PET properties
About PET
Section titled “About PET”Polyethylene terephthalate (PET or PETE) is the most common thermoplastic polyester, used massively in textile fibers, rigid and flexible packaging, and technical components due to its excellent balance of mechanical strength, light weight, and gas barrier properties. Commercial PET is available in forms ranging from amorphous and transparent to semi-crystalline and opaque, depending on the cooling rate and molecular orientation treatments. Its combination of good mechanical, electrical, and barrier properties, together with its recyclability, make it one of the most produced polymers of the 21st century.
Chemical composition
Section titled “Chemical composition”PET is obtained by polycondensation of ethylene glycol with terephthalic acid (or its dimethyl ester). The repeating unit corresponds to the formula (C10H8O4)n. The typical molar mass of injection or blow molding grades ranges from 10 to 50 kg/mol, depending on the degree of polymerization.
| Component | Description |
|---|---|
| Base monomers | Ethylene glycol + Terephthalic acid (or DMT) |
| Repeat unit formula | (C10H8O4)n |
| Typical molar mass | 10 – 50 kg/mol |
| Structure | Semi-crystalline thermoplastic, linear with ester groups in the main chain |
Mechanical properties
Section titled “Mechanical properties”The mechanical properties of PET depend strongly on the degree of molecular orientation (biaxial stretching, fiber spinning) and the crystalline state. Representative values for the most common forms are given below, with SI and imperial units.
| State / Form | Tensile strength | Tensile modulus | Elongation at break | Rockwell hardness | Izod impact |
|---|---|---|---|---|---|
| Unoriented (amorphous) | 80 MPa (11.6 ksi) | 2 – 4 GPa (290 – 580 ksi) | 2 – 10 % (typical) | M94 – M101 | 13 – 35 J/m (0.96 – 2.6 ft·lb/in) |
| Biaxially oriented film (Mylar®) | 190 – 260 MPa (27.6 – 37.7 ksi) | ~4 GPa (580 ksi) | 60 – 165 % | – | Initial tear resistance: 18 – 54 g/µm |
| Textile fiber (medium tenacity) | Tenacity 0.5 GPa (72.5 ksi) | Specific modulus 7 – 8 cN/tex | 36 % | – | – |
| High tenacity fiber | Tenacity 0.9 – 1.1 GPa (131 – 160 ksi) | Modulus 9 – 11 GPa (1305 – 1595 ksi) | 13 – 16 % | – | – |
Note: Properties in biaxial film are measured on the film itself; fiber properties are expressed in tenacity (GPa) and specific elongation.
Physical properties
Section titled “Physical properties”The density of PET varies with crystallinity: amorphous material ≈1.37 g/cm³, single crystal ≈1.455 g/cm³. The most common value for molded grades lies between 1.3 and 1.4 g/cm³. Water absorption is very low, and the material exhibits good UV resistance under normal conditions.
| Property | Value | SI units | Imperial units |
|---|---|---|---|
| Density | 1.37 – 1.38 (amorphous) / 1.455 (crystalline) | g/cm³ | 85.5 – 86.1 / 90.8 lb/ft³ |
| Water absorption (equilibrium) | <0.7 % | % | – |
| Water absorption (24 h) | 0.1 % | % | – |
| Refractive index | 1.57 – 1.58 (amorphous) | – | – |
| Flammability | Self-extinguishing | – | – |
| Limiting oxygen index | 21 % | % | – |
| UV resistance | Good | – | – |
Thermal properties
Section titled “Thermal properties”PET melts around 260 °C; thermal decomposition begins above 350 °C. It has a moderate coefficient of expansion and low thermal conductivity, typical of plastics. Specific heat and maximum service temperature values depend on the degree of crystallization.
| Property | Value | SI units | Imperial units |
|---|---|---|---|
| Melting point | 260 °C | 500 °F | – |
| Maximum continuous service temperature | 115 – 170 °C | 239 – 338 °F | – |
| Minimum working temperature | -40 to -60 °C | -40 to -76 °F | – |
| Coefficient of linear thermal expansion | 20 – 80 ×10⁻⁶ K⁻¹ | – | 11 – 44 ×10⁻⁶ in/in·°F |
| Specific heat | 1200 – 1350 J/(kg·K) | 0.287 – 0.322 BTU/(lb·°F) | – |
| Specific heat (film) | 1300 J/(kg·K) | 0.311 BTU/(lb·°F) | – |
| Thermal conductivity @ 23 °C | 0.15 – 0.4 W/(m·K) | 0.087 – 0.231 BTU/(h·ft·°F) | – |
| Thermal conductivity (film) | 0.13 – 0.15 W/(m·K) | 0.075 – 0.087 BTU/(h·ft·°F) | – |
| Heat deflection temperature @ 0.45 MPa | 115 °C | 239 °F | – |
| Heat deflection temperature @ 1.8 MPa | 80 °C | 176 °F | – |
Electrical properties
Section titled “Electrical properties”PET excels in electrical applications due to its high dielectric strength, low dielectric constant, and very high volume resistivity, making it suitable for capacitors and cable insulation.
