Ⅰ.What Is Post-Curing?
In the production workshop, the process of heating, pressurizing, and shaping the finished product in a mold is called “first-stage curing” (also known as primary curing or initial curing).
“Secondary vulcanization” (commonly referred to in the workshop as “second-stage vulcanization” or “post-cure”) refers to the process of neatly stacking rubber products that have already been demolded and shaped into a large industrial oven equipped with forced-circulation hot air, and continuing to bake them at atmospheric pressure for several hours at a specific temperature (typically 150–200°C).
Ⅱ.Which Rubbers Require Secondary Vulcanization?
Not all rubbers require secondary vulcanization. Common types such as natural rubber (NR), styrene-butadiene rubber (SBR), and butadiene rubber (BR) are generally fully cured after the initial vulcanization stage in the mold and are shipped directly from the factory.
Those requiring secondary vulcanization are often “high-end specialty rubbers” that are expensive, subject to extremely strict performance requirements, or made with special vulcanizing agents:
Ⅱ.Which Rubbers Require Secondary Vulcanization?
Not all rubbers require secondary vulcanization. Common types such as natural rubber (NR), styrene-butadiene rubber (SBR), and butadiene rubber (BR) are generally fully cured after the initial vulcanization stage in the mold and are shipped directly from the factory.
Those requiring secondary vulcanization are often “high-end specialty rubbers” that are expensive, subject to extremely strict performance requirements, or made with special vulcanizing agents:
1.Silicone Rubber (MVQ / Silicone) — Over 95% require secondary vulcanization
Reason: During compression molding or injection molding, silicone rubber uses peroxide-based curing agents (such as Di-25, Di-24, and odorless Di-25 curing agents). After these curing agents complete their reaction in the mold, they produce large amounts of acidic byproducts and volatile substances. Unless these are removed through a secondary curing process in an oven, silicone products will become brittle, yellow, or even develop a white bloom on the surface after just a few days.
2.Fluorocarbon Rubber (FKM / Viton) — 100% mandatory
Reason: Fluorocarbon rubber reacts relatively slowly. During the brief few minutes spent in the mold (the first stage of curing), it actually forms only about 70% of its chemical cross-linking network. The remaining 30% must be transferred to a high-end oven set at 200–230°C and thoroughly cured for 8 to 24 hours to fully transform into its ultimate “oil- and heat-resistant” state.
3.Acrylate Rubber (ACM) and Hydrogenated Nitrile Rubber (HNBR)
Reason: These two types of rubber are commonly used in high-end automotive oil seals and engine gaskets. Similar to fluorocarbon rubber, their reactions within the mold rarely reach full saturation. To achieve extremely low compression set, they must undergo secondary post-curing in an oven.
4.Automotive interior rubber parts with ultra-low odor and low VOC requirements (e.g., EPDM pedal covers, gaskets)
Reason: Automakers enforce extremely strict standards for in-cabin air quality (odor testing per VDA 270). Ordinary EPDM products retain pungent amine and mercaptan odors after vulcanization, so they must be placed in an oven where high-intensity hot air is used to “squeeze out and bake away” the odors in a single pass.
III. What Are the Core Benefits of Secondary Vulcanization?
Given that it is labor-intensive and energy-consuming, secondary vulcanization must offer four irreplaceable, miraculous benefits:
The Four Core Benefits of Secondary Vulcanization
1.Fills the cross-linking network (eliminates under-vulcanization, doubling rebound and tensile strength)
2.Evaporates small molecules (removes residual cross-linking agents through heating, completely eliminating odors and white bloom)
3.Eliminating Internal Stress (Prevents later issues such as curling edges, distortion, and deformation)
4.Enhancing Durability (Maximizing resistance to pressure changes at high and low temperatures)
1.Making the Cross-Linked Network More Dense: Truly “Baking” the Rubber Through
Many specialty rubbers are in a “half-baked” or “barely passable” state after the first stage of compression molding. Secondary vulcanization is like placing rice in a rice cooker for the final “steaming” process.
Effect: It allows unreacted molecular chains within the rubber to continue linking together, exponentially increasing the cross-linking density. The resulting cured rubber experiences a qualitative leap in tear strength, tensile strength, and resilience.
2.Eliminate low-molecular-weight volatiles: Purify the product, eliminate odors, and remove bloom
Toxins and odors generated by curing agents inside the mold are forcibly vaporized and extracted by the high-temperature hot air in the oven.
Effect: Completely removes the fishy, kerosene, and pungent VOC odors from new products; simultaneously, it prevents curing agent residues from migrating to the surface, thoroughly eliminating the problem of “blooming” or “whitening” on the surface. For products such as medical-grade silicone and baby pacifiers, secondary vulcanization is a mandatory requirement for obtaining food-grade certification (FDA).
3.Stabilizing Product Dimensions: Eliminating “Trapped Internal Stress”
When rubber compound is forced into the mold under high pressure, its molecular chains accumulate “internal stress” from being constrained. If shipped directly from the factory, the products will gradually shrink, deform, and warp over time.
Effect: The high temperature in the oven allows the molecular chains to relax freely, releasing all the pent-up tension (eliminating internal stress). As a result, the finished products maintain extremely stable dimensions and will not lose their shape no matter how they are positioned.
4.Quality Enhancement: Pushing Compression Set (Creep Resistance) to the Limit
High-end oil seals and O-rings, in particular, are most vulnerable to failing to rebound after being compressed.
Effect: Secondary vulcanization creates a seamless chemical network, reducing the high- and low-temperature compression set of EPDM, fluorocarbon rubber, and hydrogenated nitrile rubber to half or even one-third of their original values. This not only extends the service life of the seals but also prevents premature oil and gas leaks.
