Polyethylene PE properties
Polyethylene (PE) is a translucent, whitish semicrystalline thermoplastic with a density ranging between 0.91 and 0.97 g/cm³ depending on the grade. It is one of the highest production volume synthetic polymers, known for its excellent toughness, low-temperature impact resistance and high chemical resistance. The main variants are low-density polyethylene (LDPE, branched), linear low-density polyethylene (LLDPE), high-density polyethylene (HDPE, linear) and ultra-high molecular weight polyethylene (UHMWPE, with molecular weights between 3 and 6 million). It is processed by extrusion, blow molding and injection molding.
Chemical composition
Section titled “Chemical composition”Polyethylene is obtained by polymerization of ethylene (C₂H₄), giving rise to chains of -(CH₂-CH₂)-. The molecular structure ranges from highly branched (LDPE, approximately 2% of carbon atoms with branches) to virtually linear without branches (HDPE). This difference in branching controls crystallinity and, therefore, final properties. Commercial grades may incorporate comonomers (e.g., butene, hexene) to modify density and mechanical characteristics.
Mechanical properties
Section titled “Mechanical properties”The tensile strength of HDPE is 32 MPa and its elongation at break reaches 150%. LDPE presents significantly lower stiffness and strength values, but much higher resilience. The following table summarizes the typical mechanical properties of both grades at room temperature, without post-molding heat treatment.
| Property | HDPE | LDPE | SI Unit | Imperial Unit |
|---|---|---|---|---|
| Tensile strength | 32 | 10-15 | MPa | 4641 (HDPE) / 1450-2175 (LDPE) psi |
| Flexural modulus | 1.25 | 0.2-0.35 | GPa | 181 (HDPE) / 29-51 (LDPE) ksi |
| Elongation at break | 150 | 100-700 | % | % |
| Izod impact strength (notched) | 0.15 (HDPE) | 0.5-1.5 (LDPE, often no break) | kJ/m | 2.8 (HDPE) / 9.4-28.1 (LDPE) ft·lb/in |
| Surface hardness | SD68 (Shore D) | SD40-50 (Shore D) | – | – |
| Yield strain | 15 | – | % | % |
Physical properties
Section titled “Physical properties”The density of HDPE is 0.96 g/cm³ (59.9 lb/ft³) and that of LDPE is 0.924 g/cm³ (57.7 lb/ft³). The coefficient of linear thermal expansion is 12×10⁻⁵ /°C (6.7×10⁻⁵ /°F). Water absorption in 24 h is practically negligible (0.02% for HDPE). The limiting oxygen index is 17%, indicating ease of ignition, and the UL94 flammability rating is HB. The mold shrinkage of HDPE reaches 3%, notably higher than in other engineering thermoplastics.
Thermal properties
Section titled “Thermal properties”The maximum continuous service temperature of HDPE is 55 °C (131 °F), while LDPE can be used continuously at 65 °C (149 °F) and withstands short peaks of 90 °C (194 °F). The melting temperature of HDPE lies in the range of 120-180 °C (248-356 °F); that of LDPE is lower, typically between 105 and 115 °C. The heat deflection temperature (HDT) of HDPE at 0.45 MPa is 75 °C (167 °F) and at 1.80 MPa it drops to 46 °C (115 °F).
Electrical properties
Section titled “Electrical properties”Polyethylene, particularly HDPE, is an excellent electrical insulator. Its volume resistivity is 10¹⁷ Ω·cm (17 log Ω·cm), dielectric strength reaches 22 MV/m (559 V/mil), the dielectric constant at 1 kHz is 2.3 and the dissipation factor at the same frequency is 0.0005. These properties, combined with low moisture absorption, make it suitable for insulation applications in low-voltage cables and connectors.
Chemical resistance
Section titled “Chemical resistance”PE exhibits outstanding chemical resistance at room temperature. Based on the behavior of LDPE (also applicable to HDPE, which is usually somewhat more resistant due to its higher crystallinity), the following levels are distinguished:
- Excellent (no attack): diluted and concentrated acids, alcohols, bases and esters.
