1. Principles and Scope of the Standard
ASTM D3985 is a test method designed to determine the steady-state rate of transmission of oxygen gas through plastics in the form of films, sheeting, laminates, coextrusions, or plastic-coated papers and fabrics. Since oxygen is the primary oxidising agent responsible for the degradation of foods, pharmaceuticals and many industrial products, measuring its transmission through packaging materials is a critical parameter for shelf life management.
The principle is based on a test cell in which the specimen is sealed between two compartments. The upper compartment is purged with pure oxygen (or air), while the lower compartment is swept by a carrier gas (nitrogen) that transports oxygen molecules that have permeated through the film to a coulometric sensor. This sensor generates an electrical current proportional to the amount of oxygen detected, enabling precise quantitative measurement.
The standard provides three quantities:
- OTR (Oxygen Transmission Rate): rate of oxygen transmission, in cm³/m²/day, for the specific test conditions.
- PO2 (permeance): OTR normalised by the partial pressure of oxygen, in cm³/m²/day/atm, enabling comparison between tests performed at different pressures.
- P′O2 (permeability coefficient): permeance multiplied by thickness, in cm³·mm/m²/day/atm, applicable only to homogeneous materials and enabling comparison regardless of thickness.
1.1 Distinction from Related Test Methods
Several standards cover gas permeability measurement for packaging materials. The difference lies in the sensor type, test conditions and scope of application:
| Standard | Method | Use Case |
|---|---|---|
| ASTM D3985 | OTR by coulometric sensor, 23 °C / 0% RH (dry conditions) | Plastic films and sheeting, baseline reference |
| ASTM F1927 | OTR by coulometric sensor, 38 °C / 90% RH (humid conditions) | Barrier films sensitive to humidity (EVOH, PA) |
| ASTM F2622 | OTR of finished packages (pouches, trays, bottles) | Finished product validation, not material alone |
| ISO 15105-2 | Gas transmission by coulometric sensor (ISO reference method) | ISO equivalent of D3985, international markets |
| ASTM D1434 | Gas transmission by manometric or volumetric method | All gases (O2, CO2, N2), classical method |
D3985 is specifically designed for oxygen under dry conditions. For barrier materials whose properties vary with humidity (EVOH, polyamide), ASTM F1927 is essential as it measures OTR under real-world use conditions (elevated temperature and humidity).
2. Role in Barrier Material Evaluation
2.1 Shelf Life Prediction
The primary objective of ASTM D3985 is to quantify the oxygen barrier of a packaging material. This data is directly used in shelf life prediction models: knowing the material’s OTR, the packaging surface area, the headspace volume and the product’s sensitivity to oxygen, it is possible to calculate the duration during which the product will remain within acceptable quality limits.
For reference, typical OTR values for common materials (23 °C, 0% RH):
| Material | Typical OTR (cm³/m²/day) |
|---|---|
| PE-LD (50 µm) | 3,000 – 6,000 |
| PP (30 µm) | 1,500 – 2,500 |
| PET (12 µm) | 50 – 100 |
| PA (15 µm) | 20 – 40 |
| EVOH (5 µm, dry) | 0.01 – 0.5 |
| PVdC/Saran (25 µm) | 2 – 8 |
| Aluminium (6 µm) | < 0.001 (virtually impermeable) |
2.2 Material Selection and Optimisation
OTR is a decisive selection criterion for choosing packaging materials. By comparing the permeability coefficients (P′O2) of different polymers, engineers can design optimised multilayer structures that combine oxygen barrier, mechanical strength and sealability at the lowest cost.
The D3985 test also enables validation of surface treatments (vacuum metallisation, SiOx deposition, barrier coatings) by measuring the effective barrier improvement compared to the untreated substrate.
2.3 Interpretation Limitations
The main limitations to be aware of:
- Testing at 0% RH does not represent real-world use conditions. For humidity-sensitive materials (EVOH, PA), the OTR under dry conditions can be 10 to 100 times lower than the OTR under humid conditions.
- The permeability coefficient (P′O2) is only valid for homogeneous materials. It does not apply to laminates, coextrusions or metallised materials.
- The OTR of the material alone does not predict the barrier of the finished package: seals, folds and perforations contribute significantly to overall transmission.
- Temperature has an exponential effect on OTR (Arrhenius law). Results at 23 °C cannot be directly extrapolated to 4 °C or 38 °C without a correction model.
Important: The OTR measured at 23 °C / 0% RH is a reference value for material comparison, not a direct prediction of real-world behaviour. For humidity-sensitive materials, the ASTM F1927 test (38 °C / 90% RH) is essential.
3. Test Procedure
3.1 Specimen Preparation
Specimens are conditioned for at least 48 hours at 23 ± 2 °C and 50 ± 5% RH. Test pieces are cut to the dimensions of the test cell, avoiding any perforation, fold or surface contamination. Film integrity is critical: the slightest defect creates a leak path that distorts the measurement.
