Effect of Thermo-Oxidative, Ultraviolet and Ozone Aging on Mechanical Property Degradation of Carbon Black-Filled Rubber Materials

Abstract

Carbon black (CB)-filled rubber materials are extensively used in civil engineering seismic isolation. However, CB-filled rubber materials often experience mechanical property degradation because of exposure to environmental factors. To better understand the influences of thermo-oxidative, ultraviolet and ozone aging on mechanical property degradation, uniaxial tension and dynamic mechanical analysis (DMA) tests were carried out. In the uniaxial tension tests, the stress strength and elongation decreased with an increase in aging time. In the DMA tests, the effective temperature ranges decreased by 3.4–14%. And the neo-Hookean model was applied to simulate the hyperelasticity of CB-filled rubber materials. The relationship between the elastic modulus (a constant of the neo-Hookean model) and aging time was established, which provided a qualitative relationship between crosslink density and aging time. In addition, the dispersion of the CB aggregate was investigated using an atomic force microscope (AFM). The results indicated that the mechanical property degradation might be closely related to the aggregate diameter. This paper establishes a bridge between the microstructure and mechanical properties of CB-filled rubber materials, which can improve the understanding of the mechanical property degradation mechanisms of rubber materials and the fabrication of rubber components.

1. Introduction

Seismic isolation is regarded as a reliable method that can reduce the effect of ground motions generated by an earthquake [1]. In seismic design efforts, CB-filled rubber is a durable and reliable material that is used in rubber bearings as a seismic isolation device to mitigate structural response caused by earthquake excitations, specifically by dissipating the induced energy applied to structures under a direct axial load [2]. However, in the service process, the structure is exposed to environmental factors for a long time, leading to thermo-oxidative aging [3], ultraviolet aging [4] and ozone aging [5], which can result in the degradation of CB-filled rubber material’s mechanical properties. In addition, it is well known that rubber material directly determines the mechanical properties of rubber bearings, and the aging of rubber bearings is inevitable because of the coupled effect of the load and environmental factors [6]. Therefore, it is imperative to study the mechanical property degradation of rubber material, which is induced by environmental aging factors.

In recent years, many studies have investigated the effect of aging on the mechanical properties of rubber material used in rubber bearings. Ammineni [7] experimentally investigated the mechanical property degradation of naturally aged rubber material to analyze the structural integrity in damping applications. The results showed that the damping coefficient and loss factor deteriorated by approximately 31.25% and 31%, respectively. As for thermo-oxidative aging, Wang [8] investigated the mechanical properties of filled rubber by evaluating the changes in the chemical structure and provided a novel insight into the fabrication of damping material. In sunlight-rich areas, such as Yunnan Province in China, ultraviolet aging should be taken into consideration because of the thin atmosphere, which can result in ultraviolet radiation reaching the Earth [4]. As for ultraviolet aging, there are various methods for the investigation of the relationship between ultraviolet aging and the mechanical properties of rubber material. For example, Gin-ic-Markovic [9] investigated the accelerated ultraviolet degradation of rubber material by controlling irradiation in a weatherometer and found that ultraviolet radiation had a great effect on the durability of rubber material. As for ozone aging, rubber materials face significant challenges because of the high number of unsaturated units [5]. Wang [10] analyzed the effect of ozone aging on the mechanical properties of rubber material and found that, even at low concentrations, ozone resulted in obvious mechanical property degradation. It should be noticed that many works [11,12] have proved that thermo-oxidative aging is the main environmental factor causing the mechanical property degradation of rubber material, but ultraviolet and ozone aging cannot be ignored. Therefore, a comprehensive study on the mechanical property degradation of rubber material due to thermo-oxidative, ultraviolet and ozone aging is of great value to the field of engineering.

