Polyamide (nylon) film: how humidity changes the oxygen barrier

Polyamide film, called nylon film across most of the packaging industry, is used for puncture resistance, deep thermoforming and a mid-range oxygen barrier. Polyamide is hygroscopic. Its amide groups attract and absorb water, and that water changes the barrier, the glass transition and the mechanical behaviour. In published PA 6 data the oxygen transmission rate falls up to around 50% relative humidity, then rises again. An OTR figure quoted without a humidity value is not a usable specification.

Biaxially oriented polyamide, or BOPA, is the oriented variant used where strength, puncture resistance and dimensional stability matter. Every figure on this page carries the test method and the conditions it was measured under. Where no published measurement exists, the page says so instead of estimating.

Why packaging film is predominantly PA 6 rather than PA 6.6

Two polyamides dominate industry, and only one of them reaches polyamide film for packaging in volume.

The processing temperature settles it. Polyolefins melt between 110 and 135 °C. In coextrusion each melt has its own extruder. The streams meet in a shared feedblock and die, so the polyamide sets the temperature the polyolefin has to survive. PA 6.6 at 260 °C narrows that common window and raises the thermal stress on the polyolefin side. PA 6 at 220 °C is easier to integrate, and the copolymer at 195 °C easier still.

One published European industry dataset reports the same split for packaging film: PA 6 dominant, PA 6/6.6 second, all other polyamides marginal. The figures date from the early 2000s, so treat the ranking as current and the percentages as historical. The melting point of PA 6 is confirmed on seven separate nylon film grades from one producer, measured to ISO 3146 and ASTM D3418.

Polyamide oxygen barrier: how humidity changes OTR

The common shorthand, that polyamide barrier simply worsens as humidity rises, misses part of the curve. Blown PA 6 film, 25 µm or about 100 gauge, measured to ASTM D3985 at 23 °C:

Oriented film measures better. BOPA at 15 µm, about 60 gauge, gives 30 cm³ per m² per 24 hours per bar at 50% relative humidity and 23 °C, to DIN 53380-3. In the published comparison, the oriented grade shows roughly 30 per cent lower oxygen transmission than the non-oriented one. That figure belongs to that comparison, not to orientation as a rule.

On OTR, polyamide sits two orders of magnitude better than a polyolefin. Against EVOH it is ten to a hundred times worse. It is a mid-range oxygen barrier that brings mechanical properties, not a high barrier layer.

Barrier polyamides: amorphous PA and MXD6

Not every polyamide follows the PA 6 curve. Two specialty families behave differently enough to change a specification, and they are routinely confused with each other.

Amorphous polyamide: the barrier improves as humidity rises

Amorphous polyamide, usually PA 6I/6T, is built from hexamethylenediamine with isophthalic and terephthalic acid. It has a glass transition around 125 to 127 °C and no melting point, because there is no crystalline phase to melt.

Its barrier moves the opposite way to PA 6. One producer datasheet, measured to ISO 15105 at 23 °C, gives 30 cm³ per m² per day at 0% relative humidity falling to 10 at 85%. A patent example measured to ASTM D3985 at 30 °C on about 1 mil film gives 1.34 falling to 0.61 between 0 and 83% humidity, against a PA 6 control that rises from 1.6 to 4.64 over the same range.

Two side effects follow from having no crystalline phase. Water uptake is about 1.3 per cent against 9.5 for PA 6. And the haze that comes from spherulites scattering light does not arise, so the film is clearer. In practice it is more often blended into PA 6 than used alone, and producer literature reports that around 20 per cent is enough to make the blend behave like an amorphous polymer.

MXD6: the highest barrier, and the only co-measured comparison

MXD6 is poly(m-xylylene adipamide), from m-xylylenediamine and adipic acid. It is semi-crystalline, melting at 237 °C, with a glass transition of 85 °C. Its barrier does fall with humidity, unlike amorphous polyamide, but far less steeply than PA 6.

The producer publishes something rare: four materials measured together, at one thickness, in one table. Oxygen transmission at 20 µm and 23 °C, in cm³ per m² per day per atmosphere.

Two limits apply to both families. Neither is named anywhere in the RecyClass or CEFLEX design-for-recycling guidelines, which recognise only the PA 6/6.6 copolymer route. And MXD6 carries a tight food-contact restriction: m-xylylenediamine sits under a group specific migration limit of 0.05 mg/kg, roughly fifty times tighter than the limit on the amorphous polyamide monomers. Both polymer types are listed in 21 CFR 177.1500 for the United States.

Why polyamide is not the moisture-barrier layer

At 25 µm and 23 °C, measured from 85% to 0% relative humidity, PA 6 passes 35 g of water per m² per day. BOPA passes 30. LDPE at the same thickness passes 5.

Seven times more water than a polyolefin, at equal gauge. Polyamide is therefore not normally selected as the moisture-barrier layer, which is why it is coextruded or laminated with polyethylene rather than used alone.

How moisture changes PA 6 properties

The amide group absorbs water. Measured to a method equivalent to ISO 62, PA 6 takes up 2.6 to 3.4 per cent at 23 °C and 50% relative humidity. At saturation in water it takes up 9 to 10 per cent.

