Lin Sua,
Qi Wangb,
Ping Xianga,
Dexian Yin*b,
Xiaodong Dinga,
Li Liub and
Xiuying Zhao*b
aSystems Engineering Research Institute, Beijing, 100094, China
bKey Laboratory of Beijing City on Preparation and Processing of Novel Polymer Materials, Beijing University of Chemical Technology, Beijing, 100029, China. E-mail: yindx96@163.com; zhaoxy@mail.buct.edu.cn
First published on 2nd August 2022
Aiming at enhancing the damping and sound absorption performances of nitrile rubber (NBR) incorporated Eucommia ulmoides gum (EUG), a series of NBR/EUG composites were successfully fabricated using an open mixing mill. The co-vulcanization behaviors, fracture surface morphology observations, mechanical and thermal properties and damping and sound absorption performances of NBR/EUG composites were investigated systematically. It was shown that the crystalline area and the amorphous area in NBR/EUG composites displayed a sea-island phase distribution and most of the EUG crystals were β-form crystals. Compared to that of neat NBR, the tensile strength and storage modulus of NBR/EUG composites increased dramatically with the increasing EUG content, owing to the gradually increasing number of crystals in the NBR/EUG composites. In addition, the incorporation of EUG into the NBR matrix distinctly improved the sound absorption performance of NBR/EUG composites. This work is expected to provide a new insight into the fabrication of other composite materials with controllable damping and sound absorption properties.
Acrylonitrile-butadiene rubber, commonly called nitrile rubber (NBR), plays an important role in synthesized rubber fields and has gained considerable attention in damping application fields on account of its remarkable oil and water resistance.8–10 However, under the fast development of industry and society, the damping performance of NBR is not satisfactory, especially under extreme operating conditions.
Eucommia ulmoides gum (EUG), existing in barks, leaves and fruit coatings of Eucommia ulmoides trees, is a natural polymer with double characteristics of plastic and rubber.11 The chemical structure of EUG is trans-1, 4-polyisoprene and the chemical structure of natural rubber (NR) is cis-1, 4-polyisoprene, which displayed isomers of each other.12 Compared with NR, EUG is inclined to crystallization owing to the more regular molecular structure.13 Two crystal forms exist in EUG matrix including α-crystals and β-crystals with different melting points, and EUG owns outstanding damping properties on account of its crystal melting transition.14 In the meantime, EUG possesses a higher modulus than traditional rubber because of the existing crystals, which could resist deformation under extreme operating conditions. A hopeful strategy for obtaining diversified new-type damping elastomers with excellent comprehensive properties could be implemented via blending EUG with other synthetic rubbers.
Herein, a feasible method to fabricate NBR/EUG composites with adjustable modulus and excellent damping properties was proposed by changing the content of EUG. The effects of mixing technology and vulcanization system on the co-vulcanization of NBR and EUG were studied, and different proportions of NBR/EUG composites were prepared by using open mixing mill. The fracture surface morphology, thermal performance, mechanical property as well as damping and sound absorption behaviors of various NBR/EUG composites were systematically investigated, and the correlation between these properties and the special microstructure has been researched in this work. This work may provide a promising strategy for developing other composite materials with outstanding damping and sound absorption properties.
About 5.0 g of NBR/EUG composites were put into a vulcanizing instrument at a vulcanization temperature of 150 °C and the process positive vulcanization time T90 of NBR/EUG composites was obtained. Then various NBR/EUG composites were fabricated via an electric plate vulcanizing machine for T90 + 2 min at 25 MPa.
![]() | (1) |
![]() | (2) |
In this work, five kinds of accelerators commonly used in NBR and EUG processing were selected and showed in Fig. 2. Accelerator TMTD as a type of thiuram accelerator is widely used in NBR vulcanization, because it could give rubber outstanding permanent deformation properties of compression. But the coke safety of thiuram accelerators is froward and the spray frost effect is relatively obvious. Accelerators NS and CZ are kinds of sulfenamide accelerators, it owns the advantages of high activation performance, fast reaction rate and scorch safety. The drawback is that the single use of sulfenamide type accelerators could lead to low crosslinking density of rubber. Accelerator DTDM is also a sulphur donor with the advantages of safe operation, no spray frost and invariant color. However, the vulcanization rate is relatively slow when it used alone. Accelerator D as a part of guanidine accelerator is often utilized as accessory ingredient.
There were two considerations when designing NBR/EUG vulcanization systems. One was that good compatibility of NBR and EUG was required to optimize the overall mechanical properties. The other was that a majority of vulcanizing agents should be distributed in NBR matrix to obtain the optimum mechanical properties of NBR under the vulcanization process, in the meantime, the vulcanization process would not destroy the main crystalline region in EUG matrix. Based on the above analysises, five different vulcanization formulas were designed in this work, which were shown in Table 1. The mechanical properties of NBR/EUG composites with different formulas were displayed in Fig. 3 and Table 2.
