Detailed Study on the Mechanical Properties and Activation Energy of Natural Rubber/Chloroprene Rubber Blends during Aging Processes

Selection of a suitable thermal aging process could render desirable mechanical properties of the rubbers or blended rubbers. In this work, the effect of the aging processes on the mechanical properties and activation energies of natural rubbers (NR) and NR/ chloroprene rubbers (CR) blends with low CR contents (5–10%) was investigated. ,ree aging processes including heat aging (at 110°C for 22 hours), mechanical aging (under dynamic loading to 140% strain for 16000 cycles), and complex aging (heat and mechanical aging) were studied. ,e results revealed that the compatibility of CR in natural rubber matrix had a significant effect on the dynamic properties of the blended rubber and negligible effect on the static properties.,e changes in activation energies of the blended rubber during aging processes were calculated using Arrhenius relation.,e calculated changes (ΔUc, ΔUd, and ΔUT) in activation energies were consistent with the results of mechanical properties of the blended rubber. Interestingly, the change in activation energies using complex aging conditions (ΔUc) was mostly equal to the total changes in activation energies calculated separately from heat aging (ΔUT) and mechanical aging (ΔUd) conditions. ,is indicates that, in complex aging conditions, the heat and dynamic loading factors act independently on the properties of the blended rubber.


Introduction
Natural rubber (NR), a very important elastomer, has been widely used in wide range of technical and civil applications. However, the disadvantage of low aging resistance limits the application of natural rubber in many practical applications. In order to enhance the aging resistance of NR, one of the effective methods is to blend NR with the synthetic rubbers, which have good aging resistance such as ethylene propylene diene monomer (EPDM) and chloroprene rubber (CR) [1,2]. Among these synthetic rubbers, CR is often used to network was investigated by various methods [3,[8][9][10]. Such relationship may also be used for the prediction of rubber behavior while aging conditions were changed [11].
It is well known that structure changes commonly lead to the changes in rubber properties during the aging processes. For example, utilization of heat aging process and various accelerator systems resulted in the change of dynamic properties of rubber [12,13]. Other mechanical properties such as tensile properties [14], hardness [15], and dielectric properties [16,17] are also changed upon employment of different aging processes and accelerating systems. e aging process and mechanical loading are decisive factors on the service life of the rubber. e simultaneous effect of two factors (aging processes and mechanical loading) on the rubber service life has been studied in many works [18][19][20][21][22].
e service life of the rubber depended on several mechanical properties such as fatigue resistance, crack development, and tear strength [23][24][25]. Nevertheless, due to the complexity of the failure mechanism of rubbers in the aging process, rubber service life prediction is often based on a phenomenal approach, where the molecular mechanism of failure is not concerned. According to the phenomenal approach, two modes using Arrhenius relation or Williams-Landel-Ferry (WLF) equation are commonly employed [26,27]. Arrhenius relation is often used in the case of aging with constant temperature. Several prediction methods have been developed relying on Arrhenius model [22,[27][28][29][30][31]. However, change in activation energy of the blended rubbers during the aging processes, which lead to change in mechanical properties, has not yet fully studied.
Herein, the relation of two aging process (heat and dynamic aging processes) is investigated through change in activation energy of the NR and NR/CR blends. Based on the determination of aging coefficients and Arrhenius-type equation for tensile strength, change in activation energy will be quantitatively evaluated, while rubbers are affected by the aforementioned factors simultaneously and separately.

Experimental Section
2.1. Materials. Natural rubber was obtained from a Vietnamese RSS company. Chloroprene rubber was purchased from Japan (M120). Carbon black, antidegradants RD and 4020, stearic acid, plasticizer 112, accelerators TMTD and TBBS, and ZnO were purchased from Xilong (China). All chemicals were used as received without any further purification.
Rubber blends were prepared following formulations listed in Table 1. A reference sample of natural rubber without addition of chloroprene rubber was prepared for comparison purpose.

Sample Preparation.
Rubber and ingredients were mixed in internal mixer Baopin 8412 with rotor speed of 30 rpm at temperature of 80°C for 13 minutes. Mixtures were stabilized at 25°C for 24 hours and vulcanized at 145°C for 10 minutes.

Mechanical Properties.
e hardness (shore A) was determined by using TFCLOCKGS 790N (Japan). Tensile strength and elongation at break were defined according to ISO 37 : 2005 standard using INSTRON 5582 equipment (USA). Permanent compression was determined according to ASTM D395-03 : 2008, and rebound property was estimated followed ASTM D2632-01 standard.

