# Acrylic Rubber-Reinforced Halloysite Nanotubes/Carbon Black Hybrid Fillers for Oil Seal Applications: Thermal Stability and Dynamic Mechanical Properties

Authors: K. Senthilvel; B. Prabu; C. Moganapriya; R. Rajasekar; M. Francis Luther King; Md. Elias Uddin
Journal: Advances in Materials Science and Engineering
 (2022)

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## Abstract

In this study, we show that adding halloysite nanotubes (HNT) to carbon black (CB)-packed acrylic rubber (ACM) composites improves their thermal properties. The thermo-oxidative stability, thermal stability, and dynamic mechanical properties of ACM composites (filled simply with 70 phr CB) and ACM hybrid composites comprising a fixed amount of CB (60 phr) and a variable amount of halloysite nanotubes (HNT) (2, 4, 6, 8, and 10 phr) were investigated. As evidenced by the oxidation induction time analysis, hybrid structures support a higher degree of antioxidation in composites reinforced with twin fillers than composites reinforced just with CB. ACM composites with dual fillers had greater breakdown temperatures (temperatures at 10% weight loss (T10) and 50% weight loss (T50)) and char residue concentration at 600°C than ACM conventional composites, according to thermogravimetric tests. The activation energy of the thermal disintegration of ACM composites, as determined by the Kissinger and Flynn–Wall–Ozawa techniques, shows that the addition of HNT improves the thermal stability of ACM composites. The storage modulus of ACM composites was increased by 79 percent at 30°C when 10 phr of black filler was replaced with 6 phr of tubular HNT, according to additional viscoelastic experiments.


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## Body

## 1. Introduction

The last few decenniums have advocated a strong interest towards the development of high-performance polymeric materials with resistance to elevated or lower temperatures, flame retardation, and admissible mechanical properties. The most popular strategies for producing thermally stable materials are grafting polymeric backbones with polymers that demonstrate improved thermal stability, using high-temperature polymeric blends as the matrix, and dispersion of high-temperature-resistant filler particles. Fillers in the nanoscale range, to a greater or lesser extent less than 100 nm, have been actively explored in recent years in order to develop cutting-edge polymeric materials. Nanocomposites are made up of polymer and nanofiller combinations.Polymer nanocomposites containing nanofiller in extremely small concentrations (1–8 phr) have been shown to have significantly greater thermal stability than standard materials in studies. Among the nanofillers, layered silicates have been a popular topic of study among scientists and businesses. Intercalation of organically modified layered silicates, for example, has a considerable impact on the thermal stability of acrylonitrile butadiene nanocomposites [1]. The thermal stability of elastomeric nanocomposites with carbonaceous nanofiller, which includes various kinds of graphene and carbon nanotubes [2–7], has been studied in the literature in recent years. The enhancement in the thermal resistance of elastomers with the incorporation of nanofillers is largely due to the barrier effect. As contemplated by Vyazovkin et al. [8] and Gilman et al. [9], a polymeric-inorganic char builds upon the surface of the polymer and provides the mass and heat transfer barriers that augment the thermal resistance.The thermal deterioration of elastomeric materials is a predominant issue, as it ultimately governs the mechanical attributes, durability, and service life of rubber components. As soon as the degradation commences, the above attributes will gradually deteriorate. Hence, understanding various thermal degradation parameters of elastomers is of paramount importance for developing a rational technology for elastomer processing and higher-temperature applications. The kinetic analysis of the thermal deterioration is very essential, as it can provide a breakthrough on the energy barriers of the process and offer clues for understanding degradation mechanisms. The challenge for studying thermal degradation kinetics is to find a reliable approach. Normally, the multiheating-rate method has been extensively used to study the thermal degradation kinetics due to its reliability. Several researchers have adopted the Kissinger and Flynn–Wall–Ozawa models based on the multiheating rate method to investigate the degradation kinetics of various rubber composites [10–18].The basically existing multiwalled halloysite nanotubes (HNT), which are uniformly strong, have a composition similar to montmorillonite and a shape similar to CNT. They are comprised of silicon, aluminum, oxygen, and hydrogen and have a tubular architecture. They have a high aspect ratio and specific surface area, as well as lower surface energy and less filler aggregation [19]. The mechanical, ageing, and gas barrier properties of rubber-HNT nanocomposites [20–26] have been studied extensively in the literature. However, there are just a few exact studies showing the effect of HNT on rubber nanocomposites’ thermal behavior. In SBR nanocomposites supplemented with HNT, Rybinski et al. [27] found that the rate of heat breakdown was reduced. The effect of activated HNT and modified HNT in lowering the rate of thermal breakdown and flammability in SBR and ACM [28, 29] was also shown by the same authors.Carbon black (CB) is a typical filler that is utilized in large quantities (>30 phr) to determine the best qualities in elastomeric goods. Despite this, carbon black particles have a proclivity for aggregation due to uneven distribution during processing. As a result, the elastomeric sector will mark a watershed moment when enormous quantities of carbon black are replaced with other reinforcements in smaller volumes. The use of black filler is significantly reduced, which reduces the weight of the compound and improves its endurance [30]. Furthermore, it is widely acknowledged that combining two or more reinforcements leads to a significant improvement in the properties of elastomeric composites. As a result, elastomeric composites comprising ordinary black filler hybridized with nano-sized reinforcements (CNT, graphene nanoclay) have gained widespread acceptance among academics and megacorporations.Several recent research studies in the literature have demonstrated the synergy of CB and nanoreinforcements on the thermal behavior of rubber composites. Natural rubber (NR)/styrene butadiene rubber (SBR) blends (80 : 20) reinforced with CB and CNT hybrid fillers showed enhanced stability, according to Boonmahitthisud and Song [31]. Tagelsir et al. [32] showed that increased CB-CNT interactions improve flouroelastometer’s thermal stability. The synergy of CB and nanoclay hybrid fillers on the improved thermal degradation characteristics of butyl rubber tyre curing bladder compounds was demonstrated by Razzaghi-Kashani and Samadi [33]. Natural rubber-CB-graphene hybrid nanocomposites with superior heat stability and viscoelastic properties were created by Zang et al. [34].Nonetheless, there are few investigations on the thermal behavior of rubber nanocomposites reinforced with the HNT-CB dual-filler system. Improved thermal degradation characteristics in natural rubber nanocomposites supplemented with CB and HNT hybrid fillers were discovered by Ismail et al. [35]. The enhanced thermal properties of nitrile rubber [NBR] and polyvinyl chloride/nitrile hybrid nanocomposites [NBR/PVC] have been attributed by the authors to the synergy between the twin fillers and their enhanced interactions [36, 37].Acrylic Rubber (ACM) is a well-known synthetic elastomer that is utilized in applications requiring high fuel resistance and thermo-oxidative resistance. It is mostly utilized in the automobile industry to make seals, o-rings, and gaskets. Slusarski et al. [38] investigated the thermal behavior of acrylic rubber utilized as sealing plate components. However, there is a scarcity of research in the literature that characterizes the thermal stability of acrylic rubber. To the best of the author’s knowledge, there are no previous studies on the thermal stability, degradation kinetics, or dynamic mechanical characteristics of ACM composites reinforced with HNT-CB hybrid filler systems. The thermal properties of ACM composites (filled simply with 70 phr CB) and ACM hybrid composites comprising a fixed amount of CB (60 phr) and a variable amount of halloysite nanotubes (HNT) (2, 4, 6, 8, and 10 phr) were investigated in this study, which builds on prior research [39]. In this research work, thermo-oxidative stability, thermal decomposition kinetics, and temperature of glass transition were explored using oxidation induction time, thermogravimetric, and dynamic mechanical investigations, respectively, to evaluate the thermal characteristics.