- Good (minor attack): aldehydes, ketones and vegetable oils.
- Limited (short-term use only): aliphatic and aromatic hydrocarbons, mineral oils and oxidizing agents.
- Not recommended: halogenated hydrocarbons.
Additionally, HDPE shows susceptibility to stress cracking in contact with certain surfactants and chemical agents.
Applications by industry
Section titled “Applications by industry”Polyethylene is massively used in the following sectors:
- Packaging: bags and plastic film (LDPE), milk bottles, detergent containers, toys, tubs, drums and tanks (HDPE), lamination in beverage cartons (LDPE).
- Construction and infrastructure: pipes and fittings for potable water and gas (HDPE and MDPE), waterproofing sheets, vapor barriers.
- Automotive industry: fuel tanks, under-hood components, air ducts.
- Medical and laboratory: reagent bottles, flexible tubing, molded parts for laboratory equipment (LDPE).
- Consumer goods: flexible caps, beverage pack rings, cosmetic containers, kitchen utensils.
- Wear applications: bottle handling parts, bearings, gears and artificial joints (UHMWPE).
Comparison with similar materials
Section titled “Comparison with similar materials”Compared to polypropylene (PP), HDPE offers better low-temperature impact resistance and lower cost, but PP surpasses it in stiffness (flexural modulus ≈ 1.5 GPa vs. 1.25 GPa) and maximum service temperature (≈ 100 °C vs. 55 °C). Regarding LDPE, HDPE is much stiffer and stronger, but less flexible and with worse processability for thin film blow molding. Compared to rigid PVC, HDPE is lighter, more flexible and has better chemical resistance, although lower stiffness and flame resistance.
Processing
Section titled “Processing”Polyethylene is processed by extrusion, injection molding and blow molding, among other methods. For HDPE, the melt temperature is maintained between 200 °C and 280 °C, with a mold at 30-70 °C and shrinkage of up to 3%. LDPE is processed at lower melt temperatures (160-240 °C) and requires no pre-drying, since its moisture absorption is negligible. Due to its low continuous service temperature, cooling lines must be sized to ensure stable production cycles.
Frequently Asked Questions (FAQ)
Section titled “Frequently Asked Questions (FAQ)”What is the typical density of HDPE and LDPE?
Section titled “What is the typical density of HDPE and LDPE?”The density of HDPE is 0.96 g/cm³ (59.9 lb/ft³), while that of LDPE is 0.924 g/cm³ (57.7 lb/ft³).
What maximum continuous service temperature can polyethylene withstand?
Section titled “What maximum continuous service temperature can polyethylene withstand?”HDPE can withstand 55 °C (131 °F) continuously; LDPE can be used up to 65 °C (149 °F) continuously and 90 °C (194 °F) for short periods.
What is the tensile strength of HDPE?
Section titled “What is the tensile strength of HDPE?”The tensile strength of HDPE is 32 MPa, equivalent to 4641 psi.
What type of polyethylene is used for gas pipes?
Section titled “What type of polyethylene is used for gas pipes?”Medium-density polyethylene (MDPE) is used, a variant with density and properties intermediate between HDPE and LDPE, which optimizes slow crack growth resistance.
Is polyethylene resistant to acids?
Section titled “Is polyethylene resistant to acids?”Yes, both HDPE and LDPE exhibit excellent resistance to diluted and concentrated acids, as well as bases and alcohols.
What is the flexural modulus of HDPE?
Section titled “What is the flexural modulus of HDPE?”The flexural modulus of HDPE is 1.25 GPa (181 ksi), approximately 60% higher than that of LDPE.
References
Section titled “References”- azom.com: https://www.azom.com/article.aspx?ArticleID=421
- engineeringtoolbox.com: https://www.engineeringtoolbox.com/plastics-abbreviations-d_1778.html
- steelnumber.com: https://www.steelnumber.com/en/steel_composition_eu.php?name_id=5949