PEEK Insulation Film Tensile Strength Testing: Standards, Methods, and Technical Specifications

PEEK insulation film tensile strength testing is a critical quality control and material verification process for high-performance polymer films used in aerospace, electronics, and energy applications. This technical article explains the testing standards, specimen preparation, measurement parameters, and acceptance criteria for evaluating the tensile properties of polyetheretherketone (PEEK) insulation films according to ASTM D882 and ISO 527-3 methodologies.

Technical Overview

Polyetheretherketone (PEEK) is a semi-crystalline, high-performance engineering thermoplastic widely used as an insulation film in demanding electrical and electronic applications. PEEK films combine a continuous service temperature of approximately 240–260°C, a glass transition temperature near 143°C, and a melting point of approximately 343°C with outstanding dielectric properties, chemical inertness, flame resistance, and mechanical durability. These characteristics make PEEK film a preferred alternative to polyimide (PI) and polyethylene terephthalate (PET) films in harsh environments.

Tensile strength testing is one of the most fundamental mechanical evaluations performed on PEEK insulation film. It quantifies the film's ability to withstand uniaxial stretching forces before failure, which directly reflects the film's processability during lamination, slitting, winding, and its long-term reliability in service. Because PEEK film behavior is strongly influenced by crystallinity, molecular orientation, and thickness, standardized tensile testing provides an objective basis for material qualification, batch-to-batch consistency verification, and comparison against design requirements.

• Typical tensile strength of unfilled PEEK film: approximately 90–140 MPa depending on orientation and crystallinity
• Typical tensile modulus: approximately 2.0–4.0 GPa
• Typical elongation at break: approximately 20% to 150% depending on film grade and heat treatment
• Applicable standards: ASTM D882, ISO 527-3, GB/T 1040.3

Testing Samples

Material Grades and Forms

Testing samples may include amorphous PEEK film (produced by melt extrusion followed by rapid quenching), semi-crystalline film (annealed or slowly cooled), and biaxially oriented PEEK film. Each form exhibits distinct tensile behavior: amorphous film typically shows higher elongation and lower modulus, while crystallized or oriented film shows higher strength and modulus with reduced ductility. The film thickness commonly evaluated ranges from 5 μm to 250 μm, and the nominal thickness must be measured at multiple points before testing because tensile strength is calculated using the cross-sectional area.

Specimen Geometry and Preparation

For thin plastic sheeting, rectangular strip specimens are used rather than dumbbell shapes. According to ASTM D882, specimens are cut as strips with a width of 6 mm to 25.4 mm and a length of at least 250 mm, ensuring the length is sufficient to accommodate the gauge length plus grip sections. ISO 527-3 recommends rectangular strips of 10 mm to 25 mm width with 100 mm initial distance between grips for preferred specimen dimensions. Specimens must be cut cleanly with a sharp die or precision cutter; any nicks, burrs, or edge defects along the cut edges can initiate premature failure and invalidate results. Specimens containing visible defects should be discarded.

Conditioning Prior to Testing

PEEK film has low moisture absorption, but conditioning is still required for reproducibility. Specimens should be conditioned at 23 ± 2°C and 50 ± 5% relative humidity for a minimum of 24 hours in accordance with ISO 291 or ASTM D618 (Procedure A). Conditioning ensures that thermal history and ambient moisture are normalized across all tested samples.

Testing Items

A complete tensile evaluation of PEEK insulation film typically includes the following measured and calculated parameters:

• Tensile strength (ultimate tensile strength) — the maximum force sustained by the specimen divided by the original cross-sectional area, expressed in MPa
• Tensile stress at yield — the stress at which the stress-strain curve exhibits a zero slope or a specified deviation from proportionality
• Tensile stress at break — the stress recorded at the moment of specimen rupture
• Tensile modulus (Young's modulus) — the slope of the initial linear portion of the stress-strain curve, typically determined between strain limits of 0.05% and 0.25% using an extensometer
• Elongation at yield — the strain corresponding to the yield point
• Elongation at break — the strain at rupture, expressed as a percentage of the initial gauge length
• Anisotropy ratio — the ratio of machine direction (MD) to transverse direction (TD) strength, important for oriented films

For insulation film applications, tensile strength and elongation at break are the primary acceptance parameters, while modulus is essential for downstream processing simulations such as thermoforming and laminating.

Testing Methods

Applicable Standards

The governing standards for PEEK film tensile testing are ASTM D882 (Standard Test Method for Tensile Properties of Thin Plastic Sheeting), which applies to films less than 1.0 mm thick, and ISO 527-3 (Plastics — Determination of tensile properties — Part 3: Test conditions for films and sheets). The Chinese national standard GB/T 1040.3 is technically equivalent to ISO 527-3 and is frequently referenced for domestic qualification. Test reports must state which standard was applied because specimen dimensions, gauge length, and crosshead speeds differ between methods.

