Influence of tire-cord layers and arrangement direction on the physical properties of polyester tire cord reinforced with chloroprene rubber composite materials

2.1 Materials

CR (Bayprene 216) was obtained from Lanxess (Cologne, Germany). Other ingredients used for compounding were commercial materials usually used in the rubber industry. Resorcinol formaldehyde latex (RFL)-dipped polyester tire cord [1000/2 denier and twist of 481 (filament)] was provided by San Wu Rubber Manufacturer Co., Ltd. (Taiwan).

2.2 Basic recipes

The contents of the CR compound are shown in Table 1.

2.3 Characterization

Mooney viscosity was determined according to ASTM D1646 in a Mooney viscometer (HT-8752; Hung-Ta Instrument Co., Ltd., Taiwan). The cure times were determined by the Monsanto Moving Die Rheometer (MDR 2000) at 150°C, 160°C, and 170°C for 60 min. The hardness test was determined by a Shore A durometer from ASTM D2240. The density test was determined by a densitometer (MD 300S; Alfa Mirage Co., Japan) from ASTM D1505. The deterioration-crack growth test was according to ASTM D813-95. The numbers of repeated bending strain were 500, 1000, 1500, 2000, 2500, 5000, 10,000, 15,000, 20,000, 25,000, 30,000, 35,000, 40,000, 45,000, and 50,000, respectively. The creep test was determined by a creep tester (Model GT-F-176; Gotech Testing Machines, Inc., Taiwan) with 70°C for 22 h at compression loading of 30 kg. The Material Testing System (type MTS-810) was utilized to test compression stiffness of the experiment piece with a dimension of 29±0.5×29±0.5×12±0.5 mm under a deformation range of 7 mm. The calculation of the compression stiffness (Ks) is as follows:

(1)Ks=F/X (1)

where F is the compression force and X is the compression displacement.

3.1 Mooney viscosity

The processability of rubber can be divided into three stages: the initial rubber fluidity, the intermediate vulcanization rate, and the subsequent vulcanization reaction. The composition of the rubber influences the initial fluidity. Figure 1 shows the Mooney viscosity curve for CR. According to a Mooney viscometer test, the Mooney value of the rubber used for this experiment was 36.5 M. Generally, a Mooney value that exceeds 50 M indicates that the initial fluidity of the rubber is low. Thus, the rubber material used for this experiment possessed a low Mooney viscosity, which suggests that the raw rubber had excellent initial fluidity.

Figure 1

Mooney (M) viscosity curve of CR.

3.2 Vulcanization

Sulfur is not suitable for CR vulcanization because the double bond in this type of rubber is adjacent to the chlorine atom. Therefore, magnesium oxide (MgO) and zinc oxide (ZnO) are typically used for CR vulcanization. Following the initial flow, raw rubber undergoes a cross-linking reaction caused by temperature effects. The curve for CR vulcanization can be divided into the following three reaction stages: (a) the first stage is the region from which processability can be obtained to determine the fluidity of the rubber and the viscosity before vulcanization (the curve is similar to that of the Mooney viscosity); (b) the second is the region where the rate of rubber vulcanization can be determined; and (c) the third is the zone where the maximum torque can be measured to infer the maximum degree of vulcanization and physical properties of the rubber. In addition, the curve of the third stage can be further divided into three phases: (a) optimum vulcanization, where the curve reaches the equilibrium point and achieves the maximum torque value; (b) incomplete vulcanization, where the curve continues to rise but does not reach torque equilibrium; and (c) network degradation, where the curve reaches the maximum torque value before decreasing. An optimal vulcanization curve is one in which torque equilibrium can be rapidly obtained and maintained at a certain level for a long period of time without exhibiting a declining trend.

Figure 2 shows the vulcanization curve of CR processed under various temperatures (150°C, 160°C, and 170°C). The results demonstrate that, when CR vulcanization occurs at 150°C, vulcanization continues after the end of the reaction time and torque equilibrium is not achieved; therefore, more time is required for complete vulcanization. At 160°C, a cross-linking reaction occurred at a rate ranging between that at 150°C and 170°C. Furthermore, the time required to reach torque equilibrium exceeded that required at 170°C. Based on the vulcanization curves at the three temperatures, a vulcanization temperature of 170°C was the optimal condition. At this temperature, rubber rapidly underwent a vulcanization cross-linking reaction. In addition, when processed under the same cure time, torque equilibrium was maintained for prolonged periods of time without exhibiting a declining trend.

Figure 2

Vulcanization curve of CR with various temperatures.

Figure 3 shows the t_c90 (optimum curing time) for the vulcanization reactions at the three temperatures of 150°C, 160°C, and 170°C. The experimental results indicate that, as the processing temperature increased, less time was required for vulcanization to reach t_c90; in other words, a reaction at 170°C required the least amount of time followed by those at 160°C and 150°C. Higher processing temperatures indicate a more rapid vulcanization rate.

