3D Stereolithography of Polymer Composites Reinforced with Orientated Nanoclay

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ScienceDirect ScienceDirect Procedia Engineering 00 (2017) 000–000

Available online at www.sciencedirect.com Procedia Engineering 00 (2017) 000–000

ScienceDirect

www.elsevier.com/locate/procedia www.elsevier.com/locate/procedia

Procedia Engineering 216 (2017) 1–7

9th International Conference on Materials for Advanced Technologies (ICMAT 2017) 9th International International Conference Conference on on Materials Materials for for Advanced Advanced Technologies Technologies(ICMAT (ICMAT2017) 2017) 9th

3D Stereolithography of Polymer Composites Reinforced with 3D StereolithographyOrientated of Polymer Composites Reinforced with Nanoclay Orientated Nanoclay H. Eng1, S. Maleksaeedi*1, S. Yu*1, Y.Y.C. Choong2, F.E. Wiria1, C.L.C. Tan1, P. C. Su2, J. Wei1 1 H. Eng1, S. Maleksaeedi* , S. Yu*1, Y.Y.C. Choong2, F.E. Wiria1, C.L.C. Tan1, P. C. Su2, J. Wei1 1

Singapore Institute of Manufacturing Technology, 73 Nanyang Drive, Singapore 637662 2 Singapore Centre for 3D Printing, 50 Nanyang Ave, Singapore 639798 637662 Singapore Institute of Manufacturing Technology, 73 Nanyang Drive, Singapore

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Singapore Centre for 3D Printing, 50 Nanyang Ave, Singapore 639798

Abstract Abstract

Polymer parts fabricated by 3D stereolithography (SL) tend to have much lower mechanical properties compared to Polymer parts fabricated by 3D (SL) tend to have much properties compared to their counterparts fabricated by stereolithography conventional methods. Furthermore, due tolower crossmechanical linkage among polymer chains in their counterparts fabricated due to cross linkage among polymer chains in the thermoset polymers, the by SLconventional printed partsmethods. tend to Furthermore, have low elongation and experience brittle fracture during the thermoset the SL printed have low elongation fracture failure. Variouspolymers, types of fillers have been parts addedtend into to photopolymer to improveand theexperience mechanicalbrittle properties, withduring most failure. types of fillers have In been photopolymer nanoclays to improve(plate-shaped) the mechanicalare properties, with most of themVarious being randomly dispersed. thisadded study,into montmorillonite first exfoliated and of them being randomly In this study, nanoclays (plate-shaped) are After first exfoliated and homogenously dispersed dispersed. in the photopolymer viamontmorillonite several mixing steps, including sonification. which, using homogenously dispersed in the several mixing including sonification. which, the photopolymer dispersed withphotopolymer nanoclays in avia bottom-up SL, thesteps, descending of build platformAfter into the resinusing tank the photopolymer with nanoclays in a bottom-up SL, the descending of buildnanoclays platform in into the layer. resin tank during the printingdispersed process provides a downward force to orientate the plate-shaped each The during the will printing process provides downward force to upon orientate the plate-shaped nanoclays each layer. their The nanoclays be immobilized duringaphotopolymerization ultra-violent (UV) exposure, thusinmaintaining nanoclays immobilized photopolymerization upon (UV) exposure, thus and maintaining orientationwill andbe alignment. Thisduring process drastically improves theultra-violent elongation by more than 100% enhancestheir the orientation and process drastically improves the elongation by morethe than 100% and enhancessize the tensile stress andalignment. Young’s This Modulus in the X-Y plane. This paper will also discuss dispersion, alignment, tensile stress and Young’s Modulus in the X-Y plane. This approach paper willwill alsoopen discuss dispersion,ofalignment, size and loadings of the montorillonite nanoclays. This new up the possibilities printing high and loadingsparts of the montorillonite nanoclays. Thisitsnew approach will open up possibilities of printing high performance using stereolithography, increasing functionality. performance parts using stereolithography, increasing its functionality. © 2017 The Authors. Published by Elsevier Ltd. © 2017 The Authors. Published by Elsevier Ltd. Selection and/or peer-review under responsibility of the scientific committee of Symposium 2017 ICMAT. Selection and/or peer-review under responsibility © 2017 The Authors. Published by Elsevier Ltd. of the scientific committee of Symposium 2017 ICMAT. Selection and/or peer-review under responsibility of the scientific committee of Symposium 2017 ICMAT. Keywords: Stereolithography; Mechanical Properties; Montmorillonite Nanoclays Keywords: Stereolithography; Mechanical Properties; Montmorillonite Nanoclays 1877-7058 © 2017 The Authors. Published by Elsevier Ltd. Selection and/or peer-review under responsibility of the scientific committee of Symposium 2017 ICMAT. 1877-7058 © 2017 The Authors. Published by Elsevier Ltd. Selection and/or peer-review under responsibility of the scientific committee of Symposium 2017 ICMAT.