Swelling Behavior and Network Density Calculation.
e swelling test was carried out in toluene solvent according to ISO 1817 : 2005.
Swelling degree is defined as follows: where m 0 and m 1 are the mass of sample before and after immersion into toluene. Δmis the swelling degree, % Network density is calculated according to Flory-Rehner equation [32]: where V 1 is the mole partial volume of solvent, V 2 is the volume fraction of the sample at the saturated swell, and χ is the interaction parameter of rubber and solvent.
In the case of rubber blend, X may be calculated according to the ratio of rubbers and interaction parameters between rubber and solvent as follows [11]: where χ 1 and χ 2 are the interaction parameters of rubbers 1 and 2 with solvent and a and b are the weight part of rubbers 1 and 2.

Aging Coefficient.
Aging coefficient was determined according to ISO 188 : 1998 as follows: where k is the aging coefficient and σ o , σ are the tensile strength before and after aging.   Figure 1. From the value of saturated swelling determined from the curves in Figure 1, the "apparent" network densities of blends were calculated and the results are shown in Table 2. It can be clearly seen from Figure 1 and Table 2 that although the swelling of CR in toluene is smaller than that of NR, the addition of a small part of CR (5-10 phr) into NR leads to a higher swelling value of blends in comparison with the neat NR. It is of note that when the CR contents increase, the swelling value of NR/CR blends decreases. is is ascribed to the swelling of each rubber in solvent and the interaction of rubber on interphase surface. e NR/CR blends with low compatibility of rubbers and poor interaction on the interphase surface lead to solvent penetration into the interphase area easier than into rubbers, and as a result, the swelling property increases. Furthermore, low swelling behavior of CR might reduce the swelling of NR/CR blends. erefore, at a low percentage of CR in the blend, the swelling decreases because the decreased swelling of CR is not enough to compensate the increased swelling from rubbers incompatibility. For the same reason, the swelling of NR/CR10 is lower than that of NR/CR5 but still higher in comparison with neat NR.

Results and Discussion
at is why the "apparent" network density with neat NR is higher than of NR/CR blends. Figure 2 and Table 3 are the mechanical properties of the NR and NR/CR blends. It can be obvious that addition of CR results in the decrease of tensile strength and elongation at break of the NR/CR blends. Upon addition of 5 phr CR into the blend, the tensile strength and elongation at break decreased from 25 MPa to 620% to 23.3 MPa and 600%, respectively. e further increase of CR contents to 10 phr witnesses the slight increase in tensile strength (24.6) and elongation at break (605) in comparison with the NR/CR blends with 5 phr CR, which is still significantly lower than that of neat NR. is might be due to the incompatibility of the CR in NR matrix at low content of CR. For other mechanical properties such as hardness and permanent compression, the addition of CR slightly increases these properties of the blends as shown in Table 3. ese small changes in the mechanical properties indicate that the small amount of CR does not have a significant effect on the static mechanical properties of NR/CR blends.

Aging Coefficients.
Aging coefficients of NR and NR/ CR blends were determined following equation (4) for three aging conditions and the results are presented in Table 4. It can be seen from Table 4 that, in heat aging condition, the small amounts of CR (5-10 phr) have almost no effect on aging resistance of NR/CR blend in comparison with neat NR. is might be ascribed to the high aging resistance of CR and the incompatibility of NR and CR. is result is in well agreement with the aforementioned mechanical properties. Interestingly, under dynamic loading of the mechanical aging condition, the aging coefficients of NR/CR blends are remarkably lower than that of the neat NR. is is because, under dynamic loading, the incompatibility of rubbers might clearly appear even at a low percentage of NR (5-10 phr). It can be clearly seen in the complex aging regime that the aging coefficients of NR/CR blends are lower than that of the neat NR. us, based on the aging coefficients, it can be concluded that the addition of the CR might change activation energy of NR/CR blend under various aging conditions.

eoretical Background.
e activation energy may be considered as an energy barrier that must be overcome before materials break down. e relation between tensile strength and temperature may be described through Arrhenius-type equation [34]: where σ is the tensile strength (MPa), A is the constant, U o is the activation energy (J.mol −1 ), R is the gas constant (R � 8.314 J mol −1 K −1 ), and T is the temperature (K). Before aging, σ and T are denoted as σ o and T o , respectively . After aging, the tensile strength of the material is calculated as follows: where [34] ΔU ag is decreased value of activation energy and T ag is the aging temperature. Dividing (6) by (5) and noting that k � σ ag /σ o , one can obtain After some conversions, we have where ΔT � T ag − T o Formula (8) allows quantitatively determining the decrease in activation energy ΔU ag during the aging process.

