Closed-Cell Content Testing of Foam Plastics: Standards, Methods, and Applications

Closed-cell content is one of the most critical quality indicators for rigid foam plastics such as polyurethane (PU/PIR) and extruded polystyrene (XPS), directly governing thermal conductivity, water absorption, compressive strength, and long-term insulation performance. This article presents a professional technical overview of closed-cell content testing of cellular plastics in accordance with ISO 4590, ASTM D6226, and GB/T 10799, covering test principles, specimen preparation, procedures, calculations, and typical industrial applications.

Technical Overview

Cellular plastics (foam plastics) are two-phase materials consisting of a polymer matrix surrounding a gaseous phase distributed in the form of cells. Depending on the integrity of the cell walls, these cells are classified as closed cells (isolated, gas-tight pockets) or open cells (interconnected voids permeable to gas or liquid). The closed-cell content is defined as the volume percentage of cells that are not accessible to an external probing fluid, and it is one of the most important structural parameters of rigid foams.

A high closed-cell content provides several decisive performance benefits:

• Lower initial thermal conductivity, because closed cells retain low-conductivity blowing agents such as HFCs, HCFCs, HFOs, or CO₂ for extended periods.
• Reduced water absorption and water vapor permeability, which is essential for below-grade insulation and marine applications.
• Higher compressive strength, creep resistance, and dimensional stability.
• Improved buoyancy characteristics for flotation products.
• Slower thermal aging, since gas diffusion and air ingress are largely suppressed.

The determination of closed-cell content is standardized internationally in ISO 4590 (Rigid cellular plastics — Determination of percentage of open and closed cells), ASTM D6226 (Standard Test Method for Open Cell Content of Rigid Cellular Plastics), and GB/T 10799 (the corresponding Chinese national standard). All of these methods are based on comparing the geometric volume of a specimen with the volume measured by gas pycnometry, followed by a correction procedure to account for cells opened at cut surfaces.

Testing Samples

Rigid Polyurethane (PU) and Polyisocyanurate (PIR) Foam

Used extensively in insulation sandwich panels, spray foam, pipe insulation, and refrigerator cabinets. Product specifications and application standards for building insulation typically require a closed-cell content of at least 90%. PIR foams for fire-rated panels are also evaluated with this test.

Extruded Polystyrene (XPS) Foam

XPS boards for wall, roof, and floor insulation are produced with a fully closed-cell structure. Product standards for XPS (such as GB/T 10801.2 and EN 13164-related specifications) commonly specify a minimum closed-cell content of 90%.

Expanded Polystyrene (EPS) Foam

Although EPS contains a majority of closed cells, its bead fusion boundaries introduce a measurable fraction of interstitial voids, making the test useful for evaluating molding quality and density uniformity.

Phenolic Foam

High fire-performance insulation foam, where the closed-cell structure affects thermal conductivity, friability, and moisture behavior.

Polyolefin and Other Closed-Cell Foams

Cross-linked polyethylene (PE-X), ethyl-vinyl acetate (EVA), and PVC foams used in HVAC insulation, sports equipment, sealing, and cushioning are also characterized by closed-cell content.

Specimen Requirements

Specimens are normally cut from the core of the material, free of molded skins and surface densification. Typical specimens are cubes or rectangular prisms in the range of about 20 mm to 50 mm per side, with larger specimens preferred for coarser cell structures. At least three (preferably five) replicate specimens should be tested, and all specimens must be conditioned to equilibrium, for example at 23 ± 2 °C and 50 ± 5% relative humidity, before measurement. Cut surfaces must be smooth and free of crushed or smeared cells, since surface damage directly distorts the result.

Testing Items

The principal measurands reported in a closed-cell content test are:

• Open cell content (% by volume), uncorrected.
• Closed cell content (% by volume), uncorrected.
• Corrected open cell content and corrected closed cell content, accounting for cells opened by the cutting of the specimen.
• Supporting data such as specimen dimensions, geometric volume, pycnometric volume, and apparent core density, which are frequently reported together for traceability.

Testing Methods

Test Principle

The method is based on gas pycnometry and Boyle's law of gas expansion. The geometric volume (Vg) of the specimen is obtained from its external dimensions. The pycnometric volume (Vp) is determined by introducing a non-reacting gas (typically nitrogen, air, or helium) into a calibrated chamber and measuring pressure changes. Gas penetrates only into cells that communicate with the exterior, so Vp is smaller than Vg whenever open cells are present. The difference between the two volumes corresponds to the open cell volume.

Measurement of Geometric Volume

Specimen dimensions are measured with a calibrated caliper or micrometer at multiple positions, and the geometric volume is calculated from the mean dimensions. The specimen mass is recorded on an analytical balance so that density can be reported.

