preparation of tio2 coated silicate micro-spheres for enhancing the light diffusion property of...

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Displays xxx (2014) xxx–xxx

DISPLA 1692 No. of Pages 7, Model 5G

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Contents lists available at ScienceDirect

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Preparation of TiO2 coated silicate micro-spheres for enhancing the lightdiffusion property of polycarbonate composites

http://dx.doi.org/10.1016/j.displa.2014.06.0010141-9382/� 2014 Published by Elsevier B.V.

⇑ Corresponding authors. Tel.: +86 21 66134726.E-mail address: [email protected] (P. Ding).

Please cite this article in press as: Y. Zhao et al., Preparation of TiO2 coated silicate micro-spheres for enhancing the light diffusion proppolycarbonate composites, Displays (2014), http://dx.doi.org/10.1016/j.displa.2014.06.001

Yu Zhao a,b, Peng Ding a,⇑, Chaoqun Ba a, Anjie Tang a, Na Song a, Yimin Liu a, Liyi Shi b,⇑a Research Center of Nanoscience and Nanowater Technology, Shanghai University, 99 Shangda Road, Shanghai 200444, PR Chinab School of Environmental and Chemical Engineering, Shanghai University, Shanghai 200444, PR China

a r t i c l e i n f o a b s t r a c t

2728293031323334353637

Article history:Received 23 December 2013Received in revised form 19 June 2014Accepted 20 June 2014Available online xxxx

Keywords:TiO2

Silicate micro-spheresPolycarbonateLight diffusion

The TiO2 coated silicate micro-spheres (SMS) core–shell particles (SMS@TiO2) were synthesized using thesol–gel reaction followed by calcination. The SMS@TiO2 particles were used to enhance the light diffusionproperty of polycarbonate (PC) composites. Our experimental analysis shows that the TiO2 was coated onthe SMS particles and the optimum parameters of the reaction were 4:1 of the Si/Ti molar ratio and500 �C of the calcination temperature. The UV–Vis spectra indicate that SMS@TiO2 can absorb or hinderthe UV light, which may prolong the service life of PC light diffusion materials. Compared to that of PCcomposites physically mixed with SMS + TiO2, the haze of the PC/SMS@TiO2 composites was littleaffected, while the transmittance was obviously enhanced, which can be increased from 55.5% for PC/TiO2/SMS to 70.3% for PC/SMS@TiO2 with only 0.6 wt% filler loading.

� 2014 Published by Elsevier B.V.

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1. Introduction

Recently, considerable attraction has been concentrated on thedevelopment of light diffusion materials because they are keymaterials that can be widely used in the lighting technology oflight emitting diode (LED), liquid crystal display (LCD) backlightunits, and other fields. Light diffusion materials are optical materi-als that can homogeneously diffuse a point or line light source intoa line or surface light source, respectively. To realize this function,the materials must exhibit both good transmittance and high hazeproperties [1–14].

A light diffusion material is composed of the matrix componentand the light diffusing agent. The light diffusion material is usuallyprepared by melt-mixing, in-situ blending and solution processing[15–30]. The most common method is melt-blending. The matrixcomponent of a typical light diffusion material is polycarbonate(PC). Light diffusing agents can be inorganic (silicate microspheres(SMS), glass beads, TiO2, CaCO3, MgSiO3, BaSO4, ZnS, BaS and so on)or organic (polymethyl methacrylate (PMMA), polystyrene (PS),acrylic resin and so on) [6,31,32]. Good dispersion of a light diffus-ing agent at the micro and nano levels in PC is the most importantindicator of good optical performance. But the problem is that cur-rent light diffusing agents do not have uniform and stable disper-sion in PC. Attempts have been made to physically blend two or

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more inorganic or organic light diffusing agents, but the obtainedoptical performance of the light diffusion materials was not satis-factory. As TiO2 has a high refractive index and excellent heat resis-tance, TiO2 has been suggested as a potential candidate material[33,34].

