Thermoplastic vs. Thermosetting Resins: Key Differences Explained

Key Takeaways

• Thermoplastics soften when heated and can be remolded, while thermosetting plastics harden permanently after curing.

• Plastics are broadly classified into general-purpose, engineering, and super engineering plastics based on their performance.

• Plastics can also be categorized as crystalline or amorphous, each offering different properties such as chemical resistance, transparency, and impact strength.

Overview

Plastics are generally divided into thermoplastic resins and thermosetting resins. While both start in a liquid state during the molding, they behave very differently once exposed to high temperatures after molding.

Properties of Thermoplastic Resins

The term "plasticity" refers to a solid that, when deformed by applying force, does not return to its original shape even after the force is removed. The property of becoming pliable when heated is called thermoplasticity.

Thermoplastic resins are materials that soften when heated to their glass transition point or melting point and then harden again upon cooling. During molding, they are also solidified by cooling. Thermoplastic resins can repeatedly change their state between liquid and solid based on temperature, making them recyclable.

Thermoplastic resins are broadly divided into general-purpose plastics and engineering plastics. Common examples include polyethylene (PE), polypropylene (PP), polyamide (PA), and ABS. Due to these characteristics, many of these materials have a relatively low continuous-use temperature.

Properties of Thermosetting Resins

On the other hand, thermosetting resins, once molded, will not return to a liquid state, even when heated again. This is because they harden permanently through a chemical reaction that occurs during the molding process.

Their molecular structure is called a cross-linked bond, a very strong connection that restricts molecular movement. This makes them resistant to changes from temperature fluctuations.

This structure gives thermosetting resins excellent mechanical strength and heat resistance. However, they are not completely unaffected by heat. Some still show a slight degree of plasticity when heated.

Key examples of thermosetting resins are Bakelite (a type of phenol resin) and epoxy resins like epoxy glass.

An Easy Analogy to Understand the Difference

To understand the difference between thermoplastic and thermosetting resins, it can be helpful to visualize them with familiar items. Consider chocolate and pancakes.

• Chocolate is solid at room temperature, melts when heated, and solidifies again when cooled. This repeated change between liquid and solid states is the key characteristic of thermoplastic resins.

• In contrast, a pancake before it’s cooked starts as a liquid batter and becomes a solid when heated on a griddle. It will never return to its original liquid state, regardless of how much it's heated or cooled afterward. This irreversible hardening is the key characteristic of thermosetting resins.

General Purpose vs. Engineering Plastics

General purpose plastics are a category of thermoplastic resins that are relatively inexpensive and easy to cut and shape. They are commonly found in industrial components and everyday items, making up the majority of plastics you see in daily life.

Common examples of general-purpose plastics include:

• PE (polyethylene)

• PVC (polyvinyl chloride)

• PP (polypropylene)

• PS (polystyrene)

• ABS (acrylonitrile-butadiene-styrene)

• AS (acrylonitrile-styrene)

• PMMA (acrylic)

• PBT (polybutylene terephthalate)

• PET (polyethylene terephthalate)

Among these, PE, PP, PVC, and PS are considered the four major general-purpose resins due to their high production volume. Engineering plastics are a class of plastic with enhanced properties, such as superior heat resistance, and are widely used for industrial purposes.

Plastics with even higher performance are known as super-engineering plastics and have even higher heat resistance and mechanical strength. Within these plastics, each is classified into crystalline and non-crystalline (amorphous).

Crystalline vs. Non-Crystalline Plastics

All three types of plastics—general purpose, engineering, and super engineering—are further classified based on their molecular structure.

Crystalline Plastics

Crystalline plastics have molecules arranged in a regular, orderly structure, similar to that of a crystal lattice. Crystalline characteristics include good chemical resistance, hardness, durability, and relatively high heat resistance. Examples include POM, PE, PEEK, and Teflon.

However, the degree of crystallinity can vary slightly between plastics depending on how crystallized the molecular structure is. A higher degree of crystallinity results in more pronounced and distinctive characteristics.

Non-Crystalline Plastics

Non-crystalline (also known as amorphous) plastics, are polymers that are unable to or have difficult crystallizing. Their characteristics include transparency and excellent impact resistance. Examples include PVC, Acrylic, ABS, and PC.

CNC Machining at Yumoto Electric

Yumoto Electric delivers high-quality components fast, worldwide. We provide precision machining for a wide range of metals and engineering plastics. We have experience with over 200 materials including metals such as aluminum alloys, stainless steel, molybdenum, and titanium, and engineering plastics including PEEK, PPS, MC Nylon, and POM.

If you’re unsure about material selection or machining methods, we offer complimentary consultations to help optimize your design and production. For any questions, please feel free to contact us.