Argon or Nitrogen for Resin Post-Curing? Use What’s Available
Gas-assisted post-curing is used with many dental resins to reduce oxygen around the printed part while it is being cured.
The reason is straightforward. Oxygen can inhibit free-radical polymerisation at the surface of a photopolymer resin, leaving a softer or tacky surface layer. Replace the air around the part with another gas and that oxygen inhibition can be substantially reduced.
Nitrogen is commonly used. It turns out argon works better for me.
What Is the Gas Actually Doing?
The gas is not curing the resin. The light is doing the curing, sometimes assisted by heat.
Its job is simply to replace the oxygen-containing air around the print while polymerisation continues.
Both nitrogen and argon can do this effectively.
In fact, nitrogen, argon and helium have all been used as protective atmospheres for the polymerisation of dental light-curing resins for decades.
Is Nitrogen Actually Inert?
Not strictly.
Nitrogen is often casually described as an inert gas, but it isn't a noble gas and it is capable of chemical reactions.
N₂ is pretty unusual because of its strong triple bond between its two nitrogen atoms. This makes it very unreactive under normal conditions, which is why nitrogen is commonly used as a protective atmosphere in manufacturing, chemistry and resin curing.
Argon is different.
Argon is a noble gas and is chemically inert for practical purposes under the conditions found inside a dental curing unit.
For this application, however, that distinction doesn't matter very much. Neither gas is there to participate in the polymerisation reaction. They are there to get the oxygen out.
So Why Is Nitrogen Commonly Used?
Mostly because it is inexpensive, widely available from industrial gas suppliers and sufficiently unreactive for the job.
There is nothing particularly special about nitrogen from the resin's point of view. The objective is to create an oxygen-reduced atmosphere around the part during curing.
If nitrogen is easy for you to obtain, use nitrogen.
If argon is easier, use argon. Dental laboratories may already have argon available for laser welding, TIG welding or other equipment, which can make adding a separate nitrogen cylinder unnecessary.
Using Argon in a Nitrogen-Purge Curing Unit
A curing chamber that uses gas simply to purge the air does not particularly care whether that oxygen is displaced by nitrogen or argon.
The practical requirements are the correct regulator, fittings, gas flow and inlet pressure.
Most of the compact dental curing units I work with are designed around a gas supply of approximately 0.15–0.20 MPa, although the specified pressure for the individual curing unit should always be followed.
The curing wavelength, temperature and curing time remain determined by the resin and curing protocol.
Changing from nitrogen to argon does not mean changing those parameters.
Argon Can Be the Easier Option
For many laboratories, this is really the entire point. If you already have argon, use it.
If your local Bunnings Warehouse makes argon easier to get, use it. At the time of writing, a Coregas E-size cylinder contains 4.9 m³ of argon and costs $184 for the gas, with a $300 refundable cylinder deposit on the initial purchase. After that, you simply swap the cylinder and pay for the gas.
If nitrogen is already connected and convenient, there is no reason to change it either.
There doesn't need to be an argon-versus-nitrogen debate.
For resin post-curing, both are being used to achieve the same practical objective:
reduce the oxygen concentration around the resin while it cures.
As with any compressed gas, use the correct regulator, secure the cylinder properly and provide appropriate ventilation. Both argon and nitrogen can displace oxygen from an enclosed room, and compressed-gas cylinders must be securely restrained to prevent them falling or becoming a projectile.
