Thermoset polymers, also known as thermosets, are a type of plastic that can be molded into various shapes and forms but cannot be melted or reshaped once they are cured. They are made from monomers, or small molecules, that are linked together by covalent bonds, forming a rigid and insoluble network. Thermosets are different from thermoplastics, which can be melted and remolded multiple times.

Thermosets have many advantages over thermoplastics, such as:

• Higher strength and stiffness
• Better resistance to heat, chemicals, and corrosion
• Lower shrinkage and creep
• Better dimensional stability and durability

Some of the common examples of thermoset polymers are:

• Epoxy resin: used for adhesives, coatings, and composite materials
• Polyurethane: used for foam, elastomers, and coatings
• Phenol–formaldehyde resin: used for bakelite, plywood, and laminates
• Silicone resin: used for sealants, adhesives, and coatings
• Vulcanized rubber: used for tires, hoses, and gaskets

How Thermosets Are Made and Cured

Thermosets are usually prepared from liquid or soft solid precursors, called prepolymers, that have reactive functional groups at the ends or along the backbone. These prepolymers are mixed with curing agents, or hardeners, that initiate the cross-linking reaction. The curing process can be triggered by heat, pressure, radiation, or catalysts, depending on the type of thermoset and the desired properties. The curing process is irreversible, meaning that once the thermoset is formed, it cannot be melted or dissolved.

The curing process can be divided into three stages:

• Gelation: the formation of a soft and elastic gel that can still flow
• Vitrification: the transition of the gel to a glassy and rigid state that cannot flow
• Post-curing: the completion of the cross-linking reaction and the development of the final properties

The curing time and temperature can vary depending on the thermoset system and the application. For example, epoxy resins can be cured at room temperature or elevated temperatures, while polyurethanes can be cured by moisture or heat.

How Thermosets Are Used and Recycled

Thermosets are widely used in various industries and applications, such as aerospace, automotive, construction, electronics, and medical. They offer superior performance and functionality, such as high strength-to-weight ratio, thermal and electrical insulation, and biocompatibility. Some of the common products made from thermosets are:

• Composite materials: thermosets are combined with fibers, such as carbon, glass, or natural fibers, to form lightweight and strong materials that can be used for aircraft, wind turbines, and sports equipment
• Coatings and adhesives: thermosets are applied as protective or decorative layers on various substrates, such as metal, wood, or concrete, or as bonding agents between different materials
• Foams and elastomers: thermosets are expanded or stretched to form flexible and resilient materials that can be used for cushioning, insulation, or sealing

However, thermosets also pose a significant challenge to the environment and society, as they are difficult to recycle or reuse due to their permanent cross-linked structure. Most of the thermoset waste is either incinerated, landfilled, or mechanically ground into fillers, which only recover a small fraction or partial value of the original materials. Therefore, there is a growing interest and demand for developing more sustainable and circular solutions for thermoset recycling and upcycling.

Some of the recent trends and innovations in thermoset recycling and upcycling are:

• Physical upcycling: thermoset waste is processed into new shapes and forms, such as oil-water separation materials or 3D printable materials, by applying mechanical forces, such as shear, compression, or extrusion
• Carbonization: thermoset waste is converted into carbon materials, such as supercapacitors, photothermal conversion materials, or catalytic materials, by applying high temperature and controlled atmosphere
• Solvolysis: thermoset waste is depolymerized into monomers or oligomers, which can be used as additives, emulsifiers, or lubricants, by applying solvents, such as water, alcohol, or glycol
• Vitrimerization: thermoset waste is transformed into vitrimers, which are a new class of polymers that have reversible bonds and can be reshaped and recycled, by applying chemical modification, such as transesterification, transamination, or exchange reaction

Conclusion

Thermoset polymers are an important and versatile class of materials that have many applications and benefits but also pose significant environmental and social problems due to their non-recyclability. Therefore, it is essential to develop and implement more sustainable and circular strategies for thermoset recycling and upcycling, which can create new value and opportunities for the thermoset industry and society.

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