Comparison of Thermoset Versus Thermoplastic Materials

Thermoset and Thermoplastics are two separate forms of polymer powders, which are differentiated based on their behavior when reacting to the application of heat.

Introduction—Thermoset vs Thermoplastic

The primary difference between the two is that Thermoset is a material that strengthens when heated, but cannot be remolded or heated after the initial forming, while thermoplastics can be reheated, remolded, and cooled as necessary without causing any chemical changes. As a result of these physical and chemical properties, thermoplastic materials have low melting points while thermoset products can withstand higher temperature without loss of its structural integrity.

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In this article, the uses and applications of each of these types of thermosetting polymers will be presented, including a summary of the relative advantages and limitations of each.

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What are Thermoset Plastics?

Plastics are one of the most common materials used to manufacture goods today, from the keyboard you’re typing on to the International Space Station – plastics are here to stay. So how can plastics which can make something simple as a pen create something so complex that it can withstand rigorous physical and chemical elements such as excess heat and corrosion?  The answer is thermoset plastics or thermosetting. Thermosets are unique and quite different from traditional plastic materials such as thermoplastics, so what's the difference between thermoplastic and thermosetting? Unlike traditional thermoplastic parts that melt and disfigure when exposed to excess heat, thermoset components as implied by their name become set in their physical and chemical properties after an initial heat treatment and therefore are no longer affected by additional heat exposure. The process is very similar to hard boiling an egg. Once the egg is boiled, additional reheating does not cause disfiguration – similar to the curing process associated with a thermoset part. After initial heat forming, thermoset material’s ability to exhibit resistance to heat, corrosion, and mechanical creep make them perfectly suitable for use in components that require tight tolerances and excellent strength-to-weight characteristics, while being exposed to elevated temperatures.    

Application of Thermosetting Plastics

Thermoset plastics offer enhanced high-performance combination of thermal stability, chemical resistance, and structural integrity. Thermoset components are used extensively in a wide range of industries – and are used for applications in the automotive, appliance, electrical, lighting, and energy markets due to excellent chemical and thermal stability along with superior strength, hardness, and moldability.

Thermoset plastic composites are capable of meeting the specifications of a wide range of production materials at a very low cost. Their use allows for an assortment of small and large parts to be fabricated with high production volume while maintaining their repeatability consistently from batch to batch. Thermosets provide an alternative process when complex and geometric shapes cannot be achieved through metal fabrication or the use of thermoplastics but can be manufactured in a mold. Thermoset plastics maintain their stability in all environments and temperatures.   

Advantages of Thermosets

The use of thermoset plastics has a number of advantages. Unlike thermoplastics, they retain their strength and shape even when heated. This makes thermosetting plastics well-suited to the production of permanent components and large, solid shapes. Additionally, these components have excellent strength attributes (although they are brittle), and will not lose significant strength when exposed to higher operating temperatures. 

Thermosets plastics have gained in popularity among manufacturers, who have switched to their use as a lower cost replacement for metal components.

The benefits that thermoset plastics can offer over their metal counterparts include:

• Available molded-in tolerances
• Choice of color and surface finishes
• High strength-to-weight ratio and performance
• Outstanding dielectric strength
• Low thermal conductivity and microwave transparency
• Resistance to corrosion effects and water
• Lower tooling/set-up costs
• Reduced production costs over fabrication using metals

Disadvantages of Thermosets

There are certain disadvantages to the use of thermosets whereas the material properties are not as developed as those of thermoplastics.

The low initial viscosity of materials results in flash and the need for secondary operations. Also, low tensile strength and ductility tend to result in parts that require designs with thick walls. The compounds used in thermosets are reactive systems, which can impact the useful shelf life. Batch processes may exhibit greater variation and less consistency from lot-to-lot.  High levels of some filler in the materials may result in excessive tool wear. The product quality is dependent upon the degree of crosslinking established during the molding cycle.²

Table 1: Pros and Cons of Thermoset Materials

Thermoset Pros2	Thermoset Cons2
More Resistant against high temperature	Cannot be recycled.
Hard and rigid	Much more difficult to surface finish.
Excellent aesthetics finishes	Cannot be remolded or reshaped.
High mechanical property	Poor thermal conductivity for housing replacements.
Cost-effective	The rigidity of the material can result in product failure when used in high in vibration applications.
Excellent dimensional stability

Table credit: https://www.modorplastics.com/plastics-learning-center/thermoset-vs-thermoplastics/

Type of Thermosets & Thermoset Materials List

Thermosetting polymers are built from various types of material that serve key roles and applications in the plastics fabrication industry.  The two thermoset molding compound processes are Reaction Injection Molding (RIM) and Resin Transfer Molding (RTM).

