PEEK Melt Specific Volume Determination Experiment: pvT Testing Method, Procedure and Applications

The PEEK melt specific volume determination experiment measures how the specific volume of polyetheretherketone varies with temperature and pressure (pvT behavior), providing essential data for injection molding simulation and shrinkage prediction. As a semi-crystalline high-performance thermoplastic with a melting point near 343°C, PEEK requires precise high-temperature, high-pressure dilatometry to capture its melt density, compressibility, and pressure-dependent transitions. This article explains the sample requirements, testing items, measurement methods, and practical applications of PEEK pvT data.

Technical Overview

PEEK (polyetheretherketone) is a semi-crystalline, high-performance engineering thermoplastic widely used in aerospace, automotive, electronics, medical, and energy industries. It exhibits a glass transition temperature of approximately 143°C and a melting temperature of approximately 343°C, with typical melt processing temperatures between 360°C and 400°C. The melt specific volume determination experiment quantifies the specific volume (the reciprocal of density, typically expressed in cm³/g) of PEEK as a function of temperature and pressure, generating a pvT diagram that describes how the material expands, compresses, melts, and solidifies under realistic processing conditions.

Because PEEK is semi-crystalline, its specific volume shows a pronounced step change at the melting point during heating and a corresponding specific volume jump caused by crystallization during cooling. The magnitude and temperature of this transition shift with applied pressure, following a Clapeyron-type relationship. For unfilled PEEK, the solid specific volume at room temperature is approximately 0.76 cm³/g (density about 1.30 g/cm³), while the melt specific volume at processing temperature generally falls in the range of 0.85 to 0.95 cm³/g depending on temperature and pressure. Accurate pvT data are therefore indispensable for calculating volumetric shrinkage, optimizing packing pressure, and predicting warpage in precision-molded PEEK components.

The experiment is typically performed in accordance with ISO 17744, which specifies the piston apparatus method for determining the specific volume of plastics as a function of temperature and pressure. Modern pvT dilatometers cover temperatures up to approximately 420°C and hydrostatic pressures up to 200–250 MPa, which fully encompasses the PEEK processing window.

Testing Samples

Neat PEEK Resin

Unfilled PEEK in pellet, granule, or powder form is the baseline sample type. Both medium-viscosity general-purpose grades and high-viscosity grades can be tested to build comparative pvT databases. A sample mass of several grams is normally sufficient to fill the measuring chamber of the piston dilatometer.

Filled and Modified PEEK Compounds

Glass-fiber-reinforced PEEK (typically 30% glass fiber), carbon-fiber-reinforced PEEK (typically 30% carbon fiber), and bearing-grade compounds containing PTFE, graphite, and carbon fillers can also be characterized. Fillers significantly alter the absolute specific volume and the magnitude of the crystallization jump, so compound-specific pvT curves should always be measured rather than estimated from neat resin data.

Sample Preparation Requirements

• Dry the sample before testing, for example 3 hours at 150°C or overnight at 120°C, until the moisture content is below approximately 0.02%, because residual moisture causes voids and erroneous volume readings.
• Ensure the sample is clean, free of contamination, and representative of the production lot.
• Record the grade designation, lot number, filler content, and color, since pigments and additives measurably shift pvT behavior.
• Pre-weigh the sample with an analytical balance, because specific volume is derived from the measured volume divided by the sample mass.

Testing Items

The PEEK melt specific volume determination experiment delivers the following results:

• Specific volume as a function of temperature at constant applied pressures (isobaric heating and cooling curves).
• Specific volume as a function of pressure at constant temperatures (isothermal compression curves).
• Melt density at specified processing temperatures and pressures, for example 380°C at 80 MPa.
• Volumetric thermal expansion coefficient of the melt.
• Isothermal compressibility of the melt.
• Pressure-dependent transition temperatures, including the melting temperature during heating and the crystallization or solidification temperature during cooling at each pressure level.
• The crystallization-induced specific volume jump on cooling, which directly relates to volumetric shrinkage.
• Fitted Tait equation parameters and pvT coefficients formatted for direct import into injection molding simulation software.

Testing Methods

Piston-Die Dilatometry (ISO 17744)

The standard method confines the dried PEEK sample in a rigid measuring chamber sealed by a movable piston. A hydraulic or mechanical system applies a defined hydrostatic pressure through the piston, and the piston displacement is measured continuously with a displacement transducer while the chamber temperature is controlled by surrounding heaters and a cooling unit. The change in piston position is converted to a change in sample volume, corrected for the thermal expansion and elastic deformation of the apparatus itself, which is determined by a blank calibration run and by reference measurements on standard materials such as mercury or certified reference polymers.

