Nonwoven materials, characterized by their web-like structure and production without weaving or knitting, have found widespread application in various industries, including healthcare, packaging, and filtration. Understanding the water vapor transmission rate (WVTR) of these materials is crucial for numerous reasons. In healthcare, WVTR influences the performance of wound dressings and diapers, ensuring adequate moisture management and preventing skin irritation. In packaging, WVTR determines the shelf life of products, particularly those sensitive to moisture, such as food and pharmaceuticals. In filtration, WVTR plays a critical role in the efficiency of air and liquid filters, affecting their ability to remove contaminants.
One of the significant challenges in measuring WVTR in nonwoven materials using traditional gravimetric methods, such as ASTM E96, is the difficulty in controlling the driving force of water vapor permeation. This driving force is primarily influenced by the humidity difference between the two sides of the film and is critical to providing accurate results.
These two sides include:
The combination of uncontrolled/unknown humidity on both sides of the film can render the measurement results inaccurate and inconsistent between different laboratories.
These environmental factors significantly impact the accuracy of WVTR measurements obtained using gravimetric methods when testing very high transmitting materials. Even slight changes in these conditions during testing can lead to inconsistent results. Controlling and maintaining stable conditions throughout the testing process is crucial but can be challenging. More advanced techniques (such as those used in ASTM D6701) are needed to overcome these challenges.
Method Summary:
1. Apparatus: A diffusion cell is created using a dry chamber (with flowing nitrogen gas), guard film, and a wet chamber. With water against the guard film, this allows extremely high humidity to be applied to the test sample without physically wetting the sample surface. The dry nitrogen carrier gas flowing through the chamber is routed to a water vapor sensor.
2. Guard Film Testing: An initial test is conducted to measure the WVTR of the guard film and the air gap between it and the evaporator assembly, which generates 100% relative humidity.
3. Nonwoven Test Material Testing: The film specimen is mounted into the diffusion cell (under the guard film) and the total system WVTR (Guard Film + Test Material) is analyzed.
4. WVTR Calculation: The instrument calculates the transmission rate of the Test Material using a resistance equation. WVTRTM = [1/WVTRTotal- 1/WVTRGF ] -1
5. Equilibrium Assessment: Based on operator input, the instrument determines when the readings indicate that the samples have reached equilibrium, signaling the end of the test. Because the nonwovens are high WVTR transmitters and equilibrate quickly, the samples could complete the test in a matter of hours. This is a very fast test.
Advantages of ASTM D6701:
The equipment has proven accuracy and repeatability across multiple laboratories as demonstrated in the ASTM 6701 Interlaboratory Study (ILS) data.
The instrument can analyze up to six samples at a time, which provides high throughput for QC labs.
As the world’s largest permeation instrument manufacturer, MOCON offers the following:
To learn more about the Permatran®-W 101K Permeation Analyzer, please visit www.ametekmocon.com.
The Need for WVTR Testing
Accurately measuring WVTR in nonwoven materials is essential for many reasons:- Product Development and Quality Control: WVTR testing helps manufacturers optimize material properties to ensure their products meet specific performance requirements. It also facilitates quality control by detecting variations in material performance and identifying potential defects.
- Product Development: It helps researchers develop and optimize materials with tailored WVTR properties to meet specific application needs. Accurate results are critical when designing nonwoven structures.
- Quality Control: It facilitates quality control by identifying variations in the material’s WVTR performance, helping to identify potential defects and variability. Note: For these high WVTR materials, a test method that provides fast turnaround times and high throughput is key to efficient material evaluation.
- Regulatory Compliance: Many industries, including food, pharmaceutical, and medical device manufacturing, are subject to regulatory standards that specify WVTR limits. As such, WVTR testing is critical to demonstrating compliance with these regulations and avoiding potential legal issues.
- Shelf-Life Prediction: It can be used to predict the shelf life of packaged products by estimating the rate at which moisture will penetrate the packaging material. This information is valuable in determining appropriate storage conditions and expiration dates.
- Material Selection: It helps engineers and designers select the most appropriate nonwoven materials for specific applications based on their moisture barrier properties. This can result in cost savings and improved product performance.
- Patient Health: Medical device companies rely on WVTR testing to ensure that their nonwoven products (such as surgical masks and gowns, medical tapes, even incontinence products) remain breathable but sterile to prevent harmful microbes from leaking through. Nonwoven materials are small enough that these microbes can’t get through and the product remains sterilized.
Challenges in Water Vapor Transmission Rate Testing
Environmental FactorsOne of the significant challenges in measuring WVTR in nonwoven materials using traditional gravimetric methods, such as ASTM E96, is the difficulty in controlling the driving force of water vapor permeation. This driving force is primarily influenced by the humidity difference between the two sides of the film and is critical to providing accurate results.
