Crosslinking agents are also called curing agents, hardeners, curing agents, which can transform linear or lightly branched macromolecules into a three-dimensional network structure, thereby improving strength, heat resistance, wear resistance, and solvent resistance performance. The cross-linking agent is usually a substance containing multiple functional groups in the molecule, such as organic dibasic acid, polyhydric alcohol, etc. The crosslinkers may also be compounds containing multiple unsaturated double bonds in the molecule, such as divinylbenzene and diisocyanate, N, N-methylene bisacrylamide (MBA), etc. When polycondensation (or polymerization) reaches a certain degree, cross-linking occurs, so that the product becomes an insoluble cross-linked polymer. It can also retain a certain number of functional groups (or double bonds) in linear molecules, and then add specific substances for cross-linking, such as curing of phenolic resin and vulcanization of rubber.
Figure 1. The structural formula of cross-linking agent dicumyl peroxide.
Crosslinking agents are mainly used in polymer materials (rubber and thermosetting resins), because the molecular structure of the polymer material is like a long line. When it is not cross-linked, it has low strength and no elasticity. The role of the cross-linking agent is to create chemical bonds between the linear molecules so that the linear molecules are connected to form a network structure, which improves the strength and elasticity of the rubber.
- Crosslinking and modification of various thermoplastics: After cross-linking, it can significantly improve the heat resistance, flame retardancy, solvent resistance, mechanical strength and electrical properties of the product. It significantly improves product quality and has no odor compared to crosslinking with the peroxide system alone. Typical applications are polyethylene, polyethylene / chlorinated polyethylene, polyethylene/EVA cross-linked cables and polyethylene high and low foam products.
Figure 2. Crosslinking and modification of thermoplastics
- Special separation membrane: In terms of special separation membrane materials, ceramic ultrafiltration membranes have achieved industrial production and are widely used in the process industry. For example, MFI molecular sieve pervaporation membrane has been used in alcohol-water separation on a large scale, and gradually applied to the separation of organic matter. A series of ceramic nanofiltration membrane products, such as TiO2, ZrO2, TiO2/ZrO2, and reduced graphene oxide, developed using improved sol-gel technology, have also been successfully used in high-efficiency desalination processes.
- Ethylene propylene rubbers, various fluoro rubbers, CPE and other special rubbers for vulcanization: It can significantly shorten the vulcanization time, improve strength, wear resistance, solvent resistance and corrosion resistance.
- Crosslinking of acrylic and styrene ion exchange resins: This is a new type of ion exchange resin that has been developed recently and has excellent prospects.
- Modification of polyacrylate, polyalkyl acrylate: It can significantly improve heat resistance, optical performance and processabilit, and is typically used for heat-resistant modification of ordinary plexiglass.
- Modification of epoxy resin and DAP (polyallyl phthalate) resin: It can improve heat resistance, adhesion, mechanical strength and dimensional stability. It is typically used to modify epoxy potting compounds and encapsulants.
Figure 3. Crosslinking agent for epoxy resin
- Crosslinking and modification of unsaturated polyester and thermoplastic polyester: It can significantly improve heat resistance, chemical resistance, dimensional stability, weather resistance and mechanical properties. It is typically used to improve the heat resistance of hot-pressed unsaturated polyester fiberglass products.
- Inner plasticization of polystyrene, copolymerization modification of styrene and TAIC: Transparent and shatter-resistant products can be obtained.
- Used as an intermediate for light-curing coatings, photoresists, flame retardants and flame-retardant cross-linking agents: Typically used for the synthesis of high-efficiency flame retardant TBC and flame-retardant cross-linking agent DABC.
- Smedberg, T Hjertberg, B Gustafsson. (2004) "The role of entanglements in network formation in unsaturated low-density polyethylene." Polymer. 20(14):29-30
- A. Smedberg, T. Hjertberg.(1997) "Crosslinking reactions in an unsaturated low-density polyethylene." Polymer. 97(16):56-57