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The cure performance of modified ZrO2 coated by paraffin via projection based stereolithography
Author’s Accepted Manuscript The cure Stereolithography performance of modified ZrO2 coated by paraffin via projection based stereolithography Li Yanhui, Chen Yong, Wang Minglang, Li Lian, Wu Haidong, He Fupo, Wu Shanghua www.elsevier.com/locate/ceri
PII: DOI: Reference:
S0272-8842(18)32794-9 https://doi.org/10.1016/j.ceramint.2018.10.003 CERI19695
To appear in: Ceramics International Received date: 25 August 2018 Revised date: 28 September 2018 Accepted date: 1 October 2018 Cite this article as: Li Yanhui, Chen Yong, Wang Minglang, Li Lian, Wu Haidong, He Fupo and Wu Shanghua, The cure Stereolithography performance of modified ZrO2 coated by paraffin via projection based stereolithography, Ceramics International, https://doi.org/10.1016/j.ceramint.2018.10.003 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
The cure Stereolithography performance of modified ZrO2 coated by paraffin via projection based stereolithography Li Yanhui1*, Chen Yong2, Wang Minglang1, Li Lian1, Wu Haidong1, He Fupo1, Wu Shanghua1* 1
School of Electromechanical Engineering, Guangdong University of Technology, Guangzhou 510006, Guangdong, China
2
Viterbi School of Engineering, University of Southern California, CA 90089-0193, America E-mail:[email protected] [email protected] *
Corresponding author:
Projection based stereolithography (pSL) is widely applied to fabricate complicated ceramic parts. The preparation of ceramic suspension is critical for the process, especially, since large refractive index difference between ceramic and photosensitive resin would lead to an intense light scattering. In this paper, a ball-milling method is used to coat ZrO2 with paraffin that has a similar refractive index to that of resin, and the coated layer is measured to be around 10 nm. The modified ZrO2 is characterized by IR and TEM, and the photocuring behaviors are also investigated in terms of cure depth and excess cure width. The cure depth of modified ZrO2 increases by13.2% and 16.8% compared to the one without coating. At the same time, the excess cure width decreases by 77.1% and 62.7% using the same projection time. Moreover, ceramic ring prepared with the modified ZrO2 has a shrinkage of about 27.2%, and a density of 91.8%, smaller than the one prepared with ZrO2 without coating due to a burnout of paraffin. In conclusion, the paraffin coating layer could reduce the light scattering and thus improve the curing performance of ZrO2 suspension，especially, the excess cure width. Key words: Zirconia, refractive index, coatings, Projection based stereolithography
I. Introduction Projection based Stereolithography (pSL) such as the one using Digital Light Processing (DLP) device plays an increasingly important role in fabricating ceramic parts with complex structures. This three-dimensional (3D) printing technique has been widely used in the fields of biomedical and aerospace industries [1-7]. Photopolymerizable ceramic suspensions for pSL, composing of ceramic particles, photoinitiator and monomer, are a key for the process to build ceramic parts with complex structures [8, 9]. When the UV light illuminates the photopolymerizable suspensions, the photoinitiator will release free radicals, leading to the polymerization of monomers. The ceramic particles located in the matrix then form a green ceramic body due to polymerization. At last, the green ceramic body is debinded and sintered to a dense ceramic part with the designed structure [10]. As shown in the previous studies, during the curing process, the photocuring performance is affected by the properties of photocurable resin and ceramic powders, especially, the refractive index difference between ceramic powder and photocurable resin, which would lead to light scattering. The light scattering brings light attenuation in both the horizontal and vertical directions, which results in the decrease of cure depth and the increase of excess cure width when the ceramic suspensions are exposed by UV light [11-13]. Consequently, the building resolution and feature precision of fabricated ceramic parts are reduced. Cure depth is related to the suspension parameters, including ceramic powder and photocurable resin. According to Beer-Lambert’s semi-logarithmic equation, the cure depth is shown as [10] : Cd = Sd
=
(1)
Where E0 and Ed are the incident energy dose and the critical energy dose, respectively. Sd is photo-suspension sensitivity in cure depth, relevant to the energy attenuation by the absorption of photocurable resin and the scattering of the ceramic suspensions.
