Reading a metallized film spec sheet: OTR, WVTR and the test conditions

The short version

OTR and WVTR are measured, not looked up. The number only means something when it comes with its temperature, its humidity, its method, its unit — and says whether it is the limit the grade is sold against or what the line typically delivers. This note walks through the condition lines on our own sheets, including the one comparison we do not allow: AlOx and metallized PE side by side.

1. The four conditions

Every barrier figure sits on four legs. Take one away and the number stops being comparable.

• Temperature. Gas and water vapour transmission both rise with temperature. A WVTR taken colder than another is not a tighter film; it is an easier test.
• Humidity. On our AlOx sheets, OTR is measured at 0 % RH and WVTR at 90 % RH. Those are different moisture states, not a formatting choice.
• Method. Our AlOx sheets name GB/T 19789 / ASTM D3985 for oxygen and GB/T 26253 / ASTM E398 for water vapour. Our metallized PE sheets name ASTM D3985 and ASTM F1249. Both names are printed so a customer lab can repeat the test with the standard it works to.
• Unit. cm³/(m²·24h) for OTR and g/(m²·24h) for WVTR — a 24-hour basis, stated explicitly. Some sheets also state the oxygen pressure, as cm³/(m²·0.1MPa·24h); ours for metallized PE does, ours for AlOx states the shorter form. If the pressure is left out, ask before you assume it.

A condition line is not paperwork. It is the definition of the number. Two sheets can both say "0.6" and describe two different films, because the readings were taken at two different conditions.

2. Why the oxygen and the moisture number cannot be swapped

Oxygen and water vapour are different permeants, and on our AlOx sheets they are measured under opposite conditions. Oxygen is measured dry: 23 °C / 0 % RH. Water vapour is measured hot and humid: 38 °C / 90 % RH. The humidity in the WVTR test is not a preference — it is the condition the method defines, and the condition under which moisture flow through a thin barrier is large enough to measure.

The consequence is simple. An OTR value says nothing about how the same film behaves as a moisture barrier. The two figures cannot be swapped, averaged, or ranked against each other. And the same trap repeats inside one test: a WVTR taken at 23 °C / 50 % RH and one taken at 38 °C / 90 % RH are two different tests, and the first will look better because it is easier.

Pressure is the third trap. OTR is measured with oxygen at a stated pressure; write the unit as cm³/(m²·24h) and that pressure is implied, write it as cm³/(m²·0.1MPa·24h) and it is explicit. A reading taken at a different pressure is a different quantity, and no conversion factor fixes that without the test report behind it.

3. Standard value and typical value are two different claims

The second split on a spec sheet is not about the test — it is about which number you are holding. The standard value is the limit the grade is sold against: a ceiling the supplier undertakes to meet on every roll. The typical value is what the line usually delivers. Both are real. They answer different questions.

ALOX BOPP-A at 18 µm is the clean example from our own sheet:

Read it this way: a roll of A is specified at or below 1 for oxygen, and usually tests near 0.63. If your structure needs 0.7, the standard value tells you the film may not hold it — the typical value is not a promise. When you compare two suppliers, compare the standard limits first and the typical values second. A supplier can look better on one line and worse on the other, and both sheets can be honest.

The same coating family carries a second pair, for lamination grade B: ≤ 2 standard / 1.29 typical OTR, and ≤ 2 / 1.32 WVTR. That is a grade difference, not a test difference — same 18 µm AlOx BOPP, different grade limit.

4. Two rolls from one family, two different numbers

Our metallized PE line makes the point in a way that no amount of condition-reading can smooth over. Both grades are PE, both are metallized, both are tested by the same methods — and their numbers sit at opposite ends of the range.

VMCPE is the heat-seal grade; VMNCPE is the barrier core grade, with no heat seal. They are built for different jobs, so the gap — ≤ 80 against ≤ 1 on OTR, 2.1 against ≤ 1 on WVTR — is a product difference, not a rounding difference. Comparing the two is legitimate: same method, same unit, same gauge. It is only meaningful if you also compare the job each one does.

Now the line worth writing down. Do not compare a VMCPE or VMNCPE number with an ALOX BOPP or ALOX PET number. Our VMPE sheet prints the method and the unit but not the temperature or the humidity; our AlOx sheets print both. Two families whose condition lines differ are not horizontally comparable, and the lower number is not automatically the better barrier. If you need the missing condition for a metallized PE quote, ask us for it on that grade — we will not put a figure into a comparison table that the TDS does not support.

5. Write the condition line into your own specification

Put the condition line in the specification itself, not in the covering mail: method, temperature, humidity, unit, and whether the figure is a limit or a typical value. "OTR ≤ 1 cm³/(m²·24h) at 23 °C / 0 % RH to ASTM D3985" survives a re-quote. "High barrier" does not.

Then add the second number. A limit without its typical value invites a comparison with the next supplier's typical value, which is a different claim. Write both, and say which is which. On a laminate, the figure that qualifies the pack is the finished structure's OTR and WVTR after lamination; the film's figure is the input, not the result.

6. Six questions to ask your film supplier

Ask these before a barrier figure goes into your own specification. Five of the six are answered by a good TDS; the sixth is answered by the lot certificate.

1. At what temperature and humidity were OTR and WVTR measured — and is the condition printed beside each figure, not just once in a footnote?
2. Which method produced the number — ASTM D3985, ASTM F1249, GB/T 19789, GB/T 26253 — and was it run as written, or converted from another condition?
3. Is this the standard limit or the typical value? Ask for the other one. A limit alone cannot tell you what a roll will usually measure.
4. Is the unit complete — a 24-hour basis, and the oxygen pressure stated or explicitly implied?
5. On which grade and gauge was it measured, and on the film or on the finished laminate? A 40 µm sealant and an 18 µm AlOx film are not the same test article.
6. What does the lot certificate carry — the same method and the same condition, per roll? Can a TDS be issued for the exact grade and gauge being quoted?

Key facts

• OTR on our AlOx sheets: 23 °C / 0 % RH · GB/T 19789 / ASTM D3985 · cm³/(m²·24h). WVTR: 38 °C / 90 % RH · GB/T 26253 / ASTM E398 · g/(m²·24h).
• ALOX BOPP-A, 18 µm: OTR ≤ 1 standard, 0.63 typical; WVTR ≤ 1 standard, 0.89 typical.
• ALOX BOPP-B, 18 µm: OTR ≤ 2 / 1.29; WVTR ≤ 2 / 1.32. Same coating family, looser grade limit.
• VMCPE: OTR ≤ 80, WVTR 2.1. VMNCPE: OTR ≤ 1, WVTR ≤ 1. ASTM D3985 / ASTM F1249, cm³/(m²·0.1MPa·24h), measured on 40 µm film.
• Our VMPE TDS does not print the temperature or the humidity. Until it does, a VMPE number is an internal family comparison, not a cross-family one.
• AlOx and metallized PE numbers are not horizontally comparable. Same is true of any two sheets whose condition lines differ.
• A barrier figure without its condition line is not comparable with anything.