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Influence of 1,1′-Azobis(cyclohexanezonitrile) on the thermo-oxidative aging performance of diolefin elastomers

  • Quan Qin , Gui-Xiang Liu , Ji-Chong Wu , Wei-Xing Sun and Shuangquan Liao EMAIL logo
Published/Copyright: September 18, 2024
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e-Polymers
From the journal e-Polymers Volume 24 Issue 1

Abstract

Diolefin elastomers play an important role in production and life, but their unsaturated structure leads to extreme vulnerability to heat and oxygen attack, so research into the aging of diolefin elastomers has been a hot spot in the industry. To overcome this limitation, a strategy based on the thermal decomposition of 1,1′-Azobis(cyclohexanezonitrile) (Azo) is devised, which forms stabilized compounds with imine groups during the heat process and captures radical. The diolefin elastomer was combined with azo, and isoprene rubber (IR) is chosen as a model material. Azo was added to IR to prepare the composite material (IR-Azo), and the thermo-oxidative resistance of the composite was significantly improved. Such as, after being exposed to thermo-oxidative conditions for 24 h, IR-Azo showed a tensile strength of 14.96 MPa with a retention rate of 68.25% which exceeded that of many traditional antioxidants. This study provides new insights into the development of elastomers with excellent thermo-oxidative resistance.

Keywords: thermo-oxidative aging; structure; capturing radical; isoprene rubber; mechanical properties

1 Introduction

Diolefin elastomers such as natural rubber and isoprene rubber (IR) are indispensable in various sectors including transportation, and health care, due to their exceptional performance, playing a crucial role in the advancement of the modern technologies (1,2,3,4,5). However, in application, they are in long-term contact with oxygen and heat. Therefore, the primary mode of degradation for diolefin elastomers is thermo-oxidative aging, which is exacerbated by their high concentration of unsaturated double bonds (6,7). The diolefin elastomers undergo radical formation processes when exposed to thermal and oxidative environments, leading to significant changes in main chains and cross-linking structures (8,9,10). These changes impact the mechanical properties of diolefin elastomers, resulting in premature product failure and reduced expected lifespan (11,12,13,14). Consequently, the improvement in aging resistance of diolefin elastomers thus becomes one of the most important areas in the research studies dedicated to prolonging the serviceability of these materials in different applications (15).

To circumvent this limitation, adding antioxidants to diolefin elastomers or grafting of antioxidants onto the polymer chains or filler surface are common methods of preventing oxidative degradation of diolefin elastomers (16,17,18,19,20,21,22,23). The purpose of this strategy is to avoid or delay the aging of diolefin elastomers. Frequently used antioxidants include low molecular weight derivatives of aromatic amines and phenols. Nevertheless, these phenolic antioxidants with lower molecular weight have poor compatibility (Frosting on the surface) and long-term stability (intolerant of extraction, volatilization, etc.) in polymer, which limit their application (24,25). The application of encapsulated antioxidants requires complicated synthetic procedures for modifying fillers or polymers, which affects the efficiency of these reactions. Development of more types and highly efficient antioxidants has still been a focus. Imines linkages (carbon–nitrogen double bonds) are excellent radical acceptors in a various range of radical reactions (26,27,28), as they can be reduced to produce α-amino radical that take active parts in radical-radical coupling and reactions with electrophiles (26,29,30). Fang et al. (31) research demonstrated that carbon–nitrogen double bonds are very efficient in capturing radical. The newly modified Schiff base antioxidant compounds highlighted by their carbon–nitrogen double bonds have also been of interest as regards the mechanism of their activity in deactivating radical generated during thermo-oxidative aging (32).

