Thermally Conductive and Insulating Epoxy Composites by Synchronously Incorporating Si-sol Functionalized Glass Fibers and Boron Nitride Fillers

Glass fibers (GFs)/epoxy laminated composites always present weak interlaminar shear strength (ILSS) and low cross-plane thermal conductivity coefficient (λ⊥). In this work, silica-sol, synthesized from tetraethyl orthosilicate (TEOS) and KH-560 via sol-gel method, was employed to functionalize the surface of GFs (Si-GFs). Together with a spherical boron nitride (BNN-30), the thermally conductive BNN-30/Si-GFs/epoxy laminated composites were then fabricated. Results demonstrate that Si-sol is beneficial to the improvement of mechanical properties for epoxy laminated composites (especially for ILSS). The BNN-30/Si-GFs/epoxy laminated composites with 15 wt% BNN-30 fillers display the optimal comprehensive properties. In-plane λ(λ//) and λ⊥ reach the maximum of 2.37 and 1.07 W·m−1·K−1, 146.9% and 132.6% higher than those of Si-GFs/epoxy laminated composites (λ// = 0.96 W·m−1·K−1 and λ⊥ = 0.46 W·m−1·K−1), respectively, and also about 10.8 and 4.9 times those of pure epoxy resin (λ// = λ⊥ 0.22 W·m−1·K−1). And the heat-resistance index (THRI), dielectric constant (ε), dielectric loss (tanδ), breakdown strength (E0), surface resistivity (ρs) as well as volume resistivity (ρv) are 197.3 °C, 4.95, 0.0046, 22.3 kV·mm−1, 1.8 × 1014Ω, and 2.1 × 1014Ω·cm, respectively.


INTRODUCTION
Glass fibers (GFs)/epoxy laminated composites are one kind of light weight and outstanding design flexibility materials, which possess high specific stiffness and strength, excellent fatigue toughness, and low-cost, etc. [1][2][3] They are widely used in the fields of aerospace, automobile, wind turbine, and electronic packaging, etc. [4,5] However, low cross-plane thermal conductivity coefficient (λ ┴ ) and poor interlaminar shear strength (ILSS) have seriously limited their broader application in thermal/ structural integrated components fields. [6,7] As an important part of bearing strength inner polymer composites, GFs possess easy processability, superior insulating properties, and excellent chemical stability [8,9] in comparison with other commonly reinforced fibers (e.g. quartz, [10,11] Kevlar, [12,13] and poly(p-phenylene-2,6-benzobisoxazole) (PBO) fibers). [14,15] However, few active groups and poor wettability of GFs surface usually result in easy interlayer damage and brittle fractures of laminated composites. [16,17] Besides, thermal interfacial resistance is the key barrier for effective thermal transport. Surface treatment of GFs could help to build the thermal bridge and reduce phonon scattering. Different functionalized methods have been developed to obtain better interfacial compatibility between GFs and epoxy resin, for instance, plasma, [18,19] coupling agent, [20,21] and grafting approach, [22,23] etc. Asadi et al. [24] perpared cellulose nanocrystals (CNC) coated chopped GFs/epoxy composites. Compared with pristine GFs/epoxy composites, the tensile and flexural strength values of the 0.17 wt% CNC coated GFs/ epoxy composites were increased by 10% and 43%, respectively. In our previous work, [25] epichlorohydrin grafted GFs (f-GFs) were developed to prepare f-GFs/boron nitride (BN)/ epoxy laminated composites. When the mass fraction of GBN-100 fillers was 20 wt%, the λ ┴ and in-plane λ (λ // ) values of the f-GFs/GBN-100/epoxy laminated composites reached the maximum of 1.21 and 3.55 W·m -1 ·K -1 , respectively. Furthermore, the introduction of f-GFs could further enhance the mechanical properties of epoxy laminated composites. Nevertheless, the above approaches usually show complex process and uncontrollable grafting rate, and slight improvement in mechanical properties as well. The use of silica-sol can further simplify surface functionalization process, which can avoid using lot of corrosive solvent and causing environmental pollution. In addition, compared with traditional coupling agent, the active groups on GFs surface can be further controlled by coating amount and concentration ratio of silica-sol.
There are two main methods for enhancing the λ values of epoxy resin and their composites. One is synthesizing intrinsic thermally conductive epoxy resin, the other is fabricating thermally conductive epoxy composites via introducing thermally conductive fillers. [26] However, the preparation of intrinsic thermally conductive epoxy resin is considerably cumbersome, difficult, and relatively expensive, which is applied only in laboratory stage. [27] Therefore, directly adding thermally conductive fillers can be regarded as the easiest and simplest method for improving the λ values of the epoxy composites. [28,29] Carbon-based fillers (CNTs, [30] graphite nanoplatelets (GNPs), [31] and graphene, [32] etc.) and ceramic fillers Al 2 O 3 , [33,34] BN, [35,36] aluminum nitride (AlN), [37,38] and SiC whisker (SiC W ), [39] etc.) are the most common fillers to fabricate highly thermally conductive epoxy composites. Carbonbased fillers possess excellent electrical conductivity, and are therefore restricted in the electrical insulation field. BN fillers not only present higher λ value and high-temperature oxidation resistance, but possess ε value (about 4.0) that is the lowest among ceramic fillers. [40,41] Moreover, BN fillers are easier to connect with each other owing to their unique layered hexagonal structure, beneficial to forming more thermally conductive pathways inside epoxy resin. Hu et al. [42] obtained the epoxy/ordered 3D-BN composites via combinating icetemplating self-assembly and infiltration method, which possessed high λ values up to 4.42 W·m -1 ·K -1 with 34 vol% BN fillers. Jiang et al. [43] prepared poly(glycidyl methacrylate) grafted h-BN particles (h-BN-PGMA)/epoxy composites which achieved λ value of 1.198 W·m -1 ·K -1 with 15 vol% h-BN-PGMA fillers, 5.05 times higher than that of pure epoxy resin. In our previous work, [44] thermally conductive and self-healable 60 wt% BN/thiol-epoxy elastomer composites presented the optimal λ value of 1.058 W·m -1 ·K -1 , about 4 times that of pure epoxy resin.
In this work, silica-sol, synthesized from tetraethyl orthosilicate (TEOS) and γ-glycidoxypropyltrimethoxysilane (KH-560) via sol-gel method, was firstly developed to functionalize the surface of GFs (Si-GFs). And the thermally conductive BNN-30/Si-GFs/epoxy laminated composites were then fabricated via blending-impregnation followed by hot compression, applying the micron boron nitride with particle size of 30 μm (BNN-30) as thermally conductive fillers and bisphenol A epoxy resin (E-44) as polymer matrix. Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS) were all performed to analyze and characterize the surface component and performance of the GFs and Si-GFs. Meantime, the effects of BNN-30 filler contents on the thermal conductivities, electrical insulation, and dielectric, mechanical and thermal properties of the BNN-30/Si-GFs/epoxy laminated composites were also investigated.

