DEVELOF'MENT OF EXPLOSIVES

PHASE DIAGRAM STUDIES

8 . P . faubion

DEVELOPMENT DIVISION

OCTOBER - DECEMBER 197% SANL 900-009

Lawrence Li vemore Laboratory L i vermore , Cal i fornia

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/ This report was prepared as an accoyd o f work sponsored by the

%iTZ+-S&tes Government. N d t h e r the United States nor the United States h%tttic--&ergy” Commission, nor the i r employees, nor any of their contra+, r i ibm~troctors , or the i r employees, makes any warranty,,Asxpress x or implied, or assumeg,ary legal l iab i l i ty o r responsibilitr,-”for the accuracy, completeness or any inform&‘n, apparatus, product or process disclosed, r e p r e d i s that i t s use would not infringe privately-owned rights.

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DEVELOPMENT OF PEX EXPLOSIVES PHASE DIAGRAM STUDIES

€3. V . Faubion

DEVELOPMENT D I V I S I O N

DISCLAIMER

This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsi- bility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Refer- ence herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recom- mendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

October - December 1971 SANL 900-009

Section G

DEVELOPMENT OF pD( E~PLOSIVES k E DIAGRAM STLQIES

ABSTRACT

Experiments have been carried out to determine the solid-liquid phase diagrams of binary mixtures of FEFO, EDNP, BDNPA and BDNPF. Cooling curves for the mix- tures give no indication of crystal formation.

Penetration measurements indicate a glass transition for all of the mixtures below -40 C . visual observation with a polarizing microscope.

The existence of glassy states for the mixtures was confirmed by

Themmicroscopy has been used in the preliminary study of the phase behavior of four binary mixtures. So far no eutectic has been observed.

INTRODUCTION

In order to achieve a successful PEX explosive formulation based on dispersions of HMX in energetic liquid vehicles, the vehicles must remain fluid over a tem- perature range from -65 to 165 F. At the present time no pure substance has been found with the desired characteristics. It was therefore necessary to investi- gate the solid-liquid phase diagrams of various energetic mixes in search of a eutectic composition which is fluid at -65 F. Several materials have been pro- posed for study(1). Mixtures of four materials have been investigated so far in this study: Bis-(2,2-dinitro-2-fluoroethyl) formal (FEFO) ; Ethyl 4,4-dinitro- pentanoate (EDNP),; Bis-(2,2-dinitropropyl) formal (BDNPF) and Bis-(2,2-dinitro- propyl) acetal (BDNPA) .

EXPERIMENTAL METHODS

Several experimental techniques can be used in the investigation of a phase dia- gram. Thermal analysis, i.e., the use of cooling or heating curves, is the most common method. The temperature of the sample is measured as a function of time during heating or cooling. A change in the heating rate is observed during the disappearance or appearance of a phase due to the accompanying enthalpy change. To use this method to investigate the phase diagram of two components, it is necessary to obtain cooling curves for several mixtures. time consuming, but is particularly affected by a problem common to any method, i.e, incompletely or slowly established equilibrium. -equilibrium state in solid-liquid equilibria is that of supercooled liquids which change to glasses. larly bad in this respect.

A brief review of the various techniques is given by Haase and Schonert(2).

This method is not only

The most usual non-

The materials used in this investigation are particu-

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.......... . . . . . . . . . . . . . . , . . . . . . . . . . '>- ._ . . . . . . . . . s . . jl . , .~ . . . . . . . .

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Dilatometric methods may a l so be used t o study sol id- l iquid phase diagrams. The usual procedure is t o measure t h e volume of t he sample a s a function of tempera- ture . A var ia t ion of t h i s method w a s used i n t h i s study. The temperature a t which a 0.025-inch probe penetrated the frozen sample during heating w a s de t e r - mined using t h e thermal mechanical analyzer (TMA) attachment on the DuPont MIA. Although t h i s method i s extremely sens i t i ve , it i s s t i l l plagued by the same problems as the cooling curve method.