Based on the radical capturing properties of C═N bonds, the present study proposes an innovative method to enhance the thermo-oxidative resistance of elastomers. We used IR as a model material of diolefin elastomers and added 1,1′-Azobis(cyclohexanezonitrile) (Azo) to its formulation. Some previous studies have indicated that Azo can be transformed into stable ketenimine configurations (C═C═N) during thermal processes (33,34,35,36,37). This research made use of Fourier-transform infrared (FTIR), X-ray photoelectron spectroscopy (XPS), and other analytical techniques to investigate the role of Azo in inhibiting thermo-oxidative degradation. The effects of aging on thermo-oxidative stability of IR were studied through changes in tensile strength, elongation at break, and crosslink density before aging and after aging processes. These results show that Azo can neutralizes the generated radical generated, thus, providing a substantial improvement to the thermo-oxidative resistance of the elastomer. In particular, IR compounded with azo continued to show good mechanical properties after 24 h of aging at 100°C, with a tensile strength of 14.96 MPa and a tensile strength retention rate of 68.25%, and they were still acceptable even after 48 h of aging at 100°C. This research contributes to the development of a universal design framework for elastomers that exhibit superior resistance to thermo-oxidative aging, thereby advancing the field of rubber science and technology.

2 Materials and methods

2.1 Materials

The IR (cis-1,4-polyisoprene) designated as IR70 was supplied by Fushun Yikesi New Material Co., Ltd., a company located in the city of Fushun, within the Liaoning Province of China. From Shanghai Mayer Chemical Reagent Co., Ltd., the team procured 1,1′-Azobis (cyclohexane carbonitrile), which was noted for its 98% purity. The zinc oxide used in the experiments, recognized for its 99% purity and classified as of analytical reagent grade, was acquired from Shanghai Macklin Biochemical Co., Ltd. Additionally, N-cyclohexyl-2-benzothiazolesulfenamide (accelerator CZ), matching the 98% purity criterion, was obtained from Shanghai Aladdin Biochemical Technology Co., Ltd. The sulfur component, crucial for the study, was sourced from Guangxi Fangchenggang Wuxing Environmental Technology Co., Ltd. It is important to note that all antioxidants incorporated into this study conformed to industrial-grade standards.

2.2 Sample preparation

Stearic acid, zinc oxide, 1,1′-Azobis(cyclohexanezonitrile) (Azo), accelerator CZ, and sulfur were sequentially added to IR on an open two-roll mill, where they were thoroughly mixed. The optimal cure time was established using a moving die rheometer, and the mixture was then compression molded at 145°C under a pressure of 15 MPa to produce vulcanized specimens. The specimens that included Azo were designated as IR-Azo-x, with x indicating the Azo content in weight percentage. The vulcanization recipe comprised rubber 100 phr, stearic acid 2 phr, zinc oxide 5 phr, accelerator CZ 1.5 phr, and sulfur 1.5 phr.

2.3 Characterization

2.3.1 Electron paramagnetic resonance (EPR)

The Bruker A300-10/12 EPR spectrometer, equipped with an ER4122SHQ microwave cavity, was utilized to determine the free radical concentrations in the samples both before and after they underwent aging. It functions at a f = 9.85 GHz, with the modulation frequency set to 100 kHz. This approach was used to determine the levels of free radical in IR and IR/Azo samples before and after they had been subjected to thermal-oxidative conditions at 100°C.

2.3.2 FTIR spectroscopy

The analysis was performed of vulcanized IR and IR/Azo samples using ATR-FTIR before and after the aging process in order to reveal the changes in their functional groups. The measurements were carried out using a Bruker INVENIO-S spectrometer, the wavenumber region being in the range between 4,000 and 600 cm−1 with 32 scans.

2.3.3 XPS

XPS analysis was conducted using a K-Alpha XPS system from Thermo Fisher Scientific, USA, at an energy of 14.8 kV and a current of 1.6 A aimed at evaluating the variations in the O/C atomic ratio of samples subjected to thermo-oxidative aging at 100°C for 1 day.

