Composites Science and Technology 201 (2021) 108527 Contents lists available at ScienceDirect Composites Science and Technology journal homepage: http://www.elsevier.com/locate/compscitech Preparation of high-performance transparent glass-fiber reinforced composites based on refractive index-tunable epoxy-functionalized siloxane hybrid matrix Junho Jang, Ph.D. a,1, Hyeon-Gyun Im, Ph.D. b,1, DaeSeop Lim a, Byeong-Soo Bae, Ph.D. a,* a Wearable Materials Technology Center (WMC), Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon, 34141, Republic of Korea b Electrical Materials Research Division, Korea Electrotechnology Research Institute (KERI), Changwon-si, 51543, Republic of Korea ARTICLE INFO ABSTRACT Keywords: Fiber reinforced polymer composites (FRPs) are widely utilized in various industrial fields due to their Transparent glass-fiber reinforced composite remarkable mechanical properties and low cost fabrication. However, typical FRPs cannot concurrently fulfill Epoxy-functionalized siloxane hybrid optical transparency and high mechanical strength, which are essential for transparent electronics and auto­ Refractive index tuning motive applications. Here, we report a transparent FRP (GFRH) that exhibits high opto-mechanical properties by Optical transparency incorporating reinforced glass-fibers into refractive index-tunable epoxy-functionalized siloxane hybrid mate­ Mechanical properties rials. To achieve transparent composites, we precisely control the refractive index of the siloxane matrix by adding an epoxy cross-linkable hardener. We compare the opto-mechanical properties of the GFRP according to various conditions in terms of curing method (thermal- and UV-curing), fiber content (from 0 to 25 wt% of matrix), fiberlength (from 0.08 to 3 mm), and fibertype (filamentmat, glass-fabric, chopped strands glass-fibers, and milled glass). Finally, to obtain high-performance GFRP, we further optimize the composite structure with short fiber (5 wt% of chopped strands glass-fibers) and long fiber (10 layers of glass-fabric). The optimized composite exhibits high optical transparency (80% @ 550 nm) and mechanical properties (251 MPa flexural strength, 9 GPa flexural modulus and 3 H pencil hardness). Our transparent composite has significant potential for the application of FRPs to transparent electronics and automotive devices. 1. Introduction applications due to their high-temperature durability, chemical intert­ ness, high mechanical strength, and low thermal expansion [9–12]. Fiber reinforced polymer composites (FRPs) have been widely Despite the various advantages of glass-fiber FRPs (GFRPs), it is not a developed for automotive and electronics applications because of their simple task to prepare transparent GFRPs; a highly compatible interface promising characteristics such as structural reliability, light weight, between the fiber and matrix should be guaranteed to minimize mechanical strength, and cost-effectiveness [1–3]. FRPs consist of a light-scattering at the interface, which is critical to the optical trans­ polymeric matrix (e.g., polyester, thermostable, vinylester, and epoxy parency of the composites. Transparent GFRPs can be obtained by resin) reinforced with fibers (e.g., carbon, aramid fibers, and matching the refractive index (RI) of the glass-fiber and the polymer glass-fibers) [2–6]. By reinforcing mechanically weak polymeric mate­ matrix, which allows reduction of refraction and scattering at the rials with fibers, thermo-mechanical properties are greatly enhanced fiber-matrix interface [1,13,14]. If the refractive index (RI) of the two [3]. Typical FRPs are optically translucent or opaque; the materials is not matched, the obtained GFRPs have high optical haze light-scattering at the interface between the polymer matrix and fiber that restricts transparent applications [6,15]. Preparing transparent results in an optical haze that hinders wide use in transparent applica­ GFRPs using conventional polymer matrices and glass-fiberfillers is not tions such as flexible substrates and screen protection films [7,8]. easy due to the fact that RI-control of a typical thermosetting polymer Glass-fibers are effective reinforcing materials for various such as an epoxide is very limited for viable use [16]. Therefore, * Corresponding author. E-mail addresses: [email protected] (J. Jang), [email protected] (H.-G. Im), [email protected] (D. Lim), [email protected] (B.-S. Bae). