Abstract
To overcome the limited tribological properties of bismaleimide (BMI) resin, a kind of novel organic–inorganic hybrid silicon dioxide nanoparticles with terminal functional amidogen groups (SiO2–NH2) was prepared via a sol–gel process. The SiO2–NH2 was chosen as raw materials to react with polyformaldehyde and aniline, and the silicon dioxide-containing benzoxazine-monomer (SiO2-BOZ) can be obtained via a solvent process. The new prepared SiO2-BOZ were then as modified phase to modify BMI to prepare the SiO2-BOZ/BMI composites. With 15.0 wt-% SiO2-BOZ, the flexural strength and impact strength of SiO2-BOZ/BMI composites can reached the maximum value of 166.12 MPa and 17.93 kJ m−2, respectively, while the wear rate of SiO2-BOZ/BMI composites is only 3.9 × 10−6 mm3 N−1 m−1. The enhanced tribological properties of SiO2-BOZ/BMI can be attributed to the improvement of its heat resistance and mechanical properties, which can increase the capacity of the SiO2-BOZ/BMI to withstand external forces and heat during the friction process.
Introduction
As a class of thermosetting polyimides, bismaleimide (BMI) resins possess plenty of unique properties [1], such as high crosslinking ability, glass transition temperature, excellent fire resistance, and outstanding specific strength [2]. Hence, the BMI has the broad application prospects in electronic materials, adhesives, fibre-reinforced resin matrix materials, aerospace, and other fields [3, 4]. However, the high crosslinking ability also endows BMI with high melting point and curing temperature, higher brittleness, and lower adhesion [5, 6]. Those defects greatly restrict the utility of BMI for many industrial applications. Therefore, it is necessary to explore suitable ways to modify the BMI resin, thus these deficiencies can be resolved effectively, and the applications of BMI resin can be further extended. Blending modification is widely used in BMI modification due to its flexible material selection and convenient operation [7]. In this method, the commonly used modification systems are nanoparticles and organic resins, and the selection of blending fillers is particularly important. Many fillers had been used to improve the comprehensive performance of BMI, such as allyl compounds, binary amine chain extension, organic resins, and inorganic particles [8, 9]. Although these fillers can endow BMI with some new properties, the optimisation of their compatibility with BMI is also an urgent problem to be solved in the process of modification [10].
Silicon dioxide nanoparticles (Nano-SiO2) is a kind of nanoparticles that not only possess the advantages of traditional nanoparticles [11], such as high specific surface area, light translucence, and low cost of fabrication [10, 12] but also exhibit unique characteristics of anti-wear, reducing friction, and high load capacity [13, 14], thus they have been widely used as nano fillers to enhance the overall properties of the organic matrix [15]. However, poor lipophilicity of nano-SiO2 endows them with a strong tendency to agglomerate in the organic phase, thus their consistency with resins is extremely limited [16]. To resolve this difficulty, the nano-SiO2 should be activated by an organic phase, thus it can combine the advantages of organic and inorganic systems.
Benzoxazine (BOZ) resin is a high-performance phenolic resin that can be obtained via a ring-opening reaction without any initiator and catalyst [17, 18], and exhibits outstanding properties, such as good mechanical properties, low water absorption, high-temperature stability, flame retardant, excellent chemical, and electrical resistance [17, 19, 20]. Furthermore, when blending with the BMI resin, the amine group from the ring-opening reaction of BOZ can further catalyse the reaction of BMI and form a more dense and uniform structure of the polymer alloys [19, 21, 22]. It can be predicted that when both the nano-SiO2 and BOZ have been introduced into the BMI system, the advantages of organic and inorganic phases can endow BMI with excellent properties. The further improvement of properties of BMI can be carried out in the form of copolymers, which make them more suitable for high-performance applications [23].
