The Nature of PTFE

The use of PTFE for the sample cup in these bombs is an obvious choice, not only because of its unique inertness to strong acids and high temperatures, but also because it is transparent to microwave energy, allowing energy to flow directly to the sample while serving also as an insulator to restrict heat flow from the reaction zone. PTFE does, however, have two characteristics which make it somewhat less than perfect for this application, and the user who understands these deficiencies will be able to minimize their effect upon their work.

First, PTFE has a tendency to creep or flow under pressure or load. This tendency is present even at room temperature and it is accentuated at higher temperatures. At temperatures below 150 °C the tendency to creep will be negligible. But as temperatures rise above 150° the creep effect will become more pronounced, making it more difficult to maintain tight seals and resulting in deformation and shorter life for the PTFE components. The extent of the creep effect will be roughly proportional to the maximum operating temperature.

Secondly, PTFE is a porous material. Although the materials and designs used in Parr acid digestion bombs minimize the effects of this porosity, users of these bombs can expect to see evidence of vapor migration across the cover seal and through the wall of the liner itself. Parr is able to minimize these problems by machining these parts from virgin PTFE that has been molded at an optimum pressure selected to reduce any porosity to an absolute minimum. The thick walls and effective seals used in these bombs also help to overcome these undesirable properties. Experiments have shown that the amount of solute lost in this manner during a normal digestion is negligible, but vapor migration into the walls of the PTFE cup will occur and cannot be avoided.