Polymer Density (Specific Gravity)| Methods, Charts & Guide

In plastics engineering, polymer density is deceptively simple yet profoundly useful. At face value, it’s just mass per unit volume. In practice, it governs material selection, process design, quality control, lightweighting, buoyancy, cost modeling, and recyclability. Whether you’re specifying a film resin, optimizing a structural part, or separating mixed streams in recycling, understanding polymer density and its closely related metric specific gravity (relative density) is essential.

This guide consolidates everything you need to know about polymer density—from core definitions and units to the nuances of bulk density vs. true density, from ASTM/ISO test methods to common pitfalls (temperature, crystallinity, additives), from conversion tips to application-driven selection matrices. You’ll also find curated tables of typical density and specific gravity ranges for major polymers and reinforced/filled grades, plus worked examples you can reuse in daily engineering work.

What Is Density?

Density measures mass per unit volume:

ρ  =  mV

• ρ (rho): density
• m: mass
• V: volume

Units

• SI: kg/m³
• Plastics industry (common): g/cm³ (equivalently g/mL)
• Gases: often g/L

Quick conversions

• 1 g/cm³ = 1000 kg/m³
• 1 kg/L = 1 g/cm³
• 1 lb/ft³ ≈ 0.01602 g/cm³ (so g/cm³ × 62.43 ≈ lb/ft³)

For polymers, reporting in g/cm³ keeps comparisons intuitive.

Bulk Density vs. True Density (and Why It Matters)

True (skeletal) density refers to the density of the solid material itself, excluding open voids and inter-particle gaps. It’s what you derive from displacement/pycnometry or density gradient methods on solid, void-free specimens.

Bulk density describes the mass per bulk volume of a particulate/flake/pellet sample, including voids between particles. It’s critical for:

• Storage & handling: bin sizing, hopper angles, ratholing/arching risk
• Drying equipment sizing: residence time, airflow
• Flow behavior: dosing, blending, pneumatic conveying

Don’t confuse them. Bulk density will always be lower than true density for the same material, and it varies with particle size distribution, shape, moisture, and compaction state.

Specific Gravity vs. Density: How They’re Related

Specific gravity (SG) also called relative density—is the ratio of a material’s density to the density of a reference substance (for plastics, usually water). It has no units.

SG = ρ material  / ρ water at reference T

Because water’s density depends on temperature, you must specify the reference temperature. Common lab references are:

• 4 °C: ρ_water ≈ 1.0000 g/cm³
• 23–25 °C: ρ_water ≈ 0.997–0.998 g/cm³
• 15.6 °C (60 °F) and 21.1 °C (70 °F) are also used in legacy/industrial contexts.

Example (at 23 °C): If a polymer has ρ = 1.20 g/cm³, then
SG ≈ 1.20 / 0.9975 ≈ 1.204.

Why Polymer Density (and SG) Matter

• Design & Performance: Stiffness-to-weight, strength-to-weight, buoyancy, balance, inertia, NVH (noise/vibration/harshness) in automotive and machinery.
• Thermal Management: In foams and low-density systems, density links to insulation performance; in crystalline polymers, density trends with crystallinity and thermal resistance.
• Chemical & Barrier Behavior: Semi-crystalline polymers with higher density often exhibit improved barrier properties (e.g., HDPE vs LDPE) and different solvent uptakes.
• Processability: Melt rheology and shrinkage correlate with morphology and, indirectly, density; pellet bulk density drives hopper throughput and drying.
• Recycling & Sorting: Specific gravity separation (sink/float) helps split polyolefins (SG < 1) from heavier engineering plastics (SG > 1).
• Cost Modeling: Cost per volume depends on density; a cheaper resin by mass may not be cheaper by volume.

Factors Affecting Polymer Density

1) Crystallinity and Molecular Architecture

• Amorphous polymers (e.g., PC, PMMA, PS) have randomly arranged chains, typically resulting in lower density than their semi-crystalline counterparts at comparable chemistry.
• Semi-crystalline polymers (e.g., PE, PP, PA, PET, POM, PEEK) have ordered crystalline regions packed tightly, increasing density.
• Branching: LDPE (long-chain branching) is less dense than LLDPE (short-chain branching) which is less dense than HDPE (linear). This sequence mirrors increasing chain packing efficiency.
• Crystallinity fraction (Xc): As Xc increases, density typically increases. Engineers sometimes estimate trends using mixture-like relations between amorphous and crystalline phase densities (material-specific).

