Rubbers can be applied in several products, such as in tyres, tubes and airbags, among others (Zhang et al., Reference Zhang, Xing, Li, Xie, Huang and Wu2019; Kazemi et al., Reference Kazemi, Mighri and Rodrigue2022). However, most rubbers in their raw state have poor mechanical and thermal properties, so they need to be reinforced (Wang et al., Reference Wang, Fu, Peng, Xiao, Li and Huang2019; Sarma et al., Reference Sarma, Federico, Nzulu, Weydert, Verge and Schmidt2021; Kazemi et al., Reference Kazemi, Mighri and Rodrigue2022). A simple, popular and economical method to improve the mechanical properties of rubber compounds is using fillers (Zhang et al., Reference Zhang, Liu, Zhang, Cheng and Lu2012; Alwis et al., Reference Alwis, Kottegoda and Ratnayake2020). Fillers are also added to rubber compounds to decrease the costs of and facilitate their production (Arroyo et al., Reference Arroyo, López-Manchado and Herrero2003). Common fillers include carbon black, calcium carbonate, clay minerals, silica and calcium silicate (Moonchai et al., Reference Moonchai, Moryadee and Poosodsang2012). Among these, carbon black is widely used as a reinforcing filler in the rubber industry due to its ability to improve the mechanical properties of rubber compounds (Seo et al., Reference Seo, Lee, Lee and Song2018; Alwis et al., Reference Alwis, Kottegoda and Ratnayake2020; Utrera-Barrios et al., Reference Utrera-Barrios, Perera, León, HernándezSantana and Martínez2021). However, carbon black derives from petroleum, the use of which can cause environmental pollution and the emission of greenhouse gases (Seo et al., Reference Seo, Lee, Lee and Song2018; Kazemi et al., Reference Kazemi, Mighri and Rodrigue2022). These drawbacks have increased interest in using environmentally friendly fillers such as clay minerals (Moonchai et al., Reference Moonchai, Moryadee and Poosodsang2012; Pajtášová et al., Reference Pajtášová, Mičicová, Ondrušová, Pecušová, Feriancová and Raník2017; Alwis et al., Reference Alwis, Kottegoda and Ratnayake2020).
Montmorillonite (Mnt) is an essential clay mineral (Liu et al., Reference Liu, Zhou, Chi, Feng, Ding and Liu2019; Wójcik-Bania et al., Reference Wójcik-Bania and Matusik2021) belonging to the smectite group that is a member of 2:1 phyllosilicate family (Archibong et al., Reference Archibong, Orakwe, Ogah, Mbam, Ajah and Okechukwu2023; Safarik & Prochazkova, Reference Safarik and Prochazkova2024). Mnt is a layered crystalline material, consisting of two tetrahedral sheets of silica with an octahedral sheet of alumina (Miedzianowska et al., Reference Miedzianowska, Masłowski, Rybínski and Strzelec2021; Archibong et al., Reference Archibong, Orakwe, Ogah, Mbam, Ajah and Okechukwu2023). The octahedral sheet is sandwiched between the two tetrahedral sheets. The Mnt layer thickness is ∼1 nm along the regular space in its interlayer space (Chrissafis & Bikiaris, Reference Chrissafis and Bikiaris2011; Liu et al., Reference Liu, Zhou, Chi, Feng, Ding and Liu2019; Madejova et al., Reference Madejova, Barlog, Slany, Bashir, Scholtzova and Tunega2023); the lateral dimensions of the layers are between 30 nm and several microns or larger (Chrissafis & Bikiaris, Reference Chrissafis and Bikiaris2011). The molecular formula of Mnt is Mx(Al4–xMgx)Si8O20(OH)4, where M is a monovalent cation (Krol-Morkisz & Pielichowska, Reference Krol-Morkisz, Pielichowska, Pielichowski and Majka2018; Olszewski et al., Reference Olszewski, Lawniczak, Kosmela, Strakowski, Mielewczyk-Gryn and Hejna2023). The aluminium ions (Al3+) can be partially replaced with divalent magnesium cations (Mg2+) in the octahedral sheet, and the Si4+ can be replaced with Al3+ in the tetrahedral sheet. In these situations, negative charges are created and distributed in the platelets. These charges can be counterbalanced by positive ions such as Ca2+ and Na+ (Chen et al., Reference Chen, Sun, Zhang, Zhou, Liu and Zeng2023; Kovalchuk, Reference Kovalchuk2023; Su et al., Reference Su, Zhang, Zhou and Hou2023). The monovalent interlayer cations of Mnt cause the swelling of this substance due to water absorption in the interlayer space (Krzaczkowska et al., Reference Krzaczkowska, Fojud, Kozak and Jurga2005; Yotsuji et al., Reference Yotsuji, Tachi, Sakuma and Kawamura2021; Christidis et al., Reference Christidis, Chryssikos, Derkowski, Dohrmann, Eberl and Joussein2023).
