Which Rubbers Require Post-Curing? What Are the Functions of Post-Curing?

I. Definition of Post-Curing

In production workshops, the process of heating, pressurizing and shaping rubber products inside moulds is defined as primary vulcanization (also known as first-stage vulcanization).

In contrast, post-curing (commonly called secondary vulcanization or after-cure in workshops) refers to the procedure where demoulded and dimensionally fixed rubber goods are neatly stacked in industrial hot-air circulating ovens and baked under atmospheric pressure at constant temperatures (typically 150–200°C) for several consecutive hours.

II. Categories of Rubbers That Need Post-Curing

Post-curing is not mandatory for all rubber grades. Conventional general-purpose rubbers including Natural Rubber (NR), Styrene Butadiene Rubber (SBR) and Polybutadiene Rubber (BR) achieve full vulcanization after primary mould curing and can be dispatched directly as finished products.

Post-curing is exclusively required for high-performance specialty rubbers with strict property specifications or formulated with special curing agents, detailed as follows:

1. Silicone Rubber (MVQ / Silicone): over 95% of products require post-curing
   Reason: Peroxide curing agents (such as DBPH-50, DCBP and odourless DBPH curing agent) are adopted for silicone compression or injection moulding. These peroxides generate substantial acidic by-products and volatile residues upon primary curing inside moulds. Without subsequent oven baking for post-curing, silicone articles tend to turn brittle, yellow or develop bloom on surfaces after short-term storage.
   
2. Fluorocarbon Rubber (FKM / Viton): 100% mandatory post-curing
   Reason: FKM features slow vulcanization kinetics. Only approximately 70% of the chemical crosslink network forms within the short minutes of primary mould vulcanization. The remaining 30% crosslinking is completed via prolonged baking for 8–24 hours at 200–230°C in precision ovens, enabling FKM to attain its final state with outstanding oil and high-temperature resistance.
   
3. Acrylate Rubber (ACM) & Hydrogenated Nitrile Butadiene Rubber (HNBR)
   Reason: Both materials are widely used for premium automotive oil seals and engine gaskets. Similar to FKM, full crosslinking cannot be accomplished during primary mould curing. Post-curing in ovens is indispensable to secure ultra-low compression set performance.
   
4. Automotive interior rubber parts with ultra-low odour and low VOC requirements (e.g. EPDM pedal pads and gaskets)
   Reason: Automakers impose stringent standards on in-vehicle air quality complying with VDA 270 odour testing specifications. Standard EPDM after primary vulcanization retains pungent amine and mercaptan residues. Intensive hot-air post-baking is needed to fully eliminate volatile odorous compounds.
   

III. Core Functions of Post-Curing

Despite extra labour and power consumption, post-curing delivers four irreplaceable critical benefits for finished rubber components:

Four Core Advantages of Post-Curing

1. Complete crosslink network formation: eliminate under-vulcanization and drastically boost resilience & tensile strength
   
2. Remove low-molecular volatiles: strip residual curing agents to eradicate unpleasant odours and surface bloom
   
3. Release internal stress: avoid post-production warpage, dimensional shrinkage and deformation
   
4. Optimize long-term durability: minimize compression set across high and low temperature ranges
   

1. Perfect Crosslink Network: Complete Full Vulcanization of Rubber

Most specialty rubbers remain partially cured or insufficiently crosslinked after primary mould forming, analogous to undercooked rice needing a final simmering stage in rice cookers during post-curing.

Outcome: Unreacted molecular chains continue crosslinking under sustained heating, markedly increasing crosslink density, which delivers substantial improvements in tear strength, tensile strength and elastic recovery of vulcanized rubber.

2. Eliminate Low-Molecular Volatiles: Remove Odour and Surface Bloom

Harmful odorous by-products decomposed from curing additives inside moulds are vaporized and exhausted by circulating hot air in ovens during post-curing.

Outcome: Fishy, kerosene-like and pungent VOC odours are fully eliminated; residual curing additives are prevented from migrating to product surfaces to stop bloom formation. Post-curing serves as a compulsory precondition for medical-grade silicone and baby nipples to pass FDA food-contact certification.

3. Stabilize Dimensional Accuracy: Release Trapped Internal Stress

Rubber compound is forced into mould cavities under high compression, leaving locked-in internal stress within molecular chains. Direct delivery without post-curing leads to gradual shrinkage, twisting and distortion of finished parts during storage.

Outcome: Oven heat enables molecular chain relaxation to fully release residual internal stress, ensuring stable product dimensions free from long-term deformation.

4. Improve Durability: Reduce Compression Set to the Minimum Level

Premium oil seals and O-rings are most susceptible to permanent deformation under compressive load with poor rebound performance.

Outcome: Post-curing builds a fully integrated crosslink structure, cutting high- and low-temperature compression set of EPDM, FKM and HNBR down to 1/2 or even 1/3 of original values. This extends sealing service life and effectively prevents premature oil and gas leakage.