To this end, many studies have been conducted to analyze the aging mechanisms of rubber material used in rubber bearings. Chaabane [13] proposed a method to investigate thermal aging effects and found that the increase in free chains with aging time resulted in an increase in viscoelastic damping. Wang [4] investigated the degradation of silicone rubber for ultraviolet irradiation and found that there were two aging processes: chain scission and post-curing. Tan [14] carried out ultraviolet aging tests using rubber material and found that the crosslink density had a great effect on the mechanical properties. Nair [15] investigated the aging behavior of rubber material considering the crosslink density, which is affected by thermo-oxidative and ozone aging. As for filled rubber materials, the agglomeration of filler aggregates, formation of the filler network, and disentanglement of adsorbed chains can result in changes in the mechanical properties [16]. In addition, there is a general consensus that aging has a great effect on the crosslink density, which causes changes in mechanical behavior [17]. Recently, many studies have investigated the mechanical properties of rubber materials based on microstructure evolution using nuclear magnetic resonance (NMR), a scanning electron microscope (SEM), and an AFM [18,19,20]. Using NMR technology, Litvinov [21] found that a layer of immobilized polymer is formed at the CB surface because the chain mobility of bound rubber was less than that of the polymer matrix. However, it has been reported that NMR spectroscopy is an indirect method to characterize the existence of the bound rubber through studying chain dynamics because the chain mobility of the interphase is slower than that of the rubber matrix [22]. It is well known that macro-mechanical properties are a reflection of microstructure evolution. At present, advanced instruments have been proven to be useful in characterizing the mechanical properties of rubber material, especially an AFM.

Generally, the rubber material used in rubber bearings must have good damping performance. Among the viscoelastic materials used in rubber bearings, natural rubber (NR) is indispensable, which has numerous outstanding properties, such as mechanical and physical properties, excellent flexibility and good processing properties [23]. However, due to the nonpolar and flexible macromolecular chains, the internal friction of NR chains is relatively weak, which results in low energy dissipation and poor damping performance [24]. Therefore, the application of NR used in rubber bearings is greatly limited. Compared with NR, butyl rubber (IIR) exhibits a higher damping peak value and broader damping temperature range because of methyl side groups densely distributing around the chain backbone [25]. Based on IIR, a modified rubber, namely chlorinated butyl rubber (CIIR), has been commonly used in rubber bearings, which has the advantage of fast vulcanization speed [24]. In addition, the other rubber materials, such as nitrile butadiene rubber (NBR) and epoxidized natural rubber (ENR), can also be used in rubber bearings. Recently, in order to broaden the effective damping temperature range of rubber materials, rubber blending has become a commonly used material, especially used in high-damping rubber bearings [26]. In this paper, NR and CIIR are used as the matrix. The mechanical property degradation and the corresponding mechanisms of CB-filled NR and CB-filled CIIR under various aging conditions are discussed.

Generally, the mechanical properties of rubber bearings are highly determined by rubber materials. And the environmental factors can result in the mechanical property degradation of rubber bearings. Therefore, this paper aims to analyze the effects of thermo-oxidative, ultraviolet and ozone aging on the mechanical property degradation of rubber materials based on experimental and theoretical analysis. In uniaxial tension tests, the neo-Hookean model was used to describe the experimental results, and a qualitative analysis of crosslink density evolution with an increase in aging time was discussed. In DMA tests, the effective damping temperature range under various aging conditions was analyzed. In addition, AFM characterization tests were carried out in order to understand the mechanism of action of CB by establishing the relationship between macro-mechanical properties and microstructures.

2. Materials and Methods

2.1. General Theory

2.1.1. Hyperelastic Constitutive Model (Neo-Hookean Model)

Here, the neo-Hookean model is used to establish the relationship between crosslink density and aging time. It is well known that hyperelasticity is a main feature of CB-filled rubber. Under lager deformation, the volumetric strain is not taken into consideration compared with tensile strain. That is the so-called incompressibility. Meanwhile, the nonlinear behavior can be modelled well by the invariant-based continuum mechanics theory, as well as the statistical mechanics theory [27].

Within a unit volume, the statistical mechanics theory is proposed based on the assumption that there is an assembly of n randomly oriented long molecular chains; n represents crosslink density. In Gaussian treatment, based on the change in configurational entropy, the elastic strain energy function can be derived and expressed as [28]

$$W_{\mathrm{G}}={\displaystyle \frac{1}{2}}nkT\left({\lambda }_1^2+{\lambda }_2^2+{\lambda }_3^2-3\right)$$

(1)

where ${\lambda }_i$ is the principal stretch and T and k are the absolute temperature and Boltzmann’s constant, respectively.

