Specific Gravity of Plastic: Testing Methods, Standards, and Material Significance
What is the Specific Gravity of Plastic?
Specific gravity (SG) — also called relative density — is the ratio of the density of a plastic material to the density of water at 4°C (1.000 g/cm³). It is a dimensionless number that, for practical purposes, equals the density in g/cm³. Specific gravity is one of the most fundamental physical properties of plastic materials, providing a quick, cost-effective measure of material identity, formulation consistency, and quality conformance.
Why Specific Gravity Matters for Plastic Materials
Material Identification
Each plastic polymer has a characteristic specific gravity range. Polyethylene (0.91–0.96), polypropylene (0.90–0.91), PVC (1.16–1.58), polycarbonate (1.20), nylon 6 (1.12–1.14), and PTFE (2.14–2.20) all have distinct SG ranges that enable rapid material identification and differentiation. SG testing is a quick, inexpensive first screen for material verification.
Formulation Quality Control
Fillers, pigments, and additives in compounded plastics are typically denser than the base polymer. SG measurement detects deviations from specified filler loading — a 5–10% change in filler content is often clearly detectable by SG. Incoming material and production-batch SG monitoring identify formulation errors and supplier substitutions before they reach moulding operations.
Crystallinity Assessment
For semicrystalline polymers such as polyethylene, polypropylene, and nylon, SG reflects the degree of crystallinity — more crystalline material is denser. HDPE (high-density polyethylene, SG ~0.96) is more crystalline than LDPE (SG ~0.92). Processing conditions that affect crystallinity — such as cooling rate and nucleating agents — are reflected in measurable SG changes.
Part Weight and Dimensional Calculation
Designers use SG to calculate part weight from volume and to verify wall-thickness consistency — denser parts with correct geometry will have predictable mass. Production QC uses SG to monitor batch consistency in moulded parts.
Test Methods for Specific Gravity of Plastics
ASTM D792 — Archimedes (Hydrostatic Weighing) Method
ASTM D792 is the primary standard for measuring the specific gravity of solid plastics. The specimen is weighed in air and then in an immersion liquid (typically distilled water) using a precision analytical balance with a suspension apparatus. Specific gravity is calculated from the weight difference due to buoyancy:
SG = Wₐ / (Wₐ − Wₗ)
where Wₐ is the weight in air, and Wₗ is the weight in immersion liquid. The temperature of the immersion liquid must be controlled and recorded (typically 23°C ± 0.5°C).
ASTM D1505 — Density Gradient Column Method
A density gradient column is prepared by carefully mixing two miscible liquids of different densities to create a continuous density gradient from top to bottom. Small plastic specimens are placed in the column and sink to the position where their density matches the column density — read from calibrated floats of known density. The D1505 method achieves better precision than D792 for comparative and research applications.
ISO 1183 — Plastics Density Determination
ISO 1183 provides four methods: immersion (Method A, equivalent to ASTM D792), pyknometer (Method B), titration (Method C for sheet materials), and density gradient column (Method D). ISO 1183-A is the most widely referenced international method.
Specific Gravity Values for Common Plastics
Plastic | Specific Gravity Range |
LDPE | 0.91–0.93 |
HDPE | 0.94–0.97 |
Polypropylene (PP) | 0.90–0.91 |
PVC (rigid) | 1.30–1.45 |
Polycarbonate (PC) | 1.19–1.22 |
ABS | 1.03–1.06 |
PTFE | 2.14–2.20 |
Epoxy (cured) | 1.20–1.30 |
Industrial Applications
In the recycled plastics industry, SG measurement — combined with sink-float separation — is a primary sorting method for separating plastic types in mixed waste streams (e.g., HDPE, PET, and PVC). In compounding, production-batch SG monitoring verifies consistent filler and glass-fibre loading. In quality control of injection-moulded parts, SG measurement on a sample of parts per batch verifies material and processing consistency.
Conclusion
Specific gravity of plastics — defined as the ratio of a material’s density to that of water — is a fundamental property used for material identification, formulation control, and quality assurance. Measured using standards such as ASTM D792 and ISO 1183, it provides a quick and reliable indication of polymer type, filler content, and crystallinity. Its simplicity and sensitivity make it an essential parameter in both laboratory analysis and production environments. Selecting the appropriate test method and maintaining precise measurement conditions is critical to ensure accurate and reproducible results — making testing practice as important as the property itself.
Why Choose Infinita Lab for Specific Gravity Testing of Plastics?
Infinita Lab provides ASTM D792 and ISO 1183 specific gravity testing for all plastic types through our nationwide, accredited polymer testing laboratory network, with rapid turnaround times to support incoming material and production quality programmes.
Looking for a trusted partner to achieve your research goals? Schedule a meeting with us, send us a request, or call us at (888) 878-3090 to learn more about our services and how we can support you.
Frequently Asked Questions (FAQs)
Can SG testing alone identify an unknown plastic material? SG provides a strong indication of plastic type — particularly for materials with very distinct SG values (PTFE at 2.2, PP at 0.91). However, many plastics have overlapping SG ranges, and SG alone cannot distinguish, for example, between ABS and PVC at some formulations. Confirmation by FTIR spectroscopy provides definitive polymer identification.
Why must the immersion liquid temperature be controlled in ASTM D792 testing? Water density changes with temperature — from 0.9998 g/cm³ at 20°C to 0.9975 g/cm³ at 25°C. This change affects the buoyancy force and therefore the calculated SG. ASTM D792 specifies testing at 23°C ± 0.5°C and requires that the immersion liquid temperature be reported to enable temperature correction if needed.
How does absorbed moisture affect SG measurement of hygroscopic plastics? Hygroscopic polymers (nylon, polyurethane, PVOH) absorb atmospheric moisture, increasing their mass and therefore increasing the apparent SG above the dry material value. ASTM D792 requires that specimens of moisture-sensitive materials be measured immediately after removal from the conditioning environment or conditioned in the immersion liquid before measurement to avoid surface moisture variation.
What is the density gradient column method and when is it preferred? The density gradient column (ASTM D1505) allows simultaneous measurement of many small specimens at different densities with better precision than hydrostatic weighing. It is preferred for research applications requiring precise density mapping of crystallinity gradients across moulded parts or film cross-sections, and for quality control programmes requiring very tight density tolerance monitoring.
Is specific gravity testing required for plastic material certification? Yes. Specific gravity is a required property in most plastic material specifications under ASTM, ISO, and IEC standards. For example, ASTM D4101 (polypropylene material specification), ASTM D3350 (polyethylene pipe material), and most engineering plastic material standards list density as a mandatory certification test.