Equipment Requirements

Testing is performed on a calibrated universal testing machine (UTM) equipped with pneumatic or mechanical grips, preferably with rubber-faced or serrated inserts to prevent slippage of the smooth film surface. For thin films, slippage and premature grip failure are the most common error sources. The load cell should be selected so that the measured forces fall within 20% to 90% of its calibrated range, and a non-contact video extensometer or long-travel clip-on extensometer is required for accurate modulus and strain measurement. Grip separation or crosshead displacement may be used for elongation at break only when extensometer use is impractical.

Test Procedure

Specimens are mounted in the grips with an initial gauge length of 100 mm (ISO 527-3) or 50–250 mm depending on elongation expected (ASTM D882). The recommended crosshead speeds are as follows: 100 mm/min or 500 mm/min for modulus determination per ASTM D882 preferred parameters, 25 mm/min for ISO 527-3 preferred specimen conditions, and a lower speed of 1–25 mm/min when modulus is measured with an extensometer. A minimum of five valid specimens per direction (MD and TD) must be tested, with the median or mean value reported according to the standard. Testing is conducted at 23 ± 2°C and 50 ± 5% relative humidity. Specimens that fail within the grip region are excluded and replacement specimens are tested.

Data Calculation

Tensile strength is calculated by dividing the maximum force by the original minimum cross-sectional area (average thickness multiplied by width). Thickness should be measured at a minimum of five positions along the gauge length using a dead-weight micrometer or equivalent instrument with 1 μm resolution. Elongation values are calculated from the ratio of extensometer displacement to initial gauge length, and modulus is determined by regression of the linear segment of the stress-strain curve between the specified strain limits. Results are reported with statistical treatment (mean, standard deviation, and coefficient of variation) to demonstrate measurement reliability.

Factors Affecting Results

• Crystallinity: annealed or slowly cooled film shows higher modulus and strength but lower elongation than quenched amorphous film
• Molecular orientation: biaxially oriented film exhibits significant MD/TD anisotropy
• Thickness variation: non-uniform thickness concentrates stress and reduces measured strength
• Strain rate: higher crosshead speeds generally increase measured strength and modulus
• Temperature: elevated temperature testing (e.g., 150–260°C) dramatically reduces strength and must be controlled precisely when required
• Edge quality: die-cut edge defects cause stress concentration and premature brittle failure

Applications

Tensile strength test data for PEEK insulation film directly support qualification and quality control across multiple high-reliability industries:

• Flexible printed circuits and coverlay films: PEEK film must survive lamination pressure and dynamic flexing; tensile and elongation data establish processing windows
• Aerospace wire and cable insulation: high tensile strength ensures abrasion resistance and mechanical integrity under vibration and thermal cycling
• Electric motor and generator insulation: slot liners and phase insulation require verified tensile strength for winding and insertion operations
• Lithium-ion battery components: PEEK film serves as barrier and insulation layers where mechanical puncture and stretch resistance are safety-critical
• Semiconductor and electronics manufacturing: carrier films and high-temperature release liners rely on dimensional stability under tensile load
• Medical devices: biocompatible PEEK films used in sensors and catheters require traceable mechanical test records for regulatory submission
• High-temperature pressure sensors and acoustic components: thin PEEK diaphragms are designed using verified modulus and strength values

In supplier qualification programs, tensile test results are typically combined with dielectric strength, dissipation factor, thermal aging, and melt flow index data to build a complete material property profile.

FAQ

Q: Which standard should be used for PEEK insulation film tensile testing — ASTM D882 or ISO 527-3?

A: Both are acceptable. ASTM D882 is the prevailing standard in North America for films below 1 mm thickness, while ISO 527-3 (and GB/T 1040.3) is common in Asia and Europe. Results between the standards are not directly interchangeable due to differences in specimen dimensions and test speeds, so the applicable standard should be agreed upon in the purchase specification.

Q: What tensile strength value is expected for PEEK insulation film?

A: Unfilled, semi-crystalline PEEK film typically exhibits tensile strength in the range of 90–140 MPa. Biaxially oriented grades can exceed this range, while amorphous quenched film shows lower strength but higher elongation. The specification value should always be confirmed with the film manufacturer for the specific grade and thickness.

Q: Why do PEEK film specimens sometimes break inside the grips?

A: PEEK film has a smooth, low-friction surface combined with high strength, which promotes slippage and grip-edge stress concentration. Pneumatic grips with rubber inserts, controlled clamping pressure, and adequate specimen length minimize this failure mode. Grip-region failures must be excluded and retested per the standard.

Q: Is machine direction versus transverse direction testing necessary?

A: Yes. Extruded and oriented PEEK films are anisotropic, and MD strength can differ substantially from TD strength. Both directions should be tested with at least five specimens each whenever the film will be loaded or processed in a known direction.

Q: Can tensile testing be performed at elevated temperature?

A: Yes. Environmental chambers allow testing from below room temperature up to 300°C or higher. Because PEEK strength decreases significantly above the glass transition temperature, elevated-temperature testing is essential for applications involving sustained thermal exposure, and reported conditions must include the soak time at temperature before loading.

Q: How many specimens are required for a valid test report?

A: ASTM D882 and ISO 527 both require a minimum of five specimens per condition. For statistical quality control and lot release testing, laboratories often test ten specimens per direction and report mean, standard deviation, and coefficient of variation to demonstrate process consistency.