Maya Ellison
Cross-linked thermoplastics are an important class of materials with a wide range of applications, including heat-shrinkable tubing, rotational moulded parts, and polyolefin foams. Cross-linking is the process of setting up chemical links between the molecular chains in a polymer. This process improves the thermal stability and mechanical properties of the material, such as impact strength, tensile strength, and resistance to stress cracking. Cross-linking can be achieved through radiation, chemical means, or by using silane-grafting agents. The process of cross-linking alters the physical characteristics of the thermoplastic material, making it behave more like a thermoset. Cross-linked polymers have enhanced mechanical performance and a broader range of applications.
| Characteristics | Values |
|---|---|
| Definition | In polymer chemistry, cross-linking refers to the use of cross-links to promote a change in the polymers' physical properties. |
| Goal | To interconnect long-chain molecules through covalent bonding to improve mechanical, thermal, electrical, and chemical performance. |
| Initiation | Cross-linking is initiated through e-beam radiation (irradiation) or chemical agents. |
| Methods | Radiation cross-linking, chemical cross-linking with organic peroxides, and cross-linking using silane-grafting agents. |
| Effect on Polymers | Cross-linking makes polymers stronger and more rigid, with improved thermal stability and mechanical properties such as impact strength and tensile strength. |
| Applications | Heat-shrinkable tubing, rotational molded parts, polyolefin foams, snowmobile tracks, and catheters for medical use. |
| Recyclability | Cross-linked materials are challenging to recycle as they do not melt easily. |
| Reversibility | Some cross-links can be reversed or undone, allowing for recyclability. |
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What You'll Learn
Radiation cross-linking
One of the key applications of radiation cross-linking is in the development of ultra-high molecular weight polyethylene (UHMWPE) for orthopaedic implants, such as hip implants and total joint arthroplasty. Cross-linking UHMWPE increases its wear resistance, making it suitable for bearing surfaces in joint replacements. However, radiation can also leave behind long-lived residual free radicals, which may negatively impact the mechanical properties of the material.
Additionally, radiation cross-linking has been used to enhance the physical properties of highly plasticized PVC. By cross-linking the polymer using gamma irradiation, the wear performance of total joint replacement components can be improved. The process also improves the anti-degradability and biocompatibility of collagen, making it a promising artificial dermal substitute.
In summary, radiation cross-linking is a versatile technique that offers numerous benefits across various industries. By improving the performance and stability of plastics, this process enhances their suitability for a wide range of applications, including orthopaedic implants, joint replacements, and artificial dermal substitutes. However, it is important to carefully control the radiation dose to avoid negative effects on the mechanical properties of the cross-linked materials.
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Chemical cross-linking with organic peroxides
Cross-linked thermoplastics are an important class of materials with applications in heat-shrinkable tubing, rotational moulded parts, and polyolefin foams. Cross-linking olefins can significantly enhance their mechanical performance. One of the three main methods for achieving this is through chemical cross-linking with organic peroxides.
Organic peroxides (PO) are carbon-based chemicals that contain a peroxide functional group (R-O-O-R’). This group consists of two bonded oxygen atoms (O-O), with R and R’ being alkyl, aryl, or other organic substituents. Organic peroxides are highly reactive and unstable, and they easily break down at certain temperatures to produce free radicals. This makes them ideal for initiating chemical reactions.
When added to polymers, organic peroxides facilitate the cross-linking of polymer chains. The free radicals produced by organic peroxides can abstract hydrogen atoms from polymer chains or react with double bonds in unsaturated polymers, forming new radical sites. These new radical sites can then react with each other or with other polymer chains, resulting in the formation of covalent bonds between chains. This process creates a three-dimensional network of interconnected polymer chains.
The use of organic peroxides provides precise control over the cross-linking process, as they exhibit predictable and consistent behaviours when they decompose. The resulting crosslinked polymer exhibits superior mechanical strength, chemical resistance, and thermal stability. Crosslinking helps maintain the properties of polymers over time, reducing degradation and extending their lifespan. This is particularly beneficial for products exposed to environmental conditions such as UV radiation and oxidation.
Organic peroxides are used to enhance the resistance of plastics to high temperatures, chemicals, and mechanical stress. Crosslinked plastics, such as crosslinked polyethylene (PEX), are commonly used in plumbing, heating, and cable insulation. They also provide reliable adhesion, superior bond strength, and performance in adhesive and sealant formulations. Peroxide crosslinking results in enhanced durability, hardness, and chemical resistance in both protective and decorative coatings.
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Cross-linking using silane-grafting agents
Cross-linking is a type of polymerization reaction that branches out from the main molecular chain to form a network of chemical links. Cross-linking agents are added to resins to enable this process. Cross-linking of thermoplastics can be done through three main methods: radiation cross-linking, chemical cross-linking with organic peroxides, and cross-linking using silane-grafting agents.
Silane coupling agents can act as bonding intermediates at the interface of two dissimilar materials by altering surface properties. The silane treatment can improve the processing, performance, and durability of a mineral, silica, glass fiber, and bead. The organofunctional group of the silane reacts and bonds to the polymer backbone. Residual moisture activates the silane's alkoxy groups to the active silanol form, which then react with each other, liberating moisture and forming siloxane bonds between the polymers. The resulting Si-O-Si crosslink is highly durable and offers excellent weather, UV, temperature, chemical, and moisture resistance.
