Effect of Vulcanization Process Parameters on the Tensile ...

1. Introduction

The demand for bulk materials transportation with higher efficiency and affordable transportation cost expedites the revolution of conveyor belt technology. Since the first conveyor belt was used in the early 19th century [1], a lot has been changed; the invention of vulcanized rubber by Charles Goodyear in [2] and the introduction of thermoplastic fibers to the market in the mid-20th century has significantly boosted the development of a textile&#;rubber reinforced conveyor belt production sector.

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Conveyor belts are used for the continuous transport of lightweight to heavy materials over short to long distances. The belt&#;s construction and properties are determined based on its area of application. The heavy-duty conveyor belts are employed to convey bulk materials in mining, construction, agriculture, power, and other industries [3,4,5,6,7]. The construction of the conveyor belt comprises three main components: the carcass, skim, and covers (top and bottom covers). The carcass is a reinforcement part found inside the conveyor belt, and it can be either a textile or steel cord material. The carcass is responsible for providing the required tensile strength of the belt. The skim is a polyvinyl chloride (PVC), rubber, or urethane material used between the plies of the conveyor belt. The covers are rubber materials primarily used in conveyor belt construction to protect the carcass. Additionally, the top and bottom covers of the belt provide necessary wear resistance and adequate friction to the drive pulley, respectively.

The properties of the conveyor belt&#;s constituent materials and quality of ply adhesions substantially influence the performance, durability, and safety of the conveyor belt system [7,8,9,10]. Furthermore, the processing parameters of the conveyor belt have a significant effect on determining the properties of a conveyor belt. Several researchers have carried out various studies in order to establish optimized conveyor belt construction and processing parameters. Chou et al. [8,11] used a Taguchi method to determine optimum conditions for vulcanizing textile-reinforced conveyor belts with better elongation, adhesive strength, and abrasion resistance. Amr et al. [12] also investigated the effect of the number of plies of the reinforcing materials, loading direction, and speed on the tensile strength of conveyor belts reinforced with a woven fabric using the Taguchi method. The study shows that the tensile strength of the conveyor belt was severely affected by the loading direction and followed by the number of reinforcement plies and loading speed. The increase in the number of carcass plies increases the strength of conveyor belts, which increases the conveyor belt&#;s thickness and potentially reduces the flexibility of the belt. Ambriško et al. [13] analyzed the effect of carcass, nominal strength, and the number of plies on the tensile strength of textile&#;rubber reinforced conveyor belts using the Design of Experiment (DOE) method. Rawdha et al. [14] investigated the moisture ingression behavior of the textile-reinforced conveyor belt; the study indicates that the textile carcass is sensitive to moisture ingress, which could affect the durability of the belt. Rudawska et al. [15] studied the effect of temperature and humidity on the mechanical properties of the textile-reinforced conveyor belts by testing the samples in a climatic and thermal shock chamber, and the study reveals that the thermal shock deteriorates the mechanical properties of the conveyor belt. Fedorko et al. [16,17,18] studied in their papers the deterioration of the internal structure of textile reinforced multi-ply conveyor belts due to tensile load and dynamic wear using metro-tomography.

The application of woven fabric as reinforcement material in the conveyor belt is tremendously increasing because of its lightweight, high strength, flexibility, and corrosion resistance properties. These draw the attention of researchers to study how the structure and properties of the woven fabric can influence the properties of a conveyor belt. Barburski et al. [19] analyzed the effect of heat treatment on the internal structure of woven fabrics used for reinforcement of the conveyor belt, and the result indicates that the impact of thermal treatment on the physical properties of the fabric depends on the weave structure of the fabric and duration of thermal treatment. The influence of the fabric&#;s weave structure on the mechanical properties of the multi-layer woven fabrics used for conveyor belt reinforcement was also investigated by Witczak et al. [20]. The effect of thermal aging parameters on the physical and mechanical properties of the yarns used to produce a carcass of the textile-reinforced conveyor belt was investigated by Lemmi et al. [21]; the study revealed that the percentage elongation of the polyester yarns was adversely affected under high thermal aging temperature with a minimum aging duration, and the yarn&#;s tenacity has deteriorated at higher aging temperature (220 °C). Kabzinski [22] presented basic information about the conveyor belt design and woven fabric structure used for the purpose of conveyor belt reinforcement.

In order to produce a conveyor belt with the desired physical and mechanical properties, the components of the conveyor belt need to be laminated together. Therefore, the conveyor belt&#;s components are subjected to a vulcanization process to adhere the constituent materials together. The vulcanization process of the conveyor belt depends on three crucial parameters: vulcanization temperature, time, and pressure [23]. These parameters are primarily adjusted based on the type of rubber, carcass, the thickness of the conveyor belt, and the number of plies.

