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Description  |
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This invention relates to production of novel high strength and low density
construction materials, particularly designed for formation into panels
and other structural shapes for general construction purposes, and is
particularly concerned with production of a polymer-inorganic hybrid foam
product or construction material, which in addition to being strong and
lightweight, is load supporting, fire resistant, and thermally insulating,
and having particular utility for the construction of wall, floor and roof
panels, both for commercial and residential construction.
There has developed in recent years particularly for use in residential
construction as well as in commercial building construction a need for new
construction materials especially suitable for use in the form of large
panels for wall, floor and roof construction, which have good strength
characteristics, comparable to panels formed from cement or gypsum, but
which are of much lower density so as to permit, for example,
prefabrication of building constructions at a factory site, and
transporting of such prefabricated structures to the building site. In
addition, the construction materials or panels are required to have good
thermal insulation and high fire resistance, and particularly such
construction materials are required to be produced at reduced costs
compared to comparable dense cement and gypsum panels, with their high
labor demand, presently employed.
Sefton U.S. Pat. No. 3,272,765 is directed to a lightweight concrete
comprising closed cell expanded polymer particles such as expanded
polystyrene beads, a binder of hydraulic cement and a surface-active
additive such as alkyl aryl sulfonates, such surface-active additive being
present in the final product. The patentee also incorporates
air-entraining synergist in the form of certain hydrocarbons to augment
the air-entraining action of the surface-active additive. The concrete
composition of this patent has a substantially continuous phase of aerated
concrete with an aggregate of expanded polystyrene beads dispersed in the
concrete phase. The resulting concrete although of relatively low density,
has a relatively low strength, for example a compressive strength of the
order of about 400 psi, and is only capable of supporting light loads.
Further, the fire resistance of the concrete composition of the above
patent is not considered satisfactory, since when such composition is
subjected to elevated temperature, e.g. of the order of about 250.degree.
F., distortion and contraction of the polystyrene beads occur,
particularly in the exposed surface of the concrete, and when subjected to
fire exposure conditions at flame temperatures the exposed expanded
plastic, e.g. polystyrene, beads in the surface of the concrete matrix
ignite and burn with a very smoky flame. In addition, such concrete
composition lacks high thermal resistance. The concrete composition of the
above patent also has an undesirable heterogeneous surface texture,
particularly where the expanded plastic, e.g. polystyrene, beads are
exposed.
However, according to the present invention, there is produced a
polymer-inorganic hybrid foam which is lightweight, yet which has
substantially greater compressive strength than that of the above patent
composition, and is load supporting and can be used as a construction
material for production of wall, floor or roof panels in construction of
buildings and homes. In addition, the composition of the present invention
is highly fire resistant, has high thermal insulation characteristics and
has smooth surface characteristics, that is, it is self-skinning.
The polymer-inorganic hybrid foam of the invention consists essentially of
a plastic phase in the form of a matrix or backbone of a suitable polymer
as hereinafter defined, e.g. polyvinyl acetate, vinyl-acrylic copolymer,
or an asphalt or bituminous pitch, and an inorganic phase formed of
particles thereof, e.g. Portland cement or gypsum, and which is
substantially free of sand, rock or aggregate, substantially uniformly
distributed through the continuous plastic or polymer phase, the cells of
the resulting polymer-inorganic cellular hybrid foam, formed essentially
from the plastic matrix or phase, being highly uniform, with the particles
of the inorganic phase substantially uniformly distributed throughout such
cells.
In contrast to the concrete composition of the above patent, the
polymer-inorganic hybrid foam of the present invention does not employ
expanded polymer particles such as polystyrene beads, but rather utilizes
unexpanded polymer or resin particles. These polymer or resin particles
are initially in emulsion form, but coalesce to form a continuous phase
forming the cell walls in the final hybrid foam product, the particles of
the inorganic phase being distributed in such cell walls. In addition, the
present invention does not require addition of a surface-active additive
as an essential component as in the above patent composition. Further, the
cellular hybrid foam of the present invention is produced by a process
described hereinafter, which is distinct from that of the above patent.
The result is the production, in accordance with the present invention, of
a polymer-inorganic cellular hybrid foam of low density, but which has
substantially higher compressive strength than the concrete composition of
the above patent, and is load supporting, and having high fire resistance,
good thermal insulating characteristics and low thermal expansion,
superior to the concrete composition of the above patent.
