Improved high-temperature damping performance of nitrile-butadiene rubber/phenolic resin composites by introducing different hindered amine molecules

Abstract By introducing hindered amine GW-622 or GW-944 into nitrile-butadiene rubber/phenolic resin (NBR/PR, abbreviated as NBPR) matrix, we have prepared different hindered amine/NBR/PR ternary hybrid damping materials with high-temperature damping performance, respectively. Fourier transform infrared (FTIR) spectroscopy, scanning electron microscope (SEM), differential scanning calorimetry (DSC), and dynamic thermomechanical analysis (DMA) were used to research the microstructure, compatibility, and damping properties of the hindered amine/NBPR composites. FTIR results indicate that hydrogen bonds are formed between the hindered amine and the NBPR matrix. Both DSC and SEM results show that hindered amine has partial compatibility with the NBPR matrix. DMA results show that two loss peaks appear in the hindered amine/NBPR composite. Thereby, the composites show better damping performance at a higher temperature, and the temperature domain of high-temperature damping becomes wider with the increase in the addition of hindered amine. This study provides a theoretical support for the preparation of high-temperature damping materials. Graphical abstract To obtain high damping materials in the high-temperature domain, hindered amine GW-622 or GW-944 was introduced into nitrile-butadiene rubber/phenolic resin (NBR/PR) matrix to prepare ternary hybrid damping material. The FTIR, DSC, SEM, and DMA analyses were used to explore the microstructure, compatibility, and damping performance of the hindered amine/NBPR composite.


Introduction
Rubber is the most important damping material because of its unique viscoelastic properties (1,2). The damping principle of rubber damping material is to use the viscoelastic property of the polymer to absorb vibration energy (3). The absorbed mechanical energy or sound energy is partially converted into heat energy and dissipated to reduce vibration or reduce amplitude (4). Generally speaking, the damping factor of pure rubber matrix is small, the glass transition temperature (T g ) range is narrow, and the effective damping temperature range is in the low-temperature region below room temperature (5)(6)(7). Therefore, the material needs to be modified to achieve the required damping purpose. In recent years, the study of organic hybrid damping materials has attracted much attention (8,9). The damping property of the rubber matrix is improved by hydrogen bond (H-bond) interaction between rubber matrix and polar organic molecules (10).
To prepare organic hybrid materials, hindered phenols containing polar hydroxyl groups are often selected as polar small molecules, such as AO-80 (11), AO-60 (12), and AO-70 (13). Most of the previous studies have been on the binary hybrid damping materials (14,15). At the very beginning, Wu et al. studied the hybrid composites of AO-80/chlorinated polyethylene (CPE) (16), AO-70/CPE and AO-60/CPE (17), and found that the addition of hindered phenol could significantly increase the damping performance of CPE matrix. Zhao et al. added the hindered phenol to the polar nitrile rubber (nitrile butadiene rubber [NBR]) matrix to prepare the rubber-based damping material with high loss peak and improved tensile properties (18). Like hindered phenols, hindered amine molecules contain a large number of polar amine and imine groups. When the hindered amine is added to the polar rubber, it can interact with the polar rubber matrix, thus giving the rubber matrix excellent damping properties (19). Besides, the hindered amine has wide sources and is of lower price than hindered phenol, which can effectively reduce the production cost of organic hybrid materials. Therefore, it is of great significance to study hindered amine/rubber composites.
The damping properties of binary rubber-based organic hybrid materials tend to be improved at room temperature (20,21). However, the processing and use environment of rubber damping materials is changeable, so it is of great significance to prepare materials with high damping properties in a wide temperature range, especially in a high-temperature range. Researchers began to prepare ternary hybrid damping materials by adding polar small molecules to rubber and plastic blends. Zhang et al. prepared hindered phenol AO-80/phenol resin (PR)/ NBR ternary hybrid with long-period damping properties (22). In our previous study, by introducing AO-80 into nitrile butadiene rubber/poly(vinyl chloride) (NBR/PVC), we got ternary hybrid damping materials with high damping properties and high mechanical properties (23).
In this study, we propose a strategy that many hydroxyl polar functional groups in the phenolic resin structure, which would lead to a polar amino group on the hindered amine, can form strong H-bond with NBR and PR matrix to prepare high damping materials in the high-temperature domain. Nitrile butadiene rubber/ phenolic resin (NBR/PR [NBPR]) was selected as matrix, and different kinds of hindered amines (GW-622 and GW-944) were chosen as polar small molecules to prepare ternary hybrid damping materials. NBR exhibits excellent damping performance due to the presence of polar cyan functional groups in its structure.

