Glove Boxes : Ingress of Moisture and Oxygen
The most important thing to consider when building and maintaining a glove box is to ensure the inner atmosphere is exposed to as little moisture and oxygen as possible. One of the main ways this atmosphere can be compromised is by ingression through box materials. On this page, we have discussed how moisture and oxygen can penetrate the glove box barriers (walls, etc.) and how the amount of ingress is measured and hereby reduced.
- What Is The Importance Of An Inert Environment?
- Where Do Contaminants Come From?
- How Does Ingress Occur?
- How Can Barrier Material Affect Ingress?
- How Is Permeability Measured?
- How Would Moisture Ingress Affect A Glove box Atmosphere?
What is the Importance of an Inert Environment?
Working in an inert environment is an encouraged practice for some operations, and an essential practice for others. It is important when:
- Working with materials that oxidise, hydrolyse or degrade when exposed to air. For this reason, many researchers store air-sensitive materials under inert conditions.
- Working with materials that are hydroscopic. These may begin to absorb water as soon as they are exposed to ambient conditions, resulting in clumping of the material.
- Working with materials that react violently with air or with moisture (such as Alkali metals, metal hydrides and alkyl metal hydrides). These materials are called pyrophoroic chemicals and must be used under controlled, inert conditions, such as in a glove box.
- Working with emergent technologies that can suffer degradation if exposed to ambient conditions. For example, modern material combinations used in 3rd generation photovoltaics or lithium ion battery technology.
In order to work in these scenarios, we must create an atmosphere void of H2O and O2. One way to achieve this, is by working within a glove box. A glove box is an enclosed environment that is filled with an inert gas, such as nitrogen or argon. This atmosphere is maintained by keeping an overpressure of this inert gas in the system. Contaminants can be removed by purging the system with a continuous flow of inert gas, or by circulating the atmosphere through a filtration system that removes air and water1. However, over time contaminants can re enter the system through various pathways.
Where can Contaminants Come From?
There are three main ways that moisture and O2 can be introduced into a glove box. The first is through leaks in the system, whether via imperfect seals, through damage to the unit, or through operator error. When leaks occur, they must be identified and fixed as soon as possible. In the case of small leaks, the integrity of the glove box atmosphere can be maintained for a while, so long as an overpressure is maintained. When leaks occur the overpressure results in the flow of gas through a leak going form the inside to the outside. If the leak rate is below the fill rate that the glove box can maintain then the box can be kept at an overpressure indefinitely but with a high usage rate of inert gas.
There are two other ways that moisture and O2 levels can increase in a glove box; that’s through outgassing or ingress. Outgassing is the slow release of trapped moisture/oxygen from materials brought into the glove box that can eventually change the internal atmosphere. For this porous and high surface ratio materials such as paper, cardboard, tissue, or cloth material should be evacuated overnight to extract trapped water before taking into glove box. Materials that are either miscible with or have a high water solubility also pose problems for trapped water and require evacuating before being placed within a glove box.
Ingress, in the case of glove boxes, is the diffusion of unwanted oxygen and water into a sealed chamber. This typically occurs due to moisture and oxygen passing directly through materials used in the construction of the glove box. When talking about ingress, the property of the material that we need to examine is its permeability2. This is a measure of the total transport of matter through a system,2 in our case, O2 or moisture through the glove box walls.
How Does Ingress Occur?
When considering ingress through a material, we must split the process into different steps. Figure 1 shows the stages of a permeant travelling through a solid barrier. First (Step 1), the vapour molecules must reach the first barrier interface, B1. Then the permeant is absorbed at B1 (Step 2) before diffusing through the solid barrier (Step 3). The permeant then undergoes desorption at the B2 (Step 4) and diffuses away from the barrier (Step 5). In this article, we will only briefly consider steps 2-4. The processes in Step 2/4 are mainly determined by solubility, S, and step 3 determined by the Diffusion coefficient, D0.
Steps 2 and 4 in Figure 1 can be described by the following adaptation of Henry’s Law:
Where the Solubility, S, is determined by the partial pressure (pPM) of the permeant and Ksol is Henry’s law constant2–4. This essentially describes how quickly a permeant can be adsorbed/desorbed by the solid barrier.
The permeants movement through the solid barrier is governed by diffusion (step 3). This is often the slowest process, therefore is the rate limiting step for permeability. The diffusion process can be described by Fick’s laws. Fick’s first law denotes the relationship between concentration gradient and movement of permeants across a space dx is:
Where J is the flux of permeants moving through the barrier, D is the diffusion constant, dC/dx is the concentration difference across the barrier (from x à x+dx).
However, as permeant diffuses through a material, the concentration of the permeant at both point X and point (X+dx) will change. It is therefore necessary to consider how this process changes with time. This leads to Fick’s second law of diffusion:
Where EA is the activation energy, T is temperature and R is the gas constant. This EA essentially represents the energy required for a permeant to squeeze through a solid.5 This also means ingress depends exponentially on temperature.
The permeability coefficient, P0, is a product of a systems S and D0 values and this is the property used to describe a barriers permeability to a specific permeant.
How Can Barrier Material Affect Ingress?
Material properties of the solid barrier are hereby of massive importance when reducing oxygen and moisture permeation. Polymers are a popular choice for barrier materials as they are cheap, strong and can have a wide range of material properties. The most important thing to consider when thinking about diffusivity is the fractional free volume of the barrier material which depends on its packing factor. In the most simplistic terms the best way to think of this is the more space there is between molecules in a barrier, the more room the gas has to diffuse through. For this reason, permeability also depends on the size and structure of the permeant6.
