Shenyang Jusheng New Material Technology Co., Ltd: Boron Nitride Nanosheets Enhanced Asphalt Polyurea Coating, New Breakthrough in Multi-scenario Harsh Environment Protection
Recently, Shenyang Jusheng New Material Technology Co., Ltd. in cooperation with Prof. Meng Qingshi's team from Shenyang University of Aeronautics and Astronautics and Prof. Sherif from Nazarbayev University published a research paper titled “Enhanced Polyaspartate Polyurea Reinforced with Boron Nitride Nanosheets for Protective Coatings”.
Recently, Shenyang Jusheng New Material Technology Co., Ltd. in cooperation with Prof. Meng Qingshi's team from Shenyang University of Aeronautics and Astronautics and Prof. Sherif from Nazarbayev University published a research paper titled “Enhanced Polyaspartate Polyurea Reinforced with Boron Nitride Nanosheets for Protective Coatings”.
Link to paper: https://doi.org/10.1021/acsanm.4c04139
This work explores the mechanical and functional properties of polyaspartic acid polyurea (PAPU) protective coatings with the addition of boron nitride nanosheets (BNs). BNs with a thickness of 3.32 nm were synthesized mechanochemically and modified by ball-milling with meso-diamine. PAPU nanocomposites based on pristine BNs and modified BNs (m-BN) were prepared and systematically investigated, showing excellent mechanical properties and environmental reliability. Only 0.1 wt% of m- BN increased the tensile strength, Young's modulus and elongation at break of PAPU by 56.7%, 41% and 67.4%, respectively. While the Young's modulus of the polyurea continued to increase at filler contents higher than 0.1 wt% m-BN, the other properties began to decrease due to filler agglomeration. After coating the 0.1 wt% PAPU/m-BNs nanocomposites, the impact strength of the aluminum plate increased by 22.2%, proving that it has good metal protection properties. In addition, comprehensive environmental reliability tests showed that the PAPU nanocomposites can serve as a robust protective coating even after prolonged exposure to various chemical conditions. For example, the impact strength of pure PAPU and PAPU/m-BN composites was reduced by 25% and 9.9%, respectively, after salt spray tests. This study emphasizes the importance of adding reinforcing phases and chemical modifications to PAPU nanocomposites to obtain high mechanical properties and multifunctional protective coatings.
Figure 1 Schematic diagram of the liquid phase stripping process of boron nitride powder.
The preparation of m-BNs was divided into two stages: the stripping stage and the modification stage. In the first stage, the boron nitride powder was exfoliated using the liquid phase exfoliation method, as shown in Fig. 1. In the second stage, the BNs were noncovalently modified with meso-diamine (MXDA), a highly active surfactant; it is an aromatic diamine. It has two amino groups at both ends which can be grafted onto the surface of BNs.The aromatic ring of MXDA forms a π-π stack with the planar hexagonal structure of BNs. This interaction will increase the compatibility of BNs with PAPUs, resulting in a stronger interface with the substrate.
Fig. 2 (a) Peeling time versus levitation time; (b) BNs levitation images; (c-h) AFM images and thickness measurement curves of BNs
The optimal stripping time was determined by observing the suspension stability of BNs in anhydrous ethanol, as shown in Fig. 2a. The results showed that with high shear mixing for 20 min, the BNs were uniformly suspended in ethanol for ~ 80 h without obvious precipitation; the images of BNs suspension at different times are shown in Fig. 2b.
The thickness of BNs at different stages was measured by AFM, as shown in Fig. 2c-h. As shown in Fig. 2c, f, the initial BNs thickness was uneven and irregular, with an average thickness of 55.5 nm; notably, the maximum thickness of boron nitride was 233 nm. High-speed shear blending could strip the boron nitride powder into nanosheets with a thickness of 3.3 ± 0.2 nm, as shown in Fig. 2d, g. The AFM images confirmed the lamellar structure of the BNs after stripping. Planetary ball milling successfully modified the surface of BNs with meso-diamine (MXDA) to obtain m-BNs. m-BNs were further exfoliated into thinner flakes due to the high shear stress exerted by the ball milling; as shown in Fig. 2c, h, the average thickness of the m-BNs was 2.31 ± 0.14 nm, which was thinner than that of the unmodified BNs.
