Determination of Foaming Coefficient: Testing Procedure, Standards, and Applications

The foaming coefficient, also known as the expansion ratio or foaming multiple, quantifies the volume increase a material undergoes during the foaming process and serves as a critical quality control parameter across the polymer, elastomer, and foam-liquid industries. Accurate determination of the foaming coefficient relies on standardized procedures such as ISO 845, ASTM D1622, ASTM D3575, and GB/T 6343, which govern specimen preparation, measurement, and calculation. This article provides a step-by-step technical guide to foaming coefficient determination, covering sample types, testing items, measurement methods, calculation principles, and typical industrial applications.

Technical Overview

The foaming coefficient (also referred to as foaming multiple, blow ratio, or expansion ratio) is a dimensionless parameter that characterizes the degree of expansion achieved during a foaming process. For liquid foaming systems, it is defined as the ratio of the volume of generated foam to the volume of the original foaming solution. For cellular solid materials, it is equivalently expressed as the ratio of the density of the unfoamed base polymer to the apparent density of the foamed product. A higher foaming coefficient indicates a lighter, more highly expanded cellular structure with lower material consumption per unit volume.

Determination of the foaming coefficient is essential for process optimization, formulation development, incoming material inspection, and product certification. Because the measurement approach differs fundamentally between liquid foaming agents and solid cellular materials, two distinct test pathways are applied: a volumetric method for foam liquids and a density-based method for foamed solids. Both pathways must control temperature, timing, and specimen geometry to ensure reproducible results.

Commonly referenced standards and frameworks include:

• ISO 845 and GB/T 6343 – determination of apparent density of cellular plastics and rubbers
• ASTM D1622 – apparent density of rigid cellular plastics
• ASTM D3575 – flexible cellular materials made from olefin polymers, including density-based expansion evaluation
• ISO 7203 series and EN 1568 series – expansion ratio and drainage of low-, medium-, and high-expansion firefighting foams
• GB/T 10799 and related GB/T methods for physical property determination of rigid cellular plastics

Testing Samples

Liquid Foaming Agent Concentrates

This category includes firefighting foam concentrates, surfactant-based foaming agents, protein and synthetic foam liquids, foaming solutions for foam concrete production, and industrial cleaning or detergent foam systems. A representative sample of at least one liter should be supplied in a sealed container, protected from contamination and temperature extremes. Before testing, the concentrate is diluted with distilled water or standard hard water at the concentration specified by the formulation, typically 1%, 3%, or 6% by volume.

Thermoplastic Foam Sheets and Molded Parts

Typical samples include EVA, PE, PP, PVC, TPE, EPS, and XPS materials supplied as sheets, rolls, boards, midsole blanks, packaging inserts, or molded components. Specimens should be cut from the central region of the product, at least 10 mm away from molded edges and skins, because surface layers are denser than the core and will bias the result. Each specimen should have a regular geometry with a volume of at least 100 cubic centimeters where practical.

Rubber Sponge and Vulcanized Foam Compounds

This group covers EPDM, CR, SBR, NBR, and NBR/PVC sponge materials in the form of sealing profiles, gaskets, tubing, sheets, and molded foam pads. Vulcanized specimens should be fully cooled and stabilized after processing, and any release agents or surface coatings must be removed before measurement, as they may affect water-displacement volume determinations.

Specimen Conditioning and Quantity

All solid specimens should be conditioned at 23 ± 2 °C and 50 ± 5% relative humidity for at least 16 hours prior to testing, in accordance with ISO 291 or the equivalent conditioning clause of the governing standard. A minimum of three specimens, preferably five, should be prepared from each batch to characterize variability. Liquid foaming agents and their dilution water should be brought to the specified test temperature, typically 20 ± 1 °C, before foam generation.

Testing Items

The primary measurand is the foaming coefficient itself, expressed as a dimensionless ratio. In practice, determination of the foaming coefficient is performed together with a set of closely related items that describe foam quality and consistency:

• Foaming coefficient (expansion ratio / foaming multiple)
• Apparent (overall) density of the cellular material, in kg/m3 or g/cm3
• Foam stability, expressed as the 25% drainage time and 50% drainage time of the liquid phase
• Foam half-life for wet foam systems used in concrete and tunneling applications
• Post-foaming shrinkage rate, which verifies that the measured coefficient is stable after cooling
• Supporting characterization such as cell uniformity and skin integrity, which explains deviations in the coefficient

Testing Methods

Method A: Volumetric Method for Liquid Foaming Systems

This method directly compares the volume of generated foam against the volume of the foaming solution and is applied per the principles of ISO 7203 and EN 1568. The procedure is as follows:

• Step 1: Bring the foaming agent concentrate, dilution water, glassware, and foam generation apparatus to the specified test temperature, normally 20 ± 1 °C, and record the ambient conditions.
• Step 2: Prepare the test solution at the nominal use concentration using distilled water or defined-standard hard water, and mix gently without entraining air.
• Step 3: Transfer a precisely measured volume of solution, for example 100 mL, into the foam generation vessel, which may be a standardized stirring apparatus, blender-type generator, or column assembly.
• Step 4: Generate foam under standardized and repeatable conditions of stirring speed, agitation time, or air flow rate, exactly as prescribed by the selected method, since mixing energy strongly influences the resulting foam volume.
• Step 5: Immediately transfer the generated foam into a calibrated cylinder, allow the stated settling period of typically 10 to 30 seconds, and read the total foam volume.
• Step 6: Calculate the foaming coefficient as the foam volume divided by the initial solution volume, and repeat the complete generation sequence at least three times, reporting the mean value.
• Step 7: For stability assessment, continue observation and record the time required for 25% and 50% of the original liquid volume to drain from the foam column.

