Used tires are often retreaded or repaired, especially for heavy duty vehicles, which can lower the cost per kilometer of the tire. Developing countries practice this technique more often due to less regulations and standards. Retreading is a laborious process that is carried out manually, but it limits the consumption of raw materials and energy and reduces waste [12] [16] . Passenger vehicle tires are generally only retreaded once, while truck tires can be retreaded and reused more than six times. In some developing countries, retreading is a well-established activity. For example, in Nairobi, Kenya, more than 10,000 tires are received each week for retreading [17] [18] . In Cameroon, a Chinese company established in the two largest cities of Yaoundé and Douala carries out retreading activities and competes in the tire market with second-hand tires imported from overseas.

A common way of recycling in African countries is to cut out materials from used items to create new objects. Rubber from tires is easy to work with and can be treated like leather. Basic tools like knives, shears, pliers, and hammers are used to make new products from used tires, as well as improvised tools for specific purposes. (See Figure 1)

Products from this rubber recovery method are of medium to low quality and sold to low-income populations. These include sandals, shoe soles, doormats, water containers, pots, dustbins, and even automobile components like silent-blocks, motor mounts, seals, and washers (Figure 2). Some parts are customized by mechanics, and workshops are usually located near autoshops. Consumers generally appreciate the long-lasting effectiveness of these products, and due to the good price/quality ratio, there is a strong demand for these automotive spare parts. This consumer demand proves the great commercial outlet of the industrial production and the interest of having a good mastery of the mechanical properties of the parts thus produced.

This paper will focus on creating and characterizing a material made from used tires for producing engine sealing components, specifically O-rings (Figure 3).

An O-ring is a popular elastomer-based molded part. It is made by filling an

elastomer compound into a mold of the required shape of the part to be made [19] [20] [21] . There are two main molding processes: compression molding and injection molding. The choice of process depends on the component dimensions and the quantity to be produced.

Commercial tires are mainly composed of elastomeric rubber, carbon black, metal, textile fibers, and other additives. The composition varies depending on the type and model of the tire and its road application. Natural rubber, styrene-butadiene rubber, and butadiene rubber are commonly used elastomers, while butyl elastomer is used for inner tubes due to its impermeability to gases [12] [22] [23] [24] [25] . To make new material from used tires, the first step is to crush them using mechanical equipment to create rubber granulates or crumbs. Crumbs are particles smaller than 1 mm, while granulated aggregates range from 1 to 10 mm in size. (See Figure 4)

Industries are interested in using semi-finished products made from used tires to create molded parts. Indeed, crumbs can be re-agglomerated under heat and pressure to make items such as wheels for wheelbarrows or lawnmowers, as well as more technical spare parts like O-rings, which we will discuss in this study. Vulcanization is the process that allows the properties of the elastomer article to be suitably adjusted for proper functionality.

Vulcanization is a process that creates a three-dimensional network in raw rubber by reacting it with sulphur. This chemical reaction is slow at room temperature, so it is typically done at around 150˚C to 180˚C [23] - [30] . The most used vulcanization system is based on sulphur and applies to dienic elastomers such as NR, SBR, BR, IR, NBR, EPDM, and IIR (see Appendix for the nomenclature of elastomers). Other vulcanization systems are sometimes called crosslinking or ratification [7] [8] [9] [23] [25] . The process involves activators like zinc oxide and stearic acid, which facilitate the combination of sulphur and rubber, and accelerators that determine the kinetics of the reaction [23] . The length of the sulphur bridges created by the reaction affects the behavior of the resulting vulcanizates and can be adjusted by changing the doses of sulphur and accelerators [26] - [30] .

The choice of vulcanization system depends on the type of base elastomer for this chemical reaction, which can be either natural rubber derived from the latex sap of the Pará rubber tree (hevea brasiliensis) or synthetic rubbers derived from hydrocarbons.

Vulcanization is an irreversible chemical reaction that must be done on virgin elastomer, so used tire crumbs must be reinforced with virgin elastomer to allow them to vulcanize again. For this study, natural rubber (NR) associated with inner tube crumbs (IIR) is selected as the base elastomer, because natural rubber is abundantly produced in Africa and has a positive carbon balance as a renewable biopolymer [23] . Tire treads are not used in this study because they are made of SBR or BR elastomers, which have undesired damping characteristics for sealing [19] - [24] . However, tire treads could be a good choice as part of base elastomer for other applications that require strong damping, such as engine mounts or silent-blocks, and the methodology developed in this study for O-rings could then be used for any other technical rubbery components.

