Oxygen Transmission Rate Testing of Rubber Materials: Principles, Standards, and Applications

Oxygen transmission rate (OTR) testing measures the permeability of rubber materials to oxygen gas, a critical performance parameter for tire inner liners, pharmaceutical closures, seals, and barrier components. This article explains the testing principles, applicable international standards such as ASTM D3985, ISO 15105, and ISO 2782, sample requirements, and typical applications, providing engineers and quality professionals with a practical guide to oxygen permeability evaluation of rubber materials in accredited third-party laboratories.

Technical Overview

Oxygen transmission rate (OTR), also referred to as oxygen permeability or oxygen gas transmission rate (O2 GTR), quantifies the steady-state volume of oxygen that permeates through a unit area of a rubber material per unit time under a specified oxygen partial pressure gradient. The most common reporting unit is cm³/(m²·24h), with the permeability coefficient expressed as cm³·mm/(m²·24h·0.1 MPa) when normalized for material thickness.

Gas permeation through rubber follows the solution-diffusion mechanism: oxygen molecules first dissolve into the polymer surface on the high partial pressure side, diffuse through the free volume of the polymer matrix, and then desorb on the low partial pressure side. The permeability coefficient is therefore the product of the diffusion coefficient (D) and the solubility coefficient (S). Because elastomers are amorphous, flexible-chain polymers operating above their glass transition temperature, their free volume is relatively high, and permeability can vary by more than an order of magnitude between different rubber families.

Among common elastomers, butyl rubber (IIR) and halobutyl rubber (BIIR/CIIR) exhibit the lowest oxygen and air permeability due to the dense packing of methyl groups along the polymer backbone, which is why they are the material of choice for tire inner liners and pharmaceutical closures. Fluoroelastomers (FKM) and nitrile rubber (NBR, with higher acrylonitrile content) also provide relatively low permeability. Natural rubber (NR), styrene-butadiene rubber (SBR), and EPDM show moderate to high transmission rates, while silicone rubber (VMQ) demonstrates the highest oxygen permeability among widely used elastomers. Compounding ingredients such as carbon black and other particulate fillers reduce permeability by creating a tortuous diffusion path, while plasticizers and elevated temperatures generally increase it.

Accurate OTR measurement is essential for material selection, compound development, quality control, and product qualification. Reliable results depend on standardized test methods, careful specimen preparation, precise control of temperature and humidity, and verification of steady-state permeation conditions.

Testing Samples

Rubber Sheets and Films

Flat, defect-free sheets or calendered films are the most common specimen form. Typical specimen thickness ranges from 0.2 mm to 2 mm, with discs or squares sized to fully cover the defined test area of the permeation cell plus a sealing margin. Specimens must be free of pinholes, voids, bubbles, cracks, and surface contamination, as even microscopic defects can dominate the measured transmission rate.

Vulcanized Compounds and Molded Plaques

For compound evaluation, test plaques are vulcanized under controlled temperature, pressure, and cure time according to the material specification. The cure state, crosslink density, and post-cure conditioning should be documented because residual cure by-products and incomplete crosslinking can measurably alter oxygen permeability. Plaques should be conditioned before testing to reach equilibrium.

Finished Products and Components

Specimens may be taken directly from manufactured articles, including tire inner liner laminates, inner tubes, O-rings, gaskets, diaphragms, pharmaceutical stoppers, gloves, and rubber membranes. When testing finished parts, flat sections are cut or sectioned to fit the permeation cell, and the sampling location and orientation are recorded. Curved or irregular components may require special fixtures or the preparation of representative flat coupons from the same compound and cure conditions.

Testing Items

Typical testing items and deliverables for rubber oxygen permeability evaluation include:

• Oxygen transmission rate (OTR) at standard conditions, reported in cm³/(m²·24h)
• Oxygen permeability coefficient normalized for thickness, reported in cm³·mm/(m²·24h·0.1 MPa)
• Temperature-dependent OTR profiles and activation energy of permeation (Arrhenius analysis)
• OTR measurement under controlled relative humidity for humidity-sensitive rubbers
• Comparative permeability ranking of candidate compounds or suppliers
• Evaluation of filler loading, plasticizer content, or cure system effects on barrier performance
• OTR testing after heat aging, fluid exposure, or environmental conditioning
• Diffusion and solubility coefficients derived from time-lag analysis

Testing Methods

Coulometric Sensor Method (Equal-Pressure Method)

The coulometric method, standardized in ASTM D3985 and ISO 15105-2, is the most widely used technique for oxygen transmission rate testing. The specimen is clamped as a barrier between two chambers. High-purity oxygen or air flows through the upper chamber, while oxygen-free nitrogen carrier gas sweeps the lower chamber. Both sides are maintained at essentially equal total pressure (approximately atmospheric), so permeation is driven purely by the oxygen partial pressure difference. Oxygen that permeates through the specimen is transported by the carrier gas to a coulometric sensor, where an electrical current proportional to the quantity of oxygen is generated. The OTR is calculated from the sensor signal after calibration with a known standard. This method closely simulates real-world package conditions and, per ASTM F1927, permits testing under controlled temperature and relative humidity.

