NASA Logo

NTRS

NTRS - NASA Technical Reports Server

Collections About News Help Login
Back to Results
Macrovoid Defect Growth during Evaporative Casting of Polymeric MembranesMacrovoid (MV) formation is a significant problem in evaporatively cast polymeric membranes. MVs are large, elongated or teardrop-shaped pores (~10-50 micron) that can impair membrane structural integrity. Although MVs have been extensively studied, there is no general agreement on the mechanisms governing MV growth. Recently, our research group has formulated the solutocapillary convection (SC) hypothesis, which contends that MV growth involves three principal forces: a Marangoni force generated by surface tension gradients within the MV interface, a viscous drag force, and a gravitationally induced body force. Two sets of complementary experiments were conducted to test the SC hypothesis. Ground-based videomicroscopy flow-visualization (VMFV) was utilized to measure the flow velocities at the MV-casting solution interface and deep within the casting solution. The measurements were performed with casting solutions containing 10 wt% cellulose acetate (CA), 30 wt% H2O, 60 wt% acetone, and 200- ppm TiO2 particles for flow visualization, and the surface tension was controlled by surfactant addition. Qualitatively, the experiments indicated that MV growth occurs in three distinct phases: (1) a very rapid initial growth period, (2) a much slower growth phase, and (3) absorption of selected MVs into the expanding demixed region. The presence of tracer particles inside the MVs suggests the presence of a convective flow, which transfers the particles from the bulk solution to the MV interior. Although the VMFV experiments did not establish any surfactant effect on the interfacial velocities, a statistically significant effect on the MV number density was observed. In the second set of experiments, membranes were cast aboard a KC-135 aircraft under 0-g and 2-g conditions. Despite careful attention to the design and fabrication of the membrane casting apparatus (MCA), several problems were encountered, the most significant of which was the contamination of the casting solution by the activated carbon particles used for solvent absorption.
Document ID
20030060541
Acquisition Source
Marshall Space Flight Center
Document Type
Conference Paper
Authors
Greenberg, A. R. (Colorado Univ. Boulder, CO, United States)
Khare, V. P. (Colorado Univ. Boulder, CO, United States)
Zartman, J. (Colorado Univ. Boulder, CO, United States)
Krantz, W. B. (Cincinnati Univ. OH, United States)
Todd, P. (Space Hardware Optimization Technology, Inc. Greenville, IN, United States)
Date Acquired
September 7, 2013
Publication Date
February 1, 2003
Publication Information
Publication: 2002 Microgravity Materials Science Conference
Subject Category
Nonmetallic Materials
Funding Number(s)
CONTRACT_GRANT: NAG8-1475
Distribution Limits
Public
Copyright
Work of the US Gov. Public Use Permitted.

Related Records

IDRelationTitle20030060494Collected Works2002 Microgravity Materials Science Conference
No Preview Available