SMS is the most well-known light diffusing agent, as its constit-uent atoms are connected by Si–O bonds to form a three-dimen-sional structure. Compared with other inorganic light diffusingagents [31,32], SMS has good dispersion, good compatibility, lowwater absorption, good transparency and the ability to improvethe heat resistance of PC. But the haze improvement is not verysatisfactory for light diffusion applications. TiO2 is used in a widerange of applications, such as photocatalysis, separations, sensordevices, paints, and dye-sensitized solar cells. Because of the polarTi–O bond on the surface of TiO2, the water molecules absorbed bythe surface can dissociate to form hydroxyl groups The hydroxylgroups can improve the properties of TiO2 as an adsorbent or car-rier, as well as facilitate surface modification [35,36]. As an inor-ganic light diffusing agent, the haze of the light diffusionmaterial could be over 90% when TiO2 is composited with PC.TiO2 particles are usually hard and irregular, so it is difficult to dis-perse TiO2 particles evenly in PC and agglomeration of TiO2 parti-cles will occur. And the surface of material will be uneven if TiO2

agglomerates.SMS@TiO2 is currently a composite material of high interest

[37–41]. In this study, we prepared a series of SMS@TiO2 samplesand evaluated their light diffusing properties. The resulting light

erty of

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diffusing agent was well dispersed in PC. When PC was melt blend-ing with the light diffusing agent, the resulting light diffusionmaterial can exhibit both good transmittance and high hazeproperties.

2. Experimental section

2.1. Materials

Tetrabutyl titanate (TBT), acetylacetone (AcAc), acetic acid(HAc), and absolute ethanol (99.5%) were obtained from Sinop-harm Chemical Reagent Co., Ltd. Silicate micro-sphere (SMS,2 lm) powder was purchased from Shin-Etsu Chemical Co., Ltd.Polycarbonate (U5A12921) was purchased from Teijin Co., Ltd.

2.2. Synthesis of SMS@TiO2 core–shell particles

Titanium sol was synthesized via the sol–gel process describedbelow. Several samples were prepared according to the conditionsspecified in Table 1. For each sample, TBT was first quickly mixedwith 3.0 ml of AcAc (see Table 1). Then, 56 ml of absolute ethanolwas slowly added into the above solution with stirring at roomtemperature for 0.5 h. After that, a solution containing 1.6 ml ofH2O, 2.6 ml of HAc and 28 ml of absolute ethanol was added drop-wise into the solution with vigorous stirring at room temperaturefor 3 h to obtain the titanium sol. After evaporation of the titaniumsol to dryness, the resultant power was calcined at different tem-perature (200 �C, 300 �C, 400 �C, 500 �C and 600 �C) for 4 h toobtain the control TiO2 particles. For the batch synthesis ofSMS@TiO2 core–shell particles, 1 g of SMS powder was added tothe prepared titanium sol with stirring at room temperature for3 h. Yellow powders were obtained after the mixture was evapo-rated to dryness at room temperature. After the powder was pul-verized, it was washed and centrifuged (three times withdeionized water first, and then three times with absolute ethanol).Subsequently, the sample was dried in an oven at 110 �C to obtaina white product. After calcination at different temperatures(200 �C, 300 �C, 400 �C, 500 �C and 600 �C) for 4 h, the SMS@TiO2

core–shell particles were obtained and labeled as TS–x–y, whereTS refers to SMS@TiO2, x is the amount of TBT and y is the calcina-tion temperature.

2.3. Preparation of light diffusion materials

SMS, TiO2, SMS@TiO2 core–shell particles and the physical mix-ture of original SMS and TiO2 were used as the light diffusing fill-ers. To prepare a given light diffusion composite, 200 g of the PCwas mixed with 0.2 g, 0.4 g, 0.6 g, 0.8 g, 1.0 g, 1.2 g, 1.6 g, 2.0 g,2.4 g or 2.8 g of the light diffusing agent in the mixer. The typicalprocess was as follows: 200 g of PC was mixed with the filler in aclosed mixing machine (XSS-300, Shanghai Branch Chong plasticequipment Co., Ltd.). And the parameters were set at 260–270 �Cfor the first zone, 260–270 �C for the second zone, 30 rpm for thespeed and 1–2 min for the residence time. Then, the PC light diffu-sion material was prepared by the twin-screw extruder with a

Table 1Recipe of different samples.

