Glove Boxes: How to Detect Leaks and Reduce Ingress
Laboratory glove boxes create a sealed environment for work that involves hazardous materials, chemicals, or samples that react with air. The main chamber of such a glove box is generally filled with an inert gas, usually Argon or Nitrogen, to create an environment that is completed isolated from the atmosphere outside the glove box.
When building, maintaining, or using a glove box, it is important to 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.
Leaks within a glove box are inevitable. With effective detection, however, the source can be identified and either mitigated or prevented entirely. In order to do this, it is important to understand how and where leaks occur and how to spot and deal with them.
Ingression, where moisture and oxygen can penetrate the glove box barriers, can also be measured and the extent to which it occurs can be reduced.
- Importance Of An Inert Environment
- Where Do Contaminants Come From?
- Leaks and Leak Detection
- Why Should Leaks be Avoided?
- Causes of Leaks in Glove Boxes
- How Gloveboxes are Built to Prevent Leaks
- What to Do if You Suspect a Leak
- Where Can Leaks Form in a Glove Box?
- Ingress of Moisture and Oxygen
- How Does Ingress Occur?
- How Can Barrier Material Affect Ingress?
- How Is Permeability Measured?
- Impact of Moisture Ingress on Glove box Atmosphere
- References and Further Reading
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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 water5. However, over time contaminants can re enter the system through various pathways.
Where Do 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 permeability4. This is a measure of the total transport of matter through a system,4 in our case, O2 or moisture through the glove box walls.
Glove Box Leaks and Leak Detection
Why Should Leaks be Avoided?
Any leak in a glove box, however small, has the potential to result in contaminants entering the chamber and compromising the inert environment inside. While it is important to remember that there is no such thing as a perfectly sealed system, both the frequency and severity of leaks can be managed by choosing a well-designed glove box and following good maintenance and operational procedures.
Managing glove box leaks comes down to a few simple rules:
- Use appropriate seals for joints between surfaces
- Introduce standard glove box operating procedures to reduce occurrences of human error
- Monitor gas usage so that leaks can be detected early
- Schedule tests and inspections of seals
- Have procedures in place to reduce the impact of containment failure
Glove boxes are typically held at a positive pressure of a few millibar (mb) relative to atmosphere. This means that if there is a leak anywhere, the pressure difference will cause gas to flow outwards and a low O2/H2O atmosphere can be maintained within. A glove box with a small leak will be able to maintain overpressure by replenishing the gas within the leaking chamber, so initially the contamination levels will not increase substantially.
Leaks can have other impacts in addition to the risk of contamination. For example, increased gas flow into a glove box can result in turbulent airflow within the chamber, and for systems with HEPA filtration, this can result in stagnation points which can leave areas unfiltered.
The increased gas usage for maintaining an over pressure can also often result in depletion of the nitrogen source, the internal and external pressures equalising and water and O2/H2O entering the system. Therefore, while initially a small leak is no cause for alarm, identifying a leak, locating its origin, and fixing it are critical in maintaining the atmosphere inside the glove box.
Small leaks can also develop into larger leaks which can be dangerous not only for the glove box functionality but for the users of the system. Often glove boxes are used to isolate materials that can be dangerous if exposed to air or harmful to the user. Therefore, large leaks in a glove box can result in users being exposed to harmful materials unknowingly. Additionally, if the glove box is continually purging to maintain an overpressure, this constant outflow can affect the oxygen levels in the room. Inert gasses, such as nitrogen, cannot be detected in the air by sight or smell and the effects of oxygen depletion mean that it is difficult for those exposed to know they are in imminent danger. For this reason, it is important that glove boxes are used in rooms that are well ventilated and fitted with oxygen-level monitors.
Causes of Leaks in Glove Boxes
The most common contributor to leaks in glove boxes is human error. This can be from either following procedures incorrectly, or through accidental damage.
Transferring of samples in and out of the glove box via an antechamber
One of the most frequent causes of accidental leaks is the transferring of samples in and out of the glove box via an antechamber. Incorrect cycling or failing to purge the antechamber results in a large volume of atmospheric air mixing with the air inside the glove box, causing a large spike in O2 and H2O.
Accidental damage to the gloves
The most frequent damage by users is from holes punctured in the glove. This can be from sharp objects manipulated in the glove box – or ripped by a fingernail or jewellery catching when inserting or removing arms.
It is important that correct training is given to users of the glove box on how to work with and maintain it. It is also important to have standard glove box operating procedures for commonly performed tasks available and a plan of action in the event of issues occurring. Even with thorough procedures and careful users, leaks can still happen, so it is also important to ensure that in the event of atmospheric exposure that samples are safe and that a plan of action is in place to quickly get the system back to inert conditions.
Damage to the seals
Another common cause of leaks is where seals are present. Frequently used seals are those that pose the greatest risk of leaking as they are more likely to become damaged and develop a fault. Repeated opening and closing of seals can easily result in debris falling across a sealing surface, and even small pieces of debris such as a human hair can result in failure of a seal.
In addition, repeated expansion and contraction of any rubber or polymer seals will eventually cause fatigue resulting in less elasticity to the material and poorer sealing performance.
There are also other mechanisms where damage to the seals can occur resulting in formation of micro cracks. For example, ozone cracking of rubbers can occur at surfaces exposed to air, however this can be mostly eliminated through correct material choice. Frequently used seals should be checked often to ensure that no visible damage has occurred and they should be replaced periodically to ensure a high-quality seal is maintained.
How Gloveboxes are Built to Prevent Leaks
Preventing leaks from stationary and permanent joins
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. To get the two surfaces to a level of flatness that would allow for a gas tight seal is not economically feasible. For this reason, a softer material is compressed between the two surfaces to fill these gaps and reduce leaks (see Figure 1).