EXPERIMENTAL Surface Coating of GFs
Glass fibers (GFs, 2.45 g·cm -3 , Shanghai Jingtai Industrial Co. Ltd) were washed by absolute ethanol and deionized water successively. Silica-sol was then coated on the GFs surface and dried in the vaccum oven at 60 °C for 24 h, finally to obtain silicasol coated GFs (Si-GFs).

Fabrication of the Thermally Conductive BNN-30/ Si-GFs/Epoxy Laminated Composites
BNN-30 fillers (Ya'an Baitu High-tech Materials Co., Ltd.) were firstly dispersed in epoxy resin and stirred uniformly. Curing agent of 3,3′-diaminodiphenyl sulfone (DDS) was then added into the above solution (epoxy:DDS = 3:1, W:W) and mixed at 110 °C for 2 h. Acetone was then added to obtain transparent BNN-30/epoxy prepreg glue, which was then coated on the surface of Si-GFs, followed by air-drying to obtain the BNN-30/ Si-GFs/epoxy prepregs. The above BNN-30/Si-GFs/epoxy prepregs were then laid up and laminated under 5 MPa according to the following curing procedure: 110 °C/0.5 h + 170 °C/1 h. Si-GFs/epoxy laminated composites were also prepared by the same method. Schematic diagram for the preparation of thermally conductive BNN-30/Si-GFs/epoxy laminated composites is displayed in Fig. 1.
The information details of the "Main materials", "Synthesis of silica-sol" and "Characterizations" are presented in the electronic supplementary information (ESI). Chemical shifts at 0.87 and 1.74 ppm are attributed to protons on the methylene group directly and indirectly connected to Si in the KH-560 alkyl chains. And the signals at 2.85 and 2.97 ppm are attributed to the two protons with different chemical environments in epoxy groups of silica-sol. In addition, a few more signals at 3.30−3.85 ppm appear, corresponding to protons for -O-CH 2 -, O-CH 3 , and CH 2 -O-CH 2 , respectively. Therefore, it can be deduced that the silica-sol has been synthesized successfully.  Table S2 (in ESI).