Materials
Hindered amine powders (GW-622 and GW-944) were supplied by Beijing Additives Institute (China). NBR with an acrylonitrile weight content of 41% (N220S) was provided by Japan Synthetic Rubber Co., Ltd (Japan). Phenolic resin powders (PR 2123) were obtained from Wuxi Mingyang Bonding Material Co., Ltd (China). Other rubber processing additives were of analytical grade and used without further purification. The chemical structures of GW-622 and GW-944 are shown in Figure 1.

Preparation of hindered amine/NBPR composites
The NBPR composites were obtained by mixing the NBR and PR powders (the mass ratio of NBR and PR is 80:20) in φ152.4 mm two-roll mill at room temperature. The mixture was then added with different masses of hindered amine molecules, and the mass ratios of GW-622 and NBPR were 0/100, 17/100, 35/100, 52/100, and 70/100 (the mass ratios of GW-944 and NBPR were 0/100, 18/100, 36/100, 54/100, and 72/100). The mixtures were then kneaded at 130°C for 5 min on a two-roll mill to fully fuse the hindered amine molecules. Then, the compounding and crosslinking additives were added to the above mixtures, including 5.0 phr of zinc oxide, 2.0 phr of stearic acid, 0.2 phr of tetramethylthiuram disulfide, 0.5 phr of diphenyl guanidine, 0.5 phr of dibenzothiazole disulfide, and 2.0 phr of sulfur. Finally, the mixtures were hot pressed and vulcanized at 160°C and 15 MPa for their corresponding T90 (optimum cure time).

Characterization
Fourier transform infrared spectroscopy (FTIR) was conducted by Nicolet 8700, Thermo Fisher Scientific Inc. (USA). The spectra were obtained by using the attenuated total reflection technique in a wavenumber ranging from 4,000 cm −1 to 500 cm −1 for 32 scans at a resolution of 8 cm −1 . The GW-622 (or GW-944) powder spectra were obtained by using ultrathin disk specimens pressed from hindered amine ground in anhydrous potassium bromide. Scanning electron microscopy (SEM) images of the fracture surfaces of the hindered amine/NBPR composites were taken by Hitachi S-4800 (Japan). All SEM samples were cryogenically fractured by quenching in liquid nitrogen.
Differential scanning calorimetry (DSC) was performed on a TGA/DSC calorimeter, Mettler-Toledo Co (Switzerland). Samples were heated from −60°C to 150°C at a heating rate of 20°C/min under an N 2 atmosphere.
Dynamic mechanical performance (dynamic thermomechanical analysis [DMA]) was attained by VA 3000 dynamic mechanical analyzer, Rheometric Scientific Inc.
(USA). The sample size was 15 mm (length) × 10 mm (width) × 2 mm (thickness). The temperature range was   (19). The peak at 1,733 cm −1 wavenumber is attributed to the carbonyl groups. For GW-944 molecules, the peak at the wavenumber 3,442 cm −1 belongs to the amino groups. Figure 2b shows that pure NBPR matrix has two distinct peaks. The broad infrared absorption peak in the wavenumber ranging from 3,250 to 3,500 cm −1 is responsible for phenolic hydroxyl groups of PR (22). The -CN group of NBR is at wavenumber 2,237 cm −1 (23). With the increasing addition of GW-622 molecules, it can be found that the 3,387 peak intensity decrease and the peak shifts to the high wavenumber, which means more phenolic hydroxyl groups involves in the formation of H-bonds. Similarly, the -CN groups also change accordingly, indicating that more -CN groups are involved in the formation of H-bonds. Figure 2c shows that the addition of hindered amine GW-944 into NBPR matrix has a similar effect as GW-622. With increasing the content of GW-944 molecules, the 2,237 cm −1 peak of -CN groups decreases gradually. Also, the vibration peak of phenolic hydroxyl group moves toward high wavenumber, and the phenomenon of blue shift occurs. These phenomena indicate that the addition of GW-944 molecules increase the content of intermolecular H-bonds in the NBPR composites.

Microstructure of hindered amine/NBRR composites
To illustrate the microstructure morphology of hindered amine/NBPR composites, the SEM test is carried out, as shown in Figure 3. Figure 3a and b shows that pure NBPR samples have smooth fracture surfaces, indicating the good compatibility between the NBR and PR. When the content of GW-622 is above 35 phr, some pits begin to protrude on the surface of GW-622/NBPR composite, as shown in Figure 3c and d. The dispersion of small molecules in the NBPR has reached a saturation state. The addition of more GW-622 will cause the aggregation of small molecules. During the embrittlement and cold breaking in liquid nitrogen, the aggregates of GW-622 molecules fell off from the surface of the NBPR matrix, resulting in the formation of pits. At the same time, the partial compatibility between GW-622 molecules and the NBPR matrix is also the cause of the uneven surface.
On the basis of the microstructure morphology test, we could clearly compare with our prepared samples with GW-622/NBPR composites, and the GW-944/NBPR composites become rougher when the addition of GW-944 small molecules reaches 36 phr. This may be due to the uneven size of the hindered amine GW-944, ranging from micron to millimeter, and so it is difficult for GW-944 to be uniformly dispersed in the NBPR matrix.

