Emulsion explosive

A mixed surfactant system for use in emulsion explosives is provided which confers improved emulsion stability and comprises a surfactant and a co-surfactant, each having branched chain hydrocarbyl tail groups, the former having significantly longer tail chain groups than the latter, for which system poly[alk(en)yl] succinic anhydride based surfactants are especially preferred, said surfactants having an interaction parameter, .beta., which is less than zero.

FIELD OF THE INVENTION 
This invention relates to emulsion explosives, and in particular to 
explosives containing a mixed surfactant system. 
DESCRIPTION OF THE RELATED ART 
Water in oil emulsion explosives are well known in the explosives industry, 
and typically comprise an oxidizer salt-containing discontinuous phase 
which has been emulsified into a continuous fuel phase for which a variety 
of oils, waxes, and their mixtures have been employed. The oxidizer salt 
may be a concentrated aqueous solution of one or more suitable oxidizer 
salts or a melt of such salts containing a small proportion of water or 
even containing adventitious water only. 
Emulsion explosives have been described by, for example, Bluhm in U.S. Pat. 
No, 3,447,978 which discloses a composition comprising an aqueous 
discontinuous phase containing dissolved oxygen-supplying salts, a 
carbonaceous fuel continuous phase, an occluded gas and a water-in-oil 
emulsifier. Cattermole et al., in U.S. Pat. No. 3,674,578, describe a 
similar composition containing as part of the inorganic oxidizer phase, a 
nitrogen-base salt such as an amine nitrate. Tomic, in U.S. Pat. No. 
3,770,522 also describes a similar composition wherein the emulsifier is 
an alkali metal or ammonium stearate. Healy, in U.S. Pat. No. 4,248,644, 
describes an emulsion explosive wherein the oxidizer salt is added to the 
emulsion as a melt to form a "melt-in-fuel" emulsion. 
Selection of the emulsifier used to prepare an emulsion explosive is of 
major importance in providing an emulsion which emulsifies easily, has a 
suitable discontinuous phase droplet size, and is stable during storage to 
prevent or lower the tendency for the oxidizer salt to crystallize or 
coalesce, since crystallization or coalescence will adversely affect the 
explosive properties of the emulsion explosive. 
Australian Patent Application No. 40006/85 (Cooper and Baker) discloses 
emulsion explosive compositions in which the emulsifier is a reaction 
product of a poly[alk(en)yl] species (e.g. an alkylated succinic 
anhydride) and inter alia amines such as ethylene diamine, diethylene 
tetramine and mono- and di-ethanolamines. 
McKenzie in U.S. Pat. No. 4,931,110 describes the use of a bis(alkanolamine 
or polyol) amide and/or ester derivatives of, for example, polyalk(en)yl 
succinic anhydride compounds as suitable surfactants. Polyalk(en)yl 
succinic anhydride compounds were described by Baker in Canadian Patent 
No. 1,244,463. 
Forsberg et al. in U.S. Pat. No. 4,840,687, describe an emulsion explosive 
composition wherein the emulsifier is a nitrogen-containing emulsifier 
derived from at least one carboxylic acylating agent, a polyamine, and an 
acidic compound. 
The prior art also includes specific examples of polyalkyl succinic acid 
salts and polyalkyl phenolic derivatives. 
The formation of an emulsion explosive and the stabilization of an emulsion 
explosive once formed make a number of demands on an emulsifier system. A 
first requirement is an ability to stabilize new surfaces as the emulsion 
is formed by lowering the interfacial tension, i.e. an emulsifying 
capacity. The second requirement is an ability to form a structured 
bilayer (since an emulsion explosive is mainly composed of densely packed 
droplets of supersaturated dispersed phase in a fuel phase) so that the 
tendency, in an emulsion at rest, for droplets to coalesce and for 
crystallization of salts to spread from nucleated droplets to their 
dormant neighbours is suppressed. A third desired feature, related to the 
first but seemingly at odds with the second, would be an ability to 
preserve bilayer integrity dynamically when an emulsion explosive is 
sheared e.g. when being pumped. The industry response to these demands has 
been compromise formulations (or acceptance of operational restrictions). 
There are examples in the prior art referred to hereinabove where an 
emulsifier capable of structured packing in the bilayer is used in 
admixture with a smaller mobile surfactant that is an effective 
water-in-oil emulsifier for emulsion explosive production. 
A particularly preferred mixed emulsifier system of the prior art, as 
described, for example, in the above-mentioned Cooper/Baker reference and 
by Yates et al. in U.S. Pat. No. 4,710,248, comprises a derivitised 
polyisobutene succinic anhydride surfactant, in combination with a 
co-surfactant such as sorbitan monooleate. 
The effectiveness of emulsification of the oxidizer salts and liquid fuels 
as a promoter of explosive performance is dependent on the activity of the 
emulsifying agent chosen. The emulsifying agent aids the process of 
droplet subdivision and dispersion in the continuous phase by reducing the 
interfacial tension, and thus reducing the energy required to create new 
surfaces. The emulsifying agent also reduces the rate of coalescence by 
coating the surface of the droplet with a layer of molecules of the 
emulsifying agent. The emulsifying agents employed in the aforementioned 
prior art explosive compositions are somewhat effective in performing 
these functions, but improvements in the combination of properties 
exhibited by the emulsion system are still sought, especially for 
so-called repumpable (i.e. unpackaged) formulations of emulsion 
explosives. 
