinititor ws used s n opticl resin since it is well known tht photopolymers contining fluorine exhibit good relese chrcteristics, with the resin exhibiting very low shrinkge. The UVA Tinuvin 328 (Cib Specilty Chemicls) nd the HALS Tinuvin 292 (Cib Specilty Chemicls) were used s photostbilizers. The structures of these compounds re shown in Figure 1, nd both were used s received. The liquid formultions were pplied to glss substrte t thickness of ~100 µm using wire-wound rod, nd the wet films were exposed to UV rdition from metl hlide lmp (EFOS UV System) vi fiberoptic light pipe under n inert tmosphere such s nitrogen. The UV-light intensity t the smple ws 14 mw/cm 2 t 365 nm. Figure 1. Chemicl structures of () the UVA (Tinuvin 328) nd (b) the HALS (Tinuvin 292) used in this study. Photo-DSC. The photo-dsc experiments were performed using differentil scnning clorimeter equipped with photoclorimetric ccessory (TA 5000/DPC System). The initition light source ws 200-W high-pressure mercury lmp, which gve UV light intensity t the smple of 14 mw/cm 2 t 365 nm. Smples weighing 1.0 ± 0.1 (men ± SD) mg were plced in uncovered luminum pns, nd reference luminum pn ws left empty. The exothermic het chieved for the photocuring t 70 o C ws 273 J/g, nd ws considered s the totl het, H totl, for the fully cured resin. This vlue ws used in the subsequent nlysis. TA Instruments softwre ws employed to obtin the results from the photo-dsc experiments. FTIR-ATR Spectroscopy. FTIR-ATR spectr of the smples were recorded on Perkin-Elmer Spectrum GX device using horizontl ATR ccessory for FTIR spectroscopy (PIKE Technologies), which ws continuously purged with purified ir tht ws free of crbon dioxide nd wter (using desiccnt-bsed ir dryer from New Techniques). To increse the signl-to-noise rtio, ech set of reference nd smple spectr shown here represents the verge of 128 scns recorded t resolution of 8 cm -1. A ZnSe crystl ws used s the internl reflection element, nd the incident ngle ws 45 o. The depth of penetrtion (D p ), which depends on the wvelength (λ), cn be clculted from the following eqution: 13,14 D p λ = π n 1 sin ( θ - n 2 ) where θ is the ngle of incidence, nd n 1 nd n 2 re the refrctive indices of the reflection element nd the smple, respectively. UV-Visible Spectroscopy. The bsorption spectr of the photostbilizers (Tinuvin 328 nd Tinuvin 292) were obtined using Cry 3 Bio UV-visible spectrophotometer. Dilutions of 0.01 g/l in methylene chloride were prepred, nd qurtz cells with pth length of 1.0 cm were used in the nlysis. Photogeing Experiments. Accelerted rtificil UVinduced geing ws implemented in QUV wetherometer (Q-Pnel Lb Products) tht simultes the dmge cused by sunlight, rin, nd dew. The UV-cured smples coted onto glss substrte t thickness of ~100 µm were mounted on luminum smple holders nd subjected to UV geing by exposure to UVB-313 fluorescent UV lmps under wet cycle conditions: 8 h of UV irrdition t 70 o C, followed by 4 h of drk condenstion t 50 o C for up to 2,000 h. The UV light intensity ws 0.6 W/m 2 t 313 nm. Yellowness index (YI) ws mesured by colorimeter (Chrom Meter CR- 300, Minolt) using the CIE L**b* system (Commission Interntionle de l Eclirge, 1976), nd clculted from the following eqution: 17 (1) YI = [(1.28 X Z)/Y ] 100 (2) Mcromol. Res., Vol. 15, No. 6,

3 J.-D. Cho et l. where X, Y, nd Z re the tristimulus vlues. Ech reported mesurement represents the verge vlue of three mesurements. Results nd Discussion The UV-curble opticl resin formultions prepred by dding photostbilizers t vrious concentrtions to the opticl resin re listed in Tble I. The cure kinetics ws investigted using photo-dsc to clrify the photopolymeriztion process. Photo-DSC experiments re cpble of providing kinetics dt in which the mesured het flow cn be converted directly to the ultimte percentge conversion nd polymeriztion rte for given mount of formultion, with the obtined dt reflecting the overll curing rection of the