As for impressible hyperelastic material, the strain energy function can be obtained using continuum mechanics [27]:

$$W_{\mathrm{R}}={\displaystyle \sum _{i,j=0}^{\infty }{C}_{ij}{\left(I_1-3\right)}^i}{\left(I_2-3\right)}^j$$

(2)

where $I_1={\lambda }_1^2+{\lambda }_2^2+{\lambda }_3^2$, $I_2={\lambda }_1^2{\lambda }_2^2+{\lambda }_2^2{\lambda }_3^2+{\lambda }_3^2{\lambda }_1^2$, $I_3={\lambda }_1^2{\lambda }_2^2{\lambda }_3^2=1$ and C_ij are material parameters. This is the so-called Rivlin model. When only I₁ is taken into consideration, Equation (2) can be rewritten as

$$W_{\mathrm{NH}}=C_{10}\left(I_1-3\right)$$

(3)

Equation (3) is the neo-Hookean model. Comparing Equation (3) with Equation (1), $C_{10}={\displaystyle \frac{1}{2}}nkT$. It can be found that strain energy is a function of stretch. In uniaxial tension tests, the relationship between stress ($\sigma$) and stretch ($\lambda$) can be expressed by the use of the neo-Hookean model, which is given by the following expression [27]:

$$\sigma =2C_{10}\left(\lambda -{\lambda }^{-2}\right)$$

(4)

In Equation (4), there is only one parameter, C₁₀, which can be determined using the experimental curves of uniaxial tension. In addition, comparing Equation (3) with Equation (1), the relationship between the neo-Hookean constant C₁₀ and crosslink density can be established by the following expression:

$$n={\displaystyle \frac{2C_{10}}{kT}}$$

(5)

As for the CB-filled rubber, the shear modulus G can be written as

$$G=nkT=2C_{10}$$

(6)

Therefore, taking the impressibility into consideration, the initial elastic modulus E of CB-filled rubber can be written as

$$E=3G=3nkT=6C_{10}$$

(7)

In Equation (7), it can be observed that the elastic modulus and neo-Hookean constant are proportional to crosslink density. When a neo-Hookean constant is determined by the use of Equation (4), the crosslink density can be calculated using Equation (7). In addition, the crosslink density changes with the increase in aging time, which has a great effect on the mechanical property degradation of the CB-filled rubber. Therefore, the neo-Hookean model is used to derive the crosslink density evolution with aging time, which is meaningful in assessing the mechanical properties of CB-filled rubber.

2.1.2. Damping Loss Factor

CB-filled rubber is a typical viscoelastic material. Under dynamic loading conditions, the induced strain lags behind stress and hysteresis loss takes place, which represents energy dissipation and reflects damping performance. The hysteresis loss is represented by the hysteresis loop area, as shown in Figure 1.

Under dynamic loading conditions, the loading process is controlled by a sinusoidal strain $\epsilon \left(t\right)$, which can be written as

$$\epsilon \left(t\right)={\epsilon }_0+\Delta \epsilon \sin \left(\omega t\right)$$

(8)

where ${\epsilon }_0$ and $\Delta \epsilon$ represent the prestrain and dynamic strain, respectively, and $\omega$ represents the angular frequency. And the stress $\sigma \left(t\right)$ can be expressed as

$$\sigma \left(t\right)={\sigma }_0+\Delta \sigma \sin \left(\omega t+\delta \right)$$

(9)

where ${\sigma }_0$ and $\Delta \sigma$ are the static stress and dynamic stress, respectively, and $\delta$ is the phase angle. Based on standard methods [29], the storage modulus $E^{\prime }$ and loss modulus $E^{″}$ are obtained by the following equations:

$$E^{\prime }={\displaystyle \frac{\Delta \sigma }{\Delta \epsilon }}\cos \left(\delta \right)$$

(10)

$$E^{″}={\displaystyle \frac{\Delta \sigma }{\Delta \epsilon }}\sin \left(\delta \right)$$

(11)

As for rubber materials, the damping performance is mostly described by the damping loss factor $\tan \delta$. Generally, the change in damping energy dissipation has the same trend as the damping loss factor, which can be expressed by the following equation:

$$\tan \delta ={\displaystyle \frac{E^{″}}{E^{\prime }}}$$

(12)

In addition, rubber materials used in rubber bearings should have a broader effective damping temperature range, and the loss factor should be greater than 0.3 [24].

2.2. Materials

The materials used were CB-filled rubbers, and there were two rubber matrixes, NR and CIIR, which have been commonly used in rubber bearings. The NR (RSS3) came from Vietnam. The CIIR came from Shanxi Huojia Changhua Synthetic Rubber Co., Ltd., Changzhi, China. The particle size of the CB N330 was about 30 nm, which was provided by Cabot (Shanghai, China). The recipe was shown in

Table 1. Formulations of rubber material (Unit: phr).