is the attenuation factor to reveal the effects of ceramic suspensions
on curing. In ZrO2 suspensions, the light absorption of ZrO2 is neglected, because
there is almost no light
absorption between 400 nm and 5 μm [14]. Light scattering
of ceramic power is similar to the Mie model, and proportional to the refractive index contrast. Sd =
(2)
Q = βΔn2
(3)
Δn = ncer – n0
(4)
Therefore, cure depth of ceramic suspensions is inversely proportional to their refractive index contrast. In comparison, cure width is composed of the width of the light spot diameter (Wlight) and the excess width surrounding the light (Wex), as shown in equation (5) [10, 12]: Wcure = Wlight + Wex
(5)
As with the cure depth, Wex is closely related to the suspension parameters, specially the ceramic properties, such as the slurry particle concentration, the diameter and the refractive index, and so on. Following the quasi-Beer-Lambert model, the excess width is presented as [10]: Wex = SW
=
(6)
Where Sw, Ew and Aw are the sensitivity, apparent critical energy dose, and the width attenuation factor in the width direction, respectively. Ew and Aw are depended on the scattering of ceramic particles and the absorption of the photocurable resin. There has been no equation to interpret the light attenuation of cure width, but, in principle, the attenuation is similar to the cure depth. The excess cure width is proportional to the refractive index difference between ceramic particles and photocurable resin. To confirm the effects of light scattering due to the refractive index difference, most researchers focused on reducing the refractive index difference between ceramic particle and photocurable resin. Gentry and Halloran adjusted the refractive index difference by varying the added diluent [10]. However, no studies are found to study the effects of modifying the ceramic particles. Therefore, this paper focuses on
modifying ceramic particles by a developed coating method. The modified ZrO2 is coated by paraffin with a refractive index of 1.436 that is similar to the photocurable resin used in the study. The suspensions, prepared with pure or modified ZrO2 and photocurable resin, are illuminated with a 405 nm UV lamps to study its curing performance.
II. Experimental procedure A certain amount of paraffin dissolved in petroleum is dropped into ethyl alcohol with ZrO2 suspending, and the above suspension is ball-milled for 2 hours. ZrO2 coated by paraffin is then obtained by distillation and drying in the air. The photosensitive resin is consisted of PPTTA (ethoxylated pentaerythritol tetraacrylate, Aladdin), HDDA (1, 6-Hexanediol diacrylate, Aladdin), U600 (di-functional aliphatic urethane acrylate, Aladdin) and 1-Octanol (octanol, Aladdin). The homogeneous photopolymerizable ceramic suspensions composed of pure or modified ZrO2 and resin that are mixed by ultrasonic agitation and ball-milling for 2 hours. The green bodies of ring are fabricated using the above ceramic suspension via DLP method, and then sintered at 1450℃ in muffle furnace. The modified ZrO2 is characterized by IR and TEM, and the refractive index of photosensitive resin is measured by Abess refractometer. Photopolymerization experiments are conducted using a projection-based stereolithography printer. In this paper, the grid model is used to examine the cure depth and excess width of the built lines.
