Ammunition monitoring in field situations by stabilizer consumption and molar mass decrease as predictive tools

19 Pages·2016·0.36 MB·English

by  Bohn Manfred A.

25 - 1
Ammunition monitoring in field situations by stabilizer consumption
and molar mass decrease as predictive tools

Manfred A. Bohn

Fraunhofer-Institut für Chemische Technologie (ICT), Postfach 1240,
D-76318 Pfinztal-Berghausen

Abstract

Ammunition monitoring becomes increasingly important because of the relatively fast
change in storage conditions when the ammunition is used in so named out-of-area op-
erations, means outside the country where the ammunition was fielded by the procure-
ment agency. The need for monitoring is at one side the increased stress by elevated am-
bient temperatures, high temperature stress by often not optimal field storage condi-
tions, increased loads by transport shocks, vibrations, changing air pressure, and change
in humidity conditions. On the other side monitoring in the sense of determining on-line
the state of the ammunition with regard to readiness of use, safety and functioning in
the designed way is an increasing demand by itself. This means the traditional ammuni-
tion surveillance changes to an advanced tool to assess in short time intervals up to near
on-line the state of usability of ammunition. Optimally such a surveillance level is
achieved independently where the lot or even the piece of ammunition is located in the
world.

Generally it is preferred that ammunition should not be delaborated in order to assess its
usability in field operations. Also time consuming test firings are no suitable mean. This
focuses the demand to the application of predictive methods. The principle procedure is
the same for all types of predictive monitoring. Some actually and continuously meas-
ured stress quantities are used as input data in material descriptors which give as output
the state of ageing (state of material damage), the rate of ageing (rate of material dam-
age) and therewith in conjunction with assessment criteria, the state of present and fu-
ture usability.

Here a method is described which can be used cost effectively for selected types of am-
munition namely the nitrocellulose based propellant charges of gun and rocket ammuni-
tions. The method is based on monitoring of the actual stabilizer content and, where
necessary, also of the mechanical strength of the grain by monitoring the chain length of
NC. This monitoring is achieved by actual temperature measurements at or near the am-
munition in regular and adapted intervals. By modelling of stabilizer consumption and
molar mass decrease using the kinetics of the propellant the actual stabilizer content and
actual mean molar mass are determined without any delaboration of ammunition.

Keywords: ammunition monitoring, stabilizer consumption, molar mass degradation,
ageing of ammunition, modelling of stabilizer, modelling of chain splitting

⎯⎯⎯⎯⎯⎯⎯⎯
Paper 25 on the 37th International Annual Conference of ICT, June 27 to 30, 2006, Karlsruhe, Germany. Pro-
ceedings 2006, pages 25-1 to 25-19. Fraunhofer-Institut für Chemische Technologie (ICT), D-76318 Pfinztal.
25 - 2
1. Introduction

Propellants based on nitric acid ester (NE) compounds are intrinsically unstable because
of their chemical decomposition reactions. The following basic types of such propellants
are encountered, naming here the main type ingredients only:
single base GP NC
double base GP NC, nitrate ester plasticizers (blasting oils)
triple base GP NC, blasting oils, NQ (nitroguanidine)
semi-nitramine GP NC, nitrate ester plasticizers, crystalline energetics as RDX
new type GP NC, n,(n+2)-dinitro -n,(n+2)-diaza = DNDA type plasticizers, RDX
(also enhanced with additional high explosives as CL20, FOX7,...) /1/
double base RP NC, blasting oils
triple base RP NC, blasting oils, RDX

A further main ingredient of these types of propellants is the stabilizer, this with respect
to its effect, the amounts added are normally small. Inside such propellants chemical de-
composition reactions are on going all the time. This happens also in the presence of ac-
tive stabilizers. Decomposition reactions affect the thermal stability and the mechanical
strength of GP grains and the rocket motor charge. An essential indicator for the state of
ageing is the residual stabilizer content. It was well established that by ageing of NC-
based propellants the NC chains shorten and this also in the presence of active stabilizers.
A further effect of ageing of such propellants is there loss in energy. The decomposition
processes release energy (reaction heat) and generate gaseous decomposition products
as CO CO , N , N O, NO, NO , which causes a mass loss. Further not harmless decomposi-
2 2 2 2
tion products are water, H O, nitric acid, HNO , nitrous acid, HNO , organic acids R-COOH
2 3 2
and additional products. With the NC-based propellants the important fact is that as long
as stabilizer is active in the formulation the decomposition itself and the dangerous
products NO, NO , N O , HNO and HNO are under control. This gives the possibility to
2 2 4 3 2
use the stabilizer content and its rate of consumption as monitoring tool for assessing
the state of usability of the propellant charge. It depends on the type of propellant
charge if the monitoring is extended to the chain length of NC by using molar mass de-
crease. The chain length of NC controls the mechanical strength of the propellant grain,
which is an important quantity with rocket motors and large calibre gun propellants.

2. Methodology

The ageing looked at here is caused by chemical decomposition reactions of NC and
other nitrate esters (NE) in the formulation.
k
NE ⎯⎯N⎯E→ P + R-NE intrinsic decomposition
k
NE + P ⎯⎯au⎯⎯to→ 2 P + R-NE autocatalytic decomposition
k
S + P ⎯⎯S⎯P→ P-S stabilizing reaction

The first reaction shows the intrinsic decomposition of NE, which cannot be stopped. The
second reaction stands for the autocatalytic reaction, means one or more decomposition
products of NE open in contact with the start substance NE one or more decomposition
channels for NE. If these autocatalytically active products P could be removed the auto-
catalytic reactions will be suppressed. This is achievable with stabilizers which bind the
products P faster than these can react with NE. In reaction with P the stabilizer is con-
sumed. But as long as active stabilizer is present the propellant is (i) safe, means autoig-
nition will not occur at operation conditions and (ii) the state of quality of the propellant
is known. Further the present amount of stabilizer defines the state of ageing immedi-
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ately. By knowledge of the Arrhenius parameters of the stabilizer rate equation also the
rate of ageing is known, means the rate of stabilizer consumption is known. With these
two informations, namely the state of ageing = amount of actual stabilizer S(t) and rate
of ageing = dS(t)/dt, the residual use time of the propellant can be predicted. The ana-
logue procedure can be made with the degradation of chain length of NC. The state of
ageing for mechanical strength is given with the value of mean molar mass Mn(t) already
reached and the rate of ageing of mechanical strength = dMn(t)/dt. With the rate of age-
ing the complete rate equation is meant. The momentary rates are not enough for ad-
vanced prediction purposes, because dS(t)/dt and dMn(t)/dt are functions of time.

The decisive question in context with monitoring of the ageing is to ask for the ability to
fulfil the military mission, means at what ageing state of the propellant the fulfilment of
the military mission is no longer possible.

From the above it is clear that with NC based propellants the stabilizer content controls
ageing completely. It is enough to know the stabilizer content to make the decision
about the usability of ammunition. Limit values have to be defined for the special de-
mands in OOAO. The next important question is how to determine the stabilizer content
in OOAO field situation?

3. Determination of stabilizer content in field situations

In the following some of the possible methods to determine the actual stabilizer content
will be discussed in short.

3.1 Specially designed field laboratory to use HPLC

HPLC (high performance liquid chromatography) is today the standard method to deter-
mine the stabilizer content of NC-based propellants. In principle it can be employed in
field situations. To do this the necessary prerequisites are:
extraction of stabilizers by solvent
solvent handling
air conditioning
skilled personal
delaboration of ammunition
waste disposal
The advantage of this method is: actual and accurate values of stabilizer content obtain-
able. The error in determined content is about 2 to 5% relative, if the method is worked
out well. The disadvantage of having a full laboratory in the field is the considerable ef-
forts and burdens for the troops.

3.2 Specially designed field test using TLC

TLC (thin layer chromatography) was very helpful in developing the knowledge about
the stabilizer reactions in propellants in the years around 1960 to 1970 /2,3/. It is still very
useful as a fast qualitative analyzing tool. Quantitative analysis techniques have been
developed but they have not the capabilities of the HPLC techniques. Some test device
on the base of TLC was recently developed for field use /4,5/.
special devices and solvent handling necessary
air conditioning necessary in hot areas
skilled personal not necessary, but trained personal
delaboration of ammunition necessary
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waste disposal may be a problem
The advantage is: actual but not accurate values of stabilizer content obtainable. The
error in content is up to 50% with an instrumentally reduced field method /4/.

3.3 Specially designed field test using IR – ATR

With NIR (near infrared) absorption in ATR (attenuated total reflection) technique com-
bined with chemometric evaluation one can determine the stabilizer content /6/. This
method uses only a surface layer of about 5 to 15 µm thickness of propellant grains.
no solvent handling necessary
air conditioning necessary in hot areas
skilled personal not necessary but trained personal
delaboration of ammunition necessary
propellant amount is quite large (200-500 g)
extensive calibration is necessary before the method is usable
the propellant may not change its principal composition at the surface, otherwise
the calibration is no longer valid
> problems with surface coated propellants
> problems with graphitized propellants
Advantage: actual but medium (in)accurate values of stabilizer content obtainable
error in content is up to 15%

3.4 Conclusion about stabilizer content determination in field situation

So far no handsome procedure exists for use in OOA situations. All methods need de-
laboration of ammunition in the field. An air conditioned room seems necessary always.
At least special trained personal is necessary. Logistics and organization is necessary to
make the surveillance in the field situation. Disposal of solvents and of delaborated pro-
pellants and explosives make problems in field situation. Often no suitable waste dis-
posal is available. Methods applied as described above allow no correlation between
ageing loads and achieved ageing. As conclusion these three described methods burden
the troops and increase the mere support part of the mission but not its effectiveness to
fulfil the actual task. Therefore it is to prefer that ammunition must not be delaborated
in field situations.

4. Determination of ageing state by predictive methods

Temperature does affect the ageing of nitric acid ester based propellants at most. With
known ageing behaviour of the propellants the state of ageing can be predicted from
the known status of ageing at begin of the mission and the experienced temperature in
field situations. The only necessary work in the field is to measure the temperatures in
regular intervals. This can be done with small automated data loggers already on the
market. The ageing behaviour of the propellants used in field missions is determined at
home in the specialized surveillance laboratories. There all necessary equipment is avail-
able. Some temperature ageing is necessary to determine the Arrhenius parameters of
stabilizer decrease and if needed also of molar mass degradation of NC for selected pro-
pellants. Because the higher temperatures are important in field situations it is appropri-
ate to use accelerated temperature ageing. This is done at best in an ageing configura-
tion simulating the ammunition situation for the propellant, but a 1:1 simulation is not a
demand. Temperatures below 60°C seem not necessary. The temperature range for ac-
celerated ageing can be 70°C to 90°C. These ageing determinations are therefore quite
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fast and not expensive. Necessary are: (i) sufficient temperature control (≤ ±0.3°C) of the
ageing ovens and (ii) good description of the data with models which have a good ex-
trapolation ability of the data. The model named ‘S: first order + zero order‘ or also
named ‘S: exponential + linear’ developed at ICT /7/ has all these necessary properties,
which are: (1) to describe the data well, (2) extrapolate the data well and (3) to give an
end point of stabilizer content. Further to this it is easy to use.

4.2 Models and descriptions for prediction of stabilizer consumption

In Table 1 the most common models or descriptions of stabilizer consumption in NC / NE-
based propellants are listed. Eq.(1) is the model ‘S: exponential + linear’ just mentioned
/7/. The second equation shows a first order reaction which describes stabilizer depletion
data not well below about 20 to 35% of the original content value. The next equation is
just a decrease according to zero order kinetics, which is a rough approximation of the
true decrease. This equation is the choice if the quality of the data is so low that other
models cannot be applied usefully. The Eq.(4) is from the same author as Eq.(5) and in
principle both are the same in terms of handling /8/. But Eq.(5) must be applied with data
from at least three different temperatures otherwise the two reaction rate constants
cannot be resolved. Eq.(4) has gained some attraction recently /9/. One should be careful
in using this equation because of its variable shape behaviour, see below. The Eq.(6) is
the only one of all equations 1 to 6 shown in Table 1 with a profound mechanistic base.

Table 1: Models and descriptions for stabilizer consumption in NC / NE -based pro-
pellants, Eq.(1) to Eq.(6). The Eq.(7) is for molar mass degradation of NC.

Eq name of model
kinetic rate equation kinetic type
. suitability, how to handle
dS(t) first + zero order, ‘S: expon.+linear’
1 =−k ⋅S(t)−k
dt 1 0 two reactions /7/ suitable, easy to handle
dS(t) first order, ‘S: expon.’
2 =−k ⋅S(t)
dt 1 one reaction not suitable, easy to handle
‘S: linear’
dS(t) zero order,
3 =−k conditioned suitable,
dt 0 one reaction
easy to handle
dS(t) nth order, ‘S: nth order’
4 = −k| ⋅S(t)n
dt one reaction /8/ partly suitable, easy to handle
‘S: parallel-nth order’
nth order,
dS(t) partly suitable, not easy to
5 = −(k| +k| )⋅S(t)n two reactions, but
dt 1 2 handle, eval. with several
simply in S(t) /8/
temp. at one time necessary
two reactions, ‘S: extended’
dS(t)
6 = −k ⋅S(t)⋅(S(t)+NE(0)⋅k ⋅t) NE decomp. and partly suitable, fairly to han-
dt SP NE
consump. of S /10/ dle with advanced fit codes

’Mn: chain split-
⎛Mn(t,T)⎞ chain splitting by
7 d ⎜⎝ m ⎟⎠ =−k (T)⋅⎜⎛⎜⎛Mn(t,T)⎟⎞+⎜⎛Mn(t,T)⎟⎞2⎟⎞ element decom- tsuinitga’ ble,
dt Mn ⎜⎝ m ⎠ ⎝ m ⎠ ⎟ position /11/
⎝ ⎠ easy to handle
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Table 2: Models for prediction of stabilizer consumption and molar mass degradation. Given are rate and integrated equations and
the formulas for the times ty and ty to reach degrees of consumption y and degrees of degradation y .
S Mn S Mn

ty (T) to reach
Eq. kinetic rate equation kinetic equation, stabilizer consumption time t (T) to zero S content S
0 y = S(t,T) / S(0)
S
⎛ k (T) ⎞
⎜ 1+ 0 ⎟
1 dS(t) =−k ⋅S(t)−k S(t,T)=⎜⎛S(0) + k0(T)⎟⎞⋅exp(−k (T)⋅t) − k0(T) t (T)= 1 ⋅ln⎜⎛S(0)⋅k1(T)+1⎟⎞ ty (T)= 1 ⋅ln⎜ S(0)⋅k1(T) ⎟
dt 1 0 ⎜⎝ k1(T)⎟⎠ 1 k1(T) 0 k1(T) ⎜⎝ k0(T) ⎟⎠ S k1(T) ⎜⎜⎝yS+S(0k)0⋅(kT1)(T)⎟⎟⎠
dS(t) 1 ⎛ 1 ⎞
2 dt =−k1⋅S(t) S(t,T)=S(0)⋅exp(−k1(T)⋅t) t0(T) → ∞ tyS(T)= k1(T)⋅ln⎜⎜⎝yS ⎟⎟⎠
3 dS(t) =−k S(t,T)=S(0)−k (T)⋅t t (T)= S(0) ty (T)= S(0) ⋅(1−y )
dt 0 0 0 k (T) S k (T) S
0 0
4 dS(t) = −k| ⋅S(t)n S(t,T)=S(0)⋅[1−(1−n)⋅k(T)⋅t] ⎜⎝⎛1−1n⎟⎠⎞ t0(T)= k(1T)⎜⎝⎛1−1n⎟⎠⎞ tyS(T)= k(1T)⋅1−1(y−Sn)1−n
dt k|(T)
with k(T)=
S(0)1−n only for n<1 only for n<1
⎜⎛ 1 ⎟⎞ 1 1−(y )1−n
5 dS(t) = −(k| +k| )⋅S(t)n S(t,T)=S(0)⋅[1−(1−n)⋅(k1(T)+k2(T))⋅t] ⎝1−n⎠ t0(T)= (1−n)⋅(k (T)+k (T)) tyS(T)= (1−n)⋅(k (TS)+k (T))
dt 1 2 kI(T) 1 2 1 2
with ki(T)= S(0i)1−n only for n<1 only for n<1
( )
dS(t) S(t)= exp−NE(0)⋅kNE⋅kSP ⋅t2
6 dt = −kSP⋅S(t)⋅(S(t)+NE(0)⋅kNE⋅t) 1 + π⋅ kSP ⋅erf⎜⎛ NE(0)⋅kNE⋅kSP ⋅t⎟⎞ t0(T) → ∞ no closed form for tyS(T)
S(0) 2 NE(0)⋅k ⎜ 2 ⎟
NE ⎝ ⎠