A t h i r d method used extensively i n the study of sol id- l iquid phase diagrams is thermomicroscopy. This technique i s described i n d e t a i l by McCrone(3). To de- termine t h e sol id- l iquid phase diagram of a binary mixture, a mixed fusion i s pre- pared. A mixed fusion f o r microscopic examination is prepared by first melting the higher melting component under a cover g l a s s on t h e microscope s l i d e and allowing it t o r e so l id i fy . under the cover g l a s s . The low-melting component is then placed a t the edge of the cover g lass and heated so t h a t it m e l t s and runs under the cover g l a s s i n t o contact with the f i r s t component. during the c r y s t a l l i z a t i o n process ind ica tes t h e existence of a eu tec t i c , addi t ion compound, s o l i d so lu t ions , and unstable polymorphs. The sl ide is then placed on a hot s tage and the zone of mixing observed as the temperature i s raised. The melting poin ts of a l l so l id phases present can usual ly be measured during a s in- g l e heating. Under idea l conditions, t he e s sen t i a l cha rac t e r i s t i c s of a binary eu tec t i c diagram can be determined i n t w o to four hours. When a polar iz ing micro- scope is used, it is a l s o easy t o d is t inguish between a glassy and c rys t a l l i ne so l id . This method w a s chosen as the b e s t method fo r invest igat ing the phase diagrams of the energet ic l i qu id vehicle mixtures.

The s o l i d should idea l ly occupy only ha l f the area

Careful observation of the zone of mixing

In order t o apply t h i s method i n t h i s invest igat ion it w a s necessary t o have a microscope hot s tage capable of operating over the temperature range from -65 t o 165 F. Although no commercially ava i lab le hot s tage m e t t h i s requirement, the Lei tz cold s tage (-20 t o 350 C ) was adapted f o r use a t temperatures down t o -120 C. This w a s accomplished by cooling the s tage with dry nitrogen instead of carbon dioxide gas. The nitrogen was passed through a c o i l of copper tubing immersed i n a 2-liter dewar of l i qu id nitrogen. To prevent f r o s t from condensing on the sample and microscope objec t ive , t h e cold s tage and microscope w e r e both placed i n a Plexiglas box which w a s purged with dry nitrogen. i n one s ide of t he box allowed manipulation of the sample. The microscope tube protruded through a hole i n the top of t h e box and was equipped with a Zeiss C35M automatic camera and adaptor with viewing telescope. A Unitron MPS polar- izing microscope was used with a 1OX object ive and ocular . The temperature of the ce l l w a s measured using a Chromel-Alumel thermocouple, the output of which was p lo t ted as a function of t i m e on an X-Y Recorder (Mosley 2D-2). Photographs w e r e taken during cooling and heating of the samples using High-speed Ektachrome Type B film (EHB-135) which w a s developed t o an ASA speed of 320.

A rubber glove

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WSULTS AND DISCUSSIO'N

THERMAL ANALYSIS

A t t e m p t s t o measure the freezing poin ts of FEFO, BDNPA, BDNPF, and EDNP were un- successful. None of the curves showed a discont inui ty i n the cooling r a t e . I t appears t h a t i n a l l cases the l iqu ids supercooled and changed t o glasses . N o attempts w e r e made t o induce c r y s t a l l i z a t i o n o ther than cooling t h e sample.

PENETRATION ANALYSIS

The DuPont thermomechanical analyzer was then used t o measure the temperature a t which a 0.02s-inch probe penetrated t h e sample. Penetration measwements were made on the pure materials and on various binary mixtures. In each case one-gram samples of t h e mixtures were prepared by thoroughly mixing the carefu l ly weighed components. A sample (Q 100 mg) of each mixture w a s weighed i n an aluminum DSC sample cup and quenched on a block of d ry ice. the apparatus and immersed i n a dewar of l i qu id nitrogen. The temperature w a s then ra i sed t o above the t r a n s i t i o n point . Fig. 1 shows a typ ica l penetrat ion curve. The penetrat ion temperatures are shown i n Tables I - 111. Measurements were not made on mixtures of EDNP with the o ther mater ia ls due t o t h e lack of a pure sample a t t h e time these measurements were being made.

A l l of t h e penetrat ion temperatures are un rea l i s t i ca l ly low indicat ing t h a t t h e samples d id not c r y s t a l l i z e , but again formed glasses .

The sample was then placed i n

THERMOMICROSCOPY

Themomicroscopy proved t o be t h e most successful method f o r studying the sg l id- l iqu id phase diagrams of t he chosen materials. S t i l l , c e r t a in problems w e r e en- countered which make it d i f f i c u l t t o descr ibe t h e phase behavior with cer ta in ty . The main problem, a s previously encountered, is t h e a b i l i t y of these mater ia l s t o supercool w e l l below t h e i r freezing poin t and transform to a glassy so l id . The existence of a glassy state a t low temperatures f o r a l l of the mater ia l s has been confirmed by v isua l observation with polarized l i g h t . Considerable e f f o r t w a s required t o induce c rys t a l l i za t ion of EDNP and FEFO. repeatedly thermal cycle the sample and scratch the surface of cover g lass . This lengthened the t i m e necessary t o examine a sample t o about one week. BDNPA and BDNPF could eas i ly be c rys t a l l i zed by seeding t h e m e l t s l i g h t l y below room temperature .