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.compscitech.2020.108527 Received 18 September 2020; Received in revised form 19 October 2020; Accepted 25 October 2020 Available online 6 November 2020 0266-3538/© 2020 Elsevier Ltd. All rights reserved. J. Jang et al. Composites Science and Technology 201 (2021) 108527 Fig. 1. a) Schematic illustration of synthesis process of epoxy-Functionalized siloxane hybrid (EPSH) matrix resin. Inset photograph represents synthesized EPSH 29 matrix resin. b) Si-NMR spectrum of EPSH matrix resin. Insets: chemical structures representing Si atoms according to bonding states. R1: cycloaliphatic epoxide group; R2: phenyl group. The calculated degree of siloxane bond formation is about 90%. c) FT-Raman analysis of EPSH matrix resin. transparent GFRPs using a RI controllable matrix resin should be monohydrate (Ba(OH)2∙H2O, Sigma-Aldrich, U.S.A.), methylhexahy­ developed. In addition, inherent chromatic aberration, and other defects drophthalic anhydride (MHHPA, Sigma-Aldrich, U.S.A.), 3-ethyl-3 [(3- such as residual stress and micropores in composites have remained ethyloxetane-3-yl)methoxy]methyl oxetane (DOX, Toagosei, Japan), unsolved problems [14]. triarylsulfonium hexafluoroantimonate salt (Sigma-Aldrich, U.S.A.), Sol-gel derived siloxane-based hybrid materials have been widely octadecyl (trichloro)silane (OTS, Sigma-Aldrich, U.S.A.) and tetrabu­ studied in the past few decades due to several advantages including high tylphosphonium methanesulfonate (Sigma-Aldrich, U.S.A.) were used optical transparency, and excellent thermo-mechanical characteristics without any further purification. E-glass fibers (mats, fabrics, and [17,18]. These properties allow utilization in a broad spectrum of ap­ strands) were used as fillers, and the RT value and the density are plications, for example, optical waveguides [19], functional coatings 1.54–1.55 and 2.6 g cm 1, respectively, according to the datasheet (E- [20,21], encapsulation of nanocrystals [22], microlenses [23], and di­ glass fiber, Nitto Boseki, Japan). electrics [24]. In particular, the RI of siloxane hybrid materials can be easily controlled by changing the composition of organic moieties in the 2.2. Synthesis of epoxy-functionalized siloxane hybrid (EPSH) matrix siloxane back-bone or incorporating a cross-linker. Furthermore, their resin other physicochemical properties such as excellent thermal stability, chemical inertness, and mechanical durability can be retained while RI ESPH matrix resin was synthesized by a non-hydrolytic sol-gel re­ is tuned, and thus transparent or haze GFRPs can be prepared for a va­ action [10,25]. ECTS and DPSD were blended in a two-neck flask by a riety of uses [1,3,25]. mechanical stirrer for a few minutes. The molar ratio of ECTS to DPSD Based on the above considerations, we fabricated transparent GFRPs was 1 : 1.5. After blending, Ba(OH)2∙H2O was added in an amount of (hereafter GFRH) composed of a sol-gel derived cycloaliphatic epoxy- 0.2 mol% of the total silane precursors as a base-catalyst to promote the functionalized siloxane hybrid (EPSH) resin (as a matrix) and various sol-gel reaction between ECTS and DPSD, and further mechanically ◦ glass-fibers(as a filler).To obtain the maximum optical transparency in mixed at 80 C under N2 purging. A clear viscous resin was obtained the visible region, we manipulated the RI of the EPSH matrix by con­ after 6 h. Epoxy-hardener (MHHPA and DOX) of varying amounts was trolling the cross-linker content. Both thermal- and photo-curing were then mixed, and an initiator (tetrabutylphosphonium methanesulfonate; used to prepare the GFRHs to study physical properties of the composites thermal-initiator, triarylsulfonium hexafluoroantimonate salt; in terms of the curing mechanism. Optimizing the structure and photo-initiator) was added in an amount of 2 wt% of the total resin composition of glass fillersin the composites, we obtained a transparent mixture. In the optimization process of the transparent GFRH, composite with high optical transparency and desirable mechanical glass-fibers (chopped glass strands and milled glass) were mixed with characteristics. In addition, we investigated the structure-property the as-synthesized EPSH matrix resin. relationship between the fillers and opto-mechanical properties of the composites in terms of length, content, and type of the glass-fibers. 2.3. Fabrication of transparent glass-fiber reinforced hybrid composite (GFRH) 2. Materials and methods Using glass-fabric or mats, the entire fabrication process was similar 2.1. Materials with previously reported methods [3,10,25]. Briefly, glass-fabrics or ◦ mats were placed on the 1st donor glass plate, and heated up to 80 C. 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane (ECTS, Gelset, U.S. The EPSH resin was then dispensed onto the various glass-fibers for A.), diphenylsilanediol (DPSD, Gelest, U.S.A.), barium hydroxide impregnation. Those samples were then covered with the 2nd donor glass 2 J. Jang et al.