Sample code The amount of TBT (ml) Molar ratio of Si and Ti

TS-0.1 0.1 60:1TS-0.5 0.5 12:1TS-1.0 1.0 6:1TS-1.5 1.5 4:1TS-2.0 2.0 3:1TS-2.5 2.5 2:1

Please cite this article in press as: Y. Zhao et al., Preparation of TiO2 coatpolycarbonate composites, Displays (2014), http://dx.doi.org/10.1016/j.displa

sheet shape die (CET-20, Coperion Keya Nanjing Machinery Co.,Ltd.). The parameters of the twin-screw extruder were set at240–250 �C for the first zone, 250–260 �C for the second zone,260–270 �C for the third zone, 270–280 �C for the fourthzone, 270–280 �C for the fifth zone, 270–280 �C for the sixth zone,270–280 �C for the head zone, and 2–2.5 MPa for the pressure. The1-mm-thick plates were obtained as the final products after extru-sion and shaping.

2.4. Characterization

The morphology and microstructure of the samples wereinvestigated by scanning electron microscopy (SEM, JSM-6700F,JEOL, Japan), X-ray diffraction (XRD, D/max-2200/PC, Rigaku,Japan), and Brunauer-Emmett-Teller specific surface area analysis(BET, ASAP 2020 M + C, Micromeritics Instrument Corporation,USA). UV–Vis diffuse reflectance (DRS) electronic spectra ofsamples were collected at ambient conditions on a UV-2501PCapparatus in the wavelength range of 200–1000 nm. The prepared1-mm-thick plates were tested by an EEL57D haze meter(Shanghai Precision Instrument Co./GB/T2410-2008).

3. Result and discussion

3.1. Morphology and structure of SMS@TiO2 core–shell particles

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3.1.1. Reaction process and surface morphology of SMS@TiO2 particlesAs illustrated in Fig. 1, the SMS@TiO2 core–shell particles were

synthesized by the following two main steps: (1) preparation oftitanium sol; (2) deposition of titanium sol on the surface of SMSto form SMS@TiO2 particles through the sol–gel reaction. To verifythe materials coated on SMS were TiO2, XRD analysis wasemployed to investigate the crystal phase of TS-1.5 calcined at dif-ferent temperatures. The typical XRD pattern in Fig. 2 reveals that,with the increase of the calcination temperature, the peaks getsharper and stronger. When the powders are calcined at 200 �Cand 300 �C, a weak diffraction peak assigned to anatase at2h = 25.2� is observed, which suggests the initial phase transforma-tion is from amorphous to anatase. When the temperature reaches400 �C, the clear diffraction peaks at 2h = 25.2, 38.5, 48.3, 54.1/55.7(overlapped) and 62.8� can be indexed to (101), (004), (200),(105/211) and (204) reflections of anatase TiO2, respectively.When the temperature reaches 500 �C and 600 �C, the appearanceof weak diffraction peaks of rutile at 2h = 27.4� indicates transfor-mation from anatase to rutile has occurred. The amount of rutileTiO2 can be quantified by comparing the integrated intensities ofanatase (101) (Aanatase) and rutile (110) reflections (Arutile)[42–44]. It shows the coexistence of anatase and rutile when thepowders are calcined at 500–600 �C.

The morphology and structure of SMS and TS-1.5 samples wereinvestigated by SEM (Fig. 3). The original SMS sample shows a uni-form diameter of about 1.7 lm (Fig. 3a and b) with smooth andclean surfaces. After immersion in TBT-containing solution, dryingand calcination, it can be seen that some porous layers were coatedon the SMS surfaces, and the diameter of the microspheresincreases to about 2.1 lm (Fig. 3c and d). The BET analysis showsthat the specific surface area of the SMS was 6.0 m2/g, and theTS-1.5’s was 106.4 m2/g, indicating a 16.7-fold increase in the spe-cific surface area.