Polymers consist of long chains so therefore have a low packing factor. However, whether a polymer is classed as glassy, crystalline or rubbery will affect this, and this quality depends its structure at room temperature. A glassy polymer has glass transition temperature, Tg>room temperature, RT. This means the polymer chains are locked into one position so the material will be characteristically brittle.7 A rubbery polymer will have Tg<RT so is more malleable. In glassy polymers, solubility is increased as they have an excess non-equilibrium free volume8. In other words, the polymers chains maybe locked in awkward positions, leaving holes for moisture uptake. However, rubbery polymers tend have a higher free volume as they have a more disorganized structure. Also, their polymer chains have more room to move around, facilitating diffusion through the material. For this reason, rubbery polymers tend to have higher permeability.
Crystalline materials in general have a much better packing density than rubbery/glassy polymers therefore permeants have a harder time diffusing through them. When crystalline materials form there is a point where two crystals meet called a grain boundary. There can often be atomic mismatch at this grain boundary leading to slightly larger free volume here. Thus, there may be a small ingression of moisture or oxygen at grain boundaries5. However, in materials such as metal or glass, this amount is almost negligible.
How Is Permeability Measured?
Transmission rates of O2 and water vapor can be measured using a gas transmission cell (see Figure 2). For O2 transmission rates, a thin layer of barrier material is mounted in the centre of this cell to create a barrier dividing two chambers9. An oxygen sensor is placed in of these chambers and this chamber is purged with nitrogen gas to remove all O2. Meanwhile, high oxygen levels are maintained with a constant N2/O2 supply. As oxygen diffuses through the semi-barrier, this increase in the O2 levels is detected by the sensor and can be used to detect oxygen permeation per unit time, or the Oxygen (gas) transmission rate (OTR or O2GTR).
This method can also be used to track moisture ingress. Instead of high oxygen levels in chamber 1, wet nitrogen is used to supply chamber 1, maintaining high moisture levels, and an infrared sensor is used to detect the increase in humidity. Another common method to measure moisture vapor transmission rates is using the desiccant or water method (ASTM E96). In this method, a dish is filled with either desiccant material or water and covered with a barrier film layer. This dish is measured before and after a significant period of time. In the desiccant method, the mass gain is used to determine MVTR through the barrier material. In the water method, mass lost is used to calculate MVTR. The desiccant/water methods are much simpler to construct but the gas transmission cell will determine MVTR and OTR more accurately.
The units used for these measurements can vary depending on how they have been measured. Some metrics are oulined below:
- Moisture vapour transmission rate, MVTR, (or water vapour transmission rate, WVTR) is a measure of the rate of water passing through a film or barrier (usually per day) and is measured in g/m2/day. One way is to measure the uptake of water via the change in mass of the material.9 However, different methods can give different values for MVTR9, so to state the conditions measured in is very important. MVTR is generally described using this equation10:
Where PO is the permeation constant for the barrier, p1-p2 is the partial pressure difference of water across the material, R is the ideal gas constant, EA is the activation energy and T is temperature.
- Moisture vapour permeability is a measure of how much moisture will penetrate a barrier per given thickness. Therefore for this article, we will assume that MVTR ~ P0/(barrier thickness).
- Additionally, oxygen transmission permeability is a measure of the amount of oxygen passage through a film or barrier per day and is measured in cm3.mm/m2.day.atm.
Table 1 lists some coefficients of Oxygen and Moisture Permeability, and some MVTR for various materials. The footnotes detail the conditions under which these values are accurate.
|PTFE||222 ^9||0.006 ^13||-----|
|Silicone rubber||3940 -4330 ^9,10||
|Aluminum foil||<0.006 14||
|4 x 10-6 ^^^15|
*These materials behave as near “perfect barriers” for oxygen and moisture 16,17 – most of the ingression with these materials is at the joints, depends on the bonding method used or grain boundaries within the material .
** Permeability = (volume of permeant)*(film thickness) / (surface area)*(time)*(pressure drop across barrier)
*** At 37°C at 90% RH
^ At 25°C at 1 atm
^^ at 20 and 40°C respectively
^^^ at 30°C at 90% RH
How Would Moisture Ingress Affect Glove Box Atmosphere?
As can be seen from Table 1, the choice of material can have a significant effect on the amount of O2 and H2O ingression through glove box walls. As it is important to keep oxygen and moisture levels as low as possible the choice of material for every aspect of the glove box is important.
To show how the permeability of the material translates into a rise in the water content inside a glove box we have simulated how the water content inside of a glove box increases over time for different materials. We have used a wall thickness of 5mm, the values from Table 1 and the moisture vapour transmission rate equation to model transfer of moisture into a box that has external dimensions of 60 x 60 x 50 cm in diameter. For simplicity it is assumed that this is a closed system at a constant temperature and there is initially 0.01ppm of H2O in the system. We have also assumed that the change in concentration gradient across the walls (barriers) with time is negligible as by the time a none negligible change in concentration gradient has occurred the atmosphere in the glove box is already classed as compromised. Therefore, we have excluded the effect from this example. We have also considered separately the effect of ingress through rubber gloves using two common materials.
Figure 3 shows how easily the levels of moisture can build in a closed glove box environment. In a real glove box there would be additional purging of N2/Ar gas or atmosphere filtration, which would reduce the impact of this ingression. However, to constantly purge an environment can become expensive, and filtration media can become saturated quickly requiring frequent regeneration of the material. Therefore, the choice of materials when using a glove box is critical when working in an inert atmosphere.
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