Rubber technology is a science of precisely controlling material properties. Whether manufacturing a sealing ring or a tire, the vulcanization process is at the heart of it all. This article presents core knowledge on rubber technology in a Q&A format, covering the nature of rubber, vulcanization mechanisms, and key process control parameters.
I. Rubber Properties and Performance Changes
1. What is rubber, and what are its characteristics as a polymer material?
Answer: Rubber is an organic polymer elastomeric compound with a molecular weight typically ranging from tens of thousands to over a hundred thousand. Its unique properties stem from its molecular structure, characterized by the following key features:
Molecular Chain Flexibility: The segments of the molecular chains are highly mobile, and the chain segments can easily rotate and curl, imparting high elasticity to rubber.
Intermolecular Forces: The intermolecular forces are relatively weak, allowing molecular chains to slide and stretch under external force.
Physical State: It exists in an amorphous state at room temperature.
Crosslinkability: The molecular chains can form crosslinks (i.e., vulcanization), and both before and after crosslinking, the material can achieve the required physical and mechanical properties for practical applications.
2. Why do rubber products become hard, sticky, or cracked during use?
Answer: This phenomenon is known as aging. During processing, long-term storage, and service, rubber products are subjected to a combination of internal and external factors that gradually degrade their performance. These factors mainly include:
Chemical Action: Oxidative attack by oxygen, ozone, and variable-valence metal ions.
Physical Action: Damage from mechanical stress, light, and high-energy radiation.
Biological Action: Erosion by mold, insects, and other organisms.
Consequences: These actions cause either chain scission or excessive crosslinking of rubber molecules, macroscopically manifesting as the product becoming soft and sticky, hard and brittle, or surface-cracked, with a significant decline in physical and mechanical properties.
II. Vulcanization: From Linear to Network Structure
3. What is vulcanization, and what does it mean in rubber processing?
Answer: Vulcanization is the most critical chemical step in rubber product manufacturing. It refers to the process by which, under specific temperature, pressure, and time conditions, rubber molecules in the compound are transformed from a linear structure into a network structure (three-dimensional spatial network) through crosslinking. This transformation changes the rubber from a plastic, viscous-flow state into a vulcanizate with high elasticity and good physical properties, thus giving it practical value.
4. What is "scorch," and what harm does it cause in production?
Answer: Scorch, also known as premature vulcanization, is a problem that must be strictly avoided in rubber processing. It refers to premature crosslinking that occurs in the compound before the vulcanization stage—during various processing operations (such as mixing, calendering, extrusion) and storage—due to exposure to heat or mechanical action.
Hazards: Scorch reduces the plasticity and flowability of the compound, making subsequent molding operations difficult and potentially resulting in defective products.
III. The "Three Elements of Vulcanization": Temperature, Time, and Pressure
Effective control of the vulcanization process depends on three core process parameters, collectively known as the "Three Elements of Vulcanization": Vulcanization Temperature, Vulcanization Time, and Vulcanization Pressure.
5. Is a higher vulcanization temperature always better? How does it relate to the other elements?
Answer: Vulcanization temperature is a primary control parameter. Increasing the temperature can significantly shorten the vulcanization time and improve efficiency. For example, in the range of 130°C–170°C, for every 10°C increase in temperature, the vulcanization time is approximately halved.
However, higher temperature is not always better, due to the following limitations:
Thermal Conductivity Limitation: Rubber is a poor conductor of heat. For thick products, high temperatures can lead to uneven vulcanization between the inner and outer layers.
Heat Resistance Limitation: Different rubbers have different heat resistance capabilities. For example, natural rubber is better suited for low-temperature, long-time vulcanization; excessively high temperatures can impair its properties.
Damage to Reinforcement Materials: Excessively high temperatures can weaken the strength of fibers or textile reinforcements within the product.
6. How is vulcanization time determined, and what happens if it is too short or too long?
Answer: Vulcanization time must be precisely controlled to achieve the "optimum cure time" —the point at which the physical and mechanical properties of the compound reach their best overall balance.
Too Short (Under-cure): The crosslinking reaction is incomplete, resulting in insufficient strength, elasticity, and other performance attributes.
Too Long (Over-cure):
For natural rubber, "reversion" occurs, leading to a decline in properties.
For most synthetic rubbers, excessive crosslinking causes the material to harden.
7. Is vulcanization pressure only for shaping? What specific functions does it serve?
Answer: Vulcanization pressure is not only essential for shaping the product but also critical for ensuring its quality. During vulcanization, pressure serves the following key functions:
Improves Density: Compacts the compound and eliminates internal air bubbles, preventing a spongy or porous structure.
Promotes Flow: Drives the compound to flow within the mold cavity, ensuring it quickly and completely fills every detail of the mold.
Ensures Surface Quality: Presses the compound tightly against the mold surface, resulting in a smooth surface and clear patterns/inscriptions.
Enhances Adhesion: Improves the bonding strength between rubber and fabric layers or metal inserts, increasing the product's durability.
It is worth noting that pressure has a negligible effect on the vulcanization rate and is generally disregarded in process calculations.
IV. The Four Stages of the Vulcanization Process
The entire vulcanization process can be divided into four stages based on time. Understanding these stages helps in precisely controlling the process:
Induction Period Stage: Crosslinking has not yet begun. The compound exhibits good flowability within the mold cavity, ensuring operational safety.
Hot Curing Stage: The crosslinking reaction starts at a certain rate and accelerates. Various properties improve rapidly but have not yet reached their optimum.
Optimum Curing Stage: Physical and mechanical properties reach or approach their best overall balance. The temperature and time at this point are referred to as the "optimum cure conditions."
Overcuring Stage: The stage following optimum vulcanization. In the early part of this stage, there may be a "cure plateau" where properties remain relatively stable. Beyond this plateau, over-cure occurs, characterized by "reversion" in natural rubber or hardening in most synthetic rubbers.
Conclusion
Mastering rubber technology centers on understanding the logic of molecular structural changes and precisely balancing and controlling the relationships among the "Three Elements of Vulcanization." In actual production, the optimal vulcanization conditions must be determined based on the rubber type, product geometry, and processing method to ensure final product quality. We hope this Q&A format helps you build a clear and solid framework of knowledge in rubber technology.