## 2. Experiment

### 2.1. Materials

RK Polymers, Chennai, India, provided the elastomeric matrix Acrylic Rubber, AR801 (45 Mooney units). Sigma Aldrich in Germany provided the halloysite nanotubes (50 nm). BP Chemicals, Pune, India, provided the carbon black (FEF N550), antioxidant (Naugard 445), plasticizer (DOP), processing aid (Rubaid 2220), cure (sulphur MC), accelerators (potassium stearate, sodium stearate), and other compounding elements indicated in Table1. All the compounding ingredients used were of commercial grade.Table 1
ACM hybrid nanocomposites formulation table.
Ingredients (Phra)CompoundsCB70CB60-HNT2CB60-HNT4CB60-HNT6CB60-HNT8CB60-HNT10ACM100100100100100100HNT0246810Stearic acid1.51.51.51.51.51.5Naugard 445b222222FEF N550706060606060Rubaid222c222222DOPd555555Sulphur MC0.30.30.30.30.30.3Sodium stearate333333Potassium stearate0.30.30.30.30.30.3aParts by weight per hundreds of rubbers, b4-4′-bis (α-α-dimethyl benzyl) diphenyl amine, cpentaerythritol stearate, and ddioctyl phthalate.


### 2.2. Preparation of ACM Composites

Table1 shows the ACM composite formulas. A dual-roll mill with dimensions of 160 mm × 320 mm was used to compound the material. The two-roll mill was operated at atmospheric temperature, and the speed ratio of the rotors was maintained at 1 : 14. The blending sequence began with the introduction of ACM rubber into the mill. HNT was subsequently included after a few minutes of mastication, and the mixing was continued for 10 minutes. The antioxidant, activator (stearic acid), CB, and plasticizer were then added to the mix, which was mixed for another 20 minutes. Sulphur, potassium stearate, and sodium stearate were added last and mixed for another 2-3 minutes. It was possible to create a homogeneous compound. With atomic weights of 0, 2, 4, 6, 8, and 10% for HNT, six distinct compounds were created. These compounds are first cured in an electric press at 160°C before being postcured in a 150°C oven for 4 hours. These vulcanizates were treated for various characterisations after being kept in an ambient environment for 24 hours.


## 2.1. Materials

RK Polymers, Chennai, India, provided the elastomeric matrix Acrylic Rubber, AR801 (45 Mooney units). Sigma Aldrich in Germany provided the halloysite nanotubes (50 nm). BP Chemicals, Pune, India, provided the carbon black (FEF N550), antioxidant (Naugard 445), plasticizer (DOP), processing aid (Rubaid 2220), cure (sulphur MC), accelerators (potassium stearate, sodium stearate), and other compounding elements indicated in Table1. All the compounding ingredients used were of commercial grade.Table 1
ACM hybrid nanocomposites formulation table.
Ingredients (Phra)CompoundsCB70CB60-HNT2CB60-HNT4CB60-HNT6CB60-HNT8CB60-HNT10ACM100100100100100100HNT0246810Stearic acid1.51.51.51.51.51.5Naugard 445b222222FEF N550706060606060Rubaid222c222222DOPd555555Sulphur MC0.30.30.30.30.30.3Sodium stearate333333Potassium stearate0.30.30.30.30.30.3aParts by weight per hundreds of rubbers, b4-4′-bis (α-α-dimethyl benzyl) diphenyl amine, cpentaerythritol stearate, and ddioctyl phthalate.


## 2.2. Preparation of ACM Composites

Table1 shows the ACM composite formulas. A dual-roll mill with dimensions of 160 mm × 320 mm was used to compound the material. The two-roll mill was operated at atmospheric temperature, and the speed ratio of the rotors was maintained at 1 : 14. The blending sequence began with the introduction of ACM rubber into the mill. HNT was subsequently included after a few minutes of mastication, and the mixing was continued for 10 minutes. The antioxidant, activator (stearic acid), CB, and plasticizer were then added to the mix, which was mixed for another 20 minutes. Sulphur, potassium stearate, and sodium stearate were added last and mixed for another 2-3 minutes. It was possible to create a homogeneous compound. With atomic weights of 0, 2, 4, 6, 8, and 10% for HNT, six distinct compounds were created. These compounds are first cured in an electric press at 160°C before being postcured in a 150°C oven for 4 hours. These vulcanizates were treated for various characterisations after being kept in an ambient environment for 24 hours.


## 3. Characterisation

### 3.1. Stability of Oxidation

Oxidation induction time is used to determine the thermo-oxidative stability of the composites (OIT). OIT measurements are performed using an ISO 11357-compliant differential scanning calorimeter (DSC 6000, SII NANO Technology, Japan). The specimens are heated to 200°C in a nitrogen environment at a rate of 20°C/min. The specimens are kept at that temperature for 5 minutes to reach equilibrium before being introduced to oxygen. The commencement of oxidation in the sample is indicated by a significant increase in heat flow. The time difference between the start of oxidation and the first contact with oxygen is used to compute the sample’s OIT.


