Friction and impact sensitivity of formulations containing glycidyl azide polymer

Propellants, Explosives, Pyrotechnics 20,27-31 (1995) 27

Friction and Impact Sensitivity of Formulations Containing Glycidyl Azide Polymer

C.A. Lusby, D.C. Ferguson, and D.M. Husband

Phillips Laboratory (Propulsion Directorate), OLAC PL/RKCP Edwards AFB, California 93524-7 160 (USA)

Reib- und Schlagempfindlichkeit von Formulierungen mit Glyci- dylazid-Polymeren

In dieser Veroffentlichung wird eine Untersuchung uber mogliche Gefahrdungen durch glycidylazidhaltige (GAP) Formulierungen beschrieben. Reib- und Fallversuche wurden an unterschiedlichen GAP-Treibstoffen durchgefiihrt, wobei GAP mit verschiedenen Zusat- Zen vermischt wurde, und die Versuche wurden auch innerhalb der verschiedenen Stufen bei der Herstellung der Treibstoffmischung vor- genommen. Die Ergebnisse zeigen, dal3, unter bestimmten Bedingun- gen, GAP-Formulierungen sehr reib- und schlagempfindlich sein kon- nen.

Sensibilite a la friction et au choc de formulations contenant des gl ycid ylazide-pol y meres

Dans la prCsente publication, on dCcrit une Ctude sur les risques potentiels prCsent6s par les formulations a base de glycidylazide (GAP). Des essais de friction et de chute ont CtC rCalisCs sur diffkrents propergols GAP - le GAP &ant mClangC a diffkrents ingrkdients - et a differentes Ctapes du processus de fabrication du melange explosif. Les rksultats rnontrent que, dans certaines conditions, les formulations GAP peuvent &tre trks sensibles B la friction et au choc.

Summary

This paper describes work done to study the potential hazards of glycidyl azide polymer (GAP) based formulations. Friction and drop weight tests were performed on various GAP propellants, GAP mixed with different ingredients, and at different steps during processing of a propellant mix. The results showed that, under certain conditions, GAP formulations can be highly friction and impact sensitive.

The results have revealed several general discoveries about GAP and the various formulations in which it is commonly processed. First, the hazards of the uncured propellant andlor ingredients are much greater than that of the cured samples. In addition, formulations with coarse AP are more sensitive than those with fine AP in uncured samples. Finally, adding an inert or low hazard material such as oxamide or ammonium nitrate can significantly lower the friction and impact sensitivity of GAP-based propellants.

1. Introduction 2. Description of Test Apparatus

GAP has been examined as an energetic binder in a variety of solid propellant and explosive formulations for the last 15 years. Friction and impact tests on many of these cured formulations revealed that GAP propellants were relatively insensitive to both friction and impact. Impact levels were typically greater than 100 kg-cm and BAM friction test results were at least 6 kg.

The present study was initiated as a result of unexpect- edly high hazard sensitivity exhibited by a GAP-based high pressure, high burn rate propellant. This formulation contained a high level of fine (1.8 pm) ammonium perchlorate (AP) and trimethylolethanetrinitrate (TMETN) plasticizer. We stopped all further work on this propellant family (Table 3, propellants Al-A4) and conducted this study to evaluate the hazard properties of both the cured propellants and of GAP with a number of different constituents.

2.1 Friction Tester

The Julius Peters (BAM) Friction Tester was used to determine the friction sensitivity of the various ingredients and slurries used in this study. The tester operates by rub- bing a moving plate against a fixed pin with the material being tested placed between the plate and pin.

The instrument, shown in Fig. 1, consists of a steel base plate on which the friction device (fixed pin and moving plate) is mounted. The plate is held in a slide which is driv- en by an electric motor. The clamping device for the pin carries the load arm from which a weight is hung at any one of seven positions. There are nine load weights available, allowing the experimentor to vary the load on the pin from 0.5 kg to 36 kg.

0 VCH Verlagsgesellschaft, D-6945 1 Weinheim, 1995 0721-3 115/95/0103-027 $5.00+.25/0

28 C.A. Lusby, D.C. Ferguson, and D.M. Husband Propellants, Explosives, Pyrotechnics 20,27-3 1 (1 995)

Figure 1. Julius Peters BAM friction tester.

2.2 Drop Weight Tester

The device, shown in Fig. 2, consists of the sample hold- er assembly, the basic (1-kg) and incremental weights, and the drop weight assembly. The weights range from 1 kg to 6 kg and the height from 0 to 50 cm.

