The surface morphology of the dispersed h-BN powder was analyzed by scanning electron microscopy, Fig. 1. h-BN particles are large aggregates consisting of smaller particles of scaly shape with irregular-shaped edges, about 140 nm thick.
The fractional composition of pristine h-BN is characterized by the unimodal distribution of particles and is represented by particles from 1 to 100 μm, with a predominant number of particles with sizes of 40 μm, Fig. 2a.
The specific surface area of h-BN particles, determined by using the specific surface area and porosity analyzer (Quantachrome Nova 2200) and the low-temperature nitrogen adsorption method, is 20.3 m2/g.
As a polymer matrix we used a previously developed composition21,22, consisting of 100 parts by weight of epoxy resin brand ED-20, 40 parts by weight of TCEP and 15 parts by weight of a PEPA hardener.
TCEP performs the functions of both a plasticizer and a flame retardant. In this case, bending stress increases by 32% and toughness is 2.6 times higher, the flammability index–oxygen index (CI) increases from 19 to 27% by volume, which ensures the transition of the material to the class of hardly flammable21,22. In addition, the presence of a chemical interaction of TCEP with an epoxy oligomer during curing was previously proved22.
In plasticized epoxy composition h-BN was added as a modifying agent (0.01–1.0 parts by weight).
The conducted research has shown that the most rational content of h-BN as a structuring additive, providing maximum values of physical and mechanical properties is 0.05 parts by mass, Figs. 3, 4 and 5, at the same time the bending failure stress increases by 109% and the bending elastic modulus increases by 15%, the tensile strength increases by 25% and the tensile elastic modulus increases by 21%, impact resilience increases by 300%, the Brinell hardness increases by 85%.
To ensure the chemical interaction of the mineral filler with the polymer matrix and to increase physicomechanical characteristics of epoxy composites based on them, the h-BN surface was treated with the APTES sizing additive.
To impart hydrophilicity to the surface of h-BN and to make possibie its sizing, h-BN was annealed at 1,100 °C for 1 h. In this case, the main goal of pretreatment of boron nitride powder is to create a sufficient amount of hydroxogroups on its surface, which, when further treated with a silane modifier, will lead to the deposition and fixing of the modifier molecule on the h-BN surface.
Hydroxyl surface group formation was verified using FT-IR spectroscopy, Fig. 6, and X-ray phase analysis, Fig. 7, methods. On the IR spectrum of the h-BN sample annealed at 1,100 °C, a peak appears in the 3,400 cm−1 range, which is characteristic of the OH group. Besides, a broad line with a maximum of about 1,300–1,350 cm−1 also increases, which also indicates the formation of bonds B-OH.
The results of the X-ray phase analysis, Fig. 7, confirm the formation of a stable layer of boron hydroxide on the surface of the material. The diffraction pattern of the sample annealed at 1,100 °C clearly shows the presence of the most intense line of the B(OH)3 phase, as well as some other lines of lower intensity characteristic of this phase20.
Thus, on the base of all the data obtained, it can be concluded that annealing the initial h-BN powder at 1,100 °C for 1 h leads to the formation of a hydroxide phase, presumably on the fine faces of h-BN flakes, in the amount sufficient for the subsequent treatment. This is a simple and rather effective method of hydrophilization of the surface of hexagonal boron nitride.
The formation of strong bonds between h-BN and APTES was proved by IR Fourier spectroscopy, Fig. 6. As can be seen from the spectrum of the sample after modification, the line of hydroxogroups (3,400 cm–1), which is involved in the formation of the APTES layer, disappears almost completely. After APTES treatment, the peak separation in the range of plane stretching vibrations of the B–N bond (1,400 cm–1) can also be noted in the spectrum of the sample. The peak separation in this area may indicate an increase in the ordering of the h-BN structure as a result of the material surface treatment. One can also see a broad maximum of about 1,100 cm–1, which indicates the presence of Si–O bonds in the sample.
The chemical interaction of the APTES functional groupsand the epoxy oligomer was also proved by IR Fourier spectroscopy, Fig. 8.
APTES contains an amine group that can interact with the epoxy ring of the resin when added into the composite. After mixing APTES and ED-20, in the absence of PEPA, a significant decrease in the intensity of the absorption peak corresponding to the epoxy ring (910 cm−1) is observed, Fig. 8. This confirms the presence of theinteraction between the APTES amine groups and the ED-20 epoxy groups.
Indirect evidence of chemical interaction is a significant increase in the physic-mechanical properties of epoxy composites, which is possible only when organizing chemical interactions at the polymer matrix/filler interface.
Based on this, it can be assumed that the reactions shown in Fig. 9 take place in the system.
APTES treatment ensured the delamination and breaking of h-BN particles with a large lateral size. The fractional composition of h-BN particles modified by APTES is characterized by a unimodal particle distribution and is represented by particles from 0.1 to 25 μm, with a predominance of particles with sizes of 5–6 μm, Fig. 2b.
APTES treatment is accompanied by its intercalation into h-BN layered structures, resulting in the exfoliation of layered structures, which is confirmed by scanning electron microscopy, Fig. 10. h-BN particlesafter APTES treatment are small particles of scaly shape with irregular-shaped edges, 70–80 nm thick.
After APTES treatment h-BN particles are better wetted with the epoxy composition (ED-20 + TCEP), which is evident in a decrease in the wetting angle from 27° to 22°, their specific surface area increasing from 20.3 to 35.2 m2/g, Fig. 11.
The organization of the chemical interaction at the h-BN/polymer matrix interface, by treating the h-BN APTES surface, results in an increase in the physicomechanical properties of epoxy composites: bending stress increases by 142% and bending modulus increases by 52%, strength increases by 53% and thetensile elastic modulus increases by 37%, impact strength increases by 400% and Brinell hardness increases by 96%, in comparison with plasticized epoxy composite that does not contain h-BN, Fig. 12.
When assessing the effect of the modifying additive on network polymers, it is necessary to take into account that the curing process takes place in the presence of a developed surface of the solid material (h-BN), which can influence the kinetic characteristics of the polymerization reaction during curing, as well as the formation of the material phase structure. The role of the adsorption interaction of the components of the oligomeric composition with the solid surface of the h-BN is also great24.
The study of the curing kinetics of epoxy compositions, Fig. 13, containing pristine h-BN and h-BN after APTES treatment, show that they have a different effect on the structure formation processes of the epoxy composite, the addition of pristine h-BN reduces the duration of gelation from 45 to 36 min and the duration of curing from 53 to 48 min, while the maximum curing temperature remains practically unchanged, and the addition of h-BN after APTES treatment provides an increase in the duration of gelation from 45 to 74 min and curing from 53 to 96 min, the maximum curing temperature decreasing from 105 to 85 °C, Table 1, which is associated with the formation of thicker boundary layers due to the chemical interaction of APTES functional groups and an epoxy oligomer, which differ in their properties and structure from the bulk of the polymer, leading to a change in the ratio between the components of the binder at the filler surface21,23.
Using the method of thermogravimetric analysis, it was stated that the addition of pristine h-BN and h-BN after APTES treatment does not reduce the heat resistance of epoxy composites based on them, while the Vicat heat resistance increases from 100 to 114–170 °C, Table 2.
An important characteristic of materials used in electronic equipment is thermal conductivity. Unmodified epoxy composites have a low thermal conductivity (0.1 W/m K), therefore, when locally heated, they work as thermal insulation, which can lead to overheating and thermal decomposition of the composite18,25.
The addition of even small additives of h-BN into the epoxy composition increases the thermal conductivity coefficient by 52–217%, while a decrease in thermal resistance is noted, Table 3.