Zinc Oxide is one of the most used, and tested of all root canal sealers. It has been used and abused since we began doing endodontics.
Only recently with the advent of better chemistry and better tests have we begun to question it's use as a root canal sealer.
We have resins, silicones, acrylics etc. but we always seem to come back to plain old ZOE.
The reason seems to be that ON BALANCE it is the best, easiest, least toxic and most reliable of all sealing materials.
Each attempt to replace it has failed.
Silicone is almost totally inert but it stays rubbery and is easy to dislodge when inserting posts or restorations over it. Furthermore since it doesn't have a snap set, but instead gradually polymerizes it is difficult to actually know when it is fully set.
Epoxies set very hard and are almost impossible to re-treat once cured.
Acrylics, both methyl methacrylate and ethyl methacrylate have problems keeping the radio-opacifier emulsified during setting. It is very easy, using a syringe, to clog a needle and literally wring the radio-opaque particles out of the radio-lucent acrylic(which is a liquid until it cures) and thus be unable to determine the extent or completeness of our fill.
Some calcium hydroxides are theoretically kinder to periapical tissues but tend to be soluble in tissue fluids and blood.
So we keep coming back to ZOE.
ZOE snap sets and then continues to create a crystalline matrix ending up hard and brittle.
It mixes fairly easily from a powder and liquid form to a putty like consistency. The more eugenol mixed with the zinc oxide, the thinner and more fluid it gets.
It is available in various particle sizes depending on the manufacturing methods used and thus can be made to flow very well.
When various modifiers and radio-opacifiers are mixed with it, they stay relatively stable without settling out.
When it is injected, it flows well and adheres pretty well although, to make it adhere requires small particle sizes and thinner mixes.
When mixed very thick it will still go through a very small bore needle and adapt to the canal walls until it sets.
After setting it can be removed because it sets to a brittle but hard state and any process that essentially chips it out can be used to remove it.
There has been some concerns that eugenol is incompatible with polymerizing cements used for restorations but once set, it contains very little eugenol. And what little remains can be either washed off the interface with ethanol or covered with a material acceptable to both ZOE and composite materials.
And last but certainly not least, it is inexpensive making it a favorite in third world countries.
When mixed with either Barium Sulfate, Bismuth sub Nitrate or Titanium Dioxide it is radio opaque enough to determine the completeness of our seal and fill.
So over all, it is a good reliable, non-soluble, long lasting material.