Measurement Modes

Two complementary modes are normally executed. In the isobaric mode, the sample is heated above the melting point, pressurized to a set level (commonly 20, 40, 60, 80, 100, 120, 150, and 200 MPa), and then cooled at a controlled rate, often 5 K/min, while specific volume is recorded continuously. This mode captures the melting and crystallization transitions under pressure. In the isothermal mode, the sample is held at a fixed temperature, for example 360, 380, or 400°C, while the pressure is stepped up and down, capturing melt compressibility and any residual hysteresis.

Typical Test Procedure

• Calibrate the apparatus: determine load-frame compliance, chamber thermal expansion, piston seal friction, and temperature sensors using certified references.
• Load the dried, pre-weighed PEEK sample into the chamber, close the piston, and apply a small pre-load to exclude trapped air.
• Heat the chamber to a temperature 20–40 K above the PEEK melting point, typically 380–400°C, and hold to ensure complete melting and thermal equilibration.
• Apply the programmed pressure profile and run the isobaric cooling or isothermal pressure sequences.
• Record piston displacement and temperature continuously; subtract the blank correction and compute specific volume as chamber volume divided by sample mass.
• Repeat at each pressure level and verify repeatability between successive runs.

Data Processing and Tait Equation Fitting

The corrected specific volume data are plotted as pvT curves and fitted with the two-domain Tait equation, in which the specific volume is described by a zero-pressure volume function and a pressure term with a temperature-dependent parameter B(T). The fitted coefficients allow simulation software to interpolate specific volume at any point within the measured temperature and pressure envelope, typically with a repeatability of about 0.001 to 0.003 cm³/g. The cooling-rate dependence of the crystallization jump should be documented, because the low cooling rates used in pvT instruments differ from the rapid cooling in real molding, and appropriate adjustments may be needed for shrinkage analysis.

Applications

• Injection molding simulation: pvT curves are a mandatory material input for CAE tools such as Moldflow, Moldex3D, Cadmould, and 3D TIMON when predicting filling, packing, and volumetric shrinkage of PEEK parts.
• Shrinkage and warpage control: the specific volume jump at crystallization under packing pressure directly determines the volumetric shrinkage of semi-crystalline PEEK moldings.
• Process optimization: melt density and compressibility data are used to size holding pressure profiles, calculate shot volume, and compensate for melt compressibility in the packing phase.
• Extrusion and melt pumping calculations: true melt density at process temperature enables accurate output, residence time, and die-swell estimates.
• Grade selection and qualification: comparing pvT diagrams of unfilled, glass-filled, carbon-filled, and bearing-grade PEEK supports material substitution decisions and incoming batch consistency checks.
• Crystallization and solidification studies: pressure-shifted transition temperatures inform mold temperature and cooling strategies for achieving the target crystallinity in thick-section or high-precision parts.

FAQ

Question: Why is melt specific volume data specifically important for PEEK?

Answer: PEEK is processed at high melt temperatures and high packing pressures, and its semi-crystalline structure produces a large specific volume change upon crystallization. Without measured pvT data, shrinkage and warpage predictions for precision PEEK components in aerospace and medical applications can deviate substantially from reality.

Question: What temperature and pressure ranges are used in the experiment?

Answer: Typical conditions span room temperature to approximately 400–420°C to cover the full melt window, and hydrostatic pressures from near zero to 200 MPa, with common set points at 20, 60, 100, 150, and 200 MPa to bracket real packing pressures.

Question: What is the difference between isobaric and isothermal measurement modes?

Answer: Isobaric mode holds pressure constant while sweeping temperature, revealing melting and crystallization transitions under pressure; isothermal mode holds temperature constant while stepping pressure, revealing melt compressibility. A complete characterization uses both.

Question: How does crystallization affect the results?

Answer: During cooling, PEEK crystallization releases a specific volume jump whose magnitude depends on achievable crystallinity, which in turn depends on cooling rate and pressure. The transition temperature also shifts upward as pressure increases, and this shift is itself a valuable output of the experiment.

Question: How should PEEK samples be prepared before testing?

Answer: Samples must be thoroughly dried, for example 3 hours at 150°C, weighed accurately, and loaded so that no air is trapped in the chamber, because moisture and voids are the main sources of measurement error.

Question: What accuracy can be expected?

Answer: With proper calibration and blank correction, a well-maintained piston dilatometer achieves repeatability of roughly 0.1–0.3% in specific volume, sufficient for reliable Tait equation fitting and simulation input for PEEK.