Coextruded Film Oxygen Transmission Rate Testing: Standards, Methods, and Applications

Oxygen transmission rate (OTR) testing is the key method for verifying the barrier performance of coextruded multilayer films used in food, pharmaceutical, and industrial packaging. This technical article explains the structure and properties of coextruded films, the applicable international standards such as ASTM D3985 and ISO 15105, sample preparation requirements, coulometric and manometric test methods, and typical applications of oxygen permeability verification in quality control and research.

Technical Overview

Coextruded films are multilayer plastic films produced by simultaneously extruding several polymer melts through a single die, bonding the layers together while they are still molten. Typical structures combine polyethylene (PE), polypropylene (PP), polyamide (PA), ethylene-vinyl alcohol copolymer (EVOH), and adhesive tie layers into composite structures such as PA/tie/EVOH/tie/PE or PP/tie/EVOH/tie/PP. By placing a high-barrier resin such as EVOH or PA in the core and tough sealing resins on the outside, coextruded films achieve an excellent balance of mechanical strength, sealability, and oxygen barrier performance in a single manufacturing step.

The oxygen transmission rate (OTR) is the steady-state rate at which oxygen molecules permeate through a unit area of film per unit time under precisely controlled temperature and humidity conditions. It is typically expressed in units of cm³/(m²·24 h) under standard test conditions such as 23 °C and 0% relative humidity. Because oxygen ingress drives oxidation of fats, vitamins, pigments, and flavors, OTR is one of the most decisive quality parameters for judging whether a coextruded film can deliver the intended shelf life for sensitive products.

For coextruded barrier films, OTR values commonly range from below 1 cm³/(m²·24 h) for high-barrier EVOH-based structures to several hundred cm³/(m²·24 h) for simple polyolefin-only coextrusions. Accurate, standards-based OTR testing is therefore essential for material qualification, incoming inspection, supplier comparison, and shelf-life modeling.

Testing Samples

Multilayer Coextruded Barrier Films

Flat film samples from production rolls, including symmetrical and asymmetrical structures containing EVOH or PA barrier cores, are the most common test articles. Samples must be representative of normal production, free from wrinkles, gels, pinholes, and other visible defects, and should be cut from positions distributed across the web width to capture cross-profile variation.

Polyolefin Coextruded Films

Coextruded films composed only of PE or PP layers, used for general-purpose flexible packaging, can also be tested. Although their oxygen barrier is limited, OTR data are needed for layer-structure optimization and for verifying the effect of crystallinity and processing conditions on permeability.

Film Specimens Extracted from Finished Packaging

Flat specimens die-cut from finished pouches, bags, or lids can be tested to evaluate the combined effect of film manufacture, lamination, printing, and converting on barrier performance. For complete packages, whole-container testing may alternatively be performed using package-level test cells.

Sample Preparation and Conditioning

• Cut flat, defect-free specimens larger than the exposed test area of the permeation cell (typically a circular exposure of at least 5 cm diameter or an equivalent area of roughly 50 cm²).
• Measure and record the thickness of each specimen at multiple points, since OTR is inversely related to barrier-layer thickness.
• Condition specimens at 23 °C and 50% relative humidity for a minimum of 24 hours (or as specified) before testing to reach moisture equilibrium.
• Handle specimens with gloves and avoid stretching, folding, or scratching the barrier layer, as mechanical stress can induce micro-defects that inflate the measured OTR.

Testing Items

The core testing item is the oxygen transmission rate of the coextruded film, determined as the steady-state permeation flux under specified conditions. Typical test configurations include:

• OTR under dry conditions at 23 °C and 0% RH, the standard reference condition for material comparison and data-sheet reporting.
• OTR under humid conditions at, for example, 23 °C and 85% RH or 38 °C and 90% RH, which is critical for EVOH- and PA-containing structures because these barrier resins are humidity sensitive.
• OTR at elevated temperature (for example 38 °C or 40 °C) to simulate accelerated distribution or tropical climates and to feed Arrhenius-type shelf-life models.
• Oxygen permeability coefficient of individual homogeneous layers or materials, derived from OTR, thickness, and the oxygen partial-pressure difference across the specimen.
• Package-level oxygen transmission expressed per package (cm³/pkg·24 h) when complete pouches, bags, or rigid containers are evaluated.

Testing Methods

Coulometric Sensor Method (Equal-Pressure Method)

The most widely used method for coextruded films is the coulometric sensor technique described in ASTM D3985, ISO 15105-2, and GB/T 19789. The specimen is mounted as a barrier between two chambers: one side is swept by a stream of oxygen, while the other side is swept by oxygen-free carrier gas (nitrogen or a nitrogen-hydrogen mixture). Oxygen molecules that permeate through the film are transported by the carrier gas to a coulometric fuel-cell sensor, which generates an electrical current directly proportional to the mass of oxygen arriving per unit time. Once the signal reaches a steady state, the oxygen transmission rate is calculated from the sensor output and the exposed specimen area. Relative humidity in both chambers can be controlled, which makes the method particularly suitable for humidity-sensitive EVOH barrier films.

