Determination of the Water Vapor Permeability of Packaging Materials - METTLER TOLEDO

Determination of the Water Vapor Permeability of Packaging Materials

High demands are nowadays put on packaging materials. For example, depending on the application field, the materials must provide optimum barrier properties toward water vapor, oxygen or odorants. In addition, there are requirements regarding tear resistance, transparency and compatibility with the contents of the packaging. In this article, we show how the water vapor transmission rate of materials can be determined using a sorption test system.

 

Introduction

The water vapor transmission rate of films can be determined gravimetrically with great accuracy. To do this, sample dishes are filled with saturated salt solutions, water or a desiccant and covered with the film under investigation.

The contents of the dish define a certain vapor pressure within the dish. The difference between the partial pressures inside and outside the dish results in water molecules diffusing through the film and being absorbed by the desiccant. This leads to a change in mass of the dish1.

The mass changes can be determined by periodically weighing the dish. This allows the water vapor transmission rate of the film under investigation to be determined. Measurements like this can be carried out using a ProUmid GmbH sorption test system [1].

Figure 1 shows the preparation of a dish for an experiment. In this example, molecular sieves (about 2 g) are spread over the bottom of the dish. The material whose water vapor transmission rate is to be investigated is secured over the molecular sieves by a clamping ring. The excess film is then removed using a knife (scalpel).

 

Experimental Details

The sample tray in the sorption test system we used accommodated six sample dishes. One sample dish was empty and was used as a reference dish and to correct for any balance drift. This setup enabled up to five materials to be simultaneously measured.

In the following sections, we describe measurements of the water vapor transmission of a polyethylene (PE) film, a packaging material for cookies (biscuits), and an uncoated paper sheet. In each case, the area of the sample material was 35 cm2. The measurements were performed at relative humidities of 20, 40, 60 and 80% at 25 °C. Molecular sieves (typically 2 g) were used as the desiccant. At room temperature for example this enables a relative humidity of less than 0.5% to be maintained in the inside of the dish [2].

Conclusions

Amongst other applications, the ProUmid sorption test system can be used to investigate the water vapor transmission rate of packaging materials. Three different packaging materials (films of polyethylene, paper and a, packaging material for cookies) were investigated. The water vapor transmission behavior of the three materials was noticeably different.

 

Determination of the Water Vapor Permeability of Packaging Materials | Thermal Analysis Application No. UC 464 | Application published in METTLER TOLEDO Thermal Analysis UserCom 46