2.3.4 Cross-linking density

A sample of vulcanized rubber weighing 0.2 g was submerged in toluene for a duration of one week. Following this period, any remaining solvent on the surface was promptly removed with filter paper, after which the sample was weighed again and the mass noted. The crosslink density of the sample was then determined utilizing the Flory–Rehner equation (38):

(1) − ln ( 1 − ϕ r ) − ϕ r − χ r ϕ r 2 = n V 0 ϕ r 1 / 3 − 1 2 ϕ r

(2) M c = ρ n

Within the context of this analysis, ϕ r represents the polymer’s volume fraction in the expanded network, with the molar volume of the toluene solvent documented at 106.2 mL·mol−1. The Flory–Huggins interaction coefficient, denoted by χ r, is valued at 0.393 for the system comprising IR and toluene. Furthermore, n signifies the concentration of mobile polymer chains per volume unit, measured in mol·mL−1. M c refers to the mean molecular weight observed between crosslinks, and ρ indicates IR’s density, recorded at 0.913 g·mL−1.

2.3.5 Mechanical performance testing

The uniaxial tensile tests on the specimens were carried out at room temperature using a GOTECH AI-3000 tensile testing machine, with the strain rate set to 500 mm·min−1. The specimens prepared for tensile testing were in the form of dumbbell-shaped strips, with dimensions at the center measuring 20 mm × 4 mm × 1 mm. To obtain reliable results, IR and IR-Azo-x samples were tested in at least 5 parallel samples.

2.3.6 Thermo-oxidative aging test

To evaluate the resistance of IR to thermo-oxidative aging, vulcanized samples were placed in a thermo-oxidative aging test chamber (GOTECH-7017-EL1) and subjected to an accelerated aging process at a steady temperature of 100°C for periods of 24, 48, and 72 h. Six sample strips of IR and IR-Azo-x were taken at 24 h intervals. The samples were left in the shade for 0.5 h before being subjected to the tensile test. The assessment of aging was based on how well the samples maintained their tensile strength and elongation at break after the aging process.

3 Results and discussion

3.1 The anti-thermo-oxidative aging mechanism of Azo

Carbon–nitrogen double bonds as an excellent radical acceptor in various radical reactions (27,28), we applied EPR technology to study the variation in radical concentrations within IR and IR-Azo samples post thermo-oxidative aging, affirming their radical scavenging capability of the Azo. As shown in Figure 1a, no formation of radical in IR samples pre-aging, with the progression of aging, a notable surge in radical concentrations was detected in IR. Conversely, samples with Azo exhibited a marked reduction in radical concentrations after aging, indicating Azo’s superior radical neutralizing capability. The imine structure effectively captures peroxyl radical (such as ROO˙, RO˙), thereby preventing further radical chain reactions (Figure 1b). The complete FTIR spectra are shown in Figure S1. The attenuation in the intensity of the enone imine structure’s absorption peak with increasing aging time (33) (Figure 1c), further underscores the pivotal role of the imine structure in radical capture during aging. This mechanism not only restricts the breaking of rubber chains but also contributes to preserving the integrity of the crosslinked network structure, alleviating the detrimental impacts of aging (Figure 1d).

Figure 1 (a) The first derivative of electron paramagnetic resonance (EPR) spectra for IR, IR after aging at 100°C, and IR-Azo after aging 100°C. (b) Schematic diagram of Azo absorbing free radicals. (c) FTIR of IR-Azo before and after aging. (d) Schematic diagram of IR-Azo cross-linking network structure changes during aging.
Figure 1

(a) The first derivative of electron paramagnetic resonance (EPR) spectra for IR, IR after aging at 100°C, and IR-Azo after aging 100°C. (b) Schematic diagram of Azo absorbing free radicals. (c) FTIR of IR-Azo before and after aging. (d) Schematic diagram of IR-Azo cross-linking network structure changes during aging.

