1. Solid Loading Study of Butarez/Lead Nitrate Propellant
Funded by Hill AFB: FA X-0058 DSN: , February 2017
May 2018 CAD/PAD TEW
U.S. Export Classification: EAR99 (ref. CLS )

2. U.S. Export Classification: EAR99 (ref. CLS8621426)
Background - Problem
The properties of propellants made with Butarez and lead nitrate have deteriorated starting approximately in 2000
Philips stopped making Butarez in 1989*
Adjusting the curative no longer gets us back into the stress/strain envelope
Schedule and budget don’t allow for qualification of a replacement *
“Phillips Petroleum Chemical Plant Explosion and Fire”, US Fire Administration,
FEMA, USFA-TR-035/October 1989.
U.S. Export Classification: EAR99 (ref. CLS )

3. Background – Factors/Possible Causes
Butarez Polymer Aging
It is believed that the polymer experiences oxidative crosslinking as it ages. The change in molecular weight over time supports this theory
U.S. Export Classification: EAR99 (ref. CLS )

4. Background – Factors/Possible Causes
Mostly likely cause of the changes in mechanical properties
Butarez cure system, circa 1990
Butarez cure system, circa 2016
Unfortunately, the formulation contains only tri-functional curatives, so there’s insufficient formulation control to adjust for the changes in the Butarez
U.S. Export Classification: EAR99 (ref. CLS )

5. Background – Factors/Possible Causes
Options to make the current formulation work
Accept some loss in properties
Finite element analysis shows we cannot tolerate the low strains we obtain with the aged Butarez polymer
Change the process
Mix time, temperature, order, vacuum
Change particle size
Can the lead nitrate size be changed without affecting burning rate?
U.S. Export Classification: EAR99 (ref. CLS )

6. Investigation – Binder Cure Study
Test
A gumstock (binder without solids) was mixed. Viscosity was measured over time at different temperatures to understand the cure kinetics
Application
UTAS uses shaker mixers, which heat kinetically as they mix
If the mix gets too hot, the cure could proceed to the point where the cast will disrupt formed bonds
If the batch is too cold the mixing might be inefficient
Some lead nitrate mix processes use higher temperatures
Time to double viscosity, min.
U.S. Export Classification: EAR99 (ref. CLS )

7. Investigation – Solid Loading Study
Performed Tap Density tests on solids blends with varying sizes of lead nitrate to identify optimum size (recommended by Ray Basil)
Sample
Description
Height, mm 1
Grind 1, ~43 micron control
79.7 2
Grind 2, ~68 micron
69.1 3
Grind 3, ~80 micron
68.1 4
Grind 4, ~95 micron
63.8 5
4/1 mix of 95 & 13 micron
70.1 6
1/1 blend of Gr. 3 and 4
67.4 9
Repeat Grind 4
65.0 10
1/1 blend of Gr. 4 and Gr.4b*
66.0 11
Bldg 19 blend of ~ 92 micron X-08**
64.3
Solids: lead nitrate, potassium perchlorate and aluminum. (no binder)
Results indicated that the control (~45 micron lead nitrate) was far from optimum for solids loading. Optimum lead nitrate size was ~95 micron
(Grind sizes coarser than 95 micron were difficult to control)
U.S. Export Classification: EAR99 (ref. CLS )

8. Investigation – Solid Loading Study
A 65-lb. mix was made using 95 micron lead nitrate in place of 45 micron
Mix viscosity was much lower than any previous mix
Cured to constant Shore A hardness
Cold strain was higher than any mix in 15 years, but still 0.1% below minimum
Cold modulus was above the maximum required (probably not a real problem)
Strand burning rate and all other bulk properties were nominal.
Extending cure 8 days increased cold modulus, but also increased cold strain by 0.4% (and met strain requirements)
U.S. Export Classification: EAR99 (ref. CLS )

9. Investigation – Verification Mix
Verification mix – Repeat of the first mix
Increase batch size to full-scale
Reduce curative slightly to try to increase cold strain slightly .
Results:
Cold strain was improved: 1.6% > minimum
Cold modulus was high
Other properties were all nominal
Mix temperature was higher than the 65-lb mix and consistent with full-scale mixes made 9 years earlier
U.S. Export Classification: EAR99 (ref. CLS )

