Thermoplastics vs. Thermosetting Polymers: Properties, Processing and Applications

Thermoplastics and thermosetting polymers are types of plastic that undergo different production processes and yield a variety of properties depending on the constituent materials and production method. The terms thermoplastic and thermoset stand for how a material is or can be processed under a changed temperature [1].

The main physical difference is how they respond to high temperatures. When heated to their melting point, thermoplastics soften into a liquid form. Therefore, the curing process is reversible, which means that they can be remoulded and recycled. On the other hand, thermoset polymers form a crosslinked structure during the curing process, preventing them from being melted and remoulded.

As an analogy, think of thermosets like concrete, once they have set, they can never go back to the liquid form (irreversible process). While thermoplastics are like water, it can transition between ice and water with the application or removal of heat (reversible process).

Here, you will learn about:

  • What thermoplastics and thermosets are
  • What crosslinking is and how it differentiates thermoplastics from thermosets
  • Properties of thermoplastics and thermosets
  • Processing of thermoplastics and thermosets
  • Materials and relevant technological applications

What are thermoplastics?

A thermoplastic is a resin that is solid at room temperature but becomes plastic and soft upon heating, flowing due to crystal melting or by virtue of crossing the glass transition temperature (Tg). Upon processing, usually via injection-moulding or blow-moulding-like processes, thermoplastics take the shape of the mould within which they are poured as melt, and cool to solidify into the desired shape. The significant aspect of thermoplastics is their reversibility, the ability to undergo reheating, melt again, and change shape. This allows for additional processing of the same material, even after being prepared as a solid. Processes such as extrusion, thermoforming, and injection moulding rely on such resin behaviour. Some common thermoplastic materials include polyethylene (PE), polycarbonate (PC), and polyvinyl chloride (PVC).

However, like any other material, thermoplastics have their limitations. If subjected to extremely high temperatures, the material may unwantedly soften, deform, and lose some of its physical properties [2].

What are thermosets?

A thermosetting resin, or thermosetting polymer, is generally a liquid material at room temperature which hardens irreversibly upon heating or chemical addition. When it is placed in a mould and heated, the thermoset solidifies into the specified shape, but this solidification process includes the formation of certain bonds, called crosslinks, that hold the molecules in place and change the basic nature of the material, preventing it from melting. As a result, a thermoset, as opposed to a thermoplastic, cannot return to its initial phase, rendering the process irreversible. Thermosets, upon heating, become set, fixed in a specific form. During overheating, thermosets tend to degrade without entering a fluid phase. Processes such as compression moulding, resin transfer moulding, pultrusion, hand lay-up, and filament winding depend on thermosetting polymer behaviour.  Some common thermosets include epoxy, polyimide, and phenolic, many of which are significant in composites [2].

What is crosslinking (curing)?

Thermosets and thermoplastics differ in various ways in terms of their behaviour, but all those diverging properties result from an underlying, fundamental difference in their chemical structure. This underlying difference can be noticed in how thermoset resins, throughout the length of their polymer chain, have particular spots that can be chemically activated to be part in chemical bonding reactions with neighbouring polymer molecules. Since all thermosets carry such chemically reactive spots, it is often the case that all kinds of thermosets have the tendency to connect to one another. Such a process of forming chemical links across different thermosetting molecules is called crosslinking (or curing). Upon curing, formed crosslinks not only confine the polymer molecules from moving, but also the atoms inside those molecules are impeded to a greater degree than intermolecular attractions.

Another way of observing the behavioural difference between thermosets and thermoplastics is via their molecular weight. As we compare both polymer types, thermosets stand out in how their molecular weight drastically increases upon curing. Thermoplastics are known to have higher molecular weight values than uncured thermosets. However, when crosslinking occurs between two thermosets, a polymer network is formed of molecular weight almost double the weight when the two were separate. As the number of linked molecules increase, the molecular weight continues to rise, exceeding that of thermoplastics. This drastic increase in molecular weight causes major changes in material properties, such as an increased melting point. With a continuous increase in molecular weight due to crosslinking, the melting point can rise and reach a point that exceeds the decomposition point. In that case, a thermoset polymer would have a very high molecular weight that it would decompose before it can melt, which defines why thermoset processing is irreversible [2].

Properties of thermoplastics vs thermosets

Thermoplastics generally provide high-strength, flexibility and are resistant to shrinkage, depending on the type of resin (the polymer in melted liquid form). They are versatile materials that can be used for anything from plastic carrier bags to high-stress bearings and precision mechanical parts.

Thermosets generally yield higher chemical and heat resistance, as well as a stronger structure that does not deform easily.

Here is a list showing the difference between thermoplastics and thermosets in terms of features and properties. Notice the effect of crosslinking as an underlying factor in diverging those materials from one another.

Table 1: Thermoplastics vs thermosets [3]

Feature/Property

Thermoplastics

Thermosets

Molecular structure

Linear polymer: weak molecular bonds in a straight-chain formation

Network polymers: high level of crosslinking with strong chemical molecular bonds

Melting point

Melting point lower than the degradation temperature

Melting point higher than the degradation temperature

Mechanical

Flexible and elastic. High resistance to impact (10x more than thermosets). Strength comes from crystallinity

Inelastic and brittle. Strong and rigid. Strength comes from crosslinking.