Component (phr) | Formula 1 | Formula 2 | Formula 3 | Formula 4 | Formula 5 |
---|---|---|---|---|---|
a Notes: TMTD (tetramethyl thiuram disulfide); NS (N-tertbutyl-2-benzothiazole sulfonamide); CZ (N-cyclohexyl-2-benzothiazole sulfonamide); DTDM (4,4′-dithio-dimorpholine); D (diphenyl guanidine). | |||||
NBR | 80 | 80 | 80 | 80 | 80 |
EUG | 20 | 20 | 20 | 20 | 20 |
ZnO | 5 | 5 | 5 | 5 | 5 |
SA | 2 | 1 | 2 | 2 | 2 |
Accelerator TMTD | 0 | 0.2 | 0.2 | 0 | 0 |
Accelerator NS | 0 | 0 | 0.8 | 1.2 | 0 |
Accelerator CZ | 0 | 0 | 0 | 0 | 0.8 |
Accelerator DTDM | 3 | 0 | 0 | 0 | 0 |
Accelerator D | 1 | 1 | 0 | 0 | 0 |
S | 2 | 2 | 2.5 | 2.5 | 2.5 |
Mechanical properties | Formula 1 | Formula 2 | Formula 3 | Formula 4 | Formula 5 |
---|---|---|---|---|---|
Tensile strength (MPa) | 11.9 ± 0.3 | 12.3 ± 0.4 | 9.3 ± 0.3 | 8.8 ± 0.1 | 8.2 ± 0.2 |
Elongation at break (%) | 753 ± 13 | 675 ± 8 | 1114 ± 12 | 1035 ± 15 | 1019 ± 11 |
100% elongation stress (MPa) | 1.6 ± 0.3 | 2.0 ± 0.1 | 1.0 ± 0.2 | 1.0 ± 0.1 | 1.2 ± 0.1 |
300% elongation stress (MPa) | 2.7 ± 0.2 | 3.6 ± 0.3 | 1.5 ± 0.1 | 1.6 ± 0.1 | 2.0 ± 0.1 |
Tensile permanent deformation (%) | 20 ± 3 | 19 ± 0 | 62 ± 4 | 51 ± 3 | 55 ± 1 |
Hardness (Shore A) | 59.8 ± 0.2 | 63.2 ± 0.3 | 48.9 ± 0.1 | 49.8 ± 0.4 | 51.3 ± 0.2 |
It could be found in Fig. 3 and Table 2 that compared to other formulas, formula 2 displayed the highest tensile strength and hardness, in which the accelerators TMTD and D were utilized in this formula. Accelerator TMTD consists of two active groups and two accelerator groups (Fig. 2), in which the active sulfur atoms are precipitated to participate in the vulcanization reaction during the vulcanization process. As a kind of acid accelerator, the distribution coefficient of TMTD in NBR is higher than that of EUG. Accelerator D with relatively slow vulcanizing speed and high operation safety could be dissolved in both NBR and EUG matrix. The combination of accelerator TMTD and D could effectively vulcanize NBR while retaining part of the crystalline region of EUG, so formula 2 was chosen as the final formula to conduct the follow-up experiments. Based on formula 2, various NBR/EUG composites with NBR/EUG ratios of 100/0, 95/5, 90/10, 85/15, 80/20, 70/30, 60/40, 50/50, and 0/100 were fabricated on the hot roll mixer.