Fig. 3 (a,b) FTIR spectra of pure PAPU; (c) AFM characterization of pure PAPU; (d) UV-visible transmission spectra; (e) PAPU image showing its transparency
Figure 3a shows the FTIR spectra of pure aspartic polyurea (PAPU) films. There are two distinct characteristic peaks at 3343 and 1528 cm-1 , which correspond to the N-H stretching and bending vibrations of the amino group, respectively. In addition, Fig. 3b shows other characteristic peaks found at 1731 and 1702 cm-1 attributed to the C-O stretching vibration of the urea bond. The microphase-separated structure of PAPU was investigated by AFM, as shown in Fig. 3c.The AFM micrographs clearly show that there is a clear boundary between the dark regions (representing soft domains) and the bright regions (corresponding to hard domains). As can be seen in Fig. 3d, the PAPU film has a high transmittance of 92% in the visible light range of 400 ~ 750 nm. Fig. 3e shows an image of PAPU with a thickness of 2 mm, characterized by colorless transparency.
Figure 4 Mechanical properties of PAPU/BN and PAPU/m-BN nanocomposites: (a) tensile strength, (b) elongation at break, (c) Young's modulus, and (d) impact strength of aluminum alloy matrix coated with PAPU nanocomposites
The tensile properties of pure PAPU and its BN and m-BN nanocomposites are shown in Fig. 4a-c. The results show that either BNs or m-BNs with low filler amount can significantly improve the mechanical properties of PAPU. Chemically modified BNs have the greatest influence on the enhancement effect of PAPUs. For example, Figures 4a and 4b show that the addition of 0.1 wt% m-BNs increased the tensile strength and elongation at break by 56.62% and 67.42%, respectively, whereas these increments were much smaller when virgin BNs were used. Figure 4c depicts that the Young's modulus of the PAPU nanocomposites increased consistently with the incorporation of BNs, and the increase was significant at a content of >0.1 wt%. the increase in Young's modulus was even more significant for the PAPU/m-BN nanocomposites. For example, at 0.1 wt% m-BNs, the Young's modulus of PAPU was 170 MPa, whereas that of PAPU/BNs nanocomposites was 140 MPa. pure PAPU had a maximum Young's modulus of 195.7 MPa. 5 wt% m-BNs showed an increase of 58.39% compared to pure PAPU.
The improvement was observed to be due to the strong interfacial interaction between the m-BNs and the PAPU matrix as well as the homogeneous dispersion of the nanosheets within the matrix. Together, these factors contributed to the efficient transfer of mechanical loads to the BNs. The tensile strength and elongation at break of the composites showed a decreasing trend with further increase in the content of m-BNs. This is due to the aggregation of nanosheets within the matrix, resulting in stress concentration. This defect is more obvious in unmodified BNs-based nanocomposites. In addition, the stacked BNs would slide during load transfer, thus affecting the mechanical properties of PAPU nanocomposites. It is noteworthy that the Young's modulus does not follow a similar trend of tensile strength decrease when the BNs content is higher than 0.1 wt%. This is because Young's modulus is measured at the beginning of the elastic region of the material, where the interface between the matrix and reinforcing phases is still strong enough to share the load between the phases of the composite. In contrast, the tensile strength is measured at the maximum load in the plastic region, where the entanglement of the polymer chains and the interface between the phases are almost distorted or destroyed.
Fig. 5 Dissolution rate of PAPU nanocomposites in (a) deionized water, (b) alkaline solution, and (c) acidic solution; Impact strength and energy loss rate of PAPU nanocomposites after (d) deionized water, (e) alkaline solution, (f) acidic solution, (g) hygrothermal cycling, (h) salt spray treatment, and (i) UV aging
The samples were immersed in 5% NaOH solution, 5% H2SO4 solution and deionized water for 17 days. The swelling rate of each sample was measured and recorded daily; the results are shown in Fig. 5a-c. The impact strength was measured after 17 days to evaluate the chemical resistance. After immersion, the pure PAPU membrane had the highest swelling rate, reaching 3.83% in deionized water. With the addition of BNs or m-BNs, the swelling rate of PAPU in deionized water was small and the difference was not significant; 1.62% and 1.58%, respectively, as shown in Figure 5a. As can be seen from Fig. 5b, the dissolution rate of PAPU nanocomposites in acidic and alkaline solutions had similar trends, which were 4.21% and 4.17% in acidic and alkaline solutions, respectively.The dissolution rate of PAPU/m-BNs was the lowest among all the solutions. Overall, the PAPU nanocomposites exhibited higher swelling rates in alkaline solutions due to chemical corrosion of the PAPU molecules. In addition, the PAPU/m-BN nanocomposites showed the best chemical resistance as they were uniformly dispersed within the matrix and could act as an effective physical barrier against chemicals.