Mnt can be used as a reinforcing filler in rubber compounds (Fathurrohman et al., Reference Fathurrohman, Rugmai, Hayeemasae and Sahakaro2020; Archibong et al., Reference Archibong, Orakwe, Ogah, Mbam, Ajah and Okechukwu2023). Specific properties of rubber compound such as their flame retardancy and barrier properties can be improved by using fillers such as these (Ahmadi et al., Reference Ahmadi, Ghanbari, Shabani Moghaddam and Naghizadeh2022; Zhou et al., Reference Zhou, Niu, Liu, Chen, Li, Gates and Zhou2022; Antosik et al., Reference Antosik, Mozelewska, Musik, Miadlicki, Srenscek-Nazzal and Wilpiszewska2023). In addition, particles of Mnt have a greater surface area than other fillers, thereby reducing the amount of filler that is needed to fill such rubber compounds (Lan et al., Reference Lan, Bakar, Azahari, Ariff and Chujo2012). However, the charged nature of Mnt can produce hydrophilic surfaces, leading to incompatibility of Mnt with hydrophobic polymers (Das et al., Reference Das, Stockelhuber, Jurk, Jehnichen and Heinrich2011; Miedzianowska et al., Reference Miedzianowska, Masłowski, Rybínski and Strzelec2021). Hence, to overcome this problem and gain compatibility and uniform dispersion without agglomeration in the polymer matrix, the surface treatment of Mnt is essential (Krol-Morkisz & Pielichowska, Reference Krol-Morkisz, Pielichowska, Pielichowski and Majka2018; Gua et al., Reference Gua, Liu, Gates and Zhou2020; Wójcik-Bania et al., Reference Wójcik-Bania and Matusik2021). Various methods have been used for the treatment of Mnt. Radiation (Mohamed et al., Reference Mohamed2013), cation-exchange reactions (Xi et al., Reference Xi, Frost and He2007; Jincheng et al., Reference Jincheng, Youcheng, Kai, Yibo, Chen and Yi2013), using corrosive microorganisms (Meng et al., Reference Meng, Li, Zou, Li, Fu and Zhang2022) and applying organic modifiers are the most important such approaches. Today, particular attention is being paid to the use of organic modifiers. Various materials have been used for this purpose, such as surfactants (Zhou et al., Reference Zhou, Chen, Jiang, Lu, Zhou and Yin2009; Wójcik-Bania et al., Reference Wójcik-Bania and Matusik2021; Yu et al., Reference Yu, Liu, Wang, Li, Zhang and Huang2021; Arabmofrad et al., Reference Arabmofrad, Jafari, Lazzara, Ziaiifar, Shahiri Tabarestani and Bahlakeh2023), salts of octadecanoic acid and dodecanoic acid (Sarier & Onder, Reference Sarier and Onder2010), octadecylamine (Pattamaprom & Jiamjitsiripong, Reference Pattamaprom and Jiamjitsiripong2012), amines (Rath et al., Reference Rath, Chaki and Khastgir2012), latex (Al-Shemmari et al., Reference Al-Shemmari, Rabah, Al-Mulla and Alrahman2013), silane derivatives (Song & Sandi, Reference Song and Sandi2001; Bertuoli et al., Reference Bertuoli, Piazza, Scienza and Zattera2014; Shuai et al., Reference Shuai, Yu, Feng, Zhong, Zhao and Chen2020; Baghitabar et al., Reference Baghitabar, Jamshidi and Ghamarpoor2023), gum rosin (Esmaeili et al., Reference Esmaeili, Rounaghi and Eckert2021), diphenylmethane diisocianate (Krol-Morkisz & Pielichowska, Reference Krol-Morkisz, Pielichowska, Pielichowski and Majka2018) and polyethylenimine and poly-2-methyl-2-oxazoline (Madejova et al., Reference Madejova, Barlog, Slany, Bashir, Scholtzova and Tunega2023), among others. Industrial use of this method may be limited due to the toxicity of these organic modifiers (Moustafa et al., Reference Moustafa, Lawandy, Rabee and Zahran2020).