As the BOZ can be synthesised using formaldehyde, phenol derivative, and primary amine through the Mannich condensation, a new type of hybrid Nano-SiO2 with terminal amino groups (SiO2-NH2) was prepared in this article and used as raw materials to prepare the silicon dioxide-containing benzoxazine-monomer (SiO2-BOZ) through a solvent process. The chemical structure of the prepared SiO2-BOZ was characterised by Fourier transform infrared spectroscopy (FTIR). The SiO2-BOZ was then used as modifiers to add into the BMI resin, and the SiO2-BOZ/BMI resin was finally prepared. The curing characteristics and thermal properties of the SiO2-BOZ/BMI materials were measured, and the effect of SiO2-BOZ on the mechanical and tribological properties of SiO2-BOZ/BMI was also researched. This study aims to provide a new method for the development of a high toughness resin, which can meet the requirements of high heat resistance engineering materials, aerospace materials, micro-electronic components, and other harsh conditions.
Experimental methods
Materials
The BMI pre-polymer and the diallyl bisphenol A (a viscous transparent liquid that was industrially pure with a purity of> 95 wt-%) were purchased from Honghu Shuangma New Material Technology Co. LTD. The silane coupling agent N-(2-aminoethyl)-3-aminopropyltrimethoxysilane (KH-792, a transparent liquid that was analytically pure with a purity of> 99 wt-%) was provided by Shandong Yousuo Chemical Technology Co. LTD. The raw materials to prepare SiO2-BOZ are aniline and paraformaldehyde (the purity was >98%, purchased from Jingzhou Fine Chemical Co., Ltd). The other reagents and solvents used in the experiments without further purification were supplied by Tianjin Fuchen Chemical Reagents Factory and included tetraethyl orthosilicate (TEOS), chlorhydric acid, ethanol, and acetic acid.
Preparation of the SiO2-BOZ
The SiO2-NH2 was synthesised by the sol–gel method. TEOS and KH-792 (ratio 4:3) were added into a 100 mL three-necked flask equipped with stirring paddle, thermometer, and tunnel. The mixture was stirred with distilled water and ethanol at 60°C for 6 h to obtain a viscous transparent liquid. Then the liquid was added into a vacuum oven and dried at 50°C for 12 h, thereby completely removing water and ethanol. The SiO2–NH2 was obtained by grinding the products of the above process into nanoscale powder with a four-ball mill.
The SiO2-BOZ was synthesised by a solvent process as shown in Figure 1. The right amount of paraformaldehyde, SiO2–NH2, and 30 mL dimethylbenzene was added into a 100 mL three-neck flask fitted with paddle, thermometer, and tunnel. The mixture was stirred for 3 h in the presence of nitrogen at 40°C, and phenol was added. The temperature was raised to 110°C and stirred in the presence of nitrogen at 110°C for 5 h, to obtain SiO2-BOZ, which is a light yellow powder after washing, filtering, and drying with trichloromethane.
The preparation of SiO2-BOZ.
Preparation of the SiO2-BOZ/BMI resins
The SiO2-BOZ resins were prepared by a casting method. The BMI and diallyl bisphenol A were mixed under 150°C with a mass ratio of 4:3. The different contents of SiO2-BOZ (0.0, 5.0, 10.0, 15.0, 20.0, and 25.0 wt-% of BMI) were then added into the mixture, and heated to complete melting, and the SiO2-BOZ/BMI pre-polymer was finally obtained. These pre-polymers were poured into a pre-heated mould, which were degassed to release agent at 120°C for 40–50 min. Finally, the moulds with the mixture were treated at 120°C 2 h−1 + 140°C 2 h−1 + 180°C 4 h−1, and the post-curing process was done at 200°C 2 h−1.
Measurements
Fourier transform infrared spectroscopy
The chemical structure of SiO2-BOZ was determined by FTIR (PerkinElmer-283B FTIR Spectrometer) spectrometry and NMR (Bruker Avance 300 instrument, Newport, Virginia, USA). For the FTIR test, the SiO2-BOZ sample was pressed into a pellet with KBr, which was scanned from 400 to 4000 cm−1. For the 1H-NMR and 13C-NMR, deuterated chloroform was used as a solvent and tetramethylsilane was used as an internal standard.
