Optimization of Natural Rubber Formulation for Military Vehicle Tread Using Filler Carbon Black Type N220/N550

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Introduction
In materials science, rubber tires have been widely used in the automotive industry.However, there are some problems behind the use of these rubber tires such as durability, load capacity, and limited performance.Rubber tires have been studied for their physical and mechanical properties for various applications, one of which is the military.Rubber tire development and utilization in military applications continue to grow along with advances in technology.This aims to improve military vehicle performance, mobility, and safety in a variety of domains.
Tire technology is a complex combination of science and engineering that combines many different disciplines [1].With technological sophistication, many companies are working to develop airless tire technology.It was the Indonesian military that succeeded in creating airless tires.These tires are used on double-cabin cars that can withstand loads up to four tons [2].Airless tires manufactured by Indonesian National Armed Force have air holes.
Tires can easily overcome a variety of proven terrain such as flat roads, rocky roads, rough rocks, and even spikes during operation [3].Airless tires have a significant impact in the field of duty, particularly for Indonesian National Armed Force support vehicles.This tire has one advantage: it is resistant to punctures caused by nails and bullets.Vehicles employed in military operations must be able to overcome the problems and impediments present on the battlefield.As a result, selecting the correct vehicle tires specifically developed for the battlefield is critical to the effectiveness of military operations.However, the current structure has flaws, such as rubber tires that have a honeycomb design that easily fractures when operated and a rubber tire formulation that is not suitable for usage in harsh terrain [4].
In this study, we focused on the properties and analysis of airless tire rubber, where data collection was carried out at the National Research and Innovation Agency (BRIN) Rubber Processing Laboratory.It is thought that by developing the best rubber material for airless tires, it will be possible to use it to solve military concerns.Natural rubber has recently been enhanced with carbon black filler and other synthetic materials [5].Modified natural rubber will have much better mechanical properties than synthetic rubber.However, the optimum filler size and content for each composition must be found to ensure the best dispersion and compatibility between the filler particles and the matrix while respecting the properties required [6].
The novelty of this research is the use of natural rubber material, namely RSS 1 (Ribbed Smoked Sheet 1) and SIR 20 (Standard Indonesian Rubber 20) for the tread of military vehicle tires which consists of fillers using CB-N220 and CB-N550, and involves additives to facilitate the compound formation process.This study investigates the utilization of material formulations to generate the required mechanical qualities and compound features.The goal of this experiment is to identify a material formulation capable of producing the required mechanical qualities that can resist the most extreme operational circumstances.With this approach, it is expected that the research results will significantly contribute to the development of military vehicle tire technology that is durable and can operate effectively in a variety of difficult environmental conditions.

Materials
All of the mixing ingredients were utilized as they were obtained from PT Multi Citra Chemindo Nusa (Indonesia), which supplied the ingredients.The main ingredients used are SIR 20 and RSS 1 rubber.The two types of natural rubber commonly used in the rubber industry, particularly in tire manufacturing have complementary physicochemical characteristics.SIR 20 stands out with its resistance to extreme temperatures, high rubber content, and low moisture content.Meanwhile, RSS 1 has tensile strength and elasticity, with unique characteristics resulting from its smoking process, creating a distinctive appearance and influencing its chemical properties and mechanical strength.
The most important step is to select an appropriate recipe for preparing a suitable mixture (Table 1).The phrase "The main component is a polymer, which always represents 100 phr" refers to the practice of measuring parts per hundred rubber (phr) in the formulation of rubber compounds [7].A formula is built on a base of polymer rubber, which interacts with other ingredients.As a result, polymer rubber is always 100 PHR, and when formulas are created or updated, the other ingredients are set to a specific number of PHR in relation to the polymer content, rather than the overall batch.
Table 1.Formula of the materials in *phr *phr = per hundred rubber