Even though various studies were carried out on the conveyor belt and constituent parts of the conveyor belt, the effect of the vulcanization process on the mechanical property of the carcass of the conveyor belt has been left behind, despite its cruciality in determining the entire mechanical property of the belt. Therefore, the main objective of this study is to investigate the effect of vulcanization parameters, mainly vulcanization temperature and time, on the tensile property of the textile carcass of the conveyor belt.

3. Results and Discussion

The tensile strength of the EP fabric and textile reinforced conveyor belts under various thermal aging and vulcanization conditions have been investigated and discussed in the following sections.

3.1. Tensile Property of Greige and Dipped EP Woven Fabric

In order to analyze the effect of dipping process on the tensile property of the woven fabric, the tensile properties of the woven fabrics at the greige level and after dipping the fabric in RFL solution were investigated. There is no doubt that any type of conveyor belt has to have a proper tensile strength to withstand the maximum load exerted on the belt during the operation throughout its service life. The tensile strength of the belt primarily relies on the properties of a material used to reinforce the belt in a longitudinal direction; when it comes to textile reinforced conveyor belts, the tensile strength of the belt is determined by the tensile property of the fabric in the warp direction.

The effect of dipping process on the tensile strength and percentage elongation of the EP woven fabric in a longitudinal direction is shown in Figure 8. The result shows that impregnation of the fabric with the RFL solution has significantly influenced the fabric&#;s tensile strength and percentage elongation at break. After dipping the fabric in RFL solution, the tensile strength and percentage elongation of the fabric in a warp direction was increased by 11.41% and 30.51%, respectively. In addition, the result reveals that the fabrics were elongated differently under constant stress, as shown in Figure 8.

The stress&#;strain curve shows that at the beginning of the curve, greige fabric has shown higher elongation. However, as the stress increased, the elongation of dipped fabric surpassed the greige fabric&#;s elongation. This was because of two fundamental reasons; in the elastic region of the curve, the greige fabric was more elongated due to the high crimp percentage of the greige fabric. Nevertheless, once the warp crimp was removed, the elongation of the fabric was subsequently reduced in comparison to the dipped fabric. The other reason was that the dipped fabric has a stiffer appearance, and its ability to elongate under minor stress was low, but as the stress increases, the elongation of the fabric increases; this signifies that the dipping of EP fabric in resorcinol&#;formaldehyde&#;latex (RFL) solution has an influence on the mechanical properties of the fabric. Additionally, it can also be seen that the slopes of these curves are quite far apart from each other, which indicates that the dipping of fabric in RFL has an impact on the elastic modulus of the fabric.

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3.2. Effect of Thermal Aging on the Tensile Strength of the Woven Fabric

The impact of thermal aging temperature and duration of aging on the tensile properties of EP woven fabrics intended to be used as a carcass of the textile-reinforced conveyor belt were investigated. The tensile strength of thermally aged woven fabrics along with the respective aging temperature and duration of aging are shown in Figure 9. The results revealed that the increases in aging temperature decreased the tensile strength of the fabric regardless of aging duration.

The rise of aging temperature from 140 to 160 °C for the aging duration of 6 min and 35 min decreased the fabric tensile strength by 8.25% and 11.62%, respectively. The increase in aging temperature from 160 to 220 °C for the aging time of 6 min and 35 min decreased the fabric tensile strength by 7.05% and 22.83%. This shows that the rise in thermal aging temperature influenced the tensile strength of the EP fabric.

Additionally, the fabric samples aged at the same thermal aging temperature with different aging durations have decreased the tensile strength of the fabric as the aging duration increases. For example, as the aging duration increased from 6 to 35 min, the tensile strength of fabric aged at 140 °C decreased by 2.42%, while the tensile strength of fabric aged at 160 °C and 220 °C decreased by 11.62% and 22.83%, respectively. From the results, it can be concluded that the mechanical properties of the EP fabric are dependent on the thermal aging temperature and time; in determining the tensile properties of the fabric, the fabric&#;s fiber composition plays a major role. EP fabric is a composition of yarn made from polyester fiber and polyamide 66; these fibers&#; mechanical properties depend on the aging temperature [21,27].

3.3. Effect of Thermal Aging Parameters on the Percentage Elongation of Woven Fabric

The influence of thermal aging on the percentage elongation at break of the EP woven fabrics is shown in Figure 10. The results show that the aging temperature highly impacted the elongation property of the woven fabric for the fabric samples that underwent thermal aging above the glass transition temperature of the polyester fiber. There were no significant changes observed in the percentage elongation at break of fabric samples aged at 140 °C and 160 °C because of the variation of aging duration. However, the fabric samples that underwent thermal aging at 220 °C have shown an 8.59% increase in percentage elongation as the aging time increased from 6 to 35 min. The increment of aging temperature from 140 to 220 °C increased the elongation of the fabric irrespective of aging duration. The highest percentage elongation at break was registered for the samples aged at 220 °C for longer aging time (35 min), which is 49.78% higher than the samples aged at 140 °C.