Fig. 4 shows the evolution of the morphologies of SMS@TiO2

core–shell particles with respect to the different molar ratio of Siand Ti in the synthesis procedure. When the molar ratio is higherthan 6:1 (Fig. 4a–c), some smooth SMS surfaces can still beobserved, which indicates that the amount of TBT is not enough

ed silicate micro-spheres for enhancing the light diffusion property of.2014.06.001


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Fig. 1. The fabrication process of the SMS@TiO2 core–shell particles.

Fig. 2. XRD patterns of TS-1.5 calcined at different temperatures.

Fig. 3. SEM images of original SMS

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to completely cover all surface of SMS. When the Si/Ti molar ratioreduces to 3:1 (Fig. 4e and f), thicker coating layers and some freeTiO2 particles appear and the SMS particles do not maintain theregular spherical shape because of the overdose of TBT. Whenthe Si/Ti molar ratio is 4:1 (Fig. 3d), the sol–gel reaction and thecalcination give a relative uniform coating layers of TiO2 on thesurface of SMS spheres.

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3.1.2. Transmittance in the UV–Vis spectral rangeFig. 5 shows the UV–Vis spectra of TS-1.5 calcined at different

temperatures. In the ultraviolet region of 200–400 nm, when theTS-1.5 powders are calcined at 200–400 �C, the transmittancegradually increases as the calcination temperature is elevated;when the temperature reaches 500 �C, the transmittance is signif-icantly lowered. The reason is that the powders contain both rutileand anatase, and the rutile is conducive to the absorption of ultra-violet light. On the other hand, in the visible region, when the TS-

(a and b) and TS-1.5 (c and d).



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Fig. 4. SEM images of different molar ratio of Si: Ti, (a) 60:1, (b) 12:1, (c) 6:1, (d) 4:1, (e) 3:1, (f) 2:1, respectively.

Fig. 5. UV–Vis spectra of TS-1.5 calcined at different temperatures.

Fig. 6. Transmittance and haze of PC/TS-1.5-500.



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1.5 powders are calcined at 200–400 �C, the transmittance gradu-ally increases as the calcination temperature is elevated. Whenthe temperature reaches 500 �C, the transmittance of the samplesgets much higher which may be caused by the disturbed distribu-tion of rutile and anatase after calcination at the high temperature(P500 �C). It also indicates that SMS@TiO2 powders calcined at500 �C can absorb or hinder the UV light, which may prolong theservice life of PC light diffusion materials [45–47]. Therefore, theoptimum calcination temperature is 500 �C.

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3.2. Optical properties of the light diffusion materials

3.2.1. Optical Properties of PC/TS-1.5-500Fig. 6 gives the transmittance and haze of PC/TS-1.5-500. It can

be seen that when PC is melt-blended with the TS-1.5-500, thetransmittance of the composites decreases and the haze increaseswith the increase of the amount of TS-1.5-500. With 0.1 wt% TS-1.5-500 loading, the transmittance is 83% and the haze is only60%; and with 0.4 wt% TS-1.5-500 loading, the haze is 82%. With


0.8 wt% TS-1.5-500 loading the haze is over 90%. Beyond 0.8 wt%TS-1.5-500 loading, the haze is basically unchanged.

