Maya Ellison
Thermoset plastics are used to make parts for aircraft, cars, tires, and computer casings. They are highly durable and heat-resistant, but their very strength is what makes them difficult to dispose of or recycle. Conventional recycling methods cannot break them down, and they are often burned or buried instead. However, researchers from MIT have discovered a way to recycle thermoset plastics, which could boost sustainability in the plastics industry. This involves creating a degradable version of a thermoset plastic called pDCPD, which can be broken down into a soluble powder and then reused. Other methods of recycling thermoset plastics have also been patented, including a process that involves heating plastic scrap with a solvent in a reaction vessel.
How to Recycle Thermoset Plastic
| Characteristics | Values |
|---|---|
| Current Disposal Methods | Typically burnt or buried |
| Recycling Difficulty | Very difficult to recycle due to strong chemical bonds |
| Recycling Potential | Researchers are working on it |
| Industry Importance | Thermosets comprise about 20% of polymeric materials manufactured |
| Environmental Impact | Thermosets contribute to nearly half of all plastic waste |
| Mechanical Process | Crushing, cleaning, and feeding into an extruder |
| Thermal Process | Heating to 750-800 F, depressurizing, and cooling |
| Chemical Process | Using a solvent to break down the plastic |
| Recyclability Perception | Perceived as non-recyclable, impacting its consideration |
| Comparison to Thermoplastics | Thermosets are harder to recycle but have advantages in some applications |
| Future Prospects | Developing recyclable thermosets is crucial for waste reduction |
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What You'll Learn
Thermoset plastic recycling methods
Thermoset plastics are challenging to recycle because they are designed to be heat-resistant and durable. Once they have been melted, cooled, and set, they are difficult to remelt due to the strong chemical bonds between the polymer molecules. However, researchers and companies have developed various methods to recycle thermoset plastics.
One method is mechanical processing, which is the most commercially applicable recycling technique for thermoset plastics. It involves crushing the thermoset scrap, cleaning it using a cleaning agent and filtration, and then feeding it into an extruder. The extruder applies high temperatures and pressures to break down the crosslinking structure between the macromolecules in the thermoset plastic waste, allowing it to be recycled.
Another method is the use of a chemical linker, such as a silyl ether monomer, which is spread throughout the thermoset plastic mixture. This modification makes the plastic degradable while retaining its mechanical strength. When exposed to fluoride ions, the plastic can be broken down into a soluble powder, which can then be used to create new thermoset plastics.
Additionally, some companies have developed processes that use a combination of pressure, heat, and supercritical solvents to decrosslink thermosets. One such process, patented by GreenMantra Recycling Technologies Ltd., involves heating plastic scrap with a solvent in a reaction vessel to temperatures of up to 800 °F. The reaction vessel is then depressurized to remove contaminants, and the remaining materials are cooled. This process efficiently removes contaminants without risking thermal oxidation or degradation of the polymer material.
Furthermore, the European Union's end-of-life directive (2000/53/EC) has set targets for vehicle waste to be reusable or recyclable, which has created opportunities for the development of thermoset recycling technologies. These technologies not only help create employment and contribute to economic growth but also address the environmental concerns associated with the disposal of thermoset plastics.
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Why thermoset plastics are difficult to recycle
Thermoset plastics are difficult to recycle because of their unique chemical composition. Thermosets are made from a similar process to thermoplastics, but once they have been melted, cooled, and set, they are challenging to re-melt. This is due to the formation of strong covalent bonds between the polymer molecules during the curing process, which are difficult to break. As a result, when heated, thermoset plastics typically burn instead of melting, making them unsuitable for traditional recycling methods.
The high density of crosslinks that gives thermosets their desirable properties, such as durability, heat resistance, and tensile strength, also contributes to their recycling challenges. These strong chemical bonds are more difficult to break down compared to those in other materials, including thermoplastics. The irreversible bonding structure of thermosets makes it hard to reshape them through reheating, a common technique used for recycling thermoplastics.
Additionally, the complex composition of thermosets poses another challenge for recyclers. Thermosets used in various applications, such as wind turbines and vehicles, are often reinforced or filled composite materials. These composite materials are challenging to separate, requiring special tools and adding complexity to the recycling process. The recycling process for thermosets is often energy-intensive, costly, and time-consuming, making it unprofitable for many organizations to undertake.
Despite these challenges, some experts argue that thermoset plastics can be recycled with the right approach. Recent advancements, such as the use of chemical linkers, have been made to modify thermoset plastics, making them easier to recycle while retaining their mechanical strength. These advancements offer hope for improving the recyclability of thermoset plastics and reducing their environmental impact.
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Thermoset resins and their recyclability
Thermoset resins are cured using heat, light, or electron-beam cured systems, with catalysis, hardeners, temperature, and time controlling the curing process. They are highly cross-linked, making them resistant to thermal breakdown, even at temperatures of 3000 °C. This resistance to thermal breakdown is a result of the presence of covalent intermolecular chemical cross-links, which also increase strength and stiffness and reduce susceptibility to damage by nearby thermal and chemical impulses.
Thermosets have traditionally been considered non-recyclable due to their high thermal and mechanical stability. The difficulty in recycling thermoset plastics lies in the strong chemical bonds that form between the polymer molecules, which are difficult to break. As a result, thermoset plastics are typically burned or buried as waste. However, recent advancements have been made in the recycling of thermoset resins. For instance, researchers from MIT have discovered a way to produce a degradable version of a thermoset plastic called polydicyclopentadiene (pDCPD), which can be broken down into a soluble powder upon exposure to fluoride ions without hurting its useful mechanical properties.