Sample	NR	CIIR
CIIR	0	100
NR	100	0
N330	40	40
PO	5	5
RD	1.5	1.5
MgO	1	1
SA	1	1
TMTD	1	1
DM	2	2
S	1	1
ZnO	4	4

.

The mixing process was carried out in a mixer (Dongguan Baoding Precision Instrument Co., Ltd., Guangzhou, China). The pure NR or CIIR was first mixed for 10 min, and the mixing temperature was 55 °C. Then, CB (N330), propylene oxide (PO), magnesium oxide (MgO), antioxidant (RD), stearic acid (SA), tetramethyl thiuram disulfide (TMTD) and 2,2′-dibenzothiazole disulfide (DM) were added into the NR or CIIR, and a 10 min mixing process was performed. After this process, zinc oxide (ZnO) and sulfur (S) were added, and the mixing time was kept 5 min. The mixed material was kept at room temperature for 24 h to ensure sufficient relaxation and uniformity. In the vulcanization process, the mixed materials were hot-pressed and vulcanized at a temperature of 150 °C with a pressure of 15 MPa for 15 min. After natural cooling, CB-filled rubber materials were obtained.

2.3. Thermo-Oxidative, Ultraviolet and Ozone Aging

In the thermo-oxidative aging process, the specimens were put into a temperature-controlled air-aging oven and were hung in the oven to keep the surface temperature isothermal. When the thermo-oxidative aging temperature was lower than 100 °C, the changes in mechanical properties were not obvious [29,30]. Therefore, the thermo-oxidative aging tests were carried out at 100 °C, lasting 168 h according to the ISO 23529 standard [31].

In the ultraviolet aging process, the specimens were put into an ultraviolet aging oven for 168 h. The ultraviolet irradiation was performed by a commercial UVA-340 nm lamp. Based on the GB/T 16585-1996 standard [32], the UVA radiation power density was 550 W/m². And the temperature and humidity of ultraviolet aging were kept 50 °C and 65%, respectively. The blackboard temperature was 65 °C.

In the ozone aging process, the conditions were based on GB/T 7762-2014 [33]: temperature 40 °C, humidity 55% and ozone concentration 50 pphm. Meanwhile, the ozone aging tests lasted 144 h because of the occurrence of cracks.

2.4. Uniaxial Tension Tests

The thermo-oxidatively aged dumbbell specimens were used for uniaxial tension tests to evaluate the crosslink density evolution with aging time. The gauge length, width and thickness were 20 mm, 4 mm and 2 mm, respectively. The uniaxial tension tests were carried out with an electric testing machine (CARE S-5000, CARE Measurement & Control Co., Ltd., Tianjin, China) at room temperature with a speed of 500 mm/min. And the stress–strain curves were recorded.

2.5. DMA Tests

DMA is a useful and indispensable way to investigate the damping performance of rubber material. Before DMA tests, the specimens were cut into strips with dimensions of 25 × 5 × 2 mm³.

It is well known that the mechanical properties are unstable during the first few cycles because of the Mullins effect. In order to exclude the Mullins effect, the specimens were preloaded with 6 cycles. The DMA tests were carried out using a Gabo Eplexor 500N, Netzsch-GeräTebau GmbH, Selb, Germany. In the temperature sweep tests, based on the requirement of International Standard Technical Committee ISO/TC45, the specimens were sinusoidally loaded with a frequency of 5 Hz, prestrain of 10% and dynamic strain amplitude of 0.1%. The temperature ranged from −70 °C to 70 °C by steps of 2 °C. The damping loss factor–temperature curves were recorded.

2.6. AFM Tests

In the AFM tests, a microscope (Multimode 8, Bruker, New York, NY, USA) in Peak Force Quantative Nanomechanical Mapping (QNM) mode was used to characterize the microstructure with thermo-oxidative aging time. The AFM tests were carried out at room temperature. Calibration was carried out using a force curve obtained from the surface of a standard sapphire sample. The thermal tuning method was used to determine the sensitivity coefficient of the AFM tip. Surface topography, Young’s modulus and adhesive force images were obtained in the scanning processes.

3. Results and Discussion

3.1. Uniaxial Tension

The effects of NR and CIIR matrixes on tensile strength and elongation at break before and after thermo-oxidative aging are presented in

Table 2. Uniaxial tension properties under various thermo-oxidative aging times.