1877-7058 © 2017 The Authors. Published by Elsevier Ltd. Selection and/or peer-review under responsibility of the scientific committee of Symposium 2017 ICMAT. 10.1016/j.proeng.2018.02.080

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H. Eng et al. / Procedia Engineering 216 (2017) 1–7 Author name / Procedia Engineering 00 (2017) 000–000

Introduction 3D printed parts from SL technology tends to have much lower mechanical properties compared to parts fabricated conventionally. The thermoset nature of SL fabricated parts, with high crosslink density, generally results in low elongation with brittle fracture [1]. Enhancement to mechanical properties are required, in order to increase functionality of SL parts. Various fillers such as carbon nanotubes (CNTs) [2-4], graphene [5], clay [6, 7], nanocellulose crystal [8], etc have been added and dispersed in photopolymer resin to enhance its mechanical properties. From literature, in general, addition of fillers increases stiffness of the printed parts, which is also the case for clay reinforced parts, where the Young’s modulus increases by around 60%. However, there is a trade-off, as elongation decreases with the addition of nanoclays; and the tensile strength remains relatively similar [6]. This results in the printed part having even lower elongation, which is already an issue for SL parts. In addition, the fillers are randomly orientated in the 3D printed parts, which reinforce the part isotropically over the planes perpendicular to build direction. With orientation of the nanofillers, further optimization and enhancement of mechanical properties can be achieved in the aligned direction, which normally occurred in tensile condition. In this work, montmorillonite clays (plate-shaped) were dispersed in the photopolymer resin. The bottom-up SL printing process mechanism was utilized to align the clay fillers, achieving significant enhancement in the mechanical properties, especially the elongation, in the particularly aligned direction. Materials & Methods Surface-modified montmorillonite clay powder modified with 35-45 wt% dimethyl dialkyl (C14-18 amine) (Sigma Aldrich, US), plate-like shape, was used without any purification. The montmorillonite clay was dispersed in commercial acrylate based photopolymer resin (Spot-A-Materials HT, Spain). Various concentrations of clay loading were used to determine the optimal concentrations and comparison to the benchmark. After which, using the clay dispersed photopolymer resin as the material, parts were 3D printed in a bottom-up SL machine (DWS 029X, Italy). The downward movement of platform and spatial confinement during the bottom-up printing process assists in re-orientation and flattening the plate-like clays perpendicular to build orientation during photo-polymerization upon UV light exposure.

The montmorillonite clay powder has a plate-liked structure, initially in multi-stack layer, with average diameter of 10 µm. One dimension of the clay is in nanometer range, while the other two dimensions can be up to micrometer range. After exfoliation into individual layer, they could have a high aspect ratio (<300). This high surface area results in high surface bonding between the clays and the polymer matrix, which can enhance the mechanical properties significantly in the aligned directions. Various concentrations of montmorillonite clay loadings were formulated and studied.



H. Eng et al. / Procedia Engineering 216 (2017) 1–7 Author name / Procedia Engineering 00 (2017) 000–000

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Figure 1: SEM image of surface modified montmorillnite clay

The dispersion process is critical in achieving significant improvement in the mechanical properties. The two main objectives of dispersion process for the montmorillonite clays is to exfoliate layers of clay in order to achieve high aspect ratio while maintaining its plate-like structure; and to achieve homogenous dispersion to prevent agglomerations of exfoliated clay layers. Various mixing process, magnetic stirrer, vacuum mixing (Thinky Mixer, US) and Sonification (Sonics, US) was conducted to exfoliate the individual layers and ensure homogenous dispersion.