The improved polymer-inorganic hybrid foam composition of the invention is
produced by a process which comprises providing an aqueous resin emulsion
capable of being air whipped, whipping air into the resin emulsion and
establishing a pre-foam of polymer or resin emulsion bubbles capable of
supporting particles of an inorganic phase in the surface of said bubbles,
adding an inorganic phase selected from the group consisting of cement and
gypsum, in the form of particles, gradually to said air whipped resin
emulsion and suspending said inorganic particles in said pre-foam, that is
in the walls of said bubbles, without precipitating out said particles,
and hardening said inorganic phase to set the resulting hybrid foam of
inorganic particles and resin.
According to the process for producing the hybrid foam of the present
invention, air is whipped into the resin emulsion prior to addition of any
cement thereto. The cement or gypsum particles then gradually added to the
whipped foam tend to deposit and remain in the bubble walls of the resin
foam which enhances the stabilization of the foam. This is necessary in
order to obtain the strong lightweight homogeneous foam product of the
invention, and wherein the plastic bubbles form the cells of the final
hybrid foam composition, with the cement or gypsum particles uniformly
distributed therein.
A further feature of the present invention is that when employing cement
according to this invention, only cement powder is used, and no sand is
present, such sand being present in cement grout, and also no rock or
aggregate is incorporated in the cement employed in making the foam.
A still further feature of the invention is that fiberglass roving can be
introduced into the polymer-cement or polymer-gypsum hybrid foams of the
invention, to reinforce such hybrid foams, and such reinforced foams can
be stored over long periods of time without any evidence of fiber
degradation. This permits the use of such fiberglass roving reinforcement
having lightweight reinforcing characteristics in the invention foam.
As previously noted, a basic feature of the process is the establishment of
the polymer-emulsion system into a pre-foam, preferably of known or
predetermined composition and density, such foam consisting of
thick-walled resin emulsion bubbles capable of supporting large amounts of
the inorganic phase, e.g., Portland cement or gypsum powders, which are
subsequently vibrated onto the surface of the foam and thoroughly mixed
therewith.
To achieve such pre-foam, air is whipped into any type of resinous emulsion
which is capable of being air whipped, and which is capable when so air
whipped to maintain the inorganic phase particles in suspension therein.
By the term "whipping air" or "air whipped" as employed herein is meant
rapidly agitating the resin emulsion in the presence of air or while
forcing or bubbling air into the mixture. Thus, the aqueous resin emulsion
can be agitated in a tank open to the ambient atmosphere, or the resin
emulsion can be agitated, with compressed air simultaneously forced or
bubbled through the resin emulsion, e.g., as in a high shear single pass
continuous mixer.
Of a large number of synthetic resin emulsions tested, it has been found
that polyvinyl acetate emulsions are particularly suitable. Other aqueous
emulsion polymer systems found satisfactory are vinyl-acrylic copolymer
such as vinyl acetate-acrylic and vinyl chloride-acrylic copolymers,
acrylic homopolymer, such as acrylonitrile polymer, vinylidene chloride
polymer, butadiene copolymers and homopolymers such as
butadiene-acrylonitrile-styrene, butadiene-styrene and polybutadiene, and
silicone resins. Also, mixtures of the above polymers or resins in aqueous
emulsions also can be employed, such as an aqueous emulsion of a mixture
of about 75% vinyl acetate and about 25% vinylidene chloride based on the
polymer phase. These polymers or resins are present in the aqueous
emulsions usually in the fully polymerized state. However, where such
resins are in the partially polymerized or B-stage, curing agents,
polymerization initiators, and the like, well known in the art, can be
added just prior to use, to effect final cure of the resin during the
subsequent setting of the inorganic phase, e.g., cement.
Although the above exemplary resin emulsions are thermoplastic resin
emulsions, thermosetting resin emulsions such as phenolic, e.g.,
phenol-formaldehyde, urea, e.g., urea-formaldehyde, and melamine, e.g.,
melamine-formaldehyde resin emulsions can be employed. Also, thermoplastic
resins which are cross-linked in situ by x-ray or radioactive radiation,
or thermally with peroxides added, can be employed.