Test	NR 0 h	NR 48 h	NR 168 h	CIIR 0 h	CIIR 48 h	CIIR 168 h
Tensile strength/MPa	19.8	10.7	10.2	12.6	11.7	9.7
Elongation at break/%	505.6	256.1	195.4	757.9	604.4	489.4

. It can be seen that the tensile strengths of the CB-filled NR are moderately higher than those of the CB-filled CIIR, which is attributed to the higher tensile strength of the NR matrix. On the contrary, the elongations at break of the CB-filled NR are moderately lower than those of the CB-filled CIIR. In other words, the CB-filled CIIR is softer than the CB-filled NR. Based the Chinese Standard JG/T-118-2018 [34], the tensile strength and elongation at break of high-damping rubber materials should be above 10 MPa and 550%, respectively. Therefore, it is obvious that the CB-filled CIIR is more suitable for high-damping rubber bearings. As for the aging effect, both tensile strength and elongation at break decrease with an increase in the aging time.

To analyze the thermo-oxidative aging properties of rubber materials, the tensile product is a useful method, which is a comprehensive consideration of toughness and strength [35]. In this paper, the tensile product retention R_t represents the aging coefficient, which is expressed by the following equation [36]:

$$R_{\mathrm{t}}={\displaystyle \frac{T_{\mathrm{s}}{E}_{\mathrm{b}}}{T_{\mathrm{s}0}{E}_{\mathrm{b}0}}}$$

(13)

where T_s0 and T_s are tensile strengths before and after aging and E_b0 and E_b are elongations at break before and after aging. In the calculation of the aging coefficient, T_s and E_s correspond to the uniaxial tension properties at the aging time of 168 h. As shown in Figure 2, it can be seen that the aging coefficient of the CB-filled CIIR is higher than that of the CB-filled NR. Generally, a higher aging coefficient represents a better antiaging property [36]. Therefore, it can be concluded that CIIR can improve the thermo-oxidative resistance compared with NR, which has been verified by the uniaxial tension results in Table 2. As for the CB-filled NR, the tensile strength decreases from 19.8 MPa to 10.2 MPa with a decrease of 48.5%. As for the CB-filled CIIR, the tensile strength decreases from 12.6 MPa to 9.7 MPa with a decrease of 23%.

As for the uniaxial tension tests, the stress–stretch relationship can be expressed using the neo-Hookean model. However, it should be noticed that the neo-Hookean model is suitable only for a stretch below 1.5 [37]. Therefore, in this paper, the relationship between stress and stretch is analyzed for a stretch below 1.5. Let ${\lambda }_{\mathrm{m}}=\lambda -{\lambda }^{-2}$, and the relationship between stress and stretch is shown in Figure 3. It can be seen that there is a linear relationship between stress and stretch. With an increase in aging time, the slopes of experimental curves increase, which may contribute to the increase in crosslink density [27]. After linearly fitting the experimental data using the neo-Hookean model, the determination coefficient (the square of the correlation coefficient) R² can be obtained. It is obvious that there are high values of R², which indicates that the neo-Hookean model can describe the experimental data well.

As shown in Equation (4), the neo-Hookean constant C₁₀ can be obtained after the linear fit process. In addition, as shown in Equation (7), the crosslink density is proportional to a constant 6C₁₀. And Figure 4 exhibits the relationship between the neo-Hookean constant and aging time. It can be seen that the neo-Hookean constant increases with an increase in aging time. In other words, it can be assumed that the crosslink density increases with an increase in aging time. Based on the neo-Hookean constant, the crosslink density can be calculated using Equation (5), as shown in

Table 3. Crosslink density under various aging times.

	NR 0 h	NR 48 h	NR 168 h	CIIR 0 h	CIIR 48 h	CIIR 168 h
$n$ (10−4 mol/cm3)	3.192	4.715	7.315	1.685	1.779	2.165

. And the result has been verified by [27], as shown in Figure 5. In thermo-oxidative aging, it is believed that the tensile strength decreases with an increase in aging time, and the crosslinking can result in a decrease in the extensibility of materials [38]. In this paper, the tensile strength and elongation at break decrease with an increase in aging time, as shown in Table 2, which can prove the fact that crosslink density evolution with aging time is one of the main mechanisms of the changes in micromechanical properties.

