Design of a Dynamic Seal to Enable Movement of a Film or Filament Through a Pressurized Space While Maintaining a Desired Pressure

The solid-state process has been successful in creating microcellular foams in a number of thermoplastic polymers using sub-critical CO2 in the 3-5 MPa range. In this process, the polymer film or sheet is first saturated with CO2 in a pressure vessel. The gas saturated film is then removed from the pressure vessel and heated in a suitable medium (liquid or hot air, for example) to create a microcellular structure. The time elapsed from depressurization of the pressure vessel to the heating of the gas-saturated film, known as the desorption time, is an important process parameter that can range from a few minutes in a laboratory environment to tens of minutes in a manufacturing environment. During the desorption time, the absorbed gas leaves the polymer film. This loss of gas becomes increasingly critical as the film thickness is reduced; polymer films below 0.010" in thickness are difficult or impossible to foam in the current solid-state batch process. To successfully foam thin films it is necessary for the polymer to retain the absorbed gas prior to heating. Towards this end, a dynamic seal was developed that allows a polymer film/filament to be smoothly drawn out of a pressure vessel while preventing the pressurized gas from escaping. The dynamic seal is based on the idea of using a liquid (e.g. water) to provide a seal and keep the pressurized gas from escaping. The liquid thus experiences the same pressure as the gas. Liquid leakage is controlled to within an acceptable limit by designing sufficient resistance to the flow. The early prototypes showed that a pressure of 5 MPa could be maintained in a small pressure vessel, while a 0.4 mm nylon fishing line was drawn through the seal. Water was used to seal the gas in this case. It was quickly apparent that the key challenge would be to move, or draw, the polymer film through the seal mechanism while keeping its integrity. The key is to strike a balance between sealing forces that keep the leakage rate low and the frictional forces that result on the polymer film as it moves through the seal.;To explore the parameters that influence the seal performance, a lab scale pressure vessel and seal mechanism was designed, built, and tested. The vessel with 5 inch ID and 7.5 inch OD was machined from Aluminum. The vessel had a clear polycarbonate window of 3.5 inch diameter embedded in the lid, with 30 LEDs illuminating the inside of the vessel. This design allowed for visual observations of the seal performance during testing. Various materials, both porous and nonporous, were investigated as seal materials. It was found that porous materials allow more control over the leakage rate and the resulting pressure drop in the flow direction is linear, consistent with Darcy's law. However, the porous material (a polyurethane foam) resulted in a large frictional force on the film. On the other hand, the nonporous material (also a PU foam but with a closed-cell structure) resulted in reduced frictional resistance on the film, however there was less control on the leakage rate and the pressure distribution was non-linear.;The basic feasibility of the dynamic seal idea was demonstrated. Further work is needed to reduce the frictional resistance on the polymer film and reduce leakage rates. Several ideas for improving the seal are presented.