Physical Analysis Methods

The physical properties including hardness, elongation at the break, modulus, tear, and tensile strength of the specimens can be tested by different testing machines intended for different physical properties of the specimens.

4.3.1 Hardness

The measurement of hardness is widely utilized in quality control testing in the rubber industry. The rubber compound’s hardness can be measured in the laboratory according to the international rubber hardness degree (IRHD) by using Type A, B, C, Asker C, D, and DO of the durometer following ASTM D2240. The Shore A durometer is utilized to define the hardness of the rubber compound according to the ISO-48 standard. The hardness of the rubber compound is measured on a scale of 0 to 100, with 0 Shore A representing no modulus of elasticity, whereas 100 representing infinite elastic modulus.

The hardness tests allow for a quick assessment of changes in the mechanical properties caused by the heat treatment, compounding ingredient addition, or chemical changes in the rubber compound. The rubber compound’s hardness normally increases during the Figure 24. The thermomechanical crosslinking decompose mechanism of rubber compounds, where s is sulfide and x ≥ 3. [58]

service life of the rubber compound. The rubber compound hardens and loses its damping properties when it is exposed to the degradation agents in various conditions.

[49, 57]

Nakazono et al. [42] studied the hardness of the vulcanized styrene-butadiene rubber with various styrene, 1,2-, and 1,4-butadiene molecular weights, and contents for 0–8 weeks at 100 °C in the atmospheric conditions. All the samples were vulcanized and heat-aged in the same way. The molecular weights were defined by gel permeation chromatography (GPC) and NMR spectroscopy was utilized to determine the structure of SBR. Before and after heat aging, the mechanical characteristics of the specimens were examined. Figure 25 shows the hardness versus aging time of samples of SBR compounds. The value of hardness of samples A, C, and D increased in the same way after the heat aging. Sample B had smaller hardness values than the others. Sample E had higher hardness values both in the beginning and during the heat aging. An increase of hardness value in the samples might be because of the generated polymer radicals and their crosslinking process in the vulcanized SBR. The hardness and embrittlement developed in the rubber compound as a result of this. The hardness may increase during post-curing and thermo-oxidative aging [54]. Nonetheless, the degradation of elastomer can be caused by the build-up in the stress at elongation % and hardness. This follows the deterioration of polysulfide bonds and the creation of mono- or disulfide bonds.

Figure 25. The hardness versus aging time (week) of SBR at 100 °C, where samples A-D contained different amounts of SBR and E contained no processed oils. [42]

4.3.2 Tensile Strength

The tensile strength portrays how much stress or force (MPa) the rubber compound can resist until breaking. [57] The tensile strength of the rubber compound can be measured using a tensile tester machine followed by the ASTM D412 standard testing method. The tensile strength test is performed with the dumbbell-shaped sample which is placed in the grips of the tensometer.

Ali et al. [58] examined the impacts of heat-aging on the vulcanized natural rubber’s tensile properties. The dumbbell test specimens were aged at 100 °C and 70 °C for 5 to 168 hours in the oven. Before the tensile properties testing, the samples were kept at room temperature for one day. The tensile properties test was done following ISO 37 standard. The tensile strength decreased with an increase in the aging temperature according to the investigation. It was noticed that the tensile strength decreased fast at 100 °C (see Figure 26). This might be owing to the embrittlement process that has developed inside the rubber compound because of the absorbed heat as the temperature increased. The greater the aging temperature the more significant the degradation in the vulcanized rubber compound’s tensile strength.

4.3.3 Elongation and Modulus

The elongation at the break (%) is the maximum amount of strain that the rubber com-pound can withstand. Elongation is measured by calculating the difference in percent-ages of the initial and rupture length of the dumbbell-shaped test sample. On the other

Figure 26. The tensile strength (MPa) versus aging time (hours) of the vulcanized NR at a temperature of 100 °C, and 70 °C. [58]

hand, the modulus is commonly stated as the stress required to stretch a rubber speci-men at an elongation of 25 %, 50 %, 100 %, 200 %, or 300 %. [58] The higher the percentage value of modulus the stiffer the rubber compound. Both the elongation at the break and modulus of the rubber compound can be tested with dumbbell-shaped test pieces by the tensile tester machine following the ASTM D412 standard testing method [59].

