Water Vapor Transmission Rate (WVTR) Testing for Plastic Films

1. Introduction to WVTR
The Water Vapor Transmission Rate (WVTR) is a critical property used to assess the moisture barrier performance of plastic films and flexible packaging materials. It measures the amount of water vapor that passes through a unit area of film over a specified period, typically expressed in grams per square meter per day (g/m²/day). WVTR is especially important in applications requiring moisture-sensitive protection, such as pharmaceuticals, dehydrated foods, electronics, and medical devices.

The WVTR is calculated using the formula:

\text{WVTR} = \frac{\text{Mass of Water Vapor Transmitted}}{\text{Area} \times \text{Time}}'
 

2. Fundamental Theory – Diffusion and Permeation
Moisture permeation through polymer films is governed by molecular diffusion, described by Fick’s First Law of Diffusion:

J = -D \frac{dC}{dx}

Where:

J is the water vapor flux,

D is the diffusion coefficient of the polymer,

dC/dx is the concentration gradient across the film thickness.


In practical applications, the Permeability (P) of a material becomes more relevant:

P = D \times S

Here, S is the solubility coefficient of water vapor in the polymer. Polymers with low diffusion and solubility values offer superior moisture resistance. High-barrier materials such as PVDC, PCTFE, and SiOx-coated films exhibit exceptionally low WVTR due to their reduced permeability.
 

3. Key Factors Influencing WVTR
Film Thickness: Thicker films provide a longer diffusion path, effectively lowering the transmission rate. Multilayer films with barrier layers (e.g., EVOH, PVDC, metal oxides) significantly enhance moisture resistance.

Polymer Type: The inherent structure of the polymer greatly affects WVTR. High-barrier polymers like PVDC, EVOH, and Aclar (PCTFE) perform far better than polyolefins (PE, PP) or bioplastics like PLA. PET offers moderate resistance and is often coated to improve its barrier properties.

Temperature and Humidity: Higher temperatures increase polymer chain mobility and water molecule diffusivity, accelerating transmission. Elevated humidity also increases the driving force for vapor migration, resulting in higher WVTR.

Polymer Crystallinity: Crystalline regions hinder water vapor passage. Materials like HDPE, with high crystallinity, show lower WVTR, while amorphous polymers like LDPE or TPU allow more moisture transmission due to their looser molecular structure.
 

4. Standards for WVTR Testing
Commonly recognized standards for WVTR measurement include:

ASTM E96: Gravimetric method using weight change in a controlled cup setup.

ASTM F1249: Instrumental method with infrared sensors for detecting water vapor.

ASTM F3299: Ultra-sensitive electrolytic method using phosphorus pentoxide sensors.

Each method varies in terms of precision, sensitivity, equipment complexity, and applicability based on the film’s barrier level.
 

5. WVTR Testing Methods and Measurement Principles
Gravimetric Method (ASTM E96)
This traditional method involves sealing a test film over a cup containing either desiccant or water, placed inside a humidity-controlled chamber. The change in weight over time is used to calculate WVTR. Though widely used, it lacks precision for high-barrier films and requires long equilibration times.

Infrared Sensor Method (ASTM F1249)
In this method, humid air is applied to one side of the film, with dry nitrogen on the other. As moisture permeates, it is detected by an infrared sensor. This method offers fast, accurate, and repeatable measurements—ideal for films with medium barrier performance.

Electrolytic Sensor Method (ASTM F3299)
The most advanced method for ultra-low WVTR detection. Water vapor passing through the film reacts with phosphorus pentoxide, producing an electrical signal proportional to the moisture content. This method detects values as low as 10⁻⁶ g/m²/day, making it essential for electronics, OLEDs, and high-end pharmaceutical packaging.
 

6. Application Case Studies
Pharmaceutical Packaging:
Uncoated PVC offers minimal barrier protection. However, films coated with PVDC or made from Aclar (PCTFE) show significantly reduced WVTR, with Aclar providing the highest moisture resistance.

Food Packaging:
Compared to PET, bio-based PLA has a higher WVTR, limiting its use for moisture-sensitive products unless coated or metalized. PET is widely used for dry or semi-moist foods due to its moderate barrier properties.

Electronics:
OLED devices are extremely sensitive to moisture. While standard PET allows rapid ingress, SiOx-coated PET can reduce WVTR to less than 10⁻⁵ g/m²/day, enhancing device lifespan and reliability.

7. Conclusion
WVTR is a vital performance indicator for plastic films used in packaging and protective applications. A thorough understanding of diffusion principles, polymer characteristics, and environmental influences is key to selecting the right material and testing method.

Selecting the appropriate WVTR test standard depends on the sensitivity required. While ASTM E96 is suitable for general-purpose films, advanced technologies demand higher precision using methods such as ASTM F1249 or ASTM F3299.

For packaging engineers, material scientists, and quality assurance professionals, mastering WVTR principles ensures robust design, product safety, and long-term performance across a wide range of industries—from food and pharmaceuticals to high-tech electronics.

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Authored by:
Mr. Sanjay Saxena
Faculty, Polymerupdate Academy