III. Results and discussion For investing the coating effects, the chemical structures of ZrO2 and paraffin are presented in Fig. 1(I). Paraffin is mainly mixed by straight chain n-alkanes CH3– (CH2)(n-2)–CH3. Fig.1 (II) shows the infrared absorption spectrum of pure paraffin (a), ZrO2 coated by paraffin (b), and pure paraffin (c), respectively. The chemistry structure of paraffin is There is a wide and weak absorption peak of –OH stretching
vibration between 3300 and 3700 cm-1 in three infrared absorption spectrums. In Fig. 1(a) and (c), there exist two strong peaks at 2915 and 2854 cm-1, which belong to stretching vibration of C-H. There are another two peaks at 1464 and 1376 cm-1, corresponding to bending vibration of C-H. The characteristic peak of paraffin emerges at 728 cm-1, which is out of plane bending vibration absorption peak of C-H [15-17]. In Fig. 1(c) of modified ZrO2, the peak at 510 cm-1 attributes to Zr-O[18]. Comparing to the infrared absorption spectrums of pure paraffin (a) and pure ZrO2 (b), the peak of Zr-O and the peaks of C-H have no shift, which illustrates that paraffin physically adheres to ZrO2 surface, in accord with the chemical structure of paraffin. Fig. 2 shows the TEM images of unmodified and modified ZrO2 with paraffin. In Fig. 2(a), there are lots of clear lattice planes at the edge of ZrO2 particle. It is observed that ZrO2 is coated with a layer of paraffin in Fig. 2(b). The theoretical thickness of paraffin layer can be calculated by the following equation [18]: d=
=
⁄
(7)
The calculated layer thickness of paraffin based on the equation is 10.31 nm. It is measured to be about 9.51 nm by Digital Micograph Demo software as shown in Fig. 2(b), which is agreed well with the calculated value. In order to study the effects of coating ZrO2 particles with thin layer on the photocuring behaviors of ceramic suspension, we measure the cure depth and the width of single line of the designed grid model as shown in Fig.3. Fig. 3 shows the images of cured grid of ZrO2 without and with paraffin coating using the curing time of 10 s and 20 s. Fig. 3(a) and Fig. 3(b) show the cured grid profiles of ZrO2 without coating using the curing time of 10 s and 20 s, respectively. The cured lines are uniform and smooth with a depth of about 42.51 and 55.79 µm, correspondingly. The cured grid profiles and the micro-structure of ZrO2 coated with paraffin using the curing time of 10 s and 20 s are presented in Fig. 3(c) and 3(d). The measured cure depths of the cured line are about 48.12 and 65.16 µm, respectively. And the micro-structure of the cured line based on ZrO2 coated by paraffin is similar to that based on the ZrO2 without paraffin coating. Compared with ZrO2 without paraffin
coating, the cure depth of modified ZrO2 with paraffin coating increases by 13.2% and 16.8% using the curing time of 10 s and 20 s, respectively. To study the effects of paraffin coating on phtotocuring performance, we further characterize the excess cure width of the designed line model. Fig. 3-3 presents the cure width micrographs of ZrO2 with and without paraffin coating. Fig. 3(a, c) and 5(b, d) are the cure width images using the curing time of 10 s and 20 s, respectively. The cured lines have uniform and smooth edges for the curing time of 10 s; in comparison, the cured lines using the curing time of 20 s are jagged. The excess cure width of ZrO2 without coating increases from 193.8 μm to 491 μm, and the one with paraffin coating increases from 44.3 μm to 183.3 μm, when the cure time extends from 10 s to 20 s, as shown in Fig.3-3. Compared with ZrO2 without coating, the excess cure width of ZrO2 with paraffin coating decreases by 77.1% and 62.7% using the curing time of 10s and 20 s, respectively. Acorrding to Equation (2), ZrO2 and photocurable resin (DSM-AGI Co. Ltd., Taipei, Taiwan) have a refractive index of 2.165 and 1.455, respectively. The refractive index difference is 0.71, which leads to an intense scattering, as shown in Fig. 4(a). However, the ZrO2 suspension, prepared with modified ZrO2 with paraffin coating that has a refractive index of 1.436, which has a small refractive index difference (only 0.021 at the interface between photocurable resin and the paraffin coating layer). Accordinlgy, the optical path diagram is shown in Fig. 6(b). The incident light has a slight energy attenuation for light scattering at he interface between photocurable resin and the coating layer. As a result, the cure depth of ZrO2 coated with paraffin is deeper than the ZrO2 without paraffin coating; correspondingly, the excess cure width of ZrO2 coated with paraffin decreases compared with the ZrO2 without paraffin coating. Fig. 5 presents ZrO2 ring images of the green and sintered bodies. Fig. 5(a) shows the ring photographs of green bodies with the radius of 13.6 mm that were prepared by ZrO2 with and without paraffin coating, respectively. Correspondly, the radius of bodies obtained by thermal debinding and sintering are 9.90 and 9.85 mm, which show shrinkages of 27.2% and 27.6%, respectively. Archimedes principle is employed to
calculated the density of the rings, and the results show that their density is 91.8% and 95.8%, of the theortical density, respectively. With the same solid content, the density of ring prepared by modified ZrO2 is slightly smaller than the other one, due to the burnout of paraffin during the debinding and sintering process. In order to further examine the effect of coating layer on photocuring performance, it requires varied coating layers, such as inorganic substance. In our future work, we will examine the effects of Al2O3 coating layer on the photocuring behaviours of AlN.
