Water vapor permeation of solid media can potentially have a great impact on a number of manufactured products, with far-reaching implications across various industrial and pharmaceutical sectors. There are two main areas of interest in the transport of water vapor. The first involves preventing water vapor ingress (thus, protecting the product). The second involves almost the exact opposite, i.e., controlling the release of water vapor.
This article will explore vapor sorption, transmission, and diffusivity in further detail:
Vapor Transmission Rate
Vapor transmission refers to the measure of the manner in which vapor molecules are diffused through solid media, whether they diffuse within the solid or through the pore structure of the material. While the former is termed absorption, the latter is called adsorption.
There are three parameters that dictate the permeation or transmission rate:
- The solid dimensions
- Concentration gradient
It is important to note that a concentration gradient will always exist if the solid is not at equilibrium. Thus, there needs to be a difference in the concentration of molecules across the solid. This article describes the two most popular experimental methods to achieve a concentration gradient.
Direct Vapor Transmission Rate Determination
This method involves using the film to seal a desiccant inside an aluminum container. If the humidity differs inside and outside the container, the water vapor will diffuse through the film.
The next step involves placing the container within the IGAsorp – which is programmed to simulate relatively high humidity conditions (approximately 80% RH). This allows the rate of transfer to be measured via periodically weighing the container, which is commonly expressed as the moisture vapor transmission rate (MVTR):
MVTR= (weight change (µg)x thickness (cm))/(Area (〖cm〗^2 )x time (hours))
As the vapor permeates through the film, a linear trend is produced as a steady state is reached with a net rate of transfer. MVTR can then be determined from weight changes that range from as little as 10 µg, and the measurement can be easily replicated under different operating conditions.
Simultaneous Vapor Sorption-Diffusivity Determination
Unlike the previous method, wherein each side of the film is exposed to a different humidity, this method exposes all surfaces of the film to the same humidity. In effect, the concentration gradient is now from the center to the surface of the film.
This method also differs from the direct method in terms of the concentration gradient, which is not linear. There exists a maximum gradient when the humidity is changed, and this then reduces down to zero, in its approach towards equilibrium.
This method involves determining the rate of transfer by the chemical diffusion coefficient and the concentration gradient. Thus, the measurement of the transmission rate does not discriminate between these two properties.
Vapour Sorption, Diffusivity, and Transmission Analysis with Hiden Isochema
Hiden Ischema’s IGAsorp dynamic vapor sorption (DVS) analyzer is suitable for rapid and precise analysis of the vapor sorption characteristics of organic and inorganic media. The IGAsorp has an ultra-sensitive microbalance that assesses minute changes within the sample mass due to humidity, temperature, and time. This makes it uniquely suited to perform both direct transmission rate and simultaneous vapor sorption-diffusivity analysis.
This information has been sourced, reviewed and adapted from materials provided by Hiden Isochema.
For more information on this source, please visit Hiden Isochema.