1. JES 3-21-06 1 Dimensional Stability of Divinylbenzene (DVB) Foam HAPL Project Review Oak Ridge National Laboratory March 21-22, 2006 Presented by Jon Streit 1 Diana Schroen 2, Dan Goodin 2, Josh Gregory 1, Jared Hund 2, Katharine Nelson 1, and Nicole Petta 1 1 Schafer Corporation, Livermore, CA 2 General Atomics, Albuquerque, NM and San Diego, CA

2. JES 3-21-06 2 Why Study Dimensional Stability? We have observed shells shrinking after critical point drying. We have had poor coating yields. Large changes in foam shell dimensions could cause overcoats to fail. If we want to have a foam target with a conformal overcoat then either: – Both the foam and the overcoat do not change. – Both the foam and the overcoat change the same amount (+ or -). – The overcoat must be elastic enough to withstand the change. We needed to further investigate foam stability as it might be affecting the overcoat…

3. JES 3-21-06 3 How does crosslinking affect dimensional stability? A linear polymer (a) can be dissolved in a solvent. A crosslinked polymer (b) can swell in a good solvent and shrink in a poor solvent. The degree of crosslinking determines the dimensional stability of the system.

4. JES 3-21-06 4 Study 1: Intrinsic properties of DVB foam Can DVB be made dimensionally stable? DVB cast in rods –Advantages: ease of measurement, large volume equals large changes –Precise control Variables: polymerization time, polymerization solvent, polymerization initiator (AIBN), and the limit of theoretical crosslinking (by adding styrene). The test for stability was to place the foam in a “good solvent” (xylene) and a “poor solvent” (isopropyl alcohol) and measure dimensional changes.

5. JES 3-21-06 5 Results of Study 1: there are conditions where the foam is dimensionally stable. Most stable, 100% DVB, 6% AIBN, Diethyl phthalate Current condition, 100% DVB, 3% AIBN, Dibutyl phthalate

6. JES 3-21-06 6 This result suggests processing changes. 100% DVB is a surprise as the system should have been robust enough to have withstood the addition of some styrene, if we are achieving most of the theoretical crosslink density. 6% AIBN is a surprise, this is a very large ratio of initiator to monomer. “Most stable, 100% DVB, 6% AIBN, diethyl phthalate”

7. JES 3-21-06 7 Study 2: Stability in Current Process Conditions Rods and shells were: –Gelled for 18 and 48 hours. –Exchanged through the process solvents and measured. Purpose: –Determine conditions the overcoat must withstand or match. –Establish a baseline for future dimensional stability improvement. –Compare results of rods and shells.

8. JES 3-21-06 8 Study 2 Process Flow If the foam and the coating do not exchange and contract the same amount, overcoat failure can occur. Dibutyl Phthalate Isopropyl Alcohol Diethyl Phthalate Isopropyl Alcohol Dry

9. JES 3-21-06 9 Study 2 Results Process solvents: Dibutyl Phthalate (DBP), Isopropyl Alcohol (IPA), Diethyl Phthalate (DEP) Change too small to measure. Important exchanges for overcoat. The rod data suggests that the coating may need to withstand up to a 1% expansion and about a 2% shrinkage. Discrepancies between rod and shell data need to be understood.

10. JES 3-21-06 10 Conclusions Study 1 suggests higher AIBN & 100% DVB to minimize changes. Either need to stop shrinkage of foam or find an alternate overcoat that matches shrinkage. Next steps are to understand shell vs. rod data and to evaluate overcoats.