Today, research is moving towards the use of more accessible and easy-to-use organic modifiers. Polypropylene glycol (PPG) is a cheap and non-corrosive material with lubricating and freeze-protection effects. It is also compatible with rubber matrices. In this work, the surface treatment of Mnt by PPG was considered using various PPG dosages. The obtained surface-treated Mnt (St-Mnt) samples were applied as dual fillers with carbon black in a natural rubber (NR)/styrene butadiene rubber (SBR) compound. Using such a dual filler can reduce carbon black consumption in rubber compounds, thereby decreasing its harmful environmental effects. The cure characteristics, mechanical properties and thermal stability of the compounds were investigated and compared to rubber compounds filled with Mnt/carbon black. This new approach has not been received sufficient attention to date, and this work aims to fill this gap in the research.
Experimental
Materials
Mnt was obtained from Sigma-Aldrich (Steinheim, Germany). PPG (Mw = 2000) and ethanol (99%) were purchased from Merck (Darmstadt, Germany). Standard Malaysian rubber (SMR) 20 as the NR sample was supplied from The Ah Yau Rubber Factory (Bedong, Malaysia). SBR 1500 (23% styrene) was supplied by Bandar Imam Petrochemical (Mahshahr, Iran). Sulfur (purity ≥ 99.7%) was obtained from Tesdak (Saveh, Iran). Aromatic oil (No. 290) was purchased from Behran Oil (Tehran, Iran). 2,2,4-Trimethy-1,2-di-hydroquinoline (TMQ) was provided by Rongcheng (Rongcheng, China). Carbon black N-330 was supplied by Iran Carbon (Ahvaz, Iran). Antilux 654 was provided from Rhein Chimie (Mannheim, Germany). N-cyclohexyl-2-benzothiazole sulfenamide (CBS; purity ≥ 98.5%) was supplied by Taizhou Chemical (Taizhou, China). Zinc oxide (purity ≥ 99.0%) was obtained from Shokohie (Qom, Iran). Stearic acid (purity ≥ 95%) was obtained from Palmoleo Sdn. Bhd. (Selangor, Malaysia). N-(1,3-dimethylbutyl)-N’-phenyl-p-phenylenediamine (6PPD; active substance ≥ 97%) was supplied by Duslo (Šaľa, Slovakia).
Methods
Attenuated total reflection Fourier-transform infrared (ATR-FTIR) spectroscopy was carried out using a FTIR spectrometer with ATR cells from Bruker (Ettlingen, Germany). Thermogravimetric analysis (TGA) was performed using TA 1500 from Scinco (Seoul, Korea); heating of the samples was carried out under an inert atmosphere (nitrogen) from ambient temperature up to 900°C at a heating rate of 20°C min–1 (for rubber compounds, after ∼550°C the nitrogen atmosphere was changed to an air atmosphere, then heating was continued to obtain a constant mass loss). Scanning electron microscopy (SEM) analysis was conducted using a Yegaiitescan Analyzer (Fuveau, France); the samples were coated with a thin gold layer before the SEM analyses. X-ray diffraction (XRD) analysis was performed using a Philips X’Pert MPD device (Eindhoven, The Netherlands) with a Cu-Kα radiation source at a wavelength of 0.154056 nm, operating at voltage of 40 kV and with current values of 30 mA. The scanning step rate was 0.02° s–1 and the 2θ angle range was between 1° and 10°. Measurement of the cure properties was performed at 160°C according to ASTM D 5289 using an MDR 900 rheometer from Hiwa (Tehran, Iran). The curing of the rubber compounds for making tablets and sheets was performed at 160°C using hot press moulds. The abrasion resistance of the samples was measured using an Abrasion Tester device from Bareiss (Oberdischingen, Germany) according to ASTM D 5963. The compression set measurements were obtained according to ASTM D 395(B) using a Gotech device (Taichung, Taiwan). Rebound resilience was assessed using a Rebound Tester device from HIWA (Tehran, Iran) according to ASTM D 7121. Ozone resistance was performed using equipment from Shandong Drick Instruments (Jinan, China) according to ASTM D 1149. These properties were measured for five specimens and the median values are reported.