| Property | Value | Unit |
|---|---|---|
| Dielectric constant @ 1 MHz | 3.0 | – |
| Dissipation factor @ 1 kHz (bulk) | 0.002 | – |
| Dissipation factor @ 1 MHz (film) | 0.016 | – |
| Dielectric strength (bulk) | 17 kV/mm | – |
| Dielectric strength (25 µm film) | 300 kV/mm | – |
| Surface resistivity | 1 × 10¹³ Ω/sq | – |
| Volume resistivity | >1 × 10¹⁴ Ω·cm | – |
Chemical resistance
Section titled “Chemical resistance”PET has excellent resistance to dilute and concentrated acids, alcohols, oils, greases, and ketones. However, it is attacked by alkalis (which hydrolyze the ester bonds) and shows fair behavior towards aromatic hydrocarbons.
| Chemical agent | Behavior |
|---|---|
| Concentrated acids | Good |
| Dilute acids | Good |
| Alkalis | Poor |
| Alcohols | Good |
| Aromatic hydrocarbons | Fair |
| Fats and oils | Good |
| Halogens | Good |
| Ketones | Good |
Applications by industry
Section titled “Applications by industry”The versatility of PET allows it to cover a wide spectrum of sectors; the main formats are fiber, biaxial film, and resin for injection-blow molding.
| Industry | Typical applications |
|---|---|
| Textile | Fibers for clothing, carpets, ropes, and nonwovens (brands Dacron®, Terylene®) |
| Packaging | Bottles for carbonated drinks, food containers, blisters, and thermoformed trays |
| Electronics / Electrical | Capacitors, recording tapes, cable insulation, graphic supports (Mylar® film) |
| Energy | Photovoltaic modules, high-pressure CNG cylinders, submarine cables |
| Automotive | Interior components, connectors, glass fiber reinforcements |
Comparison with similar materials
Section titled “Comparison with similar materials”PET is often compared with other engineering thermoplastics and with polybutylene terephthalate (PBT), which has a similar chemical structure but differences in crystallization and service temperature. It also competes with polycarbonate (PC) in transparent applications and with PETG in unoriented packaging.
| Material | Advantages over PET | Disadvantages / differences with PET |
|---|---|---|
| PBT | Crystallizes faster, better moldability | Higher water absorption, lower melting temperature (≈225 °C) |
| Polycarbonate (PC) | Higher impact resistance and optical transparency | Poorer gas barrier, attacked by many solvents, more expensive |
| PETG (glycol-modified) | Better thermoformability and less brittleness in thick sections | Lower deflection temperature and mechanical strength |
| Nylon 6 | Higher wear and fatigue resistance | Absorbs more moisture, poorer dimensional stability |
Frequently Asked Questions (FAQ)
Section titled “Frequently Asked Questions (FAQ)”What is the density of PET in g/cm³ and lb/ft³?
Section titled “What is the density of PET in g/cm³ and lb/ft³?”The typical density of amorphous PET is 1.37 g/cm³ (85.5 lb/ft³), while crystalline PET can reach 1.455 g/cm³ (90.8 lb/ft³).
At what temperature does PET melt?
Section titled “At what temperature does PET melt?”PET melts at 260 °C (500 °F); thermal decomposition begins above 350 °C without passing through a true boiling point.
What is the tensile strength of PET in MPa and ksi?
Section titled “What is the tensile strength of PET in MPa and ksi?”In the unoriented state, tensile strength is around 80 MPa (11.6 ksi); biaxially oriented film reaches between 190 and 260 MPa (27.6 – 37.7 ksi).
How much water does PET absorb?
Section titled “How much water does PET absorb?”PET absorbs less than 0.7 % at equilibrium and only 0.1 % after 24 h immersion, one of the lowest values among common thermoplastics.
What is the dielectric strength of PET in kV/mm?
Section titled “What is the dielectric strength of PET in kV/mm?”The dielectric strength of solid resin is 17 kV/mm; in 25 µm thick film it can reach 300 kV/mm, ideal for capacitors.
What is the maximum continuous service temperature of PET?
Section titled “What is the maximum continuous service temperature of PET?”PET can operate continuously between 115 °C and 170 °C (239 – 338 °F), depending on the degree of crystallinity and mechanical loads.
References
Section titled “References”- azom.com: https://www.azom.com/article.aspx?ArticleID=2047
- engineeringtoolbox.com: https://www.engineeringtoolbox.com/specific-heat-polymers-d_1862.html
- steelnumber.com: https://www.steelnumber.com/en/steel_composition_eu.php?name_id=614