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 feed-throughs for services. For this, gaskets are extremely useful.
Use of gaskets 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 glovebox, 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, but the three most used in glove boxes are O-rings, flat gaskets, and knife edge gaskets. These are shown in Figure 2.
Flat gaskets are used to form a seal between two surfaces as shown in Figure 1. Flat gaskets can be cut to a wide variety of shapes, they can also be made from many types of materials with the most common being 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 milled into one of the surfaces. These are usually made of harder elastomeric materials such as nitrile or Viton. Compression of O-ring creates 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 doors of vacuum chambers integrated into glove box systems.
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. This is shown in Figure 2.
Knife-edge gaskets creates a flawless metal-metal seal. However, this results in a permanent denting of the gasket, 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 integrated vacuum chambers for connecting components.
What to Do if You Suspect a Leak
K. Curtis specifies a leak rate of 0.05 to 0.5% of box volume per hour to be acceptable in a glovebox2. However, each glove box will be built to a specified leak rate with most being below 1% of the total volume per hour. Often this is quoted as a classification rating with Class I glove boxes having less than 0.05% per hour, Class II (such as the Ossila Glove Box) having between 0.05% and 0.25%, and Class III being between 0.25% and 1%. If you suspect that your system is not achieving the value stated by the manufacturer, the first thing to do is a leak test.
Glove box leak tests
There are many versions of the leak test that can be performed depending on your system. Standard methods of testing are outlined in the ISO 10648-2 standard. The most used leak rate test is the positive pressure test which involves increasing the internal pressure of the glove box and monitoring how it changes over time. A general overview of the process is shown in Figure 3.
Each manufacturer will specify a way of testing the glove box. The Ossila Inert Atmosphere Glove Box has a built-in automated test which calculates the leak rate of the system. The automated test follows the procedure outlined below:
- The system is pressurised to a set over pressure value between 5 mbar and 10 mbar based upon the user settings.
- The internal temperature is monitored until its value has stabilised, at which point the pressure drop is measured. Once the temperature varies by less than 0.3 C inside the chamber, the system begins to monitor the pressure.
- Every 5 minutes, the internal and external pressure of the system is measured and the current leak rate is updated for the user to evaluate.
- The test can run for a maximum of one hour before allowing the over pressure to return to the normal over pressure value.
In some glove box systems, a negative pressure leak test can be performed. It is possible, but unusual, that leaks will be present at negative pressures that are not seen under positive pressure. For this, a procedure like that describes above is used, but an under pressure of between -5 mbar and –10mbar is used. If there is a leak in the system, the pressure will increase towards atmospheric pressure and the rate of this increase can be used to determine the leak rate of the system.
These leak tests can identify if there a significant leak within the system but are not thorough enough to provide exact leak rate or tell you where the leak is coming from. The exact leak rate values require specific conditions to be met around variations in external pressure, and internal and external temperature which are hard to control in laboratory conditions. In addition, leak rate tests require that glove ports be blanked off and glove box classification values stated are always in situations where gloves are not used.
Finding the leak
To test to see where a leak is coming from, a simple bubble test using a leak detecting fluid can be done. This involves putting soapy water at suspected leaky points, for example at the join between the glove and the main chamber. If this soap begins to create large bubbles then there is gas escaping from the glovebox.
Where Can Leaks Form in a Glove Box?
There are a number of places where leaks can be found in a glovebox, such as:
- Small puncture holes/tears in gloves
- Seal on door between the antechamber and the main chamber
- Seal on door between the antechamber and ambient atmosphere
- Breach in gas lines/ gas inlet/exhaust connectors
- Glove/main chamber join
- Feed-throughs for power
- Window seals/wall seals
Puncture holes and tears in gloves
The most common source of leaks in a glovebox is through small (or large) holes in the gloves. The gloves are the thinnest, most rubbery element of gloveboxes, and are therefore the most vulnerable to punctures. They are also the most heavily used part of the glovebox so the most likely to undergo significant stress. Small holes can be fixed with electrical tape3 but spare gloves should always be kept handy in case a complete replacement is needed.
Seals and joins
The seals on the doors between the main chamber, the antechamber and the outside environment can be another source of leaks. The doors between the antechamber and the main chamber will be the most heavily used, so may be the first seals to deteriorate. It is therefore very important that a tight seal is maintained between these two chambers. If the problem is between either the outside door/antechamber or antechamber/main chamber, then the antechamber will struggle to reach vacuum when cycling. Additionally, the pressure within the glove box could be affected when the antechamber is being evacuated. Check whatever seal is used between these doors (most likely O-ring) for damage or dirt that may compromise the seal. Clean them and coat with vacuum grease if necessary.
If neither of these places is the cause of the leak, check every join in the glovebox. Ensure the sealant material is not damaged, clean any seals and ensure that any bolts holding them in place are tight – vibration of the vacuum pump may cause bolts to come loose over time.
Every seal will begin to degrade over time, so it is very important that seals are regularly checked, and replaced when needed. As previously mentioned, some seals, like knife-edge gaskets, will need to be replaced after use as they are not built for repeat usage. Other gaskets, such as rubber O-rings, are more resilient to multiple uses. However, these are usually made from flexible materials so are less resistant to chemical stress or extreme temperatures or pressure. Therefore, it is important to continually assess the quality of your seals to reduce the likelihood of serious leaks. Even as simple an act as looking at the seals each time a door is opened can prevent a serious compromise in your glovebox environment.
Ingress of Moisture and Oxygen
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 constant4–6. 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.7 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.
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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 permeant8.
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.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 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 boundaries7. 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 chambers11. 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.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 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.
|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
Impact of Moisture Ingress on 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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- Jon Griffin
- Mary O'Kane