3. Test Matrix

The test matrix was divided into 3 phases. In Phase 1, various cured GAP propellants were examined. In Phase 2, tests were conducted on GAP mixed with different propellant ingredients.Following these initial tests, the most sensitive combinations were investigated more extensively. Finally, Phase 3 consisted of performing tests at the various points in the propellant mixing process.

Due to the large number of samples, tests were initially conducted at a maximum threshold value of 12.8 kg for the friction tests and 100 kg-cm for the drop weight tests. A minimum of three tests were performed at these theshold values. Samples which were negative in all three tests were considered relatively insensitive to friction and impact. Those samples which were positive at the threshold value were tested further, and the level at which 50% of the tests were positive was determined. A test was considered posi-

The Olin Mathieson Drop Weight Tester was used to determine the impact sensitivity of the samples. opera- tion, a small sample (o.03 gram) of the material is placed in a brass cup. The cup is placed in the device and a weight

tive when the propellant ignited as evidenced by a cracking noise. The standard porcelain plate and porcelain pin com- bination was used for the friction test (except in part of Phase 2, as explained later in this paper).

dropped from a predetermined height.

Phase 1: Propellant Study

Table 1 is a summary of the propellant formulations used in this phase. ANB-3066, which is the Minuteman I1 second stage propellant, was chosen as a standard for com- parison with the GAP propellants.

Table 1. Propellant Formulations for Phase 1

Propellant Family Formulation

Figure 2. Drop weight tester.

G

ANB-3066

GAP/TMETN/Carbon/AP/60% Solids GAP/BTTN/TMETN/Carbon/AN/70% Solids GAP/TEGDN/AN/Oxamide/V*05/70% Solids GAP/V205/0xamide/AP/70% Solids GAP/TMETN/Al/AN/AP/80% Solids GAP/ZrC/CarbonfTMETN/HMX/AP/7.5 % Solids GAP/BTTN/rMETN/PbCit/ZrC/Carbon/ HMX/65% Solids CTPS/Al/AP/SS% Solids

Phase 2: Ingredient Study

In this phase of the testing, GAP was mixed with various combinations of propellant ingredients. BTTN and TMETN were mixed with GAP at a plasticizer/polymer ratio of 2: 1. Table 2 lists the other ingredients used in this study.

Propellants, Explosives, Pyrotechnics 20,27-3 1 (1995)

Table 2. Ingredients Used in Phase 2

Ingredient % by Weight

A1

AP (1.8-400 Fm) Carbon black*

HMX* MNA* PbCit ZrC

,41203

cr203

7-14 2

28-70 1 2

48-50 1 2

1 ,2 ,3

* Not used after initial test series; initial results showed little or no effect on impact and friction sensitivity from these ingredients.

The influence of AP particle size on hazard sensitivity was the first variable studied in Phase 2. Based on the results of these tests, the most sensitive combinations were investigated further to determine the influence of solids loading on hazard sensitivity.

In the last part of Phase 2, stainless steel plates and pins replaced the normal porcelain plates and pins to determine the sensitivity of the samples with a material commonly used in propellant processing. In this part of the testing, the maximum threshold value used in the friction tests was raised to 36 kg.

Phase 3: Mix Cycle Study

For this phase of the testing, samples were taken before each addition step in a mix cycle. The samples were placed in a freezer and tested as soon as possible. The propellants examined were a hazard class 1.3 HTPB (R45M) propellant and a hazard class 1.3 reduced smoke GAP propellant (For- mulation F, Table 1).

4. Test Results

Phase 1 : Propellant Study

Table 3 is a summary of the Phase 1 data. Within each propellant family the friction and impact 50% levels are nearly identical. Thus, slight formulation variations between propellants within the same family had little or no effect on their sensitivity. An interesting finding is that ingredients such as HMX (the G1 propellant contains 62% HMX) do not appear to have much effect on a propellant's sensitivity. In fact the GAP/BTTN/TMETN binder matrix actually desensitizes HMX to impact because dry HMX is typically 80 kg-cm on this tester.

One of the major variables influencing hazard sensitivity appears to be AP particle size. All formulations in propellant family A contained fine AP (1.8 pm and 16 pm). Pro- pellant family F contained coarse AP (400 pm, 200 pm and 25 pm). The other formulations either did not use AP or the amount used was quite low. Ammonium nitrate and oxamide were found to desensitize GAP propellants, even those containing AP. The D propellant formulations varied from 0 to 50% AP with no change in sensitivity.

