20 September 2002

Stopcock Grease

by Gary Coyne

Stopcock grease is sort of ubiquitous, often it’s just sort of there. But selection of the various types can make a big difference in operations, performance, (and potential repair) of glass apparatus.

One expects stopcock grease to do three things: first and foremost, stopcock grease is a seal to maintain a barrier between two regions. Secondly, stopcock grease assists in preventing glass members from jamming (sticking), together. And lastly, stopcock grease provides lubricity so that an inner piece can rotate within an outer piece.

What stopcock grease cannot do is to create a barrier across a leaky region. That is, if a joint or stopcock is improperly made or worn out, stopcock grease CANNOT repair the chasm any more than paint can fill the seam between two pieces of wood. Simply using more grease WILL NOT repair what needs to be replaced.

Because not every grease is proper for every occasion and situation, there are a variety of stopcock greases. Amongst them all, there are three main classifications of stopcock greases: the fluorinated, silicon, and hydrocarbon greases.

The fluorinated greases (commercial name is Krytox®), is a wonderful grease that is not soluble in most any solvent or water. It maintains its lubricity over a wide range of temperatures, and ages slowly. There are two problems with Krytox: one, it is somewhat difficult to remove and/or clean. You can remove the bulk of it with an industrial detergent*, but in the rough surfaces of a joint or stopcock, the particulate material of the gel (a Teflon-like material) will still remain. Secondly, if you heat Krytox over 260°C, the fumes are lethal. However, if you need a stopcock grease that works wonderfully between -30°C and 220°C, does not affect elastomers, glass, or metals, and do not expect to repair and/or alter the glassware, Krytox is wonderful.

Silicon greases have one major wonderful noteworthy feature--they are cheap! Secondly, they are mostly impervious to hydrocarbon solvents. Unfortunately, they are not perfect. The biggest problem is that they are composed of two materials, a volatile and a non-volatile component, and the volatile is very volatile. In fact, once purchased, silicon grease should not be kept in the lab over 18 months as it ages in the tube. Once applied on a joint or stopcock, it should be completely removed and replaced every month or two. Even if you do not rotate the plug, a stopcock or joint may leak if left alone for several months. In addition, although silicon grease has a low vapor pressure (<10^-5), it does creep under vacuum and will spread from a joint into the apparatus itself laying a film throughout. While silicon grease is very inert and not likely to cause any problems, complications do arise if you want the items repaired and/or altered. The problem is that it is not easy to remove the non-volatile component of silicon grease: the silicon. Under high heat (>400°C), the silicon component will fuse onto the surface of the glass and if a torch is applied to the glass, the silicon will burn into the glass destroying the glass that was heated. The best way to remove silicon grease is to remove the bulk of it with methalene chloride followed by a soaking for about 20-30 minutes in a base bath.

Hydrocarbon greases have wonderful attributes and have been refined to function in a wide variety of environments. Unfortunately hydrocarbon greases can be prematurely removed by hydrocarbon solvents (although this is not as big a problem as it sounds). In addition, good quality greases (such as Apiazon®) are quite expensive. However, if you select (say) Apiazon N for stopcocks while using Apiazon M (half the cost) for all joints, considerable savings can be achieved. In addition, if you do not have environmental demands (e.g., high heat), Thomas Lubraseal is much less costly. The best aspect of the hydrocarbon greases is if any repair and/or alteration is required or needed, removal is very easily achieved by using either hydrocarbon solvents or high heat. If you are an organic chemist, you may be able to completely clean your glassware AND the greases by placing them in special ovens and brought to 400°C for four hours--that’s it--no chemicals or waste.

An alternative to stopcock greases is Teflon stopcocks, Teflon rotary valves (for high vacuum) and Teflon sleeves. If you need high vacuum and cannot use a standard Teflon sleeve, you may want to check out Teflon rings http://www.glindemann.net/.

Whatever stopcock grease you select, it is very important to completely remove it prior to addition to new grease. If not removed the layers can affect throughput and can entrap contamination that can affect subsequent chemistry. Although removal is necessary between repeated applications, complete cleaning is mandatory if you are changing grease type. That is, if you were using silicon grease and now wish to use a hydrocarbon grease, even if you place the apparatus in a base bath prior to repair, the hydrocarbon grease will protect the silicon grease and neither will be removed. The glassblower will likely let you know about this. By the way, if apparatus with silicon grease goes through a glassblowers annealing oven, the only way the (remaining) silicon grease can be removed at that point is with hydrofluoric acid. If the apparatus requires too long a soak in the HF for removal, it should be discarded. Not warning a glassblower than an apparatus had Krytox grease on it (even if it has been cleaned) should be considered criminal.

 

 

* Typo alert: On page 201 in "The Laboratory Companion," fourth line down, it states "...discovered that an industrial solvent..." This should read "...discovered that an industrial detergent..." The footnote on the bottom of the page has the same typo issue.

Gary Coyne has been a scientific glassblower for 30 years, and for the past 20 of them has been at California State University, Los Angeles. He is the author of the Laboratory Companion (see sidebar). He can be contacted at gcoyne@exchange.calstatela.edu