The investigation study by Ahagon et al. [59] revealed that the heat aging of rubber com-pounds is close to the belt-skim aging of a tire during the service time. The study was done with a laboratory aging test to the rubber sheets of the belt-skim at different tem-peratures and aging times. After the heat aging of rubber compounds, the mechanical properties including the modulus and the elongation at the break were measured with dumbbell-shaped test samples. The mechanical property results of the rubber sheets tested in the laboratory were almost identical to the results of belt-skim of the rubber in the tire aging under both test and belt-skim temperature below 100 °C.

Plotting modulus 100 % (MPa) against the square root of the aging time gives a straight line for each heat-aging temperature due to the diffusion-controlled process of oxidative aging [4, 59]. The rate of change in crosslink density can be measured by the slope of the line in the plotted figure. Figure 27 shows the results of the modulus versus the square root of the oven aging time for the laboratory-tested rubber (Figure 27a) and tire rubber (Figure 27b) sheets at a maximum temperature of 100 °C.

Figure 27. The modulus 100 % (MPa) against the square root of the aging time (s^1/2) of (a) the laboratory-tested rubber compound, and (b) tire rubber at a temperature of 70 °C (white circle), 80 °C (white circle with a black diameter), 90 °C (circle with half colored), and 100 °C (black colored circle). [59]

a) b)

4.3.4 Analysis of Aging with Ahagon Plot

Baldwin et al. [60] also studied the oxidative aging of steel belt-skim in the heated oven for 2, 4, 6, and 8 weeks at 100 °C, 90 °C, 80 °C, and 70 °C. The study found that NR’s chemical aging contains three different types of aging mechanisms based on the analy-sis of the Ahagon plot. The relationship logarithm of elongation at break (𝜆_𝐷) and modu-lus 100 % (M₁₀₀) can be seen in Figure 28. Type I mechanism of aging happens at lower temperatures aerobically. Type II happens anaerobically at elevated temperatures. Fur-thermore, type III happens at elevated temperatures aerobically. According to Figure 28, the modulus 100 % rises and the elongation at break decreases in type I aging. This follows the increase of crosslink density around the tire’s belt edge. Both the modulus 100 % and the elongation change slightly in the type II aging. This indicates the interior aging of tire rubber, particularly in the regions of the belt-skim (shoulders). This is be-cause of the diffusion of oxygen across tire rubber. The diffusion of oxygen creates a rise of oxidation and reacts with the tire rubber alongside surface-to-middle. However, the aging mechanism of type III is unusual in the tire rubber. This type of mechanism might happen in an aerobic state at elevated temperatures.

By using the modulus and elongation factors, the aging of the rubber compound can be indicated [4, 43]. Thermal-oxidative effect on the increase of modulus following the hard-ening of the rubber compound products. An increase of modulus is a key element in predicting the initiation of crack on the surface of the rubber compound [57].

Figure 28. Ahagon plot for steel belt aging of NR, which shows the ratio of strain at the break as well as modulus at 100 % strain in logarithm scales. [59]

The chemical and physical characteristics changes in the rubber compound might be assessed from Table 3. For instance, the rubber compound’s hardness can be measured with the durometer.

Table 3. The aging indication of rubber compounds by physical and chemical property changes.

Physical and

chemical properties Analysis of aging Analysis methods References

Cross-link density

A polymer chain mobility follows by increasing cross-link density. The thermo-oxidative aging causes an increase in cross-links types of mono-, and disulfidic and a decrease in polysulfidic cross-links. As a result, the rubber compound hardens with temperature increase.

TD-NMR, swelling ratio [59]

Elongation-to-break [%]

Decrease of elongation follows molecular weight loss and predicts the rubber compound aging as a result of an increase in the aging temperatures and times.

Tensile tester [60, 61]

Hardness

An increase in hardness values predicts the aging of the rubber compound because of the cross-linking formation between polymer radicals following the rubber compound's hardening.

Durometer (Shore A) [61]

Tanδ Tan δ value increases with the increase of vinyl

groups and aging period in the rubber compounds. DMA [61]

Modulus [MPa]

An increase in modulus value indicates an increase in the rate of change in the cross-links of the rubber compound. This follows the the rubber compound's hardening and the initiation of cracks on the surface of the rubber compound.

Tensile tester [56, 60]

Molecular weight loss [%]

A decrease in molecular weight loss indicates the reduction of material properties following the aging of the rubber compound.

TGA, Gel permeation chromatography/ mass

spectrometry

[52, 61]