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AQUEOUS FOAM - TECHNOLOGY & SYSTEMS DEVELOPMENT AQUEOUS FOAM -
TECHNOLOGY & SYSTEMS DEVELOPMENT
Paul A. Kittle, PhD
President
Aquafoam, Inc.
INTRODUCTION
Foam production systems are similar in that they all include a to-be-foamed liquid
phase, an expansion gas, and equipment designed to combine, mix, and
discharge the foam product.
Foams produced from these similar systems are not similar. The most obvious
difference among compressed gas foams is persistence, or lifetime, generally
defined as drain time the time required for the foam to decompose into the
original liquid and gas phases. The chemical composition does affect the drain
time.
Foam barriers can affect both heat and mass transfer utilizing only the foam
structure bubbles, which are membranes separating small gas volumes.
Foam can be utilized as a delivery system for additional ingredients, which can
produce a post-foamed medium.
Commercially successful foam systems are optimized with respect to each option
liquid composition, expansion gas, persistence, delivery equipment, and final
foam performance [1].
COMPONENTS
Liquid Phase
The most important physical characteristic of the liquid phase is surface tension,
particularly in aqueous systems, as the surface tension of pure water is 72
dynes/cm, a value too high for foaming to occur. The surface tension needs to be
reduced for foaming to occur. In general, a surfactant, added to the water in small
amounts, 2000-ppm actives, for instance, will reduce the surface tension below 30
dynes/cm a more than adequate value. Increasing the temperature can also
reduce surface tension of water [2].
Expansion Gas
The most common expansion gas in aqueous foam systems is compressed air, an
example of an insoluble expansion gas. Soluble expansion gas systems are also
possible, using low molecular weight hydrocarbons, nitric oxide, or carbon dioxide,
for instance. The dissolution of these soluble expansion gases can also contribute
to the reduction of the surface tension of the liquid system [3]. Foam Generating Equipment
There are only two important components in a compressed gas foam generating
system: (1) a device for combining the liquid phase with the non-soluble expansion
gas; and, (2) an arrangement allowing for the thorough mixing of the liquid phase
and the expansion gas.
The combining device can take many forms, but all of them are some combination
of flow control devices accommodating the flow rates and pressures of the two
components [4].
Often the thorough mixing procedure is neglected or overlooked in the foam
equipment design, probably because the real time for thorough mixing is not
recognized, appreciated, or understood the thorough mixing does not occur
instantly, so the combined liquid and expansion gas must not be discharged too
soon, or the foam quality will be less than optimum reduced drain time and/or
expansion ratio performance, for instance.
After the liquid phase and the expansion gas are combined, there are basically two
procedures to satisfy this mixing time objective: (1) pass the combination through
a sufficiently long hose or pipe, suitably sized for the flow rate, allowing the modest
turbulence to achieve complete mixing [5]; or, (2) pass the mixture through a
shorter, larger diameter, packed bed, where the residence time is reduced, but the
degree of turbulence is greatly increased [6].
Aqueous foams are metastable systems and begin draining as soon as the
formation step has been completed [1]. This draining action immediately produces
a liquid phase, which is the same composition as the original to-be-foamed liquid.
Once complete mixing has been achieved, further mixing will not affect the foam.
positively or negatively.
The balance of the equipment is "support and supply" to the above items tanks,
pumps, compressors, power supply (diesel, gasoline, electric), hoses, fittings, and
undercarriage (fixed/mounted, skid, wheels, tracks [6(a), 7]).
EXAMPLES OF FOAM SYSTEMS
Carbonated Beverages
The simplest foaming systems are carbonated beverages. The to-be-foamed
liquid phase is essentially water and the expansion gas is carbon dioxide dissolved
in the liquid under modest pressure. The carbon dioxide contributes to the surface
tension reduction [3], thereby allowing foaming to occur. The equipment is "one
time use" and disposable, simply a can or bottle capable of holding the expansion
gas pressure. On opening the container, the solubility of the expansion gas is reduced and foaming occurs. Variables affecting the result include: (1) amount of
dissolved gas; (2) temperature; (3) agitation; and, (4) chemical composition [3, 8].
