Two suppliers quote the same material and the two data sheets do not agree. One gives a thermal conductivity of 0.040 W/(m·K), the other 0.065. One says compression set is 8% or less, the other 3% or less. One quotes water absorption to one decimal, the other does not quote it at all. Neither sheet has to be wrong. What is usually missing is the information that turns a number into a specification: the grade it was measured on, the method that was used, the conditions at the time, and the specimen that went into the machine.
This page takes three rows that appear on almost every PE foam data sheet — thermal conductivity, compression set and water absorption — and shows what each one answers, what it does not answer, and how to compare two sheets that were written by two different people. It is written for the person who has to approve a purchase order, not for a laboratory.
Why Two Sheets for the Same Material Disagree
Four things have to match before two numbers can be compared. When they do not, the difference between the numbers tells you nothing about the materials.
- The grade. A property that varies across the expansion range has no single value for “XPE foam”. Thermal conductivity runs from about 0.095 W/(m·K) at 3× expansion down to about 0.040 W/(m·K) at 35× and above. A sheet quoted at 0.040 and a sheet quoted at 0.065 are not in conflict; they are two different places on the same curve.
- The method. GB/T, ASTM and ISO test standards are not interchangeable, even when they measure the same property. GB/T 1034 method 4 and an ASTM immersion test both report water absorption, and they do not have to return the same number on the same sheet. A figure without its standard is a figure you cannot check.
- The conditions. Time and temperature are part of the result. A 22-hour compression set at 23 °C and a 24-hour set at 70 °C answer different questions, and the second is much larger. A data sheet that omits the conditioning has omitted the result.
- The specimen. Thickness changes what a test sees. Closed-cell PE foam measures differently on a 2 mm sheet and on a 20 mm laminated block, and resistance and compression behave differently again on a cut part than on a loose sheet.
So the first question about two disagreeing numbers is not “which one is right” but “are these the same test”. Most of the time they are not.
Row One: Thermal Conductivity
Thermal conductivity, written λ (lambda) or k, is the rate at which heat passes through a material by conduction. In SI units it is quoted in W/(m·K): the watts crossing one square metre of a one-metre thickness for each kelvin of temperature difference. A lower number insulates better.
Values for the XPE and IXPE range in this factory are measured by GB/T 10297 and run from about 0.095 W/(m·K) at 3× expansion to about 0.040 W/(m·K) at 35× and above. The reason is not mysterious: a more highly expanded grade contains more gas and less polymer in the same volume, and gas conducts less than the cell walls do. Expansion is therefore the first thing to look for next to a conductivity figure, and its absence is the most common defect in a quoted line.
Three distinctions are worth keeping separate. Conductivity is not a U-value: a U-value describes a whole build-up, including surface films, fixings and thermal bridges, and only the assembly's designer can state it. Conductivity is not an R-value: R is thickness divided by conductivity, so the same material at two thicknesses gives two R-values. And conductivity is measured on the foam, in a steady state, at a stated mean temperature — it is not a promise about a cavity that is ventilated, compressed or wet.
Row Two: Compression Set
Compression set answers one question: after the foam has been squashed for a defined time, how much of that squash does it fail to recover? It is reported as a percentage of the original deflection. A low number means the foam springs back; a high number means it stays thinner than it started.
The measurement conditions here are GB/T 6669, 23 °C for 22 hours. Across the range the result moves with the grade: around 2% or less at 5× expansion, 5% or less at 10×, 7% or less at 15×, and 8% or less at 20×. At the light end, 35× and 40×, the figure is 11% or less. A lighter sheet has thinner cell walls, so it takes a permanent set more readily.
This is the row to read whenever a part has to hold something in place over time — a gasket under a cover, a divider between boards, a pad under a battery module. A gasket that has taken 8% set has lost 8% of the interference that was supposed to keep it sealing, and the failure appears months later as a rattle or a leak rather than as a broken part.
What the number does not tell you is how the foam behaves over years rather than 22 hours. Compression set is an accelerated indicator, not a service-life prediction, and a 22-hour test at room temperature says nothing about a part that will sit at 60 °C. If the service temperature is above ambient, ask for the set at that temperature; it will be a different, larger number.
Row Three: Water Absorption
Water absorption reports how much water a specimen takes up when it is immersed for a defined period. Here it is GB/T 1034 method 4, 22 hours at 23 °C, and it is quoted as a percentage of the dry mass.
For XPE the value is 0.2% or less through the dense end of the range (3× to 10×), 0.3% or less from 15× to 20×, and 0.4% or less from 25× upward. IXPE reaches 0.2% or less to 15× and 0.4% or less at 30× and above. Those figures are small, and they are the reason a closed-cell sheet keeps its declared conductivity after a wet season while an open-cell material measures as wet insulation from then on.