Glass transition of hindered amine/ NBPR composites
Further analysis of the thermal stability of the prepared sample was carried out, and Figure 4 shows the DSC curves of the hindered amines molecules and the hindered amine/NBPR composites with different mass ratios. As shown in Figure 4a, the glass transition temperatures of hindered amine GW-622 and GW-944 are 67.6°C and 84.8°C, respectively. As shown in Figure 4b, T g of NBPR matrix is 0.1°C, and the melting temperature of NBPR is 109.2°C. Figure 4b shows that T g of GW-622/ NBPR composite moves toward low temperature in a small extent with the addition of GW-622 molecules.
With the formation of the molecular interaction between GW-622 and NBPR, theoretically, T g should move toward high temperature. However, due to the large molecular structure of GW-622 (as shown in Figure 1), the volume of GW-622/NBPR composites becomes larger when there is an increase in the addition of GW-622, making T g move toward the low temperature. Similar results were observed for the GW-944/NBPR composite, i.e., adding GW-622 molecules and making T g to move toward the low temperature. We will further verify the aforementioned results.

Damping properties of hindered amine/ NBPR composites
The damping properties of materials are determined by the loss factor and the area of the glass transition zone (TA value) (tan δ > 0.3) (24,25). Figure 5 shows the temperature dependence of the tan δ value of hindered amine/NBPR composite. The pure NBPR matrix has only one loss peak, which again indicates the good compatibility between NBR and PR, as shown in the earlier SEM. As shown in Figure 5a, with the addition of GW-622 molecules, the GW-622/NBPR composites change from a single loss peak to two adjacent double loss peaks. Moreover, with the increasing GW-622 contents, the first loss peak in the room temperature region decreases and moves toward the low-temperature direction, as shown in the aforementioned DSC. The second loss peak in the high-temperature region increases gradually, and the range of damping temperature domain also increases gradually with the addition of GW-622 molecules. From Table 1, we can observe that the first loss peak value of GW-622/NBPR decreases from the original 1.28 to 0.87 by 32%. The TA 1 value also has a 43.6% decrement from 38.61 to 21.78. The peak position moves from 29.88 to 23.14, which is consistent with the aforementioned DSC analysis. The second loss peak value increases from the original 0.20 to 0.64 by 220%. In addition, the TA 2 value has a greater increment from 6.52 to 27.79. Figure 5b shows that the addition of GW-944 molecules also makes the GW-944/NBPR composite show two loss peaks. Compared with the addition of GW-622, the addition of GW-944 reduces the first loss peak of the NBPR matrix more and increases the second loss peak of the matrix more. From Table 2, we can observe that the first loss peak decreases by 61.7% from 1.28 to 0.49, whereas the second loss peak increases from 0.55 to 0.64. Figure 5c and d shows that the loss factor value of hindered amine/NBPR composites in the low-temperature zone (−80°C to 60°C) decreases with the addition of the hindered amine, while the loss factor value of the composite in the high-temperature zone (80-140°C) increases continuously, indicating that the addition of hindered amine GW-622 or GW-944 can effectively improve the damping performance of the NBPR matrix in the high-temperature zone. When the hindered amine molecules are added to the NBPR matrix, the H-bond interactions formed between hindered amine molecules and the NBPR matrix. At the same time, saturated small molecules are dispersed in the NBPR matrix, the addition of more hindered amines would result in the aggregation in small molecules. As a result, the volume of hindered amine/NBPR composites increases, resulting in T g of composites moving toward the direction of high temperature and decreasing of the loss factor value in the room temperature. Figure 6 shows the temperature dependence of storage modulus (E′) of hindered amine/NBPR composites. As shown in Figure 6, the higher the content of the hindered amine molecules, the greater the storage modulus of the composites (26). The E′ is a measure of the capability of the composites to store mechanical energy and resist deformation (11,27). This is due to the formation of H-bond between hindered amine molecules and the NBPR matrix, which increases the E′ of the composites. It is noteworthy that the material has a wide glass transition zone, that is, the addition of small molecules to broaden the application range of damping materials.

Conclusions
Herein, by introducing different hindered amine molecules into the NBPR matrix, we prepared different hindered amine/NBPR ternary hybrid damping materials. FTIR, DSC, SEM, and DMA were used to characterize the microstructure, compatibility, and damping performance of the hindered amine/NBPR composites as follows: 1. FTIR results show that H-bonds are formed between the hindered amine and the NBPR matrix. 2. DSC and SEM results indicate that GW-622 has partial compatibility with the NBPR matrix when the addition