Friction and Impact Sensitivity of GAP Formulations 29

Table 3. Data from Phase 1 .- - _ -

Propellant Friction Drop Weight (50% level) (50% level)

[kgl [kg-cml

A1 A2 (aged)

A3 A4 B1 B2 B3 c1 c2 c 3 D1 D2 El F1 F2 F3 F4

GI ANB-3066

2.6 2.1 2.0 1.8

12.8 12.8 12.8 12.8 12.8 12.8 12.8 12.8 12.8 6.3 7.2 5.2 5.4

11.2 12.8

42 42 55 56

I00 I00 100 100 100 I00 100 100 85 85

100 60 85 85

100

Phase 2: Ingredient Study

Table 4 shows the results of the fricition tests conducted under Phase 2. As can be seen from the table, there are two regions of high friction sensitivity. The first occurs at fine AP particle sizes and high solids loadings. The second region is at coarse AP particle sizes and extends down to a lower range of solids loading.

Table 4. Results of Phase 2

Lo

0 Lo

Y

9

Y)

0

1.8 8 16 25 50 90 200 400

AP Partlcle Size [ ~ m l

Symbol Legend Friction Sensitivity Legend

0 GAP/ ZrC/ AP 0 2 . 1 - 4 k g

0 GAP/ AP 0 - 2 k g

0 GAP/ TMETN/ AP 0 '4 kg

GAP/ TMETN/ ZrC/ AP

GAP/ TMETN/ A l / AP

0 GAP/ BTTN/ AP

0 GAP/ BTTN/ Z ~ C / AP

Figure 3 shows the friction sensitivity trend obtained during this series of tests. Several interesting things should be noted about these results. First, with the addition of

I nitrate-ester plasticizers, the friction sensitivity increases.

30 C.A. Lusby, D.C. Ferguson, and D.M. Husband

z - I -

CAP/ ZrC/ AP .___...._... ~ ........ -_ _ _ _ _ ~ - - - CAP/ TUETNI AP

CAP/ TUElN/ Z r C l AP

, . . GAP/ TMETN/ A 1 / AP

CAP/ ETlN/ AP

CAP/ B U N / ZrC/ AP

- -. - - - - - -

AP PARTICLE SIZE [pm]

Figure 3. Friction sensitivity as a function of AP particle size for various GAP formulations.

Y. SOLIDS

Figure 4. Influence of solids loading, TMETN, and zirconium carbide on the friction sensitivity of GAP/AP formulations.

1- 1

:: i

0 " ~ " " ~ ~ " " ~ " " " " " " " ~ " " ~ " " ~ " m 35 40 45 5n n 80 I 70

I SOLlDS

Figure 5. Most sensitive samples.

As expected, the more energetic plasticizer BTTN has a greater effect on the hazard sensitivity than TMETN. In addition, when added to GAPIAP, ZrC reduces friction sensitivity. However, ZrC has little influence on sensitivity in samples of either GAPRMETN or GAPBTTN. Finally, the addition of aluminum does not appreciably influence friction sensitivity.

Propellants, Explosives, Pyrotechnics 20,27-31 (1995)

Figure 6. Least sensitive samples.

Figure 4 shows the influence of adding several different ingredients on the friction sensitivity of GAP. The original sample, GAP and 25 pm AP, remains the least sensitive throughout the range of solids loading tested. With the addition of TMETN, the sensitivity increases, especially in the mid-solids loading range, between 50-60% solids. Add- ing TMETN and ZrC increases friction sensitivity at 30- 40% solids loadings. Between 50-70% solids loading, the friction sensitivity of the sample with ZrC is similar to that of the sample with TMETN alone.

Figure 5 shows results from some of the most sensitive combinations discovered in this work. Notice that the most sensitive samples contained between 55% and 65% solids. The oxidizer-to-fuel ratio at this solids loading is approxi- mately stoichiometric. Figure 6 shows results from some of the least sensitive combinations found in this investigation.

To determine if the material in contact with the sample had an influence on its friction sensitivity, stainless steel plates and pins were fabricated and several of the most sensitive samples were retested (Table 5).

Table 5. Data Using Different Plate/Pin Materials

Sample* Porc/ Porc/ Steel/ Porc Steel Steel

GAP/TMETN/25 pm AP (65% solids) 1.5 36 36 GAP/TMETN/ZrC/25 pm AP (62 % solids) 1.1 36 36 GAP/TMETN/16 pm AP (60% solids) 0.9 33.3 36 GAPDMETN/200 Lrn AP (48% solids) 1.3 36 36 GAP/BTTN/200 pm AP (60% solids) 1.2 36 36 GAP/BTTN/ZrC/25 pm AP (58% solids) 1.4 36 36 GAP/BTTN/ZrC/200 pm AP (53% solids) 1.9 36 36

Note: Porc/Porc - Porcelain plate/Porcelain pin Porc/Steel - Porcelain plate/Steel pin Steel/Steel - Steel plate/Steel pin * Uncured

It is clear that the type of materials used as plate and pin has a significant impact on the measured value of the friction sensitivity. This is only a sampling of the data; however, the remainder of the data displayed the identical trend.

