Hexagonal Boron Nitride as Filler for Silica-Based Elastomer Nanocomposites

Abstract

Two-dimensional hexagonal boron nitride (hBN) has attracted tremendous attention over the last few years, thanks to its stable structure and its outstanding properties, such as mechanical strength, thermal conductivity, electrical insulation, and lubricant behavior. This work demonstrates that hBN can also improve the rheological and mechanical properties of elastomer composites when used to partially replace silica. In this work, commercially available pristine hBN (hBN-p) was exfoliated and ball-mill treated in air for different durations (2.5, 5, and 10 h milling). Functionalization occurred with the -NH and -OH groups (hBN-OH). The functional groups were detected using Fourier-Transform Infrared pectroscopy (FT-IR) and were estimated to be up to about 7% through thermogravimetric analysis. The presence of an increased amount of oxygen in hBN-OH was confirmed using Scanning Electron Microscopy coupled with Energy-Dispersive X-ray Spectroscopy. (SEM-EDS). The number of stacked layers, estimated using WAXD analysis, decreased to 8-9 in hBN-OH (10 h milling) from about 130 in hBN-p. High-resolution transmission electron microscopy (HR-TEM) and SEM-EDS revealed the increase in disorder in hBN-OH. hBN-p and hBN-OH were used to partially replace silica by 15% and 30%, respectively, by volume, in elastomer composites based on poly(styrene-co-butadiene) from solution anionic polymerization (S-SBR) and poly(1,4-cis-isoprene) from Hevea Brasiliensis (natural rubber, NR) as the elastomers (volume (mm³) of composites released by the instrument). The use of both hBNs in substitution of 30% of silica led to a lower Payne effect, a higher dynamic rigidity, and an increase in E' of up to about 15% at 70 °C, with similar/lower hysteresis. Indeed, the composites with hBN-OH revealed a better balance of tan delta (higher at low temperatures and lower at high temperatures) and better ultimate properties. The functional groups reasonably promote the interaction of hBN with silica and with the silica's coupling agent, sulfur-based silane, and thus promoted the interaction with the elastomer chains. The volume of the composite, measured using a high-pressure capillary viscometer, increased by about 500% and 400% after one week of storage in the presence of hBN-p and hBN-OH. Hence, both hBNs improved the processability and the shelf life of the composites. Composites obtained using hBN-OH had even filler dispersion without the detachments of the filler from the elastomer matrix, as shown through TEM micrographs. These results pave the way for substantial improvements in the important properties of silica-based composites for tire compounds, used to reduce rolling resistance and thus the improve environmental impacts.

Keywords: 2D nanomaterials; h-BN functionalization; lower Payne effect; rubber compounds.

Figure 1

Block scheme of the mixing procedure.

Figure 2

Block diagram describing the preparation of the hBN-OH samples as taken from the ball-milling jar (hBN-OH_as) and after washing with water (hBN-OH_w).

Figure 3

FT-IR spectra of hBN-OH_as, milled for 2.5 (a), 5 (b), and 10 (c) h; hBN-p (d). (A) Spectra are displayed with normalized intensity. CO₂ absorption bands are labeled. (B) Spectra are displayed after baseline correction and after normalization of the scale.

Figure 4

FT-IR spectra of hBN-OH_w, milled for 2.5 (a), 5 (b), and 10 (c) h; hBN-p (d). (A) Spectra are displayed with normalized intensity. CO₂ absorption bands are labeled. (B) Spectra are displayed after baseline correction and after normalization of the scale.

Figure 5

WAXD patterns: hBN-OH_as, milled for 2.5 (a), 5 (c), and 10 (e) h; hBN-OH_w, milled for 2.5 (b), 5 (d), and 10 (f) h; hBN-p (g).

Figure 6

Micrographs of pristine hBN (a) and hBN-OH_as5h (b).

Figure 7

SEM image and EDS spectra of hBN-p.

Figure 8

SEM image and EDS spectra of hBN-OH_w10h.

Figure 9

SEM colored image and elemental imaging of hBN pristine.

Figure 10

SEM colored image and elemental imaging of hBN-OH_w10h.

Figure 11

G′ vs. strain for composites of Table 1.

Figure 12

Tan delta vs. strain for composites of Table 1.

Figure 13

Axial dynamic mechanical properties of S-SBR 4630 compounds measured in compression: (a) storage modulus vs. temperature; (b) tan delta vs. temperature.

Figure 13

Axial dynamic mechanical properties of S-SBR 4630 compounds measured in compression: (a) storage modulus vs. temperature; (b) tan delta vs. temperature.

Figure 14

Tensile curves of S-SBR 4630 compounds obtained through stress–strain experiments.

Figure 15

Measurement in mm³ of composites released by the high-pressure capillary viscometer vs. time: (a) immediately after compounding, (t = 0); (b) after 7 days (t = 7).

Figure 16

Micrographs of the following composites: silica (A,a), 15hBN-p (B,b), 30-hBN-p (C,c), 15-hBN-OH (D,d), 30-hBN-OH (E,e), at higher magnifications (A–E) and at lower magnifications (a–e).

Figure 16

Micrographs of the following composites: silica (A,a), 15hBN-p (B,b), 30-hBN-p (C,c), 15-hBN-OH (D,d), 30-hBN-OH (E,e), at higher magnifications (A–E) and at lower magnifications (a–e).