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The Influence of Organic Silicone Resin Molecules on Heat Resistance

May 28, 2024

When subjected to thermal oxidation, a stable protective layer rich in Si-0-Si chains is formed on the surface of organic silicon polymers, reducing the impact on the internal structure of the polymers. Therefore, silicone resin has a high thermal decomposition temperature, can be used for a long time at 200-250 ℃ without decomposition or discoloration, and the Si-0 Si chain only begins to break at 350 ℃; Stable at room temperature, after crosslinking and curing, it can withstand high temperatures of 350-500 ° C for a short period of time. However, most organic polymers have already been completely cracked, losing their usability. Therefore, organosilicon polymers have special thermal stability.

After being combined with heat-resistant fillers, it can withstand higher temperatures. For example, when aluminum or glass powder is mixed into silicone paint, it can withstand high temperatures of 500-600 ° C. At this time, SiO2 decomposed from silicone resin is sintered into one with aluminum or glass powder, and a part is combined with the metal matrix to form a strong and continuous inorganic coating.

The bond energy varies depending on the side groups connected to the Si atoms in the main chain.Different substituents have significant differences in half-life at 250 ° C. Those with high bond energy and long half-life have high heat resistance, which should be fully considered when designing the molecular structure of organic silicone resin.In addition, due to the high polarity of the Si-0 bond in organosilicon polymers, the Si - (} -- Si bond is prone to breakage under the attack of electrophilic or nucleophilic reagents:
Therefore, its stability for chemical drugs is relatively not good (inferior to organic nitrogen polymers). The degree is greatly influenced by the type, properties, and quantity of the connected groups on the silicon atom. If the connected group is an electron withdrawing group (such as methyl, ethyl, etc.), the Si-O-Si chain weakens and the Si-C bond strengthens. Under the attack of polar reagents, the Si - (} -- Si chain is easy to break; otherwise, if the connected group is an electron withdrawing group (such as phenyl, etc.), the Si - (} -- Si chain strengthens and the Si-C bond weakens. Under the attack of polar reagents, the Si-C bond is easy to break.

Most polysiloxanes have both methyl and phenyl substituents, and the ratio of the two has a significant impact on their performance. For example, organic silicone resins with high methyl content have fast crosslinking speed, poor storage stability, good light aging resistance, and low temperature flexibility; The organic silicone resin with high phenyl content has good high-temperature stability and storage stability, but the curing rate is slow, resulting in more weight loss and poor photostability during high-temperature curing. If a longer alkyl benzene is used to replace the organic group directly connected to silicon in organic silicon, as the chain length of the substituent on the silicon atom increases, the toughness of the resin increases, but the thermal stability and hardness decrease.

It is generally believed that the thermal stability of organosilicon polymers varies with the type of organic groups connected to silicon atoms. The thermal stability of organosilicon polymers containing aliphatic organic groups decreases with an increase in the number of organic carbon atoms, while those containing aromatic groups have better heat resistance than those containing aliphatic groups. Due to their cyclic structure, aromatic groups can exhibit higher thermal stability. But different types of organic substituents also have different heat resistance temperatures, and the heat resistance temperatures of various organic silicons are roughly:; Phenyl silicone exceeds 200℃, methyl silicone reaches 200℃, ethyl silicone reaches 140℃, and propyl silicone reaches 120℃.

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