Polymerisation

Addition polymerisation: repolymerised during manufacture (before processing)

Polycondensation polymerisation: polymerised during processing

Microstructure

Comprised of hard crystalline and elastic amorphous regions in its solid state

Comprised of thermosetting resin and reinforcing fibre in its solid state

Size

Size is expressed by molecular weight

Size is expressed by crosslink density

Recyclability

Recyclable and reusable by the application of heat and/or pressure

Non-recyclable

Chemical resistance

Highly chemical resistant

Heat and chemical resistant

Crack repair

Cracks can be repaired easily

Difficult to repair cracks

Process thermal aspect

Melting thermoplastics is endothermic

Crosslinking thermosets is exothermic

Service temperature

Lower continuous use temperature (CUT) than thermosets

Higher CUT than thermoplastics

Solubility

Can dissolve in organic solvents

Do not dissolve in organic solvents

Processing of thermoplastics vs thermosets

Thermoplastic processing

Thermoplastics can be processed in a variety of methods including extrusion moulding, injection moulding, thermoforming and vacuum forming.

Granular material is fed into the mould, usually in the form of spherical granules of approximately 3 mm diameter. These granules are then heated to melting point, which requires very high temperatures.

As thermoplastics are highly efficient thermal insulators, cooling during the curing process takes longer than other plastics. Therefore, rapid cooling is undertaken to achieve a high output rate, usually by spraying with cold water or plunging into water baths. To cool thermoplastic plastic films, cold air is blown onto the surface. The plastic shrinks upon cooling, varying between a shrinkage rate of 0.6% to 4% depending on the material. The rate of cooling and shrinkage has a distinct effect on the crystallisation of the material and internal structure, which is why the shrinkage rate is always specified for thermoplastics.

Thermosetting polymer processing

Thermosetting resins are processed in their liquid form under heat. The curing process involves adding curing agents, inhibitors, hardeners or plasticisers to the resin and reinforcement or fillers, depending on the required outcome.

The most commonly used thermosetting resins include:

  • Epoxy
  • Polyester
  • Phenolic
  • Silicone
  • Polyurethane
  • Polyamide

Thermosetting polymer composites processing

Thermosetting polymer composites are made using a laminating process, which binds together resins such as epoxy, silicone, melamine, etc. with reinforcement base materials such as glass, linen and graphite.

Prior to curing, the reinforcement substrate is dipped into the resin binder in its liquified form. Once bound, the sheets of material are passed through an oven to partially cure them. Several sheets are then piled to the required thickness, heated and pressed together to form a laminate. Alternatively, the sheets may be wrapped together and heated to create rods.

Thermoplastic and thermosetting materials and their applications

Types of thermoplastics and their applications

Thermoplastic

Properties and applications

Polyamide (nylon)

Tough and relatively hard material used for power tool casings, curtain rails, bearings, gear components and clothes

Polymethyl Methacrylate (PMMA, acrylic)

Stiff, durable and hard plastic that polishes to a sheen, used for signage, aircraft fuselage, windows, bathroom sinks and bathtubs

Polyvinyl Chloride (PVC)

Tough and durable material that is commonly used for pipes, flooring, cabinets, toys and general household and industrial fittings

Polypropylene

Light, yet hard material that scratches fairly easily, with excellent chemical resistance, used for medical and laboratory equipment, string, rope and kitchen utensils

Polystyrene (PS)

Light, stiff, hard, brittle, waterproof material used mainly for rigid packaging

Polytetrafluoroethylene (PTFE, Teflon)

Very strong and flexible material used for non-stick cooking utensils, machine components, gears and gaskets

Low-density Polythene (LDPE)

Tough, relatively soft, chemical resistant material used for packaging, toys, plastic bags and film wrap

High-density Polythene (HDPE)

Stiff, hard, chemical resistant material used for plastic bottles and casing for household goods

Types of thermosetting polymers and their applications

Thermoset

Properties and applications

Epoxy resin

Hard material that is brittle without extra reinforcement. Used for adhesives and bonding of materials

Melamine formaldehyde 

Hard, stiff and strong, with decent chemical and water resistance, used for work surface laminates, tableware and electrical insulation

Polyester resin

Hard, stiff and brittle when unlaminated. Used for encapsulation, bonding and casting

Urea formaldehyde

Hard, stiff, strong and brittle used primarily in electrical devices due to its good electrical insulation properties

Polyurethane

Hard, strong and durable material used in paint, insulating foam, shoes, car parts, adhesives and sealants

Phenol formaldehyde resin (PF)

Strong, heat and electrical-resistant material used in electrical items, sockets and plugs, car parts, cookware and precision-made industrial parts

 

Sources

[1] A.R. Rennie, "Thermoplastics and Thermosets", In G.M. Swallowe (ed.) Mechanical Properties and Testing of Polymers, Kluwer Academic Publishers, 1999. 

[2] A. Brent Strong, "Thermoplastic Composites", Fundamentals of Composites Manufacturing: Materials, Methods, and Applications, Society of Manufacturing Engineers, 2008.

[3] C.C. Ibeh, "Introduction and History of the Plastics Industry", Thermoplastic Materials: Properties, Manufacturing Methods, and Applications, FL: CRC Press, 2011.

Types of thermoplastics:

Types of thermosetting polymers: