There
are various ways of finding out the explosivity of a material. Explosivity of a
material can be identified theoretically and also practically with the help of
lab instrument (like fall-hammer test). However, liquids can’t be tested using
lab equipment. Hence, for explosivity properties for liquids, one shall
strictly depend on theoretical method and/or other literatures sources.
1.1
Identification
of high energy/ explosive substances functional groups present
Energetic
substances in general, can be identified by the presence of hazardous molecular
structures such as Peroxide
groups, nitro groups, azo groups, double and triple bonds, and ring deformation
and steric hindrance all influence the stability of a molecule. Compilations of
energetic groups have been published in “Bretherick’s Handbook of Reactive
Hazards”.
The below table lists a number of groups
that have relatively weak bonds and that release
substantial
energy upon cleavage. The list is not exhaustive. The listing does, however,
provide assistance in screening chemical structures for determining which
should be investigated further before consideration of handling, even in small
quantities.
Table 1: list
of functional groups known to release high energy on cleavage or change during
reaction
Compounds
containing |
Examples |
Carbon (no Nitrogen or Oxygen) |
•C=C-C=C dienes •C=C=C allenes •triple bonded carbons alkenynes, alkynes,
haloalkynes, polyalkynes •halo-aryl metals & haloarenemetal Ï€ complexes
(Ar-Metal-X & X-Ar-Metal) |
Compounds
containing |
Examples |
Carbon and Nitrogen |
C-N=N-C azo compounds C linking N rings triazoles, aziridines,
nitriles, diaziridines CN2 diazo compounds C-N3 alkyl, aryl azides C-N=N-N triazenes C triple bond N dicyanogen |
Carbon and Oxygen |
C linking O rings oxiranes C-O-OH alkyl hydroperoxides (-CMe2O-O-)3
trimeric acetone peroxide C-O-O-C dialkyl peroxides |
Carbon, Nitrogen and Oxygen |
C-N=O nitroso compounds C-NO2 nitro compounds C-O-NO2 alkyl nitrites C-O-NO2 alkyl nitrates C=NOH oximes C-N=N-O- arenediazoates, bis(arenediazo)
oxides C(NO2)2
-polynitroalkyl compounds CO.O-N=O acyl nitrites CO.O-NO2 acyl
nitrates C triple bond N--O nitrile oxides |
Containing Nitrogen and Oxygen |
NO nitrogen oxide NO2 Nitrogen
dioxide or N2O4 dinitrogen tetroxide H2NOH
hydroxylamine and salts N2O dinitrogen
oxide N2O5
dinitrogen pentoxide |
Nitrogen and Other Elements |
N-X N-halogen compounds N-metal N-heavy metal compounds -NF2 difluoroamino compounds -N-S- nitrogen-sulfur compounds |
Halogens, Oxygen, and Other Elements |
-O-X hypohalites -O-X-O2 halates N-Cl-O3 perchlorylamide salts
O-X-O halites, halogen oxides O-X-O3 perhalates, halogen
oxides |
Explanatory note: The presence of one of the mentioned groups in the
molecule does not necessarily imply that the substance is hazardous. For
instance, a molecule that contains a nitro group attached to a long aliphatic
chain does not show significant explosive properties. On the other hand,
tri-nitro methane, which consists of three nitro groups attached to a methane
group, does have dangerous explosive properties. "Diluting" the
active groups by increasing the molecular weight decreases the explosive
potential.
However,
the initial absence of unstable groups is no guarantee for long-term stability
of the compound. For example, some aldehydes and ethers are easily converted to
peroxides by reaction with oxygen from air. Organic peroxides represent a class
of unstable materials while monomers represent a class of substances that can
self-react by polymerization if not properly inhibited and if the temperature
is not properly maintained.
1.2
Determination
of explosivity theoretically
Oxygen balance (OB or OB%):
It is an expression
that is used to indicate the degree to which an explosive can be oxidized. If
an explosive molecule (as mentioned in section 5.1.1) contains just enough
oxygen to form carbon
dioxide from carbon, water from hydrogen atoms, all of its sulfur
dioxide from sulfur, and all metal oxides from metals with no excess, the molecule is said
to have a zero oxygen balance. The molecule is said to have
a positive oxygen balance if it contains more oxygen than is needed
and a negative oxygen balance if it contains less oxygen than is
needed; the combustion will then be incomplete, and large amount of toxic gases
like carbon
monoxide will be present. The
sensitivity, strength, and brisance of an explosive are all somewhat dependent upon oxygen balance and
tend to approach their maxima as oxygen balance approaches zero.
