Determination of explosive properties of a material (starting material, raw materials, finished products)

 

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 π 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.

Oxygen balance calculation formula is -

 

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

Therefore,

 

 

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:

Test procedure:

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.

1.4  Flow chart for explosivity screening procedure