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

2.1 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.

Figure 1

Chemical structures of GW-622 and GW-944 molecules.

2.2 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).

2.3 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 N₂ 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 measured form −100°C to 200°C at a heating rate of 5°C/min and a constant frequency of 10 Hz.

3.1 Hydrogen bonding interactions of hindered amine/NBPR composites

To test our prepared samples, the classical FTIR measurements were used to detect the intermolecular interactions between the hindered amine molecules and NBPR matrix, as shown in Figure 2. Two prominent absorption peaks appear at 3,544 and 3,453 cm⁻¹ in Figure 2a, corresponding to the free hydroxyls of GW-622 molecules and the H-bond interactions (O–H⋯O H-bonds) between GW-622 molecules (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 2

FTIR spectra of (a) GW-622 and GW-944 molecule, (b) GW-622/NBPR composites, and (c) GW-944/NBPR composites.

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.

3.2 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.

Figure 3

SEM images of NBPR composites filed with (a) 0 phr, (b) 35 phr, and (c) 52 phr GW-622, and SEM images of NBPR composites filed (d) 0 phr, (e) 36 phr, and (f) 54 phr GW-944.

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.

3.3 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.

Figure 4

DSC curves of (a) GW-622 and GW-944 molecule, (b) GW-622/NBPR composites, and (c) GW-944/NBPR composites.

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.

3.4 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 TA₁ 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 TA₂ value has a greater increment from 6.52 to 27.79.

Figure 5

Temperature dependence of tan δ of (a) GW-622/NBPR composites, (b) GW-944/NBPR composites, (c) damping parameter variation of GW-622/NBPR, and (d) damping parameter variation of GW-944/NBPR.

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.

Figure 6

Temperature dependence of storage modulus (E′) of (a) GW-622/NBPR composites and (b) GW-944/NBPR composites.