Marshmallows
Marshmallows are an example of a composition generating a post-foamed
reaction. They are prepared from a mixture of gelatin, sugar, and water, which is
warmed (reducing the surface tension [2]), injected with air for foaming, then
cooled. The cooling allows the gelatin to "gel" thereby increasing the viscosity of
the material and vastly increasing the drain time. Common experience defines
that heating marshmallows allows the gelatin to "un-gel" thereby creating a sticky,
somewhat viscous, flowable liquid. Cooling this liquid will not reproduce a
marshmallow, but will yield a gelled mass, as the foam has drained, leaving only
the liquid as a residue.
Water could be considered a special case of this example. If the surface tension
of water is reduced via the addition of a surfactant, for instance, and foaming
follows, the foam produced will start draining immediately, and completely drain
within about 30 minutes. However, if the foam is frozen, drainage will essentially
stop until the temperature has been increased above the freezing point.
Baked Goods
Baked goods - bread, cakes, etc. - are similar to marshmallows, except that the
foaming is thermally induced, during cooking. The expansion gas source is
included in the pre-cooked formulation and the heat causes the carbon dioxide
source to decompose, producing gas and generating foam. The heating process
"cross links" the foamed product, increasing the viscosity, increasing the drain
time, and thereby producing foam, which is essentially permanent. These bakery
products cannot be reversed, as they have undergone a chemical reaction, as
compared to marshmallows and ice/water, which have only changed physically.
These concepts are the basis for most (non-aqueous) foamed materials produced
from polymers, including polystyrene, polyethylene, urethanes, foamed concrete,
and gypsum board.
Fire Fighting Foam
Fire fighting foam is a particularly interesting material as it exhibits an array of
foam attributes [9]. The objective of fire fighting foam is to separate the fuel
source from the oxygen source, thereby interrupting the combustion process and
extinguishing the fire. Minimizing the heat transfer from the fire zone to the fuel
zone is also important. For all practical purposes any nonflammable foam
composition could be used for fire extinguishing, but the differences between the
"good ones" and the "others" is efficiency the best fire fighting foams extinguish
faster and with less material. All of these fire-fighting foams utilize reduced surface tension water as the liquid
phase and air as the expansion gas, regardless of whether the foam is
compressed air foam or air aspirated foam. The better fire fighting foams have the
liquid phase modified to improve efficiency better heat resistance, for instance,
among other features.
Even though the fire fighting foam is produced with air, as opposed to nitrogen, the
extinguishing procedure works well, contrary to intuition. The foam barrier
completely stops migration of the fuel to the air source as long as the barrier is
continuous.
This mass transport characteristic, exhibited by all foam systems, has been
exploited in the environmental field. A foam barrier (blanket) can be placed on a
substrate solid or liquid thereby stopping the mass transport of material on
either side of the barrier to the opposite side. Proper modification of the foam
composition allows this technology to be widely applied in many varied
applications landfill daily cover, volatile organic compound (VOC) emissions,
industrial site remediations, and other hazardous waste applications [10].
USEFUL FOAM PROPERTIES
Volume
A cubic foot of water, 62.5 pounds, will provide a cubic foot of "result" volume, but
as foam the "result" volume may be 10 to 50 cubic feet, depending on the
expansion ratio of the foaming system.
Flow Properties
All foams are thixotropic - the viscosity is shear dependent therefore, without a
shear force being applied, foam will not flow. This characteristic allows foam to be
stacked or piled, and flowing will only occur as the shear force increases with foam
depth. Altering the physical properties of the foam, mainly viscosity, can allow
foam to be piled in thick sections more than several feet, if desired.
Controlled Release Medium
Since the drain time of foam can be "designed" and the foam can be accumulated
in stacks, piles, walls, or barriers, components in the liquid phase can be
deposited in a fixed location for a time period dependent upon the drain time
property of the foam.
In a similar manner, the expansion gas used to prepare the foam is "stored" in the
foam structure. The release rate is equivalent to t