IV. Conclusions Light scattering resulted from the refractive index difference between the ceramic and photocurable resin is significant to the cure depth and excess cure width. Cure depth and excess cure width are both followed by a quasi-Beer-Lambert equation. Cure depth sensitivity Sd is inversed to the refractive index difference, while the cure width sensitivity Sw is proportional to the refractive index difference. Thus, decreasing the refractive index contrast is beneficial for both photocuring properties. The cure depth of ZrO2 coated with paraffin increases by 13.2% and 16.8% using curing time of 10 s and 20 s, respectively. Correspondingly, the excess cure width decreases by 77.1% and 62.7% using the curing time of 10 s and 20 s, respectively. In conclusion, the coating layer on ceramic particles with a material (e.g. paraffin) that has a small refractive index difference to that of photocurable resin is effective on reducing light scattering during the photocuring process.
Acknowledges This work is financially supported by Major Science and Technology Project of Guangdong Province (Grant Nos. 2017B090911011 and 2016B090915002) and Local Innovative and Research Team Project of Guangdong Pearl River Talents Program, (NO. 2017BT01C169).
References [1] T. Chartier, C.C., F. Doreau, M. Loiseau, Stereolithography of structural complex ceramic parts. J. Mater. Sci., 37(2002) 3141-3147. [2] Chen, W., S. Kirihara,Y. Miyamoto, Fabrication and Measurement of Micro Three-Dimensional Photonic Crystals of SiO2Ceramic for Terahertz Wave Applications. J. Am. Ceram. Soc., 90(2007) 2078-2081. [3] Osman, R.B., A.J. van der Veen, D. Huiberts, D. Wismeijer,N. Alharbi, 3D-printing zirconia implants; a dream or a reality? An in-vitro study evaluating the dimensional accuracy, surface topography and mechanical properties of printed zirconia implant and discs. J Mech Behav Biomed Mater, 75(2017) 521-528. [4] Liu, W., H. Wu, M. Zhou, R. He, Q. Jiang, Z. Wu, et al., Fabrication of fine-grained alumina ceramics by a novel process integrating stereolithography and liquid precursor infiltration processing. Ceram. Int., 42(2016) 17736-17741. [5] Shao, H., X. Ke, A. Liu, M. Sun, Y. He, X. Yang, et al., Bone regeneration in 3D printing bioactive ceramic scaffolds with improved tissue/material interface pore architecture in thin-wall bone defect. Biofabrication, 9(2017) 025003. [6] Yang, Y., X. Li, X. Zheng, Z. Chen, Q. Zhou,Y. Chen, 3D-Printed Biomimetic Super-Hydrophobic Structure for Microdroplet Manipulation and Oil/Water Separation. Adv. Mater., 30(2018). [7] Yang, Y., Z. Chen, X. Song, B. Zhu, T. Hsiai, P.-I. Wu, et al., Three dimensional printing of high dielectric capacitor using projection based stereolithography method. Nano Energy, 22(2016) 414-421. [8] Tomeckova, V.,J.W. Halloran, Flow behavior of polymerizable ceramic suspensions as function of ceramic volume fraction and temperature. J. Eur. Ceram. Soc., 31(2011) 2535-2542. [9] Pradhan, M.,P. Bhargava, Influence of Sucrose Addition on Rheology of Alumina Slurries Dispersed with a Polyacrylate Dispersant. J. Am. Ceram. Soc., 88(2005) 833-838. [10] Gentry, S.P.,J.W. Halloran, Light scattering in absorbing ceramic suspensions:
Effect on the width and depth of photopolymerized features. J. Eur. Ceram. Soc., 35(2015) 1895-1904. [11] Tomeckova, V.,J.W. Halloran, Cure depth for photopolymerization of ceramic suspensions. J. Eur. Ceram. Soc., 30(2010) 3023-3033. [12] Gentry, S.P.,J.W. Halloran, Depth and width of cured lines in photopolymerizable ceramic suspensions. J. Eur. Ceram. Soc., 33(2013) 1981-1988. [13] Zak C. Eckel, C.Z., John H. Martin, Alan J. Jacobsen, William B. Carter, Tobias A. Schaedle, Additive manufacturing of polymer-derived ceramics. Science, 351(2016) 58-62. [14] Emeline, A., Kataeva, GV, Litke, AS, Rudakova, AV, Ryabchuk, VK, Serpone, N., Spectroscopic and photoluminescence studies of a wide band gap insulating material Powdered and colloidal ZrO2 sols. Langmuir, 14(1998). [15] Qiu, X.Z., Y. Tao, X.Q. Xu, X.H. He,X.Y. Fu, Synthesis and