Compressive creep testing of cellular plastics quantifies the time-dependent deformation of foamed materials under sustained compressive loading, delivering essential data for the long-term design of thermal insulation boards, protective packaging, and load-bearing foam components. The determination is typically carried out by accredited third-party laboratories in accordance with ISO 7850, ASTM D3575, and the compressive creep annexes of EN 13163 and EN 13164. This article explains the test principle, sample requirements, measured parameters, procedures, and typical industrial applications of compressive creep determination for foamed plastics.
Technical Overview
Cellular plastics, also known as foamed plastics, are polymeric materials with a gas-filled cellular structure that gives them low density, low thermal conductivity, and high energy absorption. When such a foam is subjected to a constant compressive stress that is maintained over time, the deformation does not remain constant but increases progressively. This time-dependent deformation under sustained load is defined as compressive creep, and its reliable determination is critical for any application in which a foam must carry load for years or decades.
On the microstructural level, compressive creep of foamed plastics results from the viscoelastic flow of the polymer cell walls and edges, gradual cell buckling and collapse, and—in closed-cell materials—slow diffusion of the blowing gas out of the cells. These mechanisms interact, so the creep response depends strongly on polymer type, density, cell geometry, temperature, humidity, and the direction of loading relative to the foam rise or extrusion direction.
The determination of compressive creep is standardized in several widely referenced documents, including:
- ISO 7850 — Cellular plastics — Determination of compression creep, the general international method.
- ASTM D3575 — Standard test methods for flexible cellular materials made from olefin polymers, which includes a sustained-load creep test.
- EN 13163 and EN 13164 — Product standards for EPS and XPS thermal insulation boards, whose compressive creep annexes specify a long-term test with extrapolation to 50 years and a declared CC designation.
- ASTM D6817 — Specification for rigid cellular polystyrene geofoam, which controls long-term compressive creep limits for geotechnical fill applications.
- ISO 844 and GB/T 8813 — Determination of compression behaviour of rigid cellular plastics, used as complementary short-term input data.
Testing Samples
Rigid Cellular Plastics
Rigid foam materials are the most frequent subjects of compressive creep determination because they are used as load-bearing thermal insulation. Typical samples include expanded polystyrene (EPS), extruded polystyrene (XPS), rigid polyurethane (PUR) and polyisocyanurate (PIR) foams, and phenolic foam boards. Specimens are usually cut as square blocks with a standard cross-section such as 50 mm × 50 mm, with flat and parallel faces and a thickness equal to the full product thickness or a standard value in the range of 25 mm to 50 mm, depending on the governing standard.
Flexible and Semi-Rigid Foams
Flexible polyurethane foams used in seating and bedding, polyethylene foams (EPE) used in packaging, and expanded polypropylene (EPP) used in reusable transport packaging and automotive energy management are also tested. For these materials, creep determination may be combined with recovery measurements to separate elastic, delayed elastic, and permanent deformation components.
Specimen Preparation and Conditioning
All specimens must be cut cleanly without melting or crushing the cells, be free of visible defects, and be representative of the product. Before testing, specimens are conditioned in a standard atmosphere of 23 ± 2 °C and 50 ± 5 % relative humidity for at least 16 hours, or longer where the product standard requires it. Initial dimensions and mass are recorded so that apparent density can be reported together with the creep data.
Testing Items
A compressive creep determination typically delivers the following measured and derived quantities:
- Creep strain as a function of time, expressed as the change in thickness divided by the initial thickness and multiplied by 100 %.
- Total deformation under load at specified times, such as 1 day, 7 days, 28 days, or the full test duration required by the product standard.
- Relative creep, which normalizes the time-dependent deformation to the initial deformation observed immediately after loading.
- Creep compliance, obtained by dividing the measured strain by the applied constant stress.
- The stress level that produces a defined limiting deformation, for example 2 % creep strain, after the design life of 50 years, as required by the EN 13163 and EN 13164 creep annexes.
- Temperature-dependent creep behaviour at elevated temperatures such as 40 °C, 60 °C, or 80 °C for accelerated and climatic evaluations.
- A pass or fail evaluation against the declared creep class, including the CC designation for thermal insulation products.