Composite thermoset materials consist of a matrix and a dispersed, fibrous, or continuous second phase. Casting resins include a catalyst or hardener. Thermoset electrical resins and electronic-grade products are used in potting or encapsulating compounds, conductive adhesives, and dielectric sealants.¹

Thermal compounds are designed to form a thermally conductive layer on a substrate, either between components or within a finished electronic product. Thermoset purging compounds are used to clean molding machines between runs of different colors or compositions. Gap filling products are used to fill in gaps or spaces between two surfaces to be bonded or sealed.¹

Specific types of thermoset materials that are in use within the plastics fabrication industry include the following:

RIM Thermosetting Plastics:

• pDCPD (polydicyclopentadiene)
• Polyurethanes
• Polyureas
• Structural Foams

RTM Thermosetting Plastics (Fiberglass-reinforced): 

• Polyester
• Vinyl Ester
• Polyimides

Other Types of Thermosets and Thermoset Resins:

• Epoxy (e.g., carbon fiber epoxy resin)
• Phenolics
• Bismaleimide (BMI)
• Fluoropolymers
• Polytetrafluoroethylene (PTFE)
• Polyvinylidene fluoride (PVDF)
• Melamine
• Silicone
• Urea Formaldehyde

What are Thermoplastics? 

As discussed earlier, plastics are widely recognized as real-world applications of the use of chemistry. From plastic containers, bottles, and life-saving medical devices to aerospace components, plastics are a material that is used in applications all around us. Thermoplastic polymers are one such type of plastic which is known for its versatility and recyclability. Thermoplastic polymers form when repeating units called monomers link into chains or branches.

Thermoplastic pellets soften when heated and become more fluid as more heat is administered. The curing process is 100% reversible as no chemical bonding takes place. This characteristic allows thermoplastics to be remolded and recycled without negatively affecting the material’s physical properties. There are a variety of thermoplastic resins that offer various performance benefits, but the majority of materials commonly used offer high strength, shrink-resistance, and easy flexibility. Depending on the resin, thermoplastics can serve low-stress applications such as plastic bags or can be used in high-stress mechanical parts. Examples of thermoplastic polymers include polyethylene, PVC, and nylon. ¹

Image credit: Adams Plastics

Applications of Thermoplastics

In polluted, acidic, environments such as may be found in modern cities, steel piping systems are often vulnerable to rust or corrosion and therefore need special provisions for corrosion protection. The cost associated with protecting steel piping systems that are exposed to these harsh environments can be expensive. Thermoplastics are considered to be a favorable substitute to minimize these costs.  Certain properties of thermoplastics that make them a suitable substitute material are:

• Their ability to withstand corrosive materials and corrosive environments.
• Being able to carry materials of extreme temperatures (hot or cold).
• Their capacity to handle virtually any type of fluid transport application.

Common materials used to produce these pipes are PVC or CPVC. Additional materials include polypropylene, PVDF, ABS, nylon, and polyethylene. Polyethylene gas tanks are used to transport natural gas for use in residential and commercial applications.

Other common applications for thermoplastics include high-pressure polyethylene to encapsulate rigid objects like electrical equipment. Low-pressure polyethylene is very elastic and ideal for insulating electrical cables. Polyamide is most commonly associated with the production of ropes and belts.

Advantages and Disadvantages of Thermoplastics

The primary advantage of thermoplastics is their wide range of applications. Thermoplastics are high strength, lightweight materials and have relatively low processing costs. Additionally, thermoplastic components are relatively easy to manufacture with high volume and precision.

The primary disadvantage of using thermoplastics instead of materials such as metal is their relatively low melting point. Certain types of low-quality thermoplastics can melt when they're exposed to the sun for extended periods. Furthermore, thermoplastics can have poor resistance to organic solvents, hydrocarbons, and highly polar solvents.

Thermoplastics are susceptible to creep, which occurs when the material stretches and weakens under exposure to long-term stress loads. The susceptibility to creep is further exacerbated by the lower melting temperature of the material. Other types of thermoplastics, such as composites, can fracture instead of becoming deformed under high-stress conditions.

Table 2: Pros and Cons of Thermoplastic Materials

Thermoplastic Pros	Thermoplastic Cons
Extremely adhesive to metal	May soften when reheated
High recyclable
Superb impact resistance
Can be remolded and reshaped
Excellent corrosion resistance
Slip enhancement
Detergent and chemical resistance	Can be more expensive than thermoset
Flexibility and elongation of the coating film
Electrical insulation
Chip resistance
Aesthetically-superior finishes
Superb corrosion resistance