Modified PEEK Density Determination Test: Standards, Methods, and Technical Guidelines

Modified PEEK density determination is a fundamental physical characterization experiment that verifies filler loading, structural integrity, and batch consistency of glass-filled, carbon-filled, and bearing-grade polyetheretherketone materials. This technical article outlines the applicable international standards, including ISO 1183 and ASTM D792, along with sample preparation requirements, step-by-step measurement procedures, and practical guidance for interpreting test results. Engineers and quality professionals can use this guide to establish reliable density testing protocols for high-performance PEEK components.

Technical Overview

PEEK (polyetheretherketone) is a semi-crystalline thermoplastic belonging to the polyaryletherketone (PAEK) family, valued in aerospace, medical, semiconductor, and energy applications for its exceptional heat resistance, mechanical strength, and chemical inertness. Neat PEEK exhibits a density of approximately 1.30 g/cm³, and this value shifts predictably when the polymer is modified with reinforcing fibers or solid lubricants. Density determination is therefore one of the fastest and most informative quality control experiments for modified PEEK, because it indirectly reveals filler content, void formation, and degree of crystallinity.

The experiment is based on Archimedes' buoyancy principle: a specimen is weighed in air and again while immersed in a liquid of known density, and the displaced volume is calculated from the difference. Because the measurement is non-destructive, rapid, and requires only a precision analytical balance and a simple immersion setup, it is routinely performed by accredited third-party laboratories as well as in-house quality departments.

Typical density ranges for common commercial modified PEEK grades include:

• Unfilled PEEK: 1.26 to 1.32 g/cm³
• PEEK with 30% glass fiber (GF30): 1.38 to 1.44 g/cm³
• PEEK with 30% carbon fiber (CF30): 1.36 to 1.42 g/cm³
• Bearing-grade PEEK with PTFE, graphite, and carbon fiber: 1.43 to 1.50 g/cm³

Testing Samples

Neat and Unfilled PEEK

Unmodified PEEK specimens serve as the baseline reference for the experiment. These may be supplied as injection-molded plaques, compression-molded sheets, extruded rods, or machined components. Since unfilled PEEK density is sensitive to cooling history, rapidly quenched amorphous parts should be distinguished from annealed semi-crystalline parts when comparing results.

Glass-Fiber Reinforced PEEK

Glass-filled grades typically contain 10 to 30 percent by weight of chopped glass fiber, whose density of roughly 2.5 g/cm³ raises the composite density in near-linear proportion to fiber loading. Density measurement is an effective indirect check that the specified glass content has been achieved during compounding.

Carbon-Fiber Reinforced PEEK

Carbon fiber has a density of approximately 1.7 to 1.8 g/cm³, closer to that of the matrix, so the density increase is smaller than for an equivalent glass loading. A 30 percent carbon-fiber grade typically measures around 1.40 g/cm³, and deviations from the datasheet value may indicate incorrect fiber loading, fiber-rich or resin-rich regions, or internal voids.

Bearing-Grade and Lubricated PEEK

Tribologically optimized grades blend PEEK with PTFE (density about 2.1 to 2.3 g/cm³), graphite (about 2.2 g/cm³), molybdenum disulfide, or combinations of these with carbon fiber. These dense solid lubricants produce the highest densities among modified PEEK materials, often exceeding 1.45 g/cm³.

Sample Forms and Preparation Requirements

Acceptable sample forms include molded test specimens, finished machined parts, granules, and powders. For immersion testing, ISO 1183-1 recommends a specimen mass of at least 1 g. Surfaces must be clean, free of grease, and free of machining marks that could trap air bubbles. Specimens should be conditioned at 23 ± 2 °C and 50 ± 5% relative humidity in accordance with ISO 291 for a minimum of 24 hours, or dried to constant mass when moisture-sensitive comparisons are required.

Testing Items

The core deliverables of a modified PEEK density determination experiment include:

• Bulk density of solid specimens at 23 °C, expressed in g/cm³
• Specific gravity (relative density, dimensionless) referenced to water at 23 °C
• Apparent density of granules and powders for feedstock verification
• Skeletal (true) density by gas pycnometry, which excludes open porosity
• Density uniformity assessed through multi-point sampling across a part or production batch
• Estimated filler volume fraction derived from rule-of-mixtures calculations

When performed on a 0.1 mg analytical balance under controlled temperature, immersion testing typically achieves repeatability of 0.002 g/cm³ or better, and reported results should include measurement uncertainty.

Testing Methods

Several standardized methods are available, and the choice depends on sample geometry, required precision, and whether porosity must be excluded from the result.

