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 weve 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

• 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

• 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

Solvent Oil Reactant Aqueous Reactant
Dibutyl Phthalate Terephthaloyl Chloride 4,4 Biphenol (4,4), Poly(ethyleneimine) (PEI), PVP/PEI
1,2 Dichlorohexane Terephthaloyl Chloride 4,4
1,2 Dichlorohexane Isophthaloyl Dichloride Diethylene triamine, 4,4, Hexamethylene diamine HMD)
Tetrachloroethylene Isophthaloyl Dichloride PEI
Tetrachloroethylene Sebacoyl Chloride HMD
Tetrachloroethylene Toluene Diisocyanate PEI
Cyclohexane Sebacoyl Chloride PEI
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.

Polymeric Reagents used by ILE
Hydroxyethylcellulose
Methylcellulose
Poly(vinyl phenol)
Possible Polymeric Reagents
Poly(ethyleneimine)
Poly(allylamine)
Poly(4-aminostyrene)
Poly(N-methylvinylamine)
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.

Exchange Fluid Critical Temp (ºC) Critical Pressure (psi) Water Solubility
Carbon Dioxide 31 1072 Very Low
Nitrous Oxide 37 1054 High
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.