Additionally, Randy Lewis, a specialist in the field, has stated that thermoset resins can be recycled with the right imagination and desire. He notes that recycling thermosets is no more or less difficult than recycling any engineering-grade thermoplastic, except for a few specific types of thermoplastics. Furthermore, bio-based thermosetting resins have been developed with excellent properties, but their service lifetime presents a challenge due to the difficulty in processing or reforming them after the curing reaction.
Despite these advancements, the recycling of thermoset resins remains a complex task. The high density of crosslinks that gives thermosets their useful properties, such as chemical and thermal resistance and tensile strength, comes at the cost of reduced degradability and recyclability. However, with the increasing demand for sustainability, there is a growing interest in developing recyclable resins for various applications, including sporting goods, automotive, and wind turbine applications.
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Thermoset plastic recycling as a new frontier in the circular economy
Thermoset plastics are indispensable materials for modern human life. They are used in food production, health, transportation, sports, military/defence, electronics, and packaging. However, they have also contributed to environmental pollution, with millions of tons of thermoset plastics ending up in landfills each year.
Thermoset plastics are challenging to recycle because of their strong chemical bonds. Typically, they are burned or buried as recycling them is impractical. However, researchers from MIT have discovered a way to recycle thermoset plastics, which could boost sustainability in the plastics industry. This discovery paves the way for recycling a wide range of plastic products, such as car parts, tyres, and electrical appliances.
One of the key challenges in recycling thermoset plastics is breaking down their strong chemical bonds. To address this, researchers have been exploring the use of chemical linkers or cleavable co-monomers that can be triggered by external stimuli such as heat, chemicals, or optical stimuli. For example, the addition of a silyl ether monomer to polydicyclopentadiene (pDCPD), a type of thermoset plastic, makes it degradable without compromising its mechanical strength. This discovery allows pDCPD to be broken down into a soluble powder upon exposure to fluoride ions, which can then be used to create new pDCPD thermosets.
While some technologies for recycling thermoset plastics already exist, they often require a high energy input and do not consider the recycling of the thermoset matrix itself. Instead, they focus on retrieving valuable components such as fibres, fillers, or substrates. To promote circularity, studies have suggested using degradable linkages or dynamic covalent bonds, which require less energy input. Additionally, fibre-reinforced thermosetting composites can be recycled to replace virgin materials, reducing the need for both the raw thermoset matrix and raw fibres.
Thermoset plastic recycling offers new opportunities for the circular economy by converting low-value products into high-value ones. It also helps reduce the environmental footprint of the plastics industry, contributing to a more sustainable society.
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How thermoset plastics are recycled industrially
Thermoset plastics are typically challenging to recycle due to their strong chemical bonds, which make it difficult to return them to a liquid state after they have cooled and solidified. However, recent advancements have been made to improve the recyclability of thermoset plastics. Here is an overview of some industrial recycling processes for thermoset plastics:
Mechanical Recycling
The mechanical recycling process involves crushing and cleaning thermoset plastic scrap using a cleaning agent and filtration. The cleaned scrap is then fed into an extruder, where high temperatures of up to 750 °F and extrusion pressures of up to 150 bar are applied. This process breaks down the crosslinking structure between macromolecules, allowing the material to be continuously extruded. Faster screw speeds help improve decrosslinking efficiency by reducing the dwell time of the material in the extruder.
Solvent-Based Recycling
Another approach is solvent-based recycling, which involves heating plastic scrap with a solvent in a reaction vessel to temperatures up to 800 °F. The reaction vessel is then depressurized to remove contaminants suspended in the solvent, and the remaining materials are cooled. This method, known as Supercritical Fluid Extraction, effectively removes contaminants without risking thermal oxidation or degradation of the polymer material.
Chemical Recycling
Chemical recycling techniques, such as pyrolysis, are being developed to break down the cross-linked molecular structure of thermoset plastics, making them easier to recycle. One example is the incorporation of a monomer containing a silyl ether group, which can be broken down when exposed to acids, bases, or ions such as fluoride. This chemical reaction allows the thermoset plastic to degrade.
Polymer Recycling
IBM researchers have created polymers that can break down under acidic conditions, making them more amenable to recycling. Additionally, MIT chemists have developed a method to modify thermoset plastics, making them easier to break down without compromising their mechanical strength.
Resin Recycling
A company called Nano and Advanced Materials Institute Ltd. has patented a process to produce recycled resins from thermoset plastic waste. These resins have characteristics similar to virgin materials and can be returned to the manufacturing process as raw materials. This approach is characterized as harmless and effective for mass recycling.
These industrial processes highlight the ongoing efforts to improve the recyclability of thermoset plastics and promote sustainability in the plastics industry.
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Frequently asked questions
Thermosetting plastics are plastics that, once shaped and cooled, cannot return to their original form. They are used to make parts for aircraft, cars, tires, computer casings, medical equipment, etc.
Developing a thermoset plastic that can be recycled is important because otherwise, it will end up in a landfill or the ocean. This has already led to plastic waste turning up in oceans in a volume equivalent to 8 million metric tons a year.
Researchers from MIT have discovered a way to recycle thermoset plastics. The process involves crushing the plastic scrap, cleaning it using a cleaning agent, and then feeding it into an extruder. Using temperatures of up to 750 °F and extrusion pressures of up to 150 bar, the scrap can be recycled.
The strong polymer links in thermoset plastics make them difficult to recycle using conventional heating methods. They are also more challenging to recycle than thermoplastics because they cannot be fluidized through heating and remolded.
Yes, several companies are working on recycling thermoset plastics. For example, Nano and Advanced Materials Institute Ltd. has patented a new thermoset plastic recycling method to produce recycled resins with characteristics similar to virgin materials. GreenMantra Recycling Technologies Ltd. has also patented a process that involves heating plastic scrap with a solvent to temperatures of up to 800 °F to remove contaminants.
