Polyester is a specific type of synthetic polymer. To be classified as a polyester, a polymer must contain the ester functional group (-COO-) in its main molecular chain.
The commonly used engineering thermoplastics PBT (Polybutylene Terephthalate) and PET (Polyethylene Terephthalate) are both members of the polyester family because they share an identical chemical backbone containing ester functional groups. They are synthesized by reacting a terephthalate acid derivative with a glycol, making them thermoplastic polyesters with highly comparable molecular structures. Both PBT and PET are created through a condensation polymerization reaction.
PET is made by reacting terephthalic acid with ethylene glycol (a two-carbon chain).
PBT is made by reacting terephthalic acid with 1,4-butanediol (a four-carbon chain).
Because they both feature the aromatic terephthalate ring linked by ester bonds, they share the same chemical “DNA”, but differ only in the length of the hydrocarbon spacer between those bonds. This gives them distinctly different physical properties and applications in industry (Table 1).
Additionally, PET is processed differently, depending on its intended use: when cooled into solid pellets and moulded, it creates rigid items like soda and water bottles, food containers and packaging. Melted PET can be extruded through tiny holes (spinnerets) into long threads, spun into yarn and woven into fabrics for clothing, upholstery and carpets. Fabrics woven or knitted from polyester thread or yarn are used extensively in apparel and home furnishings…
Because of their excellent processing characteristics, strength, dimensional stability and resistance to heat and chemicals, PBT, PBT blends and PET also serve as engineering plastics. They are heavily used in electrical components, automotive parts and plastic reinforcements.
Key similarities Between PBT and PET
- Chemical Resistance: Both resist hydrocarbons, alcohols, ether and diluted acids.
- Mechanical Strength: Both possess high tensile strength, stiffness and dimensional stability under load.
- Electrical Insulation: Both are excellent dielectric materials, making them highly effective insulators.
- Processing Type: Both are thermoplastics, meaning they can be melted, cooled and remelted for injection moulding or extrusion.
Key Differences Between PBT and PET
Because PBT has a longer, more flexible 4-carbon chain compared to PET’s stiffer 2-carbon chain, their physical behaviours diverge significantly (see Table 2)
Materials selection tips
- Choose PET for optical clarity (clear packaging), high heat resistance, high tensile strength for fibres, or cost-effective mass production like synthetic clothing and plastic bottles.
- Choose PBT for high-precision engineering parts, rapid manufacturing cycles, superior impact resistance and components that will not warp or lose insulation properties in humid environments.
Other thermoplastic polyesters
PCT (Polycyclohexylenedimethylene Terephthalate), PEN (Polyethylene Naphthalate) and PAR (Polyarylate) are the primary other polyester variants classified as engineering thermoplastics. Liquid Crystal Polymers (LCPs) also belong to this family, representing the highest tier of high-performance polyester engineering plastics.
These materials are selected over standard PET or PBT when applications demand extreme heat resistance, superior chemical barriers, or exceptional optical clarity under stress.
PCT (Polycyclohexylenedimethylene Terephthalate)
PCT is structurally similar to PET but substitutes the ethylene glycol component with a bulky cyclohexane ring. This modification significantly stiffens the polymer chain.
Key advantages: a much higher melting point (c. 290°C), excellent hydrolytic stability and high resistance to chemical degradation.
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Engineering uses: automotive surface-mount components, high-temperature electrical connectors and LED reflectors where stability under intense heat and UV light is mandatory.
PEN (Polyethylene Naphthalate)
PEN swaps out the terephthalate ring found in PET for a dual-ring naphthalene structure. This dual-ring system provides a much more rigid molecular backbone.
Key Advantages: Glass transition temperature (approx 120C) is much higher than PET (approx 75C). It offers superior oxygen and moisture barrier properties, extreme dimensional stability and high UV resistance.
Engineering Uses: High-performance film insulation for electric vehicle batteries, aerospace components, flexible printed circuits and high-pressure industrial hoses.
PAR (Polyarylate)
PAR is an amorphous, fully aromatic polyester formed from the reaction of bisphenols with dicarboxylic acids. Because it lacks a crystalline structure, it behaves differently to PBT or PET.
Key Advantages: true transparency, exceptional elastic recovery (springiness), high flame retardancy and a very high heat deflection temperature (c. 175°C).
Engineering Uses: Optical sensors, automotive lighting bezels, safety goggles and structural outdoor equipment that requires both transparency and weather resistance.
LCP (Liquid Crystal Polymers)
LCPs are a specialised class of aromatic polyesters that exhibit highly ordered crystalline structures even while in the liquid/molten phase.