2) Temperature

Most polymers expand as temperature rises. Volume increases, so density decreases. Over moderate ranges, you can use a linear first-order approximation:

ρ(T) ≈ ρ0 1+α ΔT

• ρ₀: density at reference T₀

• α: volumetric thermal expansion coefficient (polymer-dependent)
• ΔT: T − T₀

Worked Example: Suppose ρ₀ = 1.200 g/cm³ at 23 °C and α ≈ 4.0×10⁻⁴ K⁻¹ (illustrative). At 60 °C (ΔT=37 K):

ρ(60) ≈1.2001+0.0004×37≈1.2001.0148≈1.182 g/cm3≈1+0.0004×371.200​≈1.01481.200​≈1.182 g/cm3

Takeaway: Always report test temperature with density/SG.

3) Additives, Plasticizers, and Fillers

• Plasticizers usually lower density and soften amorphous phases (e.g., plasticized PVC).
• Mineral fillers (talc, CaCO₃, glass beads) and fibers (glass, carbon) raise density, sometimes dramatically.
• Foaming agents’ lower apparent density by introducing closed/open cells.
• Moisture/Solvent uptake alters mass and sometimes volume—drying and conditioning matter.

4) Morphology & Processing History

Cooling rate, annealing, draw orientation, and recrystallization modify crystalline and free volume, nudging density up or down. Two parts from the same resin can differ if their thermal histories diverge.

Measuring Polymer Density and Specific Gravity

Standards Overview

• ASTM D792 – Density and Specific Gravity of Plastics by Displacement
  • Method A: immersion in water
  • Method B: immersion in liquids other than water (for water-sensitive materials)
  • Reports density and SG; mandates temperature control and buoyancy corrections.
• ASTM D1505 – Density by Density-Gradient Technique
  • Uses a liquid column with a continuous density gradient; sample’s equilibrium height corresponds to its density via calibration floats/beads.
• ISO 1183 series – Plastics—Methods for determining the density of non-cellular plastics
  • ISO 1183-1: immersion method (like D792)
  • ISO 1183-2: density gradient column
  • ISO 1183-3: pycnometer methods

Methods immediately

1) Immersion/Displacement (Archimedes):

• Weigh in air (m_air), then weigh immersed (m_imm) in a liquid of known density at temperature T.
• Correct for buoyancy (both on the sample in air and immersed), surface tension, and entrapped bubbles.
• Great for solid, void-free specimens (rods, plaques, injection-molded bars).

2) Density Gradient Column:

• A tall column with a stable gradient (e.g., two miscible liquids).
• Insert the specimen and calibrated glass floats; the sample’s equilibrium level indicates its density.
• Sensitive and repeatable for quality control across narrow windows.

3) Pycnometry:

• Gas (helium) pycnometer for true (skeletal) density of powders/porous solids.
• Liquid pycnometer for liquids and powders that tolerate the liquid.
• Excellent for true density when bulk voids would bias immersion.

Best Practices (to Improve Accuracy)

• Temperature Control: Condition samples and test liquid at 23 ± 2 °C (or your chosen reference); report it.
• Specimen Prep: Dry hygroscopic polymers (e.g., PAs, PET, PBT, PC) to spec; avoid surface roughness that traps bubbles; remove flash and burrs.
• Degassing: Degas liquids (especially for gradient columns) to maintain stability.
• Calibration: Verify with reference floats/standards or mass/volume artifacts; record uncertainty.
• Documentation: Report method, liquid, temperature, replicates, mean ± SD, and any corrections applied.

Conversions: Density ↔ Specific Gravity

Assuming water at 23 °C (ρ_water ≈ 0.9975 g/cm³):

• SG = ρ_material / 0.9975
• ρ (g/cm³) ≈ SG × 0.9975

At 4 °C use 1.0000 g/cm³; at 25 °C use ~0.9970 g/cm³. Always state the reference.

Typical Polymer Density & Specific Gravity Ranges

The following normalized, condensed chart summarizes typical values for common polymers and selected reinforced/filled grades. Values depend on grade, crystallinity, additives, and temperature. Treat these as engineering starting points, not specifications.

