As a raw material for silicone, what is the main role of silicon dioxide?
Silicon dioxide (usually refers to fumed silica, precipitated silica, etc.) is one of the important raw materials for silicone (silicone rubber). It does not directly constitute the main structure of silicone (the main body of silicone is composed of siloxane polymers), but it mainly plays a core role in silicone production, such as reinforcement, performance adjustment, and processability improvement. It is a key auxiliary material that determines the mechanical properties and application scenarios of silicone.
1. Core role: reinforcement - improving the mechanical properties of silicone
The main body of silicone (silicone rubber) is a linear siloxane chain (such as polydimethylsiloxane). When it is not reinforced, it is soft and has extremely low strength (tensile strength is usually <1MPa), which can hardly be used in actual industrial scenarios.
The reinforcing principle of silica is: the hydroxyl groups (-OH) on its surface can form physical or chemical bonds with the silicone rubber molecular chains, forming a "three-dimensional network structure" inside the rubber - when the silicone is subjected to stress, the silica particles can disperse the stress and prevent the molecular chains from excessively slipping, thereby significantly improving the performance:
Improving tensile strength: The tensile strength of unreinforced silicone is about 0.3-0.6MPa, which can be increased to 3-10MPa after adding silica (adjusted according to the amount and type), meeting the requirements of sealing and product molding;
Enhancing tear strength: Avoiding rapid breakage of silicone when torn by force, such as sealing rings, hoses and other products that rely on this performance;
Improving wear resistance: Reducing the loss of silicone during friction and extending the service life (such as silicone rubber rollers and gaskets). 2. Adjust the hardness and elasticity of silicone
The amount and particle size of silica directly affect the hardness of silicone:
The higher the amount added (usually 30%-80%), the greater the hardness of the silicone (from Shore A 20-30 for soft silicone to Shore A 80-90 for hard silicone);
The smaller the particle size (such as fumed silica, particle size 5-50nm), the tighter the bond with the rubber molecular chain, the more accurate the hardness adjustment, and the more balanced the elasticity (not easy to be too brittle).
For example: medical soft silicone catheters require low hardness, and a small amount of silica is added; industrial sealing silicone pads require high hardness, and the amount added needs to be increased.
3. Improve the processing performance of silicone
In the process of silicone mixing and molding (such as extrusion and molding), silica can optimize the processing performance:
Prevent sticking: silica particles can reduce the adhesion of silicone in the mold, making the demolding of the product smoother;
Improve stiffness: The fluidity of silicone without silica is too strong. After adding silica, it can increase the "thixotropy" (become softer and easier to process when stressed, and keep the shape when standing), which is convenient for extrusion molding (such as silicone tubes and sealing strips);
Inhibit shrinkage: When silicone is vulcanized (cured), it may shrink and deform due to the shrinkage of the molecular chain. Silica can fill the gap, reduce the shrinkage rate, and ensure the dimensional accuracy of the product.