Thermogravimetric analysis
The thermal stabilities of BMI and SiO2-BOZ/BMI were determined by a TG (DEL, TA, USA) at a heating rate of 20°C min−1 under a nitrogen atmosphere.
The differential scanning calorimetry
The differential scanning calorimetry (DSC) of the BMI and SiO2-BOZ/BMI was determined on a DSC-2910 (DEL, TA, USA) at a heating rate of 15°C min−1 under a nitrogen atmosphere.
Mechanical properties
The flexural strength was measured in accordance with the standard GB/T 2567-2008 at room temperature on the CMT6303 electronic universal tester. The size of the sample is 80 mm × 15 mm × 4 mm and the loading rate is 2 mm min−1. The impact strength was researched according to the GB/T 2567-2008 standard, on the ZBC-50A plastic pendulum impact test machine at room temperature, the size of the samples is 15 mm × 10 mm × 4 mm, and the impact rate in the test process is 2.9 m s−1. In each system, more than five specimens were tested.
Friction and wear tests
The friction and wear tests were performed according to GB3960-83 (Chinese Standard) on a test machine (MM-200). The machine could achieve an accuracy of ±5% in the frictional force and wear rate. Before each test, the counterpart steel ring and the materials were abraded with a No. 900 water-abrasive paper. Then, the steel ring and samples were cleaned with acetone. All the friction and wear experiments were conducted at room temperature. The friction coefficient was measured with a load of 196 N and a test duration of 120 min. The wear rate was measured using an electronic balance that had an accuracy of ±0.0001 g.
Scanning electron microscopy
The surface morphology of the fractured surface and the wear surface of the BMI and SiO2-BOZ/BMI samples were characterised with an S-3400NII scanning electron microscope (HITACHI, Japan) at room temperature.
Results and discussion
FTIR and NMR results of the SiO2-BOZ
The FTIR results of SiO2-NH2 and SiO2-BOZ are shown in Figure 2. It can be observed from the figure that, for the SiO2-NH2, the peak value of 3351 cm−1 is attributed to the tensile absorption NH2, and the peak value of 1610 cm−1 is for the bending vibration of NH2. These results confirm the existence of NH2. The band value at 1030 cm−1 is for tensile vibration of Si–O, and the absorption peak value at 772 cm−1 is for the tensile vibration of Si–C. The bBand value in 1475 cm−1 is attributed to the presence of –CH2–. These results confirm that the synthesis of SiO2–NH2 is successful. But for the SiO2-BOZ, the band with a maximum at 1261 cm−1, as well as the presence of overlapping band with a maximum at 1030 cm−1, should be attributed to the formation of the Si–O–Si network. Band with a maximum at 752 cm−1 is also present on the starting silane. The new band at 1488 cm−1 is attributed to the presence of –CH2– of the oxazine ring and the new peak at 926 cm−1 compared with SiO2–NH2, which can suggest the presence of the benzene ring. All these observations confirm that the SiO2-BOZ has been successfully synthesised.
The FTIR results of SiO2-BOZ.
The 1H-HMR and 13C-NMR results of SiO2-BOZ are shown in Figures 3 and 4, respectively. It can be observed from Figure 3 that the characteristic two singlets at 3.70 ppm (H7) and 5.01 ppm (H12) are assigned the proton resonances of the oxazine ring. The multiplets at 6.93–7.28 ppm are assigned to the protons of the aromatic ring (H8, H9, H10, H11). The singlet at 1.5 ppm is assigned to the protons of –NH–, while the singlet at 2.39 ppm can be assigned to the protons of –CH2– of the –CH2– next to the oxazine ring. These phenomena indicate that the SiO2 had already been introduced to the SiO2-BOZ monomer. The 13C-NMR result is shown in Figure 4, where it can be observed that the singlet at 52.6 ppm is assigned to the N–C–Ar, and 85.0 ppm is suggested to the existence of N–C–O of the oxazine ring. All these conclusions confirm that the SiO2-BOZ has been successfully synthesised.