characterization of paraffin/TiO2-P(MMA-co-BA) phase change material microcapsules for thermal energy storage. J. Appl. Polym. Sci., 135(2018). [16] Jiesheng, L., L. Faping, G. Xiaoqiang,Z. Rongtang, Experimental research in the phase change materials based on paraffin and expanded perlite. Phase Transitions, 91(2018) 631-637. [17] Zhang, X., X. Li, Y. Zhou, C. Hai, Y. Shen, X. Ren, et al., Calcium Chloride Hexahydrate/Diatomite/Paraffin as Composite Shape-Stabilized Phase-Change Material for Thermal Energy Storage. Energy Fuels, 32(2017) 916-921. [18] Liu, W., Z. Xie, X. Yang, Y. Wu, C. Jia, T. Bo, et al., Surface Modification Mechanism of Stearic Acid to Zirconia Powders Induced by Ball Milling for Water-Based Injection Molding. J. Am. Ceram. Soc., 94(2011) 1327-1330.
Fig.1 the chemical structure of ZrO2 and paraffin ( I ), the infrared absorption spectrums ( II ) of (a) pure paraffin, (b) pure ZrO2, and (c) ZrO2 coated by paraffin. Fig. 2 TEM micrographs of unmodified (a) and modified ZrO2 by paraffin (b) Fig.3 Cured grid profiles and images from the cure depth and width of ZrO2 without coating (a and b) and with paraffin coating (c and d), using curing time of 10 s (a and c) and 20 s(b and d). Fig.4 The scattering diagram of ZrO2 particles (a) without and (b) with paraffin coating. Fig. 5 photographs of a ring model. (a) Green bodies prepared with ZrO2 without (right) and with paraffin coating (left); and (b) sintered parts prepared with ZrO2 without (right) and with paraffin coating (left) . Figures Fig. 1
Fig. 2
Fig. 3
Fig. 4
Fig. 5
PII: DOI: Reference:
S0272-8842(18)32794-9 https://doi.org/10.1016/j.ceramint.2018.10.003 CERI19695
To appear in: Ceramics International Received date: 25 August 2018 Revised date: 28 September 2018 Accepted date: 1 October 2018 Cite this article as: Li Yanhui, Chen Yong, Wang Minglang, Li Lian, Wu Haidong, He Fupo and Wu Shanghua, The cure Stereolithography performance of modified ZrO2 coated by paraffin via projection based stereolithography, Ceramics International, https://doi.org/10.1016/j.ceramint.2018.10.003 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
The cure Stereolithography performance of modified ZrO2 coated by paraffin via projection based stereolithography Li Yanhui1*, Chen Yong2, Wang Minglang1, Li Lian1, Wu Haidong1, He Fupo1, Wu Shanghua1* 1
School of Electromechanical Engineering, Guangdong University of Technology, Guangzhou 510006, Guangdong, China
2
Viterbi School of Engineering, University of Southern California, CA 90089-0193, America E-mail:[email protected] [email protected] *
Corresponding author:
Projection based stereolithography (pSL) is widely applied to fabricate complicated ceramic parts. The preparation of ceramic suspension is critical for the process, especially, since large refractive index difference between ceramic and photosensitive resin would lead to an intense light scattering. In this paper, a ball-milling method is used to coat ZrO2 with paraffin that has a similar refractive index to that of resin, and the coated layer is measured to be around 10 nm. The modified ZrO2 is characterized by IR and TEM, and the photocuring behaviors are also investigated in terms of cure depth and excess cure width. The cure depth of modified ZrO2 increases by13.2% and 16.8% compared to the one without coating. At the same time, the excess cure width decreases by 77.1% and 62.7% using the same projection time. Moreover, ceramic ring prepared with the modified ZrO2 has a shrinkage of about 27.2%, and a density of 91.8%, smaller than the one prepared with ZrO2 without coating due to a burnout of paraffin. In conclusion, the paraffin coating layer could reduce the light scattering and thus improve the curing performance of ZrO2 suspension，especially, the excess cure width. Key words: Zirconia, refractive index, coatings, Projection based stereolithography
