Barrier improvement is the headline claim for nanoclay in packaging, and it is the claim most often quoted without stating test conditions. A film that shows a 60% oxygen transmission reduction at 23 °C and 0% relative humidity may show 25% at 38 °C and 90% RH — and the second number is the one that predicts shelf life for most real products.
Understanding how these measurements work makes it possible to read other people’s data critically and to specify your own tests so the results mean something.
What is being measured
Oxygen transmission rate (OTR) is the steady-state quantity of oxygen passing through unit area of film per unit time under a defined partial pressure difference. Typical units are cm³/(m²·day) or mol/(m²·s).
Water vapour transmission rate (WVTR) is the equivalent for water vapour, usually reported in g/(m²·day).
Both are transmission rates — properties of a specific film at a specific thickness. Permeability normalises for thickness and driving pressure and is a material property. Conflating the two is the most common error in barrier discussions: halving film thickness roughly doubles OTR without changing permeability at all, so a “barrier improvement” achieved by adding material is not a materials achievement.
For homogeneous materials the conversion is straightforward. For multilayer films it is not, and reporting permeability for a laminate is generally meaningless.
The oxygen methods
ASTM D3985 is the workhorse: oxygen gas transmission rate through plastic film and sheeting using a coulometric sensor. The film is sealed between two chambers, one containing oxygen and one purged of it, and a coulometric sensor measures oxygen arriving in the purged side. The standard notes it is suitable as a referee method where the parties have agreed on sampling, standardisation, conditions, and acceptance criteria — which is a reminder that agreeing on conditions is part of the method, not an optional extra.
ASTM F1927 covers OTR determination at controlled relative humidity using a coulometric detector. This distinction matters enormously and is the source of much of the confusion in published data. D3985 in its basic form is typically run dry; F1927 lets you specify humidity on each side.
ASTM F2476 covers carbon dioxide transmission rate using an infrared detector, relevant for modified-atmosphere packaging.
The water vapour method
ASTM F1249 covers WVTR through plastic film and sheeting using a modulated infrared sensor. It applies to sheets and films up to 3 mm thick, single or multilayer, synthetic or natural polymers and foils including coated materials, and it yields WVTR, permeance, and — for homogeneous materials — the water vapour permeability coefficient.
ISO 15106-2 is the corresponding international standard for the infrared sensor method.
Older gravimetric methods — the desiccant cup approach — still appear in some specifications. They are slower, less sensitive, and generally less repeatable, but they remain in certain regulatory contexts, so check which one a specification actually calls for before assuming instrumental results will be accepted.
Why humidity changes everything for nanoclay
This is the point that most affects how nanoclay barrier data should be read.
Clay is hydrophilic. Even organically modified montmorillonite retains affinity for water. At high humidity, water sorbs into the composite, and three things happen:
The polymer plasticises. Absorbed water increases chain mobility and free volume, raising intrinsic permeability of the matrix.
The clay–polymer interface can be disrupted. Water accumulating at the interface creates a pathway with lower resistance than the bulk matrix, partially short-circuiting the tortuous path.
Clay itself may swell. Residual hydrophilic sites take up water, and swelling changes the effective geometry.
The consequence is that nanoclay barrier improvement is consistently better in dry conditions than wet ones. A paper reporting a large improvement at 0% RH is not wrong, but it is not describing performance in a refrigerated case containing a moist food either.
For polar matrices the effect is stronger still, and for hydrophilic biopolymers — starch, PVOH, some PLA grades — the humidity dependence can dominate everything else.
Practical rule: always specify and report humidity on both sides of the film. “OTR reduced 55%” without conditions is not a usable claim.
Other conditions that move the number
Temperature. Permeation is thermally activated and follows an Arrhenius relationship over moderate ranges. A 10 °C rise commonly increases permeability by 30–70% depending on polymer. Comparisons across different test temperatures are not valid without correction.
Thickness. Report it. Always. And measure it rather than trusting the nominal value — film gauge variation of ±10% is common, and it maps directly onto the transmission rate.
Orientation. As covered elsewhere in this series, platelet alignment strongly affects barrier performance, so a cast film and an injection-moulded plaque of the same compound give quite different results.
Conditioning. Films must reach equilibrium moisture content before testing. Insufficient conditioning is a frequent cause of drifting or non-steady results, and it usually shows up as a measurement that never quite settles.
Pinholes and defects. A single pinhole dominates the measurement. For nanoclay films specifically, agglomerates can create local weak points. Anomalously high readings with high scatter between specimens should prompt an inspection for defects before any conclusion about the formulation.
Specifying a test that will be useful
For a nanoclay packaging development, a defensible minimum:
- Test at the conditions of use, not at instrument-convenient conditions. For chilled food, 5 °C and high humidity. For dry goods at ambient, 23 °C and 50% RH.
- Always run a matched control — the same polymer, the same thickness, the same process, without clay. Improvement relative to a control produced identically is the only defensible claim.
- Test at least three specimens and report the scatter, not just the mean.
- Measure and report actual thickness for every specimen.
- State the full condition set in any claim: temperature, RH on both sides, thickness, and method.
Reading other people’s data
A short checklist when evaluating a published or supplier barrier claim:
- What were the temperature and humidity? If unstated, treat the number as indicative only.
- What was the control? Improvement against a different-grade neat resin is not a like-for-like comparison.
- What thickness, and is the comparison at matched thickness?
- What process made the film, and does it match yours?
- Is it OTR or permeability, and is the distinction being used consistently?
- How many specimens, and what was the scatter?
A claim that survives all six is worth building a business case on. Most do not, and the gap between literature barrier improvements and production ones is largely explained by items 1, 3, and 4.