Does Low-volatile Liquid Silicone Molded Parts Need Post-Curing

It is the obligation for the rubber parts manufacturer to provide safe and high-performance silicone rubber parts to the customers and end-users. So it very import to ensure the materials and components meeting health and safety standard. For these molded liquid silicone parts, manufacturer should try to eliminate the volatiles as much as possible during the produtcion process.

What is Low-volatile Liquid Silicone?

Low-volatile liquid silicone is the liquid silicone with low volatiles that can meet the volatile content <0.5% without going through post-curing. By reducing the low molecular weight siloxane volatile content in the liquid silicone, there is no need to post-cure the molded silicone parts. The purity and safely are highly improved. Low-volatile liquid silicone significantly increase the production efficiency and save much energy and cost for liquid silicone molders. 

However, though the volatile content decreases and meet the standard, to get a higher purity, it is recommended to post-cure the molded low-volatile liquid silicone parts again, especially for the medical silicone parts and baby feeding industries. The mechanical properties can be further improved after post-curing.

Low-volatile liquid silicone overmolded with PC 

What Are Volatiles in Silicone Rubber?

Volatiles also known as volatile organic compounds (VOC) which are chemincal compounds that change easily into gases. Some of the volatiles are harmful to human health. They will cause eyes irritation, nause, damage to central nervous system, even cancer.

In silicone rubber molding process, the volatiles are those curing residues or impurities, like low molecular weight siloxane (Dn), and other peroxide byproducts. Small molecular weight additives, solvents or oligomers, these are common volatiles in silicone and liquid silicone molded parts. But the volatile content can be largely reduced during manufacturing process to meet the stringent regulatory and customer requirements.

For example, Wacker R240 silicone raw material that haven't been cured and post-cured. From the table, we can see that D5, D6 and D4 content exceed the limitation of REACH.

After molding and post-curing for 2 hours in 200°C high temperature. The volatile content has reduced to less than 0.05% limitation.

How to Reduce the Volatiles for Molded Silicone Rubber Parts?

There are strict standard on volatile content for the molded silicone parts. For example, volatile content of food-contact silicone parts and children’s caring products can not exceed 0.5% according to European Standard, EN14350.

In molding vulcanization process, volatiles in silicone can be reduced to some extent, but still can not achieve the required standard. So manufacturer always choose to post-cure the molded solid silicone & liquid silicone parts to eliminate the volatiles.

Post-curing is the long and energy-consuming process, which increases the production cost, and emit gas into the air. Common post-curing conditions for molded silicone & liquid silicone parts are 4 hours, 400℉ (200℃). But the post-curing time is variant according to thickness of molded silicone parts. Thicker and heavier parts need longer curing time to get rid of the volatiles.

Advantages of Post-curing for Molded Liquid Silicone Parts

After post-curing, volatiles in raw material can be largely decreased, especially the low molecular weight siloxane goes down. So purity of molded silicone parts increase. These are the chemical changes for silicone parts after post-curing.

Post-curing can also improve the mechanical properties of molded liquid silicone parts.

1. Dimensions becomes more stable after post curing, which is important for components with tight tolerance.

2. High heat speeds up the curing and adequately optimize the physical properties. For example, tensile strength, elongation, and compression set can be improved.