smple However, it should be noted tht the cure kinetics dt used in this study re essentilly empiricl nd do not provide ny mechnistic insight. Figure 2 shows the photo-dsc exotherms for the photopolymeriztion of the UV-curble opticl resin formultions. The mounts of het relesed, the ultimte percentge conversions, the pek mxim, nd the mximum polymeriztion rtes Tble I. Formultions of UV-Curble Opticl Resins Contining Photostbilizers t Vrious Concentrtions. Dt Vlues re weight Percentges Component A B C D E F Opticl resin Tinuvin 328 b Tinuvin 292 c ChemOptics. b UVA (Cib Specilty Chemicls). c HALS (Cib Specilty Chemicls). (R p, mx ) derived from Figure 2 re collected in Tble II. These kinetics dt indicted tht both the cross-link density (higher exothermic het nd higher percentge conversion men higher cross-link density) nd the cure rte (lower pek mximum nd higher R p, mx men fster cure rte) decresed with incresing concentrtion of the UVA photostbilizer (Tinuvin 328). 1,18 This result is s expected from competition for the incident photons between the photoinititor nd the UVA. In contrst, quite different behvior ws observed in the presence of the HALS photostbilizer (Tinuvin 292): in the cse of formultion F (contining only HALS t 1 wt%), there were lmost no chnges in the kinetics dt, indicting tht the HALS hrdly ffected either the cure conversion or the cure rte. This is supported by dditionl experiments performed using UV-visible spectroscopy. Figure 3 shows the bsorption spectr of the photostbilizers in the rnge nm, nd indictes tht the bsorption t 365 nm is high for UVA but very low for HALS, inferring tht the HALS does not compete with the photoinititor for the bsorption of incident photons. Other experiments were designed to obtin some mech- Tble II. Exotherm Dt Obtined by Photo-DSC on the Photopolymeriztion of the Formultions Listed in Tble I H (J/g) Conversion (%) Pek Mximum (min) R p,mx Formultion (1/min) A B C D E F Rp, mx: mximum polymeriztion rte (R p = dα/dt, where α is the frction of resin converted). Figure 2. Photo-DSC exotherms for the photopolymeriztion of UV-curble opticl resin formultions A-F t 30 o C. Smple weight: 1.0 mg, light intensity: 35 mw/cm 2 t 365 nm. Figure 3. Absorption spectr of Tinuvin 328 (the UVA) nd Tinuvin 292 (the HALS). 562 Mcromol. Res., Vol. 15, No. 6, 2007

4 Photostbiliztion nd Cure Kinetics of UV-Curble Opticl Resins Contining Photostbilizers nistic insight using FTIR-ATR spectroscopy. Figure 4 shows the FTIR-ATR spectr of formultion A ccording to the UV curing time. The bnd t 1600 cm -1 (due to romtic stretching vibrtion of C = C bonds) ws used s n internl reference pek since its intensity in the resin ws stble during the cross-linking rection, wheres the bnd t 810 cm -1 (due to deformtion modes of the = CH cryl group) ws lso used to quntify the extent of cure becuse the prevlence of this bnd decreses s the cure rection proceeds. The FTIR-ATR reltive intensity rtios of these two bnds (810 cm -1 /1600 cm -1 ) of formultions A-F fter 60 s of UV curing re presented in Figure 5, which revels tht the extent of the cure rection decresed s the concentrtion of UVA incresed, but ws hrdly ffected by HALS. This experimentl observtion is consistent with the photo-dsc Figure 6. Chnges in the yellowing index of formultions A-F subject to QUV geing. Figure 4. FTIR-ATR spectr of formultion A ccording to the UV curing time. Smple thickness: ~100 µm, light intensity: 14 mw/cm 2 t 365 nm. experimentl results. The wethering resistnce of the UV-cured opticl resins ws evluted by monitoring the yellowing occurring upon continuous exposure to fluorescent lmps in QUV wetherometer. The chnges in the yellowing index of the formultions subject to QUV geing re shown in Figure 6, which indictes tht the wether resistnce ws higher for formultions with photostbilizers (B-F) thn for tht without photostbilizer (A). Compring the results for formultions B-F, it is noteworthy tht before 200 h of QUV