The characteristics of the rubber vulcanization stage

Rubber vulcanization is an indispensable process in the production of rubber products and also a key link for fundamental changes in the properties of rubber. The vulcanization process, through a series of complex chemical reactions, transforms rubber from a linear structure to a three-dimensional structure, thereby obtaining excellent physical and mechanical properties, heat resistance, solvent resistance and corrosion resistance, significantly enhancing the application value and scope of rubber products. This article will analyze in detail the characteristics of each stage of rubber vulcanization.

I. Overview of the Vulcanization Stage

Rubber vulcanization is a chemical cross-linking process. By heating or adding a vulcanizing agent, a stable cross-linking structure is formed between the rubber molecular chains. The vulcanization process can be roughly divided into four stages: scorch stage, hot vulcanization stage (pre-vulcanization stage), flat vulcanization stage (normal vulcanization stage), and over-vulcanization stage. Each stage has its unique characteristics and significance.

Ii. Charring Stage

The scorching stage is the initial stage of the vulcanization process and is also known as the vulcanization induction stage. At this stage, crosslinking has not yet begun, and the rubber compound has good fluidity in the mold cavity. The length of scorch time directly affects the scorch performance and processing safety of the rubber compound. The scorching time is affected by various factors such as the vulcanization accelerator, the thermal history of the rubber compound itself, and the processing conditions. To achieve a longer scorch time, slow-acting accelerators are often used in the formula, and low temperatures are maintained during the mixing, storage and molding processes to reduce the operation scorch time.

Iii. Hot Vulcanization Stage (Pre-vulcanization Stage)

The hot vulcanization stage follows the scorching stage immediately and is the stage when the crosslinking reaction begins. At this stage, the crosslinking of rubber proceeds at a certain speed, but the degree of crosslinking is relatively low, and the performance has not yet met the expected requirements. However, at this time, the tear resistance and wear resistance of rubber have been superior to those of unvulcanized rubber compounds. The speed and time of hot vulcanization depend on the rubber compound formula and vulcanization temperature. At this stage, the crosslinking reaction gradually proceeds, laying the foundation for the subsequent flat vulcanization stage.

Iv. Flat vulcanization Stage (Normal vulcanization Stage)

The flat vulcanization stage is a crucial stage in the vulcanization process and also the stage when the performance of rubber products reaches the optimal value. At this stage, the cross-linking reaction is nearly complete, and the comprehensive physical and mechanical properties of the rubber products (such as elasticity, tensile strength, elongation at a given length, etc.) have reached or approached the optimal values. This stage is also known as the optimum vulcanization stage or the optimum vulcanization stage. The selection of optimum vulcanization time is crucial for ensuring the performance of rubber products. Generally, the time slightly before the tensile strength reaches its maximum value is chosen as the optimum vulcanization time for the process.

V. Overvulcanization stage

The overvulcanization stage is the final stage of the vulcanization process and also the stage when the properties of rubber may change or decline. At this stage, different rubbers perform differently. Due to the strong degree of oxidation chain breaking reaction of natural rubber, its various physical and mechanical properties may decline. Most synthetic rubbers (such as SBR and NBR) may maintain their performance constant or slightly improve. The timing of overvulcanization and the width of the vulcanization flat period depend on factors such as the formula and temperature.

Vi. Performance Changes during vulcanization

Mechanical properties: As the degree of vulcanization increases, the elasticity, tensile strength, elongation at a given length, tear strength and hardness of rubber all significantly improve, while the elongation, compression set and fatigue heat generation decrease accordingly.

2. Physical properties: After vulcanization, the air permeability and water permeability of rubber decrease. It cannot dissolve in solvents and can only swell. Meanwhile, the heat resistance of rubber has been significantly improved and the applicable temperature range has been expanded.

3. Chemical stability: The cross-linking reaction reduces the chemically active groups or atoms in rubber, and the network structure hinders the diffusion of low-molecular substances, thereby enhancing the chemical stability of rubber.

Vii. Summary

Rubber vulcanization is a key link in the production of rubber products. Through the vulcanization process, rubber transforms from a linear structure to a three-dimensional structure, obtaining excellent physical and mechanical properties as well as chemical stability. Understanding the characteristics and performance changes of each stage of vulcanization is of great significance for optimizing the vulcanization process and improving the quality of rubber products. Rubber technicians should closely monitor every link in the vulcanization process and constantly adjust the formula and processing technology to meet all the requirements of product design.