10. Investigation – “Kitchen Sink” Mix
One mix that combined several ideas for process improvements to try to achieve further improvements
1. Extend Mix to Constant Viscosity
Hypothesis: Marginal mixing could be producing less-than optimum mechanical properties
Method: Measure viscosity and repeat (Day 1) mix cycles until viscosity is constant*
2. Cool Mix
Hypothesis: High mix temperature may be starting cure, so cast may disrupt binder
Method: Cool overnight and add curative/catalyst on second day, then mix briefly
3. Vacuum Applied at Every Mix Cycle
Hypothesis: Mixing without vacuum may cause poor wetting of the solid surfaces
Method: Mix with vacuum during all steps, even oxidizer additions
4. Oxidizer added in 3 additions rather than 1
Hypothesis: Slower incorporation of powder may improve wetting of solids
5. Two-day mix with curative and cure catalyst added at the beginning of the second day
Hypothesis: No specific hypothesis, just necessary to achieve both 1 and 2 in the same mix, because the shaker mixes heats kinetically as it mixes
*In practice, constant viscosity doesn’t occur because the shaker mix produces heat, which reduces viscosity, so we mixed to minimum pseudoplastic ratio at 2 speeds.
U.S. Export Classification: EAR99 (ref. CLS )

11. Investigation – “Kitchen Sink” Mix
Unfortunately, at least one of the factors caused inconsistent hardness, so the lessons learned from the mix were limited:
● The propellant did mix on the second day, despite being colder than normal, but probably not thoroughly
● The vacuum applied during the mix following oxidizer addition did not cause the oxidizer to get sucked into the vacuum, though the absence of air allowed the dust to be thrown higher.
● Overall, the mechanical properties were inferior to those of the verification mix
● The Shore A and mechanical properties indicated that the mix was not uniform
The low temperature and short mix cycle following curative & catalyst addition probably produced the inconsistent hardness.
● Burning rate and heat of explosion were low
Dusting after oxidizer addition
The Shore A profile of the Kitchen Sink mix was inconsistent
U.S. Export Classification: EAR99 (ref. CLS )

12. Investigation – “Kitchen Sink” Mix
A 6th factor produced surprising results
6. Cool Mix
Hypothesis: 165°F cure may be too hot, lower temperature cure may improve properties
Method: Draw a propellant sample and cure at a 140°F (in an unpressurized, foil-lined box). Since the bulk sample is being cured under pressure, cure a control sample in a lined box at 165°F.
Results: Properties of the samples cured at 165°F and 140°F (in foil-lined boxes) were not significantly different…
…but the mechanical of both the samples cure in foil-lined boxes at ambient pressure were superior to the bulk propellant from the Kitchen Sink mix and the Verification mix.
Ambient pressure cure appears to produce better properties than the pressure cure !?
Stress/Strain curves of representative samples from propellant cured at 165°F and
tested at -65°F
Maximum Stress
At -65°F, the yield stress occurred at higher strain in samples cured in foil-lined boxes at ambient pressure
The block sample from the Verification Mix was better than the block sample from the Kitchen sink mix, but the box sample from the Kitchen sink mix was better than either.
U.S. Export Classification: EAR99 (ref. CLS )

13. U.S. Export Classification: EAR99 (ref. CLS8621426)
Results/Conclusions
Adjusting the particle size distribution of the solids improved the mechanical properties without adverse affects on the other properties
The production mixes will be made using the same formulation and processes as the Verification Mix
Tap density was a good tool for finding the optimum particle pack
Going Forward:
Some day we should take a better look at the relationship between pressure cure and mechanical properties
It would be interesting to see if the properties could be improved by replacing some of the trifunctional curative with difunctional curative.
U.S. Export Classification: EAR99 (ref. CLS )

14. U.S. Export Classification: EAR99 (ref. CLS8621426)
Thanks to…
Air Force
Travis Thom
Derek Madsen
Navy
Alex Woods
Harry Archer
Exodynamics
James Baglini
Durrell Boyd
Ray Bazil
UTAS
Stephen Jennings
Michael Downing
Kris Maxwell
Karl Reimer
Stephen Peterson
Jerry Lambert
U.S. Export Classification: EAR99 (ref. CLS )