geing (i.e., the initil UV exposure stge), the yellowness index decresed with n incresing concentrtion of UVA, which implies tht the UVA effectively photostbilized the UV-cured opticl resins. However, the opposite results were observed fter 500 h of QUV geing. This my be ttributble to incomplete cure conversion due to internl filtering by the UVA during the curing process (Tble II) resulting in mny unrected double-bond molecules remining in the orgnic mtrix fter UV curing, with these molecules ccelerting the yellowing of the UV-cured opticl resin. The presence of the HALS significntly increses the life of the UVA, nd thus will prolong its screening effect upon wethering. 10 Formultion D (contining equl mounts of UVA nd HALS) exhibited good resistnce to photodegrdtion. However, the best photostbiliztion nd the highest cure conversion nd durbility were obtined with formultion E (UVA nd HALS t rtio of 1 : 2). Conclusions Figure 5. FTIR-ATR reltive intensity rtios of formultions A-F fter 60 s of UV curing. Photo-DSC, UV-visible spectroscopy, FTIR-ATR spectroscopy, nd QUV wetherometer hve been used to investigte the photostbiliztion nd cure kinetics of UV-curble opticl resins contining photostbilizers in order to determine the system with the highest cure conversion nd dur- Mcromol. Res., Vol. 15, No. 6,

5 J.-D. Cho et l. bility. Photo-DSC nlysis reveled tht incresing the concentrtion of UVA decresed both the cross-link density nd the cure rte, which ws due to competition for the incident photons between the photoinititor nd the UVA, wheres the HALS hd little ffect on either the cure conversion or the cure rte due to it exhibiting little bsorption t 365 nm. This result ws confirmed by FTIR-ATR spectroscopy nd UV-visible spectroscopy nlyses. The QUV geing experiments showed tht the cure conversion nd durbility were highest in the UV-curble opticl resin with UVA nd HALS t rtio of 1 : 2 due to their synergistic ction in which the presence of HALS significntly incresed the life time of the UVA, subsequently prolong its screening effect upon wethering. Acknowledgement. This study ws supported by reserch funds from Chosun University in References (1) P. K. T. Oldring, Chemistry & Technology of UV & EB Formultion for Cotings Inks & Pints, Vols. 1-4, SITA Technology, London, (2) A. Reiser, Photorective Polymers, Wiley, New York, (3) C. G. Roffey, Photopolymeriztion of Surfce Cotings, Wiley, New York, (4) L. Dellmnn, S. Roth, C. Beuret, L. Prtte, G.-A. Rcine, H. Lorenz, M. Despont, P. Renud, P. Vettiger, nd N. de Rooij, Microsyst. Technol., 4, 147 (1998). (5) V. Seidemnn, S. Bütefisch, nd S. Büttgenbch, Sens. Actutors A: Phys., 97-98, 457 (2002). (6) T. Kyu, S. Meng, H. Durn, K. Nnjundih, nd G. R. Yndek, Mcromol. Res., 14, 155 (2006). (7) S. H. Nm, J. W. Kng, nd J. J. Kim, Mcromol. Res., 14, 114 (2006). (8) Y. Hiri, S. Hrd, S. Isk, M. Kobyshi, nd Y. Tnk, Jpn. J. Appl. Phys., 41, 4186 (2002). (9) D. I. Shin, S. H. Kim, H. S. Jeong, S. C. Lee, Y. S. Jin, J. E. Noh, H. R. Oh, K. U. Lee, D. H. Shin, nd S. H. Song, Proceedings of SPIE-IS&T Electronic Imging, 6068, 60680Q-1 (2006). (10) C. Decker nd S. Biry, Prog. Org. Cot., 29, 81 (1996). (11) C. Decker nd K. Zhouily, Polym. Degrd. Stb., 64, 293 (1999). (12) S. Zeren, Mcromol. Symp., 187, 343 (2002). (13) J. D. Cho, S. G. Kim, nd J.W. Hong, J. Appl. Polym. Sci., 99, 1446 (2006). (14) J. W. Hong nd H. K. Kim, Mcromol. Res., 14, 617 (2006). (15) S. C. Clrk, C. E. Hoyle, S. Jonsson, F. Morel, nd C. Decker, Polymer, 40, 5063 (1999). (16) J. D. Cho, Y. B. Kim, H. T. Ju, nd J. W. Hong, Mcromol. Res., 13, 362 (2005). (17) J. D. Cho, H. K. Kim, Y. S. Kim, nd J. W. Hong, Polym. Test., 22, 633 (2003). (18) J. D. Cho, H. T. Ju, nd J. W. Hong, J. Polym. Sci.; Prt A: Polym. Chem., 43, 658 (2005). (19) J. D. Cho, H. T. Ju, Y. S. Prk, nd J. W. Hong, Mcromol. Mter. Eng., 291, 1155 (2006). 564 Mcromol. Res., Vol. 15, No. 6, 2007

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