Vulcanization time control is a core technical link in the vulcanization process of conveyor belts, directly affecting the crosslinking density, physical properties and service life of rubber. To reasonably control the vulcanization time, it is necessary to combine the type of rubber, vulcanization temperature, pressure and product structure. The following is an analysis of the mechanism of action, control methods and common problem solutions of vulcanization time:
First, the mechanism by which vulcanization time affects the properties of rubber
The stages of the crosslinking reaction
The vulcanization process can be divided into the scorch stage, the hot vulcanization stage, the normal vulcanization stage and the overvulcanization stage.
Scorch stage: The rubber begins to cross-link but has not yet formed an effective network. At this stage, it is necessary to avoid temperature or pressure fluctuations that may cause premature vulcanization.
Hot vulcanization period: The cross-linking reaction accelerates, and the hardness and strength of the rubber increase rapidly, usually lasting for 3 to 8 minutes (taking natural rubber as an example).
Optimum vulcanization period: The crosslinking density reaches the optimal value, and the comprehensive properties of the rubber such as tensile strength and wear resistance are the best.
Overvulcanization period: Crosslinking bonds break or reorganize, and the performance of rubber begins to decline, manifested as increased hardness and reduced elasticity.
The relationship between vulcanization time and crosslinking density
Insufficient vulcanization time will lead to low crosslinking density and insufficient rubber strength. If the vulcanization time is too long, it may cause the crosslinking bonds to break or the molecular chains to degrade. For instance, when nitrile rubber is vulcanized at 150℃, if the vulcanization time is extended from 15 minutes to 25 minutes, the crosslinking density may increase by 10% to 15%, but the performance will decline instead after more than 30 minutes.
Second, key technologies for controlling vulcanization time
Precise setting based on the vulcanization curve
The vulcanization curve was obtained through a rotorless vulcanizing instrument (MDR) or a rheometer to determine the optimum vulcanization time (T90, that is, the time required for the torque to reach 90% of the maximum value).
Example: If the T90 of a certain rubber is 18 minutes, then the actual vulcanization time can be set to 18 to 20 minutes (considering the thermal inertia of the equipment).
Dynamic adjustment: Adjust the vulcanization time based on changes in ambient temperature. For every 10℃ decrease in ambient temperature, the vulcanization time needs to be extended by 5% to 8%.
The application of segmented vulcanization process
Pre-vulcanization stage: Pre-vulcanization is carried out at a relatively low temperature (such as 120℃) for 5 to 10 minutes to promote the initial cross-linking of rubber and reduce the formation of bubbles during subsequent high-temperature vulcanization.
High-temperature vulcanization stage: Rapidly heat up to the target temperature (such as 150℃), and maintain the T90 time to complete the crosslinking.
Cooling and setting stage: After vulcanization is completed, the temperature should be slowly reduced to below 80℃ before pressure relief to prevent joint cracking caused by thermal stress.
Online monitoring and feedback of vulcanization time
Infrared temperature measurement: Real-time monitoring of the temperature of the vulcanized plate. If the temperature fluctuation exceeds ±3℃, the vulcanization time compensation needs to be adjusted.
Pressure compensation mechanism: When the pressure drop during vulcanization exceeds 0.2MPa, extend the vulcanization time by 1-2 minutes to ensure adequate crosslinking.
Visual inspection: Observe the color changes on the rubber surface through a camera (such as from light yellow to dark brown) to assist in determining the degree of vulcanization.
Third, common problems and solutions in vulcanization time control
Insufficient vulcanization (undervulcanization
Symptoms: The rubber surface is sticky, with low hardness and insufficient tensile strength.
Reasons: Too short vulcanization time, too low temperature or insufficient pressure.
Solution: Extend the vulcanization time by 10% to 15%, and at the same time check whether the equipment temperature and pressure meet the standards.
Excessive vulcanization (overvulcanization
Symptoms: The rubber becomes brittle, its elasticity decreases, and cracks appear on the surface.
Reasons: Excessive vulcanization time, excessively high temperature or improper rubber formula.
Solution: Reduce the vulcanization time by 5% to 10%, lower the vulcanization temperature by 2 to 5 degrees Celsius, or adjust the dosage of the vulcanization accelerator.
Uneven vulcanization
Manifestation: Insufficient local strength at the joint or the appearance of bubbles.
Reason: Uneven distribution of vulcanization time or temperature.
Solution: Adopt segmented heating or electromagnetic induction heating technology to ensure that the temperature uniformity of the vulcanized plate is within ±2℃.
Fourth, the optimization direction of vulcanization time control
Intelligent control
Introduce PLC or industrial computers, and in combination with parameters such as vulcanization curves, temperature and pressure, automatically calculate and adjust the vulcanization time. For example, the optimal vulcanization time of different batches of rubber is predicted through machine learning algorithms to reduce human errors.
Rapid vulcanization technology
Develop a low-temperature rapid vulcanization system (such as using peroxide vulcanizing agents) to achieve comparable performance to the traditional 150℃ vulcanization at 120-130℃, reducing the vulcanization time by 30%-50%.
Simulation and Emulation
The cross-linking reaction and temperature distribution of rubber during the vulcanization process were simulated by finite element analysis (FEA) to optimize the vulcanization time and process parameters. For example, through simulation, it was found that the edge temperature of the vulcanized plate was relatively low, and the heating power could be adjusted specifically.