# Thermoconductive Thermosetting Composites Based on Boron Nitride Fillers and Thiol-Epoxy Matrices

^{1}

^{2}

^{3}

^{*}

## Abstract

**:**

## 1. Introduction

## 2. Materials and Methods

#### 2.1. Materials

#### 2.2. Sample Preparation

#### 2.3. Characterization Techniques

_{2}atmosphere with a gas flow of 100 mL/min. The scans were performed in the temperature range of 30 to 250 °C with a heating rate of 10 K/min. Curing enthalpies (Δh) of the different samples were calculated by integration of the calorimetric signal. Glass transition temperatures (T

_{g}) of cured samples were evaluated by a second scan as the temperature of the half-way point of the jump in the heat capacity curve. The estimated error was considered to be ±1 °C.

_{2}atmosphere (flux 50 mL/min). Pieces of cured samples of 5–10 mg were degraded between 30 and 600 °C at a heating rate of 10 K/min.

^{3}) were tested in 3-point bending mode at a heating rate of 3 K/min in the temperature range from 35 to 125 °C, with a frequency of 1 Hz and oscillation amplitude of 0.1% of sample deformation. The Young’s moduli (E) were determined at 30 °C by using a force ramp at a constant rate, 1 N/min, never exceeding 0.25% of deformation to be sure that only elasticity was evaluated. The slope between 0.1% and 0.2% of deformation was taken. E was calculated from the slope of the load deflection curve according to the following equation:

^{3}) were supported by the clamp and one silica disc to uniformly distribute the force and heated at 5 °C/min from 32 up to 120 °C by application of a minimum force of 0.01 N, to not distort the results. Two heating scans were performed, being the first to erase the thermal history and the second to determine the thermal expansion coefficients (CTEs), below and above the T

_{g}. They were calculated according to the following equation:

_{0}the initial length, t the time, T the temperature and dT/dt the heating rate.

_{P}is the projected area of indentation in mm

^{2}, C

_{P}is the indenter constant (7.028 × 10

^{−2}) relating l

^{2}to A

_{P}.

_{gel}) was determined by stopping the rheology experiment when gelation occurred and the sample was quenched in liquid N

_{2}. Then, the remaining enthalpy was evaluated by a dynamic DSC experiment at 10 K/min. The degree of conversion in the gelation was calculated according to the following equation:

_{g}is the heat released up of gelled samples, obtained by integration of the calorimetric curve, and Δh

_{T}is the heat associated with the complete curing.

^{2}of area was used. The sensor was calibrated with poly(methylmethacrylate) (PMMA), borosilicate crown glass, marble, Ti-Al alloy and titanium. Two equal rectangular samples, perfectly polished, with size of 12 × 12 × 2.3 mm

^{3}were placed at both faces of the sensor. Because of the small size of sensor, the side effects can be neglected. Measuring times of 100 s with a current of 10 mA were applied. Five measures were taken for each material.

## 3. Results and Discussion

#### 3.1. Study of the Curing Process

_{g}) determined remain practically constant for all the formulations, with a slight decrease in the enthalpy released at the highest proportion of BN, probably due to topological restrictions in curing.

#### 3.2. Rheological Study of the BN Formulations

^{2}and G″∝ω

^{1}) at low frequencies, but with the increasing content of BN, the slopes continuously decline until the maximum concentration of filler added (40 wt. %), where G′ is practically constant on varying the frequency which means that percolation threshold is overpassed.

_{c}is the mass fraction at the rheological percolation and β is the critical exponent. The threshold was calculated to be 35.5 wt. % and the critical exponent 2.4 at 1 rad/s.

^{0.5}in the terminal region in small amplitude oscillatory shear (SAOS) experiments [24,27,28]. The slope in G′ versus frequency reach a value of 0.5 between formulations with a filler content of 35% and 38% (see Table 2), agreeing this range with the previous calculations.

#### 3.3. Thermal and Mechanical Characterization of BN Composites

#### 3.4. Morphology Inspection of BN Composites

#### 3.5. Thermal Conductivity of BN Composites

## 4. Conclusions

## Acknowledgments

## Author Contributions

## Conflicts of Interest

## References

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**Scheme 1.**Chemical structures of the monomers used and the network formed during the curing process.

**Figure 1.**DSC curves showing the effect of the addition of different proportions of BN in wt. % to the formulation.

**Figure 2.**Plot of log G′ versus log % strain in oscillatory experiment (1 Hz) of uncured formulations.

**Figure 3.**Plots of G′ (filled symbols) and G″ (open symbols) against ω for all the formulations at 30 °C.

**Figure 5.**Variation of storage modulus (

**A**) and tan δ (

**B**) against temperature of the different materials prepared.

**Figure 6.**Dependence of the microindentation hardness of ECC/thiol thermosets with different weight percentages of BN.