Gas Pycnometry Procedure (ISO 4590 and ASTM D6226)

The pycnometer is calibrated using a reference object of known volume, such as a calibrated stainless-steel sphere. The specimen is then placed in the sample chamber, and the gas expansion measurement is performed. ISO 4590 offers two equivalent variants: the pressure variation method, in which the chamber pressure change after expansion is evaluated, and the volume expansion method, which determines the volume directly from the expansion element. ASTM D6226 follows the same fundamental pycnometric principle with its own calibration and calculation sequence.

Cutting Correction Procedure

Cutting a specimen from a foam block inevitably opens cells at the newly created surfaces, artificially inflating the apparent open cell content. To correct this artifact, the specimen is subdivided into smaller pieces (for example by trisecting the specimen as described in ISO 4590, or cutting into eight smaller cubes as described in ASTM D6226), which increases the surface-to-volume ratio and exposes more cut cells. The open cell content is then remeasured, and the results for different surface-to-volume ratios are extrapolated back to a zero cutting effect, yielding the true closed-cell content of the undisturbed material.

Calculation

• Open cell content (%) = (Vg − Vp) / Vg × 100, where Vg is the geometric volume and Vp is the pycnometric volume.
• Closed cell content (%) = 100 − open cell content, applied to the corrected values.
• The correction term is derived from the change in measured open cell content before and after subdivision of the specimen, extrapolated to zero additional surface area.

Equipment and Quality Control

Typical equipment includes a gas pycnometer with calibrated reference volumes, an analytical balance, precision dimensional measurement tools, fine-toothed or hot-wire cutting tools, and a conditioning chamber. Instrument calibration, chamber leak checks, and the use of reference specimens are essential to achieve the repeatability expected from the standards, which is generally on the order of 1% (by volume) for closed-cell-rich materials.

Applications

Closed-cell content testing supports a wide range of engineering and commercial purposes:

• Product certification and compliance demonstration for building insulation materials such as XPS, PU, and PIR boards against national and international product standards.
• Factory quality control to verify foaming recipes, blowing agent performance, nucleation quality, and process stability.
• Research and development of new blowing agents, catalyst systems, and foam formulations, including studies of thermal aging and gas diffusion behavior.
• Assessment of water absorption resistance and buoyancy for marine flotation, dock, and lifesaving products.
• Verification of insulation performance for refrigerated transport, cold storage, LNG carrier insulation systems, and district heating pipe networks.
• Technical documentation for automotive, aerospace, packaging, and sports equipment foam components.
• Independent verification by accredited third-party laboratories operating under ISO/IEC 17025 quality systems, providing test reports for buyers, specifiers, and regulatory bodies.

FAQ

What is the difference between open-cell and closed-cell foam?

In closed-cell foam, the majority of cells are enclosed by intact cell walls so that gas cannot pass between cells or into the foam from outside. In open-cell foam, the cell walls are ruptured and interconnected, allowing free gas movement. Closed-cell foams generally offer superior insulation, moisture resistance, and mechanical strength, while open-cell foams are softer, more absorbing, and better suited for acoustic and cushioning applications.

Why is a cutting correction necessary?

When a specimen is cut from a foam block, cells at the cut faces are opened by the cutting action itself. Without correction, this would overstate the true open cell content. The subdivision and extrapolation procedure quantifies and removes this artifact so that the reported value reflects the original, undisturbed cellular structure.

What are the key differences between ISO 4590 and ASTM D6226?

Both standards use gas pycnometry and a cutting correction. ISO 4590 provides two instrumental variants (pressure variation and volume expansion) and a specific trisection correction scheme, while ASTM D6226 prescribes its own calibration, measurement, and correction sequence. For most rigid foams, the two standards produce comparable results, but the standard used must always be stated in the test report because calculation details differ.

What closed-cell content values are typically required?

For XPS and rigid PU/PIR insulation products, specifications commonly require a closed-cell content of at least 90%. High-performance spray polyurethane foams are typically required to reach 90% or higher, while flexible polyurethane comfort foams are predominantly open-cell and are not usually evaluated by these rigid-foam standards.

How many specimens are needed and how long does the test take?

A minimum of three to five replicate specimens is recommended to characterize variability within the material. Including conditioning, dimensional measurement, pycnometric determination, and the cutting correction, a complete test sequence is typically completed within one working day in a properly equipped laboratory.

Which factors can distort the test result?

Crushed or smeared cut surfaces, insufficient conditioning after cutting (residual gas pressure within freshly exposed cells), instrument leaks, calibration errors, and non-representative sampling near skins or flow lines are the most common sources of error. Careful specimen preparation and strict adherence to the standard procedures minimize these effects.

Can the test be performed on flexible foams?

The ISO 4590, ASTM D6226, and GB/T 10799 methods are designed for rigid cellular plastics. For flexible foams with very high open-cell content, the measurement loses discrimination, and other characteristics such as airflow permeability are more appropriate for classification.