The measurement of oxygen permeability quantifies a material's intrinsic ability to allow oxygen gas to pass through it. This crucial property is commonly expressed in units of Barrer (Dk), which represents the product of the oxygen's diffusion coefficient (D) and its solubility coefficient (k) within the material.
Understanding Oxygen Permeability (Dk)
Oxygen permeability, often abbreviated as Dk, is a fundamental material characteristic that indicates how readily oxygen molecules can move through a substance. It's not just about how fast oxygen can move (diffusion), but also how much oxygen the material can hold (solubility).
- Diffusion (D): This refers to the rate at which gas molecules move through the material. A higher diffusion coefficient means oxygen moves faster.
- Solubility (k): This indicates how much oxygen dissolves into the material itself. A higher solubility coefficient means the material can hold more oxygen.
The combined effect of these two factors determines the overall permeability. Materials with high Dk values are more "breathable" or permeable to oxygen.
The Barrer Unit
The standard unit for oxygen permeability is the Barrer. One Barrer is defined as:
$$1 \text{ Barrer} = 10^{-10} \frac{\text{cm}^3 \text{(STP)} \cdot \text{cm}}{\text{cm}^2 \cdot \text{s} \cdot \text{Pa}}$$
Where:
- cm³(STP): Cubic centimeters of gas at standard temperature and pressure (0°C, 1 atm).
- cm: Centimeters (thickness of the material).
- cm²: Square centimeters (area of the material).
- s: Seconds (time).
- Pa: Pascals (pressure difference across the material).
This unit essentially describes the volume of oxygen that passes through a given area and thickness of material over time under a specific pressure difference. For practical purposes, especially in contact lens applications, the unit is often simplified to just "Barrer."
Permeability vs. Transmissibility
While permeability (Dk) is an intrinsic property of the material itself, oxygen transmissibility (Dk/t) describes the actual amount of oxygen that can pass through a specific thickness ('t') of that material.
- Permeability (Dk): A property of the material.
- Transmissibility (Dk/t): A property of the specific product or sample, considering its thickness.
To convert a measured oxygen transmissibility into the material's permeability, you must account for the specific thickness of the sample. Specifically, transmissibility needs to be divided by the harmonic mean radial thickness of the measurement area. This adjustment ensures that the reported Dk value accurately reflects the material's inherent ability to allow oxygen passage, irrespective of the sample's thickness during measurement.
Why is Oxygen Permeability Measured?
Measuring Dk is critical in various industries and applications where gas barrier properties are paramount.
Key Applications:
- Contact Lenses: This is perhaps the most well-known application. High oxygen permeability in contact lenses (measured as Dk/t, where 't' is the lens thickness) is vital for ocular health, allowing sufficient oxygen to reach the cornea. Lenses with higher Dk/t values are considered more breathable.
- Example: A daily disposable silicone hydrogel lens might have a Dk of 100-160 Barrer, providing excellent oxygen delivery.
- Food Packaging: Oxygen acts as a primary spoilage agent for many foods. Packaging materials with low oxygen permeability help extend shelf life by preventing oxygen ingress.
- Medical Devices: Beyond contact lenses, other medical devices, such as wound dressings or implantable materials, may require specific oxygen permeability characteristics.
- Construction Materials: Breathable membranes or insulation materials might be engineered with specific Dk values to manage moisture and airflow.
How is Oxygen Permeability Measured?
Various standardized methods are used to measure oxygen permeability, depending on the material and application. These methods generally involve placing a sample of the material between two chambers: one containing a known concentration of oxygen and the other containing a carrier gas or vacuum. The rate at which oxygen passes through the material into the second chamber is then measured.
Common techniques include:
- Coulometric Sensors: Often used for contact lenses and films, these sensors detect tiny amounts of oxygen that pass through the material.
- Differential Pressure: Measuring the pressure drop across the material over time.
Factors Affecting Oxygen Permeability
Several factors can influence a material's oxygen permeability:
- Material Composition: The chemical structure and polymer network play a significant role.
- Temperature: Permeability generally increases with temperature as molecular movement accelerates.
- Humidity: Water content can significantly affect the Dk of hydrophilic materials like hydrogels.
- Thickness: While Dk is intrinsic, the overall oxygen flow (transmissibility) is inversely proportional to thickness.
Key Considerations for Dk Values
| Factor | Description |
|---|---|
| Units | Always check the units, as slight variations exist (e.g., cm³(STP) vs. cm³(normal)). Barrer is the most common. |
| Standards | Measurements should adhere to recognized standards (e.g., ISO, ASTM) to ensure comparability between different materials and manufacturers. |
| Conditions | Dk values are typically reported at specific temperatures (e.g., 35°C for contact lenses) and humidity levels, as these factors influence the measurement. |
| Accuracy | The accuracy of Dk measurements is crucial for material development and regulatory compliance. |
Understanding oxygen permeability is fundamental to designing and selecting materials for a vast array of applications, ensuring product performance, safety, and longevity.