3.4. Effect of Vulcanization Conditions on the Tensile Strength of Carcass of Conveyor Belt

The carcass of the conveyor belt is the backbone of the belt in determining the tensile property of the belt. The influence of vulcanization temperature and time on the conveyor belt&#;s tensile strength has been investigated. A multi-layer conveyor belt reinforced with a textile fabric of EP 200N has been vulcanized under three different vulcanization temperatures for six and thirty-five minutes of vulcanization time, while the other vulcanizing parameters were constant. As shown in Figure 11, it is clear that vulcanizing EP fabric-reinforced conveyor belt at high temperatures has extremely affected the tensile property of the belt. The tensile strength investigation results indicate that vulcanizing EP fabric-reinforced conveyor belts at the temperature of 140 °C and 160 °C regardless of vulcanizing time used in this experiment have not shown any considerable tensile strength difference. However, the tensile strength of the conveyor belt vulcanized at 220 °C for 35 min was almost destroyed. Compared to the conveyor belt sample vulcanized at 160 °C, the tensile strength of the samples vulcanized at 220 °C for 35 min has been reduced by 89.06%. In addition to that, the tensile strength of samples vulcanized at 220 °C for 6 min was also reduced by 40.16% compared to conveyor belt vulcanized at 160 °C for 6 min. These results show that the vulcanization of EP fabric-reinforced conveyor belt at high temperature deteriorates the tensile strength of the conveyor belt regardless of vulcanization time; this shows that vulcanizing of the EP-reinforced conveyor belt above the glass transition of the polyester and polyamide 66 fibers can fully deteriorate the tensile property of the belt.

3.5. Effect of Vulcanization Conditions on the Percentage Elongation of the Textile Carcass

The elongation properties of a conveyor belt are crucial to determine heavy-duty belts&#; performance when subjected to varying stress levels. In conveyor belt design, the low extension of the belt is recommended to increase the belt&#;s service life, reduce the fluctuation of the power on the drive sharing of rollers, and prevent the driving motor from burning out because of unbalanced drive sharing [28].

The experimental test results show that the vulcanization temperature and time have an impact on determining the elongation property of a conveyor belt. As shown in Figure 12, the percentage elongation of a conveyor belt vulcanized for six minutes increased with vulcanizing temperature. In addition, the highest percentage elongation of the belt was observed for the sample vulcanized at high temperature (220 °C) for the shortest vulcanizing time (6 min), which was 20.02% higher than the conveyor belt sample vulcanized at 140 °C for 6 min. In contrast, the lowest percentage elongation of the conveyor belt was obtained for the samples vulcanized at 220 °C for thirty-five minutes of vulcanizing time, and this result was 84.95% lower than the samples vulcanized at 140 °C. The carcass of the conveyor belt vulcanized at 220 °C for 35 min was nearly destroyed, and the sample had no ability to withstand the stress imposed on the sample; this was the main reason for the droppage of the percentage elongation and tensile strength of the conveyor belt. Even though lower elongation of the conveyor belt was achieved at the vulcanization condition of 220 °C for 35 min, this cannot be considered as an optimum vulcanization temperature for the conveyor belt design due to its low tensile strength.

3.6. Comparison of the Effect of Temperature on Tensile Properties of Woven Fabric and Conveyor Belt

The effect of vulcanization temperature and duration of vulcanization process on the tensile property of the conveyor belt&#;s carcass was compared with the tensile property results of dipped woven fabric aged in the electric oven. The influence of processing parameters on the tensile strength and percentage elongation of the woven fabrics are shown in Figure 13a&#;d. As mentioned in the previous sections, the conveyor belts with three layers of EP 200 woven fabric were produced. However, to compare the tensile property difference of the fabric thermally aged in the oven with the tensile property of fabric vulcanized with the rubber, the carcass part of the conveyor belt was removed from the reinforcement, as shown in Figure 14.

As shown in Figure 13a&#;d, even though the fabrics were subjected to thermal aging in different technics under similar conditions, the property of fabrics aged in the oven is less affected in comparison to the vulcanized fabric irrespective of the duration of aging/vulcanization process. This difference arose from the following things: the fabrics aged in the oven were aged under no pressure, but the fabric used as the carcass of the conveyor belt was reinforced under high pressure; this influences the property of the vulcanized fabric. The other reason was that as the temperature of vulcanization increased to 220 °C, the tensile strength of the vulcanized fabric was almost destroyed regardless of the duration of the vulcanization process; this can also be linked with the chemical crosslinking of conveyor belt reinforcement components at high temperature.

Nevertheless, the present study shows that it is not recommendable to vulcanize the EP woven fabric-reinforced conveyor belts at higher temperature (220 °C) even if the duration of the vulcanization process is short unless a special rubber material is used for the reinforcement.

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