A Method for Applying a Permanent Teflon" Coating to Ground Class Joints Frustration, hazard, and loss of time and materials from "frozen" ground glass joints are familiar to almost all workers in chemical laboratories. Pre-formed Teflon* sleeves available wmmereially work well in many cases. However, for apparatus with a high risk of frozen joints, or wherepre-formed jointsdo not fit, aconvenient precaution is toapply a permanent coat of Teflon* to both surfaces of the joint. We have experimented with various commercial sources ofTeflon@or similar resins, and with methods of application, and wish to report a method for treating large batches of glassware. The joint surfaces are cleaned with solvent to remove grease, and then further cleaned with chromic acid, or haked a t 550°C for 8 hr, followed by slow cooling. Joint surfaces are then coated by dipping them in a colloidal suspension of Teflon" (30B. DuPont),and allowing them to drain rapidly toachieve a thin, uniform film. After the film has driedtoawhitecolor, the items are further dried in a oven a t about 120" for 30 mi". Then they are haked a t 370-380°C 10 minor more to sinter the coating to a smooth, durable film. For occasional small items, sintering can be accomplished by heating with a hightemperature electric heat gun until the film turns from white t o transparent, or by placing the items in an oven of a gas chromatograph. T h e sintering releases fumes which a r e highly toxic and must be done only i n a well-ventilated oven o r fume hood. Freouentlv . . a slieht - ridee of excess coatine" a t the edee of a ioint surface orevents a oerfect fit. This is readilv removed hv a sham knife. Damaged coatines can be removed with fine ahrasive oaoer. For inints such nr stoncocks. where contintmus ~
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to deliver a liquid that melts above room temperature. In such cases, when the Teflonm plug is warmed, i t expands more than the glass body, causing seizure. Glass stopcocks coated by the above procedure, on the other hand, do not seize up when heated. Larry Ferris of DuPont and Jack Laverdure of Miller-Stephenson Chemical Co. very helpfully supplied resin samples for evaluation.
Douglas M. JeweH James G. Lawless Ames Research Center. NASA Moffen Field, CA 94035
Volume 58
Number 11
November 1981
903