It has also been found that relatively inexpensive natural polymers such as
asphalt and bituminous pitch, the latter including coal tar and petroleum
pitch, in emulsion form, can be effectively employed alternatively to the
above-noted synthetic polymer emulsions. The resultant hybrid foams
produced employing these naturally occurring polymers have properties
comparable to those possessed by the hybrid foams obtained employing
synthetic polymer emulsions.
Additionally, auxiliary components can also be incorporated into the
aqueous resin emulsion to enhance the properties thereof, for example,
viscosity stabilizers, such as xanthate gums, accelerators such as
triethanolamine, thickeners such as methyl cellulose, carboxy methyl
cellulose, and the like. Where one of the preferred resin emulsions,
aqueous polyvinyl acetate emulsion, is employed, polyvinyl alcohol in
small amounts, e.g., about 5 to 10 percent by weight of the emulsion, can
be added to aid in maintaining the polyvinyl acetate in suspension and to
enhance the bonding strength of the resin and to confer on the resulting
hybrid foam construction superior toughness, particularly in the case of
the production of a polymer-inorganic hybrid cement foam according to the
invention, without adversely affecting any other advantageous properties
of such foam.
Although commercially available aqueous resin emulsions of the types noted
above may contain small amounts of surface active agents, it is preferred
for reasons noted hereinafter to avoid the presence of surface-active
agents in the process for producing the hybrid foam hereof. This is
contrary to the patent above, wherein a surface-active additive is
incorporated as an essential component of the concrete composition of the
patent. Specific examples of resin emulsions which are essentially free of
surface-active agents or emulsifiers are the acrylonitrile, polybutadiene,
butadiene-acrylonitrile-styrene and butadiene-styrene resin emulsions
marketed as "Hycar" emulsions by B. F. Goodrich Company.
The aqueous resin emulsion should contain sufficient water to permit the
emulsion to be air whipped in a short period of time into a foam which can
be maintained stabilized during the subsequent addition of the inorganic
phase particles, the pour time and the initial setting time. For this
purpose, for example, the aqueous resin emulsion can contain from about 10
to about 60 percent resin solids by weight, the remainder being water.
When desired, water can be added initially to a commercially available
resin emulsion to achieve desired dilution of resin or polymer in the
aqueous emulsion.
The aqueous resin emulsion is air whipped to establish a pre-foam which
preferably has a predetermined or certain volumetric expansion in relation
to the volume of the original emulsion. Thus, the initial aqueous resin
emulsion can be air whipped from about 1 to about 10 times, usually about
2 to 6 times, its initial volume. Thus, for example, an initial aqueous
resin emulsion which has a density of 8.4 pounds/ft.sup.3 and a viscosity
of 10 cps (centipoises) at 25.degree. C. can be whipped to an air
entrained foam having a density of about 1 to about 2 pounds/ft.sup.3, in
from about 2 to about 10 minutes. This can be accomplished in a device as
simple as a mixmaster equipped with standard beaters or wire whips.
When the initial aqueous resin emulsion has been air whipped to produce the
desired volumetric expansion of foam, the inorganic phase in the form of
particles of, e.g., Portland cement, gypsum, or a mixture thereof, and
which is substantially free of sand, rock or aggregate, is gradually added
to the aqueous polymer emulsion pre-foam under conditions to maintain the
foam stable and without reducing the hybrid foam volume inordinately below
the volume of the pre-foam. Ideally the rate of addition of the cement or
gypsum to the air whipped pre-foam is such as to maintain the foam volume
substantially constant. The preferred flow stream of the inorganic
particulate matter is in the form of a thin film, e.g. fed from the edge
of a vibrating hopper. Cement particles can be employed in admixture with
a minor amount of gypsum, e.g., about 1 to about 25% of the total
inorganic, by weight.
If the inorganic particles are added to the pre-foam too rapidly or in too
thick a stream, e.g., in the form of lumps, then particles of the
inorganic phase tend to precipitate or form on the bottom of the vessel.
Of particular significance in the present invention, no gravel or sand is
employed. Usually it requires only a few minutes, e.g., about 1 to 2
minutes, to add the cement to the pre-foam. Addition of the inorganic
phase particles, e.g., cement or gypsum powder, or a combination thereof,
gradually to the air whipped pre-foam as noted above, forms a thin film of
such inorganic particles in and/or on the surface of the foam bubbles or
cells, and at the intersections between the bubbles, but not in the center
of the bubbles, to form a lightweight material. The surface tension and
film thickness and strength characteristics of the polymer emulsion when
air whipped to bubble form are important factors in the ability of the
bubble or cell wall to support a large amount of inorganic powder loading.
