Rubber formula for continuous vulcanization of cables

Here’s an article for you: Determination of Process Parameters for Continuous Vulcanization of Rubber-Sheathed Cables. With the development of modern science and technology, rubber production techniques are also showing increasing characteristics of high-tech advancement. With the discovery of highly efficient and rapid vulcanizing agents and accelerators for rubber, the rapid vulcanization of wires and cables has also become possible. Continuous vulcanization evolved from extrusion vulcanization and batch vulcanization. It has many advantages: high production efficiency, an improved appearance of wires and cables compared to traditional processes, and no oxidation of the copper conductors. Rubber-sheathed cables produced using traditional methods generally cannot be used with high-end household appliances. The main reason is that these cables have an unsatisfactory appearance and their copper wires are severely oxidized, which prevents them from being suitable for use in household appliances; instead, plastic flexible cables are usually used. With China’s accession to the WTO, its product standards have been aligned with international IEC standards. For many household appliances, it is required that rubber-sheathed cables be used for power supply wires; for example, the indoor and outdoor connection wires for air conditioners must be 245IEC57 (YZW) rubber-sheathed flexible cables. Therefore, improving the visual quality of rubber-sheathed flexible cables is an urgent practical issue that needs to be addressed at present. Successive vulcanization is a new processing method for rubber-sheathed cables. By discussing various process parameters in the production of such cables, this article establishes a complete set of parameters for successive vulcanization, thereby continuously improving the quality of rubber-sheathed cables and providing a reference for professionals in the cable industry. 2 Determination of specifications: Generally, the technical specifications for continuous vulcanization units specify the range of die sizes, that is, they define the minimum diameters of the die core and die sleeve, which serves as one of the bases for determining the specifications. In addition, a range for the screw speed is specified; when the screw speed is below this specified value during rubber extrusion, the plasticity of the rubber becomes uneven, its flowability is poor, the amount of material extruded is not stable, cable processing becomes difficult, and the pressure inside the die is very high, which can easily damage the die. If it exceeds the range, the amount of rubber extruded is insufficient, and the only way to compensate is by reducing the output speed, which affects production efficiency. 3 Determination of screw speed and pulling speed: The screw speed and pulling speed determine the production efficiency as well as the outer diameter and appearance of the product. The basic principle for determining the screw speed and drawing speed is the mass balance principle, that is, the amount of rubber extruded by the screw is exactly equal to the amount of rubber required to coat the product being drawn out. Generally speaking, when determining the screw speed and drawing speed, the drawing speed is set first; that is, the vulcanization time, which is the main factor determining the various mechanical and physical properties of the product. This article will provide a detailed explanation of this. The screws of continuous vulcanization extruders all use plasticizing screws with a length-to-diameter ratio of 12:1 or higher; the amount of material returning between the screw and the barrel is small, and the screw speed is essentially proportional to the extrusion volume, that is, there is a linear relationship. For example, the technical specifications for a certain rubber extrusion unit specify that the screw speed ranges from 10 to 50 rpm/min, with a maximum extrusion capacity of 120 kg/h. Based on these data, a graph showing the relationship between screw speed and extrusion volume can be created (see Figure 1). 50 revolutions per minute 40 30 20 10 0 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 kilograms per minute Figure 1 For example, in the case of extruding a sleeve for a certain product, the theoretical amount of material required for the sleeve is 78.7 kg/km. With an output speed of 20 m/min, the amount of material used per minute can be calculated as follows: Material usage = 20 × 78.7 ÷ 1000 = 1.574 kg/min. According to the principle of material balance, the amount of rubber that needs to be extruded is also 1.574 kg/min. By referring to Figure 1, it can be seen that a screw rotation speed of around 39 revolutions per minute falls within the range specified by the equipment’s technical specifications, thus meeting those requirements. Based on this method, it is possible to determine the screw speed and pulling speed for producing screws of various specifications using this equipment. 4 Determination of the fuselage nose temperature: The temperature in the feeding section of the fuselage is generally kept low, around 30-40 degrees; if the temperature is too high, there will be a backflow of material, and the rubber is prone to burning. The head temperature should be determined based on the type of rubber and the time the compound stays inside the machine, and it is generally between 60-70 degrees. The temperatures in the intermediate sections are determined by a method that involves a gradual decrease from the temperature at the machine head, and are generally between 40-50 degrees. Continuous vulcanization feeding generally uses cold feeding, with the rubber sheet thickness controlled between 0.5–1.0 mm. If hot feeding is used and it is cut into strips, that is also very good. 5 Determination of steam pressure and wire drawing speed Steam pressure and wire drawing speed are important parameters in the sulfur bonding process; they affect the mechanical and physical properties of the product, its appearance quality, and production efficiency. Chemical reaction kinetics indicates that, when the reaction concentration remains constant, the reaction rate increases by approximately 2 to 3 times for every 10K rise in temperature. Arrhenius summarized a large number of experimental facts and identified the relationship between the reaction rate constant and temperature: k = A·e^(-E/RT), where k is the reaction rate constant ； E-----reaction activation energy ； R-----gas constant ； T-----absolute temperature, K ； e-----the base of the natural logarithm (e=2.718). This formula presents difficulties in practical use. The