Article Optical and Mechanical Properties of Highly Transparent Glass-Flake Composites Benedikt Scharfe 1,2,*, Sebastian Lehmann 1 , Thorsten Gerdes 1 and Dieter Brüggemann 3 1 Chair of Ceramic Materials Engineering, Keylab Glasstechnology, University Bayreuth, 95447 Bayreuth, Germany; [email protected] (S.L.); [email protected] (T.G.) 2 Deggendorf Institute of Technology, TAZ Spiegelau, 94518 Spiegelau, Germany 3 Department of Engineering Thermodynamics and Transport Processes (LTTT), Centre of Energy Technology (ZET), University of Bayreuth, 95447 Bayreuth, Germany; [email protected] * Correspondence: [email protected]; Tel.: +49-8553-9799-611 Received: 29 October 2019; Accepted: 18 November 2019; Published: 21 November 2019 Abstract: In this paper, the dynamic mechanic and optical properties of composites made of Polyvinyl Butyral (PVB) and Micro Glass Flakes (MGF) with matching refractive indices (RIs) are investigated. The composite is produced by a slurry-based process using additional blade casting and lamination. It can be shown that a high degree of ordering of the MGF in the polymer matrix can be achieved with this method. This ordering, combined with the platelet-like structure of the MGF, leads to very efficient strengthening of the PVB with increasing content of the MGF. By carefully adjusting the RIs of the polymer, it is shown that haze is reduced to below 2%, which has not been achieved with irregular fillers or glass fibers. Keywords: Micro Glass Flakes; PVB; Glass Particles; Transparent Composite 1. Introduction Highly transparent polymers have a very broad application spectrum. Polycarbonate (PC) and polymethyl methacrylate (PMMA) are common examples for applications that require high stiffness like displays, windows or transparent electronic packaging [1]. Low-density polyethylene (LDPE) and polyethylene terephthalate (PET) is used in applications which require high flexibility like packaging and foils for food and beverages, as well as medical applications like tubing, pipes, and bottles [2]. Lesser known are Polyvinyl Butyral (PVB) and Ethylene Vinyl Acetate (EVA), which are utilized in laminated panes and windows as interlayer materials [3]. However, typical polymer properties like high thermal expansion, low stiffness, low thermal conductivity, and low wear and scratch resistance have led to different approaches to using fillers to create a transparent composite with enhanced properties [4–8]. Employing glass particles as filler material for transparent polymers with similar refractive indices (RIs) shows promising properties for the application in transparent components [9]. Glass beads in a polymer matrix lead to increased stiffness [10,11] and reduced thermal expansion [12,13]. In theory, perfectly matched RIs between matrix polymer and glass filler should result in an unobstructed transparency [14]. However, in reality, RI matching is far from simple, as impurities and cooling rates at production have a significant impact on the RIs. Also, the RIs of glass and polymers depend on temperature (thermo-optic coefficient) and measured wavelength (dispersion), which can be used for optical temperature sensors [15,16]. The RI of typical glasses increases with increasing temperature with a thermo-optic coefficient Dn /DT between 6.7 and 24.1 10 6/K4 [17]. However, the RI of rel − × − the most common transparent polymers decreases with increasing temperature with thermo-optic coefficients which are nearly two orders of magnitude higher ( 0.9 to 3.1 10 6/K4)[18]. − − × − J. Compos. Sci. 2019, 3, 101; doi:10.3390/jcs3040101 www.mdpi.com/journal/jcs J. Compos. Sci. 2019, 3, 101 2 of 17 J. Compos. Sci. 2019, 3, 101 2 of 17 with thermo-optic coefficients which are nearly two orders of magnitude higher (−0.9 to −3.1 × 10−6/K4) [18]. Extensive research has been conducted on optica opticall and mechanical properties of glass-reinforced polymers withwith similarsimilar RI RI using using PMMA PMMA and and glass glass fibers fibers [19 ][19] or epoxyor epoxy filled filled with with irregular-shaped irregular-shaped glass particlesglass particles [20]. The [20]. influence The influence of different of particledifferent sizes, particle filler sizes, contents filler and contents influence and of RIinfluence mismatch of was RI studied.mismatch All was studies studied. show All that studies transmission show that decreases transm withission increasing decreases filler with content, increasing even withfiller identicalcontent, RI.even This with was identical traced back RI. This to the was wetting traced behavior back to of the the wetting filler, particularly behavior of when the usingfiller, moldingparticularly processes when orusing inhomogeneity molding processes in the RIor inhomogeneity within the particles in the or RI matrix. within However,the particles using or matrix. refractive However, index oilsusing in cuvettesrefractive as index a model oils matrixin cuvettes with as perfect a model wetting matrix behavior, with perfect it could wetting be shown behavior, that highit could