You’re looking at a PVC resin specification sheet and see “density” listed. But which number do you use? The true density? The bulk density? And why do they look completely different?
Getting this wrong causes real problems. Procurement teams have ordered the wrong quantity because they used bulk density instead of true density in their calculations. The result? Either too much material sitting in storage or a production line waiting for more resin to arrive.
This guide breaks down PVC resin density in plain terms. You’ll learn the standard values, understand why there are two types of density measurements, and know exactly which number to use for your situation.
What Is the Standard Density of PVC Resin?
Pure PVC resin has a density of approximately 1.4 g/cm³. This baseline value applies to the solid polymer material before any additives are mixed in.
The density changes depending on the type of PVC you’re working with:
| PVC Type | Density Range | Common Uses |
|---|---|---|
| Pure PVC Resin | ~1.4 g/cm³ | Base material |
| Rigid PVC (uPVC) | 1.3-1.45 g/cm³ | Pipes, window frames |
| Flexible PVC | 1.1-1.35 g/cm³ | Cables, hoses, flooring |
| CPVC | 1.52 g/cm³ | Hot water pipes |
One useful fact: PVC is denser than water (1.0 g/cm³), so it sinks. Polyethylene and polypropylene both float because their densities fall below 1.0 g/cm³. This simple float test helps identify unknown plastic samples quickly.
Why Are There Two Types of PVC Density Values?
What Is True Density vs. Bulk Density?
True density measures the solid material itself. Bulk density measures the powder including all the air spaces between particles.
Think of it like ice. A solid ice cube and crushed ice are the same material. But if you fill a measuring cup with crushed ice, it weighs less than the same cup filled with solid ice. The air gaps between the crushed ice pieces reduce the overall density measurement.
PVC resin powder works the same way. The true density of solid PVC is about 1.4 g/cm³. But PVC resin powder has a bulk density between 0.45-0.60 g/cc—roughly one-third of the true density.
When to use each value:
- True density (~1.4 g/cm³): Calculating weight of finished PVC products like pipes or sheets
- Bulk density (0.45-0.60 g/cc): Determining storage space, mixer capacity, or shipping weights for PVC powder
How Does K-Value Affect Bulk Density?
Higher K-value means lower bulk density. The relationship is straightforward: higher K-value resins have more porous particles with more air space inside them.
| K-Value | Typical Bulk Density | Primary Applications |
|---|---|---|
| K-57 | ~0.50 g/ml | Rigid fittings, bottles |
| K-66 | 0.55-0.60 g/cc | Rigid pipes |
| K-67 | ~0.55 g/ml | General rigid applications |
| K-70 | 0.45-0.49 g/cc | Flexible products, cables |
This inverse relationship matters for processing. A K-70 resin fills a mixer hopper differently than a K-57 resin. The K-70 powder takes up more space per kilogram because of its lower bulk density.
What Factors Change PVC Density?
Three main factors determine the final density of a PVC compound:
- Plasticizers lower density. Adding plasticizers creates flexible PVC, and these additives are less dense than the base resin. That’s why flexible PVC (1.1-1.35 g/cm³) weighs less than rigid PVC (1.3-1.45 g/cm³) for the same volume.
- Fillers go both ways. Calcium carbonate is denser than PVC, so adding it increases the compound’s density. Wood flour is lighter than PVC, so it decreases density. The type and amount of filler directly impacts the final number.
- Processing conditions matter. Temperature, pressure, and cooling rates during extrusion or molding affect the material’s final microstructure. Identical formulations can show slight density differences when processed under different conditions.
ISO 4439:1979 specifies that PVC-U pipes must have a density between 1.35 and 1.46 g/cm³. Staying within this range indicates proper formulation and processing.
Conclusion
PVC resin density isn’t complicated once you understand the two types of measurements.
For finished PVC products, use the true density of approximately 1.4 g/cm³. For PVC powder handling, use the bulk density—typically 0.45-0.60 g/cc depending on the K-value grade.
Always check whether a specification refers to true density or bulk density. Using the wrong one throws off your calculations by a factor of two or more. When in doubt, ask your supplier which measurement their data sheet references.