Polyurethane foam sealants are among the most widely used gap-filling, sealing and insulating materials in modern construction, and their expansion coefficient (foaming ratio) directly determines foam yield, joint-filling performance and application safety. This technical article explains how the foaming coefficient of one-component polyurethane foam sealant is defined, which samples and test items are involved, and how the parameter is evaluated by accredited third-party laboratories against ISO, EN, ASTM and Chinese JC/GB standards.
Technical Overview
Polyurethane foam sealant (commonly known as OCF, one-component foam) is a moisture-curing, pre-pressurized aerosol system that expands after dispensing to fill joints, cavities and penetrations. The expansion coefficient, also called the foaming coefficient or foaming ratio, is the ratio between the volume of cured foam obtained under free-rise conditions and the volume of liquid precursor dispensed. In industrial practice it is frequently expressed as foam yield in liters per can or liters per kilogram of net content.
The expansion coefficient is a decisive quality and economic indicator. A higher coefficient means greater yield per can and lower material cost per meter of joint, but excessive free rise can generate pressure on door and window frames, cause deformation, and increase secondary trimming waste. A coefficient that is too low indicates insufficient blowing agent, aged formulation or low-temperature application problems, resulting in under-filled joints, reduced thermal insulation and poor mechanical performance.
- Typical expansion coefficient of gun-grade OCF: approximately 40 to 60 times the liquid volume, depending on formulation and test conditions.
- Typical yield of a 750 mL professional can: approximately 30 to 45 liters of free-rise cured foam.
- Typical free-rise density: 12 to 25 kg/m³; in-place (filled joint) density is usually 20 to 35 kg/m³.
- Relevant reference standards: JC/T 936-2004 for one-component polyurethane foam sealant, ISO 8873 and EN 14315-1 for in-situ formed rigid polyurethane foam, and ISO 845 / ASTM D1622 for apparent density determination.
Testing Samples
Sealed Aerosol Cans of One-Component Foam Sealant
The primary test sample is the full, unopened pressurized can in its commercial packaging, typically 500 mL, 650 mL or 750 mL net volume. Both gun-type (valve adapter for professional applicator guns) and straw-type (hand-held adapter) products are tested, because the dispensing device significantly influences flow rate, bead control and post-expansion behavior. Samples must be within shelf life and stored under manufacturer-recommended conditions before testing.
Cured Free-Rise Foam Blocks
To characterize the expanded material itself, free-rise foam buns or blocks are produced by dispensing the foam onto release paper and allowing complete cure for at least 24 hours. From these blocks, cubic specimens of at least 100 cm³ are cut for density, closed-cell content, compression and dimensional stability testing.
In-Place Joint Mock-Ups
Simulated application samples, such as foam filled into standardized rectangular joints between substrate panels (concrete, PVC, aluminum or timber), are used to determine the in-place density, the overfill ratio, the secondary expansion behavior and the practical filling capacity per meter of joint.
Testing Items
Accredited third-party laboratories evaluate the foaming behavior of polyurethane foam sealant through a coordinated set of physical, mechanical and thermal parameters, of which the expansion coefficient is the core item.
- Expansion coefficient (foaming ratio) and foam yield in L/can or L/kg.
- Free-rise density and in-place density of cured foam.
- Primary expansion and secondary (post-foaming) expansion ratio.
- Tack-free time, cutting time and full cure time.
- Closed-cell content of the rigid cellular structure.
- Dimensional stability after heat, cold and humidity exposure.
- Compression strength at 10% deformation.
- Adhesion to typical substrates and cohesive failure mode.
- Thermal conductivity of the cured insulation foam.
- Temperature sensitivity of the foaming coefficient at low and high ambient temperatures.
Testing Methods
Sample Conditioning
Cans and substrates are conditioned at 23 ± 2 °C and 50 ± 5% relative humidity in accordance with ISO 291 (or ASTM D618 for alternative climate conditioning). Before dispensing, each can is shaken vigorously at least 20 times to homogenize the pre-polymer and liquefied blowing agent, and the can temperature is verified because pressure and gas solubility are strongly temperature dependent.