Key Advantages: extremely high melting points (up to 350°C), virtually zero mould shrinkage, exceptional tensile strength (similar to Kevlar polyaramid fibre) and inherently flame-retardant properties.
Engineering Uses: Micro-electronics, ultra-thin smartphone connectors, telecommunications hardware and aerospace housings where precision moulding of microscopic walls is required.
Thermosetting polyesters
Unsaturated Polyester Resins (UPRs) are the primary class of thermosetting polymers within the polyester family and the most widely used thermosetting polyesters in the world. Unlike thermoplastic polyesters (such as PET or PBT) which can be remelted, thermosetting polyesters undergo a permanent chemical cross-linking reaction when cured, forming an infusible, rigid polymer network that decomposes rather than melts when heated.
They consist of a linear polyester backbone containing carbon-carbon double bonds (C=C), which is dissolved in a reactive monomer solvent – most commonly styrene.
Engineering Advantages: UPRs offer excellent chemical resistance, high structural rigidity, low raw material costs and the unique ability to cure at room temperature without requiring high-pressure moulding machinery. Key differences between thermoplastic and thermosetting polyesters are shown in Table 3.
Fibre-reinforced plastics (FRP)
Because they are liquid before curing, thermosetting polyesters are exceptionally well-suited for wetting fibres. They serve as the primary matrix material for fibreglass and are heavily utilized across heavy industries, including:
- Marine: ship hulls, decks, yachts and personal watercraft.
- Construction: corrugated roofing sheets, industrial storage tanks, pipes and modular bathroom units.
- Automotive: body panels for heavy trucks, recreational vehicles (RVs) and wind turbine blades.
Moulding Compounds (SMC and BMC)
Thermosetting polyesters are often pre-mixed with chopped glass fibres, mineral fillers and catalysts to form intermediate manufacturing materials:
Sheet Moulding Compound (SMC): ready-to-mould sheets compressed in high-heat hydraulic presses to create structural components like automotive bumpers and electrical enclosures.
Bulk Moulding Compound (BMC): a dough-like mixture used for injection or compression moulding of intricate parts like appliance housings and circuit breakers.
Vinyl Ester Resins
While technically a distinct sub-family, vinyl ester resins are chemically related hybrids. They are produced by reacting an epoxy resin backbone with acrylic or methacrylic acid, then dissolving the product in styrene. They cure via the exact same free-radical cross-linking mechanism as standard UPRs.
Vinyl esters are chosen over standard thermoset polyesters when an application demands superior fracture toughness, higher impact resistance and maximum resistance to harsh acids and alkalis (such as in chemical processing plant piping).
Table 1 Key differences between PBT and PET
Because PBT has a longer, more flexible 4-carbon chain compared to PET’s stiffer 2 two-carbon chain, their physical behaviours diverge significantly.
| Property / Feature | PET (Polyethylene Terephthalate) | PBT (Polybutylene Terephthalate) |
| Glycol Component | Ethylene glycol (Two Carbon atoms) | 1,4-Butanediol (Four Carbon atoms) |
| Melting Point | Higher (c. 250°C to 260°C) | Lower (c. 223°C) |
| Crystallisation Rate | Slow (requires hot moulds for crystals) | Extremely fast (cools and hardens rapidly) |
| Impact Strength | Lower (more brittle when unmodified) | Higher (more flexible and impact-resistant) |
Table 2 Engineering property comparison of the various thermoplastic polyesters
| Material | Melting Point |
Glass Transition Temperature | Defining Engineering Superpower | ||
| PBT | c. 223°C | c. 60°C | Fast moulding cycles and dimensional stability |
||
| PET | c. 255°C | c. 75°C | High tensile strength and cost efficiency |
||
| PEN | c. 265°C | c. 120°C | Impressive gas barrier, structural stiffness | ||
| PCT | c. 290°C | c. 90°C | Extreme soldering heat resistance | ||
| PAR | None (Amorphous) | c.190°C | Optical clarity, combined with high heat threshold |
||
| LCP | Between 280°C and 350°C |
Between 100°C and 130°C |
Ultra-thin wall moulding and zero shrinkage |
||
Table 3 Key Differences between Thermoplastic and Thermosetting Polyesters
| Property | Thermoplastic Polyesters (PET, PBT, PEN) | Thermosetting Polyesters (UPR, Vinyl Ester) |
| Molecular Structure | Linear or branched independent polymer chains. | Highly cross-linked 3D covalent networks. |
| Response to Heat | Melts when heated; solidifies when cooled. | Does not melt; chars and decomposes at high temperatures. |
| Starting State | Solid pellets processed via injection molding. | Liquid resins or dough-like compounds cured via catalysts. |
| Recyclability | Highly recyclable via remelting. | Non-recyclable by thermal means (must be reground as filler). |
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