1. Introduction
Plastics are widely used in various industries due to their diverse properties. However, the inherent properties of some plastics may not fully meet the requirements of specific applications. This is where plastic modification comes into play, and crosslinking agents are crucial components in this process. Crosslinking agents can transform the molecular structure of plastics, thereby enhancing their performance in multiple aspects.
2. Types of Crosslinking Agents Commonly Used in Plastic Modification
2.1 Organic Peroxides
DCP (Dicumyl Peroxide): This is one of the most commonly used organic peroxides. With a density of 1.08 g/cm³, a melting point of 42°C, and a decomposition temperature range of 120-125°C, it is often used in the crosslinking of plastics. For example, in the crosslinking of polyethylene, it is frequently employed. When used for crosslinking, it is often combined with zinc oxide to improve the strength and aging resistance of the plastic products.
BPO (Benzoyl Peroxide): Appearing as a white powder, BPO has a melting point in the range of 103-106°C. It is highly unstable, insoluble in water, and slightly soluble in organic solvents. Although it has some limitations in terms of stability, it still finds applications in specific plastic crosslinking processes.
DTBP (Di-tert-butyl Peroxide): Presenting as a slightly yellow transparent liquid, DTBP has a density of 0.8 g/cm³, a boiling point of 110°C, and a flash point of 183°C. At 126°C, its half-life is 10 hours. It is also used in certain plastic crosslinking systems.
DBHP (Cumene Hydroperoxide): A light yellow liquid, DBHP is prone to decomposition when exposed to heat, acids, or alkalis. This property restricts its use to some extent, but it can still be used in specific plastic modification scenarios where the reaction conditions can be carefully controlled.
2.2 Amines
- DTA (Diethylenetriamine): Also known as diethylenetriamine, it is a colorless liquid with a boiling point of 207°C, a density of 0.954 g/cm³, and a refractive index of 1.5. Its flash point is 94°C. DTA is commonly used in epoxy resins, and the general addition amount is 5%-10%. It can effectively crosslink epoxy resins, improving their mechanical and chemical properties.
2.3 Others
- 2,5-Dimethyl-2,5-di(tert-butylperoxy)hexane (Double 25): It comes in two forms: a light yellow liquid with a purity of 90% and a density of 0.85 g/cm³, and a white powder with a purity of 50%. The decomposition temperatures are 179°C (half-life of 1 minute) and 118°C (half-life of 10 hours). As a high-temperature crosslinking agent, it is often used in materials such as ethylene-vinyl acetate copolymer, polyethylene, and chlorinated polyethylene, enhancing the strength and hardness of the products.
3. How Crosslinking Agents Modify Plastics
3.1 Changing the Molecular Structure
Crosslinking agents can form chemical bonds between polymer chains, transforming the originally linear or slightly branched polymer molecules into a three-dimensional network structure. For example, in the crosslinking of polyethylene, the crosslinking agent breaks the carbon-carbon double bonds in the polyethylene molecular chain and forms new crosslinking bonds between different chains, creating a more complex and stable network structure.
3.2 Improving Mechanical Properties
- Enhancing Strength: By forming a crosslinked network, the polymer chains are more firmly connected, which significantly increases the tensile strength, compressive strength, and flexural strength of the plastic. For instance, crosslinked polyethylene has a much higher tensile strength compared to non-crosslinked polyethylene, making it more suitable for applications that require high-strength materials, such as in the production of pipes for high-pressure fluid transportation.
- Increasing Toughness and Impact Resistance: The crosslinked structure can also improve the plastic's toughness and impact resistance. When the plastic is subjected to external forces, the crosslinked network can absorb and disperse the energy, preventing the plastic from cracking or breaking easily. This is particularly important in applications where the plastic may be exposed to impacts, such as in the manufacturing of automotive parts and sports equipment.
3.3 Improving Thermal Properties
- Raising the Heat-Distortion Temperature: Crosslinked plastics generally have a higher heat-distortion temperature. For example, polyvinyl chloride (PVC) after crosslinking can withstand higher temperatures without significant deformation. This improvement in heat resistance allows crosslinked plastics to be used in high-temperature environments, such as in electrical insulation materials for high-temperature equipment.