That water is a plasticiser, and it moves the glass transition. Dry PA 6 measures 60 °C to ISO 11357. At 1.2 per cent moisture the transition falls to 38.8 °C by DSC, refined to 40.4 °C by temperature-modulated DSC. Between a dry sample and one holding 4.9 per cent water, the reported difference exceeds 70 °C.

Elongation at break, puncture resistance, impact strength and thermoformability all rise with absorbed moisture. Stiffness and the oxygen barrier fall past the optimum. Nothing about a polyamide film is fixed until its moisture content is fixed. Any specification written for polyamide film has to state the humidity it applies at.

Cast PA, blown PA and BOPA

All oriented polyamide film starts the same way: a rapidly quenched, amorphous primary film. It is then reheated, stretched and thermofixed. The stretch ratio runs between 2.7 and 3.2, in both the machine and the transverse direction.

Two industrial routes exist, cast on a tenter frame and blown as a double bubble, and orientation can be sequential or simultaneous. Producer literature names both routes without comparing them, so this page does not publish a trade-off table between them.

The chill roll setting is documented, and it splits the film into two families. Between 20 and 40 °C the film comes out thermoformable and highly transparent. Between 80 and 120 °C it comes out dimensionally stable. The same trade-off governs cast polypropylene, for the same reason: the roll decides how the polymer freezes.

BOPA reel storage, conditioning and handling

This is the part that decides whether the film runs, and almost nobody publishes it.

A published converter guideline for BOPA film states it plainly. The edges of a reel absorb humidity and swell. The result is a poor profile and slack edges, and the effect is common to all nylon film. The windows below are that guideline’s. Another grade or structure may need different ones.

Shrinkage behaves the same way. Take a 50 µm cast PA 6 film made on a 20 °C chill roll, with crystallinity below 1 per cent by X-ray. It shrank 1 to 2 per cent within two hours at 23 °C and 65% relative humidity. The same film passed over a second heated roll at 80 to 100 °C held its dimensions in humid air. The cited guideline requires slitting to width only after the film is conditioned.

Curl appears in asymmetric structures. Copolyamides are preferred there, and the residual curl is reduced by humidifying the film in a hot water bath.

BOPA puncture, tear and flex-crack resistance

Where polyamide is used: thermoforming, vacuum packs and casings

In a thermoformable web the reason is documented directly. Polyamide gives high maximum draw depth and holds barrier layer thickness in the corners, where the draw is deepest.

It also improves the layer it protects. Take a PE / tie / PA / EVOH / PA structure. The drawability of the EVOH is significantly better than where the EVOH is not sandwiched between polyamide layers. Published thermoforming trials compare PE/PA/PE, PE/EVOH/PE and PE/PA/EVOH/PA on residual barrier thickness at draw depths of 10, 20, 40 and 60 mm.

In vacuum packs, the producer names the application list: sausage, ham, cheese, fish and nuts. As an outer layer, polyamide adds heat resistance at the sealing bar, so the operator can run hotter without the film sticking. In artificial casings the structure runs up to seven layers of polyamide, polyethylene and optionally EVOH.

Polyamide and design for recycling

The RecyClass design guidelines are precise, and the precision matters.

In the polyethylene stream, one polyamide route reaches limited compatibility. It requires four conditions at once. No more than 15 per cent, a PA 6/6.6 copolymer, a melting temperature no higher than 192 °C, and at least 10 per cent PE-g-MAH tie layers. Any other polyamide is listed as non-compatible.

In the polypropylene stream there is no threshold at all. Polyamide is listed as a non-compatible barrier layer alongside PVC and PVDC, at any percentage.

The guidelines do accept two barrier routes in a polyethylene structure. SiOx and AlOx coatings without additional coatings reach full compatibility. EVOH reaches full compatibility below 5 per cent. It needs PE-g-MAH tie layers above 0.1 per cent MAH, and an EVOH to tie layer ratio no greater than 1. The same EVOH content at a ratio above 1 drops to limited compatibility. Those are the routes to a recyclable barrier, not polyamide. The wider framework is on design for recycling.

Polyamide food-contact compliance

Caprolactam, the PA 6 monomer, is authorised in Annex I of Regulation (EU) No 10/2011, as amended, as FCM substance 212. The entries below were read against the consolidated text of 16 March 2025. Its sodium salt is FCM substance 435. Both sit under group restriction 4, with a specific migration limit of 15 mg/kg expressed as caprolactam.

That figure is a group total, shared between the two substances, not a separate limit for each. For PA 6.6, hexamethylenediamine is FCM substance 305 with a limit of 2.4 mg/kg, and adipic acid is FCM substance 303 with no specific limit. Compliance has to be established for the finished material or article under its intended conditions of use. Polymer identity alone is not sufficient. Additives, tie layers, adhesives, inks, the simulant and the contact time and temperature all enter the assessment. In the United States, nylon resins for food contact are covered separately by 21 CFR 177.1500. That section lists nylon 6, nylon 6.6 and nylon 6/66 among the authorised types.

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