Achieving temperature resistance exceeding 300°C in silicone foam boards presents significant difficulties, as standard silicone materials typically operate within a-60°C to 200°C range during long-term use, with short-term peak temperatures potentially reaching 250°C. When temperatures surpass this threshold, the polymer backbone begins to degrade, resulting in material brittleness, loss of elasticity, and eventual pulverization.
To achieve the temperature resistance of more than 300°C, it is necessary to carry out comprehensive design and optimization from the basic material, reinforcement system, heat-resistant additives, foaming process and structural design.
The main technical approaches and considerations for achieving this goal are as follows:
I. Selection and Optimization of Core Materials
This is the most fundamental step.
Use high phenyl silicone rubber:
Principle: Ordinary silicone rubber (methyl vinyl silicone rubber) has limited heat resistance. The introduction of phenyl groups (especially high content of diphenylsiloxane chain segments) in the siloxane main chain can greatly improve the heat resistance of the material.
Function: The bulky structure of the benzene ring and its conjugation effect can effectively shield and stabilize the heat-sensitive Si-O bonds, thereby inhibiting the thermal cyclization degradation and oxidative cleavage of the polymer chain. High-phenyl silicone rubber can withstand long-term temperatures up to 250-300°C and short-term temperatures exceeding 350°C.
Use fumed silica as reinforcing filler:
Necessity: Unreinforced silica gel has extremely low strength. Fumed silica (nanoscale silica) is the only reinforcing filler that can provide the strength required by silica gel.
Requirements: Must use high-purity fumed silica with special treatment, small particle size and large specific surface area. It can form a strong nano-network structure in the silica matrix and maintain certain mechanical properties even at high temperature.
II. Heat resistant additive system
It is not enough to rely on basic materials alone, and special heat-resistant additives must be added to "protect and escort".
Heat stabilizer:
1)Iron oxide (Fe₂O₃): This is the most classic and effective heat-resistant additive. The reddish-brown α-Fe₂O₃ is commonly used. It captures free radicals that attack the siloxane backbone at high temperatures and catalyzes the formation of more stable siloxane-carbon structures, thereby inhibiting backbone degradation. The dosage typically ranges from 0.5 to 3 parts.
2)Cerium oxide (CeO₂) is a highly effective rare earth-based heat-resistant stabilizer, particularly when combined with Fe₂O₃. It effectively inhibits the condensation and oxidation of silanol groups.
3)Other rare earth oxides, such as lanthanum oxide (La₂O₃), also show good thermal stability.
Select the appropriate vulcanization system:
1)Peroxide vulcanization: For foaming materials, peroxides (such as bis(2,5-tertiary)peroxide) are commonly used vulcanizing agents. It is essential to select peroxides with stable decomposition products and non-corrosive properties, while ensuring complete vulcanization to prevent residual materials from degrading under high temperatures.
2)Platinum sulfide (addition sulfide) is the preferred choice when foam-forming processes permit. Its cross-linked Si-C structure exhibits superior thermal resistance compared to the C-C bonds in peroxide-based systems, with no decomposition residues. However, platinum-based systems demand stringent process and environmental controls and are prone to poisoning.
III. Challenges of foaming processes
While pursuing high temperature resistance, it is also necessary to achieve uniform and stable foaming structure.
Chemical foaming agent: The decomposition temperature should match the vulcanization temperature, and the decomposition product should be an inert gas (such as nitrogen). The decomposition residue should not catalyze the degradation of the silica gel matrix.
Physical foaming: Technologies such as supercritical fluid foaming can avoid the problem of chemical foaming agent residue, but the equipment investment is large and the process is complex.
Key point: No matter what foaming process is used, it is necessary to ensure that the cell structure is stable and the cell wall is dense. The open cell structure or weak cell wall will collapse or break rapidly at high temperature, resulting in a sharp decline in thermal insulation performance.
IV. Structure and Post-Processing Design
Increase density/Reduce bubbles: On the premise of meeting the weight and softness requirements, appropriately increase the density of the foam board (that is, reduce the foaming ratio) can obtain thicker and more heat-resistant hole wall, thus improving the overall temperature resistance and mechanical strength.
Composite structure: Consider creating a "sandwich" structure, for example, with two sides made of high-temperature resistant non-porous silicone film or glass fiber cloth composite, and a foamed layer in the middle. This can protect the fragile porous structure from direct exposure to high temperature environment.
Post-high temperature vulcanization: The formed foam board needs to be fully post-high temperature vulcanized (e.g. baked for several hours at 200-250°C) to completely remove low molecular volatiles and stabilize the crosslinking network, which is critical for long-term high temperature use.