Thus, it is desirable to provide an emulsion explosive emulsifier with 
improved properties so that it is both effective as an emulsifier and 
capable of resisting the tendency for the oxidiser phase of the explosive 
to crystallize and/or coalesce, especially when being sheared. 
SUMMARY OF THE INVENTION 
The present invention provides an emulsion explosive having a discontinuous 
oxidizer salt phase, a continuous oil phase, and an emulsifier for 
stabilization of the emulsions characterized in that said emulsifier 
comprises a surfactant mixture of a branched polyalkyl hydrocarbon 
surfactant and a branched polyalkyl hydrocarbon surfactant and a branched 
polyalkyl hydrocarbon co-surfactant, wherein said surfactant mixture has 
an interaction parameter (.beta.) with a value below zero, preferably -2 
or lower. 
In the mixed surfactant system the interaction of the two or more 
surfactants can be measured to determine the degree of compatibility of 
the surfactants in the system. The average molecular surface area of the 
surfactant blend is measured and compared with the arithmetic mean of the 
molecular surface areas of the independent surfactants in a standard 
reference interfacial system. A reduction in average area can be 
attributed to the intermolecular attraction between the surfactant 
molecules, and an increase in area can be attributed to repulsion or 
increased disorder at the interface. These interactions can be quantified 
by a parameter, .beta., which is known as an interaction parameter, and 
determined as described hereinafter. 
For attractive interactions between surfactants, .beta. becomes negative 
which can be interpreted as positive synergism. For repulsive interaction, 
.beta. becomes positive which can be interpreted as negative synergism or 
antagonism. The larger the numerical value of .beta., the stronger the 
interaction. 
The Applicants have measured values of .beta., by the method specified 
hereinafter, for specific prior disclosed w/o emulsifier mixtures and have 
found values invariably positive for those mixtures. Generalised prior art 
disclosures to the effect that mixtures of W/O emulsifiers taken from 
given chemical classes (e.g. the same class or different classes) may be 
used in W/O explosive emulsions provide no teaching on selection and are 
wholly silent on the possibility that synergism, as reflected in negative 
.beta. values, is achievable in the demanding context of emulsion 
explosive W/O emulsifier systems. Applicants have discovered that a 
selected relatively small number of mixed surfactants that together 
function as W/O emulsifiers for an emulsion explosives show negative 
.beta. values. Applicants are not presently able to exhaustively or even 
predominantly characterise these select systems by reference to chemical 
structures of the constituent emulsifiers. Preferred chemical families of 
emulsifiers within which synergistic mixtures may be found are, however, 
identified hereins as are specific synergistic mixtures. Nevertheless a 
person skilled in the art of emulsion explosive manufacture, aided by 
persons skilled in emulsifier chemistry and interfacial tension 
measurement, can, by the methods specified herein, evaluate mixtures of 
emulsifiers to determine their .beta. values and hence the extent of any 
attractive inter-molecular interaction. 
The interaction parameter, .beta., for mixed surfactant monolayer formation 
at the liquid-liquid interface can be determined from plots of interfacial 
tension vs. total surfactant molar concentration. The method of 
determining the value of .beta., as used in this specification, is as 
follows: 
The interaction parameter .beta. is determined experimentally from a plot 
of the interfacial tension of an aqueous AN solution/oil phase interface 
versus log surfactant concentration for each of the two surfactants 
(surfactant and co-surfactant) in the system and a mixture of the two at a 
fixed mole fraction which has been previously determined to be optimum. 
The concentration of the aqueous AN solution sub phase is 35% AN m/m. The 
optimum mole fraction is determined from the minimum in the plot of 
interfacial tension versus mole fraction of one of the two surfactants 
mixed in various proportions (from 0 to 100%) in the surfactant mixtures, 
where the concentration of both of the surfactants remained above the 
critical concentration of the individual surfactants. The interfacial 
tension versus log surfactant concentration plots for single and mixed 
surfactant systems provide molar concentration values that produce a given 
interfacial tension value. This can be schematically represented in the 
FIG. 1. 
According to FIG. 1, C.sub.12.sup.M, C.sub.1.sup.M and C.sub.2.sup.M are 
the critical concentration of the mixed surfactants, pure surfactant 1 and 
pure surfactant 2 respectively. The critical surfactant concentration is 
that concentration above which no further decrease in interfacial tension 
is determined with further increase in surfactant concentration. C.sub.12, 
C.sub.1.sup.0 and C.sub.2.sup.0 are the concentrations of the surfactants 
required to produce a given interfacial tension value. The mixture of the 
two surfactants 1 and 2 at a given mole fraction produce synergism (as 
shown in A) when C.sub.12 &lt;C.sub.1.sup.0, C.sub.2.sup.0. In case of 
antagonism (as shown in B) C.sub.12 &lt;C.sub.1.sup.0, C.sub.2.sup.0. 
The interaction parameter .beta. can be calculated from the values of 
C.sub.12, C.sub.1.sup.0 and C.sub.2.sup.0 by the following equations. 
##EQU1## 
where .alpha. is the mole fraction of the surfactant 1 and (1-.alpha.) is 
the mole fraction of the surfactant 2 in the surfactant/oil mixture. 
X.sub.1 is the mole fraction of surfactant 1 in the total surfactant in 
the mixed monolayer and the value of X.sub.1 can be obtained by solving 
Equation 1. 
Interfacial tensions at a mineral oil-aqueous ammonium nitrate solution 
interface were measured by the du Nouy ring detachment method. For all the 
single and mixed surfactant systems, a number of surfactant solutions in 
mineral oil were prepared by varying the molar concentration of 
surfactants. Each solution was then separately poured onto the surface of 
a 35% m/m aqueous ammonium nitrate solution and allowed sufficient time to 
equilibrate before measuring the interfacial tensions. 
