Dimensional Stability of Divinylbenzene DVB Foam

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Dimensional Stability of Divinylbenzene (DVB) Foam
Presented by Jon Streit1 Diana Schroen2, Dan
Goodin2, Josh Gregory1, Jared Hund2, Katharine
Nelson1, and Nicole Petta1 1Schafer Corporation,
Livermore, CA 2General Atomics, Albuquerque, NM
and San Diego, CA
HAPL Project Review Oak Ridge National
Laboratory March 21-22, 2006
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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

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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.

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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.

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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
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This result suggests processing changes.
Most stable, 100 DVB, 6 AIBN, diethyl
phthalate

• 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.

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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.

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Study 2 Process Flow
Dibutyl Phthalate
Isopropyl Alcohol
Diethyl Phthalate
Isopropyl Alcohol
Dry

• If the foam and the coating do not exchange and
  contract the same amount, overcoat failure can
  occur.

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Study 2 Results
Important exchanges for overcoat.
Change too small to measure.

• Process solvents Dibutyl Phthalate (DBP),
  Isopropyl Alcohol (IPA), Diethyl Phthalate (DEP)

• 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.

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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.