Immersion Method (ISO 1183-1 Method A, ASTM D792 Method A)

This is the most widely used approach for molded and machined PEEK parts. The procedure is as follows:

• Condition the specimen to equilibrium and verify the immersion liquid temperature is 23 ± 0.5 °C
• Weigh the specimen in air and record the mass to 0.1 mg
• Immerse the specimen fully on a thin support wire, ensure complete wetting with a drop of non-ionic wetting agent, and dislodge any adhering bubbles with a fine brush
• Record the apparent immersed mass
• Calculate density using the formula ρ = (m a / (m a − m l)) × ρ L, where m a is the mass in air, m l is the apparent mass in the liquid, and ρ L is the liquid density (distilled water equals approximately 0.9976 g/cm³ at 23 °C)
• Apply sinker-wire buoyancy and air buoyancy (0.0012 g/cm³) corrections for maximum accuracy

Duplicate or triplicate specimens should be tested, and the arithmetic mean reported together with the standard deviation.

Liquid Pycnometer Method (ISO 1183-1 Method B)

This variant is preferred for granules, powder, or small irregular chips that are difficult to hang individually. The specimen is placed in a calibrated pycnometer filled with a displacement liquid, and the volume is determined from the mass of liquid displaced. The method is well suited to incoming inspection of PEEK pellets before processing.

Density Gradient Column (ISO 1183-2, ASTM D1505)

A vertical column of two miscible liquids establishes a linear density gradient calibrated with reference glass floats. Specimens settle at the height corresponding to their density, enabling resolution on the order of 0.0001 g/cm³. This technique is reserved for research work, crystallinity studies, and fine discrimination between similar grades.

Gas Pycnometer Method (ISO 1183-3)

Helium or nitrogen displacement measures the skeletal density of the polymer, excluding open pores but including closed voids. It is particularly valuable for sintered, foamed, or thick-section molded PEEK parts where internal porosity is a concern, and comparing gas pycnometry results with immersion bulk density provides a quantitative estimate of porosity.

Applications

Density determination experiments for modified PEEK support a wide range of engineering and quality functions:

• Incoming inspection of PEEK granules, rods, plates, and films against supplier datasheet specifications
• Verification of glass-fiber or carbon-fiber loading after compounding and before molding
• Detection of excessive voids or porosity in thick-section injection-molded and compression-molded components
• Assessment of crystallinity differences between as-molded and annealed parts, since higher crystallinity modestly increases density
• Batch-to-batch consistency control in regulated industries such as aerospace, automotive e-mobility, oil and gas downhole tooling, semiconductor handling, and implantable medical devices
• Input data for finite element analysis, mass-property calculations, and rule-of-mixtures verification of composite composition

Because the test is quick and non-destructive, it is frequently integrated into routine release testing and supplier qualification programs conducted by accredited third-party laboratories.

FAQ

What is the nominal density of unmodified PEEK?

Unfilled PEEK has a density of approximately 1.30 g/cm³, typically ranging from 1.26 to 1.32 g/cm³ depending on crystallinity and molding history. Rapidly quenched, largely amorphous parts fall near the lower end, while annealed semi-crystalline parts approach the upper end.

Which standards govern modified PEEK density testing?

The primary references are ISO 1183-1 (immersion, pycnometer, and titration methods), ISO 1183-2 (density gradient column), ISO 1183-3 (gas pycnometer), ASTM D792 (displacement methods), and ASTM D1505 (density gradient). Specimen conditioning follows ISO 291 or ASTM D618.

How much does filler content change PEEK density?

By the rule of mixtures, adding 30 percent by weight of glass fiber raises density to roughly 1.40 to 1.44 g/cm³, while 30 percent carbon fiber yields approximately 1.40 g/cm³. Significant deviation from the expected value usually indicates incorrect filler loading, poor fiber dispersion, or void formation.

Why is temperature control important during the test?

The density of water changes by roughly 0.0003 g/cm³ per degree Celsius near ambient temperature, and the specimen itself expands thermally. Maintaining 23 ± 0.5 °C with a calibrated thermometer keeps temperature-related errors below 0.001 g/cm³.

Can density testing detect internal voids in molded PEEK parts?

Yes. Enclosed air voids lower the measured bulk density relative to the theoretical fully dense value. Comparing immersion bulk density with gas-pycnometer skeletal density, or with the datasheet density, provides a practical quantitative estimate of porosity.

What sample size and quantity are required?

A minimum specimen mass of 1 g is recommended for immersion testing, with larger parts tested at multiple locations to check uniformity. Running at least two or three replicate specimens per condition is standard practice, and results should be reported as the mean with measurement uncertainty.