Interfacial tensions were measured by a Fisher Tensiomat (model 21) 
semi-automatic tensionmeter with a platinum-iridium ring. 
The .beta. parameters were determined by using C.sub.1.sup.0, C.sub.2.sup.0 
and C.sub.12 values taken from interfacial tension versus log 
concentration of surfactant plots at a certain value of interfacial 
tension where the slopes are almost linear. 
In a mixed surfactant system containing a major proportion of one 
surfactant, wherein .beta. is negative, the interfacial tension of the 
system will be less than the interfacial tension of a system having only 
that surfactant as the emulsifier. Preferably, the interfacial tension of 
the mixed surfactant system will be less than the interfacial tension of a 
system having any one of the surfactants of the mixture as its emulsifier. 
Thus, for a two surfactant emulsifier mixture, it is preferred that an 
emulsifier mixture is utilized in an emulsion explosive for which the 
interfacial tension of the mixture is less than the interfacial tension of 
either surfactant alone as determined by the aforedescribed method. 
It is not a necessary condition that the surfactants of the mixture should 
each be capable for forming a stable practically useful emulsion explosive 
formulation, only that the mixture should. 
The term "branched polyalkyl hydrocarbon" is used in this specification to 
mean hydrocarbon chains derived from polymerised branched hydrocarbon 
monomers, especially isobutene. These chains may be attached in a variety 
of ways to a "head" group which is the hydrophilic salt-tolerant part of 
the surfactant molecule. 
Preferably, at least one surfactant is a poly[alk(en)yl]succinic anhydride 
based compound derived from olefins preferably having from 2 to 6 carbon 
atoms which will form a branched chain hydrophobic structure preferably 
wholly free of unsaturation in the chain. Systems in which the surfactant 
and the co-surfactant have different repeat units in their chains are not 
excluded because differences do not necessarily imply antagonism and 
repulsion but preferably, however, the surfactant and co-surfactant are 
derived from the same monomer, most preferably isobutylene. 
The head group may in such cases be inserted by reacting the succinic 
anhydride (or its acid form) with an amino- or hydroxyl-function, e.g. of 
a di- or polyamine (such as the poly[ethyl amine]s) or an ethanolamine 
(such as MEA or DEA) or a di-N-alkyl ethanolamine (in which case an ester 
link forms). A 1:1 molar ratio of reacting succinic anhydride and amino 
groupings allows for imide/amide formation. Intramolecular salt linkages 
may be present also. The formation of PiBSA derivatives and their use as 
emulsifiers for emulsion explosives is fully disclosed in the prior art 
including that referenced hereinabove. An alternative linking species to 
succinic anhydride is a phenolic link as also described in the prior art. 
A linking group such as these is used because it is chemically facile to 
produce a range of emulsifiers by the route of preforming a polyalkyl 
succinic anhydride (or phenol) reagent and then derivitizing it. The 
direct joining of a polyalkyl chain to, say, an alcohol or amine is less 
straightforward but the resulting emulsifiers are effective. 
The polyalk(en)yl portion of each surfactant in a mixture of such 
surfactants will, as a consequence of its method of preparation, consist 
of a population of molecules of differing chain lengths. Typically, a 
graph of molecular weight against the amounts of constituent molecules 
having particular molecular weights will have the familiar pronounced 
"bell" shape. The molecular weight distribution may be indicated in a 
variety of ways. Preferred in the case of polymeric emulsifiers now used 
in emulsion explosives is average molecular weight because it does not 
indicate the molecular weight at and around which the bulk of the 
constituent molecules lie (the log normal distribution of molecular 
weights being relatively narrow and tall). Numerically stated, it is 
preferred that each surfactant should be one of which at least 75% of the 
polymeric tails of its constituent molecules lie in a band of molecular 
weight contributions between about 70% and about 130% of the number 
average polymeric tail molecular weight contribution as measured by the 
method of high performance size exclusion chromatography (HPSEC) with a 
photo-diode array UV-vis detector. The specific details of the method used 
to provide the data set out herein were as follows: The column set 
comprised Waters Ultra-Styragel 100, micro-styragel 500, Ultra-Styragel 
10.sup.3 micro-styragel 10.sup.4. The molecular weight standards were 
narrowly polydisperse polystyrenes from Toyo Soda Chemical Company. The 
mobile phase was tetrahydrofuran maintained under a blanket of ultra-high 
purity helium. The method produces the chromatogram, calibration curve and 
molecular weight distribution. Typical molecular weight distributions for 
PiBSA (average molecular weight 1000), PiBSA (average molecular weight 
450), and mixtures of PiBSA (MW 1000) and (MW 450) are indicated in the 
following Table II. 
