Dry-Process Polyethylene Separator Expansion Testing: Principles, Methods, and Standards

Dry-process polyethylene separator expansion testing evaluates the dimensional stability of uniaxially oriented PE lithium-ion battery separators under thermal stress and electrolyte exposure. This technical article explains specimen preparation, test procedures, calculation formulas, and relevant standards such as GB/T 36363, UL 2591, and ASTM E831, providing battery manufacturers, separator producers, and materials engineers with a practical guide to expansion characterization and quality control.

Technical Overview

Dry-process polyethylene (PE) separators are manufactured by melt extrusion of high-molecular-weight polyethylene, annealing to develop a stacked lamellar structure, cold stretching to initiate slit-like pore formation, hot stretching to enlarge and fix the pore network, and thermal setting. The result is a uniaxially oriented microporous membrane with slit-shaped pores, high tensile strength in the machine direction (MD), and pronounced anisotropic dimensional behavior. Because the polymer chains are highly oriented, the separator tends to shrink along the MD and may expand or wrinkle along the transverse direction (TD) when exposed to elevated temperatures or swelling electrolyte solvents.

Expansion testing quantifies this dimensional instability. Excessive MD shrinkage during cell operation or abuse can retract the separator from the electrode edges, exposing bare anode and cathode surfaces and triggering internal short circuits. Conversely, uncontrolled TD expansion can cause waviness, wrinkling, and poor stacking uniformity in wound or stacked cells. Since PE separators also serve a shutdown safety function—pore closure begins near the PE melting range of approximately 130–135 °C—dimensional stability data at and below this temperature are directly linked to cell safety margins. Expansion testing is therefore a mandatory element of separator qualification, incoming inspection, and change management in lithium-ion battery production.

Testing Samples

Base (Uncoated) Dry-Process PE Separator Film

Single-layer microporous PE film, typically 12 to 25 µm in thickness with porosity between 35% and 50%, cut from production rolls. Samples are taken from defined positions across the web width (near edge, quarter, center) because orientation and shrinkage behavior can vary laterally due to process conditions.

Ceramic-Coated Dry-Process PE Separator

PE base film with an alumina, boehmite, or mixed-oxide coating, either single-side or double-side. Coatings constrain dimensional movement, so coated samples are tested in the as-delivered state to verify the restraining effect and coating adhesion after thermal exposure.

Multilayer and Laminated Composite Separators

PE/PET or PE/PP laminates, thermally bonded nonwoven composites, and adhesive-bonded constructions. Each layer responds differently to heat and solvent, and expansion testing reveals delamination, curling, or differential shrinkage at layer interfaces.

Die-Cut Shapes and Converted Samples

Separator sheets die-cut to cell dimensions, including folded, tab-wrapped, or edge-coated configurations. Testing converted parts verifies that slitting, folding, and coating operations have not degraded dimensional stability.

All specimens must be conditioned before testing, typically at 23 ± 2 °C and 50 ± 5% relative humidity for at least 12 hours in accordance with ISO 291 or ASTM D618, unless otherwise specified. Electrolyte-related testing requires handling in a dry room or inert-atmosphere glovebox with dew point below −40 °C to prevent electrolyte hydrolysis.

Testing Items

A complete expansion characterization program for dry-process PE separators generally includes the following items:

• Thermal dimensional stability in MD and TD at 90 °C, 105 °C, 120 °C, and 130 °C for defined hold times (for example 30 minutes, 60 minutes, or 2 hours).
• Thickness expansion or contraction after thermal exposure, measured per ASTM D374 or equivalent micrometer/optical methods.
• Electrolyte uptake and swelling: linear expansion in MD and TD, thickness swell rate, and weight uptake percentage after immersion in carbonate-based electrolyte (for example 1 M LiPF6 in EC/DMC or EC/EMC blends).
• Coefficient of linear thermal expansion (CLTE) from ambient temperature to 130 °C by thermomechanical analysis, per ASTM E831 or ISO 11359-2.
• Air permeability change (Gurley value) before and after thermal exposure, per ISO 5636-5 or ASTM D5729, to correlate dimensional change with pore structure evolution.
• Shutdown behavior screening: Gurley value or ionic resistance monitoring during a controlled temperature ramp to confirm pore closure occurs before unacceptable shrinkage.
• Visual assessment after exposure: wrinkling, curling, color change, coating cracking, delamination, and pinhole formation.

Testing Methods

Oven-Based Thermal Dimensional Stability Method

Rectangular specimens of at least 100 mm × 100 mm are cut with precisely known MD and TD orientation. Initial lengths L0(MD) and L0(TD) are measured with a calibrated ruler, optical comparator, or image analysis system. Two reference marks are scribed or printed along each axis to define the gauge length. Specimens are laid flat without constraint on PTFE-coated or parchment-covered trays, placed in a forced-convection oven at the target temperature with tolerance of ±2 °C, and held for the specified duration. After removal, specimens are conditioned at laboratory temperature for a defined stabilization period (typically 30 to 60 minutes) before final lengths L1 are measured in the same manner. The dimensional change rate is calculated as: Dimensional change (%) = (L1 − L0) / L0 × 100. A negative value indicates shrinkage and a positive value indicates expansion; both MD and TD results are reported independently. Tests near or above 120 °C should be performed in replicate because dry-process PE approaches its melting region and scatter increases.