The row is also the one most often over-read. Closed cell resists water uptake; it does not make the sheet a vapour barrier, and it does not make an assembly waterproof. Where a design needs a vapour control layer or a sealed joint, that layer is part of the design and the foam does not replace it. A low absorption figure describes the material, not the build-up around it.
How to Compare Two Sheets in Ten Minutes
- Put the expansion ratio next to every property. If a sheet quotes a property without the grade, ask for the grade, and treat the figure as unverified until it arrives.
- Pair each number with its standard. Two figures measured to different standards are two facts about two tests, not two measurements of one thing.
- Check the units have not been converted. W/(m·K) and W/m·K are the same; kcal/(m·h·°C) is not. kg/m³, g/L and lb/ft³ are the same quantity in three hats — our guide to density and hardness sets out the conversions.
- Ask whether the figure is a limit or a measurement. “8% or less” is a specification the material is held to; “6.4%” is one result on one specimen. Both are useful, and they are not the same claim.
- Ask for the report behind the sheet. A supplier who can produce the test report can also tell you which laboratory ran it, on what date, and on which production batch.
What a Data Sheet Cannot Tell You
- It is not a batch certificate. A data sheet describes a grade. A certificate of analysis describes the material in one delivery. They are two documents, and an incoming inspection is what connects them.
- It is not a part guarantee. Every figure on the sheet was measured on sheet stock. A die-cut part with an adhesive backing, a laminated block or a cavity routed into a thick section is a different object, and some of its properties will differ from the sheet they started as.
- It does not contain what was never measured. This factory publishes no compression-strength figure for XPE, because no such measurement is held. An absent row is an honest row; a filled-in one without a method behind it is not.
- It is not comparable across methods. A sheet tested to GB/T and a sheet tested to ASTM will not line up property by property, and forcing a comparison produces a decision that looks quantitative and is not.
- It is not a design value. Design needs a safety factor, a service temperature and a load case. A data sheet gives the input to that calculation, not the answer.
What to Write in the Purchase Order
A single sentence per property removes most of the ambiguity, and it costs nothing to write:
- Material and grade: “XPE, 15× expansion, black, 5 mm sheet” — not “XPE foam”.
- Property, value, standard: “compression set 7% or less, GB/T 6669, 23 °C / 22 h”.
- Where the measurement applies: “measured on finished sheet, not on laminated stock”, if the part is laminated.
- What happens if it does not comply: the acceptance sampling plan, and who runs the test.
- The document you expect: a data sheet at quotation, a certificate of analysis with the delivery.
None of that requires a laboratory. It requires the two sides to name the same test before the material ships, which is also the only way a rejected lot can be discussed without a dispute about method.
Common Questions
Why is one supplier's conductivity lower than the other's for the same material?
Usually because the two sheets are at different expansion ratios, or because one is a laminated build and the other a single layer. Ask for the grade and the standard before treating either number as a comparison. A conductivity line with no expansion ratio cannot be compared to anything.
Is a lower compression set always better?
Not by itself. A denser, more highly cross-linked grade sets less but pushes back harder, which can distort a light component. The right figure is the one your assembly can live with, measured against the gap the foam has to fill.
Does 0.2% water absorption mean the foam is waterproof?
No. It means the material takes up very little water when immersed for the stated period. Whether an assembly keeps water out depends on joints, fixings, facings and drainage, and those are design decisions rather than material properties.
Can you supply a data sheet to a different standard?
We quote the standards the material is tested to in this factory, and we name them on every line. If your market requires a different method, say so at enquiry and the answer will be whether a report to that method exists, rather than a converted number.
Which of the three rows matters most?
Whichever one describes the job the part is doing. Insulation is a conductivity question, a fixture under sustained load is a compression-set question, and a wet or humid service is a water-absorption question. A part has one dominant requirement, and the other two are constraints on it.
What Bofeng Foam Supplies
Bofeng Foam (Zhuhai Bofeng Foam) has manufactured cross-linked PE foam since 2011 and holds ISO 9001 and ISO 14001. The XPE and IXPE ranges run from 3× to 40× expansion, single-layer 3–15 mm for XPE and 0.1–15 mm for IXPE, with heat-bonded multilayer from 15 mm to 200 mm. Every property quoted on this page is measured to the standard named beside it, and the data sheets behind them are on the XPE and IXPE material pages, with finished sheet ranges on XPE foam and IXPE foam. If your application is electrical rather than thermal, the surface-resistance rows and their measurement bases are set out in the conductive foam and ESD guide; if it is a closed-versus-open cell question, the closed cell comparison takes it from the structure rather than the table. Send the property, the value and the standard you need and the reply will name the grade, the method and the report that supports it — or say plainly that the measurement does not exist.