HDPE Plastic Properties — Mechanical, Thermal & Chemical
HDPE plastic properties reflect its semi-crystalline molecular structure: a density of 0.95 g/cc, tensile strength of 4,600 psi, continuous use temperature of 180°F, and water absorption below 0.1% combine to make HDPE one of the most cost-effective engineering materials for wet, chemical, or food-contact environments. This page covers the full property dataset—mechanical, thermal, electrical, and chemical resistance—along with notes on how grade selection (standard vs. Marine HDPE vs. borated) affects individual properties.
At a glance:
- Tensile strength: 4,600 psi (ASTM D638)
- Flexural modulus: 200,000 psi (ASTM D790)
- Continuous use temperature: 180°F (82°C)
- Melt point: 266°F (130°C)
- Water absorption: <0.1% over 24 hours (ASTM D570)
- Density: 0.95 g/cc — floats in water
- Chemical resistance: excellent to most acids, alkalis, and hydrocarbons
- UV resistance: poor on standard grades; good on Marine HDPE (Starboard)
Mechanical Properties
HDPE's mechanical profile falls between flexible LDPE and stiffer engineering thermoplastics. It is strong enough for structural applications such as dock bumpers, bin liners, and tank panels, yet soft enough to machine without tool breakage or workpiece cracking.
Tensile and Flexural Data
The wide elongation range reflects the difference between general-purpose and marine grades, as well as variations in extrusion conditions. For structural panel calculations, use the lower end of the elongation range and the flexural modulus figure.
Hardness and Abrasion
At Shore D 60–70, HDPE is noticeably softer than acetal (Shore D 80–85) and UHMW polyethylene (Shore D 60–65 but with significantly better abrasion resistance due to ultra-high molecular weight). In practice, HDPE's abrasion resistance is adequate for cutting-board and dock-bumper applications but inadequate for wear strips, chute liners, or conveyor guides where UHMW is the correct specification. See the HDPE vs. UHMW comparison for a direct side-by-side.
Thermal Properties
Operating Temperature Range
HDPE's CLTE of 7.2×10⁻⁵ in/in/°F is substantially higher than aluminum (1.3×10⁻⁵) or steel (0.65×10⁻⁵). In applications with wide temperature swings—outdoor marine structures, chemical tanks exposed to process heat—account for thermal expansion when designing mounting holes and panel joints. Slotted fastener holes are standard practice on long runs of marine HDPE decking.
Low-Temperature Performance
HDPE retains useful impact resistance well below freezing, making it suitable for cold-storage bin liners and refrigerated food-processing environments. The material does not become brittle at –40°F, unlike many rigid plastics. This low-temperature toughness is one reason HDPE is preferred over polypropylene in freezer applications—polypropylene's impact strength drops sharply below 32°F.
Chemical Resistance
Resistance Summary
HDPE offers excellent resistance to a broad range of chemicals at room temperature, with some limitations at elevated temperatures or with strong oxidizing acids.
| Chemical Class | Resistance |
|---|---|
| Dilute acids (HCl, H₂SO₄) | Excellent |
| Concentrated H₂SO₄ | Good (avoid prolonged exposure) |
| Dilute alkalis (NaOH, KOH) | Excellent |
| Aliphatic hydrocarbons | Good |
| Aromatic hydrocarbons (toluene, xylene) | Fair (some swelling at elevated temp) |
| Chlorinated solvents | Fair |
| Alcohols | Excellent |
| Ketones | Good |
| Oxidizing acids (HNO₃ >60%) | Poor |
| Halogens (Cl₂, F₂) | Poor |
| Oils and greases | Excellent |
| Saltwater | Excellent |
| UV / Ozone | Poor (standard); Good (Marine HDPE) |
For specific chemical exposures, consult the manufacturer's resistance chart and test at operating temperature before specifying for long-term immersion service.
Solvent Stress Cracking
HDPE is susceptible to environmental stress cracking (ESC) in the presence of certain surfactants, detergents, and polar solvents when under tensile stress. Injection-molded HDPE is more susceptible than extruded sheet due to residual molding stresses. Extruded sheet is the correct specification for tank liners and chemically aggressive environments.
Electrical Properties
HDPE's excellent dielectric properties make it suitable for electrical insulation, cable insulation (a major market for HDPE), and applications requiring a non-conductive structural material. The low dissipation factor is particularly useful at RF frequencies.
UV and Weathering Resistance
Standard HDPE
Standard HDPE is not UV-stabilized. Prolonged UV exposure causes photooxidative degradation: the surface turns chalky, tensile strength decreases, and the material becomes brittle over 12–24 months of outdoor exposure. Standard HDPE is not appropriate for outdoor structural applications.
Marine HDPE (Starboard / KING StarBoard)
Marine-grade HDPE incorporates UV absorbers and HALS (hindered amine light stabilizers) that significantly extend outdoor service life. King Plastic Corporation's KING StarBoard product line is formulated specifically for marine and outdoor architectural use. Accelerated weathering tests (ASTM G154, G155) show marine HDPE retaining structural properties after exposures equivalent to multiple years of Florida outdoor weathering.
UV stabilization in Marine HDPE is a bulk additive, not a surface coating. The material does not require re-treatment, painting, or UV topcoat. However, surface sanding or planing removes surface layers equally and does not affect the UV performance, since the stabilizer is distributed throughout the sheet.
Food and Regulatory Compliance Properties
HDPE compounded to FDA 21 CFR 177.1520 meets the requirements for direct food contact. Key properties relevant to food service:
- Water absorption <0.1%: Does not absorb flavors, odors, or bacteria-bearing moisture.
- Non-porous surface: Resists bacterial infiltration at low knife-score depths.
- Chemical resistance to sanitizers: Compatible with quaternary ammonium, chlorine-based sanitizers (bleach solutions), and iodophor at standard dilutions.
- NSF/ANSI 51 certification: Available on selected grades; confirms suitability for food-equipment applications.
See the HDPE FDA food-grade guide for compliance documentation details.
Comparing Properties Across HDPE Grades
| Property | Standard HDPE | Marine HDPE (Starboard) | Borated HDPE |
|---|---|---|---|
| Tensile Strength | 4,600 psi | ~4,000–4,400 psi | ~3,500–4,200 psi |
| UV Resistance | Poor | Good–Excellent | Poor |
| FDA Compliant | Yes | Varies by color | No |
| Boron Content | None | None | 5–30% (varies) |
| Color Options | Natural, black, white | 10+ marine colors | Natural |
| Neutron Shielding | None | None | Yes |
The slight tensile reduction in marine grades reflects the UV additive package and colorant load. For structural calculations, use the lower bound of the marine HDPE tensile range. Borated grades vary significantly depending on boron carbide or colemanite loading; consult the manufacturer's datasheet for the specific formulation.
Property Comparison: HDPE vs. Alternatives
For detailed analysis of each comparison, see HDPE comparisons.
Note on polypropylene: while PP offers a slightly higher continuous use temperature (200°F vs. 180°F), HDPE's advantage is low-temperature impact retention. Below 32°F, polypropylene impact strength drops sharply and the material becomes brittle. HDPE remains tough to –40°F and below, which is why it is the preferred material for freezer-environment bin liners and cold-storage food equipment. If your application involves elevated-temperature chemical service, PP may win the specification; if it involves cold or ambient temperatures with UV exposure (marine, outdoor), HDPE in the marine grade is the better choice.
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