### 3.2. Thermogravimetric Analysis

Under nitrogen atmosphere, the thermal degradation behavior of ACM composites was investigated using a thermogravimetric analyser (TG/DTA 6200, SII NANO technology, Japan). Samples weighing 7-8 mg were heated at 2.5, 5, 10, and 15°C/min under nitrogen from 30°C to 600°C.


### 3.3. Determination of Energy of Activation Energy (Ea)

To investigate the thermal behavior of any polymeric substance, it is critical to quantify its thermal degradation kinetics, which is commonly described using the Arrhenius equation (40).(1)k=Aexp−EaRT,where Ea represents energy of activation (kJ/mol), A is a representation of the preexponential factor, T denotes the absolute temperature (K), and R connotes gas constant.The following equation can be used to calculate the rate of reaction [40]:(2)dαdt=kfα.  Here, t (s) denotes the time of reaction, f indicates a function related to the reaction mechanism, K represents the rate constant, and α signifies the conversion degree which is expressed as [40](3)α=m0mtm0−mf,where mo, mi, and mf, respectively, denotes rubber composites’ initial, final, and actual weights as determined from the thermogravimetric curves.Combining equations (1) and (2) the following equation is obtained:(4)dαdt=fαexp−EaRT.The apparent activation energy for nonisothermal degradation (Ea) could be determined using the differential Kissinger or integral Flynn–Wall–Ozawa techniques. The Kissinger method, or alternatively called the peak-maximum evolution method, uses the point where the rate of reaction during heating is at its maximum (Tp) to calculate the activation energy. Furthermore, the Kissinger approach yields only a single activation energy value for the entire decomposition process. The Kissinger approach calculates the kinetic parameters that are independent of the conversion.On the other hand, the Flynn–Wall–Ozawa approach calculates the activation energy based on the temperature that corresponds to a certain conversion degree (Ta), the so called isoconversional method. This allows to calculate the apparent activation energy as a function of the conversion degree. This method also provides information on the conversion levels at which more energy is required to break the main bonds of the polymeric chain during the thermal degradation process.


#### 3.3.1. Kissinger Differential Method

The differential method endorses the following expression [41]:(5)InβTmax2=lnARgαEa−EaRTmax,where β indicates rate of heating (K/min), Tmax represents the peak temperature as determined from DTG curve α connotes samples’ mass conversion degree, and A indicates the factor preexponential R denotes the ideal gas constant. Ea symbolises activation energy.Graphs drawn using ln(β/T2max) against (1/Tmax) resulted in a fitted straight line, and the slope of the straight line (−Ea/R) can be used to determine the energy of activation Ea.


#### 3.3.2. Flynn–Wall–Ozawa Integral Method

The integral method acknowledges the following expression [42–44]:(6)logβ=logAEaRgα−2.315−0.4567EaRT.At a given conversion degreeα, graphs drawn using log β versus 1/T generates a straight line having 0.457 × −Ea/R as slope. The slope thus obtained is used to determine the activation energy (Ea).


### 3.4. Dynamic Mechanical Analysis (DMA)

The composites’ thermomechanical behavior was assessed. Using a DMA analyser, the storage modulus (E) and loss tangent were measured against temperature (6100, SII NANO Technology, Japan). In tension mode, these properties were determined at a frequency of 1 Hz and a strain of 0.2 percent. The temperature sweep trials were carried out at a rate of 5°C min−1 between −100°C and 100°C.


## 3.1. Stability of Oxidation

Oxidation induction time is used to determine the thermo-oxidative stability of the composites (OIT). OIT measurements are performed using an ISO 11357-compliant differential scanning calorimeter (DSC 6000, SII NANO Technology, Japan). The specimens are heated to 200°C in a nitrogen environment at a rate of 20°C/min. The specimens are kept at that temperature for 5 minutes to reach equilibrium before being introduced to oxygen. The commencement of oxidation in the sample is indicated by a significant increase in heat flow. The time difference between the start of oxidation and the first contact with oxygen is used to compute the sample’s OIT.


## 3.2. Thermogravimetric Analysis

Under nitrogen atmosphere, the thermal degradation behavior of ACM composites was investigated using a thermogravimetric analyser (TG/DTA 6200, SII NANO technology, Japan). Samples weighing 7-8 mg were heated at 2.5, 5, 10, and 15°C/min under nitrogen from 30°C to 600°C.


## 3.3. Determination of Energy of Activation Energy (Ea)

To investigate the thermal behavior of any polymeric substance, it is critical to quantify its thermal degradation kinetics, which is commonly described using the Arrhenius equation (40).(1)k=Aexp−EaRT,where Ea represents energy of activation (kJ/mol), A is a representation of the preexponential factor, T denotes the absolute temperature (K), and R connotes gas constant.The following equation can be used to calculate the rate of reaction [40]:(2)dαdt=kfα.  Here, t (s) denotes the time of reaction, f indicates a function related to the reaction mechanism, K represents the rate constant, and α signifies the conversion degree which is expressed as [40](3)α=m0mtm0−mf,where mo, mi, and mf, respectively, denotes rubber composites’ initial, final, and actual weights as determined from the thermogravimetric curves.Combining equations (1) and (2) the following equation is obtained:(4)dαdt=fαexp−EaRT.The apparent activation energy for nonisothermal degradation (Ea) could be determined using the differential Kissinger or integral Flynn–Wall–Ozawa techniques. The Kissinger method, or alternatively called the peak-maximum evolution method, uses the point where the rate of reaction during heating is at its maximum (Tp) to calculate the activation energy. Furthermore, the Kissinger approach yields only a single activation energy value for the entire decomposition process. The Kissinger approach calculates the kinetic parameters that are independent of the conversion.On the other hand, the Flynn–Wall–Ozawa approach calculates the activation energy based on the temperature that corresponds to a certain conversion degree (Ta), the so called isoconversional method. This allows to calculate the apparent activation energy as a function of the conversion degree. This method also provides information on the conversion levels at which more energy is required to break the main bonds of the polymeric chain during the thermal degradation process.