Characterization on GFs and Si-GFs
Table S3 (in ESI) presents the data for thermal conductivities and mechanical properties of the pristine GFs/epoxy and Si-GFs/epoxy laminated composites. Compared with pristine GFs/epoxy laminated composites, the Si-GFs/epoxy laminated composites have better thermal conductivities and mechanical properties. In-plane λ (λ // = 0.96 W·m -1 ·K -1 ) and crossplane λ (λ ┴ = 0.46 W·m -1 ·K -1 ) values of the Si-GFs/epoxy laminated composites are higher than those of pristine GFs/ epoxy laminated composites (λ // = 0.76 W·m -1 ·K -1 and λ ┴ = 0.31 W·m -1 ·K -1 ). The addition of silica-sol can effectively im-   face of Si-GFs can participate in co-cure reaction of epoxy resin with the help of DDS [45] to form effective cross-linking networks, which not only possess better interfaces bonding strength but also can improve the mechanical properties. In addition, thermal diffusion rate slows obviously and the heat flow is spread unevenly between interlayers for epoxy laminated composites. Si-GFs are beneficial to rapid diffusion of heat flow; only a small part of the heat is retained in epoxy resin, and phonons transport rapidly through Si-GFs as well. However, epoxy resin with low λ value is the main component at interlayers for BNN-30/Si-GFs/epoxy laminated composites. Phonons do random Brownian motion in epoxy resin, presenting longer mean free path and easy to be scattered. [46] Meanwhile, the addition of silica-sol can improve the interfacial compatibility between Si-GFs and epoxy resin, which would be beneficial to the improvement of the heat propaga-   In-plane temperature distribution tion. Therefore, λ // values of the BNN-30/Si-GFs/epoxy laminated composites are higher than those of λ ┴ values. The corresponding schematic diagram for thermal mechanism of the BNN-30/Si-GFs/epoxy laminated composites is shown in Fig. 5(e).

Electrical Insulation Properties of the BNN-30/ Si-GFs/Epoxy Laminated Composites
Weibull cumulative distribution function is adopted to analyze the dielectric breakdown strength of the thermally conductive BNN-30/Si-GFs/epoxy laminated composites using double parameters, defined as follows: where P is the cumulative probability of electric failure, β is the shape parameter, E is measured breakdown strength, and E 0 represents the breakdown strength at cumulative failure probability of 0.632 (i.e., 1 -1/e, where e is the exponential constant), which is also regarded as the characteristic breakdown strength. Distribution points are more evenly distributed on both sides of the fitted lines from Fig. 7(a). [47,48] β values, representing the Weibull modulus, are higher than 10, which indicates little dispersion in breakdown strength and provides high confidence degree. When the cumulative failure probability is 0.632, the characteristic breakdown strength (E 0 ) is shown in Fig. 7(b). With the increasing amount of BNN-30 fillers, the E 0 values of the BNN-30/Si-GFs/epoxy laminated composites increase firstly and then decrease. Compared with Si-GFs/ epoxy laminated composites (E 0 = 17.1 kV·mm −1 ), E 0 value of the 15 wt% BNN-30/Si-GFs/epoxy laminated composites is improved to 22.3 kV·mm −1 and increased by 30.4%.
BNN-30 fillers with low content can reduce charge accumulation at surfaces and consume carriers energy effectively, which can increase the potential energy for the electronic transition, thereby enhancing the E 0 . Nevertheless, with excessive addition of BNN-30 fillers, more reunions and inner defects are easily introduced inside the BNN-30/Si-GFs/epoxy laminated composites, and more and more charge accumulation promotes large-scale electric field distortion inside the BNN-30/Si-GFs/epoxy laminated composites, which will reduce the E 0 . Besides, the carriers gain much more energy than their loss under high field. More electrons as the starting electrons collision, thus extremely increasing the concentration of carriers, which would reduce the dielectric breakdown strength of the BNN-30/Si-GFs/epoxy laminated composites. [49,50] In Figs. 7(c) and 7(d), both the surface resistivity (ρ s ) and volume resistivity (ρ v ) of the BNN-30/Si-GFs/epoxy laminated composites gradually decrease with increasing mass fraction of BNN-30 fillers. ρ s and ρ v values of the 15 wt% BNN-30/Si-GFs/epoxy laminated composites are reduced to 1.8 × 10 14 Ω and 2.1 × 10 14 Ω·cm, respectively. The intrinsic electrical insulation of BNN-30 fillers is slightly poorer compared with epoxy resin and Si-GFs. The introduction of BNN-30 fillers leads to the improvement in space charge, which makes charge easier to achieve energy level transition, as well as to get rid of the nucleus, finally to reduce the ρ s and ρ v values. Notably, the obtained 15 wt% BNN-30/Si-GFs/epoxy laminated composites still preserve favorable electrical insulating properties, still far beyond the lower bound of electrical insulation (ρ s ≥ 1 × 10 11 Ω, ρ v ≥ 1 × 10 9 Ω·cm). [51] Meanwhile, both ε and tanδ values of the thermally conductive BNN-30/Si-GFs/epoxy laminated composites are improved with increasing the content of BNN-30 fillers as shown in Fig. S1 (in ESI).