The 1H-HMR result of the SiO2-BOZ monomer. The 13C-HMR result of the SiO2-BOZ monomer.
Mechanical properties of the materials
The influence of the SiO2-BOZ content on the impact strength of the SiO2-BOZ/BMI composite material is shown in Figure 5. It can be seen from the figure that the addition of SiO2-BOZ has a certain influence on the impact strength of the SiO2-BOZ/BMI composite material. When the content of was SiO2-BOZ gradually increased, the impact strength of SiO2-BOZ/BMI continued to steadily increase. When the maximum impact strength reaches 17.93 kJ m−2, the content of SiO2-BOZ is 15.0 wt-%. Compared with the pure BMI resin (11.57 kJ m−2), the maximum impact strength increased to as much as 55.0%. However, when the additional amount of SiO2-BOZ was further increased to 20.0 wt-%, the impact strength of the SiO2-BOZ/BMI composite material decreased, indicating that the optimal addition amount of SiO2-BOZ was 15.0 wt-%.
The impact strength of the resin with different contents of SiO2-BOZ.
The flexural strength of SiO2-BOZ/BMI resin with different contents of SiO2-BOZ is shown in Figure 6. The influence of different SiO2-BOZ doses on the flexural strength of the SiO2-BOZ/BMI composite is similar to that of the impact strength. When 15.0 wt-% of SiO2-BOZ was added, the flexural strength also reached a maximum value of 166.12 MPa, which increased by 32.3% compared with pure BMI (125.53 MPa). The increase in flexural strength may be due to the reaction between SiO2-BOZ and BMI, which can form a cross-linked structure. However, when the content of SiO2-BOZ was further increased to 20.0 wt-%, the bending strength of the SiO2-BOZ/BMI composite also decreased. Excessive SiO2-BOZ cross-linked structure may lead to mechanical deterioration. This result is consistent with the impact strength.
The flexural strength of the BMI resin with different contents of SiO2-BOZ.
In order to further study the fracture characteristics of the material, the fracture morphology of the BMI resin and its composites filled with 15.0 wt-% SiO2-BOZ in BMI is shown in Figure 7. It can be seen that after adding SiO2-BOZ, the fracture aspect ratio of SiO2-BOZ/BMI resin is lower than that of the BMI resin, and many filamentous morphologies could be observed. This phenomenon also indicates the toughening of the SiO2-BOZ/BMI resin. The possible explanation is that the reasonable addition of SiO2-BOZ can effectively react with BMI to form a homogeneous phase and construct resin matrix, which is composed of a cross-linked network structure and reduces the total free volume. Thus, the motion of the chain segment is restricted and the flexibility of matrix control is reduced. In conclusion, BMI composite materials with a reasonable ratio of SiO2-BOZ have better impact strength.
SEM of fracture surfaces taken from the resins, (A) BMI; (B) resin with 15.0 wt-% SiO2-BOZ/BMI).
Tribological properties of the materials
Friction coefficients and sliding times of composites with different SiO2-BOZ contents are shown in Figure 8. As can be seen from the figure, adding SiO2-BOZ can reduce the sliding time of the friction coefficient SiO2-BOZ/BMI resin and reduce the film conversion time of form by less than 20 min. When SiO2-BOZ is 15.0 wt-% content, a stable friction coefficient SiO2-BOZ/BMI reaches the optimal value (0.335), and decreased as much as 26.7% than the pure BMI resin (0.453), indicating that the reasonable addition of SiO2-BOZ can form a uniform deformation film during the wear process. The friction coefficient of the SiO2-BOZ/BMI resin system with different SiO2-BOZ contents first increases and then decreases after 20 min. This can be explained by the process of friction. In the initial stage of the friction process, only a few convex points on the surface of the grinding ring can contact the surface of the SiO2-BOZ/BMI resin material. At this time, the friction force is mainly represented by the plough force on the surface of the composite grinding ring, so the friction coefficient is relatively low. As the friction process continues, a large amount of heat is generated, resulting in a sharp rise in the surface temperature of the resin material. The surface of the resin material softens and exhibits viscoelastic behaviour, while the load on the resin material increases the friction between the area and the ring of the resin material. When the friction process continues, a thin and uniform transfer film will be produced on the friction surface of the resin, and the friction coefficient of the resin material tends to be stable.