Method B: Density-Based Method for Cellular Solid Materials

This method derives the foaming coefficient from apparent density measurements performed per ISO 845, GB/T 6343, ASTM D1622, or ASTM D3575. The procedure is as follows:

• Step 1: Condition the specimens to equilibrium at 23 ± 2 °C and 50 ± 5% relative humidity, and confirm that the material is fully cooled and dimensionally stable after molding or extrusion.
• Step 2: Weigh each specimen on a balance with a resolution of 0.01 g and record the mass, checking that no dust, moisture, or surface coating contaminates the specimen.
• Step 3: Determine the specimen volume. For regular shapes, measure length, width, and thickness at multiple points with calipers and compute the geometric volume. For irregular shapes, use the water-displacement technique with a thin waterproof surface film applied to prevent water ingress into open cells.
• Step 4: Calculate the apparent density by dividing the specimen mass by its measured volume, expressing the result in g/cm3 or kg/m3.
• Step 5: Establish the density of the unfoamed base polymer, either by molding an unblown control plaque of the same formulation or by using the documented resin density, for example approximately 0.94 g/cm3 for EVA, 0.92 g/cm3 for LDPE, and 1.05 g/cm3 for PS.
• Step 6: Calculate the foaming coefficient as the ratio of the unfoamed base density to the measured apparent foam density. For example, an EVA foam with an apparent density of 0.24 g/cm3 against a base density of 0.94 g/cm3 yields a foaming coefficient of approximately 3.9.
• Step 7: Test a minimum of three specimens, report individual and mean values, and note any outliers caused by cell collapse, coarse cell structure, or dense skin layers.

Calculation and Reporting

Regardless of the pathway, the test report should state the standard applied, specimen description and conditioning history, test temperature and humidity, individual measured values, the mean foaming coefficient, and any deviations from the standard procedure. For liquid systems, the drainage times should accompany the coefficient, while for solid foams the measured apparent density and the reference base density must both be documented so the ratio can be independently verified.

Applications

Foaming coefficient determination is applied throughout industries where cellular structure determines performance, cost, and regulatory compliance:

• Footwear: EVA and PU midsole development, where coefficients typically range from 2 to 5 to balance cushioning, weight, and mechanical strength.
• Thermal insulation: PU, PIR, XPS, and PE foam boards, where higher coefficients reduce thermal conductivity and material cost while maintaining compressive performance.
• Sealing and construction: EPDM and NBR/PVC sponge gaskets and expansion joint profiles, where consistent expansion ensures compression set and recovery behavior.
• Fire protection: certification testing of firefighting foam concentrates, classifying products as low expansion (up to about 20), medium expansion (20 to 200), or high expansion (above 200) per ISO 7203 and EN 1568.
• Packaging: EPS and PE protective foams, where density-derived coefficients confirm drop protection performance at minimum weight.
• Automotive and cable: foamed TPE, PVC, and polyolefin layers that must meet tight density windows for weight reduction and dielectric control.
• Foamed concrete: quality control of foaming agents used to produce lightweight backfill and thermal blocks, combining coefficient data with foam half-life.

FAQ

Q: What is the difference between the foaming coefficient and the expansion ratio?

A: The two terms are used interchangeably in most contexts. Both express the volume increase relative to the unfoamed state, whether that state is a liquid solution or a solid polymer. Terminology preference usually follows the industry, with foaming coefficient common in polymer processing and expansion ratio dominant in firefighting foam standards.

Q: What are typical foaming coefficient values for common materials?

A: EVA footwear midsoles generally fall between 2 and 4, packaging EPS between 20 and 40, structural insulation foams between 10 and 40, and firefighting foams from below 20 for low-expansion products to well above 200 for high-expansion systems. The target value always reflects the balance between lightness and mechanical requirements.

Q: Which factors most strongly affect measurement accuracy?

A: For liquid systems, the dominant factors are water temperature and hardness, mixing energy, the settling time before reading the foam volume, and ambient humidity. For solid foams, the key factors are complete removal of dense skins, accurate volume determination, correct base polymer density, and full dimensional stabilization after molding.

Q: How many specimens or replicate runs are required?

A: A minimum of three specimens or three independent foam generation runs is standard, with five preferred for certification or statistical purposes. Results should be reported as individual values together with the mean, and any specimen showing cell collapse or abnormal drainage should be investigated rather than simply discarded.

Q: How long does a typical determination take?

A: For solid foams, once specimens are conditioned, the measurement and calculation can be completed within one hour. For liquid foaming agents, including drainage observation, testing typically requires one to two hours. Accredited third-party laboratories can perform these determinations under ISO, ASTM, EN, or GB frameworks and issue reports suitable for supplier qualification and product certification.