I. Introduction Projection based Stereolithography (pSL) such as the one using Digital Light Processing (DLP) device plays an increasingly important role in fabricating ceramic parts with complex structures. This three-dimensional (3D) printing technique has been widely used in the fields of biomedical and aerospace industries [1-7]. Photopolymerizable ceramic suspensions for pSL, composing of ceramic particles, photoinitiator and monomer, are a key for the process to build ceramic parts with complex structures [8, 9]. When the UV light illuminates the photopolymerizable suspensions, the photoinitiator will release free radicals, leading to the polymerization of monomers. The ceramic particles located in the matrix then form a green ceramic body due to polymerization. At last, the green ceramic body is debinded and sintered to a dense ceramic part with the designed structure [10]. As shown in the previous studies, during the curing process, the photocuring performance is affected by the properties of photocurable resin and ceramic powders, especially, the refractive index difference between ceramic powder and photocurable resin, which would lead to light scattering. The light scattering brings light attenuation in both the horizontal and vertical directions, which results in the decrease of cure depth and the increase of excess cure width when the ceramic suspensions are exposed by UV light [11-13]. Consequently, the building resolution and feature precision of fabricated ceramic parts are reduced. Cure depth is related to the suspension parameters, including ceramic powder and photocurable resin. According to Beer-Lambert’s semi-logarithmic equation, the cure depth is shown as [10] : Cd = Sd
=
(1)
Where E0 and Ed are the incident energy dose and the critical energy dose, respectively. Sd is photo-suspension sensitivity in cure depth, relevant to the energy attenuation by the absorption of photocurable resin and the scattering of the ceramic suspensions.
is the attenuation factor to reveal the effects of ceramic suspensions
on curing. In ZrO2 suspensions, the light absorption of ZrO2 is neglected, because
there is almost no light
absorption between 400 nm and 5 μm [14]. Light scattering
of ceramic power is similar to the Mie model, and proportional to the refractive index contrast. Sd =
(2)
Q = βΔn2
(3)
Δn = ncer – n0
(4)
Therefore, cure depth of ceramic suspensions is inversely proportional to their refractive index contrast. In comparison, cure width is composed of the width of the light spot diameter (Wlight) and the excess width surrounding the light (Wex), as shown in equation (5) [10, 12]: Wcure = Wlight + Wex
(5)
As with the cure depth, Wex is closely related to the suspension parameters, specially the ceramic properties, such as the slurry particle concentration, the diameter and the refractive index, and so on. Following the quasi-Beer-Lambert model, the excess width is presented as [10]: Wex = SW
=
(6)
Where Sw, Ew and Aw are the sensitivity, apparent critical energy dose, and the width attenuation factor in the width direction, respectively. Ew and Aw are depended on the scattering of ceramic particles and the absorption of the photocurable resin. There has been no equation to interpret the light attenuation of cure width, but, in principle, the attenuation is similar to the cure depth. The excess cure width is proportional to the refractive index difference between ceramic particles and photocurable resin. To confirm the effects of light scattering due to the refractive index difference, most researchers focused on reducing the refractive index difference between ceramic particle and photocurable resin. Gentry and Halloran adjusted the refractive index difference by varying the added diluent [10]. However, no studies are found to study the effects of modifying the ceramic particles. Therefore, this paper focuses on
modifying ceramic particles by a developed coating method. The modified ZrO2 is coated by paraffin with a refractive index of 1.436 that is similar to the photocurable resin used in the study. The suspensions, prepared with pure or modified ZrO2 and photocurable resin, are illuminated with a 405 nm UV lamps to study its curing performance.