TABLE II 
______________________________________ 
Material PiBSAs 
(as purchased from 
M.sub.n (Number 
M.sub.w (weight) 
Polydispersity 
trade sources) 
average M.sub.w) 
average M.sub.w) 
(M.sub.w /M.sub.n) 
______________________________________ 
PiBSA-1000 683 993 1.45 
Nominal 
PiBSA-450 Nominal 
390 478 1.22 
1:1 mixture of 
480 720 1.50 
PiBSA-1000 and 
PiBSA-450 
(calculated M.sub.n and 
M.sub.w are 536 and 735 
respectively) 
PiBSA-1300 710 1300 1.83 
Nominal 
7:3 mixture of 
634 1024 1.61 
PiBSA-1300 and 
PiBSA-450 
(calculated M.sub.n and 
M.sub.w are 614 and 
1053 respectively) 
______________________________________ 
For practical purposes, it can be assumed that the molecules of a given 
polymeric surfactant produced with a single head-group reagent will all 
have the same head group. The molecular weight population preference 
expressed hereinabove implies a similar band of chain lengths for the 
polymeric tail of the emulsifier where it consists, as is preferred, of 
repeat units of a single monomeric hydrocarbon moiety, such as 
iso-C.sub.4. Thus a derivitised PiBSA emulsifier of which the PiBSA 
component has an average molecular weight of around 950-1000 will have an 
average carbon chain length of around 30-32 carbon atoms. The "75% 
population band" of chain lengths would then be from around 20 to around 
42 carbon atoms. 
For present purposes the mixed emulsifier system is preferably selected 
from bimodal mixtures of polymeric surfactants consisting essentially of 
1. two polymeric surfactants having branched, preferably methyl-branched 
(preferably both iso C.sub.4) hydrocarbyl repeat units in their alkyl tail 
chains; 
2. one said surfactant has a number average carbon chain length of at least 
around 30 carbon atoms, especially in the range 30 to 60 carbon atoms (and 
preferably a "75% population band" as above defined); 
3. the other said surfactant has a number average carbon chain length of at 
least 12 carbon atoms, especially in the range 12 to 30 carbon atoms (and 
preferably a "75% population band" as above defined); 
and wherein 
(i) the number average carbon chain lengths of the said surfactants differ 
by at least 10 carbon atoms, preferably at least 18 carbon atoms, and 
(ii) each said surfactant has a molecular weight contribution from the 
portion of the molecule other than the alkyl tail (i.e. the head group 
inclusive of any linkage) less than 400, preferably less than 300, and 
more preferably less than 240. 
The Applicants experience to date has shown that, for the requisite 
negative .beta. value of practically suitable emulsifier systems, the head 
groups of the mixed surfactants will likely need to be different. 
Guidance in selecting for test by the methods herein described suitable 
head groups for the mixed emulsifier is afforded by the Examples 
hereinafter. From the Examples it is reasonable to deduce: 
a) the head groups should be capable of adopting a relative spatial 
alignment in the interfacial region such that their pendant hydrocarbyl 
tails can be drawn closely together (close parallelism); 
b) the head group interactions must positively encourage the hydrocarbyl 
tails to be so drawn together; 
c) the hydrocarbyl tails should themselves be chemically and sterically 
compatible, even similar, such that they will freely associate and form an 
array of closely packed co-extensive chains (i.e. no chemical repulsion or 
steric incompatibility); 
d) there should desirably be sufficient relative mobility of one of the 
surfactants for it to be able to move into the interfacial region quickly 
to fill, and repair, gaps in the interfacial surfactant continuum. 
Acceptable relative proportions of surfactant and co-surfactant are 
determinable experimentally. Preferably, the longer tail surfactant is the 
major molar component (&gt;50% more preferably &gt;70%) because of its 
importance to bi-layer dimensions and to emulsion stability in regions of 
salt crystallisation in nucleated droplets. 
Typically, the total emulsifier component of the emulsion explosive 
comprises up to 5% by weight of the emulsion explosive composition. Higher 
proportions of the emulsifier component may be used and may serve as a 
supplemental fuel for the composition, but in general it is not necessary 
to add more than 5% by weight of emulsifier component to achieve the 
desired effect. Stable emulsions can be formed using relatively low levels 
of emulsifier component and, for reasons of economy, it is preferable to 
keep to the minimum amounts of emulsifier necessary to achieve the desired 
effect. The preferred level of emulsifier component used is in the range 
of from 0.4 to 3.0% by weight of the emulsion explosive, say 1.5 to 2.5% 
by weight. 
The oxidizer salt for use in the discontinuous phase of the emulsion is 
selected from the group consisting of ammonium and alkali and alkaline 
earth metal nitrates and perchlorates, and mixtures thereof. It is 
particularly preferred that the oxidizer salt is ammonium nitrate, or a 
mixture of ammonium and sodium nitrates. 
A very suitable oxidizer salt phase comprises a solution of about 77% 
ammonium nitrate and 11% sodium nitrate dissolved in 12% water 
(percentages being by weight of the oxidizer salt phase). 
In general the oxidizer salt phase of commercial emulsion-explosives will 
contain a significant proportion of water and is reasonably described as a 
concentrated aqueous solution of the salt or mixture of salts. However, 
the oxidizer salt phase may contain little water, say less than 5% by 
weight, and in such a case be more correctly described as a melt. 
The discontinuous phase of the emulsion explosive may be a eutectic 
composition. By eutectic composition it is meant that the melting point of 
the composition is either at the eutectic or in the region of the eutectic 
of the components of the composition. 
The oxidizer salt for use in the discontinuous phase of the emulsion may 
further contain a melting point depressant. Suitable melting point 
depressants for use with ammonium nitrate in the discontinuous phase 
include inorganic salts such as lithium nitrate, sodium nitrate, potassium 
nitrate; alcohols such as methyl alcohol, ethylene glycol, glycerol, 
mannitol, sorbitol, pentaerythritol; carbohydrates such as sugars, 
starches and dextrins; aliphatic carboxylic acids and their salts such as 
formic acid, acetic acid, ammonium formate, sodium formate, sodium 
acetates and ammonium acetate; glycine; chloracetic acid; glycolic acid; 
succinic acid; tartaric acid; adipic acid; lower aliphatic amides such as 
formamide, acetamide and urea; urea nitrate; nitrogenous substances such 
as nitroguanidine, guanidine nitrate, methylamine nitrate, and ethylene 
diamine dinitrate; and mixtures thereof. 