It has been found from experience that the presence or addition of wetting
agents generally decreases the surface tension of the emulsion polymer
cell wall, with consequent thinning of the cell wall, and while the
initial foam head may be higher upon whipping with air into foam, the
resulting foam height and stability of the foam upon addition of the
inorganic phase to the system generally is reduced. Further, for example
ammonia or simple amines tend to inhibit the crystalization of gypsum
systems, causing slow setting, and hence should be avoided in such
systems. The gypsum or cement powders appear to act as protective colloids
which enhance the stabilization of the bubble structure when suspended
within the bubble wall.
It has been found that if the inorganic phase, e.g., cement or gypsum, is
added to a resin emulsion such as those noted above, which have not been
previously air whipped, the inorganic particles are not suspended in the
bubble walls according to the invention concept, but rather tend to
nucleate or agglomerate and migrate into the interior of the bubbles
formed on mixing following addition of the inorganic phase, and the
resultant foam construction following setting does not possess the low
density, cell uniformity and high strength characteristics of the hybrid
foam construction of the present invention.
It is of interest that most industrial aqueous emulsions are prepared so as
to minimize foaming during processing, which proves troublesome, e.g.,
when vats run over, etc. In some cases minute quantities of silicone
anti-foaming agents are added to control this foaming tendency during
mixing, pumping, etc. One of the features of this invention is the
discovery that nonetheless such emulsion can be successfully emulsified
with air by proper beating to form stable re-foams.
As previously noted, a relatively large amount of inorganic particles can
be suspended in the manner noted above in the aqueous resin emulsion.
Thus, for example, from about 300 to about 1,000 parts of inorganic
particles such as cement or gypsum powder can be supported in an aqueous
resin emulsion pre-foam of the types noted above, such as a polyvinyl
acetate, asphalt or bituminous pitch emulsion, containing about 50 to
about 100 parts of resin solids and about 400 to 700 parts of water, by
weight. In such aqueous resin emulsions it is noted that hot water may be
substituted for cold water, such hot water tending to accelerate the
subsequent set of the cement or gypsum. Likewise where extended set times
are required for setting, cold or refrigerated water can be used.
It has been found that when employing, for example, polyvinyl acetate
aqeuous emulsion, such emulsion, for example, will support from 400 to 800
grams cement or gypsum loading of an aqueous resin emulsion containing
only 50 parts resin solids and 400 parts water, with relatively little
decrease in initial volume in the 400 to 500-gram loading, and less than
50 percent decrease in overall emulsion volume occurs with an 800-gram
loading. The higher the inorganic phase loading, generally the greater the
density of the resulting foam. It has been found that some systems
actually increase in volume with the addition of the inorganic powder,
e.g., cement or gypsum, showing that the inorganic particulate matter is
truly suspended in the bubble cell walls and not congragating in the
interbubble interface or migrating into the center of the bubble, thus
increasing the volume of the system.
As a further feature it has been found that the addition of iron oxide (
Fe.sub.2 O.sub.3) to the inorganic or cement particles appears to improve
the surface hardness of the resulting hybrid foam compositions. Also, the
incorporation of such iron oxide into the inorganic particles colors the
hybrid foam composition to a brick-like color.
Further, it has been found that the addition of carbon, e.g. in the form of
carbon powder, either with or without iron oxide, to the inorganic or
cement particles, prior to incorporation thereof into the whipped aqueous
resin emulsion, results in a hybrid foam composition according to the
invention having exceptional fire resistance, and rendering such hybrid
foam composition suitable for fire resistant roofs, fire walls and fire
door cores.
If desired, the iron oxide and/or the carbon, in the form of carbon powder,
can be aded separately to the whipped aqueous resin emulsion, as well as
in admixture with the inorganic or cement particles. The amount of the
iron oxide incorporated into the air whipped aqueous emulsion containing
the inorganic phase, can range from about 1 to about 50 parts of iron
oxide to about 600 parts of inorganic phase or cement particles, by
weight, and the amount of carbon, e.g. in the form of carbon powder,
incorporated into such air whipped mixture also can range from about 1 to
about 50 parts, per 600 parts of inorganic phase or cement particles, by
weight.
