Production of Divinylbenzene (DVB) Shells

1. Production of Divinylbenzene (DVB) Shells High Average Power Laser Program Workshop Sandia National Laboratory Albuquerque, NM April 9-10, 2003 Jon Streit Diana Schroen

2. Review • 4 mm Diameter Foam Shell • 300 micron DVB Foam Wall • CH Polymer • ~1-3 Micron Cell Size • 20 - 120 mg/cc • 1 micron Carbon Overcoat • Inner Water Phase • Organic Phase • Stripping Phase • Progress: • Assembled droplet generator for shell production. • Fabricated approximately 4 mm diameter shells with 300 µm walls at 100 mg/cc. • Overcoated shells with a poly(vinyl phenol) overcoat. • Problems include nonconcentricity, cracking.

3. Shell Production Flow • Shell production work is occurring in four areas simultaneously with process scale-up in mind: Formation / Gelation • Problem: Shell Nonconcentricity • Possible Solution: Density Matching, Agitation Characterization • Problem: Difficult Imaging, Cracking, Time Consuming • Possible Solution: Index Match, On-site, Automate Overcoating • Problem: Time consuming, Need surface characterization • Possible Solution: Automate, Perform SEM analysis Supercritical Drying • Problem: Overcoated shells crack • Possible Solution: Slow CO2 bleed off

4. Agitation • Shell deformation during gelation has previously been shown to increase shell concentricity • Shell path during gelation has been altered from the original configuration • Flask RPM has also been increased to intensify shell deformation • Shells have been made to study the effect of agitation, but have not yet been characterized Bottom view of shell path in full flask Bottom view of shell path in 2/3 full flask

5. 10 Hour Half-lives 30°C V-70 65°C AIBN V-70 at 50°C should react about as fast as AIBN at 85°C Density Matching / Initiator • The density of the inner water phase and the organic phase change with temperature – this can affect shell nonconcentricity • To minimize this effect a low temperature initiator, V-70, was used. • Shells made with “pre-polymerization” but at 60°C were tacky and translucent. • Shells made without “pre-polymerization” tended to agglomerate. • Benefit of V-70 is not apparent.

6. BSA DBP/ BSA DBP DBP BSA BSA After Gelation Rinse Away Water with IPA Exchange into BSA Ready to Characterize Water IPA Characterization • To increase the rate of data return and to help decrease shell cracking, an on-site characterization technique has been developed. • Two orthogonal images are taken to calculate dimensions • Benzyl Salicylate is a better index match making characterization easier and more accurate. • DBP = Dibutyl Phthalate, IPA = Isopropyl Alcohol, BSA = Benzyl Salicylate

7. Characterization Equipment • Shells are placed in an optical cell and positioned on stage of assembly station to collect images. Effective, but time consuming. • Shell images analyzed using Image Pro Plus Software • Data exported to Excel.

8. Shell Images: 16% Nonconcentricity Top View Side View Note: Color variation is due to differing lighting conditions.

9. Shell Images: 3% Nonconcentricity Top View Side View Note: Color variation is due to differing lighting conditions.

10. Nonconcentricity & Wall Thickness • Nonconcentricity % = (4Pi Offset) / (Average Wall Thickness) x 100 • 4Pi Offset is the greatest distance between the centers of the inner and outer spheres. • Example 16% Nonconcentricity: • Example 3% Nonconcentricity:

11. Microscopes (2 Views) Fail Shells Flow Through Tube Pass Optical Cell Shells Sorted Rapid Characterization Concept • A concept for rapid shell characterization is being developed. • Shell that have been exchanged into BSA flow through a tube to an optical cell on the assembly station. • Images are analyzed and shells are sorted as Pass or Fail upon exiting the cell.

12. BSA / 4CT BSA 4CT/ACl 4CT/ACl 4CT/ACl BSA PVP 4CT 4CT/ACl Water Characterized Shell Placed in PVP Solution (overcoat forms) Exchanged into 4CT Exchanged into 4CT / ACl Overcoated Shell Overcoating • A few areas need to be studied in the overcoating process: • Does reducing the concentration of the acid chloride slow the reaction and result in an improved surface finish? • A thicker overcoat can be created using a lower molecular with PVP. Would this result in the overcoat growing into the foam? • BSA = Benzyl Salicylate, 4CT = 4-Chlorotoluene, ACl = Isophthaloyl Dichloride, PVP = Poly(4-vinyl phenol)

13. Water Wash Removes Excess Organic 10% HCl Rinse PVP Solution Water Rinse Water Rinse Overcoating Reaction Occurs in Coil Overcoating Concept • Shells are currently manually overcoated. A concept is being developed to wash shells in a tube and overcoat them as they flow through a coil. = Shell containing Organic Solution of Acid Chloride = Overcoated Shell

14. Supercritical Drying • We use a semi-automated CO2 pressure vessel. • While supercritical drying of non-overcoated shells has been successful, drying of overcoated shells has been problematic - shells have ruptured. • Bleeding off CO2 over a longer period of time may eliminate the problem. • Rupture does demonstrate the seal provided by the overcoat.

15. Future Work • Continue to study agitation and density matching as a means to reduce nonconcentricity. • Investigate methods to streamline the characterization process. • Characterize shells produced in agitation study. • Continue to study and streamline the overcoating process and begin to characterize the overcoat. • Explore methods to eliminate shell rupture problem during supercritical drying.