3.2 Network structure changes before and after aging with 1,1′-Azobis(cyclohexane carbonitrile)

To explore the Azo effect on improving the thermo-oxidative aging resistance of IR, we performed high-resolution C1s XPS analysis, and the complete XPS patterns are shown in Figure S2. This analysis aimed to examine the chemical structural changes in IR and IR-Azo, both before and after accelerated thermo-oxidative aging. Illustrated in Figure 2a and c, the C1s peak for both IR and IR-Azo was dissected into four distinct peaks, which align with C═C/C−C at 284.6 eV, C−O (epoxides and alkoxy groups) at 286.8 eV, carbonyl groups (C═O) at 287.5 eV, and ester groups (O−C═O) at 289.0 eV (39). This decomposition reveals that even prior to aging, oxygen had integrated into the rubber chains, albeit at very low levels as indicated by the faint intensities of these oxygen-related functional groups. Figure 2b demonstrates a pronounced increase in the peak intensities of oxygenic carbon atoms (notably carbonyls and ester groups) for IR as the aging time extended, indicating that rubber chains were highly oxidized by O2 with heat-accelerating oxidation reaction. In contrast, Figure 2d shows that at equivalent aging durations, the peak intensities of carbonyls and ester groups for IR-Azo were much lower than IR. Moreover, there is no obvious difference in the peak intensities of carbonyls and ester groups for IR-Azo. These findings highlight that Azo showcases a better antioxidative capability, thereby imparting enhanced aging resistance to the IR.

Figure 2 (a) High-resolution C1s XPS spectra and mechanical retentions of IR before thermo-oxidative aging. (b) High-resolution C1s XPS spectra and mechanical retentions of IR after thermo-oxidative aging at 100°C. (c) High-resolution C1s XPS spectra and mechanical retentions of IR-Azo before thermo-oxidative aging. (d) High-resolution C1s XPS spectra and mechanical retentions of IR-Azo after thermo-oxidative aging at 100°C.
Figure 2

(a) High-resolution C1s XPS spectra and mechanical retentions of IR before thermo-oxidative aging. (b) High-resolution C1s XPS spectra and mechanical retentions of IR after thermo-oxidative aging at 100°C. (c) High-resolution C1s XPS spectra and mechanical retentions of IR-Azo before thermo-oxidative aging. (d) High-resolution C1s XPS spectra and mechanical retentions of IR-Azo after thermo-oxidative aging at 100°C.

The oxygen-to-carbon atomic (O/C) ratio in the samples, indicative of the extent of thermo-oxidative aging, was analyzed for both IR and IR-Azo before and after the aging process. Initially, the O/C ratio stood at 9.41% for IR and slightly higher at 10.29% for IR-Azo, as indicated in Figure 3a. Following the aging process, there was a noticeable increase in the O/C ratio to 13.56% for IR, while for IR-Azo, it rose to 11.43%, as depicted in Figure 3b. The rate of increase in the O/C ratio was significantly higher for IR at 44.10%, compared to a much lower rate of 11.11% for IR-Azo, as shown in Figure 3c. Furthermore, the crosslink density of IR experienced a notable decline across different aging durations, unlike IR-Azo, where the decrease in crosslink density was less than that for IR, suggesting enhanced stability, as illustrated in Figure 3d. These results imply that Azo can decelerate the chemical structural alterations in IR throughout the thermo-oxidative aging process and indicate better resistance to thermal oxidation.

Figure 3 (a) The O/C ratio of IR and IR-Azo inside the sample, (b) the O/C ratio of IR and IR-Azo upon thermo-oxidative, (c) the O/C specific growth rate of IR and IR-Azo during thermal oxidation, and (d) the change of M c of IR and IR-Azo vulcanized IR at different aging times.
Figure 3

(a) The O/C ratio of IR and IR-Azo inside the sample, (b) the O/C ratio of IR and IR-Azo upon thermo-oxidative, (c) the O/C specific growth rate of IR and IR-Azo during thermal oxidation, and (d) the change of M c of IR and IR-Azo vulcanized IR at different aging times.

Throughout the thermo-oxidative aging process, diene-rubber experiences transformations that lead to the creation of various kinds of oxygenic groups like carbonyls and ketones, and hence change the chemical structures of the rubber chains (40,41). FTIR testing yielded the FTIR spectra of IR and IR-Azo across various aging periods (Figure 4a and c). Analysis of these spectra showed changes in the chemical structure via the intensity of the oxygen-containing groups (C═O) stretching vibrations during aging. When comparing the intensity of the carbonyl peaks between IR and IR-Azo in the 1,750–1,600 cm−1 region of the FTIR spectra (42) (Figure 4b and d), IR samples displayed a pronounced rise in carbonyl peak intensity, while IR-Azo samples exhibited a more gradual and less marked increase. These findings indicate Azo’s significant role in delaying the oxidative aging process and hence alleviate the change in the chemical structure of the samples.

