Foam Shells: Overcoating Progress

1. Foam Shells: Overcoating Progress High Average Power Laser Program Workshop Lawrence Livermore National Lab June 20-21, 2005 Jon Streit, Diana Schroen Schafer Corporation

2. Shell Production Status Nonconcentricity Many different methods of agitation and devices have been used to produce shells. Although NC has been reduced, we can not yet produce batches of shells that consistently meet specifications. We must take a step back, look at what we’ve learned, and figure out how to apply it. Overcoat We have had continued problems with shrinkage of the PVP overcoat during drying and are now concentrating on alternative overcoat chemistries. We have also begun to focus on solutions to possible problems encountered during the shell exchange and drying processes.

3. Reducing Nonconcentricity (NC) • What have we learned? • Density matching beyond a rough match has not had a clear effect on NC. • Shells need a minimum of 1 hour of agitation before gelation to produce low NC. • Shells produced with flow disruptions have lower NC. Flow disruptions cause deformations and may produce centering forces in the shell. Disruptions are very random which may result in a wide range of NCs within a batch. • Application of shear forces may help center the shell core. • How might we apply what we have learned?

4. Concept to Cause Deformation & Shear • These conceptual ideas would incorporate deformation and shear forces to reduce NC. Two opposite spinning plates with one moving up and down to deform shells. Two opposite spinning plates with one with ridges to deform shells.

5. Review of Overcoat Work • Poly(4-vinyl phenol) (PVP) • Shrinks upon drying • Smoothest coating tested • Diethylene Triamine (DET) • Does not shrink upon drying • Not as smooth as PVP • Melamine-Formaldehyde (MF) • Does not shrink upon drying • Almost as smooth as PVP • Too thin • Other chemistries attempted, but required more preliminary work PVP 5000X DET 5000X MF 500X

6. Problems with PVP Coating • PVP appears to shrink during the final stage of the drying process • Coated and uncoated shells appear to be the same size in the CO2 dryer until the final venting stage. • Coated shells then shrink and the coating survives or the coating fails and the foam does not shrink. • Uncoated shells do not shrink to the degree of coated shells. • Failure occurred under the following different conditions: • Solvent variation: 4-chlorotoluene, diethyl phthalate, dibutyl phthalate • Exchanging into IPA, ethanol, or drying directly from diethyl phthalate. • With added crosslinkers such as tris(2-amino ethyl) amine, and diethylene triamine

7. Review of Interfacial Chemistry • Water Soluble Reactant • Organic Solvent • Oil Reactant • Surfactant • Acid Acceptor • Two solution are made: one an aqueous solution with a water soluble reactant, the other an organic solution with an organic soluble reactant. An acid acceptor must be used in the aqueous phase as acid is a byproduct of the reaction. • Interfacial microencapsulation is widely used in industry especially in pesticides and pharmaceuticals.

8. Variables Affecting Polymer Formation • Organic Solvent • Water Soluble Reactant • Organic Soluble Reactant • Acid Acceptor • Different solvents can affect surface smoothness, strength, and thickness. • Using different oil soluble and water soluble reactants will obviously result in different polymers produced and can also determine the orientation of primary and secondary growth. • Using different acid acceptors (NaOH, Na2CO3, or excess diamine) can change the properties of the polymer. • Longer reaction time produces thicker coatings, but reactions tend to be self limiting. The density of polymer produced can vary with time. • Surfactants influence reactant transport which can lead to more robust polymers.

9. Results of Overcoat Screening • We have screened many pairs of interfacial reactants in a variety of solvents in test tubes. From this study we have selected the following combinations to attempt shell overcoat:

10. Polymeric Reagents • ILE found that, “In general, membranes obtained by the low-Mw, water-soluble reagents were fragile and unable to use as ICF targets. Polymeric water-soluble reagents formed an elastic, tough membrane, but a low density layer was often seen on the outer surface.” J. Vac. Sci. Tech. A 11(5), Sep/Oct 1993. • In addition to the other reagents we have identified, we will also try overcoating with these polymeric reagents.

11. Current Availability vs. Future Possibilities Possible • DVB Foam with Interfacial Coating • Solves: • No Oxygen Content in Foam • Ease of Manufacture (less time, less cost) • Problems: • Need to control shrinkage • Need to control cracking • Requires we identify proper reactants / conditions Available • RF Foam Shell with Flash PVP Coating followed by GDP Coating • Solves: • Permeation Barrier • Problems: • High Oxygen Content • Extra Processing (more time, more cost) • Buckle Pressure

12. Some Solutions to Overcoat Problems • If osmotic pressure becomes a problem we can try other dual solvents such as tetrahydrofuran or dioxane. • We will try using supercritical nitrous oxide during the drying step to allow us to skip the foam dehydration step altogether. • We are considering working with Southwest Research Institute a major research company that has experience developing proof-of-concept industrial mass production microencapsulation processes including interfacial chemistry.

13. Summary • Build and test device that incorporates both shell deformation and shear forces to try to reduce NC. • Continue to investigate interfacial overcoating chemistries that will adhere to the shell and meet smoothness specifications. • Overcoat shells with best polymer results from overcoat screening in vial. • Overcoat shells with polymeric reactants. • Pick the smoothest overcoats and vary drying and exchange solvents to solve cracking issues. • The potential savings and ease of production require that we continue to explore interfacial overcoats as a permeation barrier.