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Thermosetting Plastics

Thermosetting plastics, with their unique chemical and physical properties, are extensively used in various fields from automotive and aerospace to everyday household appliances.
This article will provide a detailed overview of the structure, properties, production methods, and widespread applications of thermosetting plastics. Continue reading to learn more.

What Is Thermosetting Plastic?

Thermosetting plastics, also known as thermosets, are a type of polymer that undergo an irreversible chemical process when heated, resulting in a hardened state. This process, known as curing, forms cross-linked structures between polymer chains, enhancing the material’s resistance to deformation, heat, and chemicals.

Who Discovered Thermosetting Plastic?

Thermosetting plastics were discovered by Belgian-born American chemist Leo Hendrik Baekeland, who in 1909 developed the first commercially used thermoset—Bakelite.

How Is Thermosetting Plastic Produced?

Thermosetting plastics are primarily produced through liquid molding processes. Here’s a detailed overview of the production steps:

  1. Material Preparation: Polymers and other agents such as catalysts or curing agents are prepared and placed into tanks or barrels. These materials are typically in liquid form or are heated to become liquid.
  2. Molding Process: The liquid polymer and additives are then injected into a mold cavity. The specific molding techniques include:
    • Resin Transfer Molding (RTM): In this closed mold process, a resin formula mixed with a curing agent is injected into a mold containing dry fibers (such as fiberglass), hardening into the component.
    • Reaction Injection Molding (RIM): This process involves heating and injecting two liquid components (Component A and Component B) under high pressure into a mixing head. The mixed liquid then flows into the mold, where a chemical reaction occurs, forming the polymer.
  3. Curing Process: Once in the mold, the material undergoes a curing process where it solidifies and cross-links. This curing can be initiated or accelerated by heating, applying pressure, or using chemical catalysts. The process transforms the flowing liquid resin into a rigid polymer by forming a tightly interconnected network of polymer chains.
  4. Post-processing: After curing, the cured parts are removed from the mold. Any necessary post-processing steps, such as trimming excess material or further curing to achieve desired properties, are then carried out.
    The characteristic feature of the production of thermosetting plastics is the cross-linking of polymer chains during the curing process, forming an irreversible hardened structure. This cross-linking provides unique properties such as high thermal stability, chemical resistance, and mechanical strength, making them suitable for various applications in the automotive, aerospace, and electronics sectors.

What Are the Properties of Thermosetting Plastic?

Thermosetting plastics offer a range of properties suitable for various applications, especially those requiring durability, heat resistance, and chemical resistance. Here are some key characteristics:

  1. High-Temperature Resistance: Thermosets can withstand higher temperatures without losing their structural integrity. Under extreme heat, they do not melt but decompose.
  2. Chemical Resistance: These plastics have strong resistance to chemicals, making them ideal for use in harsh environments where other materials might degrade.
  3. Mechanical Strength and Rigidity: Due to their cross-linked molecular structure, thermosets have excellent mechanical properties. They are tough and rigid, which is crucial for applications involving high loads or stresses.
  4. Dimensional Stability: Thermosets maintain their shape and size even under high temperatures and mechanical forces. This stability is due to the cross-linking of their molecular chains during the curing process, preventing them from flowing or deforming under stress.
  5. Electrical Insulation: Many thermosetting plastics are excellent insulators, making them suitable for electrical and electronic applications. This property is particularly important in the manufacture of insulators and circuit breakers.
  6. Durability: The robust nature of thermosetting plastics makes them suitable for long-term applications. They exhibit resistance to weathering, aging, and long-term degradation.
  7. UV Resistance: Thermosets can withstand exposure to ultraviolet light without significant degradation, which is beneficial for outdoor applications.
  8. Flame Resistance: Some thermosetting plastics, such as melamine and phenolic plastics, are inherently flame retardant, making them suitable for applications where fire safety standards are stringent.

What Are the Common Applications of Thermosetting Plastic?

