Glove Box Design
You can use glove boxes (or gloveboxes) to either protect sensitive materials from an outside environment (isolation), or to protect yourself from infectious samples or radioactive materials (containment). The glove box is made up from a clear panel which allows you to see the contents of the chamber, while the gloves themselves allow you to manipulate it. Smaller, secondary chambers called antechambers allow you to add or remove items without exposing the inside of the glove box to the atmospheric conditions.
Inert atmosphere isolation glove boxes are the most commonly used type of glove box and are one of the most critical pieces of equipment found in any laboratory. This is because they provide you with the means to work in an environment that is completely inert, eliminating the presence of reactive species such as oxygen and water. This is crucial for applications where the presence of small quantities of these elements can result in significant chemical or physical changes in the properties of the materials that you are using. Common applications of glove boxes include chemical synthesis, organic electronics, additive manufacturing, materials handling and storage, development of battery technology, and perovskite electronics.
To maintain an inert atmosphere inside the glove box, it is important that you make sure that the inner atmosphere is exposed to as little moisture and oxygen as possible. There are two main ways that the inner atmosphere can be compromised; via physical leaks in the box, or by the ingression of moisture and oxygen through the materials that make up the chamber.
You can remove contaminants from a glove box by purging the system with a continuous flow of inert gas, or by circulating the atmosphere through a filtration system that removes air and water5. Over time, however, they can re-enter the glove box via a few different pathways.
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Gaskets and Seals
Stationary and Permanent Seals
At the interface between two hard surfaces, there will always be a pathway for gas to pass through. On a microscopic level, the two mating surfaces have a high degree of roughness relative to the size of gas molecules.
It is not economically feasible to get the two surfaces to a level of flatness that would allow for a gas tight seal. For this reason, a softer material is compressed between the two surfaces to fill these gaps and reduce leaks. For stationary and permanent joins, silicone sealants can be used along with mechanical fixtures to ensure a tight fit. Alternatively, metals can be welded together to create a seamless join.
However, there are a several parts of a glove box that require non-permanent seals, such as doors for pass-throughs and feedthroughs for service lines such as power supply. For this, gaskets are extremely useful.
Types of Gasket for Non-Permanent Seals
A gasket is a piece of material that can be placed between two surfaces to form a seal. In the context of a glove box, their main purpose is to create an air-tight seal against gas/liquids entering and leaving the system. There are many types of gasket each suited for different situations. The three most commonly used in glove boxes are O-rings, flat gaskets, and knife edge gaskets. These are shown in the figure below.
Flat gaskets are used to form a seal between two surfaces as shown in the figure above. Flat gaskets can be cut to a wide variety of shapes, and they can be made from many types of materials. They are most commonly made with rubber and expanded rubber foams. Flat gaskets are often used for washers to help make seals, or rectangular joins such as windows and doors. They are especially beneficially when low clamping forces are used.
O-rings are doughnut-shaped mechanical gaskets that sit within a groove that is milled into one of the surfaces. These are usually made out of harder elastomeric materials such as nitrile or Viton. Compression of an O-ring creates an air-tight seal that can withstand high pressures and temperatures. These are often used in regions where high-pressure differentials occur such as inlet piping, or for the doors of vacuum chambers integrated into a glove box system.
Knife-edge metal gaskets
Knife-edge metal gaskets are often used in situations where a seal needs to withstand very low vacuum pressures1. Knife-edge seals are nearly perfect seals as the materials used have extremely low ingress rates and can withstand extremely high clamping forces. To achieve this, there are "knife-edge" indentations that are on the surfaces of the two joining parts. The gasket itself is made of a much softer metal such as copper. When these two surfaces are pressed together, the knife-edges of the hard metal indent the softer metal gasket in between.
Knife-edge gaskets create a flawless metal-metal seal. However, this does result in a permanent denting of the gasket, thus limiting the amount of times the seal can be broken and resealed. In addition, the production of the knife-edge is expensive and so is limited to smaller surface couplings. In glove box systems, these are typically reserved for connecting components in integrated vacuum chambers.
Permeability and Impact of Barrier Materials
The physical properties of the materials used for the solid barriers in a glove box are the biggest determination of the amount of oxygen and moisture permeation that will occur. As we must keep these levels as low as possible, the choice of material for each aspect of the glove box is equally important.
All glove boxes are slightly permeable; moisture and oxygen can pass through the walls of the glove box and enter the atmosphere inside the chamber. Ingress, in the case of glove boxes, is the diffusion of unwanted oxygen and water into a sealed chamber. Some materials are more permeable than others. For example, stainless steel and glass have much lower ingress rates than plastic. How susceptible your glove box is to contamination from the ingress of moisture and oxygen depends on the design of the glove box.
How Does Ingress Through Barrier Materials Occur?