### 3.3.1. Kissinger Differential Method

The differential method endorses the following expression [41]:(5)InβTmax2=lnARgαEa−EaRTmax,where β indicates rate of heating (K/min), Tmax represents the peak temperature as determined from DTG curve α connotes samples’ mass conversion degree, and A indicates the factor preexponential R denotes the ideal gas constant. Ea symbolises activation energy.Graphs drawn using ln(β/T2max) against (1/Tmax) resulted in a fitted straight line, and the slope of the straight line (−Ea/R) can be used to determine the energy of activation Ea.


### 3.3.2. Flynn–Wall–Ozawa Integral Method

The integral method acknowledges the following expression [42–44]:(6)logβ=logAEaRgα−2.315−0.4567EaRT.At a given conversion degreeα, graphs drawn using log β versus 1/T generates a straight line having 0.457 × −Ea/R as slope. The slope thus obtained is used to determine the activation energy (Ea).


## 3.3.1. Kissinger Differential Method

The differential method endorses the following expression [41]:(5)InβTmax2=lnARgαEa−EaRTmax,where β indicates rate of heating (K/min), Tmax represents the peak temperature as determined from DTG curve α connotes samples’ mass conversion degree, and A indicates the factor preexponential R denotes the ideal gas constant. Ea symbolises activation energy.Graphs drawn using ln(β/T2max) against (1/Tmax) resulted in a fitted straight line, and the slope of the straight line (−Ea/R) can be used to determine the energy of activation Ea.


## 3.3.2. Flynn–Wall–Ozawa Integral Method

The integral method acknowledges the following expression [42–44]:(6)logβ=logAEaRgα−2.315−0.4567EaRT.At a given conversion degreeα, graphs drawn using log β versus 1/T generates a straight line having 0.457 × −Ea/R as slope. The slope thus obtained is used to determine the activation energy (Ea).


## 3.4. Dynamic Mechanical Analysis (DMA)

The composites’ thermomechanical behavior was assessed. Using a DMA analyser, the storage modulus (E) and loss tangent were measured against temperature (6100, SII NANO Technology, Japan). In tension mode, these properties were determined at a frequency of 1 Hz and a strain of 0.2 percent. The temperature sweep trials were carried out at a rate of 5°C min−1 between −100°C and 100°C.


## 4. Discussion of the Findings

### 4.1. Short-Term Oxidation

OIT is commonly used to verify antioxidative performance; a higher OIT value indicates that the sample is more resistant to thermal oxidation. Table2 lists the OIT values for ACM composites generated from DSC measurements (as shown in Figure 1).Table 2
OIT values of ACM composites at 200°C.
SampleCB70CB60-HNT2CB60-HNT4CB60-HNT6CB60-HNT8CB60-HNT10OIT (min)89.5104.43110.54113.31109.51108.59Figure 1
OIT values of ACM composites.ACM composites reinforced with these twin fillers have significantly better short-term oxidation values than carbon black-reinforced ACM composites. Among the numerous composites studied, the ACM composite reinforced with 6 phr had the greatest OIT value. The production of intercalated morphology (HNT-ACM-CB) and homogeneous distribution of tubular nanofiller in the ACM rubber matrix may be ascribed to the notable increase in OIT value of ACM composites [28]. These variables increase the likelihood of contact between the rubber and the antioxidants supplied during compounding and prevent the antioxidants from diffusing to the surface of the ACM matrix, even when exposed to heat and oxygen. This increases the antiaging effect by delaying the radical reaction [34]. These remarkable findings clearly show that tubular nanofiller has a major impact on delaying the onset of oxidation in ACM rubber composites.The OIT value drops with the addition of 8 phr HNT to the rubber matrix, indicating a crucial destructive effect on the thermo-oxidative stability of ACM composites. This could be due to poor filler-matrix compatibility, increased filler-filler interaction, or the production of agglomerates. These conditions limit the interfacial contacts between the rubber and antioxidants, allowing most antioxidants to diffuse to the rubber matrix’s surface under heat and oxygen, speeding up the formation of radical reactions and delaying the antiaging effect [45].


### 4.2. Analysis of Thermogravimetric Data

Figure2 represents the thermal degradation characteristics of ACM composites at 5°C/min. ACM conventional and hybrid nanocomposites have a single degradation step. The incorporation of HNT nanofiller in CB-filled ACM composites improves the degradation temperature at 10 percent weight loss and at 50 percent weight loss (T50), as shown in Figure 2 and Table 3. Furthermore, the char residue concentration of ACM composites reinforced with CB and HNT is 41.24 percent, 41.42 percent, 42.44 percent, 42.14 percent, and 41.29 percent, indicating that HNT plays an important role in enhancing the thermal degrading behavior of ACM composites.Figure 2
TGA curves of ACM composites at 10°C/min under nitrogen atmosphere.Table 3
TGA characteristics of ACM composites.
SampleTemperature at 10% weight lossT10 (°C)Temperature at 50% weight lossT50 (°C)Maximum temperature of degradationTmax (°C)Char residue weight (%) at 600 (°C)CB7035541541840.68CB60-HNT236542441941.24CB60-HNT436743142241.42CB60-HNT638043443042.44CB60-HNT837143142142.14CB60-HNT1036642441941.29As can be observed from the peak of the first derivative shown in Figure3, the temperature at which the rate of degradation (Tmax) of ACM composites filled with CB is maximum is at its maximum 418°C. ACM composite reinforced with 2 to 10 phr of HNT demonstrated Tmax values of 419°C, 422°C, 430°C, 421°C, and 419°C. It is highly evident that, among all the composites investigated, ACM composite reinforced with 6 phr HNT demonstrated the highest Tmax. The better performance of ACM composites with 6 phr HNT can be attributed to a uniform distribution of tubular nanofiller in the ACM rubber matrix, the development of intercalated morphology, superior crosslink network creation, and significant ACM-CB-HNT interactions, as evident from the highest Tg value of −22.7°C discussed in the viscoelastic studies discussed in the later part. These variables encourage a convoluted path to the ACM breakdown products, preventing the polymer from degrading further. However, it can be noticed that beyond 6 phr of HNT, Tmax decreases with further increases in HNT loading. Such a reduction in Tmax values can be ascribed to poor compatibility between ACM and HNT, agglomeration of HNT tubules, partial debonding of large agglomerates from the elastomeric matrix, and the subsequent formation of interfacial voids. Butyl rubber nanocomposites [33], NBR-CB-HNT [36], and NBR/PVC-CB-HNT [37] hybrid nanocomposites showed similar findings.Figure 3
DTG curves of ACM composites at 10°C/min under nitrogen atmosphere.