Figure 3

t_c90 curve of CR with various temperatures.

3.3 Mechanical properties

Figure 4 shows the density change curves for CR reinforced with a varying number of polyester tire-cord layers. The experimental results show that the density of CR rubber declined with increasing polyester tire-cord layers. Figure 5 shows the hardness of the CR reinforced with various numbers of tire-cord layers. The results suggest that the hardness of CR rubber exhibits an increasing trend in correlation to the number of polyester tire-cord layers; however, the amplitude of increase is not substantial. Typically, as the number of tire-cord layers increases, the constrained-layer structure of the rubber substantially increases, which alters the hardness of the rubber significantly. Nevertheless, because the transverse structures of the applied tire-cord texture are comparatively less constrained, changes in the hardness of the CR composite do not result in major differences.

Figure 4

Density curve of CR reinforced with a varying number of polyester tire-cord layers.

Figure 5

Hardness of CR reinforced with a varying number of polyester tire-cord layers.

3.4 Deterioration-crack growth

Figure 6 demonstrates the experimental results in regard to the crack growth and bending frequency of CR composites with a differing number of tire-cord layers and arrangement directions. After bending the RF-0 specimen 2800 times, a crack with a length of 25.4 mm formed, showing complete fracture. After bending the RF-1 (90°) specimen 2000 times, a crack with a length of 25.4 mm formed, exhibiting complete fracture. The RF-2 (90°, 0°) specimen (transverse and reinforced fiber in front) and RF-2 (90°, 90°) specimen (transverse double-layered) were bent 500 and 550 times, respectively, rapidly producing cracks of 25.4 mm in length, exhibiting complete fracture. Observing the cracks and fractures for the specimens, researchers have found that, after rubber is reinforced with fibers, crack growth occurs along the direction of the tire-cord reinforcement and the connection interface with the rubber. Therefore, the rate of crack growth is greater compared to that of unreinforced specimens. With double-layer reinforcements, cracks easily form because the tire-cord layer is closer to the surface of the rubber specimen (the bending side). Consequently, the damage caused by crack growth is accelerated.

Figure 6

Crack growth and bending frequency of CR composites with a differing number of tire-cord layers and arrangement directions.

Bending the RF-1-1 (0°) specimen 50,000 times led to a crack of 16.36 mm in length. Bending the RF-2-2 (0°, 90°) specimen (longitudinal front) 50,000 times created a 2.60-mm-long crack. After bending the RF-2-3 (0°, 0°) specimen 50,000 times, a crack of 3.13 mm in length formed. Based on the experimental results, reinforcement with tire cords in the 0° direction has a significant effect on the prevention of crack growth.

Comparing the crack growth of the RF-2-2 (0°, 90°) and RF-2 (90°, 0°) specimens, the results show that RF-2-2 exhibited a significant crack growth inhibition effect. Although the two samples were reinforced with two layers of tire cord, their crack growths exhibited significant differences. This was because, when RF-2 (90°, 0°) was bent, the tire cord arranged at 90° was closer to the stretching side, which caused the connection interface between the tire cord and rubber to fracture and fail rapidly.

3.5 Stiffness characteristics

Figure 7 demonstrates compression stress and strain curves for tire cord-reinforced CR rubber. Under the same compression strain, the compression stress of the RF-4 specimen was the highest followed by those for RF-3, RF-2, RF-1, and RF-0. This finding is consistent with the results of the hardness values. Figure 8 displays the compression modulus curves for the various specimens processed under 25% compression strains. The diagram clearly shows that the compression modulus of the RF-4-reinforced specimen was the highest followed by RF-3, RF-2, RF-1, and RF-0. Therefore, the tire cords significantly enhanced the stiffness of the rubber material.

Figure 7

Compression stress and strain curves for tire cord-reinforced CR rubber.

Figure 8

Compression modulus of CR composites under 25% compression strains.

3.6 Creep analysis

Creep behavior is an indicator that measures the long-term usage properties of a material. Figure 9 shows the effects that differing numbers of tire-cord layers have on the creep behavior of CR rubber. The experimental results indicate that the total deformation due to compression creep behavior for RF-0 was the largest (1.60 mm) followed by RF-1 (1.50 mm), RF-2 (1.35 mm), RF-3 (1.12 mm), and RF-4 (0.105 mm). Under a 30-kg load, the initial compression sets for RF-0, RF-1, RF-2, RF-3, and RF-4 were 0.1, 0.08, 0.075, 0.075, and 0.06 mm, respectively. This result is consistent with the findings of earlier studies [11, 12]. Overall, a comparatively greater number of fiber reinforcement layers can significantly inhibit creep behavior.

Figure 9

Creep behavior of CR reinforced with a varying number of polyester tire-cord layers.