3.2. Effective Damping Temperature Range Degradation

In the DMA tests, the damping loss factor $\tan \delta$ and temperature curves were obtained, as shown in Figure 6. It should be noticed that the rubber materials used in bearings should have a $\tan \delta$ greater than 0.3, and the effective temperature range should be greater than 60 °C [24]. However, the effective temperature ranges of most rubber materials range from 20 °C to 40 °C, which limits the application range [39]. Therefore, a broad effective temperature range is necessary for the high damping performance in rubber bearings. In Figure 6, it is obvious that the CB-filled NR has a high $\tan \delta$ peak, but the CB-filled CIIR has a broader effective temperature range. As for the CB-filled NR, the effective temperature ranges from −61.5 °C to −18.6 °C with a range of 42.9 °C. As for the CB-filled CIIR, the effective temperature ranges from −61.7 °C to 22.6 °C with a range of 84.3 °C. The results indicate that the CB-filled CIIR has good damping performance for potential applications with a broader temperature range.

To analyze damping performance degradation, the environmental factors, such as thermo-oxidative, ultraviolet and ozone aging, are taken into consideration. In thermo-oxidative aging, the damping loss factors before and after aging are shown in Figure 7. It can be observed that the effective temperature ranges of the two CB-filled rubber materials have the same trend. With an increase in aging time, the effective temperature ranges become narrow. As for the CB-filled CIIR, the effective temperature after aging for 168 h ranges from −64.2 °C to 17.2 °C with a range of 81.4 °C. As for the CB-filled NR, the effective temperature after aging 168 h ranges from −60.2 °C to −23.3 °C with a range of 36.9 °C. Compared with the unaged results, the effective temperature ranges of the CB-filled CIIR and NR decrease by 3.4% and 14%, respectively. The results show that the thermo-oxidative resistance of the CB-filled CIIR is higher than that of the CB-filled NR, which is in accordance with the results in Figure 2. The reason is that crosslinking is one of main mechanisms in the process of thermo-oxidative aging [40], which is verified by the results in Figure 4 and Figure 5.

In the ultraviolet and ozone aging tests, the damping loss factor–temperature curves are similar to the ones in the thermo-oxidative aging tests, as shown in Figure 8 and Figure 9. In addition, the effective damping temperature ranges and the corresponding decrease percentages before and after ultraviolet and ozone aging are presented in

Table 4. Effective temperature ranges under ultraviolet aging (unit: °C).

Test	NR 0 h	NR 168 h	Percentage	CIIR 0 h	CIIR 168 h	Percentage
Ultraviolet aging	42.9	37.8	11.9%	84.3	80.3	4.7%

and

Table 5. Effective temperature ranges under ozone aging (unit: °C).

Test	NR 0 h	NR 144 h	Percentage	CIIR 0 h	CIIR 144 h	Percentage
Ozone aging	42.9	38.5	10.3%	84.3	80.1	5.0%

, respectively. It can be found that the decreased percentages of the effective damping temperature ranges of the CB-filled CIIR are lower than the ones of the CB-filled NR, which indicates that the CB-filled CIIR has higher thermo-oxidative resistance. In ultraviolet aging, the rubber materials are in the crosslinking and chain scission stages, and the crosslink density will increase with an increase in aging time, which reflects that the crosslinking reaction is dominant in the aging process [41]. In addition, as for the ozone aging, crosslink density is also an important factor, which can affect the mechanical properties [42].

From the above investigations, aging conditions have great effects on the damping performances of rubber materials. Generally, rubber materials directly determine the damping performance of rubber bearings, and the aging phenomenon of rubber bearings is inevitable [6]. Therefore, it can be concluded that the damping performance degradation of rubber bearings occurs under aging conditions. For example, in thermo-oxidative aging, the high-temperature aging condition of 100 °C can result in the damping performance degradation of rubber bearings, and the equivalent stiffness (K_h), equivalent damping ratio (H_eq), yield strength (Q_d) and post-yield stiffness (K_d) before and after aging degrade by 2–8% after an aging period of 14 days, as shown in

Table 6. Comparison between the mechanical properties of isolator before aging and those after aging [24].

Test	Kh (kN/mm)	Heq (%)	Kd (kN/mm)	Qd (kN)
Before aging	1.392	11.76	0.965	13.3
After aging	1.348	11.15	0.950	12.3
After aging/before aging	0.97	0.95	0.98	0.92

[24]. The main reason is that the damping performance of rubber bearings is directly determined by rubber materials, and the mechanical properties of rubber materials are highly dependent on temperature [27].