### 4.3. Analysis of Kinetics

The kinetic parameters must be examined in order to gain a better understanding of the mechanism of ACM degradation and to conduct a comprehensive investigation into the influence of hybrid fillers. Among the several composites studied, ACM-CB60-HNT6 showed higher antiaging and TGA. As a result, nonisothermal assessments at various heating rates were used to investigate the decomposition of conventional composite (ACM-CB70) and ACM-CB60-HNT6 alone in this work. The TGA and DTG curves of ACM-CB70 at various heating rates are depicted in Figures4 and 5. Figures 6 and 7 show the TGA and DTG curves of ACM-CB60-HNT6 at various heating rates.Figure 4
TGA curves of ACM-CB70 composites at different heating rates under nitrogen atmosphere.Figure 5
DTG curves of ACM-CB70 composites at different heating rates under nitrogen atmosphere.Figure 6
TGA curves of ACM-CB60-HNT6 composites at different heating rates under nitrogen atmosphere.Figure 7
DTG curves of ACM-CB60-HNT6 composites at different heating rates under nitrogen atmosphere.


### 4.4. The Kissinger Approach

Figure8 shows the graph of ln (β/T2max) against (1/Tmax) for the evaluation of Ea using the differential method. Table 4 shows a summary of the findings. 521 kJ/mol is the activation energy of a CB-filled ACM composite reinforced with CB filler. With the addition of 6 phr HNT content, it is enhanced to 543 kJ/mol, indicating that HNT improves the thermal stability of ACM composites. The increased HNT-ACM-CB interactions and the homogeneous distribution of tubular nanofiller in the ACM matrix have resulted in such a dramatic improvement.Figure 8
Kissinger plot applied to TGA data of ACM composites at different heating rates.Table 4
Activation energy (Ea) calculated following the Kissinger method.
SampleEa (kJ/mol)R2ACM-CB705210.984ACM-CB60-HNT65430.981


### 4.5. Flynn–Wall–Ozawa Method

The integral method is used to determine theEa for a given degree of conversion (α) using graphs of log versus 1/T. The Flynn–Wall–Ozawa pattern of ACM composites reinforced with CB and ACM composites reinforced with CB and 6 phr of HNT is depicted in Figures 9 and 10. The graphs of log against 1/T generated for carbon black-filled ACM composite and dual filler reinforced ACM composites yield fitted straight lines with high linearly correlation coefficient for varied values of α. It should also be mentioned that these straight lines, which were achieved for a variety α are for the composites under consideration, which are almost parallel to one another, and all of the correlation coefficients are around 0.95, indicating that the approach is highly acceptable to ACM composites.Figure 9
Flynn–Wall–Ozawa plot of ACM-CB70 conventional composites.Figure 10
Flynn–Wall–Ozawa plot of ACM-CB60-HNT6 hybrid nanocomposites.The plot of activation energy (Ea) as a function of α are presented in Figure 11. On the basis of the trend of Ea, by which both composites achieve values that show the oxidative cross-linking process that occurs during the heat deterioration of acrylic rubber [46], it may be deduced that the levels of activation energy (Ea) acquired using the Flynn–Wall–Ozawa approach are slightly higher in comparison to the value obtained using the Kissinger approach [10–18]. Furthermore, these figures demonstrate the significance of HNT in enhancing the thermal stability of the ACM matrix. When the ACM hybrid composites were heated to disintegrate, the local (HNT-CB) hybrid filler network in the rubber formed protective layers over time to protect the underlying rubber matrix. As a result, the pace at which degradation products diffuse has slowed. As a result, the ACM rubber is protected from further deterioration, resulting in an increase in Ea [40]. Clay and carbon black-reinforced hydrogenated nitrile rubber have shown similar results [40].Figure 11
Activation esnergy vs. conversion degree (α).


### 4.6. ACM Composites’ Viscoelastic Behavior

#### 4.6.1. 1 The Influence of Temperature on Storage Modulus

The storage modulus of a composite is a measure of its stiffness. Figure12 and Table 5 show the storage modulus (E′) of the various ACM composites investigated.Figure 12
Variation of storage modulus (E′) with temperature for ACM composites.Table 5
Dynamic mechanical properties of ACM composites.
SamplesStorage modulus (MPa) at different temperatures (°C)Tg (°C)−70−50−30305070CB708348.935062.641167.0721.0315.6712.13−25.2CB60-HNT28759.216513.601441.0129.6216.6913.96−24.2CB60-HNT48844.476599.031526.4435.4119.0216.80−23.9CB60-HNT68891.726638.601566.0137.6220.6818.46−22.7CB60-HNT88834.175742.791516.0132.4018.6916.46−23.1CB60-HNT108774.266050.391456.0331.6216.6914.96−24.8It is worth noting that the addition of HNT filler to the ACM matrix results in a significant increase inE′. When compared to a CB-filled ACM composite, all ACM hybrid nanocomposites are always shown to have E′ values that are higher at all temperatures. In comparison to CB-filled ACM composites at the same temperature, the storage modulus of ACM hybrid nanocomposites containing 2, 4, 6, 8, and 10 phr of HNT increased by 41 percent, 69 percent, 79 percent, 54 percent, and 50 percent, respectively, at 30°C. Higher cross-link creation can be attributed to the superior performance of ACM composites reinforced with both HNT and CB, and it is also a strong indication of HNT’s reinforcing function. Increases in HNT content beyond 6 phr invariably result in poorHNT dispersion, poor compatibility between the filler and matrix, increased filler-filler interaction, and the eventual development of agglomerates. As a result, the free volume of the matrix enclosing these agglomerates is increased, resulting in increased polymer chain flexibility and a decrease in the storage modulus of the ACM composites [47].


