Sustainable Replacement of Petroleum Wax in Tyre Compounds

In the tyre industry, natural rubber and various synthetic rubbers are extensively utilized to meet the diverse performance demands of different vehicle types and operating conditions. The primary synthetic rubbers employed in tyre manufacturing include Styrene- Butadiene Rubber (SBR), Poly Butadiene Rubber (PBR), and both halogenated and nonhalogenated forms of isoprene isobutylene rubber. These materials are selected for their distinct mechanical properties, processing characteristics, and resistance to environmental degradation. The composition of rubber varies depending on the type of tyre; for instance, passenger car radial tyres typically consist of 19% natural rubber and 24% synthetic rubber, whereas Truck and Bus Radial (TBR) tyres, which require higher durability and load-bearing capacity, contain approximately 34% natural rubber and 11% synthetic rubber. This blend of rubber types ensures optimal balance between flexibility, durability, and performance under varying load and temperature conditions. Natural rubber, SBR, and PBR share a common structural feature-unsaturated carbon-carbon double bonds-which render them particularly vulnerable to oxidative degradation when exposed to environmental stressors such as heat and Ultra Violet (UV) radiation [1,2]. This oxidative stress initiates chain scission, where the main polymer chains break down, significantly reducing the material’s mechanical strength and overall durability [3,4]. To mitigate these degradation effects, the tyre industry employs a range of antioxidants that inhibit oxidation processes during both storage and operation. Among the most widely used antioxidants are TMQ (2,2,4-Trimethyl-1,2-Dihydroquinoline), 6PPD (N(1,3-dimethyl-butyl)-N’-phephenylenediamine), and DTPD (N, N’-ditolyl-pphenylenediamine) [5-8].

These stabilizers function by reacting with free radicals and oxygen species at the rubber surface, thereby preventing further attack on the polymer backbone. A dynamic equilibrium is maintained as these antioxidants migrate from the interior to the surface of the rubber compound whenever surface concentrations decrease, ensuring continuous protection throughout the tyre’s service life. In addition to oxidative degradation, rubber compounds are highly susceptible to attack by atmospheric ozone, which can cause surface cracking, particularly in tyre sidewalls. To combat this, petroleum-derived waxes-composed of a mixture of hydrocarbons with carbon chain lengths ranging from C16 to C60-are widely used in the tyre industry. These waxes function as physical antiozonants, forming a thin surface layer under static conditions that acts as a barrier to ozone, thereby protecting the underlying rubber polymer [9-14]. The effectiveness of these waxes is highly dependent on their molecular structure, molecular weight, concentration in the rubber matrix, and environmental factors such as ambient temperature and humidity [15-18]. Ideally, the wax should migrate at a rate sufficient to maintain a protective surface film without excessive accumulation. However, imbalances in this migration can pose aesthetic and functional challenges. Excessive wax blooming can result in a white, chalky appearance on the tyre surface, detracting from its visual appeal. Conversely, insufficient wax migration can leave the rubber vulnerable to ozone-induced cracking, compromising the tyre’s longevity and structural integrity [19-21].

To address the dual challenges of sustainability and performance, attention has increasingly turned toward renewable, non-petroleum-based alternatives that can serve similar protective functions. One promising candidate is sorbitan monostearate, a biodegradable surfactant known for its surface-active properties. Preliminary investigations have identified sorbitan monostearate as a potential substitute for traditional petroleum-based waxes in tyre compounds. Not only does this compound offer a more environmentally responsible solution, but it also has the potential to improve the visual aesthetics of tyres while maintaining or enhancing their resistance to ozone and oxidative degradation. This introduction sets the foundation for evaluating sorbitan monostearate as a viable, sustainable alternative within modern tyre formulations.