Preparation materials for rubber compounding
Basically, this experiment is divided into several phases: two-step mixing, vulcanization, surface finishing, material characterization and retrieval of data, and analysis phase.The formula relies on the type of materials used, the order of input components, the amount of material utilized, and the stirring time.A laboratory kneader (Moriyama DS3-10MWB-E, Japan) was used to create the rubber compounds.The mixing process consists of two parts.The mixing temperature was set to 100 °C, and the speed was set to 32 rpm in the first step.Furthermore, the additives used include carbon black, stearic acid, ZnO (Zinc Oxide), 6PPD (antioxidant), TMQ (antiozonant), and Processing Oil are also added to the kneader at a temperature of ± 100℃ at the same time.To strengthen physical qualities and reduce processing costs, filler can be added, while softener (oil processing) is employed to make processing easier and ensure proper mixing.The compound was run through the two-roll process six times at 70°C.After that, keep the compound aside to allow the rubber compound to rest [8].The temperature was set to 70°C for around 2 minutes, and the speed was set at 32 rpm, by adding accelerators and curing agents.Sulfur is utilized as a curing ingredient to initiate the cross-linking process, resulting in rubber goods with high tensile strength and elongation at break, while an accelerator and activator are employed to shorten the vulcanization process.After mixing, the compound was passed through the two-roll mill again, cooled, and stored at room temperature for another 24 hours before being utilized for sample preparation and subsequent analysis.After going through these two stages, the resulting sheet-shaped rubber is referred to as compound rubber.

Testing of rubber compounding
The rubber compound was arranged for testing, for example by using a hot press machine.Firstly, the cure time of the compound was determined at 140°C using a Moving Die Rheometer (MDR Professional MonTech, Germany).The vulcanization process was then carried out in accordance with the rheometer testing results and the requirements of each characterization standard.Several tests were performed in accordance with a specific standard in order to characterize the mechanical properties of rubber compounding.The sample was measured under normal conditions and after 72 hours of aging at 70°C.Shore A hardness was measured using a Durometer hardness tester (Mitutoyo, Japan) in accordance with ASTM D2240.Each sample was measured five times from different places at a distance of at least 6 mm.Mechanical characterization was measured using a Universal Testing Machine, and then tested with the incision test equipment specially that had been designed.The control samples also tested and compared.Tensile strength, tear strength, and elongation at break were measured as followed by ASTM D412.Compression set test was performed using an ASTM D395 compression device at 70°C for 22 hours and held at room temperature for 1 hour [9].

Rubber airless tire standard
The selection of reference from published literature establishes a solid foundation for the research approach, guaranteeing that the findings are reliable and useful in a scientific context.The reference standards employed in this study are based on a review of the available literature.The following reference standards are shown in Table 2.

Curing Characteristics
The curing result can be used to describe the microstructure of cured rubber composites [10].
It was feasible to extract a variety of critical data, the most important of which was curing time.In the study, curing time was examined [11].Curing time was depicted as the time required to achieve 90% of the maximal torque [12].Curing time will be followed by quick molding and excellent production efficiency, which can assist reduce energy consumption [13].The curing characteristics can be utilized to determine the rate of rubber vulcanization reactions [14].The NR compounds with expanding filler stacking expanded with the least torque (S'min).Maximum torque (S'max) can be used to calculate crosslink density.The torque difference (S) reflects the shear dynamic modulus, which is indirectly related to the crosslink density of the compound [15].The optimum cure time is required to calculate the vulcanization time for the production of completed rubber goods, whereas the scorch time indicates the flow time required by the compound to fill the mold.According to Attharangsan, the scorch and cure times in their investigation decreased as the filler quantity rose [16].The cure rate index (CRI) is a measure of the cure rate based on the difference between the optimum cure time (T90) and the incipient scorch time (TS2) [17].The following relationship is used to determine CRI: The inclusion of SIR 20 in the rubber increases the cure time because the vulcanization of SIR 20 with the sulphur bonding system is weaker due to the lower double bonding than RSS 1.In addition, smaller particle sizes have more surface area [18].This effect causes a bigger rubber-fill interaction, which tends to create resistance to the flow.Increased torque values indicate a significant restriction on macromolecular movement, presumably due to greater contacts between the fillers and the rubber matrix [19].In contrast, a composite with a higher crosslink density will have higher torque values.The greatest value obtained herein by sample AT1 (RSS 1-CB N220) was 35.5 kg-cm.As a result, integration of CB-N220 into the rubber matrix resulted in higher crosslink density [20].The addition of the filler carbon black increases the CRI.CB-N550 typically has a greater specific surface area and is generally a smaller particle than CB-N220.This means that the CB-N550 has more surfaces with which to interact with rubber compounds and vulcanizing agents.More of these interactions can hasten vulcanization and result in a higher CRI [21].The results of curing data at 140°C for 40 minutes are shown in Table 3.