3.3. Dispersion of Carbon Black Aggregate

Traditionally, most studies are likely to explain the changes in mechanical properties of rubber materials based on crosslink density evolution. Recently, AFM has become more and more popular because the microstructure can be observed directly. In this paper, the changes in microstructure before and after thermo-oxidative aging are analyzed. The Young’s modulus images are shown in Figure 10. In addition, it should be noticed that the modulus is not the true one but the relative one. The dark region represents a high modulus region, which corresponds to the CB. It should be noticed that the diameter of CB N330 is about 30 nm, which is far smaller than the diameter of the dark region. It indicates that CB disperses in a rubber matrix in the form of an aggregate. In Figure 10, it is obvious that the aggregate diameter of the CB-filled CIIR is smaller than the one of the CB-filled NR, which indicates that the CB in the CIIR matrix disperses well. It has been reported that the filler network and inter aggregate distance have a great effect on the damping loss factor [43]. As shown in Figure 6, the CB-filled CIIR has a broader effective damping temperature range, which may result from the dispersion of the CB aggregate.

On the other hand, it has been proven that the bound rubber (BR) between CB fillers and matrix is of great importance to the changes in mechanical properties [44]. BR is composed of two layers: loose bound rubber (LBR) and tight bound rubber (TBR). It has been reported that environmental factors such as temperature have a great influence on BR content, which decreases with the increase in temperature [45]. Therefore, the effective temperature ranges become narrow before and after aging, as shown in Figure 7, Figure 8 and Figure 9. In addition, the mechanical properties of the CB-filled rubber are closely related to three crucial attributes of CB: surface area or particle size, aggregate morphology and surface activity [46]. For example, it has been proven that the primary particle size is an important factor for the damping properties of rubber material, and the loss factor increases with the increase in the primary particle size [47]. In this paper, there is only one kind of CB, which has the same primary particle size. Furthermore, the CB disperses in the rubber matrix in the form of a CB aggregate, and the aggregate diameter of the CB-filled CIIR is smaller than the one of the CB-filled NR. It has been found that with the increase in the CB structure degree, the bound rubber increases [48]. The CB structure degree is related to the amount of CB per aggregate. The main reason for this is that the larger aggregates provide surfaces that are more accessible and conveniently located for the adsorption of molecular segments [49]. As shown in Figure 11, the diameter of the CB aggregate has been obtained based on the peak values of the Gaussian fitting curve. It can be observed that the peak values of the CB-filled CIIR and CB-filled NR are 833.26 nm 1052.98 nm, respectively. That is a reason why the CB-filled CIIR has a broader effective damping temperature range, as shown in Figure 6. The AFM results indicate that the decrease in BR may be one of the main factors of damping performance degradation of CB-filled rubber, which results from the mechanism of action of CB.

4. Conclusions

CB-filled rubber is a reliable and durable material used in rubber bearings to effectively reduce structural vibrations. In the service process, rubber bearings are exposed to environmental factors for a long time, such as thermo-oxidation, ultraviolet and ozone, which can result in the mechanical property degradation of CB-filled rubber materials.

In this paper, the mechanical property degradation mechanism of CB-filled rubber under thermo-oxidative, ultraviolet and ozone aging has been comprehensively investigated, especially thermo-oxidative aging. In uniaxial tension tests, the qualitative relationship between crosslink density and thermo-oxidative aging time was established using the neo-Hookean model. And the reliability of this relationship has been verified by the existing works, which can be used to explain the mechanical property degradation.

On the other hand, AFM is used to establish the relationship between microstructure evolution and macro-mechanical properties. Based on the Young’s modulus images, the mechanisms of action of CB are discussed. The results show that CB disperses in the rubber matrix in the form of an aggregate. And the aggregate diameter of the CB-filled CIIR is smaller than the one of the CB-filled NR, which indicates that the CB is dispersed in the CIIR matrix well. Furthermore, the CB structure degree has a great effect on BR. Therefore, the AFM results indicate that the decrease in BR may be one of the main factors of mechanical property degradation of CB-filled rubber, which results from the mechanism of action of CB.

In addition, rubber isolation bearings used in offshore bridges are extremely vulnerable to the action of the alternation of aging and seawater erosion caused by weather conditions, wind, waves and other factors. In future research, the mechanical property degradation of rubber material under the alternation of aging and seawater erosion is worthy of study.