Material

The sidewall of a passenger car tyre is typically composed of a blend of NR and BR, reinforced with N550-grade carbon black. To enhance durability and resistance to oxidative degradation, the compound incorporates a combination of antioxidants, including 6PPD, TMQ, and DPPD. Vulcanization of the rubber matrix is facilitated by activators such as zinc oxide and a blend of vulcanization accelerators. In this study, an optimized vulcanization system was employed, utilizing a combination of N-Tert-Butyl- 2-Benzothiazole Sulfenamide (TBBS) and N-Cyclohexyl-2- Benzothiazole Sulfenamide (CBS), along with appropriate vulcanizing agents, to achieve the desired curing rate and crosslink density. For comparative evaluation, a control compound containing conventional petroleum-based wax was prepared. In contrast, the experimental formulations involved partial and complete replacement of the petroleum-based wax with sorbitan monostearate, a sustainable, non-petroleum-based alternative. Figure 1 illustrates the chemical structures of [A] sorbitan monostearate and [B] a representative hydrocarbon commonly found in petroleum-based wax. The detailed composition of all raw materials used in the formulations is provided in Table 1.

Figure 1:Chemical structure of [A] sorbitan monostearate and [B] hydrocarbon present in wax.

Table 1:Details of raw material used.

Material

The characterization of commercial-grade petroleum-based wax and sorbitan monostearate was carried out using a range of advanced analytical techniques to gain detailed insights into their chemical and physical properties. To analyze the hydrocarbon content of the wax sample, Gas Chromatography coupled with a Flame Ionization Detector (GC-FID) was employed, following the standardized procedure outlined in ASTM D5442. Approximately 10mg of the wax was carefully dissolved in cyclohexane under controlled warm conditions to ensure complete solubilization. Once the solution reached room temperature, a 2 μL aliquot was injected into the gas chromatograph. The specific operating conditions for the gas chromatography analysis-including temperature programming, carrier gas flow rate, and detector parameters-are provided in Table 2. The commercial grade sorbitan monostearate was characterized based on several key parameters relevant to its application in rubber compounding. The melting point was determined using a melting point apparatus from M/s Mettler Toledo, offering precise insight into its thermal behavior. To evaluate its chemical characteristics, the acid and saponification numbers were measured following ASTM D1980 and ASTM D1962, respectively, both of which are standard methods for determining the chemical reactivity and quality of fatty acid esters. Additionally, the iodine number-indicative of the level of unsaturation-was determined following ASTM D1959, providing critical information on the stability and potential reactivity of the compound within rubber formulations. To further investigate the molecular structure and functional groups present in sorbitan monostearate, Fourier- Transform Infra Red (FTIR) spectroscopy was performed using the Spectrum 400 instrument from M/s Perkin Elmer, following ASTM D2702. This analysis aided in confirming the presence of functional groups relevant to antiozonant performance.

Table 2:Gas chromatography conditions during wax analysis.

The mixing process for the identified passenger car sidewall compound follows a multistage approach, comprising two primary phases: master mixing and final mixing. Both stages are carried out with precision using a 1.8-liter intermixer manufactured by Ferrel. During the master mixing phase, the process begins with the masticating of rubber components at a rotor speed of 60 Revolutions Per Minute (rpm) for 20 seconds. This step helps to soften and homogenize the rubber matrix. Subsequently, the mixer is loaded with key ingredients, including carbon black, activators, antioxidants, and processing aids. After 80 seconds of blending, process oil is added to the mixture, and mixing continues for an additional 60 seconds to ensure thorough incorporation. At this point, a ram scrap is performed to manage material flow within the mixer properly. The mixing is continued until the compound reaches a temperature in the range of 145-155 °C, at which stage the master batch is discharged from the mixer. The final mixing phase involves reintroducing the prepared master compound into the mixer, along with the accelerator and vulcanizing agents. This stage is conducted at a reduced rotor speed of 30 rpm for a total of 120 seconds to facilitate uniform dispersion of the curatives. At the 60-second mark, a ram scrap is again carried out to maintain consistent mixing conditions and avoid material accumulation. A detailed overview of the ingredient composition is provided in Table 3.

Table 3:Mixing Recipe (in PHR).