Mechanical Properties
The results of the mechanical characteristics showed significant differences between each AT specimen.Commonly, a decrease in rubber viscosity was followed by a rise in chain mobility, which led to an increase in the tensile strength of natural rubber vulcanizate.The formulation of RSS 1 with CB-N220 resulted in a tensile value of 22.31 N/mm 2 .Meanwhile, with CB-N550, the tensile value was 19.48 N/mm 2 .Then, for compound SIR 20 with CB-N220, the tensile values were 19.40 N/mm 2 and with CB-N550 were 17.46 N/mm 2 .The value of the tensile strength changed before and after aging.In AT1, there was a significant decrease/change of 7.84%, while in AT2 it was 16.24%, AT3 was 16.63%, and AT4 was 11.63%.The density of cross bonds between the material molecules affects the tensile strength value [22].Moreover, fillers and additives have an impact on the tensile strength, which is determined by the ratio of the natural rubber mixture (Table 1).
The ideal performance of a natural rubber material modification depends on tear strength.Lower tear strength values result in lower quality, as it is easily ripped [23].The number of bonds in the polymer chain is determined by the ingredient ratio (Table 1).In a composite made of natural rubber, it adds more functional groups linked to the Van der Waals style to the polymer chain [24].The addition of a filler increases the tear strength of rubber in airless tire formulations.The tear strength value, as shown in Table 4, showed that formula AT3 is higher (91.71N/mm) than other formulas, and formula AT2 has the lowest tear strength value (87.30N/mm).However, the tear strength value does not significantly increase in AT1 and AT2, as the filler types used differ between AT3 and AT4.High tear strength values reflect the high fracture energy of the polymer due to optimal cross-link density.High tear strength tire treads are more resistant to damage from friction, splitting, or foreign objects.
Crosslinking affects the elongation at break of vulcanized rubber, with increased crosslinks reducing mobility and reducing elongation at break [25].The highest value for airless tire compound, AT1, is 250.21%, with a blend of RSS 1-CB N220.This result was higher than formulas AT2, AT3, and AT4.This illustrates that in formulas AT1 and AT3, the bond density between molecules is different from formulas AT2 and AT4 due to differences in the type of carbon black.Aging treatment reduces elongation at break in vulcanizate [26], while RSS 1 has higher elongation before breaking.Elongation at break is the maximum value expressed as a percentage of the original length, and rubber used at high elongations can risk tensile rupture.The degree of elasticity of natural rubber, the density of the cross-linking created, and the binding between the molecules all have a direct impact on the elongation at break [27].Elongated rubber is crucial for tire tread, as it must stretch under load and avoid permanent deformation.To understand the effect of the cross-binding of AT specimens on the mechanical properties evaluated, the results are represented in Table 4.

Compression Set
The compression set was associated with the restoration of rubber to its original shape.Table 5. Appears to be coming about in a composite with the mechanical characteristics of the compression set test.Based on the test results, Table 5 shows how the strength of the cross-linking created by the reaction of sulfur with rubber molecules and additives affects the bond strength between composite rubber molecules.Material homogeneity, distribution, and composite composition all affect vulcanization composites' mechanical properties [28].
The results of the compression set test of the four formulas for vulcanizates show significant differences.The smallest compression test value is AT3 (RSS 1-CB N550) at 9.47%.The largest compression set value is AT1 (RSS 1-CB N220) at 12.51%.The elongation at break and compression sets increased for the majority of compounds.When overloaded, high compression set materials have a tendency to irreversibly deform.Tires on cars are part of the system that makes sure the car and the changing road surface comply.In order to enforce a stiff, cross-linked structure that inhibits compression set during brief loading intervals, rubber tires are vulcanized.

Conclusion
The fillers carbon black type N220 and N550 are alternative fillers that have been researched as natural rubber fillers.Despite the fact that CB-N220 has smaller particle size than N550 as a filler, the rubber compound mixed with N220 has much higher ideal values for airless tires.For airless tires, all testing demonstrates that RSS 1 rubber outperforms SIR 20.This is evident from the distinction between the two types of rubber, where RSS 1 is a natural rubber and SIR 20 is a synthetic rubber.RSS 1 natural rubber has good mechanical qualities and is better at maintaining its condition and elastic properties.According to the expectations of standard rubber, the ideal formulation is AT1 (RSS 1-CB N220), which can travel through terrain under extreme operating conditions, based on the results of physical testing.According to airless tire standards, the difference in test values from all optimizations for each compound has optimally strong tire tread properties.It can be noted that all of the tests performed have enhanced properties when compared to previous investigations.Carbon black's qualities will increase hardness, tensile and tear strength, and density.

Table 3 .
Curing characteristics of airless tire compound

Table 4 .
Mechanical characteristics of airless tire compound

Table 5 .
Compression set result of airless tire compound