The common applications of thermosetting plastics span various industries, showcasing their versatility and unique properties. Here are some notable applications:

  • Aerospace and Automotive: Used in industries where high thermal deformation temperatures and structural integrity are required. Applications include under-the-hood parts, body panels, seats, interior components, and carbon fiber composites for wheels and under-the-hood parts.
  • Building and Construction: Widely used in commercial and residential buildings for kitchen and bathroom fixtures, such as sinks, shower surrounds, bathtubs, doors, wall panels, roofing, windows, molding, and swimming pools.
  • Electrical and Electronics: With their excellent electrical insulating properties, thermosets are extensively used in the electrical and electronics industry for electrical casings and components, insulators, battery caps, heat shields, circuit breakers, printed circuit boards, gears, and various components of electrical equipment.
  • Appliances: Large appliances like dishwashers, refrigerators, washing machines, dryers, and small kitchen appliances often contain polyester and epoxy thermosetting composites. Components such as oven handles, ventilation decorations, control panels, and knobs are made from or contain thermosetting composites.
  • Energy Industry: Play a significant role in the energy industry, including oil, natural gas, and solar industries, where materials are required to remain stable under various environmental and temperature conditions.
  • Medical Devices: Utilizing the chemical corrosion resistance and high-temperature resistance of thermosetting plastics, the medical industry benefits from the manufacture of durable medical equipment.
  • Other Applications: Other uses include gas pipelines and water pipes, signage, storage boxes, kitchen utensils, toys, agricultural feed troughs, motor parts, disc brake pistons, and even sports equipment like archery gear, tennis racquets, golf clubs, and ski equipment. The chemical resistance, heat resistance, structural integrity, and dimensional stability of thermosetting plastics highlight their wide-ranging uses.

What Are the Types of Thermosetting Plastic?

Types of thermosetting plastics include:

These materials have unique uses and applications, providing various properties such as strength, hardness, color, flame resistance, corrosion resistance, wear resistance, and more, depending on the specific type of thermosetting plastic.

What Are the Benefits of Using Thermosetting Plastic?

Using thermosetting plastics offers many different advantages, thus they hold high value in many industrial and commercial applications. Here are the main benefits:

  1. High-Temperature Resistance: Thermosetting plastics can withstand high temperatures without losing their structural integrity. This makes them suitable for applications requiring high heat resistance, such as automotive under-the-hood components and electrical insulators.
  2. Chemical Resistance: These plastics have excellent chemical resistance, thus they can be used in corrosive or frequently chemically exposed environments, such as manufacturing storage tanks and pipelines.
  3. Mechanical Strength: Thermosetting plastics are known for their superior mechanical strength and rigidity. Therefore, they are very suitable for heavy-duty applications requiring high durability, such as construction equipment and aerospace components.
  4. Dimensional Stability: With their cross-linked molecular structure, thermosetting plastics can maintain shape and size under stress and high temperatures, which is crucial for applications requiring precision and stability in parts.
  5. Electrical Insulation: Thermosetting plastics have high dielectric strength and low conductivity, making them ideal materials for electrical insulation. In the electrical and electronics industry, components such as circuit breakers and electrical casings particularly need this property.
  6. Corrosion Resistance: Thermosetting plastics can resist corrosion caused by moisture and salt, making them suitable for outdoor and marine applications.
  7. Low Production Costs: Compared to metals, thermosetting plastics have lower mold and setup costs, and overall production costs are also lower. This makes thermosetting plastics an economical choice for manufacturing various parts.
  8. Design Flexibility: Thermosetting plastics can be molded into complex shapes and sizes, offering great design flexibility. This is beneficial for manufacturing complex parts that are difficult or impossible to achieve with other materials.
  9. Aesthetic Choices: These materials can be made in various colors and surface finishes, enhancing the aesthetics of the final product without the need for additional painting or surface treatment processes.
  10. Low Thermal Conductivity: The low thermal conductivity of thermosetting plastics makes them suitable for applications requiring thermal insulation.