The friction coefficient of the BMI resin with different contents of SiO2-BOZ.
Figure 9 shows the wear rates of resins with different SiO2-BOZ contents. As shown in the figure, the wear rate decreases with the addition of SiO2-BOZ. When the SiO2-BOZ content was 15.0 wt-%, the minimum wear rate of the composite was only 3.9 × 10−6 mm3 N−1 m−1, which was 50.6% lower than the pure BMI resin wear rate of 7.9 × 10−6 mm3 N−1 m−1. However, when the content of SiO2-BOZ was increased further, the wear rate of the composite increases. The excellent wear resistance of the SiO2-BOZ/BMI composite can be attributed to the improvement of its mechanical performance. In conclusion, a reasonable addition of SiO2-BOZ can endow SiO2-BOZ/BMI with good tribological properties. The decrease of wear rates on the SiO2-BOZ/BMI composite suggests that the composites have better wear resistance and loss reduction properties. The enhancement of tribological properties can be attributed to the improvement of their mechanical properties. With the appropriate content of SiO2-BOZ, the mechanical properties of the SiO2-BOZ/BMI composites can be greatly enhanced, which can bear more applied loads during the process of friction. Meanwhile, the Si element in the structure of SiO2-BOZ and the network structure of SiO2-BOZ/BMI composites can also endow the composite with better heat resistance to resist the damage of the composites caused by a large amount of heat generated during the friction process.
The volume wear rate of the composites with different contents of SiO2-BOZ.
It can be observed from the results of the friction performance test, when the SiO2-BOZ content is 15.0 wt-%, both the friction coefficient and volume wear rate of SiO2-BOZ/BMI achieve the lowest, this can be attributed to the chemical reaction between BMI and SiO2-BOZ, the oxazine ring of SiO2-BOZ/BMI can react with the C = C double bonds on the BMI, forming a interpenetrating network structure, so as to resist the friction process and loading charge on the SiO2-BOZ/BMI composites. However, when the content of the SiO2-BOZ continues to increase, a large number of SiO2-BOZ begin to self-polymerise, and dense crosslinking structures inside the SiO2-BOZ/BMI composites can be formed, resulting in the decrease in mechanical properties and tribological properties of the SiO2-BOZ/BMI.
The wear mechanism of materials was studied by observing the wear surface morphologies of the SiO2-BOZ/BMI resin and pure BMI resin with 15.0 wt-% SiO2-BOZ content. Under the same test conditions, SEM images of the worn surfaces of the BMI resin and SiO2-BOZ/BMI with 15.0 wt-% SiO2-BOZ are shown in Figure 8. For the BMI resin (Figure 8(A)), obvious cracking, obvious damage, and peeling areas can be observed on the worn surface, which may be caused by a fatigue wear mechanism, indicating that BMI has poor abrasion resistance to steel ring sliding. In contrast, the worn surface of the SiO2-BOZ/BMI resin (Figure 8(B)) was significantly improved, and the worn surface of the SiO2-BOZ/BMI resin is more uniform with a small amount of debris. Therefore, the wear mechanism can be attributed to adhesive wear. Appropriate SiO2-BOZ content (15.0 wt-%) reacted with the BMI resin and formed a moderately cross-linked network structure, which can improve the resistance of materials to applied pressure during friction. The improvement of wear resistance of SiO2-BOZ/BMI is not only due to the enhancement of mechanical properties but also due to the addition of SiO2, which possesses excellent self-lubrication properties.