#### 4.6.2. The Influence of Temperature on the Loss Factor

Dynamic mechanical analysis is also used to assess the effect of fillers on the mobility of adjacent elastomeric chains. During the glass transition, a large amount of energy is dissipated due to the viscous movement of long-range polymer chains. The peak height of the damping term loss factor depicts the mobility of the polymer segments (tanδ). Any decrease in the peak height of the loss factor indicates that the mobility of polymer segments is restricted. In elastomeric composites, such a restriction correlates to an increase in interfacial adhesion. As compared to composites reinforced with black filler alone, all hybrid composites had lower peak heights, as shown in Figure 13. Furthermore, ACM composites reinforced with 6 phr HNT content had the lowest peak height, indicating that a large proportion of ACM chains were restricted in their mobility. The temperature that corresponds to the highest point, tanδ, shows the glass transition (Tg). It can be seen in Figure 12 that an ACM composite filled with a single filler (CB) has a Tg value of −25.2°C. In ACM composites reinforced with twin fillers, there is also a noticeable movement in Tg values towards higher values.Figure 13
Deviation of loss factor (tanδ) vs. temperature.Furthermore, ACM composites containing 6 phr HNT loading had the highestTg value of −22.7°C of all the composites studied. The motility of ACM chains in the vicinity of the twin filler network was reduced by a homogeneous distribution of black filler and tubular HNT in the ACM matrix, as well as increased HNT-ACM-CB interfacial contacts. As a result, the Tg value of all ACM composites reinforced with hybrid reinforcement is shifting towards higher values. It was also discovered that ACM composites with 8 phr and 10 phr of HNT content had lower Tg values than ACM composites with 6 phr HNT reinforcement. Agglomerates are formed when the nanotubular filler content exceeds 6 phr. As a result, the free volume of the matrix enclosing these agglomerates is increased, resulting in easy chain motion and a reduction in the Tg of the ACM composites [48]. NBR-CB-HNT [36] and natural rubber-CB-nanoclay [49] hybrid nanocomposites showed similar results.


## 4.1. Short-Term Oxidation

OIT is commonly used to verify antioxidative performance; a higher OIT value indicates that the sample is more resistant to thermal oxidation. Table2 lists the OIT values for ACM composites generated from DSC measurements (as shown in Figure 1).Table 2
OIT values of ACM composites at 200°C.
SampleCB70CB60-HNT2CB60-HNT4CB60-HNT6CB60-HNT8CB60-HNT10OIT (min)89.5104.43110.54113.31109.51108.59Figure 1
OIT values of ACM composites.ACM composites reinforced with these twin fillers have significantly better short-term oxidation values than carbon black-reinforced ACM composites. Among the numerous composites studied, the ACM composite reinforced with 6 phr had the greatest OIT value. The production of intercalated morphology (HNT-ACM-CB) and homogeneous distribution of tubular nanofiller in the ACM rubber matrix may be ascribed to the notable increase in OIT value of ACM composites [28]. These variables increase the likelihood of contact between the rubber and the antioxidants supplied during compounding and prevent the antioxidants from diffusing to the surface of the ACM matrix, even when exposed to heat and oxygen. This increases the antiaging effect by delaying the radical reaction [34]. These remarkable findings clearly show that tubular nanofiller has a major impact on delaying the onset of oxidation in ACM rubber composites.The OIT value drops with the addition of 8 phr HNT to the rubber matrix, indicating a crucial destructive effect on the thermo-oxidative stability of ACM composites. This could be due to poor filler-matrix compatibility, increased filler-filler interaction, or the production of agglomerates. These conditions limit the interfacial contacts between the rubber and antioxidants, allowing most antioxidants to diffuse to the rubber matrix’s surface under heat and oxygen, speeding up the formation of radical reactions and delaying the antiaging effect [45].


## 4.2. Analysis of Thermogravimetric Data

Figure2 represents the thermal degradation characteristics of ACM composites at 5°C/min. ACM conventional and hybrid nanocomposites have a single degradation step. The incorporation of HNT nanofiller in CB-filled ACM composites improves the degradation temperature at 10 percent weight loss and at 50 percent weight loss (T50), as shown in Figure 2 and Table 3. Furthermore, the char residue concentration of ACM composites reinforced with CB and HNT is 41.24 percent, 41.42 percent, 42.44 percent, 42.14 percent, and 41.29 percent, indicating that HNT plays an important role in enhancing the thermal degrading behavior of ACM composites.Figure 2
TGA curves of ACM composites at 10°C/min under nitrogen atmosphere.Table 3
TGA characteristics of ACM composites.
SampleTemperature at 10% weight lossT10 (°C)Temperature at 50% weight lossT50 (°C)Maximum temperature of degradationTmax (°C)Char residue weight (%) at 600 (°C)CB7035541541840.68CB60-HNT236542441941.24CB60-HNT436743142241.42CB60-HNT638043443042.44CB60-HNT837143142142.14CB60-HNT1036642441941.29As can be observed from the peak of the first derivative shown in Figure3, the temperature at which the rate of degradation (Tmax) of ACM composites filled with CB is maximum is at its maximum 418°C. ACM composite reinforced with 2 to 10 phr of HNT demonstrated Tmax values of 419°C, 422°C, 430°C, 421°C, and 419°C. It is highly evident that, among all the composites investigated, ACM composite reinforced with 6 phr HNT demonstrated the highest Tmax. The better performance of ACM composites with 6 phr HNT can be attributed to a uniform distribution of tubular nanofiller in the ACM rubber matrix, the development of intercalated morphology, superior crosslink network creation, and significant ACM-CB-HNT interactions, as evident from the highest Tg value of −22.7°C discussed in the viscoelastic studies discussed in the later part. These variables encourage a convoluted path to the ACM breakdown products, preventing the polymer from degrading further. However, it can be noticed that beyond 6 phr of HNT, Tmax decreases with further increases in HNT loading. Such a reduction in Tmax values can be ascribed to poor compatibility between ACM and HNT, agglomeration of HNT tubules, partial debonding of large agglomerates from the elastomeric matrix, and the subsequent formation of interfacial voids. Butyl rubber nanocomposites [33], NBR-CB-HNT [36], and NBR/PVC-CB-HNT [37] hybrid nanocomposites showed similar findings.Figure 3
DTG curves of ACM composites at 10°C/min under nitrogen atmosphere.