Decrease of Activation Energy during Heat Aging
e calculated activation energies of the neat NR, NR/CR5, and NR/CR10 are determined at the temperature of 25°C to be 24500, 26134, and 25231 J mol −1 , respectively. In the experimental conditions with the aging temperature of 383 K, where ΔT � 383 − 298 � 85, formula (8) may be converted to Adding the k T values of all samples from Table 4 to (5), the decrease of activation energy during heat aging ΔU T can be calculated as shown in Table 5. It can be seen from the table that the presence of CR has almost no effect on ΔU T after heat aging, and the difference between ΔU T of NR and NR/CR blends is only in the range of 4-5%.

Decrease of Activation Energy during Mechanical Aging
(ΔU d ). During mechanical aging, the rubber samples suffered from dynamic loading at room temperature (298 K), and as a result, ΔT � 0 and (8) is converted to Adding the k d values for all samples from Table 4 to (10), ΔU d can be calculated as shown in Table 6. It is obvious that, in mechanical aging, the CR content is of significant effect on activation energy of the rubber blends. e changes in activation energies ΔU d of NR/CR blends are much higher (18-19 times) than that of the neat NR although the CR contents are small; hence, dynamic loading may strongly reduce ΔU d of NR/CR blends, while it has almost no effect on ΔU d of the neat NR.
ese results are consistent with the swelling behavior discussed previously.

Decrease of Activation Energy during Complex Aging
ΔU c . In the complex aging condition, the tensile strength of samples decreases by both factors of heat and dynamic loading. Formula (8) can be converted to Using (11) with the k c values from Table 4, ΔU c for all samples were calculated and presented in Table 7. In complex aging condition, the activation energies of NR/CR blends also decrease significantly in comparison with the activation energy of the neat NR, but lower than that obtained from mechanical aging. Even these differences in activation energy between NR and NR/CR blends are rather small (only around 10-15%), it is also demonstrated that CR has reasonable effect on the activation energy of the NR/CR blends during the complex aging. It can be also concluded that the ΔU c (complex aging) values depend on both ΔU t (from heat aging) and ΔU d (from mechanical aging). e ΔU c calculated from (11) is the sum of ΔU t + ΔU d determined from (9) and (10), and as a result, the relation of aging processes can be evaluated and the results are shown in Table 8.
It is obvious from Table 8 that low contents (5-10 phr) of CR significantly decrease activation energy of NR/CR blends during the mechanical aging process, which means that the incompatibility behavior of NR and CR rubbers is mainly expressed in dynamic conditions. e decrease of tensile strength in complex aging conditions may be from heat aging, where ΔU T is higher than ΔU d about 13-14 times. When the rubber samples are under both heat and dynamic loading, these factors may act independently.

Conclusions
In summary, the detailed investigation on the mechanical properties and the change in activation energies of the natural rubber/chloroprene rubber blends have been successfully presented. Effect of the CR content on the mechanical properties and especially activation energies of the NR/CR blends under various aging conditions of heat, mechanical, and complex aging has been studied. e incompatibility between NR and CR in NR/CR blends at low CR contents of 5-10 phr has negligible effects on static mechanical properties of the rubber blends. However, rubber incompatibility causes significant change in activation energies of NR/CR blends under mechanical aging condition. e decrease of activation energy by dynamic loading of NR/CR blends (ΔU d ) is remarkably higher (18-19 times) than that of the neat NR, which indicates that the incompatibility of rubbers affects mainly materials resistance to dynamic loading. In the complex aging, the small difference (less than 5%) between (ΔU c ) and the sum ΔU t + ΔU d indicates that heat and dynamic loading factors act independently.
is may allow evaluating the complex aging process through separate results of heat aging and mechanical aging regimes of NR/CR blends.

Data Availability
e hardness (shore A), tensile strength, elongation at break, permanent compression, and rebound property data, as well as the calculated activation energy data, used to support the findings of this study are included within the article.

Conflicts of Interest
e authors declare that they have no conflicts of interest.