The resulting pre-foam formed of aqueous resin emulsion having the
inorganic particles, and optionally iron oxide and/or carbon powder,
suspended therein preferably is poured into prepared molds and allowed to
set. The initial set time for the pre-foam should be sufficient to form a
solid mass which although not fully cured, is selfsupporting and which is
capable of being removed from the molds. Such initial set time is
generally of the order of 10 to about 60 minutes, usually about 20 to 30
minutes. A shorter set time for example of the order of about 10 minutes
can be sufficient when a continuous casting process is employed. Thus, the
process of the invention, involving rapid setting time is particularly
adapted to continuous casting of products, as for example between movable
belts.
Since it is generally well recognized that the way to retard the set of
gypsum or Portland cement is to add a small percentage of organics
thereto, it is particularly unexpected and surprising that for example, as
much as 100 parts of resin solids can be tolerated in only 600 parts of
gypsum or cement and still achieve an initial set according to the
invention of say only 10 to 30 minutes.
The molds for the initial setting of the hybrid foam containing the
inorganic particles can incorporate plastic or rubber liners. Following
the initial set as described above for a period such that the resulting
molded parts have a consistency so that such parts are handleable, the
parts re removed from the tool or mold, usually with the above-noted
plastic or rubber films attached to the parts, and such films can be
peeled off either immediately or after a subsequent additional or final
curing of the parts.
The period for additional or final setting of the hybrid foam can range,
for example, from as little as about 4 hours up to 30 days. If gypsum is
employed in the pre-foam, this additional period of setting is essentially
a drying operation to remove excess water. When using Portland cement, a
much slower hydration occurs which at room temperature may take a period
of days, e.g., about 15 to about 30 days.
For example, when employing Type V (low sulfate sea water proof) cement in
a polyvinyl acetate aqueous emulsion, this additional setting after
demolding can be accomplished according to the following schedule,
dependent upon the temperature of setting, with the following hardness
values obtained.
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.degree. F
Cure Condition
Set Time Rockwell "R" Hardness
______________________________________
75 Moist Air 5 days 0
75 Moist Air 28 days 60-70
160 Dry 17 hours 80-85
No Pressure Steam
200 Dry 6 hours 80-100
No Pressure Steam
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As previously noted, either cement, such as Portland cement, alone, or
gypsum alone, can be employed as the inorganic phase of the pre-foam resin
emulsion. Where gypsum alone is employed, the resulting structural foam
produced is not water-proof, whereas when employing Portland cement alone,
that is gypsum-free cement, the resulting hybrid foam construction is
water-proof and possesses a closed cell structure which is buoyant. In
some cases it is advantageous to employ gypsum in admixture with Portland
cement. Thus, the inorganic phase can be comprised of gypsum and about 0
to about 25% of Portland cement by weight of total inorganics. Where
cement with a small amount of gypsum is employed, such gypsum provides an
inhibiting action in setting of the inorganic phase system. The preferred
cement composition is one which is gypsum-free, permitting greately
accelerated initial sets even in the presence of substantial amounts of
organics, e.g., polyvinyl acetate.
As previously noted, the resulting polymer-inorganic hybrid foam produced
according to the invention process has substantially lower density than
corresponding cement structures produced according to the prior art, and
is of a ecllular nature yet has good strength and is load supporting, with
the additional advantages of being both thermally insulating and fire
resistant. Cement hybrid foam articles produced according to the invention
floated in water several months without sinking or apparent softening.
Gypsum hybrid foams, although softening in water, regain their entire
strength upon redrying. However, if a water-repellant gypsum hybrid foam
is desired, the outer surface of articles thus produced should be covered
with a suitable coating such as a silicone water repellant, and/or any
organic film forming protective coating. The proportions of inorganic
phase and polymer particles in the hybrid foam product are the same as
those noted above which are present in the aqueous pre-foam. The density
of the polymer or resin-inorganic hybrid foam produced herein can range
from about 10 to about 80 pounds/cu.ft., usually about 15 to about 45
pounds/cu.ft., compressive strength of form about 400 to about 2,000 psi
and Rockwell "R" hardness from about 80 to about 125, depending upon the
particular organic components employed, the setting schedule and
particularly the relative proportions of inorganic to organic components
utilized. Such foam is fire resistant and thermally insulating, with a "K
factor", e.g., ranging from about 0.15 to about 0.5. The relative
proportion or ratio of inorganic components in the final hybrid foam
construction can range from about 300 to about 1,000 parts of inorganic,
that is cement or gypsum constituents, to about 50 to about 100 parts of
resin solids, by weight.
