Oxygen transmission rate (OTR) analysis is a critical quality control and performance evaluation method for co-extruded multilayer films used in food, pharmaceutical, and industrial packaging. This article provides a comprehensive technical overview of co-extruded film OTR testing, covering common film structures, testing items, internationally recognized methods such as ASTM D3985 and ISO 15105-2, typical applications, and frequently asked questions to help packaging engineers, quality managers, and procurement specialists make informed material selection decisions.
Technical Overview
Co-extruded films are multilayer plastic structures produced by simultaneously extruding two or more polymer resins through a single die or a multi-manifold die system. By combining different materials—such as polyamide (PA) for mechanical strength and aroma barrier, polyethylene (PE) for heat sealing and moisture resistance, ethylene-vinyl alcohol copolymer (EVOH) for oxygen barrier, and tie layers (such as maleic anhydride-grafted polyolefins) for interlayer adhesion—co-extruded films achieve a balance of properties that no single polymer can provide alone.
The oxygen transmission rate (OTR) is one of the most important performance indicators for co-extruded films. Oxygen ingress into a package drives lipid oxidation, color deterioration, flavor loss, vitamin degradation, and aerobic microbial growth, all of which directly reduce product shelf life. Measuring and controlling the OTR of co-extruded films is therefore essential for:
- Verifying that barrier raw materials (EVOH, PA, PVDC) and film structures meet design specifications
- Validating extrusion process parameters such as layer ratio, melt temperature, and die uniformity
- Qualifying films for high-barrier applications such as modified atmosphere packaging (MAP) and vacuum packaging
- Supporting shelf-life studies and packaging material certifications
- Monitoring incoming material quality and supplier consistency in production environments
OTR is typically expressed in units of cm³/(m²·24h) under specified temperature and relative humidity conditions, most commonly 23°C and 0% RH or 50% RH. Because several barrier polymers—especially EVOH—are humidity sensitive, testing under multiple humidity conditions provides a more realistic picture of real-world performance.
Testing Samples
Accredited third-party laboratories and in-house packaging test centers routinely perform OTR analysis on a wide range of co-extruded film structures. Typical sample types include:
PA/PE Co-extruded Films
Nylon-polyethylene structures, such as PA/PE, PA/PE/PA/PE, and symmetric PA-tie-PE constructions, are widely used for vacuum packaging of fresh and processed meat, fish, and cheese. Polyamide provides moderate oxygen barrier properties (typically in the range of 20–80 cm³/(m²·24h) depending on thickness) along with excellent puncture resistance and aroma retention.
EVOH-Based High-Barrier Co-extruded Films
Structures containing an EVOH core layer, such as PA/tie/EVOH/tie/PA/PE or PE/tie/EVOH/tie/PE, are the workhorses of high-barrier packaging. With a properly specified EVOH layer thickness, these films can achieve OTR values below 1 cm³/(m²·24h) under dry conditions. Because EVOH loses a significant portion of its barrier performance as relative humidity increases, OTR testing at elevated humidity (85% or 90% RH) is frequently specified for these structures.
PP/EVOH/PP Thermoforming Films
Polypropylene-based co-extruded films with EVOH barrier layers are commonly used as thermoforming bottom webs for rigid and semi-rigid trays, particularly in meat, poultry, and ready-meal packaging. OTR testing verifies both the flat film barrier performance and the retention of barrier properties after deep drawing.
Barrier Lidding and Top Web Films
Co-extruded or co-extrusion-coated lidding films with peelable sealing layers are tested to ensure the complete package system, including the lid, meets the overall oxygen budget required for the target shelf life.
Converted Packages and Finished Containers
Besides flat film specimens, whole filled or unfilled packages—pouches, trays, and sealed containers—can be tested to evaluate the combined effect of film barrier, seal integrity, and any mechanical damage introduced during converting.
Testing Items
A comprehensive co-extruded film oxygen barrier evaluation typically includes the following test items and conditions:
- Oxygen transmission rate (OTR) at 23°C, 0% RH (dry condition) — the baseline barrier indicator
- OTR at 23°C, 50% RH — a common ambient reference condition
- OTR at 23°C, 85% or 90% RH — critical for EVOH-containing films used in moist environments such as chilled meat and seafood packaging
- OTR at elevated temperatures (such as 38°C or 40°C) — used for accelerated shelf-life modeling and Arrhenius-based permeation studies
- Oxygen permeance — the transmission rate normalized per unit area, expressed in cm³/(m²·24h·atm)
- Oxygen permeability coefficient — a material-intrinsic property calculated by multiplying OTR by film thickness, expressed in cm³·cm/(m²·24h·atm), useful for comparing materials independent of thickness
- Package oxygen transmission rate — measured in cm³/(package·24h) for finished pouches, trays, and containers
- Barrier retention after processing — OTR of films subjected to thermoforming, retort simulation, pasteurization, or sterilization cycles
Test reports normally document the test standard applied, specimen thickness, conditioning history, test temperature and humidity, mean OTR value of multiple specimens, and the measurement range or detection limit of the instrument.
Testing Methods
Several standardized test methods are available for measuring the oxygen transmission rate of co-extruded films. The selection depends on the expected barrier level, the required humidity control, and whether flat film or finished packages are being evaluated.
Coulometric Sensor Method (Equal-Pressure Method)
The coulometric method is the most widely used technique for co-extruded film OTR analysis and is specified in standards such as ASTM D3985, ISO 15105-2, and GB/T 19789. In this method, the specimen is mounted as a sealed barrier between two chambers. High-purity oxygen (or air) flows on one side of the film, while oxygen-free carrier gas (typically nitrogen with a small amount of hydrogen) flows on the other side. Oxygen permeating through the film is carried to a coulometric sensor, where it is reduced electrochemically; the resulting electrical current is directly proportional to the oxygen transmission rate by Faraday's law. This method offers excellent precision and is particularly suitable for medium- and high-barrier films.