Typically, the discontinuous phase of the emulsion comprises 60 to 97% by 
weight of the emulsion explosive, and preferably 86 to 95% by weight of 
the emulsion explosive. 
The continuous water-immiscible organic fuel phase of the emulsion 
explosive comprises an organic fuel. Suitable organic fuels for use in the 
continuous phase include aliphatic, alicyclic and aromatic compounds and 
mixtures thereof which are in the liquid state at the formulation 
temperature. Suitable organic fuels may be chosen from fuel oil, diesel 
oil, distillate, furnace oil, kerosene, naphtha, waxes, (e.g. 
microcrystalline wax, paraffin wax and slack wax), paraffin oils, benzene, 
toluene, xylene, asphaltic materials, polymeric oils such as the low 
molecular weight polymers of olefins, animal oils, fish oils, corn oil and 
other mineral, hydrocarbon or fatty oils, and mixtures thereof. Preferred 
organic fuels are liquid hydrocarbons, generally referred to as petroleum 
distillate, such as gasoline, kerosene, fuel oils and paraffin oils. More 
preferably the organic fuel is paraffin oil. 
Typically, the continuous water-immiscible organic fuel phase of the 
emulsion explosive (including emulsifier) comprises more than 3 to less 
than 30% by weight of the emulsion explosive, and preferably from 5 to 15% 
by weight of the emulsion explosive. 
If desired optional additional fuel materials, hereinafter referred to as 
secondary fuels, may be mixed into the emulsion explosives. Examples of 
such secondary fuels include finely divided materials such as: sulphur; 
aluminium; carbonaceous materials such as gilsonite, comminuted coke or 
charcoals carbon black, resin acids such as abietic acid, sugars such as 
glucose or dextrose and other vegetable products such as starch, nut meal, 
grain meal and wood pulp; and mixtures thereof. 
Typically, the optional secondary fuel component of the emulsion explosive 
is used in an amount up to 30% by weight based on the weight of the 
emulsion explosive. 
The explosive composition is preferably oxygen balanced or not 
significantly oxygen deficient. This provides a more efficient explosive 
composition which, when detonated, leaves fewer unreacted components. 
Additional components may be added to the explosive composition to control 
the oxygen balance of the explosive composition, such as solid particulate 
ammonium nitrate as powder or porous prill. The emulsion may also be 
blended with ANFO. 
The explosive composition may additionally comprise a discontinuous gaseous 
component which gaseous component can be utilized to vary the density 
and/or the sensitivity of the explosive composition. 
Methods of incorporating a gaseous component and the enhanced sensitivity 
of explosive compositions comprising gaseous components are well known to 
those skilled in the art. The gaseous components may, for examples be 
incorporated into the explosive composition as fine gas bubbles dispersed 
through the composition, as hollow particles which are often referred to 
as microballoons or microspheres, as porous particles of e.g. perlite, or 
mixtures thereof. 
A discontinuous phase of fine gas bubbles may be incorporated into the 
explosive composition by mechanical agitation, injection or bubbling the 
gas through the composition, or by chemical generation of the gas in situ. 
Suitable chemicals for the in situ generation of gas bubbles include 
peroxides, such as hydrogen peroxide, nitrites, such as sodium nitrite, 
nitrosoamines, such as N,N'-dinitrosopentamethylenetetramine, alkali metal 
borohydrides, such as sodium borohydride, and carbonates, such as sodium 
carbonate. Preferred chemicals for the in situ generation of gas bubbles 
are nitrous acid and its salts which decompose under conditions of acid pH 
to produce nitrogen gas bubbles. Preferred nitrous acid salts include 
alkali metal nitrites, such as sodium nitrite. These can be incorporated 
as an aqueous solutions a pre-emulsified aqueous solution in an oil phase, 
or as a water-in-oil micro emulsion comprising oil and nitrite solution. 
Catalytic agents such as thiocyanate or thiourea may be used to accelerate 
the decomposition of a nitrite gassing agent. Suitable small hollow 
particles include small hollow microspheres of glass or resinous 
materials, such as phenol-formaldehyde, urea-formaldehyde and copolymers 
of vinylidene chloride and acrylonitrile. Suitable porous materials 
include expanded minerals such as perlite, and expanded polymers such as 
polystyrene. 
The Applicants have recently shown that gas bubbles may also be added to 
the emulsion as a preformed foam of air, CO.sub.2, N.sub.2 or N.sub.2 O in 
liquid, preferably an oil phase. 
The emulsion explosives of the present invention are, preferably, made by 
preparing a first premix of water and inorganic oxidizer salt and a second 
premix of fuel/oil and a mixture of the surfactant and co-surfactant in 
accordance with the present invention. The aqueous premix is heated to 
ensure dissolution of the salts and the fuel premix is heated as may be 
necessary to provide liquidity. The premixes are blended together and 
emulsified. Common emulsification methods use a mechanical blade mixer, 
rotating drum mixer, or a passage through an in-line static mixer. 
Thereafters the property modifying materials such as, for example, glass 
microspheres, may be added along with any auxiliary fuel, e.g. aluminium 
particles, or any desired particulate ammonium nitrate. 