Figure 4 (a) FTIR of IR, (b) FTIR of 1,750–1,600 cm−1 of IR, (c) FTIR of IR-Azo, and (d) FTIR of 1,750–1,600 cm−1 IR-Azo.
Figure 4

(a) FTIR of IR, (b) FTIR of 1,750–1,600 cm−1 of IR, (c) FTIR of IR-Azo, and (d) FTIR of 1,750–1,600 cm−1 IR-Azo.

3.3 The impact of Azo on the mechanical properties of IR during thermo-oxidative aging

To further confirm the thermo-oxidative resistance effect of Azo on IR, the retention of the mechanical properties of IR-Azo system was assessed with aging times. Figure 5 displays the impact of different aging durations on the mechanical characteristics of IR and IR-Azo. As depicted in Figure 5a and b, IR’s tensile strength and elongation at break after 1 day of aging were preserved at 36% (7.61 MPa) and 73% (567.4%), respectively. With the addition of Azo, the retention of mechanical properties of IR-Azo samples was improved, when the content of Azo is 1%, IR-Azo maintained 68% (14.96 MPa) of its tensile strength and 78% (654.5%) of its elongation at break, equating to increases of approximately 1.97-fold and 1.15-fold, respectively. Following 2 days of aging, IR-Azo still showed tensile strength and elongation at break values of 38% (8.37 MPa) and 72% (607.5%). These findings affirm Azo’s role in bolstering thermo-oxidative aging resistance.

Figure 5 (a) Changes of IR tensile strength and tensile strength retention rate, (b) change of elongation at break and retention of elongation at break of IR and IR-Azo.
Figure 5

(a) Changes of IR tensile strength and tensile strength retention rate, (b) change of elongation at break and retention of elongation at break of IR and IR-Azo.

3.4 Comparison of Azo with traditional antioxidants

Compatibility and long-term stability of Azo in the matrix are important factors for the application. After aging the IR-Azo samples for 1 day, the surface was not appeared obvious frosting phenomenon, while the IR-RD sample prepared by adding traditional antioxidant (poly(1,2-dihydro-2,2,4-trimethyl-quinoline)) into the IR matrix after 1 day of ageing, there was obvious frosting on the surface (Figure 6a), indicating that the compatibility of Azo in the matrix is better than that of the antioxidant RD. After IR-Azo and IR-RD sample were aged by 60°C hot water to extract for 1 day, the water upon extraction become yellow in IR-RD, while that show no obvious change in IR-Azo (Figure 6b), indicating that it is difficult to extract Azo from the matrix but it is easy to extract RD from the matrix.

Figure 6 (a) Images of IR-Azo and IR-RD before and 1 day after thermal oxygen aging. (b) Images of IR-Azo and IR-RD before and after hot water extraction.
Figure 6

(a) Images of IR-Azo and IR-RD before and 1 day after thermal oxygen aging. (b) Images of IR-Azo and IR-RD before and after hot water extraction.

To explore Azo’s enhanced thermo-oxidative resistance, we evaluated the retention of tensile strength and elongation at break for IR-Azo compared to IR compounds with different antioxidants: IR-Amine (such as 1,2-dihydro-2,2,4-trimethyl-quinoline), IR-Phenol (such as butylated hydroxytoluene, Octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl), and IR-Heterocycle (such as 2-mercaptobenzimidazole, 1,3,5-trimethyl-2,4,6-tris(3,5-di-t-butyl-4-hydroxybenzyl)-benzen) after 1 day of thermo-oxidative aging. The retention rates for tensile strength and elongation at break were calculated for each sample (Table S1), and their results are summarized in Figure 7. After 1 day of aging, vulcanizates with Azo showed high retention rates for both tensile strength (68%) and elongation at break (78%), demonstrating Azo’s satisfactory thermo-oxidative resistance in comparison to traditional antioxidants (amine and heterocycle), thereby highlighting its exceptional antioxidative efficiency.