Surface treatment of Mnt by PPG
A total of 30 g Mnt was dispersed in 37.5 mL water. The preparation of St-Mnt with 1 wt.% PPG was performed by dissolving 0.3 mL PPG in 3.75 mL ethanol and adding it to the aqueous suspension of Mnt. After that, the obtained mixture was agitated in an ultrasonic bath for 15 min, stirred for 2 h at 40°C and filtered. The obtained particles were washed with distilled water and then dried for 6 h at 80°C. The same procedure was performed for the preparation of the 1.5 wt.% and 2.0 wt.% PPG loadings using 0.45 and 0.60 mL PPG, respectively.
Compound recipe
For preparation of the rubber compounds, the compound ingredients were put in a two-roll mill (SYM-8, Wellshayang, 8” × 20”; Zhangjiagang, China) as follows: first, a mastication process for the reducing viscosity of the NR was implemented for 1.5 min. Second, SBR was added to the masticated NR and blended for 4.5 min. Third, other ingredients such as fillers, zinc oxide and so on were added and blended for 4.0 min. Finally, aromatic oil was added and blended for 5 min (master batch). Afterward, the rubber curatives were added to the master batch in a two-roll mill and blended for 5 min at 60°C. The compound formulation process is shown in Table 1.
Table 1. Compound formulation process.
phr = parts per hundred parts of rubber.
Results and discussion
FTIR studies
The surface treatment of Mnt was considered using FTIR. The PPG spectrum (Fig. 1a) showed five characteristic bands assigned as 3312 cm–1 for O–H stretching, 2941 and 2876 cm–1 for C–H stretching, 1456 cm–1 for C–H bending and 1085 cm–1 for C–O stretching. Mnt presented five major bands (Fig. 1b). The band at 3625 cm–1 was attributed to the O–H stretching bound to aluminium or magnesium of the Mnt (structural OH groups). The broad band at ∼3389 cm–1 and the band at 1636 cm–1 corresponded to the O–H stretching of the adsorbed water and to the O–H bending of the intercalated water, respectively. The bands at 1004 and 915 cm–1 corresponded to Si–O–Si and Si–OH stretching, respectively (Sarier & Onder, Reference Sarier and Onder2010; Songurtekin et al., Reference Songurtekin, Yalcinkaya, Ag, Seleci, Demirkol and Timur2013; Bertuoli et al., Reference Bertuoli, Piazza, Scienza and Zattera2014). In the St-Mnt samples (Fig. 1c–e), especially St-Mnt with 1 wt.% PPG, there was an increase in the band intensity at ∼1100 cm–1, which corresponded to C–H stretching, and a band at ∼2954 cm–1, which corresponded to C–O stretching. These bands provided evidence of the grafting of PPG through shifting of the band at 1636 cm–1 to a lower wavenumber (1633 cm–1) due to the decrease in OH groups in St-Mnt (Sarier & Onder, Reference Sarier and Onder2010) and the absence of a band at 3389 cm–1 related to loss of adsorbed water, thereby verifying the surface treatment with PPG and decreased hydrophilicity of Mnt.
Figure 1. ATR-FTIR spectra of (a) PPG, (b) Mnt, (c) Mnt + 1.0 wt.% PPG, (d) Mnt + 1.5 wt.% PPG and (e) Mnt + 2.0 wt.% PPG.