Differential Pressure Method (Manometric Method)

The differential pressure method, described in ASTM D1434 and ISO 15105-1, divides a sealed permeation cell into two chambers with the specimen acting as the barrier. The downstream chamber is evacuated while the upstream chamber is pressurized with oxygen. The increase in pressure on the downstream side, measured by a high-sensitivity pressure transducer over time, is used to calculate the gas transmission rate and permeability coefficient. This method is free from sensor drift, accommodates a wide measurement range, and is suitable for high-permeability rubbers, though humidity control is more difficult to implement.

Constant-Volume Method for Rubber Materials

The ISO 2782 series addresses gas permeability determination specifically for vulcanized and thermoplastic rubber. A disc-shaped specimen with a defined test area is mounted between chambers; oxygen is applied at constant pressure on one side, and the pressure (or volume) change on the measurement side is recorded. Beyond the steady-state permeability coefficient, the time-lag technique allows simultaneous determination of the diffusion coefficient and the solubility coefficient, providing a complete characterization of the transport behavior of the rubber compound.

Specimen Preparation and Conditioning

Specimens are conditioned prior to testing, typically at 23 °C ± 2 °C and 50% ± 5% relative humidity for at least 24 hours in accordance with ISO 23529 or the governing material specification. Thickness is measured at a minimum of five points across the test area using a calibrated gauge accurate to 1% or 0.001 mm, whichever is greater, and the mean value is reported. Testing is typically performed at 23 °C or 37 °C, with the temperature, relative humidity, test gas, test area, and specimen thickness clearly documented in the report. Measurements continue until a steady-state transmission rate is achieved, which may take from several hours for thin, high-permeability specimens to more than 48 hours for thick, high-barrier materials. Repeatability within a single laboratory is typically in the range of 5% to 10% for properly conditioned specimens.

Applications

Oxygen transmission rate testing of rubber materials supports numerous industries and product categories:

• Tire industry: air retention performance of inner liners, inner tubes, and tubeless tire barrier layers
• Pharmaceutical packaging: oxygen barrier evaluation of elastomeric closures, stoppers, and septa for injection vials
• Medical devices: gloves, catheters, tubing, respiratory equipment membranes, and balloon components
• Fuel and chemical handling: permeation assessment of hoses, seals, bladders, and diaphragms
• Vacuum technology and aerospace: qualification of O-rings and seals where minimal gas permeation is critical
• Food and beverage packaging: barrier properties of rubber gaskets, sealing membranes, and flexible laminates
• Research and development: compound formulation optimization, filler dispersion studies, and supplier material qualification
• Quality control: incoming material inspection, batch-to-batch consistency verification, and regulatory compliance testing

FAQ

What is the difference between oxygen transmission rate and oxygen permeability coefficient?

Oxygen transmission rate (OTR) describes the total oxygen flux through a specific specimen of defined thickness under specified conditions, expressed in cm³/(m²·24h). The permeability coefficient is a material property normalized for thickness, expressed in cm³·mm/(m²·24h·0.1 MPa), allowing direct comparison between compounds of different thicknesses.

Which rubber has the lowest oxygen permeability?

Butyl rubber (IIR) and halobutyl rubber exhibit the lowest oxygen and air permeability among common elastomers, followed closely by fluoroelastomers (FKM) and high-acrylonitrile nitrile rubber (NBR). Silicone rubber (VMQ) has the highest permeability, and EPDM, NR, and SBR fall in the intermediate range.

How does specimen thickness affect the test result?

OTR is inversely proportional to specimen thickness; doubling the thickness approximately halves the transmission rate. The permeability coefficient remains constant for a homogeneous material, which is why thickness normalization and accurate thickness measurement are essential.

Can testing be performed at elevated temperature or humidity?

Yes. Temperature-controlled testing, for example at 35 °C, 40 °C, or application-specific temperatures, is routinely performed to study temperature dependence and generate Arrhenius plots. The coulometric method also supports testing under controlled relative humidity, which is important for polar rubbers whose permeability changes with moisture absorption.

How long does an oxygen transmission rate test take?

Testing duration depends on specimen thickness and barrier level. Each specimen is tested until steady-state permeation is reached, typically ranging from a few hours for thin, highly permeable rubbers to 48 hours or longer for thick or high-barrier materials such as butyl-based compounds.

What information should be included in the test report?

A complete report includes the test standard applied, material identification and compound description, specimen dimensions and mean thickness, conditioning procedure, test temperature and relative humidity, test gas and carrier gas purity, test area, steady-state OTR and/or permeability coefficient values, and the testing date and laboratory accreditation status.