The cross-linking process can be performed using a one-step or a two-step method. The two-step method is less expensive and more readily achievable. The grafting step, performed by reactive processing, is the major and key process in the two-step technique. The one-step method, on the other hand, involves free-radical grafting of unsaturated hydrolyzable alkoxy-silanes onto polyethylene chains by a peroxide initiator, followed by moisture cross-linking.
Catalysts such as tin octoate or dibutyltin dilaurate can be used at low levels to accelerate the cross-linking process. These catalyzed systems are useful for forming quick-dry skins, which are desirable for outdoor applications. The rate of crosslinking depends on the catalyst concentration and its chemical nature.
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Vulcanization
The word "vulcanization" comes from the god Vulcan, who was associated with heat and sulfur in volcanoes. In ancient Mesoamerican cultures, rubber was used to make balls, sandal soles, elastic bands, and waterproof containers. It was cured using sulfur-rich plant juices, an early form of vulcanization.
In the 1830s, Charles Goodyear worked to devise a process for strengthening rubber tires. Tires at the time would become soft and sticky with heat, accumulating road debris that punctured them. Goodyear tried heating rubber in order to mix other chemicals with it. This seemed to harden and improve the rubber, though this was due to the heating itself and not the chemicals used. Not realizing this, he repeatedly encountered setbacks when his announced hardening formulas did not work consistently. One day in 1839, when trying to mix rubber with sulfur, Goodyear accidentally dropped the mixture into a hot frying pan. To his surprise, instead of melting or vaporizing, the rubber remained firm, and as he increased the heat, it became harder. Goodyear worked out a consistent system for this hardening, and by 1844 he had patented the process and was producing rubber on an industrial scale.
During vulcanization, rubber is typically mixed with sulfur and other additives, such as accelerators and activators. This mixture is then heated to an elevated temperature, usually between 140 °C and 160 °C (284 °F–320 °F). The heat-activated process allows the sulfur to react with the polymer chains present in the rubber. As the temperature rises, the sulfur atoms form chemical bonds with specific sites on the polymer chains, creating cross-links between adjacent polymer molecules. These cross-links result in the formation of a three-dimensional network structure within the rubber material. This network of interconnected polymer chains makes the rubber material more resilient and resistant to deformation.
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Thermoset vs Thermoplastic
Thermoplastics and thermosets are two distinct types of polymers with different behaviours under heat. Thermoplastics can be heated, cooled, and reshaped repeatedly without altering their chemical structure. Thermosets, on the other hand, undergo an irreversible chemical change when heated, forming permanent bonds. This process is known as "curing" or "cross-linking", where polymers form a three-dimensional network of irreversible covalent bonds.
Thermoplastics are prone to thermal and/or oxidative degradation and creep. Cross-linking thermoplastics can improve their thermal stability and mechanical properties, such as impact strength, tensile strength, and resistance to creep. Cross-linking can be achieved through radiation, chemical cross-linking with organic peroxides, or using silane-grafting agents.
Thermosets, once cured, cannot be melted or reshaped. They are typically hard, strong, and have excellent resistance to heat and chemicals. The curing process strengthens thermosets and forms irreversible chemical bonds, making them impossible to remould or recycle. Thermosets' ability to retain their strength and geometry when exposed to elevated temperatures sets them apart from thermoplastics.
The key difference between thermosets and thermoplastics lies in their ability to produce covalent bonds. Thermosets can link at 2+ sites along the polymer, creating larger amounts of branching and stronger covalent bonds. In contrast, thermoplastics have functionality less than two, resulting in weaker intermolecular bonds.
In summary, thermoplastics are advantageous when flexibility and remouldability are required, while thermosets are preferred when dimensional stability, heat resistance, and strength are crucial. Thermosets are excellent choices for parts requiring excellent dimensional stability at elevated temperatures, while thermoplastics provide versatility in applications such as heat-shrinkable tubing, rotational moulded parts, and polyolefin foams.
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Frequently asked questions
Thermo-plastically cross-linked materials are those that have been strengthened through the process of cross-linking. Cross-linking involves using cross-links to promote a change in the polymers' physical properties, such as improving mechanical, thermal, electrical, and chemical performance.
Cross-linked thermoplastics represent an important class of materials with a wide range of applications, including heat-shrinkable tubing, rotational molded parts, polyolefin foams, and snowmobile tracks.
Cross-linking thermoplastics can improve their thermal stability and mechanical properties, such as impact strength, tensile strength, and resistance to stress cracking and creep. It also makes the material stronger and more rigid.
Cross-linking can be initiated through e-beam radiation (irradiation) or chemical agents. The three main methods for cross-linking thermoplastics are radiation cross-linking, chemical cross-linking with organic peroxides, and cross-linking using silane-grafting agents.
Thermoplastics are not initially cross-linked, meaning they can be reshaped once molded. Cross-linked materials, on the other hand, become permanent after cross-linking and cannot be melted or recycled easily.
