Testing Water Vapor Transmission Rate (WVTR) in films is a critical process for determining how well a material—such as plastic films, packaging materials, or protective coatings—resists the passage of water vapor. This property is essential for industries such as food packaging, pharmaceuticals, textiles, and electronics, where moisture control is vital for product preservation and quality. Here’s a step-by-step guide on how to test Water Vapor Transmission in films:
Methods for Testing Water Vapor Transmission in Films
The Gravimetric Method and the Infrared Method are the two primary methods used to measure WVTR in films.
1. Gravimetric Method (ASTM E96, ISO 15106)
This is the most commonly used and traditional method for measuring water vapor transmission rate, and it involves measuring the weight change of a sample exposed to a moisture gradient.
Equipment Needed:
Steps:
Sample Preparation:
- Cut the film sample into a precise size (usually a circular or square shape) according to the test standard.
- Clean the film to remove any dust or particles that could affect the test.
Assembly of Test Cell:
- Place the film sample in a test cell, which typically consists of a dish or cup containing a known amount of water or desiccant.
- Seal the edges of the test cell to ensure the film is tightly secured.
- The exposed side of the film should be in contact with either a moisture gradient (water) or a desiccant.
Environmental Conditions:
- The test should be conducted at a controlled temperature (usually 23°C) and humidity (RH) (typically 50%).
- Place the test cell in a controlled environment or a climate chamber to maintain these conditions.
Monitoring Weight Change:
- Over time, water vapor will pass through the film, leading to a change in the weight of the water container or desiccant.
- Use a precision balance to weigh the test cell at regular intervals (e.g., every 24 hours).
- Record the weight change over time. The larger the weight change, the higher the WVTR.
Calculation:
- The WVTR is calculated using the following formula:
Where:
- Δm\Delta mΔm = Change in mass (grams)
- AAA = Area of the film exposed to water vapor (cm2)
- ttt = Time of measurement (hours or days)
The result will typically be expressed in g/m2/day or g/100 in2/day.
2. Dynamic (or Infrared) Method
This method uses an infrared sensor or a similar detector to measure the vapor transmission without relying on weighing the test sample directly. It is a faster, more automated approach compared to the gravimetric method.
Equipment Needed:
- Dynamic WVTR Tester (Labthink C390H Water Vapor Transmission Rate Tester)
- Film sample (cut into the required size)
- Test chamber (usually divided into two compartments: one with water and one with dry air or desiccant)
- Environmental control (temperature and humidity chamber)
Steps:
Sample Preparation:
- Cut the film into the appropriate dimensions.
- Ensure the sample is clean and free from particles or contaminants.
Test Setup:
- Place the film between two compartments: one with a known moisture source (e.g., water) and the other with dry air or a desiccant.
- The compartments are maintained at different humidity levels, creating the vapor gradient across the film.
Detection:
- The test chamber is equipped with an infrared sensor or similar moisture detection technology.
- As water vapor passes through the film, the change in the moisture level on the dry side of the film is detected by the sensor.
Data Recording:
- The WVTR value is automatically calculated by the system based on the moisture difference over time and the area of the film exposed to the vapor.
3. Other Methods
While the gravimetric and dynamic methods are most common, some specialized applications may use the Electrolytic Method, which measures the conductivity changes in the moisture on the film's surface. you can refer to Labthink C330H Electrolytic Method Water Vapor Transmission Rate Tester
Factors Affecting WVTR Testing
- Temperature: The testing temperature should be controlled because higher temperatures generally increase the water vapor transmission rate.
- Humidity: The relative humidity on both sides of the film significantly affects the results. Ensure that environmental conditions are precisely controlled.
- Film Thickness: Thicker films may have lower WVTR values since there is more material for the moisture to diffuse through.
- Film Material: Different materials (e.g., PET, PE, PLA) have different permeabilities, with some being more resistant to vapor transmission than others.
- Test Duration: Longer test durations provide more accurate results, especially when measuring low-permeability films.
Applications of WVTR Testing
- Packaging Industry: Ensuring that food, pharmaceutical, and consumer goods packaging maintains its protective barrier against moisture.
- Textiles: Testing fabrics and membranes for waterproof and breathable performance.
- Electronics: Evaluating protective films used in electronic devices, ensuring that moisture doesn’t damage sensitive components.
- Coatings: Testing paints and coatings that protect surfaces from moisture ingress.
Conclusion
Testing the water vapor transmission rate in films is essential for assessing the moisture barrier properties of materials used in various industries. Understanding and measuring WVTR helps manufacturers choose the right materials for specific applications, ensuring product quality and longevity. Labthink has a completed product lines of water vapor transmission rate testers based on Gravimetric Method, Infrared Method and Electrolytic Methods.
Our Labthink water vapor permeability testers are recognized as the industry-leading solution for over 30 years and are the basis for many global permeability testing standards such as ASTM E96, ASTM F1249, ISO 10156 and etc. The family of Labthink WVTR analyzers offer a solution for almost any water vapor transmission rate (WVTR) testing need from sheet or barrier films to all types of packages, bottles, cartons, and more.