Commodity Polyolefins

Polymer	Density (g/cm³)	SG @ 23 °C (≈÷0.9975)	Notes
LLDPE	0.915–0.935	~0.917–0.937	Linear low-density; better toughness/barrier than LDPE
LDPE	0.917–0.930	~0.920–0.933	Long-chain branching; soft, flexible, good clarity
HDPE	0.940–0.970	~0.942–0.972	Higher crystallinity; stiffer, better barrier
UHMWPE	0.930–0.950	~0.932–0.953	Very high Mw; exceptional abrasion resistance
PP (homo/co-poly)	0.900–0.910	~0.903–0.912	Lowest among major plastics; excellent for lightweighting
PP impact modified	0.880–0.910	~0.882–0.912	Rubber modifiers lower density slightly

 

Styrene’s and Blends

Polymer	Density	SG	Notes
PS (GPPS/HIPS)	1.04–1.07	~1.04–1.07	GPPS clear; HIPS impact-modified
ABS	1.04–1.10	~1.04–1.10	Blend of SAN + butadiene; tunable impact/heat
ASA	1.05–1.07	~1.05–1.07	Weatherable alternative to ABS
SAN	1.06–1.10	~1.06–1.10	Stiffer, clearer than PS
PC/ABS, PC/PBT	1.17–1.35+	~1.17–1.35+	Density rises with PC fraction and fillers

 

Vinyl

Polymer	Density	SG	Notes
PVC (rigid)	1.35–1.50	~1.35–1.50	High SG; strong, inherently flame retardant
PVC (plasticized)	1.15–1.35+	~1.15–1.35+	Plasticizer lowers base ρ; fillers can raise it
CPVC	1.50–1.55	~1.50–1.55	Higher chlorine content; higher heat resistance

 

 

 

 

Acrylics & Polycarbonates

Polymer	Density	SG	Notes
PMMA (acrylic)	1.17–1.20	~1.17–1.20	Clear, rigid, weatherable
PC	1.20–1.22	~1.20–1.22	Clear, tough; density increases with glass/minerals

 

Polyesters

Polymer	Density	SG	Notes
PET (amorphous to semi-crystalline)	1.30–1.40	~1.30–1.40	Bottle-grade, fibers, films
PBT	1.30–1.40	~1.30–1.40	Faster crystallization than PET
PETG	1.27–1.38	~1.27–1.38	Glycol-modified PET; amorphous, good clarity

 

Polyamides (Nylons)

Polymer	Density	SG	Notes
PA6	1.12–1.14	~1.12–1.14	Hygroscopic; conditioning changes mass/volume
PA66	1.13–1.15	~1.13–1.15	Higher melting point than PA6
PA11/12 (aliphatic)	~1.01–1.14	~1.01–1.14	Lower density; flexible grades near ~1.01–1.05
GF-reinforced PA	1.25–1.60	~1.25–1.60	Density rises with GF loading & minerals

 

 

Acetals & Others

Polymer	Density	SG	Notes
POM (acetal)	1.41–1.43	~1.41–1.43	High crystallinity, good fatigue, low friction
POM impact/filled	1.30–1.60	~1.30–1.60	Modifiers can lower or increase density

 

High-Performance Thermoplastics

Polymer	Density	SG	Notes
PEEK	1.26–1.32	~1.26–1.32	High temp & chemical resistance
PEI	1.27–1.30	~1.27–1.30	Amorphous, high heat, dimensionally stable
PPS	~1.35	~1.35	Semi-crystalline; excellent chem/thermal resistance
LCP	~1.40	~1.40	Extremely high stiffness in flow direction

 

Fluoropolymers

Polymer	Density	SG	Notes
PTFE	2.10–2.20	~2.11–2.21	Very high SG; exceptional chemical resistance
FEP/PFA	2.10–2.20	~2.11–2.21	Melt-processable fluoropolymers
PVDF	1.70–1.80	~1.71–1.80	Strong, chemically resistant, electroactive

 

Selected Reinforced/Filled Examples

 

 