These two sides include:
- Humidity Control on the Wet Side: Generating a known and controlled relative humidity (RH) on the wet side of the film can be challenging, especially when dealing with highly permeable materials. The high rate of water vapor permeation of nonwovens creates a quickly varying still air gap between the water in the cup and the test specimen. This makes it difficult to maintain a consistent test RH..
- Humidity on the Dry Side: Even if the wet side is maintained at a controlled RH, the surface of the “dry” side will often have a significantly higher humidity level, creating a boundary layer that is different from the chamber humidity, yielding a higher and imprecise humidity on the “dry” side of the film.
The combination of uncontrolled/unknown humidity on both sides of the film can render the measurement results inaccurate and inconsistent between different laboratories.
These environmental factors significantly impact the accuracy of WVTR measurements obtained using gravimetric methods when testing very high transmitting materials. Even slight changes in these conditions during testing can lead to inconsistent results. Controlling and maintaining stable conditions throughout the testing process is crucial but can be challenging. More advanced techniques (such as those used in ASTM D6701) are needed to overcome these challenges.
A Water Vapor Transmission Rate Solution
ASTM D6701 is a highly accurate and reliable method for measuring WVTR in nonwoven materials. Unlike conventional gravimetric methods, such as ASTM E96, D6701 uses a controlled humidity chamber, a very high-water vapor transmitting guard film, and an electronic sensor for monitoring the transmitting water vapor.Method Summary:
1. Apparatus: A diffusion cell is created using a dry chamber (with flowing nitrogen gas), guard film, and a wet chamber. With water against the guard film, this allows extremely high humidity to be applied to the test sample without physically wetting the sample surface. The dry nitrogen carrier gas flowing through the chamber is routed to a water vapor sensor.
2. Guard Film Testing: An initial test is conducted to measure the WVTR of the guard film and the air gap between it and the evaporator assembly, which generates 100% relative humidity.
3. Nonwoven Test Material Testing: The film specimen is mounted into the diffusion cell (under the guard film) and the total system WVTR (Guard Film + Test Material) is analyzed.
4. WVTR Calculation: The instrument calculates the transmission rate of the Test Material using a resistance equation. WVTRTM = [1/WVTRTotal- 1/WVTRGF ] -1
5. Equilibrium Assessment: Based on operator input, the instrument determines when the readings indicate that the samples have reached equilibrium, signaling the end of the test. Because the nonwovens are high WVTR transmitters and equilibrate quickly, the samples could complete the test in a matter of hours. This is a very fast test.
Advantages of ASTM D6701:
- Accurate WVTR Measurement: The controlled humidity chamber and advanced sensor technology ensure accurate WVTR measurements.
- Elimination of Gravimetric Challenges: The method avoids the limitations of traditional gravimetric methods, such as controlling humidity on both sides of the film.
- Standardization: As a standardized ASTM method, D6701 provides a reliable and consistent approach to WVTR testing.
MOCON: Sole Supplier of the D6701 Apparatus
Traditional E96 testing of these very high WVTR materials can be problematic and lead to variances between laboratories. The instrumental method developed by MOCON (makers of automated instrumental permeation analyzers for nearly six decades) addresses the E96 concerns with an instrument that yields consistent results for these high transmitters. Indeed, MOCON defined the method and is currently the sole supplier of the apparatus required to perform the ASTM D6701 test.The equipment has proven accuracy and repeatability across multiple laboratories as demonstrated in the ASTM 6701 Interlaboratory Study (ILS) data.
The instrument can analyze up to six samples at a time, which provides high throughput for QC labs.
As the world’s largest permeation instrument manufacturer, MOCON offers the following:
- Permatran®-W 101K Permeation Analyzer: This instrument provides precise control of humidity and temperature within the testing chamber, allowing for accurate WVTR measurements of high transmitting nonwoven materials.
- Calibration Films and Accessories: A variety of accessories to support ASTM D6701 testing, including calibration standards and sample holders.
- Technical Support: Comprehensive technical support to customers, assisting with equipment selection, calibration, and test procedures.
Conclusion
Accurate WVTR testing is essential for understanding the performance of non-woven materials in various applications. While several challenges can arise in measuring WVTR, ASTM D6701 provides a reliable and standardized method for obtaining accurate results. MOCON’s equipment and technical support play a crucial role in enabling accurate WVTR testing, ensuring that manufacturers can meet regulatory requirements, optimize product design, and improve overall product quality.To learn more about the Permatran®-W 101K Permeation Analyzer, please visit www.ametekmocon.com.