![]() | ||
Fig. 4 AFM images of various NBR/EUG composites: (a) NBR/EUG = 95/5; (b) NBR/EUG = 90/10; (c) NBR/EUG = 80/20; (d) NBR/EUG = 70/30; (e) NBR/EUG = 60/40; (f) NBR/EUG = 50/50. |
The morphologies of polymeric composites strongly influence the properties of composites owing to the interfacial interaction of the two-phase interfaces. Thus, the fractured surface morphologies of various NBR/EUG composites were further investigated by SEM and the relevant photographs were shown in Fig. 5. As displayed in Fig. 5a, the NBR/EUG (95/5) composite presented a relatively smooth surface and a bit of EUG phase could be observed, which was uniformly distributed throughout NBR matrix. However, the size of the spherical EUG phase was visibly increased with the increase of EUG content (Fig. 5(b and c)). Especially, when the addition of EUG was more than 20%, two distinct phases could be found on the surfaces of NBR/EUG composites and the SEM photographs exhibited typical sea-island structures, in which elliptical EUG phase dispersed in continuous NBR matrix. The observations demonstrated that a small amount of EUG content could be uniformly dispersed in NBR matrix. However, with an increasing amount of EUG in NBR/EUG composites, the two-phase interface between NBR and EUG presented a weak interfacial adhesion and poor compatibility because of the crystallization of EUG, resulting in the appearance of stress concentration phenomenon and possibly poor mechanical property of NBR/EUG composites.15
![]() | ||
Fig. 5 SEM images of various NBR/EUG composites: (a) NBR/EUG = 95/5; (b) NBR/EUG = 90/10; (c) NBR/EUG = 80/20; (d) NBR/EUG = 70/30; (e) NBR/EUG = 60/40; (f) NBR/EUG = 50/50. |
![]() | ||
Fig. 6 Mechanical properties of various NBR/EUG composites: (a) elongation at break and tensile strength; (b) stress–strain curves. |
Mechanical properties | NBR/EUG | ||||||||
---|---|---|---|---|---|---|---|---|---|
100/0 | 95/5 | 90/10 | 85/15 | 80/20 | 70/30 | 60/40 | 50/50 | 0/100 | |
Tensile strength (MPa) | 4.4 ± 0.3 | 7.7 ± 0.2 | 13.0 ± 0.4 | 12.0 ± 0.2 | 11.0 ± 0.2 | 10.8 ± 0.1 | 10.3 ± 0.3 | 10.9 ± 0.4 | 17.7 ± 0.3 |
Elongation at break (%) | 409 ± 7 | 522 ± 10 | 689 ± 11 | 639 ± 14 | 606 ± 8 | 558 ± 16 | 498 ± 11 | 463 ± 6 | 495 ± 10 |
100% elongation stress (MPa) | 1.2 ± 0.1 | 1.3 ± 0.2 | 1.5 ± 0.2 | 1.7 ± 0.1 | 2.2 ± 0.1 | 2.7 ± 0.0 | 3.1 ± 0.1 | 3.5 ± 0.1 | 5.8 ± 0.1 |
300% elongation stress (MPa) | 2.5 ± 0.1 | 2.4 ± 0.1 | 2.6 ± 0.1 | 3.0 ± 0.3 | 4.2 ± 0.3 | 5.0 ± 0.1 | 5.9 ± 0.1 | 6.7 ± 0.3 | 10.7 ± 0.2 |
Tensile permanent deformation (%) | 0 ± 0 | 0 ± 0 | 4 ± 1 | 12 ± 0 | 12 ± 1 | 28 ± 1 | 28 ± 0 | 40 ± 0 | 88 ± 1 |
Hardness (Shore A) | 53 ± 1 | 54 ± 2 | 57 ± 1 | 59 ± 2 | 62 ± 0 | 70 ± 0 | 75 ± 2 | 79 ± 1 | 83 ± 2 |
As shown in Fig. 6 and Table 3, the tensile strength and elongation at break of NBR/EUG composites improved from 4.4 to 13.0 MPa and from 409% to 689%, respectively, with increasing EUG content from 0% to 10%. With EUG content in NBR matrix of more than 10%, the tensile strength and elongation at break of NBR/EUG composites decreased, which were still higher than those of NBR. It was because a small amount of EUG content dispersing in NBR matrix could be acted as physical intersection points, leading to the increase of the mechanical properties of NBR/EUG composites.18,19 Nevertheless, a large amount of EUG caused the formation of large size crystals in NBR matrix, leading to stress concentration and formation of small cracks at the two-phase interface as well as the reduction of mechanical properties of NBR/EUG composites, which could be observed in AFM and SEM photographs (in Fig. 4 and 5). Hence, the tensile strength and elongation at break of NBR/EUG composites first increased to their optimal value and then decreased gradually with EUG content increased from 0% to 50%, and the NBR/EUG (90/10) composite displayed maximum tensile strength and elongation at break values among these NBR/EUG composites. Moreover, owing to the particular rubber-plastic character of EUG, various NBR/EUG composites changed from elastic rubber materials to tough plastic materials (in Fig. 6b) and the stress at definite elongation (100% and 300%) increased gradually (in Table 3) with the increasing content of EUG in NBR matrix.
