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Thermoset vs Thermoplastic (What is the Difference?)

   

Despite the similarities in their names, thermoplastics and thermosetting plastics are different from each other with regard to their properties, applications and in how they are manufactured and processed. It is important for product designers to understand the differences in order to use the best option for their products.

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What is the Difference between Thermoset and Thermoplastic?

Thermosetting plastics and thermoplastics are both polymers, but they behave differently when exposed to heat. Thermoplastics can melt under heat after curing while thermoset plastics retain their form and stay solid under heat once cured.

Because thermoplastics have a low melting point, they are ideal for applications that use recycled materials. Thermoset plastics, in contrast, are able to withstand high temperatures without losing their shape, making them more durable.

Aesthetically, thermoplastics are seen as superior to thermoset polymers, however thermosetting materials are still deemed to have better aesthetics than alternatives such as metals. These materials allow for in-mould painting or coating, including spraying coatings directly into the mould before the thermoset polymers are injected into it. This technique offers better adhesion to the material and prevents chipping, cracking or flaking, even under harsh weather conditions.

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Thermoset Plastics

Thermoset plastics, also called thermosetting resin or thermosetting polymers, are typically liquid at room temperature and then harden once heated or with a chemical addition. They are usually produced using reaction injection moulding (RIM) or resin transfer moulding (RTM) and form permanent chemical bonds during the curing process. These chemical bonds between the monomer chains within the material, called crosslinks, hold the molecules in place and change the nature of the material, preventing it from melting and returning to a liquid state. Once heated, thermoset plastics are set into a specific form, although overheating can cause them to degrade without entering a fluid phase.

Thermoset plastics are ideal for use in situations where heat is a factor, such as with electronic housings and appliances or chemical processing equipment, due to their greater structural integrity and resistance to both heat and chemicals. Able to resist deformation and impact, common thermosets include epoxy resins, polyimide, and phenolic, which are often used in composites.

Pros

Thermoset plastics provide a wide range of benefits:

  • Able to be moulded with different tolerances
  • Allows for flexible product designs
  • Improved structural integrity through variable wall thicknesses
  • Typically cheaper than components fabricated from metals
  • Superb electrical insulation properties
  • Excellent heat resistance at high temperatures
  • Corrosion resistant
  • Strong dimensional stability
  • Low thermal conductivity
  • Cheaper setup and tooling costs than with thermoplastics
  • High strength-to-weight ratio
  • Water resistant
  • Wide range of colours and surface finishes

Cons

Despite these many benefits, there are still some drawbacks with thermoset polymers:

  • Cannot be reshaped or remoulded
  • Cannot be recycled

Thermoplastics

Thermoplastics are resins that are solid at room temperature but become soft when heated and eventually become fluid as a result of crystal melting or from crossing the glass transition temperature (Tg).

Processing thermoplastics involves no chemical bonding and they can be poured into a mould to cool and solidify into the desired shape. Thermoplastics can be reheated, recycled and remoulded without affecting the material properties. As a result, these materials are used in processes including extrusion, thermoforming, and injection moulding.

Common thermoplastics include polyethylene (PE), polycarbonate (PC), and polyvinylchloride (PVC), polypropylene (PP), polystyrene (PS), and polyethylene theraphthalate (PET), with each having varied properties. However, generally speaking, thermoplastics tend to resist shrinking while offering good elasticity and strength. Used in applications including industrial machine components and plastic bags for retail, thermoplastics can soften, deform and lose some of their physical properties if exposed to high temperatures.

Pros

The benefits of thermoplastics include:

  • Good adherence to metals
  • High quality aesthetic finish
  • Can be recycled and reshaped with little impact on material properties
  • Resistant to chemicals and detergents
  • Good electrical insulation
  • High impact resistance
  • Enhanced anti-slip properties
  • Can create both rubbery and hardened crystalline surfaces
  • Resists chipping
  • Corrosion resistant

Cons

Despite these benefits, there are a few drawbacks with these materials:

  • Not suited to all applications due to softening when heated
  • Typically more expensive than thermosetting polymers

Applications

Due to their superb chemical resistance, structural robustness and thermal stability, thermoset plastics are used for a range of applications, meeting a variety of production specifications. Easier to form into different shapes than metals, thermoset polymers offer considerable consistency in their fabrication. As a result, they are widely used in sectors including adhesives and sealants, aerospace, defence, electrical, automotive, energy, and construction.

Applications for thermosetting polymers include:

  • Pipes, fittings and cell covers for chemicals such as chlorine
  • Housings and components for electrical or medical use
  • Doors, panels and housings for heavy construction or transportation equipment
  • Agricultural products including livestock feeding troughs
  • Parts and components for vehicles, including tractors and military vehicles

Thermoplastics have found applications in a wide range of industries to create items including milk containers and piping. They are able to withstand corrosive conditions, making them a good substitute for metals, although they are limited in high temperature applications. As a result, these materials have found ready use in construction, electronics, medical, biomedical, food and beverage, chemical, automotive, plumbing and more industries.

Applications for thermoset plastics include:

  • Ropes and belts
  • Insulation for electrical cables
  • Liquid storage
  • Protective coverings for equipment

Examples

What is an example of a thermoset plastic?

Thermoset plastics and polymers include epoxy, polyurethane, phenolic and silicone as well as materials such as polyester (which can also occur in a thermoplastic form). Thermoset plastics are stored in liquid form and different thermosets offer different advantages. For example, epoxies are tough, elastic and chemical resistant while phenolic is flame resistant.

What is an example of a thermoplastic?

Any plastic that melts into a soft, pliable form at a certain temperature and then solidifies upon cooling is a thermoplastic. These materials can be re-melted and recycled and are typically stored as pellets before the moulding process. Common thermoplastics include acrylic, nylon, polyester, polypropylene, polystyrene, and Teflon. They find use in a wide range of industries and products from clothes and cookware to carpets and laboratory equipment.

FAQs

Are thermosets stronger than thermoplastics?

Thermoset polymers are generally harder and stronger than thermoplastics, which soften when heated. Thermosets do not soften due to their strong covalent crosslinks and also offer a better dimensional stability than thermoplastics.

Why are thermosets harder than thermoplastics?

Thermoset plastics are harder than thermoplastics due to the three-dimensional network of bonds, or crosslinks, that are created during the manufacturing process. Because they maintain their shape as strong covalent bonds between polymer chains, thermosets are more suited to high temperature applications too. The higher the crosslink density, the better they are able to resist heat degradation and chemical attack. Higher crosslink density also improves the mechanical strength and hardness of these materials, although this can lead to brittleness.

Conclusion

With variations in material properties, recyclability and more, there are a range of differences between thermoset and thermoplastics. This makes them suitable for different applications depending on factors such as required hardness and temperature resistance.

While thermoplastics are not as suited to higher temperature applications as thermosets, they can be recycled and re-used whereas thermosets cannot be melted and reshaped in the same manner.

Related Frequently Asked Questions (FAQs)

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