High Density Polyethylene (HDPE), Polypropylene (PP) and Polyvinylchloride (PVC) that are manufactured and supplied in stock shapes (rod, sheet etc) are three common engineering plastics that fit into a similar category, both commercially and technically. All three are weldable, thermoformable, can be machined, and are in a similar price bracket that is at the lower end of the scale compared to other engineering plastics.
How do we decide which material to use and when?
Understanding the properties and characteristics of each material will help with material selection.
Mechanical Properties
PVC is stronger, stiffer and heavier than HDPE & PP. Higher stiffness allows for larger fabricated structures such as tanks and vessels, with the ability to either eliminate or reduce reinforcement rubbing.
| HDPE | PP-H | PVC | |
|---|---|---|---|
| Specific Gravity (Density) | 0.96 (Floats in water) | 0.91 (Floats in water) | 1.44 (Sinks in water) |
| Tensile Strength | 24 MPa | > 30MPa | 55 MPa |
| Stiffness (Tensile Modulus of Elasticity) | 1000 MPa | 1500 MPa | 3000 MPa |
Thermal Properties
PVC will expand (and contract) significantly less than HDPE and PP, but also has a lower maximum allowable service temperature.
Often plastics are used for their thermal insulation characteristics. All three materials are good thermal insulators, but PVC has the lowest thermal conductivity value (closely followed by PP) which means it is the best thermal insulator out of this range.
| HDPE | PP-H | PVC | |
|---|---|---|---|
| Allowable Temperature | Long Term: -50 to 80 | Short Term: 100 Long Term: 0 to 100 | Short Term: 150 Long Term: -20 to 60 |
| Thermal Expansion Rate | 150-230 x 10-6/K 1m will expand 1.5-2.3mm per 100C increase in temperature | 120-190 x 10-6/K 1m will expand 1.2-1.9mm per 100C increase in temperature | 75 x 10-6/K 1m will expand 0.75mm per 100C increase in temperature |
| Thermal Insulation | Good (0.40 W/(m.K)) | Better (0.20 W/(m.K)) | Best (0.16 W/(m.K)) |
Food and Drinking Water
In addition to complying with food and drinking water standards as required, Ozone is used as a disinfecting agent but can be corrosive to some materials. HDPE has moderate resistance to Ozone, while Polystone P & Trovider EC N are both resistant to the degradative effects of Ozone.
| HDPE | PP-H | PVC | |
|---|---|---|---|
| Suitable for Food Contact | Yes | Yes | No |
| Suitable for Drinking Water | Yes (Polystone PG100) | Yes (Non-UV grade only) | No |
| Ozone Resistance | Moderate | Good | Good |
Other Characteristics
| HDPE | PP-H | PVC | |
|---|---|---|---|
| Flammability Resistance (UL94) | Low Resistance HB | Low Resistance HB | Highly Resistant V0 |
| Wear & Abrasion Resistance | Moderate | Low | Low |
| UV Resistant | Yes (Black & Yellow, not Natural) | Yes (UV formulated Grade) | No |
| Colours (stocked) | Natural (white), black & yellow | Beige | Grey |