time t (T) ty (T) to reach
kinetic rate equation kinetic equation, Mn degradation 0 Mn
to zero Mn y = Mn(t,T) / Mn(0)
Mn
⎛ 1 Mn(0)⎞
⎛Mn(t,T)⎞ m ⎜ + ⎟
7 d ⎜⎝ dmt ⎟⎠ =−kMn(T)⋅⎜⎜⎝⎛⎜⎝⎛Mnm(t,T)⎟⎠⎞+⎜⎝⎛Mnm(t,T)⎟⎠⎞2⎟⎟⎠⎞ Mn(t,T)= ⎜⎝⎛1+Mmn(0)⎟⎠⎞⋅exp(+kMn(T)⋅t)−1 t0(T) → ∞ tyMn(T) = kMn1(T)⋅ln⎜⎜⎜yM1n+Mn(m0) ⎟⎟⎟
⎝ m ⎠
25 - 7
Mechanistically two reaction rate constants must be considered to have a correct model
construction to describe stabilizer consumption in NC / NE- based propellants: (1) one for
the decomposition of NC / NE in forming the product P which has to be caught by the
stabilizer and (2) the stabilizing reaction itself. This is expressed with the two reaction
rate constants k for NE decomposition and k for stabilizer reaction, see reaction
NE SP
scheme in section 2. For more details about Eq.(6) see /10,11,12/. The last Eq.(7) of Table 1
is applied to describe the mean molar mass degradation of NC in the propellants. It is
mechanistically based also. This equation has proven to describe the decrease of Mn (or
for derived Mw) very well /11/. An actual application of model ‘Mn: chain splitting’ can
be found with PVN (polyvinylnitrate) /13/. Table 2 lists the models together with the in-
tegrated form of the rate equations, which are directly applicable to the measurements.
Further are given the times t to reach stabilizer content zero or Mn = 0. In the last col-
0
umn the times ty and ty respectively are given to reach a given degree of stabilizer
S Mn
consumption y and degree of mean molar mass degradation y .
S Mn

Fig. 1: Description of DPA decrease with model ‘S: extended’, Eq.(6) in Table 1 and 2.
25 - 8

Fig. 2: Description of DPA and AkII consumption with model ‘S: nth order’, separately
applied. DPA consumption is well reproduced. With AkII several modes have been
applied: AkII-1 is the most useful one, with offset in t of 60 days. AkII-2 omits the
last three points as AkII-3, but AkII-2 is used from t =0 d on and AkII-3 as AkII-1
off
from t =60 d on. Between AkII-2 and AkII-3 one has great variations. AkII-2 pro-
off
duces a negative reaction order n.