The rest of the paper is organized as follows. The framework of the investigation is depicted in Section 2. The third section describes the compounding method of masterbatches. Section 4 contains experimental tests and the results. Section 5 summarizes the findings of this paper.

The Monsanto Model R 100S Oscillating Disc Rheometer (Figure 7(a)) was used to conduct experiments, in accordance with international standards for determining the vulcanization characteristics of rubber compounds [26] - [30] . The experiments were carried out at isothermal temperatures of 150˚C, 160˚C, and 180˚C, with rotor oscillating amplitude of 3˚ and a frequency of 1.67 Hz. Raw samples of masterbatches were compressed in a sealed enclosure and maintained at the chosen vulcanization temperature. A disc embedded in the sample was used to apply a shearing force to the masterbatch, and the required torque was automatically recorded as a function of time (Figures 7(b)-(d)). The obtained rheographs allow the determination of various vulcanization parameters of the masterbatches. These include:

1) The minimum torque M_L (reflecting processability), the maximum torque M_H (corresponding to the ultimate degree of vulcanization), and the difference between the two (which provided the maximum crosslinking density). The safety

limit for optimum vulcanization retained is equal to 90% of the difference between M_H and M_L.

2) The induction time t_s2 marks the start of crosslinking and serves as an implementation security measure for the vulcanization process.

3) The vulcanization optimum time t_c(90) is the time required to obtain 90% of the maximum torque.

4) The vulcanization rate index CRI (Cure Rate Index) is determined by calculating the average slope of the ascending part of the curve and used to assess the kinetics of vulcanization. The approximate expression for CRI is thus given by the formula:

C R I = 100 t c ( 90 ) − t s 2 (1)

5) It is also noted that the constant of the chemical reaction kinetics is given by the empirical Arrhenius relationship:

K = A e − E / R T (2)

where A is the action constant; R is the ideal gas constant; and E is the activation

energy of the reaction. For rubber technologies, the constant K corresponding to 90% sulfur consumption is considered to be of first order. It is given by the formula:

K 1.90 % = L n 10 t c ( 90 ) (3)

6) Finally, the activation energy of the reaction E_a can be calculated using the Arrhenius relationship:

E a = R T 1 − T 2 T 1 T 2 L n K 1 K 2 (4)

where K₁ et K₂ are the reaction constants corresponding to T₁ et T₂ respectively.

A model for the analysis of rheographs from Figure 7 is developed to understand how vulcanization temperature and formulation affect the rubber. Table 5 shows the values found for the vulcanization parameters. These results are important in the process as they help to compare the processability, the cross-linking density, and the vulcanization kinetics of different formulations (A, B, C) at different temperatures (150˚C, 160˚C, 180˚C). Thus, the analysis can be based on selection criteria such as t_s2, t_c(90), CRI, and K₁, 90%.

- To prevent burning of the rubber, the induction time t_s2 must be long enough before vulcanization begins. The maximum value for t_s2 is 1.8 minutes for all formulations, which is obtained at 150˚C (Table 5).

- The vulcanization optimum time, t_c(90), must also be long enough to prevent surface crosslinking. The maximum value for t_c(90) is different for each formulation at each temperature, with the highest value of 4.9 minutes for formulation B at 150˚C (Table 5).

- To avoid high vulcanization kinetics parameters CRI and K_1.90%, which can lead to problems with thick parts due to poor heat conduction, the minimum values are different for each formulation. Formulation B at 150˚C has the smallest values for CRI (32.3) and K_1.90% (0.47) (Table 5).

Based on these selection criteria, we conclude that 150˚C is the best vulcanization temperature and that formulation B is most suitable for the masterbatch vulcanization. The activation energy for formulation B is also higher than the other formulations, at 15.85 KJ·mol⁻¹.

To make test specimens, a laboratory mold press (shown in Figure 8) is used to shape samples from pre-made materials A, B, and C. The press plates are heated to 150˚C and the raw material samples are placed into the mold. The process is automated with preset temperature and time parameters.

Once the samples have undergone optimal vulcanization, which takes different times for samples A, B, and C (4.1, 4.9, and 4.6 minutes respectively), they are removed from the press and quickly extracted from the molds. The samples are left at room temperature for 24 hours before being cut into five tensile bars each using a punch, following ISO 1826 standards (shown in Figure 9). Hardness and tensile tests are subsequently performed on each bar.