The oxygen balance
is calculated from the empirical formula of a compound in percentage of oxygen
required for complete conversion of carbon to carbon dioxide, hydrogen to
water, and metal to metal oxide.
The procedure for calculating oxygen balance in terms of 100 grams of
the explosive material is to determine the number of moles of oxygen
that are excess or deficient for 100 grams of a compound.
Where,
X = Number
of atoms of carbon
Y = Number of atoms of hydrogen
Z = Number of atoms of oxygen
M = Number of atoms of metal (metallic oxide
produced)
MW = Molecular weight of the material/powder
The criteria for determining the
severity from the OB calculation is:
Note: The evaluation of the hazard potential based on the oxygen balance
is not an absolute rating, but only an indicator.
Almost
all the recognized detonating explosives have oxygen balances between -100 and
+40 (e.g. glycerol trinitrate = +3.5). Any substance with an oxygen balance
more positive than -200 should be tested as a potential high risk, and explosivity
testing should be carried out (mentioned in section 5.1.3)
Exercise:
Let’s calculate the oxygen balance for TNT
(tri-nitro-toluene)
Chemical formula = C6H2(NO2)3CH3
Molecular weight = 227.1
X = 7
Y = 5
Z = 6
M = 0
Conclusion:
OB is found to be -73.97. TNT falls under the high severity (as per the scale
mentioned above) for potential explosive. TNT is a well-known explosive. But
for an unknown material even after getting the OB as high severity,
experimentally it has to be confirmed whether the material is really an
explosive or not.
Note: Why oxygen
balance shouldn’t be performed for all the chemicals can be explained by taking
an example of MDC (Dichloromethane). Using the formula, OB for MDC can be
obtained as-56.5. As per severity scale MDC falls under high severity, but in
reality, MDC is not an explosive. Hence, before taking any material for OB
exercise it has to be checked whether it is a candidate for OB exercise or not
with the help of table 1.
1.3
Determination
of explosivity or shock sensitivity of a material using BAM fall-hammer
It is very important know whether a
material is shock sensitive or not. Shock sensitive materials are to be stored
separately with certain conditions. This can be measured with the help of an
equipment called BAM Fall Hammer.
It is also known from several sources
that every powder or material is not a candidate for shock sensitivity test. A
powder/material is said to be a candidate for shock sensitivity test only if it
satisfies either of the two conditions mentioned below -
· An
exotherm or decomposition quantifying over 800 J/g from DSC result (and/or)
· Presence
of any explosive groups (or plosophores) in the chemical structure of the
molecule (e.g. nitro, azide, peroxy groups as mentioned in table 1)
BAM Fall Hammer:
A
small sample (50-60 mg) is enclosed in an impact device (Part B in the above
diagram). The sample filled impact device is placed on the anvil and the drop
weight (Part A in the above diagram) is released from a defined height. The
drop weight usually weighs in the range of 1 kg, 5 kg, 10 kg and the height can
be adjusted. The potential energy due to free fall of the drop weight is
supplied to the compound and can be calculated.
Result
and interpretation:
The
result of this experiment is either positive (shock sensitive) or negative (not
shock sensitive). But if the result is positive then the potential energy
required to create the shock sensitivity can be calculated using the formula
PE= m x g x h, where (PE: potential energy in Joules, m : mass of the drop
weight in kgs, g : acceleration due to gravity 9.8 m/sec2, h :
height of the fall of the drop weight).
Basis
of safety:
If
a material is found to be shock sensitive in nature, then following measures
can be taken-
·
Do not store at heights
·
Do not isolate the
material (if it is intermediate or final product). It is safe to keep the
material in a solution form
·
If it is a reagent or
raw material, then explore for alternative of that material
·
If it is intermediate,
then explore alternate route of synthesis if material isolation is inevitable.
·
If the material is a
final product, then it shouldn’t undergo any kind of powder processing
operations (such as milling, sifting etc.). Drying if required to be conducted
at utmost care, no FBD & spray drying operations.
·
If specific particle
size is required for final product, then during final crystallization step
itself it has to be brought and it shouldn’t be pushed to size reduction
operations like milling.