What Are the Disadvantages of Using Thermosetting Plastic?

The disadvantages of using thermosetting plastics include:

  • Non-recyclable: Once cured, thermosetting plastics cannot be melted or reshaped, making them difficult to recycle.
  • Poor Thermal Conductivity: The poor thermal conductivity of thermosetting plastics may limit their use in applications requiring heat dissipation.
  • Rigidity Leading to Product Failure: The inherent rigidity of thermosetting plastics can cause products to fail in high-vibration applications because the material cannot bend.
  • Low Tensile Strength and Flexibility: Compared to other materials, thermosetting plastics have lower tensile strength and flexibility, which may limit their use in applications requiring high flexibility and strength.
  • Complex Surface Treatment: Achieving good surface finish with thermosetting plastics is more challenging compared to other materials, increasing manufacturing complexity and cost.
  • Difficult to Repair Once Damaged: Once damaged, thermosetting plastics are difficult or even impossible to repair due to their permanent cross-linked structure.
  • Variability and Inconsistency in Batch Processing:In the batch production process of thermosetting plastics, differences between batches can be significant, leading to poor consistency and potentially affecting product quality.
  • High Filler Content Leading to Tool Wear: The high filler content in thermosetting plastics can cause excessive tool wear, increasing maintenance costs.
  • Dependent on Degree of Cross-linking: The quality of the final product is affected by the degree of cross-linking during the molding cycle, requiring precise control of the manufacturing process.

What Is The Difference Between Thermoplastic and Thermosetting Plastic?

thermal setting plastic

The main differences between thermoplastic and thermosetting plastics lie in their behavior when heated and their molecular structures:

  1. Behavior When Heated:
    • Thermoplastic: These plastics can be repeatedly melted, reshaped, and solidified. They soften when heated and harden when cooled, allowing them to be recycled and reshaped into new forms.
    • Thermosetting Plastic: Once heated and shaped, thermosetting plastics cannot be melted and reshaped. Heating cured thermosetting plastic does not cause it to melt but causes it to degrade or burn. This is because during the curing process, the polymer chains form a strong cross-link, permanently setting the material.
  2. Molecular Structure:
    • Thermoplastic: They have a linear or branched structure, with weak secondary bonds between polymer chains. This structure allows the material to be remelted and reshaped.
    • Thermosetting Plastic: They form a primary bond network between polymer chains and are fixed together by strong cross-links. These cross-links provide structural stability and rigidity, preventing the material from melting.
  3. Processing Methods:
    • Thermoplastic: Common processing methods include injection molding, extrusion, blow molding, and thermoforming.
    • Thermosetting Plastic: Typically processed using methods such as compression molding and reaction injection molding, suitable for their irreversible curing process.
  4. Applications and Properties:
    • Thermoplastic: Due to the reshaping capability of thermoplastics, they are used in applications requiring recycling and flexible design. Compared to thermosetting plastics, they typically have lower melting points and tensile strength.
    • Thermosetting Plastic: Known for their high-temperature resistance, chemical corrosion resistance, and mechanical strength, these properties make them suitable for high-performance applications requiring high durability.

Are Thermosetting Plastics Strong?

Yes, thermosetting plastics are very strong. They are typically very hard and maintain strength and shape even under heat. This makes them highly suitable for structural applications requiring high strength and stiffness to withstand high loads. Additionally, these materials possess excellent strength properties, not significantly reducing in strength at higher working temperatures.


Thermosetting plastics, with their wide range of beneficial properties and diverse applications, play a crucial role in many sectors. Their unique features, such as high heat resistance, chemical resistance, and mechanical strength, make them indispensable in high-performance environments. Despite their limitations, such as non-recyclability and the challenges associated with their production, thermosetting plastics continue to be a valuable material choice in various industries due to their superior qualities and cost-effectiveness.
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