Figure 7 After 1 day of thermal oxygen aging, tensile strength retention and elongation at break retention of IR, IR-amine, IR-phenol, IR-heterocycle, and IR-Azo.
Figure 7

After 1 day of thermal oxygen aging, tensile strength retention and elongation at break retention of IR, IR-amine, IR-phenol, IR-heterocycle, and IR-Azo.

4 Conclusion

In this work, Azo was proven to play an important role in improving the thermo-oxidative resistance of diolefin elastomers. To investigate the relationship between Azo and thermo-oxidative resistance, our group explored the effect of Azo on radical in the matrix. The findings revealed that Azo can react upon heating to form stable imino compounds, which are able to wipe out radical generated during the thermo-oxidative process. This characteristic of Azo endows IR/Azo with outstanding thermo-oxidative resistance. When subjected to the same aging conditions, IR-Azo showed a lesser extent of polymer chain degradation compared to IR. Furthermore, the maintenance of tensile strength and elasticity in aged IR-Azo samples exceeded that of IR, indicating a more robust resistance to aging in IR-Azo. This investigation provides insightful direction for the development of elastomers that exhibit outstanding resistance to thermo-oxidative aging.

Acknowledgements

This work was supported by the Strategic Priority Research Program of Financial support from the National Natural Science Foundation of China (52163010), National Key Research and Development Program of China (No. 2022YFD2301202), Hainan Provincial Natural Science Foundation (No. 521CXTD438), and Hainan Provincial Natural Science Foundation of China (No. 521RC1038).

  1. Author contributions: Quan Qin: methodology, investigation, writing – original draft. Gui-Xiang Liu: data curation, investigation. Ji-Chong Wu: formal analysis. Wei-Xing Sun: formal analysis. Shuangquan Liao: resources, conceptualization, writing – original draft, writing – review & editing.

  2. Conflict of interest: The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  3. Data availability statement: Data will be made available on request.

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Received: 2024-06-05
Revised: 2024-07-31
Accepted: 2024-08-02
Published Online: 2024-09-18

© 2024 the author(s), published by De Gruyter

This work is licensed under the Creative Commons Attribution 4.0 International License.

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https://doi.org/10.1515/epoly-2024-0058
Keywords for this article
thermo-oxidative aging; structure; capturing radical; isoprene rubber; mechanical properties
Creative Commons
BY 4.0

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  1. Research Articles
  2. Flame-retardant thermoelectric responsive coating based on poly(3,4-ethylenedioxythiphene) modified metal–organic frameworks
  3. Highly stretchable, durable, and reversibly thermochromic wrapped yarns induced by Joule heating: With an emphasis on parametric study of elastane drafts
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  11. The effects of property variation on the dripping behaviour of polymers during UL94 test simulated by particle finite element method
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  52. Surface modification of sepiolite and its application in one-component silicone potting adhesive
  53. Study on hydrophobicity and aging characteristics of epoxy resin modified with nano-MgO
  54. Optimization of baffle’s height in an asymmetric twin-screw extruder using the response surface model
  55. Effect of surface treatment of nickel-coated graphite on conductive rubber
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  58. Investigation of the compaction density of electromagnetic moulding of poly(ether-ketone-ketone) polymer powder
  59. Experimental investigation on low-velocity-impact and post-impact-tension behaviors of GFRP T-joints after hydrothermal aging
  60. The repeated low-velocity impact response and damage accumulation of shape memory alloy hybrid composite laminates
  61. Exploring a new method for high-performance TPSiV preparation through innovative Si–H/Pt curing system in VSR/TPU blends
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  69. Recent progress in properties and application of antibacterial food packaging materials based on polyvinyl alcohol
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