After dispersion, a bottom-up SL machine was used for 3D printing. Tensile bars were printed according to standard dimensions (ISO 527-1:1996) across various concentrations of the montmorillonite clays and were tested to determine the optimal concentration for the enhancement in mechanical properties. During bottom-up SL printing process, the build platform descended into the resin tray during each layer of printing. The downward movement platform, coupled with the clay’s plate-like (2D) structure, confine and align the clays along the print layer. During photo-polymerization upon Ultra-Violet (UV) exposure, the clays were fixed in the aligned orientation in the polymer matrix.

H. Eng et al. / Procedia Engineering 216 (2017) 1–7 Author name / Procedia Engineering 00 (2017) 000–000

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Figure 2: Schematic of the montmorillonite clay orientation during the bottom-up SL printing process

Results and Discussion Based on the mechanical properties results, the additional of montmorillonite clays increases the overall mechanical properties of the 3D printed samples by SL. The tensile stress generally improved slightly, up to 20%, across the various concentration and the Young‘s modulus could enhanced by up to 70% depending on the concentration. This improvement is in line or slightly better based on the other works done on using nanoclays for mechanical enhancement [6, 7]. However, the reduction in elongation base on literature was not observed. Instead, it experiences significant enhancement, where improvement of up to 100% can be achieved, with the optimal concentration at 3wt%. In addition, the elongation results became more consistent due to reduction in the standard derivation. Thus, this could be due to the alignment during the printing process.

Figure 3: Ultimate tensile stress of 3D printed samples with surface modified montmorillonite clay



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Figure 4: Elongation of 3D printed samples with surface modified montmorillonite clay

Figure 5: Young’s Modulus of 3D printed samples with surface modified montmorillonite clay

Figure 6 shows the SEM micrograph of the SL printed cross-section along the printed layer. We observe that the montmorillonite clays were aligned and orientated along the printing layer, in the polymer matrix. During tensile test, the clay and polymer matrix will generate shear stress, which help reinforce the mechanical properties [9]. With the alignment, the maximum interfacial area between the polymer matrix and clay can be achieved, which the polymer matrix and clays can withstand higher shear force, thus maximizing the mechanical properties enhancement along a particular direction [10]. However, some agglomerations were observed. Further optimization of the dispersion process could be done to further improve the mechanical properties.

H. Eng et al. / Procedia Engineering 216 (2017) 1–7 Author name / Procedia Engineering 00 (2017) 000–000

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Figure 6: SEM image of cross-section (along build layer) of 3D printed samples with surface modified clay

Conclusion The use of 2D nanofillers has successfully enhanced the mechanical properties of the 3D printed samples, especially the elongation. Montmorillonite clays were exfoliated and dispersed in the photopolymer. Leveraging on the bottom-up SL printing process, the montmorillonite clays was orientated, which was observed in the SEM image. Enhancement in the mechanical properties were achieved. In particular, the elongation improved by 100% at 3wt% concentration of clay. Consistency of properties was observed as well. With this improvement, functional parts could possibly be fabricated. Moving forward, further improvement to the dispersion process can be done to further maximize the enhancement. The work can be extended to other 2D nanofillers such as graphene, for possibility of more improvement in mechanical or other properties.

Acknowledgements This work is supported under the A*STAR TSRP–Industrial Additive Manufacturing Programme by the A*STAR Science and Engineering Research Council (SERC) [grant number 1325504107]. References [1] Salmoria, G.V., et al., Stereolithography somos 7110 resin: mechanical behavior and fractography of parts post-cured by different methods. Polymer Testing, 2005. 24(2): p. 157-162. [2] Eng, H., et al., Development of CNTs-filled photopolymer for projection stereolithography. Rapid Prototyping Journal, 2017. 23(1): p. 129-136. [3] Sandoval, J.H., et al., Nanotailoring photocrosslinkable epoxy resins with multi-walled carbon nanotubes for stereolithography layered manufacturing. Journal of Materials Science, 2007. 42(1): p. 156-165. [4] dos Santos, M.N., et al., Thermal and mechanical properties of a nanocomposite of a photocurable epoxyacrylate resin and multiwalled carbon nanotubes. Materials Science and Engineering: A, 2011. 528(13–14): p. 4318-4324. [5] Manapat, J.Z., et al., High-Strength Stereolithographic 3D Printed Nanocomposites: Graphene Oxide Metastability. ACS Applied Materials & Interfaces, 2017. 9(11): p. 10085-10093.



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