Thermal properties of the SiO2-BOZ/BMI
Thermogravimetric analysis (TGA) results of pure BMI resin and SiO2-BOZ/BMI containing 15.0 wt-% SiO2-BOZ resin are shown in Figures 10 and 11. As can be seen from the figure, the onset decomposition (the char yield is 5%) temperature of the SiO2-BOZ/BMI resin was 400°C, while for the pure BMI resin is 399°C, indicating that the addition of resin did not reduce the initial thermal stability of the BMI. However, when the temperature continues to increase to 800°C, the char yield of SiO2-BOZ/BMI is 34.4%, which is much higher than that of the pure BMI resin (29.2%), indicating that the SiO2-BOZ/BMI possesses better thermal stability. The enhancement of thermal stability on SiO2-BOZ/BMI can be attributed not only to the uniform and stable cross-linked structures between SiO2-BOZ and BMI but also to the Si–O–Si structure of SiO2, which can provide a molecular cavity to satisfy the heat transfer in the resin. In addition, there is also a synergistic effect between Si and N, endowing the SiO2-BOZ/BMI with better thermal resistance.
SEM of wear surfaces taken from the resins, (A) BMI; (B) resin with 15.0 wt-% SiO2-BOZ/BMI). The TGA curve of the BMI and with SiO2-BOZ/BMI composites with 15.0 wt-% SiO2-BOZ.
The results of derivative thermogravimetric analysis (DTG) of pure BMI resin, SiO2-BOZ/BMI, and 15.0 wt-% SiO2-BOZ resin are shown in Figure 12. As can be seen from the figure the shapes of the two curves are similar, indicating that adding SiO2-BOZ to the BMI resin does not significantly change the decomposition mechanism of the BMI resin. However, it can also be seen that the peak strength of DTG of SiO2-BOZ/BMI is much lower than that of the pure BMI resin, indicating that the thermal stability of the BMI resin can be greatly improved after adding SiO2-BOZ. In addition, the initial thermal degradation temperature of SiO2-BOZ/BMI was 450°C higher than the pure BMI (429°C). This result also proves that SiO2-BOZ/BMI has good thermal stability, owning to the appropriate addition of SiO2-BOZ. The improvement of its thermal performance can endow the SiO2-BOZ/BMI resin with excellent wear resistance, which can effectively avoid thermal damage on the SiO2-BOZ/BMI resin during the wear process. This result is
The DTG curve of the BMI and with SiO2-BOZ/BMI composites with 15.0 wt-% SiO2-BOZ.
Conclusion
A new type of silicon dioxide-containing benzoxazine-monomer (SiO2-BOZ) was prepared by a solvent process from reactive SiO2-NH2, paraformaldehyde, and tunnel. The FTIR result shows that SiO2 was successfully introduced into the SiO2-BOZ monomer. The flexural strength of the 15.0 wt-% SiO2-BOZ resin reached a maximum value of 166.12 Mpa, and the impact strength reached 17.93 kJ m−2, which increased by 32.3% and 55.0%, respectively, compared with the BMI resin. In addition, the material with the same SiO2-BOZ content has the lowest wear rate, which is only 3.9×10−6 mm3 N−1 m−1, decreased by 50.6% compared with the BMI pure resin. The wear mechanism of SiO2-BOZ/BMI was changed from adhesive wear of pure resin to abrasive wear. TGA results showed that compared with the pure BMI resin, the SiO2-BOZ/BMI resin had better thermal resistance. These results confirmed that SiO2-BOZ/BMI with 15.0 wt-% SiO2-BOZ has good thermal, mechanical and tribological properties, and can be further developed and utilised under unfavourable conditions in the aerospace field.
Footnotes
Acknowledgements
This work was financially supported by the Science and Technology Project Foundation of Xi'an (No. 2020KJWL18) and the National Natural Science Foundation of China (No. 21905226)
Disclosure statement
No potential conflict of interest was reported by the author(s).