Moisture and Water Vapor Transmission Rates in Packaging

Water Vapor Transmission

WVTR & MVTR Permeation 

Many packaged products are sensitive to moisture therefore control of water vapor into or out of the package is critical to the contained product’s quality. The permeability of packaging materials has a direct effect on a packages performance in terms of the contained products shelf-life. This permeation by moisture is measured by WVTR (water vapor transmission rate) or MVTR (moisture vapor transmission rate). The primary purpose of packaging is to get the packaged product to the customer in one piece and suitable for consumption. Packaging plays many roles, containing and protecting products during distribution, storage, and use, while also promoting, educating, and selling.

The barrier property requirements of a particular package are determined by the characteristics of the packaged product and its planned end use. Shelf-life is the length of time that packaged perishable, products including foods, pharmaceuticals, and chemicals, etc. are protected and may be still considered suitable for use. Knowledge of the barrier properties of packaging materials is necessary in order to provide a proper package, and an estimate of a contained product’s shelf-life. Packaging materials may be permeable to gases, liquids, organic vapors, and water vapor.

(O²) oxygen and water vapor get the most attention as they can readily move either from the inside of a package to the outside environment, or from the outside environment to the inside of the package. Either way they may have a detrimental effect on the quality and shelf life of a packaged product.

WVTR (water vapor transmission rate), referred to alternatively as MVTR (moisture vapor transmission rate), stands for the standard measure of the passing (permeation) of gaseous H²O through a substrate. It is the steady state rate at which gaseous water vapor passes through a substrate at specific temperature and relative humidity (RH) conditions over a period of time. Units of measurement are g/m²/24 hr. or g/100 in²/24 hr.

Considering certain packaged food products, the control of moisture permeation has a direct effect on product quality and shelf-life. Some packaged wet food products such as meat, seafood, pet food, cheese, muffins, etc., necessitate maintaining a certain moisture level inside a package. On the opposing side, other packaged dry food products including cereals, chips, snacks, pet foods, etc., require a dry, moisture-free level to maintain taste, texture and overall product quality. Products generally will gain or lose moisture rapidly without protective packaging, seeking equilibrium with environmental (the relative humidity or RH). An unsuitable permeable package for example, would likely give us unacceptable and uneatable no longer crisp chips and hard, dried out previously chewy cheese.

What factors affect WVTR/MVTR values?

  • Thickness of barrier material
  • Composition of resin
    • Crystallinity/density
    • Chain length & distribution
    • Chain orientation
    • Molecular weight distribution
  • Polymer blend
  • Coatings
  • Additives
  • Processing

Testing WVTR/MVTR
There are a number of WVTR/MVTR test methods that conform to recognized U.S. and international ASTM, ISO, and TAPPI standards.

  • Gravimetric (Cup) method
  • Electrolytic Detection Sensor Method
  • Infrared Detection Sensor Method
  • Humidity Detection Sensor Method

The moisture resistance of substrate/barrier materials is commonly measured by determining the permeation of a material. Permeation is a three-stage process. First, a gas or vapor, a permeate, is absorbed into a materials surface. Second, the gas/vapor diffuses throughout the material. Third, the gas/vapor desorbs or out gases from the materials opposite side. A driving force is a pressure differential created on each side of the material. During testing, it is important to control temperature, RH and driving force as they can materially affect the permeation rate.

In modern day testing equipment, the test cell is divided into two chambers by a mounted substrate/barrier test material. One chamber controlled by an RH probe insures that a proper constant amount of water vapor is supplied. The other chamber is charged with a nitrogen N² carrier gas. When the water vapor permeates the test material, the molecules are picked up by the N² carrier gas and are carried to a sensor detector. The supplied N² flow rate maintains the driving force throughout the test period. An infrared detector measures the increase of water vapor in the carrier gas and automatically calculates the WVTR/MVTR of the test material.

CORK FDA COMPLIANT BARRIER COATINGS

Cork, within its broad line of aqueous coatings offers a selection of barrier coatings. These are FDA compliant with, 21 CFR 176.170 Paper and Paperboard components in contact with Aqueous and Fatty Foods, and 21 CFR 176.180 Paper and Paperboard in contact with Dry Foods.


About the Author

Elmer W. Griese Jr, having accumulated 35+ years of knowledge working in the coatings and printing ink industries has now authored the Cork Tech Talk News, newsletter since 1992 producing 112 issues. He remains dedicated to educating and illuminating technological progress that offers the potential to advance coating technology and its applications.

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