- Enhancing Thermal Stability: The crosslinked structure can also enhance the thermal stability of plastics, reducing the rate of thermal degradation. This means that crosslinked plastics can maintain their properties for a longer time at elevated temperatures, extending their service life in high-temperature applications.
3.4 Improving Chemical Resistance
Crosslinking can reduce the solubility and swelling of plastics in various chemical solvents. For example, crosslinked polyethylene is more resistant to organic solvents compared to non-crosslinked polyethylene. This property makes crosslinked plastics suitable for applications in contact with chemicals, such as in the production of chemical storage tanks and pipelines for transporting corrosive substances.
4. Applications of Crosslinked Plastics in Different Industries
4.1 Construction Industry
- Pipes: Crosslinked polyethylene pipes are widely used for hot water supply, gas transportation, and sewage disposal. Their high strength, heat resistance, and chemical resistance ensure long-term and reliable use in building plumbing systems.
- Insulation Materials: Crosslinked plastics are used as insulation materials for buildings due to their good thermal insulation properties and fire resistance. They can help reduce energy consumption for heating and cooling in buildings.
4.2 Electrical and Electronics Industry
- Wire and Cable Insulation: Crosslinked polyethylene is commonly used as insulation material for wires and cables. Its excellent electrical insulation properties, heat resistance, and mechanical strength can protect the electrical conductors and ensure the safe and stable operation of electrical systems.
- Electronic Components: In the production of electronic components, crosslinked plastics are used to encapsulate and protect sensitive electronic parts. Their chemical resistance and mechanical stability can prevent damage to the components from the external environment.
4.3 Automotive Industry
- Interior Parts: Crosslinked plastics are used to manufacture automotive interior parts such as dashboards, door panels, and seat covers. Their good mechanical properties, heat resistance, and aesthetic appearance can meet the requirements of automotive interior decoration and functionality.
- Under-the-Hood Components: For components under the hood, such as engine mounts and fuel lines, crosslinked plastics are used due to their high heat resistance and chemical resistance. These components need to withstand harsh conditions in the engine compartment.
5. Considerations When Using Crosslinking Agents in Plastic Modification
5.1 Compatibility with Plastics
It is essential to ensure the compatibility between the crosslinking agent and the plastic matrix. Incompatible crosslinking agents may not be able to effectively crosslink the plastic or may cause phase separation, resulting in a decrease in the performance of the modified plastic. For example, when choosing a crosslinking agent for a specific type of plastic, factors such as the chemical structure and polarity of the crosslinking agent and the plastic need to be considered.
5.2 Dosage Control
The dosage of the crosslinking agent has a significant impact on the degree of crosslinking and the properties of the modified plastic. If the dosage is too low, the crosslinking effect may not be sufficient, and the improvement in plastic properties may be limited. On the other hand, if the dosage is too high, it may lead to excessive crosslinking, making the plastic brittle and reducing its processability. Therefore, precise control of the crosslinking agent dosage is necessary through experiments and optimization.
5.3 Reaction Conditions
The crosslinking reaction is affected by various conditions such as temperature, pressure, and reaction time. For example, some crosslinking reactions require a certain temperature range to proceed smoothly. If the temperature is too low, the reaction rate may be slow, and the crosslinking may not be complete. If the temperature is too high, it may cause side reactions or degradation of the plastic. Therefore, appropriate reaction conditions need to be determined according to the type of crosslinking agent and plastic.
6. Conclusion
Crosslinking agents play a vital role in plastic modification. They can effectively improve the mechanical, thermal, and chemical properties of plastics, enabling plastics to meet the requirements of different applications in various industries. However, when using crosslinking agents, careful consideration of factors such as compatibility, dosage, and reaction conditions is necessary to ensure the successful modification of plastics and the achievement of desired properties. As technology continues to develop, the research and application of crosslinking agents in plastic modification will also continue to progress, bringing more high-performance plastic materials to the market.