3.2 Test Execution
The procedure involves several steps:
- The specimen is sealed in the test cell, separating the oxygen compartment from the nitrogen compartment.
- Both compartments are purged: the test side is swept with ultra-pure nitrogen, the source side with pure oxygen (or air at 20.9% O2).
- The apparatus waits for steady state to be reached: the oxygen flux through the film stabilises. This phase can last from a few hours (permeable films) to several days (high-barrier films).
- Once steady state is achieved, the coulometric sensor measures the oxygen flux continuously. The value is recorded when the variation is less than 1% over a defined period.
The most widely used instrument is the MOCON OX-TRAN (now AMETEK MOCON), considered the industry standard for coulometric OTR measurements.
3.3 Data Collection and Analysis
The key parameters reported are:
| Parameter | Description |
|---|---|
| OTR | Steady-state oxygen transmission rate (cm³/m²/day) |
| PO2 | Oxygen permeance (cm³/m²/day/atm) |
| P′O2 | Permeability coefficient (cm³·mm/m²/day/atm), homogeneous materials only |
| Temperature | Test temperature (typically 23 °C) |
| Relative humidity | RH on the nitrogen side (typically 0% for D3985) |
| Thickness | Average film thickness (µm), required for P′O2 calculation |
| Test area | Exposed cell area (cm²), used for normalised OTR calculation |
The time to reach steady state should be documented, as it provides an indication of the material’s barrier capability. A high-barrier film may require several days of stabilisation before the measurement becomes usable.
4. Regulatory Framework and Industrial Applications
4.1 Standards and Regulatory References
ASTM D3985 is part of a broader standards and regulatory ecosystem:
- ASTM F1927: complementary method for OTR under humid conditions (38 °C, 90% RH), essential for humidity-sensitive materials.
- ASTM F2622: OTR measurement of finished packages (rigid and flexible containers), complementary to material-only testing.
- ISO 15105-2: equivalent ISO method for gas transmission by coulometric sensor.
- ASTM D1434: manometric/volumetric method for gas transmission, applicable to all gases (O2, CO2, N2).
- ASTM F1249: water vapour transmission rate (WVTR), complementary to OTR for complete barrier characterisation.
The current version in force is D3985-17, developed by ASTM International Subcommittee D20.19.
4.2 Industry Applications
Food Packaging
Oxygen barrier is the key parameter for the shelf life of oxidation-sensitive foods: meat, cheese, coffee, snacks, ready meals under modified atmosphere packaging (MAP). ASTM D3985 is used to qualify barrier films (EVOH, PVdC, PA, metallised PET) and optimise multilayer structures.
Pharmaceutical Industry
Many active pharmaceutical ingredients are sensitive to oxidation. Pharmaceutical blisters, pouches and vials must maintain a rigorous oxygen barrier throughout the drug’s shelf life. The OTR measured according to D3985 is integrated into validation dossiers for primary packaging materials.
Medical Devices
Sterile packaging for medical devices must maintain sterility throughout the storage period. OTR measurement contributes to validating barrier integrity and defining storage conditions.
Beverages and PET Bottles
PET bottles for beer, fruit juices and carbonated beverages are subject to strict oxygen barrier requirements. D3985 (complemented by F2622 for finished containers) validates the effectiveness of barrier treatments (SiOx coatings, PA/PET blends, oxygen scavengers).
5. Best Practices for Reliable Testing
5.1 Choosing a Laboratory
Result reliability depends directly on laboratory competence. Selection criteria:
- ISO/IEC 17025 accreditation, guaranteeing technical competence and metrological traceability.
- Reference equipment: MOCON OX-TRAN type instrument (or equivalent) with calibrated coulometric sensor, leak-tight test cells and precise temperature control.
- Humidity control: for tests at 0% RH, the carrier gas must be dried to a dew point below –40 °C.
- Cell leak verification: any leak in the system produces artificially elevated OTR values.
5.2 Practical Recommendations
To maximise the value of D3985 testing:
- Systematically specify the temperature, relative humidity, oxygen partial pressure and film thickness in every test report.
- For materials containing EVOH or PA, complement D3985 (0% RH) with an F1927 test (90% RH) to evaluate barrier loss under humid conditions.
- Use the permeability coefficient (P′O2) to compare homogeneous materials of different thicknesses, and OTR to compare complete structures (laminates, metallised films).
- Complement OTR measurements with WVTR measurements (ASTM F1249) for a complete barrier characterisation of the material.
- For shelf life prediction, measure OTR at actual storage temperatures (4 °C, 25 °C, 38 °C) and model the temperature dependence.
Food packaging, pharmaceuticals, medical devices and beverages are the main user sectors. Any sector where oxygen affects the quality or safety of the packaged product benefits from this standard for barrier material selection, production control and supplier qualification.