Cross-linking reaction during polymer molding
Cross-linking during polymer molding refers to the process by which linear polymer chains are linked through chemical covalent bonds to form a three-dimensional network structure. This reaction can significantly alter the physical and chemical properties of polymers, such as improving heat resistance, hardness, and solvent resistance. Cross-linking requires specific conditions, including temperature, pressure, time, and the presence of a cross-linking agent. For example, during the molding process of thermosetting resins (such as epoxy and phenolic resins), cross-linking agents (such as amines) are used at temperatures between 120-180°C to form C-N or C-O-C bonds between the linear chains, completing the transition from a linear structure to a network. The degree of cross-linking is typically expressed as the cross-link density, which is the number of cross-links per unit volume. Higher cross-link density indicates greater rigidity and heat resistance. For example, an epoxy resin with a cross-link density of 5×10²⁰ cross -links /cm³ has a glass transition temperature that is over 50 °C higher than its uncross-linked state , and its heat distortion temperature rises from 80 °C to 150 °C.
The mechanism of cross-linking reactions during polymer molding varies depending on the material type and can be primarily categorized as free radical cross-linking, ionic cross-linking, and condensation cross-linking. Free radical cross-linking is common in materials such as unsaturated polyesters and polyolefins. Under the action of an initiator (such as a peroxide), the polymer chain generates free radicals, which then undergo addition reactions with double bonds on other molecular chains to form cross-links. For example, when polyethylene is molded at 180°C under the initiation of peroxide, hydrogen atoms on the molecular chain are abstracted to form free radicals. These free radicals from different chains then combine to form C-C cross-links, transforming the polyethylene from a thermoplastic to a thermoset. Ionic cross-linking occurs in the reaction between epoxy resin and amine curing agents. The amino groups provided by the amine compound undergo a ring-opening reaction with the epoxy groups of the epoxy resin, forming ionic bonds that are further converted to covalent bonds, completing the cross-linking. Condensation crosslinking is more common in materials such as phenolic resins and polyurethanes. The reaction process is accompanied by the release of small molecular products (such as water and methanol). For example, when phenolic resin is formed under acidic conditions, the phenolic hydroxyl group and the aldehyde group of formaldehyde undergo a condensation reaction to generate a methylene bridge ( -CH₂- ) to connect adjacent molecular chains, while releasing water molecules, eventually forming a three-dimensional network structure.
Controlling the process parameters of the cross-linking reaction during polymer molding is crucial to the reaction rate and product performance. Temperature, time, and pressure are the three core parameters. Temperature significantly affects the cross-linking reaction rate. Generally, the reaction rate increases 2-3 times for every 10°C increase in temperature. For example, unsaturated polyester resin requires 2 hours to fully cross-link at 60°C, while it only takes 30 minutes at 80°C. However, excessively high temperatures can lead to overly vigorous cross-linking, generating internal stress or bubbles. Therefore, a suitable temperature profile should be established based on the material’s characteristics. For example, a staged temperature ramp can be used: pre-cross-linking at 80°C for 1 hour, followed by final cross-linking at 120°C for 2 hours to ensure uniform reaction. The time parameters must be matched to the temperature, ensuring sufficient reaction time at the set temperature to achieve the desired degree of cross-linking. For example, when molding a polyurethane elastomer at 100°C, it takes 30 minutes to achieve a cross-linking degree of at least 90%. Pressure promotes molecular chain contact and reduces bubbles generated by the reaction. For cross-linking reactions with significant volume shrinkage, such as phenolic resin, molding pressures of 5-10 MPa are required to ensure close contact and enhance cross-linking uniformity.
Controlling the degree of cross-linking during polymer molding is key to ensuring product quality stability. This can be characterized through gel content, cross-link density, and mechanical property testing. Gel content testing is the most commonly used method. After the molded polymer sample is placed in a good solvent (e.g., cross-linked polyethylene in boiling xylene), the mass fraction of insoluble matter is weighed after immersion for 24 hours. A higher gel content indicates a higher degree of cross-linking. Generally, a gel content of over 90% is required for thermoset products. Cross-link density can be measured using dynamic mechanical analysis (DMA) and calculated based on the relationship between storage modulus and temperature. For example, the cross-link density of cross-linked rubber can be calculated from swelling data using the Flory-Rehner equation. Ensure that the cross-link density is within the designed range (e.g., 1×10²⁰ to 1×10²¹ cells /cm³ ). Mechanical property tests (e.g., tensile strength, hardness, and impact strength) can indirectly reflect the degree of cross-linking. For example, the tensile strength of epoxy resins increases initially and then decreases with increasing cross-link density. There is an optimal cross-link density (approximately 8×10²⁰ cells /cm³ ), at which the tensile strength reaches a maximum of 80 MPa . Through the combined characterization of multiple methods, the degree of cross-linking reaction can be precisely controlled to ensure the consistency of product performance.
Cross-linking reactions are widely used in polymer molding processes, playing a vital role in plastics, rubber, and composite materials. Different applications require varying degrees of cross-linking. In structural components, such as automotive engine cylinder head gaskets, cross-linked modified nitrile rubber is used. By controlling the degree of cross-linking, the material achieves both sufficient elasticity (Shore hardness 60-70A) and the ability to withstand temperatures exceeding 150°C and oil corrosion. In the field of insulating materials, cross-linked polyethylene (XLPE) is used in cable sheathing. Radiation or chemical cross-linking enhances its heat resistance and mechanical strength. Cross-linked XLPE can withstand long-term use at 90°C, and its breakdown field strength is 30% higher than that of uncross-linked polyethylene. In the field of composite materials, glass fiber-reinforced epoxy resin forms a network structure through cross-linking, firmly bonding the glass fibers and achieving a flexural strength of over 300 MPa for the composite material. It is widely used in aerospace components. In addition, cross-linking reactions can also be used to prepare elastomers (such as vulcanized rubber), adhesives (such as epoxy resin glue), etc., by adjusting the degree of cross-linking to meet different usage requirements. For example, structural adhesives used in construction require a higher degree of cross-linking (gel content above 95%) to ensure bonding strength and durability.