transparency be shown canthat alsohigh be transparency achieved with can high alsofiller be achieved contents with [21]. high filler contents [21]. The aim of this study is to create a new type of transparent composite using Micro Glass Flakes (MGF)—a platelet-likeplatelet-like glassglass particleparticle withwith aa high high aspect aspect ratio. ratio. Micro Micro glass glass particles particles such such as as MGF MGF are are a verya very recent recent innovation innovation in glass in glass production. production. Despite Despite that, they that, have they already have gained already a broadgained application a broad spectrumapplication due spectrum to their chemicallydue to their inert chemically properties combinedinert properties with platelet-like combined morphology with platelet-like [22,23]. Theymorphology are most [22,23]. commonly They used are most as reflective commonly and eusedffect as pigments reflective in cosmeticand effect applications pigments in [24 cosmetic] and as fillerapplications material [24] to increaseand as filler the barriermaterial properties to increase of protectivethe barrier coatingsproperties [25 of]. protective coatings [25]. A method was developed to incorporate these glass flakesflakes in a model polymer via a slurry-based lab-scale castingcasting process process in in order order to increaseto increase the ordering the ordering of these of flakes. these Withflakes. this With combined this combined approach, usingapproach, platelets using in aplatelets highly orderedin a highly assembly, ordered the influenceassembly, ofthe residual influence RI mismatch of residual between RI mismatch particle andbetween matrix particle on the transparencyand matrix on and the haziness transparency of such aand structure haziness could of be such minimized, a structure as displayed could be in Figureminimized,1. At theas displayed same time, in glass Figure flakes 1. At with the a highsame aspect time, ratioglass couldflakes greatly with a improvehigh aspect the mechanicalratio could propertiesgreatly improve of the the composite mechanical in all properties in-plane directionsof the composite simultaneously. in all in-plane directions simultaneously. Figure 1. Schematics of the influence influence of glass fibers fibers and glass flakesflakes on transmittedtransmitted light in a matrix with a mismatch of refractive indices.indices. 2. Experimental 2.1. Materials 2.1. Materials 2.1.1. Micro Glass Flakes 2.1.1. Micro Glass Flakes Three different types of Micro Glass Flakes (MGF) were used in this study. Two of them are Three different types of Micro Glass Flakes (MGF) were used in this study. Two of them are commercially available from Glassflake Ltd® (GF100, GF001, Leeds, UK) [26] and one type was derived commercially available from Glassflake Ltd® (GF100, GF001, Leeds, UK) [26] and one type was by a laboratory flaker setup (GF-V8) with a boron-rich silicate glass composition (Figure2). They di ffer derived by a laboratory flaker setup (GF-V8) with a boron-rich silicate glass composition (Figure 2). in mean diameter, thickness and thus, in the aspect ratio, as well as the RI due to a different glass They differ in mean diameter, thickness and thus, in the aspect ratio, as well as the RI due to a different composition between the commercial MGF and from the laboratory flaker setup (Table1). glass composition between the commercial MGF and from the laboratory flaker setup (Table 1). J. Compos. Sci. 2019, 3, 101 3 of 17 J. Compos. Sci. 2019,, 33,, 101101 3 of 17 FigureFigure 2. Scanning 2. Scanning electron electron microscopy microscopy (SEM) (SEM) image image of commercial of commercial Micro Micro Glass Glass Flakes Flakes (MGF). (MGF). Table 1. Properties of Micro Glass Flakes used in this study. Table 1. Properties of Micro Glass Flakes used in this study. Type Mean Diameter (D50) Thickness Specific Density Type Mean Diameter (D50) Thickness Specific Density µ µ 3 GF100GF100 160 160 µmm 1.0–1.3 1.0–1.3 µm m 2.3 g/cm2.3 g3 /cm GF001 30 µm 0.9–1.3 µm 2.3 g/cm3 GF001 30 µm 0.9–1.3 µm 2.3 g/cm3 GF-V8 120 µm 2.0–5.0 µm 2.3 g/cm3 GF-V8 120 µm 2.0–5.0 µm 2.3 g/cm3 2.1.2. Polyvinyl Butyral (PVB) 2.1.2. Polyvinyl Butyral (PVB) Polyvinyl butyral (PVB) is considered to be an acetal and is formed from the reaction of an Polyvinyl butyral (PVB) is considered to be an acetal and is formed from the reaction of an aldehyde and alcohol. The general structure of PVB is shown in Figure3, whereby the shares of PVB, aldehyde and alcohol. The general structure of PVB is shownshown inin FigureFigure 3,3, wherebywhereby thethe sharesshares ofof PVB,PVB, polyvinyl alcohol (PVOH), and polyvinyl acetate segments are variable. Depending on the supposed polyvinyl alcohol (PVOH), and polyvinyl acetate segments are variable. Depending on the supposed application of the PVB, the relative amounts of these segments are controlled but they are generally application of the PVB, the relative amounts of these segments are controlled but they are generally randomly distributed through the molecular chain.