Testing Methods
Apparatus and Setup
Testing is performed using dead-load creep frames or electromechanical compression creep stations equipped with two parallel platens. The applied force is controlled within approximately ±1 % of the nominal value, and deformation is measured with dial gauges, linear variable differential transformers, or optical displacement sensors with a resolution of 0.01 mm or better. A temperature- and humidity-controlled chamber surrounds the loading station whenever conditioning other than the standard laboratory atmosphere is required.
Test Procedure
- Step 1: After conditioning, measure the initial thickness of the specimen and, where required, apply a small preload to establish full and uniform contact between the platens and the specimen faces.
- Step 2: Apply the specified constant compressive stress, chosen to represent the service load or a standardized fraction of the short-term compressive strength, smoothly and without shock, and record the initial deformation.
- Step 3: Maintain the constant load at the controlled temperature and humidity while logging the deformation at logarithmically spaced intervals, for example at 0.1, 1, 10, 100, and 1000 hours, continuing up to the required duration, which may reach 122 days or longer for insulation product qualification.
- Step 4: On completion, remove the load and, where the recovery behaviour is of interest, record the thickness recovery over a defined period.
- Step 5: Plot the creep curve, fit the data with a creep model, and evaluate the results against the acceptance criterion of the applicable standard.
Data Evaluation and Extrapolation
Creep strain at any time t is calculated as the change in thickness at time t divided by the initial thickness, multiplied by 100 %. Because foam creep approximately follows a power-law relationship between strain and time on logarithmic axes, the measured data are fitted by regression and extrapolated to the design life. In the EN 13163 and EN 13164 annexes, tests conducted at an elevated temperature of 60 °C allow the deformation after 50 years to be predicted, from which the declared compressive creep class, expressed in the CC format, is assigned. The test report must state the material and its density, specimen dimensions and orientation, applied stress, temperature, humidity, all recorded strain values, the regression equation, and the extrapolated long-term deformation.
Applications
Compressive creep determination supports design, quality control, and product qualification across many industries:
- Building insulation: verifying that XPS boards in inverted roofs, plazas, and green roofs will not deform excessively under the permanent weight of ballast, screeds, and paving.
- Floor and cold-store construction: confirming that EPS and XPS under concrete slabs and sustained racking or wheel loads retain their thickness and thermal performance.
- Geotechnical engineering: qualifying EPS geofoam used in embankments, bridge approach fills, and lightweight backfill, where decades of sustained load must be tolerated.
- Protective packaging: ensuring that cushioning foams stacked in warehouses or transported under compressive load for extended periods continue to protect the contents.
- Automotive and furniture: assessing flexible foams in seats and mattresses for long-term sag and loss of support characteristics.
- Regulatory compliance: providing the evidence needed for declarations of performance and marking of thermal insulation products under the EN harmonized system.
- Research and development: comparing resin systems, nucleation technologies, densities, and cell structures with respect to long-term load resistance.
FAQ
What is the difference between compressive creep and compression set?
Compressive creep measures how much the foam deforms while a load remains applied, whereas compression set measures the permanent thickness loss that persists after the load has been removed and a recovery period has elapsed. Compression set is governed by methods such as ASTM D3574 and ISO 1856, while creep is governed by ISO 7850 and the creep annexes of the EN insulation standards. The two tests are complementary and are frequently ordered together from accredited third-party laboratories.
How long does a compressive creep test take?
Screening tests may run for 24 hours to 7 days, whereas full qualification under the EN 13163 and EN 13164 annexes typically requires approximately 122 days of continuous loading at 60 °C so that a valid 50-year extrapolation can be produced. The exact duration is defined by the applicable standard and the declared design life.
Which stress level is applied during the test?
The stress is selected to represent the sustained service load, or a standardized multiple of it, and must remain well below the short-term compressive strength so that progressive cell collapse rather than immediate yielding dominates the response. In insulation product qualification, the objective is often to identify the highest stress that keeps the extrapolated 50-year deformation within a limit such as 2 %.
Does specimen orientation affect the result?
Yes. Foamed plastics are anisotropic, so the creep measured parallel to the rise or extrusion direction generally differs from that measured perpendicular to it. The test direction must match the direction of loading in service and must be clearly stated in the report.
Why are some creep tests performed at elevated temperature?
Because polymer viscoelastic processes accelerate with temperature, testing at an elevated temperature such as 60 °C allows long-term behaviour to be predicted from a manageable test duration. This accelerated approach underlies the 50-year extrapolation in the EN compressive creep annexes and is a well-established practice in foam material qualification.