Analysis of the influence of different oxide vulcanizing agents on the vulcanization reaction of silicone rubber
1. Organic peroxide vulcanizing agent: the leading system of free radical crosslinking
Organic peroxides are the most commonly used oxide vulcanizing agents for the vulcanization of silicone rubber. Their mechanism of action is based on free radical reaction: they decompose at high temperatures to produce free radicals, capture hydrogen atoms or vinyl on the molecular chain of silicone rubber, and form macromolecular free radicals. After the coupling reaction, a crosslinking bond is formed.The effects of different active peroxides on the vulcanization reaction are as follows：


Highly active peroxides (such as benzoyl peroxide BP, di-tert-butyl peroxide DTBP)
Advantages: Low vulcanization temperature (160-170℃), short vulcanization time, and high production efficiency.For example, BP can be quickly vulcanized at 160℃, which is suitable for molded thin-walled products.
limitations：
Risk of scorching: It is easy to cause the rubber to be crosslinked in advance without vulcanization, and the process conditions need to be strictly controlled.
Effects of decomposition products: The decomposition products contain acidic substances (such as benzoic acid), which may degrade the main chain of silicone rubber, resulting in uneven vulcanization of thick products.
Additive interference: Additives containing active hydrogen atoms such as carbon black and aromatic oils will consume free radicals and reduce the crosslinking efficiency.For example, bis-two-four and other diacyl peroxides may not be vulcanized in the presence of carbon black.
Low-activity peroxides (such as 2,5-dimethyl-2,5-di-tert-butyl hexane peroxide DBPMH)
advantage：
Good scorching safety: high vulcanization temperature (170-180℃), suitable for thick products and carbon black rubber.
Neutral decomposition product: Neutral substances such as ketones and aldehydes are generated, which have no degradation effect on silicone rubber.
Balanced physical properties: The compression permanent deformation of vulcanized products is small, and the constant tensile stress is high.
Limitations: The vulcanization time is long, and the two-stage vulcanization process needs to be optimized.
Special peroxides (such as diisopropylbenzene peroxide DCP)
Features：
Cost advantage: low price and stable vulcanization effect.
By-product problem: acetylbenzene, which produces an unpleasant odor when decomposed, needs to be improved in formula (for example, it is replaced by bis-tert-butyl peroxyisopropylbenzene (DIPB)).
Key influencing factors：


Ph and alkalinity of fillers: Acid fillers such as untreated clay can easily cause peroxide heterogeneity and reduce the crosslinking efficiency.
Oxygen interference: The combination of oxygen and polymer free radicals will inhibit the crosslinking reaction, causing the surface of the product to become sticky.
Co-crosslinking agent: Adding a co-crosslinking agent (such as trihydroxymethyl propane trimethylacrylate TMPTMA) can inhibit the disproportionation reaction and chain breaking reaction, and improve the crosslinking efficiency.
2. Metal oxide vulcanizing agent: auxiliary selection of special rubber system
Metal oxides are rarely used in the vulcanization of silicone rubber, but they play a key role in halogen-containing rubbers such as neoprene and chlorosulfonated polyethylene.Its mechanism of action is based on the reaction of metal cations with halogens in rubber molecules to form crosslinked bonds.The effects of common metal oxides are as follows：


Zinc oxide (ZnO)
effect：
Main vulcanizing agent: As the main vulcanizing agent in neoprene rubber, a crosslinking bond is formed by reacting with chlorine atoms.
Active agent: As an active agent or reinforcing agent in silicone rubber, the dosage is 5-10 parts, which can improve the heat resistance and UV resistance of the product.
Limitations: Silicone rubber cannot be vulcanized independently, and needs to be used in conjunction with other vulcanizing agents.
Magnesium oxide (MgO)
effect：
By-vulcanizing agent: Prevent early vulcanization (scorching) in neoprene rubber and neutralize hydrogen sulfide produced during vulcanization.
Improved heat resistance: The hardness and tensile strength of the product can be improved in silicone rubber, but the water resistance is poor.
Features：
High vulcanization temperature: it needs to be above 100℃ to have vulcanization effect, and the vulcanization time is long.
Physical properties: The vulcanized products have high strength, but the compression and permanent deformation are large.
Composite metal oxides (such as magnesium oxide-zinc oxide composite)
advantage：
Synergistic effects: Zinc oxide provides a crosslinking point, magnesium oxide inhibits scorching and improves vulcanization safety.
Performance optimization: In chlorosulfonated polyethylene rubber, the composite system can increase the room temperature rebound coefficient to 82%, and the resistance to permanent deformation is stable at 5.3%.
Limitations: The electrical insulation performance may decrease (for example, the volume resistance of the lead oxide system plummets by 3 orders of magnitude).
Key influencing factors：


Metal cation radius: The difference in atomic radius of zinc (1.34Å), magnesium (1.45Å), and cadmium (1.52Å) affects the cross-linking potential resistance. The zinc oxide sample produces 220,000 effective cross-linking points per cubic meter, and the spatial arrangement is tighter.
Coordination effect: The coordination reaction between the metal cation and the chlorosulfonated group affects the crosslinking density.
3. The differentiated influence of oxide vulcanizing agent on product performance
Physical and mechanical properties：
High-activity peroxide vulcanized products have high tensile strength, but their tear resistance decreases; low-activity peroxide vulcanized products have small compression permanent deformation and balanced physical and mechanical properties.
Metal oxides can improve the hardness and heat resistance of products, but the amount needs to be controlled to avoid increased brittleness.
Process adaptability：
High-activity peroxides are preferred for thin-walled products, and low-activity peroxides or segmented vulcanization processes are required for thick products.
Carbon black-containing rubber should avoid the use of aromatic acyl peroxides (such as BP) to prevent carbon black from interfering with the vulcanization reaction.
Environmental protection and safety：
Peroxide vulcanizing agents may produce volatile decomposition products (such as benzene and carbon dioxide), and ventilation conditions need to be optimized; metal oxides are non-volatile, but magnesium oxide is easy to absorb moisture and agglomeration, and it needs to be sealed and stored.
4. Future development trends
Compound vulcanizing agent: By combining high-activity and low-activity peroxides, the vulcanization efficiency and scorching safety are taken into account.
Nano-oxides: Nano-zinc oxide and magnesium oxide are used to enhance vulcanization activity, reduce dosage and improve dispersion.
Green alternative: Develop peroxides without acidic decomposition products to reduce the degradation of the main chain of silicone rubber.