In the world of silicone rubber manufacturing, understanding the importance of the post-curing process is crucial for achieving the desired performance characteristics in final products. This blog will explain the process, why it is necessary and the significance of fully post cured parts in specific applications.
By examining these aspects, manufacturers and stakeholders can make informed decisions that optimise production efficiency while maintaining the quality and reliability required for use across different industries.
What is Post-Curing?
Post-curing silicone rubber is the thermal post-treatment step that is essential for achieving superior mechanical properties, removing residual by-products and ensuring the material meets stringent safety and performance standards.
During the extrusion process, as the extrudate passes through a hot air tunnel, silicone rubber undergoes crosslinking, forming three-dimensional networks of polymer chains.This process may leave residual by-products that can adversely affect the rubber’s performance and stability if not properly addressed.
The primary objective of post-curing is to eliminate these unwanted by-products and volatile compounds, further stabilising the silicone rubber. By exposing the material to elevated temperatures for an extended duration, post-curing enhances crosslinking density and creates a more homogeneous network structure. This results in significant improvements in mechanical properties, including tensile strength, elongation at break and compression set.
These enhancements are vital for high-performance applications, such as medical devices, aerospace components and electronics, where products must meet stringent safety and performance standards. While initial curing establishes the foundational structure, post-curing fine-tunes and perfects it for demanding applications.
What does the Post Curing Process involve?
At Advanced Materials the post-curing process for the high consistency silicone rubber (HCR) that we work with involves heating the material at a specific temperature for a set duration in the presence of a controlled air supply.
The recommended standard conditions for post-curing the majority of the silicone rubber grades that we work with is to cure for four hours at 200°C, with a sufficient fresh air supply. This process is designed to enhance the physical and chemical properties of silicone rubber, making it suitable for high-performance applications, and to meet specific compliances.
To achieve the desired benefits of post-curing, manufacturers must adhere to specific conditions and best practices:
- Sufficient fresh air supply – 120 litres per kilogram of silicone rubber per minute is essential.
- Maintaining an oven temperature of 200°C for four hours – crucial for the effective post curing of silicone rubber articles. For thicker sections, additional time may be required.
- Uniform air flow – to avoid ‘dead zones’ (i.e. areas with insufficient heat circulation) which lead to incomplete curing. Packing items too close together, overloading the oven and coiling extrudate too tightly all impact effective air circulation.
- Avoiding cross-contamination – mixing peroxide-cured silicone parts with other materials can result in contamination, suboptimal performance and, in severe cases, cure reversion. It is crucial to avoid mixing silicone rubber with other rubber types, such as Ethylene Propylene Diene Monomer (EPDM), Natural Rubber (NR) or Butadiene Rubber (BR), during the post-curing process, particularly with elastomers containing sulphur. To prevent cross-contamination, platinum-cured silicone articles destined for post-curing should be processed in a dedicated oven.
- Oven maintenance and cleaning – to ensure optimal air movement, regular cleaning and maintenance are crucial. The high demand for fresh air during the post-curing process requires significant energy, making it essential to maintain the ovens properly. Implementing a thorough and consistent cleaning schedule is vital for removing evolved by-products, preventing buildup, and mitigating the risk of explosion.
By meticulously controlling the post-curing conditions, silicone rubber manufacturers and extrusion experts ensure that silicone rubber components achieve optimal performance and reliability.
Uses of Post-Cured Silicone Rubbe
Post-curing is indispensable in various industries where silicone rubber’s performance and reliability are paramount. These include:
Food Contact and Household Goods
Silicone rubber products are widely used in food contact applications, including bakeware, food processing facilities, beverage hoses, milk pumps and bottle stoppers. These products significantly benefit from post-curing in particular, which plays a crucial role in ensuring they are free from any decomposition products. This process helps them meet stringent safety standards, such as those established by the FDA (Food & Drug Administration) and BfR (Federal Institute for Risk Assessment, or Bundesinstitut für Risikobewertung in German). Post-curing specifically effectively removes short-chain silicone fragments, ensuring compliance with FDA migration limits for D4 and D5 cyclic siloxanes. By employing post-curing, manufacturers can enhance the safety and performance of silicone rubber products in food-related applications, providing consumers with reliable and high-quality options.