Humidity-Controlled OTR Testing
For EVOH-based co-extruded structures, standards such as ASTM F1927 enable testing under precisely controlled relative humidity on both sides of the specimen. Humidified oxygen streams the test side while the carrier gas at controlled humidity sweeps the sensor side. This is essential because EVOH barrier performance can degrade by an order of magnitude between 0% and 90% RH, and package performance in chilled, moist food applications must be assessed under realistic humidity exposure.
Dynamic Accumulation and Equilibrium Methods
For very high-barrier films, alternative approaches described in ISO 15105-1 and related methods use closed-volume accumulation cells in which the oxygen concentration inside a sealed chamber increases over time. Concentration changes are monitored with traceable sensors, allowing detection of extremely low transmission rates. These methods are useful when OTR values approach the detection limits of conventional coulometric instruments.
Whole-Package OTR Testing
ASTM F1307 and equivalent package-level methods measure the oxygen ingress of complete packages using an optimized coulometric detector. The sealed package is flushed or filled with an oxygen-free atmosphere, exposed to an oxygen-containing environment, and the rate of oxygen accumulation inside the package is measured. This approach captures the combined contribution of the film, seals, and any leakage paths, and is invaluable for validating complete packaging systems.
Sample Preparation and Conditioning
Proper specimen preparation is critical for reproducible results. Key practices include:
- Conditioning specimens at standard laboratory atmosphere (23°C, 50% RH) before mounting to relieve stress and stabilize moisture content
- Checking film thickness at multiple points, since OTR is inversely proportional to barrier-layer thickness
- Avoiding wrinkles, pinholes, and contamination when mounting specimens in the test cells, using appropriate sealing gaskets or vacuum grease
- Running tests to equilibrium, which may take from several hours for medium-barrier films to several days for ultra-high-barrier EVOH structures
- Testing a minimum of three specimens and reporting the mean and standard deviation
Applications
Co-extruded film OTR analysis supports quality assurance and product development across many industries:
- Processed and fresh meat packaging — vacuum skin packs, thermoformed packs, and MAP trays for sausages, ham, bacon, beef, and poultry rely on low-OTR films to prevent color loss and rancidity
- Dairy and cheese packaging — oxygen barrier films preserve flavor and inhibit mold growth in sliced and shredded cheese
- Seafood packaging — high-barrier co-extruded films prevent lipid oxidation in fatty fish such as salmon and mackerel
- Rigid and semi-rigid trays — PP/EVOH/PP bottom webs for ready meals and chilled convenience foods
- Pharmaceutical and nutraceutical packaging — barrier pouches and laminates that protect oxygen-sensitive formulations, probiotics, and active ingredients
- Medical device packaging — sterile barrier systems where oxidation of components and integrity of the barrier are both critical
- Coffee, snacks, and dry foods — films for whole-bean and ground coffee, nuts, and dehydrated products where even low oxygen ingress degrades aroma and freshness
- Industrial and electronics packaging — moisture- and oxygen-barrier films for sensitive components and chemical products
FAQ
What OTR value qualifies a co-extruded film as "high barrier"?
Although definitions vary by application, films with an OTR below approximately 1 cm³/(m²·24h) at 23°C and 0% RH are generally regarded as high barrier, while values between 1 and 10 cm³/(m²·24h) are considered medium-to-high barrier. Ultra-high-barrier structures can reach values below 0.1 cm³/(m²·24h). The required value always depends on the product's oxygen sensitivity, headspace volume, and target shelf life.
Why does EVOH lose barrier performance at high humidity?
EVOH achieves its outstanding oxygen barrier through strong hydrogen bonding and high crystallinity. Water molecules penetrate the amorphous regions at high relative humidity, plasticizing the polymer and increasing the free volume available for oxygen diffusion. As a result, OTR at 90% RH can be ten times higher or more than at dry conditions. This is precisely why EVOH layers are typically sandwiched between moisture-barrier polyolefin layers in co-extruded structures, and why testing under realistic humidity conditions is essential.
How does temperature affect OTR test results?
Oxygen permeation follows an Arrhenius-type temperature dependence: transmission rates roughly double for every 8–10°C increase in temperature within typical ranges. Tests must therefore always be reported with the exact test temperature, and accelerated shelf-life studies must account for this relationship when extrapolating results to storage conditions.
What is the difference between OTR, permeance, and permeability?
OTR is the measured oxygen quantity per unit area per day under specific conditions. Permeance is the OTR normalized by the partial pressure difference of oxygen across the film. Permeability (coefficient) additionally normalizes for film thickness, giving a material-intrinsic property that enables fair comparison between different polymers or layer ratios independent of how thick the final film happens to be.
How long does a typical OTR test take?
For medium-barrier co-extruded films, equilibrium is usually reached within several hours. High-barrier EVOH structures can require 24 to 72 hours or longer for the instrument signal to stabilize, particularly at elevated humidity. Package-level tests may run longer due to the larger internal volume and lower transmission rates involved.
Should the film be tested before or after thermoforming and filling?
Both approaches are valuable. Testing the flat film verifies the base material quality and extrusion consistency, while testing the formed package or formed tray confirms that the converting process has not thinned the barrier layer excessively or introduced defects. Many quality programs specify both flat-film and package-level verification for critical applications.
Can OTR testing be used for incoming inspection in production?
Yes. Many packaging converters and brand owners include OTR limits in raw material specifications and verify compliance through accredited third-party laboratories or in-house coulometric testers. Regular OTR monitoring helps detect raw material changes, extrusion drift, and supplier variability before they cause shelf-life failures in the market.