Tanδ curve, the ratio of loss modulus/E′, was regarded as a measurement of material damping ability (i.e., dissipation of vibration energy) and the glass transition temperature (Tg) of materials could be obtained by the maximum value of tan
δ curves. As exhibited in Fig. 7a, the tan
δ of NBR/EUG composites displayed a maximum value at the temperature of around 0 °C, and another peak could be seen at around −50 °C because of the glass transition behavior of EUG.20 With the increasing content of EUG from 0% to 50%, the Tg of NBR/EUG composites decreased gradually and shifted to a lower temperature, in the meantime, the tan
δ curves of various NBR/EUG composites changed from a single peak to double peaks (in red frame of Fig. 7a), especially in the tan
δ curves of NBR/EUG (60/40) composite, NBR/EUG (50/50) composite and NBR/EUG (0/100) composite. The appearance of double peak phenomena suggested that the compatibility of NBR and EUG phases in NBR/EUG composites decreased with the increase of EUG content.19,21
The E′ of various NBR/EUG composites were shown in Fig. 7b. As could be seen from Fig. 7b, the E′ of all NBR/EUG composites underwent two transformations, corresponding to the glass transition of NBR and the crystal melting of EUG respectively. Increasing the content of EUG, a successive increase of E′ values could be observed in various NBR/EUG composites between −20 °C to 50 °C. The reason was that EUG could crystallize in this temperature range, which significantly improved the E′ of NBR/EUG composites.15,18,22 With further increasing the temperature (above 50 °C), EUG crystals reached their melting temperature and displayed amorphous conditions, which led to a sharp decrease of E′ in NBR/EUG composites. The results indicated that the E′ of NBR/EUG composites could be effectively regulated by changing the content of EUG in the composites.
In addition, the size of the EUG crystals in NBR/EUG composites could be calculated from the XRD data via the Scherrer formula. The average crystal size of EUG determined from the diffraction peaks of 18.7°, 22.7° and 26.9° was shown in Table 4, in which the data with a large error calculated by the Scherrer formula was eliminated. As listed in Table 4, all EUG crystal sizes existed in the scope of 1 to 32 nm, indicating that the agglomeration phenomenon of EUG crystals did not occur in NBR/EUG composites. Meanwhile, it could be found in Table 4 that the χc of various NBR/EUG composites increased with the increase of EUG content.
Crystallization properties | NBR/EUG | ||||||||
---|---|---|---|---|---|---|---|---|---|
100/0 | 95/5 | 90/10 | 85/15 | 80/20 | 70/30 | 60/40 | 50/50 | 0/100 | |
Average crystal size (18.7°) (nm) | — | — | — | 8.3 | 7.0 | 14.8 | 23.6 | 22.4 | 21.4 |
Average crystal size (22.7°) (nm) | — | — | — | 1.6 | 2.9 | 16.5 | 29.7 | 20.7 | 23.9 |
Average crystal size (26.9°) (nm) | — | — | — | — | — | 23.7 | 31.8 | 30.0 | 31.8 |
χc (%) | — | — | — | 4.22 | 4.96 | 5.93 | 7.21 | 7.77 | 6.62 |
To further investigate the crystallization performances of various NBR/EUG composites, DSC was employed and the related DSC graph was given in Fig. 8b. According to the DSC results, the curve of pure NBR showed an obvious glass transition peak at about 14 °C. With the increase of EUG content, the glass transition peak became less obvious until it disappeared (in red frame of Fig. 8b), ascribing to the following two reasons. On one hand, the incorporation of EUG increased the number of crystals in NBR matrix, limiting the movement and rearrangement of chain segments during the glass transition region.28–30 On the other hand, increasing EUG content in NBR/EUG composites signified the reduction of the proportion of NBR content. Moreover, with the increase of EUG content, the melting peak of various NBR/EUG composites appeared gradually and the melting peak area was getting larger and larger (in blue frame of Fig. 8b), indicating the crystallization performances of the fabricated NBR/EUG composites were able to change by adjusting the content of EUG in NBR matrix.17
![]() | ||
Fig. 9 SAC of various NBR/EUG composites under different pressures ((a) 1.0 MPa; (b) 2.0 MPa; (c) 2.5 MPa) and (d) average SAC. |
In Fig. 9, the SAC of NBR/EUG composites distinct improved with the increase of frequency, attributing to that high frequency means short wavelength and comparatively weak penetrating power. It also could be seen from Fig. 9 that the SAC of NBR/EUG composites increased with the increasing EUG content from 0% to 20%, which was obviously regular under high-pressure conditions (e.g., 2.0 or 2.5 MPa). The reason should be attributed to two aspects. One is that the incorporated EUG crystals enhanced the modulus of NBR/EUG composites, which increased the propagation speeds of sonic waves and improved the impedance matching between the composites and water.14,31,33 The other is that the sound impedance of the crystals in NBR/EUG composites mismatched that of water, and thus the incident sound wave reflection happened and reflected many times when it met the crystals.15 As a result, the propagation path of sound waves was changed and increased, leading to the enhancement of the sound energy dissipation. However, the SAC slightly decreased when EUG content further increased, due to that the excess crystals in NBR/EUG composites would hinder the movement of molecular chains and thus affect there damping performances as well as the sound energy dissipation.
This journal is © The Royal Society of Chemistry 2022 |