## 4.3. Analysis of Kinetics

The kinetic parameters must be examined in order to gain a better understanding of the mechanism of ACM degradation and to conduct a comprehensive investigation into the influence of hybrid fillers. Among the several composites studied, ACM-CB60-HNT6 showed higher antiaging and TGA. As a result, nonisothermal assessments at various heating rates were used to investigate the decomposition of conventional composite (ACM-CB70) and ACM-CB60-HNT6 alone in this work. The TGA and DTG curves of ACM-CB70 at various heating rates are depicted in Figures4 and 5. Figures 6 and 7 show the TGA and DTG curves of ACM-CB60-HNT6 at various heating rates.Figure 4
TGA curves of ACM-CB70 composites at different heating rates under nitrogen atmosphere.Figure 5
DTG curves of ACM-CB70 composites at different heating rates under nitrogen atmosphere.Figure 6
TGA curves of ACM-CB60-HNT6 composites at different heating rates under nitrogen atmosphere.Figure 7
DTG curves of ACM-CB60-HNT6 composites at different heating rates under nitrogen atmosphere.


## 4.4. The Kissinger Approach

Figure8 shows the graph of ln (β/T2max) against (1/Tmax) for the evaluation of Ea using the differential method. Table 4 shows a summary of the findings. 521 kJ/mol is the activation energy of a CB-filled ACM composite reinforced with CB filler. With the addition of 6 phr HNT content, it is enhanced to 543 kJ/mol, indicating that HNT improves the thermal stability of ACM composites. The increased HNT-ACM-CB interactions and the homogeneous distribution of tubular nanofiller in the ACM matrix have resulted in such a dramatic improvement.Figure 8
Kissinger plot applied to TGA data of ACM composites at different heating rates.Table 4
Activation energy (Ea) calculated following the Kissinger method.
SampleEa (kJ/mol)R2ACM-CB705210.984ACM-CB60-HNT65430.981


## 4.5. Flynn–Wall–Ozawa Method

The integral method is used to determine theEa for a given degree of conversion (α) using graphs of log versus 1/T. The Flynn–Wall–Ozawa pattern of ACM composites reinforced with CB and ACM composites reinforced with CB and 6 phr of HNT is depicted in Figures 9 and 10. The graphs of log against 1/T generated for carbon black-filled ACM composite and dual filler reinforced ACM composites yield fitted straight lines with high linearly correlation coefficient for varied values of α. It should also be mentioned that these straight lines, which were achieved for a variety α are for the composites under consideration, which are almost parallel to one another, and all of the correlation coefficients are around 0.95, indicating that the approach is highly acceptable to ACM composites.Figure 9
Flynn–Wall–Ozawa plot of ACM-CB70 conventional composites.Figure 10
Flynn–Wall–Ozawa plot of ACM-CB60-HNT6 hybrid nanocomposites.The plot of activation energy (Ea) as a function of α are presented in Figure 11. On the basis of the trend of Ea, by which both composites achieve values that show the oxidative cross-linking process that occurs during the heat deterioration of acrylic rubber [46], it may be deduced that the levels of activation energy (Ea) acquired using the Flynn–Wall–Ozawa approach are slightly higher in comparison to the value obtained using the Kissinger approach [10–18]. Furthermore, these figures demonstrate the significance of HNT in enhancing the thermal stability of the ACM matrix. When the ACM hybrid composites were heated to disintegrate, the local (HNT-CB) hybrid filler network in the rubber formed protective layers over time to protect the underlying rubber matrix. As a result, the pace at which degradation products diffuse has slowed. As a result, the ACM rubber is protected from further deterioration, resulting in an increase in Ea [40]. Clay and carbon black-reinforced hydrogenated nitrile rubber have shown similar results [40].Figure 11
Activation esnergy vs. conversion degree (α).


## 4.6. ACM Composites’ Viscoelastic Behavior

### 4.6.1. 1 The Influence of Temperature on Storage Modulus

The storage modulus of a composite is a measure of its stiffness. Figure12 and Table 5 show the storage modulus (E′) of the various ACM composites investigated.Figure 12
Variation of storage modulus (E′) with temperature for ACM composites.Table 5
Dynamic mechanical properties of ACM composites.
SamplesStorage modulus (MPa) at different temperatures (°C)Tg (°C)−70−50−30305070CB708348.935062.641167.0721.0315.6712.13−25.2CB60-HNT28759.216513.601441.0129.6216.6913.96−24.2CB60-HNT48844.476599.031526.4435.4119.0216.80−23.9CB60-HNT68891.726638.601566.0137.6220.6818.46−22.7CB60-HNT88834.175742.791516.0132.4018.6916.46−23.1CB60-HNT108774.266050.391456.0331.6216.6914.96−24.8It is worth noting that the addition of HNT filler to the ACM matrix results in a significant increase inE′. When compared to a CB-filled ACM composite, all ACM hybrid nanocomposites are always shown to have E′ values that are higher at all temperatures. In comparison to CB-filled ACM composites at the same temperature, the storage modulus of ACM hybrid nanocomposites containing 2, 4, 6, 8, and 10 phr of HNT increased by 41 percent, 69 percent, 79 percent, 54 percent, and 50 percent, respectively, at 30°C. Higher cross-link creation can be attributed to the superior performance of ACM composites reinforced with both HNT and CB, and it is also a strong indication of HNT’s reinforcing function. Increases in HNT content beyond 6 phr invariably result in poorHNT dispersion, poor compatibility between the filler and matrix, increased filler-filler interaction, and the eventual development of agglomerates. As a result, the free volume of the matrix enclosing these agglomerates is increased, resulting in increased polymer chain flexibility and a decrease in the storage modulus of the ACM composites [47].


