Many who work with high-barrier film testing have the same puzzle: an ordinary plastic film gives a gas result within hours, but EVOH takes a full day, PVDC longer, and an aluminium–plastic composite shows almost no pressure change even after a long time. Why does high-barrier testing take so long?
Why EVOH takes a day
EVOH's many hydroxyl groups form a dense hydrogen-bond network, giving hundreds of times the oxygen/nitrogen barrier of ordinary PE film. In a differential-pressure test the gas must cross layer upon layer of molecular chains, so permeation is extremely slow. The low-pressure-side vacuum gauge must collect enough permeating gas to form a recognisable pressure change before a stable transmission rate can be computed; EVOH may deliver only a few pascals per hour — sometimes under 1 Pa — so it takes 20 hours or more to accumulate.
Why PVDC is slower, and aluminium–plastic almost unmeasurable
PVDC's chains are more ordered and tightly packed with very high crystallinity, so its transmission rate is an order of magnitude below EVOH; after over ten hours the low-side pressure change may still be under 10 Pa, so 30 hours or more are usually needed, and 40+ for ultra-high-barrier grades. An aluminium–plastic composite has a metal foil core whose theoretical transmission rate is essentially zero; the faint permeation comes almost entirely from pinholes, layer defects or edge leakage. The gas quantity is so small that the low-side change can stay under 1 Pa — below a conventional gauge's resolution — producing the "almost unmeasurable" situation.
Why evacuation and desorption matter
Evacuation is not a formality. In high-barrier testing, even a trace of residual gas in the chamber can exceed the total gas permeating through the sample, directly falsifying the data. Evacuation has two purposes: first, to remove air and moisture adsorbed on the chambers, tubing and sample surface so they don't slowly release and disturb the low-side monitoring; second, to pull the system's initial pressure close to vacuum and eliminate the background interference at the source.
Desorption matters equally. Under normal pressure, the chamber walls, tubing walls and sample surface adsorb large amounts of air and moisture; once pumped to vacuum, these molecules release slowly. In high-barrier testing the released quantity can exceed what permeates through the sample; starting before desorption stabilises will misread the released gas as sample permeation and produce values far above the truth.
What affects test duration
- Material barrier grade — the higher the barrier, the less gas passes per unit time and the longer the accumulation;
- Vacuum gauge resolution — a larger sensor range raises the resolution floor and needs more time to accumulate enough change;
- System desorption speed — rougher chamber walls and more adsorbed impurities mean longer stabilisation;
- Ambient temperature stability — larger swings force the instrument to spend more time cancelling thermal interference;
- System vacuum level — small leaks force longer tests for data fitting;
- Sample thickness — for the same material, thicker samples mean longer paths and slower permeation;
- Test gas type — larger molecules permeate slower and take longer;
- Chamber volume — a larger low-pressure chamber shows a smaller pressure change for the same permeation;
- Required precision — tighter repeatability demands more stable data points;
- Pre-treatment — more residual solvent/moisture on the surface means longer degassing.
How to shorten the time while keeping accuracy
Pre-drying the sample to remove surface moisture greatly reduces the desorption burden; after mounting, extend the pre-evacuation so desorption finishes early; choose a matched small-range high-precision gauge for the expected barrier grade instead of using a large-range sensor on ultra-low-permeation samples; and plan batches to use the stable overnight window for continuous runs.
For accuracy: keep ambient temperature within ±0.1 °C, verify the leak rate regularly so system leakage stays far below sample permeation, wait for full desorption stabilisation before starting, and set sensible end conditions for each barrier grade. A high-speed vacuum system, precision valves and intelligent control can nearly halve a cycle that once took tens of hours while keeping the data compliant with GB/T 1038 and similar standards.
