Thermoplastics vs Thermosets: Differences and Uses

Plastics are everywhere in engineering, from pipes and cable insulation to car parts, circuit boards and aircraft structures. Choosing the right polymer family affects processing method, cost, performance at temperature, and what happens at end of life.

Molecular structure

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Thermoplastics may be amorphous (chains randomly tangled, such as polycarbonate, PMMA and polystyrene, often transparent) or semi-crystalline (regions of ordered chains, such as polyethylene, polypropylene, nylon and PEEK, with better chemical and fatigue resistance). Their key temperatures are the glass transition temperature (Tg), where amorphous regions change from glassy to rubbery, and for semi-crystalline polymers, the melting temperature (Tm).

Thermosets are formed by a curing reaction, triggered by heat, catalysts, hardeners or UV light, that links chains together. Higher cross-link density gives higher stiffness, Tg and heat resistance but lower toughness.

Processing

Thermoplastics are supplied as pellets or sheets and processed by melting, shaping and cooling:

• Injection moulding for complex parts in high volume.
• Extrusion for pipes, profiles, films and cable insulation.
• Blow moulding for bottles and tanks; thermoforming for trays and panels.
• 3D printing, especially FDM, as in 3D printing technologies.
• Welding by hot plate, ultrasonic or laser methods; scrap can be reground.

Thermosets are shaped while still liquid or uncured, then cured in the mould:

• Compression and transfer moulding of phenolic and melamine compounds.
• Reaction injection moulding (RIM) of polyurethanes.
• Composite processes: hand lay-up, resin transfer moulding, filament winding, pultrusion and autoclave curing of prepregs for glass and carbon fibre parts.
• Casting and potting of epoxy and polyurethane for electronics and electrical insulation.

Cycle times for thermosets are usually longer because of curing, and scrap cannot be remelted.

Thermoplastics vs thermosets: comparison

Common examples and uses

Thermoplastics:

• Polyethylene (PE): pipes, tanks, films, cable insulation.
• Polypropylene (PP): car bumpers, containers, living hinges, chemical tanks.
• PVC: pipes, window frames, cable sheaths.
• Polyamide (nylon): gears, bearings, cable ties, fibres.
• Polycarbonate: safety glazing, lenses, housings.
• PEEK: aerospace, medical and high temperature parts.

Thermosets:

• Epoxy: adhesives, coatings, PCB laminates, carbon fibre composites.
• Phenolic: electrical switchgear parts, brake linings, heat resistant handles.
• Unsaturated polyester and vinyl ester: glass fibre boats, tanks, pipes, panels.
• Polyurethane: rigid insulation foams, flexible foams, coatings.
• Silicone rubber and vulcanised rubbers: seals, gaskets, hoses.

Temperature limits

A useful guide is the heat deflection temperature (HDT) or continuous use temperature. Many commodity thermoplastics are limited to roughly 60 to 100 °C, engineering thermoplastics to about 100 to 150 °C, and high performance ones such as PEEK to around 250 °C. Many thermosets maintain properties at 150 to 200 °C or more, and phenolics resist fire well.

Stiffness can be compared using the specific modulus E/ρ, as for other engineering materials:

$$\text{Specific modulus} = \frac{E}{\rho}$$

Design tips for plastic parts

Design rules differ from metals. Keep wall thickness uniform to avoid sink marks and warping in moulded thermoplastics, add ribs rather than thick sections for stiffness, use generous radii to reduce stress concentration, and allow draft angles for mould release. Remember that polymer properties depend strongly on temperature, time and humidity: long term loads cause creep, so design with creep modulus data rather than short term values.

Sustainability considerations

Thermoplastics are easier to recycle mechanically, though contamination, mixed grades and additives limit recycling in practice. Thermosets and their composites are hard to recycle, but they often have long service lives, such as wind turbine blades and pipes. Bio-based polymers, recycled content, design for disassembly and chemical recycling are growing areas, linked to circular economy principles.

Choosing between thermoplastics vs thermosets

Choose thermoplastics for high volume, fast moulding, toughness, weldability and recyclability. Choose thermosets for high temperature, dimensional stability under load, electrical and fire performance, and as matrices for high performance fibre composites. In many products both appear together, for example a thermoplastic housing around an epoxy potted circuit board.