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Divinylbenzene (DVB) Shells

Foam Buckle Pressure & Uniformity

High Average Power Laser Program Workshop

University of Wisconsin

Madison, WI

September 24-25, 2003

Jon Streit

Diana Schroen

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Review

• Status at last review:

– Fabricated approximately 4 mm diameter shells with 300 µm walls at 100 mg/cc and 50 mg/cc.

– Overcoated shells with a poly(vinyl phenol) overcoat.

– Developed in-house characterization method.

– All overcoated shell cracked during drying process.

– No characterization of overcoat.

• 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

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Shell Production Flow

• Shell production work areas:

• Problem: Low yield for dried, overcoated shells• Possible Solution: Continue drying / overcoating optimization

• Problem: Shell Nonconcentricity

• Possible Solution: Design / Obtain new apparatus for gelation with improved temperature and agitation control.

• Problem: Time Consuming• Possible Solution: Automate

• Problem: Time consuming, Poor initial surface finish• Possible Solution: Automate, Optimize chemistry, reaction conditions

Supercritical Drying

Formation / Gelation

Characterization

Overcoating

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We are Beginning to Work with GA to Develop a Flow Through System

Droplet Generator Heated Coil IPA Rinse

Benzyl Salicylate

To Shell Characterization

Sort

Microscopes (2 Views)

Shells Sorted

Fail

Pass

Optical Cell

Shells Flow Through

Tube

• Would agitation be adequate?

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Agitation & Temp During Gelation

• Shell characterization data relating non-concentricity and density matching has been scattered.

• Current generator set-up has poor temperature control and non-uniform agitation.

• To remedy this, a new generator set-up and two new agitation methods have been devised.

Current flask angle and heat source contact area.

New set-up will control stripping temp and receiving bottle temp until transfer to new agitator.

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Gelation Equipment

A hybridization oven has been ordered to gel shells using a carousel motion. The angle the tube is held in the carousel can also be adjusted to

provide a sloshing motion for periodic deformation of the shell.

A bottle rotator that is submersible in a heated bath is

being designed for superior heat transfer and uniform agitation.

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Benzyl Ether as Dibutyl Phthalate Replacement

• Benzyl ether was tested as a dibutyl phthalate substitute to allow gelation and characterization in the same solvent.

• Same density and similar solubility parameters as DBP.

• Index of refraction is higher (1.56 vs. 1.49)

Shell Gelled using DBP in Water

Shell Gelled using BE in Water

Shell Gelled using BE in BE

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Cell Size

100 mg/cc DVB Polymerized using Dibutyl Phthalate as the Solvent.

100 mg/cc DVB Polymerized Using Benzyl Ether as a Solvent.

Cell Size is Larger Using Benzyl Ether

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Fiber Size

100 mg/cc DVB

Fiber Size is difficult to Quantify for 100 mg/cc DVB. The structure resembles that of the “string of pearls”

structure of an aerogel.

TPX Foam

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Overcoating

• Overcoating experiments have begun.

• Surface roughness is greater then seen in previous work.

• We are varying reaction time, pH, reactant concentration, PVP molecular weight, crosslinker, and crosslinker concentration to try to reduce the surface roughness.

• SEM photos of initial overcoat have been taken.

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Overcoat SEM Images

5000x, 20K PVP, 5 min 25000x, 20K PVP, 5 min

At 5 minutes, the overcoat does not look complete and appears rough.

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Overcoat SEM Images

5000x, 20K PVP, 15 min 25000x, 20K PVP, 15 min

At 15 min, the overcoat shows some improvement, but still rough.

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Supercritical Drying

• Yields from drying shells are still low – at best 30%.

• Initially we thought that slowing the rate of CO2 bleed off would increase yields – but this seems to have little effect.

Dried Overcoated Shell

Dried Uncoated Shell

• Overcoating polymerization and chemistry factors that affect overcoating strength and adhesion to the capsule are now being studied to increase dry yield.

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Buckle Pressure

Hole in Foam Overcoat Failure Crack in Foam

• Shells do not dimple as expected. Failure modes are through gross foam failure, overcoat delamination, or cracks in the foam.

• Permeation is rapid for the initial overcoats – shells do not yield if pressure is increased at a moderate rate.

Buckle Pressure

0

10

20

30

40

50

60

70

80

20 40 50 60 80 100 200Pressure (psi)

Bu

ckl

e P

erc

ent

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Future Work

• Study nonconcentricity results for new droplet generator / agitator set-up with relation to density matching.

• Continue to study and streamline the overcoating process and study the effects of overcoating chemistry and reaction conditions on overcoat surface roughness.

• Explore methods to eliminate shell rupture problem during supercritical drying including overcoating chemistry and reaction condition effects.

• Continue generating buckle pressure data.