System	Density (g/cm³)	SG	Comment
PP + 30–40% GF	1.10–1.23	~1.10–1.23	Big stiffness boost vs. neat PP
PP + 10–40% talc/mineral	0.97–1.25	~0.97–1.25	Density depends on filler loading/type
PC + 20–40% GF	1.35–1.52	~1.35–1.52	Heat and stiffness rise with GF
PET/PBT + 30% GF	1.50–1.60	~1.50–1.60	Common automotive grades
PA66 + 30–50% GF	1.37–1.60	~1.37–1.60	High SG; high mechanicals
PPS + 40% GF	1.60–1.70	~1.60–1.70	Dimensional stability at high T
PTFE + 25% GF	2.20–2.30	~2.21–2.31	Very high density; wear upgrade

Worked Examples (Step-by-Step)

Example 1: Convert Density to Specific Gravity

A PC sample is measured at ρ = 1.205 g/cm³ at 23 °C.
Water at 23 °C ≈ 0.9975 g/cm³.

SG=1.2050.9975≈1.208​≈1.208

Report as: SG (23 °C, water) = 1.208.

Example 2: Predict Density Change with Temperature

An HDPE grade has ρ = 0.955 g/cm³ at 23 °C. Estimate at 60 °C using α ≈ 5.0×10⁻⁴ K⁻¹ (illustrative):

ρ(60)≈0.955 1+0.0005×37 ≈0.9551.0185≈0.937 g/cm3\rho(60) \ ≈1+0.0005×370.955​≈1.01850.955​≈0.937 g/cm3

Example 3: Bulk vs. True Density in Hopper Sizing

A PP pellet has true density 0.905 g/cm³, but bulk density 0.52 g/cm³ due to inter-pellet voids. A 100-liter hopper therefore holds ~52 kg of pellets (not 90.5 kg). This impacts dryer dwell time and material changeover planning.

Test Methods: Procedures, Pros & Cons

ASTM D792 / ISO 1183-1 (Immersion)

Procedure (simplified):

1. Condition specimens at test temperature (e.g., 23 °C).
2. Weigh dry specimen in air (m_air).
3. Weigh specimen immersed in the test liquid (m_imm).
4. Apply buoyancy corrections; compute ρ and SG.
5. Repeat (≥3) and report mean ± SD with temperature/liquid.

Pros: Widely available, good for solid parts, fast.
Cons: Sensitive to surface bubbles, roughness, and water uptake; not ideal for porous specimens.

ASTM D1505 / ISO 1183-2 (Density Gradient)

Procedure (simplified):

1. Prepare a stable gradient column and calibrate with floats.
2. Introduce small specimens and allow them to equilibrate.
3. Read height and map to density via calibration curve.

Pros: High precision for small differences; great for QA/release.
Cons: Column prep/maintenance; liquid compatibility limits; temperature stability is critical.

Pycnometry (ISO 1183-3; helium or liquid)

Pros: Best for true density, powders, porous materials.
Cons: Equipment cost (helium), liquid compatibility, operator training.

Common Pitfalls and How to Avoid Them

• Unstated Temperature: Always note test temperature (and liquid).
• Moisture: Hygroscopic polymers (PA, PET, PBT, PC) change mass and sometimes volume—dry to spec and measure rapidly.
• Surface Bubbles: Degas liquids, wet the surface, consider surfactants approved by your method, and use smoother specimens.
• Porosity: Use helium pycnometry for true density or encapsulate pores (caution) for immersion.
• Plasticizer/solvent migration: Condition samples to equilibrium; report conditioning.
• Assuming “typical” equals “your grade”: Always check your supplier’s data sheet.

Application-Driven Selection Guide

Use polymer density as a filter, not the sole decider. Properties trade off:

When to Prefer Low Density (SG < 1.0–1.1)

• Lightweighting: PP and PE families minimize mass and inertia.
• Flotation/insulation: Buoyant parts, thermal insulation foams.
• Cost-per-volume: Low density can lower material cost for volume-critical designs.

Candidates: PP (0.90–0.91), PE (0.91–0.97), PMP (~0.84), foamed variants.

When to Accept Higher Density (SG > 1.2)

• Stiffness & Strength: GF-PA, GF-PBT/PET, PC, POM, PEEK, PPS.
• Heat & Chemical Resistance: PEEK, PEI, PPS, PVDF, PTFE.
• Dimensional Stability: Mineral-filled grades, LCP.

Candidates: PA66 GF30 (≈1.37), PET/PBT GF30 (≈1.55), PC GF (≈1.4–1.5), PPS GF40 (≈1.65), PTFE (≈2.2).