3.2.2. Comparison of PC/TS-1.5-500 and PC/TiO2

The transmittance of PC/TS-1.5-500 is much higher than PC/TiO2, and the transmittance decreases with the increase of theamount of TS-1.5-500 (Fig. 7a). The haze of PC/TiO2 is generallyhigh but it remains unchanged with the increase of the amountof TiO2 (Fig. 7b). This is because the poor dispersion of TiO2 inPC. Fig. 10 gives the SEM of PC/TiO2 and PC/TS-1.5-500. It can beseen that TiO2 agglomerates when composited with PC (Fig. 8a),and TS-1.5-500 has a good dispersion in PC (Fig. 8b). As is shownin Fig. 9b, the haze of PC/TS-1.5-500 increases with TS-1.5-500loading. When the filler loading exceeds 0.5 wt%, the haze of PC/TS-1.5-500 has higher haze than PC/TiO2. Despite the poor disper-sion in the material, the addition of TiO2 can enhance the haze ofthe material obviously. And When TiO2 is composited with SMS,the resulting light diffusing agent can have a good dispersion inPC and the resulting light diffusion material can reach high haze.



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Fig. 7. Transmittance (a) and haze (b) of PC/TS-1.5-500 and PC/TiO2.Fig. 9. (a) Transmittance and (b) haze of PC/TS-1.5-500 and PC/TiO2/SMS.



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3.2.3. Comparison of PC/TS-1.5-500 and PC/TiO2/SMSFig. 9 shows the transmittance and haze of PC/TS-1.5-500 and

PC/TiO2/SMS. Compared to that of the PC composite with physi-cally mixed SMS + TiO2 (i.e. PC/TiO2/SMS), the transmittance ofPC/TS-1.5-500 is obviously higher. For example, with only0.6 wt% filler loading, the transmittance for PC/TiO2/SMS is 55.5%,while the transmittance for PC/TS-1.5-500 is 70.3% (Fig. 9a). Onthe other hand, the haze of PC/TS-1.5-500 is little affected(Fig. 9b). This shows that modified SMS with TiO2 can function aslight diffusing agents, which can improve the transmittance of PCwithout reducing the haze.

Fig. 8. SEM images of PC/TiO2


3.2.4. Comparison of PC composited with different light diffusionagents

After measurements of the haze and transmittance of the lightdiffusion materials composited with 0.6 wt% loading of differentlight diffusion agents (Fig. 10), it is found that the PC compositedwith SMS gets transmittance of 73% and haze of 87%. The transmit-tance and haze of the PC-based materials composited with TS-0.1-500–TS-1.5-500 have no obvious difference with the increase ofthe amount of Ti. And when composited with TS-2.0-500 and TS-2.5-500, the transmittance of the materials is decreased but thehaze is basically unchanged. The reason is that the amount of Tiis lacking in TS-0.1-500–TS-1.0-500 so that the transmittance

(a) and PC/TS-1.5-500 (b).



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Fig. 10. Transmittance and haze of PC composed with different light diffusionagents.



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and haze are little affected with respect to the SMS filler. Theamount of Ti is overdosed in TS-2.0-500 and TS-2.5-500, whichcontain free TiO2 that can influence the transmittance of the PC-based composites.

4. Conclusions

Light diffusing agents based on SMS@TiO2 core–shell particleswere prepared in this study. The size of original SMS particlesturned from 1.7 lm to about 2 lm after coating by TiO2. By SEMimages and XRD patterns we obtained that the optimum molarratio of Si/Ti was 4:1 and the optimum calcination temperaturewas 500 �C. The UV–Vis spectra indicated that the TS sample couldabsorb UV light and prolong the life of the material after theSMS@TiO2 powders were calcined at 500 �C, which contains bothanatase and rutile. Compared to that of PC composites with phys-ically mixed SMS + TiO2, the haze of the PC/SMS@TiO2 compositeswas little affected, while the transmittance was obviouslyenhanced, which could be increased from 55.5% for PC/TiO2/SMSto 70.3% for PC/SMS@TiO2 with only 0.6 wt% filler loading.

Acknowledgements

This work was financially supported by the National NaturalScience Foundation of China (51303101), project foundation ofShanghai (12520500100, 12521102502), Natural Science Founda-tion of Shanghai (09ZR1411600), and China Postdoctoral ScienceFoundation (20110490709). The authors thank Prof. Yuliang Chuand Prof. Weijun Yu for help with the SEM and TEMmeasurements.

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