Accordingly, in a further aspect, the present invention provides a method 
of manufacturing an emulsion explosive comprising emulsifying an oxidizer 
salt phase into an emulsifier/fuel mixture, whereins said emulsifier is a 
mixture of surfactants which has an interaction parameter (.beta.) with a 
value less than zero, preferably -2 or lower. 
In a further aspect, the present invention also provides a method of 
blasting comprising placing a emulsion explosive as described hereinabove, 
in operative contact with an initiating system including a detonator, and 
initiating said detonator and thereby said emulsion explosive.

EXAMPLES 
Various surfactants and blends of pairs of those surfactants were prepared 
as follows: 
Surfactant I 
A mixture of 40 parts of mineral oil and 60 parts of a polyisobutylene 
succinic anhydride (having an average molecular weight 1000, HPSEC), and 
6.5 parts of a diethanolamine is heated to 80.degree. C. for an hour. The 
reaction mixture is then further diluted by adding 10 parts of mineral oil 
and thus it forms the 50% active diethanolamine derivative of 
polyisobutylene succinic anhydride. 
Surfactant II 
A mixture of 40 parts of mineral oil and 60 parts of a polyisobutylene 
succinic anhydride (having an average molecular weight of 1000) was heated 
to 50.degree. C. and then 4.1 parts of ethanolamine was added dropwise 
over a period of 30 minutes. The reaction mixture is then further diluted 
by adding 20 parts of mineral oil and then it forms the 50% active 
ethanolamine derivative of polyisobutylene succinic anhydride. 
Surfactant III 
A mixture of 20 parts of mineral oil and 80 parts of polyisobutylene 
succinic anhydride (having an average molecular weight 450, HPSEC,) is 
heated to 80.degree. C. and then 18 parts of diethanolamine is slowly 
added with continuous stirring over a period of one hour. Thus it forms 
the desired diethanolamine derivative of polyisobutylene succinic 
anhydride of molecular weight 450. 
Surfactant IV 
A diethanolamine derivative of polyisobutylene succinic anhydride of 
average molecular weight 700 is prepared in a similar way as surfactant 
III by reacting the polyisobutylene succinic anhydride (80 parts) with 12 
parts of diethanolamine amine. 
Surfactant V 
A mixture of 20 parts by weight of mineral oil and 80 parts by weight of 
polyisobutylene SA (average molecular weight of 450) is heated to 
60.degree. C. and 12 parts of ethanolamine is added dropwise to the 
mixture over a period of one hour. Thus it forms the desired ethanolamine 
derivative of polyisobutylene succinic anhydride of molecular weight 450. 
Surfactant VI 
The emulsifier is synthesized by following the method used for surfactant 
V. 7.5 parts of ethanolamine was added to polyisobutylene succinic 
anhydride of molecular weight 700 (80 parts) over a period of 1 hour. 
Surfactant VII 
A mixture of 40 parts by weight of mineral oil and 60 parts by weight of 
polyisobutylene succinic anhydride of average molecular weight 1000 is 
heated to 60.degree. C. Then 5.8 parts of diethanolamine is added followed 
by the addition of 1 part of triethanolamine. The reaction mixture is then 
further diluted by adding 20 parts mineral oil and heated at 80.degree. C. 
for an hour. 
Surfactant VIII 
A mixture of 80 parts of weight of polyisobutylene succinic anhydride (of 
average molecular weight 450) and 20 parts by weight of mineral oil was 
heated to 80.degree. C. Then 16.5 parts of diethanolamine are slowly added 
followed by the addition of 2 parts of triethanolamine over a period of 
one hour. 
Blend A 
A mixed emulsifier blend of the desired composition (an optimum mixing 
ratio that has been determined by interfacial tension measurements) was 
made by mixing 70.1 parts of surfactant 1, 18.7 parts of surfactant V and 
11.2 parts of mineral oil. Thus it forms 50% active mixed emulsifier 
blend. 
Blend B 
A mixed emulsifier blend at an optimum mixing ratio (determined by 
interfacial tension measurements) was made by mixing 70.1 parts of 
surfactant II, 18.7 parts of surfactant III and 11.2 parts of mineral oil. 
Thus it forms 50% active mixed emulsifier blend. 
Blend C 
Another mixed emulsifier blend was made by mixing 70.1 parts of the 
surfactant VII, 18.7 parts of surfactant VIII and 11.2 parts of mineral 
oil. 
Blend D 
A mixed emulsifier blend was made by mixing 80 parts of surfactant 1, 12.5 
parts of surfactant VI and 7.5 parts of mineral oil. 
Blend E 
A mixed emulsifier blend was made by mixing 80 parts of surfactant II, 12.5 
parts of surfactant IV and 7.5 parts of mineral oil. 
Blend F 
A mixed emulsifier blend was made by mixing 70.1 parts of surfactant I, 
18.7 parts of surfactant III and 7.5 parts of mineral oil. 
The molecular interaction parameters of various mixed surfactant systems 
have been measured and the relevant data are given in Table II. 
TABLE II 
__________________________________________________________________________ 
Surfactant Blend 
C.sub.1.sup.0 .times. 10.sup.4 
C.sub.2.sup.0 .times. 10.sup.4 
C.sub.12 .times. 10.sup.4 
.alpha. 
X.sub.1 
.beta. 