II. Experimental procedure A certain amount of paraffin dissolved in petroleum is dropped into ethyl alcohol with ZrO2 suspending, and the above suspension is ball-milled for 2 hours. ZrO2 coated by paraffin is then obtained by distillation and drying in the air. The photosensitive resin is consisted of PPTTA (ethoxylated pentaerythritol tetraacrylate, Aladdin), HDDA (1, 6-Hexanediol diacrylate, Aladdin), U600 (di-functional aliphatic urethane acrylate, Aladdin) and 1-Octanol (octanol, Aladdin). The homogeneous photopolymerizable ceramic suspensions composed of pure or modified ZrO2 and resin that are mixed by ultrasonic agitation and ball-milling for 2 hours. The green bodies of ring are fabricated using the above ceramic suspension via DLP method, and then sintered at 1450℃ in muffle furnace. The modified ZrO2 is characterized by IR and TEM, and the refractive index of photosensitive resin is measured by Abess refractometer. Photopolymerization experiments are conducted using a projection-based stereolithography printer. In this paper, the grid model is used to examine the cure depth and excess width of the built lines.
III. Results and discussion For investing the coating effects, the chemical structures of ZrO2 and paraffin are presented in Fig. 1(I). Paraffin is mainly mixed by straight chain n-alkanes CH3– (CH2)(n-2)–CH3. Fig.1 (II) shows the infrared absorption spectrum of pure paraffin (a), ZrO2 coated by paraffin (b), and pure paraffin (c), respectively. The chemistry structure of paraffin is There is a wide and weak absorption peak of –OH stretching
vibration between 3300 and 3700 cm-1 in three infrared absorption spectrums. In Fig. 1(a) and (c), there exist two strong peaks at 2915 and 2854 cm-1, which belong to stretching vibration of C-H. There are another two peaks at 1464 and 1376 cm-1, corresponding to bending vibration of C-H. The characteristic peak of paraffin emerges at 728 cm-1, which is out of plane bending vibration absorption peak of C-H [15-17]. In Fig. 1(c) of modified ZrO2, the peak at 510 cm-1 attributes to Zr-O[18]. Comparing to the infrared absorption spectrums of pure paraffin (a) and pure ZrO2 (b), the peak of Zr-O and the peaks of C-H have no shift, which illustrates that paraffin physically adheres to ZrO2 surface, in accord with the chemical structure of paraffin. Fig. 2 shows the TEM images of unmodified and modified ZrO2 with paraffin. In Fig. 2(a), there are lots of clear lattice planes at the edge of ZrO2 particle. It is observed that ZrO2 is coated with a layer of paraffin in Fig. 2(b). The theoretical thickness of paraffin layer can be calculated by the following equation [18]: d=
=
⁄
(7)
The calculated layer thickness of paraffin based on the equation is 10.31 nm. It is measured to be about 9.51 nm by Digital Micograph Demo software as shown in Fig. 2(b), which is agreed well with the calculated value. In order to study the effects of coating ZrO2 particles with thin layer on the photocuring behaviors of ceramic suspension, we measure the cure depth and the width of single line of the designed grid model as shown in Fig.3. Fig. 3 shows the images of cured grid of ZrO2 without and with paraffin coating using the curing time of 10 s and 20 s. Fig. 3(a) and Fig. 3(b) show the cured grid profiles of ZrO2 without coating using the curing time of 10 s and 20 s, respectively. The cured lines are uniform and smooth with a depth of about 42.51 and 55.79 µm, correspondingly. The cured grid profiles and the micro-structure of ZrO2 coated with paraffin using the curing time of 10 s and 20 s are presented in Fig. 3(c) and 3(d). The measured cure depths of the cured line are about 48.12 and 65.16 µm, respectively. And the micro-structure of the cured line based on ZrO2 coated by paraffin is similar to that based on the ZrO2 without paraffin coating. Compared with ZrO2 without paraffin
