What are the differences between coupling agents, crosslinking agents, and dispersants?
I. Coupling Agents: Molecular Bridges Between Different Material Interfaces
1. Definition and Core Logic
A coupling agent is a compound with a bifunctional structure. Its core function is to act as a "translator" or "bridge," firmly connecting originally incompatible inorganic materials (such as glass fiber and calcium carbonate) with organic materials (such as plastics and rubber) through chemical bonds.
2. Main Characteristics
Asymmetric Molecular Structure: This is the physical basis of its operation. One end of the molecule has an inorganic-philic group (such as a hydrolyzable alkoxy group), and the other end has an organic-philic group (such as amino, vinyl, or epoxy groups).
Chemical Bridging Mechanism: The inorganic-philic end condenses with the hydroxyl groups on the filler surface, while the organic-philic end reacts with or entangles with the resin matrix, thus eliminating the "gap" between the inorganic and organic interfaces.
Performance Gains: It can significantly improve the mechanical strength (bending and impact resistance) of composite materials, as well as improve weather resistance and electrical insulation, while reducing interfacial hygroscopicity.
3. Typical Types
Silane Coupling Agents: The most widely used. For example, KH-550 (containing amino groups) is suitable for epoxy resins and phenolic resins; KH-570 (containing methacryloxy groups) is suitable for unsaturated polyesters.
Titanium ester coupling agents: particularly effective for filler systems such as calcium carbonate and talc, significantly reducing system viscosity and achieving high filler content.
II. Crosslinking agents: "Three-dimensional network weavers" of linear molecules
1. Definition and core logic
A crosslinking agent is a compound containing two or more active functional groups, or a substance that can initiate free radical reactions. Its core mission is to break the linear structure of polymer materials, connecting them into a three-dimensional network structure by establishing chemical bonds between molecular chains.
2. Main characteristics
Multi-functional groups/high activity: The molecule must have a "multi-claw" structure to hold more than two molecular chains.
Dimensional change: The crosslinking process is an irreversible chemical reaction. It transforms thermoplastic materials into thermosetting materials (or elastomers), causing them to lose their melt flow ability.
Performance Transformation: Imparts high elasticity (rubber), excellent heat resistance (less prone to deformation), solvent resistance (insoluble), and higher dimensional stability to materials.
3. Typical Types
Vulcanization Systems: Such as sulfur, used in natural rubber and styrene-butadiene rubber, forming polysulfide bonds, giving tires elasticity and wear resistance.
Peroxides: Such as DCP (dicumyl peroxide), used in polyethylene cable materials and ethylene propylene rubber, forming carbon-carbon crosslinks through free radical reactions.
Isocyanates: Such as MDI and TDI, used in polyurethane materials, reacting with polyols to form rigid polyurethane foams or elastomers.
III. Dispersants: The "Riot Police" of Solid Particles
1.Definition and Core Logic A dispersant is a surfactant or polymer. Its core mission is to prevent solid particles from re-aggregating (flocculating or settling) in a liquid medium, ensuring the system is in a uniform and stable dispersion state.
2. Key Characteristics
Anchoring and Dissolution: The molecular structure includes anchoring groups (which strongly adsorb onto the particle surface) and solvation chains (which are compatible with the dispersion medium and extend).
Stabilization Mechanism: Agglomeration is prevented primarily through two mechanisms:
Cellular Repulsion: Ionic dispersants impart the same charge to the particles, causing them to repel each other.
Stereotrophic Effect: The polymer chains form a physical barrier of a certain thickness around the particles, preventing them from approaching each other.
Rheological Control: Effectively reduces system viscosity, improves grinding efficiency, and prevents precipitation and agglomeration.
3. Typical Types
Ionic: Such as sodium polyacrylate (aqueous system), sodium dodecyl sulfate, relying on electrostatic repulsion.
Nonionic: Such as fatty alcohol polyoxyethylene ether, insensitive to pH, relying on steric hindrance.
Polymer: Such as polyurethane and polyacrylate dispersants, with strong anchoring power and extremely high stability, widely used in high-grade coatings and inks.