__________________________________________________________________________ 
Surfactant V + 
7.50 9.90 4.07 0.48 
0.52 
-3.00 
Surfactant I 
Surfactant III + 
6.50 9.00 4.60 0.48 
0.53 
-2.00 
Surfactant II 
Surfactant VI + 
5.00 5.20 3.60 0.32 
0.40 
-1.50 
Surfactant I 
Surfactant IV + 
4.50 5.50 3.60 0.23 
0.37 
-0.64 
Surfactant II 
Surfactant II + 
2.50 16.50 
4.48 0.48 
0.86 
0.01 
Surfactant I 
Surfactant V + 
2.50 6.80 4.06 0.48 
0.76 
0.44 
Surfactant II 
Surfactant IV + 
3.00 3.10 4.50 0.40 
0.20 
1.70 
Surfactant I 
Surfactant VI + 
3.00 3.40 4.50 0.30 
0.10 
0.86 
Surfactant II 
Sorbitan Mono-oleate + 
2.00 8.60 3.00 0.40 
0.87 
3.96 
Surfactant I 
__________________________________________________________________________ 
The molecular interaction parameters evaluated using Equations I and II are 
used to predict whether synergism or antagonism will occur when two 
surfactants are mixed and, if so, the molar ratio of the two surfactants 
at which maximum synergism or antagonism will exist. A negative value 
indicates an attractive interaction between the two surfactants a positive 
value indicates a repulsive interaction. The larger the value of .beta., 
the stronger the interaction between the surfactants. A value close to 
zero indicates no interaction. 
For the mixed surfactant systems of positive .beta. values the X.sub.1 
(mole fraction of one of the mixed surfactants present at the interface) 
values indicate that either of the two components is predominantly 
absorbed at the interface. This indicates demixing of the two surfactant 
components at the interface. In that event, the interface in which two 
components are immiscible will constitute two separate domains of single 
surfactants. Such non-homogeneity at the interface causes instability. 
The following examples are illustrative of both capsensitive packaged and 
cap-insensitive bulk explosive emulsions within the scope of invention. 
Example 1 
The following formulations (1a and 1b) of packaged emulsion explosives are 
compared where 1a represents the formulation based on a mixed emulsifier 
system of positive .beta. value, and 1b represents the formulation based 
on the mixed surfactant systems of this invention where .beta. value is 
negative. In the following table all numerical values are given in parts 
by weight. 
TABLE 1 
______________________________________ 
1a 1b 
______________________________________ 
Ammonium Nitrate 68.95 68.95 
Water 10.75 10.75 
Sodium Nitrate 9.85 9.85 
Polywax 0.57 0.57 
Microcrystalline Wax 
0.28 0.28 
Surfactant 1 1.88 -- 
Blend A -- 2.82 
Sorbitan Mono Oleate 
0.47 -- 
Paraffin Oil 2.25 1.78 
Glass Microballoons 
5.00 5.00 
______________________________________ 
The properties of the formulation 1a and 1b are compared from the data 
given in the following Table 2. 
TABLE 2 
______________________________________ 
1a 1b 
______________________________________ 
Average droplet size (micron) 
2.1 1.8 
Storage stability at room temp. (week) 
50 &gt;50 
Storage stability at 50.degree. C. (weeks) 
25 &gt;35 
Specific conductivity (pmho/m) at 
30.degree. C. 396 47 
40.degree. C. 908 111 
50.degree. C. 990 339 
60.degree. C. 1338 1036 
70.degree. C. 2075 1413 
Minimum initiator (cartridge diam. 25 mm) 
R-5 R-4 
Velocity of detonation (m/sec) 
4320 4472 
Gap sensitivity (cm) 5.5 7.5 
______________________________________ 
Although the formulations are inherently stabled, the differences in the 
longer term storage stability and in the explosives properties are readily 
noticeable. The trend in the conductivity results is also indicative of 
the improved stability of emulsion of formulation 1b based on the mixed 
emulsifiers of present invention. The lower conductivity, the higher the 
inherent storage stability. 
Example 2 
The following formulations (2a and 2b) of cap-sensitive packaged emulsion 
explosives are compared with regard to their storage stability and 
explosives properties. 2a comprises a single emulsifier system of 
surfactant II whereas 2b comprises the mixed emulsifier system of Blend A. 
Compositions are shown in Table 3 and the properties are given in Table 4. 
TABLE 3 
______________________________________ 
2a 2b 
______________________________________ 
Ammonium Nitrate 72.65 72.65 
Sodium perchlorate 8.12 8.12 
Water 9.48 9.48 
Paraffin wax 0.69 0.69 
Microcrystalline Wax 
1.06 1.06 
Surfactant II 3.00 -- 
Blend A -- 3.00 
Glass Microballoons 
5.00 5.00 
______________________________________ 
TABLE 4 
______________________________________ 
2a 2b 
______________________________________ 
Average droplet size (micron) 
2.8 2.2 
Storage stability at room temp. (week) 
35 &gt;43 
Storage stability at 50.degree. C. (weeks) 
7 &gt;10 
Specific conductivity (pmho/m) at 
30.degree. C. 122 11 
40.degree. C. 209 22 
50.degree. C. 350 140 
60.degree. C. 866 364 
70.degree. C. 1410 800 
Minimum initiator (cartridge diam. 25 mm) 
R-5 R-5 
Velocity of detonation (m/sec) 
4700 4700 
Gap sensitivity (cm) 7.0 9.5 
______________________________________ 
In this example the trend in the conductivity results, storage stability 
data and gap sensitivity data reveal the superior performance of mixed 
emulsifiers of Blend A (where the interaction parameter .beta. is 
negative) of the present invention. 