coating, the cure depth of modified ZrO2 with paraffin coating increases by 13.2% and 16.8% using the curing time of 10 s and 20 s, respectively. To study the effects of paraffin coating on phtotocuring performance, we further characterize the excess cure width of the designed line model. Fig. 3-3 presents the cure width micrographs of ZrO2 with and without paraffin coating. Fig. 3(a, c) and 5(b, d) are the cure width images using the curing time of 10 s and 20 s, respectively. The cured lines have uniform and smooth edges for the curing time of 10 s; in comparison, the cured lines using the curing time of 20 s are jagged. The excess cure width of ZrO2 without coating increases from 193.8 μm to 491 μm, and the one with paraffin coating increases from 44.3 μm to 183.3 μm, when the cure time extends from 10 s to 20 s, as shown in Fig.3-3. Compared with ZrO2 without coating, the excess cure width of ZrO2 with paraffin coating decreases by 77.1% and 62.7% using the curing time of 10s and 20 s, respectively. Acorrding to Equation (2), ZrO2 and photocurable resin (DSM-AGI Co. Ltd., Taipei, Taiwan) have a refractive index of 2.165 and 1.455, respectively. The refractive index difference is 0.71, which leads to an intense scattering, as shown in Fig. 4(a). However, the ZrO2 suspension, prepared with modified ZrO2 with paraffin coating that has a refractive index of 1.436, which has a small refractive index difference (only 0.021 at the interface between photocurable resin and the paraffin coating layer). Accordinlgy, the optical path diagram is shown in Fig. 6(b). The incident light has a slight energy attenuation for light scattering at he interface between photocurable resin and the coating layer. As a result, the cure depth of ZrO2 coated with paraffin is deeper than the ZrO2 without paraffin coating; correspondingly, the excess cure width of ZrO2 coated with paraffin decreases compared with the ZrO2 without paraffin coating. Fig. 5 presents ZrO2 ring images of the green and sintered bodies. Fig. 5(a) shows the ring photographs of green bodies with the radius of 13.6 mm that were prepared by ZrO2 with and without paraffin coating, respectively. Correspondly, the radius of bodies obtained by thermal debinding and sintering are 9.90 and 9.85 mm, which show shrinkages of 27.2% and 27.6%, respectively. Archimedes principle is employed to
calculated the density of the rings, and the results show that their density is 91.8% and 95.8%, of the theortical density, respectively. With the same solid content, the density of ring prepared by modified ZrO2 is slightly smaller than the other one, due to the burnout of paraffin during the debinding and sintering process. In order to further examine the effect of coating layer on photocuring performance, it requires varied coating layers, such as inorganic substance. In our future work, we will examine the effects of Al2O3 coating layer on the photocuring behaviours of AlN.
IV. Conclusions Light scattering resulted from the refractive index difference between the ceramic and photocurable resin is significant to the cure depth and excess cure width. Cure depth and excess cure width are both followed by a quasi-Beer-Lambert equation. Cure depth sensitivity Sd is inversed to the refractive index difference, while the cure width sensitivity Sw is proportional to the refractive index difference. Thus, decreasing the refractive index contrast is beneficial for both photocuring properties. The cure depth of ZrO2 coated with paraffin increases by 13.2% and 16.8% using curing time of 10 s and 20 s, respectively. Correspondingly, the excess cure width decreases by 77.1% and 62.7% using the curing time of 10 s and 20 s, respectively. In conclusion, the coating layer on ceramic particles with a material (e.g. paraffin) that has a small refractive index difference to that of photocurable resin is effective on reducing light scattering during the photocuring process.
Acknowledges This work is financially supported by Major Science and Technology Project of Guangdong Province (Grant Nos. 2017B090911011 and 2016B090915002) and Local Innovative and Research Team Project of Guangdong Pearl River Talents Program, (NO. 2017BT01C169).
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Fig.1 the chemical structure of ZrO2 and paraffin ( I ), the infrared absorption spectrums ( II ) of (a) pure paraffin, (b) pure ZrO2, and (c) ZrO2 coated by paraffin. Fig. 2 TEM micrographs of unmodified (a) and modified ZrO2 by paraffin (b) Fig.3 Cured grid profiles and images from the cure depth and width of ZrO2 without coating (a and b) and with paraffin coating (c and d), using curing time of 10 s (a and c) and 20 s(b and d). Fig.4 The scattering diagram of ZrO2 particles (a) without and (b) with paraffin coating. Fig. 5 photographs of a ring model. (a) Green bodies prepared with ZrO2 without (right) and with paraffin coating (left); and (b) sintered parts prepared with ZrO2 without (right) and with paraffin coating (left) . Figures Fig. 1
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Fig. 5