It was necessary t o

Another problem, pointed out by McCrone(3), is more ser ious as it can lead t o an erroneous in te rpre ta t ion of the phase diagram. equilibrium phases may not nucleate and w i l l no t be observed. eu tec t i c may not nucleate because of rapid supercooling and the system w i l l com- p l e t e ly c r y s t a l l i z e without a fine-grained eu tec t i c mixture. This makes it necessary t o confirm the microscopic observations using other experimental tech- niques.

U:nder ce r t a in conditions t h e For example, t h e

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0

0 c.l I

0 ‘;p

I

0 CD

I

0 Q

I

0 0 4 I

0 r J 4 I

4

Table I.

% FEFO

100

90

80

7 0

60

50

40

30

20

10

0

P e n e t r a t i o n Temperatures f o r FEFO/BDNPA Mixtures

% BDNPA -I_

0

10

20

30

40

50

60

70

80

90

100

T L C

-70

-65

-58

-61

-57

-54

-54

-50

-48

-46

-45

Table IT. Penetration Temperatures for FEFO/BDNPF Mixtures

% FEFO % BDNPF

0 ,100 -70

90 10

20

-65

80 -63

-59

-58

70

60 . I

30

40

-55 50

40

30 .

20

50

60 -54

70 -51

80

90

100

-50

. 10 748

* o -47

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.

Table 111. Penetration Temperatures for BDNPA/BDNPF Mixtures

% BDNPA

100

90

80

70

60

50

40

30

20

10 '

0

E % BDNPF

0

10

20

30

40

50

60

70

80

90

100

-4 5

-4 5

-46

-46

-4 5

-4 6

-46

-45

-46

-45

-47

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.

Fig. 2. Color P r i n t of FEFO C r y s t a l

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Fig. 4 . BDNPA Crystals at RT

Fig. 5. BDNPF Crystals at RT

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r

Melting poin ts of t h e four pure components have been measured and are i n agree- ment with reported values ( 2 ) . The melting points as determined microscopically are as follows:

FEFO EDNP BDNPF BDNPA

9 c - 7 c 32 C 30 C

Color photomicrographs w e r e a l so made using polarized l i g h t of each s o l i d t o a id i n ident i fying the components and dis t inguishing them from new phases. 2 - 5 show the c r y s t a l f i lms observed when each s o l i d i f i e s between the cover g lass and microscope s l ide . Fig. 2 also shows a w e l l formed i so la ted c r y s t a l of FEFO which is growing from the m e l t .

Figs.

Preliminary s tud ie s of the following binary systems have been completed:

FEFO/EDNP EDN??/BDNPA EDNP/BDNPF BDNPA/BDNPF

So f a r no e u t e c t i c has been observed i n any of these systems. This does not r u l e o u t t h e p o s s i b i l i t y t ha t eu tec t i c compositions may st i l l e x i s t f o r some of these mixtures. It is highly possible t h a t t he eu tec t ics d id not nucleate be- cause of supercooling. be observed. The four binary systems s tudies a l l exhib i t t h i s behavior. I f these systems do not i n f a c t form eutectic mixtures, it w i l l be necessary t o de- termine t h e ex ten t t o which these components a r e soluble i n the so l id s t a t e . It would then be possible t o assign these systems t o one of t he f i v e Roozeboom type composition diagrams.

I n t h i s case only t h e melting of the t w o components would

FUTURE WORK

The inves t iga t ion of the phase diagrams of energetic l iqu id vehicle mixes is continuing. Additional binary and ternary mixtures of FEFO, EDNP, BDNPA and BDNPF w i l l be examined by thermomicroscopy. When or i f a eu tec t i c is found, t he composition w i l l be determined (2,3 and 4 ) .

Cooling curves and DTA thermograms w i l l then be attempted on the eu tec t i c mixture.

Other energet ic l i qu ids w i l l also be examined t o see i f they form l o w melting eu tec t ics with t h e four mater ia ls s tudies .

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REFERENCES

1. G. Repp and R . Squire, - R a i l , UCRL-7969, October 7 , 1964.

2 . R. Haase and H. Schonert, "Solid-Liquid Equilibrium", The In te rna t iona l E n c y c l o p e d i a P h y s i c s , Volume 1, Topic 13, Pergamon Press, New York, 1969.

3 . Walter C. McCrone, Jr. , "Fusion Methods i n Chemical Microscopy", I n t e r - science Publ ishers , Inc. , New York, 1957.

4. M. Milinovsky, Chem. Zvesti 25, 92(1971).

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