Stopcock grease hydrocarbon

In particular, if a latex is to be used for coatings, adhesives, or film applications, no silicone-base stopcock greases should be used on emulsion polymerization equipment. Although hydrocarbon greases are not completely satisfactory either, there are very few alternatives. Teflon tapes, sleeves, and stoppers may be useful, although expensive. 

Table 3.4 Hydrocarbon-Based Stopcock Greases (continued)...

Tervalent phosphorus acid derivatives are normally liquids or low-melting solids which can be purified by distillation, or sometimes by recrystallization from a non-polar solvent. Most are oxidized in contact with the atmosphere, and many are easily hydrolyzed, so they must be kept under an inert atmosphere (N2 or Ar) during all manipulations. Flasks should be predried and solvents dried and deoxygenated before use. Tervalent phosphorus acid derivatives are, with few exceptions, thermally stable and can be kept indefinitely in ampoules under an inert gas (many halophosphines dissolve stopcock grease and should not be kept in stoppered flasks for prolonged periods). Inert solvents are hydrocarbons, ethers and, for most compounds, dichloromethane, ethyl acetate and tertiary amines. Aminophosphines react vigorously with tetrachloromethane and slowly with trichloromethane, and most tervalent phosphorus acid derivatives are oxidized by dimethyl sulphoxide and react with alcohols. 

Krytox seems like a product from heaven, as many of its attributes are spectacular. It is a fluorinated grease and is therefore impervious to hydrocarbon- and water-based solvents. It cannot bum, so it has no flash point and can be exposed directly to oxygen, even at high pressures. It can be used over the widest temperature range of all the greases and works equally well with stopcocks as with joints. It has remarkable stability characteristics and can withstand long-term abuse. Best of all, Krytox greases are nontoxic. 

The Pencil Test. Before performing the pencil test, completely clean the entire apparatus in question. Remove all traces of any oil or grease (if the grease is hydrocarbon-based, you can heat the glass item in an oven to approximately 400°C to bum off any remaining grease). Next, take a standard no. 2 pencil and scribble a line all over the inside surface of the outer piece and the outside surface of the inner piece, as shown at the left of Fig. 3.36. Then, wet the joint (or stopcock) members with water (for lubrication) and rotate back and forth five to ten times (do not bear down, let the water be a lubricant). If the ground sections of both faces of the joint (or stopcock) are perfectly flat, the pencil will have been... 

The lubricant in a joint or stopcock serves to protect the surface from wear, to prevent leakage, and to facilitate the separation and rotation of the two members. Petroleum jelly is used for many assemblies but is unsuitable for equipment operating at high temperature or under vacuum. Some laboratories prepare special greases for use under conditions that do not permit hydrocarbons to be used. 

All reactions are carried out using conventional vacuum-line techniques. Teflon-in-glass stopcocks and a silicone lubricant for ground-glass joints should be used because of the marked solubility of germanes in hydrocarbon greases. 

---