**Figure 9.**ESEM micrographs of the pure BN agglomerates at 400 (

**left**) and 1500 (

**right**) magnifications.

BN (wt. %) | T_{max} ^{a} (°C) | Δh ^{b} (J/g) | Δh ^{b} (kJ/ee) | T_{g} ^{c} (°C) |
---|---|---|---|---|

0 | 127 | 479 | 120 | 58 |

10 | 126 | 436 | 121 | 58 |

20 | 126 | 376 | 118 | 57 |

30 | 125 | 337 | 121 | 57 |

40 | 124 | 272 | 114 | 57 |

40 (80 µm) | 131 | 276 | 116 | 58 |

^{a}Temperature of the maximum of the curing exotherm;

^{b}Enthalpy of the curing process by gram of mixture or by epoxy equivalent;

^{c}Glass transition temperature determined by the second scan by DSC after a dynamic curing.

**Table 2.**Rheological fitting results at 30 °C and gelation data from rheometric monitoring of the curing of the formulations at 85 °C.

BN (wt. %) | G′_{slope} ^{a} (Low Freq.) | G″_{slope} ^{a} (Low Freq.) | t_{gel} ^{b} (Min.) | x_{gel} ^{c} (%) |
---|---|---|---|---|

0 | 1.81 | 1.02 | 16.3 | 59 |

10 | 1.77 | 1.06 | 17.8 | 62 |

20 | 1.34 | 0.99 | 18.3 | 60 |

30 | 0.62 | 0.84 | 18.4 | 55 |

35 | 0.57 | 0.66 | - | - |

38 | 0.36 | 0.27 | - | - |

40 | 0.22 | 0.24 | 19.5 | 55 |

40 (80 µm) | 0.11 | 0.08 | - | - |

^{a}Slopes of viscoelastic properties at low frequencies (potential functions in log-log diagrams);

^{b}Gel time determined from the frequency independent crossover of tan δ;

^{c}Determined as the conversion reached by rheometry and DSC test at 10 °C/min.

BN (wt. %) | BN (vol %) | T_{2%} ^{a} (°C) | Char Yield ^{b} (%) | CTE_{glass} ^{c} (10^{−6}·K^{−1}) | CTE_{rubber} ^{c} (10^{−6}·K^{−1}) |
---|---|---|---|---|---|

0 | 0 | 249 | 5.0 | 69 | 195 |

10 | 6.0 | 250 | 14.4 | 68 | 192 |

20 | 12.8 | 249 | 24.0 | 66 | 166 |

30 | 20.2 | 250 | 34.2 | 67 | 157 |

40 | 28.2 | 252 | 43.0 | 55 | 134 |

40 (80 µm) | 27.4 | 259 | 44.7 | 43 | 135 |

^{a}Temperature of 2% weight loss determined by TGA in N

_{2}at 10 °C/min;

^{b}Char residue at 600 °C;

^{c}Thermal expansion coefficient in the glassy state determined between 38–52 °C and in the rubbery state between 70–90 °C.

BN (wt. %) | Young’s Modulus ^{a} (GPa) | T_{tan δ} ^{b} (°C) | E’_{rubber} ^{c} (MPa) | Peak_{area} ^{d} | FWHM ^{e} (°C) |
---|---|---|---|---|---|

0 | 2.3 | 75 | 6.9 | 1.37 | 13.7 |

10 | 2.4 | 74 | 10.4 | 1.28 | 14.6 |

20 | 3.6 | 73 | 19.6 | 1.27 | 14.8 |

30 | 4.5 | 73 | 31.9 | 1.13 | 16.0 |

40 | 5.6 | 74 | 61.2 | 0.90 | 16.8 |

40 (80 µm) | 4.0 | 71 | 78.5 | 0.88 | 19.3 |

^{a}Young’s modulus determined with DMTA at 30 °C in a controlled force experiment using three point bending clamp;

^{b}Temperature of maximum of the tan δ peak at 1 Hz;

^{c}Relaxed modulus determined at the T

_{tan δ}+ 40 °C (in the rubbery state;

^{d}Area of tan δ peak between 40 and 120 °C;

^{e}FWHM stands for full width at half maximum.

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**MDPI and ACS Style**

Isarn, I.; Ramis, X.; Ferrando, F.; Serra, A.
Thermoconductive Thermosetting Composites Based on Boron Nitride Fillers and Thiol-Epoxy Matrices. *Polymers* **2018**, *10*, 277.
https://doi.org/10.3390/polym10030277

**AMA Style**

Isarn I, Ramis X, Ferrando F, Serra A.
Thermoconductive Thermosetting Composites Based on Boron Nitride Fillers and Thiol-Epoxy Matrices. *Polymers*. 2018; 10(3):277.
https://doi.org/10.3390/polym10030277

**Chicago/Turabian Style**

Isarn, Isaac, Xavier Ramis, Francesc Ferrando, and Angels Serra.
2018. "Thermoconductive Thermosetting Composites Based on Boron Nitride Fillers and Thiol-Epoxy Matrices" *Polymers* 10, no. 3: 277.
https://doi.org/10.3390/polym10030277