Thermal analysis studies
TGA was used to confirm the surface treatment of the Mnt. For PPG (Fig. 2a), the one-step decomposition was demonstrated and a mass loss of 99.87% up to 190°C was obtained. The TGA graph obtained from the Mnt sample (Fig. 2b) demonstrated the two-step decomposition that is typical of Mnt (Sarier & Onder, Reference Sarier and Onder2010; Songurtekin et al., Reference Songurtekin, Yalcinkaya, Ag, Seleci, Demirkol and Timur2013; Belousov et al., Reference Belousov, Chupalenkov, Christidis, Zakusina, Zakusin and Zakusin2021). The first step took place between ambient temperature and 300°C due to the elimination of adsorbed water from the surface and interlayer space of Mnt. The rate of mass loss up to 190°C was rapid, leading to a mass loss of 8.21%. After that, up to ∼300°C, the rate of mass loss reduced significantly, and it reached a value of 8.95%. The second step took place between 300°C and 850°C due to the dehydroxylation of the linked OH groups of the Mnt. A residual quantity of 84.65% was attained at 850°C. For the St-Mnt samples (Fig. 2c–e), a three-step decomposition was demonstrated. The first step took place between ambient temperature and 190°C. The mass losses in this temperature range of the St-Mnt samples with 1.0 wt.% PPG, 1.5 wt.% PPG and 2.0 wt.% PPG were 6.94%, 8.21% and 7.42%, respectively. The reduction of mass losses in this step, especially in the sample with 1.0 wt.% PPG, could imply a reduction of hydrophilic properties and an increase in hydrophobicity of Mnt (Zhou et al., Reference Zhou, Chen, Jiang, Lu, Zhou and Yin2009). The second step occurred between 190°C and 350°C. The mass losses of the St-Mnt samples with 1.0 wt.% PPG, 1.5 wt.% PPG and 2.0 wt.% PPG were 3.15%, 1.04% and 0.65%, respectively. These reductions, especially for the St-Mnt with 1.0 wt.% PPG, can be explained by PPG degradation that intercalated within the interlayer space of Mnt. The third step occurred between 350°C and 850°C. The mass losses of the St-Mnt samples with 1.0 wt.% PPG, 1.5 wt.% PPG and 2.0 wt.% PPG were 5.13%, 4.97% and 4.84%, respectively. The residues of the St-Mnt samples with 1.0 wt.% PPG, 1.5 wt.% PPG and 2.0 wt.% PPG at 850°C were 84.78%, 85.78% and 87.09%, respectively.
Figure 2. TGA curves: (a) PPG, (b) Mnt, (c) Mnt + 1.0 wt.% PPG, (d) Mnt + 1.5 wt.% PPG and (e) Mnt + 2.0 wt.% PPG.
SEM observations and energy-dispersive X-ray spectroscopy
SEM was applied to observe the morphology of Mnt and the St-Mnt samples. The shape and size of Mnt (Fig. 3a) showed overlapped layers that were tightly packed together and formed dense platelets. After surface treatment, the better-exfoliated state of the layers could be observed (Fig. 3b–d). This indicated the intercalation of PPG molecules in the interlayer spaces of Mnt, which resulted in increases in the interlayer spaces of the St-Mnt samples. The mass percentage of the elements that were detected using energy-dispersive X-ray spectroscopy (EDS) analyses are shown in Table 2. By comparing the structural elements of the samples, the difference between Mnt and the St-Mnt samples can be attributed to the presence of carbon in the St-Mnt samples. The presence of carbon in the St-Mnt samples could be due to the PPG grafted onto the Mnt, indicating that the surface treatment was successfully performed. It is worth noting that the highest level of carbon was obtained for the St-Mnt sample with 1.0 wt.% PPG.
Figure 3. SEM images: (a) Mnt, (b) Mnt + 1.0 wt.% PPG, (c) Mnt + 1.5 wt.% PPG and (d) Mnt + 2.0 wt.% PPG.
Table 2. Mass percentages (wt.%) of the elements detected.
XRD studies
XRD analysis was used to investigate the changes in the interlayer spaces of the Mnt after surface treatment by PPG. Mnt (Fig. 4a) showed two reflections at 1.68°2θ and 7.02°2θ, which were assigned to the interlayer platelet spaces with spaces of 6.08 and 1.46 nm, respectively. The first reflection could be attributed to the intercalation of impurities such as Fe cations into the Mnt interlayer spaces. The second reflection could be attributed to the (001) basal reflection of Mnt. In the St-Mnt samples (Fig. 4b–d), especially the St-Mnt sample with 1.0 wt.% PPG, an increase in the interlayer space of Mnt was observed due to the intercalation of PPG molecules within the interlayer space of Mnt. Hence, shifting of Mnt reflections to lower °2θ values for the St-Mnt samples occurred (Zhou et al., Reference Zhou, Chen, Jiang, Lu, Zhou and Yin2009; Songurtekin et al., Reference Songurtekin, Yalcinkaya, Ag, Seleci, Demirkol and Timur2013; Bertuoli et al., Reference Bertuoli, Piazza, Scienza and Zattera2014). These changes confirmed the successful surface treatment of Mnt by PPG. These results are shown in Table 3.