### 4.6.2. The Influence of Temperature on the Loss Factor

Dynamic mechanical analysis is also used to assess the effect of fillers on the mobility of adjacent elastomeric chains. During the glass transition, a large amount of energy is dissipated due to the viscous movement of long-range polymer chains. The peak height of the damping term loss factor depicts the mobility of the polymer segments (tanδ). Any decrease in the peak height of the loss factor indicates that the mobility of polymer segments is restricted. In elastomeric composites, such a restriction correlates to an increase in interfacial adhesion. As compared to composites reinforced with black filler alone, all hybrid composites had lower peak heights, as shown in Figure 13. Furthermore, ACM composites reinforced with 6 phr HNT content had the lowest peak height, indicating that a large proportion of ACM chains were restricted in their mobility. The temperature that corresponds to the highest point, tanδ, shows the glass transition (Tg). It can be seen in Figure 12 that an ACM composite filled with a single filler (CB) has a Tg value of −25.2°C. In ACM composites reinforced with twin fillers, there is also a noticeable movement in Tg values towards higher values.Figure 13
Deviation of loss factor (tanδ) vs. temperature.Furthermore, ACM composites containing 6 phr HNT loading had the highestTg value of −22.7°C of all the composites studied. The motility of ACM chains in the vicinity of the twin filler network was reduced by a homogeneous distribution of black filler and tubular HNT in the ACM matrix, as well as increased HNT-ACM-CB interfacial contacts. As a result, the Tg value of all ACM composites reinforced with hybrid reinforcement is shifting towards higher values. It was also discovered that ACM composites with 8 phr and 10 phr of HNT content had lower Tg values than ACM composites with 6 phr HNT reinforcement. Agglomerates are formed when the nanotubular filler content exceeds 6 phr. As a result, the free volume of the matrix enclosing these agglomerates is increased, resulting in easy chain motion and a reduction in the Tg of the ACM composites [48]. NBR-CB-HNT [36] and natural rubber-CB-nanoclay [49] hybrid nanocomposites showed similar results.


## 4.6.1. 1 The Influence of Temperature on Storage Modulus

The storage modulus of a composite is a measure of its stiffness. Figure12 and Table 5 show the storage modulus (E′) of the various ACM composites investigated.Figure 12
Variation of storage modulus (E′) with temperature for ACM composites.Table 5
Dynamic mechanical properties of ACM composites.
SamplesStorage modulus (MPa) at different temperatures (°C)Tg (°C)−70−50−30305070CB708348.935062.641167.0721.0315.6712.13−25.2CB60-HNT28759.216513.601441.0129.6216.6913.96−24.2CB60-HNT48844.476599.031526.4435.4119.0216.80−23.9CB60-HNT68891.726638.601566.0137.6220.6818.46−22.7CB60-HNT88834.175742.791516.0132.4018.6916.46−23.1CB60-HNT108774.266050.391456.0331.6216.6914.96−24.8It is worth noting that the addition of HNT filler to the ACM matrix results in a significant increase inE′. When compared to a CB-filled ACM composite, all ACM hybrid nanocomposites are always shown to have E′ values that are higher at all temperatures. In comparison to CB-filled ACM composites at the same temperature, the storage modulus of ACM hybrid nanocomposites containing 2, 4, 6, 8, and 10 phr of HNT increased by 41 percent, 69 percent, 79 percent, 54 percent, and 50 percent, respectively, at 30°C. Higher cross-link creation can be attributed to the superior performance of ACM composites reinforced with both HNT and CB, and it is also a strong indication of HNT’s reinforcing function. Increases in HNT content beyond 6 phr invariably result in poorHNT dispersion, poor compatibility between the filler and matrix, increased filler-filler interaction, and the eventual development of agglomerates. As a result, the free volume of the matrix enclosing these agglomerates is increased, resulting in increased polymer chain flexibility and a decrease in the storage modulus of the ACM composites [47].


## 4.6.2. The Influence of Temperature on the Loss Factor

Dynamic mechanical analysis is also used to assess the effect of fillers on the mobility of adjacent elastomeric chains. During the glass transition, a large amount of energy is dissipated due to the viscous movement of long-range polymer chains. The peak height of the damping term loss factor depicts the mobility of the polymer segments (tanδ). Any decrease in the peak height of the loss factor indicates that the mobility of polymer segments is restricted. In elastomeric composites, such a restriction correlates to an increase in interfacial adhesion. As compared to composites reinforced with black filler alone, all hybrid composites had lower peak heights, as shown in Figure 13. Furthermore, ACM composites reinforced with 6 phr HNT content had the lowest peak height, indicating that a large proportion of ACM chains were restricted in their mobility. The temperature that corresponds to the highest point, tanδ, shows the glass transition (Tg). It can be seen in Figure 12 that an ACM composite filled with a single filler (CB) has a Tg value of −25.2°C. In ACM composites reinforced with twin fillers, there is also a noticeable movement in Tg values towards higher values.Figure 13
Deviation of loss factor (tanδ) vs. temperature.Furthermore, ACM composites containing 6 phr HNT loading had the highestTg value of −22.7°C of all the composites studied. The motility of ACM chains in the vicinity of the twin filler network was reduced by a homogeneous distribution of black filler and tubular HNT in the ACM matrix, as well as increased HNT-ACM-CB interfacial contacts. As a result, the Tg value of all ACM composites reinforced with hybrid reinforcement is shifting towards higher values. It was also discovered that ACM composites with 8 phr and 10 phr of HNT content had lower Tg values than ACM composites with 6 phr HNT reinforcement. Agglomerates are formed when the nanotubular filler content exceeds 6 phr. As a result, the free volume of the matrix enclosing these agglomerates is increased, resulting in easy chain motion and a reduction in the Tg of the ACM composites [48]. NBR-CB-HNT [36] and natural rubber-CB-nanoclay [49] hybrid nanocomposites showed similar results.


## 5. Conclusion

The effect of HNT filler on the thermal properties of black filler-reinforced ACM composites was investigated in this study. When compared to a standard composite, composites containing twin fillers displayed excellent thermal breakdown properties and outstanding anti-oxidation performance. Furthermore, the use of differential and integral approaches to assess thermal degradation kinetic parameters revealed an increase in the activation energy of ACM composites when a tubular nanofiller was added. The storage modulus was increased by 79 percent at 30°C when 10 phr of black filler was replaced with 6 phr of tubular HNT, according to dynamic mechanical analysis. This work is expected to pave the way for the use of HNT as reinforcement in ACM rubber, and it also has great potential for rubber seal manufacturers.



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*Source: 8366665-2022-12-29.xml*