Views: 214 Author: Site Editor Publish Time: 2025-07-14 Origin: Site
Polyethylene terephthalate, commonly abbreviated as PET, is a thermoplastic polymer resin that belongs to the polyester family. Among its many applications, PET film stands out as a versatile material widely used in packaging, electronics, insulation, and labeling. Known for its high strength, chemical resistance, and transparency, PET film has become indispensable in modern industrial manufacturing.
However, one often-overlooked characteristic of PET film that plays a crucial role in product performance is its vapour permeability—a property that determines how effectively it can act as a barrier against moisture. This feature becomes particularly critical in applications like food packaging, pharmaceutical storage, and high-precision electronics, where moisture ingress can compromise integrity or product lifespan.
In this article, we dive deep into the vapour permeability of PET film, how it is measured, why it matters, and how it compares with alternative materials.
Vapour permeability, often quantified as Water Vapour Transmission Rate (WVTR), refers to the rate at which water vapor passes through a material under specific temperature and humidity conditions. It is a fundamental metric for evaluating the barrier performance of packaging films.
For PET film, vapour permeability is closely tied to its molecular structure. The tightly packed polymer chains in PET result in a semi-crystalline matrix, which inherently resists the diffusion of moisture. While PET is not entirely impermeable, it offers significantly better resistance compared to many other plastics such as polyethylene (PE) or polypropylene (PP).
Consider a moisture-sensitive pharmaceutical blister pack or a food tray sealed with PET film. Excessive water vapor ingress can result in mold formation, texture degradation, or chemical instability. In electronics, uncontrolled humidity may cause corrosion or short circuits. Thus, the vapour permeability of PET film directly influences product quality, shelf life, and even regulatory compliance.
The degree of crystallinity in PET film has a significant impact on its barrier properties. Higher crystallinity typically reduces vapor permeability as the denser structure limits the diffusion path for water molecules. On the other hand, amorphous PET exhibits higher permeability due to its loosely arranged polymer chains.
Film thickness also plays a pivotal role. As the thickness of PET film increases, the vapor path becomes longer and more tortuous, thereby reducing transmission rate. However, increasing thickness also affects flexibility, cost, and roll size, which must be balanced in practical applications.
Some PET films are engineered with special coatings or incorporated additives to enhance their barrier properties. Common enhancements include silicon oxide (SiOx) or aluminum oxide (AlOx) coatings, which dramatically lower the WVTR while maintaining transparency. Additionally, anti-fog, anti-static, or UV-resistant treatments may alter surface characteristics and influence permeability rates.
To assess PET film's performance, it’s essential to compare its WVTR values against alternative materials. The table below provides a typical range of WVTR values at 38°C and 90% RH for common packaging films:
| Film Type | Typical WVTR (g/m²/day) | Barrier Level |
|---|---|---|
| PET Film | 3–6 | Moderate Barrier |
| BOPP Film | 6–10 | Low Barrier |
| HDPE Film | 10–20 | Low Barrier |
| EVOH (Coated) | <1 | High Barrier |
| PVDC-Coated PET | <0.5 | Excellent Barrier |
| Aluminum Foil | 0 | Absolute Barrier |
As seen, standard PET film offers a moderate barrier, suitable for many dry goods and low-moisture environments. However, when enhanced with coatings or used in laminates, PET can rival high-barrier films like EVOH or PVDC composites.
In the food industry, PET film’s moderate vapour barrier makes it ideal for packaging snacks, frozen meals, and dry mixes. It prevents the ingress of moisture that could otherwise spoil the contents or reduce shelf life. When enhanced with coatings, PET can be used even for semi-moist products.
PET films with improved vapour resistance are used in blister packs, sachets, and medical trays. The goal is to ensure chemical and physical stability of sensitive substances, which could otherwise degrade due to moisture absorption.
PET’s excellent dimensional stability and vapor resistance make it suitable for circuit insulation, display protection layers, and solar back sheets. Here, water vapor ingress must be minimized to prevent corrosion and maintain functionality over time.
Vapour permeability is usually measured using standard protocols like ASTM E96 (Desiccant Method) or ISO 2528, which provide consistent test environments to derive WVTR values. During the test, a PET film is mounted between a controlled moisture source and a desiccant. The weight gain of the desiccant over time is used to calculate the transmission rate.
Testing conditions such as temperature (commonly 23°C or 38°C) and relative humidity (often 50% or 90%) are standardized, but results can vary significantly based on actual end-use environment. Thus, application-specific testing is highly recommended.
In practice, several variables can influence the measured vapour permeability of a PET film:
Pinholes or surface defects
Winding tension and storage conditions
Film orientation and manufacturing method (biaxial vs cast)
Hence, while datasheets provide nominal values, actual performance should be validated through in-house or third-party testing for critical applications.
Yes, PET film offers moderate barrier properties against water vapor, suitable for many dry and semi-moist products. For highly sensitive items, consider using PET with barrier coatings or laminate with other high-barrier layers.
Generally, uncoated PET film is recyclable. However, once coated with barrier materials like SiOx or AlOx, recyclability may become limited. Always check with local recycling guidelines and manufacturers for clarification.
Vapour transmission typically increases with temperature, as higher energy facilitates the diffusion of moisture molecules. Therefore, storage or use in high-temperature environments may necessitate enhanced PET films or multi-layered structures.
You can reduce moisture ingress by:
Increasing film thickness
Using coated PET films
Incorporating PET in multilayer laminates with other barrier layers
Ensuring proper sealing and handling to avoid punctures or surface damage
PET film remains a widely used material for a reason: it combines mechanical strength, chemical stability, and decent vapour barrier performance in a cost-effective package. Its vapour permeability, while not the lowest among polymers, can be tailored through coatings, thickness variation, and multilayer configurations to suit diverse application needs.
Whether you're packaging food, pharmaceuticals, or sensitive electronics, understanding the vapour transmission characteristics of PET film is essential for maximizing product shelf life and reliability. As sustainability and performance expectations rise, the demand for optimized barrier films like PET will only continue to grow. For professionals seeking balance between function, cost, and sustainability, PET film offers a smart, adaptable choice.
