Epoxy resins stand out for their exceptional performance, but they also face challenges such as high benzene content, high crosslink density, and brittleness. To fully realize their potential, they often require toughening and modification. In this process, polysiloxanes stand out with their unique properties. These materials offer flexibility, weather resistance, and heat resistance, as well as low surface energy, making them a perfect complement to epoxy resins.
△ Challenges of Epoxy Resins
Epoxy resins' high benzene content, high crosslink density, and brittleness limit their applications. To overcome these limitations, toughening and modification are often necessary.
△ Excellent Properties of Polysiloxanes
Polysiloxanes have extensive and far-reaching applications in epoxy resins. First, by introducing flexible Si-O bonds, they enable easier rotation of polymer chains, effectively eliminating internal stress in cured epoxy resins and significantly improving toughness. Second, the polarity and ionization potential of Si-O bonds enhance the antioxidant properties of cured epoxy resins, further improving weather resistance. Furthermore, because the bond energy of Si-O bonds is much higher than that of C-C bonds, this significantly improves the heat resistance of epoxy resin cured products. Furthermore, polysiloxanes can reduce the polarity of epoxy resin cured products by lowering surface energy and increasing hydrophobicity, thereby enhancing moisture resistance.
01 Polysiloxane Modification Methods
△ 1. Copolymerization Modification and Reaction Mechanism
To fully leverage these advantages of polysiloxanes, scientists have explored two main modification methods: copolymerization modification and blending modification. While blending modification is simple, it presents compatibility issues, potentially affecting overall performance. In contrast, copolymerization modification, through the reaction of reactive groups in epoxy resin with polysiloxane, not only resolves compatibility issues but also more effectively improves the heat resistance of epoxy resin cured products.
The table above shows the molecular structures of organosilicones containing epoxy or amino groups, all of which can be used in epoxy resin modification research. Through methods such as hydrosilylation, chain extension, and aminolysis, novel polysiloxane structures can be prepared. These polysiloxanes react with epoxy resins, thereby improving key properties of epoxy resin cured products, such as toughness and heat resistance.
Next, we will discuss copolymerization modification methods. During the copolymerization modification process, reactive groups such as epoxy, hydroxyl, and amino groups in the polysiloxane react with epoxy and hydroxyl groups in the epoxy resin to form block copolymers or graft copolymers, significantly improving the compatibility between the polysiloxane and the epoxy resin. Furthermore, copolymerization modification can further enhance key properties of the cured product, such as toughness, heat resistance, and hydrophobicity.
For the copolymerization modification of epoxy resin with polysiloxane, the following method can be used: First, the epoxy resin reacts with a polysiloxane containing hydroxyl or alkoxy groups. During the reaction, the alkoxy groups in the polysiloxane react with the secondary hydroxyl groups in the epoxy resin to form stable Si-O-C bonds and R-OH molecules. This reaction mechanism provides a solid theoretical basis for the copolymerization modification of polysiloxane and epoxy resin.
The hydroxyl groups in polysiloxane react with the secondary hydroxyl groups in epoxy resin to form a stable Si-O-C bond and release H2O molecules at the same time.
The hydroxyl groups in polysiloxane react with the epoxy groups in epoxy resin to form a ring-opening reaction, thereby forming a stable Si-O-C bond:
The reaction mechanism is explained in detail as follows: Under alkaline conditions, the terminal hydroxyl groups in polysiloxane are converted into silanol anions. Subsequently, this silanol anion reacts with the hydroxyl groups in epoxy resin to form a stable Si-O-C bond. This reaction process can be carried out through two different routes.
(2) Reaction mechanism of epoxy resin and amino-containing polysiloxane
When discussing the reaction mechanism of epoxy resin and amino-containing polysiloxane, we need to pay attention to several key steps. First, the epoxy groups in epoxy resin undergo a ring-opening reaction with the silanol anion, and this process is accompanied by the formation of Si-O-C bonds. This newly formed Si-O-C bond then reacts with the hydroxyl groups in the polysiloxane. Through this series of chemical reactions, the epoxy resin and the amino-containing polysiloxane are effectively combined to form a polymer material with specific properties.
△ 2. Blending and Modification Challenges
Blending epoxy resins with polysiloxanes can yield composite systems with excellent performance and cost-effectiveness. However, due to the significant difference in solubility parameters between epoxy resins and polysiloxanes, their compatibility is poor, significantly affecting their morphology and properties. Therefore, improving the compatibility between the two components is key to blending and modification.
To enhance the blending compatibility of polysiloxanes with epoxy resins, the following strategies can be adopted: ① First, synthesize a polysiloxane containing epoxy groups, such as epoxy groups, and simultaneously cure it with the epoxy resin; ② Introduce a coupling agent or other interfacial compatibilizer; ③ Alternatively, synthesize a polysiloxane containing epoxy curing groups, such as phenolic groups, isocyanate groups, and carboxyl groups.