During the tyre manufacturing process, the sidewall compound undergoes an extrusion stage, which shapes the material by applying heat and shear. This softens the compound, allowing it to be formed into the desired profile. To evaluate the processability of the compound, Mooney viscosity tests were conducted on three different formulations-Control, Exp-1, and Exp-2-using the Premier Mooney Viscometer instrument from Alpha Technologies, Ohio, USA, in accordance with ASTM D1646. In addition to viscosity, the scorch safety time of the compound was assessed by measuring the time required for a two-unit rise in torque (ts2), indicating the onset of vulcanization. Cure characteristics of the formulations were further analyzed by measuring the times required to reach 10%, 40%, and 90% cure levels (tc10, tc40, and tc90, respectively). These values provide crucial insight into the vulcanization behavior of the compounds. All cure data were obtained using the Premier Moving Die Rheometer, also from Alpha Technologies, following standard procedures. To determine the impact of the selected antiozonant on the mechanical properties of the tyre sidewall, various stress-strain characteristics were evaluated. These included measurements of stress at 100% and 300% strain, elongation at break, and tensile strength for each of the three formulations. Testing was carried out using the Universal Testing System Z010 from Zwick GmbH, following ASTM D412. The study also examined the dynamic mechanical properties of the sidewall compounds to assess how the experimental antiozonant influences performance under varying thermal conditions. Key parameters such as storage modulus (E′), loss modulus (E″), and tan δ (the ratio of E″ to E′) were measured at three different temperatures: 30 °C, 70 °C, and 100 °C. These tests were performed using the DMA+1000 from ACOEM, France. The selected temperatures are critical for evaluating performance attributes such as dry grip (30 °C), fuel efficiency (70 °C), and heat build-up during dynamic operation (100 °C). Finally, essential physical performance parameters of the tyre sidewall-specifically flexibility and resistance to ozone-induced degradation-were assessed. This was accomplished through fatigue-to-failure testing, which simulates long-term mechanical stress, and ozone resistance testing to evaluate the compound’s ability to withstand environmental cracking. These evaluations were conducted in strict accordance with ASTM D4482 and ASTM D1149, ensuring the validity of the tyre’s performance under expected operational conditions.

Gas chromatography analysis of commercial wax provides valuable insights into its composition, revealing a complex mixture of hydrocarbons. As illustrated in Figure 2, the carbon chain distribution ranges from C19 to C58, with C31 being the most abundant hydrocarbon present. This distribution is particularly important in understanding the behavior of wax in applications such as tyre manufacturing. Throughout the service life of a tyre, these hydrocarbons gradually migrate toward the outer surface. The migration behavior is influenced by factors such as molecular size and solubility within the rubber matrix. Smaller molecules, due to their higher solubility, exhibit reduced migration, whereas larger molecules, although more inclined to migrate, are limited by their bulky structure. Notably, hydrocarbons in the C30-C33 range migrate more efficiently and form an early-stage protective layer on the tyre surface. This thin film plays a critical role in shielding the rubber backbone from ozone attack, thereby minimizing oxidative degradation. Furthermore, these specific hydrocarbon fractions tend to crystallize at room temperature in an orthorhombic lattice structure. This crystallization leads to the formation of a dull, whitish appearance on the tyre sidewall, which, although indicative of protective film formation, can negatively affect the visual aesthetics of the tyre.

Figure 2:Carbon chain distribution profile of petroleum-based wax.

The chemical characterization of sorbitan monostearate reveals a melting point of 64 °C. This thermal property ensures that the compound melts and disperses effectively within the rubber matrix during the master mixing stage, promoting uniform distribution and consistent performance. The acid number of the identified antiozonant is 3.99mg KOH/g, indicating a minimal presence of unreacted stearic acid. In contrast, the saponification number is 153.6mg KOH/g, confirming that the compound is predominantly composed of esters. This combination of a low acid number and a high saponification number suggests that most of the compound has undergone a reaction. The unsaturation content is 1.97g/100 g, indicating that the compound is largely composed of saturated bonds, which enhances its stability in rubber applications. Fourier Transform Infra Red (FTIR) spectroscopy further validates the chemical structure. Characteristic absorption peaks at 1068cm⁻¹ and 1179cm⁻¹ confirm the presence of ether (-C-O-C-) bonds. A distinct peak at 1737cm⁻¹ indicates the presence of a carbonyl group (C=O), while the broad absorption at 3407cm⁻¹ corresponds to hydroxyl (-OH) groups. Additionally, peaks at 2850cm⁻¹, 2917cm⁻¹, and 722cm⁻¹ are consistent with the presence of saturated hydrocarbon chains. These spectral features, as illustrated in Figure 3, provide comprehensive confirmation of the chemical composition of sorbitan monostearate, which includes saturated hydrocarbons, carbonyl, ether, and hydroxyl functional groupsvalidating its suitability as a non-petroleum-based antiozonant in rubber compounds.

