What is the water vapor permeability of PE ?

Water vapor permeability is an important physical property of films, coatings and other materials. It describes the ability of water vapor to pass through a material as a result of a difference in water vapor pressure between its two sides. This property is particularly relevant in packaging applications, where controlling moisture transfer is essential for protecting products and maintaining their quality during storage and transportation.

The water vapor transmission rate (WVTR) indicates the amount of water vapor that passes through a defined area of material over a specified period under particular test conditions. It is commonly expressed in grams per square metre per day, written as g/(m²·d).

A low water vapor transmission rate indicates that a material provides an effective barrier against moisture transfer. A high rate indicates that water vapor can pass through the material more readily. When evaluating the moisture barrier properties of a film, it is therefore essential to consider both the measured transmission rate and the environmental conditions under which the measurement was performed.

Water Vapor Barrier Properties of Polyethylene

Polyethylene (PE) is widely used in packaging because of its favourable moisture barrier properties. Its non-polar molecular structure interacts relatively weakly with water, limiting the absorption and transport of water molecules through the polymer.

Unlike highly polar materials, which can interact more readily with water, polyethylene generally exhibits low water vapor permeability. This makes it particularly suitable for applications where moisture protection is required.

Polyethylene films can help prevent dry products from absorbing moisture from the surrounding environment. They can also reduce moisture loss from products that need to retain their original water content. Depending on the application, this may help preserve product quality, texture, weight and storage stability.

However, polyethylene is not completely impermeable to water vapor. Moisture can still diffuse through the polymer over time, and the rate of transmission depends on the type of polyethylene, the film thickness, the temperature, the relative humidity and the pressure difference across the film.

For this reason, the moisture barrier performance of a PE film should always be assessed in relation to the specific requirements of the intended application.

Differences Between PE-HD and PE-LD

The water vapor permeability of polyethylene depends significantly on its molecular structure and density. The two most common types used in film production are high-density polyethylene (PE-HD) and low-density polyethylene (PE-LD).

High-Density Polyethylene (PE-HD)

PE-HD has a predominantly linear molecular structure with relatively little branching. This allows the polymer chains to pack more closely together, resulting in higher density and generally greater crystallinity.

The more compact molecular structure restricts the movement of water molecules through the material. As a result, PE-HD typically provides a better water vapor barrier than PE-LD when films of comparable thickness are tested under equivalent conditions.

High-density polyethylene is therefore suitable for applications requiring effective moisture protection, good stiffness and mechanical strength. Depending on the film formulation and processing method, it can be used in packaging films, protective liners, bags and various industrial applications.

Low-Density Polyethylene (PE-LD)

PE-LD contains more branching in its molecular chains, resulting in a lower density and generally lower crystallinity than PE-HD. The less compact structure typically allows water vapor to pass through the material more readily.

Consequently, PE-LD generally has a higher water vapor transmission rate than PE-HD of comparable thickness under the same test conditions.

Despite this difference, PE-LD remains an effective moisture barrier for many applications. Its flexibility, softness, good sealability and ease of processing make it particularly suitable for flexible packaging, protective covers, bags and films used in a wide range of industries.

The choice between PE-HD and PE-LD depends on the required moisture protection and the mechanical properties needed for the finished product.

Water Vapor Transmission Rate and Film Thickness

Film thickness is one of the most important factors influencing water vapor transmission. Under otherwise identical conditions, a thicker film generally provides a longer diffusion path and reduces the amount of water vapor that passes through the material per unit area and time.

For this reason, thicker polyethylene films typically provide better moisture barrier performance than thinner films made from the same material and with an otherwise comparable structure.

The relationship between thickness and transmission rate is particularly important when designing packaging for moisture-sensitive products. A film must be thick enough to provide the required barrier performance while also meeting practical requirements for flexibility, material consumption, processing efficiency and cost.