The impact strength and the associated energy loss rate of each specimen before and after immersion were measured, as shown in Fig. 5d-f. All specimens exhibited the highest rate of impact energy loss in alkaline solution. Among them, the impact energy of PAPU/m-BN nanocomposites in deionized water, acidic and alkaline solutions decreased by 5.24%, 6.17% and 8.98%, respectively, compared to that before immersion.
Five cycle tests were performed at a temperature of 47 ± 1 °C and a relative humidity of 95 ± 1%. As shown in Fig. 5g, the impact energy of the pure PAPU specimen was significantly reduced to 272 kJ/m2 after the hygrothermal treatment, which was 30.55% lower than that of the original specimen. While the impact energy of PAPU/m-BN nanocomposites was 397 kJ/m2, which was reduced by 17.2% after the hygrothermal treatment. This is due to the plate-like structure of BNs which can effectively slow down the penetration of water vapor into the polymer matrix, thus maintaining its impact resistance under humid and hot environments. m-BNs can be uniformly dispersed into the matrix and fill up the voids inside the PAPUs, which makes the structure of the PAPUs denser and impermeable to water, thus reducing the rate of water vapor transmission. In addition, m-BNs can form strong interfacial interactions with the PAPU matrix, which enhances its resistance to water vapor penetration at the interfacial site; therefore, it retards the process of water vapor intrusion.
The impact strength of the samples before and after the salt spray test is shown in Figure 5h. After the test, the impact energy of pure PAPU was reduced by 25% compared to the untreated sample. The addition of BNs increased the impact strength of PAPU compared to pure polymer. The highest impact strength of PAPU/m-BNs nanocomposites was obtained after salt spray treatment; it was reduced by only 10%. In the salt spray environment, chloride and bromide ions can penetrate into the PAPU matrix. These ions then react with the PAPU chemical bonds, destroying them and thus its physical structure. These chemical reactions are severe when pure PAPU is exposed to a salt spray environment. Incorporating BNs into PAPUs can act as a shield against salt spray ions and reduce the acute effects of this harsh environment. When the BNs are further modified, they can interact with the PAPU molecules and uniformly disperse within the PAPU matrix. This will form a compact 3D network that acts as a physical layer against the chemical attack of salt spray ions.
The aging resistance of pure PAPU and its nanocomposites was determined using UV aging test. The impact strength of all samples was measured before and after the UV irradiation test; the results are shown in Fig. 5i. It is noted that the effect of UV radiation on PAPU is relatively small. After UV aging, the impact energy of pure PAPU was 369.5 kJ/m2 , which only decreased by 5.8% compared to the untreated samples.The impact energies of PAPU/ BN nanocomposites and PAPU/m-BN nanocomposites were 427 and 464.5 kJ/m2 , respectively.Compared to the untreated samples, their impact strengths decreased by 4.9% and 3.1%. The results indicate that PAPU has good aging resistance and can be slightly improved by adding nanofillers (e.g., BNs). Boron nitride can absorb and scatter UV rays and reduce the direct effect of UV rays on the PAPU matrix.
Summary of work
The research team successfully prepared BNs-based PAPUs and their composites, synthesized and tested the properties under various mechanical loads and chemical environments. First, BNs were produced by applying high shear energy through a liquid phase exfoliation process.The thickness of BNs was 3.32 nm on average.The surface of BNs was chemically modified by using m-BNs (m-BNs) achieved by microsphere milling with less chemical solvent; the success of the modification was verified by AFM, FTIR and TGA. Next, PAPU nanocomposites containing BNs and m-BNs were prepared and extensively investigated. The mechanical properties and environmental reliability of the PAPU nanocomposites were significantly improved by the addition of BNs; these enhancements were further enhanced by modifying the BNs with m-BNs. The tensile strength, Young's modulus and elongation at break of PAPU nanocomposites were increased by 56.62%, 41% and 67.42%, respectively, when the content of m-BNs was 0.1 wt%. Due to the homogeneous dispersion of m-BNs in the PAPU matrix, the m-BNs have reinforcing efficiency, and thus the materials have excellent mechanical properties. In addition, the nanocomposites have excellent environmental reliability properties.The PAPU nanocomposites provide an effective coating that resists prolonged exposure to different working environments such as various chemicals, salt spray, humidity-heat cycling, and UV exposure. This study provides a simple and effective method for the preparation of high performance PAPU/m-BN nanocomposites, which can be used as a protective coating to protect the substrate from harsh environmental conditions.