Why is it necessary?
During the post-curing process, vulcanizates get exposed to elevated temperatures in order to speed up the curing process, to optimize the material’s physical properties and to remove remaining volatiles, unwanted impurities or by-products resulted from cross-linking reactions. Optimal post-curing time and temperature conditions may vary and strongly depend on type of material, specific formulation, cross-linking system and application. Diffusion and evaporation of volatiles should be carried out at a temperature greater than the service temperature for the part. Always consult with your supplier!
Post-curing process is typically performed on peroxide andbisphenol curing compounds, but it can also be performed on sulphur curing compounds as well, to improve a particular physical property. In certain applications post-curing, however, is absolutely mandatory.
In peroxide curing VMQ based compounds post-curing process removes remaining volatiles, unwanted impurities and peroxide by-products, resulting in clean, odorless material. In case the material is compounded with food and water contact in mind, post-curing is absolutely mandatory in order to meet the demands of FDA, BfR, KTW or WRAS norms.
Platinum curing VMQ do not contain peroxide as such, but in order to meet the food contact demands, platinum curing VMQ compounds must be post-cured just as well, in order to remove the volatiles that remained in the material and to improve certain physical properties, especially the compression set and high temperature stability.
FKM based compounds are commonly subjected to a two-stage cure cycle in order to optimize material’s physical properties. Compression set and tensile strength are two of the properties most often improved by an oven post-cure cycle. Initial curing is typically completed under pressure in a mold or autoclave to prevent porosity or rupturing of the part due to gas formation from by-products of the cross-linking reactions. A secondary cure is accomplished during post-curing process. Typical post-curing cycles may be as short as 4 hrs or as long as 24 hrs, depending on the type of material and cross-linking agent used. More about it below.
Crucial parameters and guidelines
The majority of improvements in properties are typically obtained after the first 2–4 hrs in the oven, with smaller incremental improvements after longer post-cure times.
The rising step (or time) of the temperature inside the oven depends on the item thickness and should be altered accordingly. Articles thicker than 4mm require additional post-curing time. Adequate airflow around each individual item is critical, which is why one has to make sure that the items are not stacked. In case of post-curing long extruded profiles make sure there is minimal surface contact.
During post-curing, sufficient fresh air supply is mandatory. If ventilation is poor, high concentration levels of volatiles will occur, preventing proper out-gassing, as well as resulting in risk of fire or explosion! Poor out-gassing and high concentration levels of volatiles in the oven may also lead to reduction in strength, elongation, compression set, accompanied by chemical decomposition and sticky surface. Yellowing can also be expected in transparent and coloured VMQ compounds. To avoid aforementioned risks, we therefore recommend 120 liters of fresh air flow per kilogram of rubber per minute.
Never put articles made out of different types of material together in the same post-curing oven, even if post-curing conditions are the same. Ideally one should have separate ovens for individual material types to avoid cross-contamination due to out-gassing.
When in doubt whether post-curing process for food contact VMQ items was sufficient, measure and compare weight loss of the reference part to the one which was fully and properly post-cured. Note that not all volatiles are ever out-gassed, some still remain inside even after post-curing. Silicone material will continue to outgas for extended periods of time at elevated temperatures. But extraction levels after 4h@200°C are low enough to pass most food and water contact requirements.
Typical post-curing time temperature conditions are as follows:
- Peroxide and platinum curing VMQ and FVMQ compounds: 1 h rising time + 4 h @ 200°C
- Bisphenol curing FKM black compounds: 4 h rising time + 20 h @ 240°C
- Bisphenol curing FKM coloured compounds: 4 h rising time +20 h @ 220°C
- Peroxide curing FKM: 2h rising time + 4 h @ 230°C
- Peroxide curing FEPM: 2h rising time + 8 h @ 220°C
- Peroxide curing HNBR compounds: 1 h rising time + 2 h at 160°C
- Peroxide curing EPDM compounds: 1 h rising time + 3 h at 150°C
Some exceptions are applied to metal-bonded rubber parts, where both, metal properties (when items are designed to be elastic) or adhesion to the metal can be impaired, due to too high post-curing temperature (above 210°C).In that case the post-curing conditions must be changed accordingly.
Most common mistakes when post curing.
- Putting too much material in the oven can result in high volatile concentrations. Please avoid.
- Having low fresh air circulation can result in high volatile concentrations. Please avoid.
- Kitchen oven is used for post-curing. AVOID!
- Stacking materials one on top of the other may lead to insufficient post-cure. Please avoid.
- Putting articles made out of different materials can lead to cross-contamination and inhibition. Please avoid.