Drinking Water Applications
In processes involving drinking water, it is essential to thoroughly post-cure silicone rubber products to eliminate any residual compounds that could contribute to adverse tastes or odours. This meticulous post-curing ensures that the silicone components are safe and reliable, maintaining the purity of the drinking water and safeguarding public health.
Healthcare and Medical Devices
In healthcare, silicone rubber is used in hoses, seals, device components, membranes, bellows and medical tubing. Post-curing is essential for these applications to ensure that the material meets the necessary certifications. When USP Class VI testing (a medical device biocompatibility testing standard) is conducted on a cured silicone rubber piece, the results are valid across the grade, provided that the same curing conditions are maintained. This consistency is crucial for ensuring the safety and reliability of medical devices, ultimately safeguarding patient health and compliance with regulatory standards.
Baby Care Products
Products like pacifiers or dummies, and bottle teats demand materials that are safe, durable and free from harmful residues. Post-curing plays a vital role in ensuring that these baby care items adhere to the highest safety standards while preserving essential mechanical properties such as flexibility and tear resistance. By eliminating any residual compounds, post-curing guarantees that these products are reliable and suitable for infant use, ultimately providing peace of mind for parents and caregivers.
Applications Where Post-Curing is Not Necessary
While post curing is a critical step in many silicone rubber manufacturing processes, there are certain scenarios where it might be unnecessary.
Applications with Non-Critical Performance Requirements
For products where the mechanical properties, chemical resistance and purity are not as critical, post curing might be omitted and replaced with a non-post cure additive to prevent the ‘blooming’ or migration of additives to the surface of decomposition products. For non-critical gaskets and seals which are used in less demanding environments where specific mechanical properties are not essential, and where minor by-products or slight mechanical deficiencies do not impact performance.
Use of Addition-Cured Silicones
In cases where addition-cured (platinum catalyst) silicones are used and the formulation is designed to have no excess crosslinker, the need for post curing to degrade these residuals is reduced. This assumes that the initial curing process is sufficient to achieve the desired properties and the material does not require the post curing to meet specific certification standards.
Non-Contact Applications
For products that do not come into direct contact with sensitive substances – such as food, medical products, or potable water – the stringent standards of post-curing may be relaxed. This category includes industrial parts used in machinery where direct human contact or strict purity is not necessary, as well as automotive components that do not interact with critical fluids or systems sensitive to contamination. In these cases, the focus can shift from rigorous compliance to meeting functional performance requirements, allowing for more flexibility in manufacturing processes.
Silicone Rubber Grades Specifically Designed to Eliminate Post-Curing
Some advanced silicone rubber formulations are specifically engineered to eliminate the need for post-curing. These materials are designed to achieve complete curing at lower temperatures, which minimises the production of residual by-products during the initial curing process. As a result, they maintain excellent mechanical and chemical properties without requiring additional thermal treatment. This innovative approach not only streamlines the manufacturing process but also enhances the efficiency and performance of the final product, making them suitable for various applications.
Decision Making about Post-Curing
Pos- curing is a vital step in the silicone rubber manufacturing process, crucial for enhancing the material’s performance and ensuring its safety and reliability in various applications. By effectively removing residual by-products, improving mechanical properties, and meeting stringent regulatory standards, post curing ensures that silicone rubber products deliver the highest quality and performance. For manufacturers, implementing proper post curing practices is essential to achieve these benefits.
While post curing is a crucial process in many silicone rubber manufacturing scenarios, certain applications and material formulations allow manufacturers to skip this step without compromising the quality and performance of the final product. By understanding the application requirements and conditions, customers and manufacturers can work together to make informed decisions about the necessity of post curing, optimising their production processes and potentially reducing costs. However, it is essential to carefully evaluate the implications of omitting post curing to ensure that the final product still meets the required standards and performs as expected in its intended use.
Advanced Materials is a leading supplier of silicone rubber extrusions. To find out more about our silicone rubber manufacturing services contact our expert and friendly team on +44 (0)1903 713566 or sales@amsilicones.co.uk