Figure 3:FTIR spectrum of sorbitan monostearate.

Both the control compound, which incorporates petroleumbased wax, and the experimental compounds containing sorbitan monostearate underwent the same rigorous testing protocols to ensure the comparability of results. One of the primary parameters evaluated was the Mooney viscosity, which measures a rubber compound’s resistance to deformation. Notably, the Mooney viscosity for both the control and experimental compounds was identical, registering at 49-51 Mooney Units (MU). This similarity is likely due to the comparable melting points of the two antiozonantspetroleum- based wax and sorbitan monostearate-both of which melt at approximately 64 °C. As the viscosity testing was conducted at 100 °C, both materials exhibited similar levels of flowability. This consistency is crucial for maintaining process stability during the extrusion of tyre sidewalls. Furthermore, rheometric analysis of the rubber compounds revealed that the addition of sorbitan monostearate did not significantly alter the cure curve pattern, as illustrated in Figure 4. The rheological parameters, summarized in Table 4, show that the scorch time (ts₂) for the control compound was 4.15 minutes, while the experimental compounds Exp-1 and Exp- 2 showed slightly reduced scorch times of 4.03 and 4.05 minutes, respectively. These minor variations suggest that all formulations can be processed under the same conditions without an increased risk of premature vulcanization (scorching). In terms of optimum cure time (tc₉₀), both experimental compounds aligned closely with the control, further indicating that sorbitan monostearate does not interfere with the vulcanization process.

Figure 4:Rheology curve of control with wax, Exp-1 for partial substitution and Exp-2 representing complete substitution of wax.

Table 4:Rheological properties of control, Exp-1 and Exp-2 compounds.

Chemical characterization of sorbitan monostearate supports this observation; the compound lacks acidic functional groups that could retard vulcanization, as well as unsaturated bonds that might accelerate it. This non-reactive behavior was consistently confirmed through the rheometric evaluations of the experimental samples [22]. As illustrated in Figure 5 and summarized in Table 5, the experimental compounds-where petroleum-based wax was partially (Exp-1) and fully (Exp-2) substituted with sorbitan monostearate-exhibited stress-strain characteristics that closely matched those of the control compound. Across all three formulations, stress at 100% strain ranged from 1.2 to 1.4MPa, while stress at 300% strain fell between 4.9 and 5.5MPa. These values indicate consistent stiffness and elasticity under tensile loading. Tensile strength across all samples remained stable in the range of 15.0 to 15.4MPa, demonstrating that the overall durability and resistance to breaking under load were unaffected by the use of sorbitan monostearate. The elongation at break showed a slight increase in the experimental compounds, ranging from 659% to 684%, suggesting a modest improvement in flexibility. Shore A hardness remained consistent at 51 to 52, indicating no significant difference in surface firmness. These results collectively confirm that the incorporation of sorbitan monostearate, whether partially or fully, does not negatively affect the mechanical performance of the rubber compound.

Figure 5:Stress-strain curve of control with wax, Exp-1 for partial substitution and Exp-2 representing complete substitution of wax.

Table 5:Physical (stress-strain) properties of control with wax, Exp-1 for partial substitution and Exp-2 representing complete substitution of wax.