It is important to distinguish between the water vapor transmission rate and the water vapor permeability coefficient. The transmission rate describes the actual moisture transfer through a film of a given thickness under specified conditions. The permeability coefficient is a material-related parameter that accounts for the influence of thickness and the driving force for diffusion.

This distinction is essential when comparing materials of different thicknesses or evaluating data from different suppliers.

Typical Water Vapor Transmission Values for PE-HD and PE-LD

Under the specified measurement conditions, polyethylene films can exhibit different water vapor transmission rates depending on their density, thickness and molecular structure.

For illustration, the following approximate values may be used as reference points for films with a thickness of 100 µm:

The comparison shows that PE-HD can provide a lower water vapor transmission rate than PE-LD at the same nominal thickness. Under the conditions represented by these figures, the PE-HD film transmits approximately 60% less water vapor than the PE-LD film.

When using such values in technical documentation, the applicable test method and relative humidity conditions should also be stated. A temperature of 23 °C alone is not sufficient to make water vapor transmission results fully comparable.

Influence of Temperature and Humidity

Water vapor transmission through polyethylene is influenced by the conditions on both sides of the film. Temperature and relative humidity are particularly important because they affect the driving force for moisture diffusion and the rate at which molecules move through the polymer.

An increase in temperature generally increases the mobility of molecules within the material and can result in a higher water vapor transmission rate. Similarly, a greater difference in water vapor pressure between the two sides of the film can increase the rate of moisture transfer.

Consequently, measurements performed at different temperatures or humidity levels may produce significantly different results, even when the same polyethylene film is tested.

For meaningful comparisons, water vapor transmission rates should always be evaluated using consistent test conditions. Where the film is intended for demanding storage or transportation environments, measurements under conditions representative of actual use can provide a more reliable indication of its performance.

Applications of Polyethylene Films for Moisture Protection

Due to their favourable moisture barrier properties, polyethylene films are used in a wide range of packaging and industrial applications.

Typical applications include:

• Food packaging: Helping protect products against external moisture and reducing moisture loss from packaged goods.
• Packaging for dry products: Limiting moisture absorption in products that require dry storage conditions.
• Bags and flexible packaging: Providing a practical moisture barrier combined with flexibility and reliable sealing properties.
• Protective films and covers: Reducing exposure to environmental moisture during storage and transportation.
• Industrial liners and packaging: Helping protect moisture-sensitive materials and components.

The required level of moisture protection varies according to the product and its storage conditions. While standard polyethylene films provide sufficient protection for many applications, particularly moisture-sensitive products may require thicker films or a more advanced packaging structure.

Improving Moisture Barrier Performance

When the water vapor barrier properties of a standard polyethylene film are insufficient, several approaches can be considered to improve overall packaging performance.

Increasing the film thickness can reduce water vapor transmission, provided that the material composition and test conditions remain comparable. Selecting a different polyethylene grade may also improve performance. For example, PE-HD may be preferable when a lower water vapor transmission rate and greater stiffness are required.

In applications with particularly demanding moisture barrier requirements, polyethylene can be combined with other materials in multilayer structures. Each layer can provide a specific function, such as mechanical strength, moisture resistance, gas barrier performance or heat sealability.

The choice of a suitable solution depends on the required barrier level, the nature of the packaged product, processing conditions, material compatibility and cost considerations.

Selecting the Right PE Film for Moisture-Sensitive Applications

Selecting the right polyethylene film requires a careful balance between moisture protection and the practical demands of the application. Important considerations include the type of polyethylene, film thickness, water vapor transmission rate, temperature and humidity during use, as well as the mechanical and processing requirements.

PE-HD generally offers a better water vapor barrier than PE-LD at comparable thicknesses, while PE-LD is often preferred when flexibility and sealing performance are particularly important.

By selecting the appropriate material grade and film thickness, manufacturers can achieve reliable moisture protection tailored to the intended use. For applications with stringent requirements, the measured water vapor transmission rate should be verified under clearly defined test conditions to ensure that the selected film delivers the required performance throughout its intended service life.

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