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Aqueous PTFE Dispersion: Tetrafluoroethylene Polymerised in Soapy Water (Berry 1951)
Ordinary soaps stop tetrafluoroethylene from growing into long chains. A fluorine soap does not: with 2 % of ammonium dodecafluoroheptanoate in the water, the gas polymerises into tiny PTFE ribbons that stay suspended. The result is a milky dispersion, a third of it polymer, the raw material for PTFE fine powder.
Source: K. L. Berry (du Pont), US 2,559,752 (1951), Example III. Making the ammonium salt from our acid, and the apparatus, are ours. Picture drawn by Youblob.
Advanced
1 day
Instructions
1
1
Make the ammonium salt
Make the ammonium salt
Ours: neutralise about 1.9 parts of dodecafluoroheptanoic acid with ammonia solution until just neutral to litmus, giving 2 parts of ammonium dodecafluoroheptanoate. Make it up with distilled water to 93 parts of water in all. Parts are by weight.
Materials for this step:
Dodecafluoroheptanoic Acid1.9 parts
Ammonia Solution - 25-28%1 as required to neutralise
Distilled Water93 partsTools needed:
Litmus Paper
Glass Vessel
Balance Scale2
2
Load the pressure vessel
Load the pressure vessel
Put the solution in a pressure vessel with 0.05 part of potassium persulfate and 5 parts of paraffin wax melting at 56–58 °C. The wax protects the colloid while it is stirred.
Tools needed:
Persulfate Initiator
Paraffin Wax
Pressure ReactorBalance Scale3
3
Heat and remove the air
Heat and remove the air
Bring it to 80–85 °C, then evacuate the vessel and hold it under reduced pressure for several minutes to drive out air and dissolved oxygen.
Tools needed:
Pressure Reactor
Vacuum Pump
Thermometer4
4
Feed tetrafluoroethylene for 3.6 hours
Feed tetrafluoroethylene for 3.6 hours
Let in tetrafluoroethylene to 60 lb/sq in gauge. Agitate at 80–85 °C for 3.6 hours; each time the pressure falls to 55 lb/sq in, bring it back to 60.
Materials for this step:
Tetrafluoroethylene1 as required to hold 55–60 psiTools needed:
Pressure Reactor
Pressure Gas Cylinder
Pressure GaugeThermometer5
5
Cool and lift off the wax
Cool and lift off the wax
Cool it. The paraffin sets on top and lifts off. Below it is a colloidal PTFE dispersion with 34 % solids.
Tools needed:
Pressure Reactor
Glass Jars with LidsMaterials
4- 1.9 partsPlaceholder
- 1 as required to neutralisePlaceholder
- 93 partsPlaceholder
- 1 as required to hold 55–60 psiPlaceholder
Tools Required
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