Determination of the Expansion Coefficient
The liquid precursor is dispensed in a single continuous bead onto a horizontal release surface, with the initial and final portions discarded. The free-rise foam is allowed to expand and cure for 24 hours, after which the cured volume V1 is determined by geometric measurement or by calibrated volume displacement. The coefficient K is calculated as K = V1 / V0, where V0 is the volume of liquid dispensed, derived from the mass loss of the can divided by the liquid density (approximately 1.1 to 1.2 g/cm³). The result is reported together with ambient temperature, relative humidity, can temperature and dispensing orientation, and the test is repeated at low temperature (approximately 5 °C) and elevated temperature (approximately 35 °C) to establish the temperature dependence curve.
Secondary Expansion and Yield Verification
Bead height and width are recorded at defined intervals (typically 2, 5, 10, 30 and 60 minutes after dispensing) to quantify primary rise during extrusion and secondary post-expansion thereafter. Total yield per can is verified by completely emptying the can in controlled passes and summing the cured foam volumes, which reflects realistic consumption for estimation of on-site coverage.
Supporting Physical and Mechanical Methods
- Apparent density of rigid cellular foam: ISO 845 or ASTM D1622.
- Closed-cell content by pycnometer method: ISO 4590 or ASTM D6226.
- Compression behavior at 10% deformation: ISO 844 or ASTM D1621.
- Dimensional stability under defined temperature and humidity: ISO 2796, ASTM D2126 or EN 1604.
- Thermal conductivity by heat flow meter: ISO 8301 or ASTM C518.
- Reaction-to-fire classification of the cured foam: EN 13501-1 or GB 8624, where project specifications require it.
Applications
Expansion coefficient data is used across the entire value chain of foam sealant products, from formulation development and quality release to construction planning and acceptance inspection. Reliable coefficient values allow applicators to select the correct can size, plan joint filling sequences and avoid frame deformation caused by over-expansion.
- Installation joints of door and window frames in masonry and concrete walls.
- Sealing of curtain wall joints, façade connections and expansion gaps.
- Filling of pipe, cable and duct penetrations through walls and floors.
- Cavity insulation, sound damping and draught-proofing in renovation works.
- Industrial assembly, HVAC installation and prefabricated component manufacturing.
- Procurement specification and incoming inspection of foam sealants by contractors and distributors.
FAQ
What is the expansion coefficient of a foam sealant and how is it calculated?
It is the ratio of the cured free-rise foam volume to the volume of liquid precursor dispensed, calculated as K = V1 / V0. For gun-grade one-component polyurethane foam, values between about 40 and 60 are typical under standard laboratory conditions, corresponding to roughly 30 to 45 liters of cured foam from a 750 mL can.
What is the difference between primary and secondary expansion?
Primary expansion occurs immediately as the bead leaves the nozzle, driven by the vaporization of the liquefied blowing agent. Secondary expansion is the slower post-foaming that continues for several minutes to about one hour afterwards, and it is the main reason joints are filled to only one third to one half of their depth in a single pass.
How do temperature and humidity affect the foaming coefficient?
Low can and ambient temperatures reduce propellant pressure and slow the moisture-curing reaction, lowering the coefficient and yield. Low humidity retards curing and can produce collapsed cells, while very high humidity causes rapid skin formation and coarse cell structure. This is why laboratories report the coefficient together with the exact conditioning climate.
Which standards govern the testing?
The product is typically characterized against JC/T 936-2004 for one-component polyurethane foam sealant, with supporting methods from ISO 8873 and EN 14315-1 for in-situ rigid polyurethane foam, ISO 845 / ASTM D1622 for density, ISO 4590 / ASTM D6226 for cell structure and ISO 8301 / ASTM C518 for thermal conductivity.
Why does measured yield sometimes differ from the labeled value?
Labeled yields usually refer to free-rise volume under ideal laboratory conditions. On site, can temperature, shaking discipline, gun technique, joint geometry and the trimming of overfilled foam all reduce practical yield, so a measured expansion coefficient report is the most reliable basis for material take-off and cost estimation.