The similar stress-strain profiles indicate that sorbitan monostearate integrates well within the rubber matrix without disrupting the vulcanization process or structural cohesion. Its function appears to be that of a physically dispersed, nonreactive plasticizer-similar to petroleum-based wax-offering the same level of flexibility, durability, and hardness required for tyre sidewall applications. This consistency across all key mechanical parameters supports the potential of sorbitan monostearate as a viable, sustainable alternative to conventional waxes in tyre compounding. The evaluation of dynamic mechanical properties, depicted in Figure 6, revealed that the tan delta (tan δ) values at temperatures of 30 ℃, 70 ℃, and 100 ℃ remained consistent across the control and experimental compounds (Exp-1 and Exp-2). This indicates that the addition of sorbitan monostearate yields dynamic behavior comparable to that of petroleum-based wax. At 30 ℃, the similar tan δ values suggest that traction properties, important for wet or dry grip, are unaffected. At 70 ℃, it implies that rolling resistance, which directly impacts fuel efficiency, is not altered. Furthermore, at 100 ℃, the consistent values indicate that heat build-up during prolonged use, crucial for long-term durability, remains well-controlled. These findings demonstrate that sorbitan monostearate behaves similarly to traditional waxes in terms of dynamic mechanical performance, confirming its potential as a sustainable alternative without compromising tyre performance.

Figure 6:Dynamic mechanical properties of control with wax, Exp-1 for partial substitution and Exp-2 representing complete substitution of wax.

To thoroughly investigate the lifespan of tyre sidewalls, a fatigue-to-failure test was conducted on a control compound as well as two experimental compounds, Exp-1 and Exp-2. During this test, a 100% strain was applied at a consistent rate of 100 cycles per minute until the specimens failed. The results revealed that all compounds demonstrated impressive durability, each capable of withstanding over 150 kilocycles (kC) before failure occurred. In addition, a detailed study was carried out to assess the effects of ozone exposure on the primary properties of the chemical agents involved in safeguarding the rubber backbone against oxidative degradation. The testing was conducted at 40 ℃ with an ozone concentration of 50 Parts Per Hundred Million (pphm). After 24 hours of exposure, cracks of varying severity were observed on the surface of all samples, including the control, Exp-1, and Exp- 2, as shown in Figure 7. The exposure was then extended to 72 hours, and it became evident that the control compound exhibited the most severe ozone-induced cracking, followed closely by Exp- 1, with Exp-2 showing the least damage, demonstrating a clear difference in resistance to ozone degradation [23].

Figure 7:Optical Microscopy images of control with wax, Exp-1 for partial substitution and Exp-2 representing complete substitution of wax after 24hr (LHS) and 72hr (RHS) ozone exposure at 40 ℃/50 PPHM concentration in Bent loop condition.

The study concluded that sorbitan monostearate is a viable alternative to petroleum-based wax in tyre sidewalls made from a NR/BR rubber matrix. It exhibited comparable rheological, stressstrain, and dynamic mechanical properties to the control compound, maintaining similar performance characteristics without disrupting vulcanization. Sorbitan monostearate also showed superior ozone resistance, reducing cracking and preventing the aesthetic issues associated with hydrocarbon migration, such as surface whitening. The fatigue-to-failure tests demonstrated that all compounds with sorbitan monostearate were highly durable, with no significant difference in durability. Overall, sorbitan monostearate provides enhanced protection against ozone degradation and is a sustainable alternative to petroleum-based wax in tyre formulations. Future research should focus on its long-term diffusivity and stability in rubber matrices to fully assess its long-term effectiveness and feasibility.

