Polyethylene terephthalate (PET)

Definition

Polyethylene terephthalate (PET) is a semi-crystalline thermoplastic polyester produced by the polycondensation of ethylene glycol with terephthalic acid (PTA) or dimethyl terephthalate (DMT). Depending on processing and crystallinity, PET can be amorphous (clear) or semi-crystalline (opaque). Typical properties for unfilled grades include tensile strength around 55–75 MPa, modulus about 2.8–3.1 GPa, glass transition temperature (Tg) ~70–80 °C, and melting temperature (Tm) ~245–260 °C. Density is typically 1.33–1.40 g/cm³. PET combines good mechanical performance, dimensional stability, electrical insulation, and useful gas-barrier performance, and it is widely recyclable (resin identification code #1).

Key properties (grade- and test-dependent)

  • Mechanical and dimensional: High strength-to-weight ratio; good stiffness and creep resistance, especially in glass/mineral–reinforced grades; low moisture uptake compared with polyamides supports dimensional stability.
  • Thermal: Usable temperature range roughly –50 °C to 120–130 °C for unfilled grades; reinforced and heat-stabilized grades offer higher heat deflection temperatures. Amorphous PET softens near its Tg; crystallinity improves heat resistance.
  • Barrier and optical: Very good CO2 barrier and moderate O2 barrier; good aroma and oil barrier. Amorphous PET is optically clear; orientation (e.g., in films and bottles) further enhances clarity and strength.
  • Chemical resistance: Good resistance to dilute acids, hydrocarbons, oils, and alcohols; limited resistance to strong bases, hot water/steam (hydrolysis above Tg), and some polar solvents. Stabilized grades improve hydrolysis and heat aging.
  • Electrical: Good dielectric strength and insulating behavior; stable over a range of humidity conditions.
  • Flammability: Unfilled PET is typically UL 94 HB; halogen-free flame-retardant grades are available for electrical/electronic applications.

Processing and forms

  • Drying: Essential before melt processing to prevent hydrolytic degradation (commonly to ≤50 ppm moisture; typical drying 120–170 °C for several hours).
  • Injection molding: Primary method for engineering parts (connectors, housings, coil bobbins, relay bases); glass/mineral reinforcement increases stiffness, strength, and heat resistance.
  • Extrusion and thermoforming: Amorphous PET (A-PET) sheet for clear trays, clamshells, and protective packaging.
  • Film orientation: Biaxially oriented PET (BOPET) film provides high tensile strength, dimensional stability, and excellent barrier; used in flexible packaging, electrical insulation, graphics, labels, tapes, and solar backsheets.
  • Blow molding: Injection stretch blow molding (ISBM) for bottles and containers, leveraging chain orientation for clarity and barrier performance (e.g., carbonated beverage bottles).
  • Fiber spinning: PET fibers (“polyester”) dominate global PET consumption; used in apparel, home textiles, industrial yarns (e.g., tire cord, seat belts), nonwovens, and strapping.
  • Additive manufacturing: Neat PET is less common; glycol-modified PET (PETG) is widely used due to improved clarity, toughness, and thermoformability.

Benefits and typical use cases

  • Packaging: Beverage bottles (water, soft drinks, juices), food jars, thermoformed trays and clamshells, and multilayer flexible packaging using BOPET films (including metallized structures for enhanced barrier).
  • Fibers and textiles: Apparel, upholstery, carpets, industrial yarns, cords, and nonwovens.
  • Electrical/electronic: Insulating film and laminates, coil formers, relay bases, connectors, sensor housings, and cable management parts; flame-retardant grades support compliance with electrical standards.
  • Automotive and industrial: Precision molded components (electrical connectors, housings, brackets), fluid reservoirs and caps for select non-fuel fluids, and interior fabrics and nonwovens. PET’s insulating and dimensional-stability profile also suits many e-mobility components (e.g., high-voltage connectors, junction boxes, cable guides).
  • Other: Strapping, labels, photographic and magnetic media, and protective films.

Variants, copolymers, and related materials

  • PET (general); PETE (common packaging term); poly(ethylene terephthalate).
  • A-PET (amorphous PET) sheet; BOPET (biaxially oriented PET) film.
  • PETG (glycol-modified PET) for improved clarity, impact resistance, and easy thermoforming/3D printing.
  • Reinforced/modified grades: Glass- or mineral-filled, nucleated, impact-modified, hydrolysis-resistant, and flame-retardant PET.
  • rPET (recycled PET): Produced via mechanical recycling (often with solid-state polymerization to rebuild intrinsic viscosity) or via chemical recycling; used in bottles, fibers, films, and sheet.
  • Related polymers: PBT (polybutylene terephthalate; faster crystallization and better high-temperature dimensional stability), PEN (polyethylene naphthalate; higher thermal and barrier performance), blends (e.g., PET/PC) to tailor impact, heat, and processing behavior.
  • Typical intrinsic viscosity (IV) ranges: film ~0.60–0.65 dL/g; fiber ~0.62–0.70 dL/g; bottle grade ~0.76–0.86 dL/g; heavy-gauge or refillable bottle/engineering grades can be higher.

Limitations and design notes

  • Hydrolysis sensitivity: Prolonged exposure to hot, moist environments or inadequate pre-drying during processing reduces molecular weight and properties.
  • Alkaline sensitivity: Strong bases and some amines can cause saponification and stress cracking.
  • Thermal/UV aging: Unstabilized PET can embrittle; use heat/UV stabilizers where needed.
  • Crystallization kinetics: Relatively slow; nucleated grades and optimized mold temperatures reduce cycle times and warpage.
  • Taste/odor management: Acetaldehyde generation during processing must be controlled for beverage applications.
  • Impact/notch sensitivity: Consider radiused designs, impact modifiers, or blends where needed.

Sustainability and regulatory

  • Widely recyclable (code #1) with established mechanical and emerging chemical recycling pathways; rPET is common in bottles, fibers, films, and sheets.
  • Closed-loop systems (e.g., deposit-return) and solid-state polymerization enable high-IV rPET suitable for food-contact bottles.
  • PET is not biodegradable; litter control and appropriate recycling infrastructure are critical to minimize environmental impact.
  • Food-contact compliance is well established when materials and processes meet relevant regional regulatory requirements.

Note: All property values and service limits are typical ranges and depend on grade, reinforcement/fillers, conditioning, and test method.

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