Fire resistance, thermal degradation and heat stabilisers – what do all of these mean?
Silicone rubber has an outstanding reputation for being a reliable, durable construction material with a higher melting point and ignition point than other common polymers such as PP and PVC. These properties make silicone an excellent choice of material for use in high temperature applications. A handful of silicone manufacturers, including here at Jehbco, can enhance the high temperature performance of silicone rubber by including fire resistant additives or heat stabilisers in the silicone manufacturing process. But how do these additives affect the final silicone product, and in which applications should they be used?
When silicone rubber is exposed to high temperature environments for extended periods, as expected in automotive, refrigeration and certain electrical applications, they will lose their elasticity and flexibility, becoming brittle before ultimately tearing. In industry, this is measured by the high temperature exposure time required for the rubber to lose one half of its standard elongation at room temperature. Typically, general silicone rubber can be used indefinitely at temperatures below 150oC, for 1000 hours at temperatures around 200oC and for short periods of time at 350oC and above before breaking. Traditional heat stabilisers such as cerium oxide or titanium dioxide inhibit silicone oxidation, which slows the degradation process by over 100 orders of magnitude. This helps preserve the elasticity and elongation and increase the lifetime of the silicone product.
When exposure to fire or temperatures around silicone’s autoignition point (>450oC) becomes a possibility, then the products’ resistance to thermal degradation and fire resistance need to be considered. Resistance to thermal degradation refers to the initial ignition point of the material and describes how the rubber will behave when exposed to radiated heat. This is best pictured by thinking of a piece of silicone in a room with a very high temperature. The ability of silicone to withstand igniting at this elevated temperature is its resistance to thermal degradation, and can be quantified under standards such as the AS 1530.3 and EN 45545 standards. These standards measure the temperature and time taken to ignite, the smoke released from the material, time taken to extinguish and amount of heat released during combustion. Which of these measurements is most relevant will vary depending on application. Flame or fire resistance typically refers to the behavior of silicone when directly exposed to a flame. This is frequently standardised using the UL 94 test which measures how long the sample will take to ignite, and how long it takes to extinguished after being immersed in a hot flame. Additives can be used to influence the way silicone combusts in extreme temperatures or in contact with flame, including a variety metal hydroxides which release water vapour upon combustion to cool the subsequent flame, and other additives being developed and tested at Jehbco.
With all this in mind, deciding which blend of additives to use for any application can be difficult. Jehbco offers a unique range of fire additives and expertise in manufacturing fire resistant silicones that are can be custom engineered to suit most specific applications. Many of Jehbco’s existing products are already certified under various international fire safety standards to ensure peace of mind. For more information about heat stabilisers and fire additives, please visit our website and contact us with any questions.