Silicone is a synthetic polymer composed of a silicon-oxygen backbone, and it behaves differently than water or other common liquids when exposed to low temperatures. The short answer is that silicone does not freeze in the traditional sense of a liquid solidifying into a crystalline ice structure. Instead, silicone undergoes a dramatic change in its physical properties as the temperature drops, causing the material to transition from its typical rubbery, flexible state to a hard, rigid state.
The Science of Low Temperatures
Silicone materials are classified as amorphous polymers, meaning their molecular chains are arranged randomly. They do not have a uniform, highly ordered crystalline structure like water, which allows for a distinct, sharp freezing point. The flexibility of silicone at room temperature is due to the free movement of these long, coiled polymer chains.
When the temperature drops sufficiently, the polymer chains lose their ability to move and rotate freely, reaching a point called the Glass Transition Temperature (\(T_g\)). This transition is not a phase change, but a shift from a rubbery state to a glassy, non-flexible state. Below the \(T_g\), the silicone material becomes rigid because the chains are essentially “frozen” in place, though they have not crystallized.
The Glass Transition Temperature varies significantly depending on the specific formulation of the silicone, often ranging from approximately \(-125^\circ\text{C}\) to \(-50^\circ\text{C}\) for common elastomers. Some silicone elastomers may also undergo partial crystallization at slightly higher temperatures, around \(-45^\circ\text{C}\) to \(-60^\circ\text{C}\). This partial crystallization causes a noticeable, sudden increase in the material’s hardness and is often the practical low-temperature performance limit for commercial products.
Observable Changes in Silicone
The physical effects of approaching the glass transition or crystallization temperatures are easily observed. As the polymer chains lose mobility, the material loses its characteristic elasticity and becomes much harder. This increase in hardness is substantial and can be measured as a jump of 20 to 30 Shore A points on the durometer scale.
When the material is below its effective low-temperature limit, attempting to bend or stress it aggressively can easily cause it to crack or tear. This is especially true for gels and softer silicone compounds, where cracks may not fully self-heal even after the material is warmed up again. The material also undergoes a slight linear shrinkage as it cools, which contributes to the loss of flexibility and increased rigidity.
Handling Silicone in Cold Environments
Understanding these low-temperature property changes is important for applications like silicone bakeware, construction sealants, and automotive gaskets. For instance, if silicone bakeware is left outside in freezing conditions, it will become rigid and should be handled with care. Allowing cold silicone items to warm gradually to room temperature before attempting to flex or bend them will restore their elasticity and prevent damage.
Applying Silicone Sealants in Cold Weather
In construction, silicone sealants are favored for their wide operating temperature range, often performing well down to \(-40^\circ\text{C}\). However, the application of these sealants in cold weather requires special attention to ensure proper curing and adhesion. Low temperatures significantly slow down the curing process, which is dependent on moisture in the air.
To ensure a durable seal, it is often necessary to warm the sealant tubes before application to make them easier to dispense. The substrate surface must be completely clean and free of any frost or ice, as moisture on the surface can severely affect the sealant’s ability to bond. Though many silicone sealants can be applied in temperatures far below freezing, the total cure time may be extended from days to several weeks.