Electrolyte Immersion Swelling Method

Specimens of defined dimensions are weighed (W0) and measured, then fully immersed in electrolyte within a glovebox at a controlled temperature such as 25 °C or 45 °C for periods of 30 minutes, 2 hours, 24 hours, or longer. After removal, surface electrolyte is blotted quickly and consistently with lint-free wipes, and the specimens are reweighed (W1) and remeasured for MD, TD, and thickness. Electrolyte uptake is calculated as: Uptake (%) = (W1 − W0) / W0 × 100. Linear and thickness expansion rates follow the same length-based formula. Because carbonate electrolytes are volatile and moisture sensitive, all operations must be completed rapidly under inert atmosphere, and replicate testing is strongly recommended.

Thermomechanical Analysis (TMA) Method

For research-grade characterization, small strip specimens are tested in a TMA instrument equipped with a low-load expansion probe. Heating at 5 °C/min from ambient to 130 °C under minimal load yields the coefficient of linear thermal expansion in each direction, following ASTM E831 or ISO 11359-2. TMA curves also identify the onset of shrinkage force relaxation and provide early indication of shutdown-relevant transitions. Care must be taken that clamping and probe load do not artificially constrain or stretch the 12–25 µm film.

Data Reduction and Reporting

A compliant test report identifies the material grade, coating structure, thickness, porosity, sampling position, specimen dimensions, conditioning state, test temperature and duration or immersion protocol, individual and mean dimensional change values in MD and TD, thickness change, mass uptake where applicable, appearance observations, and the equipment and calibration references used. Typical acceptance criteria for dry-process PE separators specify MD shrinkage limits within a few percent at 90 °C and progressively evaluated limits at 120–130 °C, with TD change, whether shrinkage or expansion, held to a tighter band; exact limits depend on cell design and are agreed between separator supplier and cell manufacturer.

Applications

Expansion testing data for dry-process PE separators support multiple stages of the lithium-ion battery value chain:

• Separator manufacturing quality control, including release testing of production lots and monitoring of stretching and annealing process windows.
• New product development, comparing base film, ceramic-coated, and laminated constructions under identical thermal and electrolyte protocols.
• Cell design validation, feeding dimensional allowances for winding tension, stack pressure, and separator oversize into mechanical design.
• Incoming material inspection by battery manufacturers, using simplified oven tests as rapid acceptance screens.
• Safety and regulatory programs, including abuse testing support aligned with IEC 62660-3 for EV cells and separator requirements covered by UL 2591 for battery cell separators, as well as GB/T 36363 for PE lithium-ion battery separators.
• Change management and failure analysis, comparing retained samples when raw materials, orientation ratios, or heat-setting temperatures are modified.

Accredited third-party laboratories routinely perform these tests for suppliers seeking customer qualification, export documentation, or independent verification, reporting results against ISO, IEC, ASTM, UL, and GB frameworks as applicable.

FAQ

Why does dry-process PE separator show larger MD shrinkage than wet-process PE separator?

The dry process produces a uniaxially oriented structure in which polymer chains are aligned almost entirely along the machine direction. When heated, oriented chains relax and contract strongly along MD. Wet-process films are typically biaxially stretched, distributing orientation between MD and TD and yielding lower and more balanced shrinkage.

What test temperature should be used for expansion testing?

Common conditions include 90 °C and 105 °C to simulate high-temperature operation, 120 °C to approach abuse conditions, and 130 °C to probe behavior near the PE shutdown range. The selected condition must match the cell safety requirement defined by the end user.

Is expansion in the transverse direction a defect?

Not necessarily, but uncontrolled TD expansion is undesirable. Moderate TD dimensional change is inherent to the oriented structure, yet excessive expansion causes wrinkling and poor alignment in wound or stacked cells. Specifications therefore set limits on absolute dimensional change in both directions.

How large should specimens be?

Square specimens of 100 mm × 100 mm are standard for oven testing because they provide sufficient gauge length for accurate percentage readings while remaining easy to handle and measure flat.

Do ceramic coatings affect expansion results?

Yes. Inorganic coatings with PVDF or other binders mechanically constrain the PE substrate, typically reducing MD shrinkage and TD expansion. However, excessive shrinkage of the base film can crack or spall the coating, so visual inspection after exposure is an essential part of the evaluation.

Can electrolyte immersion tests be performed outside a glovebox?

No. LiPF6-based electrolytes react with atmospheric moisture, generating acidic species that alter swelling behavior and corrode equipment. All immersion, blotting, and weighing steps must be executed in a dry room or inert glovebox with appropriately low dew point.