1. Ossefort ZT (1958) ASTM Special Technical Publication 229: 38.
2. Lattimer RP, Layer RW (1990) Protection of rubber against ozone. Rubber Chemistry and Technology 63(3): 426-450.
3. Tucker H (1985) ASTM Special Technical Publication 229: 269.
4. Huntink NM, Datta RN, Noordermeer JWM (2004) Addressing durability of rubber compounds. Rubber Chemistry and Technology 77(3): 476-511.
5. Hu X, Zhao HN, Tian Z, Peter KT, Dodd MC, et al. (2022) Transformation product formation upon heterogeneous ozonation of the tire rubber antioxidant 6PPD (N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine). Environ Sci Technol Lett 9(5): 413-419.
6. Sugandha S, Satish A, Somaiya NKJ (2020) Optimisation and evaluation of 2,2,4-trimethyl-1,2-dihydroquinoline (TMQ) and alkyl derivative of pphenylene diamine (6PPD) and characterisation of rubber valcanisates. Research Journal of Chemistry and Environment 24(4): 107-117.
7. Budiarto (2017) The effect of antioxidant concentration of N-isopropyl-N-phenyl-p-phenylenediamine, and 2,2,4-trimethyl-1,2-dihydroquinoline and mixing time of physical properties, thermal properties, mechanical properties and microstructure on natural rubber compound. International Conference on Chemistry, Chemical Process and Engineering, Yogyakarta, Indonesia, 1823: 020068.
8. Xu J, Hao Y, Yang Z, Li W, Xie W, et al. (2022) Rubber antioxidants and their transformation products: environmental occurrence and potential impact. Int J Environ Res Public Health 19(21): 14595.
9. Ignatz-Hoover F, To BH, Datta RN, De Hoog AJ, Huntink NM, et al. (2003) Chemical additives migration in rubber. Rubber Chemistry and Technology 76(3): 747-768.
10. Choi SS (1999) Migration behaviors of wax to surface in rubber vulcanizates. J Appl Polym Sci 73(13): 2587-2593.
11. Choi SS (1999) Wax barrier effect on migration behaviors of antiozonants in NR vulcanizates. Elastomer 34(2): 147-155.
12. Torregrosa-Coque R, Álvarez-García S, Martín-Martínez JM (2011) Effect of temperature on the extent of migration of low molecular weight moieties to rubber surface. Int J Adhes Adhes 31(1): 20-28.
13. Ansarifar A, Critchlow GW, Guo R, Ellis RJ, Haile-Meskel Y (2009) Assessing effect of the migration of a paraffin wax on the surface free energy of natural rubber. Rubber Chemistry and Technology 82(1): 113-120.
14. Choi SS (1999) Migration behaviors of antiozonants to the surface in NR vulcanizates, depending on the season: The effect of wax. J Appl Polym Sci 71(12): 1987-1993.
15. Arabit J, Pajarito BB (2015) Effect of ingredient loading on surface migration of additives in a surfactant-loaded natural rubber vulcanizate. Adv Mat Res 1125: 64-68.
16. Yasin UQ, Kamarun D, Said CMS, Samsuri A (2015) Effect of compounding ingredients and crosslink concentration on blooming rate of natural rubber compounds. Adv Mat Res 1134: 50-55.
17. Li GY, Koenig JL (2005) A review of rubber oxidation. Rubber Chemistry and Technology 78(3): 355-390.
18. Choi SS, Im SH, Park JH, Kim JS (2009) Analysis of wax solubility of rubber vulcanizates using wax solution in toluene and molten wax. Polym Test 28(7): 696-701.
19. Coran AY (2003) Chemistry of the vulcanization and protection of elastomers: A review of the achievements. Journal of Applied Polymer Science 87(1): 24-30.
20. Jiang S, Li Y, Yong Z (2024) Compatibility analysis between natural rubber/polybutadiene rubber and protective waxes based on experiments and materials studio simulations. Journal of Applied Polymer Science 141(23): e55468.
21. Santiago DEO, Pajarito BB, Aparre MJA, Biolena KJE, Condez YCW (2016) Ambient blooming behavior and mechanical properties of vulcanized natural rubber loaded with non-ionic surfactants. Diffusion Foundations 9: 90-99.
22. Stiehlerz DR, Wakelin HJ (1947) Mechanism and theory of vulcanization. Ind Eng Chem 39(12): 1647-1654.
23. Sandstrom, Paul Harry Cuyahoga Falls (2014) Rubber containing hydrophilic sorbitan monostearate and tackifying resin and tire with sidewall thereof, EP2 022 823B1.