practical relationship between the reaction rate of rubber and temperature can be calculated using the following formula: V2 = V1 × √[0.1 × (t2 – t1)], where V1 is the reaction rate at time t1 ； Reaction speed at V2------t2 ； t1,t2------- Temperature (°C) ； 2--------Temperature coefficient. All these theories suggest that for every 10°C increase in the rubber vulcanization temperature, the vulcanization rate doubles, which means that the vulcanization time is reduced by half. We generally have the production and technical parameters for batch vulcanization, and we should be well aware of various vulcanization parameters for a mature rubber formula. Based on the temperature at the saturated steam pressure during tank vulcanization, the saturated steam pressure during continuous vulcanization is calculated, and then the vulcanization time and exit speed are determined. For example, in a continuous vulcanization unit where the length of the continuous vulcanization pipeline is 50 meters and cable outer sheaths are produced using established rubber formulation and vulcanization process parameters – with a pressure of 4 kg/cm2 (151°C) and a vulcanization time of 15 minutes – what would be the steam pressure required for continuous vulcanization if the sheath is produced at a output speed of 25 meters per minute? First, calculate the time required for continuous vulcanization: 50/25=2 minutes. According to the rule that for every 10-degree increase in temperature, the vulcanization time is reduced by half. The calculated time multiplier is 3 times. In other words, the temperature needs to be increased by 30 degrees. In other words, the temperature for continuous vulcanization is 181 degrees. The higher the vapor pressure during continuous vulcanization, the faster the vulcanization rate, and production efficiency also increases significantly. Practice has shown that the steam pressure cannot be increased indefinitely to boost the speed of continuous vulcanization. How to determine the maximum value of the sulfur vapor pressure (i.e., sulfidation temperature). Upon analysis, during vulcanization in a sulfur-coupling tube, the exterior of the product is exposed to high steam pressure as well as high vulcanization temperatures. The air, a small amount of water, low-boiling-point volatile substances within the product, and the gases generated during vulcanization expand. In the vulcanization tube, the internal and external pressures counteract each other, so expansion generally does not occur. When the product is released from the sealed environment into normal pressure, it does not have enough time to cool down rapidly and remains in a state of thermal expansion; the gas inside expands quickly. If the radial stress of the product cannot withstand the internal pressure, bubbles will form, and in severe cases, the product may crack. This phenomenon is more pronounced if the rubber is not mixed evenly. Assuming that the product is treated as a pressure vessel at the moment of sealing at its terminal, this paper derives a formula for the maximum allowable sulfur vapor pressure: δ = P · D_in / (2t – P). Here, δ represents the thickness of the thinnest part of the product under control, in mm ； P ------Maximum sulfurization vapor pressure, MPa ； D inner------Outer diameter of the product before rubber extrusion, mm ； σ] t ----- Radial stress of the product at the vulcanization temperature, N/mm². Among them, the radial stress t of the rubber at the vulcanization temperature is lower than the initial axial expansion strength of the product. The stress during bubbling can be used to calculate the maximum sulfurization vapor pressure at that stage, and the stress at the moment of cracking can be used to determine the maximum sulfurization vapor pressure at that point as well. t can be determined by taking into account comprehensive factors such as the specific tensile strength of the product, the thermal deformation of the rubber at high temperatures, and the relatively low radial tensile strength. Based on the maximum vapor pressure of vulcanization, which is the vulcanization temperature, the vulcanization time, that is, the maximum output speed, can be calculated. At the same time, the degree of tight filling of the cable core gaps also needs to be considered. For example, in the case of a certain heavy-duty rubber-sheathed cable, the outer diameter of the cable assembly is 38.5 mm; the average thickness of the sheath is 5.0 mm, with the thinnest part having a thickness of 4.15 mm. The sheath material is a SE3 rubber mixture made of natural/styrene-butadiene rubber, and its tensile strength is 8.4 N/mm2. The radial tensile strength is generally 60% of the axial tensile strength, so it is 5.04 N/mm2. By applying this value to the formula, the maximum continuous vulcanization steam pressure can be calculated as follows: 4.15 = P × 38.5 ÷ (2 × 5.04 – P); thus, P = 0.98 N/mm2. Therefore, the maximum steam pressure for this heavy-duty rubber-sheathed cable is 10 kg/cm2. Once the maximum steam pressure is known, the maximum drawing speed of this product can be determined using the vulcanization speed formula. For thicker products, the exit speed also needs to take into account the heat transfer rate of the rubber. The formula for heat conduction in rubber is: Q = λ A, where Q represents the heat flow rate, in watts ； λ------Rubber thermal conductivity, W/(m · °C) ； A------Heat transfer area per unit, mm2 ； t1, t2 -------external temperature, internal temperature,℃ ； δ------thickness, m. In the above formula, the thermal conductivity λ of the rubber remains constant, the heat transfer area A remains essentially unchanged, and the temperatures t1 and t2 required for vulcanization on the inside and outside of the rubber product remain constant as well. It can be seen that the heat conduction flux Q is inversely proportional to the thickness δ. That is, to double the thickness and achieve the same vulcanization effect, the wire output speed must be reduced by half. Of course, this is a theoretical statement; it still needs to be verified through practice. 6 Conclusion The process parameters for continuous vulcanization can be calculated in advance using mathematical models, which allow it to determine the appropriate range of specifications, as well as the output speed for continuous vulcanization, along with the suitable and maximum steam pressures. Other process parameters for continuous vulcanization, such as take-up and pay-off tension, cable joints, mold selection, and operating procedures at startup and shutdown, are not discussed in this article due to differences among equipment manufacturers.