Fig. 1 shows the modelling of DPA decrease in a single base GP with Eq.(6), the model ‘S:
extended’. This model is able to describe the long tailing of the stabilizer decrease which
is sometimes observed. Fig. 2 shows the application of model ‘S: nth order’ to a set of
data obtained from a single base propellant, which was simultaneously stabilized with
DPA and Ak II. Fig. 3 shows the same data modelled with model S: exponential + linear’.
It can be seen clearly that DPA has a much higher reactivity than Ak II. As long as DPA is
active AkII is not consumed. This effect must be regarded in the evaluation. It will be
mentioned here that also other stabilizer combinations will show such effects as for MNA
and 2-NO -DPA, see Fig. 4. However, to assign reactivities to the different stabilizers one
2
must consider the consecutive products of DPA also. In applying model ‘S: nth order’ one
should avoid situations with negative reaction order n, because this is mechanistically not
adequate. Further one has to regard that the stabilizer concentrations are determined
down to low concentration values. Otherwise one can encounter the situation of AkII-3
compared to AkII-1. The description AkII-3 users a reduced data set and gives already at
y about 0.4 significant deviations to too high values.
S
25 - 9

Fig. 3: Description of DPA and Ak II consumption with model ’S: exponential + linear’,
separately applied. To describe the Ak II data the start point of the description
was shifted to 60 days in order to regard the non-active part of Ak II in the begin-
ning. The achieved description is very good for both curves.

Fig. 4: Description of MNA and 2N-DPA with model ‘S: exponential + linear’, separately
applied as in Fig. 3. MNA reacts much faster than 2N-DPA.
25 - 10
5. Results from a naturally aged propellant

5.1 Determination of stabilizer consumption rate before OOAO

To make a prediction of use time the rate of stabilizer consumption must be known as
function of temperature and time. Determinations of stabilizer content as function of
time and temperature have to be made. With suitable models the rate constants are de-
termined and therewith the corresponding Arrhenius parameters. Three models have
been applied: (i) ‘S: exponential + linear’, Eq.(1), (ii) ‘S: nth order’, Eq.(4), (iii) ‘S: ex-
tended’, Eq.(6). The results of the fits can be seen in the Fig. 5, 6, and 7. The Arrhenius
parameters are compiled in Table 3. Two types of evaluation have been made: the usual
single temperature fits obtaining the rate constants and then followed by the Arrhenius
evaluation to get the parameters. In the other evaluation the fit was done simultane-
ously with all temperature data and the Arrhenius parameters itself have been fitted.
The differences are small in the data description as can be seen in the Fig. 5 and 7. With
Eq.(4) only the all temperature fit is shown in Fig. 6, but in Table 3 both data sets are
given for this model also. The all temperature fit gives an advantage, if for one tempera-
ture only a few data points are available, which even may be not enough for a single
temperature fit. Some differences in results caused by different weighing of the data
may arise between the two evaluation methods. Also between models different weigh-
ing of data occurs.

0.9
DPA [mass-%]
0.8 sb NC-propellant 90°C experiment
description with model
80°C experiment
'S: expon. + linear'
0.7 70°C experiment
zero
0.6 90°C expon.+lin.-aT to=7.05 d
80°C expon.+lin.-aT to=25.86 d
0.5
70°C expon.+lin.-aT to=102.36 d
70°C
90°C expon.+lin.-sT to=6.63 d
0.4
80°C expon.+lin.-sT to=25.83 d
80°C 70°C expon.+lin.-sT to=107.02 d
0.3
0.2 90°C
0.1 107.02
7.05 25.86 d
0
time [d]
6.63 d 25.83 d 102.36
-0.1
0 10 20 30 40 50 60 70 80 90 100 110 120 130

Fig. 5: Description of DPA consumption with model ‘S: exponential+ linear’ in two ways:
single temperature fit (sT, broken lines) and simultaneous all temperature fit (aT).