Quick “Pick-a-Polymer” Matrix

• Light + Tough packaging → LLDPE/LDPE, PP copolymer
• Rigid, clear glazing → PMMA or PC (higher SG, higher toughness)
• Low friction gears/bushings → POM (1.41–1.43)
• Hot under-hood → PPS GF, PA66 GF
• Chemical transport → PVDF, PTFE, PE (HDPE) depending on media and T
• Structural with weight limit → PP GF/LGF, PA GF (balance mass & stiffness)

Industrial Use Cases & Mini Case Studies

1) Automotive Lightweighting with PP-GF

Challenge: Replace a PC/ABS interior support (ρ ≈ 1.17–1.20) without sacrificing stiffness.
Approach: PP + 30% GF (ρ ≈ 1.15–1.23) balances density and modulus.
Outcome: ~5–10% mass reduction at equal stiffness with improved recyclability in PP stream.

2) Chemical Handling with PVDF vs. PP

Scenario: A pump housing exposed to chlorinated solvents and mild acids at 80–90 °C.
Tradeoff: PVDF (ρ ≈ 1.70–1.80) outperforms PP in chemical resistance and dimensional stability despite higher density; lifecycle wins.

3) Packaging Barrier: HDPE vs. LDPE

Context: Blow-molded container requiring hydrocarbon barrier.
Insight: HDPE’s higher crystalline and density improves barrier vs. LDPE; coextruded structures further optimize cost/barrier.

4) Wear Components: POM vs. Filled PA

Decision: POM (1.41–1.43) provides low friction and fatigue resistance; filled PA66 GF wins on stiffness/heat but brings higher moisture sensitivity and density.

5) High-Temp Structural: PEEK vs. PEI

Need: 180–220 °C service with mechanical retention.
Pick: PEEK (semi-crystalline, 1.26–1.32) for chemical resistance and long-term strength; PEI (amorphous, 1.27–1.30) for transparency/processing window.

Recycling, Sorting, and Environmental Implications

• Sink/Float Separation: SG < 1 floats in water—useful to separate polyolefins from heavier streams. Adjust brine or use other liquids to target narrow SG bands.
• Filler Awareness: Filled grades may cross SG thresholds, e.g., talc-filled PP can sink, complicating “polyolefin-floats” assumptions.
• Design for Recycling: Specify density windows compatible with your recovery method. Avoid heavy fillers when float separation is relied upon downstream.
• Environmental Impact: Lower density can aid lightweighting, reducing transport emissions. But durability (and hence lifecycle impacts) may favor higher-density engineering polymers in demanding uses.

Frequently Asked Questions (FAQ)

Q1. Is specific gravity the same as density?
No. Density has units (e.g., g/cm³). Specific gravity is a ratio to water’s density at a stated temperature and is unitless.

Q2. What temperature should I use when reporting SG?
Commonly 23 °C or 25 °C in plastics labs. State it explicitly (e.g., “SG (23 °C, water)”).

Q3. Why does my nylon’s density change day-to-day?
Moisture uptake: nylons are hygroscopic. Dry and condition consistently before measurement.

Q4. Can I compare densities from different methods?
Yes but note the method (D792 immersion vs. D1505 gradient vs. pycnometer) and conditions. True density from helium pycnometry is not directly comparable to bulk density.

Q5. Does higher density always mean higher strength?
No. Density correlates loosely with crystallinity and filler content, but strength depends on many factors (molecular weight, morphology, fiber reinforcement, processing).

Q6. How do fillers affect recycling by SG?
Fillers raise SG; filled PP/PE may sink and contaminate heavier fractions. Declare fillers and plan to sort accordingly.

Q7. What if my part has micro-voids?
Use helium pycnometry for true density, or overmold/encapsulate carefully for immersion (verify method compliance).

Q8. Can I estimate crystalline from density?
For some polymers (PE, PP, PET), empirical relationships exist, but they’re material-specific and require reference densities of amorphous and crystalline phases.

Q9. What’s the best method for films?
Thin films can be measured via D1505 gradient or by carefully prepared stacks for D792. Ensure no trapped air and stable temperature.

Q10. Why does the same resin show slightly different densities from two labs?
Differences in temperature, conditioning, equipment calibration, and specimen prep. Align protocols and report uncertainty.