Propellants, Explosives, Pyrotechnics 20,27-3 1 (1995) Friction and Impact Sensitivity of GAP Formulations 3 1

Phase 3: Mix Cycle Study

As stated earlier, this study was performed using two hazard class 1.3 propellants. The propellants were a GAP reduced smoke propellant, designated KRRS- 138, and an HTPB (R45M) aluminized propellant, MD- 102. Results are shown in Figs. 7 and 8. The processing steps for the two propellants are shown in Table 6. Table 6. Processing Steps for Figures 7 and 8

MD-102 KRRS- 138

1 Add R45M, DOA and A1 2 Add HMX (8%) 3Add400pmAP 4 Add 50% 200 pm AP 5 Add 25% 200 pm AP 6 Add 25% 200 pm AP 7 Add25 pm AP 8 Add curative

1 Add GAP, TMETN and HMX (15%) 2 Add 50% (400,200,25 pn blend) AP 3 Add 25% (400,200,25 pn blend) AP 4 Add 15% (400,200,25 pm blend) AP 5 Add 10% (400,200,25 p blend) AP 6 curative 7 Add cure catalyst

Both Figs. 7 and 8 show that the HTPB propellant remains insensitive to both impact and friction throughout the mix cycle. However, after the first AP addition, the impact and friction sensitivity of the GAP propellant dra- matically increases. Thorughout the remainder of the mix cycle, the GAP propellant remains very sensitive. Figure 7 also shows the friction sensitivity measurements of the GAP formulation tested using a porcelain pin and stainless steel plate. These values are the lowest obtained using the steel plate, and provide further evidence that great care should be used when processing GAP/AP propellants.

PROCESSING STEP

Figure 7. Mix cycle study: Friction.

5. Conclusion

The purpose of this study was to gain a greater under- standing of the hazards associated with GAP propellants so that accidents can be prevented. Initial tests on cured propellants indicated GAP was not exceptionally hazardous, but the processing of a formulation with all fine (1.8 pm) AP indicated increased friction sensitivity over previous low bum rate and/or non-AP formulations. Further testing of GAP and various propellant ingredients (AN, AP, HMX, ZrC, Al, Carbon, etc.) and tests on actual samples taken from all steps during propellant mixing showed that certain

LlI 1 I 2 3 4 5 6 7 8

PROCESSING STEP

Figure 8. Mix cycle study: Impact.

types of GAP formulations are extremely friction and impact sensitive. The major findings are: - Uncured propellants in general are much more sensitive

than the cured material. The one exception was the GAP/TMETN formulations with fine AP (1.8 pm). These propellants were not overly sensitive in the uncured state.

- In cured samples, coarse AP leads to higher friction sensitivity than fine AP.

- As expected, TEGDN, TMETN, and BTTN increase both friction and impact sensitivity, but a TMETN formulation without GAP is not friction sensitive.

- HMX at low levels does not affect friction sensitivity, but at high HMX levels sensitivity is decreased.

- ZrC, aluminum, carbon, A1203, Cr203, MNA, and lead citrate have little influence on friction or impact values at levels normally used in propellants.

- Stoichiometric solids loading generally results in the highest friction sensitivity.

- From the mix cycle study and Table 5 it is clear that GAP/nitrate ester/AP mixtures are friction sensitive at any solids level or AP particle size.

- GAP propellants without AP were not found to be friction sensitive.

Glossary

A1

AN AP BTTN

CTPB DOA GAP HMX HTPB (R45M) MNA PbCit TEGDN TMETN

ZrC

A1203

cr203

v2°5

aluminum aluminum oxide ammonium nitrate ammonium perchlorate 1,2,4-butanetriol trinitrate chromium oxide carboxy-terminated polybutadiene dioctyladipate glycidyl azide polymer cyclotetramethylene tetranitramine hydroxy-terminated polybutadiene N-methyl-p-nitroaniline lead citrate triethylenegly col dinitrate trimethylolethane trinitrate vanadium pentoxide zirconium carbide

(Received June 22,1993; Ms 59/93)

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Friction and impact sensitivity of formulations containing glycidyl azide polymer