The embrittlement of silicone rubber at 250℃ is not a "normal" manifestation of it, but rather the result of its aging mechanism under specific conditions.
Generally, silicone rubber is renowned for its heat resistance, with long-term service temperatures typically ranging from -60°C to 200-230°C. When the temperature rises to the upper limit of 250℃ or higher, several major aging processes will accelerate, eventually leading to embrittlement. The fundamental cause of embrittlement lies in the irreversible chemical degradation of the main chain and side groups of the silicone rubber polymer. The following are several core mechanisms that lead to embrittlement
1. Side group oxidation (the main mechanism) process: The most common type of silicone rubber is methyl vinyl silicone rubber. At high temperatures, oxygen in the air will attack the methyl groups (-CH₃) on the side chains of polymers. Chemical reaction: The methyl group is oxidized, first forming a hydroxyl group (-OH), which may then cross-link or generate other oxides. This process forms additional crosslinking points (oxidative crosslinking) between molecular chains.
Consequence: As the number of additional crosslinking points keeps increasing, the polymer network becomes overly dense and tight. The material loses its elasticity, becomes stiff, and eventually undergoes brittle fracture when subjected to minor stress. You can imagine a fishing net. When its nodes become too many and too dense, it loses its flexibility and breaks easily when pulled.
2.Main chain depolymerization (" unwinding "reaction) process: At a sufficiently high temperature, the siloxane main chain (-Si-O-) itself may break. Chemical reaction: The main chain breaks, generating cyclic oligomers (such as D3, D4, etc.) and other small molecules.
Consequence: The length of the polymer chain shortens and the molecular weight decreases. This directly leads to a reduction in the mechanical strength of the material. When the chain becomes too short, the material cannot effectively bear the stress and will also become brittle. This process can occur in both the presence and absence of oxygen, but it is particularly significant at extremely high temperatures.
3. Loss process of volatile substances and plasticizers: During the production of silicone rubber, some low-molecular-weight siloxanes may remain, or some additives may be added to improve processability. Under long-term high temperatures, these low-molecular-weight substances will gradually volatilize or migrate out.
Consequence: To a certain extent, these substances play a role in internal lubrication and plasticizing. Their loss will cause the material to harden, shrink and become brittle.
4.Thermal cycling and stress fatigue
process: If there are frequent cold and hot cycles in the application scenario, silicone rubber will constantly expand and contract.
Consequence: This periodic stress can cause microcracks within the material. Under the catalysis of high temperatures, these microcracks will accelerate their expansion and connection, eventually leading to macroscopic brittle cracking of the material.