Tapered density hybrid foam structures such as panels and beams, e.g., for
use in a cantilevered roof, can be made according to the invention by
varying the organic-inorganics ration, e.g., in a continuous process.
Although applicant does not know the nature of the reaction occurring or
the relation between the polymer and the inorganic phase, e.g., cement
particles, as result of the setting of the hybrid foam, it is believed
that a bonding action takes place between the polymer or resin and
inorganic phase which in conjunction with the incorporation of the
inorganic phase into an air whipped emulsion of the polymer or resin,
results in production of both a lightweight and relatively high strength
solid foam material, andthe inorganic particles are substantially
uniformly distributed throughout the plastic or polymer phase matrix of
the cellular foam structure, the cells of which are highly uniform.
It is noted that the hybrid foam product is not merely a suspension of
inorganic particulate matter in the polymer cell or bubble wall as a
filler, but rather it is believed that a reaction occurs forming
intersticial compounds or complexes between the inorganic phase, i.e.
Portland cement or gypsum, and the polymer emulsion phase. This is
demonstrated (1) by a sequestering or ion exchange which reduces the
alkalinity of the hybrid foam and (2) by the observation that the hybrid
foam adheres to or fails to separate from a large variety of commercial
polymer separators used in releasing the set foam from the mold. Such
hybrid foams even adhere to Teflon (polytetrafluoroethylene) and other
fluorinated films, which would ordinarily readily separate from the resin
phase. Also, the hybrid foam adheres to all concrete separators, which
would ordinarily separate from the concrete phase, indicating some
synergistic reaction between the polymer and inorganic phases.
The characteristics of typical polymer-inorganic hybrid foam constructions
produced according to the invention employing cement and gypsum,
respectively, are as follows:
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Properties Cement Gypsum
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Density (lbs/ft.sup.3)
43.31 37.32
Compressive Strength (lbs/in.sup.2)
997 792
Modulus 18.1 .times. 10.sup.5
7.5 .times. 10.sup.5
Flexural Strength (psi)
1440 1036.4
Resistance to Water
Excellent Poor
Chemical Resistance
Good Good
Insulating Property
(K Factor) BTU/in. .35 .28
Rockwell Hardness "R"
109 90
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The use of fiberglass fabrics and roving have in the past proved unsuitable
as reinforcement for concrete or gypsum. When employed in conventional
Portland or gypsum cement structures, within a few days following setting
of the cement, the fiberglass reinforcement loses its strength, as a
result of alkaline chemical attack, destroying the surface of the fiber
and hence its strength. A relatively heavy vinyl acetate finish on roving
has proved somewhat satisfactory in gypsum drywall products but
unsatisfactory for Portland cement. Thus, for example, due to this
alkalinity phenomenon, fiberglass roving cannot be incorporated
successfully into the concrete of the above Sefton patent.
An additional feature, according to the invention, is that fiberglass
roving can be incorporated or introduced into the polymer-Portland cement
or polymer-gypsum hybrid foams according to the invention, to form
reinforced hybrid foams having excellent strength between both types of
hybrid foams and the fiberglass roving, without evidence of degradation of
the fiberglass over extended periods when cured at room temperature. There
is believed to be an immunizing or sequestering effect of the resin
emulsion-cement or gypsum foam composition of the invention against the
alkaline nature of both gypsum and Portland cement. This avoids the
presence of active alkali in the composition when the composition is
poured over the roving in the mold. Although such mechanism of
immunization of the hybrid foam composition against the destructive
chemical effects of gypsum or Portland cement on fiber glass is not
clearly understood, its effect is, however, clearly apparent. Some samples
of fiberglass roving reinforced hybrid foams have been stored for periods
of months with no evidence of decrease in strength in the foam casting or
its fiberglass reinforcement. The present development accordingly permits
the use of fiberglass roving reinforcement rather than welded steel mesh,
with a specific gravity ratio of 2.6 for the fiberglass roving as against
6.5 for the steel mesh. An additional advantage is that the terminal
projections of the fiberglass roving outside the perimeter of a wall, roof
or floor structure composed | | |