To test the hardness of the samples, the durometer method (Shore hardness) was used following the ISO 7619-1:2010 standard. There are two measurement

a. 1 torque unit = 0.05 N m.

scales, Shore A hardness for normal to hard rubbers and Shore D hardness for high-hardness rubbers. Each of the A, B, and C samples were tested at five evenly spaced points on the specimen. Table 6 displays the results, including statistical quantities such as means, standard deviations, and coefficients of variation, which were used for analysis.

a. Standard deviation. b. Coefficient of variation.

The coefficient of variation measures the repeatability of measurements, and a lower coefficient of variation indicates better homogeneity. Formulation B has the lowest coefficient of variation at 0.9%, while A and C have coefficients of variation of 2.6% and 1.9%, respectively. Measurements for Shore A hardness are more repeatable than those for Shore D, indicating that the masterbatches are of normal-hardness range rubber material. The difference in hardness values between the three samples in each Shore scale is negligible, as the proportion of the strengthening fillers N330 is the same for all three formulations. Specifically, formulation A has 58 ± 1.5, formulation B has 60.4 ± 0.5, and formulation C has 61.4 ± 1.1 as shore A hardness.

Tensile tests are performed on masterbatches A, B, and C using the ISO 37:2017 standard. Five specimens of each masterbatch are tested by applying continuous loading to a tensile bar mounted on a dynamometer (Figure 10) at a speed of 0.25 mm/min until the material broke. This speed is chosen because elastomers are viscoelastic materials and in practice stresses occur relatively slowly. During the test, a digital display indicated the force and elongation, and the tensile curves are recorded for each specimen (Figure 11 shows curves of specimens B). The strength at 100% elongation, tensile strength at break, and ultimate length are recorded. The elongation at break, stress at break, modulus at 100% elongation, and modulus at break are then calculated and summarized in Table 7.

The initial useful length of the specimens is L₀ = 25 mm, and the rectangular section A₀ of the specimens is

A 0 = 4   mm × 3.7   mm = 14.8   mm 2 (5)

All specimens break at elongation levels beyond 300% and exhibit almost identical tensile behavior, except for specimen 4, which breaks slightly earlier. This demonstrates the masterbatch’s quasi-homogeneity.

Tensile tests show that formulation B is of higher quality than the other two. Masterbatch B can withstand a greater stress before breaking compared to masterbatch A and C, as shown in Table 7, with a value of (3.82 ± 0.64) MPa. Similarly, it can elongate more than the other two before breaking, with an elongation

at break of (319% ± 36%). In comparison, masterbatch A has an elongation at break of (245% ± 37%), and masterbatch C has an elongation at break of (262% ± 27%).

The thermo-oxidative ageing technique is used to assess the ageing of the newly developed material through accelerated ageing tests. This method, which follows the ISO 188:2011 standard, evaluates the relative resistance of vulcanizates to deterioration over time. In this technique, vulcanized specimens are placed in a cabinet oven with forced air circulation (Figure 12) for 24 hours at a temperature of 100˚C. The ventilation rate is ranged from 3 to 10 changes per hour. After

a. Strength at 100% elongation of specimens. b. The modulus at 100% elongation of the specimens.

the ageing process, the specimens undergo hardness and tensile tests, similar to the unaged ones and the resulting measurements (shown in Table 8 and Table 9) are compared with the corresponding measurements obtained from the unaged vulcanizates.

In a final phase of performance evaluation, the test results are compared to the previous measurements obtained from the unaged specimens. The change in properties after ageing is determined by the ratio of the value obtained after ageing to the value obtained before ageing, as shown in Table 10 and Table 11.

After ageing, masterbatch B maintains the main mechanical characteristic for O-ring’s materials that is its hardness (99% ± 3 in Table 10), but its elasticity (modulus, 85%) and resistance (stress at break, 61%) decrease (Table 11). This

a. Standard deviation. b. Coefficient of variation.

a. Strength at 100% elongation of specimens. b. The modulus at 100% elongation of the specimens. c. Standard deviation. d. Coefficient of variation.

a. Standard deviation.

poor retention of mechanical properties is also observed for the other masterbatches, although the trend is upward for them (mean values above 100% in Table 10 and Table 11). This result was previously expected as the chosen vulcanization system is known to sacrifice ageing resistance for good mechanical properties and fatigue resistance. To improve ageing resistance, a semi-efficient vulcanization system (SEV) that offers a good compromise between mechanical properties, resistance to fatigue and ageing should be recommended.