Differential-Pressure (Manometric) Method

An alternative technique is the pressure-based method standardized in ISO 15105-1, ASTM D1434, and GB/T 1038. The specimen separates a high-pressure oxygen chamber from an evacuated low-pressure chamber, and the increase in pressure or volume on the downstream side is monitored over time to calculate permeation. The manometric method measures total gas permeation without a sensor and is useful for very low transmission rates, but it does not reproduce humid service conditions as realistically as the coulometric method.

Test Procedure Essentials

• Verify instrument calibration using certified reference films or zero-check membranes traceable to recognized metrology practice before each test campaign.
• Mount the specimen hermetically in the test cell, masking the peripheral area so that only the defined area is exposed, and check for leaks.
• Purge both chambers until the baseline signal stabilizes, then introduce oxygen on the test side under the selected temperature and humidity conditions.
• Continue the test until a steady-state permeation plateau is reached, and report the mean OTR together with test conditions, specimen thickness, film structure, and the number of replicates.
• Test at least three replicate specimens per sample whenever possible, since localized defects in coextruded barrier layers can cause significant scatter.

Relevant Standards

• ASTM D3985 — Oxygen Gas Transmission Rate Through Plastic Film and Sheeting Using a Coulometric Sensor.
• ISO 15105-1 / ISO 15105-2 — Plastics: Film and Sheeting, Determination of Gas-Transmission Rate (differential-pressure and equal-pressure methods).
• ASTM D1434 — Standard Test Method for Determining Gas Permeability Characteristics of Plastic Film and Sheeting.
• GB/T 19789 and GB/T 1038 — Chinese national standards for packaging material gas permeability testing, harmonized with the international coulometric and manometric methods.
• ASTM F1927 and ASTM F1307 — Methods covering oxygen transmission through dry packages and through packages using a coulometric sensor.

Applications

• Food packaging: qualification of coextruded barrier films for modified-atmosphere packaging of fresh meat and cheese, vacuum packs, retort pouches, coffee packaging, and snack laminates where oxidation control defines shelf life.
• Pharmaceutical and medical packaging: verification of barrier layers in films for effervescent tablets, diagnostic reagents, parenteral containers, and sterile medical device pouches that must exclude oxygen and moisture.
• Layer-structure development: comparison of EVOH grades and mol% content, optimization of barrier-layer thickness, and evaluation of tie-layer effects on overall coextruded film permeability.
• Process control: monitoring the impact of extrusion temperature, orientation, annealing, thermoforming draw ratio, and retort sterilization on the oxygen barrier of coextruded structures.
• Shelf-life prediction and compliance: supplying OTR input data for packaging mass-balance models used to set best-before dates and to demonstrate conformity with customer barrier specifications.

FAQ

What OTR value qualifies a coextruded film as a high-barrier film? In common industry practice, films with an OTR below 1 cm³/(m²·24 h) at 23 °C and 0% RH are regarded as high-barrier materials, while structures in the range of 1 to 10 cm³/(m²·24 h) are considered medium-to-high barrier. The required value always depends on product sensitivity, headspace composition, and target shelf life.

Why must EVOH-containing coextruded films be tested at high humidity? EVOH owes its outstanding barrier to hydrogen bonding between hydroxyl groups, and absorbed water molecules disrupt this structure. At high relative humidity the OTR of an EVOH core can increase by an order of magnitude or more, so dry-condition data alone can drastically overstate real-world barrier performance. The outer polyolefin layers slow but do not stop moisture reaching the core, which is why humid-condition testing is essential.

How long does an OTR test take? Testing high-barrier coextruded films typically requires several hours to reach a steady-state plateau because of the low permeation flux; medium- and low-barrier films stabilize considerably faster. Tests at elevated humidity also include the time needed for the specimen to reach moisture equilibrium inside the cell.

Which method should be chosen — coulometric or differential pressure? The coulometric equal-pressure method is generally preferred for coextruded barrier films because it allows humidity control and closely simulates packaging service conditions. The differential-pressure method is valuable for fundamental material characterization and for confirming results with an independent technique; agreement between the two methods strengthens the reliability of reported OTR data.

Can finished pouches made from coextruded film be tested as whole packages? Yes. Accredited third-party laboratories and in-house labs equipped with package test cells can measure the oxygen transmission of complete packages, including the contribution of seals, in cm³ per package per 24 h, following package-level standards such as ASTM F1307. This complements film-level testing by capturing converting and sealing quality.