Example 3 
This example illustrates the comparison of properties of two emulsion 
explosives formulations based on the mixed surfactant systems of the 
present invention. One of the formulations is based on the mixed 
surfactant system Blend A whose interaction parameter .beta. is negative 
and the other one is based on the mixed surfactants Blend F whose 
interaction parameter is zero. The formulations are given in Table 5 and 
the properties are compared in Table 6. 
TABLE 5 
______________________________________ 
3a 3b 
______________________________________ 
Ammonium Nitrate 78.7 78.7 
Water 16.0 16.0 
Mineral Oil 2.3 2.3 
Blend A -- 3.0 
Blend F 3.0 -- 
______________________________________ 
TABLE 6 
______________________________________ 
3a 3b 
______________________________________ 
Droplet size (micron) 2.38 2.58 
Storage stability at room temp. (week) 
&lt;6 &gt;20 
Membrane conductivity (milli-mhos/m.sup.2) 
35.3 0.072 
Membrane thickness (nm) 
5.76 8.26 
______________________________________ 
The membrane conductivity and membrane thickness are measured from the 
emulsion conductivity and dielectric spectra of emulsions. The increased 
stability results if the membrane separating the droplets is thick but 
more particularly if it has an optimised molecular order. The mixed 
surfactants Blend A produce emulsions of very low membrane conductance 
suggesting good emulsion stability. 
Example 4 
The following formulations (4a, 4b, 4c and 4d) of solid fuel doped emulsion 
explosives are compared where 4a represents the formulation based on a 
mixed emulsifier system of positive .beta. value, and 4b-4d are based on 
the mixed emulsifier systems of this invention where .beta. values are 
negative. Formulations are given in Table 7 in parts by weight and 
properties are compared in Table 8. 
TABLE 7 
______________________________________ 
4a 4b 4c 4d 
______________________________________ 
Ammonium Nitrate 
75.60 74.60 74.60 74.60 
Water 15.20 15.20 15.20 15.20 
Thiourea 0.05 0.05 0.05 0.05 
Acetic Acid 0.04 0.04 0.04 0.04 
Sodium acetate 
0.08 0.08 0.08 0.08 
Surfactant II 
2.00 -- -- -- 
Sorbitan mono oleate 
0.50 -- -- -- 
Blend A -- 2.50 -- -- 
Blend B -- -- 2.50 -- 
Blend C -- -- -- 2.50 
Paraffin oil 2.47 2.47 2.47 2.47 
Ferro silicon 
5.00 5.00 5.00 5.00 
______________________________________ 
These emulsions are optionally gassed using 0.06 parts equivalent of sodium 
nitrite either in the form of aqueous solution or in the form of 
water-in-oil type microemulsion added to the premade emulsions of the 
above formulations. 
TABLE 8 
______________________________________ 
4a 4b 4c 4d 
______________________________________ 
Average droplet size (.mu.) 
2.2 1.85 2.0 1.8 
Storage stability at room temp. 
&lt;10 &gt;30 &gt;30 &gt;35 
(weeks) 
Storage stability at 50.degree. C. 
&lt;2 &gt;4 &gt;4 &gt;4 
______________________________________ 
Example 5 
In the following examples stability of the emulsion formulations (Table 9 
and 10) doped with solid ammonium nitrate prills are compared. 
TABLE 9 
______________________________________ 
5a 5b 
______________________________________ 
Ammonium Nitrate 49.35 49.35 
Water 10.08 10.08 
Thiourea 0.03 0.03 
Acetic Acid 0.03 0.03 
Sodium Acetate 0.05 0.05 
Surfactant II 1.30 -- 
Sorbitan Mono Oleate 0.33 -- 
Blend B -- 1.95 
Paraffin Oil 3.83 3.83 
Solid ammonium nitrate prills 
35.00 35.00 
______________________________________ 
The above formulations can be optionally gassed by using aqueous solutions 
of sodium nitrate or water-in-oil microemulsions of aqueous sodium nitrite 
solutions. 
TABLE 10 
______________________________________ 
5a 5b 
______________________________________ 
Average emulsion droplet size (micron) 
2.2 2.0 
Storage stability at room temp. (week) 
4 &gt;8 
Storage stability at 50.degree. C. (weeks) 
&lt;2 &gt;2 
______________________________________ 
Example 6 
In the following examples stability of the bulk repumpable emulsion 
formulations (Table 11 and 12) doped with solid chloride is compared. The 
results show a remarkable improvement in storage stability by using the 
mixed surfactant systems of the present invention having a negative .beta. 
parameter. 
TABLE 11 
______________________________________ 
6a 6b 6c 
______________________________________ 
Ammonium nitrate 57.77 57.77 57.77 
Calcium nitrate 14.00 14.00 14.00 
Water 16.34 16.24 16.24 
Thiourea 0.40 0.40 0.40 
Acetic acid 0.03 0.03 0.03 
Sodium acetate 0.06 0.06 0.06 
Sorbitan mono oleate 
0.50 -- -- 
Emulsifier of Example II 
2.00 -- -- 
Mixed emulsifiers of Example 2 
-- 3.00 -- 
Mixed emulsifiers of Example 3 
-- -- 3.00 
Paraffin oil 4.00 3.50 3.50 
Sodium chloride 5.00 5.00 5.00 
______________________________________ 
TABLE 12 
______________________________________ 
6a 6b 6c 
______________________________________ 
Average droplet size (micron) 
2.1 1.90 1.85 
Storage stability at room temp 
3 &gt;25 &gt;25 
(weeks) 
Storage stability at 50.degree. C. 
&lt;1 &gt;2 &gt;2 
(weeks) 
______________________________________