Figure 4. XRD traces: (a) Mnt, (b) Mnt + 1.0 wt.% PPG, (c) Mnt + 1.5 wt.% PPG and (d) Mnt + 2.0 wt.% PPG.
Table 3. XRD analysis results.
Dispersion of the St-Mnt samples in the rubber matrix
Dispersions of the Mnt and St-Mnt samples in the rubber compounds were studied using SEM (Fig. 5). The bright grey features in the SEM images corresponded to the Mnt aggregates in the rubber compounds. The image obtained from the Mnt (Fig. 5a) revealed undesirable agglomeration and indicated the poor dispersibility of Mnt particles in the rubber matrix due to the incompatibility of hydrophilic Mnt with the hydrophobic matrix. After surface treatment by PPG, the SEM images (Fig. 5b–d) demonstrated the smoother structure of the rubber compounds. Mnt agglomerations were removed, and the dispersions of the St-Mnt samples in the rubber matrix were improved. This confirmed the reduction in hydrophilicity and increase in hydrophobicity of Mnt. The increase in hydrophobicity resulted in improved interfacial interactions between the rubber matrix and Mnt. In addition, increasing the interlayer spaces could facilitate the dispersion of Mnt in rubber compounds.
Figure 5. SEM images of the rubber compounds based on NR/SBR filled with Mnt and St-Mnt samples: (a) Mnt, (b) Mnt + 1.0 wt.% PPG, (c) Mnt + 1.5 wt.% PPG and (d) Mnt + 2.0 wt.% PPG.
Cure characteristics
The cure characteristics of the rubber compounds are shown in Table 4.
Table 4. Rubber compound properties.
Scorch time
The St-Mnt samples filled rubber compounds, especially the St-Mnt with 1 wt.% PPG, as illustrated by the shorter scorch time (Ts2) values as compared to the Mnt-filled rubber compound. The shorter Ts2 values may be due to the exfoliated structure and uniform dispersion of St-Mnt in the rubber matrix, which could accelerate the heat transfer, facilitating the formation of cross-linkages during the cure process and speeding up the cure process.
Optimum cure time
The results showed that the optimum cure time (Tc90) values of the rubber compounds decreased with the addition of St-Mnt samples, and the shortest value was obtained when using the St-Mnt sample with 1.0 wt.% PPG. After surface treatment, the rubber–filler interaction could be improved due to the increased hydrophobicity of Mnt. Accordingly, a uniform dispersion of St-Mnt particles in the non-polar rubber matrix was obtained, which resulted in superior heat transfer through the curing process. Reducing the curing time has a significant effect on lowering energy consumption and operation costs, which is highly valuable.
Cure rate index
The cure rate index (CRI) as an indictor of the curing reaction speed was increased when using treated samples, confirming the more rapid curing system for the rubber compounds filled with St-Mnt samples, especially for the sample treated with 1.0 wt.% PPG.
Torque difference (MH – ML)
The torque difference (MH – ML) value is an indicator of the amount of pure cross-link density. It was increased when using the St-Mnt samples as fillers. The greater MH – ML values in the compounds containing the St-Mnt samples confirmed the greater cross-link densities, which were associated with increased hardness values due to the improved filler dispersion.
Mechanical properties
The mechanical properties (abrasion resistance, resilience, compression set and ozone resistance) of the compounds are reported in Table 4.
Abrasion resistances
Abrasion losses were decreased when using the St-Mnt samples; thus, abrasion resistances could be increased with surface treatment. The improved abrasion resistances for the compounds containing the treated samples, especially for St-Mnt with 1.0 wt.% PPG, are indicated by the greater hardness values and improved dispersions of the St-Mnt samples in the rubber compounds.
Resilience
The resilience value for the rubber compound with 1.0 wt.% of PPG was increased, confirming the improvement with surface treatment. PPG has a plasticizing effect that could maintain the elasticity of the rubber chains along with increasing the rubber reinforcement.
Compression set
The compression set values were decreased when applying the St-Mnt samples. The improved interfacial interaction of the rubber matrix with the St-Mnt samples as fillers resulted in improved reinforcing effects with the St-Mnt samples in comparison with Mnt, and decrease the compression set of the rubber compounds.
Ozone resistance
No cracks were observed for all of the samples, which confirmed the good ozone resistance of the compounds.
