Thermoset vs Thermoplastic: Which is Better?

When looking at thermoset vs thermoplastic, choosing the best material for your project will highly depend on the application, needed properties, and your overall budget. Both materials have been used to create products for everyday use and even specific purposes. There are many applications where either material will work, but some require the use of one over the other.

The main difference between thermosets and thermoplastics is what happens in the curing process and how they behave when heat is applied. They are also different regarding their properties, applications, and how they’re manufactured or processed.

Thermosets handle heat incredibly well after curing, as they do not melt when exposed to additional heat.

Thermoplastics begin as solids and are then heated and melted to be formed into new solid shapes once cooled. Unlike thermosets, if heat is applied to the material after it has cured, it will melt back to its liquid state.

What Is a Thermoset?

A thermoset is a high-performance polymer that cross-links during its curing process to form irreversible chemical bonds. At room temperature, it is a liquid and then hardens when heat and/or pressure is applied to make the material undergo a chemical reaction. The chemical change prevents the material from returning to a liquid state, making it impossible to reshape, recycle, or remold.

The chemical bonds also make the material stronger and more heat-resistant than its thermoplastic counterpart. The higher the cross-link density, the better the heat and chemical resistance they have. They can also be more rigid or flexible depending on the length and number of cross-link chains.

The main molding process used to make thermosets is reaction injection molding (RIM). Some materials can also be poured or sprayed. True to their name, they are set with permanent physical properties after the initial cure.

Common materials: epoxy, polyurethane, polyurea, polyaspartic, silicone

Advantages & Weaknesses

Advantages: 

• Durable. Thermosets are a good choice for parts that require dimensional stability and structural integrity at various temperatures. Due to their strong chemical bonds, they retain their strength, form, and shape in any condition, which makes them more durable.
• On-site uses. Thermosets can be applied to be cured on site without heating and/or can be retrofitted once they are in the field. They can also be sprayed through a plural component machine or injected while in the field. With easy molding characteristics, you can create large shapes, complex parts, or multipart components.
• Cost-effective. Setup and tooling costs tend to be lower. They can be molded at different tolerances, allowing for flexible product designs. Surface finishing is not required, which makes the process even more cost and time efficient.
• Versatile. There are a wide range of industries that use thermosets due to their chemical and thermal stability, as well as their various hardnesses. They have the unique advantage of being used as plastic, rubber, or foam. For example, you can make a thermoset elastomeric, but thermoplastics are incapable of achieving the same flexibility, so you would have to use a material like natural rubber. They have excellent flowability as a liquid, which allows them to fill all voids in a mold to copy small details that can’t be made with metal or thermoplastics.

Weaknesses: They might not be used over thermoplastics in instances where a recyclable or remoldable material is desired since they cannot be melted down to their original liquid state. Even though they have high strength, their rigidity can lead to reduced hardness at high temperatures. If they are overheated, they may begin to degrade but will not melt. This makes picking the correct thermoset product and the relevant HDT or heat cycling numbers important for your application.

Where Are They Used?

With good chemical resistance and thermal stability, they meet a variety of conditions for a range of applications. Their properties make them an excellent choice for high-heat applications or situations where heat is a factor. They are widely used in the aerospace, defense, electrical, automotive, and construction industries. They are a great alternative to metals and other plastics when complex, detailed parts and components are needed. Easy molding characteristics and on-site use get around many issues that you would encounter with thermoplastic materials.

What Is a Thermoplastic?

A thermoplastic is a solid polymer at room temperature (commonly stored as pellets) but becomes soft and pliable once heat is applied. There is no chemical bonding that occurs during the curing process, which means that it is reversible and only a physical change. Parts and products are made from this material by the processes of extrusion, thermoforming, or injection molding. Since they have a low melting point, they soften and deform when exposed to heat after curing, but their properties remain unaffected once the heat is removed.

Common materials: ABS, acrylic, nylon, polystyrene

Advantages & Weaknesses

Advantages: 

• Reusable. They are best known for their recyclability, as they can be melted down and molded into a new shape for reuse. Even after the material has been reshaped, its physical properties will not be negatively affected.
• Durable. They have great impact resistance and high strength while also being lightweight. They also resist shrinking as they offer good elasticity and flexibility. Since they are known for being versatile, they work well in both high and low-stress applications.
• Chemically resistant. They are desirable for their resistance to chemicals, detergents, and corrosion. They are the perfect material for applications that need protection from highly corrosive environments. While thermosets also have decent chemical resistance, it doesn’t compare to thermoplastics.

Weaknesses: Thermoplastics are not always the best or most cost-effective option, especially for low-volume or custom part production. The production process usually requires high heat and pressure, which can be more costly.

When exposed to heat and sunlight for extended periods, they experience UV degradation and soften or deform. They can’t handle heavy loads because they will stretch and weaken, which makes them more susceptible to creep and fractures.

They also struggle with application or retrofitting in the field. Primarily all work is done in the manufacturing setting and the original part may not be able to be modified in the field.

Where Are They Used?

Thermoplastics are ideal for applications that require recyclable and reusable materials. They are a good substitute for metal, as they can withstand corrosive conditions, though they are limited in high-temperature environments. They’ve also found use in the construction, electronics, medical, food and beverage, chemical, and automotive industries. They can be used to encapsulate rigid objects in electrical equipment or rope and belt production.

VFI Thermosetting Polymers

VFI has been manufacturing thermosetting polymers for almost 30 years. Our products have been used in a vast number of markets and industries. Most of our materials are two component liquids that become solid from a chemical reaction once combined and allowed to cure. They are strong, long-lasting, and third-party tested for quality assurance. If you have specifications for a needed material or questions on if you should be using a thermosetting polymer, contact VFI today.