When discussing ingress, we need to examine the permeability each material4. This is a measure of the total transport of matter through a system.4 Or in this case, the O2 or moisture through the glove box walls.
The ingress process can be split into several different steps. The figure below 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).
The processes in Step 2/4 are mainly determined by solubility, S, while step 3, is determined by the Diffusion coefficient, D0.
Steps 2 and 4 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 constant4–6. This essentially describes how quickly a permeant can be adsorbed or 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 it acts as the rate limiting step for permeability. This 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, and 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.7 This also means ingress depends exponentially on temperature.
The permeability coefficient, P0, is a product of a systems S and D0 values. This is the property used to describe a barriers permeability to a specific permeant.
How to measure the permeability of barrier materials
Transmission rates of O2 and water vapor can be measured using a gas transmission cell.
For O2 transmission rates, a thin layer of barrier material is mounted in the centre of the transmission cell to divide the two chambers11. An oxygen sensor is placed in one chamber, and this chamber is then purged with nitrogen gas to remove all O2. Meanwhile, high oxygen levels are maintained with a constant N2/O2 supply. The increase in O2 levels detected by the sensor as oxygen diffuses through the semi-barrier can be used to determine oxygen permeation per unit time, or the oxygen (gas) transmission rate (OTR or O2GTR).
You can also use this method to track moisture ingress. Instead of using high oxygen levels in the first chamber, use wet nitrogen. This maintains high moisture levels and you can then use an infrared sensor to detect the increase in humidity.
Another common method to measure moisture vapor transmission rates is by using the desiccant or water methods (ASTM E96). For these methods, fill a dish with either desiccant material or water and cover with a barrier film layer. Measure this dish before and after a significant period of time. In the desiccant method, you use mass gain to determine MVTR through the barrier material. In the water method, you use mass to calculate MVTR. The desiccant/water methods are much simpler to construct, but using a gas transmission cell will determine MVTR and OTR more accurately.
The units used for these measurements will vary depending on how they have been measured. Some metrics are outlined 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). This is measured in g/m2/day. One way to calculate this is to measure the uptake of water via the change in mass of the material.11 However, different methods can give different values for MVTR11, so to state the conditions measured in is very important. MVTR is generally described using this equation12: Where P0 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.
Permeability of various possible glove box barrier materials
The table below 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.
|Material||O2. Permeability** (cm3.mm/m2.d.atm)||H2O Permeability** (g.mm/m2.d.atm)||MVTR (g/m2.d.atm)|
|PMMA||5.8-7.2 ^12,13||5.25 ^12 55.20 ***14||-|
|PET||1.2-2.4 ^11||0.39-0.51 ***11||-|
|PTFE||222 ^11||0.006 ^15||-|
|Silicone rubber||3940 -4330 ^11,12||1.73-3.31 ***11|
|Butyl rubber||7.8-85.4||0.006 ^15||-|
|Aluminum foil||<0.006 16||-||<0.01 16|
|Glass Coverslip(0.1-0.2mm)||-||-||4 x 10-6 ^^^17|
*These materials behave as near “perfect barriers” for oxygen and moisture 18,19 – 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.
Oxygen and water ingress through polymers
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. This depends on its packing factor. Essentially, the more space there is between molecules in a barrier, the more room the gas has to diffuse through it. For this reason, permeability also depends on the size and structure of the permeant8.
Polymers consist of long chains and therefore have a low packing factor. However, whether a polymer is classed as glassy, crystalline or rubbery will affect this packing factor. Furthermore, the quality of a polymer will depend on its on its structure at room temperature. A glassy polymer has glass transition temperature, Tg>room temperature, RT, meaning that the polymer chains are locked into one position and the material will be characteristically brittle.9 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 volume10. In other words, the polymers chains may be locked in awkward positions, leaving holes for moisture uptake. However, rubbery polymers tend to 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. This is called a grain boundary. There can often be atomic mismatch at this grain boundary leading to slightly larger free volume, thus, there may be a small ingression of moisture or oxygen at grain boundaries7.
In materials such as metal or glass, as used in the production version of the Ossila Laboratory Glove Box, this amount is almost negligible.
Impact of Moisture Ingress on Glove box Atmosphere
To show how the permeability of a material will result in a rise in the water content of 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 the table above, as well as the moisture vapour transmission rate equation to model the transfer of moisture into a box with 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, with 0.01ppm of H2O in initially the system. We have also assumed that the change in concentration gradient across the walls (barriers) with time is negligible. Thus, by the time a non-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.
The resulting plots show 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, constantly purging an environment can quickly become expensive, and filtration media can become saturated - requiring frequent regeneration of the material. Therefore, when using a glove box, it is crucial to consider which materials are being used when working in this inert atmosphere.
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