Thermal properties of the rubber compounds
The curves obtained from the TGA and differential thermogravimetric (DTG) analysis of the rubber compounds (Figs 6 & 7) showed a three-step decomposition (i.e. the same as a typical TGA of rubber compounds) corresponding to volatilization of moisture and those additives with lower boiling points such as oil and plasticizers, thermal degradation of rubber chains and carbon black combustion when the nitrogen atmosphere was changed to an air atmosphere. Mineral additives such as inorganic fillers and ZnO remain as the residue (ash content; Gupta et al., Reference Gupta, Mukhopadhyay, Baranwal and Bhowmick2013). In the second step (main decomposition), the temperatures of initial decomposition and final decomposition and the temperature at the maximum rate of degradation (T p) were considered to investigate the thermal stability of the products (Table 5). In the rubber compounds filled with St-Mnt samples, significant decreases in the initial decomposition temperature could be observed, showing that the thermal degradation of rubber compounds filled with St-Mnt samples started at lower temperatures. Furthermore, the interval between the initial and final temperatures of decomposition for St-Mnt filled compounds was increased, therefore indicating that the rate of degradation in the St-Mnt filled compounds was decreased. The decrease in initial decomposition temperature of the St-Mnt filled compounds might be due to the degradation of PPG at that temperature, and hence the St-Mnt filled compounds started to degrade at lower temperatures. The degradation of PPG as a treatment agent with an organic nature could provide catalytic effects by creating Lewis acids at the Mnt edges. By contrast, the barrier properties that were obtained via the homogeneous dispersion of St-Mnt samples as fillers in the rubber matrix increased the interval between the initial and final temperatures of decomposition, which could counterbalance the catalytic effects of the St-Mnt samples and result in improved thermal properties (Carli et al., Reference Carli, Crespo and Mauler2011).
Figure 6. TGA curves of the rubber compounds: (a) Mnt, (b) Mnt + 1.0 wt.% PPG, (c) Mnt + 1.5 wt.% PPG and (d) Mnt + 2.0 wt.% PPG.
Figure 7. DTG curves of the rubber compounds: (a) Mnt, (b) Mnt + 1.0 wt.% PPG, (c) Mnt + 1.5 wt.% PPG and (d) Mnt + 2.0 wt.% PPG.
Table 5. TGA and DTG results for the rubber compounds.
As a general trend, after surface treatment, exfoliation of Mnt and the interlayer space between the plates increased; in addition, the hydrophobic properties of the Mnt increased. Therefore, interfacial interactions between the filler and the rubber were improved, and particles of filler became more compatible with the hydrophobic rubber matrix; thus, improved reinforcing effects of the St-Mnt samples were observed compared to that of the untreated Mnt.
Conclusions
A new approach to the surface treatment of Mnt using various dosages of PPG has been assessed for its potential in the utilization of Mnt as a filler in non-polar rubber compounds. The results confirmed the successful surface treatment due to the more exfoliated structure and increased interlayer space of the St-Mnt particles. The St-Mnt/carbon black-filled rubber compounds had shorter Ts2 and Tc90 values along with increased CRI values. These results lead to reduced cure costs during the curing process due to the decreased processing time. The MH – ML values for the rubber compounds filled with St-Mnt samples were increased, meaning that more reinforced compounds were formed. The mechanical properties of the rubber compounds, including abrasion resistance, resilience and compression set, all improved after surface treatment of the Mnt particles. The thermal study of the compounds indicated that the interval between the initial and final temperatures of decomposition increased after surface treatment of the Mnt particles. This treatment improved the rubber–filler interaction through increasing the hydrophobicity of the Mnt surface, resulting in a more uniform and improved dispersion of the filler particles within the rubber matrix without agglomeration. Heat transfer was facilitated under these conditions, and the cure process became more efficient. By comparing the obtained results, the St-Mnt with 1 wt.% PPG provided the best overall outcomes in terms of the curing system, mechanical properties and thermal stability of the rubber compound. These results provide new avenues for using St-Mnt as an environmentally friendly filler in the rubber industry to improve the properties of rubber goods, especially green tyres. Future work could focus on increasing the degree of dispersion and improving the compatibility of Mnt with non-polar rubber compounds by using a dual surface treatment system containing PPG and a coupling agent that is suitable for combination with PPG.
Acknowledgements
The authors thank the Iran Rubber Research Institute for its support of this experimental work.
Competing interests
The authors declare none.
